The mechanism of cone cell death in Retinitis Pigmentosa
Retinitis Pigmentosa (RP) is a group of diseases in which one of a large number of mutations causes death of rod photoreceptors. After rods die, cone photoreceptors slowly degenerate in a characteristic pattern. The mechanism of rod cell death varies depending upon the gene that is mutated and the rate that rods degenerate is an important prognostic feature, because cones do not begin to degenerate until almost all rods have been eliminated. Rod cell death causes night blindness, but visual disability and blindness result from cone degeneration and therefore it is critical to determine the mechanisms by which it occurs. The death of rods reduces oxygen consumption resulting in high tissue levels of oxygen in the outer retina. The excess oxygen stimulates superoxide radical production by mismatches in the electron transport chain in mitochondria and by stimulation of NADPH oxidase activity in cytoplasm. The high levels of superoxide radicals overwhelm the antioxidant defense system and generate more reactive species including peroxynitrite which is extremely damaging and difficult to detoxify. This results in progressive oxidative damage in cones which contributes to cone cell death and loss of function because drugs or gene transfer that reduce oxidative stress promote cone survival and maintenance of function. Compared with aqueous humor samples from control patients, those from patients with RP show significant elevation of carbonyl content on proteins indicating oxidative damage and a reduction in the ratio of reduced to oxidized glutathione indicating depletion of a major component of the antioxidant defense system from ongoing oxidative stress. The first step in clinical trials will be to identify doses of therapeutic agents that reverse these biomarkers of disease to assist in design of much longer trials with functional and anatomic endpoints.