A Novel Loss of Function Melanocortin-4-Receptor Mutation (MC4R-F313Sfs*29) in Morbid Obesity

CONTEXT: Melanocortin receptor-4 (MC4R) gene mutations are associated with early-onset severe obesity and the identification of potential pathological variants is crucial for the clinical management of patients with obesity.OBJECTIVE: To explore whether and how a novel heterozygous MC4R variant (MC4R-F313Sfs*29), identified in a young boy (BMI 38.8kg/m 2) during a mutation analysis conducted in a cohort of patients with obesity, plays a determinant pathophysiological role in the obesity development.DESIGN SETTING AND PATIENTS: The genetic screening was carried out in a total of 209 unrelated patients with obesity (BMI ≥ 35 kg/m 2). Structural and functional characterization of the F313Sfs*29-mutated MC4R was performed using computational approaches and in vitro, using HEK293 cells transfected with genetically-encoded biosensors for cAMP and Ca 2+.RESULTS: The F313Sfs*29 was the only variant identified. In vitro experiments showed that HEK293 cells transfected with the mutated form of MC4R did not increase intracellular cAMP or Ca 2+ levels after the stimulation with a specific agonist in comparison with HEK293 cells transfected with the wild type form of MC4R (∆R/R0= -90%±8%; p<0,001). In silico modelling showed that the F313Sfs*29 mutation causes a major reorganization cytosolic domain of the MC4R, thus reducing the affinity of the putative GalphaS binding site.CONCLUSIONS: The newly discovered F313Sfs*29 variant of MC4R may be involved in the impairment of alpha-MSH-induced cAMP and Ca 2+ signaling, blunting intracellular G protein mediated signal transduction. This alteration might have led to the dysregulation of satiety signaling, resulting in hyperphagia and early onset of obesity.