Melanocortin-4 receptor mutations in obesity

The current alarming spread of obesity in many parts of the world is caused by a sudden environmental shift characterized by replacement of a frugal diet with low cost, energy dense food, and little requests for physical activity during work and leisure time. Yet, not all people exposed to an obesogenic environment become obese, and individual differences in the propensity to gain weight as well as the occurrence of different obese phenotypes within the same environment indicate that the genetic heritage in this regard is significant and heterogeneous. The central melanocortin circuit has received much attention during the past decade, since mutations of genes expressing some key molecules in neurons of this system were discovered, which may cause monogenic forms of obesity in animals and humans. Within the arcuate nucleus of the hypothalamus the prohormone proopiomelanocortin is posttranslationally cleaved to produce the alpha-melanocyte stimulating hormone, a peptide with anorexigenic effects upon activation of the melanocortin-4 receptor (MC4R) expressed on the surface of target neurons. Studies regarding the frequency of MC4R mutations associated with human obesity have provided variable results (up to 6% of obese subjects). Various findings suggest an oligogenic and codominant mode of inheritance for MC4R deficiency, with modulation of expressivity and penetrance of the phenotype. The yield of MC4R testing in clinical diagnosis and treatment of obesity is at present undefined since the relatively low prevalence of MC4R pathogenic variants in the general population, along with the high number of sequence variants, has so far compromised the devising of systematic controlled intervention studies. Hopefully, in the future, MC4R testing will have practical implications for the development of new mechanism-based therapy of obesity as well as for the design of specific and more effective protocols, based on lifestyle intervention and current pharmacological or surgical approaches, for management of obesity in MC4R-mutated individuals.