Operating in the dark: a night-vision system for surgery in retinas susceptible to light damage

A Review of Complicated Cataract in Retinitis Pigmentosa: Pathogenesis and Cataract Surgery

Retinitis pigmentosa (RP) is a set of inherited retinal degenerative diseases that affect photoreceptor and retinal pigment epithelial cells (RPEs), possibly associated with some ocular complications, including cataract. The complicated cataract formation is most likely the result of RP-related inflammation response, and the most common morphology category is posterior subcapsular cataract (PSC). Despite the absence of curative pharmacologic treatment, phacoemulsification with intraocular lens implantation to deal with opacification in the lens is preferred due to the considerable visual outcomes. However, the incidence of intraocular and postoperative complications is higher in RP patients than those without, including intraoperative phototoxic retinal damage, posterior capsular opacification (PCO), capsular contraction syndrome (CCS), pseudophakic cystoid macular edema (PCME), increased postoperative intraocular pressure (IOP), and intraocular lens (IOL) dislocation. Hence, it needs much attention to surgery progress and close follow-up. In this review, we discuss the current understanding of RP patients with complicated cataracts from morphology to potential pathogenesis to cataract surgical procedure and provide a concise description and the recommended management of related surgery complications to broaden the knowledge and lower the latent risks to yield better clinical outcomes.

Mutation-independent rhodopsin gene therapy by knockdown and replacement with a single AAV vector

Inherited retinal degenerations are caused by mutations in >250 genes that affect photoreceptor cells or the retinal pigment epithelium and result in vision loss. For autosomal recessive and X-linked retinal degenerations, significant progress has been achieved in the field of gene therapy as evidenced by the growing number of clinical trials and the recent commercialization of the first gene therapy for a form of congenital blindness. However, despite significant efforts to develop a treatment for the most common form of autosomal dominant retinitis pigmentosa (adRP) caused by >150 mutations in the rhodopsin (RHO) gene, translation to the clinic has stalled. Here, we identified a highly efficient shRNA that targets human (and canine) RHO in a mutation-independent manner. In a single adeno-associated viral (AAV) vector we combined this shRNA with a human RHO replacement cDNA made resistant to RNA interference and tested this construct in a naturally occurring canine model of RHO-adRP. Subretinal vector injections led to nearly complete suppression of endogenous canine RHO RNA, while the human RHO replacement cDNA resulted in up to 30% of normal RHO protein levels. Noninvasive retinal imaging showed photoreceptors in treated areas were completely protected from retinal degeneration. Histopathology confirmed retention of normal photoreceptor structure and RHO expression in rod outer segments. Long-term (>8 mo) follow-up by retinal imaging and electroretinography indicated stable structural and functional preservation. The efficacy of this gene therapy in a clinically relevant large-animal model paves the way for treating patients with RHO-adRP.

Dog models for blinding inherited retinal dystrophies

Spontaneous canine models exist for several inherited retinal dystrophies. This review will summarize the models and indicate where they have been used in translational gene therapy trials. The RPE65 gene therapy trials to treat childhood blindness are a good example of how studies in dogs have contributed to therapy development. Outcomes in human clinical trials are compared and contrasted with the result of the preclinical dog trials.

Gene augmentation for adRP mutations in RHO

Mutations in the gene for rhodopsin, RHO, cause autosomal dominant retinitis pigmentosa, a disease characterized by death of rod photoreceptor cells. At the end stage, when most rods are gone, cones die too, taking central vision with them. One goal of gene therapy, therefore, is to preserve central vision by promoting rod survival in the vicinity of the macula. Dominance in RHO mutations is associated with two phenomena: interference with the function of normal rhodopsin and intrinsic toxicity of the mutant protein. In the case of interference, increased production of the wild-type protein may be therapeutic, but in the case of toxicity, suppression of the mutant protein may also be needed. RHO augmentation has made use of advances in gene delivery to the retina using adeno-associated virus (AAV). Several strategies have been developed for suppression of rhodopsin expression, but because of the heterogeneity of RHO mutations they are not specific for the mutant allele: They suppress both mutant and wild-type RHO. Experiments in autosomal dominant retinitis pigmentosa (adRP) mouse models suggest that both RHO augmentation and supplementation plus suppression preserve the survival of rod cells.

Gene therapy in animal models of autosomal dominant retinitis pigmentosa

Gene therapy for dominantly inherited genetic disease is more difficult than gene-based therapy for recessive disorders, which can be treated with gene supplementation. Treatment of dominant disease may require gene supplementation partnered with suppression of the expression of the mutant gene either at the DNA level, by gene repair, or at the RNA level by RNA interference or transcriptional repression. In this review, we examine some of the gene delivery approaches used to treat animal models of autosomal dominant retinitis pigmentosa, focusing on those models associated with mutations in the gene for rhodopsin. We conclude that combinatorial approaches have the greatest promise for success.

The susceptibility of the retina to photochemical damage from visible light

The photoreceptor/RPE complex must maintain a delicate balance between maximizing the absorption of photons for vision and retinal image quality while simultaneously minimizing the risk of photodamage when exposed to bright light. We review the recent discovery of two new effects of light exposure on the photoreceptor/RPE complex in the context of current thinking about the causes of retinal phototoxicity. These effects are autofluorescence photobleaching in which exposure to bright light reduces lipofuscin autofluorescence and, at higher light levels, RPE disruption in which the pattern of autofluorescence is permanently altered following light exposure. Both effects occur following exposure to visible light at irradiances that were previously thought to be safe. Photopigment, retinoids involved in the visual cycle, and bisretinoids in lipofuscin have been implicated as possible photosensitizers for photochemical damage. The mechanism of RPE disruption may follow either of these paths. On the other hand, autofluorescence photobleaching is likely an indicator of photooxidation of lipofuscin. The permanent changes inherent in RPE disruption might require modification of the light safety standards. AF photobleaching recovers after several hours although the mechanisms by which this occurs are not yet clear. Understanding the mechanisms of phototoxicity is all the more important given the potential for increased susceptibility in the presence of ocular diseases that affect either the visual cycle and/or lipofuscin accumulation. In addition, knowledge of photochemical mechanisms can improve our understanding of some disease processes that may be influenced by light exposure, such as some forms of Leber's congenital amaurosis, and aid in the development of new therapies. Such treatment prior to intentional light exposures, as in ophthalmic examinations or surgeries, could provide an effective preventative strategy.

rAAV2/5 gene-targeting to rods:dose-dependent efficiency and complications associated with different promoters

A prerequisite for using corrective gene therapy to treat humans with inherited retinal degenerative diseases that affect primarily rods is to develop viral vectors that target specifically this population of photoreceptors. The delivery of a viral vector with photoreceptor tropism coupled with a rod-specific promoter is likely to be the safest and most efficient approach to target expression of the therapeutic gene to rods. Three promoters that included a fragment of the proximal mouse opsin promoter (mOP), the human G-protein coupled receptor protein kinase 1 promoter (hGRK1), or the cytomegalovirus immediate early enhancer combined with the chicken beta actin proximal promoter CBA) were evaluated for their specificity and robustness in driving GFP reporter gene expression in rods, when packaged in a recombinant adeno-associated viral vector of serotype 2/5 (AAV2/5), and delivered via subretinal injection to the normal canine retina. Photoreceptor specific promoters (mOP, hGRK1) targeted robust GFP expression to rods, while the ubiquitously expressed CBA promoter led to transgene expression in the retinal pigment epithelium, rods, cones and rare Müller, horizontal and ganglion cells. Late onset inflammation was frequently observed both clinically and histologically with all three constructs when the highest viral titers were injected. Cone loss in the injected regions of the retinas that received the highest titers occurred with both the hGRK1 and CBA promoters. Efficient and specific rod transduction, together with preservation of retinal structure was achieved with both mOP and hGRK1 promoters when viral titers in the order of 10¹¹ vg/ml were used.

Steroids do not prevent photoreceptor degeneration in the light-exposed T4R rhodopsin mutant dog retina irrespective of AP-1 inhibition

PURPOSE

AP-1 has been proposed as a key intermediate linking exposure to light and photoreceptor cell death in rodent light-damage models. Inhibition of AP-1 associated with steroid administration also prevents light damage. In this study the role of steroids in inhibiting AP-1 activation and/or in preventing photoreceptor degeneration was examined in the rhodopsin mutant dog model.

METHODS

The dogs were dark adapted overnight, eyes dilated with mydriatics; the right eye was light occluded and the fundus of the left eye photographed ( approximately 15-17 overlapping frames) with a fundus camera. For biochemical studies, the dogs remained in the dark for 1 to 3 hours after exposure. Twenty-four hours before exposure to light, some dogs were treated with systemic dexamethasone or intravitreal/subconjunctival triamcinolone. AP-1 DNA-binding activity was determined by electrophoresis mobility shift assay (EMSA) and phosphorylation of c-Fos and activation of ERK1/2 were determined by immunoblot analyses. The eyes were collected 1 hour and 2 weeks after exposure to light, for histopathology and immunocytochemistry.

RESULTS

Inhibition of AP-1 activation, and phosphorylation of ERK1/2 and c-Fos were found after dexamethasone treatment in light-exposed T4R RHO mutant dog retinas. In contrast, increased AP-1 activity and phosphorylation of c-Fos and ERK1/2 were found in triamcinolone-treated mutant retinas. Similar extensive rod degeneration was found after exposure to light with or without treatment, and areas with surviving photoreceptor nuclei consisted primarily of cones. Only with systemic dexamethasone did the RPE cell layer remain.

CONCLUSIONS

Intraocular or systemic steroids fail to prevent light-induced photoreceptor degeneration in the T4R RHO dog retina. Finding that systemic dexamethasone prevents AP-1 activation, yet does not prevent retinal light damage, further supports the hypothesis that AP-1 is not the critical player in the cell-death signal that occurs in rods.