Mutation of a nuclear receptor gene, NR2E3, causes enhanced S cone syndrome, a disorder of retinal cell fate

Targeting Nr2e3 to Modulate Tet2 Expression: Therapeutic Potential for Depression Treatment

Epigenetic mechanisms such as DNA methylation and hydroxymethylation play a significant role in depression. This research has shown that Ten-eleven translocation 2 (Tet2) deficiency prompts depression-like behaviors, but Tet2's transcriptional regulation remains unclear. In the study, bioinformatics is used to identify nuclear receptor subfamily 2 group E member 3 (Nr2e3) as a potential Tet2 regulator. Nr2e3 is found to enhance Tet2's transcriptional activity by binding to its promoter region. Nr2e3 knockdown in mouse hippocampus leads to reduced Tet2 expression, depression-like behaviors, decreased hydroxymethylation of synaptic genes, and downregulation of synaptic proteins like postsynaptic density 95 KDa (PSD95) and N-methy-d-aspartate receptor 1 (NMDAR1). Fewer dendritic spines are also observed. Nr2e3 thus appears to play an antidepressant role under stress. In search of potential treatments, small molecule compounds to increase Nr2e3 expression are screened. Azacyclonal (AZA) is found to enhance the Nr2e3/Tet2 pathway and exhibited antidepressant effects in stressed mice, increasing PSD95 and NMDAR1 expression and dendritic spine density. This study illuminates Tet2's upstream regulatory mechanism, providing a new target for identifying early depression biomarkers and developing treatments.

Evaluating therapeutic potential of NR2E3 doses in the rd7 mouse model of retinal degeneration

Retinitis Pigmentosa is a leading cause of severe vision loss. Retinitis Pigmentosa can present with a broad range of phenotypes impacted by disease age of onset, severity, and progression. This variation is influenced both by different gene mutations as well as unique variants within the same gene. Mutations in the nuclear hormone receptor 2 family e, member 3 are associated with several forms of retinal degeneration, including Retinitis Pigmentosa. In our previous studies we demonstrated that subretinal administration of one Nr2e3 dose attenuated retinal degeneration in rd7 mice for at least 3 months. Here we expand the studies to evaluate the efficacy and longitudinal impact of the NR2E3 therapeutic by examining three different doses administered at early or intermediate stages of retinal degeneration in the rd7 mice. Our study revealed retinal morphology was significantly improved 6 months post for all doses in the early-stage treatment groups and for the low and mid doses in the intermediate stage treatment groups. Similarly, photoreceptor function was significantly improved in the early stage for all doses and intermediate stage low and mid dose groups 6 months post treatment. This study demonstrated efficacy in multiple doses of NR2E3 therapy.

Identification and Characterization of ATOH7-Regulated Target Genes and Pathways in Human Neuroretinal Development

The proneural transcription factor atonal basic helix-loop-helix transcription factor 7 (ATOH7) is expressed in early progenitors in the developing neuroretina. In vertebrates, this is crucial for the development of retinal ganglion cells (RGCs), as mutant animals show an almost complete absence of RGCs, underdeveloped optic nerves, and aberrations in retinal vessel development. Human mutations are rare and result in autosomal recessive optic nerve hypoplasia (ONH) or severe vascular changes, diagnosed as autosomal recessive persistent hyperplasia of the primary vitreous (PHPVAR). To better understand the role of ATOH7 in neuroretinal development, we created ATOH7 knockout and eGFP-expressing ATOH7 reporter human induced pluripotent stem cells (hiPSCs), which were differentiated into early-stage retinal organoids. Target loci regulated by ATOH7 were identified by Cleavage Under Targets and Release Using Nuclease with sequencing (CUT&RUN-seq) and differential expression by RNA sequencing (RNA-seq) of wildtype and mutant organoid-derived reporter cells. Additionally, single-cell RNA sequencing (scRNA-seq) was performed on whole organoids to identify cell type-specific genes. Mutant organoids displayed substantial deficiency in axon sprouting, reduction in RGCs, and an increase in other cell types. We identified 469 differentially expressed target genes, with an overrepresentation of genes belonging to axon development/guidance and Notch signaling. Taken together, we consolidate the function of human ATOH7 in guiding progenitor competence by inducing RGC-specific genes while inhibiting other cell fates. Furthermore, we highlight candidate genes responsible for ATOH7-associated optic nerve and retinovascular anomalies, which sheds light to potential future therapy targets for related disorders.

The potential key role of choroidal non-perfusion and rod degeneration in the pathogenesis of macular neovascularization type 3

Macular neovascularization type 3 (MNV3) is a multifactorial disease with distinct epidemiological, clinical, pathomorphological and topographical characteristics. This review of the literature discusses the latest experimental and clinical outcomes that could explain the pathogenesis of retinal neovascularization. Although patients with MNV3 are usually older than those with MNV1 or 2, their lesions do not coexist with, precede, or follow other types in the same eye. The regional distribution of MNV3 lesions is characterized as confined to the parafoveal macula without any involvement of the rod-free foveal area. Focal outer retinal atrophy and choroidal non-perfusion are the main structural features that occur prior to the development of retinal neovascularization. Also, histological and experimental studies of MNV3 and other non-neovascular age-related macular degeneration diseases complicated with MNV3-like lesions strongly suggest rod degeneration contributes to the pathogenesis. Therefore, the retinal neovascularization in MNV3 has a different pathogenesis from the choroidal neovascularization in MNV1 and 2 and emerging evidence indicates that choroidal non-prefusion and rod degeneration play a key role in the pathogenesis of MNV3. Accordingly, we suggest a sequence of pathological events that start with choroidal non-perfusion due to advanced age followed by hypoxia of the outer retina at the parafoveal area. This induces a remarkable degeneration of rods that triggers the growth of retinal neovascularization due to the imbalance of the angiogenic factors in the outer retina.

Genotypic and phenotypic diversity in X-linked retinoschisis: Findings from a South Indian patient cohort

PURPOSE

Retinoschisis is a distinctive condition characterized by intraretinal layer clefts, primarily associated with X-linked recessive inheritance due to RS1 gene mutations. This study aims to uncover the RS1 mutation spectrum in a cohort of 22 X-linked retinoschisis cases from South India and emphasizes the genotypic and phenotypic associations within patients harboring only RS1 mutations.

METHODS

A total of 22 probands were suspected of having X-linked retinoschisis. All study subjects underwent ophthalmic investigations, including assessments of visual acuity, fundus examination, optical coherence tomography (OCT), and electroretinogram (ERG). RS1 gene screening was conducted using Sanger sequencing, and the pathogenicity of the variants was assessed through Sorting Intolerant from Tolerant (SIFT) and PolyPhen-2 in silico tools.

RESULTS

The study found that the probands had an average visual acuity of 0.79 ± 0.39 log of minimum angle of resolution (logMAR), ranging from 0.17 to 1.77. During fundus examination, the probands exhibited a characteristic spoke wheel-like pattern in the macular region. Furthermore, OCT analysis revealed distinct alterations in the inner retinal microstructure, and ERG results consistently showed a reduction in b-wave amplitude. Eventually, Sanger sequencing results showed hemizygous mutations in the RS1 gene in only 12 probands, including a novel missense mutation in the RS1 gene's signal sequence.

CONCLUSION

This study provides valuable insights into the spectrum of RS1 mutations in X-linked retinoschisis probands from South India. It reveals distinct genotypic-phenotypic associations and highlights the clinical manifestations associated with the disease pathogenesis.

Preclinical dose response study shows NR2E3 can attenuate retinal degeneration in the retinitis pigmentosa mouse model RhoP23H+/. Gene Ther 2024;31:255-262. [PMID: 38273095 PMCID: PMC11090815 DOI: 10.1038/s41434-024-00440-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Retinitis pigmentosa (RP) is a heterogeneous disease and the main cause of vision loss within the group of inherited retinal diseases (IRDs). IRDs are a group of rare disorders caused by mutations in one or more of over 280 genes which ultimately result in blindness. Modifier genes play a key role in modulating disease phenotypes, and mutations in them can affect disease outcomes, rate of progression, and severity. Our previous studies have demonstrated that the nuclear hormone receptor 2 family e, member 3 (Nr2e3) gene reduced disease progression and loss of photoreceptor cell layers in RhoP23H-/- mice. This follow up, pharmacology study evaluates a longitudinal NR2E3 dose response in the clinically relevant heterozygous RhoP23H mouse. Reduced retinal degeneration and improved retinal morphology was observed 6 months following treatment evaluating three different NR2E3 doses. Histological and immunohistochemical analysis revealed regions of photoreceptor rescue in the treated retinas of RhoP23H+/- mice. Functional assessment by electroretinogram (ERG) showed attenuated photoreceptor degeneration with all doses. This study demonstrates the effectiveness of different doses of NR2E3 at reducing retinal degeneration and informs dose selection for clinical trials of RhoP23H-associated RP.

Specific photoreceptor cell fate pathways are differentially altered in NR2E3-associated diseases

Mutations in NR2E3, a gene encoding an orphan nuclear transcription factor, cause two retinal dystrophies with a distinct phenotype, but the precise role of NR2E3 in rod and cone transcriptional networks remains unclear. To dissect NR2E3 function, we performed scRNA-seq in the retinas of wildtype and two different Nr2e3 mouse models that show phenotypes similar to patients carrying NR2E3 mutations. Our results reveal that rod and cone populations are not homogeneous and can be separated into different sub-classes. We identify a previously unreported cone pathway that generates hybrid cones co-expressing both cone- and rod-related genes. In mutant retinas, this hybrid cone subpopulation is more abundant and includes a subpopulation of rods transitioning towards a cone cell fate. Hybrid photoreceptors with high misexpression of cone- and rod-related genes are prone to regulated necrosis. Overall, our results shed light on the role of NR2E3 in modulating photoreceptor differentiation towards cone and rod fates and explain how different mutations in NR2E3 lead to distinct visual disorders in humans.

Functional opsin patterning for Drosophila color vision is established through signaling pathways in adjacent object-detection neurons

Vision is mainly based on two different tasks, object detection and color discrimination, carried out by photoreceptor (PR) cells. The Drosophila compound eye consists of ∼800 ommatidia. Every ommatidium contains eight PR cells, six outer cells (R1-R6) and two inner cells (R7 and R8), by which object detection and color vision are achieved, respectively. Expression of opsin genes in R7 and R8 is highly coordinated through the instructive signal from R7 to R8, and two major ommatidial subtypes are distributed stochastically; pale type expresses Rh3/Rh5 and yellow type expresses Rh4/Rh6 in R7/R8. The homeodomain protein Defective proventriculus (Dve) is expressed in yellow-type R7 and in six outer PRs, and it is involved in Rh3 repression to specify the yellow-type R7. dve mutant eyes exhibited atypical coupling, Rh3/Rh6 and Rh4/Rh5, indicating that Dve activity is required for proper opsin coupling. Surprisingly, Dve activity in R1 is required for the instructive signal, whereas activity in R6 and R7 blocks the signal. Our results indicate that functional coupling of two different neurons is established through signaling pathways from adjacent neurons that are functionally different.

Bilateral helicoid peri-papillary sub-retinal fibrosis due to a biallelic NR2E3 mutation: Describing variable expressivity of a mutation

BACKGROUND

To describe different clinical presentations of a same NR2E3 recessive mutation in two families and within one family.

DESIGN

Interventional family study.

RESULTS

Our first case was a one-year-old male child with high hyperopia and refractive accommodative esotropia. In retinal examination, peri-papillary sub-retinal fibrosis with a helicoid configuration was observed in both eyes. The parents and the only sibling had no pathologic findings in the eyes. The child showed to have severely reduced responses in both photopic and scotopic electroretinogram components. In the genetic investigation, a homozygous autosomal recessive mutation in the NR2E3 gene (IVS1-2A > C) was discovered in the affected child, while the other family members were heterozygous for this mutation. We followed up with the patient for 3 years and no new lesion developed during this period. The second case was a 13-year-old male child referred to the retina clinic for decreased vision in the right eye. In retina examination, there were nummular pigmentary changes at the level of retinal pigment epithelium and along the vascular arcades with foveo-schitic changes in both eyes. A choroidal neovascularization (CNV) was noticed in the macula of his right eye. The genetic evaluation proved the same mutation in the NR2E3 gene as in the first case. Family history was remarkable for an uncle, an aunt, and two cousins with night blindness.

CONCLUSION

Same NR2E3 gene mutation can cause heterogeneous clinical manifestations such as slight retinal changes in the absence of any visual symptoms to high hyperopia associated with helicoid peri-papillary sub-retinal fibrosis.

Epigenetic Switches in Retinal Homeostasis and Target for Drug Development

Retinal homeostasis, a tightly regulated process maintaining the functional integrity of the retina, is vital for visual function. Emerging research has unveiled the critical role of epigenetic regulation in controlling gene expression patterns during retinal development, maintenance, and response to mutational loads and injuries. Epigenetic switches, including DNA methylation, histone modifications, and non-coding RNAs, play pivotal roles in orchestrating retinal gene expression and cellular responses through various intracellular, extracellular, and environmental modulators. This review compiles the current knowledge on epigenetic switches in retinal homeostasis, providing a deeper understanding of their impact on retinal structural integrity and function and using them as potential targets for therapeutic interventions.

Pediatric presentation of enhanced S-cone syndrome associated with two heterozygous NR2E3 mutations

We report the case of an otherwise healthy 10-year-old girl referred to our institution for gradually decreasing vision and nyctalopia. Based on clinical examination, she was diagnosed with inherited retinal dystrophy, presumably due to enhanced S-cone syndrome (ESCS). Subsequent genetic testing confirmed a rare combination of NR2E3 heterozygous mutations: c.119-2A>C and c.932G>A p.(Arg311Gln).

Retinal detachment in a pediatric patient with enhanced S-cone syndrome

Enhanced S-cone syndrome (ESCS), also known as Goldmann-Favre syndrome, is a retinal degeneration that presents in childhood and leads to progressive nyctalopia and visual field loss. In advanced cases, this degeneration can result in loss of central visual acuity. We describe the case of a 15-year-old boy with ESCS who presented with retinal detachment, a rare complication.

Clinical and Genetic Features of NR2E3-Associated Retinopathy: A Report of Eight Families with a Longitudinal Study and Literature Review

(1) Background: NR2E3 encodes a nuclear receptor transcription factor that is considered to promote cell differentiation, affect retinal development, and regulate phototransduction in rods and cones. This study aimed to analyze the clinical characteristics and observe the prognosis of autosomal dominant retinopathy (ADRP) and autosomal recessive retinopathy (ARRP) associated with NR2E3; (2) Methods: NR2E3 variants were collected from our exome sequencing data and identified per the American College of Medical Genetics and Genomics criteria. Data from our cohort and a systemic literature review were conducted to explore the NR2E3 variants spectrum and potential genotype-phenotype correlations; (3) Results: Nine pathogenic variants/likely pathogenic variants in NR2E3, including five novel variants, were detected in eight families (four each with ADRP and ARRP). Follow-up data showed schisis/atrophy in the macula and retinal degeneration initiation around the vascular arcades with differences in ADRP and ARRP. A systemic literature review indicated patients with ADRP presented better visual acuity (p < 0.01) and later onset age (p < 0.0001) than did those with ARRP; (4) Conclusions: Macular schisis and retinal degeneration around vascular arcades may present as the prognosis of NR2E3-retinopathy, dominant, or recessive. Our data might further enrich our understanding of NR2E3 variants and associated inherited retinopathy.

Nuclear Receptor Subfamily 2 Group E Member 3 (NR2E3): Role in Retinal Development and Disease

NR2E3 is a nuclear hormone receptor gene required for the correct development of the retinal rod photoreceptors. Expression of NR2E3 protein in rod cell precursors suppresses cone-specific gene expression and, in concert with other transcription factors including NRL, activates the expression of rod-specific genes. Pathogenic variants involving NR2E3 cause a spectrum of retinopathies, including enhanced S-cone syndrome, Goldmann-Favre syndrome, retinitis pigmentosa, and clumped pigmentary retinal degeneration, with limited evidence of genotype-phenotype correlations. A common feature of NR2E3-related disease is an abnormally high number of cone photoreceptors that are sensitive to short wavelength light, the S-cones. This characteristic has been supported by mouse studies, which have also revealed that loss of Nr2e3 function causes photoreceptors to develop as cells that are intermediate between rods and cones. While there is currently no available cure for NR2E3-related retinopathies, there are a number of emerging therapeutic strategies under investigation, including the use of viral gene therapy and gene editing, that have shown promise for the future treatment of patients with NR2E3 variants and other inherited retinal diseases. This review provides a detailed overview of the current understanding of the role of NR2E3 in normal development and disease, and the associated clinical phenotypes, animal models, and therapeutic studies.

Gene-agnostic approaches to treating inherited retinal degenerations

Most patients with inherited retinal degenerations (IRDs) have been waiting for treatments that are "just around the corner" for decades, with only a handful of seminal breakthroughs happening in recent years. Highlighting the difficulties in the quest for curative therapeutics, Luxturna required 16 years of development before finally obtaining United States Food and Drug Administration (FDA) approval and its international equivalents. IRDs are both genetically and phenotypically heterogeneous. While this diversity offers many opportunities for gene-by-gene precision medicine-based approaches, it also poses a significant challenge. For this reason, alternative (or parallel) strategies to identify more comprehensive, across-the-board therapeutics for the genetically and phenotypically diverse IRD patient population are very appealing. Even when gene-specific approaches may be available and become approved for use, many patients may have reached a disease stage whereby these approaches may no longer be viable. Thus, alternate visual preservation or restoration therapeutic approaches are needed at these stages. In this review, we underscore several gene-agnostic approaches that are being developed as therapeutics for IRDs. From retinal supplementation to stem cell transplantation, optogenetic therapy and retinal prosthetics, these strategies would bypass at least in part the need for treating every individual gene or mutation or provide an invaluable complement to them. By considering the diverse patient population and treatment strategies suited for different stages and patterns of retinal degeneration, gene agnostic approaches are very well poised to impact favorably outcomes and prognosis for IRD patients.

Enhanced S-cone Syndrome, a Mini-review

Enhanced S-cone Syndrome (ESCS) is an autosomal recessive inherited retinal disease mostly associated with disease-causing variants in the NR2E3 gene. During retinal development in ESCS, rod photoreceptor precursors are misdirected to form photoreceptors similar to short-wavelength cones, or S-cones. Compared to a normal human retina, patients with ESCS have no rods and significantly increased numbers of S-cones. Night blindness is the main visual symptom, and visual acuity and color vision can be normal at early disease stages. Histology of donor eyes and adaptive optics imaging revealed increased S-cone density outside of the fovea compared to normal. Visual function testing reveals absent rod function and abnormally enhanced sensitivity to short-wavelength light. Unlike most retinal degenerative diseases, ESCS results in a gain in S-cone photoreceptor function. Research involving ESCS could improve understanding of this rare retinal condition and also shed light on the role of NR2E3 expression in photoreceptor survival.

Disease-causing mutations in genes encoding transcription factors critical for photoreceptor development

Photoreceptor development of the vertebrate visual system is controlled by a complex transcription regulatory network. OTX2 is expressed in the mitotic retinal progenitor cells (RPCs) and controls photoreceptor genesis. CRX that is activated by OTX2 is expressed in photoreceptor precursors after cell cycle exit. NEUROD1 is also present in photoreceptor precursors that are ready to specify into rod and cone photoreceptor subtypes. NRL is required for the rod fate and regulates downstream rod-specific genes including the orphan nuclear receptor NR2E3 which further activates rod-specific genes and simultaneously represses cone-specific genes. Cone subtype specification is also regulated by the interplay of several transcription factors such as THRB and RXRG. Mutations in these key transcription factors are responsible for ocular defects at birth such as microphthalmia and inherited photoreceptor diseases such as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP) and allied dystrophies. In particular, many mutations are inherited in an autosomal dominant fashion, including the majority of missense mutations in CRX and NRL. In this review, we describe the spectrum of photoreceptor defects that are associated with mutations in the above-mentioned transcription factors, and summarize the current knowledge of molecular mechanisms underlying the pathogenic mutations. At last, we deliberate the outstanding gaps in our understanding of the genotype-phenotype correlations and outline avenues for future research of the treatment strategies.

The retinal phenotype in primary hyperoxaluria type 2 and 3

BACKGROUND

The primary hyperoxalurias (PH1-3) are rare inherited disorders of the glyoxylate metabolism characterized by endogenous overproduction of oxalate. As oxalate cannot be metabolized by humans, oxalate deposits may affect various organs, primarily the kidneys, bones, heart, and eyes. Vision loss induced by severe retinal deposits is commonly seen in infantile PH1; less frequently and milder retinal alterations are found in non-infantile PH1. Retinal disease has not systematically been investigated in patients with PH2 and PH3.

METHODS

A comprehensive ophthalmic examination was performed in 19 genetically confirmed PH2 (n = 7) and PH3 (n = 12) patients (median age 11 years, range 3-59).

RESULTS

Median best corrected visual acuity was 20/20. In 18 patients, no retinal oxalate deposits were found. A 30-year-old male with PH2 on maintenance hemodialysis with plasma oxalate (Pox) elevation (> 100 µmol/l; normal < 7.4) demonstrated bilateral drusen-like, hyperreflective deposits which were interpreted as crystallized oxalate. Two siblings of consanguineous parents with PH2 presented with retinal degeneration and vision loss; exome-wide analysis identified a second monogenic disease, NR2E3-associated retinal dystrophy.

CONCLUSIONS

Retinal disease manifestation in PH2 and PH3 is rare but mild changes can occur at least in PH2-associated kidney failure. Decline in kidney function associated with elevated plasma oxalate levels could increase the risk of systemic oxalosis. Deep phenotyping combined with genomic profiling is vital to differentiate extrarenal disease in multisystem disorders such as PH from independent inherited (retinal) disease. A higher resolution version of the Graphical abstract is available as Supplementary information.

Eyes on CHARGE syndrome: Roles of CHD7 in ocular development

The development of the vertebrate visual system involves complex morphogenetic interactions of cells derived from multiple embryonic lineages. Disruptions in this process are associated with structural birth defects such as microphthalmia, anophthalmia, and coloboma (collectively referred to as MAC), and inherited retinal degenerative diseases such as retinitis pigmentosa and allied dystrophies. MAC and retinal degeneration are also observed in systemic congenital malformation syndromes. One important example is CHARGE syndrome, a genetic disorder characterized by coloboma, heart defects, choanal atresia, growth retardation, genital abnormalities, and ear abnormalities. Mutations in the gene encoding Chromodomain helicase DNA binding protein 7 (CHD7) cause the majority of CHARGE syndrome cases. However, the pathogenetic mechanisms that connect loss of CHD7 to the ocular complications observed in CHARGE syndrome have not been identified. In this review, we provide a general overview of ocular development and congenital disorders affecting the eye. This is followed by a comprehensive description of CHARGE syndrome, including discussion of the spectrum of ocular defects that have been described in this disorder. In addition, we discuss the current knowledge of CHD7 function and focus on its contributions to the development of ocular structures. Finally, we discuss outstanding gaps in our knowledge of the role of CHD7 in eye formation, and propose avenues of investigation to further our understanding of how CHD7 activity regulates ocular and retinal development.

Non-vasogenic cystoid maculopathies

Besides cystoid macular edema due to a blood-retinal barrier breakdown, another type of macular cystoid spaces referred to as non-vasogenic cystoid maculopathies (NVCM) may be detected on optical coherence tomography but not on fluorescein angiography. Various causes may disrupt retinal cell cohesion or impair retinal pigment epithelium (RPE) and Müller cell functions in the maintenance of retinal dehydration, resulting in cystoid spaces formation. Tractional causes include vitreomacular traction, epiretinal membranes and myopic foveoschisis. Surgical treatment does not always allow cystoid space resorption. In inherited retinal dystrophies, cystoid spaces may be part of the disease as in X-linked retinoschisis or enhanced S-cone syndrome, or occur occasionally as in bestrophinopathies, retinitis pigmentosa and allied diseases, congenital microphthalmia, choroideremia, gyrate atrophy and Bietti crystalline dystrophy. In macular telangiectasia type 2, cystoid spaces and cavitations do not depend on the fluid leakage from telangiectasia. Various causes affecting RPE function may result in NVCM such as chronic central serous chorioretinopathy and paraneoplastic syndromes. Non-exudative age macular degeneration may also be complicated by intraretinal cystoid spaces in the absence of fluorescein leakage. In these diseases, cystoid spaces occur in a context of retinal cell loss. Various causes of optic atrophy, including open-angle glaucoma, result in microcystoid spaces in the inner nuclear layer due to a retrograde transsynaptic degeneration. Lastly, drug toxicity may also induce cystoid maculopathy. Identifying NVCM on multimodal imaging, including fluorescein angiography if needed, allows guiding the diagnosis of the causative disease and choosing adequate treatment when available.

Zebrafish and inherited photoreceptor disease: Models and insights

Photoreceptor dysfunctions and degenerative diseases are significant causes of vision loss in patients, with few effective treatments available. Targeted interventions to prevent or reverse photoreceptor-related vision loss are not possible without a thorough understanding of the underlying mechanism leading to disease, which is exceedingly difficult to accomplish in the human system. Cone diseases are particularly challenging to model, as some popular genetically modifiable model animals are nocturnal with a rod-dominant visual system and cones that have dissimilarities to human cones. As a result, cone diseases, which affect visual acuity, colour perception, and central vision in patients, are generally poorly understood in terms of pathology and mechanism. Zebrafish (Danio rerio) provide the opportunity to model photoreceptor diseases in a diurnal vertebrate with a cone-rich retina which develops many macular degeneration-like pathologies. Zebrafish undergo external development, allowing early-onset retinal diseases to be detected and studied, and many ophthalmic tools are available for zebrafish visual assessment during development and adulthood. There are numerous zebrafish models of photoreceptor disease, spanning the various types of photoreceptor disease (developmental, rod, cone, and mixed photoreceptor diseases) and genetic/molecular cause. In this review, we explore the features of zebrafish that make them uniquely poised to model cone diseases, summarize the established zebrafish models of inherited photoreceptor disease, and discuss how disease in these models compares to the human presentation, where applicable. Further, we highlight the contributions of these zebrafish models to our understanding of photoreceptor biology and disease, and discuss future directions for utilising and investigating these diverse models.

Clinical and genetic spectrums of 413 North African families with inherited retinal dystrophies and optic neuropathies

Background

Inherited retinal dystrophies (IRD) and optic neuropathies (ION) are the two major causes world-wide of early visual impairment, frequently leading to legal blindness. These two groups of pathologies are highly heterogeneous and require combined clinical and molecular diagnoses to be securely identified. Exact epidemiological studies are lacking in North Africa, and genetic studies of IRD and ION individuals are often limited to case reports or to some families that migrated to the rest of the world. In order to improve the knowledge of their clinical and genetic spectrums in North Africa, we reviewed published data, to illustrate the most prevalent pathologies, genes and mutations encountered in this geographical region, extending from Morocco to Egypt, comprising 200 million inhabitants.

Main body

We compiled data from 413 families with IRD or ION together with their available molecular diagnosis. The proportion of IRD represents 82.8% of index cases, while ION accounted for 17.8%. Non-syndromic IRD were more frequent than syndromic ones, with photoreceptor alterations being the main cause of non-syndromic IRD, represented by retinitis pigmentosa, Leber congenital amaurosis, and cone-rod dystrophies, while ciliopathies constitute the major part of syndromic-IRD, in which the Usher and Bardet Biedl syndromes occupy 41.2% and 31.1%, respectively. We identified 71 ION families, 84.5% with a syndromic presentation, while surprisingly, non-syndromic ION are scarcely reported, with only 11 families with autosomal recessive optic atrophies related to OPA7 and OPA10 variants, or with the mitochondrial related Leber ION. Overall, consanguinity is a major cause of these diseases within North African countries, as 76.1% of IRD and 78.8% of ION investigated families were consanguineous, explaining the high rate of autosomal recessive inheritance pattern compared to the dominant one. In addition, we identified many founder mutations in small endogamous communities.

Short conclusion

As both IRD and ION diseases constitute a real public health burden, their under-diagnosis in North Africa due to the absence of physicians trained to the identification of inherited ophthalmologic presentations, together with the scarcity of tools for the molecular diagnosis represent major political, economic and health challenges for the future, to first establish accurate clinical diagnoses and then treat patients with the emergent therapies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02340-7.

Biliverdin regulates NR2E3 and zebrafish retinal photoreceptor development

NR2E3 is an orphan nuclear receptor whose loss-of-function causes abnormal retinal photoreceptor development and degeneration. However, despite that many nuclear receptors are regulated by binding of small molecule ligands, biological small molecule ligands regulating NR2E3 have not been identified. Identification of an endogenous NR2E3 ligand might reveal a previously unrecognized component contributing to retinal development and maintenance. Here we report that biliverdin, a conserved green pigment from heme catabolism, regulates NR2E3 and is necessary for zebrafish retinal photoreceptor development. Biliverdin from retinal extracts specifically bound to NR2E3’s ligand-binding domain and induced NR2E3-dependent reporter gene expression. Inhibition of biliverdin synthesis decreased photoreceptor cell populations in zebrafish larvae, and this phenotype was alleviated by exogenously supplied biliverdin. Thus, biliverdin is an endogenous small molecule ligand for NR2E3 and a component necessary for the proper development of photoreceptor cells. This result suggests a possible role of heme metabolism in the regulation of retinal photoreceptor cell development.

Patterning and Development of Photoreceptors in the Human Retina

Humans rely on visual cues to navigate the world around them. Vision begins with the detection of light by photoreceptor cells in the retina, a light-sensitive tissue located at the back of the eye. Photoreceptor types are defined by morphology, gene expression, light sensitivity, and function. Rod photoreceptors function in low-light vision and motion detection, and cone photoreceptors are responsible for high-acuity daytime and trichromatic color vision. In this review, we discuss the generation, development, and patterning of photoreceptors in the human retina. We describe our current understanding of how photoreceptors are patterned in concentric regions. We conclude with insights into mechanisms of photoreceptor differentiation drawn from studies of model organisms and human retinal organoids.

What colour are your eyes? Teaching the genetics of eye colour & colour vision. Edridge Green Lecture RCOphth Annual Congress Glasgow May 2019

Eye colour and colour perception are excellent examples to use when teaching genetics as they encompass not simply the basic Mendelian genetics of dominant, recessive and X-linked disorders, but also many of the new concepts such as non-allelic diseases, polygenic disease, phenocopies, genome-wide association study (GWAS), founder effects, gene-environment interaction, evolutionary drivers for variations, copy number variation, insertions deletions, methylation and gene inactivation. Beyond genetics, colour perception touches on concepts involving optics, physics, physiology and psychology and can capture the imagination of the population, as we saw with social media trend of "#the dress". Television shows such as Game of Thrones focused attention on the eye colour of characters, as well as their Dire-wolves and Dragons. These themes in popular culture can be leveraged as tools to teach and engage everyone in genetics, which is now a key component in all eye diseases. As the explosion of data from genomics, big data and artificial intelligence transforms medicine, ophthalmologists need to be genetically literate. Genetics is relevant, not just for Inherited Retinal Diseases and congenital abnormalities but also for the leading causes of blindness: age-related macular degeneration, glaucoma, myopia, diabetic retinopathy and cataract. Genetics should be part of the armamentarium of every practicing ophthalmologist. We need to ask every patient about their family history. In the near future, patients will attend eye clinics with genetic results showing they are at high risk of certain eye diseases and ophthalmologists will need to know how to screen, follow and treat these patients.

Multimodal treatment of Coats-like exudative vitreoretinopathy in Goldmann-Favre syndrome

Purpose

To report a Coats-like exudative vitreoretinopathy in Goldmann-Favre syndrome.

Observations

A 64 year-old woman with prior diagnosis of retinal dystrophy presented with decreased vision in the right eye (OD). Ophthalmologic examination was remarkable for bilateral arteriolar attenuation, mid-peripheral bony-spicules, and waxy disc pallor. Coats-like exudative vitreoretinopathy and cystoid macular edema were present OD. Genetic testing showed a homozygous pathogenic mutation in gene NR2E3, variant c.932G>A (p.Arg311Gln), consistent with Goldmann-Favre syndrome. Targeted laser ablation and combination intravitreal therapy were effective in decreasing macular edema.

Conclusions and Importance

A Coats-like exudative vitreoretinopathy may occur in the setting of Goldmann-Favre syndrome. Targeted laser ablation in combination with intravitreal therapy can be efficacious in select patients.

Joint actions of diverse transcription factor families establish neuron-type identities and promote enhancer selectivity

To systematically investigate the complexity of neuron specification regulatory networks, we performed an RNA interference (RNAi) screen against all 875 transcription factors (TFs) encoded in Caenorhabditis elegans genome and searched for defects in nine different neuron types of the monoaminergic (MA) superclass and two cholinergic motoneurons. We identified 91 TF candidates to be required for correct generation of these neuron types, of which 28 were confirmed by mutant analysis. We found that correct reporter expression in each individual neuron type requires at least nine different TFs. Individual neuron types do not usually share TFs involved in their specification but share a common pattern of TFs belonging to the five most common TF families: homeodomain (HD), basic helix loop helix (bHLH), zinc finger (ZF), basic leucine zipper domain (bZIP), and nuclear hormone receptors (NHR). HD TF members are overrepresented, supporting a key role for this family in the establishment of neuronal identities. These five TF families are also prevalent when considering mutant alleles with previously reported neuronal phenotypes in C. elegans, Drosophila, and mouse. In addition, we studied terminal differentiation complexity focusing on the dopaminergic terminal regulatory program. We found two HD TFs (UNC-62 and VAB-3) that work together with known dopaminergic terminal selectors (AST-1, CEH-43, CEH-20). Combined TF binding sites for these five TFs constitute a cis-regulatory signature enriched in the regulatory regions of dopaminergic effector genes. Our results provide new insights on neuron-type regulatory programs in C. elegans that could help better understand neuron specification and evolution of neuron types.

Inherited retinal diseases: Linking genes, disease-causing variants, and relevant therapeutic modalities

Inherited retinal diseases (IRDs) are a clinically complex and heterogenous group of visual impairment phenotypes caused by pathogenic variants in at least 277 nuclear and mitochondrial genes, affecting different retinal regions, and depleting the vision of affected individuals. Genes that cause IRDs when mutated are unique by possessing differing genotype-phenotype correlations, varying inheritance patterns, hypomorphic alleles, and modifier genes thus complicating genetic interpretation. Next-generation sequencing has greatly advanced the identification of novel IRD-related genes and pathogenic variants in the last decade. For this review, we performed an in-depth literature search which allowed for compilation of the Global Retinal Inherited Disease (GRID) dataset containing 4,798 discrete variants and 17,299 alleles published in 31 papers, showing a wide range of frequencies and complexities among the 194 genes reported in GRID, with 65% of pathogenic variants being unique to a single individual. A better understanding of IRD-related gene distribution, gene complexity, and variant types allow for improved genetic testing and therapies. Current genetic therapeutic methods are also quite diverse and rely on variant identification, and range from whole gene replacement to single nucleotide editing at the DNA or RNA levels. IRDs and their suitable therapies thus require a range of effective disease modelling in human cells, granting insight into disease mechanisms and testing of possible treatments. This review summarizes genetic and therapeutic modalities of IRDs, provides new analyses of IRD-related genes (GRID and complexity scores), and provides information to match genetic-based therapies such as gene-specific and variant-specific therapies to the appropriate individuals.

ENHANCED S-CONE SYNDROME: VISUAL FUNCTION, CROSS-SECTIONAL IMAGING, AND CELLULAR STRUCTURE WITH ADAPTIVE OPTICS OPHTHALMOSCOPY

PURPOSE

To describe in detail the phenotype of a patient with enhanced S-cone syndrome.

METHODS

We describe a 13-year-old boy who presented with blurred vision, vitreous cells, cystoid macular edema refractory to steroid treatment, and a negative uveitic workup. The patient underwent a complete ophthalmic examination, full-field electroretinograms (ffERG), automatic static perimetry and multimodal imaging with spectral domain optical coherence tomography, and adaptive optics scanning laser ophthalmoscopy (AOSLO).

RESULTS

Spectral domain optical coherence tomography demonstrated cystoid macular edema and a hyperthick, delaminated midperipheral retina. Fluorescein angiography did not demonstrate macular leakage. Rod-mediated ffERGs were undetectable, and there was a supernormal response to short-wavelength stimuli compared with photopically matched longer wavelengths of light consistent with enhanced S-cone syndrome. Gene screening was positive for compound heterozygous mutations NR2E3: a known (c.119-2 A>C) and a novel (c.119-1G>A) mutation. By perimetry, sensitivities were normal or above normal for short-wavelength stimuli; there was no detectable rod-mediated vision. AOSLO demonstrated higher than normal cone densities in the perifoveal retina and evidence for smaller outer segment cone diameters.

CONCLUSION

Evidence for supernumerary cones (at least twice the normal complement) by AOSLO and spectral domain optical coherence tomography was associated with supernormal S-cone sensitivities and electroretinogram responses confirming previous in vivo findings in postmortem human specimens. Smaller than normal cones in enhanced S-cone syndrome may represent "hybrid" photoreceptors analogous to the rd7/rd7 murine model of the disease.

A reinterpretation of critical flicker-frequency (CFF) data reveals key details about light adaptation and normal and abnormal visual processing

Our ability to see flicker has an upper frequency limit above which flicker is invisible, known as the "critical flicker frequency" (CFF), that typically grows with light intensity (I). The relation between CFF and I, the focus of nearly 200 years of research, is roughly logarithmic, i.e., CFF ∝ log(I)-a relation called the Ferry-Porter law. However, why this law should occur, and how it relates to the underlying physiology, have never been adequately explained. Over the past two decades we have measured CFF in normal observers and in patients with retinal gene defects. Here, we reanalyse and model our data and historical CFF data. Remarkably, CFF-versus-I functions measured under a wide range of conditions in patients and in normal observers all have broadly similar shapes when plotted in double-logarithmic coordinates, i.e., log (CFF)-versus-log(I). Thus, the entire dataset can be characterised by horizontal and vertical logarithmic shifts of a fixed-shape template. Shape invariance can be predicted by a simple model of visual processing built from a sequence of low-pass filters, subtractive feedforward stages and gain adjustment (Rider, Henning & Stockman, 2019). It depends primarily on the numbers of visual processing stages that approach their power-law region at a given intensity and a frequency-independent gain reduction at higher light levels. Counter-intuitively, the CFF-versus-I relation depends primarily on the gain of the visual response rather than its speed-a conclusion that changes our understanding and interpretation of human flicker perception. The Ferry-Porter "law" is merely an approximation of the shape-invariant template.

Macular Rod Function in Retinitis Pigmentosa Measured With Scotopic Microperimetry

Purpose

To investigate the validity and reliability of macular rod photoreceptor function measurement with a microperimeter.

Methods

Macular sensitivity in dark-adapted retinitis pigmentosa (RP) patients (22 eyes; 9–67 years of age) and controls (five eyes; 22–55 years of age) was assessed with a modified Humphrey field analyzer (mHFA), as well as a scotopic microperimeter (Nidek MP-1S). Sensitivity loss (SL) was estimated at rod-mediated locations. All RP eyes were re-evaluated at a second visit 6 months later. The dynamic range of the MP-1S was expanded with a range of neutral-density filters (NDFs).

Results

In controls, a 4 NDF was used at all macular locations tested. In patients with RP, 0 to 3 NDFs were used, depending on the local disease severity. At rod-mediated locations (n = 281), SL estimates obtained with the MP-1S were highly correlated (r = 0.80) with those of the mHFA. The inter-perimeter difference of SL averaged less than 3 decibels (dB) with all NDFs, except those with most severe locations evaluated with a 0 NDF, where the difference averaged more than 6 dB. The results were similar on the second visit.

Conclusions

The MP-1S estimates of SL are highly correlated with those of the mHFA over a wide range of disease severity replicated at two visits; however, there was an unexplained bias in the magnitude of SL estimated by the MP-1S especially at loci with severe disease.

Translational Relevance

MP-1S scotopic microperimetry can be used to evaluate changes to macular rod function, but evaluation of treatment potential by quantitative comparison of SL to retinal structure will be more challenging.

High-Throughput Sequencing to Identify Mutations Associated with Retinal Dystrophies

Retinal dystrophies (RD) are clinically and genetically heterogenous disorders showing mutations in over 270 disease-associated genes. Several millions of people worldwide are affected with different types of RD. Studying the relevance of disease-associated sequence alterations will assist in understanding disorders and may lead to the development of therapeutic approaches. Here, we established a whole exome sequencing (WES) pipeline to rapidly identify disease-associated mutations in patients. Sanger sequencing was applied to identify deep-intronic variants and to verify the co-segregation of WES results within families. We analyzed 26 unrelated patients with different syndromic and non-syndromic clinical manifestations of RD. All patients underwent ophthalmic examinations. We identified nine novel disease-associated sequence variants among 37 variants identified in total. The sequence variants located to 17 different genes. Interestingly, two cases presenting with Stargardt disease carried deep-intronic variants in ABCA4. We have classified 21 variants as pathogenic variants, 4 as benign/likely benign variants, and 12 as variants of uncertain significance. This study highlights the importance of WES-based mutation analyses in RD patients supporting clinical decisions, broadly based genetic diagnosis and support genetic counselling. It is essential for any genetic therapy to expand the mutation spectrum, understand the genes' function, and correlate phenotypes with genotypes.

Measures of Function and Structure to Determine Phenotypic Features, Natural History, and Treatment Outcomes in Inherited Retinal Diseases

Inherited retinal diseases (IRDs) are at the forefront of innovative gene-specific treatments because of the causation by single genes, the availability of microsurgical access for treatment delivery, and the relative ease of quantitative imaging and vision measurement. However, it is not always easy to choose a priori, from scores of potential measures, an appropriate subset to evaluate efficacy outcomes considering the wide range of disease stages with different phenotypic features. This article reviews measurements of visual function and retinal structure that our group has used over the past three decades to understand the natural history of IRDs. We include measures of light sensitivity, retinal structure, mapping of natural fluorophores, evaluation of pupillary light reflex, and oculomotor control. We provide historical context and examples of applicability. We also review treatment trial outcomes using these measures of function and structure. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Management of a case of Enhanced S-cone syndrome with massive foveoschisis treated with pars plana vitrectomy with silicone oil tamponade

Introduction: Goldmann Favre Syndrome (GFS) is a vitreoretinal degenerative disease with macular retinoschisis. The current treatment of foveoschisis is topical and oral carbonic anhydrase inhibitors.Case: A 22-year-old male diagnosed with GFS presented a progressive decrease in vision of the right eye. The optical coherence tomography showed a significant macular schisis. A medical treatment with topical and oral carbonic anhydrase inhibitors was ineffective. We performed a pars plana vitrectomy and silicone oil placement which led to an improvement of the visual acuity and a reduction of the foveoschisis.Conclusion: We describe here the first case of surgical treatment for macular schisis in a patient with GFS.

Nr2e3 is a genetic modifier that rescues retinal degeneration and promotes homeostasis in multiple models of retinitis pigmentosa

Recent advances in viral vector engineering, as well as an increased understanding of the cellular and molecular mechanism of retinal diseases, have led to the development of novel gene therapy approaches. Furthermore, ease of accessibility and ocular immune privilege makes the retina an ideal target for gene therapies. In this study, the nuclear hormone receptor gene Nr2e3 was evaluated for efficacy as broad-spectrum therapy to attenuate early to intermediate stages of retinal degeneration in five unique mouse models of retinitis pigmentosa (RP). RP is a group of heterogenic inherited retinal diseases associated with over 150 gene mutations, affecting over 1.5 million individuals worldwide. RP varies in age of onset, severity, and rate of progression. In addition, ~40% of RP patients cannot be genetically diagnosed, confounding the ability to develop personalized RP therapies. Remarkably, Nr2e3 administered therapy resulted in reduced retinal degeneration as observed by increase in photoreceptor cells, improved electroretinogram, and a dramatic molecular reset of key transcription factors and associated gene networks. These therapeutic effects improved retinal homeostasis in diseased tissue. Results of this study provide evidence that Nr2e3 can serve as a broad-spectrum therapy to treat multiple forms of RP.

Recognizable Patterns of Submacular Fibrosis in Enhanced S-Cone Syndrome

PURPOSE

To highlight recognizable patterns of subretinal fibrosis in enhanced S-cone syndrome (ESCS).

DESIGN

Retrospective case series.

PARTICIPANTS

Forty-seven patients with subretinal fibrosis identified from 101 patients with clinically diagnosed ESCS, confirmed by full-field electroretinography (35/47), genetic testing (34/47), or both.

METHODS

Multimodal retinal imaging, electroretinography, and genetic analysis.

MAIN OUTCOME MEASURES

Patterns of subretinal fibrosis with angiographic, OCT, and genetic correlations.

RESULTS

Eighty-five eyes of 47 patients (24 male patients; 36 unrelated consanguineous families) had subretinal fibrosis. Mean age at presentation was 14 years. Best-corrected visual acuity ranged from 20/20 to hand movements. All 34 genetically tested patients were homozygous for pathogenic NR2E3 variants. Subretinal fibrosis was always in the macular area, although it extended beyond in some patients. Six recurrent patterns of submacular fibrosis were noted: central unifocal nodular, circumferential unifocal nodular, multifocal nodular, arcuate, helicoid, and thick geographic. Some patients showed a combination of patterns. Previous misdiagnosis as inflammatory disease was common. Fibrosis was fairly symmetrical in a given patient but not always present or identical in other affected individuals with a given homozygous mutation from the same or other families.

CONCLUSIONS

These recognizable patterns of submacular fibrosis are part of the ESCS phenotypic spectrum and strongly suggest the disease. In addition to facilitating diagnosis, recognition of these patterns can spare patients unnecessary workup for an inflammatory cause.

Genetic investigation of 211 Chinese families expands the mutational and phenotypical spectra of hereditary retinopathy genes through targeted sequencing technology

BACKGROUND

Hereditary retinopathy is a significant cause of blindness worldwide. Despite the discovery of many mutations in various retinopathies, a large number of patients remain genetically undiagnosed. Targeted next-generation sequencing of the human genome is a suitable approach for the molecular diagnosis of retinopathy.

METHODS

We describe a cohort of 211 families from central China with various forms of retinopathy; 95 patients were investigated using multigene panel sequencing, and the other 116 with suspected Leber hereditary optic neuropathy (LHON) were tested by Sanger sequencing. The detected variation of targeted sequencing was verified by PCR-based Sanger sequencing. We performed a comprehensive analysis of the cases using sequencing data and ophthalmologic examination information.

RESULTS

Potential causal mutations were identified in the majority of families with retinopathy (57.9% of 95 families) and suspected LHON (21.6% of 116 families). There were 68 variants of a certain significance distributed in 31 known disease-causing genes in the 95 families; 37 of the variants are novel and have not been reported to be related to hereditary retinopathy. The NGS panel solution provided a 45.3% potential diagnostic rate for retinopathy families, with candidate gene mutations of undefined pathogenicity revealed in another 12.6%of the families.

CONCLUSION

Our study uncovered novel mutations and phenotypic aspects of retinopathy and demonstrated the genetic and clinical heterogeneity of related conditions. The findings show the detection rate of pathogenic variants in patients with hereditary retinopathy in central China as well as the diversity and gene distribution of these variants. The significance of molecular genetic testing for patients with hereditary retinopathy is also highlighted.

Allele-Specific Knockout by CRISPR/Cas to Treat Autosomal Dominant Retinitis Pigmentosa Caused by the G56R Mutation in NR2E3

Retinitis pigmentosa (RP) is an inherited retinal dystrophy that causes progressive vision loss. The G56R mutation in NR2E3 is the second most common mutation causing autosomal dominant (ad) RP, a transcription factor that is essential for photoreceptor development and maintenance. The G56R variant is exclusively responsible for all cases of NR2E3-associated adRP. Currently, there is no treatment for NR2E3-related or, other, adRP, but genome editing holds promise. A pertinent approach would be to specifically knockout the dominant mutant allele, so that the wild type allele can perform unhindered. In this study, we developed a CRISPR/Cas strategy to specifically knockout the mutant G56R allele of NR2E3 and performed a proof-of-concept study in induced pluripotent stem cells (iPSCs) of an adRP patient. We demonstrate allele-specific knockout of the mutant G56R allele in the absence of off-target events. Furthermore, we validated this knockout strategy in an exogenous overexpression system. Accordingly, the mutant G56R-CRISPR protein was truncated and mis-localized to the cytosol in contrast to the (peri)nuclear localizations of wild type or G56R NR2E3 proteins. Finally, we show, for the first time, that G56R iPSCs, as well as G56R-CRISPR iPSCs, can differentiate into NR2E3-expressing retinal organoids. Overall, we demonstrate that G56R allele-specific knockout by CRISPR/Cas could be a clinically relevant approach to treat NR2E3-associated adRP.

Different Phenotypes in Pseudodominant Inherited Retinal Dystrophies

Retinal dystrophies (RD) are a group of Mendelian disorders caused by rare genetic variations leading to blindness. A pathogenic variant may manifest in both dominant or recessive mode and clinical and genetic heterogeneity makes it difficult to establish a precise diagnosis. In this study, families with autosomal dominant RD in successive generations were identified, and we aimed to determine the disease's molecular origin in these consanguineous families. Whole exome sequencing was performed in the index patient of each family. The aim was to determine whether these cases truly represented examples of dominantly inherited RD, or whether another mode of inheritance might be applicable. Six potentially pathogenic variants in four genes were identified in four families. In index patient with enhanced S-cone syndrome in F1, we identified a new digenetic combination: a heterozygous variant p.[G51A];[=] in RHO and a homozygous pathogenic variant p.[R311Q];[R311Q] in NR2E3. Helicoid subretinal fibrosis associated with recessive NR2E3 variant p.[R311Q];[R311Q] was identified in F2. A new frameshift variant c.[105delG];[105delG] in RDH12 was found in F3 with cone-rod dystrophy. In F4, the compound heterozygous variants p.[R964^*];[W758^*] were observed in IMPG2 with a retinitis pigmentosa (RP) phenotype. We showed that both affected parents and the offspring, were homozygous for the same variants in all four families. Our results provide evidence that in consanguineous families, autosomal recessive can be transmitted as pseudodominant inheritance in RD patients, and further extend our knowledge of pathogenic variants in RD genes.

Enhanced S-Cone Syndrome: Spectrum of Clinical, Imaging, Electrophysiologic, and Genetic Findings in a Retrospective Case Series of 56 Patients

PURPOSE

To describe the detailed phenotype, long-term clinical course, clinical variability, and genotype of patients with enhanced S-cone syndrome (ESCS).

DESIGN

Retrospective case series.

PARTICIPANTS

Fifty-six patients with ESCS.

METHODS

Clinical history, examination, imaging, and electrophysiologic findings of 56 patients (age range, 1-75 years) diagnosed with ESCS were reviewed. Diagnosis was established by molecular confirmation of disease-causing variants in the NR2E3 gene (n = 38) or by diagnostic full-field electroretinography findings (n = 18).

MAIN OUTCOME MEASURES

Age at onset of visual symptoms, best-corrected visual acuity (BCVA), quantitative age-related electrophysiologic decline, and imaging findings.

RESULTS

Mean age at onset of visual symptoms was 4.0 years, and median age at presentation was 20.5 years, with mean follow-up interval being 6.1 years. Six patients were assessed once. Disease-causing variants in NR2E3 were identified in 38 patients. Mean BCVA of the better-seeing eye was 0.32 logarithm of the minimum angle of resolution (logMAR) at baseline and 0.39 logMAR at follow-up. In most eyes (76% [76/100]), BCVA remained stable, with a mean BCVA change of 0.07 logMAR during follow-up. Nyctalopia was the most common initial symptom, reported in 92.9% of patients (52/56). Clinical findings were highly variable and included foveomacular schisis (41.1% [26/56]), yellow-white dots (57.1% [32/56]), nummular pigmentation (85.7% [48/56]), torpedo-like lesions (10.7% [6/56]), and circumferential subretinal fibrosis (7.1% [4/56]). Macular and peripheral patterns of autofluorescence were classified as (1) minimal change, (2) hypoautofluorescent (mild diffuse, moderate speckled, moderate diffuse, or advanced), or (3) hyperautofluorescent flecks. One patient showed undetectable electroretinography findings; quantification of main electroretinography components in all other patients revealed amplitude and peak time variability but with pathognomonic electroretinography features. The main electroretinography components showed evidence of age-related worsening over 6.7 decades, at a rate indistinguishable from that seen in unaffected control participants. Eighteen sequence variants in NR2E3 were identified, including 4 novel missense changes.

CONCLUSIONS

Enhanced S-cone syndrome has a highly variable phenotype with relative clinical and imaging stability over time. Most electroretinography findings have pathognomonic features, but quantitative assessment reveals variability and a normal mean rate of age-related decline, consistent with largely nonprogressive peripheral retinal dysfunction.

Incidence and Natural History of Retinochoroidal Neovascularization in Enhanced S-Cone Syndrome

OBJECTIVE

We examined the incidence and natural history of macular retinochoroidal neovascularization (RCN) in enhanced S-cone syndrome (ESCS).

DESIGN

Retrospective case series.

METHODS

This single-center study included 14 of 93 patients with ESCS who had signs of active or inactive RCN in ≥1 eye. We conducted multimodal retinal imaging, full-field electroretinography, and molecular genetic analysis of NR2E3 gene. Our main outcome measures included the cumulative incidence of RCN in ESCS, type of RCN, and mode of evolution of RCN.

RESULTS

Fourteen (15.1%) of 93 patients with ESCS had RCN in ≥1 eye at 2 to 27 years of age. All 22 RCNs (21 eyes of 14 patients) were macular. Twelve of the RCNs were active with exudates/hemorrhages. Of these, 5 appeared de novo in a subretinal location, with photographic evidence of no pre-existing lesions. The latter were compatible with type 3 neovascularization or retinal angiomatous proliferation and subsequently evolved into unifocal fibrotic nodules. The remaining active lesions all had some degree of pre-existing fibrosis and remained stable. Ten inactive fibrotic nodules, identical to end-stage de novo lesions, were found and were presumed to represent healed RCNs.

CONCLUSIONS

RCN, a treatable condition, may occur as early as 2 years of age and may be much more common in patients with ESCS than previously estimated. It may be the primary cause of the unifocal submacular fibrosis that is commonly observed in this condition. Additional research is needed to establish the pathogenesis of RCN in patients with ESCS and its optimal management.

Interspecies Correlations between Human and Mouse NR2E3-Associated Recessive Disease

NR2E3-associated recessive disease in humans is historically defined by congenital night blinding retinopathy, characterized by an initial increase in short-wavelength (S)-cone sensitivity and progressive loss of rod and cone function. The retinal degeneration 7 (rd7) murine model, harboring a recessive mutation in the mouse ortholog of NR2E3, has been a well-studied disease model and recently evaluated as a therapeutic model for NR2E3-associated retinal degenerations. This study aims to draw parallels between human and mouse NR2E3-related disease through examination of spectral domain optical coherence tomography (SD-OCT) imaging between different stage of human disease and its murine counterpart. We propose that SD-OCT is a useful non-invasive diagnostic tool to compare human clinical dystrophy presentation with that of the rd7 mouse and make inference that may be of therapeutically relevance. Additionally, a longitudinal assessment of rd7 disease progression, utilizing available clinical data from our patients as well as extensive retrospective analysis of visual acuity data from published cases of human NR2E3-related disease, was curated to identify further valuable correlates between human and mouse Nr2e3 disease. Results of this study validate the slow progression of NR2E3-associated disease in humans and the rd7 mice and identify SD-OCT characteristics in patients at or near the vascular arcades that correlate well with the whorls and rosettes that are seen also in the rd7 mouse and point to imaging features that appear to be associated with better preserved S-cone mediated retinal function. The correlation of histological findings between rd7 mice and human imaging provides a solid foundation for diagnostic use of pathophysiological and prognostic information to further define characteristics and a relevant timeline for therapeutic intervention in the field of NR2E3-associated retinopathies.

Zebrafish Models of Photoreceptor Dysfunction and Degeneration

Zebrafish are an instrumental system for the generation of photoreceptor degeneration models, which can be utilized to determine underlying causes of photoreceptor dysfunction and death, and for the analysis of potential therapeutic compounds, as well as the characterization of regenerative responses. We review the wealth of information from existing zebrafish models of photoreceptor disease, specifically as they relate to currently accepted taxonomic classes of human rod and cone disease. We also highlight that rich, detailed information can be derived from studying photoreceptor development, structure, and function, including behavioural assessments and in vivo imaging of zebrafish. Zebrafish models are available for a diversity of photoreceptor diseases, including cone dystrophies, which are challenging to recapitulate in nocturnal mammalian systems. Newly discovered models of photoreceptor disease and drusenoid deposit formation may not only provide important insights into pathogenesis of disease, but also potential therapeutic approaches. Zebrafish have already shown their use in providing pre-clinical data prior to testing genetic therapies in clinical trials, such as antisense oligonucleotide therapy for Usher syndrome.

Childhood-onset genetic cone-rod photoreceptor diseases and underlying pathobiology

Inherited retinal diseases (IRDs) were first classified clinically by history, ophthalmoscopic appearance, type of visual field defects, and electroretinography (ERG). ERGs isolating the two major photoreceptor types (rods and cones) showed some IRDs with greater cone than rod retinal dysfunction; others were the opposite. Within the cone-rod diseases, there can be phenotypic variability, which can be attributed to genetic heterogeneity and the variety of visual function mechanisms that are disrupted. Most cause symptoms from childhood or adolescence, although others can manifest later in life. Among the causative genes for cone-rod dystrophy (CORD) are those encoding molecules in phototransduction cascade activation and recovery processes, photoreceptor outer segment structure, the visual cycle and photoreceptor development. We review 11 genes known to cause cone-rod disease in the context of their roles in normal visual function and retinal structure. Knowledge of the pathobiology of these genetic diseases is beginning to pave paths to therapy.

Clinical vision and molecular loss: Integrating visual psychophysics with molecular genetics reveals key details of normal and abnormal visual processing

Over the past two decades we have developed techniques and models to investigate the ways in which known molecular defects affect visual performance. Because molecular defects in retinal signalling invariably alter the speed of visual processing, our strategy has been to measure the resulting changes in flicker sensitivity. Flicker measurements provide not only straightforward clinical assessments of visual performance but also reveal fundamental details about the functioning of both abnormal and normal visual systems. Here, we bring together our past measurements of patients with pathogenic variants in the GNAT2, RGS9, GUCA1A, RPE65, OPA1, KCNV2 and NR2E3 genes and analyse the results using a standard model of visual processing. The model treats flicker sensitivity as the result of the actions of a sequence of simple processing steps, one or more of which is altered by the genetic defect. Our analyses show that most defects slow down the visual response directly, but some speed it up. Crucially, however, other steps in the processing sequence can make compensatory adjustments to offset the abnormality. For example, if the abnormal step slows down the visual response, another step is likely to speed up or attenuate the response to rebalance system performance. Such compensatory adjustments are probably made by steps in the sequence that usually adapt to changing light levels. Our techniques and modelling also allow us to tease apart stationary and progressive effects, and the localised molecular losses help us to unravel and characterise individual steps in the normal and abnormal processing sequences.

Single-cell RNA sequencing in vision research: Insights into human retinal health and disease

Gene expression provides valuable insight into cell function. As such, vision researchers have frequently employed gene expression studies to better understand retinal physiology and disease. With the advent of single-cell RNA sequencing, expression experiments provide an unparalleled resolution of information. Instead of studying aggregated gene expression across all cells in a heterogenous tissue, single-cell technology maps RNA to an individual cell, which facilitates grouping of retinal and choroidal cell types for further study. Single-cell RNA sequencing has been quickly adopted by both basic and translational vision researchers, and single-cell level gene expression has been studied in the visual systems of animal models, retinal organoids, and primary human retina, RPE, and choroid. These experiments have generated detailed atlases of gene expression and identified new retinal cell types. Likewise, single-cell RNA sequencing investigations have characterized how gene expression changes in the setting of many retinal diseases, including how choroidal endothelial cells are altered in age-related macular degeneration. In addition, this technology has allowed vision researchers to discover drivers of retinal development and model rare retinal diseases with induced pluripotent stem cells. In this review, we will overview the growing number of single-cell RNA sequencing studies in the field of vision research. We will summarize experimental considerations for designing single-cell RNA sequencing experiments and highlight important advancements in retinal, RPE, choroidal, and retinal organoid biology driven by this technology. Finally, we generalize these findings to genes involved in retinal degeneration and outline the future of single-cell expression experiments in studying retinal disease.

Novel Pathogenic Sequence Variants in NR2E3 and Clinical Findings in Three Patients

A retrospective review of the clinical records of patients seen at the Oxford Eye Hospital identified as having NR2E3 mutations was performed. The data included symptoms, best-corrected visual acuity, multimodal retinal imaging, visual fields and electrophysiology testing. Three participants were identified with biallelic NR2E3 pathogenic sequence variants detected using a targeted NGS gene panel, two of which were novel. Participant I was a Nepalese male aged 68 years, and participants II and III were white Caucasian females aged 69 and 10 years old, respectively. All three had childhood onset nyctalopia, a progressive decrease in central vision, and visual field loss. Patients I and III had photopsia, patient II had photosensitivity and patient III also had photophobia. Visual acuities in patients I and II were preserved even into the seventh decade, with the worst visual acuity measured at 6/36. Visual field constriction was severe in participant I, less so in II, and fields were full to bright targets targets in participant III. Electrophysiology testing in all three demonstrated loss of rod function. The three patients share some of the typical distinctive features of NR2E3 retinopathies, as well as a novel clinical observation of foveal ellipsoid thickening.

Nr2e3 functional domain ablation by CRISPR-Cas9D10A identifies a new isoform and generates retinitis pigmentosa and enhanced S-cone syndrome models

Mutations in NR2E3 cause retinitis pigmentosa (RP) and enhanced S-cone syndrome (ESCS) in humans. This gene produces a large isoform encoded in 8 exons and a previously unreported shorter isoform of 7 exons, whose function is unknown. We generated two mouse models by targeting exon 8 of Nr2e3 using CRISPR/Cas9-D10A nickase. Allele Δ27 is an in-frame deletion of 27 bp that ablates the dimerization domain H10, whereas allele ΔE8 (full deletion of exon 8) produces only the short isoform, which lacks the C-terminal part of the ligand binding domain (LBD) that encodes both H10 and the AF2 domain involved in the Nr2e3 repressor activity. The Δ27 mutant shows developmental alterations and a non-progressive electrophysiological dysfunction that resembles the ESCS phenotype. The ΔE8 mutant exhibits progressive retinal degeneration, as occurs in human RP patients. Our mutants suggest a role for Nr2e3 as a cone-patterning regulator and provide valuable models for studying mechanisms of NR2E3-associated retinal dystrophies and evaluating potential therapies.