Prenylation Defect in the RPE Exacerbates Photoreceptor Degeneration in Choroideremia Mice

Abstract

Purpose: : Choroideremia (CHM) is an X-linked retinal degeneration starting in the teenage years with night blindness and progressing towards complete loss of vision within 2-3 decades. CHM gene encodes Rab Escort Protein-1 (REP1), which participates in the lipid modification of members of the Rab family. Rabs are small GTPases that act as regulators of intracellular vesicular transport and organelle motility and require addition of one or two prenyl groups for membrane association (known as prenylation). Three main layers are affected in CHM: retinal pigment epithelium (RPE), photoreceptors and choroid. Which layer is the primary site of the disease remains a controversial topic. In order to investigate this issue, we produced mice with cell-specific Chm KO.

Methods: : Chm^flox animals were crossed with Tyr-Cre transgenic line expressing Cre under control of tyrosinase promoter to produce Chm^flox, Tyr-Cre+ mice with CHM KO limited to the RPE and other pigmented cells. To achieve photoreceptor-specific CHM KO, we generated Chm^flox, IRBP-Cre⁺ mice that carry the Cre-transgene under control of the IRBP-promoter, active in photoreceptor cells. Finally we generated Chm^flox, Tyr-Cre+, IRBP-Cre+ animals to achieve CHM KO in both layers. Cre expression was studied by immunohistochemistry. Eye morphology was studied by histology and electron microscopy.

Results: : In Chm^flox, Tyr-Cre⁺ animals we observed coat colour dilution, pigmentation defect in the RPE but no pathological changes in photoreceptors. Chm^flox, IRBP-Cre⁺ animals exhibited mild degeneration of photoreceptors while RPE was not affected. In both models, only one layer degenerated with no significant secondary effect on another layer confirming our previous hypothesis of cell autonomy in CHM. In double transgenic animals, Chm^flox, Tyr-Cre⁺, IRBP-Cre⁺, we observed anincreased rate of photoreceptor degeneration in comparison with Chm^flox, IRBP-Cre⁺ siblings.

Conclusions: : Our results suggest that in CHM retinas the prenylation defect in the RPE is not the primary cause of photoreceptor degeneration, however it acts as an enhancer thus exacerbating photoreceptor degeneration. Therefore, a healthy RPE is an important determinant of the photoreceptors’ ability to withstand the degenerative process in CHM.

Keywords: transgenics/knock-outs • retinal pigment epithelium • photoreceptors