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

Two-photon excitation fluorescence in ophthalmology: safety and improved imaging for functional diagnostics

Two-photon excitation fluorescence (TPEF) is emerging as a powerful imaging technique with superior penetration power in scattering media, allowing for functional imaging of biological tissues at a subcellular level. TPEF is commonly used in cancer diagnostics, as it enables the direct observation of metabolism within living cells. The technique is now widely used in various medical fields, including ophthalmology. The eye is a complex and delicate organ with multiple layers of different cell types and tissues. Although this structure is ideal for visual perception, it generates aberrations in TPEF eye imaging. However, adaptive optics can now compensate for these aberrations, allowing for improved imaging of the eyes of animal models for human diseases. The eye is naturally built to filter out harmful wavelengths, but these wavelengths can be mimicked and thereby utilized in diagnostics via two-photon (2Ph) excitation. Recent advances in laser-source manufacturing have made it possible to minimize the exposure of in vivo measurements within safety, while achieving sufficient signals to detect for functional images, making TPEF a viable option for human application. This review explores recent advances in wavefront-distortion correction in animal models and the safety of use of TPEF on human subjects, both of which make TPEF a potentially powerful tool for ophthalmological diagnostics.

One-Year Outcomes of Oral Treatment With Alga Capsules Containing Low Levels of 9-cis-β-Carotene in RDH5-Related Fundus Albipunctatus

PURPOSE

This study aimed to clarify the effect of 1-year oral treatment with 9-cis-β-carotene-rich alga Dunaliella bardawil (Dunaliella supplementation) using full-field electroretinography (ERG) in patients with RDH5-related fundus albipunctatus (FAP).

DESIGN

Prospective, interventional case series.

PARTICIPANTS

The study included 12 patients (23 eyes) with RDH5-related FAP.

METHODS

Twelve patients (23 eyes) with RDH5-related FAP received Dunaliella supplementation (total daily dose of β-carotene was 74.0 mg, comprising 28.4 mg 9-cis-β-carotene and 45.6 mg all-trans-β-carotene at a ratio of 1:1.6) for 1 year and underwent ophthalmic examinations, including full-field ERG at baseline, 3 months, and 1 year after the initial treatment.

MAIN OUTCOME MEASURES

The main outcome was changes in the amplitudes of responses of full-field ERG before and after treatment. A linear mixed-effects model was used to evaluate the adjusted mean difference between the amplitude of each response pretreatment and posttreatment.

RESULTS

Prolonged dark adaptation (DA) responses at 3 months revealed a significant impairment in the b-wave of DA 0.01 (adjusted mean difference, -34.7, 95% CI, -66.8 to -2.73, P = .041) and a-wave of DA 3.0 (-29.0, 95% CI, -50.6 to -7.41, P = .013) and DA 10.0 (-40.4, 95% CI, -67.8 to -13.0, P = .007), which were also observed at 1 year. Additionally, prolonged DA and light adaptation (LA) responses revealed statistically significant impairment at 1 year in the b-wave of DA 3.0 (-43.8, 95% CI, -82.9 to -4.78, P = .035), DA 10.0 (-59.7, 95% CI, -101.8 to -17.61, P = .009), LA 3.0 (-7.31, 95% CI, -13.6 to -1.04, P = .029), and LA 3.0 flicker (-7.53, 95% CI, -12.7 to -2.34, P = .007).

CONCLUSIONS

Our study results suggest that Dunaliella supplementation comprising low levels of 9-cis-β-carotene compared with those reported in a previous study (1:1 ratio) adversely affects ERG amplitudes in patients with RDH5-related FAP.

From mouse to human: Accessing the biochemistry of vision in vivo by two-photon excitation

The eye is an ideal organ for imaging by a multi-photon excitation approach, because ocular tissues such as the sclera, cornea, lens and neurosensory retina, are highly transparent to infrared (IR) light. The interface between the retina and the retinal pigment epithelium (RPE) is especially informative, because it reflects the health of the visual (retinoid) cycle and its changes in response to external stress, genetic manipulations, and drug treatments. Vitamin A-derived retinoids, like retinyl esters, are natural fluorophores that respond to multi-photon excitation with near IR light, bypassing the filter-like properties of the cornea, lens, and macular pigments. Also, during natural aging some retinoids form bisretinoids, like diretinoid-pyridiniumethanolamine (A2E), that are highly fluorescent. These bisretinoids appear to be elevated concurrently with aging. Vitamin A-derived retinoids and bisretinoidss are detected by two-photon ophthalmoscopy (2PO), using a new class of light sources with adjustable spatial, temporal, and spectral properties. Furthermore, the two-photon (2P) absorption of IR light by the visual pigments in rod and cone photoreceptors can initiate visual transduction by cis-trans isomerization of retinal, enabling parallel functional studies. Recently we overcame concerns about safety, data interpretation and complexity of the 2P-based instrumentation, the major roadblocks toward advancing this modality to the clinic. These imaging and retina-function assessment advancements have enabled us to conduct the first 2P studies with humans.

Retinoid therapy restores eye-specific cortical responses in adult mice with retinal degeneration

Despite the recent emergence of multiple cellular and molecular strategies to restore vision in retinal disorders, it remains unclear to what extent central visual circuits can recover when retinal defects are corrected in adulthood. We addressed this question in an Lrat-/- mouse model of Leber congenital amaurosis (LCA) in which retinal light sensitivity and optomotor responses are partially restored by 9-cis-retinyl acetate administration in adulthood. Following treatment, two-photon calcium imaging revealed increases in the number and response amplitude of visually responsive neurons in the primary visual cortex (V1). In particular, retinoid treatment enhanced responses from the ipsilateral eye, restoring the normal balance of eye-specific responses in V1. Additionally, the treatment rescued the modulation of cortical responses by arousal. These findings illustrate the significant plasticity of the adult central visual system and underscore the therapeutic potential of retinoid administration for adults with retinal diseases.

Theoretical Study of the Photoisomerization Mechanism of All-Trans-Retinyl Acetate

The compound 9-cis-retinyl acetate (9-cis-RAc) is a precursor to 9-cis-retinal, which has potential application in the treatment of some hereditary diseases of the retina. An attractive synthetic route to 9-cis-RAc is based on the photoisomerization reaction of the readily available all-trans-RAc. In the present study, we examine the mechanism of the photoisomerization reaction with the use of state-of-the-art electronic structure calculations for two polyenic model compounds: tEtEt-octatetraene and tEtEtEc-2,6-dimethyl-1,3,5,7,9-decapentaene. The occurrence of photoisomerization is attributed to a chain-kinking mechanism, whereby a series of S₁/S₀ conical intersections associated with kinking deformations at different positions along the polyenic chain mediate internal conversion to the S₀ state, and subsequent isomerization around one of the double bonds. Two other possible photoisomerization mechanisms are taken into account, but they are rejected as incompatible with simulation results and/or the available spectroscopic data.

Gene therapy in retinal diseases: A review

Over 2 million people worldwide are suffering from gene-related retinal diseases, inherited or acquired, and over 270 genes have been identified which are found to be responsible for these conditions. This review article touches upon the mechanisms of gene therapy, various enzymes of the visual cycle responsible for different genetic diseases, Luxturna—the first US Food and Drug Administration (FDA)-approved therapeutic gene product, and several ongoing trials of gene therapy for age-related macular degeneration. Gene therapy has tremendous potential for retinal conditions due to its ease of accessibility, immune-privileged status, and tight blood-retinal barriers, limiting systemic side effects of the drug. In recent years, advances in gene therapy in retinal conditions have increasing significantly, with progress in cell-specific targeting and transduction efficiency of gene products through the use of adeno-associated viral vectors (AAVs), suggesting that even greater success in future clinical trials is possible.

The Lrat-/- Rat: CRISPR/Cas9 Construction and Phenotyping of a New Animal Model for Retinitis Pigmentosa

PURPOSE

We developed and phenotyped a pigmented knockout rat model for lecithin retinol acyltransferase (LRAT) using CRISPR/Cas9. The introduced mutation (c.12delA) is based on a patient group harboring a homologous homozygous frameshift mutation in the LRAT gene (c.12delC), causing a dysfunctional visual (retinoid) cycle.

METHODS

The introduced mutation was confirmed by DNA and RNA sequencing. The expression of Lrat was determined on both the RNA and protein level in wildtype and knockout animals using RT-PCR and immunohistochemistry. The retinal structure and function, as well as the visual behavior of the Lrat^-/- and control rats, were characterized using scanning laser ophthalmoscopy (SLO), optical coherence tomography (OCT), electroretinography (ERG) and vision-based behavioral assays.

RESULTS

Wildtype animals had high Lrat mRNA expression in multiple tissues, including the eye and liver. In contrast, hardly any expression was detected in Lrat^-/- animals. LRAT protein was abundantly present in wildtype animals and absent in Lrat^-/- animals. Lrat^-/- animals showed progressively reduced ERG potentials compared to wildtype controls from two weeks of age onwards. Vison-based behavioral assays confirmed reduced vision. Structural abnormalities, such as overall retinal thinning, were observed in Lrat^-/- animals. The retinal thickness in knockout rats was decreased to roughly 80% by four months of age. No functional or structural differences were observed between wildtype and heterozygote animals.

CONCLUSIONS

Our Lrat^-/- rat is a new animal model for retinal dystrophy, especially for the LRAT-subtype of early-onset retinal dystrophies. This model has advantages over the existing mouse models and the RCS rat strain and can be used for translational studies of retinal dystrophies.

Animals Models of Inherited Retinal Disease

Inherited retinal diseases (IRDs) are an important cause of blindness worldwide. Over 270 genes have been associated with IRD. Genetic testing can determine the cause of the clinical disease in the majority of patients. However, at least 25-50% of patients with clinical diagnosis of IRD remain unsolved even after whole genome sequencing. Animal models of IRD can be useful for expanding the set of established IRD genes, to gain biological understanding of the function of these genes in the retina, and to test advanced therapeutics prior to human clinical trials. In this chapter some small and large animal models of IRD are discussed including some of the advantages and limitations of each for various forms of retinopathy.

Gene Therapy to the Retina and the Cochlea

Vision and hearing disorders comprise the most common sensory disorders found in people. Many forms of vision and hearing loss are inherited and current treatments only provide patients with temporary or partial relief. As a result, developing genetic therapies for any of the several hundred known causative genes underlying inherited retinal and cochlear disorders has been of great interest. Recent exciting advances in gene therapy have shown promise for the clinical treatment of inherited retinal diseases, and while clinical gene therapies for cochlear disease are not yet available, research in the last several years has resulted in significant advancement in preclinical development for gene delivery to the cochlea. Furthermore, the development of somatic targeted genome editing using CRISPR/Cas9 has brought new possibilities for the treatment of dominant or gain-of-function disease. Here we discuss the current state of gene therapy for inherited diseases of the retina and cochlea with an eye toward areas that still need additional development.

DNA-Gated Graphene Field-Effect Transistors for Specific Detection of Arsenic(III) in Rice

As one of the most toxic forms of arsenic, inorganic As(III) is easy to accumulate in rice, leading to severe public health problems. Effective control of As(III) requires the development of fast analytical methods for its detection with high sensitivity and specificity. Toward this end, in this work, we report the fabrication of an As(III) electrochemical sensor based on a solution-gated graphene transistor (SGGT) platform with a novel sensing mechanism. The gold gate electrode of the SGGT was modified with DNA probes and then blocked with bovine serum albumin (BSA). The specific interaction between As(III) and gold disrupted the adsorption states of DNA probes, redistributing surface charges on the gate electrode, further leading to potential drop changes at the interfaces of the gate electrode and graphene active layer. This new mechanism based on DNA-charge-redistribution-induced SGGT current responses (denoted as "DNA-SGGT") was found to greatly improve the selectivity of the sensor: the response of DNA-SGGT to As(III) was effectively enhanced fourfold, while to other interfering cations, it was significantly reduced. The optimized sensor showed a detection limit as low as 5 nM with superior selectivity to As(III). The as-prepared DNA-SGGT-based sensor has also been successfully applied to the detection of As(III) in practical rice samples with a high recovery rate, showing great potential for heavy metal detection in many types of food samples.

Pathways and disease-causing alterations in visual chromophore production for vertebrate vision

All that we view of the world begins with an ultrafast cis to trans photoisomerization of the retinylidene chromophore associated with the visual pigments of rod and cone photoreceptors. The continual responsiveness of these photoreceptors is then sustained by regeneration processes that convert the trans-retinoid back to an 11-cis configuration. Recent biochemical and electrophysiological analyses of the retinal G-protein-coupled receptor (RGR) suggest that it could sustain the responsiveness of photoreceptor cells, particularly cones, even under bright light conditions. Thus, two mechanisms have evolved to accomplish the reisomerization: one involving the well-studied retinoid isomerase (RPE65) and a second photoisomerase reaction mediated by the RGR. Impairments to the pathways that transform all-trans-retinal back to 11-cis-retinal are associated with mild to severe forms of retinal dystrophy. Moreover, with age there also is a decline in the rate of chromophore regeneration. Both pharmacological and genetic approaches are being used to bypass visual cycle defects and consequently mitigate blinding diseases. Rapid progress in the use of genome editing also is paving the way for the treatment of disparate retinal diseases. In this review, we provide an update on visual cycle biochemistry and then discuss visual-cycle-related diseases and emerging therapeutics for these disorders. There is hope that these advances will be helpful in treating more complex diseases of the eye, including age-related macular degeneration (AMD).

Properties and Therapeutic Implications of an Enigmatic D477G RPE65 Variant Associated with Autosomal Dominant Retinitis Pigmentosa

RPE65 isomerase, expressed in the retinal pigmented epithelium (RPE), is an enzymatic component of the retinoid cycle, converting all-trans retinyl ester into 11-cis retinol, and it is essential for vision, because it replenishes the photon capturing 11-cis retinal. To date, almost 200 loss-of-function mutations have been identified within the RPE65 gene causing inherited retinal dystrophies, most notably Leber congenital amaurosis (LCA) and autosomal recessive retinitis pigmentosa (arRP), which are both severe and early onset disease entities. We previously reported a mutation, D477G, co-segregating with the disease in a late-onset form of autosomal dominant RP (adRP) with choroidal involvement; uniquely, it is the only RPE65 variant to be described with a dominant component. Families or individuals with this variant have been encountered in five countries, and a number of subsequent studies have been reported in which the molecular biological and physiological properties of the variant have been studied in further detail, including observations of possible novel functions in addition to reduced RPE65 enzymatic activity. With regard to the latter, a human phase 1b proof-of-concept study has recently been reported in which aspects of remaining vision were improved for up to one year in four of five patients with advanced disease receiving a single one-week oral dose of 9-cis retinaldehyde, which is the first report showing efficacy and safety of an oral therapy for a dominant form of RP. Here, we review data accrued from published studies investigating molecular mechanisms of this unique variant and include hitherto unpublished material on the clinical spectrum of disease encountered in patients with the D477G variant, which, in many cases bears striking similarities to choroideremia.

Noninvasive two-photon optical biopsy of retinal fluorophores

High-resolution imaging techniques capable of detecting identifiable endogenous fluorophores in the eye along with genetic testing will dramatically improve diagnostic capabilities in the ophthalmology clinic and accelerate the development of new treatments for blinding diseases. Two-photon excitation (TPE)-based imaging overcomes the filtering of ultraviolet light by the lens of the human eye and thus can be utilized to discover defects in vitamin A metabolism during the regeneration of the visual pigments required for the detection of light. Combining TPE with fluorescence lifetime imaging (FLIM) and spectral analyses offers the potential of detecting diseases of the retina at earlier stages before irreversible structural damage has occurred. The main barriers to realizing the benefits of TPE for imaging the human retina arise from concerns about the high light exposure typically needed for informative TPE imaging and the requirement to correlate the ensuing data with different states of health and disease. To overcome these hurdles, we improved TPE efficiency by controlling temporal properties of the excitation light and employed phasor analyses to FLIM and spectral data in mouse models of retinal diseases. Modeling of retinal photodamage revealed that plasma-mediated effects do not play a role and that melanin-related thermal effects are mitigated by reducing pulse repetition frequency. By using noninvasive TPE imaging we identified molecular components of individual granules in the retinal pigment epithelium and present their analytical characteristics.

Comparison between the enzymatic activity, structure and substrate binding of mouse and human lecithin retinol acyltransferase

Lecithin retinol acyltransferase (LRAT) is involved in the visual cycle where it catalyzes the formation of all-trans retinyl ester. The mouse animal model has been widely used to study LRAT. Primary sequence alignment shows 80% identity and 90% similarity between human and mouse LRAT. However, human LRAT has a proline at position 173 (hLRAT (P173)) while an arginine can be found at this position for the mouse protein (mLRAT (R173)). Moreover, residue 173 is important for the human protein since a substitution mutation of this residue to a leucine (P173L-hLRAT) caused night blindness in a patient. The present study was thus undertaken to determine whether mouse and human LRAT have a similar enzymatic activity, structure and substrate binding affinity using a truncated form of LRAT (tLRAT). The enzymatic activity and binding affinity to the substrate, all-trans retinol, of mtLRAT (R173) were found to be 2.7- and 3.9-fold lower, respectively, than that of htLRAT (P173). Moreover, the enzymatic activity of P173L-htLRAT is 6.3-fold lower compared to that of htLRAT (P173). Furthermore, a significant difference was observed between the intrinsic fluorescence emission as well as between the circular dichroism spectra of mtLRAT (R173) and htLRAT (P173). In addition, mtLRAT proteins are less thermostable than htLRAT proteins, which suggests that structural differences exist between the mouse and human proteins. Altogether, these data strongly suggest that the much lower catalytic activity of mtLRAT (R173) compared to that of htLRAT (P173) mostly results from differences between their structure, predominantly revealed by their dissimilar thermal stability, as well as their efficiency to bind all-trans retinol. Therefore, conclusions regarding the behavior of human LRAT based on measurements performed with mouse LRAT must be made with caution. Also, the much lower enzymatic activity of P173L-htLRAT compared to that of htLRAT (P173) might explain the night blindness of a patient carrying this mutation.

Gene Therapy in Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are a group of clinically and genetically heterogeneous degenerative disorders. To date, mutations have been associated with IRDs in over 270 disease genes, but molecular diagnosis still remains elusive in about a third of cases. The methodologic developments in genome sequencing techniques that we have witnessed in this last decade have represented a turning point not only in diagnosis and prognosis but, above all, in the identification of new therapeutic perspectives. The discovery of new disease genes and pathogenetic mechanisms underlying IRDs has laid the groundwork for gene therapy approaches. Several clinical trials are ongoing, and the recent approval of Luxturna, the first gene therapy product for Leber congenital amaurosis, marks the beginning of a new era. Due to its anatomical and functional characteristics, the retina is the organ of choice for gene therapy, although there are quite a few difficulties in the translational approaches from preclinical models to humans. In the first part of this review, an overview of the current knowledge on methodological issues and future perspectives of gene therapy applied to IRDs is discussed; in the second part, the state of the art of clinical trials on the gene therapy approach in IRDs is illustrated.

Gene and Induced Pluripotent Stem Cell Therapy for Retinal Diseases

Given the importance of visual information to many daily activities, retinal degenerative diseases-which include both inherited conditions (such as retinitis pigmentosa) and acquired conditions (such as age-related macular degeneration)-can have a dramatic impact on human lives. The therapeutic options for these diseases remain limited. Since the discovery of the first causal gene for retinitis pigmentosa almost three decades ago, more than 250 genes have been identified, and gene therapies have been rapidly developed. Simultaneously, stem cell technologies such as induced pluripotent stem cell-based transplantation have advanced and have been applied to the treatment of retinal degenerative diseases. Here, we review recent progress in these expanding fields and discuss the potential for precision medicine in ophthalmic care.

Chromatic Pupillometry Methods for Assessing Photoreceptor Health in Retinal and Optic Nerve Diseases

The pupillary light reflex is mediated by melanopsin-containing intrinsically-photosensitive retinal ganglion cells (ipRGCs), which also receive input from rods and cones. Melanopsin-dependent pupillary light responses are short-wavelength sensitive, have a higher threshold of activation, and are much slower to activate and de-activate compared with rod/cone-mediated responses. Given that rod/cone photoreceptors and melanopsin differ in their response properties, light stimuli can be designed to stimulate preferentially each of the different photoreceptor types, providing a read-out of their function. This has given rise to chromatic pupillometry methods that aim to assess the health of outer retinal photoreceptors and ipRGCs by measuring pupillary responses to blue or red light stimuli. Here, we review different types of chromatic pupillometry protocols that have been tested in patients with retinal or optic nerve disease, including approaches that use short-duration light exposures or continuous exposure to light. Across different protocols, patients with outer retinal disease (e.g., retinitis pigmentosa or Leber congenital amaurosis) show reduced or absent pupillary responses to dim blue-light stimuli used to assess rod function, and reduced responses to moderately-bright red-light stimuli used to assess cone function. By comparison, patients with optic nerve disease (e.g., glaucoma or ischemic optic neuropathy, but not mitochondrial disease) show impaired pupillary responses during continuous exposure to bright blue-light stimuli, and a reduced post-illumination pupillary response after light offset, used to assess melanopsin function. These proof-of-concept studies demonstrate that chromatic pupillometry methods can be used to assess damage to rod/cone photoreceptors and ipRGCs. In future studies, it will be important to determine whether chromatic pupillometry methods can be used for screening and early detection of retinal and optic nerve diseases. Such methods may also prove useful for objectively evaluating the degree of recovery to ipRGC function in blind patients who undergo gene therapy or other treatments to restore vision.

Retinal-chitosan Conjugates Effectively Deliver Active Chromophores to Retinal Photoreceptor Cells in Blind Mice and Dogs

The retinoid (visual) cycle consists of a series of biochemical reactions needed to regenerate the visual chromophore 11-cis-retinal and sustain vision. Genetic or environmental factors affecting chromophore production can lead to blindness. Using animal models that mimic human retinal diseases, we previously demonstrated that mechanism-based pharmacological interventions can maintain vision in otherwise incurable genetic diseases of the retina. Here, we report that after 9-cis-retinal administration to lecithin:retinol acyltransferase-deficient (Lrat^-/- ) mice, the drug was rapidly absorbed and then cleared within 1 to 2 hours. However, when conjugated to form chitosan-9-cis-retinal, this prodrug was slowly absorbed from the gastrointestinal tract, resulting in sustainable plasma levels of 9-cis-retinol and recovery of visual function without causing elevated levels, as occurs with unconjugated drug treatment. Administration of chitosan-9-cis-retinal conjugate intravitreally in retinal pigment epithelium-specific 65 retinoid isomerase (RPE65)-deficient dogs improved photoreceptor function as assessed by electroretinography. Functional rescue was dose dependent and maintained for several weeks. Dosing via the gastrointestinal tract in canines was found ineffective, most likely due to peculiarities of vitamin A blood transport in canines. Use of the chitosan conjugate in combination with 11-cis-6-ring-retinal, a locked ring analog of 11-cis-retinal that selectively blocks rod opsin consumption of chromophore while largely sparing cone opsins, was found to prolong cone vision in Lrat^-/- mice. Development of such combination low-dose regimens to selectively prolong useful cone vision could not only expand retinal disease treatments to include Leber congenital amaurosis but also the age-related decline in human dark adaptation from progressive retinoid cycle deficiency.

Modeling Retinal Diseases Using Genetic Approaches in Mice

Genetic mouse models mimicking human diseases have been developed and utilized for retinal research in various topics, involving anatomy, physiology, biochemistry, and pathology. The main reasons why mouse models are important for retinal research include that rodents share a key retinal homology with humans and that genetic manipulation is relatively easily applicable for mice. Here, we describe genetic mouse models, which are categorized with functions in the retina and relationship with human diseases.

Impact of LCA-Associated E14L LRAT Mutation on Protein Stability and Retinoid Homeostasis

Vitamin A (all-trans-retinol) is metabolized to the visual chromophore (11-cis-retinal) in the eyes and to all-trans-retinoic acid, a hormone like compound, in most tissues. A key enzyme in retinoid metabolism is lecithin:retinol acyltransferase (LRAT), which catalyzes the esterification of vitamin A. The importance of LRAT is indicated by pathogenic missense and nonsense mutations, which cause devastating blinding diseases. Retinoid-based chromophore replacement therapy has been proposed as treatment for these types of blindness based on studies in LRAT null mice. Here, we analyzed the structural and biochemical basis for retinal pathology caused by mutations in the human LRAT gene. Most LRAT missense mutations associated with retinal degeneration are localized within the catalytic domain, whereas E14L substitution is localized in an N-terminal α-helix, which has been implicated in interaction with the phospholipid bilayer. To elucidate the biochemical consequences of this mutation, we determined LRAT(E14L)'s enzymatic properties, protein stability, and impact on ocular retinoid metabolism. Bicistronic expression of LRAT(E14L) and enhanced green fluorescence protein revealed instability and accelerated proteosomal degradation of this mutant isoform. Surprisingly, instability of LRAT(E14L) did not abrogate the production of the visual chromophore in a cell-based assay. Instead, expression of LRAT(E14L) led to a rapid increase in cellular levels of retinoic acid upon retinoid supplementation. Thus, our study unveils the potential role of retinoic acid in the pathology of a degenerative retinal disease with important implications for the use of retinoid-based therapeutics in affected patients.

Receptor MER Tyrosine Kinase Proto-oncogene (MERTK) Is Not Required for Transfer of Bis-retinoids to the Retinal Pigmented Epithelium

Accumulation of bis-retinoids in the retinal pigmented epithelium (RPE) is a hallmark of aging and retinal disorders such as Stargardt disease and age-related macular degeneration. These aberrant fluorescent condensation products, including di-retinoid-pyridinium-ethanolamine (A2E), are thought to be transferred to RPE cells primarily through phagocytosis of the photoreceptor outer segments. However, we observed by two-photon microscopy that mouse retinas incapable of phagocytosis due to a deficiency of the c-Mer proto-oncogene tyrosine kinase (Mertk) nonetheless contained fluorescent retinoid condensation material in their RPE. Primary RPE cells from Mertk^-/- mice also accumulated fluorescent products in vitro Finally, quantification of A2E demonstrated the acquisition of retinal condensation products in Mertk^-/- mouse RPE prior to retinal degeneration. In these mice, we identified activated microglial cells that likely were recruited to transport A2E-like condensation products to the RPE and dispose of the dying photoreceptor cells. These observations demonstrate a novel transport mechanism between photoreceptor cells and RPE that does not involve canonical Mertk-dependent phagocytosis.

Molecular Basis for Vitamin A Uptake and Storage in Vertebrates

The ability to store and distribute vitamin A inside the body is the main evolutionary adaptation that allows vertebrates to maintain retinoid functions during nutritional deficiencies and to acquire new metabolic pathways enabling light-independent production of 11-cis retinoids. These processes greatly depend on enzymes that esterify vitamin A as well as associated retinoid binding proteins. Although the significance of retinyl esters for vitamin A homeostasis is well established, until recently, the molecular basis for the retinol esterification enzymatic activity was unknown. In this review, we will look at retinoid absorption through the prism of current biochemical and structural studies on vitamin A esterifying enzymes. We describe molecular adaptations that enable retinoid storage and delineate mechanisms in which mutations found in selective proteins might influence vitamin A homeostasis in affected patients.

Retinoids and Retinal Diseases

Recent progress in molecular understanding of the retinoid cycle in mammalian retina stems from painstaking biochemical reconstitution studies supported by natural or engineered animal models with known genetic lesions and studies of humans with specific genetic blinding diseases. Structural and membrane biology have been used to detect critical retinal enzymes and proteins and their substrates and ligands, placing them in a cellular context. These studies have been supplemented by analytical chemistry methods that have identified small molecules by their spectral characteristics, often in conjunction with the evaluation of models of animal retinal disease. It is from this background that rational therapeutic interventions to correct genetic defects or environmental insults are identified. Thus, most presently accepted modulators of the retinoid cycle already have demonstrated promising results in animal models of retinal degeneration. These encouraging signs indicate that some human blinding diseases can be alleviated by pharmacological interventions.

Lecithin:Retinol Acyltransferase: A Key Enzyme Involved in the Retinoid (visual) Cycle

Lecithin:retinol acyltransferase (LRAT) catalyzes the acyl transfer from the sn-1 position of phosphatidylcholine (PC) to all-trans-retinol, creating fatty acid retinyl esters (palmitoyl, stearoyl, and some unsaturated derivatives). In the eye, these retinyl esters are substrates for the 65 kDa retinoid isomerase (RPE65). LRAT is well characterized biochemically, and recent structural data from closely related family members of the NlpC/P60 superfamily and a chimeric protein have established its catalytic mechanism. Mutations in the LRAT gene are responsible for approximately 1% of reported cases of Leber congenital amaurosis (LCA). Lack of functional LRAT, expressed in the retinal pigmented epithelium (RPE), results in loss of the visual chromophore and photoreceptor degeneration. LCA is a rare hereditary retinal dystrophy with an early onset associated with mutations in one of 21 known genes. Protocols have been devised to identify therapeutics that compensate for mutations in RPE65, also associated with LCA. The same protocols can be adapted to combat dystrophies associated with LRAT. Improvement in the visual function of clinical recipients of therapy with recombinant adeno-associated virus (rAAV) vectors incorporating the RPE65 gene provides a proof of concept for LRAT, which functions in the same cell type and metabolic pathway as RPE65. In parallel, a clinical trial that employs oral 9-cis-retinyl acetate to replace the missing chromophore in RPE65 and LRAT causative disease has proven to be effective and free of adverse effects. This article summarizes the biochemistry of LRAT and examines chromophore replacement as a treatment for LCA caused by LRAT mutations.

Outer Segment Thickness Predicts Visual Field Response to QLT091001 in Patients with RPE65 or LRAT Mutations

PURPOSE

To determine whether the degree of change in Goldmann visual fields (GVFs) following oral administration of QLT091001 was related to baseline measures of retinal structure.

METHODS

Oral QLT091001 was administered once daily for 7 days in all study patients. Comprehensive ophthalmic testing, including spectral-domain optical coherence tomography (SD-OCT), was conducted in 14 patients with Leber congenital amaurosis (LCA) and 18 patients with retinitis pigmentosa (RP) at seven international sites. Average thickness of the outer segment (OS) layer was calculated over central 20°. Both eyes of each subject were evaluated separately.

RESULTS

Nineteen of 28 eyes (68%) with LCA and 13 of 36 eyes (36%) with RP responded to QLT091001. Among these responders, the average baseline thickness of the OS layer (central 20°) was 13.5 μm in the LCA cohort and 11.7 μm in the RP cohort. Nonresponders had average baseline OS thickness of less than 4.6 μm in both cohorts. The OS thickness in the central 20° was significantly shorter in nonresponders than responders in the LCA cohort (P = 0.01, t-test) and in the RP cohort (P = 0.02, Wilcoxon rank sum test). The OS thickness in the central 20° did not change significantly from baseline during the first 2 months (P = 0.09, t-test, paired).

CONCLUSIONS

The present findings suggest that there is a close parallel between the thickness of the photoreceptor layer and the potential for functional improvement in these patients.

TRANSLATIONAL RELEVANCE

SD-OCT thickness in the central retina may be useful for predicting the visual field response in the peripheral retina to QLT091001. (https://clinicaltrials.gov/ct2/show/NCT01014052 number, NCT01014052).

Gene Therapy of ABCA4-Associated Diseases

The ATP-binding cassette (ABC) transporter gene, ABCA4 (ABCR), was characterized in 1997 as the causal gene for autosomal recessive Stargardt disease (STGD1). Shortly thereafter several other phenotypes were associated with mutations in ABCA4, which now have collectively emerged as the most frequent cause of retinal degeneration phenotypes of Mendelian inheritance. ABCA4 functions as an important transporter (or "flippase") of vitamin A derivatives in the visual cycle. Several ways to alleviate the effects of the defective ABCA4 protein, which cause accumulation of 11-cis and all-trans-retinal in photoreceptors and lipofuscin in the retinal pigment epithelium, have been proposed. Although ABCA4 has proven to be a difficult research target, substantial progress through genetic, functional, and translational studies has allowed major advances in therapeutic applications for ABCA4-associated pathology, which should be available to patients in the (near) future. Here, we summarize the status of the gene therapy-based treatment options of ABCA4-associated diseases.

Supplementation with vitamin a derivatives to rescue vision in animal models of degenerative retinal diseases

The perception of light begins when photons reach retinal tissue located at the back of the eye and photoisomerize the visual chromophore 11-cis-retinal to all-trans-retinal within photoreceptor cells. Isomerization of 11-cis-retinal activates the protein rhodopsin located in photoreceptor outer segments, thereby inducing a phototransduction cascade leading to visual perception. To maintain vision, 11-cis-retinal is regenerated in the retinal pigmented epithelium (RPE) via the visual cycle and delivered back to the photoreceptor cells where it may again bind to rhodopsin. Distinct pathological mechanisms have been observed to contribute to inherited retinal degenerative diseases including severe delay in 11-cis-retinal regeneration and delayed clearance of all-trans-retinal, which leads to the accumulation of harmful retinoid by-products. In the last decade, our group has conducted several proof-of-concept (POC) studies with retinoid derivatives aimed at developing treatments for retinal degenerative diseases caused by an impaired visual cycle. Here, we will introduce experimental procedures, which have been developed for POC studies involving retinoid biology.

Highly efficient retinal gene delivery with helper-dependent adenoviral vectors

There have been significant advancements in the field of retinal gene therapy in the past several years. In particular, therapeutic efficacy has been achieved in three separate human clinical trials conducted to assess the ability of adeno-associated viruses (AAV) to treat of a type of Leber's congenital amaurosis caused by RPE65 mutations. However, despite the success of retinal gene therapy with AAV, challenges remain for delivering large therapeutic genes or genes requiring long DNA regulatory elements for controlling their expression. For example, Stargardt's disease, a form of juvenile macular degeneration, is caused by defects in ABCA4, a gene that is too large to be packaged in AAV. Therefore, we investigated the ability of helper dependent adenovirus (HD-Ad) to deliver genes to the retina as it has a much larger transgene capacity.

Using an EGFP reporter, our results showed that HD-Ad can transduce the entire retinal epithelium of a mouse using a dose of only 1 × 10⁵ infectious units and maintain transgene expression for at least 4 months. The results demonstrate that HD-Ad has the potential to be an effective vector for the gene therapy of the retina.

Oral 9-cis retinoid for childhood blindness due to Leber congenital amaurosis caused by RPE65 or LRAT mutations: an open-label phase 1b trial

BACKGROUND

Leber congenital amaurosis, caused by mutations in RPE65 and LRAT, is a severe form of inherited retinal degeneration leading to blindness. We aimed to assess replacement of the missing chromophore 11-cis retinal with oral QLT091001 (synthetic 9-cis-retinyl acetate) in these patients.

METHODS

In our open-label, prospective, phase 1b trial, we enrolled patients (aged ≥6 years) with Leber congenital amaurosis and RPE65 or LRAT mutations at McGill University's Montreal Children's Hospital. Patients received 7 days of oral QLT091001 (10-40 mg/m(2) per day). We assessed patients at baseline and days 7, 9, 14, and 30, and then 2 months and every 2 months thereafter for up to 2·2 years for safety outcomes and visual function endpoints including Goldmann visual fields (GVF), visual acuity, and functional MRI assessment. We regarded patients as having an improvement in vision if we noted at least a 20% improvement in retinal area on GVF compared with baseline or a visual acuity improvement of five or more letters compared with baseline in two consecutive study visits (or any improvement from no vision at baseline). This study is registered with ClinicalTrials.gov, number NCT01014052.

FINDINGS

Between December, 2009, and June, 2011, we enrolled and treated 14 patients aged 6-38 years who were followed up until March, 2012. Ten (71%) of 14 patients had an improvement in GVF areas (mean increase in retinal area of 28-683%). Six (43%) patients had an improvement in visual acuity (mean increase of 2-30 letters). Self-reported or parent-reported improvements in activities of daily living supported these findings. After 2 years, 11 (79%) patients had returned to their baseline GVF retinal area and ten (71%) had returned to baseline visual acuity letter values. Thus, three (21%) patients had a sustained GVF response and four (30%) had a sustained visual acuity response. Four patients had functional MRI scans, which correlated with visual response or absence of response to treatment. No serious adverse events occurred, although we noted transient headaches (11 patients), photophobia (11 patients), reduction in serum HDL concentrations (four patients), and increases in serum triglycerides (eight patients) and aspartate aminotransferase concentrations (two patients).

INTERPRETATION

Non-invasive oral QLT091001 therapy is well tolerated, and can rapidly improve visual function in some patients with Leber congenital amaurosis and RPE65 and LRAT mutations.

FUNDING

QLT, Foundation Fighting Blindness Canada, CIHR, FRSQ, Reseau Vision.

Chemistry and biology of the initial steps in vision: the Friedenwald lecture

Visual transduction is the process in the eye whereby absorption of light in the retina is translated into electrical signals that ultimately reach the brain. The first challenge presented by visual transduction is to understand its molecular basis. We know that maintenance of vision is a continuous process requiring the activation and subsequent restoration of a vitamin A-derived chromophore through a series of chemical reactions catalyzed by enzymes in the retina and retinal pigment epithelium (RPE). Diverse biochemical approaches that identified key proteins and reactions were essential to achieve a mechanistic understanding of these visual processes. The three-dimensional arrangements of these enzymes' polypeptide chains provide invaluable insights into their mechanisms of action. A wealth of information has already been obtained by solving high-resolution crystal structures of both rhodopsin and the retinoid isomerase from pigment RPE (RPE65). Rhodopsin, which is activated by photoisomerization of its 11-cis-retinylidene chromophore, is a prototypical member of a large family of membrane-bound proteins called G protein-coupled receptors (GPCRs). RPE65 is a retinoid isomerase critical for regeneration of the chromophore. Electron microscopy (EM) and atomic force microscopy have provided insights into how certain proteins are assembled to form much larger structures such as rod photoreceptor cell outer segment membranes. A second challenge of visual transduction is to use this knowledge to devise therapeutic approaches that can prevent or reverse conditions leading to blindness. Imaging modalities like optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) applied to appropriate animal models as well as human retinal imaging have been employed to characterize blinding diseases, monitor their progression, and evaluate the success of therapeutic agents. Lately two-photon (2-PO) imaging, together with biochemical assays, are revealing functional aspects of vision at a new molecular level. These multidisciplinary approaches combined with suitable animal models and inbred mutant species can be especially helpful in translating provocative cell and tissue culture findings into therapeutic options for further development in animals and eventually in humans. A host of different approaches and techniques is required for substantial progress in understanding fundamental properties of the visual system.

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.

Two-photon microscopy reveals early rod photoreceptor cell damage in light-exposed mutant mice

Atrophic age-related and juvenile macular degeneration are especially devastating due to lack of an effective cure. Two retinal cell types, photoreceptor cells and the adjacent retinal pigmented epithelium (RPE), reportedly display the earliest pathological changes. Abca4(-/-)Rdh8(-/-) mice, which mimic many features of human retinal degeneration, allowed us to determine the sequence of light-induced events leading to retinal degeneration. Using two-photon microscopy with 3D reconstruction methodology, we observed an initial strong retinoid-derived fluorescence and expansion of Abca4(-/-)Rdh8(-/-) mouse rod cell outer segments accompanied by macrophage infiltration after brief exposure of the retina to bright light. Additionally, light-dependent fluorescent compounds produced in rod outer segments were not transferred to the RPE of mice genetically defective in RPE phagocytosis. Collectively, these findings suggest that for light-induced retinopathies in mice, rod photoreceptors are the primary site of toxic retinoid accumulation and degeneration, followed by secondary changes in the RPE.

Persistence of non-viral vector mediated RPE65 expression: case for viability as a gene transfer therapy for RPE-based diseases

Mutations in the retinal pigment epithelium (RPE) gene RPE65 are associated with multiple blinding diseases including Leber's Congenital Amaurosis (LCA). Our goal has been to develop persistent, effective non-viral genetic therapies to treat this condition. Using precisely engineered DNA vectors and high capacity compacted DNA nanoparticles (NP), we previously demonstrated that both plasmid and NP forms of VMD2-hRPE65-S/MAR improved the disease phenotypes in an rpe65(-/-) model of LCA up to 6 months post-injection (PI), however the duration of this treatment efficacy was not established. Here, we test the ability of these vectors to sustain gene expression and phenotypic improvement for the life of the animal. NPs or naked DNA were subretinally injected in rpe65(-/-) mice at postnatal day (P) 16 and evaluated at 15 months PI. Quantitative real-time PCR (qRT-PCR) and immunofluorescence were performed at PI-15 months and demonstrated appreciable expression of transferred RPE65 (levels were 32% of wild-type [WT] for NPs and 44% of WT for naked DNA). No reduction in expression at the message level was observed from PI-6 month data. Spectral electroretinography (ERG) demonstrated significant improvement in cone ERG amplitudes in treated versus uninjected animals. Most importantly, we also observed reduced fundus autofluorescence in the eyes injected with NP and naked DNA compared to uninjected counterparts. Consistent with these observations, biochemical studies showed a reduction in the accumulation of toxic retinyl esters in treated mice, suggesting that the transferred hRPE65 was functional. These critical results indicate that both NP and uncompacted plasmid VMD2-hRPE65-S/MAR can mediate persistent, long-term improvement in an RPE-associated disease phenotype, and suggest that DNA NPs, which are non-toxic and have a large payload capacity, expand the treatment repertoire available for ocular gene therapy.

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.

Vitamin A derivatives as treatment options for retinal degenerative diseases

The visual cycle is a sequential enzymatic reaction for vitamin A, all-trans-retinol, occurring in the outer layer of the human retina and is essential for the maintenance of vision. The central source of retinol is derived from dietary intake of both retinol and pro-vitamin A carotenoids. A series of enzymatic reactions, located in both the photoreceptor outer segment and the retinal pigment epithelium, transform retinol into the visual chromophore 11-cis-retinal, regenerating visual pigments. Retina specific proteins carry out the majority of the visual cycle, and any significant interruption in this sequence of reactions is capable of causing varying degrees of blindness. Among these important proteins are Lecithin:retinol acyltransferase (LRAT) and retinal pigment epithelium-specific 65-kDa protein (RPE65) known to be responsible for esterification of retinol to all-trans-retinyl esters and isomerization of these esters to 11-cis-retinal, respectively. Deleterious mutations in these genes are identified in human retinal diseases that cause blindness, such as Leber congenital amaurosis (LCA) and retinitis pigmentosa (RP). Herein, we discuss the pathology of 11-cis-retinal deficiency caused by these mutations in both animal disease models and human patients. We also review novel therapeutic strategies employing artificial visual chromophore 9-cis-retinoids which have been employed in clinical trials involving LCA patients.

A review of therapeutic prospects of non-viral gene therapy in the retinal pigment epithelium

Ocular gene therapy has been extensively explored in recent years as a therapeutic avenue to target diseases of the cornea, retina and retinal pigment epithelium (RPE). Adeno-associated virus (AAV)-mediated gene therapy has shown promise in several RPE clinical trials but AAVs have limited payload capacity and potential immunogenicity. Traditionally however, non-viral alternatives have been plagued by low transfection efficiency, short-term expression and low expression levels. Recently, these drawbacks have begun to be overcome by the use of specialty carriers such as polylysine, liposomes, or polyethyleneimines, and by inclusion of suitable DNA elements to enhance gene expression and longevity. Recent advancements in the field have yielded non-viral vectors that have favorable safety profiles, lack immunogenicity, exhibit long-term elevated gene expression, and show efficient transfection in the retina and RPE, making them poised to transition to clinical applications. Here we discuss the advancements in nanotechnology and vector engineering that have improved the prospects for clinical application of non-viral gene therapy in the RPE.

S/MAR-containing DNA nanoparticles promote persistent RPE gene expression and improvement in RPE65-associated LCA

Mutations in genes in the retinal pigment epithelium (RPE) cause or contribute to debilitating ocular diseases, including Leber's congenital amaurosis (LCA). Genetic therapies, particularly adeno-associated viruses (AAVs), are a popular choice for monogenic diseases; however, the limited payload capacity of AAVs combined with the large number of retinal disease genes exceeding that capacity make the development of alternative delivery methods critical. Here, we test the ability of compacted DNA nanoparticles (NPs) containing a plasmid with a scaffold matrix attachment region (S/MAR) and vitelliform macular dystrophy 2 (VMD2) promoter to target the RPE, drive long-term, tissue-specific gene expression and mediate proof-of-principle rescue in the rpe65(-/-) model of LCA. We show that the S/MAR-containing plasmid exhibited reporter gene expression levels several fold higher than plasmid or NPs without S/MARs. Importantly, this expression was highly persistent, lasting up to 2 years (last timepoint studied). We therefore selected this plasmid for testing in the rpe65(-/-) mouse model and observe that NP or plasmid VMD2-hRPE65-S/MAR led to structural and functional improvements in the LCA disease phenotype. These results indicate that the non-viral delivery of hRPE65 vectors can result in persistent, therapeutically efficacious gene expression in the RPE.

QLT091001, a 9-cis-retinal analog, is well-tolerated by retinas of mice with impaired visual cycles

PURPOSE

Investigate whether retinas of mice with impaired retinal cycles exposed to light or kept in the dark tolerate prolonged high-dose administration of QLT091001, which contains as an active ingredient, the 9-cis-retinal precursor, 9-cis-retinyl acetate.

METHODS

Four- to six-week-old Lrat(-/-) and Rpe65(-/-) mice (n = 126) as well as crossbred Gnat1(-/-) mice lacking rod phototransduction (n = 110) were gavaged weekly for 6 months with 50 mg/kg QLT091001, either after being kept in the dark or after light bleaching for 30 min/wk followed by maintenance in a 12-hour light ≤ 10 lux)/12-hour dark cycle. Retinal health was monitored by spectral-domain optical coherent tomography (SD-OCT) and scanning laser ophthalmoscopy (SLO) every other month and histological, biochemical, and visual functional analyses were performed at the end of the experiment. Two-photon microscopy (TPM) was used to observe retinoid-containing retinosome structures in the RPE.

RESULTS

Retinal thickness and morphology examined by SD-OCT were well maintained in all strains treated with QLT091001. No significant increases of fundus autofluorescence were detected by SLO imaging of any strain. Accumulation of all-trans-retinyl esters varied with genetic background, types of administered compounds and lighting conditions but retinal health was not compromised. TPM imaging clearly revealed maintenance of retinosomes in the RPE of all mouse strains tested.

CONCLUSIONS

Retinas of Lrat(-/-), Rpe65(-/-), and crossbred Gnat1(-/-) mice tolerated prolonged high-dose QLT091001 treatment well.

Retinal degeneration in animal models with a defective visual cycle

Continuous generation of visual chromophore through the visual (retinoid) cycle is essential to maintain eyesight and retinal heath. Impairments in this cycle and related pathways adversely affect vision. In this review, we summarize the chemical reactions of vitamin A metabolites involved in the retinoid cycle and describe animal models of associated human diseases. Development of potential therapies for retinal disorders in these animal models is also introduced.

Autosomal recessive retinitis pigmentosa E150K opsin mice exhibit photoreceptor disorganization

The pathophysiology of the E150K mutation in the rod opsin gene associated with autosomal recessive retinitis pigmentosa (arRP) has yet to be determined. We generated knock-in mice carrying a single nucleotide change in exon 2 of the rod opsin gene resulting in the E150K mutation. This novel mouse model displayed severe retinal degeneration affecting rhodopsin's stabilization of rod outer segments (ROS). Homozygous E150K (KK) mice exhibited early-onset retinal degeneration, with disorganized ROS structures, autofluorescent deposits in the subretinal space, and aberrant photoreceptor phagocytosis. Heterozygous (EK) mice displayed a delayed-onset milder retinal degeneration. Further, mutant receptors were mislocalized to the inner segments and perinuclear region. Though KK mouse rods displayed markedly decreased phototransduction, biochemical studies of the mutant rhodopsin revealed only minimally affected chromophore binding and G protein activation. Ablation of the chromophore by crossing KK mice with mice lacking the critical visual cycle protein LRAT slowed retinal degeneration, whereas blocking phototransduction by crossing KK mice with GNAT1-deficient mice slightly accelerated this process. This study highlights the importance of proper higher-order organization of rhodopsin in the native tissue and provides information about the signaling properties of this mutant rhodopsin. Additionally, these results suggest that patients heterozygous for the E150K mutation should be periodically reevaluated for delayed-onset retinal degeneration.

A microparticle/hydrogel combination drug-delivery system for sustained release of retinoids

PURPOSE

To design and develop a drug-delivery system containing a combination of poly(D,L-lactide-co-glycolide) (PLGA) microparticles and alginate hydrogel for sustained release of retinoids to treat retinal blinding diseases that result from an inadequate supply of retinol and generation of 11-cis-retinal.

METHODS

To study drug release in vivo, either the drug-loaded microparticle-hydrogel combination was injected subcutaneously or drug-loaded microparticles were injected intravitreally into Lrat(-/-) mice. Orally administered 9-cis-retinoids were used for comparison and drug concentrations in plasma were determined by HPLC. Electroretinography (ERG) and both chemical and histologic analyses were used to evaluate drug effects on visual function and morphology.

RESULTS

Lrat(-/-) mice demonstrated sustained drug release from the microparticle/hydrogel combination that lasted 4 weeks after subcutaneous injection. Drug concentrations in plasma of the control group treated with the same oral dose rose to higher levels for 6-7 hours but then dropped markedly by 24 hours. Significantly increased ERG responses and a markedly improved retinal pigmented epithelium (RPE)-rod outer segment (ROS) interface were observed after subcutaneous injection of the drug-loaded delivery combination. Intravitreal injection of just 2% of the systemic dose of drug-loaded microparticles provided comparable therapeutic efficacy.

CONCLUSIONS

Sustained release of therapeutic levels of 9-cis-retinoids was achieved in Lrat(-/-) mice by subcutaneous injection in a microparticle/hydrogel drug-delivery system. Both subcutaneous and intravitreal injections of drug-loaded microparticles into Lrat(-/-) mice improved visual function and retinal structure.

AAV-mediated gene therapy in mouse models of recessive retinal degeneration

In recent years, more and more mutant genes that cause retinal diseases have been detected. At the same time, many naturally occurring mouse models of retinal degeneration have also been found, which show similar changes to human retinal diseases. These, together with improved viral vector quality allow more and more traditionally incurable inherited retinal disorders to become potential candidates for gene therapy. Currently, the most common vehicle to deliver the therapeutic gene into target retinal cells is the adenoassociated viral vector (AAV). Following delivery to the immuno-privileged subretinal space, AAV-vectors can efficiently target both retinal pigment epithelium and photoreceptor cells, the origin of most retinal degenerations. This review focuses on the AAV-based gene therapy in mouse models of recessive retinal degenerations, especially those in which delivery of the correct copy of the wild-type gene has led to significant beneficial effects on visual function, as determined by morphological, biochemical, electroretinographic and behavioral analysis. The past studies in animal models and ongoing successful LCA2 clinical trials, predict a bright future for AAV gene replacement treatment for inherited recessive retinal diseases.