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Glucagon-like peptide 1 (GLP-1) stimulated insulin secretion has a considerable heritable component as estimated from twin studies, yet few genetic variants influencing this phenotype have been identified. We performed the first genome-wide association study (GWAS) of GLP-1 stimulated insulin secretion in non-diabetic individuals from the Netherlands Twin register (n = 126). This GWAS was enhanced using a tissue-specific protein-protein interaction network approach. We identified a beta-cell protein-protein interaction module that was significantly enriched for low gene scores based on the GWAS P-values and found support at the network level in an independent cohort from Tübingen, Germany (n = 100). Additionally, a polygenic risk score based on SNPs prioritized from the network was associated (P < 0.05) with glucose-stimulated insulin secretion phenotypes in up to 5,318 individuals in MAGIC cohorts. The network contains both known and novel genes in the context of insulin secretion and is enriched for members of the focal adhesion, extracellular-matrix receptor interaction, actin cytoskeleton regulation, Rap1 and PI3K-Akt signaling pathways. Adipose tissue is, like the beta-cell, one of the target tissues of GLP-1 and we thus hypothesized that similar networks might be functional in both tissues. In order to verify peripheral effects of GLP-1 stimulation, we compared the transcriptome profiling of ob/ob mice treated with liraglutide, a clinically used GLP-1 receptor agonist, versus baseline controls. Some of the upstream regulators of differentially expressed genes in the white adipose tissue of ob/ob mice were also detected in the human beta-cell network of genes associated with GLP-1 stimulated insulin secretion. The findings provide biological insight into the mechanisms through which the effects of GLP-1 may be modulated and highlight a potential role of the beta-cell expressed genes RYR2, GDI2, KIAA0232, COL4A1 and COL4A2 in GLP-1 stimulated insulin secretion.

Understanding the physiological mechanisms by which common variants predispose to type 2 diabetes requires large studies with detailed measures of insulin secretion and sensitivity. Here we performed the largest genome-wide association study of first-phase insulin secretion, as measured by intravenous glucose tolerance tests, using up to 5,567 individuals without diabetes from 10 studies. We aimed to refine the mechanisms of 178 known associations between common variants and glycemic traits and identify new loci. Thirty type 2 diabetes or fasting glucose-raising alleles were associated with a measure of first-phase insulin secretion at P < 0.05 and provided new evidence, or the strongest evidence yet, that insulin secretion, intrinsic to the islet cells, is a key mechanism underlying the associations at the HNF1A , IGF2BP2 , KCNQ1 , HNF1B , VPS13C/C2CD4A , FAF1 , PTPRD , AP3S2 , KCNK16 , MAEA , LPP, WFS1 , and TMPRSS6 loci. The fasting glucose-raising allele near PDX1 , a known key insulin transcription factor, was strongly associated with lower first-phase insulin secretion but has no evidence for an effect on type 2 diabetes risk. The diabetes risk allele at TCF7L2 was associated with a stronger effect on peak insulin response than on C-peptide-based insulin secretion rate, suggesting a possible additional role in hepatic insulin clearance or insulin processing. In summary, our study provides further insight into the mechanisms by which common genetic variation influences type 2 diabetes risk and glycemic traits., (© 2017 by the American Diabetes Association.)

Background: A genome-wide association study (GWAS) using metabolite concentrations as proxies for enzymatic activity, suggested that two variants: rs2014355 in the gene encoding short-chain acyl-coenzyme A dehydrogenase (ACADS) and rs11161510 in the gene encoding medium-chain acyl-coenzyme A dehydrogenase (ACADM) impair fatty acid β-oxidation. Chronic exposure to fatty acids due to an impaired β-oxidation may down-regulate the glucose-stimulated insulin release and result in an increased risk of type 2 diabetes (T2D). We aimed to investigate whether the two variants associate with altered insulin release following an oral glucose load or with T2D., Methods: The variants were genotyped using KASPar® PCR SNP genotyping system and investigated for associations with estimates of insulin release and insulin sensitivity following an oral glucose tolerance test (OGTT) in a random sample of middle-aged Danish individuals (nACADS = 4,324; nACADM = 4,337). The T2D-case-control study involved a total of ~8,300 Danish individuals (nACADS = 8,313; nACADM = 8,344)., Results: In glucose-tolerant individuals the minor C-allele of rs2014355 of ACADS associated with reduced measures of serum insulin at 30 min following an oral glucose load (per allele effect (β) = -3.8% (-6.3%;-1.3%), P = 0.003), reduced incremental area under the insulin curve (β = -3.6% (-6.3%;-0.9%), P = 0.009), reduced acute insulin response (β = -2.2% (-4.2%;0.2%), P = 0.03), and with increased insulin sensitivity ISIMatsuda (β = 2.9% (0.5%;5.2%), P = 0.02). The C-allele did not associate with two other measures of insulin sensitivity or with a derived disposition index. The C-allele was not associated with T2D in the case-control analysis (OR 1.07, 95% CI 0.96-1.18, P = 0.21). rs11161510 of ACADM did not associate with any indices of glucose-stimulated insulin release or with T2D., Conclusions: In glucose-tolerant individuals the minor C-allele of rs2014355 of ACADS was associated with reduced measures of glucose-stimulated insulin release during an OGTT, a finding which in part may be mediated through an impaired β-oxidation of fatty acids.

Objective: The minor G-allele of FOXO3A rs2802292 has been associated with longevity. We aimed to investigate whether a phenotype related to healthy metabolic aging could be identified in individuals carrying the longevity-associated FOXO3A rs2802292 G-allele., Research Design and Methods: rs2802292 was genotyped in a phenotypically well-characterized population of young and elderly twins (n = 190) and in the population-based Inter99 cohort (n = 5768). All participants underwent oral glucose tolerance tests, and the twin population was additionally examined with an iv glucose tolerance test and a hyperinsulinemic, euglycemic clamp. Basal and insulin-stimulated FOXO3A mRNA expression was assessed in skeletal muscle biopsies from the twin population., Results: In the twin sample, carriers of the minor G-allele of rs2802292 showed reduced fasting plasma insulin [per allele effect (β) = -13% (-24; -1) (95% confidence interval), P = 0.03] and lower incremental area under the curve 0-120 min for insulin after an oral glucose load [β = -14% (-23; -5), P = 0.005]. The G-allele was associated with increased peripheral insulin action [glucose disposal rate clamp, β = 0.85 mg · kg(fat-free mass)(-1) · min(-1)() (0.049; 1.64), P = 0.04] and lower hepatic insulin resistance index [β = -13% (-25; -1), P = 0.03]. Furthermore, carriers of the G-allele had increased basal FOXO3A mRNA expression in skeletal muscle compared with T-allele carriers [β = 16% (0; 33), P = 0.047]. In the Inter99 sample, we found an association with reduced incremental area under the curve 0-120 min for insulin after an oral glucose load [β = -3% (-5; -0.07), P = 0.04], but this association was not significant after adjustment for body mass index., Conclusion: Our data indicate that the minor G-allele of FOXO3A rs2802292 is associated with enhanced peripheral and hepatic insulin sensitivity in our small twin cohort, which may be mediated through increased FOXO3A mRNA expression, although no major metabolic impact of rs2802292 was found in the large Inter99 cohort.

Background: Polymorphisms in the potassium channel, voltage-gated, KQT-like subfamily, member 1 (KCNQ1) have recently been reported to associate with type 2 diabetes. The primary aim of the present study was to investigate the putative impact of these KCNQ1 polymorphisms (rs2283228, rs2237892, rs2237895, and rs2237897) on estimates of glucose stimulated insulin release., Methodology/principal Findings: Genotypes were examined for associations with serum insulin levels following an oral glucose tolerance test (OGTT) in a population-based sample of 6,039 middle-aged and treatment-naïve individuals. Insulin release indices estimated from the OGTT and the interplay between insulin sensitivity and insulin release were investigated using linear regression and Hotelling T2 analyses. Applying an additive genetic model the minor C-allele of rs2237895 was associated with reduced serum insulin levels 30 min (mean+/-SD: (CC) 277+/-160 vs. (AC) 280+/-164 vs. (AA) 299+/-200 pmol/l, p = 0.008) after an oral glucose load, insulinogenic index (29.6+/-17.4 vs. 30.2+/-18.7vs. 32.2+/-22.1, p = 0.007), incremental area under the insulin curve (20,477+/-12,491 vs. 20,503+/-12,386 vs. 21,810+/-14,685, p = 0.02) among the 4,568 individuals who were glucose tolerant. Adjustment for the degree of insulin sensitivity had no effect on the measures of reduced insulin release. The rs2237895 genotype had a similar impact in the total sample of treatment-naïve individuals. No association with measures of insulin release were identified for the less common diabetes risk alleles of rs2237892, rs2237897, or rs2283228., Conclusion: The minor C-allele of rs2237895 of KCNQ1, which has a prevalence of about 42% among Caucasians was associated with reduced measures of insulin release following an oral glucose load suggesting that the increased risk of type 2 diabetes, previously reported for this variant, likely is mediated through an impaired beta cell function.

Background and Aim: Non-alcoholic fatty liver disease (NAFLD) is a common condition, associated with hepatic insulin resistance and the metabolic syndrome including hyperglycaemia and dyslipidemia. We aimed at studying the potential impact of the NAFLD-associated PNPLA3 rs738409 G-allele on NAFLD-related metabolic traits in hyperglycaemic individuals. Methods: The rs738409 variant was genotyped in the population-based Inter99 cohort examined by an oral glucose-tolerance test, and a combined study-sample consisting of 192 twins (96 twin pairs) and a sub-set of the Inter99 population (n = 63) examined by a hyperinsulinemic euglycemic clamp (ntotal = 255). In Inter99, we analyzed associations of rs738409 with components of the WHO-defined metabolic syndrome (n = 5,847) and traits related to metabolic disease (n = 5,663). In the combined study sample we elucidated whether the rs738409 G-allele altered hepatic or peripheral insulin sensitivity. Study populations were divided into individuals with normal glucose-tolerance (NGT) and with impaired glucose regulation (IGR). Results: The case-control study showed no associations with components of the metabolic syndrome or the metabolic syndrome. Among 1,357 IGR individuals, the rs738409 G-allele associated with decreased fasting serum triglyceride levels (per allele effect(β) =-9.9% [-14.4%;-4.0% (95% CI)], p = 5.1×10-5) and fasting total cholesterol (β =20.2 mmol/l [-0.3;-0.01 mmol/l(95% CI)], p = 1.5×10-4). Meta-analyses showed no impact on hepatic or peripheral insulin resistance in carriers of the rs738409 G-allele. Conclusion: Our findings suggest that the G-allele of PNPLA3 rs738409 associates with reduced fasting levels of cholesterol and triglyceride in individuals with IGR. [ABSTRACT FROM AUTHOR]