Mitochondrial myopathy due to novel missense mutation in the cytochrome c oxidase 1 gene.

We report a novel heteroplasmic mutation p.Y440C in the mitochondrial DNA-encoded subunit I of the cytochrome c oxidase (COX) gene in a patient with late onset progressive painless weakness. Her muscle biopsy showed scattered COX-negative fibers and several small collections of inflammatory cells. The mutation was detected in the patient's muscle but not in her blood. The low mutant load in muscle could explain the patient's late onset of the myopathy and milder phenotype when compared to the previously published cases with MTCO1 mutations.