Résumé :
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Centronuclear Myopathies are a group of muscular disorders characterized by the presence of abnormally large nuclei centrally localized within hypotrophic myofibres. Amongst this family of diseases, myotubular myopathy (XLMTM), the most severe condition, is associated with profound muscle weakness, respiratory deficiency and premature death of patients. XLMTM is known to be caused by inactivating mutations in the MTM1 gene encoding the lipid phosphatase myotubularin, which dephosphorylates the 3' position of the membrane phosphoinositides PI3P and PI3,5P2. Recent studies in zebrafish, mouse and dog models of the disease have shown that disturbed organization and/or function of the excitation-contraction coupling machinery may be responsible for muscle weakness. We reported the first proof of principle that local delivery of an adeno-associated virus (AAV) vector expressing myotubularin under the cytomegalovirus (CMV) promoter in symptomatic muscle-specific Mtm1-deficient (mKO) mice ameliorates the phenotype in targeted muscles (Hum Mol Genet, 2008, 17, 2132-2143). In the present study, we delivered by intravenous injection a single bolus of a rAAV2/9 vector expressing myotubularin under the desmin promoter in both constitutive (KO) and muscle-specific (mKO) Mtm1 deficient mice. Two weeks after rAAV injection, myotubularin is expressed in all analyzed muscles scattered throughout the body, including the diaphragm. Myotubularin replacement in mice leads to a strong increase in muscle volume and largely ameliorates mitochondria positioning and the distribution of triad (Ryr1, DHPRa) and sarcolemma (dysferlin) markers in myofibers. The muscle contractile force and motor activity of treated mutant mice are efficiently restored to levels statistically equivalent to that of WT animals. Most importantly, the lifespan of both AAV-treated KO and mKO mice is prolonged up to the end of the 6 and 12 monthstudy, respectively. Altogether, these results constitute the first demonstration that generalized muscle structure and function can be improved through a gene therapy approach in myotubular myopathy and pave the way for pre-clinical trials in larger animals.
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