Clinical features and molecular genetics of patients with ABCA4-retinal dystrophies

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.

A comprehensive review of experimental models for investigating blue light-induced ocular damage: Insights into parameters, limitations, and new opportunities

Light-emitting diodes (LEDs) are widely used in modern lighting and electronic devices, including smartphones, computer monitors, tablets, televisions, and vehicle lights. Blue light (BL) hazards to eye health have received increasing attention because white LED bulbs emit higher levels of BL than traditional lighting sources. At wavelengths of 400-500 nm, BL is characterized by its high energy and risks associated with prolonged exposure, which may lead to photochemical damage and morphological alterations in the retina. Recent research has revealed that the harmful effects of BL are intricately linked to light intensity and exposure frequency, with mechanisms involving the overproduction of reactive oxygen species through photooxidative processes. This growing body of knowledge deepens our understanding of photodamage and opens avenues for exploring protective strategies for our eyes. Although current clinical trials assessing the safety of BL exposure remain limited, the development of experimental models that mimic physiological conditions has revealed BL toxicity. This review categorizes and evaluates BL-induced retinopathy in vivo, providing a comprehensive overview of the associated experimental parameters, including photosensitive fluorophores, light wavelength, illuminance, irradiance, exposure duration, animal strains, and their unique lesion patterns. Moreover, this study underscores the need for further research to evaluate photoprotective agents, which may offer valuable insights to the ongoing discussion on preserving ocular health in our increasingly illuminated digital environments.

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.

Elevated cholesterol in ATAD3 mutants is a compensatory mechanism that leads to membrane cholesterol aggregation

Aberrant cholesterol metabolism causes neurological disease and neurodegeneration, and mitochondria have been linked to perturbed cholesterol homeostasis via the study of pathological mutations in the ATAD3 gene cluster. However, whether the cholesterol changes were compensatory or contributory to the disorder was unclear, and the effects on cell membranes and the wider cell were also unknown. Using patient-derived cells, we show that cholesterol perturbation is a conserved feature of pathological ATAD3 variants that is accompanied by an expanded lysosome population containing membrane whorls characteristic of lysosomal storage diseases. Lysosomes are also more numerous in Drosophila neural progenitor cells expressing mutant Atad3, which exhibit abundant membrane-bound cholesterol aggregates, many of which co-localize with lysosomes. By subjecting the Drosophila Atad3 mutant to nutrient restriction and cholesterol supplementation, we show that the mutant displays heightened cholesterol dependence. Collectively, these findings suggest that elevated cholesterol enhances tolerance to pathological ATAD3 variants; however, this comes at the cost of inducing cholesterol aggregation in membranes, which lysosomal clearance only partly mitigates.

Multimodal in-vivo maps as a tool to characterize retinal structural biomarkers for progression in adult-onset Stargardt disease

Purpose

To characterize retinal structural biomarkers for progression in adult-onset Stargardt disease from multimodal retinal imaging in-vivo maps.

Methods

Seven adult patients (29-69 years; 3 males) with genetically-confirmed and clinically diagnosed adult-onset Stargardt disease and age-matched healthy controls were imaged with confocal and non-confocal Adaptive Optics Scanning Light Ophthalmoscopy (AOSLO), optical coherence tomography (OCT), fundus infrared (FIR), short wavelength-autofluorescence (FAF) and color fundus photography (CFP). Images from each modality were scaled for differences in lateral magnification before montages of AOSLO images were aligned with en-face FIR, FAF and OCT scans to explore changes in retinal structure across imaging modalities. Photoreceptors, retinal pigment epithelium (RPE) cells, flecks, and other retinal alterations in macular regions were identified, delineated, and correlated across imaging modalities. Retinal layer-thicknesses were extracted from segmented OCT images in areas of normal appearance on clinical imaging and intact outer retinal structure on OCT. Eccentricity dependency in cell density was compared with retinal thickness and outer retinal layer thickness, evaluated across patients, and compared with data from healthy controls.

Results

In patients with Stargardt disease, alterations in retinal structure were visible in different image modalities depending on layer location and structural properties. The patients had highly variable foveal structure, associated with equally variable visual acuity (-0.02 to 0.98 logMAR). Cone and rod photoreceptors, as well as RPE-like structures in some areas, could be quantified on non-confocal split-detection AOSLO images. RPE cells were also visible on dark field AOSLO images close to the foveal center. Hypo-reflective gaps of non-waveguiding cones (dark cones) were seen on confocal AOSLO in regions with clinically normal CFP, FIR, FAF and OCT appearance and an intact cone inner segment mosaic in three patients.

Conclusion

Dark cones were identified as a possible first sign of retinal disease progression in adult-onset Stargardt disease as these are observed in retinal locations with otherwise normal appearance and outer retinal thickness. This corroborates a previous report where dark cones were proposed as a first sign of progression in childhood-onset Stargardt disease. This also supports the hypothesis that, in Stargardt disease, photoreceptor degeneration occurs before RPE cell death.

Towards Uncovering the Role of Incomplete Penetrance in Maculopathies through Sequencing of 105 Disease-Associated Genes

Inherited macular dystrophies (iMDs) are a group of genetic disorders, which affect the central region of the retina. To investigate the genetic basis of iMDs, we used single-molecule Molecular Inversion Probes to sequence 105 maculopathy-associated genes in 1352 patients diagnosed with iMDs. Within this cohort, 39.8% of patients were considered genetically explained by 460 different variants in 49 distinct genes of which 73 were novel variants, with some affecting splicing. The top five most frequent causative genes were ABCA4 (37.2%), PRPH2 (6.7%), CDHR1 (6.1%), PROM1 (4.3%) and RP1L1 (3.1%). Interestingly, variants with incomplete penetrance were revealed in almost one-third of patients considered solved (28.1%), and therefore, a proportion of patients may not be explained solely by the variants reported. This includes eight previously reported variants with incomplete penetrance in addition to CDHR1:c.783G>A and CNGB3:c.1208G>A. Notably, segregation analysis was not routinely performed for variant phasing-a limitation, which may also impact the overall diagnostic yield. The relatively high proportion of probands without any putative causal variant (60.2%) highlights the need to explore variants with incomplete penetrance, the potential modifiers of disease and the genetic overlap between iMDs and age-related macular degeneration. Our results provide valuable insights into the genetic landscape of iMDs and warrant future exploration to determine the involvement of other maculopathy genes.

Exosomal miR-4488 and miR-1273g-5p inhibit the epithelial-mesenchymal transition of transforming growth factor β2-mediated retinal pigment epithelial cells by targeting ATP-binding cassette A4

Exosomal microRNAs (miRNAs) have been shown to be involved in the regulation of many disease progression, including proliferative vitreoretinopathy (PVR). However, the roles of exosomal miR-4488 and miR-1273g-5p in PVR progression have not been demonstrated. Transforming growth factor β2 (TGF-β2)-induced ARPE-19 cells were used to stimulate the epithelial-mesenchymal transition (EMT) of cells. Exosomes derived from TGF-β2-induced ARPE-19 cells were identified by transmission electron microscopy and nanoparticle tracking analysis. The expression levels of miR-4488, miR-1273g-5p and ATP-binding cassette A4 (ABCA4) were measured by quantitative real-time PCR. The promotion levels of exosomes markers, EMT markers, apoptosis markers and ABCA4 were determined by western blot analysis. The migration, invasion and apoptosis of cells were determined by transwell assay, wound healing assay and flow cytometry. Our data showed that miR-4488 and miR-1273g-5p were lowly expressed in TGF-β2-induced ARPE-19 cells. Overexpressed exosomal miR-4488 and miR-1273g-5p could inhibit the EMT, migration, invasion, and promote apoptosis in TGF-β2-induced ARPE-19 cells. In addition, ABCA4 was a target of miR-4488 and miR-1273g-5p. Overexpressed ABCA4 also could reverse the negatively regulation of exosomal miR-4488 and miR-1273g-5p on the EMT, migration, and invasion of TGF-β2-induced ARPE-19 cells. In conclusion, our data showed that exosomal miR-4488 and miR-1273g-5p could inhibit TGF-β2-stimulated EMT in ARPE-19 cells through targeting ABCA4.

An Overview of the Genetics of ABCA4 Retinopathies, an Evolving Story

Stargardt disease (STGD1) and ABCA4 retinopathies (ABCA4R) are caused by pathogenic variants in the ABCA4 gene inherited in an autosomal recessive manner. The gene encodes an importer flippase protein that prevents the build-up of vitamin A derivatives that are toxic to the RPE. Diagnosing ABCA4R is complex due to its phenotypic variability and the presence of other inherited retinal dystrophy phenocopies. ABCA4 is a large gene, comprising 50 exons; to date > 2000 variants have been described. These include missense, nonsense, splicing, structural, and deep intronic variants. Missense variants account for the majority of variants in ABCA4. However, in a significant proportion of patients with an ABCA4R phenotype, a second variant in ABCA4 is not identified. This could be due to the presence of yet unknown variants, or hypomorphic alleles being incorrectly classified as benign, or the possibility that the disease is caused by a variant in another gene. This underlines the importance of accurate genetic testing. The pathogenicity of novel variants can be predicted using in silico programs, but these rely on databases that are not ethnically diverse, thus highlighting the need for studies in differing populations. Functional studies in vitro are useful towards assessing protein function but do not directly measure the flippase activity. Obtaining an accurate molecular diagnosis is becoming increasingly more important as targeted therapeutic options become available; these include pharmacological, gene-based, and cell replacement-based therapies. The aim of this review is to provide an update on the current status of genotyping in ABCA4 and the status of the therapeutic approaches being investigated.

Stargardt disease: Multimodal imaging: A review

Stargardt disease (STGD1) is an autosomal recessive retinal dystrophy, characterised by bilateral progressive central vision loss and subretinal deposition of lipofuscin-like substances. Recent advances in molecular diagnosis and therapeutic options are complemented by the increasing recognition of new multimodal imaging biomarkers that may predict genotype and disease progression. Unique non-invasive imaging features of STDG1 are useful for gene variant interpretation and may even provide insight into the underlying molecular pathophysiology. In addition, pathognomonic imaging features of STGD1 have been used to train neural networks to improve time efficiency in lesion segmentation and disease progression measurements. This review will discuss the role of key imaging modalities, correlate imaging signs across varied STGD1 presentations and illustrate the use of multimodal imaging as an outcome measure in determining the efficacy of emerging STGD1 specific therapies.

Genetic Dominant Variants in STUB1, Segregating in Families with SCA48, Display In Vitro Functional Impairments Indistinctive from Recessive Variants Associated with SCAR16

Variants in STUB1 cause both autosomal recessive (SCAR16) and dominant (SCA48) spinocerebellar ataxia. Reports from 18 STUB1 variants causing SCA48 show that the clinical picture includes later-onset ataxia with a cerebellar cognitive affective syndrome and varying clinical overlap with SCAR16. However, little is known about the molecular properties of dominant STUB1 variants. Here, we describe three SCA48 families with novel, dominantly inherited STUB1 variants (p.Arg51_Ile53delinsProAla, p.Lys143_Trp147del, and p.Gly249Val). All the patients developed symptoms from 30 years of age or later, all had cerebellar atrophy, and 4 had cognitive/psychiatric phenotypes. Investigation of the structural and functional consequences of the recombinant C-terminus of HSC70-interacting protein (CHIP) variants was performed in vitro using ubiquitin ligase activity assay, circular dichroism assay and native polyacrylamide gel electrophoresis. These studies revealed that dominantly and recessively inherited STUB1 variants showed similar biochemical defects, including impaired ubiquitin ligase activity and altered oligomerization properties of the CHIP. Our findings expand the molecular understanding of SCA48 but also mean that assumptions concerning unaffected carriers of recessive STUB1 variants in SCAR16 families must be re-evaluated. More investigations are needed to verify the disease status of SCAR16 heterozygotes and elucidate the molecular relationship between SCA48 and SCAR16 diseases.