A complete, homozygous CRX deletion causing nullizygosity is a new genetic mechanism for Leber congenital amaurosis

Atypic Retinitis Pigmentosa Clinical Features Associated with a Peculiar CRX Gene Mutation in Italian Patients

Purpose: To describe an atypical phenotypic pattern of late-onset retinitis pigmentosa (RP) due to the same specific c.425A>G (p.Tyr142Cys) heterozygous mutation in the cone-rod homeobox gene (CRX gene) in two unrelated Italian patients. Case 1: A 67-year-old woman (P.P.) was incidentally diagnosed with sector RP at the age of 50. The patient was initially asymptomatic and did not have any family history of retinal dystrophy. Fundus examination showed the presence of typical retinal pigmentary deposits with a peculiar pericentral/sector distribution. Genomic sequencing disclosed the missense mutation c.425A>G (p.Tyr142Cys) in the CRX gene. During the follow-up period of 7 years, the patient maintained good visual acuity and complained only of mild symptoms. Case 2: A 76-year-old man (P.E.) presented with nyctalopia and visual field constriction since the age of 50. Fundus examination showed the presence of retinal pigment deposits with a concentric pericentral and perimacular pattern. A full-field electroretinogram (ffERG) showed extinguished scotopic responses and reduced abnormal photopic and flicker cone responses. Genomic sequencing identified the same missense mutation, c.425A>G (p.Tyr142Cys), in the CRX gene. Similarly to the first case, during the whole follow-up of 7 years, the visual acuity remained stable, as did the visual field and the patient's symptoms. Conclusions: We report the first cases of late-onset retinitis pigmentosa related to a specific heterozygous CRX gene mutation in exon 4. We also report two atypical phenotypic RP patterns related to mutations in the CRX gene.

Transcriptional precision in photoreceptor development and diseases - Lessons from 25 years of CRX research

The vertebrate retina is made up of six specialized neuronal cell types and one glia that are generated from a common retinal progenitor. The development of these distinct cell types is programmed by transcription factors that regulate the expression of specific genes essential for cell fate specification and differentiation. Because of the complex nature of transcriptional regulation, understanding transcription factor functions in development and disease is challenging. Research on the Cone-rod homeobox transcription factor CRX provides an excellent model to address these challenges. In this review, we reflect on 25 years of mammalian CRX research and discuss recent progress in elucidating the distinct pathogenic mechanisms of four CRX coding variant classes. We highlight how in vitro biochemical studies of CRX protein functions facilitate understanding CRX regulatory principles in animal models. We conclude with a brief discussion of the emerging systems biology approaches that could accelerate precision medicine for CRX-linked diseases and beyond.

CRX haploinsufficiency compromises photoreceptor precursor translocation and differentiation in human retinal organoids

BACKGROUND

The CRX-associated autosomal dominant retinopathies suggest a possible pathogenic mechanism of gene haploinsufficiency. However, based on reported human patient cases and studies with mouse models, it is hard to confirm the specific weight of haploinsufficiency in pathogenesis due to the interspecies gaps between gene expression and function.

METHODS

We created monoallelic CRX by replacing one allele with tdTomato in human embryonic stem cells (hESCs) and subsequently dissect pathogenesis in hESCs-derived retinal organoids. We used transcriptome and immunofluorescence analyses to dissect phenotypic differences between CRX-monoallelic knockout and control wildtype organoids. For location analysis of CRX+ cells, a CRX-expression-tracing system was constructed in control hESCs. We implemented long-term live-cell imaging to describe the translocation of CRX+ cells between two groups in early organoid differentiation. The expression pattern of these dynamic differences was validated using RNA-seq and immunofluorescence assays.

RESULTS

We identified delayed differentiation of outer nuclear layer (ONL) stratification along with thinner ONL, serious loss of photoreceptor outer segments, as well as downregulated expression of gene for phototransduction and inner/outer segment formation. By live-cell imaging and immunostaining, we observed the overtension of actomyosin network and the arrested translocation of monoallelic CRX+ cells in the early stage of retinal differentiation.

CONCLUSIONS

We confirmed that gene haploinsufficiency is the mechanism for the dominant pathogenicity of CRX and discovered that CRX regulated postmitotic photoreceptor precursor translocation in addition to its specification of photoreceptor cell fates during human retinal development. These findings revealed a new underlying mechanism of CRX dominant pathogenesis and provided a new clue for the treatment of CRX-associated human retinopathies.

Occult Macular Dysfunction Syndrome: Identification of Multiple Pathologies in a Clinical Spectrum of Macular Dysfunction with Normal Fundus in East Asian Patients: EAOMD Report No. 5

Occult macular dystrophy (OMD) is the most prevalent form of macular dystrophy in East Asia. Beyond RP1L1, causative genes and mechanisms remain largely uncharacterised. This study aimed to delineate the clinical and genetic characteristics of OMD syndrome (OMDS). Patients clinically diagnosed with OMDS in Japan, South Korea, and China were enrolled. The inclusion criteria were as follows: (1) macular dysfunction and (2) normal fundus appearance. Comprehensive clinical evaluation and genetic assessment were performed to identify the disease-causing variants. Clinical parameters were compared among the genotype groups. Seventy-two patients with OMDS from fifty families were included. The causative genes were RP1L1 in forty-seven patients from thirty families (30/50, 60.0%), CRX in two patients from one family (1/50, 2.0%), GUCY2D in two patients from two families (2/50, 4.0%), and no genes were identified in twenty-one patients from seventeen families (17/50, 34.0%). Different severities were observed in terms of disease onset and the prognosis of visual acuity reduction. This multicentre large cohort study furthers our understanding of the phenotypic and genotypic spectra of patients with macular dystrophy and normal fundus. Evidently, OMDS encompasses multiple Mendelian retinal disorders, each representing unique pathologies that dictate their respective severity and prognostic patterns.

Cat LCA-CRX Model, Homozygous for an Antimorphic Mutation Has a Unique Phenotype

Purpose

Mutations in the CRX transcription factor are associated with dominant retinopathies often with more severe macular changes. The CRX-mutant cat (Rdy-A182d2) is the only animal model with the equivalent of the critical retinal region for high-acuity vision, the macula. Heterozygous cats (CRXRdy/+) have a severe phenotype modeling Leber congenital amaurosis. This study reports the distinct ocular phenotype of homozygous cats (CRXRdy/Rdy).

Methods

Gene expression changes were assessed at both mRNA and protein levels. Changes in globe morphology and retinal structure were analyzed.

Results

CRXRdy/Rdy cats had high levels of mutant CRX mRNA and protein. The expression of photoreceptor target genes was severely impaired although there were variable effects on the expression of other transcription factors. The photoreceptor cells remained immature and failed to elaborate outer segments consistent with the lack of retinal function. The retinal layers displayed a progressive remodeling with cell loss but maintained overall retinal thickness due to gliosis. Rapid photoreceptor loss largely occurred in the macula-equivalent retinal region. The homozygous cats developed markedly increased ocular globe length.

Conclusions

The phenotype of CRXRdy/Rdy cats was more severe compared to CRXRdy/+ cats by several metrics.

Translational Relevance

The CRX-mutant cat is the only model for CRX-retinopathies with a macula-equivalent region. A prominent feature of the CRXRdy/Rdy cat phenotype not detectable in homozygous mouse models was the rapid degeneration of the macula-equivalent retinal region highlighting the value of this large animal model and its future importance in the testing of translational therapies aiming to restore vision.

Next-generation sequencing--based genetic testing and phenotype correlation in retinitis pigmentosa patients from India

Purpose

Inherited retinal dystrophies (IRD) are a heterogeneous group of retinal diseases leading to progressive loss of photoreceptors through apoptosis. Retinitis pigmentosa (RP) is considered the most common form of IRD. Panel-based testing in RP has proven effective in identifying the causative genetic mutations in 70% and 80% of the patients. This is a retrospective, observational, single-center study of 107 RP patients who had undergone next-generation sequencing-based targeted gene panel testing for IRD genes. These patients were inspected for common phenotypic features to arrive at meaningful genotype-phenotype correlation.

Methods

Patients underwent complete ophthalmic examination, and blood was collected from the proband for DNA extraction after documenting the pedigree. Targeted Next Generation Sequencing (NGS) was done by panel-based testing for IRD genes followed by co-segregation analysis wherever applicable.

Results

Of the 107 patients, 72 patients had pathogenic mutations. The mean age of onset of symptoms was 14 ± 12 years (range: 5-55). Mean (Best Corrected Visual Acuity) BCVA was 6/48 (0.9 logMAR) (range 0.0-3.0). At presentation, over one-third of eyes had BCVA worse than 6/60 (<1 logMAR). Phenotype analysis with the gene defects showed overlapping features, such as peripheral well-defined chorioretinal atrophic patches in patients with CERKL, PROM1, and RPE65 gene mutations and large macular lesions in patients with RDH12 and CRX gene mutations, respectively. Nummular or clump-like pigmentation was noted in CRB1, TTC8, PDE6A, and PDE6B.

Conclusion

NGS-based genetic testing can help clinicians to diagnose RP more accurately, and phenotypic correlations can also help in better patient counselling with respect to prognosis and guidance regarding ongoing newer gene-based therapies.

Genotypic Profile and Clinical Characteristics of CRX-Associated Retinopathy in Koreans

This study aimed to investigate the clinical characteristics of Korean patients with retinal dystrophy associated with pathogenic variants of cone rod homeobox-containing gene (CRX). We retrospectively enrolled Korean patients with CRX-associated retinal dystrophy (CRX-RD) who visited two tertiary referral hospitals. Pathogenic variants were identified using targeted panel sequencing or whole-exome sequencing. We analyzed clinical features and phenotypic spectra according to genotype. Eleven patients with CRX-RD were included in this study. Six patients with cone-rod dystrophy (CORD), two with macular dystrophy (MD), two with Leber congenital amaurosis (LCA), and one with retinitis pigmentosa (RP) were included. One patient (9.1%) had autosomal recessive inheritance, and the other ten patients (90.9%) had autosomal dominant inheritance. Six patients (54.5%) were male, and the mean age of symptom onset was 27.0 ± 17.9 years. At the first presentation, the mean age was 39.4 ± 20.6 years, and best-corrected visual acuity (BCVA) (logMAR) was 0.76 ± 0.90 in the better eye. Negative electroretinography (ERG) was observed in seven (63.6%) patients. Nine pathogenic variants were identified, including two novel variants, c.101-1G>A and c.898T>C:p.(*300Glnext*118). Taken together with the variants reported in prior studies, all variants within the homeodomain are missense variants, whereas most variants downstream of the homeodomain are truncating variants (88%). The clinical features of pathogenic variants within the homeodomain are either CORD or MD with bull's eye maculopathy, whereas variants downstream of the homeodomain cause more diverse phenotypes, with CORD and MD in 36%, LCA in 40%, and RP in 24%. This is the first case series in Korea to investigate the CRX-RD genotype-phenotype correlation. Pathogenic variants downstream of the homeodomain of the CRX gene are present as RP, LCA, and CORD, whereas pathogenic variants within the homeodomain are mainly present as CORD or MD with bull's eye maculopathy. This trend was similar to previous genotype-phenotype analyses of CRX-RD. Further molecular biologic research on this correlation is required.

Gene Augmentation for Autosomal Dominant CRX-Associated Retinopathies

The cone-rod homeobox (CRX) protein is a key transcription factor essential for photoreceptor function and survival. Mutations in human CRX gene are linked to a wide spectrum of blinding diseases ranging from mild macular dystrophy to severe Leber congenital amaurosis (LCA), cone-rod dystrophy (CRD), and retinitis pigmentosa (RP). These diseases are still incurable and mostly inherited in an autosomal dominant form. Dysfunctional mutant CRX protein interferes with the function of wild-type CRX protein, demonstrating the dominant negative effect. At present, gene augmentation is the most promising treatment strategy for hereditary diseases. This study aims to review the pathogenic mechanisms of various CRX mutations and propose two therapeutic strategies to rescue sick photoreceptors in CRX-associated retinopathies, namely, Tet-On-hCRX system and adeno-associated virus (AAV)-mediated gene augmentation. The outcome of proposed studies will guide future translational research and suggest guidelines for therapy evaluation in terms of treatment safety and efficacy.

Late-Onset Autosomal Dominant Macular Degeneration Caused by Deletion of the CRX Gene

PURPOSE

To characterize the phenotype observed in a case series with macular disease and determine the cause.

DESIGN

Multicenter case series.

PARTICIPANTS

Six families (7 patients) with sporadic or multiplex macular disease with onset at 20 to 78 years, and 1 patient with age-related macular degeneration.

METHODS

Patients underwent ophthalmic examination; exome, genome, or targeted sequencing; and/or polymerase chain reaction (PCR) amplification of the breakpoint, followed by cloning and Sanger sequencing or direct Sanger sequencing.

MAIN OUTCOME MEASURES

Clinical phenotypes, genomic findings, and a hypothesis explaining the mechanism underlying disease in these patients.

RESULTS

All 8 cases carried the same deletion encompassing the genes TPRX1, CRX, and SULT2A1, which was absent from 382 control individuals screened by breakpoint PCR and 13 096 Clinical Genetics patients with a range of other inherited conditions screened by array comparative genomic hybridization. Microsatellite genotypes showed that these 7 families are not closely related, but genotypes immediately adjacent to the deletion breakpoints suggest they may share a distant common ancestor.

CONCLUSIONS

Previous studies had found that carriers for a single defective CRX allele that was predicted to produce no functional CRX protein had a normal ocular phenotype. Here, we show that CRX whole-gene deletion in fact does cause a dominant late-onset macular disease.

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.

Allele-specific gene editing to rescue dominant CRX-associated LCA7 phenotypes in a retinal organoid model

Cases of Leber congenital amaurosis caused by mutations in CRX (LCA7) exhibit an early form of the disease and show signs of significant photoreceptor dysfunction and eventual loss. To establish a translational in vitro model system to study gene-editing-based therapies, we generated LCA7 retinal organoids harboring a dominant disease-causing mutation in CRX. Our LCA7 retinal organoids develop signs of immature and dysfunctional photoreceptor cells, providing us with a reliable in vitro model to recapitulate LCA7. Furthermore, we performed a proof-of-concept study in which we utilize allele-specific CRISPR/Cas9-based gene editing to knock out mutant CRX and saw moderate rescue of photoreceptor phenotypes in our organoids. This work provides early evidence for an effective approach to treat LCA7, which can be applied more broadly to other dominant genetic diseases.

Novel clinical presentation of a CRX rod-cone dystrophy

We describe a novel clinical presentation of a CRX rod-cone dystrophy in a single family. Two boys ages 6 and 12 years presented with clinical and optical coherence tomography features suggestive of X-linked retinoschisis, but with optic nerve swelling without increased intracranial pressure. One patient had an electronegative electroretinogram (ERG) and the other had rod-cone dysfunction. Neither had retinoschisin (RS1) gene mutations. Biological mother and sister presented with retinal pigment epithelium (RPE) changes and abnormal cone-rod ERG responses. On further testing, next generation sequencing with array comparative genomic hybridisation showed a deletion in exon 4 of the CRX gene. Cystoid maculopathy in young male children can be difficult to distinguish from RS1-associated schisis. Phenotypic variants within a family must prompt a thorough retinal dystrophy evaluation even with electronegative ERG in the presenting child. This novel phenotype for CRX presents with optic nerve swelling and cystoid maculopathy in men, and RPE changes in women.

Gene Therapy of Dominant CRX-Leber Congenital Amaurosis using Patient Stem Cell-Derived Retinal Organoids

Mutations in the photoreceptor transcription factor gene cone-rod homeobox (CRX) lead to distinct retinopathy phenotypes, including early-onset vision impairment in dominant Leber congenital amaurosis (LCA). Using induced pluripotent stem cells (iPSCs) from a patient with CRX-I138fs48 mutation, we established an in vitro model of CRX-LCA in retinal organoids that showed defective photoreceptor maturation by histology and gene profiling, with diminished expression of visual opsins. Adeno-associated virus (AAV)-mediated CRX gene augmentation therapy partially restored photoreceptor phenotype and expression of phototransduction-related genes as determined by single-cell RNA-sequencing. Retinal organoids derived from iPSCs of a second dominant CRX-LCA patient carrying K88N mutation revealed the loss of opsin expression as a common phenotype, which was alleviated by AAV-mediated augmentation of CRX. Our studies provide a proof-of-concept for developing gene therapy of dominant CRX-LCA and other CRX retinopathies.

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Leber congenital amaurosis caused by CRX mutations is modeled in retinal organoids

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Patient iPSCs-derived organoids show impaired expression of visual opsins

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AAV-mediated CRX delivery partially restores expression of phototransduction genes

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Gene therapy is applicable to mutations in DNA-binding and transactivation domains

Clinical and Genetic Characteristics of 18 Patients from 13 Japanese Families with CRX-associated retinal disorder: Identification of Genotype-phenotype Association

Inherited retinal disorder (IRD) is a leading cause of blindness, and CRX is one of a number of genes reported to harbour autosomal dominant (AD) and recessive (AR) causative variants. Eighteen patients from 13 families with CRX-associated retinal disorder (CRX-RD) were identified from 730 Japanese families with IRD. Ophthalmological examinations and phenotype subgroup classification were performed. The median age of onset/latest examination was 45.0/62.5 years (range, 15-77/25-94). The median visual acuity in the right/left eye was 0.52/0.40 (range, -0.08-2.00/-0.18-1.70) logarithm of the minimum angle of resolution (LogMAR) units. There was one family with macular dystrophy, nine with cone-rod dystrophy (CORD), and three with retinitis pigmentosa. In silico analysis of CRX variants was conducted for genotype subgroup classification based on inheritance and the presence of truncating variants. Eight pathogenic CRX variants were identified, including three novel heterozygous variants (p.R43H, p.P145Lfs*42, and p.P197Afs*22). A trend of a genotype-phenotype association was revealed between the phenotype and genotype subgroups. A considerably high proportion of CRX-RD in ADCORD was determined in the Japanese cohort (39.1%), often showing the mild phenotype (CORD) with late-onset disease (sixth decade). Frequently found heterozygous missense variants located within the homeodomain underlie this mild phenotype. This large cohort study delineates the disease spectrum of CRX-RD in the Japanese population.

Association of CRX genotypes and retinal phenotypes confounded by variable expressivity and electronegative electroretinogram

IMPORTANCE

A framework for understanding the phenotypic features of CRX retinopathy was established.

BACKGROUND

To perform a phenotype-genotype correlation analysis in two groups of patients with heterozygous mutations in distinct locations of the CRX gene, encoding the cone-rod homeobox.

DESIGN

Multicentre retrospective study.

PARTICIPANTS

Twenty-one Japanese patients from 14 families with a heterozygous CRX mutation.

METHODS

Retrospective data analysis.

MAIN OUTCOME MEASURES

Clinical records on CRX mutation, symptoms, best-corrected visual acuity (BCVA), visual field, fundus photography, fundus auto-fluorescence, optical coherence tomography and electroretinograms (ERGs).

RESULTS

Six different CRX heterozygous mutations were identified in the subjects. Twelve patients from 9 families shared the p.R41W mutation and 1 patient had the p.R43C mutation, both of which affect the homeobox domain of CRX. These patients often displayed adult-onset retinal dystrophy with macular degeneration. In contrast, five patients with downstream mutations (p.S204fs, p.S213fs, p.G243X and p.L299F) displayed retinal degeneration or macular degeneration with bone-spicule pigmentation. Three asymptomatic carriers with different mutations (p.R41W, p.S213fs and p.G243X) were present in both groups. Nearly all patients and carriers had an electronegative ERG in response to a bright flash under dark adaptation. There was no cross-sectional association between patients' age and BCVA, despite progressive decline in BCVA.

CONCLUSIONS AND RELEVANCE

Heterozygous mutations within or downstream of the homeobox domain in CRX relate to the difference associated retinal phenotypes, which was confounded by variable expressivity and electronegative ERGs. CRX mutations should be considered in patients with an electronegative ERG with minimal or no macular changes.

Inherited Eye Diseases with Retinal Manifestations through the Eyes of Homeobox Genes

Retinal development is under the coordinated control of overlapping networks of signaling pathways and transcription factors. The paper was conceived as a review of the data and ideas that have been formed to date on homeobox genes mutations that lead to the disruption of eye organogenesis and result in inherited eye/retinal diseases. Many of these diseases are part of the same clinical spectrum and have high genetic heterogeneity with already identified associated genes. We summarize the known key regulators of eye development, with a focus on the homeobox genes associated with monogenic eye diseases showing retinal manifestations. Recent advances in the field of genetics and high-throughput next-generation sequencing technologies, including single-cell transcriptome analysis have allowed for deepening of knowledge of the genetic basis of inherited retinal diseases (IRDs), as well as improve their diagnostics. We highlight some promising avenues of research involving molecular-genetic and cell-technology approaches that can be effective for IRDs therapy. The most promising neuroprotective strategies are aimed at mobilizing the endogenous cellular reserve of the retina.

Copy number variations and multiallelic variants in Korean patients with Leber congenital amaurosis

Purpose

We comprehensively evaluated the mutational spectrum of Leber congenital amaurosis (LCA) and investigated the molecular diagnostic rate and genotype-phenotype correlation in a Korean cohort.

Methods

This single-center retrospective case series included 50 Korean patients with LCA between June 2015 and March 2019. Molecular analysis was conducted using targeted panel-based next-generation sequencing, including deep intronic and regulatory variants or whole exome sequencing. The molecular diagnosis was made based on the inheritance pattern, zygosity, and pathogenicity.

Results

Among the 50 patients, 27 patients (54%) were male, and 11 (22%) showed systemic features. Genetic variants highly likely to be causative were identified in 78% (39/50) of cases and segregated into families. We detected two pathogenic or likely pathogenic variants in a gene linked to a recessive trait without segregation analysis in three cases (6.0%). GUCY2D (20%), NMNAT1 (18%), and CEP290 (16%) were the most frequently mutated genes in Korean LCA. Copy number variations were found in three patients, which accounted for 6% of LCA cases. A possible dual molecular diagnosis (Senior-Løken syndrome along with Leigh syndrome, and Joubert syndrome with transposition of the great arteries) was made in two patients (4%). Three of 50 patients were medically or surgically actionable: one patient for RPE65 gene therapy and two patients with WDR19 Senior-Løken syndrome for early preparation for kidney and liver transplantations.

Conclusions

This study demonstrated that approximately 4% of patients may have dual molecular diagnoses, and 6% were surgically or medically actionable in LCA. Therefore, accurate molecular diagnosis and careful interpretation of next-generation sequencing results can be of great help in patients with LCA.

Pathogenicity discrimination and genetic test reference for CRX variants based on genotype-phenotype analysis

The cone-rod homeobox (CRX) gene is specifically expressed in developing and mature photoreceptors and is relatively conserved, with limited polymorphisms in coding regions. Rare variants in CRX are usually considered causative for different forms of retinal degeneration, but this might be problematic based on recent data. This study aimed to classify CRX variants based on a genotype-phenotype analysis of our data and the literature. Twenty-four CRX variants, including 14 novel variants, were detected in 37 Chinese families based on exome sequencing data obtained from 4971 Chinese probands with different forms of eye diseases. After detailed phenotypic analysis and cosegregation analysis in families with CRX variants, the 24 variants could be classified into three groups: benign (six), likely benign (six), and pathogenic (12). Somatic mosaicism was identified in a family with unaffected parents (the father had a mutant allele that was detected in approximately 17% of his leukocyte DNA) and two affected sons. Furthermore, a thorough reassessment was systematically performed for all 113 heterozygous variants as well as for their associated phenotypes from our cohort and patients previously reported. Two critical findings on the pathogenicity of CRX variants were obtained based on the genotype-phenotype correlation, family segregation and ensemble predicting methods: 1) approximately half of heterozygous missense variants are likely benign, and 2) heterozygous truncating variants affecting the homeodomain are likely benign. Truncating mutations after the homeodomain are likely associated with a more severe phenotype. Although most heterozygous pathogenic variants in CRX are associated with autosomal dominant retinal degeneration, a homozygous c.268C> T (p.Arg90Trp) substitution and homozygous complete deletion of CRX have been reported to cause Leber congenital amaurosis. In conclusion, many rare missense variants and some truncating variants in CRX are likely benign, although previously, they might have been predicted to be damaging by some online tools. Evaluation of the pathogenicity of a CRX variant should consider both its nature and location. The information obtained in this study is critical in the era of routine clinical genetic test, not only for CRX but also for many other genes with many more variants. Functional studies and additional genotype-phenotype analyses are expected to confirm these associations.

A novel CRX variant (p.R98X) is identified in a Chinese family of Retinitis pigmentosa with atypical and mild manifestations

BACKGROUND

Retinitis pigmentosa (RP) is the most common form of hereditary retinal degeneration that can cause inherited blindness. RP has extreme genetic and clinical heterogeneity, which brings a major obstacle to obtaining an accurate molecular diagnosis.

OBJECTIVE

To analyze the genetic defect in a Chinese family of RP with a few atypical manifestations.

METHODS

Whole-exome sequencing (WES) was applied to identify the disease-associated genes. Sanger sequencing was performed to validate the variants of candidate genes in the patient and his parents. In vitro expression analysis was further conducted to examine the potential biological function of the gene variant.

RESULTS

A heterozygous nonsense variant c.292C > T (p.R98X) of CRX gene was identified to be present in the affected male. The c.292C > T variant of CRX was absent in all of the searched databases, including the 10,000 Chinese exome database. The nonsense variant was supposed to result in a truncated CRX protein with a destroyed homedomain (HD), which is essential for CRX translation. Interestingly, the following assay showed that the potential truncated protein was not detected, indicating that the variant may cause a loss-of-function mutation of CRX gene.

CONCLUSION

We identified a novel heterozygous null mutation in the CRX gene which was the first evidence of a nonsense mutation in the HD domain of CRX. Our finding suggested that the haploinsufficiency mutation of CRX gene contributed to the atypical and mild manifestations of the autosomal dominant RP in the Chinese family.