Inherited retinal dystrophies (IRD) are due to various gene mutations. Each mutated gene instigates a specific cell homeostasis disruption, leading to a modification in gene expression and retinal degeneration. We previously demonstrated that the polycomb-repressive complex-1 (PRC1) markedly contributes to the cell death process. To better understand these mechanisms, we herein study the role of PRC2, specifically EZH2, which often initiates the gene inhibition by PRC1. We observed that the epigenetic mark H3K27me3 generated by EZH2 was progressively and strongly expressed in some individual photoreceptors and that the H3K27me3-positive cell number increased before cell death. H3K27me3 accumulation occurs between early (accumulation of cGMP) and late (CDK4 expression) events of retinal degeneration. EZH2 hyperactivity was observed in four recessive and two dominant mouse models of retinal degeneration, as well as two dog models and one IRD patient. Acute pharmacological EZH2 inhibition by intravitreal injection decreased the appearance of H3K27me3 marks and the number of TUNEL-positive cells revealing that EZH2 contributes to the cell death process. Finally, we observed that the absence of the H3K27me3 mark is a biomarker of gene therapy treatment efficacy in XLRPA2 dog model. PRC2 and PRC1 are therefore important actors in the degenerative process of multiple forms of IRD.

In normal mice, the lentiviral vector (LV) is very efficient to target the RPE cells, but transduces retinal neurons well only during development. In the present study, the tropism of LV has been investigated in the degenerating retina of mice, knowing that the retina structure changes during degeneration. We postulated that the viral transduction would be increased by the alteration of the outer limiting membrane (OLM). Two different LV pseudotypes were tested using the VSVG and the Mokola envelopes, as well as two animal models of retinal degeneration: light-damaged Balb-C and Rhodopsin knockout (Rho-/-) mice. After light damage, the OLM is altered and no significant increase of the number of transduced photoreceptors can be obtained with a LV-VSVG-Rhop-GFP vector. In the Rho-/- mice, an alteration of the OLM was also observed, but the possibility of transducing photoreceptors was decreased, probably by ongoing gliosis. The use of a ubiquitous promoter allows better photoreceptor transduction, suggesting that photoreceptor-specific promoter activity changes during late stages of photoreceptor degeneration. However, the number of targeted photoreceptors remains low. In contrast, LV pseudotyped with the Mokola envelope allows a wide dispersion of the vector into the retina (corresponding to the injection bleb) with preferential targeting of Müller cells, a situation which does not occur in the wild-type retina. Mokola-pseudotyped lentiviral vectors may serve to engineer these glial cells to deliver secreted therapeutic factors to a diseased area of the retina.