Bioenergetic consequences of accumulating the common 4977-bp mitochondrial DNA deletion.
Mutations and deletions in mitochondrial DNA (mtDNA) lead to a number of human diseases characterized by neuromuscular degeneration. Accumulation of truncated mtDNA molecules (delta-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 delta-mtDNA present increases with age in a range of tissues. Consequently, there is considerable interest in the effects of the accumulation of delta-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 delta-mtDNA affects oxidative phosphorylation we constructed a series of cybrids by fusing a human osteosarcoma cell line depleted of mtDNA (rho0) with enucleated skin fibroblasts from a CPEO patient. The ensuing cybrids contained 0-86% 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 those of cybrids with intact mtDNA. However, once the proportion of delta-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 delta-mtDNA in the target tissues of patients with mtDNA diseases.
Porteous
W K
WK
Department of Biochemistry, University of Otago, Dunedin, New Zealand.
James
A M
AM
Sheard
P W
PW
Porteous
C M
CM
Packer
M A
MA
Hyslop
S J
SJ
Melton
J V
JV
Pang
C Y
CY
Wei
Y H
YH
Murphy
M P
MP
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000096:biosynthesis / Q000378:metabolism; D004272:DNA, Mitochondrial / Q000235:genetics* / Q000378:metabolism; D004734:Energy Metabolism; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials; D008854:Microscopy, Electron; D008928:Mitochondria / Q000378:metabolism / Q000648:ultrastructure; D017384:Sequence Deletion; D014407:Tumor Cells, Cultured
D004272:DNA, Mitochondrial; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate