Bioenergetic consequences of accumulating the common 4977‐bp mitochondrial DNA deletion

Abstract

Mutations and deletions in mitochondrial DNA (mtDNA) lead to a number of human diseases characterized by neuromuscular degeneration. Accumulation of truncated mtDNA molecules (∆-mtDNA) lacking a specific 4977-bp fragment, the common deletion, leads to three related mtDNA diseases : Pearson's syndrome; Kearns-Sayre syndrome; and chronic progressive external ophthalmoplegia (CPEO). In addition, the proportion of ∆-mtDNA present increases with age in a range of tissues. Consequently, there is considerable interest in the effects of the accumulation of ∆-mtDNA on cell function. The 4977-bp deletion affects genes encoding 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 ∆-mtDNA affects oxidative phosphorylation we constructed a series of cybrids by fusing a human osteosarcoma cell line depleted of mtDNA (ρ 0 ) with enucleated skin fibroblasts from a CPEO patient. The ensuing cybrids contained 0Ϫ86 % ∆-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 ∆-mtDNA were assessed by measuring the mitochondrial membrane potential, rate of ATP synthesis and ATP/ADP ratio. In cybrids containing less than 50Ϫ55% ∆-mtDNA, these bioenergetic functions were equivalent to those of cybrids with intact mtDNA. However, once the proportion of ∆-mtDNA exceeded this threshold, the mitochondrial membrane potential, rate of ATP synthesis, and cellular ATP/ADP ratio decreased. These bioenergetic deficits will contribute to the cellular pathology associated with the accumulation of ∆-mtDNA in the target tissues of patients with mtDNA diseases.

Quantitative analysis of mitochondrial DNA 4977-bp deletion in sporadic breast cancer and benign breast diseases

“…Our finding is also biologically plausible, due to the fact that cancer cells are metabolically adapted for rapid growth and proliferation under conditions of low pH and oxygen tension, solid cancer cells generate energy by glycolysis in strong preference to oxidative phosphorylation (33)(34)(35). Furthermore, previous in vitro studies have suggested that mitochondrial protein synthesis is defective and the activity of cytochrome oxidase decreases when the proportion of the mtDNA 4977 mutation was greater than 60% in the cancer cell (23). The amount of the mtDNA 4977 mutation that accumulates may not be sufficient, by itself, to cause a significant defect or dysfunction in breast cancer cells.…”

“…The mtDNA 4977 mutation affects genes encoding 7 polypeptide components of the mitochondrial respiratory chain and 5 of the 22 tRNAs necessary for mitochondrial protein synthesis (23). Thus, the mutated mtDNA 4977 may represent a strong functional disadvantage, possibly thereby repressing the growth of cancer cells harboring deleted mtDNA.…”

Mitochondrial DNA deletions inhibit proteasomal activity and stimulate an autophagic transcript

“…Single mtDNA deletions cause KSS, CPEO, or PS, whose affected tissues show impaired electron transport activity, ATP production, and mitochondrial protein synthesis [26,56,57]. Muscle biopsies from patients with KSS or CPEO show ragged-red fibers and cytochrome oxidase-negative fibers [58,59].…”

Pathological mechanisms underlying single large‐scale mitochondrial DNA deletions

“…Although a specific deletion encompassing 4,977bp is commonly reported,6 single, large‐scale mtDNA deletions vary in size (between 1.3 and 10kb), removing a number of genes encoding both mt‐tRNAs and structural mRNAs 7, 8, 9, 10, 11. They are typically mutational events that occur during the replication/repair of the mitochondrial genome and are invariably heteroplasmic in human tissues ‐ a condition where both mutated and wild‐type mtDNA genomes coexist in the same cell.…”

“…They are typically mutational events that occur during the replication/repair of the mitochondrial genome and are invariably heteroplasmic in human tissues ‐ a condition where both mutated and wild‐type mtDNA genomes coexist in the same cell. In postmitotic tissues, such as skeletal muscle, the proportion of deleted to wild‐type mtDNA must exceed a reported threshold (50–90%)9, 10, 11, 12, 13 for the expression of a biochemical defect, leading to a marked mosaicism between different fibers and well‐described cytochrome c oxidase (COX)‐deficient, ragged‐red fibers 13…”