Résumé :
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Communication n° 381. Mitochondrial DNA (mtDNA) deletions were the first mtDNA alterations identified as the cause of mitochondrial diseases. They have been associated with three clinical presentations : two severe multisystemic diseases with early onset (Pearson and Kearns Shy Sayre (KSS) syndromes) and a milder adult disease with only muscle involvement (Chronic Progressive External Ophthalmoplegia, CPEO). However, many adult cases present with a multisystemic disease but an evolution much less severe than typical KSS (CPEO+). Potential molecular mechanisms underlying the difference of severity encompass tissue distribution, proportion, and location of mtDNA deletion as well as mechanisms associated with the defect of the respiratory chain activities such as apoptosis and overproduction of Reactive Oxygen Species (ROS). We have analysed the muscle biopsy of 16 patients with mtDNA deletion and diverse clinical presentation. Serial sections were investigated by histochemistry for the presence of defect of respiratory chain activity, mitochondrial proliferation, and by immunohistochemistry for apoptosis and overproduction of ROS. The study of more than 2000 muscle fibres showed that apoptosis and ROS production are associated with respiratory chain defect and mitochondrial proliferation. To correlate these parameters with their genetic cause, the amounts of deleted and wild type mtDNA were evaluated using real time PCR in muscle fragment and in individual muscle fibres. We found a correlation between proportion of apoptotic fibres and deletion percentage. In Ragged-Red Fibres (RRF), amount of wild type mtDNA was lower in apoptotic fibres than in non apoptotic ones and percentage of deletion showed tendency to increase in apoptotic fibres. The same morphological and molecular study was performed on 4 mtDNA point mutation cases (A3244G tRNA Leu). Apoptotic features were also localized in cytochrome-c oxydase negative, RRF. Molecular studies are ongoing. Our study suggests that apoptosis plays a pathogenic role, probably late in the course of mitochondrial diseases.
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