Résumé :
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Autosomal dominant centronuclear myopathy (AD-CNM) is a rare congenital myopathy, clinically characterized by delayed motor milestones and muscle weakness and often associated with ptosis and ophthalmoplegia. The gene responsible for AD-CNM has been identified as DNM2, which encodes dynamin 2 (DNM2). This protein is a member of the large GTPase family that includes three dynamin-encoding genes (DNM1, 2 and 3), with distinct tissue expression patterns and several isoforms. RNA was extracted from human brain, skeletal muscles and cultured primary muscle cells. RT-PCR was performed to determine the dynamin isoforms expressed in these tissues. The 3 genes are expressed in brain, whereas only DNM2 is expressed in skeletal muscle, human primary myoblasts and myotubes. In DNM2, alternative forms of exons 10 and 13 can be differentially spliced leading potentially to 4 different transcripts. By RT-PCR and sequencing we found that only 2 of these transcripts were detected in skeletal muscle: transcripts 1 (containing exons 10a and 13b), and 2 (containing 10b and 13b), corresponding to the longest sequences i.e. 870 amino acids. However, in brain and in primary cultured myoblasts and myotubes only transcript 1 is expressed. The cellular localisation of DNM2 in skeletal muscle was unclear from standard immunohistochemical staining of muscle sections. To investigate this further, a construct encoding wild type (WT) DNM2 transcript 1 tagged with GFP was electro-transferred into mouse tibialis anterior muscle. In transfected muscles GFP-tagged WT DNM2 was distributed homogenously in the cytoplasm and localized to the sarcolemma. In contrast, when GFP-tagged mutant R465W DNM2 was transfected, staining was restricted to the sarcolemma. The R465W mutation is the most common AD-CNM mutation reported. Consequences of this alteration on the function of DNM2 into skeletal muscle remain to be determined.
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