Characterization of the benign mutations, R247W and R249W, associated with B-hexosaminidase A pseudodeficiency
Deficient activity of $\beta$-hexosaminidase A (Hex A), resulting from mutations in the HEX4 gene, typically causes Tay-Sachs disease. However, healthy individuals lacking Hex A activity against synthetic substrates (i.e. individuals who are pseudodeficient) have been described. Most of these individuals have a C739T (R247W) mutation in compound heterozygosity with a Tay-Sachs disease-causing allele. We identified a second benign mutation, C745T (R249W) in a pseudodeficient subject and showed that it accounted for approximately 6% (4/63) of enzyme-defined non-Jewish Tay-Sachs disease carriers. Taken together, the two benign mutations accounted for approximately 38% of non-Jewish enzyme-defined carriers, making their identification an important component of Tay-Sachs disease prevention programs. To confirm the relationship between benign mutations and Hex A pseudodeficiency and to determine how the benign mutations reduce Hex A activity, each of the benign mutations and other mutations associated with infantile, juvenile, and adult-onset forms of Gm2 gangliosidosis were transiently expressed as Hex S $(\alpha\alpha)$ and Hex A $(\alpha\beta)$ in Cos-7 cells. The benign mutations decreased the expressed Hex A and Hex S activities toward the synthetic substrate 4-methylumbelliferyl-6-sulfo-$\beta$-N-acetylglucosaminide (4-MUGS) by 60 to 80%, indicating they are the primary cause of Hex A pseudodeficiency. Western blot analysis showed that the benign mutations reduced enzymatic activity by reducing the $\alpha$-subunit protein level. No change in Hex A heat sensitivity, catalytic activity, or specificity toward 4-MUG and 4-MUGS, were detected in the studies. The effects of benign mutations on Hex A were further analysed in fibroblasts, and during transient expression, using pulse-chase metabolic labelling. These studies showed that (1) the benign mutations reduced the $\alpha$-subunit protein by affecting its stability in vivo, not by affecting the processing of the $\alpha$-subunit, ie. phosphorylation, targeting, or secretion, and (2) these benign mutations could be readily differentiated from disease-causing mutations using a transient expression system.