Diabetes and mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS): radiolabeled polymerase chain reaction is necessary for accurate detection of low percentages of mutation.

A 6-yr-old boy presented with muscle weakness, lactic acidemia, and insulin-dependent diabetes mellitus (IDDM). Using PCR and restriction enzyme analysis, he was found to have the classical A3248G mitochondrial DNA (mtDNA) mutation frequently associated with mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). The mutation was confirmed by sequencing muscle mtDNA. The mutation in mtDNA from muscle, lymphoblasts, and blood was clearly demonstrable by standard methods using ethidium bromide staining. His mother also had IDDM, but no A3243G mutation could be detected in her blood or transformed lymphoblasts using the same PCR technique. When PCR was carried out in the presence of [32P]deoxycytidine triphosphate, subsequent autoradiography detected the presence of the mutation at low levels in mtDNA from the mother's lymphoblasts and blood. Study of the mother's muscle showed a mitochondrial myopathy, despite the fact that she was asymptomatic. We emphasize that the increased sensitivity of radiolabeled PCR may be necessary to detect small percentages of heteroplasmic A3243G mtDNA mutation in blood from diabetic subjects. Otherwise the incidence of mtDNA mutations in both IDDM and non-insulin dependent diabetes may be underestimated.