How to Detect A Faulty Gene

Many babies have a standard reaction to a sudden, sharp noise. They fling up their arms, and thrust out their legs. This "startle response" normally disappears by the time a baby is four months old. But if it persists and gradually intensifies, it is probably an indication that the baby has Tay-Sachs disease. This is a rare genetic defect that leaves children completely paralyzed, deaf and blind by the time they are two, and is usually fatal by the age of four. Modern medicine knows no cure for Tay-Sachs (named for the physicians who first described the condition), but two scientists at the University of California's San Diego School of Medicine have now provided a means for detecting and avoiding it.

In a Tay-Sachs victim, the system fails to produce an enzyme crucial to a chemical process within cells: the metabolizing of fats (technically, "lipids"). As a result, excess fats accumulate in the brain cells and block normal activity. Earlier researchers suspected that the missing enzyme was hexosaminidase. Yet substantial amounts of hexosaminidase are found in Tay-Sachs victims. Neuroscientists John O'Brien and Shintaro Okada investigated hexosaminidase more intensively and discovered that it actually consisted of two enzymes, Hex-A and Hex-B. Both are present in normal tissue but, they found, only Hex-B occurs in the tissue of Tay-Sachs victims. So, they concluded, it is the absence of Hex-A that prevents the metabolism of fats in brain cells, and this results in the fatal disease.

This enzyme deficiency is caused by an inborn genetic defect that has been traced back 500 years to Ashkenazic Jews who lived in Lithuania and Poland. Because Jews usually marry within their own faith, the genetic defect--and the dread disease--are still largely confined to Jews. In the U.S., for example, Tay-Sachs occurs once in every 5,000 Jewish births, but only once in every 400,000 non-Jewish babies.

One-in-Four Risk. A single defective Tay-Sachs gene cannot afflict its carrier with the disease. The paired, normal gene orders the production of enough Hex-A to allow the necessary brain-cell metabolism. But if both parents carry a Tay-Sachs gene, there is a one-in-four risk that the baby will receive two abnormal genes--one from each parent--and succumb to the disease. If he receives only one, his body will produce less Hex-A than it should, but he will be able to lead a normal life. Like his parents, of course, he will be a carrier.

The identification of Hex-A will enable doctors to detect both the carriers and victims of Tay-Sachs disease. If blood tests reveal that both a man and his wife have less than normal amounts of Hex-A and are thus carriers of Tay-Sachs genes, they can be warned of their 25% risk of producing a Tay-Sachs child and perhaps be discouraged from having children of their own. By inserting a needle through a woman's abdomen when she is 16 weeks pregnant and extracting fluid from the amniotic sac, doctors can determine if the unborn child will have Tay-Sachs disease. Cells shed by the developing fetus into the fluid will be analyzed for traces of HexA. If the enzyme is missing, doctors could advise an abortion that would save the parents from the heartbreak of having a doomed, Tay-Sachs child.

This file is automatically generated by a robot program, so reader's discretion is required.