Two novel mitochondrial DNA mutations in muscle tissue of a patient with limb-girdle myopathy.

Patients with an oxidative phosphorylation (OXPHOS) disorder often present with complex clinical features, including neurological and muscular dysfunction. Mutations underlying these diseases are located in the mitochondrial or nuclear genome. Here, we report on a forty-eight-year-old woman suffering from mild limb girdle myopathy. The enzymatic activities of the OXPHOS enzymes in skeletal muscle tissue of the patient were within the control ranges. However, activity staining following BN-PAGE showed a decreased complex III activity and the presence of complex V subcomplexes. Immunocytochemistry demonstrated a mosaic staining pattern in a minority (about 20%) of the muscle fibers for complex I (subunit 20kd) and complex IV (subunit I). Molecular analyses identified two novel heteroplasmic mitochondrial DNA nucleotide aberrations in muscle tissue : an insertion, m.5888insA in the mt-tRNATyr gene and an alteration, m.14639A>G, changing a leucine into a serine residue in the ND6 gene. Less than 1% of both mutant alterations were present in the patient's fibroblasts, while they were undetectable in her blood and in blood and fibroblasts from her mother.
Single muscle fiber analyses clearly demonstrated that COX-deficient fibers, as compared to COX-positive fibers, harbored a significantly higher level of both mtDNA mutations. These results, together with previously defined canonical criteria determining the pathogenic character of mtDNA analyses, suggest that both nucleotide changes are pathogenic mutations.