Functional and Molecular Characterization of Centrally Expressed Genes Associated with Obesity

Sammanfattning: Obesity is a complex disorder that has reached epidemic proportions in the Western world, currently affection more than two billion people. The evidence for the genetic influence on obesity has been estimated to be as high as 70% based on twin studies. Subsequent application of genome wide association studies has identified more than 90 genes to be associated with BMI. Despite great efforts the majority of the identified genetic variants have an unknown link to BMI and lack scientific basis explaining how they affect energy balance resulting in altered body weight. This thesis aims to characterize seven BMI-associated genes, Coronin 7, Etv5, Mtch2, Nudt3, Raptor, Sh2b1 and Vps13B by performing a molecular and functional profiling in mouse, zebrafish and fruit fly. A screen analysing the regulation of the selected genes under different dietary conditions revealed altered transcript levels in mouse, zebrafish and fruit fly including a conserved regulation in all species for some of the genes. Using genetic tools the Nudt3 homolog Aps in the Drosophila CNS was knocked down and showed that Aps has a role in the regulation of insulin signaling which could explain the robust association to obesity in humans. A comprehensive in situ hybridization revealed abundant Nudt3 mRNA and protein expression throughout the brain, including in reward and feeding related regions of the hypothalamus while Nudt3 mRNA expression was significantly up-regulated in the same region of food-deprived mice. Furthermore, we were able to identify a novel molecular link between obesity and bipolar disorder. The Drosophila homologue Ets96B regulates the expression of a cellular system with links to obesity and bipolar disorder, including the expression of a conserved endoplasmic reticulum molecular chaperone complex. A connection between the obesity-linked gene ETV5 and bipolar disorder emphasizes a functional relationship between obesity and bipolar disorder at the molecular level. Furthermore, as the BMI associated genetic variants does not fully explain the heritability of obesity we decided to perform a genome wide DNA methylation profile where we compared normal-weight and obese preadolescent children. We found a CpG site located near Coronin 7 to have significantly lower methylation levels in obese children. Further studies showed Coronin 7 to be highly expressed in important brain regions involved in energy balance. Strong immunostaining was also seen in locus coeruleus, the main site for noradrenergic production, and injecting mice with an appetite suppressant increased the number of Coronin 7 neurons within the very same brain region. An evolutionary conserved metabolic function in Drosophila was also demonstrated by knocking down the Coronin 7 homologue Pod1 in the fruit fly adult nervous system.