RPGR isoform imbalance causes ciliary defects due to exon ORF15 mutations in X-linked retinitis pigmentosa (XLRP). (23rd December 2020) - Moreno-Leon, Laura
Abstract: Mutations in retinitis pigmentosa GTPase regulator ( RPGR ) cause severe retinal ciliopathy, X-linked retinitis pigmentosa. Although two major alternatively spliced isoforms, RPGR ex1-19 and RPGR ORF15, are expressed, the relative importance of these isoforms in disease pathogenesis is unclear. Here, we analyzed fibroblast samples from eight patients and found that all of them form longer cilia than normal controls, albeit to different degrees. Although all mutant RPGR ORF15 messenger RNAs (mRNAs) are unstable, their steady-state levels were similar or higher than those in the control cells, suggesting there may be increased transcription. Three of the fibroblasts that had higher levels of mutant RPGR ORF15 mRNA also exhibited significantly higher levels of RPGR ex1-19 mRNA. Four samples with unaltered RPGR ex1-19 levels carried mutations in RPGR ORF15 that resulted in this isoform being relatively less stable. Thus, in all cases, the RPGR ex1-19 / RPGR ORF15 isoform ratio was increased, and this was highly correlative to the cilia extension defect. Moreover, overexpression of RPGR ex1-19 (mimicking the increase in RPGR ex1-19 to RPGR ORF15 isoform ratio) or RPGR ORF15 (mimicking reduction of the ratio) resulted in significantly longer or shorter cilia, respectively. Notably, the cilia length defect appears to be attributable to both the loss of the wild-type RPGR ORF15 protein and to the higher levels of the RPGR ex1-19 isoform, indicating that the observed defect is due to the altered isoform ratios. These results suggest that maintaining the optimal RPGR ex1-9 to RPGR ORF15 ratio is critical for cilia growth and that designing strategies that focus on the best ways to restore the RPGR ex1-19 / RPGR ORF15 ratio may lead to better therapeutic outcomes.