ANALYSIS OF A SPLICE-SITE MUTATION IN THE SAP-PRECURSOR GENE OF A PATIENT WITH METACHROMATIC LEUKODYSTROPHY

Sphingolipid activator proteins (SAPs) are small, nonenzymatic glycopr oteins required for the lysosomal degradation of various sphingolipids with a short oligosaccharide chain by their exohydrolases. Four of th e five known activator proteins (sap-A-sap-D), also called ''saposins, '' are derived from a common precursor by proteolytic processing. sap- B stimulates hydrolysis of sulfatides by arylsulfatase A in vivo. Its recessively inherited deficiency results in a metabolic disorder simil ar to classical metachromatic leukodystrophy, which is caused by a def ect of arylsulfatase A, Here we report on a patient with sap-B deficie ncy. Reverse-transcription-PCR studies on the patient's mRNA revealed the occurrence of two distinct mutant species: one with an in-frame de letion of the first 21 bases of exon 6, the other with a complete in-f rame deletion of this exon. The patient was homozygous for the underly ing mutation, which was found to be a G-->T transversion within the ac ceptor splice site between intron e and exon 6, abolishing normal RNA splicing. Allele-specific oligonucleotide hybridization revealed that the parents and both grandfathers of the patient were carriers of this mutation. In order to analyze the fate of the mutant precursor protei ns, both abnormal cDNAs were stably expressed in baby hamster kidney c ells. Pulse-chase experiments showed that the deletion of 21 bp had no effect on the transport and the maturation of the encoded precursor. All sap forms except sap-B were detectable by immunochemical methods. The cDNA bearing a complete deletion of exon 6 encoded a shortened pre cursor of only 60 kD, and no mature SAPs were detectable. The carbohyd rate chains of this polypeptide were of the high-mannose and hybrid ty pe, indicating no transport of the mutant precursor beyond early Golgi apparatus. An endoplasmic-reticulum localization of this polypeptide was supported by indirect immunofluorescence analysis.