Two novel (1334delC and 1363G to A, G455R) mutations in exon 12 of the beta-hexosaminidase alpha-chain gene in two Portuguese patients

Specific mutations in the HEXA gene among Iraqi Jewish Tay-Sachs disease carriers: dating of founder ancestor

The incidence of Tay-Sachs disease (TSD) carriers, as defined by enzyme assay, is 1:29 among Ashkenazi Jews and 1:110 among Moroccan Jews. An elevated carrier frequency of 1:140 was also observed in the Iraqi Jews (IJ), while in other Israeli populations the world's pan-ethnic frequency of approximately 1:280 has been found. Recently a novel mutation, G749T, has been reported in 38.7% of the IJ carriers (24/62). Here we report a second novel HEXA mutation specific to the IJ TDS carriers: a substitution of cytosine 1351 by guanosine (C1351G), resulting in the change of leucine to valine in position 451. This mutation was found in 33.9% (21/62) of the carriers and in none of 100 non-carrier IJ. In addition to the two specific mutations, 14.5% (9/62) of the IJ carriers bear a known "Jewish" mutation (Ashkenazi or Moroccan) and 11.3% (7/62) carry a known "non-Jewish" mutation. In 1 DNA sample no mutation has yet been detected. To investigate the genetic history of the IJ-specific mutations (C1351G and G749T), the allelic distribution of four polymorphic markers (D15S131, D15S1025, D15S981, D15S1050) was analyzed in IJ heterozygotes and ethnically matched controls. Based on linkage disequilibrium, recombination factor (theta) between the markers and mutated loci, and the population growth correction, we deduced that G749T occurred in a founder ancestor 44.8 +/- 14.2 generations (g) ago [95% confidence interval (CI) 17.0-72.6 g] and C1351G arose 80.4 +/- 35.9 g ago (95% CI 44.5-116.3 g). Thus, the estimated dates for introduction of mutations are: 626 +/- 426 A.D. (200-1052 A.D.) for G749T and 442 +/- 1077 B.C. (1519 B.C. to 635 A.D.) for C1351G.

Tay-Sachs disease carrier screening: a model for prevention of genetic disease

Tay-Sachs disease (TSD) is an autosomal-recessive, progressive, and ultimately fatal neurodegenerative disorder. Within the last 30 years, the discovery of the enzymatic basis of the disease, namely deficiency of the enzyme hexosaminidase A, made possible both enzymatic diagnosis of TSD and heterozygote identification. In the last decade, the cloning of the HEXA gene and the identification of more than 80 associated TSD-causing mutations has permitted molecular diagnosis in many instances. TSD was the first genetic condition for which community-based screening for carrier detection was implemented. As such, the TSD experience can be viewed as a prototypic effort for public education, carrier testing, and reproductive counseling for avoiding fatal childhood disease. More importantly, the outcome of TSD screening over the last 28 years offers convincing evidence that such an effort can dramatically reduce incidence of the disease.