A quote commonly heard these days is that in the history of medicine, the greatest advancements in the treatment of patients have occurred within the past 50 years. But what if a doctor could prevent a disease from occurring, treating the cause rather than the symptoms? We all agree this would be wonderful, but how could a doctor predict a patient's medical future? This dream is now within the realm of possibility. In fact, the greatest change in the history of medicine since the discovery of antibiotics is anticipated to occur over the next several generations. It is the movement of medicine from a discipline that reacts to and treats a disease to one that is focused on preventing disease from occurring.
This feat will be accomplished using genetic and genomic information gained from the Human Genome Project to tailor treatments and medicine to each individual patient. The individual's risk of getting a disease, or even infections, is almost always at least partly due to the combination of genes he or she was born with. Therefore, by using genetic information doctors will be able to predict, with different degrees of certainty, what diseases their patients are at risk of developing. They will then be able to use this information to reduce the chance of the disease from occurring in an individual, or prevent it entirely. This new approach of using genetics in the regular practice of medicine has been given the name "genomic medicine."
The Importance of SNPs
Genomic medicine will be applied to patients through several mechanisms. The primary tool will be the detection of single nucleotide polymorphisms (SNPs). These are small variations in DNA sequences that are found in every person. These normal polymorphisms are very frequent, occurring in approximately one out of every 1,000 base pairs. The majority of these SNPs occurs in regions of the genome that do not code for proteins, and usually do not contribute to a person's disease susceptibility. However, they may serve as "markers" for a disease, lying close to an important susceptibility gene. When they occur directly in a protein-coding area of a gene (exon ), they can cause the protein variation that helps make each of us unique. This variation in proteins is one of the primary reasons that each of us differs in our risk for disease, infections, and drug tolerance.
It is important to realize that these SNPs are not mutations . The term "mutation" suggests a nucleotide change that prevents a protein from performing its normal function. These mutation-caused changes are rare, occurring in less than 1 percent of the population. This type of genetic mutation is almost always severe enough that its presence alone is enough to cause the disease. In contrast, SNPs are much more common, certainly occurring in more than 1 percent of the population. Their presence may make it easier for a person to get a specific disease, but they do not cause the disease by themselves. Instead, disease can occur only in the presence of the correct environment, other gene combinations, drugs, and other such factors. This is very important, for it suggests that by knowing that an individual is at risk for a disease, a doctor can take actions to prevent it. These actions may be as simple as a dietary or lifestyle change, or taking a medicine or vaccine.
Doctors will be able to use this same identification of SNPs to screen patients prior to prescribing drugs. Drug side effects are usually due to a specific, uncommon polymorphism in a gene that produces a protein that interacts with the drug. These gene interactions may not have anything to do with how the drug helps the patient. More likely, the gene is involved in how the body metabolizes the drug. This field of studying genes and their interaction with drugs is called pharmacogenetics. And pharmacogenetics does not just study drug side effects. Every doctor knows that some drugs work better for some patients than others. This difference is also likely due to normal polymorphisms, or different genetic forms of the disease.
The Future of Medicine
The idea of preventive medicine is not new, but until the completion of the Human Genome Project medicine did not have a way of accomplishing it for common medical problems. For example, for years almost every baby born in the United States has had its urine or blood screened early in life for phenylketonuria (PKU), a rare genetic disease in which affected individuals cannot metabolize the amino acid phenyalanine, a common amino acid in our food. Untreated, PKU patients develop severe mental and psychomotor retardation. However, if a child is identified early in life to have the PKU mutation, the disease can be prevented by placing the child on a special diet that lacks phenylalanine.
Genomic medicine technique will not look at just one gene or protein in an individual. It will also look at the interaction of thousands of genes at once, using DNA chips or microarrays. These microarrays are already being used in cancer chemotherapy to predict which drugs will work best on each patient's specific tumor.
Therefore, in the future an individual will be able to go to the doctor for a regular checkup and give a small sample of blood, or maybe even just let the doctor take a mouth swab. This will provide a DNA sample that would then be placed on a genetic "microchip" or other device and quickly be genotyped to give a genetic profile of the patient. The doctor will use the information to tailor the medical treatment for that patient. Lifestyle changes or medicine will be suggested to prevent the occurrence of diseases to which the patient is genetically susceptible, or at least to reduce the risk or severity of such diseases. If medication must be prescribed, the doctor will also use this genetic profile before choosing the medicine, to make sure that "the right medicine for the right patient" is chosen: one that works and will not harm the patient or cause side effects.
These changes in medicine are likely to take place over the first half of the twenty-first century. They will be exciting, but they will require that both patient and doctor have a strong understanding of genetics, the most powerful future tool of medicine.
see also DNA Microarrays; Genetic Testing; Pharmacogenetics and Pharmacogenomics; Polymorphisms.
Jeffery M. Vance
Guttmacher, A., et al. "Genomic Medicine: Who Will Practice It? A Call to Open Arms." American Journal of Medical Geneticists 106 (2001): 216-222.
"Making the Vision of Genomic Medicine a Reality." Centers for Disease Control. (March 2001). <http:www.cdc.gov/genomics/info/perspectives/vision.htm>.
"Medicine and the New Genetics." Human Genome Project Information. <http://www.ornl.gov/hgmis/medicine/medicine.html>.
Pistoi, Sergio. "Facing Your Genetic Destiny." Scientific American (February 18, 2002). <http://www.sciam.com/article.cfm?articleID=00016A09-BE5F-1CDAB4A8809EC588EEDF>.