Low arylsulphatase A activity in a family without metachromatic leukodystrophy.

A low arylsulphatase A activity was noted in the leukocytes and cultured skin fibroblasts of a child without any other symptoms of metachromatic leukodystrophy. Although the mother had a level of arylsulphatase commensurate with heterozygosity for the classical metachromatic leukodystrophy gene, the father had a variant gene giving an unusually low in vitro level of this enzyme. In combination (the proband), these two genes gave rise to a very low in vitro activity without any apparent disease. In screening for metachromatic leukodystrophy, a low arylsulphatase A level is not necessarily indicative of this disease, if a clinically normal parent shows an unusually low level of this enzyme.