Identification and characterization of positional candidate genes for obesity in selected mouse lines

Quantitative traits are in most cases complex traits, which are controlled by many genes and influenced by the environment. Growth and obesity are examples for complex traits. For breeders of farm animal, body weight and fat deposition have major impact on an efficient animal production, carcass and meat quality. In humans, obesity is often associated with other severe diseases, for example, hypertension, diabetes, cardiovascular diseases, osteoarthritis, and certain cancers. The primary aim of this study was the identification of differentially expressed positional candidate genes underlying quantitative trait loci (QTL) effects, which have recently been identified for body weight and obesity. In this study, we used the high body weight selected mouse line DU6 and its inbred derivate DU6i as animal models for obesity. High body weight selected DU6 mice are twice as heavy and three times as fat as unselected DUK and DBA/2 control mice. In the fine-mapped QTL on chromosomes 5 and 7, which have the largest effect on body weight and obesity, we have identified allele fixations and significant differences in allele frequencies in the high body weight selected mouse line DU6. Using microarray genechips and semi-quantitative RT-PCR analyses, ten positional candidate genes located on previously identified QTL regions and differently expressed among selected and control mouse lines were identified. To identify DNA sequence variations which might be a cause for the different gene expression, we sequenced 5´ and 3´ -flanking, putatively regulatory regions of the positional candidate genes in selected and control lines. We identified 32 single nucleotide polymorphisms (SNP) some of which are located in putative transcription factor binding sites. Additionally, 22 different haplotypes in 5´ and 3´ flanking regions of 4 differentially expressed positional candidate genes were identified between the high body weight selected DU6 mice, its founder population FzT: DU and 25 phylogenetic diverse inbred mouse strains. The unified usage of different approaches, which are QTL mapping, fine-mapping, gene expression analysis and search for regulatory region DNA variations, is a powerful way to dissect QTL effects and to identify candidate genes that underlie complex traits.