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

Bethlem myopathy and Ullrich congenital muscular dystrophy (UCMD) sit at opposite ends of a clinical spectrum caused by mutations in the extracellular matrix protein collagen VI. Bethlem myopathy is relatively mild, and patients remain ambulant in adulthood while many UCMD patients lose ambulation by their teenage years and require respiratory interventions. Dominant and recessive mutations are found across the entire clinical spectrum; however, recessive Bethlem myopathy is rare, and our understanding of the molecular pathology is limited. We studied a patient with Bethlem myopathy. Electron microscopy of his muscle biopsy revealed abnormal mitochondria. We identified a homozygous COL6A2 p. D871N amino acid substitution in the C-terminal C2 A-domain. Mutant alpha 2(VI) chains are unable to associate with alpha 1(VI) and alpha 3(VI) and are degraded by the proteasomal pathway. Some collagen VI is assembled, albeit more slowly than normal, and is secreted. These molecules contain the minor alpha 2(VI) C2a splice form that has an alternative C terminus that does include the mutation. Collagen VI tetramers containing the alpha 2(VI) C2a chain do not assemble efficiently into microfibrils and there is a severe collagen VI deficiency in the extracellular matrix. We expressed wildtype and mutant alpha 2(VI) C2 domains in mammalian cells and showed that while wild-type C2 domains are efficiently secreted, the mutant p.D871N domain is retained in the cell. These studies shed new light on the protein domains important for intracellular and extracellular collagen VI assembly and emphasize the importance of molecular investigations for families with collagen VI disorders to ensure accurate diagnosis and genetic counseling.