Assessing the Severity of the Small Inframe Deletion Mutation in the Alpha-Subunit of Beta-Hexosaminidase A Found in the Turkish Population By Reproducing It in the More Stable Beta-Subunit

Abstract

GM(2) gangliosidoses are a group of panethnic lysosomal storage diseases in which GM(2) ganglioside accumulates in the lysosome due to a defect in one of three genes, two of which encode the alpha- or beta-subunits of beta-N-acetylhexosaminidase (Hex) A. A small inframe deletion mutation in the catalytic domain of the alpha-subunit of Hex has been found in five Turkish patients with infantile Tay-Sachs disease. To date it has not been detected in other populations and is the only mutation to be found in exon 10. It results in detectable levels of inactive alpha-protein in its precursor form. Because the alpha- and beta-subunits share 60% sequence identity, the Hex A and Hex B genes are believed to have arisen from a common ancestral gene. Thus the subunits must share very similar three-dimensional structures with conserved functional domains. Hex B, the beta-subunit homodimer is more stable than the heterodimeric HexA, and much more stable than Hex S, the alpha homodimer. Thus, mutations that completely destabilize the alpha-subunit can often be partially rescued if expressed in the aligned positions in the beta-subunit. To better understand the severity of the Turkish HEXA mutation, we reproduced the 12 bp deletion mutation (1267-1278) in the beta-subunit cDNA. Western blot analysis of permanently transfected CHO cells expressing the mutant detected only the pro-form of the beta-subunit coupled with a total lack of detectable Hex B activity. These data indicate that the deletion of the four amino acids severely affects the folding of even the more stable beta-subunit, causing its retention in the endoplasmic reticulum and ultimate degradation.