A heterozygous mutation in the third transmembrane domain causes a dominant-negative effect on signalling capability of the MC4R.

BACKGROUND Heterozygous MC4R mutation is the most frequent cause of monogenic obesity. For most MC4R mutations a gene dosage effect seems to be the underlying mechanism. However, a dominant negative effect of a heterozygous MC4R mutation was recently identified, pointing to an additional mechanism of MC4R inactivation. METHODS The complete loss-of-function mutation (Ser136Phe), identified in a cohort of obese Austrian patients, was characterized for cell surface expression, signal transduction and ligand binding properties. Co-transfection studies tested for a dominant negative effect. Dimerization was investigated by a sandwich ELISA and by fluorescence resonance energy transfer (FRET) approach. Potential intramolecular interactions of Ser136 were studied by homologous receptor modelling based on the crystal structure of the beta2-adrenergic receptor. RESULTS The Ser136Phe mutation showed a dominant negative effect. The sandwich ELISA and FRET approach demonstrated dimerization of mutant and wild type receptor. Receptor modelling revealed an essential function of Ser136 at transmembrane helix 3 (TMH3) for establishing H-bonds between TMH2, TMH3, and TMH7. The mutation Ser136Phe most likely disrupts this network and leads to an incompetent helix-helix arrangement in the mutated receptor. CONCLUSION Identification of dominant negative MC4R mutations is important to fully understand receptor function and to determine receptor regions that are involved in MC4R dimer activation.