Characterization of two Turkish β-hexosaminidase mutations causing Tay–Sachs disease

Two homoallelic mutations have recently been identified in the α-subunit of hexosaminidase A (EC 3.2.1.52) causing the infantile form of Tay–Sachs disease in Turkish patients. Both of these mutations, a 12 bp deletion (1096–1107 or 1098–1108 or 1099–1109) in exon 10 and a point mutation (G1362 to A, Gly454 to Asp) in exon 12, are located in the catalytic domain of the hexosaminidase α-chain. In order to determine whether these mutations affect the function of the catalytic domain or result in an instable protein, both mutant cDNAs were overexpressed in COS-1 cells. As judged by Western blotting, transfections of wild-type cDNA produced pro-α-chain and mature α-chain in parallel with a fivefold increase in cellular hexosaminidase activity using the synthetic substrate 4-methylumbelliferyl β-N-acetylglucosamine 6-sulfate (MUGS). However, both mutants produced only pro-α-chains, although no mature form or detectable hexosaminidase activity towards two different synthetic substrates was observed. These data are consistent with the biochemical phenotype of infantile Tay–Sachs disease. We conclude that the overexpressed mutant pro-α-chains were misfolded and could not undergo further proteolytic processing to the active form of the enzyme in the lysosome.