Title : A novel mutation in
UDP-N-acetylglucosamine-1-phosphotransferase gamma
subunit subunit (
GNPTAG ) in two siblings with mucolipidosis type III alters a used glycosylation
site
Abstract :
- The N-acetylglucosaminyl-1- phosphotransferase (termed phosphotransferase ) catalyzes the initial step in the formation of mannose 6-phosphate (M6P) residues required for the efficient transport of soluble lysosomal enzymes
- The phosphotransferase is a multi subunit enzyme composed of three subunits ( alpha2beta2gamma2 ) that are products of two genes
- The gene encoding the gamma- subunit subunit ( GNPTAG ) appears to be defective in patients with mucolipidosis type III (ML III)
- We have analyzed the GNPTAG gene in two siblings with ML III showing elevated activities of several lysosomal enzymes in cultured fibroblasts serum and diminished activities in cultured fibroblasts
- Immunoprecipitation of metabolically labeled cathepsin D ( CtsD ) from fibroblasts revealed that the sorting/transport of this lysosomal protease was affected
- Addition of ammonium chloride inhibiting pH-dependent processes, such as the CtsD-M6P receptor interaction, indicated that 15 to 20% of the newly synthesized CtsD is transported in ML III fibroblasts in an M6P-dependent manner
- By direct sequencing a novel homozygous mutation, c.347_349delACA (p.Asn116del), was identified affecting a potential N-linked glycosylation site
- Western blot analysis of extracts from control fibroblasts detect a 97 kDa glycosylated dimer whereas ML III cells contain a GNPTAG dimer of reduced molecular mass
- These data suggest that the loss of the used glycosylation site in the gamma subunit subunit may affect the intracellular localization of GNPTAG and the overall efficiency of M6P formation