Rhodopsin-associated retinal dystrophy: Disease mechanisms and therapeutic strategies

Recent Advances on Modeling Retinal Disease: Towards Efficient Gene/Drug Therapy

Advanced modeling biotechnologies are required to understand retinal diseases and develop effective treatments based on the patient's genetic background, lifestyle, and environment. In this work, recent advances in different types of study models that are used in the retinal disease area of research will be explored. The retinal models to be covered are: in vivo systems (human and animal), in vitro organisms (cell lines, primary cells, patient-derived stem cells, microfluidics, organoids, and spheroids), ex vivo models (explant cultures and retinal tissue preparations), and in silico models (computational and mathematical). Moreover, the unique comprehension of models of retinal disease, advantages, and disadvantages will be scrutinized. Finally, innovations/improvements derived from models towards gene and pharmacological therapy that display promise for treating retinal illnesses are elucidated.

Sex-specific attenuation of photoreceptor degeneration by reserpine in a rhodopsin P23H rat model of autosomal dominant retinitis pigmentosa

Inherited retinal degenerations (IRDs) constitute a group of clinically and genetically diverse vision-impairing disorders. Retinitis pigmentosa (RP), the most common form of IRD, is characterized by gradual dysfunction and degeneration of rod photoreceptors, followed by the loss of cone photoreceptors. Recently, we identified reserpine as a lead molecule for maintaining rod survival in mouse and human retinal organoids as well as in the rd16 mouse, which phenocopy Leber congenital amaurosis caused by mutations in the cilia-centrosomal gene CEP290 (Chen et al., 2023). Here, we show the therapeutic potential of reserpine in a rhodopsin P23H rat model of autosomal dominant RP. At postnatal day (P) 68, when males and females are analyzed together, the reserpine-treated rats exhibit higher rod-derived scotopic b-wave amplitudes compared to the controls with little or no change in scotopic a-wave or cone-derived photopic b-wave. Interestingly, the reserpine-treated female rats display enhanced scotopic a- and b-waves and photopic b-wave responses at P68, along with a better contrast threshold and increased outer nuclear layer thickness. The female rats demonstrate better preservation of both rod and cone photoreceptors following reserpine treatment. Retinal transcriptome analysis reveals sex-specific responses to reserpine, with significant upregulation of phototransduction genes and proteostasis-related pathways, and notably, genes associated with stress response. This study builds upon our previously reported results reaffirming the potential of reserpine for gene-agnostic treatment of IRDs and emphasizes the importance of biological sex in retinal disease research and therapy development.

Retina-Targeted 17β-Estradiol by the DHED Prodrug Rescues Visual Function and Actuates Neuroprotective Protein Networks After Optic Nerve Crush in a Rat Model of Surgical Menopause

The association between 17β-estradiol (E2) deprivation, seen in menopause, and a risk for developing glaucoma has been shown. Thus, exogenous supplementation of E2 may protect against retinal ganglion cell (RGC) degradation and vision loss. Here, we investigated the utility of topical 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED), a prodrug of E2 that selectively produces the neuroprotective hormone in the retina, on visual function after optic nerve crush (ONC) and ovariectomy (OVX). We used female Brown Norway rats that underwent either Sham or OVX surgeries. After ONC, OVX animals received DHED or vehicle eye drops for 12 weeks. Visual function, via the optomotor reflex, and retinal thickness, via optical coherence tomography, were followed longitudinally. Afterward, we performed mass spectrometry-based label-free retina proteomics to survey retinal protein interaction networks in our selected animal model and to identify E2-responsive proteins after OVX on neurodegeneration. We found that ONC with OVX caused a significant decline in visual functions that were ameliorated by DHED treatments. Discovery-driven retina proteomics identified numerous proteins associated with neurodegenerative processes due to ONC that were remediated by DHED eye drops. Altogether, our three-pronged phenotypic preclinical evaluation of the topical DHED in the OVX + ONC model of glaucoma reveals the therapeutic potential of the prodrug to prevent visual deficits after glaucomatous retinal injury.

Evaluation of subretinally delivered Cas9 ribonucleoproteins in murine and porcine animal models highlights key considerations for therapeutic translation of genetic medicines

Genetic medicines, including CRISPR/Cas technologies, extend tremendous promise for addressing unmet medical need in inherited retinal disorders and other indications; however, there remain challenges for the development of therapeutics. Herein, we evaluate genome editing by engineered Cas9 ribonucleoproteins (eRNP) in vivo via subretinal administration using mouse and pig animal models. Subretinal administration of adenine base editor and double strand break-inducing Cas9 nuclease eRNPs mediate genome editing in both species. Editing occurs in retinal pigmented epithelium (RPE) and photoreceptor cells, with favorable tolerability in both species. Using transgenic reporter strains, we determine that editing primarily occurs close to the site of administration, within the bleb region associated with subretinal injection. Our results show that subretinal administration of eRNPs in mice mediates base editing of up to 12% of the total neural retina, with an average rate of 7% observed at the highest dose tested. In contrast, a substantially lower editing efficiency was observed in minipigs; even with direct quantification of only the treated region, a maximum base editing rate of 1.5%, with an average rate of <1%, was observed. Our data highlight the importance of species consideration in translational studies for genetic medicines targeting the eye and provide an example of a lack of translation between small and larger animal models in the context of subretinal administration of Cas9 eRNPs.

In vivo adenine base editing ameliorates Rho-associated autosomal dominant retinitis pigmentosa

Mutations in the Rhodopsin (RHO) gene are the main cause of autosomal dominant retinitis pigmentosa (adRP), 84% of which are pathogenic gain-of-function point mutations. Treatment strategies for adRP typically involve silencing or ablating the pathogenic allele, while normal RHO protein replacement has no meaningful therapeutic benefit. Here, we present an adenine base editor (ABE)-mediated therapeutic approach for adRP caused by RHO point mutations in vivo. The correctable pathogenic mutations are screened and verified, including T17M, Q344ter, and P347L. Two adRP animal models are created carrying the class 1 (Q344ter) and class 2 (T17M) mutations, and dual AAV-delivered ABE can effectively repair both mutations in vivo. The early intervention of ABE8e efficiently corrects the Q344ter mutation that causes a severe form of adRP, delays photoreceptor death, and restores retinal function and visual behavior. These results suggest that ABE is a promising alternative to treat RHO mutation-associated adRP. Our work provides an effective spacer-mediated point mutation correction therapy approach for dominantly inherited ocular disorders.

Prevalence Estimates and Genetic Diversity for Autosomal Dominant Retinitis Pigmentosa Due to RHO, c.68C>A (p.P23H) Variant

OBJECTIVE

To provide the most up-to-date clinical prevalence estimate for autosomal dominant retinitis pigmentosa (adRP) patients due to RHO c.68C>A, (p.P23H) in the United States, supported by two independent approaches; literature based meta-analysis of reported patients and population genetics modeling.

DESIGN

Systematic review and meta-analysis plus population genetics modeling.

METHODS

Systematic review of the literature describing RP patients attributed to RHO variants was conducted to support a meta-analysis used to estimate the clinical prevalence of the RHO P23H patients diagnosed in the US. In parallel, large-scale genetic diversity studies describing the US population and non-European cohorts of the Americas (PAGE II), were evaluated to ascertain the allele frequencies of variant RHO c.68C>A, (p.P23H). The genetic prevalence for variant RHO c.68C>A, (p.P23H) was calculated using Hardy-Weinberg equilibrium. Further demographic data, including age and average age of onset for visual impairment were incorporated into a basic distribution model to estimate clinical prevalence of genetically predisposed persons.

RESULTS

The estimated clinical prevalence of adRP due to RHO P23H based on literature review was approximately 2000-3000 patients. In comparison the genetic prevalence of persons with RHO c.68C>A, (p.P23H) in the United States was an estimated 6176 (90% CI: 3333-11398) and only half of them are expected to cluster with European genetic ancestry. This variant was found enriched in subgroups of African American or other non-European biogeographic ancestries. Of the estimated 6200 persons carrying this variant in the US, ∼3500 (estimate range: 1900-6500) are expected to show clinical signs of visual impairment as modeled by average age of onset previously reported for patients with this variant.

CONCLUSIONS

We utilized two independent approaches to estimate the total number of adRP patients due to RHO c.68C>A, (p.P23H) in the United States; systematic literature review based meta-analysis and population genetics modeling. Both approaches yielded similar, overlapping estimates of adRP patients due to RHO P23H. However, comparison of these estimates provides some indication for a diagnosis gap. Unexpectedly, this variant is present at relatively higher frequency in some predominantly non-European genetic ancestries in the US. While this genetic analysis supports our estimates of clinical prevalence of adRP due to RHO P23H in the United States, it also has implications for diagnosing potential adRP patients due to this variant, raising questions of genotype-phenotype correlation and access to genetic testing.

Screening of Inherited Retinal Disease Patients in a Low-Resource Setting Using an Augmented Next-Generation Sequencing Panel

BACKGROUND

Inherited retinal diseases (IRDs) are a clinically and genetically heterogeneous group of disorders affecting millions worldwide. Despite the widespread adoption of next-generation sequencing (NGS) panels, there remains a critical gap in the genetically diverse and understudied African populations.

METHODS

One hundred and thirty-five South African patients affected by various IRDs underwent NGS using a custom-targeted panel sequencing over 100 known genes. The panel was supplemented by in silico screening for a MAK-Alu insertion and screening of seven functionally established deep intronic variants.

RESULTS

Through our combined screening strategy, we obtained a probable genetic diagnosis for 56% of the cohort. We identified 83 unique variants in 29 IRD genes underlying the disease, including 16 putative novel variants. Molecular findings prompted recommendations for clinical re-examination in ten patients. Resolution rates varied across clinical classifications and population groups.

CONCLUSIONS

This study reports the first use of a targeted NGS panel for IRDs in southern Africa, demonstrating a cost-effective, customisable approach that optimises both diagnostic yield and resource efficiency, making it a valuable tool for IRD molecular characterisation in resource-limited settings. Augmenting the panel by screening for variants relevant to South African patients allowed us to achieve a resolution rate in line with international studies. Our study underscores the importance of investigating diverse populations to bridge disparities in genomic research and improve diagnostic outcomes for underrepresented population groups.

Optimization of HITI-Mediated Gene Insertion for Rhodopsin and Peripherin-2 in Mouse Rod Photoreceptors: Targeting Dominant Retinitis Pigmentosa

Purpose

Among the genome-editing methods for repairing disease-causing mutations resulting in autosomal dominant retinitis pigmentosa, homology-independent targeted integration (HITI)-mediated gene insertion of the normal form of the causative gene is useful because it allows the development of mutation-agnostic therapeutic products. In this study, we aimed for the rapid optimization and validation of HITI-treatment gene constructs of this approach in developing HITI-treatment constructs for various causative target genes in mouse models of retinal degeneration.

Methods

We constructed the Cas9-driven HITI gene cassettes in plasmid vectors to treat the mouse Rho gene. A workflow utilizing in vivo electroporation was established to validate the efficacy of these constructs. Single-cell genotyping was conducted to evaluate allelic donor gene insertion. The therapeutic potency of HITI-treatment plasmid and adeno-associated virus (AAV) vectors was examined by section immunohistochemistry and optomotor response (OMR) in Rho+/P23H mutant mice. We also targeted mouse Prph2 to examine the workflow.

Results

The optimized HITI-treatment constructs for mouse Rho genes achieved gene insertion in 80% to 90% of transduced mouse rod photoreceptor cells. This construct effectively suppressed degeneration and induced visual restoration in mutant mice. HITI-treatment constructs for the Rhodopsin gene demonstrated efficacy in AAV vectors and are adaptable for the mouse Prph2 gene locus.

Conclusions

The study showcases a workflow for the rapid optimization and validation of highly effective HITI-treatment gene constructs against dominant-negative inheritance in inherited retinal dystrophy. These findings suggest the potential utility of this approach in developing HITI-treatment constructs for various target genes, advancing gene therapy products for diverse genetic disorders.

Retigabine increases the conformational stability of the visual photoreceptor rhodopsin

Rhodopsin is the key photoreceptor protein that mediates vision in low-light conditions. Mutations in rhodopsin are the cause of retinal degenerative diseases such as retinitis pigmentosa. Some of these mutations cause a decreased stability of the receptor. It is, therefore, of interest to find new approaches that can help improving rhodopsin conformational stability. In this study, we have analyzed the effect of retigabine, an anticonvulsant formerly used to treat epilepsy, on rhodopsin thermal stability, regeneration capacity, and signal transduction by means of UV-visible and fluorescence spectroscopic techniques. We find that retigabine enhances the thermal stability of dark-state rhodopsin and improves chromophore regeneration without disrupting the photobleaching process. Furthermore, retigabine does not significantly affect transducin activation. These results provide novel insights into the potential therapeutic applications of retigabine in the treatment of retinitis pigmentosa caused by rhodopsin mutations that cause a decreased stability of the mutated receptors.

RHO Variants and Autosomal Dominant Retinitis Pigmentosa: Insights from the Italian Genetic Landscape

Autosomal dominant retinitis pigmentosa (AD-RP) is caused by several genes, among which RHO is one of the most investigated. This article will be focused on RHO and its role in explaining AD-RP cases in the Italian population, taking advantage of the experience of the Genomic Medicine Laboratory UILDM at the Santa Lucia Foundation IRCCS. The retrospective evaluation of the distribution of RHO variants in the Italian patients with a clinical suspicion of RP pointed out eight variants. Of them, four variants (c.632A>T, c.1040C>T, c.1030C>T, c.383_392del) were pathogenic and made it possible to confirm the diagnosis of AD-RP in nine affected patients, highlighting a lower frequency (17%) of RHO variants compared to previous studies (30-40%). In addition, this study identified four variants classified as Variants of Uncertain Significance (VUS). In conclusion, the experience of the Genomic Medicine Laboratory provides an overview of the distribution of RHO variants in the Italian population, highlighting a slightly lower frequency of these variants in our cases series compared to previous reports. However, further studies on RHO variants are essential to characterize peculiar RP phenotypes and extend the spectrum of disease associated with this gene.

Minocycline prevents photoreceptor degeneration in Retinitis pigmentosa through modulating mitochondrial homeostasis

Minocycline, a broad-spectrum tetracycline antibiotic, has been shown to possess anti-inflammatory and antioxidative effects in various neurodegenerative diseases. However, its specific effects on retinitis pigmentosa (RP) have not been thoroughly investigated. Therefore, the objective of this study was to explore the potential role of minocycline in treating RP. In this investigation, we used rd1 to explore the antioxidant effect of minocycline in RP. Minocycline therapy effectively restored retinal function and structure in rd1 mice at 14 days postnatal. Additionally, minocycline inhibited the activation of microglia. Moreover, RNA sequencing analysis revealed a significant downregulation in the expression of mitochondrial genes within the retina of rd1 mice. Further KEGG and GO pathway analysis indicated impaired oxidative phosphorylation and electron transport chain processes. TEM confirmed the presence of damaged mitochondria in photoreceptors, while JC-1 staining demonstrated a decrease in mitochondrial membrane potential, accompanied by an increase in mitochondrial reactive oxygen species (ROS) levels. However, treatment with minocycline successfully reversed the abnormal expression of mitochondrial genes and reduced the levels of mitochondrial ROS, thereby providing protection against photoreceptor degeneration. Collectively, minocycline demonstrated the ability to rescue photoreceptor cells in RP by effectively modulating mitochondrial homeostasis and subsequently inflammation. These findings hold significant implications for the development of potential therapeutic strategies for RP.

Expression of red/green-cone opsin mutants K82E, P187S, M273K result in unique pathobiological perturbations to cone structure and function

Long-and middle-wavelength cone photoreceptors, which are responsible for our visual acuity and color vision, comprise ~95% of our total cone population and are concentrated in the fovea of our retina. Previously, we characterized the disease mechanisms of the L/M-cone opsin missense mutations N94K, W177R, P307L, R330Q and G338E, all of which are associated with congenital blue cone monochromacy (BCM) or color-vision deficiency. Here, we used a similar viral vector-based gene delivery approach in M-opsin knockout mice to investigate the pathogenic consequences of the BCM or color-vision deficient associated L-cone opsin (OPN1LW) mutants K82E, P187S, and M273K. We investigated their subcellular localization, the pathogenic effects on cone structure, function, and cone viability. K82E mutants were detected predominately in cone outer segments, and its expression partially restored expression and correct localization of cone PDE6α' and cone transducin γ. As a result, K82E also demonstrated the ability to mediate cone light responses. In contrast, expression of P187S was minimally detected by either western blot or by immunohistochemistry, probably due to efficient degradation of the mutant protein. M273K cone opsin appeared to be misfolded as it was primarily localized to the cone inner segment and endoplasmic reticulum. Additionally, M273K did not restore the expression of cone PDE6α' and cone transducin γ in dorsal cone OS, presumably by its inability to bind 11-cis retinal. Consistent with the observed expression pattern, P187S and M273K cone opsin mutants were unable to mediate light responses. Moreover, expression of K82E, P187S, and M273K mutants reduced cone viability. Due to the distinct expression patterns and phenotypic differences of these mutants observed in vivo, we suggest that the pathobiological mechanisms of these mutants are distinct.

Molecular mechanisms underlying inherited photoreceptor degeneration as targets for therapeutic intervention

Retinitis pigmentosa (RP) is a form of retinal degeneration characterized by primary degeneration of rod photoreceptors followed by a secondary cone loss that leads to vision impairment and finally blindness. This is a rare disease with mutations in several genes and high genetic heterogeneity. A challenging effort has been the characterization of the molecular mechanisms underlying photoreceptor cell death during the progression of the disease. Some of the cell death pathways have been identified and comprise stress events found in several neurodegenerative diseases such as oxidative stress, inflammation, calcium imbalance and endoplasmic reticulum stress. Other cell death mechanisms appear more relevant to photoreceptor cells, such as high levels of cGMP and metabolic changes. Here we review some of the cell death pathways characterized in the RP mutant retina and discuss preclinical studies of therapeutic approaches targeting the molecular outcomes that lead to photoreceptor cell demise.