The molecular lesion in the α-N-acetylgalactosaminidase gene that causes angiokeratoma corporis diffusum with glycopeptiduria

Angiokeratoma corporis diffusum with glycopeptiduria is a recently recognized inborn error of glycoprotein catabolism resulting from the deficient activity of human α-N-acetylgalactosaminidase (E.C.3.2.1.49; α- GalNAc). The first patient with this autosomal recessive disorder, a 46-yr- old consanguineous Japanese woman, presented with diffuse angiokeratoma, mild intellectual impairment, and peripheral neuroaxonal degeneration. Deficient α-GalNAc activity also has been reported in consanguineous brothers with an infantile-onset form of neuroaxonal dystrophy resulting from a missense mutation (designated E325K) in the α-GalNAc gene. To identify the mutation causing the phenotypically distinct adult-onset disorder, Southern and Northern hybridization analyses of DNA and RNA from the affected homozygote were performed which revealed a grossly normal α-GalNAc gene structure and normal transcript size and abundancy. Reverse transcription, amplification, and sequencing of the α-GalNAc transcript identified a single C to T transition at nucleotide (nt) 985 that predicted an arginine to tryptophan substitution in residue 329 (designated R329W) of the α-GalNAc polypeptide. This base substitution was confirmed by hybridization of PCR-amplified genomic DNA from family members with allele-specific oligonucleotides. Transient expression of an α-GalNAc construct containing the R329W mutation resulted in the expression of an immunoreactive polypeptide which had no detectable α-GalNAc activity. Comparison of the biosynthesis and stabilities of the transiently expressed and radiolabeled normal, E325K (infantile- onset) and R329W (adult-onset) α-GalNAc polypeptides in COS-1 cells indicated that both the mutant precursors were processed to the mature form; however, the E325K mutant polypeptide was more rapidly degraded than the R329W subunit, thereby providing a basis for the distinctly different infantile- and adult-onset phenotypes.