A meta-analysis of quantitative trait loci associated with body weight and adiposity in mice

Glo1 reduction in mice results in age- and sex-dependent metabolic dysfunction

Objectives

Advanced glycation end products (AGEs) have been implicated as an important mediator of metabolic disorders including obesity, insulin resistance, and coronary artery disease. Glyoxalase 1 (Glo1) is a critical enzyme in the clearance of toxic dicarbonyl such as methylglyoxal, precursors of AGEs. The role of AGE-independent mechanisms that underly Glo1-induced metabolic disorders have yet to be elucidated.

Methods

We performed a longitudinal study of female and male Glo1 heterozygous knockdown (Glo1 +/- ) mice with ~50% gene expression and screened metabolic phenotypes such as body weight, adiposity, glycemic control and plasma lipids. We also evaluated atherosclerotic burden, AGE levels, and gene expression profiles across cardiometabolic tissues (liver, adipose, muscle, kidney and aorta) to identify pathway perturbations and potential regulatory genes of Glo1 actions.

Results

Partial loss of Glo1 resulted in obesity, hyperglycemia, dyslipidemia, and alterations in lipid metabolism in metabolic tissues in an age- and sex-dependent manner. Glo1 +/- females displayed altered glycemic control and increased plasma triglycerides, which aligned with significant perturbations in genes involved in adipogenesis, PPARg, insulin signaling, and fatty acid metabolism pathways in liver and adipose tissues. Conversely, Glo1 +/- males developed increased skeletal muscle mass and visceral adipose depots along with changes in lipid metabolism pathways. For both cohorts, most phenotypes manifested after 14 weeks of age. Evaluation of methylglyoxal-derived AGEs demonstrated changes in only male skeletal muscle but not in female tissues, which cannot explain the broad metabolic changes observed in Glo1 +/- mice. Transcriptional profiles suggest that altered glucose and lipid metabolism may be partially explained by alternative detoxification of methylglyoxal to metabolites such as pyruvate. Moreover, transcription factor (TF) analysis of the tissue-specific gene expression data identified TFs involved in cardiometabolic diseases such as Hnf4a (all tissues) and Arntl (aorta, liver, and kidney) which are female-biased regulators and whose targets are altered in response to Glo1 +/- .

Conclusions

Our results indicate that Glo1 reduction perturbs metabolic health and metabolic pathways in a sex- and age-dependent manner without significant changes in AGEs across metabolic tissues. Rather, tissue-specific gene expression analysis suggests that key transcription factors such as Hfn4a and Arntl as well as metabolite changes from alternative methylglyoxal detoxification such as pyruvate, likely contribute to metabolic dysregulation in Glo1 +/- mice.

The AGE-RAGE Axis and the Pathophysiology of Multimorbidity in COPD

Chronic obstructive pulmonary disease (COPD) is a disease of the airways and lungs due to an enhanced inflammatory response, commonly caused by cigarette smoking. Patients with COPD are often multimorbid, as they commonly suffer from multiple chronic (inflammatory) conditions. This intensifies the burden of individual diseases, negatively affects quality of life, and complicates disease management. COPD and comorbidities share genetic and lifestyle-related risk factors and pathobiological mechanisms, including chronic inflammation and oxidative stress. The receptor for advanced glycation end products (RAGE) is an important driver of chronic inflammation. Advanced glycation end products (AGEs) are RAGE ligands that accumulate due to aging, inflammation, oxidative stress, and carbohydrate metabolism. AGEs cause further inflammation and oxidative stress through RAGE, but also through RAGE-independent mechanisms. This review describes the complexity of RAGE signaling and the causes of AGE accumulation, followed by a comprehensive overview of alterations reported on AGEs and RAGE in COPD and in important co-morbidities. Furthermore, it describes the mechanisms by which AGEs and RAGE contribute to the pathophysiology of individual disease conditions and how they execute crosstalk between organ systems. A section on therapeutic strategies that target AGEs and RAGE and could alleviate patients from multimorbid conditions using single therapeutics concludes this review.

Genetics of murine type 2 diabetes and comorbidities

ABSTRAC

Type 2 diabetes (T2D) is a polygenic and multifactorial complex disease, defined as chronic metabolic disorder. It's a major global health concern with an estimated 463 million adults aged 20-79 years with diabetes and projected to increase up to 700 million by 2045. T2D was reported to be one of the four leading causes of non-communicable disease (NCD) deaths in 2012. Environmental factors play a part in the development of polygenic forms of diabetes. Polygenic forms of diabetes often run-in families. Fortunately, T2D, which accounts for 90-95% of the entire four types of diabetes including, Type 1 diabetes (T1D), T2D, monogenic diabetes syndromes (MGDS), and Gestational diabetes mellitus, can be prevented or delayed through nutrition and lifestyle changes as well as through pharmacologic interventions. Typical symptom of the T2D is high blood glucose levels and comprehensive insulin resistance of the body, producing an impaired glucose tolerance. Impaired glucose tolerance of T2D is accompanied by extensive health complications, including cardiovascular diseases (CVD) that vary in morbidity and mortality among populations. The pathogenesis of T2D varies between populations and/or ethnic groupings and is known to be attributed extremely by genetic components and environmental factors. It is evident that genetic background plays a critical role in determining the host response toward certain environmental conditions, whether or not of developing T2D (susceptibility versus resistant). T2D is considered as a silent disease that can progress for years before its diagnosis. Once T2D is diagnosed, many metabolic malfunctions are observed whether as side effects or as independent comorbidity. Mouse models have been proven to be a powerful tool for mapping genetic factors that underline the susceptibility to T2D development as well its comorbidities. Here, we have conducted a comprehensive search throughout the published data covering the time span from early 1990s till the time of writing this review, for already reported quantitative trait locus (QTL) associated with murine T2D and comorbidities in different mouse models, which contain different genetic backgrounds. Our search has resulted in finding 54 QTLs associated with T2D in addition to 72 QTLs associated with comorbidities associated with the disease. We summarized the genomic locations of these mapped QTLs in graphical formats, so as to show the overlapping positions between of these mapped QTLs, which may suggest that some of these QTLs could be underlined by sharing gene/s. Finally, we reviewed and addressed published reports that show the success of translation of the identified mouse QTLs/genes associated with the disease in humans.

Analysis of independent cohorts of outbred CFW mice reveals novel loci for behavioral and physiological traits and identifies factors determining reproducibility

Combining samples for genetic association is standard practice in human genetic analysis of complex traits, but is rarely undertaken in rodent genetics. Here, using 23 phenotypes and genotypes from two independent laboratories, we obtained a sample size of 3076 commercially available outbred mice and identified 70 loci, more than double the number of loci identified in the component studies. Fine-mapping in the combined sample reduced the number of likely causal variants, with a median reduction in set size of 51%, and indicated novel gene associations, including Pnpo, Ttll6, and GM11545 with bone mineral density, and Psmb9 with weight. However, replication at a nominal threshold of 0.05 between the two component studies was low, with less than one-third of loci identified in one study replicated in the second. In addition to overestimates in the effect size in the discovery sample (Winner's Curse), we also found that heterogeneity between studies explained the poor replication, but the contribution of these two factors varied among traits. Leveraging these observations, we integrated information about replication rates, study-specific heterogeneity, and Winner's Curse corrected estimates of power to assign variants to one of four confidence levels. Our approach addresses concerns about reproducibility and demonstrates how to obtain robust results from mapping complex traits in any genome-wide association study.

Diet and Obesity-Induced Methylglyoxal Production and Links to Metabolic Disease

The obesity rate in the United States is 42.4% and has become a national epidemic. Obesity is a complex condition that is influenced by socioeconomic status, ethnicity, genetics, age, and diet. Increased consumption of a Western diet, one that is high in processed foods, red meat, and sugar content, is associated with elevated obesity rates. Factors that increase obesity risk, such as socioeconomic status, also increase consumption of a Western diet because of a limited access to healthier options and greater affordability of processed foods. Obesity is a public health threat because it increases the risk of several pathologies, including atherosclerosis, diabetes, and cancer. The molecular mechanisms linking obesity to disease onset and progression are not well understood, but a proposed mechanism is physiological changes caused by altered lipid peroxidation, glycolysis, and protein metabolism. These metabolic pathways give rise to reactive molecules such as the abundant electrophile methylglyoxal (MG), which covalently modifies nucleic acids and proteins. MG-adducts are associated with obesity-linked pathologies and may have potential for biomonitoring to determine the risk of disease onset and progression. MG-adducts may also play a role in disease progression because they are mutagenic and directly impact protein stability and function. In this review, we discuss how obesity drives metabolic alterations, how these alterations lead to MG production, the association of MG-adducts with disease, and the potential impact of MG-adducts on cellular function.

The Glyoxalase System in Age-Related Diseases: Nutritional Intervention as Anti-Ageing Strategy

The glyoxalase system is critical for the detoxification of advanced glycation end-products (AGEs). AGEs are toxic compounds resulting from the non-enzymatic modification of biomolecules by sugars or their metabolites through a process called glycation. AGEs have adverse effects on many tissues, playing a pathogenic role in the progression of molecular and cellular aging. Due to the age-related decline in different anti-AGE mechanisms, including detoxifying mechanisms and proteolytic capacities, glycated biomolecules are accumulated during normal aging in our body in a tissue-dependent manner. Viewed in this way, anti-AGE detoxifying systems are proposed as therapeutic targets to fight pathological dysfunction associated with AGE accumulation and cytotoxicity. Here, we summarize the current state of knowledge related to the protective mechanisms against glycative stress, with a special emphasis on the glyoxalase system as the primary mechanism for detoxifying the reactive intermediates of glycation. This review focuses on glyoxalase 1 (GLO1), the first enzyme of the glyoxalase system, and the rate-limiting enzyme of this catalytic process. Although GLO1 is ubiquitously expressed, protein levels and activities are regulated in a tissue-dependent manner. We provide a comparative analysis of GLO1 protein in different tissues. Our findings indicate a role for the glyoxalase system in homeostasis in the eye retina, a highly oxygenated tissue with rapid protein turnover. We also describe modulation of the glyoxalase system as a therapeutic target to delay the development of age-related diseases and summarize the literature that describes the current knowledge about nutritional compounds with properties to modulate the glyoxalase system.

Dicarbonyl Stress at the Crossroads of Healthy and Unhealthy Aging

Dicarbonyl stress occurs when dicarbonyl metabolites (i.e., methylglyoxal, glyoxal and 3-deoxyglucosone) accumulate as a consequence of their increased production and/or decreased detoxification. This toxic condition has been associated with metabolic and age-related diseases, both of which are characterized by a pro-inflammatory and pro-oxidant state. Methylglyoxal (MGO) is the most reactive dicarbonyl and the one with the highest endogenous flux. It is the precursor of the major quantitative advanced glycated products (AGEs) in physiological systems, arginine-derived hydroimidazolones, which accumulate in aging and dysfunctional tissues. The aging process is characterized by a decline in the functional properties of cells, tissues and whole organs, starting from the perturbation of crucial cellular processes, including mitochondrial function, proteostasis and stress-scavenging systems. Increasing studies are corroborating the causal relationship between MGO-derived AGEs and age-related tissue dysfunction, unveiling a previously underestimated role of dicarbonyl stress in determining healthy or unhealthy aging. This review summarizes the latest evidence supporting a causal role of dicarbonyl stress in age-related diseases, including diabetes mellitus, cardiovascular disease and neurodegeneration.

A collective diabetes cross in combination with a computational framework to dissect the genetics of human obesity and Type 2 diabetes

To explore the genetic determinants of obesity and Type 2 diabetes (T2D), the German Center for Diabetes Research (DZD) conducted crossbreedings of the obese and diabetes-prone New Zealand Obese mouse strain with four different lean strains (B6, DBA, C3H, 129P2) that vary in their susceptibility to develop T2D. Genome-wide linkage analyses localized more than 290 quantitative trait loci (QTL) for obesity, 190 QTL for diabetes-related traits and 100 QTL for plasma metabolites in the outcross populations. A computational framework was developed that allowed to refine critical regions and to nominate a small number of candidate genes by integrating reciprocal haplotype mapping and transcriptome data. The efficiency of the complex procedure was demonstrated for one obesity QTL. The genomic interval of 35 Mb with 502 annotated candidate genes was narrowed down to six candidates. Accordingly, congenic mice retained the obesity phenotype owing to an interval that contains three of the six candidate genes. Among these the phospholipase PLA2G4A exhibited an elevated expression in adipose tissue of obese human subjects and is therefore a critical regulator of the obesity locus. Together, our broad and complex approach demonstrates that combined- and comparative-cross analysis exhibits improved mapping resolution and represents a valid tool for the identification of disease genes.

Dissection of Leaf Angle Variation in Maize through Genetic Mapping and Meta-Analysis

Maize ( L.) hybrids have transitioned to upright leaf angles (LAs) over the last 50 yr as maize yields and planting densities increased concurrently. Genetic mapping and a meta-analysis were conducted in the present study to dissect genetic factors controlling LA variation. We developed mapping populations using inbred lines B73 (Iowa Stiff Stalk Synthetic), PHW30 (Iodent, expired plant variety protection inbred), and Mo17 (Non-Stiff Stalk) that have distinct LA architectures and represent three important heterotic groups in the United States. These populations were genotyped using genotyping-by-sequencing (GBS), and phenotyped for LA in the F and F generation. Inclusive composite interval mapping across the two generations of the mapping populations revealed 12 quantitative trait loci (QTL), and a consistent QTL on chromosome 1 explained 10 to 17% of the phenotypic variance. To gain a comprehensive understanding of natural variations underlying LA variation, these detected QTL were compared with results from 19 previous studies. In total, 495 QTL were compiled and mapped into 143 genomic bins. A meta-analysis revealed that 58 genomic bins were associated with LA variation. Thirty-three candidate genes were identified in these genomic bins. Together, these results provide evidence of QTL controlling LA variation from inbred lines representing three important heterotic groups in the United States and a useful resource for future research into the molecular variants underlying specific regions of the genome associated with LA variation.

Glyoxalase 1 Modulation in Obesity and Diabetes

Significance: Obesity and type 2 diabetes mellitus are increasing globally. There is also increasing associated complications, such as non-alcoholic fatty liver disease (NAFLD) and vascular complications of diabetes. There is currently no licensed treatment for NAFLD and no recent treatments for diabetic complications. New approaches are required, particularly those addressing mechanism-based risk factors for health decline and disease progression. Recent Advances: Dicarbonyl stress is the abnormal accumulation of reactive dicarbonyl metabolites such as methylglyoxal (MG) leading to cell and tissue dysfunction. It is a potential driver of obesity, diabetes, and related complications that are unaddressed by current treatments. Increased formation of MG is linked to increased glyceroneogenesis and hyperglycemia in obesity and diabetes and also down-regulation of glyoxalase 1 (Glo1)-which provides the main enzymatic detoxification of MG. Glo1 functional genomics studies suggest that increasing Glo1 expression and activity alleviates dicarbonyl stress; slows development of obesity, related insulin resistance; and prevents development of diabetic nephropathy and other microvascular complications of diabetes. A new therapeutic approach constitutes small-molecule inducers of Glo1 expression-Glo1 inducers-exploiting a regulatory antioxidant response element in the GLO1 gene. A prototype Glo1 inducer, trans-resveratrol (tRES)-hesperetin (HESP) combination, in corrected insulin resistance, improved glycemic control and vascular inflammation in healthy overweight and obese subjects in clinical trial. Critical Issues: tRES and HESP synergize pharmacologically, and HESP likely overcomes the low bioavailability of tRES by inhibition of intestinal glucuronosyltransferases. Future Directions: Glo1 inducers may now be evaluated in Phase 2 clinical trials for treatment of NAFLD and vascular complications of diabetes.

Burly1 is a mouse QTL for lean body mass that maps to a 0.8-Mb region of chromosome 2

To fine map a mouse QTL for lean body mass (Burly1), we used information from intercross, backcross, consomic, and congenic mice derived from the C57BL/6ByJ (host) and 129P3/J (donor) strains. The results from these mapping populations were concordant and showed that Burly1 is located between 151.9 and 152.7 Mb (rs33197365 to rs3700604) on mouse chromosome 2. The congenic region harboring Burly1 contains 26 protein-coding genes, 11 noncoding RNA elements (e.g., lncRNA), and 4 pseudogenes, with 1949 predicted functional variants. Of the protein-coding genes, 7 have missense variants, including genes that may contribute to lean body weight, such as Angpt41, Slc52c3, and Rem1. Lean body mass was increased by the B6-derived variant relative to the 129-derived allele. Burly1 influenced lean body weight at all ages but not food intake or locomotor activity. However, congenic mice with the B6 allele produced more heat per kilogram of lean body weight than did controls, pointing to a genotype effect on lean mass metabolism. These results show the value of integrating information from several mapping populations to refine the map location of body composition QTLs and to identify a short list of candidate genes.

Global Transcriptome Analysis of Brown Adipose Tissue of Diet-Induced Obese Mice

Consumption of a high-fat diet (HFD) promotes the development of obesity, a disease resulting from an imbalance between energy intake and energy expenditure. Brown adipose tissue (BAT) has thermogenic capacity that burns calories to produce heat, and it is a potential target for the treatment and prevention of obesity. There is limited information regarding the impact of HFD on the BAT transcriptome. We hypothesized that HFD-induced obesity would lead to transcriptional regulation of BAT genes. RNA sequencing was used to generate global transcriptome profiles from BAT of lean mice fed with a low-fat diet (LFD) and obese mice fed with a HFD. Gene Ontology (GO) analysis identified increased expression of genes involved in biological processes (BP) related to immune responses, which enhanced molecular function (MF) in chemokine activity; decreased expression of genes involved in BP related to ion transport and muscle structure development, which reduced MF in channel and transporter activity and structural binding. Kyoto Encyclopedia of Genes and Genomes (KEGG) functional pathway analysis indicated that pathways associated with innate immunity were enhanced by HFD, while pathways associated with muscle contraction and calcium signaling were suppressed by HFD. Collectively, these results suggest that diet-induced obesity changes transcriptomic signatures of BAT, leading to dysfunction involving inflammation, calcium signaling, ion transport, and cell structural development.

Methylglyoxal-induced dicarbonyl stress in aging and disease: first steps towards glyoxalase 1-based treatments

Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in aging and disease. It is produced by increased formation and/or decreased metabolism of dicarbonyl metabolites. MG (methylglyoxal) is a dicarbonyl metabolite of relatively high flux of formation and precursor of the most quantitatively and functionally important spontaneous modifications of protein and DNA clinically. Major MG-derived adducts are arginine-derived hydroimidazolones of protein and deoxyguanosine-derived imidazopurinones of DNA. These are formed non-oxidatively. The glyoxalase system provides an efficient and essential basal and stress-response-inducible enzymatic defence against dicarbonyl stress by the reduced glutathione-dependent metabolism of methylglyoxal by glyoxalase 1. The GLO1 gene encoding glyoxalase 1 has low prevalence duplication and high prevalence amplification in some tumours. Dicarbonyl stress contributes to aging, disease and activity of cytotoxic chemotherapeutic agents. It is found at a low, moderate and severe level in obesity, diabetes and renal failure respectively, where it contributes to the development of metabolic and vascular complications. Increased glyoxalase 1 expression confers multidrug resistance to cancer chemotherapy and has relatively high prevalence in liver, lung and breast cancers. Studies of dicarbonyl stress are providing improved understanding of aging and disease and the basis for rational design of novel pharmaceuticals: glyoxalase 1 inducers for obesity, diabetes and cardiovascular disease and glyoxalase 1 inhibitors for multidrug-resistant tumours. The first clinical trial of a glyoxalase 1 inducer in overweight and obese subjects showed improved glycaemic control, insulin resistance and vascular function.

Genomewide association of piglet responses to infection with one of two porcine reproductive and respiratory syndrome virus isolates

Porcine reproductive and respiratory syndrome (PRRS) is a devastating disease in the swine industry. Identification of host genetic factors that enable selection for improved performance during PRRS virus (PRRSV) infection would reduce the impact of this disease on animal welfare and production efficiency. We conducted genomewide association study (GWAS) analyses of data from 13 trials of approximately 200 commercial crossbred nursery-age piglets that were experimentally infected with 1 of 2 type 2 isolates of PRRSV (NVSL 97-7985 [NVSL] and KS2006-72109 [KS06]). Phenotypes analyzed were viral load (VL) in blood during the first 21 d after infection (dpi) and weight gain (WG) from 0 to 42 dpi. We accounted for the previously identified QTL in the region on SSC4 in our models to increase power to identify additional regions. Many regions identified by single-SNP analyses were not identified using Bayes-B, but both analyses identified the same regions on SSC3 and SSC5 to be associated with VL in the KS06 trials and on SSC6 in the NVSL trials ( < 5 × 10); for WG, regions on SSC5 and SSC17 were associated in the NVSL trials ( < 3 × 10). No regions were identified with either method for WG in the KS06 trials. Except for the region on SSC4, which was associated with VL for both isolates (but only with WG for NVSL), identified regions did not overlap between the 2 PRRSV isolate data sets, despite high estimates of the genetic correlation between isolates for traits based on these data. We also identified genomic regions whose associations with VL or WG interacted with either PRRSV isolate or with genotype at the SSC4 QTL. Gene ontology (GO) annotation terms for genes located near moderately associated SNP ( < 0.003) were enriched for multiple immunologically (VL) and metabolism- (WG) related GO terms. The biological relevance of these regions suggests that, although it may increase the number of false positives, the use of single-SNP analyses and a relaxed threshold also increased the identification of true positives. In conclusion, although only the SSC4 QTL was associated with response to both PRRSV isolates, genes near associated SNP were enriched for the same GO terms across PRRSV isolates, suggesting that host responses to these 2 isolates are affected by the actions of many genes that function together in similar biological processes.

Quantitative trait loci that control body weight in DDD/Sgn and C57BL/6J inbred mice

Inbred DDD/Sgn mice are heavier than inbred C57BL/6J mice. In the present study, we performed quantitative trait loci (QTL) mapping for body weight using R/qtl in reciprocal F₂ male populations between the two strains. We identified four significant QTL on Chrs 1, 2, 5, and 17 (proximal region). The DDD/Sgn allele was associated with increased body weight at QTL on Chrs 1 and 5, and the DDD/Sgn allele was associated with decreased body weight at QTL on Chrs 2 and 17. A multiple regression analysis indicated that the detected QTL explain 30.94 % of the body weight variation. Because DDD/Sgn male mice have extremely high levels of circulating testosterone relative to other inbred mouse strains, we performed QTL mapping for plasma testosterone level to examine the effect of testosterone levels on body weight. We identified one suggestive QTL on Chr 5, which overlapped with body weight QTL. The DDD/Sgn allele was associated with increased testosterone level. Thus, we confirmed that there was a genetic basis for the changes in body weight and testosterone levels in male mice. These findings provide insights into the genetic mechanism by which body weight is controlled in male mice.

Dicarbonyls and glyoxalase in disease mechanisms and clinical therapeutics

The reactive dicarbonyl metabolite methylglyoxal (MG) is the precursor of the major quantitative advanced glycation endproducts (AGEs) in physiological systems - arginine-derived hydroimidazolones and deoxyguanosine-derived imidazopurinones. The glyoxalase system in the cytoplasm of cells provides the primary defence against dicarbonyl glycation by catalysing the metabolism of MG and related reactive dicarbonyls. Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in ageing and disease. It is produced endogenously by increased formation and/or decreased metabolism of dicarbonyl metabolites. Dicarbonyl stress contributes to ageing, disease and activity of cytotoxic chemotherapeutic agents. It contributes to ageing through age-related decline in glyoxalase 1 (Glo-1) activity. Glo-1 has a dual role in cancer as a tumour suppressor protein prior to tumour development and mediator of multi-drug resistance in cancer treatment, implicating dicarbonyl glycation of DNA in carcinogenesis and dicarbonyl-driven cytotoxicity in mechanism of action of anticancer drugs. Glo-1 is a driver of cardiovascular disease, likely through dicarbonyl stress-driven dyslipidemia and vascular cell dysfunction. Dicarbonyl stress is also a contributing mediator of obesity and vascular complications of diabetes. There are also emerging roles in neurological disorders. Glo-1 responds to dicarbonyl stress to enhance cytoprotection at the transcriptional level through stress-responsive increase of Glo-1 expression. Small molecule Glo-1 inducers are in clinical development for improved metabolic, vascular and renal health and Glo-1 inhibitors in preclinical development for multidrug resistant cancer chemotherapy.

Dicarbonyl stress in clinical obesity

The glyoxalase system in the cytoplasm of cells provides the primary defence against glycation by methylglyoxal catalysing its metabolism to D-lactate. Methylglyoxal is the precursor of the major quantitative advanced glycation endproducts in physiological systems - arginine-derived hydroimidazolones and deoxyguanosine-derived imidazopurinones. Glyoxalase 1 of the glyoxalase system was linked to anthropometric measurements of obesity in human subjects and to body weight in strains of mice. Recent conference reports described increased weight gain on high fat diet-fed mouse with lifelong deficiency of glyoxalase 1 deficiency, compared to wild-type controls, and decreased weight gain in glyoxalase 1-overexpressing transgenic mice, suggesting a functional role of glyoxalase 1 and dicarbonyl stress in obesity. Increased methylglyoxal, dicarbonyl stress, in white adipose tissue and liver may be a mediator of obesity and insulin resistance and thereby a risk factor for development of type 2 diabetes and non-alcoholic fatty liver disease. Increased methylglyoxal formation from glyceroneogenesis on adipose tissue and liver and decreased glyoxalase 1 activity in obesity likely drives dicarbonyl stress in white adipose tissue increasing the dicarbonyl proteome and related dysfunction. The clinical significance will likely emerge from on-going clinical evaluation of inducers of glyoxalase 1 expression in overweight and obese subjects. Increased transcapillary escape rate of albumin and increased total body interstitial fluid volume in obesity likely makes levels of glycation of plasma protein unreliable indicators of glycation status in obesity as there is a shift of albumin dwell time from plasma to interstitial fluid, which decreases overall glycation for a given glycemic exposure.

Activity, regulation, copy number and function in the glyoxalase system

Molecular, catalytic and structural properties of glyoxalase pathway enzymes of many species are now known. Current research has focused on the regulation of activity and expression of Glo1 (glyoxalase I) and Glo2 (glyoxalase II) and their role in health and disease. Human GLO1 has MRE (metal-response element), IRE (insulin-response element), E2F4 (early gene 2 factor isoform 4), AP-2α (activating enhancer-binding protein 2α) and ARE (antioxidant response-element) regulatory elements and is a hotspot for copy number variation. The human Glo2 gene, HAGH (hydroxyacylglutathione hydrolase), has a regulatory p53-response element. Glo1 is linked to healthy aging, obesity, diabetes and diabetic complications, chronic renal disease, cardiovascular disease, other disorders and multidrug resistance in cancer chemotherapy. Mathematical modelling of the glyoxalase pathway predicts that pharmacological levels of increased Glo1 activity markedly decrease cellular methylglyoxal and related glycation, and pharmacological Glo1 inhibition markedly increases cellular methylglyoxal and related glycation. Glo1 inducers are in development to sustain healthy aging and for treatment of vascular complications of diabetes and other disorders, and cell-permeant Glo1 inhibitors are in development for treatment of multidrug-resistant tumours, malaria and potentially pathogenic bacteria and fungi.

Obesity genetics in mouse and human: back and forth, and back again

Obesity is a major public health concern. This condition results from a constant and complex interplay between predisposing genes and environmental stimuli. Current attempts to manage obesity have been moderately effective and a better understanding of the etiology of obesity is required for the development of more successful and personalized prevention and treatment options. To that effect, mouse models have been an essential tool in expanding our understanding of obesity, due to the availability of their complete genome sequence, genetically identified and defined strains, various tools for genetic manipulation and the accessibility of target tissues for obesity that are not easily attainable from humans. Our knowledge of monogenic obesity in humans greatly benefited from the mouse obesity genetics field. Genes underlying highly penetrant forms of monogenic obesity are part of the leptin-melanocortin pathway in the hypothalamus. Recently, hypothesis-generating genome-wide association studies for polygenic obesity traits in humans have led to the identification of 119 common gene variants with modest effect, most of them having an unknown function. These discoveries have led to novel animal models and have illuminated new biologic pathways. Integrated mouse-human genetic approaches have firmly established new obesity candidate genes. Innovative strategies recently developed by scientists are described in this review to accelerate the identification of causal genes and deepen our understanding of obesity etiology. An exhaustive dissection of the molecular roots of obesity may ultimately help to tackle the growing obesity epidemic worldwide.

Dicarbonyl stress in cell and tissue dysfunction contributing to ageing and disease

Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in ageing and disease. Enzymes metabolising dicarbonyls, glyoxalase 1 and aldoketo reductases, provide an efficient and stress-response enzyme defence against dicarbonyl stress. Dicarbonyl stress is produced by increased formation and/or decreased metabolism of dicarbonyl metabolites, and by exposure to exogenous dicarbonyls. It contributes to ageing, disease and activity of cytototoxic chemotherapeutic agents.

Identification of co-expression gene networks, regulatory genes and pathways for obesity based on adipose tissue RNA Sequencing in a porcine model

BACKGROUND

Obesity is a complex metabolic condition in strong association with various diseases, like type 2 diabetes, resulting in major public health and economic implications. Obesity is the result of environmental and genetic factors and their interactions, including genome-wide genetic interactions. Identification of co-expressed and regulatory genes in RNA extracted from relevant tissues representing lean and obese individuals provides an entry point for the identification of genes and pathways of importance to the development of obesity. The pig, an omnivorous animal, is an excellent model for human obesity, offering the possibility to study in-depth organ-level transcriptomic regulations of obesity, unfeasible in humans. Our aim was to reveal adipose tissue co-expression networks, pathways and transcriptional regulations of obesity using RNA Sequencing based systems biology approaches in a porcine model.

METHODS

We selected 36 animals for RNA Sequencing from a previously created F2 pig population representing three extreme groups based on their predicted genetic risks for obesity. We applied Weighted Gene Co-expression Network Analysis (WGCNA) to detect clusters of highly co-expressed genes (modules). Additionally, regulator genes were detected using Lemon-Tree algorithms.

RESULTS

WGCNA revealed five modules which were strongly correlated with at least one obesity-related phenotype (correlations ranging from -0.54 to 0.72, P < 0.001). Functional annotation identified pathways enlightening the association between obesity and other diseases, like osteoporosis (osteoclast differentiation, P = 1.4E-7), and immune-related complications (e.g. Natural killer cell mediated cytotoxity, P = 3.8E-5; B cell receptor signaling pathway, P = 7.2E-5). Lemon-Tree identified three potential regulator genes, using confident scores, for the WGCNA module which was associated with osteoclast differentiation: CCR1, MSR1 and SI1 (probability scores respectively 95.30, 62.28, and 34.58). Moreover, detection of differentially connected genes identified various genes previously identified to be associated with obesity in humans and rodents, e.g. CSF1R and MARC2.

CONCLUSIONS

To our knowledge, this is the first study to apply systems biology approaches using porcine adipose tissue RNA-Sequencing data in a genetically characterized porcine model for obesity. We revealed complex networks, pathways, candidate and regulatory genes related to obesity, confirming the complexity of obesity and its association with immune-related disorders and osteoporosis.

The genetic basis of obesity-associated type 2 diabetes (diabesity) in polygenic mouse models

Obesity-associated diabetes (“diabesity”) in mouse strains is characterized by severe insulin resistance, hyperglycaemia and progressive failure, and loss of beta cells. This condition is observed in inbred obese mouse strains such as the New Zealand Obese (NZO/HlLt and NZO/HlBomDife) or the TALLYHO/JngJ mouse. In lean strains such as C57BLKS/J, BTBR T+tf/J or DBA/2 J carrying diabetes susceptibility genes (“diabetes susceptible” background), it can be induced by introgression of the obesity-causing mutations Lep<ob> (ob) or Lepr<db> (db). Outcross populations of these models have been employed in the genome-wide search for mouse diabetes genes, and have led to positional cloning of the strong candidates Pctp, Tbc1d1, Zfp69, and Ifi202b (NZO-derived obesity) and Sorcs1,Lisch-like, Tomosyn-2, App, Tsc2, and Ube2l6 (obesity caused by the ob or db mutation). Some of these genes have been shown to play a role in the regulation of the human glucose or lipid metabolism. Thus, dissection of the genetic basis of obesity and diabetes in mouse models can identify regulatory mechanisms that are relevant for the human disease.

Copy number variation of glyoxalase I

The glyoxalase I gene GLO1 is a hotspot for copy number variation in the human and mouse genomes. The additional copies are often functional, giving rise to 2–4-fold increased glyoxalase I expression and activity. The prevalence of GLO1 copy number increase in the human population appears to be approximately 2% and may be linked to a risk of obesity, diabetes and aging. Increased GLO1 copy number has been found in human tumour cell lines and primary human tumours. The minimum common copy number increase region was approximately 1 Mb and it contained GLO1 and seven other genes. The increased copy number was generally functional, being associated with increased glyoxalase I protein and multidrug resistance in cancer chemotherapy. Glo1 duplication in the mouse genome is found within approximately 0.5 Mb of duplicated DNA. It was claimed to be linked to anxiety phenotypes, but other related discordant findings have doubted the association with glyoxalase I and further investigation is required.

Identification and Association of SNPs in TBC1D1 Gene with Growth Traits in Two Rabbit Breeds

The TBC1D1 plays a key role in body energy homeostasis by regulating the insulin-stimulated glucose uptake in skeletal muscle. The present study aimed to identify the association between genetic polymorphisms of TBC1D1 and body weight (BW) in rabbits. Among the total of 12 SNPs detected in all 20 exons, only one SNP was non-synonymous (c.214G>A. p.G72R) located in exon 1. c.214G>A was subsequently genotyped among 491 individuals from two rabbit breeds by the high-resolution melting method. Allele A was the predominant allele with frequencies of 0.7780 and 0.6678 in European white rabbit (EWR, n = 205) and New Zealand White rabbit (NZW, n = 286), respectively. The moderate polymorphism information content (0.250.05). Our results implied that the c.214G>A of TBC1D1 gene might be one of the candidate loci affecting the trait of 35 d BW in the rabbit.

Conventional knockout of Tbc1d1 in mice impairs insulin- and AICAR-stimulated glucose uptake in skeletal muscle

In the obesity-resistant SJL mouse strain, we previously identified a naturally occurring loss-of-function mutation in the gene for Tbc1d1. Characterization of recombinant inbred mice that carried the Tbc1d1(SJL) allele on a C57BL/6J background indicated that loss of TBC1D1 protects from obesity, presumably by increasing the use of fat as energy source. To provide direct functional evidence for an involvement of TBC1D1 in energy substrate metabolism, we generated and characterized conventional Tbc1d1 knockout mice. TBC1D1-deficient mice showed moderately reduced body weight, decreased respiratory quotient, and an elevated resting metabolic rate. Ex vivo analysis of intact isolated skeletal muscle revealed a severe impairment in insulin- and AICAR-stimulated glucose uptake in glycolytic extensor digitorum longus muscle and a substantially increased rate of fatty acid oxidation in oxidative soleus muscle. Our results provide direct evidence that TBC1D1 plays a major role in glucose and lipid utilization, and energy substrate preference in skeletal muscle.

Comparative genomics and functional study of lipid metabolic genes in Caenorhabditis elegans

BACKGROUND

Animal models are indispensable to understand the lipid metabolism and lipid metabolic diseases. Over the last decade, the nematode Caenorhabditis elegans has become a popular animal model for exploring the regulation of lipid metabolism, obesity, and obese-related diseases. However, the genomic and functional conservation of lipid metabolism from C. elegans to humans remains unknown. In the present study, we systematically analyzed genes involved in lipid metabolism in the C. elegans genome using comparative genomics.

RESULTS

We built a database containing 471 lipid genes from the C. elegans genome, and then assigned most of lipid genes into 16 different lipid metabolic pathways that were integrated into a network. Over 70% of C. elegans lipid genes have human orthologs, with 237 of 471 C. elegans lipid genes being conserved in humans, mice, rats, and Drosophila, of which 71 genes are specifically related to human metabolic diseases. Moreover, RNA-mediated interference (RNAi) was used to disrupt the expression of 356 of 471 lipid genes with available RNAi clones. We found that 21 genes strongly affect fat storage, development, reproduction, and other visible phenotypes, 6 of which have not previously been implicated in the regulation of fat metabolism and other phenotypes.

CONCLUSIONS

This study provides the first systematic genomic insight into lipid metabolism in C. elegans, supporting the use of C. elegans as an increasingly prominent model in the study of metabolic diseases.

Moo1 obesity quantitative trait locus in BTBR T+ Itpr3tf/J mice increases food intake

The rising prevalence of obesity is one of the greatest health challenges facing the world today. Discovery of genetic factors affecting obesity risk will provide important insight to its etiology that could suggest new therapeutic approaches. We have previously identified the Modifier of obese 1 (Moo1) quantitative trait locus (QTL) in a cross between leptin-deficient BTBR T(+) Itpr3(tf)/J (BTBR) and C57BL/6J (B6) mice. Understanding the mechanism by which this locus acts will aid in the identification of candidate genes. Here we refined the location of this QTL and sought to determine the mechanism by which Moo1 affects body weight. We found that the effects of Moo1 also alter high fat diet-induced obesity in mice having functional leptin. In detailed metabolic analyses we determined that this locus acts by increasing food intake in BTBR mice, without affecting energy expenditure. The expression levels of the main molecular mediators of food intake in the hypothalamus were not altered, suggesting this locus affects an independent pathway, consistent with its identification in mice lacking functional leptin. Finally, we show that the increased adiposity resulting from Moo1 is sufficient to affect glucose tolerance. These studies show that the Moo1 obesity QTL affects food intake, likely through a novel mechanism, and indicate that modulation of the underlying pathway may not only ameliorate obesity but also its clinical consequences.

Nutrition-/diet-induced changes in gene expression in pancreatic β-cells

β-Cell dysfunction is a critical component in the development of type 2 diabetes. Whilst both genetic and environmental factors contribute to the development of the disease, relatively little is known about the molecular network that is responsible for diet-induced functional changes in pancreatic β-cells. Recent genome-wide association studies for diabetes-related traits have generated a large number of candidate genes that constitute possible links between dietary factors and the genetic susceptibility for β-cell failure. Here, we summarize recent approaches for identifying nutritionally regulated transcripts in islets on a genome-wide scale. Polygenic mouse models for type 2 diabetes have been instrumental for investigating the mechanism of diet-induced β-cell dysfunction. Enhanced oxidative metabolism, triggered by a combination of dietary carbohydrates and fat, appears to play a critical role in the pathophysiology of diet-induced impairment of islets. More systematic studies of gene-diet interactions in β-cells of rodent models in combination with genetic profiling might reveal the regulatory circuits fundamental for the understanding of diet-induced impairments of β-cell function in humans.

Inferring genetic architecture of complex traits using Bayesian integrative analysis of genome and transcriptome data

Background

To understand the genetic architecture of complex traits and bridge the genotype-phenotype gap, it is useful to study intermediate -omics data, e.g. the transcriptome. The present study introduces a method for simultaneous quantification of the contributions from single nucleotide polymorphisms (SNPs) and transcript abundances in explaining phenotypic variance, using Bayesian whole-omics models. Bayesian mixed models and variable selection models were used and, based on parameter samples from the model posterior distributions, explained variances were further partitioned at the level of chromosomes and genome segments.

Results

We analyzed three growth-related traits: Body Weight (BW), Feed Intake (FI), and Feed Efficiency (FE), in an F₂ population of 440 mice. The genomic variation was covered by 1806 tag SNPs, and transcript abundances were available from 23,698 probes measured in the liver. Explained variances were computed for models using pedigree, SNPs, transcripts, and combinations of these. Comparison of these models showed that for BW, a large part of the variation explained by SNPs could be covered by the liver transcript abundances; this was less true for FI and FE. For BW, the main quantitative trait loci (QTLs) are found on chromosomes 1, 2, 9, 10, and 11, and the QTLs on 1, 9, and 10 appear to be expression Quantitative Trait Locus (eQTLs) affecting gene expression in the liver. Chromosome 9 is the case of an apparent eQTL, showing that genomic variance disappears, and that a tri-modal distribution of genomic values collapses, when gene expressions are added to the model.

Conclusions

With increased availability of various -omics data, integrative approaches are promising tools for understanding the genetic architecture of complex traits. Partitioning of explained variances at the chromosome and genome-segment level clearly separated regulatory and structural genomic variation as the areas where SNP effects disappeared/remained after adding transcripts to the model. The models that include transcripts explained more phenotypic variance and were better at predicting phenotypes than a model using SNPs alone. The predictions from these Bayesian models are generally unbiased, validating the estimates of explained variances.

A role for microchimerism in obesity and evolution?

Cells exchanged between individuals, such as those passing the placenta from the mother to the child and vice versa, may survive in the fetal or maternal circulation and tissues for decades and result in microchimerism. Microchimeric cells may play a role in tissue repair, but they have also been implicated as inducers of chronic inflammation, leading to autoimmunity or even cancer. Here we propose that microchimerism may play a more fundamental role in health and evolution by setting a limit to genomic variability within populations. This means that microchimerism allows immune recognition of genomic differences between donor and host which may, depending on the level of variability, cause chronic inflammation. Since chronic inflammation has been experimentally linked to metabolic syndrome, we propose that genomic variability could affect the individual's weight. Thus, metabolic syndrome, which is a growing health problem, may not only result from our lifestyle, but in part be caused by global migration and the increasingly diverse origin of the present human population. Moreover, since in nature weight gain is associated with an increased risk of predation, we discuss the possibility that immunological incompatibility normally promotes the continuous development of new species.

The TALLYHO mouse as a model of human type 2 diabetes

The TALLYHO/Jng (TH) mouse is an inbred polygenic model for type 2 diabetes (T2D) with moderate obesity. Both male and female TH mice are characterized by increased body and fat pad weights, hyperleptinemia, hyperinsulinemia, and hyperlipidemia. Glucose intolerance and hyperglycemia are exhibited only in males. Reduced 2-deoxy-glucose uptake occurs in adipose tissue and skeletal muscle of male TH mice. While both sexes of TH mice exhibit enlarged pancreatic islets, only males have degranulation and abnormal architecture in islets. Endothelial dysfunction and considerably decreased bone density are also observed in male TH mice. The blood pressure of male TH mice is normal. Genetic outcross experiments with non-diabetic strains revealed multiple susceptibility loci (quantitative trait loci) for obesity, hypertriglyceridemia, hypercholesterolemia, and hyperglycemia. In conclusion, TH mice encompass many aspects of polygenic human diabetes and are a very useful model for T2D.

Detection of novel SNPs and mapping of the fatness QTL on pig chromosome 7q1.1-1.4 region

Many QTLs for fatness traits have been mapped on pig chromosome 7q1.1-1.4 in various pig resource populations. Eight novel markers, including seven SNPs and one insertion or deletion within BTNL1, COL21A1, PPARD, GLP1R, MDFI, GNMT, ABCC10, and PLA2G7 genes, as well as two previously reported SNPs in SLC39A7 and HMGA1 genes, were genotyped in Large White and Meishan pig breeds. Except for two SNPs in HMGA1 and ABCC10 genes, allele frequencies of the other eight markers are highly significant different between Chinese indigenous Meishan breeds and Large White pig breeds. Eight polymorphic sites were then used for linkage and QTL mapping to refine the fatness QTL in a Large White × Meishan F(2) resource population. Five chromosome-wise significant QTLs were detected, of which the QTLs for leaf fat weight, backfat thickness at 6-7th rib and rump, and mean backfat thickness were narrowed to the interval between PPARD and GLP1R genes and the QTL for backfat thickness at thorax-waist between GNMT and PLA2G7 genes on SSC7p1.1-q1.4.

Advances in comparative genetics: influence of genetics on obesity

Obesity has reached epidemic proportions and is recognised as a significant global health problem. Increased food intake and decreased physical activity are traditionally to blame for the development of obesity; however, many variables such as behaviour, diet, environment, social structures and genetics also contribute to this multifactorial disease. Complex interactions among these variables (for example, gene-environment, gene-diet and gene-gene) contribute not only to individual differences in the development of obesity, but also in treatment response. Mouse models have historically played valuable roles in understanding the genetics of traits related to energy balance and obesity. In the present review, we survey past use and examine new advances in mouse models designed to uncover the genetic architecture of obesity and its component traits. We discuss traditional models such as inbred strains and selectively bred lines and their contributions and shortcomings. We consider the evolution of mouse models into more informative resources such as outbred crosses and the Hybrid Mouse Diversity Panel, as well as novel next-generation approaches such as the Collaborative Cross. Moreover, the genetic architecture of voluntary exercise and the interactive relationship between host genetics and the gut microbiome are presented as novel phenotypes that augment studies using body weight and body fat percentage as endpoints. Understanding the intricate network of phenotypic, genotypic and environmental variables that predispose individuals to obesity will elucidate biological networks involved in the development of obesity. Knowledge obtained from advances in mouse models will inform human health and provide insight into inter-individual variability in the aetiology of obesity-related diseases.

Gene-diet interactions in childhood obesity

Childhood overweight and obesity have reached epidemic proportions worldwide, and the increase in weight-associated co-morbidities including premature type 2 diabetes mellitus (T2DM) and atherosclerotic cardiovascular disease will soon become major healthcare and economic problems. A number of studies now indicate that the childhood obesity epidemic which has emerged during the past 30 years is a complex multi-factorial disease resulting from interaction of susceptibility genes with an obesogenic environment. This review will focus on gene-diet interactions suspected of having a prominent role in promoting childhood obesity. In particular, the specific genes that will be presented (FTO, MC4R, and NPC1) have recently been associated with childhood obesity through a genome-wide association study (GWAS) and were shown to interact with nutritional components to increase weight gain. Although a fourth gene (APOA2) has not yet been associated with childhood obesity, this review will also present information on what now represents the best characterized gene-diet interaction in promoting weight gain.

A comparative assessment of mandible shape in a consomic strain panel of the house mouse (Mus musculus)--implications for epistasis and evolvability of quantitative traits

Background

Expectations of repeatedly finding associations between given genes and phenotypes have been borne out by studies of parallel evolution, especially for traits involving absence or presence of characters. However, it has rarely been asked whether the genetic basis of quantitative trait variation is conserved at the intra- or even at the interspecific level. This question is especially relevant for shape, where the high dimensionality of variation seems to require a highly complex genetic architecture involving many genes.

Results

We analyse here the genetic effects of chromosome substitution strains carrying M. m. musculus chromosomes in a largely M. m. domesticus background on mandible shape and compare them to the results of previously published QTL mapping data between M. m. domesticus strains. We find that the distribution of genetic effects and effect sizes across the genome is consistent between the studies, while the specific shape changes associated with the chromosomes are different. We find also that the sum of the effects from the different M. m. musculus chromosomes is very different from the shape of the strain from which they were derived, as well as all known wild type shapes.

Conclusions

Our results suggest that the relative chromosome-wide effect sizes are comparable between the long separated subspecies M. m. domesticus and M. m. musculus, hinting at a relative stability of genes involved in this complex trait. However, the absolute effect sizes and the effect directions may be allele-dependent, or are context dependent, i.e. epistatic interactions appear to play an important role in controlling shape.

Fat maintenance is a predictor of the murine lifespan response to dietary restriction

Dietary restriction (DR), one of the most robust life-extending manipulations, is usually associated with reduced adiposity. This reduction is hypothesized to be important in the life-extending effect of DR, because excess adiposity is associated with metabolic and age-related disease. Previously, we described remarkable variation in the lifespan response of 41 recombinant inbred strains of mice to DR, ranging from life extension to life shortening. Here, we used this variation to determine the relationship of lifespan modulation under DR to fat loss. Across strains, DR life extension correlated inversely with fat reduction, measured at midlife (males, r= -0.41, P<0.05, n=38 strains; females, r= -0.63, P<0.001, n=33 strains) and later ages. Thus, strains with the least reduction in fat were more likely to show life extension, and those with the greatest reduction were more likely to have shortened lifespan. We identified two significant quantitative trait loci (QTLs) affecting fat mass under DR in males but none for lifespan, precluding the confirmation of these loci as coordinate modulators of adiposity and longevity. Our data also provide evidence for a QTL previously shown to affect fuel efficiency under DR. In summary, the data do not support an important role for fat reduction in life extension by DR. They suggest instead that factors associated with maintaining adiposity are important for survival and life extension under DR.

Four out of eight genes in a mouse chromosome 7 congenic donor region are candidate obesity genes

We previously identified a region of mouse chromosome 7 that influences body fat mass in F2 littermates of congenic × background intercrosses. Current analyses revealed that alleles in the donor region of the subcongenic B6.C-D7Mit318 (318) promoted a twofold increase in adiposity in homozygous lines of 318 compared with background C57BL/6ByJ (B6By) mice. Parent-of-origin effects were discounted through cross-fostering studies and an F1 reciprocal cross. Mapping of the donor region revealed that it has a maximal size of 2.8 Mb (minimum 1.8 Mb) and contains a maximum of eight protein coding genes. Quantitative PCR in whole brain, liver, and gonadal white adipose tissue (GWAT) revealed differential expression between genotypes for three genes in females and two genes in males. Alpha-2,8-sialyltransferase 8B (St8sia2) showed reduced 318 mRNA levels in brain for females and males and in GWAT for females only. Both sexes of 318 mice had reduced Repulsive guidance molecule-a (Rgma) expression in GWAT. In brain, Family with sequence similarity 174 member b (Fam174b) had increased expression in 318 females, whereas Chromodomain helicase DNA binding protein 2 (Chd2-2) had reduced expression in 318 males. No donor region genes were differentially expressed in liver. Sequence analysis of coding exons for all genes in the 318 donor region revealed only one single nucleotide polymorphism that produced a nonsynonymous missense mutation, Gln7Pro, in Fam174b. Our findings highlight the difficulty of using expression and sequence to identify quantitative trait genes underlying obesity even in small genomic regions.

Intersubspecific subcongenic mouse strain analysis reveals closely linked QTLs with opposite effects on body weight

A previous genome-wide QTL study revealed many QTLs affecting postnatal body weight and growth in an intersubspecific backcross mouse population between the C57BL/6J (B6) strain and wild Mus musculus castaneus mice captured in the Philippines. Subsequently, several closely linked QTLs for body composition traits were revealed in an F(2) intercross population between B6 and B6.Cg-Pbwg1, a congenic strain on the B6 genetic background carrying the growth QTL Pbwg1 on proximal chromosome 2. However, no QTL affecting body weight has been duplicated in the F(2) population, except for mapping an overdominant QTL that causes heterosis of body weight. In this study, we developed 17 intersubspecific subcongenic strains with overlapping and nonoverlapping castaneus regions from the B6.Cg-Pbwg1 congenic strain in order to search for and genetically dissect QTLs affecting body weight into distinct closely linked loci. Phenotypic comparisons of several developed subcongenic strains with the B6 strain revealed that two closely linked but distinct QTLs that regulate body weight, named Pbwg1.11 and Pbwg1.12, are located on an 8.9-Mb region between D2Mit270 and D2Mit472 and on the next 3.6-Mb region between D2Mit205 and D2Mit182, respectively. Further analyses using F(2) segregating populations obtained from intercrosses between B6 and each of the two selected subcongenic strains confirmed the presence of these two body weight QTLs. Pbwg1.11 had an additive effect on body weight at 6, 10, and 13 weeks of age, and its castaneus allele decreased it. In contrast, the castaneus allele at Pbwg1.12 acted in a dominant fashion and surprisingly increased body weight at 6, 10, and 13 weeks of age despite the body weight of wild castaneus mice being 60% of that of B6 mice. These findings illustrate the complex genetic nature of body weight regulation and support the importance of subcongenic mouse analysis to dissect closely linked loci.

The genetic basis of obesity and type 2 diabetes: lessons from the new zealand obese mouse, a polygenic model of the metabolic syndrome

The New Zealand obese (NZO) mouse is a polygenic model of severe obesity and type 2 diabetes-like hyperglycaemia. Outcross experiments with lean strains have led to the identification of numerous susceptibility loci (quantitative trait loci (QTL)) for adiposity and/or hyperglycaemia. Several major QTL were successfully introgressed into lean strains, and two responsible genes, the RabGAP Tbc1d1 and the transcription factor Zfp69, were so far identified by a conventional strategy of positional cloning. Tbc1d1 controls substrate utilization in muscle; SJL mice carry a loss-of-function variant that shifts substrate oxidation from glucose to fat and suppresses adiposity as well as development of diabetes. The zinc finger domain transcription factor Zfp69 appears to regulate triglyceride storage in adipose tissue. Its normal allele Zfp69 causes a redistribution of triglycerides from gonadal stores to liver, and consequently enhances diabetes when introgressed from SJL into NZO, whereas the loss-of-function variant present in NZO and C57BL/6J reduces the prevalence of diabetes. Data from human patients suggest that the orthologs of both genes may play a role in the pathogenesis of the human metabolic syndrome. In addition to Tbc1d1 and Zfp69, variants of Lepr, Pctp, Abcg1, and Nmur2 located in other QTL were identified as potential candidates by sequencing and functional studies. These results indicate that dissection of the genetic basis of obesity and diabetes in mouse models can identify novel regulatory mechanisms that are relevant for the human disease.

Genetic and genomic analysis of hyperlipidemia, obesity and diabetes using (C57BL/6J × TALLYHO/JngJ) F2 mice

BACKGROUND

Type 2 diabetes (T2D) is the most common form of diabetes in humans and is closely associated with dyslipidemia and obesity that magnifies the mortality and morbidity related to T2D. The genetic contribution to human T2D and related metabolic disorders is evident, and mostly follows polygenic inheritance. The TALLYHO/JngJ (TH) mice are a polygenic model for T2D characterized by obesity, hyperinsulinemia, impaired glucose uptake and tolerance, hyperlipidemia, and hyperglycemia.

RESULTS

In order to determine the genetic factors that contribute to these T2D related characteristics in TH mice, we interbred TH mice with C57BL/6J (B6) mice. The parental, F1, and F2 mice were phenotyped at 8, 12, 16, 20, and 24 weeks of age for 4-hour fasting plasma triglyceride, cholesterol, insulin, and glucose levels and body, fat pad and carcass weights. The F2 mice were genotyped genome-wide and used for quantitative trait locus (QTL) mapping. We also applied a genetical genomic approach using a subset of the F2 mice to seek candidate genes underlying the QTLs. Major QTLs were detected on chromosomes (Chrs) 1, 11, 4, and 8 for hypertriglyceridemia, 1 and 3 for hypercholesterolemia, 4 for hyperglycemia, 11 and 1 for body weight, 1 for fat pad weight, and 11 and 14 for carcass weight. Most alleles, except for Chr 3 and 14 QTLs, increased phenotypic values when contributed by the TH strain. Fourteen pairs of interacting loci were detected, none of which overlapped the major QTLs. The QTL interval linked to hypercholesterolemia and hypertriglyceridemia on distal Chr 1 contains Apoa2 gene. Sequencing analysis revealed polymorphisms of Apoa2 in TH mice, suggesting Apoa2 as the candidate gene for the hyperlipidemia QTL. Gene expression analysis added novel information and aided in selection of candidates underlying the QTLs.

CONCLUSIONS

We identified several genetic loci that affect the quantitative variations of plasma lipid and glucose levels and obesity traits in a TH × B6 intercross. Polymorphisms in Apoa2 gene are suggested to be responsible for the Chr 1 QTL linked to hypercholesterolemia and hypertriglyceridemia. Further, genetical genomic analysis led to potential candidate genes for the QTLs.

Identification of genes and networks driving cardiovascular and metabolic phenotypes in a mouse F2 intercross

To identify the genes and pathways that underlie cardiovascular and metabolic phenotypes we performed an integrated analysis of a mouse C57BL/6JxA/J F2 (B6AF2) cross by relating genome-wide gene expression data from adipose, kidney, and liver tissues to physiological endpoints measured in the population. We have identified a large number of trait QTLs including loci driving variation in cardiac function on chromosomes 2 and 6 and a hotspot for adiposity, energy metabolism, and glucose traits on chromosome 8. Integration of adipose gene expression data identified a core set of genes that drive the chromosome 8 adiposity QTL. This chromosome 8 trans eQTL signature contains genes associated with mitochondrial function and oxidative phosphorylation and maps to a subnetwork with conserved function in humans that was previously implicated in human obesity. In addition, human eSNPs corresponding to orthologous genes from the signature show enrichment for association to type II diabetes in the DIAGRAM cohort, supporting the idea that the chromosome 8 locus perturbs a molecular network that in humans senses variations in DNA and in turn affects metabolic disease risk. We functionally validate predictions from this approach by demonstrating metabolic phenotypes in knockout mice for three genes from the trans eQTL signature, Akr1b8, Emr1, and Rgs2. In addition we show that the transcriptional signatures for knockout of two of these genes, Akr1b8 and Rgs2, map to the F2 network modules associated with the chromosome 8 trans eQTL signature and that these modules are in turn very significantly correlated with adiposity in the F2 population. Overall this study demonstrates how integrating gene expression data with QTL analysis in a network-based framework can aid in the elucidation of the molecular drivers of disease that can be translated from mice to humans.

Combining QTL data for HDL cholesterol levels from two different species leads to smaller confidence intervals

Quantitative trait locus (QTL) analysis detects regions of a genome that are linked to a complex trait. Once a QTL is detected, the region is narrowed by positional cloning in the hope of determining the underlying candidate gene-methods used include creating congenic strains, comparative genomics and gene expression analysis. Combined cross analysis may also be used for species such as the mouse, if the QTL is detected in multiple crosses. This process involves the recoding of QTL data on a per-chromosome basis, with the genotype recoded on the basis of high- and low-allele status. The data are then combined and analyzed; a successful analysis results in a narrowed and more significant QTL. Using parallel methods, we show that it is possible to narrow a QTL by combining data from two different species, the rat and the mouse. We combined standardized high-density lipoprotein phenotype values and genotype data for the rat and mouse using information from one rat cross and two mouse crosses. We successfully combined data within homologous regions from rat Chr 6 onto mouse Chr 12, and from rat Chr 10 onto mouse Chr 11. The combinations and analyses resulted in QTL with smaller confidence intervals and increased logarithm of the odds ratio scores. The numbers of candidate genes encompassed by the QTL on mouse Chr 11 and 12 were reduced from 1343 to 761 genes and from 613 to 304 genes, respectively. This is the first time that QTL data from different species were successfully combined; this method promises to be a useful tool for narrowing QTL intervals.

Genetic analysis of abdominal fat distribution in SM/J and A/J mice

Each abdominal fat depot, such as mesenteric or epididymal, differently contributes to the development of insulin resistance. The aim of this study was to identify the genetic regions that contribute to fat accumulation in epididymal/mesenteric fat and to examine whether or not the genetic regions that affect glucose metabolism and body fat distribution are coincident. We previously mapped a major quantitative trait locus (QTL) (T2dm2sa) for impaired glucose tolerance on chromosome 2 and revealed that SM.A-T2dm2sa congenic mice showed not only glucose tolerance but also fat accumulation. In the present study, to identify the loci/genes that control the accumulation of abdominal fat, we performed QTL analyses of epididymal/mesenteric fat weight by using (A/J x SM.A-T2dm2sa)F2 mice in which the effect of T2dm2sa was excluded. As a result, two highly significant QTLs for mesenteric fat, as well as three significant QTLs for epididymal/mesenteric fat, were mapped on the different chromosomal regions. This suggests that the fat accumulations in individual fat depots are controlled by distinct genomic regions. Our comparison of these QTLs for abdominal fat distribution with those for glucose metabolism revealed that the major genetic factors affecting body fat distribution do not coincide with genetic factors affecting glucose metabolism in (A/J x SM.A-T2dm2sa)F2.

A unique genetic defect on chromosome 3 is responsible for juvenile obesity in the Berlin Fat Mouse

OBJECTIVE

This study aimed at the mapping and estimation of genetic and sex effects contributing to the obese phenotype of the Berlin Fat Mouse Inbred line 860 (BFMI860). This mouse line is predisposed for juvenile obesity. BFMI860 mice accumulate 24% total fat mass at 10 weeks of age under a standard maintenance diet.

DESIGN

A total of 471 mice of a (BFMI860 x C57BL/6NCrl) F₂ intercross population were fed a standard maintenance diet and were analysed for body composition at 10 weeks when they finished their rapid growth phase.

RESULTS

The most striking result was the identification of a novel obesity locus on chromosome 3 (Chr 3) at 40 Mb, explaining 39% of the variance of total fat mass in the F₂ population under a standard diet. This locus was named jObes1 (juvenile obesity 1). The BFMI860 allele effect was recessive. Males and females homozygous at jObes1 had on average 3.0 and 3.3 g more total fat mass at 10 weeks than the other two genotype classes, respectively. The effect was evident in all white adipose tissues, brown adipose tissue and also in liver. The position of the Chr 3 effect is syntenic to an obesity locus in humans. Additional loci for total fat mass and different white adipose tissue weights with minor effects were detected on mouse Chr 5 and 6. Another locus on Chr 4 had influence especially on liver weight. Many loci including jObes1 affected males and females to a different extent.

CONCLUSION

The major locus on Chr 3 for juvenile obesity and its interaction with sex is unique and makes the BFMI860 mice an interesting resource for the discovery of novel genetic factors predisposing obesity, which might also contribute to obesity in humans. The results suggested that metabolic and regulatory pathways differed between the sexes.

Congenic and bioinformatics analyses resolved a major-effect Fob3b QTL on mouse Chr 15 into two closely linked loci

We previously identified a Chr 15 quantitative trait locus (QTL) Fob3b in lines of mice selected for high (Fat line) and low (Lean line) body fat content that represent a unique model of polygenic obesity. Here we genetically dissected the Fob3b interval by analyzing the phenotypes of eight overlapping congenic lines and four F(2) congenic intercrosses and prioritized candidates by bioinformatics approaches. Analyses revealed that the Fob3b QTL consists of at least two separate linked QTLs Fob3b1 and Fob3b2. They exhibit additive inheritance and are linked in coupling with alleles originating from the Lean line, decreasing obesity-related traits. In further analyses, we focused on Fob3b1 because it had a larger effect on obesity-related traits than Fob3b2, e.g., the difference between homozygotes for adiposity index (ADI) percentage was 1.22 and 0.77% for Fob3b1 and Fob3b2, respectively. A set of bioinformatics tools was used to narrow down positional candidates from 85 to 4 high-priority Fob3b1 candidates. A previous single Fob3b QTL was therefore resolved into another two closely linked QTLs, confirming the fractal nature of QTLs mapped at low resolution. The interval of the original Fob3b QTL was narrowed from 22.39 to 4.98 Mbp for Fob3b1 and to 7.68 Mbp for Fob3b2, which excluded the previously assigned candidate squalene epoxidase (Sqle) as the causal gene because it maps proximal to refined Fob3b1 and Fob3b2 intervals. A high-resolution map along with prioritization of Fob3b1 candidates by bioinformatics represents an important step forward to final identification of the Chr 15 obesity QTL.

Metabolic syndrome components in murine models

Animal models have enriched understanding of the physiological basis of metabolic disorders and advanced identification of genetic risk factors underlying the metabolic syndrome (MetS). Murine models are especially appropriate for this type of research, and are an excellent resource not only for identifying candidate genomic regions, but also for illuminating the possible molecular mechanisms or pathways affected in individual components of MetS. In this review, we briefly discuss findings from mouse models of metabolic disorders, particularly in light of issues raised by the recent flood of human genome-wide association studies (GWAS) results. We describe how mouse models are revealing that genotype interacts with environment in important ways, indicating that the underlying genetics of MetS is highly context dependant. Further we show that epistasis, imprinting and maternal effects each contribute to the genetic architecture underlying variation in metabolic traits, and mouse models provide an opportunity to dissect these aspects of the genetic architecture that are difficult if not impossible to ascertain in humans. Finally we discuss how knowledge gained from mouse models can be used in conjunction with comparative genomic methods and bioinformatic resources to inform human MetS research.