Recessive COL6A2 C-globular Missense Mutations in Ullrich Congenital Muscular Dystrophy

Abstract

Ullrich congenital muscular dystrophy (UCMD) is a disabling and life-threatening disorder resulting from either recessive or dominant mutations in genes encoding collagen VI. Although the majority of the recessive UCMD cases have frameshift or nonsense mutations in COL6A1, COL6A2, or COL6A3, recessive structural mutations in the COL6A2 C-globular region are emerging also. However, the underlying molecular mechanisms have remained elusive. Here we identified a homozygous COL6A2 E624K mutation (C1 subdomain) and a homozygous COL6A2 R876S mutation (C2 subdomain) in two UCMD patients. The consequences of the mutations were investigated using fibroblasts from patients and cells stably transfected with the mutant constructs. In contrast to expectations based on the clinical severity of these two patients, secretion and assembly of collagen VI were moderately affected by the E624K mutation but severely impaired by the R876S substitution. The E624K substitution altered the electrostatic potential of the region surrounding the metal ion-dependent adhesion site, resulting in a collagen VI network containing thick fibrils and spots with densely packed microfibrils. The R876S mutation prevented the chain from assembling into triple-helical collagen VI molecules. The minute amount of collagen VI secreted by the R876S fibroblasts was solely composed of a faster migrating chain corresponding to the C2a splice variant with an alternative C2 subdomain. In transfected cells, the C2a splice variant was able to assemble into short microfibrils. Together, the results suggest that the C2a splice variant may functionally compensate for the loss of the normal COL6A2 chain when mutations occur in the C2 subdomain.Ullrich congenital muscular dystrophy (UCMD, 2 OMIM 254090) has recently emerged as one of the most prevalent forms of congenital muscular dystrophy (1, 2). The disorder is characterized by severe muscle weakness in conjunction with proximal joint contractures and co-existing distal joint hypermobility (3). Children afflicted with UCMD may never achieve independent ambulation or may be able to walk for only a limited period of time. They may suffer from respiratory insufficiency, leading to early demise if not treated. UCMD was initially thought to be a recessive condition caused by mutations in the COL6A1, COL6A2, or COL6A3 genes encoding three collagen VI chains (4 -6). We have demonstrated that the disease can also result from a dominant mutation in COL6A1 (7). It is now apparent that dominant mutations in the three collagen VI genes are common in UCMD patients (8, 9).Collagen VI is a ubiquitously expressed extracellular matrix protein composed of ␣1(VI), ␣2(VI), and ␣3(VI) chains encoded by the COL6A1, COL6A2, and COL6A3 genes, respectively (10). Each chain is made up of a triple-helical domain flanked by N-and C-globular regions that consist mainly of von Willebrand factor, type A (vWF-A) modules (11,12) (Fig. 1A). There are two vWF-A modules in the C-globular region of each chain, named C1 and C2 subdomains, where C2 is m...

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

“…Secreted tetramers are not able to form microfibrils and pathology results because there are no collagen VI supramolecular structures in the extracellular matrix. Monomer, dimer, and tetramer formation are also not affected by the ␣2(VI) C1 domain substitution, p.E624K (38). In contrast to UCMD21, secreted tetramers with the ␣2(VI) p.E624K mutation form microfibrils in the extracellular matrix, although when viewed by electron microscopy they appear disorganized.…”

“…To date, the mutations in this class are all in the ␣2(VI) C2 domain and include p.L873P, p.R876S, p.N897del, and p.P932L (17,18,38). ␣2(VI) chains expressing these mutations are not able to assemble into triple helical structures.…”

Aberrant Mitochondria in a Bethlem Myopathy Patient with a Homozygous Amino Acid Substitution That Destabilizes the Collagen VI α2(VI) Chain

“…For example, a homozygous ␣2(VI) p.R876S mutation destabilizes the mutant chain and the small amount of secreted collagen VI contains the ␣2(VI) C2a chain (31). We reported a patient with recessive ␣2(VI) C2 domain mutations, p.L873P and p.N897del, and showed that the mutant chains were unstable and only small amounts of collagen VI were secreted (3).…”

The collagen VI-related myopathies: muscle meets its matrix

“…Such mutations presumably affect crucial functions of the collagen VI protein and its interactions. 16,90 …”

“…32,90 In addition, mutation analysis of the collagen VI genes has led to identification of a growing number of polymorphic variants of unknown significance. These variants require careful genetic and biochemical analysis to prove or disprove their pathogenetic roles.…”