Genome wide meta-analysis highlights the role of genetic variation in RARRES2 in the regulation of circulating serum chemerin

The Role of Adipokines and Myokines in the Pathogenesis of Different Obesity Phenotypes-New Perspectives

Obesity is a characteristic disease of the twenty-first century that is affecting an increasing percentage of society. Obesity expresses itself in different phenotypes: normal-weight obesity (NWO), metabolically obese normal-weight (MONW), metabolically healthy obesity (MHO), and metabolically unhealthy obesity (MUO). A range of pathophysiological mechanisms underlie the occurrence of obesity, including inflammation, oxidative stress, adipokine secretion, and other processes related to the pathophysiology of adipose tissue (AT). Body mass index (BMI) is the key indicator in the diagnosis of obesity; however, in the case of the NWO and MONW phenotypes, the metabolic disturbances are present despite BMI being within the normal range. On the other hand, MHO subjects with elevated BMI values do not present metabolic abnormalities. The MUO phenotype involves both a high BMI value and an abnormal metabolic profile. In this regard, attention has been focused on the variety of molecules produced by AT and their role in the development of obesity. Nesfatin-1, neuregulin 4, myonectin, irisin, and brain-derived neurotrophic factor (BDNF) all seem to have protective effects against obesity. The primary mechanism underlying the action of nesfatin-1 involves an increase in insulin sensitivity and reduced food intake. Neuregulin 4 sup-presses lipogenesis, decreases lipid accumulation, and reduces chronic low-grade inflammation. Myonectin lowers the amount of fatty acids in the bloodstream by increasing their absorption in the liver and AT. Irisin stimulates the browning of white adipose tissue (WAT) and consequently in-creases energy expenditure, additionally regulating glucose metabolism. Another molecule, BDNF, has anorexigenic effects. Decorin protects against the development of hyperglycemia, but may also contribute to proinflammatory processes. Similar effects are shown in the case of visfatin and chemerin, which may predispose to obesity. Visfatin increases adipogenesis, causes cholesterol accumulation in macrophages, and contributes to the development of glucose intolerance. Chemerin induces angiogenesis, which promotes the expansion of AT. This review aims to discuss the role of adipokines and myokines in the pathogenesis of the different obesity phenotypes.

Genetic Markers of Insulin Resistance and Atherosclerosis in Type 2 Diabetes Mellitus Patients with Coronary Artery Disease

Type 2 diabetes mellitus (T2DM) is characterized by impaired insulin secretion on a background of insulin resistance (IR). IR and T2DM are associated with atherosclerotic coronary artery disease (CAD). The mechanisms of IR and atherosclerosis are known to share similar genetic and environmental roots. Endothelial dysfunction (ED) detected at the earliest stages of IR might be the origin of atherosclerosis progression. ED influences the secretion of pro-inflammatory cytokines and their encoding genes. The genes and their single nucleotide polymorphisms (SNPs) act as potential genetic markers of IR and atherosclerosis. This review focuses on the link between IR, T2DM, atherosclerosis, CAD, and the potential genetic markers CHI3L1, CD36, LEPR, RETN, IL-18, RBP-4, and RARRES2 genes.

Association of serum levels of Visfatin, Intelectin-1, RARRES2 and their genetic variants with bone mineral density in postmenopausal females

Background

Adipokines are engaged in bone physiology and regulate bone mineral density (BMD) by playing protective or cynical role in bone metabolism. The study is designed to measure and compare BMD, adipokines (retinoic acid receptor responder protein-2 RARRES2, visfatin and Intelectin-1) and their genetic variants in postmenopausal osteoporotic, osteopenic and non-osteoporotic females.

Methods

This comparative study included postmenopausal non-osteoporotic (n=72), osteopenic (n=72) and osteoporotic (n=100) females with two years of amenorrhea and age between 50 to 70 years. Gold standard DXA was used to measure BMD. Hardy-Weinberg equilibrium was established. Kruskal-Wallis test for comparisons, logistic and multivariate regression analysis were used to rule out the predictors of BMD.

Results

On comparing the three groups, significant differences were observed in serum RARRES2 (p <0.001) and serum visfatin (p=0.050). The significant positive predictor of BMD at lumbar spine and total hip was serum visfatin. BMD at right and left femoral neck was predicted negatively by serum chemerin while BMD at left femoral neck was also predicted positively by serum calcium levels. There was significant difference in BMD at right femoral neck (p = 0.033) between rs7806429 genotypes. The odds of having low BMD increases with increasing serum levels of chemerin and decreasing serum levels of visfatin and calcium.

Conclusion

The adipokines RARRES2 and visfatin are associated with BMD. RARRES2 is an independent negative and visfatin is positive predictor of BMD in postmenopausal females. BMD at right femoral neck was significantly low in RARRES2 rs7806429 TC heterozygotes.

Chemerin Forms: Their Generation and Activity

Chemerin is the product of the RARRES2 gene which is secreted as a precursor of 143 amino acids. That precursor is inactive, but proteases from the coagulation and fibrinolytic cascades, as well as from inflammatory reactions, process the C-terminus of chemerin to first activate it and then subsequently inactivate it. Chemerin can signal via two G protein-coupled receptors, chem1 and chem2, as well as be bound to a third non-signaling receptor, CCRL2. Chemerin is produced by the liver and secreted into the circulation as a precursor, but it is also expressed in some tissues where it can be activated locally. This review discusses the specific tissue expression of the components of the chemerin system, and the role of different proteases in regulating the activation and inactivation of chemerin. Methods of identifying and determining the levels of different chemerin forms in both mass and activity assays are reviewed. The levels of chemerin in circulation are correlated with certain disease conditions, such as patients with obesity or diabetes, leading to the possibility of using chemerin as a biomarker.

Divergence of Chemerin Reduction by an ATS9R Nanoparticle Targeting Adipose Tissue In Vitro vs. In Vivo in the Rat

Nanoparticles (NPs) can enable delivery of a drug to a targeted tissue. Previous studies have shown that an NP utilizing an adipose targeting sequence (ATS) peptide in conjunction with a drug can selectively deliver the drug to mouse adipose tissues, using the prohibitin protein expressed in adipose tissue as the target of the ATS. Adipose tissue is a major source of the adipokine chemerin, a prohypertensive protein. Liver-derived chemerin, the largest source of circulating chemerin, is biologically inactive in blood pressure regulation. Our goal is to understand if chemerin produced in adipose tissue contributes to blood pressure/hypertension. We hypothesize the ATS drug delivery system could be used specifically to reduce the levels of adipose tissue-derived chemerin. We created an NP consisting of an antisense oligonucleotide (ASO) against chemerin and a FITC-labeled ATS with a nine arginine sequence (ATS9R). In vitro studies showed that the ASO is functional when incorporated into an NP with ATS9R as it reduced chemerin mRNA expression in isolated epidydimal (Epi) and retroperitoneal (RP) fat adipocytes from Dahl SS rats. This same NP reduced chemerin in isolated whole fats. However, this NP was unable to selectively deliver the ASO to adipose tissue in vivo; liver delivery was dominant. Varying NP doses, administration route, and the concentration of components constituting the NP showed no improvement in ASO delivery to fats vs. the liver. Further studies are therefore needed to develop the ATS9R system to deliver an ASO to adipose beds in rats.

Causal Associations Between Circulating Adipokines and Cardiovascular Disease: A Mendelian Randomization Study

CONTEXT

Observational studies have suggested associations between adipokines and cardiovascular disease (CVD), but the roles of certain adipokines remain controversial, and these associations have not yet been ascertained causally.

OBJECTIVE

To investigate whether circulating adipokines causally affect the risk of CVD using 2-sample Mendelian randomization (MR).

METHODS

Independent genetic variants strongly associated with adiponectin, resistin, chemerin, and retinol binding protein 4 (RBP4) were selected from public genome-wide association studies. Summary-level statistics for CVD, including coronary artery disease (CAD), myocardial infarction, atrial fibrillation (AF), heart failure (HF), and stroke and its subtypes were collected. The inverse-variance weighted and Wald ratio methods were used for the MR estimates. The MR pleiotropy residual sum and outlier, weighted median, MR-Egger, leave-one-out analysis, MR Steiger, and colocalization analyses were used in the sensitivity analysis.

RESULTS

Genetically predicted resistin levels were positively associated with AF risk (odds ratio [OR] 1.09; 95% confidence interval [CI], 1.04-1.13; P = 4.1 × 10-5), which was attenuated to null after adjusting for blood pressure. We observed suggestive associations between higher genetically predicted chemerin levels and an increased risk of CAD (OR 1.27; 95% CI, 1.01-1.60; P = 0.040), higher genetically predicted RBP4 levels and an increased risk of HF (OR 1.14; 95% CI, 1.02-1.27; P = 0.024). There was no causal association between genetically predicted adiponectin levels and CVD risk.

CONCLUSIONS

Our findings reveal the causal association between resistin and AF, probably acting through blood pressure, and suggest potential causal associations between chemerin and CAD, RBP4, and HF.

Poly(A) RNA sequencing reveals age-related differences in the prefrontal cortex of dogs

Dogs may possess a unique translational potential to investigate neural aging and dementia because they are prone to age-related cognitive decline, including an Alzheimer’s disease–like pathological condition. Yet very little is known about the molecular mechanisms underlying canine cognitive decline. The goal of the current study was to explore the transcriptomic differences between young and old dogs’ frontal cortex, which is a brain region often affected by various forms of age-related dementia in humans. RNA isolates from the frontal cortical brain area of 13 pet dogs, which represented 7 different breeds and crossbreds, were analyzed. The dogs were euthanized for medical reasons, and their bodies had been donated by their owners for scientific purposes. The poly(A) tail RNA subfraction of the total transcriptome was targeted in the sequencing analysis. Cluster analyses, differential gene expression analyses, and gene ontology analyses were carried out to assess which genes and genetic regulatory mechanisms were mostly affected by aging. Age was the most prominent factor in the clustering of the animals, indicating the presence of distinct gene expression patterns related to aging in a genetically variable population. A total of 3436 genes were found to be differentially expressed between the age groups, many of which were linked to neural function, immune system, and protein synthesis. These findings are in accordance with previous human brain aging RNA sequencing studies. Some genes were found to behave more similarly to humans than to rodents, further supporting the applicability of dogs in translational aging research.

Association of serum uromodulin with adipokines in dependence of type 2 diabetes

BACKGROUND

The renal tubular glycoprotein uromodulin is associated with obesity and type 2 diabetes, but the underlying mechanisms are elusive. We investigated the association of serum uromodulin with adipokines and tested the effect modification by diabetes status.

METHODS

The associations of serum uromodulin with eight adipokines were assessed in 795-1080 participants of the KORA F4 study aged 62-81 years using linear regression models adjusted for sex, age, BMI, estimated glomerular filtration rate and diabetes. Significant associations were assessed for effect modification by diabetes status. We further tested using logistic regression whether adjustment for the significant adipokines affected the association of uromodulin with type 2 diabetes.

RESULTS

Serum uromodulin was inversely associated with chemerin and retinol-binding protein-4 after multivariable adjustment (p < 0.001) and Bonferroni correction for multiple testing. No significant association was observed between uromodulin and the other adipokines (leptin, adiponectin, secreted frizzled-related protein 5, progranulin, omentin-1 and vaspin) after correcting for multiple testing. The association of uromodulin with chemerin and retinol-binding protein-4 was stronger in participants with type 2 diabetes than in participants without diabetes (p for interaction < 0.05). However, inclusion of chemerin and retinol-binding protein-4 in logistic regression models did not attenuate the association of serum uromodulin with diabetes.

CONCLUSIONS

Serum uromodulin was inversely associated with the predominantly pro-inflammatory adipokines chemerin and retinol-binding protein-4. The associations were stronger in participants with type 2 diabetes compared to participants without diabetes. However, the association of serum uromodulin with type 2 diabetes was independent of chemerin and retinol-binding protein-4.

Causal association of adipokines with osteoarthritis: a Mendelian randomization study

OBJECTIVE

This two-sample Mendelian randomization study aimed to delve into the effects of genetically predicted adipokine levels on OA.

METHODS

Summary statistic data for OA originated from a meta-analysis of a genome-wide association study with an overall 50 508 subjects of European ancestry. Publicly available summary data from four genome-wide association studies were exploited to respectively identify instrumental variables of adiponectin, leptin, resistin, chemerin and retinol-blinding protein 4. Subsequently, Mendelian randomization analyses were conducted with inverse variance weighted (IVW), weighted median and Mendelian randomization-Egger regression. Furthermore, sensitivity analyses were then conducted to assess the robustness of our results.

RESULTS

The positive causality between genetically predicted leptin level and risk of total OA was indicated by IVW [odds ratio (OR): 2.40, 95% CI: 1.13-5.09] and weighted median (OR: 2.94, 95% CI: 1.23-6.99). In subgroup analyses, evidence of potential harmful effects of higher level of adiponectin (OR: 1.28, 95% CI: 1.01-1.61 using IVW), leptin (OR: 3.44, 95% CI: 1.18-10.03 using IVW) and resistin (OR: 1.18, 95% CI: 1.03-1.36 using IVW) on risk of knee OA were acquired. However, the mentioned effects on risk of hip OA were not statistically significant. Slight evidence was identified supporting causality of chemerin and retinol-blinding protein 4 for OA. The findings of this study were verified by the results from sensitivity analysis.

CONCLUSIONS

An association between genetically predicted leptin level and risk of total OA was identified. Furthermore, association of genetically predicted levels of adiponectin, leptin and resistin with risk of knee OA were reported.

Chemerin causes lipid metabolic imbalance and induces passive lipid accumulation in human hepatoma cell line via the receptor GPR1

AIMS

Chemerin is abundant in patients with high body mass index and metabolic syndrome possibly due to its activation in adipogenesis and glucose intolerance. It has reported that sera chemerin is positively associated with fatty liver with little known underlying mechanisms. Our aim is to study the role of chemerin in hepatic lipid metabolism.

MAIN METHODS

Oil Red O staining and TG quantitative assay were used to detect intracellular lipid accumulation. PCR, QPCR and western blot were applied to measure lipid metabolism-related genes, CMKLR1, GPR1 and inflammation marker genes. Luciferase reporter assay was employed to uncover the down-regulation of proximate promoter activities of CMKLR1 and GPR1 by SREBP1c. Antibody neutralization assay was used to address the effects of chemerin on hepatic lipid synthesis.

KEY FINDINGS

Over-expression of chemerin led to passive lipid accumulation, in human hepatoma cell line HepG2. The disable form of chemerin (chemerin 21-158) and active chemerin (chemerin 21-157) performed strongly effects on lipid metabolism in HepG2 cells. Heterologous expression of CMKLR1 or G-protein coupled receptor1 (GPR1) played similar roles in hepatocyte lipid metabolism as chemerin. Chemerin exerted its effects on lipid metabolism via GPR1 in HepG2 cells. Furthermore, free fatty acids and high concentration insulin inhibited chemerin expression. Consistently, the key lipogenic transcription factor Sterol regulatory element binding protein 1c suppressed chemerin mRNA expression and proximate promoter activities of CMKLR1 and GPR1.

SIGNIFICANCE

It implied the existence of negative feed-back regulation and further confirmed the involvement of chemerin in hepatic lipid metabolism.

The Effect of FGF21 and Its Genetic Variants on Food and Drug Cravings, Adipokines and Metabolic Traits

Fibroblast growth factor 21 (FGF21) is a regulator of addictive behavior. Increasing evidence suggests an impact of FGF21 on eating behavior, food and drug cravings and on other adipokines like insulin-like growth factor 1 (IGF-1) or adiponectin. We investigated the association of serum FGF21 and genetic variants with aspects of food and drug craving and obesity related metabolic parameters including serum adipokine levels. Standardized questionnaires, blood samples and anthropometric data of the Sorbs cohort (n = 1046) were analyzed using SPSS. For genetic analyses, the FGF21-locus ±10 kb was genotyped and analyzed using PLINK. Validation was conducted in a second independent cohort (n = 704). FGF21 was significantly associated with alcohol and coffee consumption, smoking and eating behavior (disinhibition). We confirmed correlations of FGF21 serum levels with IGF-1, adiponectin, pro-enkephalin, adipocyte fatty-acid-binding protein, chemerin and progranulin. FGF21 genetic variants were associated with anthropometric and metabolic parameters, adipokines, food and drug craving while strongest evidence was seen with low-density lipoprotein cholesterol (LDL-C). We highlight the potential role of FGF21 in food and drug cravings and provide new insights regarding the link of FGF21 with other adipokines as well as with metabolic traits, in particular those related to lipid metabolism (LDL-C).

Circulating chemerin levels are determined through circulating platelet counts in nondiabetic Taiwanese people: A bidirectional Mendelian randomization study

BACKGROUND AND AIMS

Platelet count (PLT) is a predictor of metabolic and inflammation-related disorders. Platelets can release prochemerin, which acts as a link between coagulation and inflammation and between innate and adaptive immunity. The causal effect between PLT and circulating chemerin level has not been elucidated.

METHODS

Nondiabetic participants with samples in the Taiwan Biobank were recruited for a genome-wide association study (GWAS) based on PLT (17,037 participants) and chemerin levels (3887 participants). A bidirectional Mendelian randomization (MR) study was conducted to determine the association between circulating PLT and chemerin levels.

RESULTS

For a GWAS of PLT, 11 gene loci were found to have genome-wide significance. For a GWAS of chemerin levels, two gene loci, RARRES2 and HLADQA2-HLADQB1, were found to have genome-wide significance. Age, sex, body mass index, leukocyte count, hemoglobin, mean blood pressure, hemoglobin A1C, serum total bilirubin, aspartate aminotransferase, triglyceride, and low-density-lipoprotein cholesterol levels, estimated glomerular filtration rate, and circulating chemerin level were found to be independently associated with PLT through a stepwise regression analysis. A bidirectional MR study revealed weighted genetic risk scores (WGRSs) for PLT were significantly associated with chemerin levels by using a two-stage least-square method in a multivariate analysis (p = 0.0031), and no significant association between chemerin level WGRSs and PLT was noted. Sensitivity analysis further revealed no violation of the exclusion-restriction assumption with PLT-determining genotypes on chemerin levels.

CONCLUSIONS

Through a bidirectional MR analysis, our data revealed that chemerin levels were determined based on circulating PLT. Circulating chemerin levels can be intermediates between PLT and future metabolic and inflammation-related disorders.

m6A Regulators in Human Adipose Tissue - Depot-Specificity and Correlation With Obesity

BACKGROUND

N⁶-methyladenosine (m6A) is one of the most abundant post-transcriptional modifications on mRNA influencing mRNA metabolism. There is emerging evidence for its implication in metabolic disease. No comprehensive analyses on gene expression of m6A regulators in human adipose tissue, especially in paired adipose tissue depots, and its correlation with clinical variables were reported so far. We hypothesized that inter-depot specific gene expression of m6A regulators may differentially correlate with clinical variables related to obesity and fat distribution.

METHODS

We extracted intra-individually paired gene expression data (omental visceral adipose tissue (OVAT) N=48; subcutaneous adipose tissue (SAT) N=56) of m6A regulators from an existing microarray dataset. We also measured gene expression in another sample set of paired OVAT and SAT (N=46) using RT-qPCR. Finally, we extracted existing gene expression data from peripheral mononuclear blood cells (PBMCs) and single nucleotide polymorphisms (SNPs) in METTL3 and YTHDF3 from genome wide data from the Sorbs population (N=1049). The data were analysed for differential gene expression between OVAT and SAT; and for association with obesity and clinical variables. We further tested for association of SNP markers with gene expression and clinical traits.

RESULTS

In adipose tissue we observed that several m6A regulators (WTAP, VIRMA, YTHDC1 and ALKBH5) correlate with obesity and clinical variables. Moreover, we found adipose tissue depot specific gene expression for METTL3, WTAP, VIRMA, FTO and YTHDC1. In PBMCs, we identified ALKBH5 and YTHDF3 correlated with obesity. Genetic markers in METTL3 associate with BMI whilst SNPs in YTHDF3 are associated with its gene expression.

CONCLUSIONS

Our data show that expression of m6A regulators correlates with obesity, is adipose tissue depot-specific and related to clinical traits. Genetic variation in m6A regulators adds an additional layer of variability to the functional consequences.

Association of Serum Chemerin Levels with Coronary Artery Disease: Pathogenesis and Clinical Research

Recent studies have revealed that chemerin plays an essential role in the development of cardiovascular diseases. Autopsy studies found a strong correlation between the secretion of chemerin in peripheral tissues and aortic and coronary atherosclerosis. Plasma chemerin is a marker of systemic inflammation and is associated with metabolic syndrome. Chemerin plays a vital role in vascular inflammation and atherogenesis. Plasma chemerin levels are increased in patients with dilated cardiomyopathy, and chemerin is associated with left ventricular dysfunction. In this review, we focus on chemerin expression, chemerin processing, its biological function, and its role in the diagnosis of cardiovascular diseases.

Genetic variation, adipokines, and cardiometabolic disease

Adipokines are adipocyte-secreted cell signalling proteins that travel to distant target organs and tissues, where they regulate a variety of biological actions implicated in cardiometabolic health. In the past decade, genome-wide association studies have identified multiple genetic variants associated with circulating levels of adipokines, providing new instruments for examining the role of adipokines in cardiometabolic pathologies. Currently, there is limited genetic evidence of causal relationships between adipokines and cardiometabolic disease, which is consistent with findings from randomized clinical trials that have thus far shown limited success for adipokine-based treatments in improving cardiometabolic health. Incorporating human genetic data in early phases of target selection is essential for enhancing the success of adipokine-based therapies for cardiometabolic disease.

Atg7 Knockdown Reduces Chemerin Secretion in Murine Adipocytes

CONTEXT

In individuals with obesity, adipocyte endocrine function is affected by altered autophagy. Genetic variants in autophagy-related gene 7 (ATG7) correlated with serum chemerin (RARRES2) concentrations.

OBJECTIVES

To investigate a functional interplay between chemerin and ATG7, how it may relate to autophagy-mediated adipocyte dysfunction in obesity, and the relevance of genetic ATG7 variants in chemerin physiology.

DESIGN

Adipose ATG7 mRNA expression and adiposity measures were available in two human study cohorts. The effect of a high-calorie diet on adipose Rarres2 and Atg7 expression was investigated in mice. In 3T3-L1 adipocytes, the effect of Atg7 knockdown on chemerin expression and secretion was studied. The influence of single nucleotide polymorphisms on ATG7 transcription and chemerin physiology was investigated using a luciferase assay.

SETTING

Mouse model, clinical trials, in vitro studies.

PARTICIPANTS

Native American (n = 83) and white (n = 100) cohorts.

MAIN OUTCOME MEASURE

Adipocyte chemerin expression and secretion.

RESULTS

In mice fed a high-calorie diet, adipose Atg7 mRNA expression did not parallel an increase in Rarres2 mRNA expression. ATG7 mRNA expression in human subcutaneous adipose tissue correlated with body mass index, fat mass (r > 0.27; P < 0.01), and adipocyte cell size (r > 0.24; P < 0.02). Atg7 knockdown in 3T3-L1 adipocytes decreased chemerin secretion by 22% (P < 0.04). Rs2606729 in ATG7 was predicted to alter ATG7 transcription and induced higher luciferase activity in vitro (P < 0.0001).

CONCLUSIONS

Human adipose ATG7 mRNA expression relates to measures of adiposity. Atg7 knockdown reduces chemerin secretion from adipocytes in vitro, supportive of a functional interplay between ATG7 and chemerin in autophagy-mediated adipocyte dysfunction.

Single Nucleotide Polymorphisms at a Distance from Aryl Hydrocarbon Receptor (AHR) Binding Sites Influence AHR Ligand-Dependent Gene Expression

Greater than 90% of significant genome-wide association study (GWAS) single-nucleotide polymorphisms (SNPs) are in noncoding regions of the genome, but only 25.6% are known expression quantitative trait loci (eQTLs). Therefore, the function of many significant GWAS SNPs remains unclear. We have identified a novel type of eQTL for which SNPs distant from ligand-activated transcription factor (TF) binding sites can alter target gene expression in a SNP genotype-by-ligand–dependent fashion that we refer to as pharmacogenomic eQTLs (PGx-eQTLs)—loci that may have important pharmacotherapeutic implications. In the present study, we integrated chromatin immunoprecipitation-seq with RNA-seq and SNP genotype data for a panel of lymphoblastoid cell lines to identify 10 novel cis PGx-eQTLs dependent on the ligand-activated TF aryl hydrocarbon receptor (AHR)—a critical environmental sensor for xenobiotic (drug) and immune response. Those 10 cis PGx-eQTLs were eQTLs only after AHR ligand treatment, even though the SNPs did not create/destroy an AHR response element—the DNA sequence motif recognized and bound by AHR. Additional functional studies in multiple cell lines demonstrated that some cis PGx-eQTLs are functional in multiple cell types, whereas others displayed SNP-by-ligand–dependent effects in just one cell type. Furthermore, four of those cis PGx-eQTLs had previously been associated with clinical phenotypes, indicating that those loci might have the potential to inform clinical decisions. Therefore, SNPs across the genome that are distant from TF binding sites for ligand-activated TFs might function as PGx-eQTLs and, as a result, might have important clinical implications for interindividual variation in drug response.

(Epi)genetic regulation of CRTC1 in human eating behaviour and fat distribution

BACKGROUND

In brain, CREB-regulated transcription co-activator 1 (CRTC1) is involved in metabolic dysregulation. In humans a SNP in CRTC1 was associated to body fat percentage and two SNPs affected RNA Pol II binding and chromatin structure, implying epigenetic regulation of CRTC1. We sought to understand the relevance of CRTC1 SNPs, DNA methylation and expression in human eating behaviour and its relationship to clinical variables of obesity in blood and adipose tissue.

METHODS

13 CRTC1 SNPs were included to analyze eating behaviour. For rs7256986, follow up association analyses were applied on DNA methylation, CRTC1 expression and clinical parameters. Linear regression was used throughout the study adjusted for age, sex and BMI. Besides data extraction from previous work, rs7256986 was de-novo genotyped and DNA methylation was evaluated by using pyrosequencing.

FINDINGS

We found several SNPs in the CRTC1 locus nominally associated with human eating behaviour or 2hr postprandial insulin levels and observed a correlation with alcohol and coffee intake (all P < 0.05). G-allele carriers of rs7256986 showed slightly increased hip circumference. We showed that rs7256986 represents a methylation quantitative trait locus (meQTL) in whole blood and adipose tissue. The presence of the SNP and/or DNA methylation correlated with CRTC1 gene expression which in turn, related to BMI and fat distribution.

INTERPRETATION

Our data support the known role of CRCT1 regulating energy metabolism in brain. Here, we highlight relevance of CRTC1 regulation in blood and adipose tissue. FUND: IFB AdiposityDiseases (BMBF); n609020-Scientia Fellows; Helse-SørØst; DFG: CRC 1052/1 and/2; Kompetenznetz Adipositas, German Diabetes Association.

Circulating Chemerin Levels, but not the RARRES2 Polymorphisms, Predict the Long-Term Outcome of Angiographically Confirmed Coronary Artery Disease

Chemerin, a novel adipokine, has been associated with metabolic, inflammatory, and atherosclerotic diseases. We aimed to determine the genetic basis of chemerin levels by conducting a genome-wide association study (GWAS) and to investigate the role of RARRES2 polymorphisms and circulating chemerin levels in the long-term outcome of coronary artery disease (CAD). A total of 2197 participants from the Taiwan Biobank (TWB) were recruited for the GWAS analysis, and 481 patients with angiographically confirmed CAD were enrolled for long-term outcome analysis. One locus of genome-wide significance with a single independent association signal was identified in the GWAS for chemerin levels with the peak association at the RARRES2 gene promoter region polymorphism rs3735167 (p = 2.35 × 10⁻²¹). In the CAD population, borderline significance was noted between RARRES2 polymorphisms and chemerin levels, whereas high chemerin levels were associated with obesity, female sex, diabetes mellitus, hypertension, current smoking, high platelet and leukocyte counts, anemia, impaired renal function, high C-reactive protein (CRP) levels, and multi-vessel disease. Kaplan–Meier survival curves indicated that the patients with high chemerin and CRP levels, but not those with RARRES2 polymorphisms, had a lower survival rate and higher combined cerebral and cardiovascular event rates. Combined chemerin and CRP levels further revealed a stepwise increase in poor clinical outcomes from low- to high-risk subgroups. In conclusion, rs3735167 is the lead RARRES2 polymorphism for chemerin levels in Taiwanese. Chemerin levels, but not the rs3735167 genotypes, predicted the long-term outcome of CAD, especially when combined with CRP levels.

Chemerin Isoforms and Activity in Obesity

Overweight and adiposity are risk factors for several diseases, like type 2 diabetes and cancer. White adipose tissue is a major source for adipokines, comprising a diverse group of proteins exerting various functions. Chemerin is one of these proteins whose systemic levels are increased in obesity. Chemerin is involved in different physiological and pathophysiological processes and it regulates adipogenesis, insulin sensitivity, and immune response, suggesting a vital role in metabolic health. The majority of serum chemerin is biologically inert. Different proteases are involved in the C-terminal processing of chemerin and generate diverse isoforms that vary in their activity. Distribution of chemerin variants was analyzed in adipose tissues and plasma of lean and obese humans and mice. The Tango bioassay, which is suitable to monitor the activation of the beta-arrestin 2 pathway, was used to determine the ex-vivo activation of chemerin receptors by systemic chemerin. Further, the expression of the chemerin receptors was analyzed in adipose tissue, liver, and skeletal muscle. Present investigations assume that increased systemic chemerin in human obesity is not accompanied by higher biologic activity. More research is needed to fully understand the pathways that control chemerin processing and chemerin signaling.

Genome-wide meta-analysis identifies novel determinants of circulating serum progranulin

Progranulin is a secreted protein with important functions in processes including immune and inflammatory response, metabolism and embryonic development. The present study aimed at identification of genetic factors determining progranulin concentrations. We conducted a genome-wide association meta-analysis for serum progranulin in three independent cohorts from Europe: Sorbs (N = 848) and KORA (N = 1628) from Germany and PPP-Botnia (N = 335) from Finland (total N = 2811). Single nucleotide polymorphisms (SNPs) associated with progranulin levels were replicated in two additional German cohorts: LIFE-Heart Study (Leipzig; N = 967) and Metabolic Syndrome Berlin Potsdam (Berlin cohort; N = 833). We measured mRNA expression of genes in peripheral blood mononuclear cells (PBMC) by micro-arrays and performed mRNA expression quantitative trait and expression-progranulin association studies to functionally substantiate identified loci. Finally, we conducted siRNA silencing experiments in vitro to validate potential candidate genes within the associated loci. Heritability of circulating progranulin levels was estimated at 31.8% and 26.1% in the Sorbs and LIFE-Heart cohort, respectively. SNPs at three loci reached study-wide significance (rs660240 in CELSR2-PSRC1-MYBPHL-SORT1, rs4747197 in CDH23-PSAP and rs5848 in GRN) explaining 19.4%/15.0% of the variance and 61%/57% of total heritability in the Sorbs/LIFE-Heart Study. The strongest evidence for association was at rs660240 (P = 5.75 × 10-50), which was also associated with mRNA expression of PSRC1 in PBMC (P = 1.51 × 10-21). Psrc1 knockdown in murine preadipocytes led to a consecutive 30% reduction in progranulin secretion. In conclusion, the present meta-GWAS combined with mRNA expression identified three loci associated with progranulin and supports the role of PSRC1 in the regulation of progranulin secretion.

The evolutionary dynamics of microRNAs in domestic mammals

MiRNAs are crucial regulators of gene expression found across both the plant and animal kingdoms. While the number of annotated miRNAs deposited in miRBase has greatly increased in recent years, few studies provided comparative analyses across sets of related species, or investigated the role of miRNAs in the evolution of gene regulation. We generated small RNA libraries across 5 mammalian species (cow, dog, horse, pig and rabbit) from 4 different tissues (brain, heart, kidney and testis). We identified 1676 miRBase and 413 novel miRNAs by manually curating the set of computational predictions obtained from miRCat and miRDeep2. Our dataset spanning five species has enabled us to investigate the molecular mechanisms and selective pressures driving the evolution of miRNAs in mammals. We highlight the important contributions of intronic sequences (366 orthogroups), duplication events (135 orthogroups) and repetitive elements (37 orthogroups) in the emergence of new miRNA loci. We use this framework to estimate the patterns of gains and losses across the phylogeny, and observe high levels of miRNA turnover. Additionally, the identification of lineage-specific losses enables the characterisation of the selective constraints acting on the associated target sites. Compared to the miRBase subset, novel miRNAs tend to be more tissue specific. 20 percent of novel orthogroups are restricted to the brain, and their target repertoires appear to be enriched for neuron activity and differentiation processes. These findings may reflect an important role for young miRNAs in the evolution of brain expression plasticity. Many seed sequences appear to be specific to either the cow or the dog. Analyses on the associated targets highlight the presence of several genes under artificial positive selection, suggesting an involvement of these miRNAs in the domestication process. Altogether, we provide an overview on the evolutionary mechanisms responsible for miRNA turnover in 5 domestic species, and their possible contribution to the evolution of gene regulation.

Pleiotropic Associations of RARRES2 Gene Variants and Circulating Chemerin Levels: Potential Roles of Chemerin Involved in the Metabolic and Inflammation-Related Diseases

Chemerin, an adipokine and inflammatory mediator, is associated with metabolic, inflammation- and immune-mediated diseases. The genetic, clinical, and biomarker correlates of circulating chemerin levels have not been completely elucidated. We analyzed the determinants and correlates of retinoic acid receptor responder 2 (RARRES2; encoding chemerin) gene variants and chemerin levels in the Taiwanese population. In total, 612 individuals were recruited. Clinical and metabolic phenotypes, 13 inflammatory markers, 5 adipokines, and 6 single-nucleotide polymorphisms (SNPs) covering the RARRES2 region were analyzed. High chemerin levels and chemerin level tertiles were positively associated with multiple metabolic phenotypes and circulating inflammatory marker and adipokine levels and negatively associated with high-density lipoprotein cholesterol and adiponectin levels and estimated glomerular filtration rates (eGFRs). Genotype and haplotype analyses showed that RARRES2 SNPs were significantly associated with chemerin, fibrinogen, interleukin 6, and lipocalin 2 levels. Stepwise logistic regression analysis showed that C-reactive protein level, leptin level, triglyceride level, eGFR, rs3735167 genotypes, sex, and soluble P-selectin level were independently associated with chemerin levels. In conclusion, pleiotropic associations were noted between RARRES2 variants, circulating chemerin levels and multiple metabolic phenotypes and inflammatory marker levels. This study provides further evidence for the potential roles of chemerin in metabolic and inflammation-related diseases.

On the impact of relatedness on SNP association analysis

BACKGROUND

When testing for SNP (single nucleotide polymorphism) associations in related individuals, observations are not independent. Simple linear regression assuming independent normally distributed residuals results in an increased type I error and the power of the test is also affected in a more complicate manner. Inflation of type I error is often successfully corrected by genomic control. However, this reduces the power of the test when relatedness is of concern. In the present paper, we derive explicit formulae to investigate how heritability and strength of relatedness contribute to variance inflation of the effect estimate of the linear model. Further, we study the consequences of variance inflation on hypothesis testing and compare the results with those of genomic control correction. We apply the developed theory to the publicly available HapMap trio data (N=129), the Sorbs (a self-contained population with N=977 characterised by a cryptic relatedness structure) and synthetic family studies with different sample sizes (ranging from N=129 to N=999) and different degrees of relatedness.

RESULTS

We derive explicit and easily to apply approximation formulae to estimate the impact of relatedness on the variance of the effect estimate of the linear regression model. Variance inflation increases with increasing heritability. Relatedness structure also impacts the degree of variance inflation as shown for example family structures. Variance inflation is smallest for HapMap trios, followed by a synthetic family study corresponding to the trio data but with larger sample size than HapMap. Next strongest inflation is observed for the Sorbs, and finally, for a synthetic family study with a more extreme relatedness structure but with similar sample size as the Sorbs. Type I error increases rapidly with increasing inflation. However, for smaller significance levels, power increases with increasing inflation while the opposite holds for larger significance levels. When genomic control is applied, type I error is preserved while power decreases rapidly with increasing variance inflation.

CONCLUSIONS

Stronger relatedness as well as higher heritability result in increased variance of the effect estimate of simple linear regression analysis. While type I error rates are generally inflated, the behaviour of power is more complex since power can be increased or reduced in dependence on relatedness and the heritability of the phenotype. Genomic control cannot be recommended to deal with inflation due to relatedness. Although it preserves type I error, the loss in power can be considerable. We provide a simple formula for estimating variance inflation given the relatedness structure and the heritability of a trait of interest. As a rule of thumb, variance inflation below 1.05 does not require correction and simple linear regression analysis is still appropriate.

Genetic Determination of Serum Levels of Diabetes-Associated Adipokines

Adipose tissue secretes an abundance of proteins. Some of these proteins are known as adipokines and adipose-derived hormones which have been linked with metabolic disorders, including type 2 diabetes, and even with cancer. Variance in serum adipokine concentration is often closely associated with an increase (obesity) or decrease (lipodystrophy) in fat tissue mass, and it is affected by age, gender, and localization of the adipose tissue. However, there may be genetic variants which, in consequence, influence the serum concentration of a certain adipokine, and thereby promote metabolic disturbances or, with regard to the "protective" allele, exert beneficial effects. This review focuses on the genetic determination of serum levels of the following adipokines: adiponectin, chemerin, leptin, progranulin, resistin, retinol binding protein 4, vaspin, adipsin, apelin, and omentin. The article reports on the latest findings from genome-wide association studies (GWAS) and candidate gene studies, showing variants located in/nearby the adipokine genes and other (non-receptor) genes. An extra chapter highlights adipokine-receptor variants. Epigenetic studies on adipokines are also addressed.

Integration of Genome-Wide SNP Data and Gene-Expression Profiles Reveals Six Novel Loci and Regulatory Mechanisms for Amino Acids and Acylcarnitines in Whole Blood

Profiling amino acids and acylcarnitines in whole blood spots is a powerful tool in the laboratory diagnosis of several inborn errors of metabolism. Emerging data suggests that altered blood levels of amino acids and acylcarnitines are also associated with common metabolic diseases in adults. Thus, the identification of common genetic determinants for blood metabolites might shed light on pathways contributing to human physiology and common diseases. We applied a targeted mass-spectrometry-based method to analyze whole blood concentrations of 96 amino acids, acylcarnitines and pathway associated metabolite ratios in a Central European cohort of 2,107 adults and performed genome-wide association (GWA) to identify genetic modifiers of metabolite concentrations. We discovered and replicated six novel loci associated with blood levels of total acylcarnitine, arginine (both on chromosome 6; rs12210538, rs17657775), propionylcarnitine (chromosome 10; rs12779637), 2-hydroxyisovalerylcarnitine (chromosome 21; rs1571700), stearoylcarnitine (chromosome 1; rs3811444), and aspartic acid traits (chromosome 8; rs750472). Based on an integrative analysis of expression quantitative trait loci in blood mononuclear cells and correlations between gene expressions and metabolite levels, we provide evidence for putative causative genes: SLC22A16 for total acylcarnitines, ARG1 for arginine, HLCS for 2-hydroxyisovalerylcarnitine, JAM3 for stearoylcarnitine via a trans-effect at chromosome 1, and PPP1R16A for aspartic acid traits. Further, we report replication and provide additional functional evidence for ten loci that have previously been published for metabolites measured in plasma, serum or urine. In conclusion, our integrative analysis of SNP, gene-expression and metabolite data points to novel genetic factors that may be involved in the regulation of human metabolism. At several loci, we provide evidence for metabolite regulation via gene-expression and observed overlaps with GWAS loci for common diseases. These results form a strong rationale for subsequent functional and disease-related studies.

High plasma chemerin is associated with renal dysfunction and predictive for cardiovascular events - Insights from phenotype and genotype characterization

The novel adipokine chemerin, encoded by the RARRES2 gene, has been suggested to be linked to insulin resistance and to the metabolic syndrome (MetS). However, no well-defined cardiovascular profile has been reported and the association with coronary artery disease (CAD) is a matter of debate. Because there is a relation between renal dysfunction and CAD, we analyzed plasma chemerin levels and the estimated glomerular filtration rate (eGFR) in 495 patients undergoing coronary angiography for the evaluation of established or suspected stable CAD. Chemerin levels were higher in patients with Type 2 diabetes mellitus (T2DM, n=111) and the metabolic syndrome (MetS, n=147) than in subjects without T2DM (191.5±72.9 vs. 169.7±64.7ng/ml, p=0.001) or the MetS (201.2±71.0 vs. 163,1ng/ml, p<0.001), but did not differ significantly between patients with significant CAD (n=247) and those without significant CAD (177.1±67.0 vs. 171.7±67.2ng/ml, p=0.193). Analysis of covariance using age, sex, and BMI as covariates showed that chemerin was significantly and independently associated with eGFR (F=49.6, p<0.001). After an 8-year follow-up period, patients with high chemerin levels were more often affected by cardiovascular events (HR=1.72 [95% CI 1.19-2.47], p=0.004), even after appropriate adjustment for age, gender, BMI, as well as eGFR (adjusted HR 1.51 [95% CI 1.03-2.23], p=0.037). Given the cardiometabolic role of chemerin, we also applied a Cardio-Metabo Chip analysis and revealed a genome-wide significant association with SNPs (rs55709438, rs2444030, and rs3098423) located at chromosomal region 15q15-23, which were associated with metabolic traits and eGFR. This study for the first time demonstrates that high chemerin concentrations are significantly associated with renal impairment and predictive of cardiovascular events and that 15q15-23 might have an impact on chemerin levels beyond common genetic variations in RARRES2.