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This book will contain the proceedings of the XIV International Symposium on Retinal Degeneration (RD2010), held July 13-17, 2010, in Mont-Tremblant, Quebec, Canada. The volume will present representative state-of-the-art research in almost all areas of retinal degenerations, ranging from cytopathologic, physiologic, diagnostic and clinical aspects; animal models; mechanisms of cell death; candidate genes, cloning, mapping and other aspects of molecular genetics; and developing potential therapeutic measures such as gene therapy and neuroprotective agents for potential pharmaceutical therapy.

Retinal degenerations (RD) are a complex heterogeneous group of diseases in which retinal photoreceptors and the supporting retinal pigment epithelial cells die irreversibly, causing visual loss for millions of people. Mutations on more than 150 genes have been discovered for RD and there are many forms that possess complex etiology involving more than one gene and environmental effect. For years, many have searched for some common intracellular second messenger for these many forms of cell death which could be targeted for therapy. Ceramide is a novel cellular second messenger which signals for apoptosis. Several lines of evidence suggest an integral role of ceramide in photoreceptor apoptosis and cell death. Understanding their role in the pathogenic pathways of retinal degenerative diseases is important for development of targeted therapeutics.

Ribozymes targeted to a point mutation (histidine for proline substitution at position 23) in rhodopsin were evaluated as therapeutic agents for photoreceptor degeneration in an animal model of autosomal dominant retinitis pigmentosa (ADRP). Recombinant adeno-associated virus (AAV) vectors incorporating a proximal bovine rod opsin promoter were used to transfer either hairpin or hammerhead ribozyme genes to photoreceptors. AAV was administered by subretinal injection at postnatal day (P) 15 to rats expressing a rod opsin transgene with a proline-to-histidine substitution in exon 1 at codon 23 of the rhodopsin gene (P23H). In vivo levels of normal and mutant RNAs were measured by allele-specific RT-PCR at P60. Eyes were examined by histopathology and morphometric analysis, as well as by ERG at approximately P60, P75 or P90. Expression of either a hammerhead or hairpin ribozyme markedly slowed the rate of photoreceptor degeneration for at least 3 months as determined by outer nuclear layer thickness as well as by inner and outer segment length. Catalytically inactive control ribozymes had a significantly smaller effect on the retinal degeneration. Minimal rescue effects were seen in retinas injected with either AAV containing a gene for green fluorescent protein rather than the ribozyme gene or buffer only. The level of mutant opsin RNA relative to wild-type RNA was also reproducibly lower in ribozyme-treated retina. Finally, ERG comparisons of ribozyme-treated versus control eyes demonstrated functional rescue. We conclude that ribozyme-directed cleavage of mutant mRNAs appears to be a potentially effective therapy for autosomal dominant retinal degeneration.

Age related macular degeneration (AMD) is the leading cause of blindness among the elderly in the United States (Klein et al., 1992; Klein et al., 2002), and choroidal neovascularization (CNV) accounts for the majority of severe vision loss (Ferris et al., 1984). The current standard treatment for CNV is verteporfin photodynamic therapy (PDT) (Landy and Brown, 2003), which uses a laser to activate a photosensitizing dye accumulated within the CNV. Although PDT causes less damage to the retina overlying CNV than thermal laser, in normal primate (Husain et al., 1996; Kramer et al., 1996; Reinke et al., 1999; Peyman et al., 2000), rabbit (Peyman et al., 2000) and rat (Zacks et al., 2002) models, PDT damages photoreceptors and retinal pigment epithelial (RPE) cells. Although there has been no histologic evidence of damage to normal human retinal cells after PDT (Schlotzer-Schrehardt et al., 2002), patients treated with PDT experience visual disturbances and acute severe vision loss significantly more often than patients receiving placebo (Arnold et al., 2004; Azab et al., 2004). Because neurotrophic agents, such as brain-derived neurotrophic factor (BDNF) have been proven effective in reducing retinal damage in rodents after exposure to constant light (LaVail et al., 1992; Okoye et al., 2003), we hypothesized that BDNF treatment prior to PDT might reduce collateral damage to retinal and RPE cells in normal rats.

Abnormalities of the retinal pigment epithelial (RPE) cells are seen in several inherited degenerations such as Best disease (Petrukhin et al., 1998; Marmorstein et al., 2000), Stargardt disease (Weng et al., 1999), Sorsby’s fundus dystrophy (Steinmetz et al., 1992; Jacobson et al., 1995), and childhood-onset severe retinal dystrophy (Gu et al., 1997) and Leber congenital amaurosis due to RPE65 mutations (Marlhens et al., 1997; Morimura et al., 1998; Lotery et al., 2000; Thompson et al., 2000). RCS rats have been studied extensively as a model of photoreceptor (PR)-RPE cell interactions (Mullen and LaVail, 1976; LaVail, 2001) and retinal degeneration (Strauss et al., 1998; LaVail, 2001). Retinal degeneration in RCS results from failure of the RPE to phagocytize shed rod outer segments (OS) (Herron et al., 1969; Bok and Hall, 1971). The unphagocytized OS membranes form membranous whorls at the RPE surface, and the rod OS grow abnormally long (Dowling and Sidman, 1962; LaVail and Battelle, 1975). Eventually, the OS layer degenerates into a debris zone with subsequent PR cell death (Dowling and Sidman, 1962).

Transgenic animals, in which the transgene has been engineered to mimic mutations known to cause photoreceptor degenerations in humans, provide powerful models of the diseases known as retinitis pigmentosa. Here we describe the time course of photoreceptor death in rats in which degeneration is caused by a rhodopsin transgene containing the P23H or S334ter mutations.

Preface. I: Specific Gene Defects Cause Photoreceptor Degeneration. II: Gene Therapy for Photoreceptor Rescue. III: The Involvement of Light in Retinal Degeneration. IV: Animal Models with Photoreceptor Degeneration. V: Molecular and Cell Biological Studies.VI: Retinal Prosthesis as a Replacement for Photoreceptors. VII: Neuroprotective Agents Promote Photoreceptor Survival. VIII: Transplantation Studies.

Constant light damage in albino rats has become a useful model for testing the therapeutic potential of survival factors to protect photoreceptors from cell damage and death. Considerable variation occurs among animals in light damage experiments. An additional complication with the light damage model is that less damaged photoreceptors show greater protection by a given survival factor than those that are more severely damaged by light. A method is described that allows for comparison of relatively small numbers of animals and considers variability, relative degree of protection by survival factors and other qualitative measures of damage.

In 1990, Faktorovich and co-workers1 demonstrated that intraocularly injected basic fibroblast growth factor (bFGF) dramatically delayed photoreceptor degeneration in Royal College of Surgeons (RCS) rats with inherited retinal dystrophy. This showed that at least one growth factor, bFGF, could act as a survival-promoting agent in one form of hereditary retinal degeneration, and was consistent with its action as a survival-promoting neurotrophic agent in some other regions of the central and peripheral nervous system.2–7

Photoreceptor destruction is known to occur as a result of exposure for various lengths of time to low (less than about 1000 lux) (1, 2), moderate (about 1000–3000 lux) (3, 4) and higher (5, 6) levels of visible light, as well as by the radiant energy from lasers of different wavelengths (7, 8). In the case of high levels of visible light (6) and laser-induced damage to the retina, the mechanism appears to be the absorption of energy by melanosomes in the pigment epithelium and, indeed, the extent of laser-induced retinal destruction is directly related to the degree of retinal pigmentation (7, 8).