The Discovery of Genetic Markers for Disease
The Discovery of Genetic Markers for Disease
Genetic markers are sequences of DNA located near defective or disease-causing genes that can be used to indicate the presence or absence of these genes. Genetic markers are always at the same place on a chromosome.
Several scientists hit upon the idea of markers at the same time. The association of a gene with a particular chromosome forms the basis of the field of cytogenetics. Cytogentics is a subdiscipline within genetics that links chromosomal variations to specific traits. Beginning with single-gene diseases, such as Duchenne muscular dystrophy, Huntington's disease, and cystic fibrosis, the search for genetic markers has mushroomed to an all-out hunt.
Along with these discoveries have come social and ethical debates over the use of genetic markers. To these debates has been added the idea of changing the problematic gene using gene therapy, creating one of the most controversial issues of the twentieth century.
The idea of mapping the human genome actually began in 1950 with the goal of associating a particular chromosome with a specific physical trait. Geneticists created linkage maps, looking especially at large families with an aberration in a chromosome.
The development of technology has underlain the search for genetic markers. Restriction enzymes were first used to map DNA in 1971. In 1975 two simple techniques sequenced DNA in gels and in transformed bacteria. E.M. Southern developed a technique for gel electrophoresis, in which, using an electric field, light bases travel farther along a gel plate than heavier bases. More complicated techniques were later developed for protein eletrophoresis. For example, high throughput analysis combines many techniques involving microchips and spectrographic analysis. In addition, each October the magazine Science produces a full-color page of known gene locations on the genome. The map is thus slowly becoming complete.
In 1978 two scientists, Y.M. Kan and Andrea Dozy, from the University of California Los Angeles (UCLA) noted that a harmless piece of DNA was always inherited along with sickle cell anemia a blood condition. This variation marked the possibility of a faulty gene, aiding researchers to develop the idea of "reverse genetics." Instead of starting with a product such as a protein and then searching for the gene, members of a family who have the gene are compared with relatives who do not. This method was still time consuming because of the massive amount of DNA to be searched through.
Beginning in 1979, Raymond White began work on restriction fragment length polymorphisms or RFLPs. Certain enzymes, called restriction enzymes, cut DNA out of a chromosome at specific sites. For example, one may cut at a site with the sequence G-A-A-T-T-C and not cut if the sequence has mutated to G-A-C-T-T-C. The fragments can thus be cut into different lengths and separated through gel electrophoresis. Variations in their lengths are called RFLPs. These RFLPs are scattered across the 23 human chromosomes, which have over 100,000 genes. White began by eliminating parts of the genome that were identical. This procedure narrowed the search down to five sequences of DNA between 15,000 to 20,000 bases. Using DNA from 56 donors and radioactive labels, he was able to find eight RFLPs.
Throughout the 1980s, White worked on the theory that RFLPs could be used as genetic markers. In 1981 a team of British researchers studying Duchenne muscular dystrophy (DMD), a condition that affects the muscles of young boys (usually killing them before age 20), found the first RFLP on the X chromosome. In 1986 Louis Kunkel found the gene responsible for Duchenne muscular dystrophy. White also worked with markers for neurofibromatosis 1, a gene related to colon cancer, and the BRCA1 genes related to breast cancer.
In 1982 Canadians Lap-Chee Tsui and Manuel Buchwald started looking at a statistical analysis of factors from blood samples of over 50 families with two or more children with cystic fibrosis (CF). CF is a fatal disease in which the mucus lining thickens, causing major lung congestion and other problems. For two years they searched without finding a marker. A biotechnology firm in Massachusetts offered to provide them with labeled fragments of single-strand DNA involving 200 additional markers. Within a few weeks, a link was found between one of the markers and the DNA from the sample. The firm traced the marker to chromosome 7. White then found a close link between CF and "met," a cancer-causing gene. The English researcher Robert Williamson discovered another marker on chromosome 7. Seven teams had been looking for the CF gene; they decided to meet in Toronto and pool their knowledge. In 1986 Jean-Mark Laloud and White determined that two markers were on either side of the CF gene. In April 1987 Willamson's team announced that they had found a candidate gene but additional tests revealed an error.
Tsui continued to bombard chromosome 7 with markers. Hei obtained membranes from sweat glands, which are associated with CF, and began to compare CF cells with normal cells. In the September 1989 issue of Science, they reported that their new gene had 27 sequences of DNA; only three base pairs were missing—a small number for such a lethal condition.
In 1872 a physician named George Huntington (1850-1916) had described a disease that starts in middle age and then destroys the person both physically and mentally. The disease was later called Huntington's disease (HD). Over one hundred years later, Nancy Wexler, a Columbia University psychologist whose mother had HD, saw a photo of natives in a village on Lake Maricaibo in Venezuela, whose gait resembled that of individuals with HD. Traveling to the remote area, she heard the story of a Portuguese sailor in the 1800s who always walked as if he were drunk. He had married a local woman and had many children. Seven generations later, 250 out of 5,000 of his descendants now had HD. This large family was a boon to geneticists, who could now map a huge family tree. (Wexler gave them blue jeans in exchange for 2,000 blood and skin samples.)
Back in the United States, James Gusella and a team at Massachusetts General Hospital had been sampling tissue from an Iowa family with HD to look for an RFLP. When he received the blood from the Venezuelans, Gusella expected to look for years for the RFLP. A team in Indiana led by P. Michael Conneally, however, joined the information from the Iowan family and the Venezuelan families. In May 1983 the computer found an RFLP that matched; this RFLP was the genetic marker for HD.
Although people walk around carrying genes for lethal diseases, few realize it. In 1989, after the gene for CF was found, some clinics developed a test for the disease, but many problems were encountered. Actually, general screening of the population identifies only 70-75% of CF carriers because many mutations cause the disease. The National Institutes of Health (NIH) recommended not to test until there was a 95% accuracy level. What to do with genetic markers has presented many practical and ethical problems. Having a test that will tell if a person carries a defective gene involves a question regarding benefit. What purpose will it serve and what if the test is wrong? For example, there is a prenatal test for Down's syndrome, a condition involving mental retardation and physical disabilities that is caused by an extra chromosome 21. The test, however, does not reveal how severe the condition is and what—if any—physical impairments will develop.
Another problem with genetic testing involves detecting a gene for a disease that has no cure. Would knowing about the gene help, especially given the possibility for error or misinterpretation? In the summer of 1999, a panel at Stanford University convened to consider a genetic test for Alzheimer's disease. Because there is no cure for the disease, a disease which has major social and personal ramifications, the committee refused to support the initiative.
If screening does become available for a disease, the question then becomes who should be tested. Should ethnic groups displaying a higher proportion of the condition be targeted? Another potential problem is misuse of the information. For example, one fear is that insurance companies may deny health coverage to a person who carries a certain gene. In 1970 the United States pushed an aggressive program to test for sickle cell anemia, a blood condition found primarily in individuals of African descent. The results of this program were a disaster. Misunderstanding was rampant as people who were carriers of the gene thought that they were going to die. Some people were denied jobs or health insurance. Couples who carried the gene were told not to have children. A screening program without counseling and follow-up information thus courts disaster.
In 1985 the NIH approved guidelines for gene therapy experiments in humans. In 1990 the first such treatment was used on a four-yearold girl with adenosine deaminase deficiency. While the therapy appeared to work, its use set off a storm of ethical debate.
Gene therapy uses a vector to send the corrective DNA to cells. Viruses may be used as vectors. Because of the complexity of the procedure and the variety of individual reactions, the progress in gene therapy has been slow. Additionally, in the fall of 1999, the death of a healthy-looking eighteen-year-old with a rare genetic disorder involving ammonia metabolism caused an ethical fury to erupt. Nevertheless, gene therapy remains an area of promise.
The search for genetic markers continues as does the work to complete the human genome. Ethical questions surrounding genetic testing and gene therapy will no doubt likewise continue.
EVELYN B. KELLY
Lewis, Ricki. Human Genetics: Concepts and Applications. Dubuque: Wm. C. Brown, 1997.
Rabinow, Paul. Making PCR: A Story of Biotechnology. Chicago: University of Chicago Press, 1997.
Wexler, Alice. Mapping Fate. New York: Random House, 1995.