Brief Genetics Report Fine-Mapping Gene-by-Diet Interactions on Chromosome 13 in a LG/J  SM/J Murine Model of Obesity Thomas H. Ehrich, 1 Tomas Hrbek, 1 Jane P. Kenney-Hunt, 1 L. Susan Pletscher, 1 Bing Wang, 1 Clay F. Semenkovich, 2 and James M. Cheverud 1 Obesity is one of the most serious threats to human health today. Although there is general agreement that environmental factors such as diet have largely caused the current obesity pandemic, the environmental changes have not affected all individuals equally. To model gene-by-environment interactions in a mouse model system, our group has generated an F 16 advanced intercross line (AIL) from the SM/J and LG/J inbred strains. Half of our sample was fed a low-fat (15% energy from fat) diet while the other half was fed a high-fat (43% energy from fat) diet. The sample was assayed for a variety of obesity- and diabetes-related phenotypes such as growth rate, response to glucose challenge, organ and fat pad weights, and serum lipids and insulin. An examination in the F 16 sample of eight adiposity quantitative trait loci previously identified in an F 2 intercross of SM/J and LG/J mouse strains reveals locus-by-diet interactions for all previously mapped loci. Adip7, located on proximal chromosome 13, dem- onstrated the most interactions and therefore was se- lected for fine mapping with microsatellite markers. Three phenotypic traits, liver weight in male animals, serum insulin in male animals, and reproductive fat pad weight, show locus-by-diet interactions in the 127-kb region between markers D13Mit1 and D13Mit302. The phosphofructokinase (PFK) C (Pfkp) and the pitrilysin metalloprotease 1 (Pitrm1) genes are compelling posi- tional candidate genes in this region that show coding sequence differences between the parental strains in functional domains. Diabetes 54:1863–1872, 2005 E stimates of the heritability of obesity in humans range as high as 70% based on twin studies (1). However, obesity in the developed world is increasing too rapidly to be caused by changes in genetic background (2). Moreover, some human geno- types are apparently resistant to an obesogenic environ- ment and do not become obese (3). Genotype-by- environment interactions such as these have long been of concern to evolutionary biologists and have been com- monly noted as important for evolutionary processes (4 – 6). Likewise, gene-by-diet interactions are known to influence obesity- and diabetes-related traits in humans (7–9). The elucidation of the biochemistry of appetite control in mouse models generally depends on transgenic or knockout mice (7– 8). Because of the complexity of the obese phenotype, it is unlikely that all gene-by-environ- ment interactions will be discovered using these methods (9). Human studies examining gene-by-diet interactions rely on epidemiological data collected from large samples (10 –12). While these and other studies make valuable contributions to our understanding of gene-by-diet inter- actions, large-scale human studies can be difficult due to unknown genetic lineages (13) as well as data collection biases (11). With carefully controlled lineages and environments, samples derived from inbred mouse strains are amenable to powerful statistical dissection of complex genetic and environmental interactions (15). Crosses derived from the SM/J and LG/J mouse strains are particularly attractive for these types of studies. At 60 days of age, there is an 24-g difference in weight between the two strains (16,17). Later experiments have demonstrated that this considerable weight difference is due to the many different genes of individually small effect (18). Furthermore, SM/J and LG/J respond differently to increased amounts of dietary fat (19,20). To follow up on these earlier findings, we divided the 16th generation of an advanced intercross line (AIL) derived from SM/J and LG/J by diet, feeding half of the mice from each family a high-fat diet and the other half a low-fat diet. A quantitative genetic analysis of a number of obesity- and diabetes-related phenotypes, including growth rates, fat pad and organ weights, and serum levels of triglycerides, free fatty acids, cholesterol, and insulin, From the 1 Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri; and the 2 Department of Medicine, Washington University School of Medicine, St. Louis, Missouri. Address correspondence and reprint requests to James M. Cheverud, Department of Anatomy and Neurobiology, Washington University School of Medicine, 660 S. Euclid Ave., St. Louis, MO 63110. E-mail: cheverud@ wustl.edu. Received for publication 15 September 2004 and accepted in revised form 18 February 2005. Additional information for this article can be found in an online appendix at http://diabetes.diabetesjournals.org. AIL, advanced intercross line; AUC, area under the curve; LOD, logarithm of odds; PFK, phosphofructokinase; QTL, quantitative trait locus; SNP, single nucleotide polymorphism. © 2005 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. DIABETES, VOL. 54, JUNE 2005 1863