Mutations in the collagen XII gene define a new form of extracellular matrix-related myopathy.

Bethlem myopathy (BM) [MIM 158810] is a slowly progressive muscle disease characterized by contractures and proximal weakness, which can be caused by mutations in one of the collagen VI genes (COL6A1, COL6A2 and COL6A3). However, there may be additional causal genes to identify as in ∼50% of BM cases no mutations in the COL6 genes are identified. In a cohort of -24 patients with a BM-like phenotype, we first sequenced 12 candidate genes based on their function, including genes for known binding partners of collagen VI, and those enzymes involved in its correct post-translational modification, assembly and secretion. Proceeding to whole-exome sequencing (WES), we identified mutations in the COL12A1 gene, a member of the FACIT collagens (fibril-associated collagens with interrupted triple helices) in five individuals from two families. Both families showed dominant inheritance with a clinical phenotype resembling classical BM. Family 1 had a single-base substitution that led to the replacement of one glycine residue in the triple-helical domain, breaking the Gly-X-Y repeating pattern, and Family 2 had a missense mutation, which created a mutant protein with an unpaired cysteine residue. Abnormality at the protein level was confirmed in both families by the intracellular retention of collagen XII in patient dermal fibroblasts. The mutation in Family 2 leads to the up-regulation of genes associated with the unfolded protein response (UPR) pathway and swollen, dysmorphic rough-ER. We conclude that the spectrum of causative genes in extracellular matrix (ECM)-related myopathies be extended to include COL12A1.

Macrophages: a minimally invasive tool for monitoring collagen VI myopathies.

INTRODUCTION: Collagen VI expression was tested in peripheral blood macrophages from patients with collagen VI-related myopathies and compared with muscle biopsy. METHODS: RNA and protein studies were performed in blood macrophages from 5 patients previously diagnosed with either Ullrich congenital muscular dystrophy (UCMD) or Bethlem myopathy (BM). The full spectrum of possible genotypes was considered, including both dominant and recessive UCMD and BM cases. RESULTS: In the dominant BM patient, no collagen VI alterations were detectable in macrophages or muscle biopsy. In the remaining patients, the protein defect caused by the selected mutations, as well as the transcriptional abnormalities, were readily detectable in macrophages, at levels comparable to those observed in muscle biopsy samples and cultured skin fibroblasts. CONCLUSIONS: Our data support the suitability of peripheral blood macrophages as a reliable, minimally invasive tool for supplementing or replacing muscle/skin biopsies in the diagnosis and monitoring of collagen VI-related myopathies.

Expression of the collagen VI α5 and α6 chains in normal human skin and in skin of patients with collagen VI-related myopathies.

Collagen VI is an extracellular matrix protein with critical roles in maintaining muscle and skin integrity and function. Skin abnormalities, including predisposition to keratosis pilaris and abnormal scarring, were described in Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy (BM) patients carrying mutations in COL6A1, COL6A2, and COL6A3 genes, whereas COL6A5, previously designated as COL29A1, was linked to atopic dermatitis. To gain insight into the function of the newly identified collagen VI α5 and α6 chains in human skin, we studied their expression and localization in normal subjects and in genetically characterized UCMD and BM patients. We found that localization of α5, and to a lesser extent α6, is restricted to the papillary dermis, where the protein mainly colocalizes with collagen fibrils. In addition, both chains were found around blood vessels. In UCMD patients with COL6A1 or COL6A2 mutations, immunolabeling for α5 and α6 was often altered, whereas in a UCMD and in a BM patient, each with a COL6A3 mutation, expression of α5 and α6 was apparently unaffected, suggesting that these chains may substitute for α3, forming α1α2α5 or α1α2α6 heterotrimers.