In Vivo Functions of Myotubularins and Therapy for Myotubular Myopathy

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The long-term goals of this project are to understand the molecular basis for the myotubular/ centronuclear myopathies and their defects in muscle function, and to use this information to develop therapies for patients with these neuromuscular diseases. X-linked myotubular myopathy (XLMTM) is caused by mutations of myotubularin (MTM1), the prototypic member of a novel family of lipid phosphatases that includes both catalytically active, and inactive (adaptor) family members (myotubularin-related proteins or "MTMRs"). There is strong evidence that, despite having similar biochemical activities, different myotubularin family members play physiologically distinct roles. For example, mutations of two related genes (MTMR2 and MTMR13) both cause forms of the inherited neuropathy, Charcot-Marie-Tooth (CMT) disease. The primary goals of this Project are 1) to develop and exploit a vertebrate model system (zebrafish) to determine the physiological function(s), of MTM1 and several of it's family members, 2) to utilize candidate gene mutation studies to identify genes for related forms of centronuclear myopathy and 3) to utilize a mouse Mtm1 knockout model of X-linked myotubular myopathy to develop AAV8-mediated gene therapy approaches to treat this fatal disease. Information resulting from the zebrafish studies will allow us to determine the degree of potential functional redundancy between family members and to identify the specific protein domains responsible for any tissue or molecular specificity. Identification of these functionally complementary MTMR genes may identify appropriate candidate genes to aid in finding the gene(s) for a severe autosomal recessive form of centronuclear myopathy. Observations on recovery from transient transcriptional knockdowns of MTM1 in the fish, will also be important in design of effective therapies using the mouse model. Gene replacement therapy experiments will address questions of efficacy and the appropriate therapeutic window for this approach to treating XLMTM, and will set the stage for possible future human clinical trials and animal studies that may attempt treatment via up-regulation of functionally equivalent MTMRs identified through the zebrafish experiments. Overall, success in this project will lead to 1) new insights into the similarities and differences in function between myotubularin and some of it's related family members, 2) an increased understanding of the molecular pathophysiology underlying myotubular myopathy in human patients, and 3) the first indications as to whether a gene therapy approach is likely to be effective in treating this devastating childhood disease.