Résumé :
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Collagen VI-myopathies, caused by mutations in any of the three genes encoding collagen type VI (ColVI), represent a clinical continuum with Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy at each end of the spectrum. The severe, early-onset and progressive forms of ColVI-myopathy classically present with hypotonia, muscle weakness and motor delay in the first months of life, in association with proximal joint contractures and a striking distal hyperlaxity, as well as respiratory insufficiency. ColVI-myopathies may be inherited as autosomal recessive or dominant disorders, but numerous dominant de novo mutations have now been reported. ColVI is a major component of connective tissues, composed of 3 alpha chainswhose structures are highly similar. These chains assemble intracellularly as monomers, dimers and tetramers consecutively, prior to secretion and alignment to form microfibrils in the extracellular space. ColVI is modified and assembled within the endoplasmic reticulum (ER) where the 3 chains are subjected to extensive post-traductional modifications. Mutations in the ColVI-encoding genes generally alter its secretion in cultured fibroblasts from patients and often result in intracellular retention of unassembled ColVI chains. We further characterized this altered secretion of mutated ColVI in cultured fibroblasts from 8 patients with early onset ColVI-myopathy harbouring distinct mutations in the COL6A genes (5 homozygotes, 2 compound heterozygotes and 1 dominant de novo). Using co-immunolabelling with ER markers, as well as transmission electronic microscopy in 2 instances, we established that unassembled ColVI chains are specifically retained in this subcellular compartment. In order to test the hypothesis that this accumulation of ColVI polypeptides could perturb ER homeostasis and function, we investigated the expression of ER stress markers in fibroblasts. By RT-PCR and immunoblotting, we showed that ColVI accumulation in the ER does not cause cleavage of XBP1 transcripts, nor does it induce over-expression of the BiP/Grp78 chaperone and the transcription factor CHOP.Taken together, our data show that ER stress and the unfolded protein response do not play a significant pathogenic role in cultured fibroblasts, but whether this holds true in other cell types and in vivo remains to be further investigated.
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