A non-glycosylated and functionally deficient mutant (N215H) of the sphingolipid activator protein B (SAP-B) in a novel case of metachromatic leukodystrophy (MLD)

The lysosomal degradation of sphingolipids with short oligosaccharide chain s depends on small glycosylated non-enzymatic sphingolipid activator protei ns (SAPs, saposins). Four of the five known SAPs, SAP-A, -B, -C and -D, are derived by proteolytic processing from a common precursor protein (SAP-pre cursor) that is encoded by a gene on chromosome 10 consisting of 15 exons a nd 14 introns. SAP-B is a non-specific glycolipid binding protein that stim ulates in vitro the hydrolysis of about 20 glycolipids by different enzymes . In vivo SAP-B stimulates in particular the degradation of sulphatides by arylsulphatase A. So far, four different point mutations have been identifi ed on the SAP-B domain of the SAP-precursor gene. The mutations result in a loss of mature SAP-B, causing the lysosomal accumulation of sulphatides an d other sphingolipids, resulting in variant forms of metachromatic leukodys trophy (MLD). Here we report on a patient with SAP-B deficiency that is cau sed by a new homoallelic point mutation that has been identified by mRNA an d DNA analysis. A 643A > C transversion results in the exchange of asparagi ne 215 to histidine and eliminates the single glycosylation site of SAP-B. Metabolic labelling experiments showed that the mutation had no effect on t he intracellular transport of the encoded precursor to the acidic compartme nts and its maturation in the patient's cells. All four SAPs (SAP-A to SAP- D) were detectable by immunochemical methods. SAP-B in the patient's cells was found to be slightly less stable than the protein in normal cells and c orresponded in size to the deglycosylated form of the wild-type SAP-B. Feed ing studies with non-glycosylated SAP-precursor, generating non-glycosylate d SAP-B, showed that the loss of the carbohydrate chain reduced the intrace llular activity of the protein significantly. The additional structural cha nge of the patient's SAP-B, caused by the change of amino acid 215 from asp aragine to histidine, presumably resulted in an almost completely inactive protein.