Résumé :
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X-linked myotubular myopathy (XLMTM) is a severe congenital disease that affects the skeletal musculature leading to early postnatal death of most patients. The gene responsible for the disorder, MTM1, encodes a lipid phosphatase named myotubularin. Mtm1 knockout mice develop a progressive generalized myopathy with reduced life expectancy and present a muscle pathology that resembles that of XLMTM patients. The aim of our study is to identify the molecular mechanisms leading to XLMTM pathogenesis. For this, we have analyzed the transcriptome of Mtm1 knockout (KO) skeletal muscle before (2 week-old) and after (5 weeks) the appearance of muscle weakness by using Affymetrix arrays and found a highly active transcriptional response in mice at symptomatic stages of the disease. Since we have observed that overexpressed myotubularin is associated to the sarcolemma and triads and muscle weakness is very severe in KO mice, we focused on genes involved in calcium homeostasis. We found that genes linked to the excitation-contraction (E-C) coupling machinery are deregulated at the transcriptional (Q-PCR) and/or protein level in muscles of 5 week-old Mtm1 KO mice, with some of them being already altered at early stages of the disease. We thus measured intracellular Ca2+ transients elicited by voltage-clamp depolarisations in muscle fibres from 4-5 week-old control and Mtm1 KO mice. The peak Ca2+ transient was strongly reduced in myotubularin deficient fibres whereas resting Ca2+ and time course of Ca2+ removal from the cytoplasm remained essentially unaffected. These results strongly suggest that failure of excitation-contraction coupling due to alteration of calcium release from the sarcoplasmic reticulum accounts for muscle weakness in Mtm1 deficient mice. We also found ultrastructural alterations of the triads in Mtm1-deficient muscle that may contribute to the E-C uncoupling. These results provide novel insights into the pathomechanism of XLMTM and may open novel therapeutic strategies for patients.
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