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Course of the Disease
The enzyme involved in Tay-Sachs is called hexosaminidase A. Its absence allows a lipid called GM2 ganglioside to build up in the brain, destroying the nerve cells. The process starts in the fetus; the disease is clinically apparent in the first few months of life. Initial symptoms vary, but usually include a general slowing of development and loss of peripheral vision. By the age of one, most children are experiencing convulsions. The damage to the nervous system progresses inexorably, bringing with it an inability to swallow, difficulty in breathing, blindness, mental retardation, and paralysis.
In late-onset Tay-Sachs, which is often misdiagnosed, the symptoms (ataxia, dysarthria, and muscle weakness) usually become apparent late in childhood or early in adulthood. About 40% of the patients display symptoms of bipolar disorder. Life expectancy does not appear to be affected.
Genetic Basis and Screening
Tay-Sachs disease occurs primarily among Jews of Eastern European descent but is also found in French Canadians whose roots are in the St. Lawrence region, certain Cajuns in Louisiana, and some Amish communities. Tay-Sachs is an autosomal recessive disorder; a person must have two defective genes (one from each parent) in order for the disease to occur. Carriers, people with only one gene for the disorder, are physically unaffected. When both parents are carriers, each child has a 25% chance of getting the disease. If only one parent is a carrier, there is no chance that the child will get the disease, but there is a 50% chance that the child will be a carrier. The gene may be carried by many generations without a manifestation. For this reason, plus the historical lack of accurate diagnosis and routinely high infant mortality of past generations, there is often no known family history of the disease.
Genetic screening (see genetic testing) for the disease has been possible since the early 1970s and is encouraged in high-risk populations. Blood tests of carriers reveal a reduced amount of the hexosaminidase A. If a couple elects to go forward with a pregnancy, fetal status (again utilizing hexosaminidase A levels) can be ascertained via chorionic villus sampling or amniocentesis. Genetic screening and counseling has greatly reduced the incidence of the disease.
See M. M. Kaback, ed., Tay-Sachs Disease: Screening and Prevention (1977); W. Stockton, Altered Destinies (1979).