Freya M. Mowat

BACKGROUND: Mutations in RPE65 cause Leber's congenital amaurosis, a progressive retinal degenerative disease that severely impairs sight in children. Gene therapy can result in modest improvements in night vision, but knowledge of its efficacy in humans is limited. METHODS: We performed a phase 1-2 open-label trial involving 12 participants to evaluate the safety and efficacy of gene therapy with a recombinant adeno-associated virus 2/2 (rAAV2/2) vector carrying the RPE65 complementary DNA, and measured visual function over the course of 3 years. Four participants were administered a lower dose of the vector, and 8 were administered a higher dose. In a parallel study in dogs, we investigated the relationship among vector dose, visual function, and electroretinography (ERG) findings. RESULTS: Improvements in retinal sensitivity were evident, to varying extents, in six participants for up to 3 years, peaking at 6 to 12 months after treatment and then declining. No associated improvement in retinal function was detected by means of ERG. Three participants had intraocular inflammation, and two had clinically significant deterioration of visual acuity. The reduction in central retinal thickness varied among participants. In dogs, RPE65 gene therapy with the same vector at lower doses improved vision-guided behavior, but only higher doses resulted in improvements in retinal function that were detectable with the use of ERG. CONCLUSIONS: Gene therapy with rAAV2/2 RPE65 vector improved retinal sensitivity, albeit modestly and temporarily. Comparison with the results obtained in the dog model indicates that there is a species difference in the amount of RPE65 required to drive the visual cycle and that the demand for RPE65 in affected persons was not met to the extent required for a durable, robust effect. (Funded by the National Institute for Health Research and others; ClinicalTrials.gov number, NCT00643747.).

Cell-cell adhesion regulates the development and function of epithelia by providing mechanical support and by guiding cell proliferation and differentiation. The tight junction (TJ) protein zonula occludens (ZO)-1 regulates cell proliferation and gene expression by inhibiting the activity of the Y-box transcription factor ZONAB in cultured epithelial cells. We investigated the role of this TJ-associated signalling pathway in the retinal pigment epithelium (RPE) in vivo by lentivirally-mediated overexpression of ZONAB, and knockdown of its cellular inhibitor ZO-1. Both overexpression of ZONAB or knockdown of ZO-1 resulted in increased RPE proliferation, and induced ultrastructural changes of an epithelial-mesenchymal transition (EMT)-like phenotype. Electron microscopy analysis revealed that transduced RPE monolayers were disorganised with increased pyknosis and monolayer breaks, correlating with increased expression of several EMT markers. Moreover, fluorescein angiography analysis demonstrated that the increased proliferation and EMT-like phenotype induced by overexpression of ZONAB or downregulation of ZO-1 resulted in RPE dysfunction. These findings demonstrate that ZO-1 and ZONAB are critical for differentiation and homeostasis of the RPE monolayer and may be involved in RPE disorders such as proliferative vitroretinopathy and atrophic age-related macular degeneration.

PURPOSE: The canine is an important large animal model of human retinal genetic disorders. Studies of ganglion cell distribution in the canine retina have identified a visual streak of high density superior to the optic disc with a temporal area of peak density known as the area centralis. The topography of cone photoreceptors in the canine retina has not been characterized in detail, and in contrast to the macula in humans, the position of the area centralis in dogs is not apparent on clinical funduscopic examination. The purpose of this study was to define the location of the area centralis in the dog and to characterize in detail the topography of rod and cone photoreceptors within the area centralis. This will facilitate the investigation and treatment of retinal disease in the canine. METHODS: We used peanut agglutinin, which labels cone matrix sheaths and antibodies against long/medium wavelength (L/M)- and short wavelength (S)-cone opsins, to stain retinal cryosections and flatmounts from beagle dogs. Retinas were imaged using differential interference contrast imaging, fluorescence, and confocal microscopy. Within the area centralis, rod and cone size and density were quantified, and the proportion of cones expressing each cone opsin subtype was calculated. Using a grid pattern of sampling in 9 retinal flatmounts, we investigated the distribution of cones throughout the retina to predict the location of the area centralis. RESULTS: We identified the area centralis as the site of maximal density of rod and cone photoreceptor cells, which have a smaller inner segment cross-sectional area in this region. L/M opsin was expressed by the majority of cones in the retina, both within the area centralis and in the peripheral retina. Using the mean of cone density distribution from 9 retinas, we calculated that the area centralis is likely to be centered at a point 1.5 mm temporal and 0.6 mm superior to the optic disc. For clinical funduscopic examination, this represents 1.2 disc diameters temporal and 0.4 disc diameters superior to the optic disc. CONCLUSIONS: We have described the distribution of rods and cone subtypes within the canine retina and calculated a predictable location for the area centralis. These findings will facilitate the characterization and treatment of cone photoreceptor dystrophies in the dog.

BACKGROUND: Hypoxia plays a key role in ischaemic and neovascular disorders of the retina. Cellular responses to oxygen are mediated by hypoxia-inducible transcription factors (HIFs) that are stabilised in hypoxia and induce the expression of a diverse range of genes. The purpose of this study was to define the cellular specificities of HIF-1alpha and HIF-2alpha in retinal ischaemia, and to determine their correlation with the pattern of retinal hypoxia and the expression profiles of induced molecular mediators. METHODOLOGY/PRINCIPAL FINDINGS: We investigated the tissue distribution of retinal hypoxia during oxygen-induced retinopathy (OIR) in mice using the bio-reductive drug pimonidazole. We measured the levels of HIF-1alpha and HIF-2alpha proteins by Western blotting and determined their cellular distribution by immunohistochemistry during the development of OIR. We measured the temporal expression profiles of two downstream mediators, vascular endothelial growth factor (VEGF) and erythropoietin (Epo) by ELISA. Pimonidazole labelling was evident specifically in the inner retina. Labelling peaked at 2 hours after the onset of hypoxia and gradually declined thereafter. Marked binding to Müller glia was evident during the early hypoxic stages of OIR. Both HIF-1alpha and HIF-2alpha protein levels were significantly increased during retinal hypoxia but were evident in distinct cellular distributions; HIF-1alpha stabilisation was evident in neuronal cells throughout the inner retinal layers whereas HIF-2alpha was restricted to Müller glia and astrocytes. Hypoxia and HIF-alpha stabilisation in the retina were closely followed by upregulated expression of the downstream mediators VEGF and EPO. CONCLUSIONS/SIGNIFICANCE: Both HIF-1alpha and HIF-2alpha are activated in close correlation with retinal hypoxia but have contrasting cell specificities, consistent with differential roles in retinal ischaemia. Our findings suggest that HIF-2alpha activation plays a key role in regulating the response of Müller glia to hypoxia.