Therapeutic benefit derived from RNAi-mediated ablation of IMPDH1 transcripts in a murine model of autosomal dominant retinitis pigmentosa (RP10)

Abstract

Mutations within the inosine 5'-monophosphate dehydrogenase 1 (IMPDH1) gene cause the RP10 form of autosomal dominant retinitis pigmentosa (adRP), an early-onset retinopathy resulting in extensive visual handicap owing to progressive death of photoreceptors. Apart from the prevalence of RP10, estimated to account for 5-10% of cases of adRP in United States and Europe, two observations render this form of RP an attractive target for gene therapy. First, we show that while recombinant adeno-associated viral (AAV)-mediated expression of mutant human IMPDH1 protein in the mouse retina results in an aggressive retinopathy modelling the human counterpart, expression of a normal human IMPDH1 gene under similar conditions has no observable pathological effect on retinal function, indicating that over-expression of a therapeutic replacement gene may be relatively well tolerated. Secondly, complete absence of IMPDH1 protein in mice with a targeted disruption of the gene results in relatively mild retinal dysfunction, suggesting that significant therapeutic benefit may be derived even from the suppression-only component of an RNAi-based gene therapy. We show that AAV-mediated co-expression in the murine retina of a mutant human IMPDH1 gene together with short hairpin RNAs (shRNA) validated in vitro and in vivo, targeting both human and mouse IMPDH1, substantially suppresses the negative pathological effects of mutant IMPDH1, at a point where, in the absence of shRNA, expression of mutant protein in the RP10 model essentially ablates all photoreceptors in transfected areas of the retina. These data strongly suggest that an RNAi-mediated approach to therapy for RP10 holds considerable promise for human subjects.

C1q enhances cone photoreceptor survival in a mouse model of autosomal recessive retinitis pigmentosa

“…[3][4][5] In addition, therapeutic modalities have been demonstrated for other genetic subtypes of RP and LCA in a growing number of animal models, both of recessive and dominant forms of disease. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Notwithstanding such progress, the genetic complexity of this group of diseases represents a formidable logistic and economic hurdle in developing viable methods of prevention. Given this caveat, parameters effecting photoreceptor survival that are independent of the primary genetic lesion are critically important to identify.…”

Gene-based therapies for dominantly inherited retinopathies

“…Therefore, for IMPDH1-adRP, a mutation-independent 'suppression only' strategy may potentially provide benefit as demonstrated at the histological level in an AAV-induced mouse model of IMPDH1-adRP. 51 In this study, AAV2/ 5-delivered mutant IMPDH was shown to induce a retinopathy in mice akin to RP. However, when co-injected with a second AAV expressing an siRNA targeting IMPDH, the outer nuclear layer thickness of co-injected mice at 4 weeks post-injection resembled wild-type outer nuclear layer thicknesses.…”

An experimental platform for systemic drug delivery to the retina

“…Although degenerative retinopathies, including age-related macular degeneration (AMD), diabetic retinopathy, and RP represent major causes of world blindness, treatment options are currently exceedingly limited (15,(21)(22)(23)(24). No form of prevention is available for the nonexudative (dry) form of AMD, apart from dietary intervention.…”

“…First, we chose a model of autosomal recessive RP, the IMPDH1 Ϫ/Ϫ mouse (14). These animals lack one of the two rate-limiting enzymes of de novo synthesis of GTP, a central mediator of visual phototransduction (15). With increasing age, these mice develop a mild retinopathy, as evidenced by decreases in electrophysiological readout from the retina in tandem with a thinning of the photoreceptor ONL in animals older than 10 months.…”

“…RNAi-mediated suppression of claudin-5 in the retinas of adult mice causes a transient and size-selective increase in paracellular permeability of microvessels without the development of retinal edema or changes in retinal function as assayed by electrophysiology. Moreover, we demonstrate the therapeutic potential of this technique by enhancing the delivery of two distinct lowmolecular weight molecules to the neural retina of two rodent models of retinopathy: IMPDH1 Ϫ/Ϫ mice (14), a model of auto-somal recessive retinitis pigmentosa (RP) that lacks the key enzyme involved in the de novo synthesis of GTP (15), and a mouse model of light-induced retinal degeneration (16). These results highlight the safety and efficacy of selective iBRB modulation as an experimental tool for the systemic screening of a wide range of potentially therapeutic molecules in rodent models of retinopathies.…”