Characterization of the melanocortin-4-receptor nonsense mutation W16X in vitro and in vivo

Several genetic diseases are triggered by nonsense mutations leading to the formation of truncated and defective proteins. Aminoglycosides have the capability to mediate a bypass of stop mutations during translation thus resulting in a rescue of protein expression. So far no attention has been directed to obesity-associated stop mutations as targets for nonsense suppression. Herein, we focus on the characterization of the melanocortin-4-receptor (MC4R) nonsense allele W16X identified in obese subjects. Cell culture assays revealed a loss-of-function of Mc4r X16 characterized by impaired surface expression and defect signaling. The aminoglycoside G-418 restored Mc4r X16 function in vitro demonstrating that Mc4r X16 is susceptible to nonsense suppression. For the evaluation of nonsense suppression in vivo, we generated a Mc4r X16 knock-in mouse line by gene targeting. Mc4r X16 knock-in mice developed hyperphagia, impaired glucose tolerance, severe obesity and an increased body length demonstrating that this new mouse model resembles typical characteristics of Mc4r deficiency. In a first therapeutic trial, the aminoglycosides gentamicin and amikacin induced no amelioration of obesity. Further experiments with Mc4r X16 knock-in mice will be instrumental to establish nonsense suppression for Mc4r as an obesity-associated target gene expressed in the central nervous system.