Structure and function of ABCA4 and its role in the visual cycle and Stargardt macular degeneration

Recent updates on drug delivery approaches for improved ocular delivery with an insight into nanostructured drug delivery carriers for anterior and posterior segment disorders

Ocular diseases have a major impact on patient's vision and quality of life, with approximately 2.2 billion people have visual impairment worldwide according to the findings from the World Health Organization (WHO). The eye is a complex organ with unique morphology and physiology consisting of numerous ocular barriers which hinders the entry of exogenous substances and impedes drug absorption. This in turn has a substantial impact on effective drug delivery to treat ocular diseases, especially intraocular disorders which has consistently presented a challenge to eye care professionals. The most common method of delivering medications to the eye is topical instillation of eye drops. Although this approach is a viable option for treating many ocular diseases remains a major challenge for the effective treatment of posterior ocular conditions. Up till now, incessant efforts have been committed to design innovative drug delivery systems with the hopes of potential clinical application. Modern developments in nanocarrier's technology present a potential chance to overcome these obstacles by enabling targeted delivery of the loaded medication to the eyes with improved solubility, delayed release, higher penetration and increased retention. This review covers the anatomy of eye with associated ocular barriers, ocular diseases and administration routes. In addition it primarily focuses on the latest progress and contemporary applications of ophthalmic formulations providing specific insight on nanostructured drug delivery carriers reported over the past 5 years highlighting their values in achieving efficient ocular drug delivery to both anterior and posterior segments. Most importantly, we outlined in this review the macro and nanotechnology based ophthalmic drug formulations that are being patented or marketed so far for treating ocular diseases. Finally, based on current trends and therapeutic concepts, we highlighted the challenges faced by novel ocular drug delivery systems and provided prospective future developments for further research in these directions. We hope that this review will serve as a source of motivation and ideas for formulation scientists in improving the design of innovative ophthalmic formulations.

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.

Inhibition of JNK signaling attenuates photoreceptor ferroptosis caused by all-trans-retinal

The disruption of the visual cycle leads to the accumulation of all-trans-retinal (atRAL) in the retina, a hallmark of autosomal recessive Stargardt disease (STGD1) and dry age-related macular degeneration (AMD), both of which cause retinal degeneration. Although our previous studies have shown that atRAL induces ferroptosis and activates c-Jun N-terminal kinase (JNK) signaling in the retina, the relationship between JNK signaling and ferroptosis in atRAL-mediated photoreceptor damage remains unclear. Here, we reported that JNK activation by atRAL drove photoreceptor ferroptosis through ferritinophagy. In photoreceptor cells loaded with atRAL, activated JNK phosphorylated c-Jun, which facilitated its nuclear translocation and promoted the expression of the nuclear receptor coactivator 4 (NCOA4). Elevated NCOA4 induced ferritin degradation via lysosomal processing, a process known as ferritinophagy, thereby releasing a large amount of labile iron. Iron overload led to the generation of reactive oxygen species (ROS) and lipid peroxidation, ultimately culminating in ferroptosis. Treatment with the JNK inhibitor JNK-IN-8, as well as the knockout of Jnk1 and Jnk2 genes, significantly rescued atRAL-loaded photoreceptor cells from ferritinophagy-induced ferroptosis. Abca4-/-Rdh8-/- mice, which exhibit atRAL accumulation in the retina following light exposure, are commonly used to study the pathological processes of STGD1 and dry AMD. In these mice, light exposure activated the JNK/c-Jun/NCOA4 axis, resulting in ferritinophagy in the neural retina. Importantly, intraperitoneal administration of JNK-IN-8 significantly rescued retinal function and photoreceptors from ferritinophagy-induced ferroptosis and effectively mitigated retinal degeneration in light-exposed Abca4-/-Rdh8-/- mice. This study underscores the critical role of the JNK/c-Jun/NCOA4 axis in mediating atRAL-induced ferritinophagy, which drives ferroptosis and retinal atrophy, suggesting that targeting this pathway may offer a potential therapeutic approach for STGD1 and dry AMD.

The central retinal thickness and its related genotype in ABCA4-related retinopathy

PURPOSE

To further explore the influence of genotype, including mutation type and structural domain, on the severity of macular atrophy, we measured the central retinal thickness (CRT) in patients with ABCA4-related retinopathy.

METHODS

A total of 66 patients were included in the cohort. This was a retrospective investigation. The patients were tested using whole exon sequencing and ophthalmic exams, including slip lamp exams, best-corrected visual acuity, optical coherence tomography, fundus photo, and fundus autofluorescence.

RESULTS

In this study, we discovered that mutations on nucleotide binding domains (NBD) lead to less CRT (45.00 ± 25.25μm, 95% CI: 31.54-58.46) had significantly less CRT than the others (89.75 ± 71.17μm, 95% CI: 30.25-149.25, p = 0.032), and could accelerate the rate of CRT decrease.

CONCLUSIONS

Our study provides new perspectives in the understanding of ABCA4-related retinopathy.

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.

How Advanced are Nanocarriers for Effective Subretinal Injection?

Subretinal injection (SR injection) is a commonly used method of ocular drug delivery and has been mainly applied for the treatment of neovascular age-associated macular degeneration (nAMD) and sub-macular hemorrhage (SMH) caused by nAMD, as well as various types of hereditary retinopathies (IRD) such as Stargardt's disease (STGD), retinitis pigmentosa (RP), and a series of fundus diseases such as Leber's congenital dark haze (LCA), choroidal defects, etc. The commonly used carriers of SR injection are mainly divided into viral and non-viral vectors. Leber's congenital amaurosis (LCA), choroidal agenesis, and a series of other fundus diseases are also commonly treated using SR injection. The commonly used vectors for SR injection are divided into two categories: viral vectors and non-viral vectors. Viral vectors are a traditional class of SR injection drug carriers that have been extensively studied in clinical treatment, but they still have many limitations that cannot be ignored, such as poor reproduction efficiency, small loading genes, and triggering of immune reactions. With the rapid development of nanotechnology in the treatment of ocular diseases, nanovectors have become a research hotspot in the field of non-viral vectors. Nanocarriers have numerous attractive properties such as low immunogenicity, robust loading capacity, stable structure, and easy modification. These valuable features imply greater safety, improved therapeutic efficacy, longer duration, and more flexible indications. In recent years, there has been a growing interest in nanocarriers, which has led to significant advancements in the treatment of ocular diseases. Nanocarriers have not only successfully addressed clinical problems that viral vectors have failed to overcome but have also introduced new therapeutic possibilities for certain classical disease types. Nanocarriers offer undeniable advantages over viral vectors. This review discusses the advantages of subretinal (SR) injection, the current status of research, and the research hotspots of gene therapy with viral vectors. It focuses on the latest progress of nanocarriers in SR injection and enumerates the limitations and future perspectives of nanocarriers in the treatment of fundus lesions. Furthermore, this review also covers the research progress of nanocarriers in the field of subretinal injection and highlights the value of nanocarrier-mediated SR injection in the treatment of fundus disorders. Overall, it provides a theoretical basis for the application of nanocarriers in SR injection.

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.

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.

The New Era of Therapeutic Strategies for the Management of Retinitis Pigmentosa: A Narrative Review of the Pathomolecular Mechanism for Gene Therapies

Retinitis pigmentosa, or RP, is a group of inherited retinal degenerations involving progressive loss of photoreceptor cells- rods and cones- ultimately causing severe vision loss and blindness. RP, although a very common ailment, continues to be an incurable disease with little to be done medically. However, with the breakthroughs in gene therapy and stem cell transplantation in recent years, a new door has been opened to the treatment of RP. This narrative review summarizes the pathomolecular mechanisms of RP, focusing on the genetic and molecular abnormalities that lead to the process of retinal degeneration. In this section, we talk about the current theories of how RP develops, gene mutations, oxidative stress, and inflammation. We also delve into new therapeutic approaches such as gene therapy, stem cell transplantation and genome surgery, which are designed to either replace or repair the damaged photoreceptors to restore vision and ultimately enhance the life of the RP patient. Another topic covered is the obstacles and research frontiers of these revolutionary treatments. This article is intended to give a complete overview of the molecular processes of RP and the promising treatment strategies that could change the way this devastating disease is treated.

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.

Impaired cathepsin D in retinal pigment epithelium cells mediates Stargardt disease pathogenesis

Recessive Stargardt disease (STGD1) is an inherited juvenile maculopathy caused by mutations in the ABCA4 gene, for which there is no suitable treatment. Loss of functional ABCA4 in the retinal pigment epithelium (RPE) alone, without contribution from photoreceptor cells, was shown to induce STGD1 pathology. Here, we identified cathepsin D (CatD), the primary RPE lysosomal protease, as a key molecular player contributing to endo-lysosomal dysfunction in STGD1 using a newly developed "disease-in-a-dish" RPE model from confirmed STGD1 patients. Induced pluripotent stem cell (iPSC)-derived RPE originating from three STGD1 patients exhibited elevated lysosomal pH, as previously reported in Abca4-/- mice. CatD protein maturation and activity were impaired in RPE from STGD1 patients and Abca4-/- mice. Consequently, STGD1 RPE cells have reduced photoreceptor outer segment degradation and abnormal accumulation of α-synuclein, the natural substrate of CatD. Furthermore, dysfunctional ABCA4 in STGD1 RPE cells results in intracellular accumulation of autofluorescent material and phosphatidylethanolamine (PE). The altered distribution of PE associated with the internal membranes of STGD1 RPE cells presumably compromises LC3-associated phagocytosis, contributing to delayed endo-lysosomal degradation activity. Drug-mediated re-acidification of lysosomes in the RPE of STGD1 restores CatD functional activity and reduces the accumulation of immature CatD protein loads. This preclinical study validates the contribution of CatD deficiencies to STGD1 pathology and provides evidence for an efficacious therapeutic approach targeting RPE cells. Our findings support a cell-autonomous RPE-driven pathology, informing future research aimed at targeting RPE cells to treat ABCA4-mediated retinopathies.

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.

Understanding and Rescuing the Splicing Defect Caused by the Frequent ABCA4 Variant c.4253+43G>A Underlying Stargardt Disease

Pathogenic variants in ABCA4 are the underlying molecular cause of Stargardt disease (STGD1), an autosomal recessive macular dystrophy characterized by a progressive loss of central vision. Among intronic ABCA4 variants, c.4253+43G>A is frequently detected in STGD1 cases and is classified as a hypomorphic allele, generally associated with late-onset cases. This variant was previously reported to alter splicing regulatory sequences, but the splicing outcome is not fully understood yet. In this study, we attempted to better understand its effect on splicing and to rescue the aberrant splicing via antisense oligonucleotides (AONs). Wild-type and c.4253+43G>A variant-harboring maxigene vectors revealed additional skipping events, which were not previously detected upon transfection in HEK293T cells. To restore exon inclusion, we designed a set of 27 AONs targeting either splicing silencer motifs or the variant region and screened these in maxigene-transfected HEK293T cells. Candidate AONs able to promote exon inclusion were selected for further testing in patient-derived photoreceptor precursor cells. Surprisingly, no robust splicing modulation was observed in this model system. Overall, this research helped to adequately characterize the splicing alteration caused by the c.4253+43G>A variant, although future development of AON-mediated exon inclusion therapy for ABCA4 is needed.

ABCA4 variant screening in a Turkish cohort with Stargardt disease

PURPOSE

This study aims to evaluate the ABCA4 variants in patients diagnosed with Stargardt disease.

METHODS

This is a retrospective study designed to investigate variants in the ABCA4 in Stargardt disease and the clinical findings of the cases. Sex, age, age of onset of symptoms, best-corrected visual acuity, color fundus photography, optical coherence tomography, and visual field test of the patients were recorded. Genetic analyses were screened, and patients with at least two variants in the ABCA4 were included in this study.

RESULTS

Twenty-seven patients diagnosed with Stargardt disease with the ABCA4 variants were included in this study. Twelve of them (44.4%) were female and fifteen (55.5%) were male. The mean age of the cases was 27.44 years (ranging from 8 to 56 years). Thirty different variants were detected in 54 ABCA4 alleles of 27 patients. The two most common pathogenic variants were c.5882 G>A p.(Gly1961Glu) and c.52C>T p.(Arg18Trp) in this cohort. Two novel variants were identified (c.3855_3856dup, c.1554 + 3_1554 + 4del) and the patient with the c.1554 + 3_1554 + 4del variant additionally had a different ABCA4 variant in trans. The other novel variant was homozygous.

CONCLUSIONS

In this study, two novel variants were described in a Turkish cohort with Stargardt disease. The variant c.52C>T p.(Arg18Trp) was the most common disease-causing variant besides the c.5882 G>A p.(Gly1961Glu) which was identified frequently in the previous studies. A larger sample size is necessary for describing different pathogenic variants and understanding the phenotype-genotype correlations.

Preclinical Development of Antisense Oligonucleotides to Rescue Aberrant Splicing Caused by an Ultrarare ABCA4 Variant in a Child with Early-Onset Stargardt Disease

Precision medicine is rapidly gaining recognition in the field of (ultra)rare conditions, where only a few individuals in the world are affected. Clinical trial design for a small number of patients is extremely challenging, and for this reason, the development of N-of-1 strategies is explored to accelerate customized therapy design for rare cases. A strong candidate for this approach is Stargardt disease (STGD1), an autosomal recessive macular degeneration characterized by high genetic and phenotypic heterogeneity. STGD1 is caused by pathogenic variants in ABCA4, and amongst them, several deep-intronic variants alter the pre-mRNA splicing process, generally resulting in the insertion of pseudoexons (PEs) into the final transcript. In this study, we describe a 10-year-old girl harboring the unique deep-intronic ABCA4 variant c.6817-713A>G. Clinically, she presents with typical early-onset STGD1 with a high disease symmetry between her two eyes. Molecularly, we designed antisense oligonucleotides (AONs) to block the produced PE insertion. Splicing rescue was assessed in three different in vitro models: HEK293T cells, fibroblasts, and photoreceptor precursor cells, the last two being derived from the patient. Overall, our research is intended to serve as the basis for a personalized N-of-1 AON-based treatment to stop early vision loss in this patient.

Light-responsive polymeric nanoparticles for retinal drug delivery: design cues, challenges and future perspectives

A multitude of sight-threatening retinal diseases, affecting hundreds of millions around the globe, lack effective pharmacological treatments due to ocular barriers and common drug delivery limitations. Polymeric nanoparticles (PNPs) are versatile drug carriers with sustained drug release profiles and tunable physicochemical properties which have been explored for ocular drug delivery to both anterior and posterior ocular tissues. PNPs can incorporate a wide range of drugs and overcome the challenges of conventional retinal drug delivery. Moreover, PNPs can be engineered to respond to specific stimuli such as ultraviolet, visible, or near-infrared light, and allow precise spatiotemporal control of the drug release, enabling tailored treatment regimens and reducing the number of required administrations. The objective of this study is to emphasize the therapeutic potential of light-triggered drug-loaded polymeric nanoparticles to treat retinal diseases through an exploration of ocular pathologies, challenges in drug delivery, current production methodologies and recent applications. Despite challenges, light-responsive PNPs hold the promise of substantially enhancing the treatment landscape for ocular diseases, aiming for an improved quality of life for patients.

Comprehensive genetic analysis reveals the mutational landscape of ABCA4-associated retinal dystrophy in a Chinese cohort

PURPOSE

To depict the variant profiles of the ABCA4 gene in a large Chinese cohort of patients with ABCA4-associated retinal dystrophy (ABCA4-RD).

METHODS

We recruited 290 unrelated Chinese patients with ABCA4-RD and did ABCA4 mutational screening by a combination of Sanger sequencing, targeted exome sequencing, entire ABCA4 locus sequencing, and whole genome sequencing (WGS). The pathogenicity of variants was assessed using in silico tools or in vitro splicing assays following the American College of Medical Genetics and Genomics guidelines.

RESULTS

Two hundred sixty-eight distinct pathogenic variants were identified, and 57 were novel. In 580 alleles, 22 noncoding region variants outside canonical splice sites and 4 structural variations were found in 44 alleles accounting for 7.6% of all alleles. Bioinformatics analysis showed the complex mechanism of aberrant splicing productsnatural splice site disruption, branch point destruction, and cryptic splice site activation. Correspondingly, minigene assays validated the various abnormal splicing products, including exon skipping, exon elongation, partial exon deletion, and pseudoexon insertion. WGS identified the first inversion variation in ABCA4.

CONCLUSIONS

This study systematically depicted the variant profiles of ABCA4 and revealed the missing alleles of patients with ABCA4-RD in a large Chinese cohort. Our findings demonstrated the complexity of molecular diagnosis of Mendelian diseases and the efficiency of WGS for detecting structural variants.

GENETIC FACTORS AND CHARACTERISTICS ON SPECTRAL-DOMAIN OPTICAL COHERENCE TOMOGRAPHY ARE ASSOCIATED WITH CHOROIDAL THICKNESS IN ABCA4 -RELATED RETINOPATHY

PURPOSE

To investigate the possible correlation factors of choroidal thickness in ABCA4 -related retinopathy.

METHODS

A total of 66 patients were included in the cohort. It is a retrospective, cross-sectional laboratory investigation. The patients were tested using whole-exon sequencing and ophthalmic examinations, including slit-lamp examinations, best-corrected visual acuity, spectral-domain optical coherence tomography, fundus photograph, and fundus autofluorescence.

RESULTS

Besides demographic characteristics (age, onset age, duration), we selected genetic factors and ocular characteristics on spectral-domain optical coherence tomography as the candidates related to choroidal thickness. Mutation type (inframe mutation or premature termination codon), epiretinal membrane, retinal pigment epithelium- Bruch membrane integrity, and macular curvature changes were identified as related factors to choroidal thickness in ABCA4 -related retinopathy after the adjustment of Logistic LASSO regression.

CONCLUSION

Mutation type, epiretinal membrane, retinal pigment epithelium-Bruch membrane integrity, and macular curvature changes are related factors to choroidal thinning. These findings could provide us a further understanding for the pathological process and clinical features of ABCA4 mutation.

Scavenging of Cation Radicals of the Visual Cycle Retinoids by Lutein, Zeaxanthin, Taurine, and Melanin

In the retina, retinoids involved in vision are under constant threat of oxidation, and their oxidation products exhibit deleterious properties. Using pulse radiolysis, this study determined that the bimolecular rate constants of scavenging cation radicals of retinoids by taurine are smaller than 2 × 107 M-1s-1 whereas lutein scavenges cation radicals of all three retinoids with the bimolecular rate constants approach the diffusion-controlled limits, while zeaxanthin is only 1.4-1.6-fold less effective. Despite that lutein exhibits greater scavenging rate constants of retinoid cation radicals than other antioxidants, the greater concentrations of ascorbate in the retina suggest that ascorbate may be the main protectant of all visual cycle retinoids from oxidative degradation, while α-tocopherol may play a substantial role in the protection of retinaldehyde but is relatively inefficient in the protection of retinol or retinyl palmitate. While the protection of retinoids by lutein and zeaxanthin appears inefficient in the retinal periphery, it can be quite substantial in the macula. Although the determined rate constants of scavenging the cation radicals of retinol and retinaldehyde by dopa-melanin are relatively small, the high concentration of melanin in the RPE melanosomes suggests they can be scavenged if they are in proximity to melanin-containing pigment granules.

Lipofuscin, Its Origin, Properties, and Contribution to Retinal Fluorescence as a Potential Biomarker of Oxidative Damage to the Retina

Lipofuscin accumulates with age as intracellular fluorescent granules originating from incomplete lysosomal digestion of phagocytosed and autophagocytosed material. The purpose of this review is to provide an update on the current understanding of the role of oxidative stress and/or lysosomal dysfunction in lipofuscin accumulation and its consequences, particularly for retinal pigment epithelium (RPE). Next, the fluorescence of lipofuscin, spectral changes induced by oxidation, and its contribution to retinal fluorescence are discussed. This is followed by reviewing recent developments in fluorescence imaging of the retina and the current evidence on the prognostic value of retinal fluorescence for the progression of age-related macular degeneration (AMD), the major blinding disease affecting elderly people in developed countries. The evidence of lipofuscin oxidation in vivo and the evidence of increased oxidative damage in AMD retina ex vivo lead to the conclusion that imaging of spectral characteristics of lipofuscin fluorescence may serve as a useful biomarker of oxidative damage, which can be helpful in assessing the efficacy of potential antioxidant therapies in retinal degenerations associated with accumulation of lipofuscin and increased oxidative stress. Finally, amendments to currently used fluorescence imaging instruments are suggested to be more sensitive and specific for imaging spectral characteristics of lipofuscin fluorescence.

Generation of an iPSC line (RMCGENi020-A) from a patient with Stargardt disease harboring the recurrent intronic ABCA4 variant c.4253+43G>A

Pathogenic variants in ABCA4 are associated with Stargardt disease (STGD1), an autosomal recessive macular dystrophy characterized by bilateral central vision loss due to a progressive degeneration of retinal cells. An induced pluripotent stem cell (iPSC) line was generated from late-onset STGD1 patient-derived fibroblasts harboring bi-allelic ABCA4 variants by lentivirus-induced reprogramming. The obtained iPSC line (RMCGENi020-A) showed pluripotent features after the reprogramming process. The generation of this iPSC line facilitates its use to differentiate it into relevant retinal-like cell models, with the aim to adequately evaluate the effects of the ABCA4 variants.

Stargardt disease-associated in-frame ABCA4 exon 17 skipping results in significant ABCA4 function

BACKGROUND

ABCA4, the gene implicated in Stargardt disease (STGD1), contains 50 exons, of which 17 contain multiples of three nucleotides. The impact of in-frame exon skipping is yet to be determined. Antisense oligonucleotides (AONs) have been investigated in Usher syndrome-associated genes to induce skipping of in-frame exons carrying severe variants and mitigate their disease-linked effect. Upon the identification of a STGD1 proband carrying a novel exon 17 canonical splice site variant, the activity of ABCA4 lacking 22 amino acids encoded by exon 17 was examined, followed by design of AONs able to induce exon 17 skipping.

METHODS

A STGD1 proband was compound heterozygous for the splice variant c.2653+1G>A, that was predicted to result in in-frame skipping of exon 17, and a null variant [c.735T>G, p.(Tyr245*)]. Clinical characteristics of this proband were studied using multi-modal imaging and complete ophthalmological examination. The aberrant splicing of c.2653+1G>A was investigated in vitro in HEK293T cells with wild-type and mutant midigenes. The residual activity of the mutant ABCA4 protein lacking Asp864-Gly885 encoded by exon 17 was analyzed with all-trans-retinal-activated ATPase activity assay, along with its subcellular localization. To induce exon 17 skipping, the effect of 40 AONs was examined in vitro in WT WERI-Rb-1 cells and 3D human retinal organoids.

RESULTS

Late onset STGD1 in the proband suggests that c.2653+1G>A does not have a fully deleterious effect. The in vitro splice assay confirmed that this variant leads to ABCA4 transcripts without exon 17. ABCA4 Asp864_Gly863del was stable and retained 58% all-trans-retinal-activated ATPase activity compared to WT ABCA4. This sequence is located in an unstructured linker region between transmembrane domain 6 and nucleotide-binding domain-1 of ABCA4. AONs were designed to possibly reduce pathogenicity of severe variants harbored in exon 17. The best AON achieved 59% of exon 17 skipping in retinal organoids.

CONCLUSIONS

Exon 17 deletion in ABCA4 does not result in the absence of protein activity and does not cause a severe STGD1 phenotype when in trans with a null allele. By applying AONs, the effect of severe variants in exon 17 can potentially be ameliorated by exon skipping, thus generating partial ABCA4 activity in STGD1 patients.

Genome editing in the treatment of ocular diseases

Genome-editing technologies have ushered in a new era in gene therapy, providing novel therapeutic strategies for a wide range of diseases, including both genetic and nongenetic ocular diseases. These technologies offer new hope for patients suffering from previously untreatable conditions. The unique anatomical and physiological features of the eye, including its immune-privileged status, size, and compartmentalized structure, provide an optimal environment for the application of these cutting-edge technologies. Moreover, the development of various delivery methods has facilitated the efficient and targeted administration of genome engineering tools designed to correct specific ocular tissues. Additionally, advancements in noninvasive ocular imaging techniques and electroretinography have enabled real-time monitoring of therapeutic efficacy and safety. Herein, we discuss the discovery and development of genome-editing technologies, their application to ocular diseases from the anterior segment to the posterior segment, current limitations encountered in translating these technologies into clinical practice, and ongoing research endeavors aimed at overcoming these challenges.

Retinal-phospholipid Schiff-base conjugates and their interaction with ABCA4, the ABC transporter associated with Stargardt Disease

N-retinylidene-phosphatidylethanolamine (N-Ret-PE), the Schiff-base conjugate formed through the reversible reaction of retinal (Vitamin A-aldehyde) and phosphatidylethanolamine, plays a crucial role in the visual cycle and visual pigment photoregeneration. However, N-Ret-PE can react with another molecule of retinal to form toxic di-retinoids if not removed from photoreceptors through its transport across photoreceptor membranes by the ATP-binding-cassette transporter ABCA4. Loss-of-function mutations in ABCA4 are known to cause Stargardt disease (STGD1), an inherited retinal degenerative disease associated with the accumulation of fluorescent di-retinoids and severe loss in vision. A larger assessment of retinal-phospholipid Schiff-base conjugates in photoreceptors is needed, along with further investigation of ABCA4 residues important for N-Ret-PE binding. In this study we show that N-Ret-PE formation is dependent on pH and phospholipid content. When retinal is added to liposomes or photoreceptor membranes, 40-60% is converted to N-Ret-PE at physiological pH. Phosphatidylserine and taurine also react with retinal to form N-retinylidene-phosphatidylserine (N-Ret-PS) and N-retinylidene-taurine, respectively, but at significantly lower levels. N-Ret-PS is not a substrate for ABCA4 and reacts poorly with retinal to form di-retinoids. Additionally, amino acid residues within the binding pocket of ABCA4 that contribute to its interaction with N-Ret-PE were identified and characterized using site-directed mutagenesis together with functional and binding assays. Substitution of arginine residues and hydrophobic residues with alanine or residues implicated in STGD1 significantly reduced or in some cases eliminated substrate-activated ATPase activity and substrate binding. Collectively, this study provides important insight into conditions which affect retinal-phospholipid Schiff-base formation and mechanisms underlying the pathogenesis of STGD1.

Antisense oligonucleotide therapy corrects splicing in the common Stargardt disease type 1-causing variant ABCA4 c.5461-10T>C

Stargardt disease type 1 (STGD1) is the most common hereditary form of maculopathy and remains untreatable. STGD1 is caused by biallelic variants in the ABCA4 gene, which encodes the ATP-binding cassette (type 4) protein (ABCA4) that clears toxic byproducts of the visual cycle. The c.5461-10T>C p.[Thr1821Aspfs∗6,Thr1821Valfs∗13] variant is the most common severe disease-associated variant, and leads to exon skipping and out-of-frame ABCA4 transcripts that prevent translation of functional ABCA4 protein. Homozygous individuals typically display early onset STGD1 and are legally blind by early adulthood. Here, we applied antisense oligonucleotides (AONs) to promote exon inclusion and restore wild-type RNA splicing of ABCA4 c.5461-10T>C. The effect of AONs was first investigated in vitro using an ABCA4 midigene model. Subsequently, the best performing AONs were administered to homozygous c.5461-10T>C 3D human retinal organoids. Isoform-specific digital polymerase chain reaction revealed a significant increase in correctly spliced transcripts after treatment with the lead AON, QR-1011, up to 53% correct transcripts at a 3 μM dose. Furthermore, western blot and immunohistochemistry analyses identified restoration of ABCA4 protein after treatment. Collectively, we identified QR-1011 as a potent splice-correcting AON and a possible therapeutic intervention for patients harboring the severe ABCA4 c.5461-10T>C variant.

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

O-GlcNAcylation regulates phagocytosis by promoting Ezrin localization at the cell cortex

O-GlcNAcylation is a post-translational modification that serves as a cellular nutrient sensor and participates in multiple physiological and pathological processes. However, it remains uncertain whether O-GlcNAcylation is involved in the regulation of phagocytosis. Here, we demonstrate a rapid increase in protein O-GlcNAcylation in response to phagocytotic stimuli. Knockout of O-GlcNAc transferase or pharmacological inhibition of O-GlcNAcylation dramatically blocks phagocytosis, resulting in the disruption of retinal structure and function. Mechanistic studies reveal that O-GlcNAc transferase interacts with Ezrin, a membrane-cytoskeleton linker protein, to catalyze its O-GlcNAcylation. Our data further show that Ezrin O-GlcNAcylation promotes its localization to the cell cortex, thereby stimulating the membrane-cytoskeleton interaction needed for efficient phagocytosis. These findings identify a previously unrecognized role for protein O-GlcNAcylation in phagocytosis with important implications in both health and diseases.

ABC transporters: human disease and pharmacotherapeutic potential

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are a 48-member superfamily of membrane proteins that actively transport a variety of biological substrates across lipid membranes. Their functional diversity defines an expansive involvement in myriad aspects of human biology. At least 21 ABC transporters underlie rare monogenic disorders, with even more implicated in the predisposition to and symptomology of common and complex diseases. Such broad (patho)physiological relevance places this class of proteins at the intersection of disease causation and therapeutic potential, underlining them as promising targets for drug discovery, as exemplified by the transformative CFTR (ABCC7) modulator therapies for cystic fibrosis. This review will explore the growing relevance of ABC transporters to human disease and their potential as small-molecule drug targets.

Functional characterization of ABCA4 genetic variants related to Stargardt disease

The ATP-binding cassette subfamily 4 (ABCA4), a transporter, is localized within the photoreceptors of the retina, and its genetic variants cause retinal dystrophy. Despite the clinical importance of the ABCA4 transporter, a few studies have investigated the function of each variant. In this study, we functionally characterized ABCA4 variants found in Korean patients with Stargardt disease or variants of the ABCA4 promoter region. We observed that four missense variants-p.Arg290Gln, p.Thr1117Ala, p.Cys1140Trp, and p.Asn1588Tyr-significantly decreased ABCA4 expression on the plasma membrane, which could be due to intracellular degradation. There are four major haplotypes in the ABCA4 proximal promoter. We observed that the H1 haplotype (c.-761C>A) indicated significantly increased luciferase activity compared to that of the wild-type, whereas the H3 haplotype (c.-1086A>C) indicated significantly decreased luciferase activity (P < 0.01 and 0.001, respectively). In addition, c.-900A>T in the H2 haplotype exhibited significantly increased luciferase activity compared with that of the wild-type. Two transcription factors, GATA-2 and HLF, were found to function as enhancers of ABCA4 transcription. Our findings suggest that ABCA4 variants in patients with Stargardt disease affect ABCA4 expression. Furthermore, common variants of the ABCA4 proximal promoter alter the ABCA4 transcriptional activity, which is regulated by GATA-2 and HLF transcription factors.

cGMP Analogues with Opposing Actions on CNG Channels Selectively Modulate Rod or Cone Photoreceptor Function

The vertebrate retina harbors rod and cone photoreceptors. Human vision critically depends on cone photoreceptor function. In the phototransduction cascade, cGMP activates distinct rod and cone isoforms of the cyclic nucleotide-gated (CNG) channel. Excessive cGMP levels initiate a pathophysiological rollercoaster, which starts with CNG channel over-activation, typically in rod photoreceptors. This triggers cell death of rods first, and then cones, and is the root cause of many blinding retinal diseases, including Retinitis pigmentosa. While targeting of CNG channels has been proposed for therapeutic purposes, thus far, it has not been possible to inhibit rod CNG channels without compromising cone function. Here, we present a novel strategy, based on cGMP analogues with opposing actions on CNG channels, which enables the selective modulation of either rod or cone photoreceptor activity. The combined treatment with the weak rod-selective CNG-channel inhibitor (Rp-8-Br-PET-cGMPS) and the cone-selective CNG-channel activator (8-pCPT-cGMP) essentially normalized rod CNG-channel function while preserving cone functionality at physiological and pathological cGMP levels. Hence, combinations of cGMP analogues with desired properties may elegantly address the isoform-specificity problem in future pharmacological therapies. Moreover, this strategy may allow for improvements in visual performance in certain light environments.

Effective gene therapy of Stargardt disease with PEG-ECO/pGRK1-ABCA4-S/MAR nanoparticles

Stargardt disease (STGD) is the most common form of inherited retinal genetic disorders and is often caused by mutations in ABCA4. Gene therapy has the promise to effectively treat monogenic retinal disorders. However, clinically approved adeno-associated virus (AAV) vectors do not have a loading capacity for large genes, such as ABCA4. Self-assembly nanoparticles composed of (1-aminoethyl)iminobis[N-(oleoylcysteinyl-1-amino-ethyl)propionamide (ECO; a multifunctional pH-sensitive/ionizable amino lipid) and plasmid DNA produce gene transfection comparable with or better than the AAV2 capsid. Stable PEG-ECO/pGRK1-ABCA4-S/MAR nanoparticles produce specific and prolonged expression of ABCA4 in the photoreceptors of Abca4 -/- mice and significantly inhibit accumulation of toxic A2E in the eye. Multiple subretinal injections enhance gene expression and therapeutic efficacy with an approximately 69% reduction in A2E accumulation in Abca4 -/- mice after 3 doses. Very mild inflammation was observed after multiple injections of the nanoparticles. PEG-ECO/pGRK1-ABCA4-S/MAR nanoparticles are a promising non-viral mediated gene therapy modality for STGD type 1 (STGD1).

Polymeric biomaterials in the treatment of posterior segment diseases

Polymeric biomaterials are biological or synthetic substances which can be engineered to interact with biological systems for the diagnosis or treatment of diseases. These biomaterials have immense potential for treating eyes diseases, particularly the retina—a site of many inherited and acquired diseases. Polymeric biomaterials can be engineered to function both as an endotamponade agent and to prevent intraocular scarring in retinal detachment repair surgeries. They can also be designed as a drug delivery platform for treatment of retinal diseases. Finally, they can be used as scaffolds for cellular products and provide non-viral gene delivery solutions to the retina. This perspective article explains the role of polymeric biomaterials in the treatment of retinal conditions by highlighting recent advances being translated to clinical practice. The article will also identify potential hurdles to clinical translation as future research directions in the field.

Is There an Optimal Combination of AREDS2 Antioxidants Zeaxanthin, Vitamin E and Vitamin C on Light-Induced Toxicity of Vitamin A Aldehyde to the Retina?

Vitamins C and E and zeaxanthin are components of a supplement tested in a large clinical trial-Age-Related Eye Disease Study 2 (AREDS2)-and it has been demonstrated that they can inhibit the progression of age-related macular degeneration. The aim of this study was to determine the optimal combinations of these antioxidants to prevent the phototoxicity mediated by vitamin A aldehyde (ATR), which can accumulate in photoreceptor outer segments (POS) upon exposure to light. We used cultured retinal pigment epithelial cells ARPE-19 and liposomes containing unsaturated lipids and ATR as a model of POS. Cells and/or liposomes were enriched with lipophilic antioxidants, whereas ascorbate was added just before the exposure to light. Supplementing the cells and/or liposomes with single lipophilic antioxidants had only a minor effect on phototoxicity, but the protection substantially increased in the presence of both ways of supplementation. Combinations of zeaxanthin with α-tocopherol in liposomes and cells provided substantial protection, enhancing cell viability from ~26% in the absence of antioxidants to ~63% in the presence of 4 µM zeaxanthin and 80 µM α-tocopherol, and this protective effect was further increased to ~69% in the presence of 0.5 mM ascorbate. The protective effect of ascorbate disappeared at a concentration of 1 mM, whereas 2 mM of ascorbate exacerbated the phototoxicity. Zeaxanthin or α-tocopherol partly ameliorated the cytotoxic effects. Altogether, our results suggest that the optimal combination includes upper levels of zeaxanthin and α-tocopherol achievable by diet and/or supplementations, whereas ascorbate needs to be at a four-fold smaller concentration than that in the vitreous. The physiological relevance of the results is discussed.

Cholesterol occupies the lipid translocation pathway to block phospholipid scrambling by a G protein-coupled receptor

Class A (rhodopsin-like) G protein-coupled receptors (GPCRs) are constitutive phospholipid scramblases as evinced after their reconstitution into liposomes. Yet phospholipid scrambling is not detectable in the resting plasma membrane of mammalian cells that is replete with GPCRs. We considered whether cholesterol, a prominent component of the plasma membrane, limits the ability of GPCRs to scramble lipids. Our previous Markov State Model (MSM) analysis of molecular dynamics simulations of membrane-embedded opsin indicated that phospholipid headgroups traverse a dynamically revealed hydrophilic groove between transmembrane helices (TM) 6 and 7 while their tails remain in the bilayer. Here, we present comparative MSM analyses of 150-μs simulations of opsin in cholesterol-free and cholesterol-rich membranes. Our analyses reveal that cholesterol inhibits phospholipid scrambling by occupying the TM6/7 interface and stabilizing the closed groove conformation while itself undergoing flip-flop. This mechanism may explain the inability of GPCRs to scramble lipids at the plasma membrane.