Inherited retinal diseases in Norway - Studies on phenotype and genotype characteristics

Inherited retinal diseases are a leading cause of vision loss in children and young adults. The majority of inherited retinal diseases are untreatable, consequently leading to irreversible retinal damage and blindness. Recent advances in the understanding of the underlying molecular causes, has increased knowledge of the diseases and facilitated the development of the first gene therapy for an inherited retinal disease.1 Inherited retinal diseases are a heterogeneous group of diseases, consisting of approximately 60 clinical diagnoses,2 caused by monogenetic variants in over 270 genes.3 Consequently, the clinical expression and visual prognosis of inherited retinal diseases varies significantly. Few studies on the inherited retinal disease population in Norway have been performed, therefore increased knowledge of this patient population was warranted. The aim of this thesis was to increase knowledge of inherited retinal diseases in Norway by estimating prevalence and providing a detailed analysis of the clinical expressions (phenotypes) and the molecular causes (genotypes) of inherited retinal disease in Norway. To achieve this aim, a patient registry of inherited retinal diseases was established, including detailed clinical and genetic data of 900 patients. A population based study was performed by utilising phenotype and genotype data from the registry. Prevalence of inherited retinal disease in the South-East region of Norway was estimated with a minimum prevalence of 1:3,856, meaning that an estimated minimum of 1,400 patients in Norway live with an inherited retinal disease. The clinical spectrum demonstrated heterogeneity, with over 40 clinical diagnoses registered. Non-syndromic retinitis pigmentosa was the most common clinical diagnosis and was further observed as the most genetically heterogenic diagnosis, associated with more than 20 genes. In contrast, the prevalence of patients with clinical diagnosis choroideremia was low, in comparison to other countries.4-10 The diagnostic yield of routine genetic testing from 2007 to 2018 was low (32%), compared to other genetic studies of inherited retinal diseases.2, 5-9, 11-18 The most common genes found in the study population were ABCA4, USH2A, BEST1, RHO and RS1. Additional descriptive studies were performed on the clinical subgroups autosomal dominant retinitis pigmentosa and ABCA4-retinal dystrophies. In total 15 patients with autosomal dominant retinitis pigmentosa were analysed with gene panels that resulted in a genetic diagnosis in 11 of 15 patients (diagnostic yield 73%). The genetic cause was found in 10 genes, one of which was the ARL3 gene. The ARL3 gene had previously been suspected of causing autosomal dominant retinitis pigmentosa. The study was able to confirm the phenotype association to the ARL3 p.(Tyr90Cys) variant, resulting in a change in the phenotype classification of ARL3. The phenotype was given the name retinitis pigmentosa type 83 [MIM# 618173]. The study of ABCA4-retinal dystrophies was a national collaboration, including in total 106 patients. The study demonstrated that the clinical diagnosis of Stargardt disease did not correlate well with the prognosis for development of peripheral degeneration in the retina. A phenotype-genotype subclassification system was implemented for classification of long-term prognosis based on the clinical expression, disease duration and the underlying genetic variants. Further, the study demonstrated a high carrier frequency of the loss of function splice variant c.5461-10T>C in the Norwegian population, compared to other studies of the ABCA4 gene.19 A genetic diagnostic yield of 89% was established, which was in the high range compared to other ABCA4 studies.20-23 The diagnostic yield was achieved by facilitating a tiered genetic analysis, including whole-gene sequencing of the ABCA4 gene. In total 84 variants were detected, of those 15 were novel, including a novel deletion and several novel rare deep intronic variants. This thesis describes the first ever population based study that includes all inherited retinal disease diagnoses in Norway. The studies significantly increase our knowledge of this patient population and ensures knowledge required for updated clinical recommendations for genetic diagnostics, clinical follow-up as well as improved prognostic guidance

Dominant ARL3-related retinitis pigmentosa

Purpose: To clinically and genetically characterise a second family with dominant ARL3-related retinitis pigmentosa due to a specific ARL3 missense variant, p.(Tyr90Cys). Methods: Clinical examination included optical coherence tomography, electroretinography, and ultra-wide field retinal imaging with autofluorescence. Retrospective data were collected from the registry of inherited retinal diseases at Oslo university hospital. DNA was analysed by whole-exome sequencing and Sanger sequencing. The ARL3 missense variant was visualized in a 3D-protein structure. Results: The phenotype was non-syndromic retinitis pigmentosa with cataract associated with early onset of decreased central vision and central retinal thinning. Sanger sequencing confirmed the presence of a de novo ARL3 missense variant p.(Tyr90Cys) in the index patient and his affected son. We did not find any other cases with rare ARL3 variants in a cohort of 431 patients with retinitis pigmentosa-like disease. By visualizing Tyr90 in the 3D protein structure, it seems to play an important role in packing of the α/β structure of ADP-ribosylation factor-like 3 (ARL3). When changing Tyr90 to cysteine, we observe a loss of interactions in the core of the α/β structure that is likely to affect folding and stability of ARL3. Conclusion: Our study confirms that the ARL3 missense variant p.(Tyr90Cys) causes retinitis pigmentosa. In 2016, Strom et al. reported the exact same variant in a mother and two children with RP, labelled ?RP83 in the OMIM database. Now the questionmark can be removed, and ARL3 should be added to the list of genes that may cause non-syndromic dominant retinitis pigmentosa

Inherited retinal disease in Norway - a characterization of current clinical and genetic knowledge

Purpose The purpose of this study was to characterize current clinical and genetic knowledge of patients with inherited retinal disease in Norway and give an estimate of the prevalence. These data are necessary to identify patients eligible for new personalized medicines, to facilitate genetic counselling for their families and to plan clinical follow‐up. Methods A patient registry including clinical and genetic data was established. Clinical data were retrieved during 2003–2018. Genetic testing was performed in the period 2007–2018. Results The material included 866 patients with 41 clinical diagnoses at the cut‐off date. The most prevalent diseases were as follows: retinitis pigmentosa (54%), Stargardt macular dystrophy (6.5%) and Leber congenital amaurosis (5.2%). A genetic diagnosis was identified in 32% of patients. In total, 207 disease‐causing variants in 56 genes were reported. The most commonly reported disease‐causing genes were ABCA4, USH2A and BEST1. The estimated adjusted minimum prevalence of inherited retinal disease in the south‐east region of Norway was 1: 3,856 (2.6/10 000). Conclusion This population‐based study demonstrated an estimated prevalence for all inherited retinal diseases in south‐east Norway and described the distribution of clinical diagnoses, onset of symptoms, inheritance patterns and genetic data and thereby expands our knowledge of inherited retinal disease in Norway. The newly established registry and biobank will support patient feasibility for future clinical trials, treatment selection and counselling of families

Dominant ARL3-related retinitis pigmentosa

Purpose: To clinically and genetically characterise a second family with dominant ARL3-related retinitis pigmentosa due to a specific ARL3 missense variant, p.(Tyr90Cys). Methods: Clinical examination included optical coherence tomography, electroretinography, and ultra-wide field retinal imaging with autofluorescence. Retrospective data were collected from the registry of inherited retinal diseases at Oslo university hospital. DNA was analysed by whole-exome sequencing and Sanger sequencing. The ARL3 missense variant was visualized in a 3D-protein structure. Results: The phenotype was non-syndromic retinitis pigmentosa with cataract associated with early onset of decreased central vision and central retinal thinning. Sanger sequencing confirmed the presence of a de novo ARL3 missense variant p.(Tyr90Cys) in the index patient and his affected son. We did not find any other cases with rare ARL3 variants in a cohort of 431 patients with retinitis pigmentosa-like disease. By visualizing Tyr90 in the 3D protein structure, it seems to play an important role in packing of the α/β structure of ADP-ribosylation factor-like 3 (ARL3). When changing Tyr90 to cysteine, we observe a loss of interactions in the core of the α/β structure that is likely to affect folding and stability of ARL3. Conclusion: Our study confirms that the ARL3 missense variant p.(Tyr90Cys) causes retinitis pigmentosa. In 2016, Strom et al. reported the exact same variant in a mother and two children with RP, labelled ?RP83 in the OMIM database. Now the questionmark can be removed, and ARL3 should be added to the list of genes that may cause non-syndromic dominant retinitis pigmentosa