Assessing the severity of the small inframe deletion mutation in the α-subunit of β-hexosaminidase A found in the Turkish population by reproducing it in the more stable β-subunit

GM₂ gangliosidoses are a group of panethnic lysosomal storage diseases in which GM₂ ganglioside accumulates in the lysosome due to a defect in one of three genes, two of which encode the α- or β-subunits of β-N-acetylhexosaminidase (Hex) A. A small inframe deletion mutation in the catalytic domain of the α -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 α-protein in its precursor form. Because the α- and β-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 β-subunit homodimer is more stable than the heterodimeric Hex A, and much more stable than Hex S, the α homodimer. Thus, mutations that completely destabilize the α-subunit can often be partially rescued if expressed in the aligned positions in the β ;-subunit. To better understand the severity of the Turkish HEXA mutation, we reproduced the 12 bp deletion mutation (1267-1278) in the β-subunit cDNA. Western blot analysis of permanently transfected CHO cells expressing the mutant detected only the pro-form of the β ;-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 β-subunit, causing its retention in the endoplasmic reticulum and ultimate degradation.