Short-interfering RNAs induce retinal degeneration via TLR3 and IRF3.
ABSTRACT The discovery of sequence-specific gene silencing by endogenous double-stranded RNAs (dsRNA) has propelled synthetic short-interfering RNAs (siRNAs) to the forefront of targeted pharmaceutical engineering. The first clinical trials utilized 21-nucleotide (nt) siRNAs for the treatment of neovascular age-related macular degeneration (AMD). Surprisingly, these compounds were not formulated for cell permeation, which is required for bona fide RNA interference (RNAi). We showed that these "naked" siRNAs suppress neovascularization in mice not via RNAi but via sequence-independent activation of cell surface Toll-like receptor-3 (TLR3). Here, we demonstrate that noninternalized siRNAs induce retinal degeneration in mice by activating surface TLR3 on retinal pigmented epithelial cells. Cholesterol conjugated siRNAs capable of cell permeation and triggering RNAi also induce the same phenotype. Retinal degeneration was not observed after treatment with siRNAs shorter than 21-nts. Other cytosolic dsRNA sensors are not critical to this response. TLR3 activation triggers caspase-3-mediated apoptotic death of the retinal pigment epithelium (RPE) via nuclear translocation of interferon regulatory factor-3. While this unexpected adverse effect of siRNAs has implications for future clinical trials, these findings also introduce a new preclinical model of geographic atrophy (GA), a late stage of dry AMD that causes blindness in millions worldwide.
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ABSTRACT: Age-related macular degeneration is the most common cause of irreversible visual impairment in the developed world. Advanced age-related macular degeneration consists of geographic atrophy and choroidal neovascularization. The specific genetic variants that predispose patients to geographic atrophy are largely unknown. We tested for an association between the functional toll-like receptor 3 gene (TLR3) variant rs3775291 (involving the substitution of phenylalanine for leucine at amino acid 412) and age-related macular degeneration in Americans of European descent. We also tested for the effect of TLR3 Leu and Phe variants on the viability of human retinal pigment epithelial cells in vitro and on apoptosis of retinal pigment epithelial cells from wild-type mice and Tlr3-knockout (Tlr3(-/-)) mice. The Phe variant (encoded by the T allele at rs3775291) was associated with protection against geographic atrophy (P=0.005). This association was replicated in two independent case-control series of geographic atrophy (P=5.43x10(-4) and P=0.002). No association was found between TLR3 variants and choroidal neovascularization. A prototypic TLR3 ligand induced apoptosis in a greater fraction of human retinal pigment epithelial cells with the Leu-Leu genotype than those with the Leu-Phe genotype and in a greater fraction of wild-type mice than Tlr3(-/-) mice. The TLR3 412Phe variant confers protection against geographic atrophy, probably by suppressing the death of retinal pigment epithelial cells. Since double-stranded RNA (dsRNA) can activate TLR3-mediated apoptosis, our results suggest a role of viral dsRNA in the development of geographic atrophy and point to the potential toxic effects of short-interfering-RNA therapies in the eye.New England Journal of Medicine 09/2008; 359(14):1456-63. · 53.30 Impact Factor