DNA damage and repair in light-induced photoreceptor degeneration

Intense light causes photoreceptor death that is greatest in the superior central retina. Short-duration treatment in a light-damage model results in TUNEL-positive photoreceptor nuclei within this region. However, cells lost 10 days after light treatment are fewer than the TUNEL-labeled cells observed earlier. Therefore, this study was undertaken to monitor DNA fragmentation and cell death to explain the discrepancy.

Eyes of dark-adapted rats were light damaged for 4 or 5 hours. DNA fragmentation was measured by TUNEL, laddering, and highly repetitive short interspersed nuclear element (SINE) analysis in dark-adapted, nondamaged control (dark-control) retinas and in retinas collected at 6-hour intervals after light treatment. TUNEL-positive photoreceptor nuclei were counted in these samples along a superior-to-inferior meridian and compared with control and damaged 10-day retinas. Monocytes and DNA polymerase beta were monitored by immunohistochemistry.

TUNEL-positive staining of photoreceptors was centered around the superior central retina. At 10 days, photoreceptor loss had occurred in this region. In graphs of 6-hour-interval data, two DNA-fragmentation peaks, 24 hours apart, were evident. Monocytes appeared after nuclear damage. Total TUNEL-positive cells under both peaks exceeded the number of photoreceptors lost. The DNA-repair enzyme, polymerase beta, was induced in the superior central retina, within photoreceptor inner segments, 24 hours after light treatment, but declined thereafter.

One population of damaged cells may mend DNA until the repair mechanism is exceeded and then revert to apoptosis, or, alternatively, two populations may undergo DNA fragmentation 24 hours apart. Either DNA fragmentation is masked at midpoint by temporary repair, or two waves of damage occur, but repair rescues the first set, not the second. Photoreceptors lost are fewer than TUNEL-positive cells. Thus, both possibilities suggest photoreceptor DNA repair. The transient appearance of DNA polymerase beta in photoreceptors under these experimental conditions further suggests nuclear repair. Thus, maintenance of in-house DNA-repair mechanisms may provide an alternate approach for the rescue of photoreceptors, as well as other neurons with stress-induced damage. These events may provide useful drug targets to promote photoreceptor survival in various forms of retinal degeneration.