Citing rising Jewish intermarriage rates, the leading organization devoted to combating Tay-Sachs is urging doctors to encourage the use of more comprehensive testing methodology to identify carriers of the deadly genetic disease.
The National Tay-Sachs & Allied Diseases Association issued its position statement on Tay-Sachs carrier screening in September 2009. The statement stressed the importance of using enzyme testing to discover whether individuals are Tay-Sachs carriers, rather than relying solely on DNA testing, which it warned is becoming “less sensitive” as “the Ashkenazi Jewish population becomes less homogeneous.”
In the 1970s, when the first Tay-Sachs screening programs were set up, the only carrier-detection method was enzyme testing, which uses a blood sample to look for the low levels of Hexosaminidase A enzyme that suggest someone is a Tay-Sachs carrier. In recent years, however, DNA testing has enabled screening for a battery of Jewish genetic diseases all at once via a saliva or buccal sample.
DNA testing geared toward Ashkenazi Jews searches for specific genetic mutations that cause Tay-Sachs and are prevalent among this population. But it does not turn up other Tay-Sachs mutations that are more prevalent among non-Jews. Rising levels of intermarriage mean that the traditional Ashkenazi mutations increasingly are not the only ones that Jews have to worry about.
“As Jews have become a more mixed group, there is less and less chance that those mutations are the only ones that can be found in Jews,” said Dr. Jodi Hoffman of the Tufts Medical Center, who led the NTSAD advisory panel that recommended updating the guidelines. “Anyone who has ever had someone marry into the family who’s not Ashkenazi can have a different mutation.”
When two parents are both carriers of mutations that cause Tay-Sachs, each of their children will have a 25% chance of being born with the disease.
Despite common misconceptions, Tay-Sachs does not affect only Jews. One in 30 Ashkenazi Jews is a carrier, but carriers are equally common in non-Jewish French-Canadian and Cajun popultions, and Irish people also have a relatively high carrier rate. The carrier rate in the general population is about 1 in 300.
According to the NTSAD statement, DNA testing first became available in the 1990s and screening for three mutations has yielded a 92% to 98% success rate in identifying carriers among people of entirely Ashkenazi descent.
Intermarriage, however, is making the Jewish community more heterogeneous. “The genetic mix in our society is not as segregated as it once was, so the simple DNA test may not be effective anymore,” said NTSAD’s executive director, Susan Kahn.
DNA testing for three mutations among non-Ashkenazi individuals is reported to have yielded a detection rate of only about 20%. Even when the DNA test looked for five different mutations, the success rate climbed to just 60%.
Enzyme testing, which studies have shown to be 98% reliable in detecting Tay-Sachs carriers, works by measuring the Hex-A activity in serum or white-blood cells. In babies born with Tay-Sachs, the absence of Hex-A enzyme causes lipid buildup in nervous system cells, especially in the brain, which almost always leads to death by age 5. Carriers, though asymptomatic, have lower levels of Hex-A that can be detected in a blood sample.
Because enzyme testing measures actual rates of Hex-A rather than looking for specific genetic mutations — which can be different in different populations — intermarriage does not compromise its reliability. The decreased level of Hex-A is analogous to a fever: It is easy and straightforward to measure, even though its underlying cause — in this case, a mutation — may be difficult to pinpoint.
The NTSAD statement expressed concern about people who have had a DNA test opting out of enzyme testing. While it stated that “DNA testing is the appropriate screening modality for many other genetic disorders,” it urged medical professionals to emphasize the importance of enzyme analysis, which it recommended “as the primary testing modality for identifying carriers for Tay-Sachs disease.”
The statement suggested that “DNA testing can and should be used to confirm Tay-Sachs enzyme results, to clarify indeterminate enzyme results, to identify cases of pseudodeficiency, as well as to provide molecular information for reproductive procedures and genetic counseling.”
“DNA testing is nice because it can be confirmatory,” Hoffman said. “It provides added information, but enzyme testing is the gold standard.”