Phenotypic heterogeneity in families with the myoclonic epilepsy and ragged‐red fiber disease point mutation in mitochondrial DNA

Abstract

Two families with a point mutation in mtDNA associated with myoclonic epilepsy and ragged‐red fiber disease showed pronounced clinical heterogeneity. The mothers of the two families had adult‐onset myopathy with ragged‐red fibers, partial deficiency of cytochrome c oxidase, and sensory neuropathy. Members of the first family had variable clinical features of progressive ataxic‐myoclonic encephalomyopathy and of the other family, primarily adult‐onset myiopathy. There was a point mutation from A to G at uncleotide pair 8344 located in the tRNA^Lys gene of the tmDNA of all patients tested, three in family 1, and the mother of Family 2.This clinical heterogeneity may reflect the effects of varying proportions of mutant and wild‐type mt DNA in the different organ systems in each individual.