[Mitochondrial DNA mutations and three major forms of mitochondrial myopathies: CPEO, MELAS and MERRF]

Mitochondrial DNA (mtDNA) mutations were discovered during a few years around 1990 and thought to be assigned to the three major clinical forms of mitochondrial myopathies. Then, several mtDNA mutations were detected in each disease and the common mutations were found in more than one disease, suggesting genotype and phenotype heterogeneity. Heteroplasmy, tissue/cellular specificity and threshold effect have been extensively studied using skeletal muscle and culture cells from the patients, but the major part of the relations up between mtDNA genotype and clinical phenotype remains unknown. Because of the facts that mtDNA mutations have been found in other clinical forms (not myopathies), and frataxin, the product of the responsible gene for Freidreich ataxia, may be associated with mtDNA biogenesis, disease entity accompanied by mtDNA abnormalities could expand more in the future.