HEALTH - JANUARY 14, 2004

Genetic testing: when is it beneficial?

I don’t normally consider The Wall Street Journal a great medical journal, but an article there by Michael Waldholz (Dec. 3) did a good job of summarizing some newer genetic tests and of giving some important tips about their proper uses.

We’ve all heard about the great scientific breakthrough of the last few years – the successful mapping of the more-or-less complete human genome. The success was accompanied by promises of all sorts of revolutions in health care that would be spawned by the new genomics. The revolution, so far, has come to us out of sync. What’s clearly upon us right now is the ability to test people for the presence or absence of just about any gene you might want to know about. What appears as of now to be some distance off is the ability to design any new treatments of genetic diseases that will actually fix what’s wrong with our genes. This out-of-sync nature of genomics has important implications for patients trying to decide whether or not to undergo genetic testing.

The big kids on the gene testing block are:

– Breast and ovarian cancer. About 5 percent of these nasty cancers are caused by mutations in the BRCA1 and BRCA2 genes. Tests can tell if the cancer-causing mutation is present in high-risk women.

– Colon Cancer. Similarly, a rare form of colon cancer runs in families and usually strikes at a young age. A new test, costing several thousand dollars, can detect this rare mutation.

– Diabetes. Another new test seems to offer a good prediction of which children in diabetes prone families are at highest risk of developing the juvenile-onset form of the disease. The test appears about 85 percent accurate, but there is no way of knowing when the disease would actually appear.

– Alzheimer’s Disease. About 60 percent of those who inherit a particular mutation of the APOe gene will develop Alzheimer’s by age 70.

Who, if anybody, should have these tests? The actual value of being tested hinges on several factors. One of the most important, but often forgotten, is whether early knowledge of a risk can be translated into an action plan that prevents or delays the disease.

Breast, ovarian, and colon cancer genetic tests pass this criterion. People known to have the bad genes for breast or colon cancer can have more frequent screening tests and catch a new cancer early enough for it to be successfully treated. Many women with high genetic risk for ovarian cancer will elect to have ovaries surgically removed, since effective screening tests for ovarian cancer are basically non-existent.

By contrast, there is no proven way to prevent people carrying genes predictive of Alzheimer’s or juvenile diabetes from developing those diseases. In those cases, advance knowledge provides no helpful medical strategy for effectively doing something.

Another factor that we’ve discussed in earlier columns on prevention is whether it makes sense to use a genetic test in a selected high-risk group, or on everyone. Since women know that breast cancer is common, they might conclude that a genetic test for breast cancer is of general use to everyone. That’s why it’s so important to realize that some of these tests detect relatively rare genetic disorders that are responsible for only a small percentage of all (say) breast and colon cancers. People at high risk for the bad gene will probably have warning signs – relatives who have developed those cancers at relatively young ages. It makes the most sense today to restrict the use of these genetic tests to those high-risk families, and not to implement them as routine screening tests the way we now do Pap smears, for example.

That’s basically the upside of how to use these new tests wisely. (Incidentally, the basic principles apply to any screening test and are not restricted to tests that carry the “genomics” cachet.) In a later column I’ll touch on some of the downside of these screening tests.

Howard Brody, MD, teaches family practice and medical ethics at Michigan State University. You can reach him at brody@msu.edu.

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