Alexandre Belisle

A genome-wide association study identifies novel risk loci for type 2 diabetes

Type 2 diabetes mellitus results from the interaction of environmental factors with a combination of genetic variants, most of which were hitherto unknown. A systematic search for these variants was recently made possible by the development of high-density arrays that permit the genotyping of hundreds of thousands of polymorphisms. We tested 392,935 single-nucleotide polymorphisms in a French case–control cohort. Markers with the most significant difference in genotype frequencies between cases of type 2 diabetes and controls were fast-tracked for testing in a second cohort. This identified four loci containing variants that confer type 2 diabetes risk, in addition to confirming the known association with the TCF7L2 gene. These loci include a non-synonymous polymorphism in the zinc transporter SLC30A8, which is expressed exclusively in insulin-producing β-cells, and two linkage disequilibrium blocks that contain genes potentially involved in β-cell development or function (IDE–KIF11–HHEX and EXT2–ALX4). These associations explain a substantial portion of disease risk and constitute proof of principle for the genome-wide approach to the elucidation of complex genetic traits.

Genetic variant near IRS1 is associated with type 2 diabetes, insulin resistance and hyperinsulinemia

Genome-wide association studies have identified common variants that only partially explain the genetic risk for type 2 diabetes (T2D). Using genome-wide association data from 1,376 French individuals, we identified 16,360 SNPs nominally associated with T2D and studied these SNPs in an independent sample of 4,977 French individuals. We then selected the 28 best hits for replication in 7,698 Danish subjects and identified 4 SNPs showing strong association with T2D, one of which (rs2943641, P = 9.3 × 10−12, OR = 1.19) was located adjacent to the insulin receptor substrate 1 gene (IRS1). Unlike previously reported T2D risk loci, which predominantly associate with impaired beta cell function, the C allele of rs2943641 was associated with insulin resistance and hyperinsulinemia in 14,358 French, Danish and Finnish participants from population-based cohorts; this allele was also associated with reduced basal levels of IRS1 protein and decreased insulin induction of IRS1-associated phosphatidylinositol-3-OH kinase activity in human skeletal muscle biopsies.

Genetic Risk Factors for Post-Infectious Irritable Bowel Syndrome Following a Waterborne Outbreak of Gastroenteritis

BACKGROUND & AIMS Acute gastroenteritis is the strongest risk factor for irritable bowel syndrome (IBS). In May 2000, >2300 residents of Walkerton, Ontario, developed gastroenteritis from microbial contamination of the municipal water supply; a longitudinal study found that >36.2% of these developed IBS. We used this cohort to study genetic susceptibility to post-infectious (PI)-IBS. METHODS We screened 79 functional variants of genes with products involved in serotoninergic pathways, intestinal epithelial barrier function, and innate immunity and performed fine mapping in regions of interest. We compared data from Walkerton residents who developed gastroenteritis and reported PI-IBS 2 to 3 years after the outbreak (n = 228, cases) with data from residents who developed gastroenteritis but did not develop PI-IBS (n = 581, controls). RESULTS Four variants were associated with PI-IBS, although the association was not significant after correction for the total number of single nucleotide polymorphisms. Two were located in TLR9, which encodes a pattern recognition receptor (rs352139, P545P; P = .0059 and rs5743836, -T1237C; P = .0250; r(2) < 0.14); 1 was in CDH1, which encodes a tight junction protein (rs16260, -C160A; P = .0352); and 1 was in IL6, which encodes a cytokine (rs1800795, -G174C; P = .0420). Denser mapping of these 3 regions revealed 1 novel association in IL6 (rs2069861; P = .0069) and 14 associations that could be accounted for by linkage disequilibrium with the 4 original variants. The TLR9, IL6, and CDH1 variants all persisted as independent risk factors for PI-IBS when controlling for previously identified clinical risk factors. CONCLUSION This is the first descriptive study to assess potential genetic determinants of PI-IBS. Genes that encode proteins involved in epithelial cell barrier function and the innate immune response to enteric bacteria are associated with development of IBS following acute gastroenteritis.

Genetic Variants of FTO Influence Adiposity, Insulin Sensitivity, Leptin Levels, and Resting Metabolic Rate in the Quebec Family Study

OBJECTIVE—A genome-wide association study conducted by the Wellcome Trust Case Control Consortium recently associated single nucleotide polymorphisms (SNPs) in the FTO (fatso/fat mass and obesity associated) gene with type 2 diabetes. These associations were shown to be mediated by obesity. Other research groups found similar results in Europeans and Hispanics but not African Americans. The mechanism by which FTO influences obesity and type 2 diabetes is currently unknown. The present study investigated the role of two FTO SNPs (rs17817449 and rs1421085) in adiposity, insulin sensitivity, and body weight regulation, including energy intake and expenditure. RESEARCH DESIGN AND METHODS—We genotyped 908 individuals from the Quebec City metropolitan area that participated in the Quebec Family Study, a long-term study of extensively phenotyped individuals designed to investigate factors involved in adiposity. RESULTS—We found significant associations for both SNPs with several obesity-related phenotypes. In particular, rs17817449 was associated with BMI (P = 0.0014), weight (P = 0.0059), and waist circumference (P = 0.0021) under an additive model. In addition, this FTO SNP influenced fasting insulin (P = 0.011), homeostasis model assessment of insulin resistance (P = 0.038), and an insulin sensitivity index derived from an oral glucose tolerance test (P = 0.0091). Associations were also found with resting metabolic rate (RMR) (P = 0.042) and plasma leptin levels (P = 0.036). Adjustment for BMI abolished the associations with insulin sensitivity, RMR, and plasma leptin levels. CONCLUSIONS—These results confirm that genetic variation at the FTO locus contributes to the etiology of obesity, insulin resistance, and increased plasma leptin levels.

DPP4 Gene DNA Methylation in the Omentum is Associated With Its Gene Expression and Plasma Lipid Profile in Severe Obesity

Severely obese subjects with the metabolic syndrome (MS) have higher dipeptidyl peptidase‐4 (DPP4) expression in their visceral adipose tissue (VAT) compared to obese individuals without MS. We tested the hypothesis that methylation level of CpG sites in the DPP4 promoter CpG island in VAT was genotype‐dependent and associated with DPP4 mRNA abundance and MS‐related phenotypes. The VAT DNA was extracted in 92 severely obese premenopausal women undergoing biliopancreatic derivation for the treatment of obesity. Women were nondiabetic and none of them used medication to treat MS features. Cytosine methylation rates (%) of 102 CpG sites in the DPP4 CpG island were assessed by pyrosequencing of sodium bisulfite‐treated DNA. Methylation rates were >10% for CpG sites 94–102. Their mean methylation rate (%Meth94–102) was different between genotypes for DPP4 polymorphisms rs13015258 (P = 0.001), rs17848915 (P = 0.0004), and c.1926 G>A (P = 0.001). The %Meth94–102 correlated negatively with DPP4 mRNA abundance (r = −0.25, P < 0.05) and positively with plasma high‐density lipoprotein (HDL) cholesterol concentrations (r = 0.22, P < 0.05), whereas DPP4 mRNA abundance correlated positively with plasma total‐/HDL‐cholesterol ratio (r = 0.25; P < 0.05). In the VAT of nondiabetic severely obese women, genotype‐dependent methylation levels of specific CpG sites in the DPP4 promoter CpG island were associated with DPP4 gene expression and variability in the plasma lipid profile. Higher DPP4 gene expression in VAT and its relationship with the plasma lipid profile may be explained by actually unknown DPP4 biological effect or, to another extent, may also be a marker of VAT inflammation known to be associated with metabolic disturbances.

LINE-1 methylation in visceral adipose tissue of severely obese individuals is associated with metabolic syndrome status and related phenotypes

BackgroundEpigenetic mechanisms may be involved in the regulation of genes found to be differentially expressed in the visceral adipose tissue (VAT) of severely obese subjects with (MetS+) versus without (MetS-) metabolic syndrome (MetS). Long interspersed nuclear element 1 (LINE-1) elements DNA methylation levels (%meth) in blood, a marker of global DNA methylation, have recently been associated with fasting glucose, blood lipids, heart diseases and stroke.AimTo test whether LINE-1%meth levels in VAT are associated with MetS phenotypes and whether they can predict MetS risk in severely obese individuals.MethodsDNA was extracted from VAT of 34 men (MetS-: n = 14, MetS+: n = 20) and 152 premenopausal women (MetS-: n = 84; MetS+: n = 68) undergoing biliopancreatic diversion for the treatment of obesity. LINE-1%meth levels were assessed by pyrosequencing of sodium bisulfite-treated DNA.ResultsThe mean LINE-1%meth in VAT was of 75.8% (SD = 3.0%). Multiple linear regression analyses revealed that LINE-1%meth was negatively associated with fasting glucose levels (β = -0.04; P = 0.03), diastolic blood pressure (β =  -0.65; P = 0.03) and MetS status (β = -0.04; P = 0.004) after adjustments for the effects of age, sex, waist circumference (except for MetS status) and smoking. While dividing subjects into quartiles based on their LINE-1%meth (Q1 to Q4: lower %meth to higher %meth levels), greater risk were observed in the first (Q1: odds ratio (OR) = 4.37, P = 0.004) and the second (Q2: OR = 4.76, P = 0.002) quartiles compared to Q4 (1.00) when adjusting for age, sex and smoking.ConclusionsThese results suggest that lower global DNA methylation, assessed by LINE-1 repetitive elements methylation analysis, would be associated with a greater risk for MetS in the presence of obesity.

The type I diabetes association of the IL2RA locus

To confirm and fine map previous reports of association, the Type I Diabetes (T1D) Genetics Consortium (T1DGC) assembled a large collection of DNA samples from affected sib-pair (ASP) families with T1D (5003 affected individuals) and genotyped polymorphic markers. One of these loci, involving the IL2RA gene, had been reported to be due to three independent effects. The T1DGC genotyped 69 single-nucleotide polymorphisms (SNPs) that span ∼88 kb from the 5′ flanking to 3′ flanking region of the IL2RA locus. The most highly associated SNP reported earlier (ss52580101) was not included in the genotyping list; however, a 5-SNP (rs3134883, rs3118470, rs7072793, rs4749955 and rs12251307) haplotype (H5) was identified that strongly tagged its minor allele with r2=0.869 (95% CI, 0.850–0.885). This haplotype was significantly protective (P=3.2 × 10−5) in the T1D ASP families, with an odds ratio virtually identical to that reported for ss52580101. The SNP marking the second independent locus, (rs11594656) showed no association in the T1DGC set and the third (rs2104286) could not be distinguished, by conditional regression, from H5. Instead, the most significant independent effect was detected from the 5′ flanking IL2RA SNP rs4749955, which remained significant after regression for H5. Thus, we confirm independent effects at the IL2RA locus.

Parental Effect of DNA (Cytosine-5) Methyltransferase 1 on Grandparental-Origin-Dependent Transmission Ratio Distortion in Mouse Crosses and Human Families

Transmission ratio distortion (TRD) is a deviation from the expected Mendelian 1:1 ratio of alleles transmitted from parents to offspring and may arise by different mechanisms. Earlier we described a grandparental-origin-dependent sex-of-offspring-specific TRD of maternal chromosome 12 alleles closely linked to an imprinted region and hypothesized that it resulted from imprint resetting errors in the maternal germline. Here, we report that the genotype of the parents for loss-of-function mutations in the Dnmt1 gene influences the transmission of grandparental chromosome 12 alleles. More specifically, maternal Dnmt1 mutations restore Mendelian transmission ratios of chromosome 12 alleles. Transmission of maternal alleles depends upon the presence of the Dnmt1 mutation in the mother rather than upon the Dnmt1 genotype of the offspring. Paternal transmission mirrors the maternal one: live-born offspring of wild-type fathers display 1:1 transmission ratios, whereas offspring of heterozygous Dnmt1 mutant fathers tend to inherit grandpaternal alleles. Analysis of allelic transmission in the homologous region of human chromosome 14q32 detected preferential transmission of alleles from the paternal grandfather to grandsons. Thus, parental Dnmt1 is a modifier of transmission of alleles at an unlinked chromosomal region and perhaps has a role in the genesis of TRD.

Comparison of the dipeptidyl peptidase-4 gene methylation levels between severely obese subjects with and without the metabolic syndrome

BackgroundThe dipeptidyl peptidase-4 (DPP4) enzyme is a novel adipokine potentially involved in the development of the metabolic syndrome (MetS). Previous observations demonstrated higher visceral adipose tissue (VAT) DPP4 gene expression in non-diabetic severely obese men with (MetS+) vs. without (MetS−) MetS. DPP4 mRNA abundance in VAT correlated also with CpG site methylation levels (%Meth) localized within and near its exon 2 (CpG94 to CpG102) in non-diabetic severely obese women, regardless of their MetS status. The actual study tested whether DPP4 %Meth levels in VAT are different between MetS− and MetS+ non-diabetic severely obese subjects, whether variable metabolic and plasma lipid profiles are observed between DPP4 %Meth quartiles, and whether correlation exists in DPP4 %Meth levels between VAT and white blood cells (WBCs).MethodsDNA was extracted from the VAT of 26 men (MetS−: n=12, MetS+: n=14) and 79 women (MetS−: n=60; MetS+: n=19), as well as from WBCs in a sub-sample of 17 women (MetS−: n=9; MetS+: n=8). The %Meth levels of CpG94 to CpG102 were assessed by pyrosequencing of sodium bisulfite-treated DNA. ANOVA analyses were used to compare the %Meth of CpGs between MetS− and MetS+ groups, and to compare the metabolic phenotype and plasma lipid levels between methylation quartiles. Pearson correlation coefficient analyses were computed to test the relationship between VAT and WBCs CpG94-102 %Meth levels.ResultsNo difference was observed in CpG94-102 %Meth levels between MetS− and MetS+ subjects in VAT (P=0.67), but individuals categorized into CpG94-102 %Meth quartiles had variable plasma total-cholesterol concentrations (P=0.04). The %Meth levels of four CpGs in VAT were significantly correlated with those observed in WBCs (r=0.55−0.59, P≤0.03).ConclusionsThis study demonstrated that %Meth of CpGs localized within and near the exon 2 of the DPP4 gene in VAT are not associated with MetS status. The actual study also revealed an association between the %Meth of this locus with plasma total-cholesterol in severe obesity, which suggests a link between the DPP4 gene and plasma lipid levels.