Retrovirus-mediated suicide gene transduction in the vitreous cavity of the eye: feasibility in prevention of proliferative vitreoretinopathy

Gene transfer to the outflow tract

Elevated intraocular pressure is the primary cause of open angle glaucoma. Outflow resistance exists within the trabecular meshwork but also at the level of Schlemm's canal and further downstream within the outflow system. Viral vectors allow to take advantage of naturally evolved, highly efficient mechanisms of gene transfer, a process that is termed transduction. They can be produced at biosafety level 2 in the lab using protocols that have evolved considerably over the last 15-20 years. Applied by an intracameral bolus, vectors follow conventional as well as uveoscleral outflow pathways. They may affect other structures in the anterior chamber depending on their transduction kinetics which can vary among species when using the same vector. Not all vectors can express long-term, a desirable feature to address the chronicity of glaucoma. Vectors that integrate into the genome of the target cell can achieve transgene function for the life of the transduced cell but are mutagenic by definition. The most prominent long-term expressing vector systems are based on lentiviruses that are derived from HIV, FIV, or EIAV. Safety considerations make non-primate lentiviral vector systems easier to work with as they are not derived from human pathogens. Non-integrating vectors are subject to degradation and attritional dilution during cell division. Lentiviral vectors have to integrate in order to express while adeno-associated viral vectors (AAV) often persist as intracellular concatemers but may also integrate. Adeno- and herpes viral vectors do not integrate and earlier generation systems might be relatively immunogenic. Nonviral methods of gene transfer are termed transfection with few restrictions of transgene size and type but often a much less efficient gene transfer that is also short-lived. Traditional gene transfer delivers exons while some vectors (lentiviral, herpes and adenoviral) allow transfer of entire genes that include introns. Recent insights have highlighted the role of non-coding RNA, most prominently, siRNA, miRNA and lncRNA. SiRNA is highly specific, miRNA is less specific, while lncRNA uses highly complex mechanisms that involve secondary structures and intergenic, intronic, overlapping, antisense, and bidirectional location. Several promising preclinical studies have targeted the RhoA or the prostaglandin pathway or modified the extracellular matrix. TGF-β and glaucoma myocilin mutants have been transduced to elevate the intraocular pressure in glaucoma models. Cell based therapies have started to show first promise. Past approaches have focused on the trabecular meshwork and the inner wall of Schlemm's canal while new strategies are concerned with modification of outflow tract elements that are downstream of the trabecular meshwork.

In vivo models of proliferative vitreoretinopathy

We outline current in vitro and in vivo models for experimental proliferative vitreoretinopathy (PVR) and provide a detailed protocol of our standardized in vivo PVR model. PVR is the leading cause of failed surgical procedures for the correction of rhegmatogenous retinal detachment. The pathogenesis of this multifactorial condition is still not completely understood. Experimental models for PVR help us understand the factors that play a role in the pathogenesis of the disease process in a controlled manner and allow for reproducible preclinical assessment of novel therapeutic interventions. We describe a cell injection model in detail that uses homologous retinal pigment epithelial (RPE) cell cultures to induce PVR over a 2-8 week period.

Long-term safety of GDNF gene delivery in the retina

PURPOSE

To examine retinal function after the long-term, gene therapy-delivered expression of exogenous glial cell line-derived neurotrophic factor (GDNF).

METHODS

Forty Sprague-Dawley rats each received an intravitreal injection of recombinant adeno-associated virus expressing GDNF (rAAV-GDNF) in their right eye. The left eye was untreated. One year after viral transduction in ocular tissues, retinal morphology and function were compared between rAAV-GDNF-injected and normal naïve eyes. Synthesis and accumulation of GDNF within the retina were immunohistologically confirmed using enzyme-linked immunosorbent assay. Morphological analyses included light microscope examination of retinal sections and the counting of retinal ganglion cells. Inflammation by infiltration of leukocytes in retina was assessed immunohistochemically. Retinal function was assessed using electroretinography.

RESULTS

GDNF expression was confirmed. There was no obvious abnormality in retinal section or increased infiltration by leukocytes after retinal transduction of rAAV-GDNF for 1 year. Counts of retinal ganglion cells were not decreased in rAAV-GDNF-injected eyes. There were no statistical differences in amplitude as well as latency of the electroretinogram-determined a- and b-waves between transduced and untreated eyes.

CONCLUSIONS

Long-term expression of GDNF within the eyes can be achieved by intravitreal injection of rAAV vectors in the absence of morphological or functional deficits in the retina.

Recent developments in ocular gene therapy

The past two to three years have witnessed a remarkable increase in the number of gene therapy studies to treat almost every disease of the eye. All types of delivery systems, viral and non-viral, have been used. Experiments have begun to move from the use of reporters, to genes with potential therapeutic value. In this paper, rather than giving an overview from the beginning of ocular gene therapy, I have chosen to review its most recent advances. Although numerous issues remain to be solved, the emerging picture is encouraging. Within the experimental setting, conditions in the anterior and posterior segments have been improved by the administration of genes encoding beneficial proteins. In one case, vision has been restored in a congenitally blind animal. Limitations do exit, however a greater understanding of the molecular biology of eye tissues coupled with the development of low immunogenicity vectors will continue edging the way for a future use of gene therapy in the clinical setting.

Gene therapy for genetic and acquired retinal diseases

We present an overview of the current status of basic science and translational research being applied to gene therapy for eye disease, focusing on diseases of the retina. We discuss the viral and nonviral methods being used to transfer genes to the retina and retinal pigment epithelium, and the advantages and disadvantages of each approach. We review the various genetic and somatic treatment strategies that are being used for genetically determined and acquired diseases of the retina, including gene replacement, gene silencing by ribozymes and antisense oligonucleotides, suicide gene therapy, antiapoptosis, and growth factor therapies. The rationales for the specific therapeutic approaches to each disease are discussed. Schematics of gene transfer methods and therapeutic approaches are presented together with a glossary of gene transfer terminology.

Comparative analysis of the uptake and expression of plasmid vectors in human ciliary and retinal pigment epithelial cells in vitro

The retinal pigment epithelium is uniquely suited to gene therapy that uses lipid-mediated DNA transfer due to its high phagocytic activity in situ. We compared the relative efficacy of phagocytosis on the uptake of labeled plasmid vectors by retinal pigment epithelial and ciliary epithelial cells in vitro. Relative levels of endocytosis were then compared with the efficiency of marker transgene expression in these cells. Human retinal pigment epithelial and ciliary epithelial cells from a single donor were isolated and expanded in vitro. Polyplex-mediated transfections were performed using a rhodamine-labeled expression vector for green fluorescent protein. Rhodamine-labeled endosomes were examined by fluorescence microscopy at different time points. Rhodamine labeling and green fluorescent protein expression were analyzed by flow cytometry 48 h after transfection. These gene transfer studies showed that expression of transgenes does occur in both human retinal pigment epithelial and ciliary epithelial cells in vitro. Endocytosis of labeled plasmid vectors occurs at a significantly higher number and density in retinal pigment epithelial cells than in ciliary epithelial cells (P < 0.04). However, the efficiency of marker transgene expression is similar in the two cell types. These studies demonstrate that the higher intrinsic phagocytic activity does not enhance the efficacy of transgene expression in retinal pigment epithelial cells in vitro. Both human retinal pigment epithelial and ciliary epithelial cells are competent recipients for lipid-mediated gene transfer, and transgene expression occurs at similar levels in both cell types.

Adenoviral-mediated gene transfer to the filtering bleb in rabbits

PURPOSE

To determine if genes can be transferred to fibroblasts in the filtering bleb using adenoviral vectors.

MATERIALS AND METHODS

Twelve New Zealand albino rabbits underwent bilateral full-thickness sclerostomies. On postoperative day 1 an adenoviral vector carrying a reporter gene (lacZ) was injected into the right-eye bleb and saline was injected into the left eye bleb of each rabbit. Three rabbits were euthanized on each of the after days (days 3, 7, 14, and 21). The eyes were enucleated and tissue samples from the filtering bleb were processed for beta-galactosidase activity (a marker for lacZ gene expression) and expression of vimentin (a fibroblast marker). The median number of cells per high-power field with both beta-galactosidase activity and vimentin expression on days 3, 7, 14, and 21 in the right and left eyes were counted to determine adenoviral-mediated gene expression in fibroblasts.

RESULTS

In the adenoviral vector-treated eyes, the median number of cells per high-power field with both beta-galactosidase and vimentin expression on days 3, 7, 14, and 21 was 83,100, 1, and 0, respectively. No beta-galactosidase activity was noted in the saline-treated eyes.

CONCLUSIONS

Adenoviral vectors can transfer genes to fibroblasts in filtering blebs after glaucoma surgery. The peak efficiency of gene transfer to fibroblasts occurred 7 days after glaucoma surgery. These studies show a potential for genetic manipulation of fibroblast activity in filtering blebs after glaucoma surgery.

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.

Polyplex-mediated gene transfer into human retinal pigment epithelial cells in vitro

The human retinal pigment epithelium (RPE) is a potential target tissue for directed transfer of candidate genes to treat age-related macular degeneration (AMD). The RPE is uniquely suited to gene therapy protocols that use liposome-mediated DNA transfer because of its high intrinsic phagocytic function in vivo. In these studies, we examined the efficacy of human RPE cell uptake and expression of the green fluorescent protein (GFP) and neomycin resistance marker genes by polyplex-mediated gene transfer in vitro. The effects of varying DNA and polyplex concentration and ratios on GFP transgene expression were examined. A narrow range of experimental conditions were found to maximize transgene expression; most important were the DNA concentration and the DNA:polyplex ratio. The transfection efficiency for human RPE cells was reproducibly 20% in vitro by this method and reached a maximum level of expression after 48 h. There was a rapid decline in gene expression over 2 weeks following polyplex-mediated gene transfer, but stable integration does occur at low frequencies with and without selection.

Gene therapy in the treatment of ocular inflammation

Gene therapy may become a powerful therapeutic modality in the treatment of both ocular inflammatory disease and as a means of preventing rejection following tissue transplantation. By directly introducing into ocular cells genes that encode proteins capable of down-regulating the immune response, gene therapy has potential for both therapy and as a method for studying mechanisms of disease. While marked and rapid advances in the study of gene therapy have been realized, technical questions regarding the appropriate vector or the choice of efficacious immunomodulatory protein still remain.

Retinal functional change caused by adenoviral vector-mediated transfection of LacZ gene

We examined the effect of insertion of an exogenous gene on retinal function to assess the rationale of adenoviral vector-mediated gene transfer for future gene therapy. An adenoviral vector expressing bacterial LacZ (AdCALacZ) was injected into the eyes of adult rats either intravitreally (group A) or subretinally (group B), and the gene expression and retinal function were thus examined at different time points after gene transfer for 3 weeks. X-Gal histostaining showed that neural retinal cells were transfected in group A and that retinal pigment epithelial cells were transfected in group B. The gene transfer was more efficient in group B (54.4% of the fixed retinal area was stained) than in group A (10.4%). The electroretinogram (ERG) revealed retinal dysfunction in the AdCALacZ-transfected rats even at the stage in which the histological damage was not apparent by electron microscopy and immunohistochemical studies for cytokeratin, S-100 protein, and glial fibrillary acidic protein. The ERG change was correlated with the intensity of inflammation, and retinal function recovered to the original level by 3 weeks, along with a diminution of inflammation. Functional changes were more evident in eyes treated with AdCALacZ than in those infected with adenoviral vector with no exogenous gene; however, no histological difference was observed between these groups, indicating that the insertion of exogenous gene itself affects retinal function. The results showed that different kinds of retinal cells could be gene-transferred by an adenoviral vector, depending on the application method. The retinal dysfunction caused by each adenoviral transfection method was caused by inflammation and the insertion of exogenous gene, and this retinal dysfunction was recoverable. In future gene therapy, special attention should be given to the method of exogenous gene insertion in the retina.

Post-traumatic proliferative vitreoretinopathy. The epidemiologic profile, onset, risk factors, and visual outcome

PURPOSE

The purpose of the study was to characterize the clinical development of proliferative vitreoretinopathy (PVR) after trauma in the human eye.

METHODS

A chart review was performed on the records of 1564 patients with ocular trauma seen at a large metropolitan hospital. The frequency, type of ocular trauma, time to onset, potential risk factors, and visual outcome for PVR were evaluated.

RESULTS

Proliferative vitreoretinopathy occurred in 71 (4%) of 1654 injured eyes. Of these 71 injured eyes, 30 (42%) resulted from rupture, 15 (21%) from penetration, 13 (18%) from perforation, and 7 (10%) from confusion. Six (9%) were associated with an intraocular foreign body (IOFB). The frequency of PVR following perforation, rupture, penetration, IOFB, and contusion was 43%, 21%, 15%, 11%, and 1%, respectively. Overall, those eyes that developed PVR had a poorer visual outcome, with PVR being the primary reason for visual loss. The time from injury to onset of PVR was shortest after perforation (median, 1.3 months), followed by rupture (2.1 months), IOFB (3.1 months), penetration (3.2 months), and contusion (5.7 months). Vitreous hemorrhage was the strongest independent predictive factor for the development of PVR. A long, posteriorly located wound and persistent intraocular inflammation were also important risk factors for PVR.

CONCLUSIONS

These results suggest that PVR is a common complication following a variety of ocular injuries, and that it is associated with a poor visual outcome. Its frequency, onset, and outcome are strongly dependent on the nature of the trauma. Specific high-risk groups are identified as candidates for more aggressive therapy.