Amelioration of both functional and morphological abnormalities in the retina of a mouse model of ocular albinism following AAV-mediated gene transfer

Management of albinism: French guidelines for diagnosis and care

Albinism is a worldwide genetic disorder caused by mutations in at least 20 genes, identified to date, that affect melanin production or transport in the skin, hair and eyes. Patients present with variable degrees of diffuse muco-cutaneous and adnexal hypopigmentation, as well as ocular features including nystagmus, misrouting of optic nerves and foveal hypoplasia. Less often, albinism is associated with blood, immunological, pulmonary, digestive and/or neurological anomalies. Clinical and molecular characterizations are essential in preventing potential complications. Disease-causing mutations remain unknown for about 25% of patients with albinism. These guidelines have been developed for the diagnosis and management of syndromic and non-syndromic forms of albinism, based on a systematic review of the scientific literature. These guidelines comprise clinical and molecular characterization, diagnosis, therapeutic approach and management.

Novel Biallelic Variants and Phenotypic Features in Patients with SLC38A8-Related Foveal Hypoplasia

Biallelic pathogenic variants in solute carrier family 38 member 8, SLC38A8, cause a pan-ocular autosomal recessive condition known as foveal hypoplasia 2, FVH2, characterised by foveal hypoplasia, nystagmus and optic nerve chiasmal misrouting. Patients are often clinically diagnosed with ocular albinism, but foveal hypoplasia can occur in several other ocular disorders. Here we describe nine patients from seven families who had molecularly confirmed biallelic recessive variants in SLC38A8 identified through whole genome sequencing or targeted gene panel testing. We identified four novel sequence variants (p.(Tyr88*), p.(Trp145*), p.(Glu233Gly) and c.632+1G>A). All patients presented with foveal hypoplasia, nystagmus and reduced visual acuity; however, one patient did not exhibit any signs of chiasmal misrouting, and three patients had features of anterior segment dysgenesis. We highlight these findings in the context of 30 other families reported to date. This study reinforces the importance of obtaining a molecular diagnosis in patients whose phenotype overlap with other inherited ocular conditions, in order to support genetic counselling, clinical prognosis and family planning. We expand the spectrum of SLC38A8 mutations which will be relevant for treatment through future genetic-based therapies.

Current and emerging treatments for albinism

Albinism is a group of rare inherited disorders arising from impairment of melanin biosynthesis. The reduction of melanin synthesis leads to hypopigmentation of the skin and eyes. A wide range of ophthalmic manifestations arise from albinism, including reduction of visual acuity, nystagmus, strabismus, iris translucency, foveal hypoplasia, fundus hypopigmentation, and abnormal decussation of retinal ganglion cell axons at the optic chiasm. Currently, albinism is incurable, and treatment aims either surgically or pharmacologically to optimize vision and protect the skin; however, novel therapies that aim to directly address the molecular errors of albinism, such as l-dihydroxyphenylalanine and nitisinone, are being developed and have entered human trials though with limited success. Experimental gene-based strategies for editing the genetic errors in albinism have also met early success in animal models. The emergence of these new therapeutic modalities represents a new era in the management of albinism. We focus on the known genetic subtypes, clinical assessment, and existing and emerging therapeutic options for the nonsyndromic forms of albinism.

miR-181a/b downregulation exerts a protective action on mitochondrial disease models

Mitochondrial diseases (MDs) are a heterogeneous group of devastating and often fatal disorders due to defective oxidative phosphorylation. Despite the recent advances in mitochondrial medicine, effective therapies are still not available for these conditions. Here, we demonstrate that the microRNAs miR-181a and miR-181b (miR-181a/b) regulate key genes involved in mitochondrial biogenesis and function and that downregulation of these miRNAs enhances mitochondrial turnover in the retina through the coordinated activation of mitochondrial biogenesis and mitophagy. We thus tested the effect of miR-181a/b inactivation in different animal models of MDs, such as microphthalmia with linear skin lesions and Leber's hereditary optic neuropathy. We found that miR-181a/b downregulation strongly protects retinal neurons from cell death and significantly ameliorates the disease phenotype in all tested models. Altogether, our results demonstrate that miR-181a/b regulate mitochondrial homeostasis and that these miRNAs may be effective gene-independent therapeutic targets for MDs characterized by neuronal degeneration.

A novel iris transillumination grading scale allowing flexible assessment with quantitative image analysis and visual matching

PURPOSE

To develop a sensitive scale of iris transillumination suitable for clinical and research use, with the capability of either quantitative analysis or visual matching of images.

METHODS

Iris transillumination photographic images were used from 70 study subjects with ocular or oculocutaneous albinism. Subjects represented a broad range of ocular pigmentation. A subset of images was subjected to image analysis and ranking by both expert and nonexpert reviewers. Quantitative ordering of images was compared with ordering by visual inspection. Images were binned to establish an 8-point scale. Ranking consistency was evaluated using the Kendall rank correlation coefficient (Kendall's tau). Visual ranking results were assessed using Kendall's coefficient of concordance (Kendall's W) analysis.

RESULTS

There was a high degree of correlation among the image analysis, expert-based and non-expert-based image rankings. Pairwise comparisons of the quantitative ranking with each reviewer generated an average Kendall's tau of 0.83 ± 0.04 (SD). Inter-rater correlation was also high with Kendall's W of 0.96, 0.95, and 0.95 for nonexpert, expert, and all reviewers, respectively.

CONCLUSIONS

The current standard for assessing iris transillumination is expert assessment of clinical exam findings. We adapted an image-analysis technique to generate quantitative transillumination values. Quantitative ranking was shown to be highly similar to a ranking produced by both expert and nonexpert reviewers. This finding suggests that the image characteristics used to quantify iris transillumination do not require expert interpretation. Inter-rater rankings were also highly similar, suggesting that varied methods of transillumination ranking are robust in terms of producing reproducible results.

Rhodopsin targeted transcriptional silencing by DNA-binding

Transcription factors (TFs) operate by the combined activity of their DNA-binding domains (DBDs) and effector domains (EDs) enabling the coordination of gene expression on a genomic scale. Here we show that in vivo delivery of an engineered DNA-binding protein uncoupled from the repressor domain can produce efficient and gene-specific transcriptional silencing. To interfere with RHODOPSIN (RHO) gain-of-function mutations we engineered the ZF6-DNA-binding protein (ZF6-DB) that targets 20 base pairs (bp) of a RHOcis-regulatory element (CRE) and demonstrate Rho specific transcriptional silencing upon adeno-associated viral (AAV) vector-mediated expression in photoreceptors. The data show that the 20 bp-long genomic DNA sequence is necessary for RHO expression and that photoreceptor delivery of the corresponding cognate synthetic trans-acting factor ZF6-DB without the intrinsic transcriptional repression properties of the canonical ED blocks Rho expression with negligible genome-wide transcript perturbations. The data support DNA-binding-mediated silencing as a novel mode to treat gain-of-function mutations.

DOI:http://dx.doi.org/10.7554/eLife.12242.001

Proteins called transcription factors bind to sections of DNA known as regulatory elements to activate or deactivate nearby genes. In animals, transcription factors typically have two sections: a “DNA-binding domain” that attaches to DNA, and an “effector domain” that is responsible for interacting with other proteins to regulate the gene’s expression.

Rhodopsin is a gene that encodes the instructions needed to make a light-sensitive protein in the eyes of humans and other animals. Botta et al. have now used this gene as an example to investigate whether proteins that contain a DNA-binding domain – but not an effector domain – can repress gene expression.

The experiments show that only a small section of the regulatory elements in the human Rhodopsin gene is actually required for the gene to be expressed. Botta et al. designed an artificial protein – referred to as ZF6-DB – that is able to bind to this section of DNA. The binding of ZF6-DB to this short DNA section was sufficient to switch off a Rhodopsin gene in living pig cells, and, unlike conventional transcription factors, seemed to have minimal impact other genes.

Next, Botta et al. used a virus to insert both the gene that encodes ZF6-DB and a normal copy of Rhodopsin into pigs. In these animals, ZF6-DB switched off the existing copy of Rhodopsin, but not the inserted copy so the cells produced a working form of the light-sensitive protein. Further experiments were carried out in mice that have both a faulty version and a normal copy of the Rhodopsin gene. ZF6-DB switched off the faulty Rhodopsin gene, which allowed the normal Rhodopsin gene to work without any interference from the faulty copy.

Mutations in Rhodopsin can cause an eye disease that leads to severe loss of vision in humans. These new findings could now guide future efforts to develop treatments for people with this condition. It will also be important to investigate how ZF6-DB binds to the regulatory elements in the Rhodopsin gene and whether a similar strategy could be used to alter the expression of other genes.

DOI:http://dx.doi.org/10.7554/eLife.12242.002

Conjunctivally Applied BDNF Protects Photoreceptors from Light-Induced Damage

PURPOSE

To test whether the topical eye treatment with BDNF prevents the effects of continuous light exposure (LE) in the albino rat retina.

METHODS

Two groups of albino rats were used. The first group of rats received an intraocular injection of BDNF (2 μL, 1 μg/μL) before LE, while the second group was treated with one single drop of BDNF (10 μL, 12 μg/μL) dissolved in different types of solutions (physiological solution, the polysaccharide fraction of Tamarind gum, TSP, and sodium carboxy methyl cellulose), at the level of conjunctival fornix before LE. The level of BDNF in the retina and optic nerve was determined by enzyme-linked immunosorbent assay. We recorded the flash electroretinogram (fERG) in dark adapted rats 1 week after LE. At the end of the recording session, the retinas were removed and labeled so that the number of photoreceptors nuclear rows and thickness of the outer nuclear layer was analyzed.

RESULTS

Intravitreal injection of BDNF before LE prevented fERG impairment. Different ophthalmic preparations were used for topical eye application; the TSP resulted the most suitable vehicle to increase BDNF level in the retina and optic nerve. Topical eye application with BDNF/TSP before LE partially preserved both fERG response and photoreceptors.

CONCLUSIONS

Topical eye treatment with BDNF represents a suitable, noninvasive tool to increase the retinal content of BDNF up to a level capable of exerting neuroprotection toward photoreceptors injured by prolonged LE.

TRANSLATIONAL RELEVANCE

A collyrium containing BDNF may serve as an effective, clinically translational treatment against retinal degeneration.

Mutational analysis of oculocutaneous albinism: a compact review

Oculocutaneous albinism (OCA) is an autosomal recessive disorder caused by either complete lack of or a reduction of melanin biosynthesis in the melanocytes. The OCA1A is the most severe type with a complete lack of melanin production throughout life, while the milder forms OCA1B, OCA2, OCA3, and OCA4 show some pigment accumulation over time. Mutations in TYR, OCA2, TYRP1, and SLC45A2 are mainly responsible for causing oculocutaneous albinism. Recently, two new genes SLC24A5 and C10orf11 are identified that are responsible to cause OCA6 and OCA7, respectively. Also a locus has been mapped to the human chromosome 4q24 region which is responsible for genetic cause of OCA5. In this paper, we summarized the clinical and molecular features of OCA genes. Further, we reviewed the screening of pathological mutations of OCA genes and its molecular mechanism of the protein upon mutation by in silico approach. We also reviewed TYR (T373K, N371Y, M370T, and P313R), OCA2 (R305W), TYRP1 (R326H and R356Q) mutations and their structural consequences at molecular level. It is observed that the pathological genetic mutations and their structural and functional significance of OCA genes will aid in development of personalized medicine for albinism patients.

Gene therapies for inherited retinal disorders

Significant advances have been made over the last decade or two in the elucidation of the molecular pathogenesis of inherited ocular disorders. In particular, remarkable successes have been achieved in exploration of gene-based medicines for these conditions, both in preclinical and in clinical studies. Progress in the development of gene therapies targeted toward correcting the primary genetic defect or focused on modulating secondary effects associated with retinal pathologies are discussed in the review. Likewise, the recent utilization of genes encoding light-sensing molecules to provide new functions to residual retinal cells in the degenerating retina is discussed. While a great deal has been learned over the last two decades, the next decade should result in an increasing number of preclinical studies progressing to human clinical trial, an exciting prospect for patients, those active in research and development and bystanders alike.

Myosin7a deficiency results in reduced retinal activity which is improved by gene therapy

Mutations in MYO7A cause autosomal recessive Usher syndrome type IB (USH1B), one of the most frequent conditions that combine severe congenital hearing impairment and retinitis pigmentosa. A promising therapeutic strategy for retinitis pigmentosa is gene therapy, however its pre-clinical development is limited by the mild retinal phenotype of the shaker1 (sh1^−/−) murine model of USH1B which lacks both retinal functional abnormalities and degeneration. Here we report a significant, early-onset delay of sh1^−/− photoreceptor ability to recover from light desensitization as well as a progressive reduction of both b-wave electroretinogram amplitude and light sensitivity, in the absence of significant loss of photoreceptors up to 12 months of age. We additionally show that subretinal delivery to the sh1^−/− retina of AAV vectors encoding the large MYO7A protein results in significant improvement of sh1^−/− photoreceptor and retinal pigment epithelium ultrastructural anomalies which is associated with improvement of recovery from light desensitization. These findings provide new tools to evaluate the efficacy of experimental therapies for USH1B. In addition, although AAV vectors expressing large genes might have limited clinical applications due to their genome heterogeneity, our data show that AAV-mediated MYO7A gene transfer to the sh1^−/− retina is effective.

Gene therapy of inherited retinopathies: a long and successful road from viral vectors to patients

Inherited retinopathies (IRs) are common and untreatable blinding conditions inherited mostly as monogenic due to mutations in genes expressed in retinal photoreceptors (PRs) and in retinal pigment epithelium (RPE). Over the last two decades, the retina has emerged as one of the most favorable target tissues for gene therapy given its small size and its enclosed and immune-privileged environment. Different types of viral vectors have been developed, especially those based on the adeno-associated virus (AAV), which efficiently deliver therapeutic genes to PRs or RPE upon subretinal injections. Dozens of successful proofs of concept of the efficacy of gene therapy for recessive and dominant IRs have been generated in small and large models that have paved the way to the first clinical trials using AAV in patients with Leber congenital amaurosis, a severe form of childhood blindness. The results from these initial trials suggest that retinal gene therapy with AAV is safe in humans, that vision can be improved in patients that have suffered from severe impairment of visual function, in some cases for decades, and that readministration of AAV to the subretinal space is feasible, effective, and safe. However, none of the trials could match the levels of efficacy of gene therapy observed in a dog model of the disease, suggesting that there is room for improvement. In conclusion, these results bode well for further testing of AAV-mediated retinal gene therapy in patients with other monogenic and complex forms of blindness.

Gene delivery to the retina: from mouse to man

With the recent progress in identifying disease-causing genes in humans and in animal models, there are more and more opportunities for using retinal gene transfer to learn more about retinal physiology and also to develop therapies for blinding disorders. Success in preclinical studies for one form of inherited blindness have led to testing in human clinical trials. This paves the way to consider a number of other retinal diseases as ultimate gene therapy targets in human studies. The information presented here is designed to assist scientists and clinicians to use gene transfer to probe the biology of the retina and/or to move appropriate gene-based treatment studies from the bench to the clinic.

Molecular and clinical characterization of albinism in a large cohort of Italian patients

PURPOSE

The purpose of this study was to identify the molecular basis of albinism in a large cohort of Italian patients showing typical ocular landmarks of the disease and to provide a full characterization of the clinical ophthalmic manifestations.

METHODS

DNA samples from 45 patients with ocular manifestations of albinism were analyzed by direct sequencing analysis of five genes responsible for albinism: TYR, P, TYRP1, SLC45A2 (MATP), and OA1. All patients studied showed a variable degree of skin and hair hypopigmentation. Eighteen patients with distinct mutations in each gene associated with OCA were evaluated by detailed ophthalmic analysis, optical coherence tomography (OCT), and fundus autofluorescence.

RESULTS

Disease-causing mutations were identified in more than 95% of analyzed patients with OCA (28/45 [62.2%] cases with two or more mutations; 15/45 [33.3%] cases with one mutation). Thirty-five different mutant alleles were identified of which 15 were novel. Mutations in TYR were the most frequent (73.3%), whereas mutations in P occurred more rarely (13.3%) than previously reported. Novel mutations were also identified in rare loci such as TYRP1 and MATP. Mutations in the OA1 gene were not detected. Clinical assessment revealed that patients with iris and macular pigmentation had significantly higher visual acuity than did severe hypopigmented phenotypes.

CONCLUSIONS

TYR gene mutations represent a relevant cause of oculocutaneous albinism in Italy, whereas mutations in P present a lower frequency than that found in other populations. Clinical analysis revealed that the severity of the ocular manifestations depends on the degree of retinal pigmentation.

Gene delivery in the equine cornea: a novel therapeutic strategy

OBJECTIVE

To determine if hybrid adeno-associated virus serotype 2/5 (AAV5) vector can effectively deliver foreign genes into the equine cornea without causing adverse side effects. The aims of this study were to: (i) evaluate efficacy of AAV5 to deliver therapeutic genes into equine corneal fibroblasts (ECFs) using enhanced green fluorescent protein (EGFP) marker gene, and (ii) establish the safety of AAV5 vector for equine corneal gene therapy.

MATERIAL

Primary ECF cultures were harvested from healthy donor equine corneas. Cultures were maintained at 37°C in humidified atmosphere with 5% CO(2).

PROCEDURE

AAV5 vector expressing EGFP under control of hybrid cytomegalovirus + chicken β-actin promoter was applied topically to ECF. Expression of delivered EGFP gene in ECF was quantified using fluorescent microscopy. Using fluorescent staining, the total number of cells and transduction efficiency of tested AAV vector was determined. Phase contrast microscopy, trypan blue and TUNEL assays were used to determine toxicity and safety of AAV5 for ECFs.

RESULTS

Topical AAV5 application successfully transduced significant numbers of ECFs. Transduction efficiency was 13.1%. Tested AAV5 vector did not cause phenotype change or significant cell death and cell viability was maintained.

CONCLUSIONS

Tested AAV5 vector is effective and safe for gene therapy in ECFs in vitro.

Recent advances in ocular gene therapy

PURPOSE OF REVIEW

Ocular gene therapy has made significant advances due to improvements in viral vector delivery methods. Recent clinical trials for the treatment of Leber's congenital amaurosis have provided evidence for successful in-vivo gene therapy in humans.

RECENT FINDINGS

Gene therapy for ocular disease has been under investigation just over 15 years. Recently, the first human gene therapy trials for retinal degeneration were undertaken with encouraging preliminary safety and efficacy findings. Building on previous success in reversing blindness in animal models of Leber's congenital amaurosis, several groups proceeded with adeno-associated virus-mediated gene replacement. Many of the humans demonstrated increases in light sensitivity and in visual acuity. Subjective improvements in vision were corroborated in some cases by objective tests such as pupillary light response and nystagmography. Although much of the work in ocular gene therapy has involved retinal applications, significant progress has been seen in other aspects of ophthalmology.

SUMMARY

Ongoing human clinical trials support the safety and efficacy of adeno-associated virus-mediated gene therapy for retinal disease. These and other studies will establish the foundation for methodology to treat additional ocular diseases using gene therapy strategies.

AAV-mediated tyrosinase gene transfer restores melanogenesis and retinal function in a model of oculo-cutaneous albinism type I (OCA1)

Oculo-cutaneous albinism type 1 (OCA1) is characterized by congenital hypopigmentation and is due to mutations in the TYROSINASE gene (TYR). In this study, we have characterized the morpho-functional consequences of the lack of tyrosinase activity in the spontaneous null mouse model of OCA1 (Tyr(c-2j)). Here, we show that adult Tyr(c-2j) mice have several retinal functional anomalies associated with photoreceptor loss. To test whether these anomalies are reversible upon TYR complementation, we performed intraocular administration of an adeno-associated virus (AAV)-based vector, encoding the human TYR gene, in adult Tyr(c-2j) mice. This resulted in melanosome biogenesis and ex novo synthesis of melanin in both neuroectodermally derived retinal pigment epithelium (RPE) and in neural crest-derived choroid and iris melanocytes. Ocular melanin accumulation prevented progressive photoreceptor degeneration and resulted in restoration of retinal function. Our results reveal novel properties of pigment cells and show that the developmental anomalies of albino mice are associated with defects occurring in postnatal life, adding novel insights on OCA1 disease pathogenesis. In addition, we provide proof-of-principle of an effective gene-based strategy relevant for future application in albino patients.

Retinal function in X-linked ocular albinism (OA1)

PURPOSE

To characterize retinal function in human recessive X-linked ocular albinism (OA1) across the normal lifespan.

METHODS

Retinal function was evaluated in 14 OA1 patients (ages 11 to 71 years) and five obligate carriers (ages 41 to 50 years) and compared to normal controls using full-field and multi-focal electroretinograms (ERG and mERG, respectively) and electro-oculography (EOG).

RESULTS

No consistent differences in ERG response parameters were observed when OA1 patients were compared as a group to normal controls. A trend in the direction of better correlations of response parameters with age was, however, observed in OA1. EOG Arden ratios were normal or hypernormal for all patients, but were uncorrelated with age. Central retinal function measured with the mERG suggested a flat response topography with depressed macular function compared to normal controls.

CONCLUSIONS

Panretinal function in OA1 is within normal limits at all ages, consistent with previous reports in generalized albinism. The stronger correlations with age in OA1 may suggest a different rate of age-related change in OA1 compared to normal populations, but the precise nature of this change must await an appropriate prospective study. The topography of mERG amplitudes in OA1 is relatively flat across the central retina with a reduction in amplitude in the macular region consistent with anatomical studies demonstrating an underdeveloped macular region in albinism.

AAV-mediated gene therapy for retinal disorders: from mouse to man

A wide range of retinal disorders can potentially be treated using viral vector-mediated gene therapy. The most widely used vectors for ocular gene delivery are based on adeno-associated virus (AAV), because they elicit minimal immune responses and mediate long-term transgene expression in a variety of retinal cell types. Proof-of-concept experiments have demonstrated the efficacy of AAV-mediated transgene delivery in a number of animal models of inherited and acquired retinal disorders. Following extensive preclinical evaluation in large animal models, gene therapy for one form of inherited retinal degeneration due to RPE65 deficiency is now being tested in three concurrent clinical trials. Here, we review different approaches for treating inherited retinal degenerations and more common acquired retinal disorders using AAV-based vectors.

Involvement of OA1, an intracellular GPCR, and G alpha i3, its binding protein, in melanosomal biogenesis and optic pathway formation

PURPOSE

Ocular albinism type 1 (OA1) is characterized by abnormalities in retinal pigment epithelium (RPE) melanosomes and misrouting of optic axons. The OA1 gene encodes a G-protein-coupled receptor (GPCR) that coimmunoprecipitates with the G alpha i-subunit of heterotrimeric G-proteins from human melanocyte extracts. This study was undertaken to test whether one of the G alpha i proteins, G alpha i3, signals in the same pathway as OA1 to regulate melanosome biogenesis and axonal growth through the optic chiasm.

METHODS

Adult G alpha i3(-/-) and Oa1(-/-) mice were compared with their respective control mice (129Sv and B6/NCrl) to study the effects of the loss of G alpha i3 or Oa1 function. Light and electron microscopy were used to analyze the morphology of the retina and the size and density of RPE melanosomes, electroretinograms to study retinal function, and retrograde labeling to investigate the size of the uncrossed optic pathway.

RESULTS

Although G alpha i3(-/-) and Oa1(-/-) photoreceptors were comparable to those of the corresponding control retinas, the density of their RPE melanosomes was significantly lower than in control RPEs. In addition, the RPE cells of G alpha i3(-/-) and Oa1(-/-) mice showed abnormal melanosomes that were far larger than the largest 129Sv and B6/NCrl melanosomes, respectively. Although G alpha i3(-/-) and Oa1(-/-) mice had normal results on electroretinography, retrograde labeling showed a significant reduction from control in the size of their ipsilateral retinofugal projections.

CONCLUSIONS

These results indicate that G alpha i3, like Oa1, plays an important role in melanosome biogenesis. Furthermore, they suggest a common Oa1-G alpha i3 signaling pathway that ultimately affects axonal growth through the optic chiasm.

Novel adeno-associated virus serotypes efficiently transduce murine photoreceptors

Severe inherited retinal diseases, such as retinitis pigmentosa and Leber congenital amaurosis, are caused by mutations in genes preferentially expressed in photoreceptors. While adeno-associated virus (AAV)-mediated gene transfer can correct retinal pigment epithelium (RPE) defects in animal models, approaches for the correction of photoreceptor-specific diseases are less efficient. We evaluated the ability of novel AAV serotypes (AAV2/7, AAV2/8, AAV2/9, AAV2rh.43, AAV2rh.64R1, and AAV2hu.29R) in combination with constitutive or photoreceptor-specific promoters to improve photoreceptor transduction, a limiting step in photoreceptor rescue. Based on a qualitative analysis, all AAV serotypes tested efficiently transduce the RPE as well as rod and cone photoreceptors after subretinal administration in mice. Interestingly, AAV2/9 efficiently transduces Müller cells. To compare photoreceptor transduction from different AAVs and promoters in both a qualitative and quantitative manner, we designed a strategy based on the use of a bicistronic construct expressing both enhanced green fluorescent protein and luciferase. We found that AAV2/8 and AAV2/7 mediate six- to eightfold higher levels of in vivo photoreceptor transduction than AAV2/5, considered so far the most efficient AAV serotype for photoreceptor targeting. In addition, following subretinal administration of AAV, the rhodopsin promoter allows significantly higher levels of photoreceptor expression than the other ubiquitous or photoreceptor-specific promoters tested. Finally, we show that AAV2/7, AAV2/8, and AAV2/9 outperform AAV2/5 following ex vivo transduction of retinal progenitor cells differentiated into photoreceptors. We conclude that AAV2/7 or AAV2/8 and the rhodopsin promoter provide the highest levels of photoreceptor transduction both in and ex vivo and that this may overcome the limitation to therapeutic success observed so far in models of inherited severe photoreceptor diseases.

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.

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.