Suzanne de Bruijn

78 Chapter 2 ABSTRACT Retinitis pigmentosa (RP) shows substantial genetic heterogeneity. It has been estimated that in approximately 60-80% of RP cases the genetic diagnosis can be found using whole exome sequencing (WES). In this study, the purpose was to identify causative variants in individuals with genetically unexplained retinal disease, which included one consanguineous family with two affected siblings and one case with RP. To identify the geneticdefect,WESwasperformed inbothprobands andclinical analysiswasperformed. To obtain insight into the function of KIAA1549 in photoreceptors, mRNA expression, knockdown and protein localization studies were performed. Through analysis of WES data, based on population allele frequencies, and in silico prediction tools we identified a homozygous missense variant and a homozygous frameshift variant in KIAA1549 that segregate in two unrelated families. KIAA1549 was found to localize at the connecting cilium of the photoreceptor cells and the synapses of the mouse retina. Both variants affect the long transcript of KIAA1549 which encodes a 1950-amino acid protein and shows prominent brain expression. The shorter transcript encodes a 734-amino acid protein with a high retinal expression and is affected by the identified missense variant. Strikingly, knockdown of the long transcript also leads to decreased expression of the short transcript likely explaining the nonsyndromic retinal phenotype caused by the two variants targeting different transcripts. In conclusion, our results underscore the causality of segregating variants in KIAA1549 for autosomal recessive RP. Moreover, our data indicate that KIAA1549 plays a role in photoreceptor function.