SIGNIFICANCE OF 2 POINT MUTATIONS PRESENT IN EACH HEXB ALLELE OF PATIENTS WITH ADULT G(M2) GANGLIOSIDOSIS (SANDHOFF DISEASE) - HOMOZYGOSITYFOR THE ILE(207)-]VAL SUBSTITUTION IS NOT ASSOCIATED WITH A CLINICAL OR BIOCHEMICAL PHENOTYPE

The molecular defects in the HEXB gene encoding the common beta-subuni t of lysosomal P-hexosaminidase At beta-Hex A, alpha beta) and beta-He x B (beta beta) were investigated in a Portuguese family affected with late onset Sandhoff disease (G(M2)-gangliosidosis variant 0). This fa mily comprised two unaffected daughters and three affected sibs who de veloped at about age 17 cerebellar ataxia and mental deficiency. Their parents were consanguineous and clinically asymptomatic. There was no detectable beta-Hex B activity and a profound reduction in the activi ty of beta-Hex A in the leukocytes and transformed lymphoid cell lines from the affected sibs. The expected intermediate values were observe d in the parents as well as in one daughter and her children. Western analysis revealed the presence of reduced, but detectable amounts of m ature beta-chain protein in cell lysates from the probands and interme diate levels in the parents. Nucleotide sequencing of amplified, rever se-transcribed beta-chain mRNA demonstrated the presence of two single point mutations: an A(619) to G transition in exon 5 (Ile(207)-->Val) , and a G(1514) to A transition in exon 13 (Arg(505)-->Gln). Both of t hese two mutations have been previously linked to the adult form of Sa ndhoff disease in compound heterozygote patients, All three affected s ibs were found to be homoallelic for both mutations. Interestingly, wh ile the mother was heterozygous for each mutation, the father was homo zygote for the A(619)-->G substitution and heterozygote for the G(1514 )-->A transition. Since the father is homozygote for the A(619)-->G mu tation but expresses a biochemical phenotype consistent with a carrier of Sandhoff disease and is clinically asymptomatic, this substitution is likely a neutral mutation. We confirmed this hypothesis by finding this transition present in 4 of 30 alleles from normal individuals. W e conclude that homozygosity for the G(1514)-->A mutation is exclusive ly responsible for the adult form of Sandhoff disease in this family, and that the A(619)-->G substitution is not a deleterious mutation but rather a common HEXB polymorphism.