MC4R Variants Modulate α-MSH and Setmelanotide Induced Cellular Signaling at Multiple Levels

Context: The melanocortin-4 receptor (MC4R) plays an important role in body weight regulation. Pathogenic MC4R variants are the most common cause of monogenic obesity. Objective: We have identified 17 MC4R variants in adult and pediatric patients with obesity. Here we aimed to functionally characterize these variants by analyzing 4 different aspects of MC4R signaling. In addition, we aimed to analyze the effect of setmelanotide, a potent MC4R agonist, on these MC4R variants. Materials and Methods: Cell surface expression and alpha-melanocyte stimulating hormone (alpha-MSH)- or setmelanotide-induced cAMP response, beta-arrestin-2 recruitment, and ERK activation were measured in cells expressing either wild type or variant MC4R. Results: We found a large heterogeneity in the function of these variants. We identified variants with a loss of response for all studied MC4R signaling, variants with no cAMP accumulation or ERK activation but normal beta-arrestin-2 recruitment, and variants with normal cAMP accumulation and ERK activation but decreased beta-arrestin-2 recruitment, indicating disrupted desensitization and signaling mechanisms. Setmelanotide displayed a greater potency and similar efficacy as alpha-MSH and induced significantly increased maximal cAMP responses of several variants compared to alpha-MSH. Despite the heterogeneity in functional response, there was no apparent difference in the obesity phenotype in our patients. Conclusion: We show that these obesity-associated MC4R variants affect MC4R signaling differently yet lead to a comparable clinical phenotype. Our results demonstrate the clinical importance of assessing the effect of MC4R variants on a range of molecular signaling mechanisms to determine their association with obesity, which may aid in improving personalized treatment.