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Genetic Testing for Tay-Sachs Disease

Source: www.cigna.com
Topic: Tay-Sachs
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Sort Desciption: Tay-Sachs disease (TSD) is an autosomal recessive lysosomal storage disease in which acidic fatty acids called Gm2 gangliosides accumulate in neuronal cells, resulting in progressive destruction of the central nervous system. ...

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Tay-Sachs disease (TSD) is an autosomal recessive lysosomal storage disease in which acidic fatty acids called Gm2 gangliosides accumulate in neuronal cells, resulting in progressive destruction of the central nervous system. The enzyme hexosaminidase (HEX) normally catalyzes the biogradation of these lipids and occurs in two principal forms, HEX A and HEX B. HEX A is composed of one alpha (a) and one beta (B) subunit, while HEX B is composed of two B subunits. TSD occurs when mutations of the HEX A gene encoding the a subunit result in a deficiency of the HEX A enzyme. Mutations in the gene coding for the B subunit lead to a deficiency of both the HEX A and HEX B form of the enzyme producing a variant type of lysosomal storage disease known as Sandhoff disease. Sandhoff disease usually leads to death at about age three. The enzyme deficiency in Sandhoff disease arises from mutations in a B subunit gene on chromosome 5, instead of the a subunit gene on chromosome 13 as in TSD. Mutations in the B subunit gene affect both HEX A and HEX B activity. Sandhoff disease is inherited as an autosomal recessive disorder occurring more commonly in the non-Jewish population (Hayes, 2003).

The phenotypes of HEX A deficiency include the following:
* acute infantile TSD with rapid progression and death before age four
* juvenile (subacute)TSD with later onset and survival into late childhood or adolescence
* chronic and adult onset TSD with long-term survival

TSD is most common among people who are of Ashkenazi Jewish, French-Canadian or Cajun descent, but it can occur in all ethnic and racial groups.

The common clinical findings in patients with TSD are progressive weakness and loss of motor skills beginning between ages three to six months, associated with decreased attentiveness and an increased startle response. Typical findings on physical examination are a cherry red spot of the macula, a normal- sized liver and spleen, and hyperreflexia with sustained ankle clonus. These findings are followed by signs of progressive neurodegeneration, seizures, blindness and spasticity, usually leading to death before age four. Individuals with the juvenile, chronic and adult onset forms have later onset, slower progression and more variable neurological findings.

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