BIOENERGETIC CONSEQUENCES OF ACCUMULATING THE COMMON 4977-BP MITOCHONDRIAL-DNA DELETION

Mutations and deletions ill mitochondrial DNA (mtDNA) lead to a number of human diseases characterized by neuromuscular degeneration, Accumu lation of truncated mtDNA molecules (Delta-mtDNA) lacking a specific 4 977-bp fragment, the common deletion, leads to three related mtDNA dis eases: Pearson's syndrome: Kearns-Sayre syndrome; and chronic progress ive external ophthalmoplegia (CPEO). In addition. the proportion of De lta-mtDNA present increases with age in a range of tissues. Consequent ly, there is considerable interest in the effects of the accumulation of Delta-mtDNA on cell function. The 4977-bp deletion affects genes en coding 7 polypeptide components of the mitochondrial respiratory chain , and 5 of the 22 tRNAs necessary for mitochondrial protein synthesis. To determine how the :accumulation of Delta-mtDNA affects oxidative p hosphorylation we constructed a series of cybrids by fusing a human os teosarcoma cell line depleted of mtDNA (rho degrees) with enucleated s kin fibroblasts from a CPEO patient. The ensuing cybrids contained 0-8 6% Delta-mtDNA and all had volumes, protein contents, plasma-membrane potentials and mitochondrial contents similar to those of the parental cell line. The bioenergetic consequences of accumulating Delta-mtDNA were assessed by measuring the mitochondrial membrane potential, rate of ATP synthesis and ATP/ADP ratio. In cybrids containing less than 50 -55% Delta-mtDNA, these bioenergetic functions were equivalent to thos e of cybrids with intact mtDNA. However, once the proportion of Delta- mtDNA exceeded this threshold, the mitochondrial membrane potential, r ate of ATP synthesis. and cellular ATP/ADP ratio decreased. These bioe nergetic deficits will contribute to the cellular pathology associated with the accumulation of Delta-mtDNA in the target tissues of patient s with mtDNA diseases.