Pharmacological and rAAV gene therapy rescue of visual functions in a blind mouse model of Leber congenital amaurosis

A homozygous frameshift mutation in LRAT causes retinitis punctata albescens

PURPOSE

To determine the genetic defect and to describe the clinical characteristics in patients with retinitis punctata albescens (RPA) and fundus albipunctatus (FAP).

DESIGN

Case series/observational study.

PARTICIPANTS

We included 13 patients affected by RPA or FAP.

METHODS

Thirteen patients were collected from 8 families with a retinal dystrophy characterized by tiny, yellow-white dots on funduscopy, typical for FAP or RPA. All patients underwent full ophthalmologic examinations, including visual field assessment. Fundus photography, and electroretinography were performed in 12 patients, and optical coherence tomography and fundus autofluorescence were performed in 4 patients. DNA samples of all patients were screened for mutations in RLBP1 and for mutations in RDH5 in patients who did not carry mutations in RLBP1. DNA samples of 2 sibling pairs of nonconsanguineous families who carried mutations neither in RLBP1 nor in RDH5 were analyzed by genome-wide homozygosity mapping. Sequence analysis was performed of LRAT, a candidate gene in a shared homozygous region.

MAIN OUTCOME MEASURES

We assessed DNA sequence variants, best-corrected visual acuity, fundus appearance, visual field measurements, electroretinogram responses, optical coherence tomography, and fundus autofluorescence.

RESULTS

A homozygous frameshift mutation was identified in LRAT in 4 patients with RPA. Mutations in RLBP1 were identified in 7 patients with RPA and in 1 patient with FAP and cone dystrophy. One patient had compound heterozygous mutations in RDH5 and suffered from FAP with mild maculopathy.

CONCLUSIONS

A genetic defect was identified in LRAT as a novel cause of RPA. LRAT is therefore the fourth gene involved in the visual cycle that may cause a white-dot retinopathy. We also revealed that mutations in RLBP1 may lead to FAP with cone dystrophy.

Hepatic metabolism of retinoids and disease associations

The liver is the most important tissue site in the body for uptake of postprandial retinoid, as well as for retinoid storage. Within the liver, both hepatocytes and hepatic stellate cells (HSCs) are importantly involved in retinoid metabolism. Hepatocytes play an indispensable role in uptake and processing of dietary retinoid into the liver, and in synthesis and secretion of retinol-binding protein (RBP), which is required for mobilizing hepatic retinoid stores. HSCs are the central cellular site for retinoid storage in the healthy animal, accounting for as much as 50-60% of the total retinoid present in the entire body. The liver is also an important target organ for retinoid actions. Retinoic acid is synthesized in the liver and can interact with retinoid receptors which control expression of a large number of genes involved in hepatic processes. Altered retinoid metabolism and the accompanying dysregulation of retinoid signaling in the liver contribute to hepatic disease. This is related to HSCs, which contribute significantly to the development of hepatic disease when they undergo a process of cellular activation. HSC activation results in the loss of HSC retinoid stores and changes in extracellular matrix deposition leading to the onset of liver fibrosis. An association between hepatic disease progression and decreased hepatic retinoid storage has been demonstrated. In this review article, we summarize the essential role of the liver in retinoid metabolism and consider briefly associations between hepatic retinoid metabolism and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

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.

Subretinal delivery and electroporation in pigmented and nonpigmented adult mouse eyes

Subretinal injection offers one of the best ways to deliver many classes of drugs, reagents, cells and treatments to the photoreceptor, Müller, and retinal pigment epithelium (RPE) cells of the retina. Agents delivered to this space are placed within microns of the intended target cell, accumulating to high concentrations because there is no dilution due to transport processes or diffusion. Dilution in the interphotoreceptor space (IPS) is minimal because the IPS volume is only 10-20 μl in the human eye and less than 1 μl in the mouse eye. For gene delivery purposes, we wished to transfect the cells adjacent to the IPS in adult mouse eyes. Others transfect these cells in neonatal rats to study the development of the retina. In both neonates and adults, electroporation is found to be effective. Here we describe the optimization of electroporation conditions for RPE cells in the adult mouse eye with naked plasmids. However, both techniques, subretinal injection and electroporation, present some technical challenges that require skill on the part of the surgeon to prevent untoward damage to the eye. Here we describe methods that we have used for the past 10 years (Johnson et al. Mol Vis 14: 2211-2226, 2008).

Chemistry and biology of vision

Visual perception in humans occurs through absorption of electromagnetic radiation from 400 to 780 nm by photoreceptors in the retina. A photon of visible light carries a sufficient amount of energy to cause, when absorbed, a cis,trans-geometric isomerization of the 11-cis-retinal chromophore, a vitamin A derivative bound to rhodopsin and cone opsins of retinal photoreceptors. The unique biochemistry of these complexes allows us to reliably and reproducibly collect continuous visual information about our environment. Moreover, other nonconventional retinal opsins such as the circadian rhythm regulator melanopsin also initiate light-activated signaling based on similar photochemistry.

Dietary 9-cis-β,β-carotene fails to rescue vision in mouse models of leber congenital amaurosis

Synthetic 9-cis-stereoisomers of vitamin A (all-trans-retinol) are especially promising agents for the fight against blinding diseases. Several studies suggested that 9-cis-β,β-carotene (9-cis-BC), a natural and abundant β-carotene isomer in the diet, could be the precursor of 9-cis-retinoids and thus could have therapeutic applications. Here we showed that 9-cis-BC is metabolized both in vitro and in vivo by two types of mouse carotenoid oxygenases, β,β-Carotene monooxygenase 1 (BCMO1), and β,β-carotene dioxygenase 2 (BCDO2). In the symmetric oxidative cleavage reaction at C15,C15' position by BCMO1, part of the 9-cis-double bond was isomerized to the all-trans-stereoisomer, yielding all-trans-retinal and 9-cis-retinal in a molar ratio of 3:1. The asymmetric cleaving enzyme BCDO2 preferentially removed the 9-cis-ring site at the C9,C10 double bond from this substrate, providing an all-trans-β-10'-apocarotenal product that can be further metabolized to all-trans-retinal by BCMO1. Studies in knockout mouse models confirmed that each carotenoid oxygenase can metabolize 9-cis-BC. Therefore, treatment of mouse models of Leber congenital amaurosis with 9-cis-BC and 9-cis-retinyl-acetate, a well established 9-cis-retinal precursor, showed that the cis-carotenoid was far less effective than the cis-retinoid in rescuing vision. Thus, our in vitro and in vivo studies revealed that 9-cis-BC is not a major source for mouse 9-cis-retinoid production but is mainly converted to all-trans-retinoids to support canonical vitamin A action.

Clinical gene therapy for the treatment of RPE65-associated Leber congenital amaurosis

INTRODUCTION

The positive results of pioneering clinical trials using gene therapy as treatment for patients with Leber congenital amaurosis (LCA) have ushered in a new era of molecular retinal therapeutics for LCA, other blinding retinal disorders and gene therapy applications.

AREAS COVERED

This review describes the role of retinal pigment epithelium-specific 65 kDa protein (RPE65) in the visual cycle and how RPE65 deficiency results in LCA; the extensive preclinical studies with recombinant adeno-associated virus (rAAV)-RPE65 gene vectors; and the human rAAV-RPE65 and related gene therapy clinical trials and studies. The literature search included a review of primary sources (e.g., journal articles) that reported study data results and key secondary sources such as meta-reviews available through PubMed, as well as reviews of clinical trial descriptions and results as reported in clinicaltrials.gov, conference publications and news releases.

EXPERT OPINION

LCA-RPE65 gene therapy is an example of successful, innovative, translational research. Further research is needed regarding how retinal gene therapy can be improved.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Focus on vision: 3 decades of remarkable contributions to biology and medicine

The FASEB Journal is a pillar among biomedical publications, contributing greatly by disseminating the results of vision research during its lifetime. Progress over this period has been remarkable. George Wald provided the first chemical understanding of the fundamental processes governing vision: the photoisomerization of 11-cis-retinal to all-trans-retinal and the enzymatic regeneration of this chromophore. Contributions of this extraordinary scientist set the stage for discoveries ranging from gross recording of various electrical responses to light to elucidation of signal transduction at a structural level, and from characterization of retinal diseases to successful treatments.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

Probing mechanisms of photoreceptor degeneration in a new mouse model of the common form of autosomal dominant retinitis pigmentosa due to P23H opsin mutations

Rhodopsin, the visual pigment mediating vision under dim light, is composed of the apoprotein opsin and the chromophore ligand 11-cis-retinal. A P23H mutation in the opsin gene is one of the most prevalent causes of the human blinding disease, autosomal dominant retinitis pigmentosa. Although P23H cultured cell and transgenic animal models have been developed, there remains controversy over whether they fully mimic the human phenotype; and the exact mechanism by which this mutation leads to photoreceptor cell degeneration remains unknown. By generating P23H opsin knock-in mice, we found that the P23H protein was inadequately glycosylated with levels 1-10% that of wild type opsin. Moreover, the P23H protein failed to accumulate in rod photoreceptor cell endoplasmic reticulum but instead disrupted rod photoreceptor disks. Genetically engineered P23H mice lacking the chromophore showed accelerated photoreceptor cell degeneration. These results indicate that most synthesized P23H protein is degraded, and its retinal cytotoxicity is enhanced by lack of the 11-cis-retinal chromophore during rod outer segment development.

Noninvasive multiphoton fluorescence microscopy resolves retinol and retinal condensation products in mouse eyes

Multi–photon excitation fluorescence microscopy (MPM) can image certain molecular processes in vivo. In the eye, fluorescent retinyl esters in sub–cellular structures called retinosomes mediate regeneration of the visual chromophore, 11–cis–retinal, by the visual cycle. But harmful fluorescent condensation products were also identified previously. We report that in wild type mice, excitation with λ ~730 nm identified retinosomes in the retinal pigment epithelium, whereas excitation with λ ~910 nm revealed at least one additional retinal fluorophore. The latter fluorescence was absent in eyes of genetically modified mice lacking a functional visual cycle, but accentuated in eyes of older WT mice and mice with defective clearance of all–trans–retinal, an intermediate in the visual cycle. MPM, a noninvasive imaging modality that facilitates concurrent monitoring of retinosomes along with potentially harmful products in aging eyes, has the potential to detect early molecular changes due to age–related macular degeneration and other defects in retinoid metabolism.

Retinoids for treatment of retinal diseases

Knowledge about retinal photoreceptor signal transduction and the visual cycle required for normal eyesight has increased exponentially over the past decade. Substantial progress in human genetics has facilitated the identification of candidate genes and complex networks underlying inherited retinal diseases. Natural mutations in animal models that mimic human diseases have been characterized and advanced genetic manipulation can now be used to generate small mammalian models of human retinal diseases. Pharmacological repair of defective visual processes in animal models not only validates their involvement in vision, but also provides great promise for the development of improved therapies for millions who are progressing towards blindness or are almost completely robbed of their eyesight.

An acyl-covalent enzyme intermediate of lecithin:retinol acyltransferase

Synthesis of fatty acid retinyl esters determines systemic vitamin A levels and provides substrate for production of visual chromophore (11-cis-retinal) in vertebrates. Lecithin:retinol acyltransferase (LRAT), the main enzyme responsible for retinyl ester formation, catalyzes the transfer of an acyl group from the sn-1 position of phosphatidylcholine to retinol. To delineate the catalytic mechanism of this reaction, we expressed and purified a fully active, soluble form of this enzyme and used it to examine the possible formation of a transient acyl-enzyme intermediate. Detailed mass spectrometry analyses revealed that LRAT undergoes spontaneous, covalent modification upon incubation with a variety of phosphatidylcholine substrates. The addition of an acyl chain occurs at the Cys(161) residue, indicating formation of a thioester intermediate. This observation provides the first direct experimental evidence of thioester intermediate formation that constitutes the initial step in the proposed LRAT catalytic reaction. Additionally, we examined the effect of increasing fatty acyl side chain length in phosphatidylcholine on substrate accessibility in this reaction, which provided insights into the function of the single membrane-spanning domain of LRAT. These observations are critical to understanding the catalytic mechanism of LRAT protein family members as well as other lecithin:acyltransferases wherein Cys residues are required for catalysis.

Effective and sustained delivery of hydrophobic retinoids to photoreceptors

PURPOSE

Delivery of hydrophobic compounds to the retina/RPE has been challenging. The purpose of this study was to develop an effective method for the sustained delivery of retinoids to rod and cone photoreceptors of young mice lacking a normal supply of 11-cis retinal.

METHODS

Solubilized basement membrane matrix (Matrigel; BD Biosciences, San Jose, CA) loaded with 9-cis retinal was administered subcutaneously into Rpe65(-/-) mouse pups for assessment of delivery to rods and cones and to Rpe65(-/-)Rho(-/-) mouse pups for assessment of delivery to cones. Intraperitoneal injections of 9-cis retinal were used for comparison. Cone density and opsin localization were evaluated with immunohistochemistry. Cone opsin protein levels were assayed with immunoblots, and cone function was analyzed by electroretinography (ERG) recordings. Retinoid content was determined by high-performance liquid chromatography analysis of retinal extracts. Pigment levels were quantified in homogenized retinas by absorption spectroscopy before and after light exposure.

RESULTS

Single administration of Matrigel loaded with 9-cis retinal to Rpe65(-/-) mice increased cone densities in all analyzed regions of the retina compared with mice treated using intraperitoneal delivery. Cone opsin levels increased to near wild-type levels. Similar treatment in Rpe65(-/-)Rho(-/-) mice increased b-wave ERG amplitudes significantly, indicating the maintenance of cone function. Matrigel was shown to continuously release 9-cis retinal for periods up to 1 week.

CONCLUSIONS

As a method for sustained drug delivery, subcutaneous administration using Matrigel proved more efficacious than intraperitoneal injection for in vivo delivery of retinoids to cone photoreceptors. These experiments are the first to show a sustained delivery of retinoids in mice and suggest a strategy for potential clinical therapeutic development.

Palmitoylation stabilizes unliganded rod opsin

S-palmitoylation is a conserved feature in many G protein-coupled receptors (GPCRs) involved in a broad array of signaling processes. The prototypical GPCR, rhodopsin, is S-palmitoylated on two adjacent C-terminal Cys residues at its cytoplasmic surface. Surprisingly, absence of palmitoylation has only a modest effect on in vitro or in vivo signaling. Here, we report that palmitoylation-deficient (Palm(-/-)) mice carrying two Cys to Thr and Ser mutations in the opsin gene displayed profound light-induced retinal degeneration that first involved rod and then cone cells. After brief bright light exposure, their retinas exhibited two types of deposits containing nucleic acid and invasive phagocytic macrophages. When Palm(-/-) mice were crossed with Lrat(-/-) mice lacking lecithin:retinol acyl transferase to eliminate retinoid binding to opsin and thereby rendering the eye insensitive to light, rapid retinal degeneration occurred even in 3- to 4-week-old animals. This rapid degeneration suggests that nonpalmitoylated rod opsin is unstable. Treatment of 2-week-old Palm(-/-)Lrat(-/-) mice with an artificial chromophore precursor prevented this retinopathy. In contrast, elimination of signaling to G protein in Palm(-/-)Gnat1(-/-) mice had no effect, indicating that instability of unpalmitoylated opsin lacking chromophore rather than aberrant signal transduction resulted in retinal pathology. Together, these observations provide evidence for a structural role of rhodopsin S-palmitoylation that may apply to other GPCRs as well.

Phagocytosis of retinal rod and cone photoreceptors

Photoreceptor cells maintain a roughly constant length by continuously generating new outer segments from their base while simultaneously releasing mature outer segments engulfed by the retinal pigment epithelium (RPE). Thus postmitotic RPE cells phagocytose an immense amount of material over a lifetime, disposing of photoreceptor cell waste while retaining useful content. This review focuses on current knowledge of outer segment phagocytosis, discussing the steps involved along with their critical participants as well as how various perturbations in outer segment (OS) disposal can lead to retinopathies.

Molecular biology and analytical chemistry methods used to probe the retinoid cycle

The retinoid (visual) cycle is a complex enzymatic pathway essential for regeneration of the visual chromophore, 11-cis-retinal, a component of rhodopsin that undergoes activation by light in vertebrate eyes. Pathogenic mutations within genes encoding proteins involved in the retinoid cycle lead to abnormalities in retinoid homeostasis and numerous congenital blinding diseases of humans. Thus, elucidation of disease-specific changes in enzymatic activities and retinoid content of the retina can provide important insights into the mechanisms of disease initiation and progression. Here, we use the protein RPE65 as an example to describe generally applicable methods for determining the stability and enzymatic activity of proteins and their mutants involved in retinoid metabolism. Additionally, we introduce a range of analytical techniques involving high-performance liquid chromatography and mass spectrometry to detect and quantify retinoids and their derivatives in eye extracts. Biochemical protocols combined with advanced mass spectrometry should facilitate fundamental biological studies of vision.

A functional kinase homology domain is essential for the activity of photoreceptor guanylate cyclase 1

Phototransduction is carried out by a signaling pathway that links photoactivation of visual pigments in retinal photoreceptor cells to a change in their membrane potential. Upon photoactivation, the second messenger of phototransduction, cyclic GMP, is rapidly degraded and must be replenished during the recovery phase of phototransduction by photoreceptor guanylate cyclases (GCs) GC1 (or GC-E) and GC2 (or GC-F) to maintain vision. Here, we present data that address the role of the GC kinase homology (KH) domain in cyclic GMP production by GC1, the major cyclase in photoreceptors. First, experiments were done to test which GC1 residues undergo phosphorylation and whether such phosphorylation affects cyclase activity. Using mass spectrometry, we showed that GC1 residues Ser-530, Ser-532, Ser-533, and Ser-538, located within the KH domain, undergo light- and signal transduction-independent phosphorylation in vivo. Mutations in the putative Mg(2+) binding site of the KH domain abolished phosphorylation, indicating that GC1 undergoes autophosphorylation. The dramatically reduced GC activity of these mutants suggests that a functional KH domain is essential for cyclic GMP production. However, evidence is presented that autophosphorylation does not regulate GC1 activity, in contrast to phosphorylation of other members of this cyclase family.

An analysis of retinal gene therapy clinical trials

In 2008, the initial results from the first three gene therapy trials to use adeno-associated viral vectors to treat an inherited retinal degeneration were published. These trials demonstrated no significant vector-related side effects and provided evidence of successful gene transfer with improved vision in several patients. The success of these trials heralds the beginning of a new era in the treatment of retinal diseases. Much can be learnt by comparing the results of the individual studies, as each has subtle differences, both in surgical technique and vector design. In contrast to laboratory models, humans generally have missense rather than null mutations and are treated later in the disease process than experimental models, when recipient cells are compromised. Intracellular stress responses, such as those regulated by endoplasmic reticulum protein kinase (PERK) and the mTOR pathways, are likely to inhibit the translation of transgenic mRNA by mechanisms that are not evident in null laboratory models treated early in the disease process. Understanding methods to overcome stress responses is likely to be a critical step in translating the applications of gene therapy from animal models to other human retinal diseases.

Gene therapy for retinitis pigmentosa and Leber congenital amaurosis caused by defects in AIPL1: effective rescue of mouse models of partial and complete Aipl1 deficiency using AAV2/2 and AAV2/8 vectors

Defects in the photoreceptor-specific gene encoding aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) are clinically heterogeneous and present as Leber Congenital Amaurosis, the severest form of early-onset retinal dystrophy and milder forms of retinal dystrophies such as juvenile retinitis pigmentosa and dominant cone-rod dystrophy. [Perrault, I., Rozet, J.M., Gerber, S., Ghazi, I., Leowski, C., Ducroq, D., Souied, E., Dufier, J.L., Munnich, A. and Kaplan, J. (1999) Leber congenital amaurosis. Mol. Genet. Metab., 68, 200-208.] Although not yet fully elucidated, AIPL1 is likely to function as a specialized chaperone for rod phosphodiesterase (PDE). We evaluate whether AAV-mediated gene replacement therapy is able to improve photoreceptor function and survival in retinal degeneration associated with AIPL1 defects. We used two mouse models of AIPL1 deficiency simulating three different rates of photoreceptor degeneration. The Aipl1 hypomorphic (h/h) mouse has reduced Aipl1 levels and a relatively slow degeneration. Under light acceleration, the rate of degeneration in the Aipl1 h/h mouse is increased by 2-3-fold. The Aipl1-/- mouse has no functional Aipl1 and has a very rapid retinal degeneration. To treat the different rates of degeneration, two pseudotypes of recombinant adeno-associated virus (AAV) exhibiting different transduction kinetics are used for gene transfer. We demonstrate restoration of cellular function and preservation of photoreceptor cells and retinal function in Aipl1 h/h mice following gene replacement therapy using an AAV2/2 vector and in the light accelerated Aipl1 h/h model and Aipl1-/- mice using an AAV2/8 vector. We have thus established the potential of gene replacement therapy in varying rates of degeneration that reflect the clinical spectrum of disease. This is the first gene replacement study to report long-term rescue of a photoreceptor-specific defect and to demonstrate effective rescue of a rapid photoreceptor degeneration.

Evaluation of 9-cis-retinyl acetate therapy in Rpe65-/- mice

PURPOSE

Mice lacking retinal pigment epithelium-specific 65-kDa protein (RPE65) develop retinopathy and blindness resembling Leber congenital amaurosis. Effects of 9-cis-retinyl acetate (9-cis-R-Ac) on visual function and retinopathy progression were tested in Rpe65(-/-) mice.

METHODS

Young C57Bl/6 mice were given 9-cis-R-Ac in each of four different oil-based vehicle solutions by gastric gavage to identify the vehicle most suitable for drug delivery by measuring retinoid levels in plasma. Then doses of 9-cis-R-Ac ranging from 1 to 100 mg/kg were administered to 5- to 12-week-old Rpe65(-/-) mice by different treatment regimens, including single doses and either intermittent or daily doses for various periods up to 8 weeks. Retinoid effects on visual function were evaluated by electroretinography, retinoid analyses, histologic methods, and vision-dependent behavioral testing.

RESULTS

Soybean oil vehicle provided the highest 9-cis-R-Ac metabolite levels in plasma. Single doses of 9-cis-R-Ac (6.25-50 mg/kg) provided significant dose-dependent improvement in electroretinographic responses. Well-tolerated daily doses (1-12.5 mg/kg) for 2 weeks induced remarkable improvement of retinal function. Significant dose-dependent improvement of electroretinographic responses was observed 6 days after administration of 9-cis-R-Ac daily for 3 days at 1 to 12.5 mg/kg. Mice given either daily or intermittent 9-cis-R-Ac treatment at 1 and 4 mg/kg and evaluated 8 weeks later displayed dose-dependent improvement of retinal function and morphology, whereas retinal function deteriorated in control animals. Treated mice also performed better than control animals in vision-dependent behavioral tests.

CONCLUSIONS

Treatment with 9-cis-R-Ac improves visual function and preserves retinal morphology in Rpe65(-/-) mice.

Loss of cone photoreceptors caused by chromophore depletion is partially prevented by the artificial chromophore pro-drug, 9-cis-retinyl acetate

Inactivating mutations in the retinoid isomerase (RPE65) or lecithin:retinol acyltransferase (LRAT) genes cause Leber congenital amaurosis (LCA), a severe visual impairment in humans. Both enzymes participate in the retinoid (visual) cycle, the enzymatic pathway that continuously generates 11-cis-retinal, the chromophore of visual pigments in rod and cone photoreceptor cells needed for vision. We investigated human RPE65-LCA patients and mice with visual cycle abnormalities to determine the impact of chronic chromophore deprivation on cones. Young patients with RPE65 mutations showed foveal cone loss along with shortened inner and outer segments of remaining cones; cone cell loss also was dramatic in young mice lacking Rpe65 or Lrat gene function. To selectively evaluate cone pathophysiology, we eliminated the rod contribution to electroretinographic (ERG) responses by generating double knockout mice lacking Lrat or Rpe65 together with an inactivated rod-specific G protein transducin gene (Gnat1-/-). Cone ERG responses were absent in Gnat1-/-Lrat-/- mice which also showed progressive degeneration of cones. Cone ERG responses in Gnat1-/-Rpe65-/- mice were markedly reduced and declined over weeks. Treatment of these mice with the artificial chromophore pro-drug, 9-cis-retinyl acetate, partially protected inferior retinal cones as evidenced by improved ERGs and retinal histochemistry. Gnat1-/- mice chronically treated with retinylamine, a selective inhibitor of RPE65, also showed a decline in the number of cones that was ameliorated by 9-cis-retinyl acetate. These results suggest that chronic lack of chromophore leads to progressive loss of cones in mice and humans. Therapy for LCA patients should be geared toward early adequate delivery of chromophore to cone photoreceptors.

Involvement of all-trans-retinal in acute light-induced retinopathy of mice

Exposure to bright light can cause visual dysfunction and retinal photoreceptor damage in humans and experimental animals, but the mechanism(s) remain unclear. We investigated whether the retinoid cycle (i.e. the series of biochemical reactions required for vision through continuous generation of 11-cis-retinal and clearance of all-trans-retinal, respectively) might be involved. Previously, we reported that mice lacking two enzymes responsible for clearing all-trans-retinal, namely photoreceptor-specific ABCA4 (ATP-binding cassette transporter 4) and RDH8 (retinol dehydrogenase 8), manifested retinal abnormalities exacerbated by light and associated with accumulation of diretinoid-pyridinium-ethanolamine (A2E), a condensation product of all-trans-retinal and a surrogate marker for toxic retinoids. Now we show that these mice develop an acute, light-induced retinopathy. However, cross-breeding these animals with lecithin:retinol acyltransferase knock-out mice lacking retinoids within the eye produced progeny that did not exhibit such light-induced retinopathy until gavaged with the artificial chromophore, 9-cis-retinal. No significant ocular accumulation of A2E occurred under these conditions. These results indicate that this acute light-induced retinopathy requires the presence of free all-trans-retinal and not, as generally believed, A2E or other retinoid condensation products. Evidence is presented that the mechanism of toxicity may include plasma membrane permeability and mitochondrial poisoning that lead to caspase activation and mitochondria-associated cell death. These findings further understanding of the mechanisms involved in light-induced retinal degeneration.

Gene therapy for retinitis pigmentosa and Leber congenital amaurosis caused by defects in AIPL1: effective rescue of mouse models of partial and complete Aipl1 deficiency using AAV2/2 and AAV2/8 vectors

Defects in the photoreceptor-specific gene encoding aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) are clinically heterogeneous and present as Leber Congenital Amaurosis, the severest form of early-onset retinal dystrophy and milder forms of retinal dystrophies such as juvenile retinitis pigmentosa and dominant cone-rod dystrophy. [Perrault, I., Rozet, J.M., Gerber, S., Ghazi, I., Leowski, C., Ducroq, D., Souied, E., Dufier, J.L., Munnich, A. and Kaplan, J. (1999) Leber congenital amaurosis. Mol. Genet. Metab., 68, 200-208.] Although not yet fully elucidated, AIPL1 is likely to function as a specialized chaperone for rod phosphodiesterase (PDE). We evaluate whether AAV-mediated gene replacement therapy is able to improve photoreceptor function and survival in retinal degeneration associated with AIPL1 defects. We used two mouse models of AIPL1 deficiency simulating three different rates of photoreceptor degeneration. The Aipl1 hypomorphic (h/h) mouse has reduced Aipl1 levels and a relatively slow degeneration. Under light acceleration, the rate of degeneration in the Aipl1 h/h mouse is increased by 2-3-fold. The Aipl1-/- mouse has no functional Aipl1 and has a very rapid retinal degeneration. To treat the different rates of degeneration, two pseudotypes of recombinant adeno-associated virus (AAV) exhibiting different transduction kinetics are used for gene transfer. We demonstrate restoration of cellular function and preservation of photoreceptor cells and retinal function in Aipl1 h/h mice following gene replacement therapy using an AAV2/2 vector and in the light accelerated Aipl1 h/h model and Aipl1-/- mice using an AAV2/8 vector. We have thus established the potential of gene replacement therapy in varying rates of degeneration that reflect the clinical spectrum of disease. This is the first gene replacement study to report long-term rescue of a photoreceptor-specific defect and to demonstrate effective rescue of a rapid photoreceptor degeneration.

Ocular gene therapy: current progress and future prospects

As gene therapy begins to produce its first clinical successes, interest in ocular gene transfer has grown owing to the favorable safety and efficacy characteristics of the eye as a target organ for drug delivery. Important advances also include the availability of viral and non-viral vectors that are able to efficiently transduce various ocular cell types, the use of intraocular delivery routes and the development of transcriptional regulatory elements that allow sustained levels of gene transfer in small and large animal models after a single administration. Here, we review recent progress in the field of ocular gene therapy. The first experiments in humans with severe inherited forms of blindness seem to confirm the good safety and efficacy profiles observed in animal models and suggest that gene transfer has the potential to become a valuable therapeutic strategy for otherwise untreatable blinding diseases.

Technical brief: subretinal injection and electroporation into adult mouse eyes

PURPOSE

Our goal was to improve and standardize the procedure for subretinal injection of mouse eyes. Also, we wished to optimize conditions for electroporation of retinal pigment epithelium (RPE) cells in the mouse eye with naked plasmids.

METHODS

Mouse eyes were injected subretinally with reporter plasmid DNA, nanobeads, or buffer. A blunt needle was introduced across the cornea, through the pupil, into the vitreous, until it made contact with the neural retina. Gentle pressure was applied to the needle, forcing it to puncture the retina and enter the subretinal space. Fluid was injected subretinally, raising large blebs evident on the mouse fundus. Subretinal injection surgery and outcomes were monitored and evaluated by video recording. Vidisic aided in fundus examination of the blebs. Pores in RPE cells, across which the plasmid in the fluid could diffuse, were created by several short electrical bursts. Expression of the delivered gene, tdTomato, in the plasmid was examined under fluorescence microscopy to identify targeted cells. Electroporation conditions were varied from 0 to 200 V, 5 to 10 pulses, 0.1 ms to 100 ms duration of each pulse, and a space of 1.5 to 2 mm between electrodes on the cornea and sclera.

RESULTS

A critical sign of surgical success was the appearance and persistence of three large blebs in the mouse eye. This was illustrated by video recordings from two different systems. Application of Vidisic to the cornea made immediate examination of the fundus possible at the end of the surgery. An 80% success rate was readily achieved by following this method. Once a successful subretinal injection technique was established, electroporation conditions were evaluated. Parameters of 50 V, 1 ms pulse duration, 5-10 pulses, 1 s apart and electrodes spaced 1.5-2 mm apart with the anode placed on the sclera in the vicinity of the blebs produced a tight pattern of RPE transfection at approximately 30% efficiency.

CONCLUSIONS

A standardized surgical method and a fast distinct indicator of successful surgery were essential to establishing a gene delivery system based on subsequent electroporation. With the surgery better controlled, electroporation was adequate to transfect a substantial number of RPE cells in a defined position in the globe.

Animal models for target diseases in gene therapy--using DNA and siRNA delivery strategies

Nanoparticles, including lipopolyamines leading to lipoplexes, liposomes, and polyplexes are targeted drug carrier systems in the current search for a successful delivery system for polynucleic acids. This review is focused on the impact of gene and siRNA delivery for studies of efficacy, pharmacodynamics, and pharmacokinetics within the setting of the wide variety of in vivo animal models now used. This critical appraisal of the recent literature sets out the different models that are currently being investigated to bridge from studies in cell lines through towards clinical reality. Whilst many scientists will be familiar with rodent (murine, fecine, cricetine, and musteline) models, few probably think of fish as a clinically relevant animal model, but zebrafish, madake, and rainbow trout are all being used. Larger animal models include rabbit, cat, dog, and cow. Pig is used both for the prevention of foot-and-mouth disease and human diseases, sheep is a model for corneal transplantation, and the horse naturally develops arthritis. Non-human primate models (macaque, common marmoset, owl monkey) are used for preclinical gene vector safety and efficacy trials to bridge the gap prior to clinical studies. We aim for the safe development of clinically effective delivery systems for DNA and RNAi technologies.

Effects of long-term administration of 9-cis-retinyl acetate on visual function in mice

PURPOSE

Long-term effects of treatment with 9-cis-retinyl acetate (9-cis-R-Ac), an artificial retinoid prodrug, were tested on changes in rod and cone visual functions in mice.

METHODS

The acetyl ester of the functional geometric chromophore 9-cis-retinal was delivered by oral gavage to C57BL/6 female mice. In initial experiments, 10-month-old mice were used for the single treatment with 9-cis-R-Ac or the control vehicle. In long-term experiments, 4-month-old mice were treated with 9-cis-R-Ac monthly for 6 and 10 months. Photoreceptor status was evaluated by various electroretinographic (ERG) techniques, retinoid analyses, and retinal morphology. Opsin, the predicted target of oxidized 9-cis-R-Ac, was purified and its chromophore was characterized.

RESULTS

Age-related changes observed in vehicle-treated mice at 10 months of age, compared with those in 4-month-old mice, included a progressive decline in ERG responses, such as a decreased rate of dark adaptation and a lowered rhodopsin/opsin ratio. Administration of 9-cis-R-Ac increased the rhodopsin regeneration ratio, and improved ERG responses and dark adaptation. Compared with vehicle-treated control animals, 10- and 14-month-old mice treated monthly with 9-cis-R-Ac for 6 or 10 months exhibited improved dark adaptation. In 14-month-old mice treated monthly, changes in the expression of retina-specific genes in the eye were detected by mRNA expression profiling, but no significant effects in gene expression were detected in the liver and kidney.

CONCLUSIONS

Deteriorating photoreceptor function documented in mice at 10 and 14 versus 4 months of age was improved significantly by long-term, monthly administration of 9-cis-R-Ac. These findings suggest a potential therapeutic approach to prevent age-related retinal dysfunction.

Correction of the disease phenotype in the mouse model of Stargardt disease by lentiviral gene therapy

Autosomal recessive Stargardt disease (STGD1) is a macular dystrophy caused by mutations in the ABCA4 (ABCR) gene. The disease phenotype that is most recognized in STGD1 patients, and also in the Abca4-/- mouse (a disease model), is lipofuscin accumulation in retinal pigment epithelium. Here, we tested whether delivery of the normal (wt) human ABCA4 gene to the subretinal space of the Abca4 -/- mice via lentiviral vectors would correct the disease phenotype; that is, reduce accumulation of the lipofuscin pigment A2E. Equine infectious anemia virus (EIAV)-derived lentiviral vectors were constructed expressing either the human ABCA4 gene or the LacZ reporter gene under the control of the constitutive (CMV) or photoreceptor-specific (Rho) promoters. Abca4-/- mice were injected subretinally with 1 microl ( approximately 5.0 x 10(5) TU) of each EIAV vector in one eye at postnatal days 4 and 5. An injection of saline, an EIAV-null vector, or an uninjected contralateral eye served as a control. Mice were killed at various times after injection to determine photoreceptor (PR) transduction efficiency and A2E concentrations. EIAV-LacZ vectors transduced from 5 to 20% of the PRs in the injected area in mice. Most importantly, a single subretinal injection of EIAV-CMV-ABCA4 to Abca4-/- mouse eyes substantially reduced disease-associated A2E accumulation compared to untreated and mock-treated control eyes. Treated eyes of Abca4-/- mice accumulated 8-12 pmol per eye (s.d.=2.7) of A2E 1 year after treatment, amounts comparable to wt controls, whereas mock-treated or untreated eyes had 3-5 times more A2E (27-39 pmol per eye, s.d.=1.5; P=0.001-0.005). Although extrapolation to humans requires caution, the high transduction efficiency of both rod and cone photoreceptors and the statistically significant reduction of A2E accumulation in the mouse model of STGD1 suggest that lentiviral gene therapy is a potentially efficient tool for treating ABCA4-associated diseases.

Trafficking of membrane-associated proteins to cone photoreceptor outer segments requires the chromophore 11-cis-retinal

Lecithin retinol acyl transferase (LRAT) and retinal pigment epithelium protein 65 (RPE65) are key enzymes of the retinoid cycle. In Lrat(-/-) and Rpe65(-/-) mice, models of human Leber congenital amaurosis, the retinoid cycle is disrupted and 11-cis-retinal, the chromophore of visual pigments, is not produced. The Lrat(-/-) and Rpe65(-/-) retina phenotype presents with rapid sectorial cone degeneration, and the visual pigments, S-opsin and M/L-opsin, fail to traffic to cone outer segments appropriately. In contrast, rod opsin traffics normally in mutant rods. Concomitantly, guanylate cyclase 1, cone T alpha-subunit, cone phosphodiesterase 6alpha' (PDE6alpha'), and GRK1 (G-protein-coupled receptor kinase 1; opsin kinase) are not transported to Lrat(-/-) and Rpe65(-/-) cone outer segments. Aberrant localization of these membrane-associated proteins was evident at postnatal day 15, before the onset of ventral and central cone degeneration. Protein levels of cone T alpha and cone PDE6alpha' were reduced, whereas their transcript levels were unchanged, suggesting posttranslational degradation. In an Rpe65(-/-)Rho(-/-) double knock-out model, trafficking of cone pigments and membrane-associated cone phototransduction polypeptides to the outer segments proceeded normally after 11-cis-retinal administration. These results suggest that ventral and central cone opsins must be regenerated with 11-cis-retinal to permit transport to the outer segments. Furthermore, the presence of 11-cis-retinal is essential for proper transport of several membrane-associated cone phototransduction polypeptides in these cones.

Metabolic basis of visual cycle inhibition by retinoid and nonretinoid compounds in the vertebrate retina

In vertebrate retinal photoreceptors, the absorption of light by rhodopsin leads to photoisomerization of 11-cis-retinal to its all-trans isomer. To sustain vision, a metabolic system evolved that recycles all-trans-retinal back to 11-cis-retinal. The importance of this visual (retinoid) cycle is underscored by the fact that mutations in genes encoding visual cycle components induce a wide spectrum of diseases characterized by abnormal levels of specific retinoid cycle intermediates. In addition, intense illumination can produce retinoid cycle by-products that are toxic to the retina. Thus, inhibition of the retinoid cycle has therapeutic potential in physiological and pathological states. Four classes of inhibitors that include retinoid and nonretinoid compounds have been identified. We investigated the modes of action of these inhibitors by using purified visual cycle components and in vivo systems. We report that retinylamine was the most potent and specific inhibitor of the retinoid cycle among the tested compounds and that it targets the retinoid isomerase, RPE65. Hydrophobic primary amines like farnesylamine also showed inhibitory potency but a short duration of action, probably due to rapid metabolism. These compounds also are reactive nucleophiles with potentially high cellular toxicity. We also evaluated the role of a specific protein-mediated mechanism on retinoid cycle inhibitor uptake by the eye. Our results show that retinylamine is transported to and taken up by the eye by retinol-binding protein-independent and retinoic acid-responsive gene product 6-independent mechanisms. Finally, we provide evidence for a crucial role of lecithin: retinol acyltransferase activity in mediating tissue specific absorption and long lasting therapeutic effects of retinoid-based visual cycle inhibitors.

Rescue of sight by gene therapy - closer than it may appear

This review will cover the state of the field in retinal degeneration and gene therapy with a focus on the great strides that have been made in retina gene therapy. Topics ranging from the development of animal models to clinical trials (for the treatment of Leber congenital amaurosis, age-related macular degeneration, and retinoblastoma) will be discussed. In addition, the results of gene therapy studies targeting the photoreceptors will be presented. Finally, strategies and progress in overcoming the challenges of photoreceptor-directed gene therapy will be presented.

Guanylate cyclase-activating proteins and retina disease

Detailed biochemical, structural and physiological studies of the role of Ca2(+)-binding proteins in mammalian retinal neurons have yielded new insights into the function of these proteins in normal and pathological states. In phototransduction, a biochemical process that is responsible for the conversion of light into an electrical impulse, guanylate cyclases (GCs) are regulated by GC-activating proteins (GCAPs). These regulatory proteins respond to changes in cytoplasmic Ca2+ concentrations. Disruption of Ca2+ homeostasis in photoreceptor cells by genetic and environmental factors can result ultimately in degeneration of these cells. Pathogenic mutations in GC1 and GCAP1 cause autosomal recessive Leber congenital amaurosis and autosomal dominant cone dystrophy, respectively. This report provides a recent account of the advances, challenges, and possible future prospects of studying this important step in visual transduction that transcends to other neuronal Ca2+ homeostasis processes.

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.

Three-dimensional architecture of murine rod outer segments determined by cryoelectron tomography

The rod outer segment (ROS) of photoreceptor cells houses all components necessary for phototransduction, a set of biochemical reactions that amplify and propagate a light signal. Theoretical approaches to quantify this process require precise information about the physical boundaries of the ROS. Dimensions of internal structures within the ROS of mammalian species have yet to be determined with the precision required for quantitative considerations. Cryoelectron tomography was utilized to obtain reliable three-dimensional morphological information about this important structure from murine retina. Vitrification of samples permitted imaging of the ROS in a minimally perturbed manner and the preservation of substructures. Tomograms revealed the characteristic highly organized arrangement of disc membranes stacked on top of one another with a surrounding plasma membrane. Distances among the various membrane components of the ROS were measured to define the space available for phototransduction to occur. Reconstruction of segments of the ROS from single-axis tilt series images provided a glimpse into the three-dimensional architecture of this highly differentiated neuron. The reconstructions revealed spacers that likely maintain the proper distance between adjacent discs and between discs and the plasma membrane. Spacers were found distributed throughout the discs, including regions that are distant from the rim region of discs.

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.

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.

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.

Low prevalence of lecithin retinol acyltransferase mutations in patients with Leber congenital amaurosis and autosomal recessive retinitis pigmentosa

PURPOSE

To determine the the prevalence of pathogenic mutations in the gene encoding lecithin retinol acyltransferase (LRAT) in patients from North America with either Leber congenital amaurosis (LCA) or autosomal recessive retinitis pigmentosa (ARRP).

METHODS

Exon 1, exon 2, and the coding region of exon 3 of LRAT were PCR-amplified and directly sequenced from the leukocyte DNA of 82 unrelated patients with LCA and 190 unrelated patients with ARRP.

RESULTS

One isocoding change was found in this screen of LRAT (Glu114 GAG>GAA; c.342), and 5 other sequence changes were found in intronic or untranslated regions of the gene. None of these changes were predicted to affect the encoded protein and were therefore deemed non-pathogenic.

CONCLUSIONS

LRAT mutations are likely a rare cause of LCA among patients from North America.

Delivery of retinoid-based therapies to target tissues

Through its various metabolites, vitamin A controls essential physiological functions. Both naturally occurring metabolites and novel retinoid analogues have shown effectiveness in many clinical settings that include skin diseases and cancer, and in animal models of human conditions affecting vision. In this review, we analyze several potential retinoid-based therapies from the point of view of drug metabolism and transport to target tissues. We focus on the endogenous factors that affect the absorption, transport, and metabolism of retinoids by taking into account data obtained from the analysis of animal models that lack the enzymes or proteins involved in the storage and absorption of retinoids. We also discuss findings of toxicity associated with retinoids in an effort to improve the outcome of retinoid-based therapies. In this context, we review evidence that esterification of retinol and retinol-based drugs within target tissues provides one of the most efficient means to improve the absorption and to reduce the toxicity associated with pharmacological doses of retinoids. Future retinoid-based therapeutic strategies could involve targeted delivery mechanisms leading to lower toxicity and improved effectiveness of retinoids.

From vivarium to bedside: lessons learned from animal models

In this review, we focus primarily on information obtained by studying mouse models of heritable ocular diseases. These models have proven to be important in advancing our understanding of disease etiology and of pathological consequences of heritable disorders. Careful phenotypic analyses of these models have lead to hypotheses regarding the function of various molecules as well as the mechanisms underlying the observed pathologies. Specific examples of the utility of mouse models in vision research are discussed.

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.

Improvement in rod and cone function in mouse model of Fundus albipunctatus after pharmacologic treatment with 9-cis-retinal

PURPOSE

To assess changes in rod and cone visual functions in a mouse model of Fundus albipunctatus with disrupted 11-cis-retinol dehydrogenase (RDH) genes after pharmacologic treatment with an artificial retinal chromophore.

METHODS

Retinoid levels and photoreceptor functions of Rdh5-/-Rdh11-/- mice at a variety of light intensities were analyzed with normal-phase HPLC and ERG techniques. Production of 11-cis-retinal, the visual pigment chromophore, was suppressed with a potent inhibitor of the retinoid cycle, all-trans-retinylamine (Ret-NH2). The chromophore was replaced by a functional geometric isomer, 9-cis-retinal, delivered by oral gavage.

RESULTS

Aberrant cone responses were detected in 12-month-old Rdh5-/-Rdh11-/- mice raised in a 12-hour light/12-hour dark cycle. This cone defect was exacerbated in conditions of low levels of 11-cis-retinal. Administration of 9-cis-retinal increased the rate of dark adaptation and improved cone function in Rdh5-/-Rdh11-/- mice.

CONCLUSIONS

Disruption of 11-cis-RDHs causes a slowly developing cone dystrophy caused by inefficient cone pigment regeneration. Rod and cone visual function improved significantly in the mouse model of F. albipunctatus after treatment with 9-cis-retinal, suggesting a potential approach to slow the progression of cone dystrophy in affected humans.