Gregory M. Acland

Gene therapy restores vision in a canine model of childhood blindness

The relationship between the neurosensory photoreceptors and the adjacent retinal pigment epithelium (RPE) controls not only normal retinal function, but also the pathogenesis of hereditary retinal degenerations. The molecular bases for both primary photoreceptor and RPE diseases that cause blindness have been identified. Gene therapy has been used successfully to slow degeneration in rodent models of primary photoreceptor diseases, but efficacy of gene therapy directed at photoreceptors and RPE in a large-animal model of human disease has not been reported. Here we study one of the most clinically severe retinal degenerations, Leber congenital amaurosis (LCA). LCA causes near total blindness in infancy and can result from mutations in RPE65 (LCA, type II; MIM 180069 and 204100). A naturally occurring animal model, the RPE65−/− dog, suffers from early and severe visual impairment similar to that seen in human LCA. We used a recombinant adeno-associated virus (AAV) carrying wild-type RPE65 (AAV-RPE65) to test the efficacy of gene therapy in this model. Our results indicate that visual function was restored in this large animal model of childhood blindness.

An Expressed Fgf4 Retrogene Is Associated with Breed-Defining Chondrodysplasia in Domestic Dogs

Going Retro In a year celebrating Darwin, the question of how new functional genes arise during evolution is of particular interest. Through a multibreed genetic analysis of the domestic dog, Parker et al. (p. 995, published online 16 July; see the Perspective by Kaessmann) find that the short-legged phenotype that characterizes at least 19 common dog breeds, including the corgi, dachshund, and basset hound, is specifically associated with the expression in developing bone of a gene encoding fibroblast growth factor 4 (fgf4), a member of a gene family previously implicated in dwarfism in humans. Interestingly, the culprit fgf4 gene in dogs has the hallmarks of a “retrogene,” a gene that arises when a parental gene is duplicated through an RNA-based copying mechanism. The short legs that characterize certain dog breeds are associated with a gene that arose recently by RNA-based gene duplication. Retrotransposition of processed mRNAs is a common source of novel sequence acquired during the evolution of genomes. Although the vast majority of retroposed gene copies, or retrogenes, rapidly accumulate debilitating mutations that disrupt the reading frame, a small percentage become new genes that encode functional proteins. By using a multibreed association analysis in the domestic dog, we demonstrate that expression of a recently acquired retrogene encoding fibroblast growth factor 4 (fgf4) is strongly associated with chondrodysplasia, a short-legged phenotype that defines at least 19 dog breeds including dachshund, corgi, and basset hound. These results illustrate the important role of a single evolutionary event in constraining and directing phenotypic diversity in the domestic dog.

Gene therapy rescues cone function in congenital achromatopsia

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 vectors to mutant rods and/or cones will be required. In this study, the primary cone photoreceptor disorder achromatopsia 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.

Mutations associated with neutropenia in dogs and humans disrupt intracellular transport of neutrophil elastase

Cyclic hematopoiesis is a stem cell disease in which the number of neutrophils and other blood cells oscillates in weekly phases. Autosomal dominant mutations of ELA2, encoding the protease neutrophil elastase, found in lysosome-like granules, cause cyclic hematopoiesis and most cases of the pre-leukemic disorder severe congenital neutropenia (SCN; ref. 3) in humans. Over 20 different mutations of neutrophil elastase have been identified, but their consequences are elusive, because they confer no consistent effects on enzymatic activity. The similar autosomal recessive disease of dogs, canine cyclic hematopoiesis, is not caused by mutations in ELA2 (data not shown). Here we show that homozygous mutation of the gene encoding the dog adaptor protein complex 3 (AP3) β-subunit, directing trans-Golgi export of transmembrane cargo proteins to lysosomes, causes canine cyclic hematopoiesis. C-terminal processing of neutrophil elastase exposes an AP3 interaction signal responsible for redirecting neutrophil elastase trafficking from membranes to granules. Disruption of either neutrophil elastase or AP3 perturbs the intracellular trafficking of neutrophil elastase. Most mutations in ELA2 that cause human cyclic hematopoiesis prevent membrane localization of neutrophil elastase, whereas most mutations in ELA2 that cause SCN lead to exclusive membrane localization.

Reversal of Blindness in Animal Models of Leber Congenital Amaurosis Using Optimized AAV2-mediated Gene Transfer

We evaluated the safety and efficacy of an optimized adeno-associated virus (AAV; AAV2.RPE65) in animal models of the RPE65 form of Leber congenital amaurosis (LCA). Protein expression was optimized by addition of a modified Kozak sequence at the translational start site of hRPE65. Modifications in AAV production and delivery included use of a long stuffer sequence to prevent reverse packaging from the AAV inverted-terminal repeats, and co-injection with a surfactant. The latter allows consistent and predictable delivery of a given dose of vector. We observed improved electroretinograms (ERGs) and visual acuity in Rpe65 mutant mice. This has not been reported previously using AAV2 vectors. Subretinal delivery of 8.25 x 10(10) vector genomes in affected dogs was well tolerated both locally and systemically, and treated animals showed improved visual behavior and pupillary responses, and reduced nystagmus within 2 weeks of injection. ERG responses confirmed the reversal of visual deficit. Immunohistochemistry confirmed transduction of retinal pigment epithelium cells and there was minimal toxicity to the retina as judged by histopathologic analysis. The data demonstrate that AAV2.RPE65 delivers the RPE65 transgene efficiently and quickly to the appropriate target cells in vivo in animal models. This vector holds great promise for treatment of LCA due to RPE65 mutations.

Persistent transgene product in retina, optic nerve and brain after intraocular injection of rAAV

Recombinant adeno-associated virus (rAAV) is a promising vector for retinal application as it transduces photoreceptors and retinal pigment epithelium cells efficiently and in a stable fashion. Because rAAV also transduces retinal ganglion cells, we reasoned that ocular application of rAAV might result in delivery of transgenic protein to the CNS. Here we describe high levels of green fluorescent protein (GFP) persisting at least 6 months in optic nerves and brains of mice and dogs after intravitreal delivery of rAAV-GFP. There was no clinical or histological evidence of inflammatory response although a mild humoral Th-2 response to viral capsid proteins was detected. These findings have important implications with respect to therapeutic applications of rAAV.

Naturally occurring rhodopsin mutation in the dog causes retinal dysfunction and degeneration mimicking human dominant retinitis pigmentosa.

Rhodopsin is the G protein-coupled receptor that is activated by light and initiates the transduction cascade leading to night (rod) vision. Naturally occurring pathogenic rhodopsin (RHO) mutations have been previously identified only in humans and are a common cause of dominantly inherited blindness from retinal degeneration. We identified English Mastiff dogs with a naturally occurring dominant retinal degeneration and determined the cause to be a point mutation in the RHO gene (Thr4Arg). Dogs with this mutant allele manifest a retinal phenotype that closely mimics that in humans with RHO mutations. The phenotypic features shared by dog and man include a dramatically slowed time course of recovery of rod photoreceptor function after light exposure and a distinctive topographic pattern to the retinal degeneration. The canine disease offers opportunities to explore the basis of prolonged photoreceptor recovery after light in RHO mutations and determine whether there are links between the dysfunction and apoptotic retinal cell death. The RHO mutant dog also becomes the large animal needed for preclinical trials of therapies for a major subset of human retinopathies.

Genome-wide association identifies a deletion in the 3' untranslated region of striatin in a canine model of arrhythmogenic right ventricular cardiomyopathy.

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a familial cardiac disease characterized by ventricular arrhythmias and sudden cardiac death. It is most frequently inherited as an autosomal dominant trait with incomplete and age-related penetrance and variable clinical expression. The human disease is most commonly associated with a causative mutation in one of several genes encoding desmosomal proteins. We have previously described a spontaneous canine model of ARVC in the boxer dog. We phenotyped adult boxer dogs for ARVC by performing physical examination, echocardiogram and ambulatory electrocardiogram. Genome-wide association using the canine 50k SNP array identified several regions of association, of which the strongest resided on chromosome 17. Fine mapping and direct DNA sequencing identified an 8-bp deletion in the 3′ untranslated region (UTR) of the Striatin gene on chromosome 17 in association with ARVC in the boxer dog. Evaluation of the secondary structure of the 3′ UTR demonstrated that the deletion affects a stem loop structure of the mRNA and expression analysis identified a reduction in Striatin mRNA. Dogs that were homozygous for the deletion had a more severe form of disease based on a significantly higher number of ventricular premature complexes. Immunofluorescence studies localized Striatin to the intercalated disc region of the cardiac myocyte and co-localized it to three desmosomal proteins, Plakophilin-2, Plakoglobin and Desmoplakin, all involved in the pathogenesis of ARVC in human beings. We suggest that Striatin may serve as a novel candidate gene for human ARVC.

Canine and Human Visual Cortex Intact and Responsive Despite Early Retinal Blindness from RPE65 Mutation

Background RPE65 is an essential molecule in the retinoid-visual cycle, and RPE65 gene mutations cause the congenital human blindness known as Leber congenital amaurosis (LCA). Somatic gene therapy delivered to the retina of blind dogs with an RPE65 mutation dramatically restores retinal physiology and has sparked international interest in human treatment trials for this incurable disease. An unanswered question is how the visual cortex responds after prolonged sensory deprivation from retinal dysfunction. We therefore studied the cortex of RPE65-mutant dogs before and after retinal gene therapy. Then, we inquired whether there is visual pathway integrity and responsivity in adult humans with LCA due to RPE65 mutations (RPE65-LCA). Methods and Findings RPE65-mutant dogs were studied with fMRI. Prior to therapy, retinal and subcortical responses to light were markedly diminished, and there were minimal cortical responses within the primary visual areas of the lateral gyrus (activation amplitude mean ± standard deviation [SD] = 0.07% ± 0.06% and volume = 1.3 ± 0.6 cm3). Following therapy, retinal and subcortical response restoration was accompanied by increased amplitude (0.18% ± 0.06%) and volume (8.2 ± 0.8 cm3) of activation within the lateral gyrus (p < 0.005 for both). Cortical recovery occurred rapidly (within a month of treatment) and was persistent (as long as 2.5 y after treatment). Recovery was present even when treatment was provided as late as 1–4 y of age. Human RPE65-LCA patients (ages 18–23 y) were studied with structural magnetic resonance imaging. Optic nerve diameter (3.2 ± 0.5 mm) was within the normal range (3.2 ± 0.3 mm), and occipital cortical white matter density as judged by voxel-based morphometry was slightly but significantly altered (1.3 SD below control average, p = 0.005). Functional magnetic resonance imaging in human RPE65-LCA patients revealed cortical responses with a markedly diminished activation volume (8.8 ± 1.2 cm3) compared to controls (29.7 ± 8.3 cm3, p < 0.001) when stimulated with lower intensity light. Unexpectedly, cortical response volume (41.2 ± 11.1 cm3) was comparable to normal (48.8 ± 3.1 cm3, p = 0.2) with higher intensity light stimulation. Conclusions Visual cortical responses dramatically improve after retinal gene therapy in the canine model of RPE65-LCA. Human RPE65-LCA patients have preserved visual pathway anatomy and detectable cortical activation despite limited visual experience. Taken together, the results support the potential for human visual benefit from retinal therapies currently being aimed at restoring vision to the congenitally blind with genetic retinal disease.

ANCHORING OF CANINE LINKAGE GROUPS WITH CHROMOSOME-SPECIFIC MARKERS

Abstract. A high-resolution genetic map with polymorphic markers spaced frequently throughout the genome is a key resource for identifying genes that control specific traits or diseases. The lack of rigorous selection against genetic disorders has resulted in many breeds of dog suffering from a very high frequency of genetic diseases, which tend to be breed-specific and usually inherited as autosomal recessive or apparently complex genetic traits. Many of these closely resemble human genetic disorders in their clinical and pathologic features and are likely to be caused by mutations in homologous genes. To identify loci important in canine disease genes, as well as traits associated with morphological and behavioral variation, we are developing a genetic map of the canine genome. Here we report on an updated version of the canine linkage map, which includes 341 mapped markers distributed over the X and 37 autosomal linkage groups. The average distance between markers on the map is 9.0 cM, and the linkage groups provide estimated coverage of over 95% of the genome. Fourteen linkage groups contain either gene-associated or anonymous markers localized to cosmids that have been assigned to specific canine chromosomes by FISH. These 14 linkage groups contain 150 microsatellite markers and allow us to assign 40% of the linkage groups to specific canine chromosomes. This new version of the map is of sufficient density and characterization to initiate mapping of traits of interest.