Two variant hexosaminidase beta-chain alleles segregating in a South African family

A family is described in which alleles for two different hexosaminidase beta-chain variants are segregating. When they co-exist in the same individual Sandhoff disease results. In the heterozygous state one of the variant alleles results in the production of an unstable Hex B and a Hex A with an altered Km for the substrate 4-MU-acetamido-2-deoxy-beta-D-galactopyranoside. The other allele when heterozygous with a normal allele does not produce unstable isozymes with altered kinetics. Like many rare recessive diseases the affected children in this family would appear to have been compound heterozygotes and not true homozygotes.