Résumé :
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In addition to their key role in the cell energy production, mitochondria are critical actors in apoptosis and oxidative stress. Increased susceptibility to apoptosis and oxidative stress has been shown in cellular models of defects in the oxidative phosphorylation pathway similar to those encountered in mitochondrial diseases. However extent of both apoptosis and oxidative stress as well as their responsibility in the clinical outcome of human mitochondrial diseases is till controversial and remains to be elucidated in the prospect of potential therapeutic approaches of mitochondrial diseases. Using an in situ approach to address in muscle from patients with mitochondrial myopathies, we address the relationship between respiratory defect, mitochondrial proliferation and mutation load on the one hand and apoptosis and oxidative stress on the other hand. Eleven biopsies from patients with typical mitochondrial myopathy and large size mitochondrial DNA (mtDNA) deletion were analyzed with respect to the presence of apoptosis (immunoreaction of activated caspase 3), and of oxidative lesions (immunological demonstration of the presence of 8-hydroxy-deoxyguanosine or nitrotyrosine, and overexpression of superoxide dismutase 1 and 2). Serial muscle sections allowed correlative analysis of these anomalies with respiratory defect (cytochrome c oxidase activity) and mitochondrial proliferation (succinate dehydrogenase activity) in 3000 individual muscle fibres. They demonstrate that oxidative stress occurs early in the sequence of events that link respiratory defect to oxidative stress to mitochondrial proliferation to apoptosis. Amount of normal and mutant mtDNA in individual muscle fibres will now be addressed by a quantitative PCR approach.
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