A human-specific RPGR isoform and a clinically approved Rho/ROCK inhibitor ameliorate defects associated with RPGR...

Pathogenic variants in the RPGR gene are the primary cause of photoreceptor degeneration in X-linked retinitis pigmentosa (RP). Previous studies have linked RPGR dysfunction to defects in ciliary structure and actin turnover. RPGR encodes three major isoforms-RPGR ^1-19 , RPGR ^ORF15 , and the human-specific RPGR ^s14/15 -yet the function of RPGR ^s14/15 remains poorly understood. There is an urgent unmet need for effective treatments targeting RPGR-associated RP. We generated RPGR mutant hTERT-RPE1 cell lines and found that the loss of all RPGR isoforms resulted in pronounced ciliary defects, including aberrant cilia length and segmentation, along with disrupted actin turnover. Strikingly, cells expressing only the human-specific isoform RPGR ^s14/15 closely resembled wild-type controls and were largely protected from these defects, underscoring a critical role for RPGR ^s14/15 in maintaining ciliary integrity and actin dynamics. Furthermore, we demonstrated that pharmacological disruption of actin polymerization with cytochalasin D (CytoD) in control cells mimicked the ciliary abnormalities seen in RPGR_KO cells. Conversely, treatment with ripasudil-a clinically approved Rho/ROCK inhibitor-rescued both ciliary and actin-related defects in RPGR_KO lines without observable cellular side effects. In summary, our findings highlight the therapeutic relevance of the human-specific isoform RPGR ^s14/15 and identify ripasudil as a promising candidate for treating RPGR-associated retinal degeneration.