Protein Misfolding as an Underlying Molecular Defect in Mucopolysaccharidosis III Type C

Department of Medical Genetics, CHU Sainte-Justine University of Montreal, Montreal, Canada.
PLoS ONE (Impact Factor: 3.23). 10/2009; 4(10):e7434. DOI: 10.1371/journal.pone.0007434
Source: PubMed


Mucopolysaccharidosis type IIIC or Sanfilippo syndrome type C (MPS IIIC, MIM #252930) is an autosomal recessive disorder caused by deficiency of the lysosomal membrane enzyme, heparan sulfate acetyl-CoA: α-glucosaminide N-acetyltransferase (HGSNAT, EC, which catalyses transmembrane acetylation of the terminal glucosamine residues of heparan sulfate prior to their hydrolysis by α-N-acetylglucosaminidase. Lysosomal storage of undegraded heparan sulfate in the cells of affected patients leads to neuronal death causing neurodegeneration and is accompanied by mild visceral and skeletal abnormalities, including coarse facies and joint stiffness. Surprisingly, the majority of MPS IIIC patients carrying missense mutations are as severely affected as those with splicing errors, frame shifts or nonsense mutations resulting in the complete absence of HGSNAT protein.
In order to understand the effects of the missense mutations in HGSNAT on its enzymatic activity and biogenesis, we have expressed 21 mutant proteins in cultured human fibroblasts and COS-7 cells and studied their folding, targeting and activity. We found that 17 of the 21 missense mutations in HGSNAT caused misfolding of the enzyme, which is abnormally glycosylated and not targeted to the lysosome, but retained in the endoplasmic reticulum. The other 4 mutants represented rare polymorphisms which had no effect on the activity, processing and targeting of the enzyme. Treatment of patient cells with a competitive HGSNAT inhibitor, glucosamine, partially rescued several of the expressed mutants.
Altogether our data provide an explanation for the severity of MPS IIIC and suggest that search for pharmaceutical chaperones can in the future result in therapeutic options for this disease.

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Available from: Alexey Pshezhetsky, Oct 30, 2014
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    • "HGSNAT transfers an acetyl group from cytoplasmic acetyl-CoA to terminal N-glucosamine residues of heparan sulphate within the lysosomes (Klein et al., 1978). To date, more than 80 HGSNAT mutations have been identified including 18 missense which all result in misfolding of the mutant enzyme (Feldhammer et al., 2009). "
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    ABSTRACT: Severe progressive neurological paediatric disease mucopolysaccharidosis III type C is caused by mutations in the HGSNAT gene leading to deficiency of acetyl-CoA: α-glucosaminide N-acetyltransferase involved in the lysosomal catabolism of heparan sulphate. To understand the pathophysiology of the disease we generated a mouse model of mucopolysaccharidosis III type C by germline inactivation of the Hgsnat gene. At 6-8 months mice showed hyperactivity, and reduced anxiety. Cognitive memory decline was detected at 10 months and at 12-13 months mice showed signs of unbalanced hesitant walk and urinary retention. Lysosomal accumulation of heparan sulphate was observed in hepatocytes, splenic sinus endothelium, cerebral microglia, liver Kupffer cells, fibroblasts and pericytes. Starting from 5 months, brain neurons showed enlarged, structurally abnormal mitochondria, impaired mitochondrial energy metabolism, and storage of densely packed autofluorescent material, gangliosides, lysozyme, phosphorylated tau, and amyloid-β. Taken together, our data demonstrate for the first time that deficiency of acetyl-CoA: α-glucosaminide N-acetyltransferase causes lysosomal accumulation of heparan sulphate in microglial cells followed by their activation and cytokine release. They also show mitochondrial dysfunction in the neurons and neuronal loss explaining why mucopolysaccharidosis III type C manifests primarily as a neurodegenerative disease. © The Author (2015). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email:
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    • "This hypothesis would be strengthened by the severe neurodegenerative phenotype of the Cln10 mouse model deficient in the aspartyl protease cathepsin D, which mediates protein degradation and activates precursor forms of proteases [55]. Disease-causing mutations in polytopic lysosomal membrane proteins , such as CLN3 and HGSNAT, are typically retained in the endoplasmic reticulum due to misfolding of the proteins [56] [57]. However, we and others have shown that several disease-causing mutations in the CLN7/MFSD8 gene did not alter subcellular localization of CLN7 in lysosomal compartments [9] [16], suggesting that these mutations cause alterations in protein stability and/or biological function in lysosomes, rather than retention of misfolded CLN7 proteins in the endoplasmic reticulum or intracellular missorting. "
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