EXTREMELY HIGH-LEVELS OF MUTANT MTDNAS CO-LOCALIZE WITH CYTOCHROME-C OXIDASE-NEGATIVE RAGGED-RED FIBERS IN PATIENTS HARBORING A POINT MUTATION AT NT-3243

A single mtDNA point mutation at nt 3243 has been associated with two different clinical phenotypes:mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes ('MELAS(3243)') and progressive ext ernal ophthalmoplegia ('PEO(3243)'). If has been shown that there is a much higher proportion of ragged-red fibers (RRF) with cytochrome c o xidase (COX) deficiency in PEO(3243) than in MELAS(3243). Using PCR/RF LP analysis of isolated individual skeletal muscle fibers from patient s with both syndromes, we found a direct correlation between the local ized concentration of the nt 3243 mutation and impairment of COX funct ion at the single muscle fiber level: we found relatively low levels o f mutant mtDNAs (56+/-21%) in 'normal' fibers; high levels (90+/-6%) i n COX-positive RRF; and an almost complete segregation of mutant mtDNA s (95+/-3%) in COX-negative RRF. Thus, the differential distribution o f fibers with extremely high concentrations of mutant mtDNAs character izes, and probably distinguishes, the skeletal muscle of PEO and MELAS patients harboring the same nt-3243 mutation.