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The mutation mechanism causing juvenile‐onset Tay‐Sachs disease among Lebanese

Expression of the hexosaminidase isozymes was evaluated in fibroblast cell lines obtained from two sibs of Lebanese‐Christian origin who presented with juvenile‐onset Tay‐Sachs disease. In the normal control fibroblasts the α subunit of hexosaminidase A (hex A) is synthesized as a 67 KD precursor which is cleaved in lysosomes to a mature 54 KD peptide. The patients' fibroblasts were capable of synthesizing the 67 KD precursor but failed to convert it to the mature subunit. The α subunit precursor synthesized by patients' cells could not be phosphorylated, nor was the patients' α subunit precursor secreted into the medium in response to NH4Cl, which caused accumulation of both α and β subunit precursor in the medium of the normal control fibroblasts. The measurement of residual enzyme activity in the fibroblasts of patients which best correlated with the onset of the illness was the ion exchange chromatographic separation of Hex A‐associated hydrolysis of the synthetic substrate 4‐methylumbelliferyl N‐acetyl‐β‐D‐glucosamine‐6‐sulfate (4MUGS). The patients had 0.32% and 0.36% of Hex Aassociated 4MUGS cleaving activity compared to normal control fibroblasts as compared to <0.016% for infantile Tay‐Sachs disease fibroblasts. The residual Hex A activity in patients cells had a pH optimum identical with normal enzyme (pH 3.9–4.0), a reduced specific activity for 4MUGS (relative to hydrolysis of unsulfated synthetic substrate), and a greatly enhanced thermal stability. The occurrence of this form of Tay‐Sachs disease in Lebanon, the fact that the condition has been described in three unrelated Lebanese immigrant families in Canada, together with the fact that the grandparents of the unrelated probands come from villages in both the northern and southern regions of Lebanon, leads us to speculate that a gene causing juvenile‐onset Tay‐Sachs disease may not be infrequent in Lebanon.