AAV mediated expression of myotubularin in muscle corrects the myotubular myopathy phenotype in a mouse model and suggests a function in membrane remodeling at the sarcolemma. A. Buj-Bello1,3, F. Fougerousse2, Y. Schwab1, N. Messaddeq1, D. Spehner1, P. Schultz1, O. Danos2, J. Laporte1,3, A-M. Douar2, J-L. Mandel1,3 1) IGBMC, CNRS/INSERM/University of Strasbourg, 67404 Illkirch, France; 2) Genethon, CNRS UMR8115, 91000 Evry, France; 3) Genetique Humaine, College de France.
X-linked myotubular myopathy (XLMTM) is a severe congenital disease due to mutations affecting the phosphoinositide phosphatase myotubularin (MTM1 gene), and characterized by small skeletal muscle fibers with frequent occurence of central nuclei. The pathophysiology of the disease is still poorly understood and specific treatment is unavailable. We have constructed a recombinant serotype 1 adeno-associated virus (rAAV2/1) vector expressing myotubularin and injected it into skeletal muscle to analyze the subcellular localization of myotubularin in myofibers and test its therapeutic potential in a faithful XLMTM mouse model (Mtm1 conditional KO). We show that a substantial proportion of myotubularin associates to the sarcolemma and I band, including triads. Transgene expression in Mtm1 KO muscle halts the progression of the histological phenotype, leading to a large increase in muscle weight and myofiber area and to a decrease in the percentage of fibers with internal nuclei. Mislocalization of other organelles such as mitochondria was also corrected. A single injection in Mtm1 KO muscles leads to a full rescue of the contractile force in the injected muscle. Overexpression of myotubularin in wild-type muscle causes myofiber vacuolation and accumulation of packed membrane saccules (myelin-like) close to the sarcolemma. This suggests that myotubularin is involved in plasma membrane remodeling and/or homeostasis, like the two other known genes implicated in autosomal forms of centronuclear myopathy. Such a role fits also with the implication of MTM1 paralogs MTMR2 and MTMR13 in forms of recessive demyelinating CMT (CMT4B). This study has provided insights into the function of myotubularin in muscle and a proof of principle that viral-mediated MTM1 gene delivery may be an effective therapeutic approach for patients with myotubular myopathy.