Collagen VI Microfibril Formation Is Abolished by an α2(VI) von Willebrand Factor Type A Domain Mutation in a Patient with Ullrich Congenital Muscular Dystrophy

Abstract

Collagen VI is an extracellular protein that most often contains the three genetically distinct polypeptide chains, ␣1(VI), ␣2(VI), and ␣3(VI), although three recently identified chains, ␣4(VI), ␣5(VI), and ␣6(VI), may replace ␣3(VI) in some situations. Each chain has a triple helix flanked by N-and C-terminal globular domains that share homology with the von Willebrand factor type A (VWA) domains. During biosynthesis, the three chains come together to form triple helical monomers, which then assemble into dimers and tetramers. Tetramers are secreted from the cell and align end-to-end to form microfibrils. The precise molecular mechanisms responsible for assembly are unclear. Mutations in the three collagen VI genes can disrupt collagen VI biosynthesis and matrix organization and are the cause of the inherited disorders Bethlem myopathy and Ullrich congenital muscular dystrophy. We have identified a Ullrich congenital muscular dystrophy patient with compound heterozygous mutations in ␣2(VI). The first mutation causes skipping of exon 24, and the mRNA is degraded by nonsensemediated decay. The second mutation is a two-amino acid deletion in the C1 VWA domain. Recombinant C1 domains containing the deletion are insoluble and retained intracellularly, indicating that the mutation has detrimental effects on domain folding and structure. Despite this, mutant ␣2(VI) chains retain the ability to associate into monomers, dimers, and tetramers. However, we show that secreted mutant tetramers containing structurally abnormal C1 VWA domains are unable to associate further into microfibrils, directly demonstrating the critical importance of a correctly folded ␣2(VI) C1 domain in microfibril formation.Collagen VI is a microfibrillar protein broadly expressed in connective tissues. It is composed of three genetically distinct polypeptide chains, ␣1(VI), ␣2(VI), and, most often, ␣3(VI), encoded by the genes COL6A1, COL6A2, and COL6A3, respectively. Three recently identified chains, ␣4(VI), ␣5(VI), and ␣6(VI), may replace ␣3(VI) in some situations (1, 2). Each chain consists of a short Gly-X-Y triple helical sequence (3) flanked on either side by a number of globular domains, the majority of which share homology with the von Willebrand factor type A (VWA) 4 domains (4 -6). Each chain has two C-terminal VWA domains (C1 and C2). In addition, the ␣3(VI) chain has three C-terminal domains that are not found in ␣1(VI) or ␣2(VI); C3 is proline-rich and similar to some salivary proteins, C4 has homology to fibronectin type III repeats, and C5 shows homology to the Kunitz family of serine protease inhibitors (6). The ␣1(VI) and ␣2(VI) chains each have one N-terminal VWA domain (N1), whereas the ␣3(VI) chain has up to 10 N-terminal VWA domains (N1-N10), some of which are subject to alternative splicing (7,8).Collagen VI assembly begins with the intracellular association of ␣1(VI), ␣2(VI), and ␣3(VI) chains to form a monomer that consists of a 105-nm triple helical segment flanked on either side by the N-and C-terminal globular domains (9...

Genetic and clinical findings in a Chinese cohort of patients with collagen VI‐related myopathies

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Selecting biologically informative genes in co-expression networks with a centrality score

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Mitochondrial Dysfunction and Defective Autophagy in the Pathogenesis of Collagen VI Muscular Dystrophies

“…Based on the mutation, different mechanisms can be predicted such as: (1) loss-of-function for mutations perturbing triple helix formation, intracellular assembly or ColVI secretion; (2) dominant negative effect for mutations giving rise to abnormal ColVI polypeptides secreted in the ECM; and (3) haploinsufficiency for mutations affecting mRNA stability, with a decreased synthesis of normal ColVI. The effect of specific COL6 mutations has been investigated by in vitro studies (Lamande et al 1999(Lamande et al , 2002Sasaki et al 2000;Zhang et al 2002;Jimenez-Mallebrera et al 2006;Baker et al 2007;Tooley et al 2010), which provided relevant clues on their effect at the protein level. However, there is no conclusive evidence on the genotype -phenotype correlation for BM and UCMD, which may represent a clinical continuum rather than strictly separate entities Lampe and Bushby 2005;Bönnemann 2011).…”