Oxidative DNA damage is thought to contribute to aging
17 18 and to a host of degenerative diseases of aging, including cancer.
19 20 DNA damage can occur intrinsically as a consequence of normal metabolism. The rate of oxidative DNA damage is directly related to the metabolic rate and is inversely related to the lifespan of the organism.
21 Leukocyte DNA 8-OHdG levels are increased in cigarette smokers,
22 patients with diabetes,
23 and patients on chronic hemodialysis.
24 The 8-OHdG content in cybrids increases as the proportion of mtDNA with the 4977 deletion increases.
25 The present study indicated that the leukocyte DNA 8-OHdG content was significantly increased in patients with LHON. This is powerful evidence supporting the presence of oxidative stress in the pathogenesis of LHON.
Others have implicated oxidative stress in LHON.
13 26 27 28 29 30 In in vitro studies, cybrid cell lines bearing the pathogenic LHON 11778 mutation were much more sensitive than the parental cell line to oxidative stress, which causes cell death in a Ca
2+-dependent manner.
26 Biochemical studies of LHON suggest that the cytotoxicity induced by the loss of complex-I activity was not from reductions in oxidative phosphorylation, but was due to increased production of ROS. Chronic overproduction of ROS may be an important consequence of the pathogenic mtDNA mutations.
27 ROS production is increased in cybrids carrying the three primary mutations associated with LHON and different mutations in mtDNA result in a modified pattern of the antioxidant machinery.
28 In animal studies, optic neuropathy induced by reductions in mitochondrial superoxide dismutase, the essential antioxidant that catalyzes the dismutation of superoxide radicals, was strikingly similar to the histopathology of LHON.
29 Patients with LHON and asymptomatic carriers have a reduced α-tocopherol/lipid ratio in their plasma, which most probably reflects increased free radical generation and α-tocopherol consumption.
30 In the present study, mtDNA damage was increased in patients with the 11778 mtDNA mutation, which may reflect the increased ROS production.
How does optic neuropathy in LHON occur? The pathogenic mtDNA mutations of complex-I genes result in defective respiration, inhibiting the electron transport chain, thereby generating ROS to levels beyond the capability of endogenous antioxidants present within the mitochondria. The selective vulnerability of the optic nerve in LHON, however, remains a mystery. Using a neuronal precursor cell line NT2 containing mitochondria bearing the 11778 and 3460 mutations, differentiation significantly reduced LHON cells (by 30%) compared with control subjects, indicating either a decreased proliferative potential or increased cell death of the LHON-NT2 cells.
31 There are increased mitochondrial ROS observed in differentiated LHON-NT2 cells. The findings suggest that the LHON phenotype might be the result of an increase in mitochondrial ROS, which is caused by LHON mutations, possibly mediated through neuron-specific alterations in the complex-I structure.
In summary, the present study indicated that the 8-OHdG content in leukocyte DNA was significantly increased in patients with LHON. This provides further evidence linking oxidative stress to the pathogenesis of LHON.