W474C amino acid substitution affects early processing of the α‐subunit of β‐hexosaminidase A and is associated with subacute GM2 gangliosidosis

Abstract

Mutations in the HEXA gene, encoding the α-subunit of β-hexosaminidase A (Hex A), that abolish Hex A enzyme activity cause Tay-Sachs disease (TSD), the fatal infantile form of GM2 gangliosidosis, Type 1. Less severe, subacute (juvenile-onset) and chronic (adult-onset) variants are characterized by a broad spectrum of clinical manifestations and are associated with residual levels of Hex A enzyme activity. We identified a 1422 G→C (amino acid W474C) substitution in the first position of exon 13 of HEXA of a non-Jewish proband who manifested a subacute variant of GM2 gangliosidosis. On the second maternally inherited allele, we identified the common infantile disease-causing 4-bp insertion, +TATC 1278, in exon 11. Pulse-chase analysis using proband fibroblasts revealed that the W474C-containing α-subunit precursor was normally synthesized, but not phosphorylated or secreted, and the mature lysosomal α-subunit was not detected. When the W474C-containing α-subunit was transiently co-expressed with the β-subunit to produce Hex A (αβ) in COS-7 cells, the mature α-subunit was present, but its level was much lower than that from normal α-subunit transfections, although higher than in those cells transfected with an α-subunit associated with infantile TSD. Furthermore, the precursor level of the W474C α-subunit was found to accumulate in comparison to the normal α-subunit precursor levels. We conclude that the 1422 G→C mutation is the cause of Hex A enzyme deficiency in the proband. The resulting W474C substitution clearly interferes with α-subunit processing, but because the base substitution falls at the first position of exon 13, aberrant splicing may also contribute to Hex A deficiency in this proband. Hum Mutat 11:432–442, 1998. © 1998 Wiley-Liss, Inc.