Visual Cycle Function Is Not Limited by Rdh12-Deficiency in Knockout Mice

Abstract

Purpose:: Mutations in RDH12, encoding a short chain dehydrogenase/reductase, cause a severe and progressive form of childhood-onset autosomal recessive retinal dystrophy (arRD) diagnosed in many cases as Leber congenital amaurosis (LCA). To determine the role of RDH12 in photoreceptor physiology, we generated and characterized the retinal phenotype of Rdh12 knockout mice.

Methods:: RDH12-deficient mice were generated by homologous recombination using Cre-lox technology to delete Rdh12 exons 1-3. Retinal histology was evaluated by light and electron microscopy. Retinoid content was determined by normal phase HPLC. Visual function was assessed by ERG analysis. Lipid peroxidation products were assayed as TBARs reactive substances. RDH12 expression in the retina was localized by immunohistochemical analysis.

Results:: RDH12-deficient mice exhibited grossly normal retinal histology at 10 months of age. Levels of all-trans and 11-cis retinoids in dark- and light-adapted animals, and scotopic and photopic electroretinogram (ERG) responses were similar to wild type, as was recovery of the ERG response following bleaching in animals matched for an Rpe65 polymorphism (p.L450M). Lipid peroxidation products, a measure of oxidative stress, did not appear to be elevated. RDH12 was localized to photoreceptor inner segments and the outer nuclear layer in both mouse and human retinas.

Conclusions:: These unexpected findings indicate that RDH12 deficiency alone does not limit visual cycle throughput under the experimental conditions tested. In fact, this situation appears similar to that for mice deficient in RDH8(prRDH) or RDH11, two other RDH isoforms expressed in rods, with earlier studies showing only minor deficits in visual cycle function (delayed dark-adaptation) and no pathology. These findings suggest that the activity of a single RDH isoform, or perhaps even RDH activity itself, is not rate limiting in the visual response in mouse. It is also possible that RDH12 is needed to perform oxidation-reduction reactions involved in other biosynthetic pathways in the retina. Understanding the role of RDH12 in retinal physiology and pathology, and the mechanism of the visual cycle itself, will require the definition of the unique function of RDH12 relative to that of other RDHs present.

Keywords: retinal degenerations: hereditary • photoreceptors • proteins encoded by disease genes