Low levels of mtDNA deletion mutations in ETS normal fibers from aged rats.

The objectives were (1) to determine whether deletion mutations occur in phenotypically normal type I and type II fibers, (2) to quantify the levels of both deletion mutant and wild-type (wt) mtDNA (nondeletion) within single normal fibers containing mutant mtDNA, and (3) to quantify the amount of wt mtDNA in genotypically and phenotypically normal type I and type II fibers. Deletion mutations in normal fibers are not restricted to specific fiber types, although clonal accumulation of mtDNA deletion mutations and subsequent ETS abnormalities occur exclusively in type II fibers.