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Whether mutations in the melanocortin-4 receptor (MC4R) are the cause of binge eating disorder was controversial. In addition, the penetrance of mutations in the MC4R in causing obesity was debated.We investigated whether MC4R variants identified from obese patients with binge eating disorder (T11A, F51L, T112M, and M200V) and variants identified in nonobese (I102T, F202L, and N240S) or obese (I102S, A154D, and S295P) subjects cause loss-of-function and what are the defects.Variant or wild-type MC4Rs were expressed in HEK293 cells and examined for their pharmacological characteristics.The study setting was in vitro bench-top laboratory experiments.Ligand binding, signaling, and cell surface expression of the variant MC4Rs were compared with wild-type MC4R.Our data clearly show a loss-of-function phenotype in vitro for I102T and N240S variants identified in nonobese individuals. Furthermore, not all MC4R variants identified in obese subjects exhibit a loss-of-function phenotype in vitro. Finally, the MC4R variants T11A, F51L, T112M, and M200V identified from patients with binge eating disorder displayed normal function with regards to the parameters measured in our study.Patients harboring loss-of-function MC4R mutations do not always exhibit obesity. Novel MC4R variant identified from an obese patient cannot be assumed to be the cause of obesity without demonstrating a loss-of-function phenotype in vitro for the variant MC4R. Whether MC4R mutations are involved in the pathogenesis of binge eating disorder needs additional investigation.

Genetic and pharmacological studies have shown that the melanocortin-4 receptor (MC4R) is an important regulator of food intake and energy homeostasis. Consistent with these studies, several mutations of the MC4R gene have been identified as being associated with early-onset severe obesity. We report here the first in-frame deletion mutation of the MC4R gene (delta88-92) in an obese female patient with onset of obesity at less than 5 yr of age. Functional analysis revealed that the mutant receptor is expressed well on the cell surface but completely devoid of ligand binding and cAMP generation in response to agonist stimulation. We conclude that this novel mutation is the cause of obesity of this patient.

Activating mutations of the human lutropin/choriogonadotropin receptor (hLHR), a Gs-coupled receptor, have been identified in young boys with gonadotropin-independent precocious puberty (testotoxicosis). The properties of these mutants have typically been characterized in heterologous cells transfected with recombinant mutant receptor and compared with those expressing wild-type (wt) receptor. The affected individuals, however, are heterozygous and, therefore, express wt receptor in addition to the mutant receptor. The present studies were undertaken to determine what effects, if any, coexpression of a constitutively active hLHR might have on hLHR(wt). HEK 293 cells were cotransfected with hLHR(wt) and hLHR(L457R), a mutant that we have previously shown to be both constitutively active and unresponsive to further hormonal stimulation as determined in both intact cells and isolated membranes. When coexpressed at submaximal concentrations, L457R does not decrease the cell surface expression of hLHR(wt). Coexpression of L457R, however, causes an attenuation of human choriogonadotropin-stimulated cAMP production by hLHR(wt). We show that this attenuation is caused by an activation of the phosphodiesterase (PDE)4D3. Additional experiments demonstrate that the coexpression of L457R with the human beta(2)-adrenergic receptor causes an attenuation of isoproterenol-stimulated cAMP and that other activating mutations of the hLHR also induce PDE activation. Taken together, these data demonstrate that the activation of PDE is a compensatory mechanism common to hLHR constitutively active mutants and that cellular responses to agonists that stimulate Gs-coupled receptors may be blunted in tissues expressing these activating mutants. This novel desensitizing effect of constitutively active hLHRs on hormone-stimulated cAMP production has not been noticed before and would typically not be detected because of the routine inclusion of PDE inhibitors in experiments determining cAMP accumulation. Importantly, however, this mechanism of desensitization would be expected to occur in a physiological context in which PDE inhibitors are not present and thus may influence hormonal signaling in cells expressing the activating hLHR mutant.