Tay-Sachs disease
Tay-Sachs disease is a fatal genetic disorder in which harmful quantities of a fatty substance called ganglioside GM2 accumulate in the nerve cells in the brain. Infants with Tay-Sachs disease appear to develop normally for the first few months of life. Then, as nerve cells become distended with fatty material, a relentless deterioration of mental and physical abilities occurs. The child becomes blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in. A much rarer form of the disorder which occurs in patients in their twenties and early thirties is characterized by unsteadiness of gait and progressive neurological deterioration. Patients with Tay-Sachs have a "cherry-red" spot in the back of their eyes. The condition is caused by insufficient activity of an enzyme called hexosaminidase A that catalyzes the biodegradation of acidic fatty materials known as gangliosides. Gangliosides are made and biodegraded rapidly in early life as the brain develops. Patients and carriers of Tay-Sachs disease can be identified by a simple blood test that measures hexosaminidase A activity. Both parents must be carriers in order to have an affected child. When both parents are found to carry a genetic mutation in hexosaminidase A, there is a 25 percent chance with each pregnancy that the child will be affected with Tay-Sachs disease. Prenatal monitoring of pregnancies is available if desired.Presently there is no treatment for Tay-Sachs. Even with the best of care, children with Tay-Sachs disease usually die by age 5.
To expand somewhat on the genetic basis, Tay-Sachs is an autosomal recessive genetic condition resulting from mutation of the HEXA gene encoding the alpha-subunit of the lysosomal enzyme alpha-N-acetylhexosaminidase. This enzyme is necessary for breaking down N-galactosamine from GM2 gangliosides in brain and nerve cells. More than thirty mutations have been identified in the HEXA gene. These consist of base pair insertions, base pair deletions, splice site mutations, and point mutations. All of these mutations alter the protein product. For example, a four base pair insertion in exon 11 results in an altered reading frame for the HEXA gene while a three base pair deletion eliminates the amino acid phenylalinine from the protein product at position 304. A G to C point mutation at amino acid 180 changes the codon UAC to UAG causing termination of the polypeptide. A G to A point mutation at amino acid 170 changes the codon CGA to CAA and CGG to CAG which produces glutamine instead of arginine. A G to C mutation in the splice site of intron 12 has also been identified. This mutation creates a recognition site for the restriction enzyme DdeI resulting in abnormal splicing and the production of aberrant mRNA species.
Ashkenazi Jews have a high incidence of Tay-Sachs and other lipid storage diseases (in the United States, 1 in 27 Ashkenazi Jews is a recessive carrier, compared to 1 in 100 in the general population [1]). It is theorized that people who are heterozygous for Tay-Sachs are resistant to tuberculosis, which was common in the Eastern European towns where they lived. Puzzlingly, it has been estimated that it would take more than 300 generations to reach the observed frequency of TSD heterozygosity, yet the Ashkenazim have only been a separate group for 70 generations (Shaw and Smith, 1969). Also, one would expect a higher frequency of TSD in other ethnic groups living in the same conditions, which is not the case. More research is needed to answer these questions.
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