Limited evidence in medical literature suggests that Americans of Irish descent have an elevated carrier frequency, but estimates vary wildly. A 1992 study in Chicago found more Tay-Sachs genes in Irish Catholic spouses of Jews than in the Jews themselves.
Adele Schneider, Einstein's director of clinical genetics, decided to do a large enough study - testing 1,000 adults who have at least three Irish grandparents - to make a reliable carrier estimate. The poignant catalyst: In the last five years, she has seen three Philadelphia-area tots of Irish descent with Tay-Sachs.
Of the three, only Nathan Harney, 2, of Downingtown, is alive.
His parents, Kathryn and Aaron Harney, had never heard of Tay-Sachs until their blue-eyed baby was found to have it. Like other Tay-Sachs babies, he appeared healthy in his early months. Then, because his broken gene could not make a vital enzyme, toxic waste began building up in his brain and spinal cord.
Nathan can no longer swallow, is partly paralyzed, and needs oxygen and seizure medication.
"People we told who knew about Tay-Sachs thought it was a Jewish genetic disease," Kathryn Harney recalled. "A lot of doctors we talked to thought it was a Jewish disease."
Mutations can arise out of the blue in people of any ethnicity, and each year there are about a dozen Tay-Sachs cases in the United States. For the general population, about one in 250 people are carriers, according to the National Human Genome Research Institute.
The carrier rate is much higher - about one in 27 - in Jews of Eastern European (Ashkenazi) descent. So at least 60 Jewish babies with Tay-Sachs were born each year in the United States before screening became routine.
Why do some ethnic groups have a heavy genetic burden? The "founder effect" theory posits that a community suddenly contracts because of, say, genocide, famine, or emigration, and then remaining members intermarry, reducing the gene pool.
In the last two decades, the founder effect has been offered to explain various mutations of the Tay-Sachs gene found in French Canadians, Pennsylvania Amish, and Louisiana Cajuns.
Schneider's study began advertising around St. Patrick's Day, but the official launch is this Saturday at the Irish Immigration Center of Philadelphia, 7 S. Cedar Lane, Upper Darby. From 11 a.m. to 2 p.m., free screening - involving a blood draw - will be offered to people over 18 with three or four Irish grandparents.
More than a million people in the metropolitan Philadelphia area claim some Irish blood, but "some" is not enough.
"As much as possible, we want people descended from Ireland," not from Britain, Schneider said.
After Nathan's Tay-Sachs diagnosis, Kathryn Harney's two sisters were screened. Both carry the harmful gene.
"My sisters' husbands don't have the gene, but their children could still be carriers," Harney said.
Research into gene therapy and other potential Tay-Sachs treatments is advancing. If the new study shows that the Irish American carrier rate is higher than one in 100, the researchers will ask professional medical organizations to recommend routine carrier screening.
"Had we known we were at risk, it would have been a simple blood test for me and Aaron," Harney said.
Contact Marie McCullough at 215-854-2720 or firstname.lastname@example.org.