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Mutations in the melanocortin-4 receptor (MC4R) gene are the most frequent monogenic causes of severe obesity. Most mutations have been described as heterozygous with loss of function, suggesting that haploinsufficiency is the most likely mechanism of dominant inheritance. We detected a heterozygous mutation, D90N, in a patient with severe early-onset obesity. Functional characterization of the mutant receptor revealed normal cell-surface expression and binding properties but loss of signal transduction activity. In coexpression studies of wild-type (WT)-MC4R and D90N, the mutant receptor had a dominant-negative effect on WT-receptor function. Further investigation of this effect with sandwich enzyme-linked immunosorbent assays and fluorescence resonance energy transfer studies showed that the WT-MC4R and the D90N mutant form homodimers and heterodimers. We hypothesize that the dominant-negative effect of the D90N mutation is caused by a functionally altered WT-MC4R/D90N receptor heterodimer. These findings necessitate the reinvestigation of other heterozygous MC4R missense mutations to discriminate between haploinsufficiency and a dominant-negative effect. The finding of receptor dimerization highlights a more complex hypothalamic signaling network governing the regulation of body weight. Diabetes 52:2984-2988, 2003
Several genes that play a role in monogenic forms of obesity have been identified. In humans, mutations or disruptions of genes of the leptin-melanocortin pathway (1-4) were identified as rare causes for recessively inherited obesity. The gene for the melanocortin-4 receptor (MC4R) is an exception because mutations were detected in 3-5% of the studied population of severely obese patients (5,6). The MC4R belongs to the large superfamily of G protein-coupled receptors (GPCRs) (7) and is activated by proopiomelanocortin-derived peptides. The activated receptor couples to the G^sub s^/adenylyl cyclase system, resulting in decreased food intake and increased energy expenditure (8). Inactivating mutations of the MC4R gene were found to be mostly inherited in an autosomal-dominant manner. The molecular mechanism of this dominant manifestation of obesity as a result of heterozygous loss-of-function MC4R mutations is still unclear. However, a clear dosage effect on body weight has been shown in heterozygous versus homozygous MC4R knockout mice (9). Therefore, for the manifestation of an obese phenotype in heterozygous loss-of-function mutation carriers, haploinsufficiency is the widely accepted hypothesis (10,11). We report a heterozygous D90N mutation resulting in complete loss of function but normal cell-surface expression and ligand-binding affinity. Cotransfection studies revealed a dominant-negative effect...