The genetic landscape of inherited retinal dystrophies in Arabs

Systematic review of genotype-phenotype associations in CRX-associated retinal dystrophies

BACKGROUND AND OBJECTIVES

CRX-associated retinal dystrophies (CRX-RDs) exhibit significant genotype-phenotype heterogeneity. This study aimed to elucidate the genotype-phenotype associations of CRX through a systematic analysis of the reported cases.

METHODS

84 studies, including 373 worldwide participants, were reviewed. These studies were checked for quality using Murad's tool for methodological quality and synthesis of case series and case reports. Clinical data, fundus imaging characteristics and genetic pathogenic variants were analysed.

RESULTS

The quality analysis revealed an overall good quality of the dataset, with some exceptions that do not detract from this trend. A predominance of cone-rod dystrophy (CRD) and Leber congenital amaurosis (LCA) among CRX-RDs (43% and 27%, respectively) was noted. Missense pathogenic variants were significantly associated with macular pigmentation, an absence of peripheral atrophy, an absence of peripheral pigmentation and CRD (p<0.05). In contrast, the indels (98% frameshifts) were associated with pale optic discs, attenuated optic vessels, and peripheral bone spicules, and more severe phenotypes, such as LCA (p<0.05). Pathogenic variants in the homeodomain were associated with cone and/or CRD; others in the OTX tail were linked to LCA.

CONCLUSION

CRX pathogenic variants are associated with specific phenotypic features.

Deciphering the Genetic and Epidemiological Landscape of Inherited Retinal Diseases (IRDs) in a Cohort of Eastern Iranian Patients

Inherited retinal diseases (IRDs) may have significant diagnostic challenges due to their genetic complexity and diverse inheritance patterns. Advanced genotyping tools like exome sequencing (ES) offer promising opportunities for identifying causative variants and improving disease management. This retrospective study was aimed to present prevalent pathogenic and novel variants in patients diagnosed with IRDs using ES. We investigated 154 patients diagnosed clinically with IRDs, of which non-syndromic IRDs were more prevalent than syndromic form (~56% vs. ~44%). Out of 154 unrelated patients, 133 (~86%) were genetically resolved, where retinitis pigmentosa was the most common subtype (26% of all resolved patients). Fifty-three previously known and also 56 novel variants across known IRD genes were identified. Autosomal recessive inheritance predominated in both resolved forms (112/133, 84.21%), with 46 novel variants. This could be due to high rate of consanguinity in the studied families (114/133 patients, 85.71%). The two previously reported ancestral founder pathogenic variants in TMEM67 (c.725A > G) and BBS2 (c.471G > A) genes, as well as the most common variant in AIPL1 gene (c.834G > A), were also prevalent in our patients. Interestingly, identical novel compound heterozygote of the CEP290 gene (c.3167C > A and c.7024C > T) were identified in two unrelated cases. This retrospective study was the first attempt in terms of sample size and diversity to add more to our current knowledge of the genetic makeup of IRDs in a population from the East of Iran. Our findings can facilitate genetic counselling and subtype classification of IRDs, especially in challenging cases.

Exploring Molecular Pathways in Refractive Errors Associated with Inherited Retinal Dystrophies

The term inherited retinal dystrophies (IRDs) refers to a diverse range of conditions characterized by retinal dysfunction, and mostly deterioration, leading to a gradual decay of the visual function and eventually to total vision loss. IRDs have a global impact on about 1 in every 3000 to 4000 individuals. However, the prevalence statistics might differ significantly depending on the exact type of dystrophy and the demographic being examined. The cellular pathophysiology and genetic foundation of IRDs have been extensively studied, however, knowledge regarding associated refractive errors remain limited. This review aims to clarify the cellular and molecular processes that underlie refractive errors in IRDs. We did a thorough search of the current literature (Pubmed, accession Feb 2024), selecting works describing phenotypic differences among genes-related to IRDs, particularly in relation to refractive errors. First, we summarize the wide range of IRDs and their genetic causes, describing the genes and biological pathways connected to the etiology of the disease. We then explore the complex relationship between refractive errors and retinal dysfunction, including how the impairment of the vision-related mechanisms in the retina can affect ocular biometry and optical characteristics. New data about the involvement of aberrant signaling pathways, photoreceptor degeneration, and dysfunctional retinal pigment epithelium (RPE) in the development of refractive errors in IRDs have been examined. We also discuss the therapeutic implications of refractive defects in individuals with IRD, including possible approaches to treating visual impairments. In addition, we address the value of using cutting-edge imaging methods and animal models to examine refractive errors linked to IRDs and suggest future lines of inquiry for identifying new targets for treatment. In summary, this study presents an integrated understanding of the cellular and molecular mechanisms underlying refractive errors in IRDs. It illuminates the intricacies of ocular phenotypes in these conditions and offers a tool for understanding mechanisms underlying isolated refractive errors, besides the IRD-related forms.

Clinical and genetic landscape of IRD in Portugal: pooled data from the nationwide IRD-PT registry

This study aims to characterize the clinical spectrum and genetic landscape of IRDs in Portugal. Multicentre, cross-sectional, cohort study comprising consecutive patients with a clinical diagnosis of IRD and available genetic results, enroled in the IRD-PT registry (retina.com.pt). Among the 1369 patients enroled from 1125 families, the most frequently observed phenotype was non-syndromic retinitis pigmentosa (40.8%). A genetically confirmed diagnosis was achieved in 72.3% of families. Consanguinity was observed in one-fifth of cases, contributing to a higher frequency of homozygous variants within this cohort. Disease-causing genotypes were distributed across 136 different genes, with ABCA4 (13.0%), EYS (10.0%) and USH2A (6.9%) being the most frequently mutated genes. Overall, these results from a nationwide cohort significantly advance our understanding of the clinical and genetic spectrum of IRDs in Portugal, laying the groundwork for future studies to identify patients eligible for targeted therapies and to describe the natural history of these diseases.

Usher syndrome in the United Arab Emirates

PURPOSE

Usher syndrome, a common form of syndromic inherited retinal dystrophy in the Arabian Gulf, has not been molecularly defined in the United Arab Emirates. The current study addresses this gap in knowledge.

METHODS

A retrospective case series of Emirati patients referred to the Ocular Genetics Clinic of Cleveland Clinic Abu Dhabi who (1) were clinically diagnosed with Usher syndrome and underwent genetic testing (whole exome sequencing, 2019 to 2023, inclusive) and (2) were identified to have biallelic pathogenic variants in Usher syndrome genes during the same time period.

RESULTS

Ten probands (thirteen patients) were identified-seven probands (nine patients) with clinically diagnosed Usher syndrome and three additional probands (four patients) with biallelic homozygous USH2A variants. Among the seven probands initially diagnosed with Usher syndrome, six had different homozygous variants (three in MYO7A, one in ADGRV1, and one in CLRN1), one had dual diagnoses rather than Usher syndrome (i.e. separate cause for retinal dystrophy and deafness), and one had no identifiable genetic cause. Regarding the three additional probands identified with homozygous USH2A variants, all three had retinitis pigmentosa only rather than Usher syndrome and all three had different variants.

DISCUSSION

Clinically diagnosed Usher syndrome was genetically heterogenous without evidence for founder effect in this Emirati cohort. MYO7A was the most common associated gene. Dual diagnosis rather than single cause can mimic Usher syndrome. Homozygous USH2A variants were not identified as a cause for Usher syndrome in this cohort but were a recurrent cause for retinitis pigmentosa without hearing impairment and without founder effect.

The Landscape of Genomic Services for Inherited Retinal Degenerations (IRDs) Across Europe

Purpose

To map the existing genomic services available for patients with IRDs across Europe.

Methods

A survey was conducted to 24 ophthalmic and/or genetic specialists across 19 European countries. The survey was conducted in an interview style via zoom for participants from 17 out of 19 countries. Interviewees were clinical/medical/ophthalmic geneticists, ophthalmologists/retina specialists and internal medicine specialists. The survey focused on referral pathways, genetic counseling, insurance coverage, awareness of genetic testing and counseling for IRDs among practitioners and patients, and preferred testing methodologies.

Results

Genomic services (testing and counselling) for IRDs vary among countries from an awareness, availability and insurance coverage perspective. Affordability could be a barrier for patients in countries without any payment scheme (eg, Poland) and in countries where only a targeted population is covered (eg, Bulgaria). Genetic counseling via qualified genetic counsellors did not exist in many countries. The level of awareness regarding the benefits of genetic testing in IRDs among healthcare professionals (HCPs) and patients was perceived as low in some countries. Panel-based next-generation sequencing (NGS) was the first test of choice for genetic testing in 68% of the studied countries.

Conclusion

There is some disparity in the approach to genetic testing for IRDs across Europe. Greater awareness of genetic testing services is required among the eye care professional community. A revised approach to the provision of genetic testing services such as centralized free genetic testing with associated interpretation and genetic counselling may help in ensuring equitable access and reimbursement, which will empower patients through improved access to clinical trials, expedite innovation, improve access to therapy and the delivery of care.

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.