SAT027 Novel Obesity-Associated Melanocortin 4 Receptor Variants Modulate Signaling Differently
Authors: Rodriguez Rondon Alejandra V , Welling Mila Sofie , van den Akker Erica L T , van Rossum Elisabeth F C , Delhanty Patric John , Visser Jenny A 
Published: October 2023

Abstract
Disclosure: A.V. Rodriguez Rondon: None. M.S. Welling: None. E.L. van den Akker: None. E.F. van Rossum: None. P.J. Delhanty: None. J.A. Visser: None.
Melanocortin-4 receptor (MC4R) is a G-protein-coupled receptor expressed in the hypothalamus regulating appetite and energy expenditure. Extensive evidence from genetic and biological studies show that MC4R is a key player in the homeostatic regulation of body weight. MC4R loss of function (LoF) variants are the most common cause of monogenic obesity. We have identified 1 homozygous and 16 heterozygous MC4R variants in patients with obesity at our academic Obesity Center CGG, Erasmus MC. The impact of these variants on MC4R signaling and the association with obesity are unknown. Here, we functionally characterized these variants by analyzing four different aspects of MC4R signaling. HEK293 cells were transiently transfected with expression plasmids encoding wild type (WT) or variant MC4R and stimulated with melanocyte stimulating hormone. Cyclic AMP response (cAMP) was measured using GloSensor cAMP bioluminescence assay, and β-arrestin-2 recruitment (βarr2) was measured using NanoBiT complementation luminescence assay. Furthermore, HiBiT Detection System was used to measure cell surface expression, and serum response element (SRE) reporter assay was used to measure extracellular signal-regulated kinase (ERK) activation. Variants were classified according to the percent of maximum response (Emax) relative to WT: complete LoF (cLoF): 0-20%; partial LoF (pLoF): 20-80%; WT: 80-120%; gain of function (GoF): &gt;120%. The MC4R variants were identified by sequencing the MC4R gene of adult and pediatric patients with obesity using an obesity gene panel. Eleven of these variants have not been functionally studied previously. The median age of onset obesity for all patients is 3 years (range 0 – 6). Furthermore, the median BMI of the adult patients was 48.8 kg/m2 (range 36 - 58.8) and the median BMI-SDS of the pediatric patients was +4.2 SDS (range +3 – 7). Twelve out of 17 patients presented with hyperphagia. MC4R variants had differential effects on signaling. In 7 out of 17 variants, cLoF for cAMP was observed. This could be explained in 6 out of 7 of these variants due to lack of cell surface expression. Intriguingly, five out of 17 variants had a normal or GoF cAMP response and normal ERK activation. However, three out of these 5 variants showed cLoF or pLoF for βarr2, indicating a disrupted desensitization mechanism. In addition, the remaining five variants displayed pLoF for most studied pathways. We show that these MC4R variants identified in our patients with obesity affect signaling differently, through modulation of cAMP, βarr2, cell surface expression, and ERK pathways. This demonstrates the value of examining different aspects of MC4R signaling to understand possible biased effects of variants on these pathways. Overall, our results show the clinical importance of assessing the effect of MC4R variants on a range of molecular signaling mechanisms to determine their association with obesity.
Presentation: Saturday, June 17, 2023