RP1L1 and inherited photoreceptor disease: A review

Varied clinical presentations of RP1L1 variants in Chinese patients: a study of occult macular dystrophy and vitelliform macular dystrophy

BACKGROUND

Occult Macular Dystrophy (OMD), primarily caused by retinitis pigmentosa 1-like 1 (RP1L1) variants, is a complex retinal disease characterised by progressive vision loss and a normal fundus appearance. This study aims to investigate the diverse phenotypic expressions and genotypic correlations of OMD in Chinese patients, including a rare case of Vitelliform Macular Dystrophy (VMD) associated with RP1L1.

METHODS

We analysed seven OMD patients and one VMD patient, all with heterozygous pathogenic RP1L1 variants. Clinical assessments included Best Corrected Visual Acuity (BCVA), visual field testing, Spectral Domain Optical Coherence Tomography (SD-OCT), multifocal Electroretinograms (mfERGs), and microperimetry. Next-generation sequencing was utilised for genetic analysis.

RESULTS

The OMD patients displayed a range of phenotypic variability. Most (5 out of 7) had the RP1L1 variant c.133 C > T; p.R45W, associated with central vision loss and specific patterns in SD-OCT and mfERG. Two patients exhibited different RP1L1 variants (c.3599G > T; p.G1200V and c.2880G > C; p.W960C), presenting milder phenotypes. SD-OCT revealed photoreceptor layer changes, with most patients showing decreased mfERG responses in the central rings. Interestingly, a unique case of VMD linked to the RP1L1 variant was observed, distinct from traditional OMD presentations.

CONCLUSIONS

This study highlights the phenotypic diversity within OMD and the broader spectrum of RP1L1-associated macular dystrophies, including a novel association with VMD. The findings emphasise the complexity of RP1L1 variants in determining clinical manifestations, underscoring the need for comprehensive genetic and clinical evaluations in macular dystrophies.

Biallelic occult macular dystrophy

PURPOSE

Occult macular dystrophy (OMD) is a cause of visual loss in young adults with a grossly normal fundus appearance. It is considered an autosomal dominant disorder, related to heterozygous pathogenic variants in the gene RP1L1. The purpose of this study is to report a biallelic form of the disease.

RESULTS

A 29-year-old female had undergone neurological workup and ophthalmic examinations for transient visual loss in her left eye over the past two years but there was no definitive diagnosis. The best-corrected visual acuity was 20/30, 20/20. Indirect ophthalmoscopy with a 78D lens revealed subtle central retinal pigment epithelium mottling and optical coherence tomography confirmed subtle central thickening of the ellipsoid zone. Full-field electroretinography was normal, but pattern electroretinography showed decreased p50 responses. OMD was suspected. Retinal gene panel testing was significant only for a homozygous variant in RP1L1 (NM_178857.6: c.3571 G>T; p.Glu1191*). The parents and older brother were unavailable for segregation analysis. By history they did not have visual complaints other than a need for glasses.

CONCLUSIONS

This report presents the clinical and genetic findings of a biallelic form of OMD associated with a novel pathogenic variant in RP1L1. It would be of interest to carefully assess macular function in heterozygotes with this variant.

The genomic mosaic of mitochondrial dysfunction: Decoding nuclear and mitochondrial epigenetic contributions to maternally inherited diabetes and deafness pathogenesis

Aims

Maternally inherited diabetes and deafness (MIDD) is a complex disorder characterized by multiorgan clinical manifestations, including diabetes, hearing loss, and ophthalmic complications. This pilot study aimed to elucidate the intricate interplay between nuclear and mitochondrial genetics, epigenetic modifications, and their potential implications in the pathogenesis of MIDD.

Main methods

A comprehensive genomic approach was employed to analyze a Sicilian family affected by clinically characterized MIDD, negative to the only known causative m.3243 A > G variant, integrating whole-exome sequencing and whole-genome bisulfite sequencing of both nuclear and mitochondrial analyses.

Key findings

Rare and deleterious variants were identified across multiple nuclear genes involved in retinal homeostasis, mitochondrial function, and epigenetic regulation, while complementary mitochondrial DNA analysis revealed a rich tapestry of genetic diversity across genes encoding components of the electron transport chain and ATP synthesis machinery. Epigenetic analyses uncovered significant differentially methylated regions across the genome and within the mitochondrial genome, suggesting a nuanced landscape of epigenetic modulation.

Significance

The integration of genetic and epigenetic data highlighted the potential crosstalk between nuclear and mitochondrial regulation, with specific mtDNA variants influencing methylation patterns and potentially impacting the expression and regulation of mitochondrial genes. This pilot study provides valuable insights into the complex molecular mechanisms underlying MIDD, emphasizing the interplay between nucleus and mitochondrion, tracing the way for future research into targeted therapeutic interventions and personalized approaches for disease management.

Genomics and tumor microenvironment of breast mucoepidermoid carcinoma based on whole-exome and RNA sequencing

Mammary mucoepidermoid carcinoma (MEC) is a rare entity. The molecular characteristics of breast MEC have not been fully investigated due to its rarity. We performed a retrospective study among 1000 patients with breast carcinomas and identified four cases of breast MEC. Clinical and demographic data were collected. Immunohistochemistry panels which were used to diagnose salivary gland MEC and breast carcinomas were also performed. MAML2 rearrangements were detected by FISH and fusion partners were identified by RNA sequencing. Whole-exome sequencing (WES) was used to reveal the genomes of these four breast MEC. Then, the biological functions and features of breast MEC were further compared with those of invasive breast carcinomas and salivary gland MEC.According to Ellis and Auclair's methods, these four breast MEC could be classified as low-grade breast MEC. All the patients were alive, and disease-free survival (PFS) ranged from 20 months to 67 months. Among these four breast MEC, two cases were triple-negative, and the other two cases were found to be ER positive, with one also showing HER2 equivocal by immunohistochemical staining, but no amplification in FISH. FISH analysis confirmed the presence of the MAML2 translocation in three of four tumors, and CRTC1-MAML2 fusion was confirmed in two of them by RNA-sequencing. The average coverage size of WES for the tumor mutation burden estimation was 32 Mb. MUC4, RP1L1 and QRICH2 mutations were identified in at least three tumors, and these mutation also existed in breast invasive carcinoma databases (TCGA, Cell 2015; TCGA, Nature 2012). The results showed that there were many genes in breast MEC overlapping with the breast invasive carcinoma databases mentioned above, range from 5 to 63 genes (median:21 genes). Next, we assessed immune cell infiltration levels in these tumors. In all these tumors, M2 macrophages and plasma cell were in the high infiltration group. Our breast MEC showed different results from the salivary gland MEC, whose plasma cells were in the low infiltration group. Overall, we first analyzed the genomics and tumor microenvironment of breast mucoepidermoid carcinoma and proposed our hypothesis that although MECs arising in the breast resemble their salivary gland counterparts phenotypically, our findings indicate that breast MECs probably resemble invasive breast carcinomas at the genetic level and immune cell infiltration levels. More cases and in deep research need to be done to further understand this rare carcinoma.

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.

Genomics of adaptive evolution in the woolly mammoth

Ancient genomes provide a tool to investigate the genetic basis of adaptations in extinct organisms. However, the identification of species-specific fixed genetic variants requires the analysis of genomes from multiple individuals. Moreover, the long-term scale of adaptive evolution coupled with the short-term nature of traditional time series data has made it difficult to assess when different adaptations evolved. Here, we analyze 23 woolly mammoth genomes, including one of the oldest known specimens at 700,000 years old, to identify fixed derived non-synonymous mutations unique to the species and to obtain estimates of when these mutations evolved. We find that at the time of its origin, the woolly mammoth had already acquired a broad spectrum of positively selected genes, including ones associated with hair and skin development, fat storage and metabolism, and immune system function. Our results also suggest that these phenotypes continued to evolve during the last 700,000 years, but through positive selection on different sets of genes. Finally, we also identify additional genes that underwent comparatively recent positive selection, including multiple genes related to skeletal morphology and body size, as well as one gene that may have contributed to the small ear size in Late Quaternary woolly mammoths.

Cellular and Molecular Mechanisms of Pathogenesis Underlying Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are congenital retinal degenerative diseases that have various inheritance patterns, including dominant, recessive, X-linked, and mitochondrial. These diseases are most often the result of defects in rod and/or cone photoreceptor and retinal pigment epithelium function, development, or both. The genes associated with these diseases, when mutated, produce altered protein products that have downstream effects in pathways critical to vision, including phototransduction, the visual cycle, photoreceptor development, cellular respiration, and retinal homeostasis. The aim of this manuscript is to provide a comprehensive review of the underlying molecular mechanisms of pathogenesis of IRDs by delving into many of the genes associated with IRD development, their protein products, and the pathways interrupted by genetic mutation.

Extensive set of African ancestry-informative markers (AIMs) to study ancestry and population health

Introduction: Human populations are often highly structured due to differences in genetic ancestry among groups, posing difficulties in associating genes with diseases. Ancestry-informative markers (AIMs) aid in the detection of population stratification and provide an alternative approach to map population-specific alleles to disease. Here, we identify and characterize a novel set of African AIMs that separate populations of African ancestry from other global populations including those of European ancestry. Methods: Using data from the 1000 Genomes Project, highly informative SNP markers from five African subpopulations were selected based on estimates of informativeness (In) and compared against the European population to generate a final set of 46,737 African ancestry-informative markers (AIMs). The AIMs identified were validated using an independent set and functionally annotated using tools like SIFT, PolyPhen. They were also investigated for representation of commonly used SNP arrays. Results: This set of African AIMs effectively separates populations of African ancestry from other global populations and further identifies substructure between populations of African ancestry. When a subset of these AIMs was studied in an independent dataset, they differentiated people who self-identify as African American or Black from those who identify their ancestry as primarily European. Most of the AIMs were found to be in their intergenic and intronic regions with only 0.6% in the coding regions of the genome. Most of the commonly used SNP array investigated contained less than 10% of the AIMs. Discussion: While several functional annotations of both coding and non-coding African AIMs are supported by the literature and linked these high-frequency African alleles to diseases in African populations, more effort is needed to map genes to diseases in these genetically diverse subpopulations. The relative dearth of these African AIMs on current genotyping platforms (the array with the highest fraction, llumina's Omni 5, harbors less than a quarter of AIMs), further demonstrates a greater need to better represent historically understudied populations.

Seroreactivity against retinal proteins in a case of POC1B gene associated cone dystrophy with normal funduscopic appearance: a systematic approach to diagnosis

PURPOSE

To report a case of cone dystrophy, associated with autosomal recessive homozygote POC1B gene variant, mimicking autoimmune retinopathy.

CASE

A 45-year-old female presented with a complaint of decreased vision in both eyes. Her best corrected visual acuity was 20/32 in the right eye and 20/50 in the left eye. Anterior segment and dilated fundus examinations were unremarkable. Spectral domain optical coherence tomography showed a subfoveal blurred dome-shaped ellipsoid zone and an extinguished interdigitation zone affecting the entire macula. Full field electroretinography revealed reduced cone responses. The differential diagnosis included inflammatory chorioretinopathies, autoimmune retinopathies (paraneoplastic or nonparaneoplastic), and hereditary retinal dystrophies. No remarkable finding was observed on combined fluorescein and indocyanine green angiographies. Paraneoplastic autoimmune antibody panel revealed nothing; however, aldolase, enolase, pyruvate kinase M2, and glyceraldehyde-3-phosphate dehydrogenase antibodies were positive on autoimmune retinopathy panel. To exclude hereditary retinal dystrophies, whole-exome sequencing (WES) was applied. WES identified an autosomal recessive homozygote POC1B gene variant (c.680A>G, p.His227Arg). Cone dystrophy diagnosis was given.

CONCLUSION

Cone dystrophy associated with POC1B gene variant may present without visible fundus abnormalities. It should be kept in mind that retinal autoantibodies may be positive in such a hereditary dystrophy case due to long-term exposure of the immune system to self-antigens. Therefore, autoimmune retinopathy is a diagnosis of exclusion and should not be diagnosed until all other causes, including hereditary dystrophies, have been ruled out.

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.

Genetic dissection of non-syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) belongs to a group of pigmentary retinopathies. It is the most common form of inherited retinal dystrophy, characterized by progressive degradation of photoreceptors that leads to nyctalopia, and ultimately, complete vision loss. RP is distinguished by the continuous retinal degeneration that progresses from the mid-periphery to the central and peripheral retina. RP was first described and named by Franciscus Cornelius Donders in the year 1857. It is one of the leading causes of bilateral blindness in adults, with an incidence of 1 in 3000 people worldwide. In this review, we are going to focus on the genetic heterogeneity of this disease, which is provided by various inheritance patterns, numerosity of variations and inter-/intra-familial variations based upon penetrance and expressivity. Although over 90 genes have been identified in RP patients, the genetic cause of approximately 50% of RP cases remains unknown. Heterogeneity of RP makes it an extremely complicated ocular impairment. It is so complicated that it is known as “fever of unknown origin”. For prognosis and proper management of the disease, it is necessary to understand its genetic heterogeneity so that each phenotype related to the various genetic variations could be treated.

Occult Macular Dystrophy: a case report and major review

BACKGROUND

Occult Macular Dystrophy (OMD), a rare autosomal dominant disorder caused by mutations in the retinitis pigmentosa 1-like protein 1 gene (RP1L1), is characterized by loss of central visual acuity in the absence of fundoscopic abnormalities. In patients suspected of having OMD based on unexplained central vision loss and/or photophobia, changes may be detected with spectral-domain optical coherence tomography. Subsequently, the diagnosis can be confirmed with genetic analysis.We report a case of an 18-year-old White male whose suspected diagnosis of OMD was confirmed by molecular testing. We conducted an extensive review of the literature of previously reported patients with OMD to date.

METHODS

A PubMed search of "RP1L1 and Occult Macular Dystrophy" revealed 34 papers. There were 225 individuals with genetically confirmed, symptomatic OMD; an additional 15 had a confirmed mutation but were asymptomatic and discovered incidentally.

RESULTS

Our patient presented with a 10-year history of unexplained loss of central visual acuity and photophobia. Genetic analysis confirmed the presence of a p.R45W substitution on the RP1L1 gene, the most common pathologic mutation in OMD.

CONCLUSIONS

Due to the lack of appreciable fundoscopic changes, correct identification of the disease can be difficult. Incomplete penetrance has been associated with the condition, and the age of onset is highly variable. Much of the research discussing OMD has come from Eastern Asia, but whether this is due to a heightened awareness and screening protocols, or increased incidence is unclear. Additional research and increased awareness globally will help with more timely and accurate diagnoses.

Multimodal imaging evaluation of occult macular dystrophy associated with a novel RP1L1 variant

Purpose

Occult Macular Dystrophy (OMD) is an autosomal dominant inherited retinal dystrophy caused by mutations in the retinitis pigmentosa 1-like 1 (RP1L1) gene. The present study describes a novel RP1L1 variant, identified for the first time in two Italian sisters diagnosed with OMD, along with multimodal imaging features, including Optical Coherence Tomography (OCT) Angiography.

Methods

We performed multimodal imaging including spectral-domain OCT, blue light autofluorescence (BAF), infrared autofluorescence (IRAF), swept-source OCT Angiography (OCTA), full-field and multifocal electroretinography. Genetic analysis was performed using Next-Generation Sequencing. Pathogenic potential of nonsynonymous novel variants was scored with two in silico algorithms.

Results

Proband 1 (P1) and proband 2 (P2) were two Italian sisters of 61 and 56 years old. Both reported a history of progressive visual loss without fundoscopic alterations. P1 reported a 4-year history of rapid visual function worsening, and her best-corrected visual acuity (BCVA) was counting fingers in both eyes. P2 reported a 20-year history of mild but progressive visual acuity loss, and her BCVA was 1/10 and 2/10 respectively in her right and left eye. Structural OCT displayed disorganization of outer retinal bands at the macula and foveal cavitation; loss of foveal photoreceptors was remarkably evident on en-face OCT slabs. OCTA quantitative analysis found that vessel density was reduced both at SCP and DCP while choriocapillaris blood flow was relatively spared. Genetic analysis found the same rare dominant c.2873G > C, p.Arg958Pro variant in the RP1L1 gene. The substitution was regarded as moderately radical according to Grantham score while PolyPhen2 classified the amino acidic substitution as probably damaging.

Conclusions and importance

Our study expands the mutational spectrum of RP1L1 gene: the rare c.2873G > C, p.Arg958Pro missense variant may be considered a new pathogenic variant for OMD, the first to be identified exclusively in an Italian family. Moreover, our quantitative OCTA data suggest that OMD is characterized by a rarefaction of superficial and deep capillary plexus.

Whole exome sequencing analysis identifies novel Stargardt disease-related gene mutations in Chinese Stargardt disease and retinitis pigmentosa patients

OBJECTIVES

To delineate the disease-causing mutations of the Stargardt disease-related genes in Chinese patients diagnosed with Stargardt disease or retinitis pigmentosa (RP) by whole exome sequencing analysis.

METHODS

A total of 123 sporadic RP or Stargardt disease patients and 2 Stargardt disease families were recruited. All sporadic patients and the probands of the families were subjected to whole exome sequencing analysis. The candidate mutations were verified by direct sequencing based on the cosegregation pattern and in 200 control subjects and by the bioinformatics analyses.

RESULTS

A total of three reported ABCA4 mutations were identified in the probands of the two Stargardt disease families. The probands and the affected family members with either homozygous or compound heterozygous mutations showed typical Stargardt disease features, which was absent in their unaffected family members. The cosegregation pattern confirmed the mode of recessive inheritance. Moreover, two sporadic Stargardt disease patients were identified to carry two novel ABCA4 and one PROM1 mutations. In addition, 13 novel variants were found in 119 sporadic RP patients in 7 Stargardt disease-related genes, and 8 novel missense variants were conserved across different species and predicted to be damaging to the protein. All 15 novel variants were absent in our 200 control subjects.

CONCLUSIONS

This study revealed 22.4% study subjects carrying Stargardt disease-related gene mutations with total 15 novel variants in seven Stargardt disease-related genes, assuring that targeted next-generation sequencing analysis is a high throughput strategy to facilitate the clinical diagnosis from suspicious patients and recommended as a routine examination for inherited retinal dystrophies.

An extended phenotype of RP1L1 maculopathy - case report

BACKGROUND

To report the ophthalmological findings of a new phenotypical variant of RP1L1 maculopathy in an Indian patient with a homozygous variant in the RP1L1 gene.

MATERIALS AND METHODS

A 39-year-old male presented with complaints of disturbance in the central field of vision in both eyes (BE) for a duration of 6 months. He underwent ophthalmic examinations and diagnostic imaging. A complete retinal degeneration panel consisting of 228 genes was evaluated for pathologic variations using next-generation sequencing (NGS), which showed a variant in the RP1L1 gene.

RESULTS

On fundus examination, he was found to have ill-defined foveal mottling in BE. Spectral domain optical coherence tomography (SD-OCT) showed sub-foveal hyper-reflective deposits and outer retinal layer disruption. A provisional diagnosis of the atypical variant of adult-onset foveomacular vitelliform dystrophy (AOFVD) was made on the basis of clinical, OCT, Fundus autofluorescence (FAF) and electrophysiological features. Genetic assessment of the proband revealed the presence of a homozygous base pair deletion in exon 4 of RP1L1 gene (chr8:g.10468194_10468195del), which results in frameshift and premature truncation of the protein 24 amino acids downstream to codon 1138 (p.Lys1138SerfsTer24). This variant was confirmed in the proband's parents by Sanger sequencing. The diagnosis was revised to RP1L1 maculopathy, as the RP1L1 gene variant is most commonly associated with this entity.

CONCLUSION

This report presents the multimodal imaging of a previously unreported phenotype of RP1L1 maculopathy associated with a genetic variant of RP1L1 gene, thereby expanding the spectrum associated with RP1L1 maculopathy.

Electrical response of retinal ganglion cells in an N-methyl-N-nitrosourea-induced retinal degeneration porcine model

Retinal prosthesis is regarded as the treatment for vision restoration in the blind with retinal degeneration (RD) due to the loss of photoreceptors. A strategy for retinal prosthesis is to electrically activate surviving neurons. The retina’s response to electrical stimulation in a larger RD model has not been studied yet. Therefore, in this study, we investigated electrically evoked retinal responses in a previously validated N-methyl-N-nitrosourea (MNU)-induced porcine RD model. Electrically evoked responses were evaluated based on the number of retinal ganglion cell (RGC) spikes via multichannel recordings. Stimulation pulses were applied to degenerative and wild-type retinas with pulse modulation. Compared to wild-type retinas, degenerative retinas showed higher threshold values of pulse amplitude and pulse duration. The rate of increase in the number of RGC spikes relative to stimulus intensity was lower in degenerative retinas than in normal retinas. In severely degenerated retinas, few RGCs showed electrically evoked spikes. Our results suggest that the degenerative porcine retina requires a higher charge than the normal porcine retina. In the early stage of RD, it is easier to induce RGC spikes through electrical stimulation using retinal prosthesis; however, when the degeneration is severe, there may be difficulty recovering patient vision.

A rare case of RGR/CDHR1 haplotype identified in Bulgarian patient with cone-rod dystrophy

AIM

To present a rare clinical case of CDHR1-related retinopathy with cone and rod involvementconfirmed clinically, electrophysiologically and genetically as a cone-rod dystrophy.

MATERIAL AND METHODS

A 26-year-old woman underwent detailed ophthalmic examinationincluding fundus photography, full-field and multifocal electroretinography, visual field testing, optical coherence tomography and fluorescein angiography, which established the clinical diagnosis. Next-generation sequencing of a custom panel including 140 of the most common genes for inherited retinal degenerations was used for mutation screening.

RESULTS

The symptoms onset was two years ago included gradual loss of vision and photophobia. The clinical findings were reduced visual acuity, central and peripheral scotomas, sporadic pigmentary cells localized mainly in the peripheral retina, a thinner retina in the macula and peripherally, moderate retinal vessels attenuation and reduced cone and rod ERG responses. The genetic analysisfound that the patient was homozygous for two already reported mutations: RGR-c.196A>C (p.Ser66Arg) variant and a co-segregating frame-shift deletion in CDHR1-c.2522_2528delTCTCTGA (p.Ile841Serfs119*). Segregation analysis showed that the two mutations were transmitted by the asymptomatic heterozygous parents.

CONCLUSION

The rare haplotype of RGR mutation co-segregating incis- with CDHR1 mutation in our patient has been previously described in Albanian patients with recessive retinal dystrophy. Our findings add further support to the hypothesis of a common ancestral haplotype spread in the Balkan population. The comprehensive clinical, electrophysiological and genetic testing of patients with rare hereditary retinal dystrophies is essential for the correct diagnosis and the choice of potential novel therapies.

On the Wrong Track: Alterations of Ciliary Transport in Inherited Retinal Dystrophies

Ciliopathies are a group of heterogeneous inherited disorders associated with dysfunction of the cilium, a ubiquitous microtubule-based organelle involved in a broad range of cellular functions. Most ciliopathies are syndromic, since several organs whose cells produce a cilium, such as the retina, cochlea or kidney, are affected by mutations in ciliary-related genes. In the retina, photoreceptor cells present a highly specialized neurosensory cilium, the outer segment, stacked with membranous disks where photoreception and phototransduction occurs. The daily renewal of the more distal disks is a unique characteristic of photoreceptor outer segments, resulting in an elevated protein demand. All components necessary for outer segment formation, maintenance and function have to be transported from the photoreceptor inner segment, where synthesis occurs, to the cilium. Therefore, efficient transport of selected proteins is critical for photoreceptor ciliogenesis and function, and any alteration in either cargo delivery to the cilium or intraciliary trafficking compromises photoreceptor survival and leads to retinal degeneration. To date, mutations in more than 100 ciliary genes have been associated with retinal dystrophies, accounting for almost 25% of these inherited rare diseases. Interestingly, not all mutations in ciliary genes that cause retinal degeneration are also involved in pleiotropic pathologies in other ciliated organs. Depending on the mutation, the same gene can cause syndromic or non-syndromic retinopathies, thus emphasizing the highly refined specialization of the photoreceptor neurosensory cilia, and raising the possibility of photoreceptor-specific molecular mechanisms underlying common ciliary functions such as ciliary transport. In this review, we will focus on ciliary transport in photoreceptor cells and discuss the molecular complexity underpinning retinal ciliopathies, with a special emphasis on ciliary genes that, when mutated, cause either syndromic or non-syndromic retinal ciliopathies.

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.

Progressive Photoreceptor Dysfunction and Age-Related Macular Degeneration-Like Features in rp1l1 Mutant Zebrafish

Photoreceptor disease results in irreparable vision loss and blindness, which has a dramatic impact on quality of life. Pathogenic mutations in RP1L1 lead to photoreceptor degenerations such as occult macular dystrophy and retinitis pigmentosa. RP1L1 is a component of the photoreceptor axoneme, the backbone structure of the photoreceptor's light-sensing outer segment. We generated an rp1l1 zebrafish mutant using CRISPR/Cas9 genome editing. Mutant animals had progressive photoreceptor functional defects as determined by electrophysiological assessment. Optical coherence tomography showed gaps in the photoreceptor layer, disrupted photoreceptor mosaics, and thinner retinas. Mutant retinas had disorganized photoreceptor outer segments and lipid-rich subretinal drusenoid deposits between the photoreceptors and retinal pigment epithelium. Our mutant is a novel model of RP1L1-associated photoreceptor disease and the first zebrafish model of photoreceptor degeneration with reported subretinal drusenoid deposits, a feature of age-related macular degeneration.