Mitochondrial myopathy and complex III deficiency in a patient with a new stop-codon mutation (G339X) in the cytochrome b gene.

A 19-year-old woman complained of life-long exercise intolerance and had chronic lactic acidosis. Neurological examination was normal, but muscle biopsy showed cytochrome c oxidase-positive fibers and marked complex III deficiency. Sequence analysis showed a novel stop-codon mutation (G15761A) in the mitochondrial DNA (mtDNA)-encoded cytochrome b gene, resulting in loss of the last 41 amino acids of the protein. By PCR/restriction fragment-length polymorphism (RFLP) analysis, the G15761A mutation was very abundant (73%) in the patient's muscle, barely detectable (less than 1%) in her urine, and absent in her blood; it was also absent in muscle, urine and blood from the patient's mother. This mutation fulfills all accepted criteria for pathogenicity.