The lysosomal disease caused by mutant VPS33A

Abstract

A rare lysosomal disease resembling a mucopolysaccharidosis with unusual systemic features, including renal disease and platelet dysfunction, caused by the defect in a conserved region of the VPS33A gene on human chromosome 12q24.31, occurs in Yakuts—a nomadic Turkic ethnic group of Southern Siberia. VPS33A is a core component of the class C core vacuole/endosome tethering (CORVET) and the homotypic fusion and protein sorting (HOPS) complexes, which have essential functions in the endocytic pathway. Here we show that cultured fibroblasts from patients with this disorder have morphological changes: vacuolation with disordered endosomal/lysosomal compartments and—common to sphingolipid diseases—abnormal endocytic trafficking of lactosylceramide. Urine glycosaminoglycan studies revealed a pathological excess of sialylated conjugates as well as dermatan and heparan sulphate. Lipidomic screening showed elevated β-D-galactosylsphingosine with unimpaired activity of cognate lysosomal hydrolases. The 3D crystal structure of human VPS33A predicts that replacement of arginine 498 by tryptophan will de-stabilize VPS33A folding. We observed that the missense mutation reduced the abundance of full-length VPS33A and other components of the HOPS and CORVET complexes. Treatment of HeLa cells stably expressing the mutant VPS33A with a proteasome inhibitor rescued the mutant protein from degradation. We propose that the disease is due to diminished intracellular abundance of intact VPS33A. Exposure of patient-derived fibroblasts to the clinically approved proteasome inhibitor, bortezomib, or inhibition of glucosylceramide synthesis with eliglustat, partially corrected the impaired lactosylceramide trafficking defect and immediately suggest therapeutic avenues to explore in this fatal orphan disease.

VPS41 recessive mutation causes ataxia and dystonia with retinal dystrophy and mental retardation by inhibiting HOPS function and mTORC1 signaling

“…Currently, patients with mutations in HOPS core components VPS11, VPS16 and VPS33A have been reported in literature, which all show a neurodegenerative phenotype (11,(67)(68)(69)(70)(71)(72)(73)(74). Moreover, during our studies, a third patient with compound heterozygote mutations in VPS41 was identified (Personal communication by I. Stolte-Dijkstra, University Medical Center Groningen), bearing the VPS41 R662* mutation as well as a missense mutation (VPS41 H466R ), and displaying a similar neurological phenotype.…”

Mucopolysaccharidosis-Plus Syndrome

“…Most patients develop hematopoietic disorders including anemia, thrombocytopenia and leukocytopenia. An increase in serum IgM and decrease of IgG levels were found in MPSPS patients [49]. Bone marrow aspiration analysis showed hypoplastic bone marrow without any storage cells.…”

“…Examination of urine can reveal elevated levels of GAGs, specifically heparan and dermatan sulfate [4,5,49]. Accumulation of heparan sulfate was also detected in patient's plasma and skin fibroblasts.…”

“…Pavlova et al [49] showed that patient-derived fibroblasts have increased vacuolization and abnormal endocytic trafficking of lactosylceramide. Quantity of enlarged vacuoles (>500 nm diameter/cell) was increased in patients cells.…”

“…Pathophysiology of the disease is still unknown, however possible treatments of MPSPS are being investigated. In experimental study of Pavlova et al [49] was shown that treatment of MPSPS patient-derived fibroblasts with proteasomal inhibitor (bortezomib), and inhibitor of glucosylceramide synthesis (eliglustat) partially corrected the impaired lactosylceramide trafficking defect.…”