A major task in dissecting the genetics of complex traits is to identify causal genes for disease phenotypes. We previously developed a method to infer causal relationships among genes through the integration of DNA variation, gene transcription, and phenotypic information. Here we validated our method through the characterization of transgenic and knockout mouse models of candidate genes that were predicted to be causal for abdominal obesity. Perturbation of eight out of the nine genes, with Gas7, Me1 and Gpx3 being novel, resulted in significant changes in obesity related traits. Liver expression signatures revealed alterations in common metabolic pathways and networks contributing to abdominal obesity and overlapped with a macrophage-enriched metabolic network module that is highly associated with metabolic traits in mice and humans. Integration of gene expression in the design and analysis of traditional F2 intercross studies allows high confidence prediction of causal genes and identification of involved pathways and networks.

Little is known about how genetic variation affects the capacity for exercise to change body composition. We examined the extent to which voluntary exercise alters body composition in several lines of selectively bred mice compared to controls. Lines studied included high runner (HR) (selected for high wheel running), M16 (selected for rapid weight gain), Institute of Cancer Research (ICR) (randomly bred as control for M16), M16i (an inbred line derived from M16), HE (selected for high percentage of body fat while holding body weight constant), LF (selected for low percentage of body fat), C57BL/6J (common inbred line), and the F1 between HR and C57BL/6J. Body weight and body fat were recorded before and after 6 days of free access to running wheels in males and females that were individually caged. Total food intake was measured during this 6‐day period. All pre‐ and postexercise measures showed significant strain effects. While HR mice predictably exercised at higher levels, all other selection lines had decreased levels of wheel running relative to ICR. The HR × B6 F1 ran at similar levels to HR demonstrating complete dominance for voluntary exercise. Also, all strains lost body fat after exercise, but the relationships between exercise and changes in percent body were not uniform across genotypes. These results indicate that there is significant genetic variation for voluntary exercise and its effects on body composition. It is important to carefully consider genetic background and/or selection history when using mice to model effects of exercise on body composition, and perhaps, other complex traits as well.

Previous quantitative trait locus mapping (QTL) identified multigenic obesity (MOB) loci on mouse Chromosome (Chr) 2 that influence the interrelated phenotypes of obesity, insulin resistance, and dyslipidemia. To better localize and characterize the MOB locus, three congenic mouse strains were created. Overlapping genomic intervals from the lean CAST/Ei (CAST) strain were introgressed onto an obesity-susceptible C57BL/6 (BL6) background to create proximal (15 Mb-73 Mb), middle (63 Mb-165 Mb), and distal (83 Mb-182 Mb) congenic strains. The congenic strains showed differences in obesity, insulin, and lipid traits consistent with the original QTL analysis for the locus. Importantly, characterization of the MOB congenics localized the effects of genes that underlie obesity-related traits to an introgressed interval (73-83 Mb) unique to the middle MOB congenic. Conversely, significant differences between the lipid and insulin profiles of the middle and distal MOB congenics implicated the presence of at least two genes that underlie these traits. When fed an atherogenic diet, several traits associated with metabolic syndrome were observed in the distal MOB congenic, while alterations in plasma lipoproteins were observed in the middle MOB congenic strain.

Traits related to energy balance and obesity are exceptionally complex, with varying contributions of genetic susceptibility and interacting environmental factors. The use of mouse models has been a powerful driving force in understanding the genetic architecture of polygenic traits such as obesity. However, the use of mouse models for analysis of complex traits is at an important crossroad. Genome-wide association studies in humans are now leading to direct identification of obesity genes. In this review, we focus on three areas representing the current and future roles of mouse models regarding genetics of complex obesity. First, we summarize increasingly powerful ways to harness the strength of mouse models for discovery of genes affecting polygenic obesity. Second, we examine the status of using a systems biology approach to dissect the genetic architecture of obesity. And third, we explore the effects of recent findings indicating increasing levels of complexity in the nature of variation underlying, and the heritability of, complex traits such as obesity.

Previous characterization of mouse chromosome 2 identified genomic intervals that influence obesity, insulin resistance, and dyslipidemia. For this, resistant CAST/Ei (CAST) alleles were introgressed onto a susceptible C57BL/6J background to generate congenic strains with CAST alleles encompassing 67-162 Mb (multigenic obesity 6 [MOB6]) and 84 -180 Mb (MOB5) from mouse chromosome 2. To examine the effects of each congenic locus on atherosclerosis and glucose disposal, we bred each strain onto a sensitizing LDL receptor-null (LDLR ؊/؊ ) C57BL/6J background to predispose them to hypercholesterolemia and insulin resistance. LDLR ؊/؊ congenics and controls were characterized for measures of atherogenesis, insulin sensitivity, and obesity. We identified a genomic interval unique to the MOB6 congenic (72-84 Mb) that dramatically decreased atherosclerosis by approximately threefold and decreased insulin resistance. This region also reduced adiposity twofold. Conversely, the congenic region unique to MOB5 (162-180 Mb) increased insulin resistance but had little effect on atherosclerosis and adiposity. The MOB congenic intervals are concordant to human and rat quantitative trait loci influencing diabetes and atherosclerosis traits. Thus, our results define a strategy for studying the poorly understood interactions between diabetes and atherosclerosis and for identifying genes underlying the cardiovascular complications of insulin resistance. Diabetes 55:2265-2271, 2006 I nsulin resistance increases the risk of hypertension, obesity, and dyslipidemia and is thereby a principal risk factor for cardiovascular disease (1-4), but the mechanisms involved are largely unknown (3-5). Loci on mouse chromosome 2 are concordant (6) for insulin resistance and atherosclerosis quantitative trait loci from homologous genomic intervals in rats, mice, and humans (supplemental data, which is detailed in the online appendix [available at http://diabetes.diabetesjournals. org]). Previously, we characterized overlapping multigenic obesity (MOB)5 and -6 congenic mice (7) for these traits to better understand their impact. Here, chromosome 2 alleles from CAST/Ei (CAST) mice that are resistant to obesity were introgressed onto an obesity-susceptible C57BL/6J background (available online from http://phenome.jax.org/ pub-cgi/phenome). We showed that MOB5 congenics became insulin resistant, suggesting that genes underlying this trait resided in the congenic interval unique to this strain (CAST alleles from 162 to 180 Mb). By contrast, MOB6 congenics displayed an atherosclerosis-susceptible cholesterol profile when fed a diet high in fat, cholesterol, and cholic acid, suggesting that genes in the interval unique to this strain (CAST alleles from 72 to 84 Mb) underlie the atherogenic cholesterol profile (7). However, the experimental diet used in that study, though enriched in lipid and cholesterol, did not trigger atherogenesis or obesity in these strains.C57BL/6J (B6) mice with homozygous null alleles for the LDL receptor (LDLR Ϫ/Ϫ ) become obese a...

M16 mice are polygenic obese models created by 27 generations of selection for weight gain from an ICR background. Recent transcriptional analysis of adipose and hypothalamus from ~300 M16 × ICR F2 animals identified expression variants (eQTL) and cross‐tissue co‐expression networks of coordinately expressed genes that regulate many processes including satiety, circadian oscillations and stress responses.The purpose of this project is to experimentally validate effects of eQTL and genes in co‐expression networks on obesity phenotypes. Approaches include a Zebrafish screen of morhpholinos, over‐expression and expression knock down by RNAi in adipocyte differentiation, and sequence analysis of regulatory regions. Preliminary results suggest a significant effect of several candidate genes on fat utilization and storage in Zebrafish morpholinos. Sequence analysis of regulatory intervals show variations that alter transcription factor binding sites. These differences may underlie changes in the regulation of gene expression in obese mice. Studies on changes in adipocyte differentiation as a consequence of altered expression levels are just being initiated. Given the biological complexity of fat accretion and energy regulation, a systems biology approach utilizing cumulative statistical and experimental methods may provide needed insight into the molecular basis of obesity.