We performed morphological, biochemical, and genetic studies, including single-fiber PCR (sf PCR), on muscle biopsies obtained from a mother and daughter with
MELAS syndrome due to the A3243G transition of
mitochondrial DNA (
mtDNA). The severity of muscle involvement appeared quite distinct, in spite of the fact that both patients segregated similar mutant
mtDNA levels on total muscle
DNA. The daughter did not show any clinical muscle involvement: muscle biopsy revealed many ragged red fibers (RRFs) mostly positive for
cytochrome-c oxidase (COX) activity. In contrast, her mother had developed a generalized
myopathy without
progressive external ophthalmoplegia (PEO), morphologically characterized by many COX-negative RRFs. Single-muscle fiber PCR demonstrated in both patients significantly higher percentages of wild-type
mtDNA in normal fibers (daughter: 23.25 +/- 15.22; mother: 43.13 +/- 26.11) than in COX-positive RRFs (daughter: 11.25 +/- 5.22, P < 0.005; mother: 9.12 +/- 5.9, P < 0.001) and in COX-negative RRFs (daughter: 8.9 +/- 4.2, P < 0.001 mother: 4.8 +/- 2.8, P < 0.001). Wild-type
mtDNA levels resulted higher also in COX-positive vs. COX-negative RRFs (daughter: P < 0.05; mother: P < 0.001). Our data confirm a direct correlation between A3243G levels and impairment of COX function at the single-muscle fiber level. Moreover, the evidence of a clinical
myopathy in the patient with higher amounts of COX-negative RRFs bolsters the concept that a differential distribution of mutant mtDNAs at the cellular level may have effects on the clinical involvement of individual tissues. However, the occurrence of a similar morphological and biochemical muscle phenotype also in PEO(3243) patients suggests that other genetic factors involved in the interaction between mitochondrial and nuclear
DNA, rather than the stochastic distribution of
mtDNA genomes during embryogenesis, are primarily implicated in determining the various clinical expressions of the A3243G of
mtDNA.