The importance of context to the genetic architecture of diabetes-related traits is revealed in a genome-wide scan of a LG/J × SM/J murine model.

Variations in diabetic phenotypes are caused by complex interactions of genetic effects, environmental factors, and the interplay between the two. We tease apart these complex interactions by examining genome-wide genetic and epigenetic effects on diabetes-related traits among different sex, diet, and sex-by-diet cohorts in a Mus musculus model. We conducted a genome-wide scan for quantitative trait loci that affect serum glucose and insulin levels and response to glucose stress in an F(16) Advanced Intercross Line of the LG/J and SM/J intercross (Wustl:LG,SM-G16). Half of each sibship was fed a high-fat diet and half was fed a relatively low-fat diet. Context-dependent genetic (additive and dominance) and epigenetic (parent-of-origin imprinting) effects were characterized by partitioning animals into sex, diet, and sex-by-diet cohorts. We found that different cohorts often have unique genetic effects at the same loci, and that genetic signals can be masked or erroneously assigned to specific cohorts if they are not considered individually. Our data demonstrate that the effects of genes on complex trait variation are highly context-dependent and that the same genomic sequence can affect traits differently depending on an individual's sex and/or dietary environment. Our results have important implications for studies of complex traits in humans.

Diet-dependent genetic and genomic imprinting effects on obesity in mice.

Although the current obesity epidemic is of environmental origin, there is substantial genetic variation in individual response to an obesogenic environment. In this study, we perform a genome-wide scan for quantitative trait loci (QTLs) affecting obesity per se, or an obese response to a high-fat diet in mice from the LG/J by SM/J Advanced Intercross (AI) Line (Wustl:LG,SM-G16). A total of 1,002 animals from 78 F16 full sibships were weaned at 3 weeks of age and half of each litter placed on high- and low-fat diets. Animals remained on the diet until 20 weeks of age when they were necropsied and the weights of the reproductive, kidney, mesenteric, and inguinal fat depots were recorded. Effects on these phenotypes, along with total fat depot weight and carcass weight at necropsy, were mapped across the genome using 1,402 autosomal single-nucleotide polymorphism (SNP) markers. Haplotypes were reconstructed and additive, dominance, and imprinting genotype scores were derived every 1 cM along the F16 map. Analysis was performed using a mixed model with additive, dominance, and imprinting genotype scores, their interactions with sex, diet, and with sex-by-diet as fixed effects and with family and its interaction with sex, diet, and sex-by-diet as random effects. We discovered 95 trait-specific QTLs mapping to 40 locations. Most QTLs had additive effects with dominance and imprinting effects occurring at two-thirds of the loci. Nearly every locus interacted with sex and/or diet in important ways demonstrating that gene effects are primarily context dependent, changing depending on sex and/or diet.

Genetic, epigenetic, and gene-by-diet interaction effects underlie variation in serum lipids in a LG/JxSM/J murine model.

Variation in serum cholesterol, free-fatty acids, and triglycerides is associated with cardiovascular disease (CVD) risk factors. There is great interest in characterizing the underlying genetic architecture of these risk factors, because they vary greatly within and among human populations and between the sexes. We present results of a genome-wide scan for quantitative trait loci (QTL) affecting serum cholesterol, free-fatty acids, and triglycerides in an F(16) advanced intercross line of LG/J and SM/J (Wustl:LG,SM-G16). Half of the population was fed a high-fat diet and half was fed a relatively low-fat diet. Context-dependent genetic (additive and dominance) and epigenetic (imprinting) effects were characterized by partitioning animals into sex, diet, and sex-by-diet cohorts. Here we examine genetic, environmental, and genetic-by-environmental interactions of QTL overlapping previously identified loci associated with CVD risk factors, and we add to the serum lipid QTL landscape by identifying new loci.

Genetic variation and correlation of dietary response in an advanced intercross mouse line produced from two divergent growth lines.

Levels of human obesity have increased over the past 20 years worldwide, primarily due to changes in diet and activity levels. Although environmental changes are clearly responsible for the increasing prevalence of obesity, individuals may show genetic variation in their response to an obesogenic environment. Here, we measure genetic variation in response to a high-fat diet in a mouse model, an F16 Advanced Intercross Line derived from the cross of SM/J and LG/J inbred mouse strains. The experimental population was separated by sex and fed either a high-fat (42% of energy from fat) or low-fat (15% of energy from fat) diet. A number of phenotypic traits related to obesity and diabetes such as growth rate, glucose tolerance traits, organ weights and fat pad weights were collected and analysed in addition to serum levels of insulin, free fatty acids, cholesterol and triglycerides. Most traits are different between the sexes and between dietary treatments and for a few traits, including adult growth, fat pad weights, insulin and glucose tolerance, the dietary effect is stronger in one sex than the other. We find that fat pad weights, liver weight, serum insulin levels and adult growth rates are all phenotypically and genetically correlated with one another in both dietary treatments. Critically, these traits have relatively low genetic correlations across environments (average r =0.38). Dietary responses are also genetically correlated across these traits. We found substantial genetic variation in dietary response and low cross environment genetic correlations for traits aligned with adiposity. Therefore, genetic effects for these traits are different depending on the environment an animal is exposed to.

Maternal genotype affects adult offspring lipid, obesity, and diabetes phenotypes in LGXSM recombinant inbred strains.

Maternal effects on offspring phenotypes occur because mothers in many species provide an environment for their developing young. Although these factors are correctly "environmental" with respect to the offspring genome, their variance may have both a genetic and an environmental basis in the maternal generation. Here, reciprocal crosses between C57BL/6J and 10 LGXSM recombinant inbred (RI) strains were performed, and litters were divided at weaning into high-fat and low-fat dietary treatments. Differences between reciprocal litters were used to measure genetic maternal effects on offspring phenotypes. Nearly all traits, including weekly body weights and adult blood serum traits, show effects indicative of genetic variation in maternal effects across RI strains, allowing the quantitative trait loci involved to be mapped. Although much of the literature on maternal effects relates to early life traits, we detect strong and significant maternal effects on traits measured at adulthood (as much as 10% of the trait variance at 17 or more weeks after weaning). We also found an interaction affecting adult phenotype between the effects of maternal care between RI strain mothers and C57BL/6J mothers and a later environmental factor (dietary fat intake) for some age-specific weights.

Fine-mapping gene-by-diet interactions on chromosome 13 in a LG/J x SM/J murine model of obesity.

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, demonstrated the most interactions and therefore was selected 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 positional candidate genes in this region that show coding sequence differences between the parental strains in functional domains.

Quantitative trait loci for obesity- and diabetes-related traits and their dietary responses to high-fat feeding in LGXSM recombinant inbred mouse strains.

Genetic variation in response to high-fat diets is important in understanding the recent secular trends that have led to increases in obesity and type 2 diabetes. The examination of quantitative trait loci (QTLs) for both obesity- and diabetes-related traits and their responses to a high-fat diet can be effectively addressed in mouse model systems, including LGXSM recombinant inbred (RI) mouse strains. A wide range of obesity- and diabetes-related traits were measured in animals from 16 RI strains with 8 animals of each sex fed a high- or low-fat diet from each strain. Marker associations were measured at 506 microsatellite markers spread throughout the mouse genome using a nested ANOVA. Locations with significant effects on the traits themselves and/or trait dietary responses were identified after correction for multiple comparisons by limiting the false detection rate. Nonsyntenic associations of marker genotypes were common at QTL locations so that the significant results were limited to loci still significant in multiple QTL models. We discovered 91 QTLs at 39 locations. Many of these locations (n = 31) also showed genetic effects on dietary response, typically because the loci produced significantly larger effects on the high-fat diet. Fat depot weights, leptin levels, and body weight at necropsy tended to map to the same locations and were responsible for a majority of the dietary response QTLs. Basal glucose levels and the response to glucose challenge mapped together in locations distinct from those affecting obesity. These QTL locations form a panel for further research and fine mapping of loci affecting obesity- and diabetes-related traits and their responses to high-fat feeding.

Genetic evidence for discordance between obesity- and diabetes-related traits in the LGXSM recombinant inbred mouse strains.

Obesity and its comorbidities, particularly type 2 diabetes, have become serious public health problems over the past few decades. Although the current pandemic is largely caused by societal environmental changes in diet, variation in response to these changes have, in part, a genetic basis. Here we address the genetic basis for both obesity- and diabetes-related traits themselves and dietary fat responses for these traits in a set of recombinant inbred mouse strains formed from the cross of LG/J with SM/J (LGXSM lines) fed a standard low-fat (15% calories from fat) or high-fat (42% calories from fat) diet. We found substantial genetic variation for most of the traits studied. Weight at time of death, liver weight, and weight of the reproductive fat pad had especially high heritabilities, whereas heart weight and serum levels of free fatty acids and triglycerides had low heritabilities. Genetic correlations were very high among fat pad weights and serum leptin, indicating shared genetic variation between fat levels and hormonal appetite control. These obesity traits were moderately correlated with adult growth, liver weight, and serum insulin and cholesterol levels. A majority of traits also displayed genetic variation in response to a high-fat diet, especially the weight of the reproductive and renal fat pads as well as the liver. Genetic correlations in dietary response followed a pattern similar to that found for the traits themselves. Several strains manifested discordant responses for obesity, glucose, and insulin, consistent with the presence of genotypes protective for diabetes in the presence of obesity. These recombinant inbred strains represent potentially valuable new models for dissecting the complex physiological relationships among obesity and diabetes.

Pleiotropic effects on mandibular morphology II: differential epistasis and genetic variation in morphological integration.

The evolution of morphological modularity through the sequestration of pleiotropy to sets of functionally and developmentally related traits requires genetic variation in the relationships between traits. Genetic variation in relationships between traits can result from differential epistasis, where epistatic relationships for pairs of loci are different for different traits. This study maps relationship quantitative trait loci (QTLs), specifically QTLs that affect the relationship between individual mandibular traits and mandible length, across the genome in an F2 intercross of the LG/J and SM/J inbred mouse strains (N = 1045). We discovered 23 relationship QTLs scattered throughout the genome. All mandibular traits were involved in one or more relationship QTL. When multiple traits were affected at a relationship QTL, the traits tended to come from a developmentally restricted region of the mandible, either the muscular processes or the alveolus. About one-third of the relationship QTLs correspond to previously located trait QTLs affecting the same traits. These results comprise examples of genetic variation necessary for an evolutionary response to selection on the range of pleiotropic effects.

Diet, obesity, and hyperglycemia in LG/J and SM/J mice.

OBJECTIVE: To examine the differential response of obesity- and diabetes-related traits to a high- or low-fat diet in LG/J and SM/J mice. We also examined food consumption in these strains. RESEARCH METHODS AND PROCEDURES: Mice were placed on a high- or low-fat diet after weaning. Animals were weighed once per week and subjected to glucose tolerance tests at 20 weeks. At sacrifice, fat pads and internal organs were removed along with serum samples. For food consumption, LG/J and SM/J mice of each sex were assigned to a high-fat or low-fat diet after reaching maturity. Mice were weighed three times per week, and food consumed was determined by subtraction. RESULTS: LG/J animals consume more total food, but SM/J animals consume more food per gram of body weight. LG/J mice grow faster to 10 weeks but slower from 10 to 20 weeks, have higher cholesterol and free fatty acid levels, and have lower basal glucose levels and better response to a glucose challenge than SM/J mice. For most traits, SM/J mice respond more strongly to a high-fat diet than LG/J mice, including body weight and growth, basal glucose levels, organ weights, fat distribution, and circulating triglycerides and cholesterol levels. DISCUSSION: Obesity-related phenotypes, as well as response to increased dietary fat, differ genetically between LG/J and SM/J and can, therefore, be mapped. This study indicates that the cross of SM/J and LG/J mice would be an excellent model system for the study of gene-by-diet interaction in obesity.

Pleiotropic effects on mandibular morphology I. Developmental morphological integration and differential dominance.

Pleiotropy refers to a single genetic locus that affects more than one phenotypic trait. Pleiotropic effects of genetic loci are thought to play an important role in evolution, reflecting functional and developmental relationships among phenotypes. In a previous study, we examined pleiotropic effects displayed by quantitative trait loci (QTLs) on murine mandibular morphology in relation to mandibular structure and function. In replicating most of our previous QTLs and increasing our sample size, this study strengthens and extends our earlier results. As in our previous study, we find that QTL effects tend to be restricted to developmentally or functionally related traits. In addition, we examine patterns of differential dominance for pleiotropic QTL effects. Differential dominance occurs when dominance patterns for a single locus vary among traits. We find that multivariate overdominance is a common and substantial phenomenon, and may potentially provide an explanation for the persistence of heterozygosity in natural populations.