No abstract

OBJECTIVE-Melanocortin-4 receptor (MC4R) deficiency is the most frequent genetic cause of obesity. However, there is uncertainty regarding the degree of penetrance of this condition, and the putative impact of the environment on the development of obesity in MC4R mutation carriers is unknown.RESEARCH DESIGN AND METHODS-We determined the MC4R sequence in 2,257 obese individuals and 2,677 nonobese control subjects of European origin and established the likely functional impact of all variants detected. We then included relatives of probands carriers and studied 25 pedigrees, including 97 carriers and 94 noncarriers from three generations.RESULTS-Of the MC4R nonsynonymous mutations found in obese subjects, 68% resulted in a loss of function in vitro. They were found in 1.72% of obese versus 0.15% of nonobesed subjects (P ϭ 6.9 ϫ 10 Ϫ10 ). Among the families, abnormal eating behavior was more frequent in both MC4R-deficient children and adults than in noncarriers. Although BMI was inversely associated with educational status in noncarrier adults, no such relationship was seen in MC4R mutation carriers. We observed a generational effect, with a penetrance of 40% in MC4R-deficient adults aged Ͼ52 years, 60% in 18-to 52-year-old adults, and 79% in children. The longitudinal study of adult carriers showed an increasing age-dependent penetrance (37% at 20 years versus 60% at Ͼ40 years).CONCLUSIONS-We have established a robust estimate of age-related penetrance for MC4R deficiency and demonstrated a generational effect on penetrance, which may relate to the development of an "obesogenic" environment. It remains to be seen whether appropriate manipulation of environmental factors may contribute to preventing the development of obesity even in those strongly genetically predisposed to it.

The melanocortin-4 receptor (MC4R) gene pathogenic mutations are the most prevalent forms of monogenic obesity, responsible for approximately 2% of obesity cases, but its role in common obesity is still elusive. We analyzed the contribution of non-synonymous mutations V103I (rs2229616, c.307G > A) and I251L (no rs, c.751A > C) to obesity in 16 797 individuals of European origin from nine independent case-control, population-based and familial cohorts. We observed a consistent negative association of I251L variant (prevalence ranging 0.41-1.21%) with both childhood and adult class III obesity [odds ratio (OR) ranging from 0.25 to 0.76, 0.001 < P-value < 0.05] and with modulation of body mass index (BMI) in general populations, in eight out of nine studies, whereas only one study showed an association between V103I and BMI. Meta-analyses of previous published data with the current ones provided strong evidence of the protective effect of I251L toward obesity (OR = 0.52, P = 3.58 10-5), together with a modest negative association between V103I and obesity (OR = 0.80, P = 0.002). Taken together, gain-of-function mutations I251L and V103I may be responsible for a preventive fraction of obesity of 2%, which mirrors the prevalence of monogenic obesity due to MC4R haploinsufficiency. These results also emphasize the importance of the MC4R signalling tonus to prevent obesity, even in the context of our current obesogenic environment.