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 T
urkic 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.
Dursun A, etal., Clin Dysmorphol. 2017 Jan;26(1):1-12. doi: 10.1097/MCD.0000000000000149.
We present a novel multisystem disease in two siblings with clinical features resembling a lysosomal storage disease. These included coarse face, dysostosis multiplex, respiratory difficulty, proteinuria with glomerular foamy cells, neurological involvement with developmental delays, pyramidal signs
, and severe chronic anemia. Detailed enzymatic analysis for lysosomal diseases and whole-exome sequencing studies excluded known lysosomal storage diseases in the proband. Subsequently, genome-wide genotyping and exome sequencing analysis of the family indicated two large homozygous regions on chromosomes 5 and 12, and strongly suggested that a homozygous p. R498W missense mutation in the VPS33A gene might be responsible for this novel disease. Segregation analysis in family members and mutation prediction tools' results also supported the damaging effect of the missense mutation on the function of the Vps33a protein, which plays a role in the vesicular transport system. Electron microscopic studies of the cornea of the proband showed findings supportive of dysfunction in vesicular transport. The clinical phenotype and genetic studies support the suggestion that the siblings most probably have a novel disease very likely caused by a VPS33A gene defect.
Zhen Y and Li W, Autophagy. 2015;11(9):1608-22. doi: 10.1080/15548627.2015.1072669.
The HOPS (homotypic fusion and protein sorting) complex functions in endocytic and autophagic pathways in both lower eukaryotes and mammalian cells through its involvement in fusion events between endosomes and lysosomes or autophagosomes and lysosomes. However, the differential molecular mechanism
s underlying these fusion processes are largely unknown. Buff (bf) is a mouse mutant that carries an Asp251-to-Glu point mutation (D251E) in the VPS33A protein, a tethering protein and a core subunit of the HOPS complex. Bf mice showed impaired spontaneous locomotor activity, motor learning, and autophagic activity. Although the gross anatomy of the brain was apparently normal, the number of Purkinje cells was significantly reduced. Furthermore, we found that fusion between autophagosomes and lysosomes was defective in bf cells without compromising the endocytic pathway. The direct association of mutant VPS33A(D251E) with the autophagic SNARE complex, STX17 (syntaxin 17)-VAMP8-SNAP29, was enhanced. In addition, the VPS33A(D251E) mutation enhanced interactions with other HOPS subunits, namely VPS41, VPS39, VPS18, and VPS11, except for VPS16. Reduction of the interactions between VPS33A(Y440D) and several other HOPS subunits led to decreased association with STX17. These results suggest that the VPS33A(D251E) mutation plays dual roles by increasing the HOPS complex assembly and its association with the autophagic SNARE complex, which selectively affects the autophagosome-lysosome fusion that impairs basal autophagic activity and induces Purkinje cell loss.
Mucopolysaccharidoses (MPS) are a group of genetic deficiencies of lysosomal enzymes that catabolize glycosaminoglycans (GAG). Here we describe a novel MPS-like disease caused by a specific mutation in the VPS33A gene. We identified several Yakut patients showin
g typical manifestations of MPS: coarse facial features, skeletal abnormalities, hepatosplenomegaly, respiratory problems, mental retardation, and excess secretion of urinary GAG. However, these patients could not be diagnosed enzymatically as MPS. They showed extremely high levels of plasma heparan sulphate (HS, one of GAG); 60 times the normal reference range and 6 times that of MPS patients. Additionally, most patients developed heart, kidney, and hematopoietic disorders, which are not typical symptoms for conventional MPS, leading to a fatal outcome between 1 and 2-years old. Using whole exome and Sanger sequencing, we identified homozygous c.1492C > T (p.Arg498Trp) mutations in the VPS33A gene of 13 patients. VPS33A is involved in endocytic and autophagic pathways, but the identified mutation did not affect either of these pathways. Lysosomal over-acidification and HS accumulation were detected in patient-derived and VPS33A-depleted cells, suggesting a novel role of this gene in lysosomal functions. We hence propose a new type of MPS that is not caused by an enzymatic deficiency.
