Inherited Retinal Dystrophies in Spain: three decades of epidemiological, clinical, and genetic study

Inherited Retinal Dystrophies (IRDs) are a group of rare diseases with a prevalence of 1:3000-4000 people. They are genetic, primarily affecting retinal photoreceptors and epithelial pigmentary cells, and lead to neurodegeneration and finally apoptosis. In 2021, we published our global results obtained in our registry at the Fundación Jiménez Díaz University Hospital (Madrid, Spain) from 1991 to August 2019. Now, we aimed to update these results until August 2022. Thus, we conducted a retrospective hospital-based cross-sectional study on 4.794 IRD-affected unrelated families from all the Spanish autonomous communities. Families were classified into three phenotypic categories: a) “NON-RP” for cone-dominated phenotypes, b) “RP” (retinitis pigmentosa) for primary rod involvement, and c) “syndromic IRD” when visual plus extra-ocular symptoms are present. Molecular studies included: single-gene studies, clinical exome, whole exome or whole genome sequencing. Overall, 62% (2962/4794) of the families were genetically characterized, in which 1.997 different likely causative variants (5.064 different alleles) were identified in 188 genes. The most common phenotype encountered was RP (59% of families, 2465/4794). Regarding the types of causative alleles, missenses were the most frequent (51%), followed by truncating (nonsense, frameshift, indels and splicing; 44%), while copy number variations were only 2%. The most recurrently mutated genes were PRPH2, ABCA4 and RS1 in autosomal dominant (AD), autosomal recessive (AR) and X-linked (XL) NON-RP families, respectively; RHO, USH2A and RPGR in AD, AR and XL for non-syndromic RP; and MYO7A, USH2A and BBS1 in syndromic IRD. Pathogenic variants ABCA4:p.Arg1129Leu and USH2A:p.Cys759Phe were the most frequent. Our study provides the overall genetic landscape for IRD in Spain, reporting the largest cohort ever presented and a high number of causal genes involved in these diseases. Furthermore, our findings have important implications for genetic diagnosis and counseling, but also for possible therapeutic management