The Johns Hopkins scientists whose research led to the first blood tests for colon cancer predisposition now have developed a technology that dramatically improves the accuracy of such tests. They now can detect-- nearly 100 percent of the time--genetic mutations associated with certain hereditary diseases. A report of their work is in the Feb. 17 issue of Nature.
"Of those who seek genetic testing, up to 50 percent will walk away with no clear answer, not because a gene mutation wasn't there but because the technology was not sophisticated enough to detect it," says Bert Vogelstein, Clayton Professor of Oncology at Johns Hopkins and investigator, Howard Hughes Medical Institute. "Now we can tell people who seek testing, with much greater certainty, whether or not they have inherited specific genetic predispositions to colon cancer."
The team from the Johns Hopkins Oncology Center and the Howard Hughes Medical Institute overcame a major obstacle to testing for genetic mutations in inherited diseases by unmasking mutated genes. The new technology, called Conversion, takes advantage of what has long been a weakness in genetic testing.
Every person carries two copies of a gene--scientifically known as alleles--one of them inherited from the father, one from the mother. Normal genes can "mask" or hide defective ones. Conventional genetic tests analyze both copies of potentially mutated genes at the same time. Conversion separates the two copies of the gene, allowing them to be individually analyzed.
"With current tests, if a portion of a gene was deleted, that deletion mutation could be masked or hidden by the normal copy of the gene. By separating and looking at each copy individually, we can now detect these and other kinds of genetic alterations that were previously missed," explains Kenneth W. Kinzler, professor of oncology and co-director of the study.
Researchers uncovered masked mutant genes by fusing human cells with specially designed mouse cells to create mouse-human cell lines. Each cell line contained a copy of the gene they wanted to study. Then, they looked for gene mutations by using conventional DNA sequencing methods. "Conversion does not replace conventional technologies for genetic testing," Vogelstein notes, "but, by providing separated alleles, Conversion markedly enhances these technologies, making it possible for them to detect mutations that would otherwise be masked."
In the Nature report, the scientists said they used blood samples from 22 patients with a hereditary form of colorectal cancer, known as hereditary nonpolyposis colorectal cancer. Conventional genetic testing was unable to verify mutations in 10 of the 22 patients. Conversion allowed the researchers to identify mutations in all 22 patients.
"This technology does not change who is eligible to receive a genetic test (i.e., people at high risk for a known mutation), but it will make the tests much more useful for those who decide to take them," says Hai Yan, research fellow at the Oncology Center and first author of the study.
Although Conversion has been tested so far only on colon cancer mutations, the technique should apply to other hereditary cancers, including breast and kidney cancer, as well as to a wide variety of neurological and cardiovascular disease genes.
The new gene separation technology described in the Nature report is expected to be available this summer through Hopkins in conjunction with genetic counseling and risk assessment for individuals with two types of inherited predisposition for colorectal cancer: hereditary nonpolyposis colorectal cancer and familial adenomatous polyposis. Anyone with a family history of colorectal cancer can call 410-955-4041 for information.
In addition to Vogelstein, Kinzler and Yan, other research participants from Hopkins included Karin Berg, James R. Eshleman, Weishi Yuan, Steven J. Laken and Christoph Lengauer.