NUCLEIC ACIDS, PROTEIN SYNTHESIS, AND MOLECULAR GENETICS
Bethlem Myopathy and Engineered Collagen VI Triple Helical Deletions Prevent Intracellular Multimer Assembly and Protein Secretion*

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Mutations in the genes that code for collagen VI subunits, COL6A1, COL6A2, and COL6A3, are the cause of the autosomal dominant disorder, Bethlem myopathy. Although three different collagen VI structural mutations have previously been reported, the effect of these mutations on collagen VI assembly, structure, and function is currently unknown. We have characterized a new Bethlem myopathy mutation that results in skipping of COL6A1 exon 14 during pre-mRNA splicing and the deletion of 18 amino acids from the triple helical domain of the α1(VI) chain. Sequencing of genomic DNA identified a G to A transition in the +1 position of the splice donor site of intron 14 in one allele. The mutant α1(VI) chains associated intracellularly with α2(VI) and α3(VI) to form disulfide-bonded monomers, but further assembly into dimers and tetramers was prevented, and molecules containing the mutant chain were not secreted. This triple helical deletion thus resulted in production of half the normal amount of collagen VI. To further explore the biosynthetic consequences of collagen VI triple helical deletions, an α3(VI) cDNA expression construct containing a 202-amino acid deletion within the triple helix was produced and stably expressed in SaOS-2 cells. The transfected mutant α3(VI) chains associated with endogenous α1(VI) and α2(VI) to form collagen VI monomers, but dimers and tetramers did not form and the mutant-containing molecules were not secreted. Thus, deletions within the triple helical region of both the α1(VI) and α3(VI) chains can prevent intracellular dimer and tetramer assembly and secretion. These results provide the first evidence of the biosynthetic consequences of structural collagen VI mutations and suggest that functional protein haploinsufficiency may be a common pathogenic mechanism in Bethlem myopathy.

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This work was supported by grants from the National Health and Medical Research Council of Australia and the Royal Children's Hospital Research Institute.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.