Haplotypes of transcription factor 7-like 2 (TCF7L2) gene and its upstream region are associated with type 2 diabetes and age of onset in Mexican Americans

TCF7L2 acts as both a repressor and transactivator of genes, as directed by the Wnt signaling pathway. Recently, several highly correlated sequence variants located within a haplotype block of the TCF7L2 gene were observed to associate with type 2 diabetes in three Caucasian cohorts. We previously reported linkage of type 2 diabetes in the San Antonio Family Diabetes Study (SAFADS) cohort consisting of extended pedigrees of Mexican Americans to the region of chromosome 10q harboring TCF7L2. We therefore genotyped 11 single nucleotide polymorphisms (SNPs) from nine haplotype blocks across the gene in 545 SAFADS subjects (178 diabetic) to investigate their role in diabetes pathogenesis. We observed nominal association between four SNPs (rs10885390, rs7903146, rs12255372, and rs3814573) in three haplotype blocks and type 2 diabetes, age at diagnosis, and 2-h glucose levels (P = 0.001-0.055). Furthermore, we identified a common protective haplotype defined by these four SNPs that was significantly associated with type 2 diabetes and age at diagnosis (P = 4.2 x 10(-5), relative risk [RR] 0.69; P = 6.7 x 10(-6), respectively) and a haplotype that confers diabetes risk that contains the rare alleles at SNPs rs10885390 and rs12255372 (P = 0.02, RR 1.64). These data provide evidence that variation in the TCF7L2 genomic region may affect risk for type 2 diabetes in Mexican Americans, but the attributable risk may be lower than in Caucasian populations.     

Common single nucleotide polymorphisms in TCF7L2 are reproducibly associated with type 2 diabetes and reduce the insulin response to glucose in nondiabetic individuals

Recently, common noncoding variants in the TCF7L2 gene were strongly associated with increased risk of type 2 diabetes in samples from Iceland, Denmark, and the U.S. We genotyped 13 single nucleotide polymorphisms (SNPs) across TCF7L2 in 8,310 individuals in family-based and case-control designs from Scandinavia, Poland, and the U.S. We convincingly confirmed the previous association of TCF7L2 SNPs with the risk of type 2 diabetes (rs7903146T odds ratio 1.40 [95% CI 1.30-1.50], P = 6.74 x 10(-20)). In nondiabetic individuals, the risk genotypes were associated with a substantial reduction in the insulinogenic index derived from an oral glucose tolerance test (risk allele homozygotes have half the insulin response to glucose of noncarriers, P = 0.003) but not with increased insulin resistance. These results suggest that TCF7L2 variants may act through insulin secretion to increase the risk of type 2 diabetes.     

Evidence that Rho guanine nucleotide exchange factor 11 (ARHGEF11) on 1q21 is a type 2 diabetes susceptibility gene in the Old Order Amish

Rho guanine nucleotide exchange factor 11 (ARHGEF11), located on chromosome 1q21, is involved in G protein signaling and is a pathway known to play a role in both insulin secretion and action. We genotyped 52 single nucleotide polymorphims (SNPs) in ARHGEF11 and compared the genotype frequencies of subjects with type 2 diabetes (n = 145) or type 2 diabetes/impaired glucose tolerance (IGT) (n = 293) with those of control subjects with normal glucose tolerance (NGT) (n = 358). Thirty SNPs, spanning the entire gene, were significantly associated with type 2 diabetes or type 2 diabetes/IGT. The most significantly associated SNP was rs6427340 (intron 2), in which the less common allele was the risk allele (odds ratio [OR] 1.82 [95% CI 1.20-2.70], P = 0.005 for type 2 diabetes vs. NGT and 1.79 [1.27-2.50], P = 0.0008 for type 2 diabetes/IGT vs. NGT). In an expanded set of nondiabetic subjects (n = 754), most of the type 2 diabetes-and IGT-associated SNPs were significantly associated with glucose levels during an oral glucose tolerance test, with the same SNP (rs6427340) showing the most significant associations (P = 0.007). All type 2 diabetes-and IGT-associated SNPs were in high linkage disequilibrium and constitute a single 133-kb haplotype block. These results, coupled with similar findings in Pima Indians, suggest that sequence variation in ARHGEF11 may influence risk of type 2 diabetes.     

Polymorphism in the calsequestrin 1 (CASQ1) gene on chromosome 1q21 is associated with type 2 diabetes in the old order Amish

Calsequestrin (CASQ)1 is involved in intracellular storage and release of calcium, a process that has been shown to mediate glucose transport in muscle. Its gene, CASQ1, is encoded on chromosome 1q21, a region that has been linked to type 2 diabetes in the Amish and several other populations. We screened all 11 exons, exon-intron junctions, and the proximal regulatory region of CASQ1 for mutations. We detected four novel single nucleotide polymorphisms (SNPs) (-1470C-->T, -1456delG, -1366insG, and 593C-->T). Ten informative SNPs within CASQ1 were genotyped in Amish subjects with type 2 diabetes (n = 145), impaired glucose tolerance (n = 148), and normal glucose tolerance (n = 358). Rs2275703 and rs617698 in introns 4 and 2 were significantly associated with type 2 diabetes (P = 0.008 and 0.04, respectively); three other SNPs showed borderline evidence for association to type 2 diabetes (P = 0.076-0.093). Furthermore, in nondiabetic subjects (n = 754), both rs2275703 and rs617698 were significantly associated with glucose area under the curve during an oral glucose tolerance test (P = 0.035 and 0.013, respectively). Haplotype analysis suggested that no haplotype could explain these associations better than rs2275703. These findings, coupled with similar findings in Utah Caucasians, suggest that sequence variation in CASQ1 may influence risk of type 2 diabetes.     

Genetic variation in adiponectin receptor 1 and adiponectin receptor 2 is associated with type 2 diabetes in the Old Order Amish

Adiponectin receptor 1 (ADIPOR1) and adiponectin receptor 2 (ADIPOR2) are newly identified receptors for adiponectin, an adipocytokine with anti-inflammatory and insulin-sensitizing properties. We screened for polymorphisms by performing sequence analysis on all eight exons, splice junctions, and approximately 2 kb of the 5' flanking regions of each receptor. We detected 5 single nucleotide polymorphisms (SNPs) in ADIPOR1 and 16 SNPs in ADIPOR2. We genotyped these SNPs in Amish subjects with type 2 diabetes (n = 137), impaired glucose tolerance (IGT) (n = 139), and normal glucose tolerance (n = 342) to test for association with type 2 diabetes. Three intronic SNPs in ADIPOR1 were significantly associated with type 2 diabetes (P = 0.014-0.007; odds ratio [OR] 1.61-1.65) and in high linkage disequilibrium (r2 = 0.97-1.0). In ADIPOR2, we found that five SNPs delineated one large haplotype block (r2= 0.9-1.0) spanning >98 kb of the gene and promoter region, which was strongly associated with the combined type 2 diabetes/IGT trait (P < or = 0.001; OR 1.64-1.71). To our knowledge, these data provide the first evidence for association between variation in the adiponectin receptors and type 2 diabetes.     

Polymorphisms in the SLC2A2 (GLUT2) gene are associated with the conversion from impaired glucose tolerance to type 2 diabetes: the Finnish Diabetes Prevention Study

Impaired insulin secretion is a fundamental defect in type 2 diabetes. The aim of this study was to investigate whether single nucleotide polymorphisms (SNPs) in the genes regulating insulin secretion (SLC2A2 [encoding GLUT2], GCK, TCF1 [encoding HNF-1alpha], HNF4A, GIP, and GLP1R) are associated with the conversion from impaired glucose tolerance (IGT) to type 2 diabetes in participants of the Finnish Diabetes Prevention Study. With the exception of SLC2A2, other genes were not associated with the risk of type 2 diabetes. All four SNPs of SLC2A2 predicted the conversion to diabetes, and rs5393 (AA genotype) increased the risk of type 2 diabetes in the entire study population by threefold (odds ratio 3.04, 95% CI 1.34-6.88, P = 0.008). The risk for type 2 diabetes in the AA genotype carriers was increased in the control group (5.56 [1.78-17.39], P = 0.003) but not in the intervention group. We conclude that the SNPs of SLC2A2 predict the conversion to diabetes in obese subjects with IGT.     

Single nucleotide polymorphisms of PPARD in combination with the Gly482Ser substitution of PGC-1A and the Pro12Ala substitution of PPARG2 predict the conversion from impaired glucose tolerance to type 2 diabetes: the STOP-NIDDM trial

Peroxisome proliferator-activated receptor (PPAR)-delta regulates fatty acid oxidation and improves insulin sensitivity. We screened six single nucleotide polymorphisms (SNPs) of the PPAR-delta gene (PPARD) for an association with the conversion from impaired glucose tolerance (IGT) to type 2 diabetes in 769 subjects participating in the STOP-NIDDM trial. A 2.7-fold increase in the risk of diabetes was observed in female carriers of the C allele of rs6902123 (95% CI 1.44-5.30; adjusted P = 0.002). In the placebo group, subjects possessing both the 482Ser allele of the PPAR-gamma coactivator-1alpha gene (PGC-1A) and the rare allele of two SNPs of PPARD (rs6902123 and rs3734254) had up to 2.5-fold increased risk for diabetes. Furthermore, women carrying the C allele of rs6902123 of PPARD and the Pro12Pro genotype of the PPAR-gamma2 gene (PPARG2) had a 3.9-fold (95% CI 1.79-8.63; P = 0.001)-higher risk for diabetes than women with protective genotypes. Expression levels of PPAR-delta in subcutaneous adipose tissue of 87 offspring of Finnish patients with type 2 diabetes did not differ among the genotype groups of SNPs of PPARD. We conclude that SNPs in PPARD modify the conversion from IGT to type 2 diabetes, particularly in combination with the SNPs of PGC-1A and PPARG2.     

Insulin secretion and insulin sensitivity in relation to glucose tolerance: lessons from the Botnia Study

Recently, a new stage in glucose tolerance, impaired fasting glucose (IFG) (fasting plasma glucose level of 6.1-6.9 mmol/l), was introduced in addition to impaired glucose tolerance (IGT) (2-h glucose level of 7.8-11.0 mmol/l). It is not clear whether IFG and IGT differ with respect to insulin secretion or sensitivity. To address this question, we estimated insulin secretion (by measuring both insulin levels and the ratio of insulin-to-glucose levels in 30-min intervals) and insulin sensitivity (by using the homeostasis model assessment [HOMA] index) from an oral glucose tolerance test (OGTT) in 5,396 individuals from the Botnia Study who had varying degrees of glucose tolerance. There was poor concordance between IFG and IGT: only 36% (303 of 840) of the subjects with IFG had IGT, whereas 62% (493 of 796) of the subjects with IGT did not have IFG. Compared with subjects with normal glucose tolerance (NGT), subjects with IFG were more insulin resistant (HOMA-insulin resistance [IR] values 2.64 +/- 0.08 vs. 1.73 +/- 0.03, P < 0.0005), had greater insulin responses during an OGTT (P = 0.0001), had higher waist-to-hip ratios (P < 0.005), had higher triglyceride and total cholesterol concentrations (P < 0.0005), and had lower HDL cholesterol concentrations (P = 0.0001). Compared with subjects with IFG, subjects with IGT had a lower incremental 30-min insulin-to-glucose area during an OGTT (13.8 +/- 1.7 vs. 21.7 +/- 1.7, P = 0.0008). Compared with subjects with IGT, subjects with mild diabetes (fasting plasma glucose levels <7.8 mmol/l) showed markedly impaired insulin secretion that could no longer compensate for IR and elevated glucose levels. A progressive decline in insulin sensitivity was observed when moving from NGT to IGT and to subjects with diabetes (P < 0.05 for trend), whereas insulin secretion followed an inverted U-shaped form. We conclude that IFG is characterized by basal IR and other features of the metabolic syndrome, whereas subjects with IGT have impaired insulin secretion in relation to glucose concentrations. An absolute decompensation of beta-cell function characterizes the transition from IGT to mild diabetes.     

Beta-cell function across the spectrum of glucose tolerance in obese youth

The profile of insulin secretion and the role of proinsulin processing across the spectrum of glucose tolerance in obese youth have not been studied. The aims of this study were to define the role of insulin secretion and proinsulin processing in glucose regulation in obese youth. We performed hyperglycemic clamps to assess insulin secretion, applying a model of glucose-stimulated insulin secretion to the glucose and C-peptide concentration data. Thirty obese youth with normal glucose tolerance (NGT), 22 with impaired glucose tolerance (IGT), and 10 with type 2 diabetes were studied. The three groups had comparable anthropometric measures and insulin sensitivity. The glucose sensitivity of first-phase secretion showed a significant stepwise decline from NGT to IGT and from IGT to type 2 diabetes. The glucose sensitivity of second-phase secretion was similar in NGT and IGT subjects yet was significantly lower in subjects with type 2 diabetes. Proinsulin-to-insulin ratios were comparable during first- and second-phase secretion between subjects with NGT and IGT and were significantly increased in type 2 diabetes. Obese youth with IGT have a significant defect in first-phase insulin secretion, while a defect in second-phase secretion and proinsulin processing is specific for type 2 diabetes in this age-group.     

Beta-cell function is a major contributor to oral glucose tolerance in high-risk relatives of four ethnic groups in the U.S

First-degree relatives of individuals with type 2 diabetes are at increased risk of developing hyperglycemia. To examine the prevalence and pathogenesis of abnormal glucose homeostasis in these subjects, 531 first-degree relatives with no known history of diabetes (aged 44.1 +/- 0.7 years; BMI 29.0 +/- 0.3 kg/m(2)) underwent an oral glucose tolerance test (OGTT). Newly identified diabetes was found in 19% (n = 100), and impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT) was found in 36% (n = 191). Thus, only 45% (n = 240) had normal glucose tolerance (NGT). The homeostasis model assessment of insulin resistance (HOMA-IR) was used to estimate insulin sensitivity; beta-cell function was quantified as the ratio of the incremental insulin to glucose responses over the first 30 min during the OGTT (DeltaI(30)/DeltaG(30)). This latter measure was also adjusted for insulin sensitivity as it modulates beta-cell function ([DeltaI(30)/DeltaG(30)]/HOMA-IR). Decreasing glucose tolerance was associated with increasing insulin resistance (HOMA: NGT 12.01 +/- 0.54 pmol/mmol; IFG/IGT 16.14 +/- 0.84; diabetes 26.99 +/- 2.62; P < 0.001) and decreasing beta-cell function (DeltaI(30)/DeltaG(30): NGT 157.7 +/- 9.7 pmol/mmol; IFG/IGT 100.4 +/- 5.4; diabetes 57.5 +/- 7.3; P < 0.001). Decreasing beta-cell function was also identified when adjusting this measure for insulin sensitivity ([DeltaI(30)/DeltaG(30)]/HOMA-IR). In all four ethnic groups (African-American, n = 55; Asian-American, n = 66; Caucasian, n = 217; Hispanic-American, n = 193), IFG/IGT and diabetic subjects exhibited progressively increasing insulin resistance and decreasing beta-cell function. The relationships of insulin sensitivity and beta-cell function to glucose disposal, as measured by the incremental glucose area under the curve (AUCg), were examined in the whole cohort. Insulin sensitivity and AUCg were linearly related so that insulin resistance was associated with poorer glucose disposal (r(2) = 0.084, P < 0.001). In contrast, there was a strong inverse curvilinear relationship between beta-cell function and AUCg such that poorer insulin release was associated with poorer glucose disposal (log[DeltaI(30)/DeltaG(30)]: r(2) = 0.29, P < 0.001; log[(DeltaI(30)/DeltaG(30))/HOMA-IR]: r(2) = 0.45, P < 0.001). Thus, abnormal glucose metabolism is common in first-degree relatives of subjects with type 2 diabetes. Both insulin resistance and impaired beta-cell function are associated with impaired glucose metabolism in all ethnic groups, with beta-cell function seeming to be more important in determining glucose disposal.     

Endoneurial capillary abnormalities presage deterioration of glucose tolerance and accompany peripheral neuropathy in man

To explore whether microangiopathy is associated with disturbed glucose tolerance and peripheral neuropathy, we assessed endoneurial capillary morphology in sural nerve biopsies from men with diabetes, impaired glucose tolerance (IGT), and normal glucose tolerance (NGT). Baseline morphology was related to glucose tolerance and neuropathy at baseline and at follow-up 6 years later. Capillary density (in number per millimeters squared) at baseline was higher in subjects with diabetes (n = 10) compared with those with NGT (n = 5) at follow-up (median [interquartile range]) (86.0 [24.3] vs. 54.9 [17.1]; P = 0.0200) and in those progressing from IGT to diabetes (n = 4) compared with those with persistent IGT (n = 4) (86.7 [25.2] vs. 54.1 [14.6]; P = 0.0433). The capillary luminal area (in micrometers squared) was lower in subjects with NGT progressing to IGT (n = 2) or subjects with IGT progressing to diabetes (n = 3) compared with subjects with constant NGT (n = 6) or constant IGT (n = 4) (11.9 [2.4] vs. 20.8 [7.8]; P = 0.0201). The capillary basement membrane area (in micrometers squared) was increased in patients with peripheral neuropathy (n = 10) compared with those without (n = 7) (114.6 [68.8] vs. 75.3 [28.7]; P = 0.0084). In conclusion, increased capillary density was associated with current or future diabetes, decreased capillary luminal area with future deterioration in glucose tolerance, and increased basement membrane area with peripheral neuropathy.     

Metabolic factors contributing to increased resting metabolic rate and decreased insulin-induced thermogenesis during the development of type 2 diabetes.

Previous studies have indicated that individuals with type 2 diabetes have an increased resting metabolic rate (RMR) but decreased insulin-induced thermogenesis (IIT) compared with those with normal glucose tolerance (NGT). When and by which mechanisms these abnormalities occur during the development of diabetes remain unknown. In 560 Pima Indians, sleeping metabolic rate (respiratory chamber) was higher not only in subjects with diabetes (+4.9%, P < 0.001) but also in those with impaired glucose tolerance (IGT) (+2.7%, P < 0.01) compared with subjects with NGT. Longitudinally, RMR (ventilated hood) increased progressively in 17 subjects in whom glucose tolerance deteriorated from NGT to IGT (+4.2%) to diabetes (+2.6%) over 5.1 +/- 1.4 years (P < 0.05 for trend). In parallel, IIT (% increase in metabolic rate during an insulin/glucose infusion) decreased during the transition from NGT (11.7%) to IGT (7.3%) to diabetes (6.5%) (P < 0.05 for trend). In 151 subjects, basal endogenous glucose output (3-3H-glucose), fasting insulin and free fatty acid concentrations, and glucose disposal (hyperinsulinemic clamp) were significant determinants of RMR, independent of body composition, age, and sex. Nonoxidative and oxidative glucose disposal, RMR, and fasting insulin and glucose concentrations were determinants of IIT. Differences in RMR and IIT between glucose tolerance groups decreased after adjusting for these factors. These findings indicate that increases in RMR and decreases in IIT occur early in the development of type 2 diabetes, and that both changes are related to the progressive metabolic abnormalities that occur during the development of the disease.     

Decrease of stimulated amylin release precedes impairment of insulin secretion in type II diabetes.

Amylin, a 37-amino acid polypeptide, has been identified as the major protein component of pancreatic amyloid deposits in patients with non-insulin-dependent (type II) diabetes mellitus. Amylin is stored and released together with insulin and has been proposed to play a major role in the pathogenesis of type II diabetes. To compare amylin release and its proportion to insulin secretion under different metabolic conditions, oral and intravenous glucose tolerance tests (OGTT and IVGTT, respectively) were performed in healthy, lean control subjects, obese patients with normal and impaired glucose tolerance (NGT and IGT, respectively), and obese type II diabetic patients. Compared with control subjects, basal and stimulated amylin secretion during OGTT was significantly higher in obese patients with NGT and IGT but not in type II diabetic patients. The integrated amylin response was significantly higher in obese patients with NGT than lean control subjects and type II diabetic patients matched for degree of obesity. The amylin-insulin ratio decreased slightly in obese subjects with NGT and IGT and significantly in type II diabetic patients. Amylin secretion was significantly stimulated during IVGTT in control subjects and obese patients with NGT and IGT but not in type II diabetic patients. These findings suggest that amylin is physiologically released by pancreatic beta-cells in a constant ratio to insulin in nondiabetic subjects. Glucose-stimulated amylin secretion is increased in obese subjects with NGT and IGT. In type II diabetes mellitus, amylin secretion relative to that of insulin is decreased, and amylin is not stimulated by IVGTT.