Rare variants in the melanocortin 4 receptor gene (MC4R) are associated with abdominal fat and insulin resistance in youth with obesity

Abstract

Background: Rare variants in melanocortin 4 receptor gene (MC4R) result in a severe form of early-onset obesity; however, it is unclear how these variants may affect abdominal fat distribution, intrahepatic fat accumulation, and related metabolic sequelae.

Methods: Eight hundred seventy-seven youth (6-21 years) with overweight/obesity, recruited from the Yale Pediatric Obesity Clinic in New Haven, CT, underwent genetic analysis to screen for functionally damaging, rare variants (MAF < 0.01) in MC4R. Participants were assigned to a Pathogenic Variant or No Pathogenic Variant group and completed a 10-timepoint 180-min oral glucose tolerance test (OGTT) and abdominal MRI.

Results: Compared to the No Pathogenic Variant group, the Pathogenic Variant group demonstrated significantly greater glucose concentrations (AUC_tot: 24.7 ± 1.22 g/dL × 180 min vs. 21.9 ± 1.41 g/dL × 180 min; p = 0.001), insulin levels (AUC_tot: 57.4 ± 11.5 mU/mL × 180 min vs. 35.5 ± 8.90 mU/mL × 180 min; p = 0.002), and lower insulin sensitivity (WBISI: 1.01 ± 0.137 vs. 1.85 ± 0.036; p = 0.0008) during the OGTT. The Pathogenic Variant group also presented with greater visceral adipose tissue (VAT) (85.1 cm² ± 10.3 vs. 56.1 cm² ± 1.64; p = 0.003) and intrahepatic fat content (HFF%) (19.4% ± 4.94 vs. 8.21% ± 0.495; p = 0.012) than the No Pathogenic Variant group despite the two groups having similar BMI z-scores (p = 0.255), subcutaneous adipose tissue (SAT) (p = 0.643), and total body fat (p = 0.225).

Conclusions: Pathogenic variants in MC4R are associated with increased VAT, HFF%, and insulin resistance, independent from the degree of obesity in youth.

Fig. 1. In vitro characterization of wild-type (WT) and novel partial loss of function (pLoF) variants in MC4R.

Mean ± SEM of cAMP response curves of A WT MC4R, B I104V, and C C277G to the addition of increasing amounts of ligand on a logarithmic scale. Experiments were performed three independent times in duplicate. α-MSH: naturally occurring POMC-derived agonist for MC4R. NDP-MSH: high affinity synthetic ligand. MTII: superpotent α-MSH analog. γ₂-MSH: Endogenous POMC-derived agonist for MC3R and MC4R. Ac-His-DPhe-Arg-Trp-NH2: Molecular recognition sequence common to the endogenous melanocortin agonists that binds MC4R.

Fig. 2. Abdominal fat deposition measured by MRI in subjects with or without pathogenic variants in MC4R.

A Visceral fat, B subcutaneous fat, and C visceral/(visceral + subcutaneous) fat ratio. Subjects without pathogenic variants in MC4R are depicted in blue and subjects with pathogenic variants in MC4R are depicted in red. p value is from a Mann–Whitney U test.

Fig. 3. Measure of intrahepatic fat content with MRI and index of insulin sensitivity in subjects with or without pathogenic variants in MC4R.

A Intrahepatic fat content (HFF%) and B whole-body insulin sensitivity index (WBISI). Subjects without pathogenic variants in MC4R are depicted in blue and subjects with pathogenic variants in MC4R are depicted in red. p value is from a Mann–Whitney U test.

Fig. 4. Outcomes during a 180-min oral glucose tolerance test in subjects with or without pathogenic variants in MC4R.

A Plasma concentrations of glucose, B plasma concentrations of insulin, C plasma concentrations of C-peptide, D insulin secretion rate from C-peptide deconvolution, E model-derived β-cell glucose sensitivity (β-GS), F β-cell rate sensitivity (β-RS), G potentiation factor ratio, and H insulin clearance during an OGTT. Subjects without pathogenic variants in MC4R are depicted in blue and subjects with pathogenic variants in MC4R are depicted in red. In A–D, p value is from a Mann–Whitney U test comparing the total AUC between the two groups. In E–H, p value is from a Mann–Whitney U test. Error bars represent standard deviation in (A–D). Boxplot in E–H reported the median [25th–75th] of the distributions along with the scatter plot for each group.