"Diagnosis" means finding the cause of a disorder, not just giving it a name.
—Sydney Walker III, A Dose of Sanity: Mind, Medicine, and Misdiagnosis (1996)
Over the course of the last decade the definitions of health and disease have been transformed by advances in genetics. Genetic testing has enabled researchers and clinicians to detect inherited traits, diagnose heritable conditions, determine and quantify the likelihood that a heritable disease will develop, and identify genetic susceptibility to familial disorders. Many of the strides made in genetic diagnostics are direct results of the Human Genome Project, an international thirteen-year effort begun in 1990 by the U.S. Department of Energy and the National Institutes of Health, which mapped and sequenced the human genome in its entirety. The increasing availability of genetic testing has been one of the most immediate applications of this groundbreaking research.
A genetic test is the analysis of human deoxyribonucleic acid (DNA), ribonucleic acid (RNA), chromosomes, and proteins to detect heritable disease-related genotypes, mutations, phenotypes, or karyotypes (standard pictures of the chromosomes in a cell) for the purposes of diagnosis, treatment, and other clinical decision making. Most genetic testing is performed by drawing a blood sample and extracting DNA from white blood cells. Genetic tests may detect mutations at the chromosomal level, such as additional, absent, or rearranged chromosomal material, or even subtler abnormalities such as a substitution in one of the bases that make up the DNA. There is a broad range of techniques that can be used for genetic testing. Genetic tests have diverse purposes, including screening for and diagnosis of genetic disease in newborns, children, and adults; the identification of future health risks; the prediction of drug responses; and the assessment of risks to future children.
There is a difference between genetic tests performed to screen for disease and testing conducted to establish a diagnosis. Diagnostic tests are intended to definitively determine whether a patient has a particular problem. They are generally complex tests and commonly require sophisticated analysis and interpretation. They may be expensive and are generally performed only on people believed to be at risk, such as patients who already have symptoms of a specific disease.
In contrast, screening is performed on healthy, asymptomatic (showing no symptoms of disease) people and often to the entire relevant population. A good screening test is relatively inexpensive, easy to use and interpret, and helps identify which individuals in the population are at higher risk of developing a specific disease. By definition, screening tests identify people who need further testing or those who should take special preventive measures or precautions. For example, people who are found to be especially susceptible to genetic conditions with specific environmental triggers are advised to avoid the environmental factors linked to developing the disease. Examples of genetic tests used to screen relevant populations include those that screen people of Ashkenazi Jewish heritage (the East European Jewish population primarily from Germany, Poland, and Russia, as opposed to the Sephardic Jewish population primarily from Spain, parts of France, Italy, and North Africa) for Tay-Sachs disease, African-Americans for sickle-cell disease, and the fetuses of expectant mothers over age thirty-five for Down syndrome.
QUALITY AND UTILITY OF GENETIC TESTS
Like all diagnostic and screening tests, the quality and utility of genetic tests depend on their reliability, validity, sensitivity, specificity, positive predictive value, and negative predictive value. Reliability of testing refers to the test's ability to be repeated and to produce equivalent results in comparable circumstances. A reliable test is consistent and measures the same way each time it is used with the same patients in the same circumstances. For example, a well-calibrated balance scale is a reliable instrument for measuring body weight.
Validity is the accuracy of the test. It is the degree to which the test correctly identifies the presence of disease, blood level, or other quality or characteristic it is intended to detect. For example, if you put an object you knew weighed ten pounds on a scale and the scale said it weighed ten pounds, then the scale's results are valid. There are two components of validity: sensitivity and specificity.
Sensitivity is the test's ability to identify people who have the disease. Mathematically speaking, it is the percentage of people with the disease who test positive for the disease. Specificity is the test's ability to identify people who do not have the disease—it is the percentage of people without the disease who test negative for the disease. Ideally, diagnostic and screening tests should be highly sensitive and highly specific, thereby accurately classifying all people tested as either positive or negative. In practice, however, sensitivity and specificity are frequently inversely related—most tests with high levels of sensitivity have low specificity, and the reverse is also true.
The likelihood that a test result will be incorrect can be gauged based on the sensitivity and specificity of the test. For example, if a test's sensitivity is 95%, then when one hundred patients with the disease are tested, ninety-five will test positive and five will test false negative—they have the disease but the test has failed to detect it. For example, disorders such as Charcot-Marie-Tooth disease (a group of inherited, slowly progressive disorders that result from progressive damage to nerves; its symptoms include numbness and wasting of muscle tissue in the feet and legs, then in the hands and arms) can arise from mutations in one of many different genes, and because some of these genes have not yet been identified, they will not be detected and a false negative result might be reported. By contrast, if a test is 90% specific, when one hundred healthy, disease-free people are tested, ninety will receive negative test results and ten will be given false-positive results, meaning that they do not have the disease but the test has inaccurately classified them as positive.
The positive predictive value is the percentage of people that actually have the disease of all those with positive test results. The negative predictive value measures the percent of all the people with negative test results who do not have the disease.
PREGNANCY, CHILDBIRTH, AND GENETIC TESTING
There are thousands of genetic diseases, such as sickle-cell anemia, cystic fibrosis, and Tay-Sachs disease, that may be passed from one generation to the next. Many tests have been developed to help screen parents at risk of passing on genetic disease to their children as well as to identify embryos, fetuses, and newborns who suffer from genetic diseases.
Carrier identification is the term for genetic testing to determine whether a healthy individual has a gene that may cause disease if passed on to his or her offspring. It is usually performed on people considered to be at higher than average risk, such as those of Ashkenazi Jewish descent, who have a 1 in 27 chance of being Tay-Sachs carriers (in other populations the risk is 1 out of 250), according to the National Tay-Sachs and Allied Diseases Association (2007, http://www.tay-sachs.org/taysachs.php). Testing is necessary because many carriers have just one copy of a gene for an autosomal recessive trait and are unaffected by the trait or disorder. Only someone with two copies of the gene will actually have the disorder. So while it is widely assumed that everyone is an unaffected carrier of at least one autosomal recessive gene, it only presents a problem in terms of inheritance when two parents have the same recessive disorder gene (or both are carriers). In this instance the offspring would each have a one in four chance of receiving a defective copy of the gene from each parent and developing the disorder. Figure 6.1 shows the pattern of inheritance of an autosomal recessive disorder.
Carrier testing is offered to individuals who have family members with a genetic condition, people with family members who are identified carriers, and members of racial and ethnic groups known to be at high risk. Figure 6.2 shows how carrier testing would be used for a family affected by cystic fibrosis and among African-Americans who may carry the gene for sickle-cell anemia.
Preimplantation Genetic Diagnosis
Preimplantation diagnosis is a newer genetic test that enables parents undergoing in vitro fertilization (fertilization that takes place outside the body) to screen an embryo for specific genetic mutations when it is no larger than six or eight cells and before it is implanted in the uterus to grow and develop. Figure 6.3 shows how preimplantation genetic diagnosis is performed.
Prenatal genetic testing enables physicians to diagnose diseases in the fetus. Most genetic tests examine blood or other tissue to detect abnormalities. An example of a blood test is the triple marker screen. This test measures levels of alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol and can identify some birth defects such as Down syndrome and neural tube defects. (Two of the most common neural tube defects are anencephaly—absence of most of the brain—and spina bifida—incomplete development of the back and spine.)
The fetal yolk sac and the fetal liver make AFP, which is continuously processed by the fetus and excreted into the amniotic fluid. A small amount crosses the placenta and can be found in maternal blood. Maternal screening for AFP levels is based on maternal age, fetal gestation, and the number of fetuses the mother is carrying. Elevated levels of AFP are associated with conditions such as spina bifida and low levels are found with Down syndrome. Because AFP levels alone may not always adequately detect disorders, two other blood serum tests have been developed. hCG is a glycoprotein produced by the placenta. Normally, hCG is elevated at the time of implantation, but decreases at about eight weeks of gestation, and then drops again at approximately twelve weeks of gestation. Elevated levels of hCG are found with Down syndrome. The placenta also produces unconjugated estriol. As with AFP, lower unconjugated estriol maternal serum levels are also found with Down syndrome. Triple marker screen results are usually available within several days and women with abnormal results are often advised to undergo additional diagnostic testing such as chorionic villus sampling (CVS), amniocentesis, or percutaneous umbilical blood sampling (withdrawing blood from the umbilical cord).
CVS enables obstetricians and perinatologists (physicians specializing in evaluation and care of high-risk expectant mothers and infants) to assess the progress of pregnancy during the first trimester (the first three months). A physician passes a small, flexible tube called a catheter through the cervix to extract chorionic villi tissue—cells that will become the placenta and are genetically identical to the baby's cells. The chorion develops from trophoblasts, or the same cells as the fetus, and contains the same DNA and chromosomes. The cells obtained via CVS are examined in the laboratory for indications of genetic disorders such as cystic fibrosis, Down syndrome, Tay-Sachs, and thalassemia, and the results of testing are available within seven to fourteen days. Table 6.1 describes CVS and other prenatal diagnostic tests and specifies when during pregnancy they are performed.
Amniocentesis involves taking a sample of the fluid that surrounds the fetus in the uterus for chromosome analysis. An amniocentesis is usually performed at fifteen to twenty weeks of gestation, although it can be done as early as twelve weeks. (See the description of early amniocentesis in Table 6.1.) About twenty milliliters of amniotic fluid is obtained when the physician inserts a hollow needle through the abdominal wall and the wall of the uterus. Fetal karyotyping, DNA analysis, and biochemical testing may be performed on the isolated fetal cells. Like CVS, amniocentesis samples and analyzes cells derived from the baby to enable parents to learn of chromosomal abnormalities, as well as the gender of the unborn child, about two weeks after the test is performed.
Using samples of genetic material obtained from amniocentesis or CVS, physicians can detect disease in an unborn child. Down syndrome (also known as trisomy 21, because it is caused by an extra copy of chromosome 21) is the genetic disease most often identified using this technique. Down syndrome is rarely inherited; most cases result from an error in the formation of the ovum (egg) or sperm, leading to the inclusion of an extra chromosome 21 at conception. As with prenatal diagnosis for most inherited genetic diseases, this use of genetic testing is focused on reproductive decision making.
The most invasive prenatal procedure for genetic testing is periumbilical blood sampling. Table 6.1 describes the technique. Periumbilical blood sampling poses the greatest risk to the unborn child—one in fifty miscarriages occurs as a result of this procedure. It is used when a diagnosis must be made quickly. For example, when an expectant mother is exposed to an infectious agent with the potential to produce birth defects, it may be used to examine fetal blood for the presence of infection.
Until 2006 it was thought that women undergoing CVS were more likely to miscarry than those who had amniocentesis; however, Aaron B. Caugey, Linda M. Hopkins, and Mary E. Norton, in "Chorionic Villus Sampling Compared with Amniocentesis and the Difference in the Rate of Pregnancy Loss" (Obstetrics and Gynecology, September 2006), refute this notion. Caugey, Hopkins, and Norton analyzed the outcomes of nearly 10,000 CVS and 32,000 amniocentesis procedures and found that CVS was no more likely than amniocentesis to lead to pregnancy loss. They attribute previously reported higher rates of miscarriage resulting from CVS to the fact that clinicians were not yet experienced at performing the newer procedure.
|Prenatal diagnostic testing methods|
|Procedure||Technique (ultrasound guided)||Sample||Timing in gestation*|
|*Menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.|
|Source: "Prenatal Diagnosis," in GeneTests Data Base: GeneReviews Illustrated Glossary, National Institutes of Health, U.S. National Library of Medicine, National Human Genome Research Institute, http://www.genetests.org/servlet/access?qry=ALLTERMS&db=genestar&fcn=term>report2=true&id=08888891&key=mbmumbAd9rOWs (accessed October 24,2006)|
|Chorionic villus sampling (CVS)||Needle inserted through mother's abdomen or catheter through cervix||Chorionic villus||10-12 weeks|
|Early amniocentesis||Needle inserted through mother's abdomen into amniotic sac||Amniotic fluid and/or amniocytes||<15 weeks|
|Amniocentesis||Needle inserted through mother's abdomen into amniotic sac||Amniotic fluid and/or amniocytes||15-20 weeks|
|Placental biopsy||Needle inserted through mother's abdomen into placenta||Placental tissue||>12 weeks|
|Periumbilical blood sampling (PUBS) (aka cordocentesis)||Needle inserted through mother's abdomen into fetal umbilical vein||Fetal blood||>18 weeks|
|Fetoscopy with fetal skin biopsy||Needle inserted through mother's abdomen, camera used to facilitate biopsy||Fetal skin||>18 weeks|
Genetic Testing of Newborns
The most common form of genetic testing is the screening of blood taken from newborn infants for genetic abnormalities. The Maternal and Child Health Bureau (September 2004, http://mchb.hrsa.gov/programs/genetics/presentations/comments/meeting2comments.htm) reports that in the United States more than four million newborns are screened every year for specific genetic disorders, such as phenylketonuria (PKU), and other medical conditions that are only indirectly genetically linked, such as congenital hypothyroidism (underactive thyroid gland). PKU is an inherited error of metabolism resulting from a deficiency of an enzyme called phenylalanine hydroxylase. The lack of this enzyme can produce mental retardation, organ damage, and postural problems. Children born with PKU must pay close attention to their diets to lead healthy, normal lives.
Laboratory Techniques for Genetic Prenatal Testing
Genetic testing is performed on chromosomes, genes, or gene products to determine whether a mutation is causing or may cause a specific condition. Direct testing examines the DNA or RNA that makes up a gene. Linkage testing looks for disease-causing gene markers in family members from at least two generations. Biochemical testing assays certain enzymes or proteins, which are the products of genes. Cytogenetic testing examines the chromosomes.
Generally, a blood sample or buccal smear (cells from the mouth) is used for genetic tests. Other tissues used include skin cells from a biopsy, fetal cells, or stored tissue samples. (Table 6.1 lists the tissue samples used in each procedure.) Testing requires highly trained, certified technicians and laboratories because the procedures are complex and varied, the technology is new and evolving, and hereditary conditions are often rare, so many testing techniques require special expertise. In the United States laboratories performing clinical genetic tests must be approved under the Clinical Laboratory Improvement Amendments (CLIA), passed by Congress in 1988 to establish the standards with which all laboratories that test human specimens must comply to receive certification. CLIA standards determine the qualifications of laboratory personnel, categorize the complexity of various tests, and oversee quality improvement and assurance. In 2005 about 600 laboratories in the United States were performing genetic testing to detect and diagnose more than 1,200 conditions. Figure 6.4 shows the tremendous growth of both laboratories and diseases for which testing is available from 1993 to 2005.
The polymerase chain reaction (PCR) technique permits rapid cloning and DNA analysis and allows selective amplification of specific DNA sequences. A polymerase chain reaction can be performed in hours and is a sensitive test that may be used to screen for altered genes, but it is limited by the size and length of the DNA sequences that can be cloned. Figure 6.5 shows how PCR is performed.
Fluorescence in situ hybridization (FISH), in which a fluorescent label is attached to a DNA probe that will bind to the complementary DNA strands, provides a unique opportunity to view specific genetic codes. The FISH technique may be used on cells and fluid obtained by CVS and amniocentesis as well as on maternal blood. With this technique, a single strand of DNA is used to create a probe that attaches at the specific gene location. To separate the double-stranded DNA, heat or chemicals are used to break the chemical bonds of the DNA and obtain a single strand. (See Figure 6.6.)
There are three kinds of chromosome-specific probes: repetitive probes, painting probes, and locus-specific probes. Repetitive probes produce intense signals by creating tandem repeats of base pairs. Painting probes are collections of specific DNA sequences that may extend along either part or all of an individual chromosome. These probe labels are most useful for identifying complex rearrangements of genetic material in structurally abnormal chromosomes. Probes that can hybridize to a single gene locus are called locus-specific probes. They can be used to identify a gene in a particular region of a chromosome. Locus-specific probes are used to identify a deletion or duplication of genetic material. The FISH procedure allows a signal to be visualized that indicates the presence or absence of DNA. The entire process can be completed in fewer than eight hours using five to seven probes. For this reason, FISH is frequently used as a rapid screen for trisomies and genetic disorders.
Although FISH is the most commonly used and most readily available prenatal diagnostic cytogenetic technique, it has limited ability to detect translocations, deletions, and inversions. Microdissection FISH (also used for prenatal diagnosis) is another method that is more sensitive to these alterations. Microdissection FISH constructs probes to define specific regions of the human chromosome. Figure 6.7 shows how the microdissection technique is used to identify structurally abnormal chromosomes.
Newer technologies allow for the identification of all human chromosomes, thereby expanding the FISH application to the entire genome. Multiplex FISH applies combinations of probes to color components of fluorescent dye during metaphase to visualize each chromosome. This type of FISH procedure evolved from the whole-chromosome painting probes and uses a combination of fluorochromes to identify each chromosome. Just five fluorophores are needed to decode the entire complement of human chromosomes. Multicolor spectral karyotyping uses computer imaging and Fourier spectroscopy to increase the analysis of genetic markers. (See Figure 6.8.) Although FISH techniques are considered highly reliable, there are limitations to multiplex and multicolor FISH, in that inversion or subtle deletions may be overlooked. The greatest advant-age of multiplex FISH is its application in cases of dysmorphic infants. Spectral karyotyping techniques can detect cryptic unbalanced translocations that cannot be identified by other techniques.
GENETIC TESTING FOR SICKLE-CELL ANEMIA
Sickle-cell anemia is an autosomal recessive disease that results when hemoglobin S is inherited from both parents. (When hemoglobin S is inherited from only one parent, the individual is a sickle-cell carrier.) Because normal and sickle hemoglobins differ at only one amino acid in the hemoglobin gene, a test called hemoglobin electrophoresis is used to establish the diagnosis.
Genetic testing for sickle-cell anemia involves restriction fragment length polymorphism (DNA sequence variant) that uses specific enzymes to cut the DNA. (See Figure 6.9.) These enzymes cut the DNA at a specific base sequence on the normal gene but not on a gene in which a mutation is present. As a result of this technique, there are longer fragments of sickle hemoglobin. Another technique known as gel electrophoresis sorts the DNA fragments by size. Autoradiography renders the DNA fragments by generating an image after radioactive probes have labeled the DNA fragments that contain the specific gene sequence. The location of the fragments distinguishes carrier status (heterozygous) from sickle-cell anemia (homozygous), or normal blood.
New Techniques Detect Fetal Gene Mutations
In 2005 Ying Li et al. announced success with a technique that identifies fetal gene mutations such as beta-thalassemia from samples of maternal blood and reported their discovery in "Detection of Paternally Inherited Fetal Point Mutations for Beta-Thalassemia Using Size-Fractionated Cell-Free DNA in Maternal Plasma" (Journal of the American Medical Association, February 16, 2005). The technique relies on the fact that circulatory fetal DNA sequences comprise fewer than 300 base pairs, whereas maternal DNA exceeds 500 base pairs. Li and the other researchers used PCR amplification to select for paternally inherited DNA sequences. Presence of the paternal mutant alleles for beta-thalassemia was then detected by allele-specific PCR.
They tested the new technique on maternal blood samples from expectant mothers whose fetuses were at risk for beta-thalassemia because the father was a carrier for one of four beta-globin gene mutations. The results were verified by comparing the findings from chorionic villus sampling. Li et al. also reported the cost-effectiveness of the new technique. Because it does not require complex machinery and relies on currently available technology, they estimate the cost of a single analysis might be just $8.
Another new technique reported in 2005 was analysis of amniotic fluid using oligonucleotide hybridization and microarray data analysis. (Figure 6.10 shows how microarray technology is performed.) In "Global Gene Expression Analysis of the Living Human Fetus Using Cell-Free Messenger RNA in Amniotic Fluid" (Journal of the American Medical Association, February 16, 2005), Paige B. Larrabee et al. note that they found that gene expression patterns correlated with the gender of the fetus, gestational age, and disease status. Larrabee and her collaborators assert that this technology could assist in advancing human developmental research and in identifying new biomarkers for prenatal assessment.
GENETIC DIAGNOSIS IN CHILDREN AND ADULTS
Genetic testing can also be performed postnatally (after birth) to determine which children and adults are at increased risk of developing specific diseases. By 2006 scientists could perform predictive genetic testing to identify which individuals were at risk for cystic fibrosis, Tay-Sachs disease, Huntington's disease, amyotrophic lateral sclerosis (a degenerative neurological condition commonly known as Lou Gehrig's disease), and several types of cancers, including some cases of breast, colon, and ovarian cancer.
More than 1,200 genetic tests were available in 2006, but public health professionals did not consider it practical to screen for conditions that are rare, have only minor health consequences, or those for which there is still no effective treatment. The most frequently performed genetic tests were those considered most useful in terms of their potential to screen populations for diseases that occur relatively frequently, have serious medical consequences (including death) if untreated, and for which effective treatment is available.
A positive test result (the presence of mutation—a defective or altered gene) from predictive genetic testing does not guarantee that the individual will develop the disease; it simply identifies the individual as genetically susceptible and at an increased risk for developing the disease. For example, a woman who tests positive for the BRCA1 gene has about an 80% chance of developing breast cancer before age sixty. It is also important to note that, like other types of diagnostic medical testing, genetic tests are not 100% predictive—the results rely on the quality of laboratory procedures and accuracy of interpretations. Furthermore, because tests vary in their sensitivity and specificity, there is always the possibility of false-positive and false-negative test results.
Researchers hope that positive test results will encourage people who are at higher than average risk of developing a disease to be especially vigilant about disease prevention and screening for early detection, when many diseases are most successfully treated. There is an expectation that genetic information will increasingly be used in routine population screening to determine individual susceptibility to common disorders such as heart disease, diabetes, and cancer. This type of screening will identify groups at risk so that primary prevention efforts such as diet and exercise or secondary prevention efforts such as early detection can be initiated.
Diagnostic Genetic Testing
Most genetic testing is performed on people who are asymptomatic (people who are apparently healthy). The objective of screening is to determine if people are carriers of a genetic disease or to identify their susceptibility or risk of developing a specific disease or disorder. There is, however, some testing performed on people with symptoms of a disease to clarify or establish the diagnosis and calculate the risk of developing the disease for other family members. This type of testing is known as diagnostic genetic testing or symptomatic genetic testing. It may also assist in directing treatment for symptomatic patients in whom a mutation in a single gene (or in a gene pair) accounts for a disorder. Cystic fibrosis and myotonic dystrophy are examples of disorders that may be confirmed or ruled out by diagnostic genetic testing and other methods (such as the sweat test for cystic fibrosis or a neurological evaluation for myotonic dystrophy).
One issue involved in diagnostic genetic testing is the appropriate frequency of testing in view of rapidly expanding genetic knowledge and identification of genes linked to disease. Physicians frequently see symptomatic patients for whom there is neither a definitive diagnosis nor a genetic test. The as-yet-unanswered question is: Should such people be recalled for genetic testing each time a new test becomes available? Although clinics and physicians who perform genetic testing counsel patients to maintain regular contact so they may learn about the availability of new tests, there is no uniform guideline or recommendation about the frequency of testing.
New Genetic Tests Help Patients Choose Optimal Treatment
In August 2005 the Food and Drug Administration (FDA) approved marketing of a new genetic test that will help physicians make personalized drug treatment decisions for some patients ("FDA Clears Genetic Test That Advances Personalized Medicine: Test Helps Determine Safety of Drug Therapy," August 22, 2005, http://www.fda.gov/bbs/topics/NEWS/2005/NEW01220.html). The Invader UGT1A1 Molecular Assay detects variations in a gene that affects how certain drugs are broken down and cleared by the body. Using this information, physicians can determine the optimal drug dosage for each patient and minimize the harsh and potentially life-threatening side effects of drug treatment. The new test joins a growing list of genetic tests used to customize treatment decisions, including the Roche AmpliChip, used to personalize dosage of antidepressants, antipsychotics, beta-blockers, and some chemotherapy drugs, and TRUGENE HIV-1 Genotyping Kit, used to detect variations in the genome of the human immunodeficiency virus (which can cause acquired immunodeficiency syndrome) that make the virus resistant to some antiretroviral drugs.
In 2006 Anil Potti et al. reported in "A Genomic Strategy to Refine Prognosis in Early-Stage Non-Small-Cell Lung Cancer" (New England Journal of Medicine, August 10, 2006) about a new genetic test that predicts with up to 90% accuracy which early-stage lung cancer patients are likely to experience recurrence and therefore would benefit from chemotherapy (drug treatment for cancer). Currently, patients diagnosed with early-stage non-small-cell lung cancer undergo surgery followed by observation, without chemotherapy. About one-third of such patients experience a relapse. Potti and his coauthors describe the test as "a fingerprint unique to the individual patient [that] predicts survival changes."
The National Cancer Institute announced in 2006 another new genetic test that was in clinical trials and that promised to distinguish which breast cancer patients can safely skip chemotherapy (May 23, 2006, http://www.cancer.gov/clinicaltrials/digestpage/TAILORx). The Onco-type DX test examines the surgically removed tumor for twenty-one different genes whose interactions can predict the likelihood of a relapse and calculates the odds of a relapse on a scale from 0 to 100. A score greater than 30 indicates the benefits of chemotherapy, whereas a score lower than 18 suggests forgoing chemotherapy.
Population screening for heritable diseases is one potentially lifesaving application of molecular genetics technology. Prenatal screening has demonstrated benefits and gained widespread use; however, genetic screening has not yet become part of routine medical practice for adults. Geneticists have identified at least seven genes that might be candidates for use as population screening tests in adults in the United States. The genes include HFE, for hereditary hemochromatosis; apolipoprotein E-4, linked to Alzheimer's disease; CYP2D6, linked to ankylosing spondylitis (arthritis of the spine); BRCA1 and BRCA2, genes for hereditary breast and ovarian cancer; familial adenomatous polyposis, associated with precancerous growths in the colon; and factor V Leiden, the most common hereditary blood clotting disorder in the United States. As of 2007, screening for variants in these genes had not entered into routine medical practice because there was considerable controversy about the predictive value of testing for these genes and how to monitor and care for people who test positive for them.
Susceptibility testing, also known as predictive testing, determines the likelihood that a healthy person with a family history of a disorder will develop the disease. Testing positive for a specific genetic mutation indicates an increased susceptibility to the disorder but does not establish a diagnosis. For example, a woman may choose to undergo testing to find out whether she has genetic mutations that would indicate likelihood of developing hereditary cancer of the breast or ovary. If she tests positive for the genetic mutation, she may then decide to undergo some form of preventive treatment. Preventive measures may include increased surveillance such as more frequent mammography, chemoprevention—prescription drug therapy intended to reduce risk—or surgical prophylaxis, such as mastectomy and/or oophorectomy (surgical removal of the breasts and ovaries, respectively).
Testing Children for Adult-Onset Disorders
In 2000 the American Academy of Pediatrics Committee on Genetics recommended genetic testing for people under age eighteen only when testing would offer immediate medical benefits or when there is a benefit to another family member and there is no anticipated harm to the person being tested. The committee considered genetic counseling before and after testing as essential components of the process.
The American Academy of Pediatrics Committee on Bioethics and Newborn Screening Task Force recommended the inclusion of tests in the newborn-screening battery based on scientific evidence. The academy advocated informed consent for newborn screening. (To date, most states do not require informed consent.) The Committee on Bioethics did not endorse carrier screening in people under eighteen years of age, except in the case of a pregnant teenager. It also recommended against predictive testing for adult-onset disorders in people under eighteen.
The American College of Medical Genetics, the American Society of Human Genetics (ASHG), and the World Health Organization have also weighed in about genetic testing of asymptomatic children, asserting that decision making should emphasize the child's well-being. One issue involves the value of testing asymptomatic children for genetic mutations associated with adult-onset conditions such as Huntington's disease. Because no treatment can begin until the onset of the disease, and at present there is no treatment to alter the course of the disease, it may be ill advised to test for it. Another concern is testing for carrier status of autosomal recessive or X-linked conditions such as cystic fibrosis or Duchenne muscular dystrophy. Experts caution that children might confuse carrier status with actually having the condition, which in turn might provoke needless anxiety.
There are, however, circumstances in which genetic testing of children may be appropriate and useful. Examples are children with symptoms of suspected hereditary disorders or those at risk for cancers in which inheritance plays a primary role.
ETHICAL CONSIDERATIONS—CHOICES AND CHALLENGES
Rapid advances in genetic research since the 1980s have challenged scientists, health care professionals, ethicists, government regulators, legislators, and consumers to stay abreast of new developments. Understanding the scientific advances and their implications is critical for everyone involved in making informed decisions about the ways in which genetic research and information will affect the lives of current and future generations. American citizens, scientists, ethicists, legislators, and regulators share this responsibility. According to the Human Genome Project Information Web site (February 5, 2003, http://genome.rtc.riken.go.jp/hgmis/elsi/elsi.html), the pivotal importance of these societal decisions was underscored by the allocation of 3% to 5% of the budget of the Human Genome Project for the study of ethical, legal, and social issues related to genetic research. To date, consideration of these issues has not produced simple or universally applicable answers to the many questions posed by the increasing availability of genetic information. Ongoing public discussion and debate is intended to inform, educate, and help people in every walk of life make personal decisions about their health and participate in decisions that concern others.
As researchers learn more about the genes responsible for a variety of illnesses, they can design more tests with increased accuracy and reliability to predict whether an individual is at risk of developing specific diseases. The ethical issues involved in genetic testing have turned out to be far more complicated than originally anticipated. Initially, physicians and researchers believed that a test to determine in advance who would develop or escape a disease would be welcomed by at-risk families, who would be able to plan more realistically about having children, choosing jobs, obtaining insurance, and going about their daily lives. Nevertheless, many people with family histories of a genetic disease have decided that not knowing is better than anticipating a grim future and an agonizing, slow death. They prefer to live with the hope that they will not develop the disease rather than having the certain knowledge that they will.
The discovery of genetic links and the development of tests to predict the likelihood or certainty of developing a disease raise ethical questions for people who carry a defective gene. Should women who are carriers of Huntington's disease or cystic fibrosis have children? Should a fetus with a defective gene be carried to term or aborted?
There are also concerns about privacy and the confidentiality of medical records and the results of genetic testing and possible stigmatization. Some people are reluctant to be tested because they fear they may lose their health, life, and disability insurances, or even their jobs if they are found to be at risk for a disease. Genetic tests are sometimes costly, and some insurers agree to reimburse for testing only if they are informed of the results. The insurance companies feel they cannot risk selling policies to people they know will become disabled or die prematurely.
The fear of discrimination by insurance companies or employers who learn the results of genetic testing is often justified. An insurance carrier may charge a healthy person a higher rate or disqualify an individual based on test results, and an employer might choose not to hire or to deny an affected individual a promotion. The American Society of Medical Genetics and most other medical professional associations agree that people should not be forced to choose between having a genetic test that could provide lifesaving information and avoiding a test to save a job or retain health insurance coverage.
Unanticipated Information and Results of Genetic Testing
Sometimes genetic testing yields unanticipated information, such as paternity, or other unexpected results, such as the presence of a disorder that was not sought directly. Many health professionals consider disclosure of such information, particularly when it does not influence health or medical treatment decision making, as counterproductive and even potentially harmful.
The ASHG is the primary professional organization for human geneticists in the United States. Its 8,000 members include researchers, academicians, clinicians, laboratory practice professionals, genetic counselors, nurses, and others involved in or with a special interest in human genetics. The ASHG recommends that family members not be informed of misattributed paternity unless the test requested was determination of paternity. The ASHG offers comparable guidelines regarding other unexpected results such as associations among diseases. For example, while performing screening for one disease, information about another disease may be discovered. Although the person may have requested screening for the first disorder, the presence of the second disorder may be unanticipated and may lead to stigmatization and discrimination on the part of insurance companies and employers. The ASHG encourages all health professionals to educate, counsel, and obtain informed consents that include cautions regarding unexpected findings before performing genetic testing.
Advertising Genetic Testing Directly to Consumers
Although pharmaceutical drugs have been advertised directly to consumers for more than two decades, direct-to-consumer advertising of genetic tests is a relatively recent phenomenon. Especially controversial are direct-to-consumer DNA tests sold in retail stores and via the Internet that promise personalized nutritional counseling based on genetic makeup. In Nutrigenetic Testing: Tests Purchased from Four Web Sites Mislead Consumers (July 27, 2006, http://www.gao.gov/new.items/d06977t.pdf), Gregory Kutz of the U.S. Government Accountability Office asserts that these tests are of no medical value. He notes that by promising results they cannot deliver, they often deceive people. Although nutritional genomics and nutrigenetic testing, which consider how complex interactions between genes and diet may affect the risk of future illnesses, is a legitimate discipline, Kutz believes the firms offering these services make health claims that are not supported by credible scientific evidence. Kutz decries the current regulatory environment, which provides only limited oversight of firms developing and marketing new types of genetic tests.
FDA Moves to Regulate Genetic Testing
Experts have been urging either the FDA or the Centers for Medicare and Medicaid Services, which regulates clinical labs, to strengthen regulation of diagnostic genetic tests. Andrew Pollack reports in "F.D.A. Seeks to Regulate New Types of Diagnostic Tests" (New York Times, September 6, 2006) that the FDA took its first steps to regulate such tests by issuing in September 2006 a draft guideline that would require FDA approval before the test kits can be marketed. The FDA intends to regulate at least one category of genetic tests: those that measure multiple genes, proteins, or other pieces of clinical information taken from a patient and then analyze the data. The best known of such tests is Oncotype DX, which costs $3,500 and analyzes twenty-one genes in a sample of breast tumor and then computes a score that predicts whether the cancer will recur and whether the patient would benefit from chemotherapy.
Personal Choices and Psychological Consequences of Genetic Testing
The results of genetic tests may be used to make decisions such as whether to have children or end a pregnancy. Results that predict the likelihood that an individual will develop a disease may affect decisions about education, marriage, family, or career choices. The decision to undergo genetic testing and the results of the tests not only affect the individual tested but also his or her family members. For example, when an unaffected patient requests a genetic susceptibility test, another test of an affected relative may be required to accurately calculate probability. Family members may vary in their willingness to share genetic information and their desire to know about genetic risks.
There are psychological consequences of genetic testing and coming to terms with the results. People may be relieved or distressed when they learn the results of a genetic test. The results can change the way they feel about themselves and can influence their relationships with relatives. For example, family members who discover they are carriers for cystic fibrosis may feel isolated or estranged from siblings who have opted not to be tested. By contrast, those who find out they do not have a genetic mutation for Huntington's disease may feel guilty because they were spared and other family members were not, or they may worry about assuming responsibility for family members who develop the disease.
The complexity of genetic testing and the uncertainty of many results pose an additional psychological challenge. The results of predictive genetic tests are often expressed in probabilities rather than certainties, and, even for people with a high probability of developing a disease, there are often conflicting opinions about the most appropriate course of action. For example, a woman who tests positive for BRCA1 or BRCA2 has an increased lifetime risk of developing breast or ovarian cancer or both, but it does not mean that her risk of developing either or both is 100%. Depending on her personal circumstances and the medical advice she receives, she may opt to intensify screening to detect disease; use prescription medication intended to reduce risk, such as tamoxifen; or undergo preventive procedures, such as mastectomy and oophorectomy.
Even a negative test result can be stressful, creating nearly as many questions as it does answers about disease risk. A negative test result for BRCA1 or BRCA2 in a woman with an affected family member (one who has the genetic mutation) means that even though she does not have the genetic mutation, her risk is the same as that of the general population. There are multiple factors associated with the risk of developing breast cancer that are not identified through genetic testing, such as the age at which a woman has her first child. In addition, most breast cancer is not believed to be hereditary, and most women with diagnosed breast cancer do not test positive for BRCA1 or BRCA2 genetic mutations.
LEGISLATION TO PROTECT GENETIC INFORMATION AND PREVENT DISCRIMINATION
By 2007 most state legislatures had taken steps to safeguard genetic information beyond the protections provided for other types of health information. However, absent comprehensive federal legislation, not all people will be protected from discrimination based on genetic information.
Federal Executive Order 13145, "To Prohibit Discrimination in Federal Employment Based on Genetic Information," was signed on February 8, 2000. The executive order prohibits federal government agencies from obtaining genetic information from employees or job applicants and from using genetic information in hiring and promotion decisions. The executive order defines genetic information as information about an individual's genetic tests; information about the genetic tests of an individual's family members; and information about the occurrence of a disease, medical condition, or disorder in family members of the individual.
On October 14, 2003, the U.S. Senate passed the Genetic Information Nondiscrimination Act (S. 1053), which would have prohibited discrimination on the basis of genetic information in health insurance coverage and the workplace. Francis S. Collins, the director of the National Human Genome Research Institute, published a statement (October 14, 2003, http://genome.gov/11009127) expressing his contention that:
No one should lose his job because of the genes he inherited. No one should be denied health insurance because of her DNA. But genetic discrimination affects more than jobs and insurance. It slows the pace of science. We know that many people have refused to participate in genetic research for fear of genetic discrimination. This means that without the kind of legal protections offered by this bill, our clinical research protocols will lack participants, and those who do participate will represent a self-selected group.
The Genetic Information Nondiscrimination Act of 2005 (S. 306) was unanimously passed on February 17, 2005. President George W. Bush expressed his support for the bill in a Statement of Administrative Policy. A virtually identical bill, the Genetic Information Nondiscrimination Act (H.R. 1227), was considered by the U.S. House of Representatives during 2005 but by the close of 2006 it had not made it out of committee.
Health Insurance Portability and Accountability Act of 1996
On August 21, 1996, President Bill Clinton signed the Health Insurance Portability and Accountability Act (HIPAA; PL 104-191). This legislation aims to provide better portability (transfer) of employer-sponsored insurance from one job to another. By preventing job lock—the need to remain in the same position or with the same employer for fear of losing health care coverage—the act was designed to afford American workers greater career mobility and the freedom to pursue job opportunities. Industry observers and policy makers viewed HIPAA as an important first step in the federal initiative to significantly reduce the number of uninsured people in the United States. They also hoped it would provide a measure of protection from genetics-based discrimination.
HIPAA stipulates that American workers who have previous insurance coverage are immediately eligible for new coverage when changing jobs. The law prohibits group health plans from denying new coverage based on past or present poor health and guarantees that employees can retain their health care coverage even after they leave their jobs. New employers can still require a routine waiting period (usually no more than three months) before paying for health benefits, but the new employee who applies for insurance coverage can be continuously covered during the waiting period. HIPPA also prohibits excluding an individual from group coverage because of past or present medical problems, including genetic information.
HIPAA does not, however, prohibit the use of genetic information as a basis for charging a group more for health insurance. It neither limits the collection of genetic information by insurers nor prohibits insurers from requiring an individual to take a genetic test. The act does not limit the disclosure of genetic information by insurers, and it does not apply to individual health insurers unless they are covered by the portability provision.
Generally, patients receive explicit counseling before undergoing genetic testing to ensure that they are able to make informed decisions about choosing to have the tests and the consequences of testing. Most genetic counseling is informative and nondirective—it is intended to offer enough information to allow families or individuals to determine the best courses of action for themselves but avoids making testing recommendations.
Patients undergoing tests to improve their care and treatment have different pretest counseling needs from those choosing susceptibility or predictive testing. In such instances genetic counselors do offer testing recommendations, particularly when a test offers an opportunity to prevent disease. As tests for genetic risk factors increasingly become routine in clinical medical practice, they are likely to be offered without formal pretest counseling. Genetic counselors urge physicians, nurses, and other health care professionals not to discount or rush through the process of obtaining informed consent to conduct a genetic test. They caution that potential psychological, social, and family implications should be acknowledged and addressed in advance of testing, including the potential for discrimination on the basis of genetic-risk status and the possibility that the predictive value of genetic information may be overestimated. The potentially life-changing consequences of genetic testing suggest that all health care professionals involved in the process should not only adhere to thoughtful informed consent procedures for genetic testing but also offer or make available genetic counseling when patients and families receive the results of testing.
"Genetic Testing." Genetics and Genetic Engineering. 2008. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3049300012.html
"Genetic Testing." Genetics and Genetic Engineering. 2008. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3049300012.html
A genetic test examines the genetic information contained inside a person's cells, called DNA, to determine if that person has or will develop a certain disease or could pass a disease to his or her offspring. Genetic tests also determine whether or not couples are at a higher risk than the general population for having a child affected with a genetic disorder.
Some families or ethnic groups have a higher incidence of a certain disease than the population as a whole. For example, individuals from Eastern European, Ashkenazi Jewish descent are at higher risk for carrying genes for rare conditions that occur much less frequently in populations from other parts of the world. Before having a child, a couple from such a family or ethnic group may want to know if their child would be at risk of having that disease. Genetic testing for this type of purpose is called genetic screening.
During pregnancy, a baby's cells can be studied for certain genetic disorders or chromosomal problems such as Down syndrome. Chromosome testing is most commonly offered when the mother is 35 years or older at the time of delivery. When there is a family medical history of a genetic disease or there are individuals in a family affected with developmental and physical delays, genetic testing also may be offered during pregnancy. Genetic testing during pregnancy is called prenatal diagnosis.
Prior to becoming pregnant, couples who are having difficulty conceiving a child or who have suffered multiple miscarriages may be tested to see if a genetic cause can be identified.
A genetic disease may be diagnosed at birth by doing a physical evaluation of the baby and observing characteristics of the disorder. Genetic testing can help to confirm the diagnosis made by the physical evaluation. In addition, genetic testing is used routinely on all newborns to screen for certain genetic diseases that can affect a newborn baby's health shortly after birth.
There are several genetic diseases and conditions in which the symptoms do not occur until adulthood. One such example is Huntington's disease. This is a serious disorder affecting the way in which individuals walk, talk and function on a daily basis. Genetic testing may be able to determine if someone at risk will in fact develop the disease.
Genetic testing may take on new emphasis in the near future as genetic research continues to advance. In April 2003, the Human Genome Project announced completion of mapping the entire human genetic makeup. The project identified more than 1,400 disease genes and completed study of the ethical, legal, and social issues raised by this expanded knowledge of human genetics. As knowledge expands and scientists discover more methods to identify and treat various diseases, people will face more difficult decisions about their own genetic information. In fact, the amount of genetic testing was increasing internationally in 2003, especially for rare diseases.
Some genetic defects may make a person more susceptible to certain types of cancer. Testing for these defects can help predict a person's risk. Other types of genetic tests help diagnose and predict and monitor the course of certain kinds of cancer, particularly leukemia and lymphoma.
Because genetic testing is not always accurate and because there are many concerns surrounding insurance and employment discrimination for the individual receiving a genetic test, genetic counseling should always be performed prior to genetic testing. A genetic counselor is an individual with a master's degree in genetic counseling. A medical geneticist is a physician specializing and board certified in genetics.
A genetic counselor reviews the person's family history and medical records and the reason for the test. The counselor explains the likelihood that the test will detect all possible causes of the disease in question (known as the sensitivity of the test), and the likelihood that the disease will develop if the test is positive (known as the positive predictive value of the test).
Learning about the disease in question, the benefits and risks of both a positive and a negative result, and what treatment choices are available if the result is positive, will help prepare the person undergoing testing. During the genetic counseling session, the individual interested in genetic testing will be asked to consider how the test results will affect his or her life, family, and future decisions.
After this discussion, the person should have the opportunity to indicate in writing that he or she gave informed consent to have the test performed, verifying that the counselor provided complete and understandable information.
Genes and chromosomes
Deoxyribonucleic acid (DNA) is a long molecule made up of two strands of genetic material coiled around each other in a unique double helix structure. This structure was discovered in 1953 by Francis Crick and James Watson.
DNA is found in the nucleus, or center, of most cells (Some cells, such as a red blood cell, don't have a nucleus). Each person's DNA is a unique blueprint, giving instructions for a person's physical traits, such as eye color, hair texture, height, and susceptibility to disease. DNA is organized into structures called chromosomes.
The instructions are contained in DNA's long strands as a code spelled out by pairs of bases, which are four chemicals that make up DNA. The bases occur as pairs because a base on one strand lines up with and is bound to a corresponding base on the other strand. The order of these bases form DNA's code. The order of the bases on a DNA strand is important to ensuring that we are not affected with any genetic diseases. When the bases are out of order, or missing, our cells often do not produce important proteins which can lead to a genetic disorder. While our genes are found in every cell of our body, not every gene is functioning all of the time. Some genes are turned on during critical points in development and then remain silent for the rest of our lives. Other genes remain active all of our lives so that our cells can produce important proteins that help us digest food properly or fight off the common cold.
The specific order of the base pairs on a strand of DNA is important in order for the correct protein to be produced. A grouping of three base pairs on the DNA strand is called a codon. Each codon, or three base pairs, comes together to spell a word. A string of many codons together can be thought of as a series of words all coming together to make a sentence. This sentence is what instructs our cells to make a protein that helps our bodies function properly.
Our DNA strands, containing a hundred to several thousand copies of genes, are found on structures called chromosomes. Each cell typically has 46 chromosomes arranged into 23 pairs. Each parent contributes one chromosome to each pair. The first 22 pairs are called autosomal chromosomes, or non-sex chromosomes, and are assigned a number from 1-22. The last pair are the sex chromosomes and include the X and Y chromosomes. If a child receives an X chromosome from each parent, the child is female. If a child receives an X from the mother, and a Y from the father, the child is male.
Just as each parent contributes one chromosome to each pair, so each parent contributes one gene from each chromosome. The pair of genes produces a specific trait in the child. In autosomal dominant conditions, it takes only one copy of a gene to influence a specific trait. The stronger gene is called dominant; the weaker gene, recessive. Two copies of a recessive gene are needed to control a trait while only one copy of a dominant gene is needed. Our sex chromosomes, the X and the Y, also contain important genes. Some genetic diseases are caused by missing or altered genes on one of the sex chromosomes. Males are most often affected by sex chromosome diseases when they inherit an X chromosome with missing or mutated genes from their mother.
TYPES OF GENETIC MUTATIONS. Genetic disease results from a change, or mutation, in a chromosome or in one or several base pairs on a gene. Some of us inherit these mutations from our parents, called hereditary or germline mutations, while other mutations can occur spontaneously, or for the first time in an affected child. For many of the adult on-set diseases, genetic mutations can occur over the lifetime of the individual. This is called acquired or somatic mutations and these occur while the cells are making copies of themselves or dividing in two. There may be some environmental effects, such as radiation or other chemicals, which can contribute to these types of mutations as well.
There are a variety of different types of mutations that can occur in our genetic code to cause a disease. And for each genetic disease, there may be more than one type of mutation to cause the disease. For some genetic diseases, the same mutation occurs in every individual affected with the disease. For example, the most common form of dwarfism, called achondroplasia, occurs because of a single base pair substitution. This same mutation occurs in all individuals affected with the disease. Other genetic diseases are caused by different types of genetic mutations that may occur anywhere along the length of a gene. For example, cystic fibrosis, the most common genetic disease in the caucasian population is caused by over hundreds of different mutations along the gene. Individual families may carry the same mutation as each other, but not as the rest of the population affected with the same genetic disease.
Some genetic diseases occur as a result of a larger mutation which can occur when the chromosome itself is either rearranged or altered or when a baby is born with more than the expected number of chromosomes. There are only a few types of chromosome rearrangements which are possibly hereditary, or passed on from the mother or the father. The majority of chromosome alterations where the baby is born with too many chromosomes or missing a chromosome, occur sporadically or for the first time with a new baby.
The type of mutation that causes a genetic disease will determine the type of genetic test to be performed. In some situations, more than one type of genetic test will be performed to arrive at a diagnosis. The cost of genetic tests vary: chromosome studies can cost hundreds of dollars and certain gene studies, thousands. Insurance coverage also varies with the company and the policy. It may take several days or several weeks to complete a test. Research testing where the exact location of a gene has not yet been identified, can take several months to years for results.
Types of Genetic Testing
Direct DNA mutation analysis
Direct DNA sequencing examines the direct base pair sequence of a gene for specific gene mutations. Some genes contain more than 100,000 bases and a mutation of any one base can make the gene nonfunctional and cause disease. The more mutations possible, the less likely it is for a test to detect all of them. This test usually is done on white blood cells from a person's blood but also can be performed on other tissues. There are different ways in which to perform direct DNA mutation analysis. When the specific genetic mutation is known, it is possible to perform a complete analysis of the genetic code, also called direct sequencing. There are several different lab techniques used to test for a direct mutation. One common approach begins by using chemicals to separate DNA from the rest of the cell. Next, the two strands of DNA are separated by heating. Special enzymes (called restriction enzymes) are added to the single strands of DNA and then act like scissors, cutting the strands in specific places. The DNA fragments are then sorted by size through a process called electrophoresis. A special piece of DNA, called a probe, is added to the fragments. The probe is designed to bind to specific mutated portions of the gene. When bound to the probe, the mutated portions appear on x-ray film with a distinct banding pattern.
Indirect DNA Testing
Family linkage studies are done to study a disease when the exact type and location of the genetic alteration is not known, but the general location on the chromosome has been identified. These studies are possible when a chromosome marker has been found associated with a disease. Chromosomes contain certain regions that vary in appearance between individuals. These regions are called polymorphisms and do not cause a genetic disease to occur. If a polymorphism is always present in family members with the same genetic disease, and absent in family members without the disease, it is likely that the gene responsible for the disease is near that polymorphism. The gene mutation can be indirectly detected in family members by looking for the polymorphism.
To look for the polymorphism, DNA is isolated from cells in the same way it is for direct DNA mutation analysis. A probe is added that will detect the large polymorphism on the chromosome. When bound to the probe, this region will appear on x-ray film with a distinct banding pattern. The pattern of banding of a person being tested for the disease is compared to the pattern from a family member affected by the disease.
Linkage studies have disadvantages not found in direct DNA mutation analysis. These studies require multiple family members to participate in the testing. If key family members choose not to participate, the incomplete family history may make testing other members useless. The indirect method of detecting a mutated gene also causes more opportunity for error.
Various genetic syndromes are caused by structural chromosome abnormalities. To analyze a person's chromosomes, his or her cells are allowed to grow and multiply in the laboratory until they reach a certain stage of growth. The length of growing time varies with the type of cells. Cells from blood and bone marrow take one to two days; fetal cells from amniotic fluid take seven to 10 days.
When the cells are ready, they are placed on a microscope slide using a technique to make them burst open, spreading their chromosomes. The slides are stained: the stain creates a banding pattern unique to each chromosome. Under a microscope, the chromosomes are counted, identified, and analyzed based on their size, shape, and stained appearance.
A karyotype is the final step in the chromosome analysis. After the chromosomes are counted, a photograph is taken of the chromosomes from one or more cells as seen through the microscope. Then the chromosomes are cut out and arranged side-by-side with their partner in ascending numerical order, from largest to smallest. The karyotype is done either manually or using a computer attached to the microscope. Chromosome analysis also is called cytogenetics.
Applications for Genetic Testing
Genetic testing is used most often for newborn screening. Every year, millions of newborn babies have their blood samples tested for potentially serious genetic diseases.
An individual who has a gene associated with a disease but never exhibits any symptoms of the disease is called a carrier. A carrier is a person who is not affected by the mutated gene he or she possesses, but can pass the gene to an offspring. Genetic tests have been developed that tell prospective parents whether or not they are carriers of certain diseases. If one or both parents are a carrier, the risk of passing the disease to a child can be predicted.
To predict the risk, it is necessary to know if the gene in question is autosomal or sex-linked. If the gene is carried on any one of chromosomes 1-22, the resulting disease is called an autosomal disease. If the gene is carried on the X or Y chromosome, it is called a sex-linked disease.
Sex-linked diseases, such as the bleeding condition hemophilia, are usually carried on the X chromosome. A woman who carries a disease-associated mutated gene on one of her X chromosomes, has a 50% chance of passing the gene to her son. A son who inherits that gene will develop the disease because he does not have another normal copy of the gene on a second X chromosome to compensate for the mutated copy. A daughter who inherits the disease associated mutated gene from her mother on one of her X chromosomes will be at risk for having a son affected with the disease.
The risk of passing an autosomal disease to a child depends on whether the gene is dominant or recessive. A prospective parent carrying a dominant gene has a 50% chance of passing the gene to a child. A child needs to receive only one copy of the mutated gene to be affected by the disease.
If the gene is recessive, a child needs to receive two copies of the mutated gene, one from each parent, to be affected by the disease. When both prospective parents are carriers, their child has a 25% chance of inheriting two copies of the mutated gene and being affected by the disease; a 50% chance of inheriting one copy of the mutated gene, and being a carrier of the disease but not affected; and a 25% chance of inheriting two normal genes. When only one prospective parent is a carrier, a child has a 50% chance of inheriting one mutated gene and being an unaffected carrier of the disease, and a 50% chance of inheriting two normal genes.
Cystic fibrosis is a disease that affects the lungs and pancreas and is discovered in early childhood. It is the most common autosomal recessive genetic disease found in the caucasian population: one in 25 people of Northern European ancestry are carriers of a mutated cystic fibrosis gene. The gene, located on chromosome 7, was identified in 1989.
The gene mutation for cystic fibrosis is detected by a direct DNA test. More than 600 mutations of the cystic fibrosis gene have been found; each of these mutations causes the same disease. Tests are available for the most common mutations. Tests that check for 86 of the most common mutations in the Caucasian population will detect 90% of carriers for cystic fibrosis. (The percentage of mutations detected varies according to the individual's ethnic background). If a person tests negative, it is likely, but not guaranteed that he or she does not have the gene. Both prospective parents must be carriers of the gene to have a child with cystic fibrosis.
Tay-Sachs disease, also autosomal recessive, affects children primarily of Ashkenazi Jewish descent. Children with this disease die between the ages of two and five. This disease was previously detected by looking for a missing enzyme. The mutated gene has now been identified and can be detected using direct DNA mutation analysis.
Not all genetic diseases show their effect immediately at birth or early in childhood. Although the gene mutation is present at birth, some diseases do not appear until adulthood. If a specific mutated gene responsible for a late-onset disease has been identified, a person from an affected family can be tested before symptoms appear.
Huntington's disease is one example of a late-onset autosomal dominant disease. Its symptoms of mental confusion and abnormal body movements do not appear until middle to late adulthood. The chromosome location of the gene responsible for Huntington's chorea was located in 1983 after studying the DNA from a large Venezuelan family affected by the disease. Ten years later the gene was identified. A test now is available to detect the presence of the expanded base pair sequence responsible for causing the disease. The presence of this expanded sequence means the person will develop the disease.
The specific genetic cause of Alzheimer's disease, another late onset disease, is not as clear. Although many cases appear to be inherited in an autosomal dominant pattern, many other cases exist as single incidents in a family. Like Huntington's, symptoms of mental deterioration first appear in adulthood. Genetic research has found an association between this disease and genes on four different chromosomes. The validity of looking for these genes in a person without symptoms or without family history of the disease is still being studied.
CANCER SUSCEPTIBILITY TESTING. Cancer can result from an inherited (germline) mutated gene or a gene that mutated sometime during a person's lifetime (acquired mutation). Some genes, called tumor suppressor genes, produce proteins that protect the body from cancer. If one of these genes develops a mutation, it is unable to produce the protective protein. If the second copy of the gene is normal, its action may be sufficient to continue production, but if that gene later also develops a mutation, the person is vulnerable to cancer. Other genes, called oncogenes, are involved in the normal growth of cells. A mutation in an oncogene can cause too much growth, the beginning of cancer.
Direct DNA tests currently are available to look for gene mutations identified and linked to several kinds of cancer. People with a family history of these cancers are those most likely to be tested. If one of these mutated genes is found, the person is more susceptible to developing the cancer. The likelihood that the person will develop the cancer, even with the mutated gene, is not always known because other genetic and environmental factors also are involved in the development of cancer.
Cancer susceptibility tests are most useful when a positive test result can be followed with clear treatment options. In families with familial polyposis of the colon, testing a child for a mutated APC gene can reveal whether or not the child needs frequent monitoring for the disease. In 2003, reports showed that genetic testing for high-risk colon cancer patients has improved risk assessment. In families with potentially fatal familial medullary thyroid cancer or multiple endocrine neoplasia type 2, finding a mutated RET gene in a child provides the opportunity for that child to have preventive removal of the thyroid gland. In the same way, MSH1 and MSH2 mutations can reveal which members in an affected family are vulnerable to familiar colorectal cancer and would benefit from aggressive monitoring.
In 1994, a mutation linked to early-onset familial breast and ovarian cancer was identified. BRCA1 is located on chromosome 17. Women with a mutated form of this gene have an increased risk of developing breast and ovarian cancer. A second related gene, BRCA2, was later discovered. Located on chromosome 13, it also carries increased risk of breast and ovarian cancer. Although both genes are rare in the general population, they are slightly more common in women of Ashkenazi Jewish descent.
When a woman is found to have a mutation of one of these genes, the likelihood that she will get breast or ovarian cancer increases, but not to 100%. Other genetic and environmental factors influence the outcome.
Testing for these genes is most valuable in families where a mutation has already been found. BRCA1 and BRCA2 are large genes; BRCA1 includes 100,000 bases. More than 120 mutations to this gene have been discovered, but a mutation could occur in any one of the bases. Studies show tests for these genes may miss 30% of existing mutations. The rate of missed mutations, the unknown disease likelihood in spite of a positive result, and the lack of a clear preventive response to a positive result, make the value of this test for the general population uncertain.
Prenatal and postnatal chromosome analysis
Chromosome analysis can be done on fetal cells primarily when the mother is age 35 or older at the time of delivery, experienced multiple miscarriages, or reports a family history of a genetic abnormality. Prenatal testing is done on the fetal cells from a chorionic villus sampling (from the baby's developing placenta) at 9-12 weeks or from the amniotic fluid (the fluid surrounding the baby) at 15-22 weeks of pregnancy. Cells from amniotic fluid grow for seven to 10 days before they are ready to be analyzed. Chorionic villi cells have the potential to grow faster and can be analyzed sooner.
Chromosome analysis using blood cells is done on a child who is born with or later develops signs of mental retardation or physical malformation. In the older child, chromosome analysis may be done to investigate developmental delays.
Extra or missing chromosomes cause mental and physical abnormalities. A child born with an extra chromosome 21 (trisomy 21) has Down syndrome. An extra chromosome 13 or 18 also produce well known syndromes. A missing X chromosome causes Turner syndrome and an extra X in a male causes Klinefelter syndrome. Other abnormalities are caused by extra or missing pieces of chromosomes. Fragile X syndrome is a sex-linked disease, causing mental retardation in males.
Chromosome material also may be rearranged, such as the end of chromosome 1 moved to the end of chromosome 3. This is called a chromosomal translocation. If no material is added or deleted in the exchange, the person may not be affected. Such an exchange, however, can cause infertility or abnormalities if passed to children.
Evaluation of a man and woman's infertility or repeated miscarriages will include blood studies of both to check for a chromosome translocation. Many chromosome abnormalities are incompatible with life; babies with these abnormalities often miscarrry during the first trimester. Cells from a baby that died before birth can be studied to look for chromosome abnormalities that may have caused the death.
Cancer diagnosis and prognosis
Certain cancers, particularly leukemia and lymphoma, are associated with changes in chromosomes: extra or missing complete chromosomes, extra or missing portions of chromosomes, or exchanges of material (translocations) between chromosomes. Studies show that the locations of the chromosome breaks are at locations of tumor suppressor genes or oncogenes.
Chromosome analysis on cells from blood, bone marrow, or solid tumor helps diagnose certain kinds of leukemia and lymphoma and often helps predict how well the person will respond to treatment. After treatment has begun, periodic monitoring of these chromosome changes in the blood and bone marrow gives the physician information as to the effectiveness of the treatment.
A well-known chromosome rearrangement is found in chronic myelogenous leukemia. This leukemia is associated with an exchange of material between chromosomes 9 and 22. The resulting smaller chromosome 22 is called the Philadelphia chromosome.
Most tests for genetic diseases of children and adults are done on blood. To collect the 5-10 mL of blood needed, a healthcare worker draws blood from a vein in the inner elbow region. Collection of the sample takes only a few minutes.
Prenatal testing is done either on amniotic fluid or a chorionic villus sampling. To collect amniotic fluid, a physician performs a procedure called amniocentesis. An ultrasound is done to find the baby's position and an area filled with amniotic fluid. The physician inserts a needle through the woman's skin and the wall of her uterus and withdraws 5-10 mL of amniotic fluid. Placental tissue for a chorionic villus sampling is taken through the cervix. Each procedure takes approximately 30 minutes. A 2003 study comparing the two tests reported that chorionic villus sampling resulted in fewer cases of pregnancy loss, amniotic fluid leakage, and birth defects.
Bone marrow is used for chromosome analysis in a person with leukemia or lymphoma. The person is given local anesthesia. Then the physician inserts a needle through the skin and into the bone (usually the sternum or hip bone). One-half to 2 mL of bone marrow is withdrawn. This procedure takes approximately 30 minutes.
After blood collection the person can feel discomfort or bruising at the puncture site or may become dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs to the puncture site relieve discomfort.
Chorionic villus sampling, amniocentesis and bone marrow procedures are done under a physician's supervision. The person is asked to rest after the procedure and is watched for weakness and signs of bleeding.
Collection of amniotic fluid and chorionic villus sampling, have the risk of miscarriage, infection, and bleeding; the risks are higher for the chorionic villus sampling. Because of the potential risks for miscarriage, 0.5% following the amniocentesis and 1% following the chorionic villus sampling procedure, both of these prenatal tests are offered to couples, but not required. A woman should tell her physician immediately if she has cramping, bleeding, fluid loss, an increased temperature, or a change in the baby's movement following either of these procedures.
After bone marrow collection, the puncture site may become tender and the person's temperature may rise. These are signs of a possible infection.
Genetic testing involves other nonphysical risks. Many people fear the possible loss of privacy about personal health information. Results of genetic tests may be reported to insurance companies and affect a person's insurability. Some people pay out-of-pocket for genetic tests to avoid this possibility. Laws have been proposed to deal with this problem. Other family members may be affected by the results of a person's genetic test. Privacy of the person tested and the family members affected is a consideration when deciding to have a test and to share the results.
A positive result carries a psychological burden, especially if the test indicates the person will develop a disease, such as Huntington's chorea. The news that a person may be susceptible to a specific kind of cancer, while it may encourage positive preventive measures, also may negatively shadow many decisions and activities.
A genetic test result may also be inconclusive, meaning no definitive result can be given to the individual or family. This may cause the individual to feel more anxious and frustrated and experience psychological difficulties.
Prior to undergoing genetic testing, individuals need to learn from the genetic counselor the likelihood that the test could miss a mutation or abnormality.
A normal result for chromosome analysis is 46, XX or 46, XY. This means there are 46 chromosomes (including two X chromosomes for a female or one X and one Y for a male) with no structural abnormalities. A normal result for a direct DNA mutation analysis or linkage study is no gene mutation found.
There can be some benefits from genetic testing when the individual tested is not found to carry a genetic mutation. Those who learn with certainty they are no longer at risk for a genetic disease may choose not to undergo preventive therapies and may feel less anxious and relieved.
An abnormal chromosome analysis report will include the total number of chromosomes and will identify the abnormality found. Tests for gene mutations will report the mutations found.
There are many ethical issues to consider with an abnormal prenatal test result. Many of the diseases tested for during a pregnancy cannot be treated or cured. In addition, some diseases tested for during pregnancy may have a late-onset of symptoms or have minimal effects on the affected individual.
Before making decisions based on an abnormal test result, the person should meet again with a genetic counselor to fully understand the meaning of the results, learn what options are available based on the test result, and the risks and benefits of each of those options.
Autosomal disease— A disease caused by a gene located on a chromosome other than a sex chromosome (autosomal chromosome).
Carrier— A person who possesses a gene for an abnormal trait without showing signs of the disorder. The person may pass the abnormal gene on to offspring.
Chromosome— A microscopic thread-like structure found within each cell of the body that consists of a complex of proteins and DNA. Humans have 46 chromosomes arranged into 23 pairs. Changes in either the total number of chromosomes or their shape and size (structure) may lead to physical or mental abnormalities.
Deoxyribonucleic acid (DNA)— The genetic material in cells that holds the inherited instructions for growth, development, and cellular functioning.
Dominant gene— A gene, whose presence as a single copy, controls the expression of a trait.
Enzyme— A protein that catalyzes a biochemical reaction or change without changing its own structure or function.
Gene— A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.
Karyotype— A standard arrangement of photographic or computer-generated images of chromosome pairs from a cell in ascending numerical order, from largest to smallest.
Mutation— A permanent change in the genetic material that may alter a trait or characteristic of an individual, or manifest as disease, and can be transmitted to offspring.
Positive predictive value (PPV)— The probability that a person with a positive test result has, or will get, the disease.
Recessive gene— A type of gene that is not expressed as a trait unless inherited by both parents.
Sensitivity— The proportion of people with a disease who are correctly diagnosed (test positive based on diagnostic criteria). The higher the sensitivity of a test or diagnostic criteria, the lower the rate of 'false negatives,' people who have a disease but are not identified through the test.
Sex-linked disorder— A disorder caused by a gene located on a sex chromosome, usually the X chromosome.
"Best Early Test." Fit Pregnancy (October-November, 2003): 37.
Bodenhorn, Nancy, and Gerald Lawson. "Genetic Counseling: Implications for Community Counselors." Journal of Counseling and Development (Fall 2003): 497-495.
"Genetic Testing for High-risk Colon Cancer Patients has Improved Risk Assessment." Genomics & Genetics Weekly (August 1, 2003): 18.
"Genetic Testing Increasing Internationally." Health & Medicine Week (September 29, 2003): 283.
Wechsler, Jill. "From Genome Exploration to Drug Development." Pharmaceutical Technology Europe (June 2003): 18-23.
Yan, Hai. "Genetic Testing-Present and Future." Science (September 15, 2000): 1890-1892.
Alliance of Genetic Support Groups. 4301 Connecticut Ave. NW, Suite 404, Washington, DC 20008. (202) 966-5557. Fax: (202) 966-8553. 〈http://www.geneticalliance.org〉.
American College of Medical Genetics. 9650 Rockville Pike, Bethesda, MD 20814-3998. (301) 571-1825. 〈http://www.faseb.org/genetics/acmg/acmgmenu.htm〉.
American Society of Human Genetics. 9650 Rockville Pike, Bethesda, MD 20814-3998. (301) 571-1825. 〈http://www.faseb.org/genetics/ashg/ashgmenu.htm〉.
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. firstname.lastname@example.org. 〈http://www.modimes.org〉.
National Human Genome Research Institute. The National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892. (301) 496-2433. 〈http://www.nhgri.nih.gov〉.
National Society of Genetic Counselors. 233 Canterbury Dr., Wallingford, PA 19086-6617. (610) 872-1192. 〈http://www.nsgc.org/GeneticCounselingYou.asp〉.
Blazing a Genetic Trail. Online genetic tutorial. 〈http://www.hhmi.org/GeneticTrail/〉.
The Gene Letter. Online newsletter. 〈http://www.geneletter.org〉.
Online Mendelian Inheritance in Man. Online genetic testing information sponsored by National Center for Biotechnology Information. 〈http://www.ncbi.nlm.-nih.gov/Omim/〉.
Understanding Gene Testing. Online brochure produced by the U.S. Department of Health and Human Services. 〈http://www.gene.com/ae/AE/AEPC/NIH/index.html〉.
Hunt, Katherine; Odle, Teresa. "Genetic Testing." Gale Encyclopedia of Medicine, 3rd ed.. 2006. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3451600699.html
Hunt, Katherine; Odle, Teresa. "Genetic Testing." Gale Encyclopedia of Medicine, 3rd ed.. 2006. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3451600699.html
Genetic testing is a process which involves examining individuals' genetic material for the presence of a change that indicates why they may have developed a disease or disorder. Genetic testing may also tell patients if they are at increased risk for developing a disease such as cancer in the future, but currently do not have any symptoms of that particular disease.
Genetic testing is usually done by taking a sample of a person's blood. The changes in the genetic material that can be detected by this testing vary in size. Sometimes parts or even entire chromosomes may be altered or missing completely. Other times, a mutation is present on a gene that causes it to malfunction. One type of mutation is known as a hereditary mutation. Hereditary mutations may also be called germline mutations because they are found in all the cells of a person's body, including the reproductive or germ cells, the sperm for a male and the egg for a female. This is why hereditary mutations can be inherited, or passed from a parent to a child. Genetic testing often looks for the presence or absence of these types of mutations in genes.
Genes and cancer
Cancer is defined as one cell that grows out of control and subsequently invades nearby cells and tissue. There are several steps involved in the process that causes a normal cell to become malignant (cancerous). It is believed that different genes play a role in this specialized process. Oncogenes typically promote or encourage cell growth. However, if they are overexpressed or mutated, they may cause cancer to arise. Tumor-suppressor genes, when working properly, prevent cells from growing too quickly or out of control. They are often compared to brakes in a car. If these genes cannot perform their function because of the presence of a mutation, cells may grow out of control and become cancerous. Finally, cancer may also be caused by faulty DNA repair genes. These genes usually correct the common mistakes that are made by the body as the DNA copies itself, a normally occurring process. However, if these genes can't correct mistakes, the mistakes may accumulate and lead to cancer.
It is very important to remember that while all cancer is genetic, or caused by changes in genes, just a small amount of cancer is hereditary, or passed from parent to child. It is thought that only about 5-10% of cancer falls into this category. Therefore, the majority of cancer is not hereditary. Most cancer is due to other causes, such as environmental exposures. Usually it is very difficult to determine the exact cause of cancer that is not known to be the result of an altered gene.
Identifying at-risk individuals and families for hereditary cancer
Although scientists have identified genetic tests for common cancers, like breast and colon cancer , genetic testing is not an option that should be offered to all people with cancer, or even to those who may have cancer in their family. This is primarily due to the fact that most cancer does not run in families. Therefore, genetic testing will not be helpful for many people. In order to determine those who may benefit from undergoing genetic testing for cancer, health care providers need to be aware of certain aspects of an individual's personal and family history of cancer.
A person who is thinking about having a genetic test for cancer often meets with a genetic counselor, a specially trained health care provider. When a patient meets with a genetic counselor, the counselor will ask the patient about their personal and family history of cancer. The counselor will also draw a very detailed family tree, also known as a pedigree. The counselor will then examine the family tree to determine if there are certain "clues" that the cancer may be hereditary.
The clues that may be observed in a family tree are listed below, with breast cancer used as an example.
- Multiple relatives in more than one generation with the same type of cancer, or related cancers. For example, a grandmother, mother and daughter with breast cancer. Or, relatives with both breast and ovarian cancer .
- Cancer occurring in the family at younger ages than is typically observed in the general population. For example, breast cancer usually occurs in women as they get older, most commonly in their 60's to 70's. However, in families that may have an alteration in a gene increasing their risk for developing breast cancer, the disease may occur in women at much younger ages.
- Cancer that occurs in paired organs. For example, breast cancer that occurs in both of a woman's breasts. This is also called bilateral breast cancer.
- Development of more than one type of related cancer in the same person within a family. For example, a female relative with both breast and ovarian cancer diagnosed at young ages.
- Specific ethnic background. Mutations in certain cancer susceptibility genes may be more likely to occur in individuals of specific ethnic backgrounds. For this reason, it is very important that a complete family tree includes the country where a person's relatives originally lived.
If a genetic counselor or other health care provider observes one or more of the above features in an individual's family tree, he or she may talk about the option of genetic testing with the patient. In the case of cancer genetic testing, it is only offered to a patient if there are options available to screen for the certain cancer and detect it early, or to possibly prevent it from occurring at all.
The process of genetic testing
The process of genetic testing for genes that may increase risk for cancer is different from other medical tests. Genes involved in cancer are called cancer susceptibility genes. If a mutation is identified in one of these genes, it does not reveal that a person has cancer, but rather whether an individual has an increased risk to develop cancer in the future. In addition, if the person undergoing genetic testing has already had cancer, genetic testing may tell them whether they are at increased risk for developing cancer again. However, the risk for developing cancer is not 100%. The likelihood that a person will develop cancer if they carry an altered gene is called penetrance. Penetrance may differ even among relatives in the same family, and the reasons are not well understood. For example, a mother with a mutation in a cancer susceptibility gene may never develop cancer, but may pass this mutation on to her daughter, who is then diagnosed with cancer at a young age.
For a family in which an inherited mutation has not been previously identified, it is best to begin genetic testing by obtaining a blood sample from a person who has had cancer at a young age. From this blood sample, scientists will be able to extract some DNA. There are a number of different ways that they can then look at the DNA to determine if a mutation is present. The most common is known as sequencing, whereby the chemical sequence of a patient's DNA is compared to DNA that is known to be normal. Scientists will look for any differences, such as missing or extra pieces of DNA in the patient's gene.
Testing can be very expensive and it may take several weeks or months to obtain results. Also, insurance companies will sometimes not cover the cost of testing. Some families are able to participate in research studies where genetic counseling and testing is offered at a lower cost or free of charge.
Categories of results
A positive result indicates the presence of a genetic mutation that is known to be associated with an increased risk for developing cancer. Once this kind of mutation has been found in an individual, it is possible to test this person's relatives, like their children, for the presence or absence of that particular mutation. This testing can be done in a relatively short period of time and provides results that are clearly positive or negative for a particular mutation.
If a relative in a family is tested for a mutation in a cancer susceptibility gene that was previously identified in their family, and they are not found to have this mutation, this type of test result is called a true negative. This means that they did not inherit the mutation in the gene that is the reason why their relative (s)developed cancer. If a person receives a true negative test result, their risk for developing cancer is generally considered to be reduced to that of someone in the general population. Also, because they did not inherit the mutation, they cannot pass it down to any of their children. The term true negative is used to distinguish this test result from a negative or indeterminate result, which is described below.
If the first person tested within a family is not found to have an alteration in a cancer susceptibility gene, this result is negative. However, this result is often called indeterminate. This is because a negative test result cannot completely rule out the possibility of hereditary cancer still being present within a family. The interpretation of this type of result can be very complex. For example, a negative result may mean that the method used to detect mutations may not be sensitive enough to identify all mutations in the gene, or perhaps the mutation is in a part of the gene that is difficult to analyze. It may also mean that a person has a mutation in another cancer susceptibility gene that has not yet been discovered or is very rare. Finally, a negative result could mean that the person tested does not have an increased risk for developing cancer because of a mutation in a single cancer susceptibility gene.
Finally, sometimes mutations are identified in cancer genes and scientists do not know what they mean. They do not know if these types of mutations affect the functioning of the gene and thereby increase a person's risk for cancer, or if they are normal changes in the DNA that just make one person's gene a little bit different from another person's. When this occurs, the genetic counselor may work with the laboratory to determine if future research can be done to find out the meaning of the patient's test result.
In general, a genetic counselor will help a patient to understand the meaning of his or her genetic test result, whether positive, negative, or indeterminate.
Benefits and limitations of undergoing genetic testing for cancer susceptibility genes
There are potential benefits for patients who undergo genetic testing, but there are also possible limitations and risks regarding the information that is obtained. A genetic counselor will discuss these issues in detail with a patient. Before undergoing genetic testing, a patient will also sign a consent form. This is a written agreement indicating that the patient understands the benefits and risks of genetic testing and has made an independent decision to undergo the testing. The informed consent process is a very important part of genetic counseling and testing. With the exception of FAP, where polyps and subsequently colon cancer can occur at young ages or in the teens, the cancers associated with carrying an altered breast or colon cancer susceptibility gene do not typically occur at very young ages. Therefore, genetic testing for mutations in these genes is usually only offered to those men and women who are 18 years of age or older. In addition, individuals who are 18 or older are considered legally able to provide informed consent.
Benefits of participating in genetic testing for alterations in cancer susceptibility genes:
- Results of genetic testing may provide additional information about the increased risk for developing cancer in the future. It may also provide relief from anxiety if a person learns that they do not carry an altered gene.
- If a person finds out that they are at increased risk for developing cancer, they may choose to be screened for this cancer at a younger age and more often than someone without an altered cancer susceptibility gene. Results may also help men and women decide about prophylactic surgery.
- Testing may provide information about cancer risks for children, brothers and sisters, and other relatives.
- Genetic testing may help a person understand why they and/or their family members developed cancer. This may relieve a person from the emotional burden surrounding their cancer diagnosis.
Limitations and risks of participating in genetic testing for cancer susceptibility genes:
- It is possible that the results of genetic testing may be difficult to interpret. Even if a patient receives a positive test result, this does not mean that he/she will definitely develop cancer.
- During the process of undergoing genetic testing, a person may learn information about themselves or their family members. For example, they may learn about an adoption or that an individual is not the biological father of a child. This kind of information may cause strained relationships among relatives.
- Some patients may become sad, angry or anxious if they learn that they have a mutation in a cancer susceptibility gene. If these feelings are very intense, psychological counseling may be helpful.
- Results of genetic testing may place a person at risk for discrimination by health or life insurers, or their employer. There are some laws in effect that provide limited protection to people who undergo genetic testing. The completion of the Human Genome Project, which has mapped all the genes in the human body, will increase the number of genetic tests that are available. Therefore, additional laws need to be passed to completely protect all people who undergo genetic testing from any type of discrimination.
Genes and cancer types
As of 2001, genes have been discovered that are associated with or responsible for several types of cancer, including Chronic myelocytic leukemia , Burkitt's lymphoma , retinoblastoma , Wilms' tumor , and breast and colon cancers. The remainder of this entry will focus only on genetic testing for two of the most common cancers, breast and colon cancer.
Breast cancer genetic testing
Breast and ovarian cancer statistics
All women have a risk for developing breast and ovarian cancer during their lifetime. While breast cancer is a common cancer among women in the United States, ovarian cancer is not. Most women are diagnosed with breast or ovarian cancer after the age of 50, and the great majority of cases are not hereditary. But, of the 5-10% of breast and ovarian cancer that does run in families, most is due to mutations in two genes, the BReast CAncer-1 gene (BRCA1) and the BReast CAncer-2 gene (BRCA2). The BRCA1 gene is located on chromosome 17, and was discovered in 1994. The BRCA2 gene is on chromosome 13, and was discovered in 1995.
BRCA1 and BRCA2 genes
BRCA1 and BRCA2 genes are tumor suppressor genes and are inherited in a dominant fashion. This means that children of a parent with a mutation in one of the breast cancer genes have a 50% chance to inherit this mutation. These mutations can be passed from either mother or father, and can be inherited by both males and females. The mutations may be detected by performing genetic testing on a patient's blood sample.
Mutations in these genes are more common in people who are Ashkenazi (Eastern or Central European) Jewish. While these mutations may be more common in this specific population, they can be identified in a person of any ethnic background.
Females who inherit a mutation in the BRCA1 or the BRCA2 gene have an increased risk for developing breast and/or ovarian cancer over their lifetime. The lifetime risk for breast cancer may be as high as 85%, as compared to about 13% in the general population. The lifetime risk for developing ovarian cancer may be as high as 60%, as compared to 1.5% in the general population. Males who inherit a mutation in one of these genes are also at increased risk for developing certain cancers, including prostate, colon and breast cancer.
Men and women who inherit an alteration in the BRCA2 gene also have an increased risk to develop more rare cancers, such as pancreatic and stomach cancer . However, these risks are much lower than those observed for breast, ovarian, and prostate cancer .
Screening and prevention options
It is recommended that individuals who are at increased risk for developing breast cancer undergo increased surveillance. This means that they may choose to see their physicians for medical screening tests at an earlier age and more often than they would if they did not have an altered gene. For example, it is recommended that women with an altered BRCA1 or BRCA2 gene undergo mammograms at a younger age than is recommended in the general population. It is also recommended that these women see their doctors more often to do a breast exam and also perform breast self-exams regularly. Because women who have a mutation in BRCA1 or BRCA2 are also at increased risk for developing ovarian cancer, they may also choose to be screened closely for this cancer. This screening involves undergoing a test, called a CA-125, which looks for protein levels in a woman's blood. Women may also undergo a pelvic ultrasound to look at the size and shape of the ovaries to determine if cancer may be growing in that area. It is important to mention that ovarian cancer is a difficult cancer to detect, and these screening methods may not be able to find the cancer at an early stage when a woman can undergo successful treatment.
Men with an altered BRCA1 or BRCA2 gene may also choose to be screened earlier and more frequently for the cancers they are at increased risk to develop. Prostate screening consists of a test called prostate specific antigen (PSA) that looks for protein levels in a man's blood. Men may also undergo an examination by a physician. There are no standard screening recommendations for males who are at increased risk for breast cancer. It is usually recommended that they learn to do breast self-exams and talk with their doctors if they find any changes in their breast tissue.
Some women at increased risk for developing breast or ovarian cancer may decide to have prophylactic or preventive surgery. This means that they may choose to have their healthy breasts or ovaries removed before cancer develops. However, even the very best surgeon cannot remove all of the breast or ovarian tissue. Therefore, even if a woman has her breasts or ovaries removed preventively, she may still develop cancer in the remaining tissue, but this risk is believed to be small.
Finally, some healthy women who are at increased risk for breast or ovarian cancer may decide to take certain medications that have been shown to reduce risk. As some of these medications have been studied only in the general population, further research is underway to find out how effective these medications are for women with an inherited risk for developing cancer.
Colon cancer genetic testing
Colon cancer statistics
Males and females in the general population have a 6% risk for developing colon cancer over their lifetime, and the average individual is diagnosed in their 60s to 70s. Similar to breast and ovarian cancer, most colon cancer does not run in families. However, some colon cancer is hereditary, and may be due to a mutation in a colon cancer susceptibility gene. Three of the more common hereditary colon cancer syndromes are described below.
Familial Adenomatous Polyposis (FAP)
FAP is a syndrome in which individuals develop numerous polyps (growths) in their colon or rectum. This disorder may also be called familial polyposis or Gardner's syndrome. Males or females with FAP often have hundreds of precancerous polyps at young ages, such as when they are teenagers or young adults.
FAP is due to a mutation in a gene called APC. Mutations in this gene are dominantly inherited. In about 80% of families genetic testing performed on a blood sample can find the alteration in the APC gene that is causing this disorder. It is believed that 2/3 of the people with FAP have inherited a mutated gene from their parent. The other 1/3 of individuals with FAP are believed to be new (sporadic) mutations, meaning that the alteration in the APC gene was not inherited from a parent. Individuals with sporadic mutations can pass the mutation on to their children.
Due to the fact that individuals with FAP develop so many polyps in their colon, there is a very high risk that these polyps, if not removed, will develop into colon cancer. Individuals with FAP may also develop precancerous polyps in other organs, such as their stomach or small intestine. Young people with FAP may also be at increased risk for developing a tumor in the liver, known as a hepatoblastoma. They are also at increased risk for developing tumors in other parts of the body, such as the thyroid gland or pancreas. Males or females with FAP may also have other manifestations of the disease. For example, they may have cysts or bumps on their skin or on the bones of their legs or arms, or freckle-like spots in their eyes.
APC I1307K mutation
In 1997 scientists identified another mutation on the APC gene, known as I1307K. This mutation is found only in individuals who are of Ashkenazi Jewish descent. It is estimated that about 6% of individuals who are Jewish have this particular mutation. The I1307K mutation itself does not cause an increased risk for colon cancer, but rather makes the APC gene more likely to undergo other genetic changes. These other genetic changes increase a person's risk for developing colon cancer. Genetic testing can be performed on a blood sample to determine if an individual carries the I1307K mutation. A person with this mutation has a 50% chance of passing it on to his/her children.
Individuals who carry the I1307K mutation have an 18%-30% risk for developing colon cancer over their lifetime. Research is ongoing to determine if individuals with this mutation may also be at risk for developing other types of cancer, such as breast cancer.
Hereditary Non-Polyposis Colorectal Cancer (HNPCC)
HNPCC, also known as Lynch Syndrome, is a condition in which individuals have an increased risk for developing colon cancer, even if there are very few or no polyps present in the colon. It is believed that mutations in one of five cancer susceptibility genes are associated with most cases of HNPCC. These genes are known as hMSH2, hPMS1, MSH6 (all on chromosome 2), hMLH1 (chromosome 1) and hPMS2 (chromosome 7). It is possible that other genes may be found which are also associated with HNPCC. Mutations in these genes are dominantly inherited, and may be able to be detected through genetic testing performed on a patient's blood sample.
Individuals with an altered HNPCC gene have a much higher risk for developing colon cancer, often at a younger age (less than 50) than people in the general population. Those with an HNPCC mutation are at increased risk for developing other types of cancer, including stomach, urinary tract, bile duct, uterine and ovarian cancer. It is recommended that men and women also be screened closely for these cancers.
Screening and prevention options
It is recommended that all individuals who are at increased risk for developing colon cancer undergo screening for this cancer. Screening for colon cancer consists of two main types of tests. The first test is called a sigmoidoscopy . It is performed by inserting a flexible tube, called a sigmoidoscope into the anus to look at the rectum and the lower colon. The doctor can use the scope to see whether polyps may be present, but these growths can not be removed with this test. The second test is known as a colonoscopy . While it is very similar to a sigmoidoscopy, it allows a doctor to see the entire colon. Also, with the use of a colonoscope a polyp can be easily removed at the same time a person is undergoing the test. However, because a colonoscopy is a more invasive test, patients have to be sedated. For patients who are at increased risk for developing colon cancer, it is recommended that they undergo this screening at younger ages and more often then individuals in the general population. For example, because cancer can occur at such young ages for individuals with FAP, it is recommended that they have a sigmoidoscopy beginning at age 11.
Finally, men and women with a mutation in a colon cancer susceptibility gene may take certain medications that have been approved for use in individuals with an increased risk for developing colon cancer.
The only way to prevent colon cancer from developing is to remove the colon entirely. If a person with FAP, HNPCC or the I1307K mutation develops colon cancer he/she may choose to have the colon removed. In addition, if an individual is very anxious about developing colon cancer he or she may choose to have the colon removed before cancer develops. There are several different procedures for removing the colon that allow a person to function normally. Women with an HNPCC mutation may also consider prophylactic removal of their ovaries and uterus.
See Also Cancer genetics; Familial cancer syndromes
Offitt, Kenneth, M.D., M.P.H., Clinical Cancer Genetics. New York:Wiley Liss Inc., 1998.
Burke, W., et al. "Recommendations for Follow-up Care of Individuals With an Inherited Predisposition to Cancer-Hereditary Nonpolyposis Colon Cancer." Journal of the American Medical Association 277 (March 1997): 915-19.
Burke, W., et al. "Recommendations for Follow-up Care of Individuals With an Inherited Predisposition to Cancer-BRCA1 and BRCA2." Journal of the American Medical Association 277 (March 1997): 997-03.
Cummings, S., and O. Olopade. "Predisposition Testing for Inherited Breast Cancer." Oncology 12 (August 1998): 1227-41.
Laken, S.J., et al. "Familial Colorectal Cancer in Ashkenazim due to a hymermutable tract in APC." Nature Genetics 17 (1997): 79-83.
Lindor, N.M., et al. "The Concise Handbook of Family Cancer Syndromes." Journal of the National Cancer Institute 90 (July 1998): 1039-71.
National Cancer Institute. 31 Center Drive, MSC 2580 Bethesda, MD 20892-2580. (800)4-CANCER.<http://www.nci.nih.gov>.
National Society of Genetic Counselors. 233 Canterbury Drive, Wallingford, PA 19086-6617. (610) 872-7608. <http://www.nsgc.og>.
Johns Hopkins Hereditary Colorectal Cancer Resources. 23March 2001 <http://www.hopkins-coloncancer.org/>.
Genetic Health. 27 March 2001 <http://www.genetichealth.com/>.
Tiffani A. DeMarco, M.S.
—The process by which cells grow out of control and subsequently invade nearby cells and tissue.
Cancer susceptibility gene
—The type of genes involved in cancer. If a mutation is identified in this type of gene it does not reveal whether or not a person has cancer, but rather whether an individual has an increased risk (is susceptible) to develop cancer (or develop cancer again) in the future.
—A screening test performed with a tube called a colonoscope that allows a doctor to view a patient's entire colon and rectum.
—Structures found in the center of a human cell on which genes are located.
DNA repair genes
—A type of gene that usually corrects the common mistakes that are made by the body as DNA copies itself. If these genes are mutated and cannot correct these mistakes they may accumulate and lead to cancer.
—Packages of DNA that control the growth, development and normal function of the body.
—A specially trained health care provider who helps individuals understand if a disease (such as cancer)is running in their family and their risk for inheriting this disease. Genetic counselors also discuss the benefits, risks and limitations of genetic testing with patients.
—A cancerous tumor of the liver. Individuals with FAP have an increased risk for developing this type of tumor at a young age.
—A screening test that uses X-rays to look at a woman's breasts for any abnormalities, such as cancer.
—An alteration in the number or order of the DNA sequence of a gene.
—Genes that typically promote cell growth. If mutated, they may encourage cancer to develop.
—A family tree. Often used by a genetic counselor to determine if a disease may be passed from a parent to a child.
—The likelihood that a person will develop a disease (such as cancer), if they have a mutation in a gene that increases their risk for developing that disorder.
—A growth that may develop in the colon. These growths may be benign or cancerous.
—The preventive removal of an organ or tissue before a disease such as cancer develops.
—A method of performing genetic testing where the chemical order of a patient's DNA is compared to that of normal DNA.
—A screening test performed with a flexible scope called a sigmoidoscope, that allows a doctor to view a limited portion of a patient's colon or rectum for the presence of polyps.
Tumor suppressor gene
—Genes that typically prevent cells from growing out of control and forming tumors that may be cancerous.
QUESTIONS TO ASK THE DOCTOR
- What is the likelihood that the cancer in my family is due to a mutation in a cancer susceptibility gene?
- If the cancer in my family is hereditary, what is the chance that I carry a mutation in a cancer susceptibility gene?
- What are the benefits, limitations and risks of undergoing genetic testing?
- What is the cost of genetic testing and how long will it take to obtain results?
- If I undergo genetic testing, will my insurance company pay for testing? If so, will I want to share my results with them?
- What does a positive test result mean for me?
- What does a negative test result mean for me?
- If I test positive for a mutation in a cancer susceptibility gene, what are the best options available for screening and prevention? What research studies may I be eligible to participate in?
- What legislation is in effect to protect me against discrimination by my insurer or employer?
DeMarco, Tiffani A. "Genetic Testing." Gale Encyclopedia of Cancer. 2002. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3405200200.html
DeMarco, Tiffani A. "Genetic Testing." Gale Encyclopedia of Cancer. 2002. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3405200200.html
Genetic testing involves examining a person's DNA in order to find changes or mutations that might put an individual, or that individual's children, at risk for a genetic disorder. These changes might be at the chromosomal level, involving extra, missing, or rearranged chromosome material. Or the changes might be extremely small, affecting just one or more of the chemical bases that make up the DNA. In a broader sense, genetic testing includes other types of testing that provide information about a person's genetic makeup, such as enzyme testing to diagnose or identify carriers for a genetic condition such as Tay-Sachs disease.
With hundreds of genetic tests available, determining who should be offered testing and under what circumstances testing should occur is relatively complicated. In general, testing is offered to those at highest risk based on their ethnic background, family history, or symptoms. However, just because genetic testing is possible and a person is at risk, this does not mean it should be offered or will be useful to that person. Genetic testing is unlike other medical tests in that individual results may also provide information about relatives, may be able to predict the likelihood of a future illness for which treatment may or may not be available, may put the person at risk for harm such as discrimination, or may have limited accuracy. There are a number of settings in which genetic testing occurs and within each setting there are a variety of indications and considerations for testing.
Prenatal Genetic Testing
Prenatal (before birth) genetic testing refers to testing the fetus for a potential genetic condition. The pregnant woman is considered the patient and makes decisions regarding prenatal testing. There are a variety of circumstances under which a woman might be offered prenatal genetic testing. The parents of the fetus may have a genetic disorder, or they may be what is known as carriers . An abnormality or birth defect may be detected on ultra-sound that could indicate a genetic condition. The fetus may be identified to be at increased risk for a chromosome abnormality, such as Down syndrome, or a birth defect, such as spina bifida, based on the result of a maternal serum screening test performed on the mother. This is a test that looks at several proteins made by the fetus that are found in a woman's bloodstream while she is pregnant. Or, the mother might be at increased risk for having a baby with a chromosome abnormality because of her age. While all women are at risk for having a baby with a chromosome abnormality, women who are age thirty-five or older are offered prenatal chromosome testing because the chance their fetus has a chromosome abnormality is equal to or higher than the chance she will have a miscarriage due to the sampling procedure.
As with most genetic testing, prenatal genetic testing should occur in conjunction with genetic counseling. The genetic counselor provides supportive, nondirective counseling and information. Nondirective counseling means that while the counselor will try to facilitate decisions regarding testing and future pregnancy management, she will not make specific recommendations. Because the decision to undergo testing is personal and must take into account differences in beliefs, life circumstances, and the risk of the procedure, the decisions regarding testing and pregnancy management must be made by the patient. This encounter is also likely to include information about risk of the fetus being affected, the disorder in question, and available testing options.
Most genetic tests are performed on tissue or a blood sample. For obvious reasons, obtaining a sample from a fetus is not the same as obtaining one from a child or adult. Prenatal testing procedures are invasive, and there is a risk of miscarriage with every procedure. For this reason, specially trained physicians perform these tests. Prenatal testing can be accomplished using three different methods: amniocentesis, chorionic villus sampling, and percutaneous umbilical blood sampling. These tests differ in the type of fetal tissue studied, the timing of the testing during pregnancy, and in their risks and benefits.
Amniocentesis is the most common, and it carries the lowest risk of miscarriage (about one in two hundred pregnancies). It is typically performed between sixteen and eighteen weeks into the pregnancy and involves collecting a small amount of amniotic fluid that contains cells of the developing fetus, which can be used for testing. Chorionic villus sampling is performed earlier than amniocentesis, typically between ten and twelve weeks of pregnancy, but about one in one hundred pregnancies are miscarried as a result of this procedure. It involves obtaining a small sample of chorionic villi (fingerlike projections of the chorion, a membrane that will later develop into the placenta), which should contain cells of the fetus. Percutaneous umbilical blood sampling, typically performed after eighteen weeks, is the most difficult to perform and carries the highest risk of miscarriage (about one in fifty pregnancies). It involves withdrawing blood from the umbilical cord and is primarily used when results are needed extremely quickly, or when only a fetal blood sample can provide a given answer about the fetus. For example, it may be used to test the fetus if the mother has been exposed to an infectious organism known to cause birth defects.
While in vitro fertilization has been available for over two decades, more recently it has become possible to test the resulting embryos for genetic disorders when the embryos are between eight and sixteen cells in size. In this procedure, one to two cells are removed, and the section of DNA containing the gene in question is replicated and tested. Only those embryos identified to be free of risk (based on the DNA results) for developing the genetic disorder are implanted in the uterus. This technique is not widely available, however, and it is both expensive and time consuming. Thus, it is used only infrequently.
Newborn screening is unique in being the only genetic testing that it is mandated by the state. The premise of newborn screening is that, for some disorders very early detection and initiation of treatment will prevent health problems, often mental retardation. All newborn infants are tested for a variety of genetic disorders. Each state determines for itself for what disorders to test their newborns. Disorders are chosen based on severity, incidence, ease and accuracy of testing, cost, and benefit of early diagnosis. All states test newborns for phenylketonuria (PKU), a metabolic disorder that is almost never evident at birth. Individuals with PKU are missing an enzyme called phenylalanine hydroxylase, which results in the buildup of phenylalanine. If left untreated, severe mental retardation develops. However, infants with PKU who are placed on a diet low in phenylalanine immediately after birth are expected to develop normally, making PKU an excellent candidate for newborn screening.
Symptomatic Genetic Testing
Genetic testing of individuals who are exhibiting symptoms of a genetic disorder is relatively straightforward. Testing is necessary to either make or confirm a diagnosis, which may improve treatment and establish risk estimates for other family members.
A concern related to symptomatic testing is the duty to recontact the patient in the future if more information becomes available. Often, symptomatic individuals, usually children, present with a variety of symptoms for which no diagnosis can be made clinically and for which there is no genetic test. However, with the completion of the Human Genome Project and the wealth of research being conducted, new genes are discovered regularly, which may result in new testing possibilities. Most physicians inform their patients that more information may be available in the future and ask their patients to contact the clinic periodically to inquire about such updates. It is unclear whether this is sufficient to fulfill the physicians' obligation; however, no clear standards exist on this issue.
Another common testing situation is carrier testing for autosomal recessive disorders. Autosomal recessive disorders are caused by the inheritance of two nonfunctioning genes, one from each carrier parent. The parents are referred to as carriers because they carry only one nonfunctioning gene and are, therefore, not affected by the disorder. Every individual is thought to be an unaffected carrier of some autosomal recessive disorder. This is only a problem, however, if two individuals who both carry the same recessive disorder conceive a child together. Under this circumstance, the child would have a one in four (25%) chance of inheriting a nonworking copy from each parent, thereby inheriting the disorder.
There are hundreds of genetic tests available, but it is not practical to perform every available test on each person. Carrier testing is typically offered only to those individuals who are at increased risk based on family history or ethnic background. While family history bears an obvious correlation, ethnic background is important because those who descend from the same group of ancestors are more likely to carry the same genetic changes. For example, individuals of Ashkenazi Jewish descent have about a one in thirty chance of being a carrier for a condition called Tay-Sachs disease, whereas the carrier frequency is only about one in 300 in other populations. In instances such as this, population screening is often recommended.
Presymptomatic testing (that is, testing a healthy person before symptoms appear) may be considered for a genetic disorder for which there is a family history. The decision to undergo this type of testing is not usually straightforward and should always be accompanied by genetic counseling. There are a number of considerations to take into account when deciding whether to proceed with testing. The first is the usefulness of the information. How will knowing the genetic information benefit the person? Testing is more favorable when preventive treatment is available, when results might have a significant impact upon life decisions, such as having children or getting married, or if it will ease extreme anxiety to learn one's genetic status. If no treatment is available, as in the case of Huntington's disease and other triplet repeat diseases, the information may be of less benefit. In some cases it may even be psychologically harmful.
The second consideration is accuracy, not only of the actual test result, but also of its ability to predict the development of the disorder. Some disorders are caused by more than one gene, or by multiple changes or mutations in the same gene. A given test might not be able to look at all mutations or every gene that causes a disorder, leaving a person who tests negative with doubt as to whether they are truly mutation-free. Some genetic tests, particularly those for complex disorders, are for susceptibility genes. As the name implies, these are genes that make a person susceptible to developing a disorder, but do not guarantee it. An example of this is breast cancer. When deciding whether or not to test for such a disorder, it is important to ask how it would feel to test positive for a susceptibility gene for a serious genetic disorder that may never develop.
The third consideration is risk of personal harm. Testing positive for a disorder may put a person at risk of economic or social harm. Although rare, there have been instances where individuals have been denied insurance, employment, or both based on the results of genetic testing. Also, a person may be at risk of experiencing psychological or emotional problems after undergoing genetic testing. There have been instances, particularly with Huntington's disease testing, where individuals have committed suicide following a positive test result. All of these factors must be presented to, discussed with, and weighed by the individual considering testing, in the context of genetic counseling and prior to making a decision about testing.
Presymptomatic Testing of Children
Presymptomatic testing of children has somewhat different considerations. It is typically considered only when the onset of the disorder occurs in childhood, or when knowing the genetic status will significantly benefit the child, for example by enabling him to receive early preventive treatment. For example, children at risk for inheriting the gene that causes retinoblastoma (cancer of the retina) may be tested because the disease usually presents before age five. With early treatment, the long-term outcome is favorable. Knowing whether the child has inherited the gene will allow physicians to know whether to aggressively screen the child for signs of cancer development.
Many genetic professional organizations have developed position statements regarding genetic testing of children that discourage testing for disorders that do not pose a risk in childhood and for which early identification poses no benefit to the child. This includes adult-onset disorders, such as Huntington's disease, but also pertains to carrier testing of females for X-linked recessive disorders, such as muscular dystrophy. In most X-linked (sometimes referred to as sex-linked) recessive disorders, females who inherit a mutation on one of their two X chromosomes are usually unaffected carriers because the second X chromosome is able to compensate for the loss. However, because males have only one X chromosome (the other sex chromosome is a Y), they will be affected if they inherit a mutation. In genetic medicine, personal autonomy is a priority. Individuals have the right to make their own decision regarding genetic testing. If a child is tested for an adult-onset disorder or to determine carrier status, their right to make their own decision as an adult has essentially been taken away.
see also Alzheimer's Disease; Breast Cancer; Chromosomal Aberrations; Cystic Fibrosis; Down Syndrome; Genetic Counseling; Genetic Counselor; Genetic Discrimination; Genetic Testing: Ethical Issues; Inheritance Patterns; Metabolic Diseases; Muscular Dystrophy; Population Screening; Prenatal Diagnosis; Tay-Sachs Disease; Triplet Repeat Diseases.
Susan E. Estabrooks
Holtzman, Neil A., et al. "Predictive Genetic Testing: From Basic Research to Clinical Practice." Science 24, no. 278 (1997): 602-605.
Martindale, Diane. "Pink Slip in Your Genes." Scientific American 284 (2001): 19-20.
Ostrer, Harry, Richard H. Scheuermann, and Louis J. Picker. "Benefits and Dangers of Genetic Tests." Nature 392 (1998): 14.
Ponder, Bruce. "Genetic Testing for Cancer Risk." Science 278 (1997): 1050-1054.
Rennie, John. "Grading the Gene Tests." Scientific American 270 (1994): 88-96.
"Secretary's Advisory Committee for Genetic Testing." <http://www4.od.nih.gov/oba/sacgt.htm>.
"Understanding Gene Testing." U.S. Department of Health and Human Services. <http://rex.nci.nih.gov/PATIENTS/INFO_TEACHER/bookshelf/NIH_gene-testing/gene00.html>.
Estabrooks, Susan E.. "Genetic Testing." Genetics. 2003. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3406500123.html
Estabrooks, Susan E.. "Genetic Testing." Genetics. 2003. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3406500123.html
The term genetic testing refers to the molecular analysis of DNA for genetic markers associated with particular genetic conditions, to tests for enzymes or proteins related to gene function, and to chromosomal analysis. Social science studies of genetic testing practices and their social and cultural implications involve, for example, considerations of community, family, kinship, and health and address issues such as informed consent, intellectual property rights, and privacy. Critical assessments of screening and testing practices and of the interpretations of genetic information illuminate the normative effects and premises, past and present, that occur both in institutional settings and in everyday life and are reflected in policy.
One of the earliest and most enduring areas of social science research on genetic testing concerns the social implications of prenatal diagnostic testing (PND). PND involves genetic testing during pregnancy for genetic conditions and chromosomal anomalies in the developing fetus using practices such as chorionic villus sampling (testing placental blood cells) and amniocentesis (testing fetal cells from amniotic fluid). Studies of prenatal diagnostic testing have illustrated the influence it has on experiences of pregnancy, conceptualizations of disability, mediations of genetic information, and knowledge and performances of parental and civil responsibility. Barbara Katz Rothman’s The Tentative Pregnancy: How Amniocentesis Changes the Experience of Motherhood (1986) was among the first studies following the introduction of amniocentesis (at that time done between sixteen and twenty weeks) to illustrate the complexity of women’s experiences of mediating the possibility of undergoing the test, abortion, and/or results indicating a possible disability during the first four to five months of pregnancy. By the late 1990s, amniocentesis had become a routine testing possibility during pregnancy, concurrent with the emergence of an understanding of the fetus as patient (Casper 1998). Published in 1999, Rayna Rapp’s book Testing Women, Testing the Fetus speaks to the duality of women and fetuses as the subjects of prenatal testing. Employing a multi-sited approach, including research with lab technicians, genetic counselors, and pregnant women, Rapp explores the meanings of amniocentesis within this shifting context. Critical decisions stemming from the availability of PND involve the interpretation of genetic information and embodied experience by medical practitioners, genetic counselors, lab technicians, and prospective parents.
In 1989 the first child born as a result of the application of preimplantation genetic diagnosis (PGD), involving the molecular analysis of one or two cells taken from an embryo created by in vitro fertilization prior to transferring the embryo to a woman’s uterus, was reported. In most cases, PGD involves tests for specific genetic mutations, which are offered on the basis of an understanding that one or both potential parents (or egg or sperm donor) is a carrier of a genetically inheritable condition or has a prior history of having a child with a genetic condition. In common parlance and media coverage, children born following PGD have been called designer babies, a term referring to the growing potential to influence the genetic make-up of one’s child and reflecting concerns over the procedure’s implications for social perceptions of normalcy and disability. Aneuploidy screening, generally testing embryos for chromosomal anomalies rather than for specific genetic mutations, is referred to as preimplantation genetic screening (PGS), rather than PGD.
In 2000 Adam Nash became the first child reported to be born as a result of the use of PGD not only for the purpose of selecting out embryos with a particular genetic marker, but also for selecting in embryos whose HLA tissue type directly matches that of an already existing sibling, rendering the child-to-be a compatible stem cell donor. In the media, such children have been referred to as savior siblings, invoking associations with sacrifice as well as life-saving. The use of PGD, or PID as it is sometimes called, is banned in many countries (for example, Germany) and restricted to use in relation to particular genetic conditions in others (for example, the United Kingdom).
Preimplantation genetic diagnosis is also applied to test embryos for late-onset disorders and genetic susceptibility to particular conditions, including Huntington’s disease, breast cancer, and hereditary colon cancer. Debates in this area highlight the perceptions and cultural management of “genetic risk status” in relation to disorders with variable degrees of penetrance (or the degree to which the genetic mutation corresponds to the manifestation of the condition) and for which preventative measures or treatment may be available. For example, the genetic mutation associated with Huntington’s disease is highly penetrant, corresponding to an expected certainty of developing the disease over the course of a lifetime. In contrast, the mutations on the BRCA 1 and BRCA 2 genes, associated with hereditary forms of breast cancer (accounting for 5 to 10 percent of breast cancer cases), have a penetrance level of approximately 80 percent. This means that not everyone who carries the BRCA 1 and BRCA 2 gene mutations will develop breast cancer in their lifetime and that the likelihood of developing this form of breast cancer increases with age.
The term predictive genetic testing (PGT) refers to testing that occurs prior to the appearance of symptoms. It is used in situations where there is a known history of a perceived genetic condition among genetically related individuals. While issues involving the individual are often the focus of studies of genetic testing—for example, the autonomy of the individual to make an informed decision regarding a test and follow-up actions or the specificity and uniqueness of an individual’s “genetic code”—social science research on PGT provides critical data and insight into the construction and experience of hereditary risk and the familial and social context of genetic testing. In the case of testing for genetically inherited mutations, testing may require the prior or parallel testing of a family member and the communication of their potential genetic risk status. Results of genetic tests for one family member may implicate the “risk” status of other genetically related family members, who may or may not choose to undergo genetic testing.
The Human Genome Project, which ran from 1991 to 2003, resulted in the identification of increasing numbers of specific genes and genetic mutations implicated in genetic conditions, as well as the function of enzymes and proteins. There is increasing emphasis placed on the use of predictive genetic testing among a broader population with no prior awareness or familial history of specific genetic conditions. This form of predictive testing is proposed with respect to the implementation of preventative measures and personalized medicine. Routine genetic screening, for example of newborns for cystic fibrosis, raises additional questions regarding the disclosure and use of information about the gene carrier status of individuals, as well as about potential implications for individuals who did not consent to genetic testing. While in many cultural contexts, predictive testing of children is advised against, research demonstrates that testing occurs and, in the name of preventative medicine, is promoted. The availability of genetic tests and their use on a broader scale raises concerns regarding genetic discrimination, specifically in the areas of health insurance and employment.
The field of pharmocogenomics (sometimes referred to as personalized medicine ) focuses on the development of medical and preventative care tailored to an individual’s genetic makeup. In 2005 the U.S. Food and Drug Agency approved the use of BiDil, a post-heart-attack drug treatment marketed by NitroMed, for black individuals following a clinical trial exclusively involving self-identifying black patients. Because women of Ashkenazi Jewish descent are seen to have a significantly higher percentage of mutations at particular points along the BRCA 1 and BRCA 2 genes, in some jurisdictions they may be given access to breast cancer gene testing without the involvement of another living family member with breast cancer, which is often otherwise required. This relationship between genetic testing and ethnic identification has manifested differently in Europe, where in 2005 Myriad Genetics won the right to a patent on a particular mutation associated with the Ashkenazi Jewish population, requiring physicians to ask breast cancer gene test candidates whether they are of Ashkenazi Jewish descent. While variations in the manifestation, management, and recognition of various conditions have been recognized in comparative and cross-cultural studies of health, reducing these variations to genetic difference runs the risk of reifying and geneticizing concepts such as race and ethnicity and obscuring social determinants of health.
The turn toward genomics, the study of the interaction between genes and the environment, has resulted in the implementation of biobanks, or population genetic databases, as genetic research resource centers. Members of communities and nation-states are requested to donate blood (DNA) samples for research purposes. When genetic testing is conducted within the framework and for the purpose of health care, there is a normative expectation that individuals are aware of the tests being carried out and also of how resulting genetic information will be managed. Studies of biobanking reveal, however, that research participants (those who donated DNA samples) do not have a clear idea of what will be done with the sample or the information derived from it and are not aware of whom it could be distributed to or for how long it will be retained (see Hoeyer 2003). Children are also being included as participants in larger genomic studies, enrolled at birth via the donation of umbilical cord blood by consenting parents.
The advent of genetic testing in the form of PND, PGD, predictive testing, and population-based genetic research is captured by the phrase new genetics, within which is embedded a distinction from eugenics and other previous uses and abuses of genetics. Whereas eugenics is depicted as the imposition on individuals of decisions made by states or other authorities, the new genetics is associated with choice, information, knowledge, autonomy, and responsibility. Social science research and analysis of the new genetics is needed in order to address, for example, how concepts of normalcy are mobilized in new ways, how new forms of genetic information may lead to present and future discrimination, and how concepts such as reproductive choice and civil responsibility are experienced in the context of increasing emphasis on the significance of genetic information, genetic health, and genetic research.
SEE ALSO Bioethics; Eugenics; Genomics
Casper, Monica. 1998. The Making of the Unborn Patient. A Social Anatomy of Fetal Surgery. New Brunswick, NJ: Rutgers University Press.
Cox, Susan M., and William H. McKellin. 1999. “There’s This Thing in Our Family”: Predictive Testing and the Social Construction of Risk for Huntington Disease. Sociology of Health and Illness 21 (5): 622–646.
Hoeyer, Klaus. 2003. “Science Is Really Needed—That’s All I Know”: Informed Consent and the Non-Verbal Practices of Collecting Blood for Genetic Research in Northern Sweden. New Genetics and Society 22 (3): 229–244.
Koch, Lene. 2004. The Meaning of Eugenics: Reflections on the Government of Genetic Knowledge in the Past and the Present. Science in Context 17 (3): 315–331.
Rapp, Rayna. 1999. Testing Women, Testing the Fetus: The Social Impact of Amniocentesis in America. New York: Routledge.
Rothman, Barbara Katz. 1986. The Tentative Pregnancy: How Amniocentesis Changes the Experience of Motherhood. New York: Viking.
Jacquelyne Marie Luce
"Genetic Testing." International Encyclopedia of the Social Sciences. 2008. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3045300913.html
"Genetic Testing." International Encyclopedia of the Social Sciences. 2008. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3045300913.html
The scientific procedure of examining genetic makeup to determine if an individual possesses genetic traits that indicate a tendency toward acquiring or carrying certain diseases or conditions. In 2001, scientists first published the complete human genome map (a human's genetic blueprint), greatly advancing the capability and use of genetic screening, manipulation, and replication.
Genetic testing of humans facilitates the discovery and treatment of genetic defects, both before and after birth. civil rights proponents, employers, and those who suffer from genetic diseases have debated genetic screening because the procedure poses practical and theoretical legal, economic, and ethical problems. Some theorists, for example, have suggested that genetic screening could improve society if it were made mandatory before hiring or marriage. Others say that this practice would be unconstitutional. Genetic screening is a dynamic rather than static field of medical and scientific experimentation and application that clearly involves scientific, legal, and ethical interests which may differ or compete. Accordingly, each new milestone or discovery warrants commensurate review of these interests for both beneficial and potentially detrimental consequences.
Federal and State Legislation
The earliest national and state legislation concerning genetic screening was enacted in the 1970s. The legislation focused on voluntary genetic testing. The laws generally protect the interests of those who suffer from genetic disease, offer federal and state subsidies for counseling, and support research in genetic diseases.
Congress enacted in 1976 the National Sickle Cell Anemia, Cooley's Anemia, Tay-Sachs, and Genetic Diseases Act (42 U.S.C.A. § 300b-1 et seq.), which permitted the use of public funds for voluntary genetic screening and counseling programs. State legislatures passed measures, with certain exceptions, requiring genetic screening of school-age children for sickle cell anemia. New York enacted a law that provides for premarital testing to identify carriers of the defective sickle cell gene (N.Y. Dom. Rel. Law §13aa [McKinney 1977]). Other states provided for voluntary premarital testing for the sickle cell disease (e.g., Cal. Health & Safety Code § 325-331 [West 1978]); Ga. Code Ann. § 19-3-40 ). Such legislation often included provisions for voluntary, funded counseling (see Va. Code Ann. § 32.1-68 [Michie]).
With the advent of new technology in genetics came increasing concern about its application. In 1996, Congress passed the all-encompassing Health Insurance Portability and Accountability Act (P.L.104-191). One key provision barred group insurance plan administrators from using individual employees' genetic information as a factor when writing group policies (unless such information already resulted in the diagnosis of a illness). However, the bill addressed neither individual policies and premiums nor the use of genetic information in the workplace.
Consequently, in 2000, President bill clinton signed executive order 13145, prohibiting discrimination in federal employment based on genetic information. As of early 2003, no similar federal law covered the private sector workplace. However, according to the National Human Genome Research Institute (a division of the National Institutes of Health), 39 states had enacted bills addressing genetic discrimination in health insurance (see, e.g. Alabama Code §27–53–2,4; Alaska Statutes Annotated §21.54.100; Louisiana Revised Statutes Annotated §22.213.6,7, and so on). Another 27 states had passed bills addressing genetic discrimination in the workplace.
The Constitution, Civil Rights, and Scientific Theory
In 1981 and again in 2002, Congress held hearings to identify potential problems of widespread genetic screening. Subsequent legal and medical discussion has focused on the ethics of certain practices such as eugenics, a form of genetic engineering that involves the systematic programming of genes to create a specific life form or the use of living animals for experimentation. Both House and Senate committees had pending bills before Congress (S 318, S 382) hoping to create national legislation addressing prohibited uses of genetic screening.
One potential problem with genetic screening arises in its use by employers. Although an employer considering hiring an individual with a genetic disease often relies primarily on economic issues, the practice of screening prospective employees and eliminating those with defective genes may be discriminatory because some genetic diseases afflict certain ethnic and racial groups more often than others. G-6-PD deficiency, for example, occurs most frequently in blacks and persons of Mediterranean descent. If screening excludes persons with G-6-PD deficiency, it will have a stronger effect on those groups. This practice could violate Title VII of the civil rights act of 1964 (42 U.S.C.A. §§ 2000e et seq.).
In early 2001, the first federal court lawsuit of its kind was filed against a private company alleging violations under the Americans with Disabilities Act (ADA), P.L. 101-336 and several state laws. According to the suit, employer Burlington Sante Fe Railroad began furtively testing the blood of workers with carpal tunnel syndrome. At least 18 employees claimed to have been subjected to nonconsensual genetic testing. Still, other courts have permitted limited use of genetic screening as an adjudicatory aid in disputes. In a South Carolina child custody case, a judge ordered a woman to undergo genetic testing for Huntington's Disease, because the result could impact her ability to care for the children. While some experts would argue that these factors are important to proper legal and personal decision making, others question where the line will be drawn.
Nevertheless, some legal scholars maintain that compulsory genetic screening programs violate the Constitution. They assert, for example, that taking a child's blood sample constitutes a physical invasion of the body in violation of the fourth amendment. Compulsory counseling programs for parents, they say, interfere with the fundamental rights to marry and procreate. The critics of screening propose that less intrusive voluntary programs together with education could accomplish the same objectives.
Even though genetic screening involves at least a minor intrusion into an individual's body and may involve a search within the meaning of the Fourth Amendment, proponents of genetic science maintain that such searches are not unreasonable if executed in a proper manner and justified by a legitimate state interest (see Schmerber v. California, 384 U.S. 757, 86 S. Ct. 1826, 16 L. Ed. 2d 908  [holding that a compulsory blood test to determine intoxication of an automobile driver is not an unreasonable search]). Proponents of mandatory screening and counseling agree that these practices could interfere with the right to procreate. However, they suggest that the state's interests in improving the quality of a population's genetic pool in order to minimize physical suffering and reduce the number of economically dependent persons justifies the infringement on the civil liberties of individuals.
Amniocentesis and the Abortion Debate
A specific form of genetic screening known as amniocentesis raised fundamental constitutional issues when first introduced; in the twenty-first century, however, it is considered standard operating procedure for older women to undergo amniocentesis when they have conceived for the first time. Amniocentesis consists of inserting a needle through the abdominal wall of a pregnant woman into the amniotic sac containing the fetus, withdrawing a sample of the sac fluid, analyzing it for genetic characteristics, and determining whether the fetus has certain genetic defects. If amniocentesis reveals a genetically defective fetus, the parents may choose to abort it or carry it to term. Children born with genetic defects have brought legal claims against their parents for the tort of wrongful life, or wrongful birth.
Before the advent of amniocentesis, wrongful life actions generally failed (Pinkney v. Pinkney, 198 So. 2d 52, [Fla. App. 1967] [refusing to recognize tort of wrongful life for extramarital child plaintiff against father]; and Zepeda v. Zepeda, 41 Ill. App. 2d 240, 190 N.E.2d 849 , cert. denied, 379 U.S. 945, 85 S. Ct. 444, 13 L. Ed. 2d 545 ). The development of procedures such as amniocentesis, coupled with a shift in societal attitudes toward abortion, has led to successful claims for wrongful life. For example, in Haymon v. Wilkerson, 535 A.2d 880 (D.C. App. 1987), a mother brought a
wrongful birth action against a physician after her child was born with Down's syndrome. The court of appeals held that the mother was entitled to recover extraordinary medical and health care expenses incurred as a result of the child's mental and physical abnormalities. As a result of cases such as Haymon, doctors have increased their use of genetic counseling and prenatal testing.
The Future of Genetic Screening
In 1993, the Nobel Prize for chemistry was awarded to Kary Mullis for his development of a technique known as polymerase chain reaction, a method for rapidly isolating and copying any DNA sequence out of a sample that may contain thousands of other genes. This technology is rapidly developing for application not only in eugenics but also for gene manipulation to correct defective gene sequences in many diseases or conditions (nanotechnology). Researchers at Oxford University's Wellcome Trust Centre for Human Genetics announced in 2003 the development of a methodology for concurrently evaluating the functional significance of millions of noncoding polymorphisms that exist in the human genome. This development is expected to contribute greatly to the determination of genetic susceptibility to disease and assessing future health risk through genetic screening.
Beauchamp, Tom L., and James F. Childress. 1983. Principles of Biomedical Ethics. New York: Oxford Univ. Press.
Bettelheim, Adriel. 2001. "Cures May Arise from Genome Mapping, but Congress Anticipates Headaches." CG Weekly 59.
"A Comparison of Enacted State Genetic Discrimination Legislation." 2001. Council for Responsible Genetics (summer). Available online at <www.gene-watch.org> (accessed July 26, 2003).
Darvall, Leanna. 1993. Medicine, Law, and Social Change. Aldershot, England; Brookfield, Wis.: Dartmouth.
Hawkins, Dana. 2001. "The Dark Side of Genetic Testing." U.S. News & World Report 130.
Higgins, Michael. 1998. "Tempest in a Tube." ABA Journal 84.
Mason, John Kenyon, and R. A. McCall-Smith. 1994. Law and Medical Ethics. London: Butterworths.
——. 1987. Butterworths Medico-Legal Encyclopedia. London: Butterworths.
Reilly, Philip R. 1993. "Public Policy and Legal Issues Raised by Advances in Genetic Screening and Testing." Suffolk University Law Review (winter).
"Researchers Describe Advances in Determining Genetic Susceptibility to Disease." 2003. Genomics and Genetics Weekly. Available online at <NewsRx.com> (accessed July 26, 2003).
Smith, George P., II. 1993. Bioethics and the Law. Lanham, Md.: Univ. Press of America.
——. 1981. Genetics, Ethics, and the Law. Gaithersburg, Md.: Associated Faculty Press.
Stever, Pamela S. 1989. "Haymon v. Wilkerson: The Wrongful
Birth Cause of Action Emerges in the District of Columbia." Contemporary Health Law and Policy 51.
"Genetic Screening." West's Encyclopedia of American Law. 2005. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3437701975.html
"Genetic Screening." West's Encyclopedia of American Law. 2005. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3437701975.html
What is genetic testing, what does it involve, and how is it done? We all have 22 pairs of chromosomes, one of each pair from each parent plus an X and Y chromosome in the male and two X chromosomes in the female. Paired genes are arrayed along the length of each pair of chromosome. Thus we all have two copies of each gene, one from each parent, plus either an X and a Y or two X chromosomes. Suppose the particular gene on a particular chromosome carries a defect responsible for a disease, while the other gene on the paired chromosome is normal, yet the person has the disease. Then the mutant gene is said to be dominant. If no disease is present then the gene is recessive; the person is a carrier, but may pass on the defective gene to the offspring. It is important to know in genetic testing whether an individual has two normal genes or one normal and one abnormal gene. Some genetic diseases are associated with X and Y chromosomes and are described as sex-linked, while genetic diseases due to gene defects on the non-sex chromosomes are called autosomal; i.e. these can affect males and females alike. Genetic testing therefore consists of looking for the gene mutation, together with the normal gene in a DNA sample, from a patient. Providing a sample is very easy; for example, simply lightly brushing the inside of the mouth yields enough cells to examine the DNA. Techniques are available to amplify (copy) the DNA in the sample, and, since the sequence of the gene involved is known, probes are used to look for the presence of normal and mutant genes. If only one type of gene is found then the sample is from a homozygous individual, who may have two normal or two abnormal genes. If both types of gene are found then the individual is heterozygous, that is they have one normal and one abnormal gene. Given this information, the consequences for the offspring follow clearly defined genetic rules. Examples of dominant inherited disorders are Huntington's chorea and retinoblastoma; of recessive disorders are cystic fibrosis (CF), sickle cell anaemia, and thalassaemia; and of sex-linked inherited disorders are haemophilia and muscular dystrophy.
Consider an example. A perfectly healthy pregnant female is offered a genetic test and discovers that she is a carrier of CF. It is most important that confidentiality is maintained and that the patient is not made to feel somehow responsible. If the father is also a carrier then the chances are 1 in 4 that the child will have CF. The chances that the child will also be a carrier, like the parents, is 50%, and the chance that the child inherits a normal gene from both parents is again 1 in 4. Testing the father involves some delay and, of course, concern for the parents. In some instances the father will be unknown or parentage of the child may be in doubt, raising further embarrassments for those involved. If the father cannot be found or refuses to take part it is possible to test the fetus by amniocentesis. In this procedure a small volume of fluid is drawn from around the foetus. This contains sufficient fetal cells to collect the DNA. If, when all the tests are done, the unborn child is found to have CF, then termination may be offered, again an ethical problem which the originally unsuspecting parents had never envisaged. Counselling is very important at this stage.
Looking to the future, it is possible that babies will be genotyped at birth and that this may allow prediction of diseases that might arise in later life. In this way prophylactic measures taken early may be able to prevent the disease appearing, or delay its onset. Furthermore, if the disease does develop, knowledge of the genetic profile will allow the most appropriate drug regimens to be prescribed. This sort of benefit will be most useful for polygenic diseases, such as high blood pressure, various forms of cancer, and asthma. It must be remembered, however, that complex diseases of this type have causes that are due both to nature and nurture. Clearly, much disease prevention can be achieved by appropriate attention to nurture — lifestyle, diet, and the like.
Alan W. Cuthbert
COLIN BLAKEMORE and SHELIA JENNETT. "genetic testing." The Oxford Companion to the Body. 2001. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1O128-genetictesting.html
COLIN BLAKEMORE and SHELIA JENNETT. "genetic testing." The Oxford Companion to the Body. 2001. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O128-genetictesting.html
Advances in the science of human genetics since 1980 (particularly the Human Genome Project) have prompted the development of techniques that identify a growing range of deleterious traits or predispositions. As researchers gain greater knowledge about the genetic components of many diseases or disorders, individuals are enabled to take precautionary measures reducing the chances of contracting an illness or mitigating its effects. In addition, genetic testing may be used to prevent the birth of offspring with a severely debilitating illness or disability.
Individuals, for example, may be tested for genetic traits indicating a proclivity for various forms of cancer or heart disease. If these genetic indications are present, individuals can avoid certain lifestyles or diets, take prescribed medications, or undergo invasive surgical techniques (in rare instances), which may help prevent the onset of cancer or heart disease. Moreover, individuals may also be tested for genetic abnormalities that may be passed on to offspring. Individuals may use this knowledge to inform their reproductive decisions. An individual carrying a recessive gene for cystic fibrosis, for instance, may avoid reproducing, limit mate selection to individuals not carrying the same recessive gene, or use a reproductive technology that employs donated gametes.
Genetic testing may also be used to prevent the birth of offspring with a debilitating illness or disability. Using amniocentesis or chorionic villus sampling, for example, a fetus can be tested for a genetically based condition such as Tay-Sachs syndrome. If the test is positive, parents may choose to prepare themselves to care for a terminally ill child or terminate the pregnancy. In addition, preimplantation genetic diagnosis in conjunction with in vitro fertilization may be employed to test a number of embryos, implanting only those that are unaffected by the deleterious gene.
As genetic testing becomes more sophisticated, it offers great promise for advances in diagnostic and preventive techniques. As the understanding of the complex relationship between genes and environmental factors increases, it is hoped that drugs can be developed that will prevent a wide range of late onset diseases. Some envision a day, for example, when individuals with a genetic predisposition for Alzheimer's disease will be able to take prescribed drugs preventing the onset of the disease, or at least mitigating its effects.
Although genetic testing undoubtedly benefits many people, it also raises a number of important ethical, pastoral, and religious issues. There are concerns over privacy. Some worry that genetic testing will be used to discriminate against individuals in employment or insurance coverage. There are concerns over the moral status of the fetus and embryo. Although prenatal testing may prevent the birth of children suffering from severely debilitating illnesses, the techniques also entail destruction of selected fetuses and embryos. More broadly, genetic testing raises intriguing implications for theological anthropology. How will a burgeoning knowledge of human genetics, as well as the ability to manipulate genes, inform religious accounts of what it means to be human?
See also Biotechnology; DNA; Genetic Determinism; Genetics; Human Genome Project; Nature versus Nurture; Sociobiology
chapman, audrey r. unprecedented choices: religious ethics at the frontiers of genetic science. minneapolis, minn.: fortress press, 1999.
cole-turner, ronald. the new genesis: theology and the genetic revolution. louisville, ky.: westminster john knox press, 1993.
cole-turner, ronald, and waters, brent. pastoral genetics: theology and care at the beginning of life. cleveland, ohio: pilgrim press, 1996.
peterson, james c. genetic turning points: the ethics of human genetic intervention. grand rapids, mich.: eerdmans, 2001.
WATERS, BRENT. "Genetic Testing." Encyclopedia of Science and Religion. 2003. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1G2-3404200234.html
WATERS, BRENT. "Genetic Testing." Encyclopedia of Science and Religion. 2003. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3404200234.html
ge·net·ic test·ing • n. the sequencing of human DNA in order to discover genetic differences, anomalies, or mutations that may prove pathological: genetic testing to identify HLA status in the fetuses of 49 couples considered at risk.
"genetic testing." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. (July 28, 2016). http://www.encyclopedia.com/doc/1O999-genetictesting.html
"genetic testing." The Oxford Pocket Dictionary of Current English. 2009. Retrieved July 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-genetictesting.html