Clinical spectrum, genetic complexity and therapeutic approaches for retinal disease caused by ABCA4 mutations

Functional and structural polypharmacology of indazole-based privileged ligands to tackle the undruggability of membrane transporters

Despite the significant roles of solute carrier (SLC) and ATP-binding cassette (ABC) transporters in human health and disease, most remain poorly characterized as intrinsic and/or xenobiotic ligands are unknown, rendering them as 'undruggable'. Polypharmacology, defined as the simultaneous engagement of multiple targets by a single ligand, offers a promising avenue for discovering novel lead compounds addressing these emerging pharmacological challenges - a major focus in contemporary medicinal chemistry. While common structural motifs among phylogenetically diverse proteins have been proposed to underlie polypharmacology through the concept of 'multitarget binding sites', a comprehensive analysis of these functional and structural aspects from a medicinal chemistry perspective has yet to be undertaken. In our study, we synthesized 65 distinct indazole derivatives and evaluated their activity across a broad biological assessment platform encompassing 17 specific and polyspecific SLC and ABC transporters. Notably, ten indazoles exhibited cross-target activity against challenging transporter targets associated with neurodegeneration (ABCA1), metabolic reprogramming (MCT4), and cancer multidrug resistance (ABCC10). Furthermore, molecular blind docking experiments and advanced binding site analyses revealed, for the first time, conserved binding motifs across monocarboxylate transporters (MCTs), organic anion transporting polypeptides (OATPs), organic cation transporters (OCTs), and ABC transporters, characterized by specific and recurring residues of tyrosine, phenylalanine, serine, and threonine. These findings highlight not only the potential of polypharmacology in drug discovery but also provide insights into the structural underpinnings of ligand binding across membrane transporters.

Exploring retinal conditions through blue light reflectance imaging

Blue light reflectance (BLR) imaging offers a non-invasive, cost-effective method for evaluating retinal structures by analyzing the reflectance and absorption characteristics of the inner retinal layers. By leveraging blue light's interaction with retinal tissues, BLR enhances visualization beyond the retinal nerve fiber layer, improving detection of structures such as the outer plexiform layer and macular pigment. Its diagnostic utility has been demonstrated in distinct retinal conditions, including hyperreflectance in early macular telangiectasia, hyporeflectance in non-perfused areas indicative of ischemia, identification of pseudodrusen patterns (notably the ribbon type), and detection of peripheral retinal tears and degenerative retinoschisis in eyes with reduced retinal pigment epithelial pigmentation. Best practices for image acquisition and interpretation are discussed, emphasizing standardization to minimize variability. Common artifacts and mitigation strategies are also addressed, ensuring image reliability. BLR's clinical utility, limitations, and future research directions are highlighted, particularly its potential in automated image analysis and quantitative assessment. Different BLR acquisition methods, such as fundus photography, confocal scanning laser ophthalmoscopy, and broad line fundus imaging, are evaluated for their respective advantages and limitations. As research advances, BLR's integration into multimodal workflows is expected to improve early detection and precise monitoring of retinal diseases.

Superotemporal predisposition to traumatic subretinal fibrosis in Stargardt disease: A case report

Purpose

Subretinal fibrosis has been reported as a presumed late sequela of orbital trauma in those with Stargardt disease (STGD). This case report highlights the sequential pathologic changes in response to trauma utilizing multimodal imaging.

Observations

An asymptomatic 19-year-old female with no significant ocular history presented for possible drusen. Initial imaging noted yellow-white pisciform perifoveal flecks in both eyes with corresponding hyper-and hypo-fluorescent lesions on fundus autofluorescence and hyperreflective deposits on near-infrared and spectral-domain optical coherence tomography (SD-OCT). A few months later, the patient presented with a new onset "black shadow" in the right eye after a traumatic periorbital injury, with multi-modal imaging revealing sequelae of commotio retinae superotemporally. Follow-up imaging three months later revealed a large patch of hyperpigmented chorioretinal scar corresponding to the region of commotio. SD-OCT delineated findings consistent with subretinal fibrosis. Given the constellation of findings and subsequent genetic testing, the patient was diagnosed with STGD.

Conclusions and importance

Multimodal imaging allows for the detection of traumatic transformation of STGD and monitoring for early signs of massive lipofuscin release within the immediate post-traumatic period. Given the impact of minor orbital trauma on prognosis, caution should be taken to minimize and prevent orbital trauma in patients with STGD.

MULTIMODAL IMAGING OF A NOVEL MFSD8/CLN7 MUTATION ASSOCIATED WITH NONSYNDROMIC SYMMETRIC ADULT-ONSET MACULAR DYSTROPHY

PURPOSE

The objective of this study was to report multimodal imaging features of a novel MFSD8/CLN7 pathogenic variant associated with bilateral and symmetric nonsyndromic macular dystrophy.

METHODS

A 63-year-old female patient presented complaining of a gradual subjective decline in VA in both eyes over the previous months. This patient underwent a comprehensive ophthalmological assessment, including multimodal retinal imaging and electrophysiological testing. Given suspicion for a hereditary retinal disorder, genetic testing was pursued.

RESULTS

The eye examination revealed blunted foveal reflexes and no lesions or abnormalities in the equatorial or anterior retinal periphery. Multimodal imaging showed a bilateral and almost symmetrical subfoveal interruption of the outer retinal layers, including an optical gap. Genetic testing revealed that the MFSD8/CLN7 gene exhibited a homozygous variant, specifically p.Ala484Val (c.1451C>T). This variant was identified as the likely causative factor for the condition.

CONCLUSION

In this study, the authors describe the clinical findings of a previously unreported homozygous variant in the MFSD8/CLN7 gene, resulting in a nonsyndromic form of bilateral central macular dystrophy.

Longitudinal Assessment of Structural and Functional Changes in Rod-cone Dystrophy: A 10-year Follow-up Study

Purpose

Emerging clinical trials for inherited retinal disease (IRD) require an understanding of long-term progression. This longitudinal study investigated the genetic diagnosis and change in retinal structure and function over 10 years in rod-cone dystrophies (RCDs).

Design

Longitudinal observational follow-up study.

Participants

Individuals initially diagnosed with retinitis pigmentosa who underwent baseline assessment between 2010 and 2013.

Methods

Baseline and follow-up assessments included best-corrected visual acuity (VA), Goldmann visual field (GVF) perimetry, spectral-domain OCT imaging, electroretinogram, and panel-based genetic testing. Linear mixed models were used to investigate disease progression and interaction between progression rate and baseline measurement. Interocular symmetry in disease progression was assessed using intraclass correlation coefficients (ICCs).

Main Outcome Measures

Change in VA, GVF area, and ellipsoid zone (EZ) width over 10 years in RCD.

Results

A total of 23 participants attended follow-up (mean age 63 ± 15 years at follow-up; 48% female), with 20 classified as having RCD and 3 reclassified as having cone-rod dystrophy based on genetic diagnosis. At 10-year follow-up, only 60% of RCD participants showed progression of ≥15 letters in either or both eyes, and 40% did not meet the criteria in either eye. Between the eye with poorer versus better VA at baseline, high symmetry in disease progression was observed for GVF area (ICC = 0.87; 95% confidence interval [CI]: 0.68-0.95), and moderate interocular symmetry in disease progression was observed for VA (ICC = 0.50 [95% CI: 0.07-0.77]) and EZ width (ICC = 0.64 [95% CI: 0.25-0.85]). Baseline values influenced progression for VA and percentage change in GVF area, whereas total percentage change in EZ width did not differ across baseline values.

Conclusions

Many individuals with RCD did not have a significant 15-letter decline in VA over a 10-year follow-up, highlighting the challenges of relying on VA as a measure of disease progression. Symmetry between eyes varies, emphasizing a key consideration for selection of outcome measures in IRD clinical trials.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Retina-on-chip: engineering functional in vitro models of the human retina using organ-on-chip technology

The retina is a complex and highly metabolic tissue in the back of the eye essential for human vision. Retinal diseases can lead to loss of vision in early and late stages of life, significantly affecting patients' quality of life. Due to its accessibility for surgical interventions and its isolated nature, the retina is an attractive target for novel genetic therapies and stem cell-based regenerative medicine. Understanding disease mechanisms and evaluating new treatments require relevant and robust experimental models. Retina-on-chip models are microfluidic organ-on-chip systems based on human tissue that capture multi-cellular interactions and tissue-level functions in vitro. Various retina-on-chip models have been described in literature. Some of them capture basic retinal barrier functions while others replicate key events underlying vision. In addition, some of these cellular systems have also been used in studies to explore their added value in retinal disease modeling. Most existing retina-on-chip models capture limited aspects of the phenotypic complexity of human diseases. This limitation arises primarily from the challenges related to controlled recapitulation of retinal function, including the relevant multi-cellular interactions and functional read-outs. In this review, we provide an update on recent advancements in the field of retina-on-chip, and we discuss the biotechnical strategies to further enhance the physiological relevance of the models. We emphasize that developers and researchers should prioritize the incorporation of the full spectrum of retinal complexity to effectuate a direct impact of retina-on-chip models in disease modeling and development of therapeutic strategies.

Exposure of A2E to blue light promotes ferroptosis in the retinal pigment epithelium

BACKGROUND

Age-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE) is closely related to the etiology of autosomal recessive Stargardt's disease (STGD1) and dry age-related macular degeneration (AMD). N-retinylidene-N-retinylethanolamine (A2E) is a leading component of RPE lipofuscin that is highly susceptible to blue light. Ferroptosis is an iron-dependent form of non-apoptotic cell death characterized by the accumulation of lipid peroxides to a lethal level, which plays an important role in retinal diseases. However, it remains unknown whether A2E functions as a physiological trigger for eliciting blue light-induced ferroptosis of RPE cells.

METHODS

A2E-loaded RPE cells and Abca4-/-Rdh8-/- mice were exposed to blue light, respectively. Western blotting, immunofluorescence staining, reactive oxygen species (ROS) staining, intracellular iron staining, lipid peroxidation staining, fundus imaging, optical coherence tomography (OCT), hematoxylin-eosin (HE) staining, and electroretinography (ERG) were utilized to elucidate the role of blue light in A2E induced ferroptosis in the RPE and its potential mechanisms.

RESULTS

Exposure of A2E to blue light promoted ferroptotic cell death in RPE cells by elevating ferrous ion (Fe2+) levels and inhibiting the solute carrier family 7 membrane 11 (SLC7A11)-glutathione (GSH)-glutathione peroxidase 4 (GPX4) axis. GPX4 inactivation and ROS generated by Fe2+ overload and GSH depletion precipitated lipid peroxidation and subsequent ferroptosis in A2E-containing RPE cells upon exposure to blue light. In addition to GSH supplement, repressing either Fe2+ by deferiprone (DFP) or lipid peroxidation with ferrostatin-1 (Fer-1) significantly protected RPE cells against ferroptosis caused by blue light illumination of A2E. Abca4-/-Rdh8-/- mice featured by an accelerated deposition of A2E in the RPE is an animal model for STGD1 and dry AMD. It was observed that ferroptosis was indeed present in the RPE of Abca4-/-Rdh8-/- mice following exposure to blue light. Notably, alleviating ferroptosis by intraperitoneally injected Fer-1 effectively rescued retinal function and ameliorated RPE/photoreceptor degeneration in blue light-exposed Abca4-/-Rdh8-/- mice.

CONCLUSIONS

Our results suggest the importance of blue light in A2E-mediated ferroptosis in the RPE, and deeply broaden the understanding of mechanisms underlying RPE atrophy arising from lipofuscin accumulation in STGD1 and dry AMD.

Unravelling genotype-phenotype correlations in Stargardt disease using patient-derived retinal organoids

Stargardt disease is an inherited retinopathy affecting approximately 1:8000 individuals. It is characterised by biallelic variants in ABCA4 which encodes a vital protein for the recycling of retinaldehydes in the retina. Despite its prevalence and impact, there are currently no treatments available for this condition. Furthermore, 35% of STGD1 cases remain genetically unsolved. To investigate the cellular and molecular characteristics associated with STGD1, we generated iPSCs from two monoallelic unresolved (PT1 & PT2), late-onset STGD1 cases with the heterozygous complex allele - c.[5461-10 T > C;5603 A > T]. Both patient iPSCs and those from a biallelic affected control (AC) carrying -c.4892 T > C and c.4539+2001G > A, were differentiated to retinal organoids, which developed all key retinal neurons and photoreceptors with outer segments positive for ABCA4 expression. We observed patient-specific disruption to lamination with OPN1MW/LW+ cone photoreceptor retention in the retinal organoid centre during differentiation. Photoreceptor retention was more severe in the AC case affecting both cones and rods, suggesting a genotype/phenotype correlation. scRNA-Seq suggests retention may be due to the induction of stress-related pathways in photoreceptors. Whole genome sequencing successfully identified the missing alleles in both cases; PT1 reported c.-5603A > T in homozygous state and PT2 uncovered a rare hypomorph - c.-4685T > C. Furthermore, retinal organoids were able to recapitulate the retina-specific splicing defect in PT1 as shown by long-read RNA-seq data. Collectively, these results highlight the suitability of retinal organoids in STGD1 modelling. Their ability to display genotype-phenotype correlations enhances their utility as a platform for therapeutic development.

Involvement of STAT3 activation in ameliorating all-trans-retinal-induced ferroptosis in photoreceptor-derived 661W cells in vitro

Ferroptosis, a form of iron-dependent programmed cell death, has emerged as a critical player in various diseases, including retinal degenerative disorders. Previous studies have highlighted that ferroptosis, triggered by all-trans-retinal (atRAL) accumulation in photoreceptor cells, contributes significantly to the pathogenesis of dry age-related macular degeneration (AMD) and autosomal recessive Stargardt's disease (STGD1). However, the underlying molecular mechanisms regulating this process remain poorly understood. In this study, we explore the involvement of signal transducer and activator of transcription 3 (STAT3) in the regulation of atRAL-induced 661W photoreceptor cells (mouse-derived photoreceptor cells) ferroptosis. We found that atRAL treatment induces phosphorylation of STAT3 in 661W photoreceptor cells. Meanwhile, we also discovered that the accumulation of Reactive oxygen species (ROS) induced by atRAL partly contributes to the activation of STAT3 in 661W photoreceptor cells. Importantly, our data suggest that inhibition of STAT3 phosphorylation, resulting in increased lipid peroxidation through upregulation of the acyl-CoA synthetase long-chain family member 4 (ACSL4) and prostaglandin-endoperoxide synthase 2 (PTGS2) gene, exacerbates ferroptosis in atRAL-loaded 661W photoreceptor cells. Additionally, our findings further confirm that STAT3 activator Colivelin may significantly reduce ferroptosis in 661W photoreceptor cells exposed to atRAL by regulating the ACSL4 and PTGS2 gene. Overall, these results revealed that activated STAT3 mitigates atRAL-induced ferroptosis in photoreceptor cells, possibly by reducing ACSL4 and PTGS2 gene expression. This pathway highlights the therapeutic potential of STAT3 as a novel target for treating dry AMD and STGD1.

Insights into eye genetics and recent advances in ocular gene therapy

The rapid advancements in the field of genetics have significantly propelled the development of gene therapies, paving the way for innovative treatments of various hereditary disorders. This review focuses on the genetics of ophthalmologic conditions, highlighting the currently approved ophthalmic gene therapy and exploring emerging therapeutic strategies under development. Inherited retinal dystrophies represent a heterogeneous group of genetic disorders that manifest across a broad spectrum from infancy to late middle age. Key clinical features include nyctalopia (night blindness), constriction of the visual field, impairments in color perception, reduced central visual acuity, and rapid eye movements. Recent technological advancements, such as multimodal imaging, psychophysical assessments, and electrophysiological testing, have greatly enhanced our ability to understand disease progression and establish genotype-phenotype correlations. Additionally, the integration of molecular diagnostics into clinical practice is revolutionizing patient stratification and the design of targeted interventions, underscoring the transformative potential of personalized medicine in ophthalmology. The review also covers the challenges and opportunities in developing gene therapies for other ophthalmic conditions, such as age-related macular degeneration and optic neuropathies. We discuss the viral and non-viral vector systems used in ocular gene therapy, highlighting their advantages and limitations. Additionally, we explore the potential of emerging technologies like CRISPR/Cas9 in treating genetic eye diseases. We briefly address the regulatory landscape, concerns, challenges, and future directions of gene therapy in ophthalmology. We emphasize the need for long-term safety and efficacy data as these innovative treatments move from bench to bedside.

High-efficiency base editing in the retina in primates and human tissues

Stargardt disease is a currently untreatable, inherited neurodegenerative disease that leads to macular degeneration and blindness due to loss-of-function mutations in the ABCA4 gene. We have designed a dual adeno-associated viral vector encoding a split-intein adenine base editor to correct the most common mutation in ABCA4 (c.5882G>A, p.Gly1961Glu). We optimized ABCA4 base editing in human models, including retinal organoids, induced pluripotent stem cell-derived retinal pigment epithelial (RPE) cells, as well as adult human retinal explants and RPE/choroid explants in vitro. The resulting gene therapy vectors achieved high levels of gene correction in mutation-carrying mice and in female nonhuman primates, with average editing of 75% of cones and 87% of RPE cells in vivo, which has the potential to translate to a clinical benefit. No off-target editing was detectable in human retinal explants and RPE/choroid explants. The high editing rates in primates show promise for efficient gene editing in other ocular diseases that are targetable by base editing.

Preclinical assessment of splicing modulation therapy for ABCA4 variant c.768G>T in Stargardt disease

BACKGROUND

Stargardt disease type 1 (STGD1) is a progressive retinal disorder caused by bi-allelic variants in the ABCA4 gene. A recurrent variant at the exon-intron junction of exon 6, c.768G>T, causes a 35-nt elongation of exon 6 that leads to premature termination of protein synthesis.

METHODS

To correct this aberrant splicing, twenty-five 2'-O-methoxyethyl antisense oligonucleotides (AONs) were designed, spanning the entire exon elongation.

RESULTS

Testing of these AONs in patient-derived photoreceptor precursor cells and retinal organoids allow the selection of a lead candidate AON (A7 21-mer) that rescues on average 52% and 50% expression of wild-type ABCA4 transcript and protein, respectively. In situ hybridization and probe-based ELISA demonstrate its distribution and stability in vitro and in vivo. No major safety concerns regarding off-targets, immunostimulation and toxicity are observed in transcriptomics analysis, cytokine stimulation assays in human primary immune cells, and cytotoxicity assays.

CONCLUSIONS

Additional optimization and in vivo studies will be performed to further investigate the lead candidate. Considering the high prevalence of this variant, a substantial number of patients are likely to benefit from a successful further development and implementation of this therapy.

ABCA4 Deep Intronic Variants Contributed to Nearly Half of Unsolved Stargardt Cases With a Milder Phenotype

Purpose

The purpose of this study was to investigate the contribution and natural progression of ABCA4 deep intronic variants (DIVs) among a Chinese Stargardt disease (STGD) cohort.

Methods

For unsolved STGD probands, DIVs in ABCA4 were detected by next-generation sequencing, and splicing effects were evaluated by in silico tools and validated through minigene experiments. Comprehensive ocular examinations, especially fundus changes, were carried out and analyzed. These and long-term follow-up data were compared with data of patients carrying biallelic coding variants of ABCA4.

Results

Seven DIVs in ABCA4 were identified in 18 of 40 (45.0%) unsolved STGD probands, involving 2 novel and 5 known variants. Four DIVs were confirmed to effect splicing through minigene assay. The c.161-395G>A was the most prevalent DIV allele (30.6%, 11/36). In the early 5-year duration, localized maculopathy was predominant, accounting for 51.9% (14/27) of fundus recordings. Expanded macular lesion within the vascular arch, with or without flecks, was observed in 75.0% (12/16) of recordings beyond the 5-year duration, whereas generalized retinal dystrophy was rarely observed. Compared with those in the non-DIV group, the patients in the DIV group manifested milder fundus change at all disease stages (P < 0.05). Follow-up visits utilizing wide-field fundus autofluorescence (FAF) further validated the slower development of lesions. Optical coherence tomography angiography (OCTA) documented a gradual reduction in perfusion in each layer's capillaries and high-reflective deposits below the sub-RPE layer.

Conclusions

DIVs contribute to nearly half of STGD cases with missing heritability, totally occupying 7.8% of all STGD families. Based on optimized grading criteria, patients with DIV alleles manifested localized macular lesions with slow progress. The amount of residual correctly spliced mRNA might play a role, suggesting that adding or enhancing normal ABCA4 expression might be a potential approach of intervention.

Progression Rate of Macular Retinal Pigment Epithelium Atrophy in Geographic Atrophy and Selected Inherited Retinal Dystrophies. A Systematic Review and Meta-Analysis

PURPOSE

To compare the macular retinal pigment epithelium (RPE) atrophy progression rate of selected degenerative and macular inherited retinal diseases (IRD).

DESIGN

Systematic review and meta-analysis.

METHODS

The protocol was registered on the PROSPERO database. Medline, Embase, Web of Science, Cochrane Central Register of Controlled Trials, and Google Scholar were searched up to September 15, 2023 for articles reporting the RPE atrophy growth rate in treatment-naïve eyes with geographic atrophy (GA), Stargardt disease (STGD1), Best disease, pseudoxanthoma elasticum (PXE), central areolar choroidal dystrophy (CACD), or pattern dystrophies with no previous or current macular neovascularization and a minimum follow-up time of 12 months. Meta-analyses determined mean RPE atrophy growth rates per disease, imaging modality (fundus autofluorescence [FAF], optical coherence tomography [OCT], or color fundus photography [CFP]) and metric (mm2/y or mm/y). The Newcastle-Ottawa scale and the Cochrane Risk-of-Bias tool assessed the risk of bias, and funnel plots were used to evaluate small-study effects.

RESULTS

From 4354 publications, 85 were included for meta-analysis: 69 studies (7815 eyes) on GA, 15 (1367 eyes) on STGD1, and one on both. Two studies on PXE were only eligible for review. No studies for other diseases met our eligibility criteria. The overall mean RPE atrophy growth rate for GA using FAF was 1.65 mm2/y (95% confidence interval [CI], 1.49-1.81) and 0.35 mm/y (95% CI, 0.28-0.41); using OCT, it was 1.46 mm2/y (95% CI, 1.28-1.64) and 0.34 mm/y (95% CI, 0.28-0.40); and on CFP it was 1.76 mm2/y (95% CI, 1.56-1.97) and 0.30 mm/y (95% CI, 0.28-0.31). For STGD1, using FAF it was 1.0 mm2/y (95% CI, 0.77-1.23) and 0.20 mm/y (95% CI, 0.17-0.23); on OCT, it was 0.80 mm2/y (95% CI, 0.72-0.88). No studies on STGD1 reported the growth rate with other imaging modalities or metrics. Growth rates in GA were faster than in STGD1 (p < .05). A larger baseline area of atrophy was generally associated with faster growth rates.

CONCLUSIONS

The RPE atrophy growth rate in GA is faster than in STGD1 but with great variation between studies and imaging modalities. Limited information was available for other macular IRD, suggesting further research is needed.

Abca4, mutated in Stargardt disease, is required for structural integrity of cone outer segments

Stargardt disease (STGD), the leading cause of inherited childhood blindness, is primarily caused by mutations in the ABCA4 gene; yet, the underlying mechanisms of photoreceptor degeneration remain elusive, partly due to limitations in existing animal disease models. To expand our understanding, we mutated the human ABCA4 paralogues abca4a and abca4b in zebrafish, which has a cone-rich retina. Our study unveiled striking dysmorphology and elongation of cone outer segments (COS) in abca4a;abca4b double mutants, alongside reduced phagocytosis by the retinal pigmented epithelium (RPE). We report that zebrafish Abca4 protein forms a distinctive stripe along the length of COS, suggesting a potential structural role. We further show that, in wild-type zebrafish, outer segments of cone cells constitutively present externalized phosphatidylserine, an apoptotic 'eat-me' signal, and that this pattern is disrupted in abca4a;abca4b double mutants, potentially contributing to reduced RPE phagocytic activity. More broadly, constitutive presentation of the 'eat-me' signal by COS - if conserved in humans - might have important implications for other retinal degenerative diseases, including age-related macular degeneration. Our zebrafish model provides novel insights into cone dysfunction and presents a promising platform for unraveling the mechanisms of STGD pathogenesis and advancing therapeutic interventions.

Interruption of the visual cycle in a novel animal model induces progressive vision loss resembling Stargardts Disease

Mutations in the gene ABCA4 coding for photoreceptor-specific ATP-binding cassette subfamily A member 4, are responsible for Stargardts Disease type 1 (STGD1), the most common form of inherited macular degeneration. STGD1 typically declares early in life and leads to severe visual handicap. Abca4 gene-deletion mouse models of STGD1 accumulate lipofuscin, a hallmark of the disease, but unlike the human disease show no or only moderate structural changes and no functional decline. The human macula is highly enriched in cones, and reasoning that the low cone percentage in mice retinas (< 3%) might compromise faithful modelling of human maculopathies, we performed sub-retinal injections of CRISPR/Cas9-abca4 Adeno-Associated Virus constructs into young Sand Rats (Psammomys obesus), a diurnal rodent containing > 30% cones. Compared to control injections of AAV-abca4-GFP, treated eyes exhibited extensive retinal degeneration by two months. Sanger sequencing of the CRISPR targeted sequence show a clear edition of Abca4 gene. Non-invasive fundus imaging showed widespread photoreceptor loss, confirmed by ocular coherence tomography. Functional recording by single flash and flicker electroretinography showed significant decline in photopic (cone) light responses. Post-mortem real-time PCR, immunohistochemistry and western blotting showed significant decrease of cone-specific (MW cone opsin) but not rod-specific (rhodopsin) markers. Transmission electron microscopy showed large numbers of lipid inclusions in treated but not control retinal pigmented epithelium. Finally, ultra-high performance liquid chromatography analysis of whole P. obesus eyes showed the presence of all-trans retinal-dimer, not detected in rod-rich rat eyes. In conclusion, Abca4 knockout in P. obesus results in a predominantly cone degeneration phenotype, more accurately reflecting the etiology of human STGD1, and should be valuable for characterizing pathogenic pathways and exploring treatment options.

Syndromic Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a progressive inherited retinal dystrophy, characterized by the degeneration of photoreceptors, presenting as a rod-cone dystrophy. Approximately 20-30% of patients with RP also exhibit extra-ocular manifestations in the context of a syndrome. This manuscript discusses the broad spectrum of syndromes associated with RP, pathogenic mechanisms, clinical manifestations, differential diagnoses, clinical management approaches, and future perspectives. Given the diverse clinical and genetic landscape of syndromic RP, the diagnosis may be challenging. However, an accurate and timely diagnosis is essential for optimal clinical management, prognostication, and potential treatment. Broadly, the syndromes associated with RP can be categorized into ciliopathies, inherited metabolic disorders, mitochondrial disorders, and miscellaneous syndromes. Among the ciliopathies associated with RP, Usher syndrome and Bardet-Biedl syndrome are the most well-known. Less common ciliopathies include Cohen syndrome, Joubert syndrome, cranioectodermal dysplasia, asphyxiating thoracic dystrophy, Mainzer-Saldino syndrome, and RHYNS syndrome. Several inherited metabolic disorders can present with RP including Zellweger spectrum disorders, adult Refsum disease, α-methylacyl-CoA racemase deficiency, certain mucopolysaccharidoses, ataxia with vitamin E deficiency, abetalipoproteinemia, several neuronal ceroid lipofuscinoses, mevalonic aciduria, PKAN/HARP syndrome, PHARC syndrome, and methylmalonic acidaemia with homocystinuria type cobalamin (cbl) C disease. Due to the mitochondria's essential role in supplying continuous energy to the retina, disruption of mitochondrial function can lead to RP, as seen in Kearns-Sayre syndrome, NARP syndrome, primary coenzyme Q10 deficiency, SSBP1-associated disease, and long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Lastly, Cockayne syndrome and PERCHING syndrome can present with RP, but they do not fit the abovementioned hierarchy and are thus categorized as 'Miscellaneous'. Several first-in-human clinical trials are underway or in preparation for some of these syndromic forms of RP.

Lipopeptide-mediated Cas9 RNP delivery: A promising broad therapeutic strategy for safely removing deep-intronic variants in ABCA4

Deep-intronic (DI) variants represent approximately 10%-12% of disease-causing genetic defects in ABCA4-associated Stargardt disease (STGD1). Although many of these DI variants are amenable to antisense oligonucleotide-based splicing-modulation therapy, no treatment is currently available. These molecules are mostly variant specific, limiting their applicability to a broader patient population. In this study, we investigated the therapeutic potential of the CRISPR-Cas9 system combined with the amphipathic lipopeptide C18:1-LAH5 for intracellular delivery to correct splicing defects caused by different DI variants within the same intron. The combination of these components facilitated efficient editing of two target introns (introns 30 and 36) of ABCA4 in which several recurrent DI variants are found. The partial removal of these introns did not affect ABCA4 splicing or its expression levels when assessed in two different human cellular models: fibroblasts and induced pluripotent stem cell-derived photoreceptor precursor cells (PPCs). Furthermore, the DNA editing in STGD1 patient-derived PPCs led to a ∼50% reduction of the pseudoexon-containing transcripts resulting from the c.4539+2001G>A variant in intron 30. Overall, we provide proof-of-concept evidence of the use of C18:1-LAH5 as a delivery system for therapeutic genome editing for ABCA4-associated DI variants, offering new opportunities for clinical translation.

The value of age of onset and family history as predictors of molecular diagnosis in a Swedish cohort of inherited retinal disease

PURPOSE

This study aimed to characterize clinical and genetic findings in a Swedish cohort with inherited retinal disease (IRD), identify predictors for achieving a molecular diagnosis and evaluate the effects of increased genetic testing over time.

METHODS

Clinical and genetic data from 324 nonrelated IRD index individuals referred for genetic testing in the Stockholm region between 2016 and 2023 were collected retrospectively and analysed by clinical subtype, age of onset and testing period (2016-2020 vs. 2021-2023). Logistic regression was used to calculate odds ratios for age of onset and family history on the likelihood of achieving a molecular diagnosis.

RESULTS

The diagnostic yield was 55% and involved 56 genes. In 10% of solved individuals, the molecular diagnosis refined the clinical diagnosis. For each 1-year increase in age of onset, the odds of achieving a molecular diagnosis decreased by 3% (odds ratio 0.97, 95% confidence interval 0.96-0.98). A positive family history doubled the odds (odds ratio 2.1, 95% confidence interval 1.3-3.4). The use of genetic testing increased 2.1-fold and the number of molecular diagnoses increased 1.6-fold relative to the population of the Stockholm region between the two testing periods.

CONCLUSION

This study adds to the knowledge of the clinical and genetic landscape of IRDs in Sweden and establishes age of onset and family history as significant predictors for achieving a molecular diagnosis. Increased genetic testing on a population level substantially increased the number of individuals receiving a molecular diagnosis with a high diagnostic yield compared to other rare diseases.

Efficacy and safety of minocycline in retinitis pigmentosa: a prospective, open-label, single-arm trial

Retinitis pigmentosa (RP) is characterized by progressive photoreceptor cells death accelerated by the proliferation and activation of microglia pathologically. No consensus exists on the treatment. Minocycline is recognized as a microglia inhibitor with great anti-inflammatory and neuro-protective functions. However, efficacy of minocycline in RP patients is lacking. We conducted a prospective, open-label, and single-arm trial, in which daily oral minocycline of 100 mg was administered for 12 months in RP patients with light-adapted 30 Hz flicker electroretinography (ERG) amplitude >0 µV in at least one eye (NCT04068207). The primary outcome was the proportion of participants with improvement in the ERG amplitude at month 12. The secondary outcomes included improvements of the following items: other ERGs amplitudes, visual field, best-corrected visual acuity, contrast sensitivity, color vision, and NEI-VFQ-25. 35 of 288 patients met inclusive criteria were enrolled (median [IQR] age, 36 [31-45] years; 17 female [48.6%]). 32 participants completed all examinations, while 3 participants completed the 12-month online visit via conducting NEI-VFQ-25. The primary outcome showed improvement was 34.3% (12 of 35 [95% CI 19.1-52.2]). Similarly, all secondary outcomes showed improvements. Adverse events were reported in 22 participants (62.9%) and were all resolved without extra medication during the study period. No severe adverse events were recorded. Our findings identified daily oral minocycline of 100 mg for 12 months was beneficial in improving the visual function of RP patients with good safety. This study indicates minocycline may be a promising therapy for RP, but a randomized controlled trial is still needed of further exploration.

Distinguishing ABCA4 from PRPH2-related disease: qualitative analysis of examination and imaging features

INTRODUCTION

ABCA4 and PRPH2-related diseases are both phenotypically heterogeneous and clinically difficult to differentiate. There may be examination and imaging features that can aid in establishing a clinical diagnosis.

METHODS

A single-center, retrospective, consecutive case series including patients with a molecular confirmation of pathologic variants in either the ABCA4 or PRPH2 were included. Chi-square analysis, Fisher exact test, and Student's t-test comparing prevalence of specific examination and imaging features between ABCA4 and PRPH2.

RESULTS

Of the 127 eyes from 64 patients included, the ABCA4 group was more significantly associated with peripapillary sparing on both fundus imaging (73% vs. 40%; p = 0.006) and FAF (71% vs. 44%; p = 0.025), macular (64% vs. 12%; p < 0.001) and peripheral pisciform flecks (22% vs. 3.6%; p = 0.025). The PRPH2 group was more highly associated with macular chorioretinal atrophy (86% vs. 55%; p = 0.003).

CONCLUSIONS

Peripapillary sparing and pisciform flecks are more highly associated with ABCA4-related disease, while macular chorioretinal atrophy is more highly associated with PRPH2-related disease. Logistic regression demonstrates that bull's eye maculopathy and macular flecks are predictive of the ABCA4 genotype.

Rescue of Aberrant Splicing Caused by a Novel Complex Deep-intronic ABCA4 Allele

Background/Objectives: Stargardt disease (STGD1) is an autosomal recessive disorder caused by pathogenic variants in ABCA4 that affects the retina and is characterised by progressive central vision loss. The onset of disease manifestations varies from childhood to early adulthood. Methods: Whole exome (WES), whole gene, and whole genome sequencing (WGS) were performed for a patient with STGD1. Results: WES revealed a heterozygous pathogenic missense variant in ABCA4, but no second pathogenic variant was found. ABCA4 whole-gene sequencing, subsequent WGS, and segregation analysis identified a complex deep-intronic allele (NM_000350.2(ABCA4):c.[1555-5882C>A;1555-5784C>G]) in trans to the missense variant. Minigene assays combined with nanopore sequencing were performed to characterise this deep-intronic complex allele in more detail. Surprisingly, the reference minigene revealed the existence of two pseudoexons in intron 11 of the ABCA4 gene that are included in low-abundance (<1%) transcripts. Both pseudoexons could be confirmed in cDNA derived from wildtype retinal organoids. Despite mild splicing predictions, the variant minigene revealed that the complex deep-intronic allele substantially increased the abundance of transcripts that included the pseudoexon overlapping with the variants. Two antisense oligonucleotides (AONs) were designed to rescue the aberrant splicing events. Both AONs increased the proportion of correctly spliced transcripts, and one of them rescued correct splicing to reference levels. Conclusions: Minigene assays combined with nanopore sequencing proved instrumental in identifying low-abundance transcripts including pseudoexons from wildtype ABCA4 intron 11, one of which was substantially increased by the complex allele.

Characterization of the Subclinical Perilesional Zone in the Macula of Early-Stage ABCA4 Disease

Purpose

To characterize photoreceptor layer thinning in clinically unremarkable regions adjacent to the atrophic lesion in early-stage ABCA4 disease eyes.

Methods

27 patients with confined atrophic lesions (≤ 3.5mm in diameter) were included. Two pathogenic alleles were confirmed by sequencing of the ABCA4 locus. Multimodal imaging included color fundus photography, short wavelength-autofluorescence (SW-AF) and near infrared-autofluorescence (NIR-AF) imaging. Total receptor+ (TREC+) thickness was segmented in spectral domain-optical coherence tomography (SD-OCT) scans in patient eyes (n=27) along with age-matched healthy control eyes (n=20).

Results

μ age of the study cohort was 24.1 years and 15/27 (55.6%) patients harbored genotypes consisting of the p.(Gly1961Glu) variant in ABCA4. Atrophic lesions in the central macula ranged from 0.61 to 3.13 mm in diameter (μ = 1.73, σ = 0.70). Six patients had mild RPE mottling adjacent to the lesion on NIR-AF. The atrophic lesion corresponded to a disruption of photoreceptor-attributable bands on SD-OCT while all layers were visibly intact outside the lesion. TREC+ thickness in patient eyes were <0.15 mm or below 4σ of normal control eyes immediately adjacent to the lesion edge and gradually normalized to within ± 2σ at ≈ 1.2 mm eccentricity from the fovea.

Conclusion

A uniform subclinical perilesional zone (SPZ) of photoreceptor thinning extends around the perimeter of early-stage atrophic lesions in ABCA4 disease. This region spatially maps to known regions of vision loss and more accurately approximates the extent of photoreceptor abnormality compared to the disease changes visible on standard fundus imaging.

Translational relevance

Semi-automated segmentation of SD-OCT scans identifies a consistent subclinical biomarker relevant to early photoreceptor degeneration in ABCA4 disease.

Advances in retinal pigment epithelial cell transplantation for retinal degenerative diseases

Retinal degenerative diseases are a leading cause of vision loss and blindness globally, impacting millions. These diseases result from progressive damage to retinal pigment epithelial (RPE) cells for which no curative or palliative treatments exist. Cell therapy, particularly RPE transplantation, has emerged as a promising strategy for vision restoration. This review provides a comprehensive overview of the recent advancements in clinical trials related to RPE transplantation. We discuss scaffold-free and scaffold-based approaches, including RPE cell suspensions and pre-organized RPE monolayers on biomaterial scaffolds. Key considerations, such as the form and preparation of RPE implants, delivery devices, strategies, and biodegradability of scaffolds, are examined. The article also explores the challenges and opportunities in RPE scaffold development, emphasising the crucial need for functional integration, immunomodulation, and long-term biocompatibility to ensure therapeutic efficacy. We also highlight ongoing efforts to optimise RPE transplantation methods and their potential to address retinal degenerative diseases.

Presentation and Clinical Features of Stargardt Disease in a Series of Nigerian Patients

Stargardt disease (SD) is a common inherited macular dystrophy. It exhibits a high degree of phenotypic and genotypic heterogeneity. Yellow-white flecks are often found in the posterior pole in the early stages of the disease with a reduction in central vision from foveal atrophy as it progresses. A characteristic dark choroid appearance is seen on fundus fluorescein angiography (FFA) in many cases, with occasional reports of choroidal neovascular membranes. We report a series of four Nigerian patients, with varied presentations diagnosed with SD in our facility. One patient had good vision, while the other three had variable degrees of reduced vision. All patients had macular atrophy and flecks, while three patients had a dark choroid appearance on FFA and one patient developed a choroidal neovascular membrane in one eye.

Virus-like particles as robust tools for functional assessment: Deciphering the pathogenicity of ABCA4 genetic variants of uncertain significance

The retina-specific ABCA transporter, ABCA4, is essential for vision, and its genetic variants are associated with a wide range of inherited retinal degenerative diseases, leading to blindness. Of the 1630 identified missense variants in ABCA4, ∼50% are of unknown pathogenicity (variants of unknown significance, VUS). This genetic uncertainty presents three main challenges: (i) inability to predict disease-causing variants in relatives of inherited retinal degenerative disease patients with multiple ABCA4 mutations; (ii) limitations in developing variant-specific treatments; and (iii) difficulty in using these variants for future disease prediction, affecting patients' life-planning and clinical trial participation. To unravel the clinical significance of ABCA4 genetic variants at the level of protein function, we have developed a virus-like particle-based system that expresses the ABCA4 protein and its variants. We validated the efficacy of this system in the enzymatic characterization (ATPase activity) of VLPs harboring ABCA4 and two variants of established pathogenicity: p.N965S and p.C1488R. Our results were consistent with previous reports and clinical phenotypes. We also applied this platform to characterize the VUS p.Y1779F and observed a functional impairment, suggesting a potential pathogenic impact. This approach offers an efficient, high-throughput method for ABCA4 VUS characterization. Our research points to the significant promise of the VLP-based system in the functional analysis of membrane proteins, offering important perspectives on the disease-causing potential of genetic variants and shedding light on genetic conditions involving such proteins.

Nanopore Deep Sequencing as a Tool to Characterize and Quantify Aberrant Splicing Caused by Variants in Inherited Retinal Dystrophy Genes

The contribution of splicing variants to molecular diagnostics of inherited diseases is reported to be less than 10%. This figure is likely an underestimation due to several factors including difficulty in predicting the effect of such variants, the need for functional assays, and the inability to detect them (depending on their locations and the sequencing technology used). The aim of this study was to assess the utility of Nanopore sequencing in characterizing and quantifying aberrant splicing events. For this purpose, we selected 19 candidate splicing variants that were identified in patients affected by inherited retinal dystrophies. Several in silico tools were deployed to predict the nature and estimate the magnitude of variant-induced aberrant splicing events. Minigene assay or whole blood-derived cDNA was used to functionally characterize the variants. PCR amplification of minigene-specific cDNA or the target gene in blood cDNA, combined with Nanopore sequencing, was used to identify the resulting transcripts. Thirteen out of nineteen variants caused aberrant splicing events, including cryptic splice site activation, exon skipping, pseudoexon inclusion, or a combination of these. Nanopore sequencing allowed for the identification of full-length transcripts and their precise quantification, which were often in accord with in silico predictions. The method detected reliably low-abundant transcripts, which would not be detected by conventional strategies, such as RT-PCR followed by Sanger sequencing.

Guidance for estimating penetrance of monogenic disease-causing variants in population cohorts

Penetrance is the probability that an individual with a pathogenic genetic variant develops a specific disease. Knowing the penetrance of variants for monogenic disorders is important for counseling of individuals. Until recently, estimates of penetrance have largely relied on affected individuals and their at-risk family members being clinically referred for genetic testing, a 'phenotype-first' approach. This approach substantially overestimates the penetrance of variants because of ascertainment bias. The recent availability of whole-genome sequencing data in individuals from very-large-scale population-based cohorts now allows 'genotype-first' estimates of penetrance for many conditions. Although this type of population-based study can underestimate penetrance owing to recruitment biases, it provides more accurate estimates of penetrance for secondary or incidental findings. Here, we provide guidance for the conduct of penetrance studies to ensure that robust genotypes and phenotypes are used to accurately estimate penetrance of variants and groups of similarly annotated variants from population-based studies.

Structural and functional characterization of the nucleotide-binding domains of ABCA4 and their role in Stargardt disease

ABCA4 is an ATP-binding cassette (ABC) transporter that prevents the buildup of toxic retinoid compounds by facilitating the transport of N-retinylidene-phosphatidylethanolamine across membranes of rod and cone photoreceptor cells. Over 1500 missense mutations in ABCA4, many in the nucleotide-binding domains (NBDs), have been genetically linked to Stargardt disease. Here, we show by cryo-EM that ABCA4 is converted from an open outward conformation to a closed conformation upon the binding of adenylyl-imidodiphosphate. Structural information and biochemical studies were used to further define the role of the NBDs in the functional properties of ABCA4 and the mechanisms by which mutations lead to the loss in activity. We show that ATPase activity in both NBDs is required for the functional activity of ABCA4. Mutations in Walker A asparagine residues cause a severe reduction in substrate-activated ATPase activity due to the loss in polar interactions with residues within the D-loops of the opposing NBD. The structural basis for how disease mutations in other NBD residues, including the R1108C, R2077W, R2107H, and L2027F, affect the structure and function of ABCA4 is described. Collectively, our studies provide insight into the structure and function of ABCA4 and mechanisms underlying Stargardt disease.

Emerging Therapeutic Approaches and Genetic Insights in Stargardt Disease: A Comprehensive Review

Stargardt disease, one of the most common forms of inherited retinal diseases, affects individuals worldwide. The primary cause is mutations in the ABCA4 gene, leading to the accumulation of toxic byproducts in the retinal pigment epithelium (RPE) and subsequent photoreceptor cell degeneration. Over the past few years, research on Stargardt disease has advanced significantly, focusing on clinical and molecular genetics. Recent studies have explored various innovative therapeutic approaches, including gene therapy, stem cell therapy, and pharmacological interventions. Gene therapy has shown promise, particularly with adeno-associated viral (AAV) vectors capable of delivering the ABCA4 gene to retinal cells. However, challenges remain due to the gene's large size. Stem cell therapy aims to replace degenerated RPE and photoreceptor cells, with several clinical trials demonstrating safety and preliminary efficacy. Pharmacological approaches focus on reducing toxic byproduct accumulation and modulating the visual cycle. Precision medicine, targeting specific genetic mutations and pathways, is becoming increasingly important. Novel techniques such as clustered regularly interspaced palindromic repeats (CRISPR)/Cas9 offer potential for directly correcting genetic defects. This review aims to synthesize recent advancements in understanding and treating Stargardt disease. By highlighting breakthroughs in genetic therapies, stem cell treatments, and novel pharmacological strategies, it provides a comprehensive overview of emerging therapeutic options.

Functional Characterization of ABCA4 Missense Variants Aids Variant Interpretation and Phenotype Prediction in Patients With ABCA4-Retinal Dystrophies

Purpose

Biallelic pathogenic variants in the gene encoding the ATP-binding cassette transporter ABCA4 are the leading cause of irreversible vision loss in inherited retinal dystrophies (IRDs). Interpretation of ABCA4 variants is challenging, due to cis-modifying and hypomorphic variants. We have previously detected 10 missense variants of unknown significance (VUS) in patients with suspected ABCA4-retinal dystrophies (ABCA4-RDs) in Norway. In this study, we functionally characterized the VUS to aid interpretation of the variants and to determine if they are associated with the disease.

Methods

The ABCA4 VUS were expressed in HEK293T cells and the ABCA4 expression level and ATPase activity were determined and correlated with the patients' phenotype. The functional data further used for reclassification of the VUS following the American College of Medical Genetics and Genomics (ACMG) guidelines.

Results

Of the 10 VUSs, 2 variants, Cys205Phe and Asn415Thr, were categorized as functionally severe. The age at presentation in the 2 patients carrying these variants was divergent and seemed to be driven by the patients' second pathogenic variants Gly1961Glu and c.5461-10T>C, respectively. Three variants, Val643Gly, Pro799Leu, and Val1433Ile were categorized as functionally moderate, and were found in patients with intermediate/late age at presentation. The remaining five variants were categorized as functionally normal/mild. Based on our data, c.614G>T p.(Cys205Phe), c.1244A>C p.(Asn415Thr), and c.2396C>T p.(Pro799Leu) were reclassified to (likely) pathogenic, while 4 of the functionally normal/mild variants could be reclassified to likely benign.

Conclusions

Functional analyses of ABCA4 variants are a helpful tool in variant classification and enable us to better predict the disease severity in patients with ABCA4-RDs.

Genetic and Clinical Features of ABCA4-Associated Retinopathy in a Japanese Nationwide Cohort

PURPOSE

To clarify the genetic and clinical features of Japanese patients with ABCA4-associated retinopathy.

DESIGN

Retrospective, multicenter cohort study.

METHODS

Patients with retinal degeneration and biallelic ABCA4 variants were recruited from 13 different hospitals. Whole exome sequencing analysis was used for genetic testing. Comprehensive ophthalmic examinations were performed on matched patients. The primary outcome measure was identifying multimodal retinal imaging findings associated with disease progression.

RESULTS

This study included 63 patients: 19 with missense/missense, 23 with missense/truncation, and 21 with truncation/truncation genotypes. In total, 62 variants were identified, including 29 novel variants. Six patients had a mild phenotype characterized by foveal-sparing or preserved foveal structure, including 4 with missense/missense and 2 with missense/truncation genotypes. The p.Arg212His variant was the most frequent in patients with mild phenotypes (4/12 alleles). Clinical findings showed a disease duration-dependent worsening of the phenotypic stage. Patients with the truncation/truncation genotype exhibited rapid retinal degeneration within a few years and definite fundus autofluorescence imaging patterns, including hyper autofluorescence at the macula and few or no flecks.

CONCLUSIONS

Our results indicate that missense/missense or missense/truncation genotypes, including the p.Arg212His variant, are associated with a relatively mild phenotype. In contrast, the truncation/truncation genotype causes rapid and severe retinal degeneration in Japanese patients with ABCA4-associated retinopathy. These data are vital in predicting patient prognosis, guiding genetic counseling, and stratifying patients for future clinical trials.

Novel Variants in ABCA4-Related Retinopathies with Structural Re-Assessment of Variants of Uncertain Significance

INTRODUCTION

Conclusive molecular genetic diagnoses in inherited retinal diseases remains a major challenge due to the large number of variants of uncertain significance (VUS) identified in genetic testing. Here, we determined the genotypic and phenotypic spectrum of ABCA4 gene variants in a cohort of Canadian inherited retinal dystrophy subjects.

METHODS

This retrospective study evaluated 64 subjects with an inherited retinal dystrophy diagnosis with variants in the ABCA4 gene. Pathogenicity of variants was assessed by comparison to genetic databases and in silico modelling. ABCA4 variants classified as VUS were further evaluated using a cryo-electron structural model of the ABCA4 protein to predict impact on protein function and were also assessed for evolutionary conservation.

RESULTS

Conclusive disease-causing biallelic ABCA4 variants were detected in 52 subjects with either Stargardt's disease, cone-rod dystrophy, macular dystrophy, or pattern dystrophy. A further 14 variants were novel comprising 1 nonsense, 1 frameshift, 3 splicing, and 9 missense variants. Based on in silico modelling, protein modelling and evolutionary conservation from human to zebrafish, we re-classified 5 of these as pathogenic and a further 3 as likely pathogenic. We also added to the ABCA4 phenotypic spectrum seen with four known pathogenic variants (c.2161-2A>G; Leu296Cysfs*4; Arg1640Gln; and Pro1380Leu).

CONCLUSIONS

This study expands the genotypic and phenotypic spectrum of ABCA4 disease-associated variants. By panel-based genetic testing, we identified 14 novel ABCA4 variants of which 8 were determined to be disease-causing or likely disease-causing. These methodologies could circumvent somewhat the need for labour intensive in vitro and in vivo assessments of novel ABCA4 variants.

Whole genome sequencing identifies elusive variants in genetically unsolved Italian inherited retinal disease patients

Inherited retinal diseases (IRDs) are a group of rare monogenic diseases with high genetic heterogeneity (pathogenic variants identified in over 280 causative genes). The genetic diagnostic rate for IRDs is around 60%, mainly thanks to the routine application of next-generation sequencing (NGS) approaches such as extensive gene panels or whole exome analyses. Whole-genome sequencing (WGS) has been reported to improve this diagnostic rate by revealing elusive variants, such as structural variants (SVs) and deep intronic variants (DIVs). We performed WGS on 33 unsolved cases with suspected autosomal recessive IRD, aiming to identify causative genetic variants in non-coding regions or to detect SVs that were unexplored in the initial screening. Most of the selected cases (30 of 33, 90.9%) carried monoallelic pathogenic variants in genes associated with their clinical presentation, hence we first analyzed the non-coding regions of these candidate genes. Whenever additional pathogenic variants were not identified with this approach, we extended the search for SVs and DIVs to all IRD-associated genes. Overall, we identified the missing causative variants in 11 patients (11 of 33, 33.3%). These included three DIVs in ABCA4, CEP290 and RPGRIP1; one non-canonical splice site (NCSS) variant in PROM1 and three SVs (large deletions) in EYS, PCDH15 and USH2A. For the previously unreported DIV in CEP290 and for the NCCS variant in PROM1, we confirmed the effect on splicing by reverse transcription (RT)-PCR on patient-derived RNA. This study demonstrates the power and clinical utility of WGS as an all-in-one test to identify disease-causing variants missed by standard NGS diagnostic methodologies.

Spectrum of Genetic Variants in the Most Common Genes Causing Inherited Retinal Disease in a Large Molecularly Characterized United Kingdom Cohort

PURPOSE

Inherited retinal disease (IRD) is a leading cause of blindness. Recent advances in gene-directed therapies highlight the importance of understanding the genetic basis of these disorders. This study details the molecular spectrum in a large United Kingdom (UK) IRD patient cohort.

DESIGN

Retrospective study of electronic patient records.

PARTICIPANTS

Patients with IRD who attended the Genetics Service at Moorfields Eye Hospital between 2003 and July 2020, in whom a molecular diagnosis was identified.

METHODS

Genetic testing was undertaken via a combination of single-gene testing, gene panel testing, whole exome sequencing, and more recently, whole genome sequencing. Likely disease-causing variants were identified from entries within the genetics module of the hospital electronic patient record (OpenEyes Electronic Medical Record). Analysis was restricted to only genes listed in the Genomics England PanelApp R32 Retinal Disorders panel (version 3.24), which includes 412 genes associated with IRD. Manual curation ensured consistent variant annotation and included only plausible disease-associated variants.

MAIN OUTCOME MEASURES

Detailed analysis was performed for variants in the 5 most frequent genes (ABCA4, USH2A, RPGR, PRPH2, and BEST1), as well as for the most common variants encountered in the IRD study cohort.

RESULTS

We identified 4415 individuals from 3953 families with molecularly diagnosed IRD (variants in 166 genes). Of the families, 42.7% had variants in 1 of the 5 most common IRD genes. Complex disease alleles contributed to disease in 16.9% of affected families with ABCA4-associated retinopathy. USH2A exon 13 variants were identified in 43% of affected individuals with USH2A-associated IRD. Of the RPGR variants, 71% were clustered in the ORF15 region. PRPH2 and BEST1 variants were associated with a range of dominant and recessive IRD phenotypes. Of the 20 most prevalent variants identified, 5 were not in the most common genes; these included founder variants in CNGB3, BBS1, TIMP3, EFEMP1, and RP1.

CONCLUSIONS

We describe the most common pathogenic IRD alleles in a large single-center multiethnic UK cohort and the burden of disease, in terms of families affected, attributable to these variants. Our findings will inform IRD diagnoses in future patients and help delineate the cohort of patients eligible for gene-directed therapies under development.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Nationwide Prevalence of Inherited Retinal Diseases in the Israeli Population

Importance

Data regarding the prevalence of various inherited retinal diseases (IRDs) are limited and vary across populations; moreover, nationwide prevalence studies may be limited to a specific IRD phenotype, potentially leading to inaccurate prevalence estimations. Therefore, nationwide prevalence data are needed.

Objective

To determine the prevalence of 67 IRD phenotypes in the Israeli population.

Design, Setting, and Participants

This cohort study collected nationwide data regarding the number of individuals affected with IRD phenotypes assessed in 10 clinical and academic centers in Israel as part of the research activity of the Israeli inherited retinal disease consortium. Data were collected in May 2023 on 9396 individuals residing in Israel who were diagnosed by an ophthalmologist with an IRD using either electroretinography or retinal imaging where included. Individuals with retinal diseases known to have a nonmendelian basis or without a clear genetic basis and those who were reported as deceased at the time of data collection were excluded from this study.

Main Outcomes and Measures

Prevalence of 67 IRD phenotypes.

Results

Among the 9396 participants in our cohort, the most common IRD in Israel was retinitis pigmentosa with a disease prevalence of approximately 1:2400 individuals, followed by cone-rod dystrophy (approximately 1:14 000), Stargardt disease (approximately 1:16 000), Usher syndrome (approximately 1:16,000), and congenital stationary night blindness (approximately 1:18 000). The prevalence of all IRDs combined was 1:1043 individuals.

Conclusions and Relevance

The current study provides large prevalence dataset of 67 IRD phenotypes, some of which are extremely rare, with only a single identified case. This analysis highlights the potential importance of performing additional nationwide prevalence studies to potentially assist with determining the prevalence of IRDs worldwide.

Genetic Characterization of 191 Probands with Inherited Retinal Dystrophy by Targeted NGS Analysis

Inherited retinal diseases (IRDs) represent a frequent cause of blindness in children and adults. As a consequence of the phenotype and genotype heterogeneity of the disease, it is difficult to have a specific diagnosis without molecular testing. To date, over 340 genes and loci have been associated with IRDs. We present the molecular finding of 191 individuals with IRD, analyzed by targeted next-generation sequencing (NGS). For 67 of them, we performed a family segregation study, considering a total of 126 relatives. A total of 359 variants were identified, 44 of which were novel. Genetic diagnostic yield was 41%. However, after stratifying the patients according to their clinical suspicion, diagnostic yield was higher for well-characterized diseases such as Stargardt disease (STGD), at 65%, and for congenital stationary night blindness 2 (CSNB2), at 64%. Diagnostic yield was higher in the patient group where family segregation analysis was possible (68%) and it was higher in younger (55%) than in older patients (33%). The results of this analysis demonstrated that targeted NGS is an effective method for establishing a molecular genetic diagnosis of IRDs. Furthermore, this study underlines the importance of segregation studies to understand the role of genetic variants with unknow pathogenic role.

PCYT1A deficiency disturbs fatty acid metabolism and induces ferroptosis in the mouse retina

BACKGROUND

Inherited retinal dystrophies (IRDs) are a group of debilitating visual disorders characterized by the progressive degeneration of photoreceptors, which ultimately lead to blindness. Among the causes of this condition, mutations in the PCYT1A gene, which encodes the rate-limiting enzyme responsible for phosphatidylcholine (PC) de novo synthesis via the Kennedy pathway, have been identified. However, the precise mechanisms underlying the association between PCYT1A mutations and IRDs remain unclear. To address this knowledge gap, we focused on elucidating the functions of PCYT1A in the retina.

RESULTS

We found that PCYT1A is highly expressed in Müller glial (MG) cells in the inner nuclear layer (INL) of the retina. Subsequently, we generated a retina-specific knockout mouse model in which the Pcyt1a gene was targeted (Pcyt1a-RKO or RKO mice) to investigate the molecular mechanisms underlying IRDs caused by PCYT1A mutations. Our findings revealed that the deletion of Pcyt1a resulted in retinal degenerative phenotypes, including reduced scotopic electroretinogram (ERG) responses and progressive degeneration of photoreceptor cells, accompanied by loss of cells in the INL. Furthermore, through proteomic and bioinformatic analyses, we identified dysregulated retinal fatty acid metabolism and activation of the ferroptosis signalling pathway in RKO mice. Importantly, we found that PCYT1A deficiency did not lead to an overall reduction in PC synthesis within the retina. Instead, this deficiency appeared to disrupt free fatty acid metabolism and ultimately trigger ferroptosis.

CONCLUSIONS

This study reveals a novel mechanism by which mutations in PCYT1A contribute to the development of IRDs, shedding light on the interplay between fatty acid metabolism and retinal degenerative diseases, and provides new insights into the treatment of IRDs.

The Extraordinary Phenotypic and Genetic Variability of Retinal and Macular Degenerations: The Relevance to Therapeutic Developments

Inherited retinal diseases (IRDs) are a clinically and genetically heterogeneous group of rare conditions leading to various degrees of visual handicap and to progressive blindness in more severe cases. Besides visual rehabilitation, educational, and socio-professional support, there are currently limited therapeutic options, but the approval of the first gene therapy product for RPE65-related IRDs raised hope for therapeutic innovations. Such developments are facing obstacles intrinsic to the disease and the affected tissue including the extreme phenotypic and genetic variability of IRDs and the fine tuning of visual processing through the complex architecture of the postmitotic neural retina. A precise phenotypic characterization is required prior to genetic testing, which now relies on high-throughput sequencing. Their challenges will be discussed within this article as well as their implications in clinical trial design.

Novel and Recurrent Copy Number Variants in ABCA4-Associated Retinopathy

ABCA4 is the most frequently mutated gene leading to inherited retinal disease (IRD) with over 2200 pathogenic variants reported to date. Of these, ~1% are copy number variants (CNVs) involving the deletion or duplication of genomic regions, typically >50 nucleotides in length. An in-depth assessment of the current literature based on the public database LOVD, regarding the presence of known CNVs and structural variants in ABCA4, and additional sequencing analysis of ABCA4 using single-molecule Molecular Inversion Probes (smMIPs) for 148 probands highlighted recurrent and novel CNVs associated with ABCA4-associated retinopathies. An analysis of the coverage depth in the sequencing data led to the identification of eleven deletions (six novel and five recurrent), three duplications (one novel and two recurrent) and one complex CNV. Of particular interest was the identification of a complex defect, i.e., a 15.3 kb duplicated segment encompassing exon 31 through intron 41 that was inserted at the junction of a downstream 2.7 kb deletion encompassing intron 44 through intron 47. In addition, we identified a 7.0 kb tandem duplication of intron 1 in three cases. The identification of CNVs in ABCA4 can provide patients and their families with a genetic diagnosis whilst expanding our understanding of the complexity of diseases caused by ABCA4 variants.

Antisense Oligonucleotide-Based Rescue of Complex Intronic Splicing Defects in ABCA4

The ABCA4 gene, involved in Stargardt disease, has a high percentage of splice-altering pathogenic variants, some of which cause complex RNA defects. Although antisense oligonucleotides (AONs) have shown promising results in splicing modulation, they have not yet been used to target complex splicing defects. Here, we performed AON-based rescue studies on ABCA4 complex splicing defects. Intron 13 variants c.1938-724A>G, c.1938-621G>A, c.1938-619A>G, and c.1938-514A>G all lead to the inclusion of different pseudo-exons (PEs) with and without an upstream PE (PE1). Intron 44 variant c.6148-84A>T results in multiple PE inclusions and/or exon skipping events. Five novel AONs were designed to target these defects. AON efficacy was assessed by in vitro splice assays using midigenes containing the variants of interest. All screened complex splicing defects were effectively rescued by the AONs. Although varying levels of efficacy were observed between AONs targeting the same PEs, for all variants at least one AON restored splicing to levels comparable or better than wildtype. In conclusion, AONs are a promising approach to target complex splicing defects in ABCA4.

Comparative 3D genome analysis between neural retina and retinal pigment epithelium reveals differential cis-regulatory interactions at retinal disease loci

BACKGROUND

Vision depends on the interplay between photoreceptor cells of the neural retina and the underlying retinal pigment epithelium (RPE). Most genes involved in inherited retinal diseases display specific spatiotemporal expression within these interconnected retinal components through the local recruitment of cis-regulatory elements (CREs) in 3D nuclear space.

RESULTS

To understand the role of differential chromatin architecture in establishing tissue-specific expression at inherited retinal disease loci, we mapped genome-wide chromatin interactions using in situ Hi-C and H3K4me3 HiChIP on neural retina and RPE/choroid from human adult donor eyes. We observed chromatin looping between active promoters and 32,425 and 8060 candidate CREs in the neural retina and RPE/choroid, respectively. A comparative 3D genome analysis between these two retinal tissues revealed that 56% of 290 known inherited retinal disease genes were marked by differential chromatin interactions. One of these was ABCA4, which is implicated in the most common autosomal recessive inherited retinal disease. We zoomed in on retina- and RPE-specific cis-regulatory interactions at the ABCA4 locus using high-resolution UMI-4C. Integration with bulk and single-cell epigenomic datasets and in vivo enhancer assays in zebrafish revealed tissue-specific CREs interacting with ABCA4.

CONCLUSIONS

Through comparative 3D genome mapping, based on genome-wide, promoter-centric, and locus-specific assays of human neural retina and RPE, we have shown that gene regulation at key inherited retinal disease loci is likely mediated by tissue-specific chromatin interactions. These findings do not only provide insight into tissue-specific regulatory landscapes at retinal disease loci, but also delineate the search space for non-coding genomic variation underlying unsolved inherited retinal diseases.

ABCA4-related retinopathies in Lebanon

Variants in ATP-binding cassette transporter type A4 (ABCA4) have been linked to several forms of inherited retinal diseases (IRDs) besides the classically defined Stargardt disease (STGD), known collectively as ABCA4 retinopathies. ABCA4 is a sizable locus harboring 50 exons; thus, its analysis has revealed over 2,400 variants described, of which more than 2,000 are causal. Due to the clinical and genetic heterogeneity, diagnosing ABCA4 retinopathies is challenging. To date, no ABCA4-related retinopathy has been detected in Lebanon. Using next-generation sequencing, we analyzed our IRDs' cohort retrospectively (61 families) and identified five with ABCA4-related retinopathies, making it a relatively abundant cause of IRDs (about 8 %). Three families were diagnosed with rod-cone dystrophy (RCD), two with STGD, and one with cone-rod dystrophy (CRD). In conclusion, our study showed the presence of ABCA4 variants with a high degree of heterogeneity in Lebanon.

Representation of Women Among Individuals With Mild Variants in ABCA4-Associated Retinopathy: A Meta-Analysis

Importance

Previous studies indicated that female sex might be a modifier in Stargardt disease, which is an ABCA4-associated retinopathy.

Objective

To investigate whether women are overrepresented among individuals with ABCA4-associated retinopathy who are carrying at least 1 mild allele or carrying nonmild alleles.

Data Sources

Literature data, data from 2 European centers, and a new study. Data from a Radboudumc database and from the Rotterdam Eye Hospital were used for exploratory hypothesis testing.

Study Selection

Studies investigating the sex ratio in individuals with ABCA4-AR and data from centers that collected ABCA4 variant and sex data. The literature search was performed on February 1, 2023; data from the centers were from before 2023.

Data Extraction and Synthesis

Random-effects meta-analyses were conducted to test whether the proportions of women among individuals with ABCA4-associated retinopathy with mild and nonmild variants differed from 0.5, including subgroup analyses for mild alleles. Sensitivity analyses were performed excluding data with possibly incomplete variant identification. χ2 Tests were conducted to compare the proportions of women in adult-onset autosomal non-ABCA4-associated retinopathy and adult-onset ABCA4-associated retinopathy and to investigate if women with suspected ABCA4-associated retinopathy are more likely to obtain a genetic diagnosis. Data analyses were performed from March to October 2023.

Main Outcomes and Measures

Proportion of women per ABCA4-associated retinopathy group. The exploratory testing included sex ratio comparisons for individuals with ABCA4-associated retinopathy vs those with other autosomal retinopathies and for individuals with ABCA4-associated retinopathy who underwent genetic testing vs those who did not.

Results

Women were significantly overrepresented in the mild variant group (proportion, 0.59; 95% CI, 0.56-0.62; P < .001) but not in the nonmild variant group (proportion, 0.50; 95% CI, 0.46-0.54; P = .89). Sensitivity analyses confirmed these results. Subgroup analyses on mild variants showed differences in the proportions of women. Furthermore, in the Radboudumc database, the proportion of adult women among individuals with ABCA4-associated retinopathy (652/1154 = 0.56) was 0.10 (95% CI, 0.05-0.15) higher than among individuals with other retinopathies (280/602 = 0.47).

Conclusions and Relevance

This meta-analysis supports the likelihood that sex is a modifier in developing ABCA4-associated retinopathy for individuals with a mild ABCA4 allele. This finding may be relevant for prognosis predictions and recurrence risks for individuals with ABCA4-associated retinopathy. Future studies should further investigate whether the overrepresentation of women is caused by differences in the disease mechanism, by differences in health care-seeking behavior, or by health care discrimination between women and men with ABCA4-AR.

Phenotyping and genotyping inherited retinal diseases: Molecular genetics, clinical and imaging features, and therapeutics of macular dystrophies, cone and cone-rod dystrophies, rod-cone dystrophies, Leber congenital amaurosis, and cone dysfunction syndromes

Inherited retinal diseases (IRD) are a leading cause of blindness in the working age population and in children. The scope of this review is to familiarise clinicians and scientists with the current landscape of molecular genetics, clinical phenotype, retinal imaging and therapeutic prospects/completed trials in IRD. Herein we present in a comprehensive and concise manner: (i) macular dystrophies (Stargardt disease (ABCA4), X-linked retinoschisis (RS1), Best disease (BEST1), PRPH2-associated pattern dystrophy, Sorsby fundus dystrophy (TIMP3), and autosomal dominant drusen (EFEMP1)), (ii) cone and cone-rod dystrophies (GUCA1A, PRPH2, ABCA4, KCNV2 and RPGR), (iii) predominant rod or rod-cone dystrophies (retinitis pigmentosa, enhanced S-Cone syndrome (NR2E3), Bietti crystalline corneoretinal dystrophy (CYP4V2)), (iv) Leber congenital amaurosis/early-onset severe retinal dystrophy (GUCY2D, CEP290, CRB1, RDH12, RPE65, TULP1, AIPL1 and NMNAT1), (v) cone dysfunction syndromes (achromatopsia (CNGA3, CNGB3, PDE6C, PDE6H, GNAT2, ATF6), X-linked cone dysfunction with myopia and dichromacy (Bornholm Eye disease; OPN1LW/OPN1MW array), oligocone trichromacy, and blue-cone monochromatism (OPN1LW/OPN1MW array)). Whilst we use the aforementioned classical phenotypic groupings, a key feature of IRD is that it is characterised by tremendous heterogeneity and variable expressivity, with several of the above genes associated with a range of phenotypes.

Clinical, Genotypic, and Imaging Characterization of the Spectrum of ABCA4 Retinopathies

PURPOSE

To investigate the clinical and genotypic differences in the spectrum of ABCA4-associated retinopathies (ABCA4Rs).

DESIGN

Observational, cross sectional case series.

PARTICIPANTS

Sixty-six patients (132 eyes) carrying biallelic ABCA4 variants.

METHODS

Patients underwent visual acuity measurement and multimodal imaging. Clinical records were reviewed for age at onset, presenting symptoms, genetic variants, and electroretinogram (ERG). Each eye was assigned to a phenotype based on age at onset, imaging and ERG: cone dystrophy-bull's-eye maculopathy (CD-BEM, 40 eyes), cone-rod dystrophy (CRD, 12 eyes), Stargardt disease (SD, 28 eyes), late-onset SD (LO-SD, 38 eyes), and fundus flavimaculatus (14 eyes). Images were analyzed for: peripapillary sparing, retinal pigment epithelium (RPE) atrophy (definitely decreased autofluorescence, DDAF), flecks patterns using autofluorescence; type of atrophy according to Classification of Atrophy Meeting reports, macular and choroidal thickness on OCT; and choriocapillaris flow deficits on OCT angiography.

MAIN OUTCOME MEASURES

Primary outcome was to report the demographic, genotypic, and imaging characteristics of the different ABCA4R phenotypes. Secondary objectives included the assessment of imaging biomarkers as outcome measures for clinical trials.

RESULTS

Age at onset was lower in CRD (12 ± 8 years) and higher in patients with LO-SD (59 ± 9 years) (all P < 0.01). Central vision loss was a common presenting symptom in CD-BEM and SD, whereas patients with LO-SD primarily complained of difficult dark adaptation. Missense variants were more frequent in CD-BEM, and splice site in CRD and LO-SD (P < 0.05). Peripapillary sparing was absent in 3 eyes with LO-SD (8%). Cone dystrophy-bull's-eye maculopathy eyes typically had complete outer retinal atrophy alterations (98%), whereas CRD and SD eyes showed both complete outer retinal atrophy and complete RPE and outer retinal atrophy (cRORA) (71%-100%). Patients with LO-SD had larger areas of DDAF (100% cRORA) and of choriocapillaris flow deficits (all P < 0.01). Repeatability of DDAF measurements was low for some phenotypes (CD-BEM and CRD) and atrophic areas <7.5 mm2. Resorbed flecks were significantly associated with CRD and LO-SD (P < 0.01).

CONCLUSIONS

This research provides a thorough evaluation of the spectrum of ABCA4R. Our findings suggest that certain phenotypes show preferential photoreceptor degeneration (e.g., CD-BEM), whereas others have substantial RPE and choriocapillaris alterations (e.g., LO-SD). We recommend that clinical trial end points take into consideration these imaging features to improve the interpretation of their results.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Optical Coherence Tomography in Inherited Macular Dystrophies: A Review

Macular dystrophies (MDs) constitute a collection of hereditary retina disorders leading to notable visual impairment, primarily due to progressive macular atrophy. These conditions are distinguished by bilateral and relatively symmetrical abnormalities in the macula that significantly impair central visual function. Recent strides in fundus imaging, especially optical coherence tomography (OCT), have enhanced our comprehension and diagnostic capabilities for MD. OCT enables the identification of neurosensory retinal disorganization patterns and the extent of damage to retinal pigment epithelium (RPE) and photoreceptor cells in the dystrophies before visible macular pathology appears on fundus examinations. It not only helps us in diagnostic retinal and choroidal pathologies but also guides us in monitoring the progression of, staging of, and response to treatment. In this review, we summarize the key findings on OCT in some of the most common MD.

A Proximity Complementation Assay to Identify Small Molecules That Enhance the Traffic of ABCA4 Misfolding Variants

ABCA4-related retinopathy is the most common inherited Mendelian eye disorder worldwide, caused by biallelic variants in the ATP-binding cassette transporter ABCA4. To date, over 2200 ABCA4 variants have been identified, including missense, nonsense, indels, splice site and deep intronic defects. Notably, more than 60% are missense variants that can lead to protein misfolding, mistrafficking and degradation. Currently no approved therapies target ABCA4. In this study, we demonstrate that ABCA4 misfolding variants are temperature-sensitive and reduced temperature growth (30 °C) improves their traffic to the plasma membrane, suggesting the folding of these variants could be rescuable. Consequently, an in vitro platform was developed for the rapid and robust detection of ABCA4 traffic to the plasma membrane in transiently transfected cells. The system was used to assess selected candidate small molecules that were reported to improve the folding or traffic of other ABC transporters. Two candidates, 4-PBA and AICAR, were identified and validated for their ability to enhance both wild-type ABCA4 and variant trafficking to the cell surface in cell culture. We envision that this platform could serve as a primary screen for more sophisticated in vitro testing, enabling the discovery of breakthrough agents to rescue ABCA4 protein defects and mitigate ABCA4-related retinopathy.