Connecting cilium, stress response, and proteostasis abnormalities inform variant and therapy assessment in RPGRIP1 retinal organoids.

RPGRIP1 encodes a connecting cilium (CC) protein essential for normal photoreceptor cell development and maintenance. Damaging variants in RPGRIP1 cause severe inherited retinal disease (IRD) and currently incurable vision loss, with mouse studies showing promising preclinical gene augmentation therapy results. Almost one-half of variants in RPGRIP1 in the ClinVar database are variants of uncertain significance (VUS), hindering genetic diagnosis for affected individuals and, hence, access to clinical trials of novel therapies and other management options. Here, we use human induced pluripotent stem cell (iPSC)-derived retinal organoids to model RPGRIP1-associated IRD, detecting biomarkers of disease including CC interactome dysfunction, stress response, and proteostasis abnormalities. In parallel, utilizing these novel disease biomarkers, we demonstrate the pathogenicity of a missense VUS, RPGRIP1 c.2108T>C p.(Ile703Thr). In addition, RPGRIP1 gene augmentation therapy rescued disease phenotypes, further supporting the utility of these biomarkers of RPGRIP1-IRD for reclassifying VUS and testing response to therapy.