A shortened beta-hexosaminidase alpha-chain in an Italian patient with infantile Tay-Sachs disease.

Fibroblasts derived from a beta-hexosaminidase A (HexA)-deficient infant with clinically classic Tay-Sachs disease synthesized a precursor alpha-chain that was smaller than its normal counterpart. Fibroblasts from the infant's parents, who were consanguinous, produced both normal and mutant alpha-chains. The size difference, estimated to be 2-3 kilodaltons on the basis of sodium dodecyl sulfate-polyacrylamide-gel electrophoresis, persisted after removal of oligosaccharides with endo-H and is therefore attributable to a shortened polypeptide. The mutant alpha-chain did not undergo the further posttranslational modifications characteristic of its normal counterpart--i.e., synthesis of the mannose phosphate recognition marker, association with the beta-chain to give HexA, and proteolytic conversion to the mature form. Nor was it secreted, even in the presence of NH4Cl. Instead, it disappeared in the course of a 20-h chase. These results suggest that the mutant alpha-chain was trapped in an early biosynthetic compartment, either the endoplasmic reticulum or the cis-Golgi. The mutation appears to be different from all those previously described in patients with clinically classic Tay-Sachs disease.