Background Defects in the oxidative phosphorylation system can cause a broad spectrum of clinical symptoms ranging from an isolated myopathy to a multisystemic disorder. Objective To study and identify the underlying molecular defect in a patient with limb-girdle myopathy. Design Biochemical, histochemical, and immunocytochemical analyses were performed in combination with polymerase chain reaction–single-strand conformation polymorphism and restriction fragment length polymorphism–polymerase chain reaction techniques. Setting University hospital. Patient A 48-year-old woman with limb-girdle myopathy. Main Outcome Measures The pathogenic characteristics of the identified nucleotide alterations were defined using single-muscle fiber analysis. Results A complex III deficiency was detected using blue native–polyacrylamide gel electrophoresis, while immunocytochemical results showed a mosaic staining pattern for complexes I and IV. After molecular analyses, 2 novel heteroplasmic mitochondrial DNA (mtDNA) nucleotide aberrations, m.5888insA and m.14639A>G, were identified in muscle tissue. Single-muscle fiber analyses demonstrated that cytochrome c oxidase–deficient fibers, compared with cytochrome c oxidase–positive fibers, harbored statistically significantly higher levels of both mtDNA mutations ( P t test). Conclusions These results, together with previously defined canonical criteria determining the pathogenic characteristics of mtDNA mutations, suggest that both nucleotide changes are pathogenic mutations. To our knowledge, this is only the third report of the coexistence of 2 pathogenic mtDNA mutations present in different genes within individual skeletal muscle fibers of a patient.
