SUN-LB107 Functional Characterisation of Human Heterozygous Non-Synonymous MC3R Variants

Background: Melanocortin 3 receptor (MC3R) is a member of the melanocortin family of G-protein coupled receptors predominantly expressed in hypothalamic tissue. Rodent models have implicated MC3R in the pathogenesis of obesity, however in humans the relationship between obesity phenotypes and impaired MC3R function is less well established than that observed for loss of function mutations in the homologous MC4R. Aim: Recently three heterozygous MC3R variants identified from candidate gene sequencing of an obese human cohort were reported to be pathogenic. We sought to functionally characterise the these MC3R non-synonymous variants (p.Q11X, p.I50T and p.A149V) in vitro and examine their associations to body mass index in an unselected population-based cohort. Methodology: Functional characterisation of each variant was undertaken in HEK293 cells transiently overexpressing either wild-type MC3R or the respective MC3R mutant where cAMP-dependant luciferase activity in response to alpha-melanocyte stimulating hormone (α-MSH) was measured. Body mass index (BMI) was compared between heterozygous carriers of the MC3R variants of interest and control participants (matched for age and sex and ethnicity) identified within the UK Biobank whole exome sequencing dataset. Results: Impairment of the canonical MC3R cAMP signalling response to α-MSH was observed for both p.Q11X and p.A149V MC3R variants when compared to wild-type MC3R receptor function in vitro. In contrast the cAMP signalling response of MC3R p.I50T to α-MSH was non-inferior to wild-type. Thirty-nine (male=18) heterozygous carriers of MC3R p.I50T were identified in the UK Biobank whole exome cohort. There was no statistical difference in median (IQR) BMI for female carriers 27.1(7.8) kg/m(2) vs. matched female control participants 26.2(5.5) kg/m(2) or male carriers 27.7(5.4) kg/m(2) vs. matched male control participants 27.4(4.9) kg/m(2). A single participant heterozygous for the MC3R p.Q11X variant (BMI= 28.3 kg/m(2)) and two participants heterozygous for MC3R p.A149V (BMI= 24.7 and 25.1 kg/m(2) respectively) variant were identified in the UK Biobank whole exome cohort. BMI did not significantly differ from the match control population median for either of these variants. Conclusion: In vitro assessment and phenotype correlates suggest that MC3R p.I50T is not an obesity causing mutation. Despite evidence of impaired receptor function in vitro, MC3R p.Q11X and p.A149V did not associate with obesity in this unselected population and further study are required to elucidate their pathogenicity in humans.