Metachromatic leukodystrophy: Subtype genotype phenotype correlations and identification of novel missense mutations (P148L and P191T) causing the juvenile-onset disease

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease resulting from the deficient activity of arylsulfatase A (ASA) and the accumulation of sulfatides. The disease is characterized by several subtypes, designated by age at onset: the late-infantile-, juvenile-, and adult-onset variants. Mutation analysis of genomic DNA from. a proband with each variant was per formed to identify and characterize their causative ASA mutations. Two sist ers with the infantile-onset disease were homoallelic for the missense muta tion D335V, a juvenile-onset proband was heteroallelic for two novel missen se mutations, P148L and P191T, and an adult-onset patient was heteroallelic for the H397Y and P426L mutations, The novel mutations were not identified in 108 normal alleles indicating that these base substitutions were not co mmon polymorphisms. To further characterize the mutant gene products, the m utant enzymes were partially purified from cultured fibroblasts and their m olecular weights and charges were compared by immunoblotting; following SDS -PAGE or isoelectric focusing (IEF). Normal fibroblast ASA had a single, br oad band at 54 kDa. The enzyme from the late-infantile-onset patient had di stinct bands elf 36 and 78 kDa, but lacked the normal 54-kDa species. The j uvenile- and adult-onset patients each had a faint band of 54 kDa and sever al other bands ranging from 29 to 64 kDa. IEF revealed several bands for th e partially purified normal enzyme with a relatively narrow ph range around 4.0, whereas numerous bands with a wider range of isoelectric points were observed with the enzymes from the juvenile- and adult-onset fibroblasts. I n contrast, the enzyme from the late-infantile-onset proband had four bands with more acidic isoelectric points, none corresponding to those of the no rmal enzyme. These results document changes in both size and charge of the mutant enzymes from patients with different mutations and MLD subtypes. (C) 1999 Academic Press.