A 61-year-old woman with a five-year history of progressive muscle weakness and atrophy had a muscle biopsy characterized by a combination of dystrophic features (necrotic fibers and endomysial fibrosis) and mitochondrial alterations [ragged-red cytochrome c oxidase (COX)-negative fibers]. Sequencing of the whole mtDNA, assessment of the mutation load in muscle and in accessible non-muscle tissues, and single fiber polymerase chain reaction (PCR) were performed. Muscle mitochondrial DNA (mtDNA) sequencing revealed a novel heteroplasmic mutation (m.4403G>A) in the gene (MTTM) that encodes tRNAMet. The mutation was not present in accessible non-muscle tissues from the patient or 2 asymptomatic sisters. The clinical features and muscle morphology in this patient are very similar to those described in a previous patient with a different mutation, also in MTTM, which suggests that mutations in this gene confer a distinctive “dystrophic” quality. This may be a diagnostic clue in patients with isolated mitochondrial myopathy.
