Clinical Characteristics of POC1B-Associated Retinopathy and Assignment of Pathogenicity to Novel Deep Intronic and Non-Canonical Splice Site Variants

Mutations in POC1B are a rare cause of inherited retinal degeneration. In this study, we present a thorough phenotypic and genotypic characterization of three individuals harboring putatively pathogenic variants in the POC1B gene. All patients displayed a similar, slowly progressive retinopathy (cone dystrophy or cone-rod dystrophy) with normal funduscopy but disrupted outer retinal layers on optical coherence tomography and variable age of onset. Other symptoms were decreased visual acuity and photophobia. Whole genome sequencing revealed a novel homozygous frameshift variant in one patient. Another patient was shown to harbor a novel deep intronic variant in compound heterozygous state with a previously reported canonical splice site variant. The third patient showed a novel nonsense variant and a novel non-canonical splice site variant. We aimed to validate the effect of the deep intronic variant and the non-canonical splice site variant by means of in vitro splice assays. In addition, direct RNA analysis was performed in one patient. Splicing analysis revealed that the non-canonical splice site variant c.561-3T>C leads to exon skipping while the novel deep intronic variant c.1033-327T>A causes pseudoexon activation. Our data expand the genetic landscape of POC1B mutations and confirm the benefit of genome sequencing in combination with downstream functional validation using minigene assays for the analysis of putative splice variants. In addition, we provide clinical multimodal phenotyping of the affected individuals.