Juvenile Mucopolysaccharidosis plus disease caused by a missense mutation in VPS33A

Abstract

BackgroundA rare and fatal disease resembling mucopolysaccharidosis in infants, is caused by impaired intracellular endocytic trafficking due to deficiency of core components of the intracellular membrane-tethering protein complexes, HOPS and CORVET. MethodsWhole Exome Sequencing identified a novel VPS33A mutation in a patient suffering from a variant form of mucopolysaccharidosis. Electron and confocal microscopy, immunoblotting, and glycosphingolipid trafficking experiments were undertaken to investigate the effects of the mutant VPS33A in patient-derived skin fibroblasts. ResultsWe describe an attenuated juvenile form of VPS33A-related syndrome - mucopolysaccharidosis plus in a man who is homozygous for a hitherto unknown missense mutation (NM_022916.4: c.599 G>C; R200P) in a conserved region of the VPS33A gene. Urinary glycosaminoglycan analysis revealed increased heparan, dermatan sulphates and hyaluronic acid. We showed decreased abundance of VPS33A in patient derived fibroblasts and provided evidence that the R200P mutation leads to destabilisation of the protein and proteasomal degradation. As in the infantile form of mucopolysaccharidosis plus, the endocytic compartment in the fibroblasts also expanded - a phenomenon accompanied by increased endolysosomal acidification and impaired intracellular glycosphingolipid trafficking. Experimental treatment of the patients cultured fibroblasts with the proteasome inhibitor, bortezomib, or exposure to an inhibitor of glucosylceramide synthesis, eliglustat, improved glycosphingolipid trafficking. ConclusionTo our knowledge this is the first report of an attenuated juvenile form of VPS33A insufficiency characterised by appreciable residual endosomal-lysosomal trafficking and a milder mucopolysaccharidosis plus than the disease in infants. Our findings expand the proof of concept of redeploying clinically approved drugs for therapeutic exploitation in patients with juvenile as well as infantile forms of mucopolysaccharidosis plus disease.