title: Muscular dystrophy begins early in embryonic development deriving from stem cell loss and disrupted skeletal muscle formation.
abstract: Examination of embryonic myogenesis of two distinct, but functionally related, skeletal muscle dystrophy mutants (mdx and cav-3(-/-)) establishes for the first time that key elements of the pathology of Duchenne muscular dystrophy (DMD) and limb-girdle muscular dystrophy type 1C (LGMD-1c) originate in the disruption of the embryonic cardiac and skeletal muscle patterning processes. Disruption of myogenesis occurs earlier in mdx mutants, which lack a functional form of dystrophin, than in cav-3(-/-) mutants, which lack the Cav3 gene that encodes the protein caveolin-3; this finding is consistent with the milder phenotype of LGMD-1c, a condition caused by mutations in Cav3, and the earlier [embryonic day (E)9.5] expression of dystrophin. Myogenesis is severely disrupted in mdx embryos, which display developmental delays; myotube morphology and displacement defects; and aberrant stem cell behaviour. In addition, the caveolin-3 protein is elevated in mdx embryos. Both cav-3(-/-) and mdx mutants (from E15.5 and E11.5, respectively) exhibit hyperproliferation and apoptosis of Myf5-positive embryonic myoblasts; attrition of Pax7-positive myoblasts in situ; and depletion of total Pax7 protein in late gestation. Furthermore, both cav-3(-/-) and mdx mutants have cardiac defects. In cav-3(-/-) mutants, there is a more restricted phenotype comprising hypaxial muscle defects, an excess of malformed hypertrophic myotubes, a twofold increase in myonuclei, and reduced fast myosin heavy chain (FMyHC) content. Several mdx mutant embryo pathologies, including myotube hypotrophy, reduced myotube numbers and increased FMyHC, have reciprocity with cav-3(-/-) mutants. In double mutant (mdxcav-3(+/-)) embryos that are deficient in dystrophin (mdx) and heterozygous for caveolin-3 (cav-3(+/-)), whereby caveolin-3 is reduced to 50% of wild-type (WT) levels, these phenotypes are severely exacerbated: intercostal muscle fibre density is reduced by 71%, and Pax7-positive cells are depleted entirely from the lower limbs and severely attenuated elsewhere; these data suggest a compensatory rather than a contributory role for the elevated caveolin-3 levels that are found in mdx embryos. These data establish a key role for dystrophin in early muscle formation and demonstrate that caveolin-3 and dystrophin are essential for correct fibre-type specification and emergent stem cell function. These data plug a significant gap in the natural history of muscular dystrophy and will be invaluable in establishing an earlier diagnosis for DMD/LGMD and in designing earlier treatment protocols, leading to better clinical outcome for these patients.
authors.affiliation: School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
authors.forename: Deborah
authors.initials: D
authors.lastname: Merrick
authors.forename: Lukas Kurt Josef
authors.initials: LK
authors.lastname: Stadler
authors.forename: Dean
authors.initials: D
authors.lastname: Larner
authors.forename: Janet
authors.initials: J
authors.lastname: Smith
mesh_terms: D000818:Animals; D051244:Caveolin 3 / Q000235:genetics; D049109:Cell Proliferation; D004195:Disease Models, Animal; D016189:Dystrophin / Q000235:genetics; D053595:Embryonic Stem Cells / Q000166:cytology*; D018507:Gene Expression Regulation, Developmental; D020022:Genetic Predisposition to Disease; D006321:Heart / Q000196:embryology; D051379:Mice; D008810:Mice, Inbred C57BL; D018485:Muscle Fibers, Skeletal / Q000166:cytology; D009136:Muscular Dystrophies / Q000235:genetics* / Q000503:physiopathology*; D010641:Phenotype; D013234:Stem Cells / Q000166:cytology
chemical_list: D051244:Caveolin 3; D016189:Dystrophin