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

Summary: 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 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.