Knowing the Risks for a Deadly Disorder
Tay-Sachs is an inherited disease that usually proves fatal by age 5. Since there is no cure, medical experts stress the importance of genetic testing before a woman becomes pregnant. Health spoke to Dr. Laird Jackson, a professor of medical genetics and director of the Tay-Sachs Prevention Program at Thomas Jefferson University in Philadelphia, to learn what else can be done.
Question: Who is most at risk for Tay-Sachs?
Answer: Roughly one of every 27 persons of Ashkenazi, or Eastern European, Jewish descent is a carrier of the Tay-Sachs gene. But the Cajun community of Louisiana and French Canadians, especially those centered in eastern Quebec, are at the same risk as Ashkenazi Jews. And those with Irish ancestry are also at increased risk. For them, the carrier rate is about one in 50.
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Q: So just because you come from a diverse gene pool doesn’t mean you’re not at risk. For instance, the children of an Irish American man married to a Louisiana Cajun woman are also at elevated risk for Tay-Sachs.
A: That’s absolutely true. Jews are pretty well-informed about Tay-Sachs and get tested, so that at this point in time, there are probably as many people who aren’t Jewish who get Tay-Sachs. If there’s any suspicion that either person came from a susceptible genetic population, then both parents should be tested to make sure they don’t carry the Tay-Sachs gene. It’s a simple blood test that was developed by Dr. Michael Kaback in the early 1970s.
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Q: What makes certain ethnic groups more susceptible?
A: Two things have to happen. First, a small group of “pioneers” moves to a new geographic area. Fifty people move and by random chance, two of them have the Tay-Sachs gene whereas only 10 of their original population of 5,000 have it. In the new area these “pioneers” remain socially distinct and do not reproduce outside their group. Thus as the new group expands its numbers, 1 in 25 of them will have the gene, while only 1 in 500 at their original home possessed it. It is the combination of the original “sampling” accident and the selective breeding that produces the increased genetic susceptibility.
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Q: What causes Tay-Sachs?
A: Children with Tay-Sachs lack a vital enzyme called hexosaminidase A, which the body needs to break down a fatty waste substance found in brain cells. Without the enzyme, hexosaminidase accumulates abnormally and causes progressive damage to the nervous system that eventually causes death. These children are normal at birth, but they soon begin to fall off the development curve. Most kids sit up by 6 months of age. Tay-Sachs kids don’t. That’s usually the first critical sign. After that, everything starts to go downhill. They lose motor skills and mental functions and become paralyzed, blind, deaf and mentally retarded. Then they die.
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Q: What are the odds of giving birth to a child with Tay-Sachs?
A: If both parents are carriers, there’s a one in four chance the child will have Tay-Sachs. And there is a one in two chance that the child will carry the Tay-Sachs gene. That means the child won’t be affected but could pass the disease on to his or her children.
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Q: I know that 35,000 people a year are now tested for Tay-Sachs in the U.S. and Canada. But what are the options for people at risk who get pregnant accidentally?
A: The latest development is that it’s possible in some families now to be able to do the prenatal testing noninvasively, from the mother’s blood alone. We can’t do that in all cases yet, although I think we will be able to soon. Otherwise, parents can choose either amniocentesis around the 16th week of pregnancy or chorionic villus sampling (CVS) performed between the 10th and 13th week. That will reveal whether the fetus has Tay-Sachs and give parents the option of choosing a therapeutic abortion or carrying the fetus to term. There is no cure.
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Q: But with such good testing available, the number of children born with Tay-Sachs must be shrinking.
A: That’s right. But it hasn’t disappeared, and people need to be aware of that. The best thing to do if you and your spouse suspect you’re in one of the high-risk populations is to get tested before you get pregnant.
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For more information, call the National Tay-Sachs & Allied Diseases Assn at (800) 906-8723 or visit the Web site at https://www.ntsad.org.
