Niemann-Pick type B disease: Identification of a single codon deletion in the acid sphingomyelinase gene and genotype/phenotype correlations in Type A and B patients

Types A and B Niemann-Pick disease both result from the deficient activity of the lysosomal hydrolase, acid sphingomyelinase (E.C. 3.1.4.12). Type A Niemann-Pick disease is a severe neurodegenerative disorder of infancy which leads to death by three years of age, whereas Type B disease has a later age at onset, little or no neurologic involvement, and most patients survive into adulthood. To investigate the molecular basis for the remarkable phenotypic heterogeneity, the nature of the mutations causing Type B Niemann-Pick disease in Ashkenazi Jewish patients was determined. The entire acid sphingomyelinase coding region from an Ashkenazi Jewish Type B patient was polymerase chain reaction-amplified, subcloned, and completely sequenced. A three-base deletion was identified of nucleotides 1821-1823 in the cDNA which predicted the removal of an arginine residue from position 608 of the acid sphingomyelinase polypeptide (ΔR608). The other cDNA clones from this patient had the R496L mutation previously identified in Type A Niemann-Pick disease patients. Both Ashkenazi Jewish Type B patients were heteroallelic for the AR608 mutation, whereas this allele was not present in 15 unrelated non-Jewish Type B patients, with the notable exception of one mildly affected patient of Arabic descent who was homoallelic for the ΔR608 mutation. These results indicate that the ΔR608 mutation predicts the Type B Niemann-Pick disease phenotype, even in the presence of the R496L Type A allele, thereby providing the first genotype/phenotype correlation for this lysosomal storage disease. Although only two patients have been studied, it appears that the ΔR608 mutation occurs frequently in Type B Niemann-Pick disease patients of Ashkenazi Jewish descent.