An unusual genotype in an Ashkenazi Jewish patient with Tay-Sachs disease

Three novel mutations in Iranian patients with Tay-Sachs disease

BACKGROUND

Tay-Sachs disease (TSD), or GM2 gangliosidosis, is a lethal autosomal recessive neurodegenerative disorder, which is caused by a deficiency of beta-hexosaminidase A (HEXA), resulting in lysosomal accumulation of GM2 ganglioside. The aim of this study was to identify the TSD-causing mutations in an Iranian population.

METHODS

In this study, we examined 31 patients for TSD-causing mutations using PCR, followed by restriction enzyme digestion.

RESULTS

Molecular genetics analysis of DNA from 23 patients of TSD revealed mutations that has been previously reported, including four-base duplications c.1274_1277dupTATC in exon 11 and IVS2+1G>A, deletion TTAGGCAAGGGC in exon 10 as well as a few novel mutations, including C331G, which altered Gln>Glu in HEXB, A>G, T>C, and p.R510X in exon 14, which predicted a termination codon or nonsense mutation.

CONCLUSION

In conclusion, with the discovery of these novel mutations, the genotypic spectrum of Iranian patients with TSD disease has been extended and could facilitate definition of disease-related mutations.

Nonsense-mediated decay of human HEXA mRNA

Nonsense-mediated mRNA decay (NMD), the loss of mRNAs carrying premature stop codons, is a process by which cells recognize and degrade nonsense mRNAs to prevent possibly toxic effects of truncated peptides. Most mammalian nonsense mRNAs are degraded while associated with the nucleus, but a few are degraded in the cytoplasm; at either site, there is a requirement for translation and for an intron downstream of the early stop codon. We have examined the NMD of a mutant HEXA message in lymphoblasts derived from a Tay-Sachs disease patient homozygous for the common frameshift mutation 1278ins4. The mutant mRNA was nearly undetectable in these cells and increased to approximately 40% of normal in the presence of the translation inhibitor cycloheximide. The stabilized transcript was found in the cytoplasm in association with polysomes. Within 5 h of cycloheximide removal, the polysome-associated nonsense message was completely degraded, while the normal message was stable. The increased lability of the polysome-associated mutant HEXA mRNA shows that NMD of this endogenous mRNA occurred in the cytoplasm. Transfection of Chinese hamster ovary cells showed that expression of an intronless HEXA minigene harboring the frameshift mutation or a closely located nonsense codon resulted in half the normal mRNA level. Inclusion of multiple downstream introns decreased the abundance further, to about 20% of normal. Thus, in contrast to other systems, introns are not absolutely required for NMD of HEXA mRNA, although they enhance the low-HEXA-mRNA phenotype.

Tay-Sachs disease and HEXA mutations among Moroccan Jews

Moroccan Jewry (N>750,000) is the only non-Ashkenazi Jewish community in which Tay-Sachs disease (TSD) is not extremely rare. Previous studies among Moroccan Jewish TSD families identified three HEXA mutations. In this study, extended to enzyme-defined and new obilgate TSD carriers, we found four additional mutations. One of them is a novel, IVS5-2(A-->G) substitution, resulting in exon skipping, and it was found only among enzyme-defined carriers. The seven HEXA identified mutations among Moroccan Jews are: deltaF(304/305), R170Q, IVS-2(A-->G), Y180X, E482K, 1278+TATC, and IVS12+1(G-->C). Their respective distribution among 51 unrelated enzyme-defined and obligate carriers is 22:19:6:1:1:1:1. The mutation(s) remain unknown in only three enzyme-defined carriers. Five of the seven Moroccan mutations, including the three most common ones, were not found among Ashkenazi Jews. Compared with the much larger and relatively homogeneous Ashkenazi population, the finding among Moroccan Jews probably reflects their much longer history.

Tay-Sachs disease-causing mutations and neutral polymorphisms in the Hex A gene

Tay-Sachs disease is an autosomal recessive disorder affecting the central nervous system. The disorder results from mutations in the gene encoding the alpha-subunit of beta-hexosaminidase A, a lysosomal enzyme composed of alpha and beta polypeptides. Seventy-eight mutations in the Hex A gene have been described and include 65 single base substitutions, one large and 10 small deletions, and two small insertions. Because these mutations cripple the catalytic activity of beta-hexosaminidase to varying degrees, Tay-Sachs disease displays clinical heterogeneity. Forty-five of the single base substitutions cause missense mutations; 39 of these are disease causing, three are benign but cause a change in phenotype, and three are neutral polymorphisms. Six nonsense mutations and 14 splice site lesions result from single base substitutions, and all but one of the splice site lesions cause a severe form of Tay-Sachs disease. Eight frameshift mutations arise from six deletion- and two insertion-type lesions. One of these insertions, consisting of four bases within exon 11, is found in 80% of the carriers of Tay-Sachs disease from the Ashkenazi Jewish population, an ethnic group that has a 10-fold higher gene frequency for a severe form of the disorder than the general population. A very large deletion, 7.5 kilobases, including all of exon 1 and portions of DNA upstream and downstream from that exon, is the major mutation found in Tay-Sachs disease carriers from the French Canadian population, a geographic isolate displaying an elevated carrier frequency. Most of the other mutations are confined to single pedigrees. Identification of these mutations has permitted more accurate carrier information, prenatal diagnosis, and disease prognosis. In conjunction with a precise tertiary structure of the enzyme, these mutations could be used to gain insight into the structure-function relationships of the lysosomal enzyme.

Biochemical consequences of mutations causing the GM2 gangliosidoses

The hydrolysis of GM2-ganglioside is unusual in its requirements for the correct synthesis, processing, and ultimate combination of three gene products. Whereas two of these proteins are the alpha- (HEXA gene) and beta- (HEXB) subunits of beta-hexosaminidase A, the third is a small glycolipid transport protein, the GM2 activator protein (GM2A), which acts as a substrate specific co-factor for the enzyme. A deficiency of any one of these proteins leads to storage of the ganglioside, primarily in the lysosomes of neuronal cells, and one of the three forms of GM2-gangliosidosis, Tay-Sachs disease, Sandhoff disease or the AB-variant form. Studies of the biochemical impact of naturally occurring mutations associated with the GM2 gangliosidoses on mRNA splicing and stability, and on the intracellular transport and stability of the affected protein have provided some general insights into these complex cellular mechanisms. However, such studies have revealed little in the way of structure-function information on the proteins. It appears that the detrimental effect of most mutations is not specifically on functional elements of the protein, but rather on the proteins' overall folding and/or intracellular transport. The few exceptions to this generalization are missense mutations at two codons in HEXA, causing the unique biochemical phenotype known as the B1-variant, and one codon in both the HEXB and GM2A genes. Biochemical characterization of these mutations has led to the localization of functional residues and/or domains within each of the encoded proteins.

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.