Pseudodeficiency of arylsulfatase A: a common genetic polymorphism with possible disease implications

At the locus for arylsulfatase A (ASA) at least four to five alleles exist: besides the normal ASA+ and at least two to three deficiency alleles (ASA-), a pseudodeficiency allele, ASAp, is known. On SDS-PAGE the ASAp enzyme migrates slightly faster than ASA+. Treatment of extracts from cells with ASA+/ASA+, ASAp/ASAp, or ASA+/ASAp genotypes with endoglycosidase F leads to the same deglycosylated subunit pattern. Presumably the degree of glycosylation is lower in ASAp than in ASA+. In a large-scale screening project we determined a gene frequency of 7.3% for ASAp. Thus, the ASA locus is polymorphic. In seven families, ASAp showed a codominant mode of inheritance with ASA+. Homozygosity for ASAp has no obvious clinical consequences. In subjects with the compound genotype ASA-/ASAp, the residual enzyme activity may fall below a critical threshold, so that the substrate can no longer be hydrolyzed sufficiently. Since these compounds are not so rare (estimated frequency 0.073%), this mechanism could be of importance in neuropsychiatric disorders with late onset.