The FOXO3A rs2802292 G-allele associates with improved peripheral and hepatic insulin sensitivity and increased skeletal muscle-FOXO3A mRNA expression in twins.

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 (ß) = -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 [ß = -14% (-23; -5), P = 0.005]. The G-allele was associated with increased peripheral insulin action [glucose disposal rate clamp, ß = 0.85 mg · kg(fat-free mass)(-1) · min(-1)() (0.049; 1.64), P = 0.04] and lower hepatic insulin resistance index [ß = -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 [ß = 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 [ß = -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.

Genetic and metabolic effects on skeletal muscle AMPK in young and older twins.

The protein complex AMP-activated protein kinase (AMPK) is believed to play an important role in the regulation of skeletal muscle glucose and lipid metabolism. Defects in the AMPK system might therefore be an important factor in the pathogenesis of type 2 diabetes. We aimed to identify genetic and environmental mechanisms involved in the regulation of AMPK expression and activity and to examine the association between AMPK protein levels and activity on the one hand, and glucose and fat metabolism on the other. We investigated skeletal muscle biopsies from 100 young and 82 older mono- and dizygotic nondiabetic twins excised during the basal and insulin-stimulated states of a physiological hyperinsulinemic-euglycemic clamp. AMPKalpha1, -alpha2, and -gamma3 mRNA expression was investigated using real-time PCR, and Western blotting was employed to measure protein levels. Multiple regression analyses indicated that skeletal muscle AMPK mRNA and protein expression as well as activity were regulated by sex, age, obesity, and aerobic capacity. Comparison of intraclass correlations on AMPK measurements from mono- and dizygotic twins suggested that skeletal muscle AMPK expression was under minor genetic influence. AMPKgamma3 protein expression and activity were negatively related to whole body glucose uptake through the nonoxidative metabolic pathway and positively related to phosphorylation of glycogen synthase. Our results suggest that skeletal muscle AMPK expression is under minor genetic control but regulated by age and sex and associated with obesity and aerobic capacity. Furthermore, our results indicate a role for gamma3-containing AMPK complexes in downregulation of insulin-stimulated nonoxidative glucose metabolism possibly through inhibition of glycogen synthase activity.

G-allele of intronic rs10830963 in MTNR1B confers increased risk of impaired fasting glycemia and type 2 diabetes through an impaired glucose-stimulated insulin release: studies involving 19,605 Europeans.

OBJECTIVE: Genome-wide association studies have identified several variants within the MTNR1B locus that are associated with fasting plasma glucose (FPG) and type 2 diabetes. We refined the association signal by direct genotyping and examined for associations of the variant displaying the most independent effect on FPG with isolated impaired fasting glycemia (i-IFG), isolated impaired glucose tolerance (i-IGT), type 2 diabetes, and measures of insulin release and peripheral and hepatic insulin sensitivity. RESEARCH DESIGN AND METHODS: We examined European-descent participants in the Inter99 study (n = 5,553), in a sample of young healthy Danes (n = 372), in Danish twins (n = 77 elderly and n = 97 young), in additional Danish type 2 diabetic patients (n = 1,626) and control subjects (n = 505), in the Data from the Epidemiological Study on the Insulin Resistance Syndrome (DESIR) study (n = 4,656), in the North Finland Birth Cohort 86 (n = 5,258), and in the Haguenau study (n = 1,461). RESULTS: The MTNR1B intronic variant, rs10830963, carried most of the effect on FPG and showed the strongest association with FPG (combined P = 5.3 x 10(-31)) and type 2 diabetes. The rs10830963 G-allele increased the risk of i-IFG (odds ratio [OR] 1.64, P = 5.5 x 10(-11)) but not i-IGT. The G-allele was associated with a decreased insulin release after oral and intravenous glucose challenges (P < 0.01) but not after injection of tolbutamide. In elderly twins, the G-allele associated with hepatic insulin resistance (P = 0.017). CONCLUSIONS: The G-allele of MTNR1B rs10830963 increases risk of type 2 diabetes through a state of i-IFG and not through i-IGT. The same allele associates with estimates of beta-cell dysfunction and hepatic insulin resistance.

Impact of TCF7L2 rs7903146 on insulin secretion and action in young and elderly Danish twins.

OBJECTIVE: We investigated the regulation and metabolic effects of TCF7L2 gene expression in human sc fat and skeletal muscle and the impact of the TCF7L2, rs7903146, T-allele on gene expression and measures of glucose metabolism including insulin secretion and peripheral and hepatic insulin action. RESEARCH DESIGN AND METHODS: The rs7903146 was genotyped in 1) a population-based sample of 587 twins (55-64 yr) with glucose tolerance ranging from normal to type 2 diabetes and 2) a population of 196 nondiabetic young (22-31 yr) and elderly (57-66 yr) twins. All subjects underwent oral glucose tolerance tests, and population 2 was additionally examined with iv glucose tolerance tests and hyperinsulinemic, euglycemic clamps. RESULTS: Elderly T-allele carriers had decreased plasma insulin responses and lower disposition index, whereas insulinogenic index was similar between genotype groups. Elderly nondiabetic T-allele carriers had increased peripheral insulin sensitivity (P = 0.03). Young T-allele carriers had impaired hepatic insulin sensitivity (P = 0.04) independent of plasma insulin levels. TCF7L2 gene expression in skeletal muscle and adipose tissue was not explained by genotype, sex, aerobic capacity, birth, or adult anthropometry and was not associated with in vivo glucose metabolism. CONCLUSIONS: The rs7903146 T-allele associates with hepatic insulin resistance and diminished glucose-stimulated plasma insulin secretion. Our study does not provide evidence of a role of TCF7L2 gene expression in sc fat tissue and muscle tissue in the regulation of glucose homeostasis. This suggests that the primary defect of rs7903146 T-allele carriers is impairment of insulin secretion rather than a defect in insulin action in peripheral tissues.

Low physical activity accentuates the effect of the FTO rs9939609 polymorphism on body fat accumulation.

OBJECTIVE: Three independent studies have shown that variation in the fat mass and obesity-associated (FTO) gene associates with BMI and obesity. In the present study, the effect of FTO variation on metabolic traits including obesity, type 2 diabetes, and related quantitative phenotypes was examined. RESEARCH DESIGN AND METHODS: The FTO rs9939609 polymorphism was genotyped in a total of 17,508 Danes from five different study groups. RESULTS: In studies of 3,856 type 2 diabetic case subjects and 4,861 normal glucose-tolerant control subjects, the minor A-allele of rs9939609 associated with type 2 diabetes (odds ratio 1.13 [95% CI 1.06-1.20], P = 9 x 10(-5)). This association was abolished when adjusting for BMI (1.06 [0.97-1.16], P = 0.2). Among 17,162 middle-aged Danes, the A-allele associated with overweight (1.19 [1.13-1.24], P = 1 x 10(-12)) and obesity (1.27 [1.20-1.34], P = 2 x 10(-16)). Furthermore, obesity-related quantitative traits such as body weight, waist circumference, fat mass, and fasting serum leptin levels were significantly elevated in A-allele carriers. An interaction between the FTO rs9939609 genotype and physical activity (P = 0.007) was found, where physically inactive homozygous risk A-allele carriers had a 1.95 +/- 0.3 kg/m(2) increase in BMI compared with homozygous T-allele carriers. CONCLUSIONS: We validate that variation in FTO is associated with type 2 diabetes when not adjusted for BMI and with an overall increase in body fat mass. Furthermore, low physical activity seems to accentuate the effect of FTO rs9939609 on body fat accumulation.

Common variation in LMNA increases susceptibility to type 2 diabetes and associates with elevated fasting glycemia and estimates of body fat and height in the general population: studies of 7,495 Danish whites.

Mutations in LMNA encoding lamin A and C proteins cause monogenic syndromes characterized by muscular dystrophy and familial partial lipodystrophy. Eight tag single nucleotide polymorphisms in the LMNA locus were genotyped in 7,495 Danish whites and related to metabolic and anthropometric traits. The minor T-allele of rs4641 was nominally associated with type 2 diabetes (odds ratio 1.14 [95% CI 1.03-1.26], P = 0.01) in a study of 1,324 type 2 diabetic patients and 4,386 glucose-tolerant subjects and with elevated fasting plasma glucose levels in a population-based study of 5,395 middle-aged individuals (P = 0.008). The minor T-allele of rs955383 showed nominal association with obesity in a study of 5,693 treatment-naïve subjects (1.25 [1.07-1.64], P = 0.01), and after dichotomization of waist circumference, the minor alleles of rs955383 and rs11578696 were nominally associated with increased waist circumference (1.14 [1.04-1.23], P = 0.003; 1.12 [1.00-1.25], P = 0.04). The minor G-allele of rs577492 was associated with elevated fasting serum cholesterol and short stature (P = 3.0 . 10(-5) and P = 7.0 . 10(-4)). The findings are not corrected for multiple comparisons and are by nature exploratory. However, if replicated, these findings suggest that less severe variation in a gene locus known to harbor severe mutations causing monogenic syndromes may modestly increase susceptibility to common metabolic and anthropometrical phenotypes of polygenic origin.

PGC-1alpha Gly482Ser polymorphism associates with hypertension among Danish whites.

PGC-1alpha is a coactivator of numerous transcription factors and is expressed in tissues with high energy demands and abundant in mitochondria. It is induced in the myocardium on fasting and physical exercise, and cardiac-specific overexpression stimulates mitochondrial biogenesis in mice. The common Gly482Ser polymorphism of PGC-1alpha has previously shown association with arterial hypertension among Austrian men. Thus, we aimed at investigating this relationship in the Danish white population. The Gly482Ser polymorphism was genotyped in a total of 2562 Danish white subjects using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) and a GenoView locked nucleic acid assay (LNA), and the relationships of this variant with blood pressure levels and arterial hypertension were analyzed. Furthermore, we performed a combined analysis of the data from the present study in combination with previously published results. The Ser/Ser genotype was significantly associated with a reduced risk of hypertension and with lower systolic, diastolic, and mean arterial blood pressure levels, predominantly among women. Finally, in a combined analysis using data obtained in both sexes, the Ser/Ser genotype group had an estimated odds ratio of 0.70 (95% confidence interval, 0.56 to 0.86) for hypertension compared with Gly/X carriers (P=0.001). In conclusion, the Ser allele of PGC-1alpha Gly482Ser confers a significantly reduced risk of hypertension in whites. Further studies are needed to elucidate the differential role of this polymorphism in men and women.

Variation in NCB5OR: studies of relationships to type 2 diabetes, maturity-onset diabetes of the young, and gestational diabetes mellitus.

Recent data show that homozygous Ncb5or(-/-) knock-out mice present with an early-onset nonautoimmune diabetes phenotype. Furthermore, genome-wide scans have reported linkage to the chromosome 6q14.2 region close to the human NCB5OR. We therefore considered NCB5OR to be a biological and positional candidate gene and examined the coding region of NCB5OR in 120 type 2 diabetic patients and 63 patients with maturity-onset diabetes of the young using denaturing high-performance liquid chromatography. We identified a total of 22 novel nucleotide variants. Three variants [IVS5+7del(CT), Gln187Arg, and His223Arg] were genotyped in a case-control design comprising 1,246 subjects (717 type 2 diabetic patients and 529 subjects with normal glucose tolerance). In addition, four rare variants were investigated for cosegregation with diabetes in multiplex type 2 diabetic families. The IVS5+7del(CT) variant was associated with common late-onset type 2 diabetes; however, we failed to relate this variant to any diabetes-related quantitative traits among the 529 control subjects. Thus, variation in the coding region of NCB5OR is not a major contributor in the pathogenesis of nonautoimmune diabetes.