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Dangers hidden in genetic codes

While screenings are increasing control over inherited illnesses, a number of factors

By TOM VALEO
Published June 14, 2005

On March 29, 2002, Sean Leer was born two months premature. He spent the first 10 weeks of his life in the intensive care unit of All Children's Hospital in St. Petersburg and eventually went home with a feeding tube still in his belly.

His parents, Angel and Erik Leer, weren't too worried. Two years earlier their first child, Alexis, was born one month early, and she was doing fine. Sean gained weight steadily, and by the time the feeding tube was removed at seven months, he seemed healthy and alert.

Then, just before his first birthday, while he was playing on the living room carpet, Sean had a seizure. He was readmitted to All Children's, where 10 days of testing and examination revealed nothing wrong. Doctors decided Sean could be released, but first they wanted to give him an eye test.

"A half hour later we were told he couldn't leave," said Angel. "The eye doctor noticed a cherry-red spot at the back of his eye, which he thought could be a sign of a genetic disorder."

That's precisely what it was. Sean had Tay-Sachs disease, a rare genetic disorder that prevents the body's producing an enzyme needed to break down a fatty substance secreted by nerve cells in the brain.

As the fatty substance accumulates, brain neurons start to malfunction, producing relentless deterioration. Children with Tay-Sachs gradually become paralyzed, blind and deaf, and lose their ability to swallow. Most die by age 4. Sean died at 19 months, after struggling with pneumonia for days.

Tay-Sachs cannot be cured, but it can be detected through genetic testing of prospective parents. Among Ashkenazi Jews, those with European roots, the number of people carrying the gene for Tay-Sachs is 10 times higher than in the general population. Other groups, including Cajuns in Louisiana and French Canadians living near the St. Lawrence River, also have a higher-than-average number of Tay-Sachs carriers.

Yet, among these groups the number of babies born with Tay-Sachs has dropped nearly to zero because doctors and genetic counselors advise couples to undergo genetic screening before having children.

The Leers, however, had no reason to suspect they were carriers of the Tay-Sachs gene. Angel's father is Irish, and her mother is English and Irish. Erik's mother was born in Canada, but her ancestry, along with her husband's, is English. As a result, their odds of having a baby with Tay-Sachs were about 1 in 360,000, according to Virginia Corson, a genetic counselor at Johns Hopkins Hospital in Baltimore. The odds were so remote that "when we got married they told us not to bother getting tested for Tay-Sachs," Angel said.

As knowledge of the human genome grows, and scientists discover the genetic cause of more diseases, more tests will emerge for genetic defects in parents and fetuses.

Genetic testing, however, has significant limits.

For example, screening every prospective parent for an array of genetic disorders would be expensive and would uncover few cases.

"The biggest obstacle to widespread testing is the detection rate - it's not high enough to justify testing everyone," said Troy A. Becker, a genetic counselor at All Children's Hospital in St. Petersburg.

Since 1970, for example, more than 1.4-million members of the U.S. Jewish population have been tested for Tay-Sachs. The effort has reduced the number of new cases of the disease from about 60 a year to fewer than five. The low detection rate is the reason screening is usually limited to people considered to be at higher-than-average risk.

"Usually we screen based on ethnicity," said Andrea Wray, a genetic counselor at Georgetown University Hospital in Washington, D.C. "We test Caucasians for cystic fibrosis. With African-Americans, we worry about sickle cell anemia and other hemoglobin problems. People with Mediterranean ancestry, and certain types of Hispanic ancestry, are at greater risk for beta thalassemia, another hemoglobin problem. With Asians, especially Southeast Asians, we worry about alpha thalassemia," Wray said.

After conception, genetic defects in the fetus can be detected through amniocentesis, but even though using a needle to draw amniotic fluid from the mother's womb carries a small risk, that risk is considered too large unless there's reason to expect a problem, and Tay-Sachs is a very remote possibility.

"We don't test the fetus unless both parents are known carriers," said Corson.

In addition, amniocentesis is not usually done unless the parents are willing to face the prospect of terminating the pregnancy.

"We talk about that option," said Corson. "It's a factor in whether they even have the test. If they wouldn't consider ending the pregnancy, they may decide not to have the testing."

The Leers are considering having another child, and, if Angel became pregnant, she would have amniocentesis or a similar test done at eight weeks to see if the child had Tay-Sachs. She would have the test knowing how much Sean suffered as a result of the disease, and remembering the cost of his treatment - more than $1.2-million - which, even with insurance, left them with more than $45,000 in hospital bills.

And she would remember the anguish that Sean's death caused her and Erik.

"I read an article about signs of depression, and I fit almost every one," Angel said, recalling the weeks after her son died. "I was still carrying pictures of him. I was talking about him all the time. My husband shut down when our son died. One day he snapped and started crying. He said, "I feel like I made my son die; I feel like I didn't do my job.' He had guilt. But we did so much."

So, if the fetus turned out to have Tay-Sachs, they wouldn't hesitate to terminate the pregnancy.

"I wouldn't put any child through that," Angel said emphatically. "I wouldn't put us through that."

-- Tom Valeo is a freelance writer in St. Petersburg.

[Last modified June 13, 2005, 16:28:03]