A founder Alu insertion in RP1 gene in Japanese patients with retinitis pigmentosa

Clinical Exome-Based Redefinition and Reclassification of Retinitis Pigmentosa

BACKGROUND

Because of the low prevalence of inherited retinal diseases, reports on the distribution of retinitis pigmentosa (RP)-related genes in Korean patients are scarce. The aim of this study was to determine the mutation spectrum and allele frequency and observe the final diagnoses in a Korean cohort clinically diagnosed with RP.

METHODS

We used whole-exome sequencing (WES) to analyze a Korean cohort of 100 unrelated patients clinically diagnosed with RP. The possible pathogenicity of each variant was assessed based on the guidelines of the American College of Medical Genetics and Genomics and Association for Molecular Pathology, in-silico prediction tools, known clinical phenotypes, and inheritance patterns.

RESULTS

Definite causative genes were detected in 60/100 patients (60.0%). Of these 60 cases, USH2A was the most common causative gene (14/60, 23.3%), followed by EYS (13/60, 21.7%) and RP1 (6/60, 10.0%). The clinical diagnosis was redefined in 9 of the 60 probands (15.0%) with causative genes after WES. Five of the 60 patients (8.3%) carried a causative variant in CHM, and the clinical diagnosis was redefined as choroideremia. Leber congenital amaurosis was diagnosed in 2/60 probands (3.3%), and RDH12 and RPGRIP1 were the causative genes in each patient. One patient (1/60, 1.7%) was diagnosed with Bietti's crystalline dystrophy, with CYP4V2 identified as the causative gene. In another patient (1/60, 1.7%), ABCA4 variants were detected with clinical findings suggestive of cone-rod dystrophy.

CONCLUSION

This study reports the mutational spectrum of a cohort of Korean patients with a clinical diagnosis of RP who were referred for genetic testing. This study adds valuable data regarding the frequency of genes as well as their relation to the age of symptom onset and relation to other inherited retinal degenerations.

Phenotypic variability of RP1-related inherited retinal dystrophy associated with the c.5797 C > T (p.Arg1933*) variant in the Japanese population

The phenotypes of RP1-related inherited retinal dystrophies (RP1-IRD), causing autosomal dominant (AD) and autosomal recessive (AR) diseases, vary depending on specific RP1 variants. A common nonsense mutation near the C-terminus, c.5797 C > T (p.Arg1933*), is associated with RP1-IRD, but the exact role of this mutation in genotype-phenotype correlation remains unclear. In this study, we retrospectively analyzed patients with RP1-IRD (N = 42) from a single center in Japan. AR RP1-IRD patients with the c.5797 C > T mutation (N = 14) mostly displayed macular dystrophy but rarely retinitis pigmentosa or cone-rod dystrophy. Conversely, AR RP1-IRD patients without the c.5797 C > T mutation, including those with other pathogenic RP1 variants, were mostly diagnosed with severe retinitis pigmentosa. Full-field electroretinograms were significantly better in patients homozygous or compound heterozygous for the c.5797 C > T mutation than in those without this mutation, corresponding to their milder phenotypes. Clinical tests also revealed a slower onset of age and a better mean deviation value with the static visual field in AR RP1-IRD patients with the c.5797 C > T mutation compared to those without. Therefore, the presence of c.5797 C > T may partly account for the phenotypic variety of RP1-IRD and may yield milder phenotypes. These findings may be useful for predicting the prognosis of RP1-IRD patients.

Identification of diagnostic challenges in RP1 Alu insertion and strategies for overcoming them

Recently, a founder Alu insertion in exon 4 of RP1 was detected in Japanese and Korean patients with inherited retinal diseases (IRDs). However, carrier frequency and diagnostic challenges for detecting AluY insertion are not established. We aim to investigate the frequency of AluY in individuals with or without IRDs and to overcome common diagnostic pitfalls associated with AluY insertion. A total of 1,072 subjects comprising 411 patients with IRD (IRD group) and 661 patients with other suspected Mendelian genetic disease (non-IRD group) was screened for AluY insertion. Targeted panel sequencing and whole-genome sequencing were used for detection of AluY insertion, and an optimized allele-specific PCR (AS-PCR) was used for validation. The AluY insertion was detected in 1.5% in IRD group (6/411). The AluY insertion was not observed in non-IRD group (0/661). All patients with AluY were confirmed to have RP1 pathogenic variants on the paired allele. We identified AluY allele dropout leading to false homozygosity for c.4196del pathogenic variant in Sanger sequencing. The allelic relationship between variants of RP1 was accurately determined by AluY AS-PCR. Delineating diagnostic challenges of AluY insertion and strategies to avoid potential pitfalls could aid clinicians in an accurate molecular diagnosis for patients with IRD.

Detecting Alu Element Insertion Variant in RP1 Gene Using Whole Genome Sequencing in Patients with Retinitis Pigmentosa

Background/Objectives:Alu element insertion in the exon 4 of the RP1 gene was newly identified through whole genome sequencing (WGS). This was not detected in previous next-generation sequencing (NGS) analysis. We report three cases of Korean retinitis pigmentosa (RP) patients with compound heterozygous variants including Alu element insertion in the RP1 gene, indicating that Alu element insertion could be a cause of RP; Methods: Among patients diagnosed with RP having variants in the RP1 gene in the Asan Medical Center, WGS was additionally performed for genetically unsolved cases in previous NGS analysis to detect any presence of Alu element insertion. For cases detected to have Alu element insertion in the exon 4 of the RP1 gene, genetic and clinical characteristics were analyzed; Results: Among 16 patients with RP, 3 patients were detected to have Alu element insertion in the RP1 gene. Alu element insertion in the RP1 gene was also detected using WGS. It was revealed to be a pathogenic variant. Therefore, RP1 gene mutation was the confirmed genetic cause of RP for these three cases and genetic counseling was enabled for them; Conclusions: Alu element insertion in the RP1 gene could be a genetic cause of autosomal recessive RP patients with compound heterozygous variants. Through WGS, the identification of this pathogenic variant was possible. Confirmation is needed to check the presence of Alu element insertion in patients with compound heterozygous variants in the RP1 gene.

Late-Onset Slowly Progressing Cone/Macular Dystrophy in Patients With the Biallelic Hypomorphic Variant p.Arg1933Ter in RP1

Purpose

Homozygous hypomorphic variants of the RP1 gene, including c.5797C>T, p.Arg1933Ter, have traditionally been considered non-pathogenic. This study aimed to elucidate the clinical manifestations of late-onset, slowly progressive cone/macular dystrophy in patients homozygous for p.Arg1933Ter in the RP1 gene.

Methods

Five patients with biallelic p.Arg1933Ter in RP1 were retrospectively recruited, and their clinical profiles were analyzed. Copy number variation analysis and Alu insertion assessment of genes associated with inherited retinal diseases were conducted. The results of comprehensive ophthalmological examinations, multimodal imaging, and full-field electroretinogram tests were analyzed.

Results

No specific sequencing errors or structural variations associated with the clinical phenotypes were identified. Alu element insertion in RP1 was not detected. The mean ± SD age at the first visit was 62.2 ± 9.8 years, with symptoms typically starting between 45 and 50 years of age. Two patients exhibited a mild form of cone/macular dystrophy, characterized by a relatively preserved fundus appearance and blurring of the ellipsoid zone on optical coherence tomography. Three patients had late-onset cone/macular dystrophy with significant atrophy.

Conclusions

To our knowledge, this study is the first to report that a homozygous hypomorphic variant of RP1, previously considered non-pathogenic, leads to cone/macular dystrophy.

Translational Relevance

The study introduces novel possibilities suggesting that the homozygous hypomorphic variant of RP1 may be linked to variant pathogenicity.

Gene Therapy for Retinitis Pigmentosa: Current Challenges and New Progress

Retinitis pigmentosa (RP) poses a significant threat to eye health worldwide, with prevalence rates of 1 in 5000 worldwide. This genetically diverse retinopathy is characterized by the loss of photoreceptor cells and atrophy of the retinal pigment epithelium. Despite the involvement of more than 3000 mutations across approximately 90 genes in its onset, finding an effective treatment has been challenging for a considerable time. However, advancements in scientific research, especially in gene therapy, are significantly expanding treatment options for this most prevalent inherited eye disease, with the discovery of new compounds, gene-editing techniques, and gene loci offering hope for more effective treatments. Gene therapy, a promising technology, utilizes viral or non-viral vectors to correct genetic defects by either replacing or silencing disease-causing genes, potentially leading to complete recovery. In this review, we primarily focus on the latest applications of gene editing research in RP. We delve into the most prevalent genes associated with RP and discuss advancements in genome-editing strategies currently employed to correct various disease-causing mutations.

Disease-specific variant interpretation highlighted the genetic findings in 2325 Japanese patients with retinitis pigmentosa and allied diseases

BACKGROUND

As gene-specific therapy for inherited retinal dystrophy (IRD) advances, unified variant interpretation across institutes is becoming increasingly important. This study aims to update the genetic findings of 86 retinitis pigmentosa (RP)-related genes in a large number of Japanese patients with RP by applying the standardised variant interpretation guidelines for Japanese patients with IRD (J-IRD-VI guidelines) built upon the American College of Medical Genetics and Genomics and the Association for Molecular Pathology rules, and assess the contribution of these genes in RP-allied diseases.

METHODS

We assessed 2325 probands with RP (n=2155, including n=1204 sequenced previously with the same sequencing panel) and allied diseases (n=170, newly analysed), including Usher syndrome, Leber congenital amaurosis and cone-rod dystrophy (CRD). Target sequencing using a panel of 86 genes was performed. The variants were interpreted according to the J-IRD-VI guidelines.

RESULTS

A total of 3564 variants were detected, of which 524 variants were interpreted as pathogenic or likely pathogenic. Among these 524 variants, 280 (53.4%) had been either undetected or interpreted as variants of unknown significance or benign variants in our earlier study of 1204 patients with RP. This led to a genetic diagnostic rate in 38.6% of patients with RP, with EYS accounting for 46.7% of the genetically solved patients, showing a 9% increase in diagnostic rate from our earlier study. The genetic diagnostic rate for patients with CRD was 28.2%, with RP-related genes significantly contributing over other allied diseases.

CONCLUSION

A large-scale genetic analysis using the J-IRD-VI guidelines highlighted the population-specific genetic findings for Japanese patients with IRD; these findings serve as a foundation for the clinical application of gene-specific therapies.

Long-read sequencing improves the genetic diagnosis of retinitis pigmentosa by identifying an Alu retrotransposon insertion in the EYS gene

BACKGROUND

Biallelic variants in EYS are the major cause of autosomal recessive retinitis pigmentosa (arRP) in certain populations, a clinically and genetically heterogeneous disease that may lead to legal blindness. EYS is one of the largest genes (~ 2 Mb) expressed in the retina, in which structural variants (SVs) represent a common cause of disease. However, their identification using short-read sequencing (SRS) is not always feasible. Here, we conducted targeted long-read sequencing (T-LRS) using adaptive sampling of EYS on the MinION sequencing platform (Oxford Nanopore Technologies) to definitively diagnose an arRP family, whose affected individuals (n = 3) carried the heterozygous pathogenic deletion of exons 32-33 in the EYS gene. As this was a recurrent variant identified in three additional families in our cohort, we also aimed to characterize the known deletion at the nucleotide level to assess a possible founder effect.

RESULTS

T-LRS in family A unveiled a heterozygous AluYa5 insertion in the coding exon 43 of EYS (chr6(GRCh37):g.64430524_64430525ins352), which segregated with the disease in compound heterozygosity with the previously identified deletion. Visual inspection of previous SRS alignments using IGV revealed several reads containing soft-clipped bases, accompanied by a slight drop in coverage at the Alu insertion site. This prompted us to develop a simplified program using grep command to investigate the recurrence of this variant in our cohort from SRS data. Moreover, LRS also allowed the characterization of the CNV as a ~ 56.4kb deletion spanning exons 32-33 of EYS (chr6(GRCh37):g.64764235_64820592del). The results of further characterization by Sanger sequencing and linkage analysis in the four families were consistent with a founder variant.

CONCLUSIONS

To our knowledge, this is the first report of a mobile element insertion into the coding sequence of EYS, as a likely cause of arRP in a family. Our study highlights the value of LRS technology in characterizing and identifying hidden pathogenic SVs, such as retrotransposon insertions, whose contribution to the etiopathogenesis of rare diseases may be underestimated.

Characterisation of a LINE-1 Insertion in the RP1 Gene by Targeted Adaptive Nanopore Sequencing in a Family with Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a group of inherited degenerative retinal disorders affecting more than 1.5 million people worldwide. For 30-50% of individuals with RP, the genetic cause remains unresolved by current clinical diagnostic gene panels. It is likely explained by variants in novel RP-associated genes or noncoding regulatory regions, or by complex genetic alterations such as large structural variants. Recent developments in long-read sequencing techniques have opened an opportunity for efficient analysis of complex genetic variants. We analysed a Finnish family with dominantly inherited RP affecting six individuals in three generations. Two affected individuals underwent a comprehensive clinical examination in combination with a clinical diagnostic gene panel, followed by whole exome sequencing in our laboratory. They exhibited typical signs of RP, yet initial sequence analysis found no causative variants. Reanalysis of the sequencing data detected a LINE-1 (L1) retrotransposon insertion of unknown size in exon 4 of the RP1 axonemal microtubule-associated (RP1) gene. The large chimeric L1 insertion that segregated with the disease was further characterised using targeted adaptive nanopore sequencing of RP1, allowing us to identify a 5.6 kb L1 transposable element insertion in RP1 as the cause of RP in this family with dominantly inherited RP.

Detailed analysis of an enriched deep intronic ABCA4 variant in Irish Stargardt disease patients

Over 15% of probands in a large cohort of more than 1500 inherited retinal degeneration patients present with a clinical diagnosis of Stargardt disease (STGD1), a recessive form of macular dystrophy caused by biallelic variants in the ABCA4 gene. Participants were clinically examined and underwent either target capture sequencing of the exons and some pathogenic intronic regions of ABCA4, sequencing of the entire ABCA4 gene or whole genome sequencing. ABCA4 c.4539 + 2028C > T, p.[= ,Arg1514Leufs*36] is a pathogenic deep intronic variant that results in a retina-specific 345-nucleotide pseudoexon inclusion. Through analysis of the Irish STGD1 cohort, 25 individuals across 18 pedigrees harbour ABCA4 c.4539 + 2028C > T and another pathogenic variant. This includes, to the best of our knowledge, the only two homozygous patients identified to date. This provides important evidence of variant pathogenicity for this deep intronic variant, highlighting the value of homozygotes for variant interpretation. 15 other heterozygous incidents of this variant in patients have been reported globally, indicating significant enrichment in the Irish population. We provide detailed genetic and clinical characterization of these patients, illustrating that ABCA4 c.4539 + 2028C > T is a variant of mild to intermediate severity. These results have important implications for unresolved STGD1 patients globally with approximately 10% of the population in some western countries claiming Irish heritage. This study exemplifies that detection and characterization of founder variants is a diagnostic imperative.

A comprehensive WGS-based pipeline for the identification of new candidate genes in inherited retinal dystrophies

To enhance the use of Whole Genome Sequencing (WGS) in clinical practice, it is still necessary to standardize data analysis pipelines. Herein, we aimed to define a WGS-based algorithm for the accurate interpretation of variants in inherited retinal dystrophies (IRD). This study comprised 429 phenotyped individuals divided into three cohorts. A comparison of 14 pathogenicity predictors, and the re-definition of its cutoffs, were performed using panel-sequencing curated data from 209 genetically diagnosed individuals with IRD (training cohort). The optimal tool combinations, previously validated in 50 additional IRD individuals, were also tested in patients with hereditary cancer (n = 109), and with neurological diseases (n = 47) to evaluate the translational value of this approach (validation cohort). Then, our workflow was applied for the WGS-data analysis of 14 individuals from genetically undiagnosed IRD families (discovery cohort). The statistical analysis showed that the optimal filtering combination included CADDv1.6, MAPP, Grantham, and SIFT tools. Our pipeline allowed the identification of one homozygous variant in the candidate gene CFAP20 (c.337 C > T; p.Arg113Trp), a conserved ciliary gene, which was abundantly expressed in human retina and was located in the photoreceptors layer. Although further studies are needed, we propose CFAP20 as a candidate gene for autosomal recessive retinitis pigmentosa. Moreover, we offer a translational strategy for accurate WGS-data prioritization, which is essential for the advancement of personalized medicine.

Long-term Outcomes of Cataract Surgery in Patients with Retinitis Pigmentosa

PURPOSE

To investigate the long-term outcomes of cataract surgery in patients with retinitis pigmentosa (RP).

DESIGN

Retrospective, observational study.

PARTICIPANTS

Sixty-four patients with typical RP (22 males, 42 females, average age 62.8 ± 10.1 years) who underwent cataract surgery at Kyushu University Hospital between May 2007 and October 2015 and who were followed up for ≥3 years post-surgery.

METHODS

Differences between pre- and post-surgery visual function, including best-corrected visual acuity (BCVA) and parameters in the Humphrey Field Analyzer (HFA) examinations using the central 10-2 program were investigated. The pre-surgery conditions of the foveal ellipsoid zone (EZ) were classified into three grades (grade 1: invisible; grade 2: abnormal; grade 3: normal) based on optical coherence tomography findings.

MAIN OUTCOME MEASURES

BCVA, the retinal sensitivity in HFA 10-2 tests.

RESULTS

Cataract surgery was performed in 96 eyes, with an average follow-up period of 5.8 ± 2.4 years. The mean pre-surgery BCVA was 0.64 ± 0.52 logarithm of the minimum angle of resolution (logMAR), and the final post-surgery BCVA was 0.61 ± 0.67 logMAR (p=0.57). Significant improvement of post-surgery BCVA was observed only in the eyes with preserved foveal EZ (grade 3) (p<0.01). In 62 eyes of the 45 patients who received HFA 10-2 tests, the mean values of deviation, macular sensitivity and foveal sensitivity at the final visit were significantly decreased compared with preoperative values (p<0.01), while those in the grade 3 eyes did not change significantly post-surgery (p=0.13).

CONCLUSIONS

In the long-term course after cataract surgery in RP patients, many cases experienced vision loss with progression of the disease. A preoperative finding of preserved foveal EZ was associated with a better visual prognosis, suggesting that EZ evaluation is useful to predict the long-term visual outcome after cataract surgery in RP patients.

In Silico identification of a common mobile element insertion in exon 4 of RP1

Mobile element insertions (MEIs) typically exceed the read lengths of short-read sequencing technologies and are therefore frequently missed. Recently, a founder Alu insertion in exon 4 of RP1 has been detected in Japanese patients with macular dystrophy by PCR and gel electrophoresis. We aimed to develop a grep search program for the detection of the Alu insertion in exon 4 of RP1 using unprocessed short reads. Among 494 unrelated Korean patients with inherited eye diseases, 273 patients with specific retinal phenotypes who were previously genotyped by targeted panel or whole exome sequencing were selected. Five probands had a single heterozygous truncating RP1 variant, and one of their unaffected parents also carry this variant. To find a hidden genetic variant, whole genome sequencing was performed in two patients, and it revealed AluY c.4052_4053ins328/p.(Tyr1352Alafs*9) insertion in RP1 exon 4. This AluY insertion was additionally identified in other 3 families, which was confirmed by PCR and gel electrophoresis. We developed simplified grep search program to detect this AluY insertion in RP1 exon 4. The simple grep search revealed a median variant allele frequency of 0.282 (interquartile range, 0.232–0.383), with no false-positive results using 120 control samples. The MEI in RP1 exon 4 was a common founder mutation in Korean, occurring in 1.8% of our cohort. The RP1-Alu grep program efficiently detected the AluY insertion, without the preprocessing of raw data or complex installation processes.

Genotype-Phenotype Correlations in RP1-Associated Retinal Dystrophies: A Multi-Center Cohort Study in JAPAN

Background: Little is known about genotype–phenotype correlations of RP1-associated retinal dystrophies in the Japanese population. We aimed to investigate the genetic spectrum of RP1 variants and provide a detailed description of the clinical findings in Japanese patients. Methods: In total, 607 patients with inherited retinal diseases were examined using whole-exome/whole-genome sequencing (WES/WGS). PCR-based screening for an Alu element insertion (c.4052_4053ins328/p.Tyr1352AlafsTer9) was performed in 18 patients with autosomal-recessive (AR)-retinitis pigmentosa (RP) or AR-cone dystrophy (COD)/cone-rod dystrophy (CORD), including seven patients with heterozygous RP1 variants identified by WES/WGS analysis, and 11 early onset AR-RP patients, in whom no pathogenic variant was identified. We clinically examined 25 patients (23 families) with pathogenic RP1 variants, including five patients (five families) with autosomal-dominant (AD)-RP, 13 patients (11 families) with AR-RP, and seven patients (seven families) with AR-COD/CORD. Results: We identified 18 pathogenic RP1 variants, including seven novel variants. Interestingly, the Alu element insertion was the most frequent variant (32.0%, 16/50 alleles). The clinical findings revealed that the age at onset and disease progression occurred significantly earlier and faster in AR-RP patients compared to AD-RP or AR-COD/CORD patients. Conclusions: Our results suggest a genotype–phenotype correlation between variant types/locations and phenotypes (AD-RP, AR-RP, and AR-COD/CORD), and the Alu element insertion was the most major variant in Japanese patients with RP1-associated retinal dystrophies.

A hypomorphic variant in EYS detected by genome-wide association study contributes toward retinitis pigmentosa

The genetic basis of Japanese autosomal recessive retinitis pigmentosa (ARRP) remains largely unknown. Herein, we applied a 2-step genome-wide association study (GWAS) in 640 Japanese patients. Meta-GWAS identified three independent peaks at P < 5.0 × 10⁻⁸, all within the major ARRP gene EYS. Two of the three were each in linkage disequilibrium with a different low frequency variant (allele frequency < 0.05); a known founder Mendelian mutation (c.4957dupA, p.S1653Kfs*2) and a non-synonymous variant (c.2528 G > A, p.G843E) of unknown significance. mRNA harboring c.2528 G > A failed to restore rhodopsin mislocalization induced by morpholino-mediated knockdown of eys in zebrafish, consistent with the variant being pathogenic. c.2528 G > A solved an additional 7.0% of Japanese ARRP cases. The third peak was in linkage disequilibrium with a common non-synonymous variant (c.7666 A > T, p.S2556C), possibly representing an unreported disease-susceptibility signal. GWAS successfully unraveled genetic causes of a rare monogenic disorder and identified a high frequency variant potentially linked to development of local genome therapeutics.

Koji Nishiguchi et al. identify three genetic variants within the EYS gene that are associated with retinitis pigmentosa using a genome-wide association study. They demonstrate that one of these variants (G843E) causes retinal dysfunction in zebrafish, suggesting a causal role for EYS in retinitis pigmentosa.