RP9 revisited; RP9 p.(H137L) remains a likely cause of dominant splicing factor-Retinitis Pigmentosa

Variants in six pre-mRNA processing factors cause autosomal dominant Retinitis Pigmentosa (adRP). The RP9 gene encodes a seventh splicing factor, and in 2002, we published RP9 variants c.410A>T; p.(H137L) and c.509A>G; p.(D170G) as likely causes of adRP in a large multigenerational RP9-linked family and a single case, respectively. It has since been suggested these variants might be artefacts due to simultaneous amplification of the RP9P pseudogene, and no further pathogenic variants have been reported. We therefore rescreened two members of the RP9-linked family by genome sequencing. Examination of the 2 Mb locus defined by crossovers in the original family revealed no other plausible causative variants. Alignment of both short and long-read sequences confirmed that p.(H137L) is in the RP9 gene, not the pseudogene. Screening for p.(H137L) in 1961 RP/Rod-cone dystrophy (RCD) cases from the Leeds patient cohort and UK 100,000 Genomes Project (100kGP) database revealed four further carriers. Including the original family, this variant was therefore present in 5/1962 RP/RCD probands, and is absent from gnomAD, constituting statistically significant enrichment in RP cases. Long-read sequencing of p.(H137L) in available carriers showed this is a UK founder allele. The RP9 p.(D170G) allele was also confirmed as gene, not pseudogene, derived, but is present in 22 individuals in the 100kGP cohort, none with RP, as well as >200 individuals in gnomAD and Biobank, suggesting it is non-pathogenic. In conclusion, RP9 p.(H137L) is strongly associated with RP and remains the only plausible variant accounting for the condition in a large multi-generation adRP family.