Duplications of mitochondrial DNA in mitochondrial myopathy

Restriction enzyme analysis was done on total cellular DNA extracted from whole blood in two patients with mitochondrial myopathy and multisystem involvement and their families. The two patients had an abnormal mitochondrial genome with a large (about 8 kb) duplication present in several tissues. Normal mitochondrial DNA (mtDNA) was also present, but within each maternal lineage the abnormal mitochondrial genome was confined to clinically affected individuals. This observation, together with the failure of extensive population surveys to identify such abnormalities of mtDNA, suggests that these mutations cause the disease.