Ocular gene therapy: experimental studies and clinical possibilities

Soluble CX3CL1 gene therapy improves cone survival and function in mouse models of retinitis pigmentosa

Retinitis pigmentosa (RP) is a disease that initially presents as night blindness due to genetic deficits in the rod photoreceptors of the retina. Rods then die, causing dysfunction and death of cone photoreceptors, the cell type that mediates high acuity and color vision, ultimately leading to blindness. We investigated immune responses in mouse models of RP and found evidence of microglia activation throughout the period of cone degeneration. Using adeno-associated vectors (AAVs), delivery of genes encoding microglial regulatory signals led to the identification of AAV serotype 8 (AAV8) soluble CX3CL1 (sCX3CL1) as a promising therapy for degenerating cones. Subretinal injection of AAV8-sCX3CL1 significantly prolonged cone survival in three strains of RP mice. Rescue of cones was accompanied by improvements in visual function. AAV8-sCX3CL1 did not affect rod survival, microglia localization, or inflammatory cytokine levels in the retina. Furthermore, although RNA sequencing of microglia demonstrated marked transcriptional changes with AAV8-sCX3CL1, pharmacological depletion of up to ∼99% of microglia failed to abrogate the effect of AAV8-sCX3CL1 on cone survival. These findings indicate that AAV8-sCX3CL1 can rescue cones in multiple mouse models of RP via a pathway that does not require normal numbers of microglia. Gene therapy with sCX3CL1 is a promising mutation-independent approach to preserve vision in RP and potentially other forms of retinal degeneration.

N-acetylcysteine suppresses retinal detachment in an experimental model of proliferative vitreoretinopathy

Proliferative vitreoretinopathy (PVR) is a complication that develops in 5% to 10% of patients who undergo surgery to correct a detached retina. The only treatment option for PVR is surgical intervention, which has a limited success rate that diminishes in patients with recurring PVR. Our recent studies revealed that antioxidants prevented intracellular signaling events that were essential for experimental PVR. The purpose of this study was to test whether N-acetyl-cysteine (NAC), an antioxidant used in a variety of clinical settings, was capable of protecting rabbits from PVR. Vitreous-driven activation of PDGFRalpha and cellular responses intrinsic to PVR (contraction of collagen gels and cell proliferation) were blocked by concentrations of NAC that were well below the maximum tolerated dose. Furthermore, intravitreal injection of NAC effectively protected rabbits from developing retinal detachment, which is the sight-robbing phase of PVR. Finally, these observations with an animal model appear relevant to clinical PVR because NAC prevented human PVR vitreous-induced contraction of primary RPE cells derived from a human PVR membrane. Our observations demonstrate that antioxidants significantly inhibited experimental PVR, and suggest that antioxidants have the potential to function as a PVR prophylactic in patients undergoing retinal surgery to repair a detached retina.

Characterization of adenovirus p21 gene transfer, biodistribution, and immune response after local ocular delivery in New Zealand white rabbits

Previous studies suggest that local gene therapy with rAd-p21(WAF1/Cip-1) [. Arch. Ophthalmol. 120, (2002) 941-949] may provide an effective adjunctive anti-proliferative treatment to prevent glaucoma surgery failure. To further investigate rAd-p21 in this indication, we have characterized several parameters of local gene delivery to conjunctiva including, vector delivery and transgene expression in target tissue, inflammatory response, biodistribution to non-target tissues, and immune response. Quantitative PCR and RT-PCR assays were employed to evaluate rAd-p21 dissemination and gene transfer following a single subconjunctival injection. In target tissue, significant levels of rAd-p21 DNA were found in 6/6 animals 1 and 4 days after injection. rAd-p21 DNA and RNA could be detected in the un-injected contralateral eye but at levels that were 10000-100000 lower than in the injected eye. Expression of human p21 transgene in conjunctival fibroblasts was confirmed by immunohistochemistry. Biodistribution of rAd-p21 following subconjunctival injection was substantially limited to ocular tissue. In 1/6 rabbits, rAd-p21 DNA was found in whole blood, liver, and spleen at levels that were barely detectable. All non-target organs were negative on day 4. In contrast, in a rabbit injected intravenously as a positive control, all blood samples and tissues samples were positive. rAd-p21 delivery to conjunctiva followed by filtration surgery caused an early acute inflammatory response, which by day 14 was indistinguishable from placebo-treated eyes. Neutralizing anti-adenovirus antibodies were detected following administration of rAd-p21 to conjunctiva, however, vector delivery and transgene expression were unaffected in a subsequent administration to the contralateral eye in the same animal. These results show that local delivery to conjunctiva may be a suitable delivery mode for ocular gene therapy.

Targeted gene transfer to corneal stroma in vivo by electric pulses

This study was conducted to develop a method of targeted gene transfer by electric pulses to a selected area of corneal stroma in vivo. Plasmid DNA with a green fluorescent protein (GFP) gene under a cytomegalovirus promoter was injected through the corneal pocket into the corneal stroma of the adult Brown Norway rat, and various intensities of electric pulses ranging from 10 to 30 V were delivered to the corneal epithelial side with an electric probe. Direct stereomicroscopy of the fluorescent using real-time imaging was used to determine in vivo gene expression on days 1, 2, 4, 6, 8, 10, 15, and 20 after gene transfer. Transgene expression was detected in the corneal stroma as early as day 1 and until day 15. The most intense expression was noted on days 4 and 6. Gene transfer was most effective using eight electric pulses of 20 V for 50 msec. Histologic study disclosed GFP expression in keratocytes within the targeted area. There was no apparent cell damage in the gene transferred cells. No apparent inflammation was found in the anterior chamber or trabecular cells when electric pulses less than 30 V were used. In summary, the present technique transferred the gene of interest to a highly selected area of corneal stroma with no apparent damage. This method will likely be useful not only for developing gene therapy for corneal diseases but also for corneal research in general.

Looking into anti-angiogenic gene therapies for disorders of the eye

Age-related macular degeneration (AMD) and proliferative diabetic retinopathy (DR) are the most common causes of visual impairment in the developed world. Because the key factor in AMD and DR is aberrant neovascularization in the retina (DR) or in the choroid (AMD), strategies to inhibit abnormal neovascularization represent a compelling therapeutic approach. Here we review various anti-angiogenic strategies for the treatment of ocular neovascular diseases with special emphasis on gene transfer as a way of achieving high, sustained concentrations of anti-angiogenic proteins in the back of the eye without concomitant systemic toxicity.

Gene transfer by viral vectors into blood vessels in a rat model of retinopathy of prematurity

AIMS

To test the feasibility of gene transfer into hyaloid blood vessels and into preretinal neovascularisation in a rat model of retinopathy of prematurity (ROP), using different viral vectors.

METHODS

Newborn rats were exposed to alternating hypoxic and hyperoxic conditions in order to induce ocular neovascularisation (ROP rats). Adenovirus, herpes simplex, vaccinia, and retroviral (MuLV based) vectors, all carrying the beta galactosidase (beta-gal) gene, were injected intravitreally on postnatal day 18 (P18). Two sets of controls were also examined: P18 ROP rats injected with saline and P18 rats that were raised in room air before the viral vectors or saline were injected. Two days after injection, the rats were killed, eyes enucleated, and beta-gal expression was examined by X-gal staining in whole mounts and in histological sections.

RESULTS

Intravitreal injection of the adenovirus and vaccinia vectors yielded marked beta-gal expression in hyaloid blood vessels in the rat ROP model. Retinal expression of beta-gal with these vectors was limited almost exclusively to the vicinity of the injection site. Injection of herpes simplex yielded a punctuate pattern of beta-gal expression in the retina but not in blood vessels. No significant beta-gal expression occurred in rat eyes injected with the retroviral vector.

CONCLUSIONS

Adenovirus is an efficient vector for gene transfer into blood vessels in an animal model of ROP. This may be a first step towards utilising gene transfer as a tool for modulating ocular neovascularisation for experimental and therapeutic purposes.

Immunosuppression by IL-10-transfected human retinal pigment epithelial cells in vitro

PURPOSE

Retinal pigment epithelium (RPE) transplantation seems to be a possible therapy for restoring vision in the case of retinal degeneration. As there is a risk of allergic rejection, a gene-transfer of immunosuppressive cytokines into the graft may diminish this reaction. Therefore, we investigated the transfer of interleukin-10 (IL-10) into an immortalised human RPE cell line (hTERT-RPE1) and its effect on the proliferation of allogeneic immune competent cells.

METHODS

The hTERT-RPE1 cells were transiently transfected with the cDNA of human IL-10 using a lipid-based transfection reagent. The expression of IL-10 mRNA was ana-lysed by reverse transcription-polymerase chain reaction and an enzyme-linked immunosorbent assay measured the secretion of the cytokine over 7 days. The effect of the secreted IL-10 on the proliferation of allogeneic T cells with and without homologous macrophages was investigated colorimetrically. To enhance this reaction, RPE cells were pre-activated with interferon-gamma (IFN-gamma). Anti-IL-10 antibodies were used in a neutralising assay.

RESULTS

A transfection efficiency of 23.3 +/- 9.03% was achieved. IL-10 mRNA could only be shown in IL-10-transfected hTERT-RPE1 cells. The same was found for the level of cytokine, with a maximum on day 3 (10.34 +/- 0.09 ng/ml). A significant suppressive effect of the secreted IL-10 on T-cell proliferation was detectable on days 5 and 6. This effect could be significantly abolished with anti-IL-10 antibodies.

CONCLUSIONS

The IL-10-producing hTERT-RPE1 cells had an immunosuppressive action on T-cell proliferation in vitro. A gene-transfer into RPE allografts before transplantation may be able to promote graft survival.

Adenovirus-mediated gene transfer to human lens epithelial cells in organ culture

PURPOSE

To assess the feasibility of using recombinant adenovirus vectors to transduce the human lens epithelial cells (LECs) involved in posterior capsule opacification (PCO).

SETTING

Department of Ophthalmology and Molecular Medicine Unit, University of Manchester, Manchester, United Kingdom.

METHODS

Seventeen human lens capsules were maintained in organ culture to allow LECs to proliferate onto the posterior capsule. Partly covered and completely covered capsules were infected with a recombinant adenovirus vector RAd35, encoding for the marker gene beta-galactosidase at plaque-forming units per milliliter (pfu/mL) ranging from 10(7) to 10(10) for up to 48 hours. Assessment of infection and transduction of the marker gene were achieved by calculating the percentage of cells exhibiting X-gal staining both macroscopically and microscopically.

RESULTS

Staining appeared to be dependent on virus dose, with most intense staining at doses of 10(8) and 10(9) pfu/mL with decreased staining at higher and lower viral doses. Microscopic assessment demonstrated that all cells expressed beta-galactosidase when infected with 10(9) pfu, 84% at 10(8) pfu, and 45% at 10(7) pfu. At 10(10) pfu, some cytotoxicity was observed.

CONCLUSIONS

These results indicate that recombinant adenoviruses can be used to transfer genes to the LECs involved in PCO. The transfer of cytotoxic genes after cataract surgery may be considered a preventive measure for PCO.

Treatment of age-related macular degeneration

Age-related macular degeneration (AMD), while rapidly becoming more prevalent due to an aging population, is still poorly understood and treatment modalities are limited. Fortunately, advances are being made in the treatment of AMD that may greatly alter the outcome of this debilitating disease. Treatments for both wet and dry AMD are reviewed.

Retrovirus-mediated transfer of a suicide gene into lens epithelial cells in vitro and in an experimental model of posterior capsule opacification

PURPOSE

The most common complication of cataract surgery is the development of posterior capsule opacification (PCO). Hyperplasia of the lens epithelium is one of the main cellular events following phacoemulsification and was found to be an important feature contributing to opacification of the posterior capsule. We investigated the feasibility of killing the residual lens epithelial cells by retroviral-mediated transfer of the herpes simplex virus-thymidine kinase (HSV-tk) gene, a well-studied suicide gene, into rabbit lens epithelial cells followed by ganciclovir (GCV) treatment.

METHODS

The capacity of retroviral vectors to transfer genes into rabbit lens epithelial cells was determined either in vitro (culture of rabbit lens epithelial cells) or in vivo (experimental model of PCO in rabbits) using cDNA encoding the beta-galactosidase (LacZ) reporter gene. To evaluate the efficiency of suicide gene therapy (infection with retroviral vectors encoding the HSV-tk gene followed by GCV treatment) we determined the sensitivity of HSV-tk infected lens epithelial cells to different concentrations of GCV in vitro. Then, in an experimental model of PCO, rabbits were treated with HSV-tk retroviral vectors at the end of the surgery and they received repeated intracameral and intravitreal injections of GCV at the concentration determined by the in vitro experiments.

RESULTS

Infection efficiency using LacZ retroviral vectors was about 29% in vitro and 10% in vivo. After infection of the HSV-tk cDNA in vitro, the cell killing effect of GCV was evaluated. A significant enhancement (four- to five-fold) of the cell sensitivity to GCV was shown in FLY-DFGtk as compared with mock infected (P < 0.01) cells even without selection of the HSV-tk positive cells. The GCV concentration leading to 50% reduction in cell number (IC50) was 50 microg/ml. In vivo infection with a HSV-tk vector led to the tk gene transfer into lens epithelial cells. Despite this local HSV-tk gene expression, we could not prevent capsule opacification.

CONCLUSIONS

Lens epithelial cells were successfully infected both in vitro and in vivo by beta-galactosidase and HSV-tk genes via retroviral vectors. In vitro infected lens epithelial cells displayed a strong sensitivity to GCV treatment. In vivo, we could not prevent capsule opacification in the rabbit model, very likely due to the limited level of the HSV-tk gene expression. However, our results suggest that virus-mediated suicide gene therapy might be a feasible treatment strategy to prevent capsule opacification with a more powerful vector.

Adenovirus-mediated suicide gene transduction: feasibility in lens epithelium and in prevention of posterior capsule opacification in rabbits

The most common complication of cataract surgery is the development of posterior capsule opacification (PCO). Hyperplasia of the lens epithelium is one of the main cellular events following phacoemulsification, and has been found to be an important feature contributing to opacification of the posterior capsule. Adenoviral vector-mediated transfer is a suitable method for transducing the herpes simplex virus thymidine kinase gene (HSV-tk) into proliferating cells, allowing for the selective killing of these cells by ganciclovir (GCV) treatment. To determine the potential of gene transduction for lens epithelial cells, we studied the transduction of rabbit lens epithelial cells with adenoviral vectors containing either the Escherichia coli beta-galactosidase (lacZ) gene or the HSV-tk gene in vitro and in vivo in an experimental model of PCO. The efficiency of lacZ gene transfer in rabbit lens epithelial cells was at least 95% both in vitro and in vivo. In vivo transduction with HSV-tk adenoviral vector followed by GCV treatment significantly inhibited the development of PCO (p<0.001). These results suggest that adenoviral vector-mediated transfer of HSV-tk into the proliferating lens epithelial cells is feasible and may provide a novel therapeutic strategy for PCO.

Management of hereditary retinal degenerations: present status and future directions

Research on hereditary retinal degenerations has considerably improved our understanding of these disorders, although much remains to be learned about the exact mechanism involved in the pathogenesis. The advent of recombinant DNA technology will refine diagnostic capabilities, which have so far been based on the manifestations of the disease to localization of the molecular defects. The correlation of the molecular defects with the phenotype of the disease will result in better prognostic counseling for patients. In certain forms of retinitis pigmentosa, such as Refsum disease, gyrate atrophy of the choroid and retina, and abetalipoproteinemia, exact biochemical defects have been identified and specific treatments have been applied with some success. In other forms of retinitis pigmentosa, various investigations have suggested the possibilities of arresting the progress of degeneration by means such as the use of growth factors and controlling apoptosis. Efforts to alter the expression of the mutated gene or to introduce a normal gene into the genome are in their infancy, but results are encouraging. Vitamin A has been tried in patients with retinitis pigmentosa, and the results demonstrate statistically significant beneficial effects of this vitamin, suggesting that the course of the disease can be decelerated to some extent. Another interesting research area with potential for therapeutic application is the replacement of the retinal pigment epithelium or the degenerated neural retina by transplantation of the respective cell types. Clinical trials are being conducted both with retinal pigment epithelium and neuroretinal transplants.