Mitochondrial DNA deletion mutations are concomitant with ragged red regions of individual, aged muscle fibers: analysis by laser-capture microdissection

Laser-capture microdissection was coupled with PCR to define the mitochondr ial genotype of aged muscle fibers exhibiting mitochondrial enzymatic abnor malities. These electron transport system (ETS) abnormalities accumulate wi th age, are localized segmentally along muscle fibers, are associated with fiber atrophy and may contribute to age-related fiber loss. DNA extracted f rom single, 10 mum thick, ETS abnormal muscle fibers, as well as sections f rom normal fibers, served as templates for PCR-based deletion analysis. Lar ge mitochondrial (mt) DNA deletion mutations (4.4-9.7 kb) were detected in all 29 ETS abnormal fibers analyzed. Deleted mtDNA genomes were detected on ly in the regions of the fibers with ETS abnormalities; adjacent phenotypic ally normal portions of the same fiber contained wild-type mtDNA. In additi on, identical mtDNA deletion mutations were found within different sections of the same abnormal region. These findings demonstrate that large deletio n mutations are associated with ETS abnormalities in aged rat muscle and th at, within a fiber, deletion mutations are clonal. The displacement of wild -type mtDNAs with mutant mtDNAs results in concomitant mitochondrial enzyma tic abnormalities, fiber atrophy and fiber breakage.