Purpose: To describe a sensitivity to light-induced damage associated with expression of a T17M mutant human rhodopsin (hT17M) transgene in mice, with the goal of minimizing retinal injury during the subretinal delivery of rAAV-mediated gene therapy.
Methods: Mice were bred to express the hT17M rhodopsin transgene in a line that was hemizygous null for wild-type mouse rhodopsin (mrho(+/-)), and the eyes of transgenic mice and nontransgenic littermates were exposed for 2.5 minutes to unilateral illumination with fiber-optic light ranging from 5,000 to 10,000 lux. Funduscopic images were made with a handheld camera (Genesis; Kowa Company, Ltd., Tokyo, Japan). Full-field scotopic electroretinographic analysis (ERG) was performed to measure loss of retinal function. Morphometry in the light microscope was used to measure loss of rod photoreceptors. TUNEL staining and a nucleosome release assay were used to measure levels of apoptosis in retinal specimens.
Results: mrho(+/-);hT17M mice exhibited a sensitivity to light-induced damage that caused severe loss of a- and b-wave ERG responses. hT17M transgenic mice on the mrho(+/+) background were equally sensitive to light-induced damage. Histologic analysis showed a concomitant loss of photoreceptors and TUNEL labeling of fragmented DNA in rod photoreceptor cells, demonstrating that the damage occurred via an apoptotic pathway. Nontransgenic littermate mice were not affected by this exposure to light. Mice expressing an hP23H mutant human rhodopsin transgene were minimally sensitive to light-induced damage at these intensities, in comparison to hT17M mice. Treating the hT17M mice with an equivalent regimen of exposure to red light was less damaging to the retina, as measured by ERG and histology.
Conclusions: Expression of a human hT17M mutant rhodopsin transgene in mice is associated with photoreceptor apoptosis in response to moderate exposure to light. This phenotype was not observed in nontransgenic littermates or in mice expressing an hP23H mutant human rhodopsin transgene. The results suggest that elimination of the glycosylation site at N15 is associated with increased sensitivity to light-induced damage.