Long-term restoration of rod and cone vision by single dose rAAV-mediated gene transfer to the retina in a canine model of childhood blindness

Gene transfer of an engineered zinc finger protein enhances the anti-angiogenic defense system

Zinc finger protein transcription factors (ZFP TFs) have been shown to positively or negatively regulate the expression of endogenous genes involved in a number of different disease processes. In this study we investigated whether gene transfer of an engineered ZFP TF designed to up-regulate expression of the chromosomal pigment epithelium-derived factor (Pedf) gene could suppress experimentally induced choroidal neovascularization (CNV). Transient transfection with engineered ZFP TFs significantly increased both Pedf messenger RNA (mRNA) and secreted PEDF protein levels in cell culture. Six weeks after intravitreous or subretinal injection of an adeno-associated viral (AAV) vector expressing the PEDF-activating ZFP TF in mice, we observed increased retinal Pedf mRNA, and a significant reduction in the size of CNV at Bruch's membrane rupture sites, assessed in vivo by fluorescein angiography or by postmortem measurements on choroidal flat mounts. Importantly, the anti-angiogenic activity persisted at 3 months after intravitreous injection. These data suggest that ZFP TF-driven enhancement of the endogenous anti-angiogenic defense system may provide a new approach for prophylaxis and treatment of neovascular diseases of the eye.

Process optimization of large-scale production of recombinant adeno-associated vectors using dielectric spectroscopy

A well-characterized manufacturing process for the large-scale production of recombinant adeno-associated vectors (rAAV) for gene therapy applications is required to meet current and future demands for pre-clinical and clinical studies and potential commercialization. Economic considerations argue in favor of suspension culture-based production. Currently, the only feasible method for large-scale rAAV production utilizes baculovirus expression vectors and insect cells in suspension cultures. To maximize yields and achieve reproducibility between batches, online monitoring of various metabolic and physical parameters is useful for characterizing early stages of baculovirus-infected insect cells. In this study, rAAVs were produced at 40-l scale yielding ~1 x 10(15) particles. During the process, dielectric spectroscopy was performed by real time scanning in radio frequencies between 300 kHz and 10 MHz. The corresponding permittivity values were correlated with the rAAV production. Both infected and uninfected reached a maximum value; however, only infected cell cultures permittivity profile reached a second maximum value. This effect was correlated with the optimal harvest time for rAAV production. Analysis of rAAV indicated the harvesting time around 48 h post-infection (hpi), and 72 hpi produced similar quantities of biologically active rAAV. Thus, if operated continuously, the 24-h reduction in the production process of rAAV gives sufficient time for additional 18 runs a year corresponding to an extra production of ~2 x 10(16) particles. As part of large-scale optimization studies, this new finding will facilitate the bioprocessing scale-up of rAAV and other bioproducts.

RPE transplantation and its role in retinal disease

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

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 cm(3)). Following therapy, retinal and subcortical response restoration was accompanied by increased amplitude (0.18% +/- 0.06%) and volume (8.2 +/- 0.8 cm(3)) 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 cm(3)) compared to controls (29.7 +/- 8.3 cm(3), p < 0.001) when stimulated with lower intensity light. Unexpectedly, cortical response volume (41.2 +/- 11.1 cm(3)) was comparable to normal (48.8 +/- 3.1 cm(3), 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.

Novel RDH12 mutations associated with Leber congenital amaurosis and cone-rod dystrophy: biochemical and clinical evaluations

The purpose of this study was to determine the role of the retinol dehydrogenase 12 (RDH12) gene in patients affected with Leber congenital amaurosis (LCA), autosomal recessive retinitis pigmentosa (arRP) and autosomal dominant/recessive cone-rod dystrophies (CORD). Changes in the promoter region, coding regions and exon/intron junctions of the RDH12 gene were evaluated using direct DNA sequencing of patients affected with LCA (n=36 cases), RP (n=62) and CORD (n=21). The allele frequency of changes observed was assessed in a multiethnic control population (n=159 individuals). Detailed biochemical and structural modeling analysis of the observed mutations were performed to assess their biological role in the inactivation of Rdh12. A comprehensive clinical assessment of retinal structure and function in LCA patients carrying mutations in the RDH12 gene was completed. Of the six changes identified, three were novel including a homozygous C201R change in a patient affected with LCA, a heterozygous A177V change in patients affected with CORD and a heterozygous G46G change in a patient affected with LCA. A novel compound heterozygote T49M/A269fsX270 mutation was also found in a patient with LCA, and both homozygous and heterozygous R161Q changes were seen in 26 patients affected with LCA, CORD or RP. These R161Q, G46G and the A177V sequence changes were shown to be polymorphic. We found that Rdh12 mutant proteins associated with LCA were inactive or displayed only residual activity when expressed in COS-7 and Sf9 cells, whereas those mutants that were considered polymorphisms were fully active. Thus, impairment of retinal structure and function for patients carrying these mutations correlated with the biochemical properties of the mutants.

The proteome of the mouse photoreceptor sensory cilium complex

Primary cilia play critical roles in many aspects of biology. Specialized versions of primary cilia are involved in many aspects of sensation. The single photoreceptor sensory cilium (PSC) or outer segment elaborated by each rod and cone photoreceptor cell of the retina is a classic example. Mutations in genes that encode cilia components are common causes of disease, including retinal degenerations. The protein components of mammalian primary and sensory cilia have not been defined previously. Here we report a detailed proteomics analysis of the mouse PSC complex. The PSC complex comprises the outer segment and its cytoskeleton, including the axoneme, basal body, and ciliary rootlet, which extends into the inner segment of photoreceptor cells. The PSC complex proteome contains 1968 proteins represented by three or more unique peptides, including approximately 1500 proteins not detected in cilia from lower organisms. This includes 105 hypothetical proteins and 60 proteins encoded by genes that map within the critical intervals for 23 inherited cilia-related disorders, increasing their priority as candidate genes. The PSC complex proteome also contains many cilia proteins not identified previously in photoreceptors, including 13 proteins produced by genes that harbor mutations that cause cilia disease and seven intraflagellar transport proteins. Analyses of PSC complexes from rootletin knock-out mice, which lack ciliary rootlets, confirmed that 1185 of the identified PSC complex proteins are derived from the outer segment. The mass spectrometry data, benchmarked by 15 well characterized outer segment proteins, were used to quantify the copy number of each protein in a mouse rod outer segment. These results reveal mammalian cilia to be several times more complex than the cilia of unicellular organisms and open novel avenues for studies of how cilia are built and maintained and how these processes are disrupted in human disease.

Leber Congenital Amaurosis Caused by an RPGRIP1 Mutation Shows Treatment Potential

PURPOSE

To determine the treatment potential in Leber congenital amaurosis (LCA) resulting from an RPGRIP1 (retinitis pigmentosa GTPase regulating-interacting protein 1) mutation, a form of LCA with recent gene therapy success in an animal model.

DESIGN

Case report of a rare genetic eye disease investigated for intervention potential.

PARTICIPANTS

A 19-year-old man with LCA.

METHODS

We studied the retinal structure and function in an LCA patient with a novel homozygous Val1211Glu mutation in the RPGRIP1 gene using optical coherence tomography and colocalized dark-adapted thresholds.

MAIN OUTCOME MEASURE

Optical coherence tomography results.

RESULTS

Central retinal laminar architecture was preserved, and there was a measurable outer nuclear layer. The retained retinal structure corresponded to the region of visual sensitivity. With increasing eccentricity, there was no measurable visual function, and retinal laminar disorganization suggested a remodeling process.

CONCLUSIONS

The RPGRIP1-LCA patient has treatment potential for a gene replacement strategy if targeted to central, but not pericentral or peripheral, retina. The results differ from similarly studied RPE65-LCA and CRB1-LCA patients. Preclinical progress toward therapy in LCA patients warrants detailed structure-function studies in humans to determine feasibility and candidacy for clinical trials.

Recombinant adeno-associated virus targets passenger gene expression to cones in primate retina

Recombinant adeno-associated virus (rAAV) is a promising vector for gene therapy of photoreceptor-based diseases. Previous studies have demonstrated that rAAV serotypes 2 and 5 can transduce both rod and cone photoreceptors in rodents and dogs, and it can target rods, but not cones in primates. Here we report that using a human cone-specific enhancer and promoter to regulate expression of a green fluorescent protein (GFP) reporter gene in an rAAV-5 vector successfully targeted expression of the reporter gene to primate cones, and the time course of GFP expression was able to be monitored in a living animal using the RetCam II digital imaging system.

Diseases caused by defects in the visual cycle: retinoids as potential therapeutic agents

Absorption of a photon by an opsin pigment causes isomerization of the chromophore from 11-cis-retinaldehyde to all-trans-retinaldehyde. Regeneration of visual chromophore following light exposure is dependent on an enzyme pathway called the retinoid or visual cycle. Our understanding of this pathway has been greatly facilitated by the identification of disease-causing mutations in the genes coding for visual cycle enzymes. Defects in nearly every step of this pathway are responsible for human-inherited retinal dystrophies. These retinal dystrophies can be divided into two etiologic groups. One involves the impaired synthesis of visual chromophore. The second involves accumulation of cytotoxic products derived from all-trans-retinaldehyde. Gene therapy has been successfully used in animal models of these diseases to rescue the function of enzymes involved in chromophore regeneration, restoring vision. Dystrophies resulting from impaired chromophore synthesis can also be treated by supplementation with a chromophore analog. Dystrophies resulting from the accumulation of toxic pigments can be treated pharmacologically by inhibiting the visual cycle, or limiting the supply of vitamin A to the eyes. Recent progress in both areas provides hope that multiple inherited retinal diseases will soon be treated by pharmaceutical intervention.

AAV-mediated gene transfer for retinal diseases

Vectors based on the adeno-associated virus (rAAV) are able to transduce the retina of animal models, including non-human primates, for a long-term period, safely and at sustained levels. The ability of the various rAAV serotypes to transduce retinal target cells has been exploited to successfully transfer genes to photoreceptors, retinal pigment epithelium and the inner retina, which are affected in many inherited and non-inherited blinding diseases. rAAV-mediated, constitutive and regulated gene expression at therapeutic levels has been achieved in the retina of animal models, thus providing proof-of-principle of gene therapy efficacy and safety in models of dominant and recessive retinal disorders. In addition, gene transfer of molecules with either neurotrophic or antiangiogenic properties provides useful alternatives to the classic gene replacement for treatment of both mendelian and complex traits affecting the retina. Years of successful rAAV-mediated gene transfer to the retina have resulted in restoration of vision in dogs affected with congenital blindness. This has paved the way to the first attempts at treating inherited retinal diseases in humans with rAAV. Although the results of rAAV clinical trials for non-retinal diseases give a warning that the outcome of viral-mediated gene transfer in humans may be different from that predicted based on results in other species, the immune privilege of the retina combined with the versatility of rAAV serotypes may ultimately provide the first successful treatment of human inherited diseases using rAAV.

Light at the end of the tunnel? Advances in the understanding and treatment of glaucoma and inherited retinal degeneration

Glaucoma and inherited retinal degeneration/dystrophy are leading causes of blindness in veterinary patients. Currently, there is no treatment for the loss of vision that characterizes both groups of diseases. However, this reality may soon change as recent advances in understanding of the disease processes allow researchers to develop new therapies aimed at preventing blindness and restoring vision to blind patients. Elucidating the molecular mechanisms of retinal ganglion cell death in glaucoma patients has led to the development of neuroprotective drugs which protect retinal cells and their function from the disastrous effects of elevated pressure. Identification of the genetic mutation responsible for inherited degenerations and dystrophies of the outer retina has enabled researchers using gene therapy to restore vision to blind dogs. Other patients may benefit from retinal transplantation, stem cell therapy, neuroprotective drugs, nutritional supplementation and even retinal prostheses. It is possible that soon it will be possible to restore sight to some blind patients.

Lentiviral gene replacement therapy of retinas in a mouse model for Usher syndrome type 1B

One of the most disabling forms of retinal degeneration occurs in Usher syndrome, since it affects patients who already suffer from deafness. Mutations in the myosin VIIa gene (MYO7A) cause a major subtype of Usher syndrome, type 1B. Owing to the loss of function nature of Usher 1B and the relatively large size of MYO7A, we investigated a lentiviral-based gene replacement therapy in the retinas of MYO7A-null mice. Among the different promoters tested, a CMV-MYO7A chimeric promoter produced wild-type levels of MYO7A in cultured RPE cells and retinas in vivo. Efficacy of the lentiviral therapy was tested by using cell-based assays to analyze the correction of previously defined, MYO7A-null phenotypes in the mouse retina. In vitro, defects in phagosome digestion and melanosome motility were rescued in primary cultures of RPE cells. In vivo, the normal apical location of melanosomes in RPE cells was restored, and the abnormal accumulation of opsin in the photoreceptor connecting cilium was corrected. These results demonstrate that a lentiviral vector can accommodate a large cDNA, such as MYO7A, and mediate correction of important cellular functions in the retina, a major site affected in the Usher syndrome. Therefore, a lentiviral-mediated gene replacement strategy for Usher 1B therapy in the retina appears feasible.

Application of a new subretinal injection device in the dog

The use of a new subretinal injection device (RetinaJect Subretinal Cannula, SurModics, Inc., Eden Prairie, MN) to access the subretinal space in the canine model was evaluated. Subretinal injections were performed in 33 mongrel dogs between 2 and 52 months of age (median = 9 months). In 5 normal dogs the injection of 150 microl saline or India ink occurred by using a conventional subretinal injection device (CSID) with a 30-gauge anterior chamber irrigating cannula. The sclera had to be surgically exposed and penetrated before the subretinal injection with the CSID could occur. After removing the CSID, the conjunctiva over the sclerotomy site had to be closed. In a second group of 28 dogs [16 normals, 10 RPE65 mutants, and 2 with progressive rod cone degeneration (prcd)], the 25-gauge needle of the RetinaJect was used to penetrate the conjunctiva and the sclera. Once the tip of the needle was close to the retinal surface, a 39-gauge polyimide cannula was extended and brought into apposition with the retina for the subsequent subretinal injection of 150 microl saline, India ink, or adeno-associated virus (AAV). No closure of the conjunctiva was required. The animals were clinically monitored between 1 and 59 weeks after surgery. From this second group 25 eyes were harvested for routine histological analysis either immediately after surgery or after a clinical observation time of between 1 and 40 weeks. Both devices provided equally successful access to the subretinal space. The main advantage of the RetinaJect was that no surgical dissection was required; this led to a shorter procedure time and milder postoperative conjunctival swelling. In summary, the use of the RetinaJect can be recommended as an alternative to the CSID for subretinal injections in dogs.

Efficient non-viral ocular gene transfer with compacted DNA nanoparticles

Background

The eye is an excellent candidate for gene therapy as it is immune privileged and much of the disease-causing genetics are well understood. Towards this goal, we evaluated the efficiency of compacted DNA nanoparticles as a system for non-viral gene transfer to ocular tissues. The compacted DNA nanoparticles examined here have been shown to be safe and effective in a human clinical trial, have no theoretical limitation on plasmid size, do not provoke immune responses, and can be highly concentrated.

Methods and Findings

Here we show that these nanoparticles can be targeted to different tissues within the eye by varying the site of injection. Almost all cell types of the eye were capable of transfection by the nanoparticle and produced robust levels of gene expression that were dose-dependent. Most impressively, subretinal delivery of these nanoparticles transfected nearly all of the photoreceptor population and produced expression levels almost equal to that of rod opsin, the highest expressed gene in the retina.

Conclusions

As no deleterious effects on retinal function were observed, this treatment strategy appears to be clinically viable and provides a highly efficient non-viral technology to safely deliver and express nucleic acids in the retina and other ocular tissues.

Retinitis pigmentosa

Hereditary degenerations of the human retina are genetically heterogeneous, with well over 100 genes implicated so far. This Seminar focuses on the subset of diseases called retinitis pigmentosa, in which patients typically lose night vision in adolescence, side vision in young adulthood, and central vision in later life because of progressive loss of rod and cone photoreceptor cells. Measures of retinal function, such as the electroretinogram, show that photoreceptor function is diminished generally many years before symptomic night blindness, visual-field scotomas, or decreased visual acuity arise. More than 45 genes for retinitis pigmentosa have been identified. These genes account for only about 60% of all patients; the remainder have defects in as yet unidentified genes. Findings of controlled trials indicate that nutritional interventions, including vitamin A palmitate and omega-3-rich fish, slow progression of disease in many patients. Imminent treatments for retinitis pigmentosa are greatly anticipated, especially for genetically defined subsets of patients, because of newly identified genes, growing knowledge of affected biochemical pathways, and development of animal models.

Induction of melanocytes from embryonic stem cells and their therapeutic potential

Embryonic stem (ES) cells from many organisms have the capacity to generate in vitro a wide variety of cell types depending on their environment. Understanding precisely how such toti- or pluripotent cells may be driven towards a specific lineage represents a major challenge if our ambition of using ES cells to generate a ready supply of healthy cells for cell-based therapies for a range of diseases is to be realized. Recent advances have demonstrated that melanocytes and retinal pigmented epithelial (RPE) cells exhibiting the characteristics of their natural counterparts can be induced from undifferentiated ES cells grown on monolayers of specific stromal cell lines or by using a combination of Wnt3a, Endothelin-3 and SCF. The ability to induce pigment cells from ES cells promises to facilitate our understanding of the precise molecular mechanisms underlying this process and moreover enable us to distinguish the program of gene expression that underpins the choice made between generating a nerual crest-type melanocyte versus an RPE cell. Moreover, once the combination of signals required to induce a particular type of pigment cell are characterized, the way may be open for future cell-based therapy for various diseases caused by defective pigment cells.

Retinitis pigmentosa

Retinitis pigmentosa (RP) is an inherited retinal dystrophy caused by the loss of photoreceptors and characterized by retinal pigment deposits visible on fundus examination. Prevalence of non syndromic RP is approximately 1/4,000. The most common form of RP is a rod-cone dystrophy, in which the first symptom is night blindness, followed by the progressive loss in the peripheral visual field in daylight, and eventually leading to blindness after several decades. Some extreme cases may have a rapid evolution over two decades or a slow progression that never leads to blindness. In some cases, the clinical presentation is a cone-rod dystrophy, in which the decrease in visual acuity predominates over the visual field loss. RP is usually non syndromic but there are also many syndromic forms, the most frequent being Usher syndrome. To date, 45 causative genes/loci have been identified in non syndromic RP (for the autosomal dominant, autosomal recessive, X-linked, and digenic forms). Clinical diagnosis is based on the presence of night blindness and peripheral visual field defects, lesions in the fundus, hypovolted electroretinogram traces, and progressive worsening of these signs. Molecular diagnosis can be made for some genes, but is not usually performed due to the tremendous genetic heterogeneity of the disease. Genetic counseling is always advised. Currently, there is no therapy that stops the evolution of the disease or restores the vision, so the visual prognosis is poor. The therapeutic approach is restricted to slowing down the degenerative process by sunlight protection and vitaminotherapy, treating the complications (cataract and macular edema), and helping patients to cope with the social and psychological impact of blindness. However, new therapeutic strategies are emerging from intensive research (gene therapy, neuroprotection, retinal prosthesis).

Restoration of vision in RPE65-deficient Briard dogs using an AAV serotype 4 vector that specifically targets the retinal pigmented epithelium

Previous studies have tested gene replacement therapy in RPE65-deficient dogs using recombinant adeno-associated virus 2/2 (rAAV2/2), -2/1 or -2/5 mediated delivery of the RPE65 gene. They all documented restoration of dark- and light-adapted electroretinography responses and improved psychophysical outcomes. Use of a specific RPE65 promoter and a rAAV vector that targets transgene expression specifically to the RPE may, however, provide a safer setting for the long-term therapeutic expression of RPE65. Subretinal injection of rAAV2 pseudotyped with serotype 4 (rAAV2/4) specifically targets the RPE. The purpose of our study was to evaluate a rAAV2/4 vector carrying a human RPE65cDNA driven by a human RPE65 promoter, for the ability to restore vision in RPE65-/- purebred Briard dogs and to assess the safety of gene transfer with respect to retinal morphology and function. rAAV2/4 and rAAV2/2 vectors containing similar human RPE65 promoter and cDNA cassettes were generated and administered subretinally in eight affected dogs, ages 8-30 months (n = 6 with rAAV2/4, n = 2 with rAAV2/2). Although fluorescein angiography and optical coherence tomography examinations displayed retinal abnormalities in treated retinas, electrophysiological analysis demonstrated that restoration of rod and cone photoreceptor function started as soon as 15 days post-injection, reaching maximal function at 3 months post-injection, and remaining stable thereafter in all animals treated at 8-11 months of age. As assessed by the ability of these animals to avoid obstacles in both dim and normal light, functional vision was restored in the treated eye, whereas the untreated contralateral eye served as an internal control. The dog treated at a later age (30 months) did not recover retinal function or vision, suggesting that there might be a therapeutic window for the successful treatment of RPE65-/- dogs by gene replacement therapy.

Lentiviral gene transfer of RPE65 rescues survival and function of cones in a mouse model of Leber congenital amaurosis

BACKGROUND

RPE65 is specifically expressed in the retinal pigment epithelium and is essential for the recycling of 11-cis-retinal, the chromophore of rod and cone opsins. In humans, mutations in RPE65 lead to Leber congenital amaurosis or early-onset retinal dystrophy, a severe form of retinitis pigmentosa. The proof of feasibility of gene therapy for RPE65 deficiency has already been established in a dog model of Leber congenital amaurosis, but rescue of the cone function, although crucial for human high-acuity vision, has never been strictly proven. In Rpe65 knockout mice, photoreceptors show a drastically reduced light sensitivity and are subject to degeneration, the cone photoreceptors being lost at early stages of the disease. In the present study, we address the question of whether application of a lentiviral vector expressing the Rpe65 mouse cDNA prevents cone degeneration and restores cone function in Rpe65 knockout mice.

METHODS AND FINDINGS

Subretinal injection of the vector in Rpe65-deficient mice led to sustained expression of Rpe65 in the retinal pigment epithelium. Electroretinogram recordings showed that Rpe65 gene transfer restored retinal function to a near-normal pattern. We performed histological analyses using cone-specific markers and demonstrated that Rpe65 gene transfer completely prevented cone degeneration until at least four months, an age at which almost all cones have degenerated in the untreated Rpe65-deficient mouse. We established an algorithm that allows prediction of the cone-rescue area as a function of transgene expression, which should be a useful tool for future clinical trials. Finally, in mice deficient for both RPE65 and rod transducin, Rpe65 gene transfer restored cone function when applied at an early stage of the disease.

CONCLUSIONS

By demonstrating that lentivirus-mediated Rpe65 gene transfer protects and restores the function of cones in the Rpe65(-/-) mouse, this study reinforces the therapeutic value of gene therapy for RPE65 deficiencies, suggests a cone-preserving treatment for the retina, and evaluates a potentially effective viral vector for this purpose.

Cortical visual function in the rd12 mouse model of Leber Congenital Amarousis (LCA) after gene replacement therapy to restore retinal function

One eye of rd12 mice received a sub-retinal injection of a vector carrying normal human RPE65 cDNA at post-natal day 18, and at 6- and 13-months of age. Electroretinograms (ERGs) and visual-evoked potentials (VEPs) were recorded to luminance, and to spatially and temporally modulated stimuli to assess the consequences of delayed treatment on visual pathway function. Early treatment resulted in better overall retinal rescue and better rescue of cone-mediated function. VEPs to low temporal frequency luminance modulation were well preserved at all but the oldest treatment age and corresponded to predictions based on the amount of retinal rescue. In contrast, VEPs to high frequency spatially and temporally modulated stimuli were impaired even at the earliest age. These results provide further support that early treatment in human LCA will have the most hope for optimal visual performance.

Safety in Nonhuman Primates of Ocular AAV2-RPE65, a Candidate Treatment for Blindness in Leber Congenital Amaurosis

Leber congenital amaurosis (LCA) is a molecularly heterogeneous disease group that leads to blindness. LCA caused by RPE65 mutations has been studied in animal models and vision has been restored by subretinal delivery of AAV-RPE65 vector. Human ocular gene transfer trials are being considered. Our safety studies of subretinal AAV-2/2.RPE65 in RPE65-mutant dogs showed evidence of modest photoreceptor loss in the injection region in some animals at higher vector doses. We now test the hypothesis that there can be vectorrelated toxicity to the normal monkey, with its human-like retina. Good Laboratory Practice safety studies following single intraocular injections of AAV-2/2.RPE65 in normal cynomolgus monkeys were performed for 1-week and 3-month durations. Systemic toxicity was not identified. Ocular-specific studies included clinical examinations, electroretinography, and retinal histopathology. Signs of ocular inflammation postinjection had almost disappeared by 1 week. At 3 months, electroretinography in vector-injected eyes was no different than in vehicle-injected control eyes or compared with presurgical recordings. Healed sites of retinal perforation from subretinal injections were noted clinically and by histopathology. Foveal architecture in subretinally injected eyes, vector or vehicle, could be abnormal. Morphometry of central retina showed no photoreceptor layer thickness abnormalities occurring in a dose-dependent manner. Vector sequences were present in the injected retina, vitreous, and optic nerve at 1 week but not consistently in the brain. At 3 months, there were no vector sequences in optic nerve and brain. The results allow for consideration of an upper range for no observed adverse effect level in future human trials of subretinal AAV-2/2.RPE65. The potential value of foveal treatment for LCA and other retinal degenerations warrants further research into how to achieve gene transfer without retinal injury from surgical detachment of the retina.

Quantitation of encapsidated recombinant adeno-associated virus DNA in crude cell lysates and tissue culture medium by quantitative, real-time PCR

Recombinant AAV vectors are produced by transient transfection of mammalian cells. The virus is usually purified from a combination of lysed cells and spent culture medium by HPLC. We have developed a quantitative, real-time PCR assay for quantifying encapsidated single-stranded viral DNA (i.e. DNA-containing virions) in cell lysates and the spent culture medium. This requires extensive DNaseI digestion to reduce the amount of AAV replicative DNA, as well as plasmid and cellular DNA, to negligible amounts. To demonstrate the utility of this assay, we produced recombinant AAV in HeLa cells and five different types of 293 cells. We used primers to the EGFP transgene to detect the production of a recombinant AAV. We assayed the cell lysates and media by both our quantitative PCR assay and a functional transduction assay. The quantitative PCR assay data correlated well with the transduction assay data. Because this assay only requires standard PCR primers and SYBR Green I dye to detect the amplification of the PCR template, it will readily adapt to any target DNA sequence within the recombinant AAV genome. The recombinant AAV vector does not need to express a reporter gene, such as EGFP or beta-galactosidase in order to assay the amount of virus produced.

From basic to clinical research: a journey with the retina, the retinal pigment epithelium, the cornea, age-related macular degeneration and hereditary degenerations, as seen in the rear view mirror

PURPOSE

This Acta Ophthalmologica Award and Gold Medal Honorary Lecture (the Lundsgaard Gold Medal Honorary Lecture) reviews some of the work I have carried out with my mentors and many of my wonderful collaborators and research students over more than 40 years, also including related work by other groups. It concentrates on the basic electrophysiology and ultrastructure of the retina and the retinal pigment epithelium (RPE), as well as covering basic and clinical aspects of the cornea, contact lenses, age-related macular degeneration (AMD) and hereditary diseases.

METHODS

The review describes research performed using light and electron microscopy, basic and clinical electrophysiology, genetics and biochemistry in animal experiments and in research on patients. It also outlines clinically used techniques, such as laser and photodynamic treatment and scanning laser ophthalmoscopy.

RESULTS

The paper reports on the following subjects: the mechanisms behind some of the electrical potentials originating in the retina and the RPE and the use of these potentials in hereditary diseases; corneal receptors for lectins and presumably for bacteria; the turnover of the photoreceptor outer segment and the formation of lipofuscin, including the relation of these processes to AMD; certain treatments for AMD, and hereditary degenerations in animal models, such as the RPE65 gene mutation in Briard dogs, which makes them a model of Leber's congenital amaurosis. The dogs are now treated successfully with gene therapy in the USA, and a clinical trial is in preparation.

CONCLUSIONS

During the last 40 years we have had the good fortune to experience a dramatic growth in knowledge and understanding within ophthalmic science of basic mechanisms. Huge progress has been made in diagnostics and clinical ophthalmological treatments, much to the benefit of our patients. Even a small contribution made by my group to these developments has been well worth the effort, particularly as scientific work is not just deeply satisfying: it is also fun!

Gene therapy progress and prospects: the eye

The eye has unique advantages as a target organ for gene therapy of both inherited and acquired ocular disorders and offers a valuable model system for gene therapy. The eye is readily accessible to phenotypic examination and investigation of therapeutic effects in vivo by fundus imaging and electrophysiological techniques. Considerable progress has been made in the development of gene replacement therapies for retinal degenerations resulting from gene defects in photoreceptor cells (rds, RPGRIP, RS-1) and in retinal pigment epithelial cells (MerTK, RPE65, OA1) using recombinant adeno-associated virus and lentivirus-based vectors. Gene therapy also offers a potentially powerful approach to the treatment of complex acquired disorders such as those involving angiogenesis, inflammation and degeneration, by the targeted sustained intraocular delivery of therapeutic proteins. Proposals for clinical trials of gene therapy for early-onset retinal degeneration owing to defects in the gene encoding the visual cycle protein RPE65 have recently received ethical approval.

Lentiviral expression of retinal guanylate cyclase-1 (RetGC1) restores vision in an avian model of childhood blindness

BACKGROUND

Leber congenital amaurosis (LCA) is a genetically heterogeneous group of retinal diseases that cause congenital blindness in infants and children. Mutations in the GUCY2D gene that encodes retinal guanylate cyclase-1 (retGC1) were the first to be linked to this disease group (LCA type 1 [LCA1]) and account for 10%-20% of LCA cases. These mutations disrupt synthesis of cGMP in photoreceptor cells, a key second messenger required for function of these cells. The GUCY1*B chicken, which carries a null mutation in the retGC1 gene, is blind at hatching and serves as an animal model for the study of LCA1 pathology and potential treatments in humans.

METHODS AND FINDINGS

A lentivirus-based gene transfer vector carrying the GUCY2D gene was developed and injected into early-stage GUCY1*B embryos to determine if photoreceptor function and sight could be restored to these animals. Like human LCA1, the avian disease shows early-onset blindness, but there is a window of opportunity for intervention. In both diseases there is a period of photoreceptor cell dysfunction that precedes retinal degeneration. Of seven treated animals, six exhibited sight as evidenced by robust optokinetic and volitional visual behaviors. Electroretinographic responses, absent in untreated animals, were partially restored in treated animals. Morphological analyses indicated there was slowing of the retinal degeneration.

CONCLUSIONS

Blindness associated with loss of function of retGC1 in the GUCY1*B avian model of LCA1 can be reversed using viral vector-mediated gene transfer. Furthermore, this reversal can be achieved by restoring function to a relatively low percentage of retinal photoreceptors. These results represent a first step toward development of gene therapies for one of the more common forms of childhood blindness.

Safety of recombinant adeno-associated virus type 2-RPE65 vector delivered by ocular subretinal injection

AAV2 delivery of the RPE65 gene to the retina of blind RPE65-deficient animals restores vision. This strategy is being considered for human trials in RPE65-associated Leber congenital amaurosis (LCA), but toxicity and dose efficacy have not been defined. We studied ocular delivery of AAV-2/2.RPE65 in RPE65-mutant dogs. There was no systemic toxicity. Ocular examinations showed mild or moderate inflammation that resolved over 3 months. Retinal histopathology indicated that traumatic lesions from the injection were common, but thinning within the injection region occurred only at the two highest vector doses. Biodistribution studies at 3 months postinjection showed no vector in optic nerve or visual centers in the brain and only isolated non-dose-related detection in other organs. We also performed biodistribution studies in normal rats at about 2 weeks and 2 months postinjection and vector was not widespread outside the injected eye. Dose-response results in RPE65-mutant dogs indicated that the highest 1.5-log unit range of vector doses proved efficacious. The efficacy and toxicity limits defined in this study lead to suggestions for the design of a subretinal AAV-2/2.RPE65 human trial of RPE65-associated LCA.

Controlled and localized genetic manipulation in the brain

Brain structure and function are determined in part through experience and in part through our inherited genes. A powerful approach for unravelling the balance between activity-dependent neuronal plasticity and genetic programs is to directly manipulate the genome. Such molecular genetic studies have been greatly aided by the remarkable progress of large-scale genome sequencing efforts. Sophisticated mouse genetic manipulations allow targeted point-mutations, deletions and additions to the mouse genome. These can be regulated through inducible promoters expressing in genetically specified neuronal cell types. However, despite significant progress it remains difficult to target specific brain regions through transgenesis alone. Recent work suggests that transduction vectors, like lentiviruses and adeno-associated viruses, may provide suitable additional tools for localized and controlled genetic manipulation. Furthermore, studies with such vectors may aid the development of human genetic therapies for brain diseases.