A pan-cancer analysis of RGR opsin expression and its downregulation associated with poor prognosis in glioma

Retinal G protein-coupled receptor (RGR) serves a retinal photoisomerase function to mediate retinoid metabolism and visual chromophore regeneration in the human eyes. Retinoids display critical functions in cell proliferation, differentiation, and apoptosis. Abnormal retinoid metabolism may contribute to tumor development. However, in human tumor tissues, the expression of RGR remains uncharacterized. Herein, we performed the analysis of RGR expression in 620 samples from 24 types of tumors by immunohistochemistry (IHC) and 33 cancer types from the Cancer Genome Atlas (TCGA), the Chinese Glioma Genome Atlas (CGGA), and Gene Expression Omnibus (GEO) databases by bioinformatic analyses. Furthermore, the biological role of RGR in glioma cells was investigated using molecular biology approaches in vitro. Notably, we found that brain lower grade glioma (LGG), in contrast to other tumor types, had the highest median score of IHC and RNA level of RGR expression. Survival analysis showed that low RGR expression was associated with worse overall survival in LGG (p<0.0001). RGR expression levels in glioma were also associated with pathological subtypes, grades, and isocitrate dehydrogenase (IDH) mutations. Moreover, its molecular function was closely associated with cadherin-related family member 1 (CDHR1), a tumor suppressive protein in glioma, suggesting that RGR might negatively regulate the tumorigenesis and progression of LGG through interacting with CDHR1. Our findings provide new insight into the role of RGR in human cancer, especially in glioma.