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Ординатура / Офтальмология / Английские материалы / Retinal Degenerative Diseases Laboratory and Therapeutic Investigations_Anderson_2008.pdf
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L.C.S. Tam et al.

on the delivery of functional transgenes to the retinal pigment epithelium (RPE). Similarly in retinitis pigmentosa (RP), which is mainly caused by mutations in genes expressed exclusively in rod photoreceptor cells, specific delivery into the outer segment (OS) and outer nuclear layer (ONL) of the retina would be required. Furthermore, acquired retinopathies such as diabetic retinopathy or age-related macular degeneration (AMD), which are caused by complex pathological mechanisms, would require delivery into multiple cell types. Recently, adeno-associated viral (AAV) vectors have become one of the most successful delivery systems because of the wide variety of retinal cell types that can be selectively transduced using these vectors without causing significant toxicity over a long period of time in small and large animal models. Furthermore, the capsid genes can be interchanged between different AAV serotypes to create hybrid AAV vectors that benefit from enhanced in vivo efficacy and unique cellular tropism of the various serotypes (Auricchio et al. 2001; Rabinowitz et al. 2002). Therefore, one can package the best studied genome of AAV2 into the capsid of any AAV serotype (Rabinowitz et al. 2002). Currently there are many examples in the literature illustrating the use of AAV pseudotyping strategy to achieve specific cellular transduction within the retina. For example, recombinant AAV2/1 and 2/4 vectors (rAAVs) have been shown to transduce the pigment epithelium efficiently in rodents, canines and nonhuman primates (Auricchio et al. 2001; Weber et al. 2003; Acland et al. 2005; Le Meur et al. 2007), while rAAV2/5 was reported to be ideal for photoreceptor transduction (Auricchio et al. 2001; Lotery et al. 2003; O’Reilly et al. 2007)

Here in this report, we describe the development of a therapeutic strategy that combines the use of recombinant AAV vectors and RNA interference (RNAi) for treating the RP10 form of RP. The molecular elucidation of RP10 began in 1993, when an autosomal dominant RP gene (RP10 locus) that segregated in a large Spanish family was mapped to chromosome 7 through genetic linkage mapping (Jordan et al. 1993). Twelve years later, comparative transcriptional profiling between wild-type (WT) and degenerating mouse retinas (Rho–/– and Crx–/–) (Kennan et al. 2002; Bowne et al. 2002) identified mutations in the gene encoding inosine 5 -monophosphate dehydrogenase type 1 (IMPDH1) to be associated with the disease. Further studies on the molecular pathology of the disease indicated that combined suppression of both normal and mutant IMPDH1 transcripts in the human form of the disease may hold great therapeutic potential (Aherne et al. 2004). We therefore explored the feasibility of using rAAV2/5 vectors for the delivery of IMPDH1-targeting shRNAs in vivo to alleviate the pathological effect of mutant IMPDH1.

64.2 Results

64.2.1 Evaluation of Optimal IMPDH1 Suppressors

Initially a series of short hairpin RNAs (shRNAs) targeting distinct regions of human and mouse IMPDH1 transcripts were evaluated in mammalian cell cultures.

64 Protection of Photoreceptors in a Mouse Model of RP10

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Quantitative Real-Time RT-PCR and western blot analysis showed that one particular shRNA, shImp1, was capable of eliciting up to 84% suppression of IMPDH1 at both the mRNA and protein levels in HeLa cells, as compared to non-targeting controls. Furthermore, the use of a dual expression vector system directing the synthesis of shImp1 and enhanced green fluorescent protein (EGFP) demonstrated efficient knockdown of endogenous IMPDH1 transcripts following in vitro electroporation in murine retinal explants. Following this, rAAV2/5 vectors carrying shImp1 and EGFP were generated to evaluate suppression efficiency in vivo. Similar to previous observations, subretinal inoculation of rAAV2/5 vectors expressing shImp1 (3 μl of 6.85 × 1012 vp/ml per eye) into WT mice resulted in greater than 70% suppression of IMPDH1 at both the mRNA (Fig. 64.1a) and protein (Fig. 64.1b) levels in vivo. Moreover, rAAV2/5 vectors specifically transduced the outer segment and outer nuclear layer of WT mouse retinas as shown in Fig. 64.1c.

Fig. 64.1 rAAV-mediated suppression of IMPDH1 in vivo. (a) Quantitative Real-time RT-PCR analysis showing shRNA significant reduced IMPDH1 mRNA expression to 22 ± 3% following subretinal injection of rAAV targeting shRNA. P<0.05. Error bar: standard error of the mean. (b) Western blot analysis illustrating potent suppression of mouse IMPDH1 (55 kDa) in vivo. (c) Specific transduction of the OS and ONL (pointed arrows) in mouse retinal sections (12 μm) following subretinal inoculation of rAAV2/5 expressing shRNA and EGFP. Scale bar, 40 μm. RPE: retinal pigment epithelium; OS: outer segment; ONL: outer nuclear layer; OPL; outer plexiform layer; INL: inner nuclear layer; IPL: inner plexiform layer; GCL: ganglion cell layer

64.2.2 RP10 Mouse Model

Animal models are valuable tools to study disease pathogenesis and to evaluate experimental therapies. A mouse model displaying the pathological effect of mutant IMPDH1 was generated by subretinal inoculation of rAAV2/5 expressing human mutant IMPDH1 bearing the missense mutation (Arg224Pro) in adult WT mice (3 μl of 1.4 × 1012 vp/ml per eye). Four weeks post-injection, mice receiving rAAV2/5 expressing WT IMPDH1 showed normal retinal structure (Fig. 64.2a). In stark contrast, delivery of mutant IMPDH1 completely ablated the outer nuclear layer (Fig. 64.2b). rAAV-mediated delivery of human mutant IMPDH1 via

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Fig. 64.2 Protection of ONL structure by rAAV-mediated suppression of mutant IMPDH1. (a) Representative images of eyes treated with rAAV WT IMPDH1 displayed normal ONL structure, (b) whereas eyes treated with rAAV mutant IMPDH1 alone, or with non-targeting shRNA showed dramatic ONL degeneration. (c) In contrast, eyes treated with rAAV mutant IMPDH1 and targeting shRNA protected the ONL from degeneration. Scale bar, 40 μm. RPE: retinal pigment epithelium; OS: outer segment; ONL: outer nuclear layer; OPL; outer plexiform layer; INL: inner nuclear layer; IPL: inner plexiform layer; GCL: ganglion cell layer

subretinal inoculation induced a rapid and aggressive retinopathy in WT mice, and thus provided a rapid method for generating a disease animal model for RP10.

64.2.3Rescue of Photoreceptor Cells by rAAV-Mediated Downregulation of Mutant IMPDH1

In the final stage of this study, we tested our hypothesis namely that the photoreceptor cells could be rescued through RNAi-mediated downregulation of mutant IMPDH1 protein. rAAV2/5 vectors carrying human mutant IMPDH1 cDNA or shImp were co-injected subretinally at an experimentally derived viral particle

Fig. 64.3 Synaptic connectivity in RP10 mice following rAAV inoculation. (a) Eyes treated with rAAV mutant IMPDH1 and non-targeting shRNA showed an absence of synaptic marker staining. (b) In contrast, eyes treated with rAAV mutant IMPDH1 and shImp1 stained positively for synaptophysin (pointed arrow) and (c) bassoon (pointed arrow) across the outer plexiform layer (OPL). Scale bar, 20 μm. ONL: outer nuclear layer; OPL: outer plexiform layer; INL: inner nuclear layer