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Gene Therapy Rescues Cone Function in Congenital Achromatopsia

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Gene Therapy Rescues Cone Function in Congenital Achromatopsia Human Molecular Genetics, 2010, Vol. 19, No. 13 2581–2593 doi:10.1093/hmg/ddq136 Advance Access published on April 8, 2010 Gene therapy rescues cone function in congenital achromatopsia Andra´s M. Koma´romy1,∗, John J. Alexander2,3,4, Jessica S. Rowlan1, Monique M. Garcia1,5, Vince A. Chiodo3, Asli Kaya5, Jacqueline C. Tanaka5, Gregory M. Acland6, William W. Hauswirth2,3 and Gustavo D. Aguirre1 1Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA, 2Department of Molecular Genetics and Microbiology and 3Department of Ophthalmology and Powell Gene Therapy Center, University of Florida, Gainesville, FL 32610, USA, 4Vision Science Research Center, University of Alabama, Birmingham, AL 35294, USA, 5Department of Biology, Temple University, Philadelphia, PA 19122, USA and 6Baker Institute, Cornell University, Ithaca, NY 14853, USA Received January 19, 2010; Revised March 12, 2010; Accepted April 5, 2010 The successful restoration of visual function with recombinant adeno-associated virus (rAAV)-mediated gene replacement therapy in animals and humans with an inherited disease of the retinal pigment epithelium has ushered in a new era of retinal therapeutics. For many retinal disorders, however, targeting of therapeutic vec- tors to mutant rods and/or cones will be required. In this study, the primary cone photoreceptor disorder achro- matopsia served as the ideal translational model to develop gene therapy directed to cone photoreceptors. We demonstrate that rAAV-mediated gene replacement therapy with different forms of the human red cone opsin promoter led to the restoration of cone function and day vision in two canine models of CNGB3 achromatopsia, a neuronal channelopathy that is the most common form of achromatopsia in man. The robustness and stability of the observed treatment effect was mutation independent, but promoter and age dependent. Subretinal administration of rAAV5–hCNGB3 with a long version of the red cone opsin promoter in younger animals led to a stable therapeutic effect for at least 33 months. Our results hold promise for future clinical trials of cone-directed gene therapy in achromatopsia and other cone-specific disorders. INTRODUCTION have been translated to the clinic and been shown to be safe and successful for a human retinal degeneration caused by Most vision-impairing disorders in humans result from genetic RPE65 mutations (4–11). defects, and, to date, mutations have been identified in 150 However, this example of an RPE disease is rare and genes out of 200 mapped retinal disease loci (RetNet: http:// complex in that loss of visual function and eventual photo- www.sph.uth.tmc.edu/RetNet/). This wealth of genetic infor- receptor degeneration is secondary to a biochemical blockade mation has provided fundamental understanding of the mul- of the RPE visual cycle. While it has been an ideal test plat- tiple and specialized roles played by photoreceptors and the form for the first clinical gene therapy trial for blindness in retinal pigment epithelium (RPE) in the visual process, and man, RPE65 disease is not representative of the diversity of how mutations in these genes result in disease. Together more common retinal disorders that could be considered for with the development of gene transfer technologies, it is gene therapy where the target cells are the rods and/or now possible to realistically consider the use of gene therapy cones. For these, successful targeting of therapeutic vectors to treat these previously untreatable disorders. A new era of to mutant photoreceptors to restore function and preserve retinal therapeutics has been started by recent studies, structure will be required, and achieving this initially in exper- showing that treatment of animal models with gene therapy imental animals is necessary for eventual application to successfully restored visual function (1–3). These results humans. ∗To whom correspondence should be addressed at: Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, 3900 Delancey Street, Philadelphia, PA 19104-6010, USA. Tel: +1 2155732695; Fax: +1 2155732162; Email: [email protected] # The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected] 2582 Human Molecular Genetics, 2010, Vol. 19, No. 13 Congenital achromatopsia, also called rod monochromacy red/green and blue pigments (39). These are termed long/ or total congenital color blindness, is a rare autosomal reces- medium-wavelength-absorbing cones (L/M-cones) and sive disorder with an estimated prevalence of about 1 in short-wavelength-absorbing cones (S-cones), and have 30 000–50 000 (12–15). Because it primarily affects cones, maximal sensitivities of 555 and 429–435 nm, respectively it serves as the ideal translational model to develop gene thera- (39,40). The number, distribution and ratios of L/M- and pies directed to photoreceptors. Even though cones are S-cones in the normal canine retina have recently been numerically small, 5% of the photoreceptors in man (16), reported, and L/M-cones outnumber S-cones by a ratio of they are essential for color vision, central visual acuity and 8:1 in the areas selected for therapy [see Materials and most daily visual activities, functions that are severely Methods (41)]. impaired or absent in affected patients. Thus far, four genes Dogs with either the CNGB32/2 or CNGB3m/m mutation have been identified to cause achromatopsia in human are blind in bright light after 8 weeks of age, a time point patients, all of them encoding principal components of the when vision can be reproducibly evaluated (35–38,42). At cone phototransduction cascade: the alpha subunit of cone this age, cone-mediated function is absent, but rod responses transducin [GNAT2 (17–19)], the catalytic alpha’ subunit of are normal (Fig. 1). Despite the loss of cone function, both the cone phosphodiesterase (PDE6C) (20,21) and the alpha L/M- and S-cones are present, and express markers character- [CNGA3 (22–24)] and beta [CNGB3 (25–28)] subunits of istic of their differentiated state (Fig. 2). Cone arrestin is the cone cyclic nucleotide-gated (CNG) channel located in expressed in both L/M- and S-cones, but expression in the the plasma membrane of the cone outer segment. The majority outer segments appears to be more intense in the former of human achromatopsia (70–92%) are considered channe- (Fig. 2). Although there has been quantification of the rate lopathies as they are caused by mutations in either CNGA3 or of cone loss in canine CNGB3-achromatopsia with time CNGB3 (29–32). The CNGB3 gene appears to be the most (unpublished data), we find that the process is slow with prevalent causal gene for achromatopsia in patients of North- many cones still present in older affected dogs (Supplementary ern European descent, with reported prevalence between 50 Material, Fig. S1). Thus the time window for cone-targeted and 87% (29,32). However, it is suspected that CNGA3 gene replacement therapy deserves examination. mutations are more prevalent among patients from the Middle East (33). Based on the very high prevalence of the CNGB3 mutations Gene expression can be targeted to cones in autosomal recessive achromatopsia (29,32), our identifi- in CNGB3-achromatopsia by rAAV cation of two independent canine models with distinct In an effort to direct treatment to cones, we first asked whether CNGB3 mutations has provided us with a unique, physiologi- mutant cones could be transduced with vectors convey- cally relevant system in which to assess potential gene therapy ing reporter constructs and promoters that are derived from strategies designed to restore visual function (34). These L/M-cones. We previously reported the effectiveness and models result from either a missense mutation in exon 6 specificity of rAAV5 and different versions of the human (CNGB3m/m) or a genomic deletion of the entire CNGB3 red cone opsin promoter to direct cell class-specific GFP gene (CNGB32/2, ‘null mutation’) (34). The canine disease expression to L/M-cones in normal and diseased (primary, caused by the null mutation has been characterized, and early rod degeneration caused by PDE6B mutation) canine shares the same clinical phenotype as human patients (35– retina (43). In preparation for rAAV5-mediated gene replace- 38). The classic achromatopsia phenotype develops between ment therapy in CNGB3-achromatopsia, we confirmed that 8 and 12 weeks of age, soon after retinal differentiation is the longer (PR2.1) and shorter (3LCR-PR0.5) variants of completed (35–38); however, other than the salient functional the human red cone opsin promoter were also able to target and structural abnormalities in affected cones, the retina L/M-cones in mutant canine retinas (Table 1). We found remains normal throughout life (36,38). that gene expression could be targeted specifically to nonfunc- Here we report the successful restoration of cone function tional L/M-cones in the CNGB3-mutant retina, and that the and associated photopic vision in both canine achromatopsia robustness of expression was promoter dependent. While the models by gene replacement therapy. We used recombinant PR2.1 led to strong GFP expression that could easily be visu- adeno-associated virus serotype 5 (rAAV5) with different alized by direct GFP fluorescence, expression with the shorter forms of the human red cone opsin promoter. The robustness 3LCR-PR0.5 promoter was weaker, and required immunohis- and stability of the observed treatment effect was mutation tochemical enhancement to visualize the transduced cones independent, but promoter and age dependent. These results (Fig. 3). We also verified that the transfection of cones only hold promise for future clinical trials in human patients with occurred in the area of the subretinal bleb, and that CNGB3-achromatopsia. GFP-gene expression levels diminished sharply beyond the bleb margins (Fig. 3). Both PR2.1 and 3LCR-PR0.5 promoters were subsequently used for rAAV5-mediated gene replace- RESULTS ment therapies.
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