THE VAL(192)LEU MUTATION IN THE ALPHA-SUBUNIT OF BETA-HEXOSAMINIDASE-A IS NOT ASSOCIATED WITH THE B1-VARIANT FORM OF TAY-SACHS-DISEASE

Substitution mutations adversely affecting the alpha-subunit of beta-h exosaminidase A (alpha beta) (Ec 3.2.1.52) result in Tay-Sachs disease . The majority affect the initial folding of the pro-alpha chain in th e endoplasmic reticulum, resulting in its retention and degradation. A much less common occurrence is a mutation that specifically affects a n ''active-site'' residue necessary for substrate binding and/or catal ysis. In this case, hexosaminidase A is present in the lysosome, but i t lacks all alpha-specific activity. This biochemical phenotype is ref erred to as the ''B1-variant form'' of Tay-Sachs disease. Kinetic anal ysis of suspected B1-variant mutations is complex because hexosaminida se A is heterodimeric and both subunits possess similar active sites. In this report, we examine a previously identified B1-variant mutation , alpha-Val(192)Leu. Chinese hamster ovary cells were permanently cotr ansfected with an alpha-cDNA-construct encoding the substitution and a mutant beta-cDNA (beta-Arg(211)Lys), encoding a beta-subunit that is inactive but normal in all other respects. We were surprised to find t hat the Val(192)Leu substitution produced a pro-alpha chain that did n ot form alpha-beta dimers and was not transported to the lysosome. Fin ally, we reexamined the hexosaminidase activity and protein levels in the fibroblasts from the original patient. These data were also not co nsistent with the biochemical phenotype of the B1 variant of Tay-Sachs disease previously reported to be present. Thus, we conclude that the Val(192)Leu substitution does not specifically affect the alpha-activ e site.