Altered splicing of the BIN1 muscle-specific exon in humans and dogs with highly progressive centronuclear myopathy.

Amphiphysin 2, encoded by BIN1, is a key factor for membrane sensing and remodelling in different cell types. Homozygous BIN1 mutations in ubiquitously expressed exons are associated with autosomal recessive centronuclear myopathy (CNM), a mildly progressive muscle disorder typically showing abnormal nuclear centralization on biopsies. In addition, misregulation of BIN1 splicing partially accounts for the muscle defects in myotonic dystrophy (DM). However, the muscle-specific function of amphiphysin 2 and its pathogenicity in both muscle disorders are not well understood. In this study we identified and characterized the first mutation affecting the splicing of the muscle-specific BIN1 exon 11 in a consanguineous family with rapidly progressive and ultimately fatal centronuclear myopathy. In parallel, we discovered a mutation in the same BIN1 exon 11 acceptor splice site as the genetic cause of the canine Inherited Myopathy of Great Danes (IMGD). Analysis of RNA from patient muscle demonstrated complete skipping of exon 11 and BIN1 constructs without exon 11 were unable to promote membrane tubulation in differentiated myotubes. Comparative immunofluorescence and ultrastructural analyses of patient and canine biopsies revealed common structural defects, emphasizing the importance of amphiphysin 2 in membrane remodelling and maintenance of the skeletal muscle triad. Our data demonstrate that the alteration of the muscle-specific function of amphiphysin 2 is a common pathomechanism for centronuclear myopathy, myotonic dystrophy, and IMGD. The IMGD dog is the first faithful model for human BIN1-related CNM and represents a mammalian model available for preclinical trials of potential therapies.

Molecular and cellular insights into a distinct myopathy of Great Dane dogs.

A myopathy in the Great Dane dog with characteristic pathological and molecular features is reported. Young adults present with progressive weakness and generalised muscle atrophy. To better define this condition, an investigation using histopathology, confocal microscopy, biochemistry and microarray analysis was undertaken. The skeletal muscles of affected dogs exhibited increased oxidative fibre phenotype and core fibre lesions characterised by the disruption of the sarcomeric architecture and the accumulation of mitochondrial organelles. Affected muscles displayed co-ordinated expression of genes consistent with a slow-oxidative phenotype, which was possibly a compensatory response to chronic muscle damage. There was disruption of Z-lines in affected muscles which, at the molecular level, manifested as transcriptional dysregulation of several Z-line associated genes, including alpha-actinin, myotilin, desmin, vimentin and telethonin. The pathology of this canine myopathy is distinct from that of human central core myopathies that are characterised by cores devoid of mitochondria and by the presence of myofibrillar breakdown products.