Spinal muscular atrophy is not the result of mutations at the beta–hexosaminidase or GM₂–activator locus

The disease locus for the clinically heterogeneous childhood spinal muscular atrophies (SMA) maps to the chromosome 5 subregion, 5q11.2–13.3. The beta-subunit of beta-D-N-acetylhexosaminidase (hexosaminidase) (EC 3.2.1.52) (Hex B) maps to the same region, and the protein required for substrate recognition by this enzyme, GM₂-activator protein, likewise maps to chromosome 5. We have investigated the possibility of allelic variation among some forms of SMA and hexosaminidase deficiency. Recombination between the Hex B and SMA loci eliminates this enzyme as a candidate site for defects causing the illness. Furthermore, we show that, despite previous evidence to the contrary, the GM₂-activator locus does not map to chromosome 5, thereby eliminating it as a candidate gene for SMA.