Minimal model-based insulin sensitivity has greater heritability and a different genetic basis than homeostasis model assessment or fasting insulin

Novel regulators of islet function identified from genetic variation in mouse islet Ca2+ oscillations

Insufficient insulin secretion to meet metabolic demand results in diabetes. The intracellular flux of Ca2+ into β-cells triggers insulin release. Since genetics strongly influences variation in islet secretory responses, we surveyed islet Ca2+ dynamics in eight genetically diverse mouse strains. We found high strain variation in response to four conditions: (1) 8 mM glucose; (2) 8 mM glucose plus amino acids; (3) 8 mM glucose, amino acids, plus 10 nM glucose-dependent insulinotropic polypeptide (GIP); and (4) 2 mM glucose. These stimuli interrogate β-cell function, α- to β-cell signaling, and incretin responses. We then correlated components of the Ca2+ waveforms to islet protein abundances in the same strains used for the Ca2+ measurements. To focus on proteins relevant to human islet function, we identified human orthologues of correlated mouse proteins that are proximal to glycemic-associated single-nucleotide polymorphisms in human genome-wide association studies. Several orthologues have previously been shown to regulate insulin secretion (e.g. ABCC8, PCSK1, and GCK), supporting our mouse-to-human integration as a discovery platform. By integrating these data, we nominate novel regulators of islet Ca2+ oscillations and insulin secretion with potential relevance for human islet function. We also provide a resource for identifying appropriate mouse strains in which to study these regulators.

The usefulness of the estimated average glucose/fasting blood glucose ratio for pancreatic β-cell function assessment in hyperglycemia during health checkups

Background

Type 2 diabetes (T2DM) is a disease marked by inadequate insulin secretion by pancreatic beta‐cell function (BCF) failure and insulin resistance (IR). Assessing and managing the BCF and IR should be started early to prevent or delay the progression of the disease. The aim of this study was to determine the usefulness of the estimated average glucose (eAG)/fasting blood glucose (FBG) ratio for pancreatic BCF in hyperglycemia.

Methods

This cross‐sectional study consecutively selected 10,594 subjects who underwent a health checkup at 16 health checkup centers in 13 Korean cities between 2019 and 2021. The subjects consisted of 3003 patients with normoglycemia, 3413 with impaired fasting glucose and 4178 with T2DM. The eAG was calculated using Nathan's regression equation. BCF and IR were estimated by the homeostasis model assessment (HOMA)‐β and HOMA‐IR, respectively. Multivariate (adjusted) regression analysis was performed to evaluate the association between the eAG/FBG ratio and HOMA.

Results

The median values among FBG groups for the eAG/FBG ratio, HOMA‐β, ‐IR and insulin differed significantly (p < 0.001). The second‐, third‐ and fourth‐quartile groups of the eAG/FBG ratio had positive higher correlation coefficients [9.533, 10.080 and 12.021, respectively (all p < 0.001)] for HOMA‐β than the first quartile group, and higher negative coefficients for HOMA‐IR [−0.696, −0.727 and −0.598, respectively (all p = 0.001)].

Conclusion

The eAG/FBG ratio was significantly correlated with both HOMA‐β and ‐IR, which suggests that eAG/FBG ratio reveals BCF and IR in hyperglycemia. Measurement of this ratio could be useful for monitoring BCF and IR in prediabetes and T2DM.

Associations of complete blood count parameters with pancreatic beta-cell function and insulin resistance in prediabetes and type 2 diabetes mellitus

INTRODUCTION

Previous studies found controversial associations of CBC parameters with pancreatic beta-cell function (BCF) and insulin resistance (IR). The aim of this was to determine the independent associations of CBC parameters with BCF and IR in prediabetes and type 2 diabetes mellitus (T2DM).

METHODS

This study selected subjects who underwent health checkups at 16 health-promotion centers in 13 Korean cities during 2021. The subjects comprised 1470 patients with normoglycemia, 1124 with prediabetes, and 396 with T2DM. BCF and IR were assessed using the homeostasis model assessment (HOMA)-β and HOMA-IR, respectively. Correlation and multiple linear regression analyses were used to determine the correlation between CBC parameters and HOMA.

RESULTS

While HOMA-IR gradually increased according to red blood cell count quartiles (1.22, 1.40, 1.47, and 1.91, in the first, second, third, and fourth quartiles, respectively; p < 0.001), there was no correlation after adjusting for waist circumference (WC) and HbA1c. The red blood cell distribution width (RDW) was associated with HOMA-β [coefficient (β) = 15.527, p = 0.002], but not with HOMA-IR. White blood cells (WBCs) were associated with HOMA-IR and HOMA-β, which was stronger in HOMA-β (β = 0.505 vs 15.171, p = 0.002) after adjusting for WC and HbA1c. The platelet count was correlated with HOMA-IR and HOMA-β, which only remained in HOMA-β (β = 15.581, p = 0.002) after adjusting for WC and HbA1c.

CONCLUSION

RDW, WBC, and platelet counts were independently associated with only HOMA-β in prediabetes and T2DM. This suggests that these CBC parameters could represent BCF in prediabetes and T2DM.

Early Impairment of Insulin Sensitivity, β-Cell Responsiveness, and Insulin Clearance in Youth with Stage 1 Type 1 Diabetes

CONTEXT

Clinical onset of type 1 diabetes (Stage 3 T1D) is preceded by a presymptomatic phase characterized by multiple islet autoantibodies with normal glucose tolerance (Stage 1 T1D).

OBJECTIVE

The aim was to explore the metabolic phenotypes of β-cell function and insulin sensitivity and clearance in normoglycemic youth with Stage 1 T1D and compare them with healthy nonrelated peers during a 3-hour oral glucose tolerance test (OGTT).

METHODS

Twenty-eight lean youth, 14 with ≥2 islet autoantibodies (cases) and 14 healthy controls underwent a 3-hour 9-point OGTT with measurement of glucose, C-peptide, and insulin. The oral minimal model was used to quantitate β-cell responsiveness (φtotal) and insulin sensitivity (SI), allowing assessment of β-cell function by the disposition index (DI=φtotal×SI). Fasting insulin clearance (CL0) was calculated as the ratio between the fasting insulin secretion rate (ISR) and plasma insulin levels (ISR0/I0), while postload clearance (CL180) was estimated by the ratio of AUC of ISR over the plasma insulin AUC for the 3-hour OGTT (ISRAUC/IAUC). Participants with impaired fasting glucose, impaired glucose tolerance, or any OGTT glucose concentration ≥200 mg/dL were excluded.

RESULTS

Cases (10.5 years [8, 15]) exhibited reduced DI (P < .001) due to a simultaneous reduction in both φtotal (P < 0.001) and SI (P = .008) compared with controls (11.5 years [10.4, 14.9]). CL0 and CL180 were lower in cases than in controls (P = .005 and P = .019).

CONCLUSION

Presymptomatic Stage 1 T1D in youth is associated with reduced insulin sensitivity and lower β-cell responsiveness, and the presence of blunted insulin clearance.

Serum vitamin D content is associated with semen parameters and serum testosterone levels in men

The present study aimed to evaluate the influence of serum vitamin D levels on semen quality and testosterone levels. This is a cross-sectional study conducted at Androscience, Science and Innovation Center in Andrology and High-Complex Clinical and Andrology Laboratory in Sao Paulo, Brazil, with 508 male patients, aged 18-60 years, from 2007 to 2017. Seminal parameters and serum sexual hormones were correlated with serum vitamin D concentrations in 260 men selected by strict selection criteria. Patients were divided into normozoospermic group (NZG, n = 124) and a group with seminal abnormalities (SAG, n = 136). Evaluation included complete physical examination, past medical history, habits and lifestyle factors, two complete seminal analysis with sperm functional tests, serum levels of 25-hydroxy-vitamin D3(25(OH)VD3), total and free testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH), sex hormone-binding globulin (SHBG), total cholesterol, homeostatic model assessment of insulin resistance (HOMA-IR) index, and karyotype. The mean concentration of 25(OH)VD3was significantly lower in the SAG (P < 0.001) and positively correlated with all baseline seminal parameters and total testosterone levels. In addition, serum vitamin D3concentration was found to be positively correlated with sperm concentration (β= 2.103; P < 0.001), total number of spermatozoa with progressive motility (β = 2.069; P = 0.003), total number of motile spermatozoa (β = 2.571; P = 0.015), and strict morphology (β = 0.056; P = 0.006), regardless of other variables. This is the first comparative study to address the issue of serum vitamin D3content between normozoospermic patients and those with sperm abnormalities. It clearly demonstrates a direct and positive relationship between serum vitamin D level and overall semen quality, male reproductive potential, and testosterone levels.

Longitudinal associations between ambient air pollution and insulin sensitivity: results from the KORA cohort study

BACKGROUND

Impaired insulin sensitivity could be an intermediate step that links exposure to air pollution to the development of type 2 diabetes. However, longitudinal associations of air pollution with insulin sensitivity remain unclear. Our study investigated the associations of long-term air pollution exposure with the degree and rate of change of insulin sensitivity.

METHODS

In this longitudinal study, we analysed data from the Cooperative Health Research in the Region of Augsburg (KORA) cohort from Augsburg, Germany, which recruited participants aged 25-74 years in the survey between 1999 and 2001 (KORA S4), with two follow-up examinations in 2006-08 (KORA F4) and 2013-14 (KORA FF4). Serum concentrations of fasting insulin and glucose, and homoeostasis model assessment of insulin resistance (HOMA-IR, a surrogate measure of insulin sensitivity) and β-cell function (HOMA-B, a surrogate marker for fasting insulin secretion) were assessed at up to three visits between 1999 and 2014. Annual average air pollutant concentrations at the residence were estimated by land-use regression models. We examined the associations of air pollution with repeatedly assessed biomarker levels using mixed-effects models, and we assessed the associations with the annual rate of change in biomarkers using quantile regression models.

FINDINGS

Among 9620 observations from 4261 participants in the KORA cohort, we included 6008 (62·5%) observations from 3297 (77·4%) participants in our analyses. Per IQR increment in annual average air pollutant concentrations, HOMA-IR significantly increased by 2·5% (95% CI 0·3 to 4·7) for coarse particulate matter, by 3·1% (0·9 to 5·3) for PM_2·5, by 3·6% (1·0 to 6·3) for PM_{2·5absorbance}, and by 3·2% (0·6 to 5·8) for nitrogen dioxide, and borderline significantly increased by 2·2% (-0·1 to 4·5) for ozone, whereas it did not significantly increase for the whole range of ultrafine particles. Similar positive associations in slightly smaller magnitude were observed for HOMA-B and fasting insulin levels. In addition, air pollutant concentrations were positively associated with the annual rate of change in HOMA-IR, HOMA-B, and fasting insulin. Neither the level nor the rate of change of fasting glucose were associated with air pollution exposure.

INTERPRETATION

Our study indicates that long-term air pollution exposure could contribute to the development of insulin resistance, which is one of the key factors in the pathogenesis of type 2 diabetes.

FUNDING

German Federal Ministry of Education and Research.

Classification of Type 2 Diabetes Genetic Variants and a Novel Genetic Risk Score Association With Insulin Clearance

CONTEXT

Genome-wide association studies have identified more than 450 single nucleotide polymorphisms (SNPs) for type 2 diabetes (T2D).

OBJECTIVE

To facilitate use of these SNPs in future genetic risk score (GRS)-based analyses, we aimed to classify the SNPs based on physiology. We also sought to validate GRS associations with insulin-related traits in deeply phenotyped Mexican Americans.

DESIGN, SETTING, AND PARTICIPANTS

A total of 457 T2D SNPs from the literature were assigned physiologic function based on association studies and cluster analyses. All SNPs (All-GRS), beta-cell (BC-GRS), insulin resistance (IR-GRS), lipodystrophy (Lipo-GRS), and body mass index plus lipids (B + L-GRS) were evaluated for association with diabetes and indices of insulin secretion (from oral glucose tolerance test), insulin sensitivity and insulin clearance (from euglycemic clamp), and adiposity and lipid markers in 1587 Mexican Americans.

RESULTS

Of the 457 SNPs, 52 were classified as BC, 30 as IR, 12 as Lipo, 12 as B + L, whereas physiologic function of 351 was undefined. All-GRS was strongly associated with T2D. Among nondiabetic Mexican Americans, BC-GRS was associated with reduced insulinogenic index, IR-GRS was associated with reduced insulin sensitivity, and Lipo-GRS was associated with reduced adiposity. B + L-GRS was associated with increased insulin clearance. The latter did not replicate in an independent cohort wherein insulin clearance was assessed by a different method.

CONCLUSIONS

Supporting their utility, BC-GRS, IR-GRS, and Lipo-GRS, based on SNPs discovered largely in Europeans, exhibited expected associations in Mexican Americans. The novel association of B + L-GRS with insulin clearance suggests that impaired ability to reduce insulin clearance in compensation for IR may play a role in the pathogenesis of T2D. Whether this applies to other ethnic groups remains to be determined.

Shared genetic architecture and casual relationship between leptin levels and type 2 diabetes: large-scale cross-trait meta-analysis and Mendelian randomization analysis

OBJECTIVE

We aimed to estimate genetic correlation, identify shared loci and test causality between leptin levels and type 2 diabetes (T2D).

RESEARCH DESIGN AND METHODS

Our study consists of three parts. First, we calculated the genetic correlation of leptin levels and T2D or glycemic traits by using linkage disequilibrium score regression analysis. Second, we conducted a large-scale genome-wide cross-trait meta-analysis using cross-phenotype association to identify shared loci between trait pairs that showed significant genetic correlations in the first part. In the end, we carried out a bidirectional MR analysis to find out whether there is a causal relationship between leptin levels and T2D or glycemic traits.

RESULTS

We found positive genetic correlations between leptin levels and T2D (R_g=0.3165, p=0.0227), fasting insulin (FI) (R_g=0.517, p=0.0076), homeostasis model assessment-insulin resistance (HOMA-IR) (R_g=0.4785, p=0.0196), as well as surrogate estimates of β-cell function (HOMA-β) (R_g=0.4456, p=0.0214). We identified 12 shared loci between leptin levels and T2D, 1 locus between leptin levels and FI, 1 locus between leptin levels and HOMA-IR, and 1 locus between leptin levels and HOMA-β. We newly identified eight loci that did not achieve genome-wide significance in trait-specific genome-wide association studies. These shared genes were enriched in pancreas, thyroid gland, skeletal muscle, placenta, liver and cerebral cortex. In addition, we found that 1-SD increase in HOMA-IR was causally associated with a 0.329 ng/mL increase in leptin levels (β=0.329, p=0.001).

CONCLUSIONS

Our results have shown the shared genetic architecture between leptin levels and T2D and found causality of HOMA-IR on leptin levels, shedding light on the molecular mechanisms underlying the association between leptin levels and T2D.

Hyperinsulinemia: An Early Indicator of Metabolic Dysfunction

Hyperinsulinemia is strongly associated with type 2 diabetes. Racial and ethnic minority populations are disproportionately affected by diabetes and obesity-related complications. This mini-review provides an overview of the genetic and environmental factors associated with hyperinsulinemia with a focus on racial and ethnic differences and its metabolic consequences. The data used in this narrative review were collected through research in PubMed and reference review of relevant retrieved articles. Insulin secretion and clearance are regulated processes that influence the development and progression of hyperinsulinemia. Environmental, genetic, and dietary factors are associated with hyperinsulinemia. Certain pharmacotherapies for obesity and bariatric surgery are effective at mitigating hyperinsulinemia and are associated with improved metabolic health. Hyperinsulinemia is associated with many environmental and genetic factors that interact with a wide network of hormones. Recent studies have advanced our understanding of the factors affecting insulin secretion and clearance. Further basic and translational work on hyperinsulinemia may allow for earlier and more personalized treatments for obesity and metabolic diseases.

Arsenic exposure induces glucose intolerance and alters global energy metabolism

Environmental pollutants acting as endocrine-disrupting chemicals (EDCs) are recognized as potential contributors to metabolic disease pathogenesis. One such pollutant, arsenic, contaminates the drinking water of ~100 million people globally and has been associated with insulin resistance and diabetes in epidemiological studies. Despite these observations, the precise metabolic derangements induced by arsenic remain incompletely characterized. In the present study, the impact of arsenic on in vivo metabolic physiology was examined in 8-wk-old male C57BL/6J mice exposed to 50 mg/l inorganic arsenite in their drinking water for 8 wk. Glucose metabolism was assessed via in vivo metabolic testing, and feeding behavior was analyzed using indirect calorimetry in metabolic cages. Pancreatic islet composition was assessed via immunofluorescence microscopy. Arsenic-exposed mice exhibited impaired glucose tolerance compared with controls; however, no difference in peripheral insulin resistance was noted between groups. Instead, early insulin release during glucose challenge was attenuated relative to the rise in glycemia. Despite decreased insulin secretion, pancreatic β-cell mass was not altered, suggesting that arsenic primarily disrupts β-cell function. Finally, metabolic cage analyses revealed that arsenic exposure induced novel alterations in the diurnal rhythm of food intake and energy metabolism. Taken together, these data suggest that arsenic exposure impairs glucose tolerance through functional impairments in insulin secretion from β-cells rather than by augmenting peripheral insulin resistance. Further elucidation of the mechanisms underlying arsenic-induced behavioral and β-cell-specific metabolic disruptions will inform future intervention strategies to address this ubiquitous environmental contaminant and novel diabetes risk factor.

Shared Genetic Control of Brain Activity During Sleep and Insulin Secretion: A Laboratory-Based Family Study

Over the past 20 years, a large body of experimental and epidemiologic evidence has linked sleep duration and quality to glucose homeostasis, although the mechanistic pathways remain unclear. The aim of the current study was to determine whether genetic variation influencing both sleep and glucose regulation could underlie their functional relationship. We hypothesized that the genetic regulation of electroencephalographic (EEG) activity during non-rapid eye movement sleep, a highly heritable trait with fingerprint reproducibility, is correlated with the genetic control of metabolic traits including insulin sensitivity and β-cell function. We tested our hypotheses through univariate and bivariate heritability analyses in a three-generation pedigree with in-depth phenotyping of both sleep EEG and metabolic traits in 48 family members. Our analyses accounted for age, sex, adiposity, and the use of psychoactive medications. In univariate analyses, we found significant heritability for measures of fasting insulin sensitivity and β-cell function, for time spent in slow-wave sleep, and for EEG spectral power in the delta, theta, and sigma ranges. Bivariate heritability analyses provided the first evidence for a shared genetic control of brain activity during deep sleep and fasting insulin secretion rate.

Physiologic Interpretation of GWAS Signals for Type 2 Diabetes

This chapter reviews both statistical and physiologic issues related to the pathophysiologic effects of genetic variation in the context of type 2 diabetes. The goal is to review current methodologies used to analyze disease-related quantitative traits for those who do not have extensive quantitative and physiologic background, as an attempt to bridge that gap. We leverage mathematical modeling to illustrate the strengths and weaknesses of different approaches and attempt to reinforce with real data analysis. Topics reviewed include phenotype selection, phenotype specificity, multiple variant analysis via the genetic risk score, and consideration of multiple disease-related phenotypes. Type 2 diabetes is used as the example, not only because of the extensive existing knowledge at the genetic, physiologic, clinical, and epidemiologic levels, but also because type 2 diabetes has been at the forefront of complex disease genetics, with many examples to draw from.

Genome-Wide Association Study of the Modified Stumvoll Insulin Sensitivity Index Identifies BCL2 and FAM19A2 as Novel Insulin Sensitivity Loci

Genome-wide association studies (GWAS) have found few common variants that influence fasting measures of insulin sensitivity. We hypothesized that a GWAS of an integrated assessment of fasting and dynamic measures of insulin sensitivity would detect novel common variants. We performed a GWAS of the modified Stumvoll Insulin Sensitivity Index (ISI) within the Meta-Analyses of Glucose and Insulin-Related Traits Consortium. Discovery for genetic association was performed in 16,753 individuals, and replication was attempted for the 23 most significant novel loci in 13,354 independent individuals. Association with ISI was tested in models adjusted for age, sex, and BMI and in a model analyzing the combined influence of the genotype effect adjusted for BMI and the interaction effect between the genotype and BMI on ISI (model 3). In model 3, three variants reached genome-wide significance: rs13422522 (NYAP2; P = 8.87 × 10(-11)), rs12454712 (BCL2; P = 2.7 × 10(-8)), and rs10506418 (FAM19A2; P = 1.9 × 10(-8)). The association at NYAP2 was eliminated by conditioning on the known IRS1 insulin sensitivity locus; the BCL2 and FAM19A2 associations were independent of known cardiometabolic loci. In conclusion, we identified two novel loci and replicated known variants associated with insulin sensitivity. Further studies are needed to clarify the causal variant and function at the BCL2 and FAM19A2 loci.

Improved Performance of Dynamic Measures of Insulin Response Over Surrogate Indices to Identify Genetic Contributors of Type 2 Diabetes: The GUARDIAN Consortium

Type 2 diabetes (T2D) is a heterogeneous disorder with contributions from peripheral insulin resistance and β-cell dysfunction. For minimization of phenotypic heterogeneity, quantitative intermediate phenotypes characterizing basal glucose homeostasis (insulin resistance and HOMA of insulin resistance [HOMAIR] and of β-cell function [HOMAB]) have shown promise in relatively large samples. We investigated the utility of dynamic measures of glucose homeostasis (insulin sensitivity [SI] and acute insulin response [AIRg]) evaluating T2D-susceptibility variants (n = 57) in Hispanic Americans from the GUARDIAN Consortium (n = 2,560). Basal and dynamic measures were genetically correlated (HOMAB-AIRg: ρG = 0.28-0.73; HOMAIR-SI: ρG = -0.73 to -0.83) with increased heritability for the dynamic measure AIRg Significant association of variants with dynamic measures (P < 8.77 × 10(-4)) was observed. A pattern of superior performance of AIRg was observed for well-established loci including MTNR1B (P = 9.46 × 10(-12)), KCNQ1 (P = 1.35 × 10(-4)), and TCF7L2 (P = 5.10 × 10(-4)) with study-wise statistical significance. Notably, significant association of MTNR1B with AIRg (P < 1.38 × 10(-9)) was observed in a population one-fourteenth the size of the initial discovery cohort. These observations suggest that basal and dynamic measures provide different views and levels of sensitivity to discrete elements of glucose homeostasis. Although more costly to obtain, dynamic measures yield significant results that could be considered physiologically "closer" to causal pathways and provide insight into the discrete mechanisms of action.

An Islet-Targeted Genome-Wide Association Scan Identifies Novel Genes Implicated in Cytokine-Mediated Islet Stress in Type 2 Diabetes

Genome-wide association studies in human type 2 diabetes (T2D) have renewed interest in the pancreatic islet as a contributor to T2D risk. Chronic low-grade inflammation resulting from obesity is a risk factor for T2D and a possible trigger of β-cell failure. In this study, microarray data were collected from mouse islets after overnight treatment with cytokines at concentrations consistent with the chronic low-grade inflammation in T2D. Genes with a cytokine-induced change of >2-fold were then examined for associations between single nucleotide polymorphisms and the acute insulin response to glucose (AIRg) using data from the Genetics Underlying Diabetes in Hispanics (GUARDIAN) Consortium. Significant evidence of association was found between AIRg and single nucleotide polymorphisms in Arap3 (5q31.3), F13a1 (6p25.3), Klhl6 (3q27.1), Nid1 (1q42.3), Pamr1 (11p13), Ripk2 (8q21.3), and Steap4 (7q21.12). To assess the potential relevance to islet function, mouse islets were exposed to conditions modeling low-grade inflammation, mitochondrial stress, endoplasmic reticulum (ER) stress, glucotoxicity, and lipotoxicity. RT-PCR revealed that one or more forms of stress significantly altered expression levels of all genes except Arap3. Thapsigargin-induced ER stress up-regulated both Pamr1 and Klhl6. Three genes confirmed microarray predictions of significant cytokine sensitivity: F13a1 was down-regulated 3.3-fold by cytokines, Ripk2 was up-regulated 1.5- to 3-fold by all stressors, and Steap4 was profoundly cytokine sensitive (167-fold up-regulation). Three genes were thus closely associated with low-grade inflammation in murine islets and also with a marker for islet function (AIRg) in a diabetes-prone human population. This islet-targeted genome-wide association scan identified several previously unrecognized candidate genes related to islet dysfunction during the development of T2D.

High heritability and genetic correlation of intravenous glucose- and tolbutamide-induced insulin secretion among non-diabetic family members of type 2 diabetic patients

AIMS/HYPOTHESIS

The aim of this study was to estimate the heritability of quantitative measures of glucose regulation obtained from a tolbutamide-modified frequently sampled IVGTT (t-FSIGT) and to correlate the heritability of the glucose-stimulated beta cell response to the tolbutamide-induced beta cell response. In addition, single nucleotide polymorphisms (SNPs) having an exclusive effect on either glucose- or tolbutamide-stimulated insulin release were identified.

METHODS

Two hundred and eighty-four non-diabetic family members of patients with type 2 diabetes underwent a t-FSIGT with intravenous injection of glucose at t = 0 min and tolbutamide at t = 20 min. Measurements of plasma glucose, serum insulin and serum C-peptide were taken at 33 time points from fasting to 180 min. Insulin secretion rate, acute insulin response (AIR), disposition index (DI) after glucose and disposition index after tolbutamide (DIT), insulin sensitivity (SI), glucose effectiveness (SG) and beta cell responsiveness to glucose were calculated. A polygenic variance component model was used to estimate heritability, genetic correlations and associations.

RESULTS

We found high heritabilities for acute insulin secretion subsequent to glucose stimulation (AIRglucose h (2) ± SE: 0.88 ± 0.14), but these were slightly lower after tolbutamide (AIRtolbutamide h (2) ± SE: 0.69 ± 0.14). We also estimated the heritabilities for SI (h (2) ± SE: 0.26 ± 0.12), SG (h (2) ± SE: 0.47 ± 0.13), DI (h (2) ± SE: 0.56 ± 0.14), DIT (h (2) ± SE: 0.49 ± 0.14) and beta cell responsiveness to glucose (h (2) ± SE: 0.66 ± 0.12). Additionally, strong genetic correlations were found between measures of beta cell response after glucose and tolbutamide stimulation, with correlation coefficients ranging from 0.77 to 0.88. Furthermore, we identified five SNPs with an exclusive effect on either glucose-stimulated (rs5215, rs1111875, rs11920090) or tolbutamide-stimulated (rs10946398, rs864745) insulin secretion.

CONCLUSIONS/INTERPRETATION

Our data demonstrate that both glucose- and tolbutamide-induced insulin secretions are highly heritable traits, which are largely under the control of the same genes.

Failure of homeostatic model assessment of insulin resistance to detect marked diet-induced insulin resistance in dogs

Accurate quantification of insulin resistance is essential for determining efficacy of treatments to reduce diabetes risk. Gold-standard methods to assess resistance are available (e.g., hyperinsulinemic clamp or minimal model), but surrogate indices based solely on fasting values have attractive simplicity. One such surrogate, the homeostatic model assessment of insulin resistance (HOMA-IR), is widely applied despite known inaccuracies in characterizing resistance across groups. Of greater significance is whether HOMA-IR can detect changes in insulin sensitivity induced by an intervention. We tested the ability of HOMA-IR to detect high-fat diet-induced insulin resistance in 36 healthy canines using clamp and minimal model analysis of the intravenous glucose tolerance test (IVGTT) to document progression of resistance. The influence of pancreatic function on HOMA-IR accuracy was assessed using the acute insulin response during the IVGTT (AIRG). Diet-induced resistance was confirmed by both clamp and minimal model (P < 0.0001), and measures were correlated with each other (P = 0.001). In striking contrast, HOMA-IR ([fasting insulin (μU/mL) × fasting glucose (mmol)]/22.5) did not detect reduced sensitivity induced by fat feeding (P = 0.22). In fact, 13 of 36 animals showed an artifactual decrease in HOMA-IR (i.e., increased sensitivity). The ability of HOMA-IR to detect diet-induced resistance was particularly limited under conditions when insulin secretory function (AIRG) is less than robust. In conclusion, HOMA-IR is of limited utility for detecting diet-induced deterioration of insulin sensitivity quantified by glucose clamp or minimal model. Caution should be exercised when using HOMA-IR to detect insulin resistance when pancreatic function is compromised. It is necessary to use other accurate indices to detect longitudinal changes in insulin resistance with any confidence.

Insulin sensitivity and insulin clearance are heritable and have strong genetic correlation in Mexican Americans

OBJECTIVE

The GUARDIAN (Genetics UndeRlying DIAbetes in HispaNics) consortium is described, along with heritability estimates and genetic and environmental correlations of insulin sensitivity and metabolic clearance rate of insulin (MCRI).

METHODS

GUARDIAN is comprised of seven cohorts, consisting of 4,336 Mexican-American individuals in 1,346 pedigrees. Insulin sensitivity (SI ), MCRI, and acute insulin response (AIRg) were measured by frequently sampled intravenous glucose tolerance test in four cohorts. Insulin sensitivity (M, M/I) and MCRI were measured by hyperinsulinemic-euglycemic clamp in three cohorts. Heritability and genetic and environmental correlations were estimated within the family cohorts (totaling 3,925 individuals) using variance components.

RESULTS

Across studies, age, and gender-adjusted heritability of insulin sensitivity (SI , M, M/I) ranged from 0.23 to 0.48 and of MCRI from 0.35 to 0.73. The ranges for the genetic correlations were 0.91 to 0.93 between SI and MCRI; and -0.57 to -0.59 for AIRg and MCRI (all P < 0.0001). The ranges for the environmental correlations were 0.54 to 0.74 for SI and MCRI (all P < 0.0001); and -0.16 to -0.36 for AIRg and MCRI (P < 0.0001-0.06).

CONCLUSIONS

These data support a strong familial basis for insulin sensitivity and MCRI in Mexican Americans. The strong genetic correlations between MCRI and SI suggest common genetic determinants.

Effect of preeclampsia on insulin sensitivity

OBJECTIVE

The objective of this study is to investigate whether preeclampsia is associated with exacerbation of insulin resistance.

MATERIALS AND METHODS

The study was conducted over a period of 7 months from November 2011 to May 2012, in a tertiary care hospital attached to a medical college. A total of 14 pregnant women in the third trimester with preeclampsia were recruited for this study and 14 well-matched normotensive women in the third trimester were taken as control. 15 g, 50% dextrose load was given intravenously and blood sampling was carried out for glucose and insulin levels up to 3 h afterward. Minimal model analysis of glucose and insulin levels was performed to arrive at results.

RESULTS

No significant changes in mean age, body mass index, gestation, serum lipid and progesterone, cortisol and androgen concentrations were recognized. No significant difference was found between the glucose decay curves and between the glucose clearance rate K, in the two groups (preeclamptic vs. normotensive: 2.1 ± 0.2 vs. 2.2 ± 0.3; P = 0.48). Therefore, there was a small but prolonged decrease in the insulin response of women with preeclampsia compared with women in the normotensive group.

CONCLUSION

Preeclampsia per se is not a risk factor for development of insulin resistance.

HOMA and Matsuda indices of insulin sensitivity: poor correlation with minimal model-based estimates of insulin sensitivity in longitudinal settings

AIMS/HYPOTHESIS

Little is known about the performance of surrogates in assessing changes in insulin sensitivity over time. This report compared updated HOMA of insulin sensitivity (HOMA2-%S) and the Matsuda index from OGTTs with minimal model-based estimates of insulin sensitivity (SI) from frequently sampled IVGTTs (FSIGTs) in longitudinal settings and cross-sectional settings.

METHODS

Two longitudinal studies were used: one a natural observational study in which 338 individuals were followed for a median of 4 years; one a clinical treatment study in which 97 individuals received pioglitazone treatment and were followed for 1 year. Pairs of OGTTs and FSIGTs were performed at baseline and follow-up. Correlations were computed. Impact of measurement uncertainty was investigated through simulation studies.

RESULTS

Correlations between HOMA2-%S and SI from baseline or follow-up data were in the range reported previously (0.61-0.69). By contrast, correlations for changes over time were only 0.35-0.39. The corresponding correlations between the Matsuda index and SI were 0.66-0.72 for cross-sectional data and 0.40-0.48 for longitudinal change. Correlations for changes were significantly lower than the cross-sectional correlations in both studies (p < 0.03). Simulation results demonstrated that the reduced correlations for change were not explained by error propagation, supporting a real limitation of surrogates to fully capture longitudinal changes in insulin sensitivity.

CONCLUSIONS/INTERPRETATION

HOMA and Matsuda indices derived from cross-sectional data should be used cautiously in assessing longitudinal changes in insulin sensitivity.

Diabetes mellitus in patients with thalassemia major

BACKGROUND

Diabetes mellitus is a major endocrinopathy for patients with thalassemia major. Although diabetes mellitus is multifactorial, iron loading is its primary cause and its management poses a clinical challenge. Detecting the pre-diabetes stage is critical because clinical diabetes can potentially be reversed or prevented.

PROCEDURE

Patients with thalassemia major who received regular blood transfusion therapy from 1994 to 2010 were evaluated for the incidence of diabetes mellitus and glucose dysregulation. The association between patients' clinical, biochemical, and image parameters was also evaluated.

RESULTS

The patients with diabetes were significantly older, had higher ferritin levels, a smaller pancreas volume, and lower cardiac T2* magnetic resonance imaging (MRI) values than the patients without diabetes. The pancreas T2* MRI values were higher in the patients without diabetes, but the difference was not statistically significant. The liver iron concentration did not differ between the patients with and without diabetes. The prevalence of hepatitis C infection and hypogonadism was also higher in the patients with diabetes. In the patients without diabetes, the cardiac T2* MRI values were higher in patients with normal fasting glucose levels (P = 0.03), and the homeostasis model assessment of insulin resistance level was associated with hepatitis C infection (P = 0.024, r = 0.32) and hypogonadism (P = 0.034, r = 0.301).

CONCLUSIONS

Fasting glucose and insulin levels were appropriate screening tools for evaluating glucose dysregulation and complemented the MRI findings. The cardiac T2* and pancreas volumes were significant predictors of diabetes.

Genetics of glucose homeostasis: implications for insulin resistance and metabolic syndrome

This review summarizes the current understanding of the genetic basis of glucose homeostasis through genome-wide association scans and candidate gene studies of case-control and family-based designs. We highlight the implications of phenotype-direct (euglycemic clamp or frequently sampled intravenous glucose tolerance test) and indirect (fasting insulin and fasting glucose) measures on the determinants of insulin resistance and β-cell response that precede and contribute to the development of type 2 diabetes mellitus (T2DM) and the metabolic syndrome. Finally, we examine future approaches that may aid in understanding the biology of insulin resistance and T2DM. Over the past 2 decades, the prevalence of insulin resistance, the metabolic syndrome, and T2DM has increased. Ethnic differences in T2DM and insulin resistance are evident, with nonwhite populations having the greatest risk. There continue to be significant gaps in our knowledge regarding the metabolic, behavioral, and genetic determinants of these conditions. Understanding the genetic basis of glucose homeostasis, insulin resistance, and T2DM should provide insight on known and novel metabolic pathways that identify potential therapeutic targets and mechanisms for intervention.

Diabetes technology: markers, monitoring, assessment, and control of blood glucose fluctuations in diabetes

People with diabetes face a life-long optimization problem: to maintain strict glycemic control without increasing their risk for hypoglycemia. Since the discovery of insulin in 1921, the external regulation of diabetes by engineering means has became a hallmark of this optimization. Diabetes technology has progressed remarkably over the past 50 years-a progress that includes the development of markers for diabetes control, sophisticated monitoring techniques, mathematical models, assessment procedures, and control algorithms. Continuous glucose monitoring (CGM) was introduced in 1999 and has evolved from means for retroactive review of blood glucose profiles to versatile reliable devices, which monitor the course of glucose fluctuations in real time and provide interactive feedback to the patient. Technology integrating CGM with insulin pumps is now available, opening the field for automated closed-loop control, known as the artificial pancreas. Following a number of in-clinic trials, the quest for a wearable ambulatory artificial pancreas is under way, with a first prototype tested in outpatient setting during the past year. This paper discusses key milestones of diabetes technology development, focusing on the progress in the past 10 years and on the artificial pancreas-still not a cure, but arguably the most promising treatment of diabetes to date.

Interactions between genetic background, insulin resistance and β-cell function

An interaction between genes and the environment is a critical component underlying the pathogenesis of the hyperglycaemia of type 2 diabetes. The development of more sophisticated techniques for studying gene variants and for analysing genetic data has led to the discovery of some 40 genes associated with type 2 diabetes. Most of these genes are related to changes in β-cell function, with a few associated with decreased insulin sensitivity and obesity. Interestingly, using quantitative traits based on continuous measures rather than dichotomous ones, it has become evident that not all genes associated with changes in fasting or post-prandial glucose are also associated with a diagnosis of type 2 diabetes. Identification of these gene variants has provided novel insights into the physiology and pathophysiology of the β-cell, including the identification of molecules involved in β-cell function that were not previously recognized as playing a role in this critical cell.

Fasting and oral glucose-stimulated levels of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are highly familial traits

AIMS/HYPOTHESIS

Heritability estimates have shown a varying degree of genetic contribution to traits related to type 2 diabetes. Therefore, the objective of this study was to investigate the familiality of fasting and stimulated measures of plasma glucose, serum insulin, serum C-peptide, plasma glucose-dependent insulinotropic polypeptide (GIP) and plasma glucagon-like peptide-1 (GLP-1) among non-diabetic relatives of Danish type 2 diabetic patients.

METHODS

Sixty-one families comprising 193 non-diabetic offspring, 29 non-diabetic spouses, 72 non-diabetic relatives (parent, sibling, etc.) and two non-related relatives underwent a 4 h 75 g OGTT with measurements of plasma glucose, serum insulin, serum C-peptide, plasma GIP and plasma GLP-1 levels at 18 time points. Insulin secretion rates (ISR) and beta cell responses to glucose, GIP and GLP-1 were calculated. Familiality was estimated based on OGTT-derived measures.

RESULTS

A high level of familiality was observed during the OGTT for plasma levels of GIP and GLP-1, with peak familiality values of 74 ± 16% and 65 ± 15%, respectively (h (2) ± SE). Familiality values were lower for plasma glucose, serum insulin and serum C-peptide during the OGTT (range 8-48%, 14-44% and 15-61%, respectively). ISR presented the highest familiality value at fasting reaching 59 ± 16%. Beta cell responsiveness to glucose, GLP-1 and GIP also revealed a strong genetic influence, with peak familiality estimates of 62 ± 13%, 76 ± 15% and 70 ± 14%, respectively.

CONCLUSIONS/INTERPRETATION

Our results suggest that circulating levels of GIP and GLP-1 as well as beta cell response to these incretins are highly familial compared with more commonly investigated measures of glucose homeostasis such as fasting and stimulated plasma glucose, serum insulin and serum C-peptide.

Investigating the role of T-cell avidity and killing efficacy in relation to type 1 diabetes prediction

During the progression of the clinical onset of Type 1 Diabetes (T1D), high-risk individuals exhibit multiple islet autoantibodies and high-avidity T cells which progressively destroy beta cells causing overt T1D. In particular, novel autoantibodies, such as those against IA-2 epitopes (aa1-577), had a predictive rate of 100% in a 10-year follow up (rapid progressors), unlike conventional autoantibodies that required 15 years of follow up for a 74% predictive rate (slow progressors). The discrepancy between these two groups is thought to be associated with T-cell avidity, including CD8 and/or CD4 T cells. For this purpose, we build a series of mathematical models incorporating first one clone then multiple clones of islet-specific and pathogenic CD8 and/or CD4 T cells, together with B lymphocytes, to investigate the interaction of T-cell avidity with autoantibodies in predicting disease onset. These models are instrumental in examining several experimental observations associated with T-cell avidity, including the phenomenon of avidity maturation (increased average T-cell avidity over time), based on intra- and cross-clonal competition between T cells in high-risk human subjects. The model shows that the level and persistence of autoantibodies depends not only on the avidity of T cells, but also on the killing efficacy of these cells. Quantification and modeling of autoreactive T-cell avidities can thus determine the level of risk associated with each type of autoantibodies and the timing of T1D disease onset in individuals that have been tested positive for these autoantibodies. Such studies may lead to early diagnosis of the disease in high-risk individuals and thus potentially serve as a means of staging patients for clinical trials of preventive or interventional therapies far before disease onset.

The genetics of insulin resistance: Where's Waldo?

The physiologic hallmarks of type 2 diabetes are insulin resistance in hepatic and peripheral tissues and pancreatic β-cell dysfunction. Thus, genetic loci underlying susceptibility to type 2 diabetes are likely to map to one of these endophenotypes. Genome-wide association studies have now identified up to 38 susceptibility loci for type 2 diabetes and a number of other loci underlying variation in type 2 diabetes-related quantitative traits. The majority are of unknown biology or map to pancreatic β-cell dysfunction. A seemingly disproportionate minority map to insulin resistance. We briefly discuss the known insulin resistance loci identified from genome-wide association, and then discuss reasons why additional insulin resistance loci have not been identified. We present alternative views that may partly explain the apparent dearth of insulin resistance loci contributing to genetic susceptibility to type 2 diabetes, rather than focus on traditional issues such as study design and sampling, which have been addressed elsewhere.

The genetics of type 2 diabetes: what have we learned from GWAS?

Type 2 diabetes mellitus has been at the forefront of human diseases and phenotypes studied by new genetic analyses. Thanks to genome-wide association studies, we have made substantial progress in elucidating the genetic basis of type 2 diabetes. This review summarizes the concept, history, and recent discoveries produced by genome-wide association studies for type 2 diabetes and glycemic traits, with a focus on the key notions we have gleaned from these efforts. Genome-wide association findings have illustrated novel pathways, pointed toward fundamental biology, confirmed prior epidemiological observations, drawn attention to the role of β-cell dysfunction in type 2 diabetes, explained ~10% of disease heritability, tempered our expectations with regard to their use in clinical prediction, and provided possible targets for pharmacotherapy and pharmacogenetic clinical trials. We can apply these lessons to future investigation so as to improve our understanding of the genetic basis of type 2 diabetes.

Pharmacogenetics of Anti-Diabetes Drugs

A variety of treatment modalities exist for individuals with type 2 diabetes mellitus (T2D). In addition to dietary and physical activity interventions, T2D is also treated pharmacologically with nine major classes of approved drugs. These medications include insulin and its analogues, sulfonylureas, biguanides, thiazolidinediones (TZDs), meglitinides, α-glucosidase inhibitors, amylin analogues, incretin hormone mimetics, and dipeptidyl peptidase 4 (DPP4) inhibitors. Pharmacological treatment strategies for T2D are typically based on efficacy, yet favorable responses to such therapeutics are oftentimes variable and difficult to predict. Characterization of drug response is expected to substantially enhance our ability to provide patients with the most effective treatment strategy given their individual backgrounds, yet pharmacogenetic study of diabetes medications is still in its infancy. To date, major pharmacogenetic studies have focused on response to sulfonylureas, biguanides, and TZDs. Here, we provide a comprehensive review of pharmacogenetics investigations of these specific anti-diabetes medications. We focus not only on the results of these studies, but also on how experimental design, study sample issues, and definition of 'response' can significantly impact our interpretation of findings. Understanding the pharmacogenetics of anti-diabetes medications will provide critical baseline information for the development and implementation of genetic screening into therapeutic decision making, and lay the foundation for "individualized medicine" for patients with T2D.

Detailed physiologic characterization reveals diverse mechanisms for novel genetic Loci regulating glucose and insulin metabolism in humans

OBJECTIVE Recent genome-wide association studies have revealed loci associated with glucose and insulin-related traits. We aimed to characterize 19 such loci using detailed measures of insulin processing, secretion, and sensitivity to help elucidate their role in regulation of glucose control, insulin secretion and/or action. RESEARCH DESIGN AND METHODS We investigated associations of loci identified by the Meta-Analyses of Glucose and Insulin-related traits Consortium (MAGIC) with circulating proinsulin, measures of insulin secretion and sensitivity from oral glucose tolerance tests (OGTTs), euglycemic clamps, insulin suppression tests, or frequently sampled intravenous glucose tolerance tests in nondiabetic humans (n = 29,084). RESULTS The glucose-raising allele in MADD was associated with abnormal insulin processing (a dramatic effect on higher proinsulin levels, but no association with insulinogenic index) at extremely persuasive levels of statistical significance (P = 2.1 x 10(-71)). Defects in insulin processing and insulin secretion were seen in glucose-raising allele carriers at TCF7L2, SCL30A8, GIPR, and C2CD4B. Abnormalities in early insulin secretion were suggested in glucose-raising allele carriers at MTNR1B, GCK, FADS1, DGKB, and PROX1 (lower insulinogenic index; no association with proinsulin or insulin sensitivity). Two loci previously associated with fasting insulin (GCKR and IGF1) were associated with OGTT-derived insulin sensitivity indices in a consistent direction. CONCLUSIONS Genetic loci identified through their effect on hyperglycemia and/or hyperinsulinemia demonstrate considerable heterogeneity in associations with measures of insulin processing, secretion, and sensitivity. Our findings emphasize the importance of detailed physiological characterization of such loci for improved understanding of pathways associated with alterations in glucose homeostasis and eventually type 2 diabetes.

Determinants of impaired fasting glucose versus glucose intolerance in polycystic ovary syndrome

OBJECTIVE

To determine insulin resistance and response in patients with polycystic ovary syndrome (PCOS) and normal glucose tolerance (NGT), impaired fasting glucose (IFG), impaired glucose tolerance, and combined glucose intolerance (CGI).

RESEARCH DESIGN AND METHODS

In this cross-sectional study, 143 patients with PCOS (diagnosed on the basis of National Institutes of Health criteria) underwent oral glucose tolerance testing (OGTT), and 68 patients also had frequently sampled intravenous glucose tolerance tests. Changes in plasma glucose, insulin, cardiovascular risk factors, and androgens were measured.

RESULTS

Compared with patients with NGT, those with both IFG and CGI were significantly insulin resistant (homeostasis model assessment 3.3 +/- 0.2 vs. 6.1 +/- 0.9 and 6.4 +/- 0.5, P < 0.0001) and hyperinsulinemic (insulin area under the curve for 120 min 973 +/- 69 vs. 1,470 +/- 197 and 1,461 +/- 172 pmol/l, P < 0.0001). Insulin response was delayed in patients with CGI but not in those with IFG (2-h OGTT, insulin 1,001 +/- 40 vs. 583 +/- 45 pmol/l, P < 0.0001). Compared with the NGT group, the CGI group had a lower disposition index (1,615 +/- 236 vs. 987 +/- 296, P < 0.0234) and adiponectin level (11.1 +/- 1.1 vs. 6.2 +/- 0.8 ng/ml, P < 0.0096). Compared with the insulin-resistant tertile of the NGT group, those with IFG had a reduced insulinogenic index (421 +/- 130 vs. 268 +/- 68, P < 0.05). Compared with the insulin-sensitive tertile of the NGT group, the resistant tertile had higher triglyceride and high-sensitivity C-reactive protein (hs-CRP) and lower HDL cholesterol and sex hormone-binding globulin (SHBG). In the entire population, insulin resistance correlated directly with triglyceride, hs-CRP, and the free androgen index and inversely with SHBG.

CONCLUSIONS

Patients with PCOS develop IFG and CGI despite having significant hyperinsulinemia. Patients with IFG and CGI exhibit similar insulin resistance but very different insulin response patterns. Increases in cardiac risk factors and free androgen level precede overt glucose intolerance.

Gene--environment-wide association studies: emerging approaches

Despite the yield of recent genome-wide association (GWA) studies, the identified variants explain only a small proportion of the heritability of most complex diseases. This unexplained heritability could be partly due to gene--environment (G×E) interactions or more complex pathways involving multiple genes and exposures. This Review provides a tutorial on the available epidemiological designs and statistical analysis approaches for studying specific G×E interactions and choosing the most appropriate methods. I discuss the approaches that are being developed for studying entire pathways and available techniques for mining interactions in GWA data. I also explore methods for marrying hypothesis-driven pathway-based approaches with 'agnostic' GWA studies.

New genetic loci implicated in fasting glucose homeostasis and their impact on type 2 diabetes risk

Levels of circulating glucose are tightly regulated. To identify new loci influencing glycemic traits, we performed meta-analyses of 21 genome-wide association studies informative for fasting glucose, fasting insulin and indices of beta-cell function (HOMA-B) and insulin resistance (HOMA-IR) in up to 46,186 nondiabetic participants. Follow-up of 25 loci in up to 76,558 additional subjects identified 16 loci associated with fasting glucose and HOMA-B and two loci associated with fasting insulin and HOMA-IR. These include nine loci newly associated with fasting glucose (in or near ADCY5, MADD, ADRA2A, CRY2, FADS1, GLIS3, SLC2A2, PROX1 and C2CD4B) and one influencing fasting insulin and HOMA-IR (near IGF1). We also demonstrated association of ADCY5, PROX1, GCK, GCKR and DGKB-TMEM195 with type 2 diabetes. Within these loci, likely biological candidate genes influence signal transduction, cell proliferation, development, glucose-sensing and circadian regulation. Our results demonstrate that genetic studies of glycemic traits can identify type 2 diabetes risk loci, as well as loci containing gene variants that are associated with a modest elevation in glucose levels but are not associated with overt diabetes.

Candidate loci for insulin sensitivity and disposition index from a genome-wide association analysis of Hispanic participants in the Insulin Resistance Atherosclerosis (IRAS) Family Study

AIMS/HYPOTHESIS

The majority of type 2 diabetes genome-wide association studies (GWAS) to date have been performed in European-derived populations and have identified few variants that mediate their effect through insulin resistance. The aim of this study was to evaluate two quantitative, directly assessed measures of insulin resistance, namely insulin sensitivity index (S(I)) and insulin disposition index (DI), in Hispanic-American participants using an agnostic, high-density single nucleotide polymorphism (SNP) scan, and to validate these findings in additional samples.

METHODS

A two-stage GWAS was performed in Hispanic-American samples from the Insulin Resistance Atherosclerosis Family Study. In Stage 1, 317,000 SNPs were assessed using 229 DNA samples. SNPs with evidence of association with glucose homeostasis and adiposity traits were then genotyped on the entire set of Hispanic-American samples (n = 1,190). This report focuses on the glucose homeostasis traits: S(I) and DI.

RESULTS

Although evidence of association did not reach genome-wide significance (p = 5 x 10(-7)), in the combined analysis SNPs had admixture-adjusted p values of p (ADD) = 0.00010-0.0020 with 8 to 41% differences in genotypic means for S(I) and DI.

CONCLUSIONS/INTERPRETATION

Several candidate loci were identified that are nominally associated with S(I) and/or DI in Hispanic-American participants. Replication of these findings in independent cohorts and additional focused analysis of these loci is warranted.

Serum fatty acid binding protein 4, free fatty acids, and metabolic risk markers

Fatty acid binding protein (FABP) 4 chaperones free fatty acids (FFAs) in the adipocytes during lipolysis. Serum FFA relates to metabolic syndrome, and serum FABP4 is emerging as a novel risk marker. In 36 overweight/obese women, serum FABP4 and FFA were measured hourly during 5-hour oral glucose tolerance test. Insulin resistance was determined using frequently sampled intravenous glucose tolerance test. Serum lipids and inflammation markers were measured at fasting. During oral glucose tolerance test, serum FABP4 decreased by 40%, reaching its nadir at 3 hours (from 45.3 +/- 3.1 to 31.9 +/- 1.6 ng/mL), and stayed below the baseline at 5 hours (35.9 +/- 2.2 ng/mL) (P < .0001 for both, compared with the baseline). Serum FFA decreased by 10-fold, reaching a nadir at 2 hours (from 0.611 +/- 0.033 to 0.067 +/- 0.004 mmol/L), then rebounded to 0.816 +/- 0.035 mmol/L at 5 hours (P < .001 for both, compared with baseline). Both fasting FABP4 and nadir FABP4 correlated with obesity. Nadir FABP4 correlated also with insulin resistance parameters from frequently sampled intravenous glucose tolerance test and with inflammation. Nadir FFA, but not fasting FFA, correlated with the metabolic syndrome parameters. In conclusion, fasting FABP4 related to metabolic risk markers more strongly than fasting FFA. Nadir FABP4 and nadir FFA measured after glucose loading may provide better risk assessment than the fasting values.

A genome-wide association scan for acute insulin response to glucose in Hispanic-Americans: the Insulin Resistance Atherosclerosis Family Study (IRAS FS)

AIMS/HYPOTHESIS

This study sought to identify genes and regions in the human genome that are associated with the acute insulin response to glucose (AIRg), an important predictor of type 2 diabetes, in Hispanic-American participants from the Insulin Resistance Atherosclerosis Family Study (IRAS FS).

METHODS

A two-stage genome-wide association scan (GWAS) was performed in IRAS FS Hispanic-American samples. In the first stage, 317K single nucleotide polymorphisms (SNPs) were assessed in 229 Hispanic-American DNA samples from 34 families from San Antonio, TX, USA. SNPs with the most significant associations with AIRg were genotyped in the entire set of IRAS FS Hispanic-American samples (n = 1,190). In chromosomal regions with evidence of association, additional SNPs were genotyped to capture variation in genes.

RESULTS

No individual SNP achieved genome-wide levels of significance (p < 5 x 10(-7)); however, two regions (chromosomes 6p21 and 20p11) had multiple highly ranked SNPs that were associated with AIRg. Additional genotyping in these regions supported the initial evidence of variants contributing to variation in AIRg. One region resides in a gene desert between PXT1 and KCTD20 on 6p21, while the region on 20p11 has several viable candidate genes (ENTPD6, PYGB, GINS1 and RP4-691N24.1).

CONCLUSIONS/INTERPRETATION

A GWAS in Hispanic-American samples identified several candidate genes and loci that may be associated with AIRg. These associations explain a small component of variation in AIRg. The genes identified are involved in phosphorylation and ion transport, and provide preliminary evidence that these processes are important in beta cell response.

Cardiometabolic risk among African American women: a pilot study

OBJECTIVE

This study aimed to determine the associations of the Homeostatic Model of Assessment-insulin resistance (HOMA-ir), acanthosis nigricans, high-sensitivity C-reactive protein (hs-CRP), and plasminogen activator inhibitor-1 (PAI-1) with 2 of the commonly used definitions of metabolic syndrome (Adult Treatment Panel III [ATP III] and International Diabetes Federation [IDF]) among reproductive-age, healthy, free-living African American women.

METHODS

A pilot study with a cross-sectional design examined 33 African American women aged 20 to 46 years (mean [SD], 31.24 [7.25] years) for the presence of metabolic syndrome determined by ATP III and IDF criteria, insulin resistance (HOMA-ir and/or acanthosis nigricans), degree of inflammation (hs-CRP), and presence of dysfibrinolysis (PAI-1).

RESULTS

HOMA-ir identified insulin resistance in 27 (81.8%) women, whereas the presence of acanthosis nigricans indicated that 16 (48%) of these women manifested insulin resistance. Metabolic syndrome was found in 7 women (21.2%) by ATP III or in 9 (27.3%) women by IDF criteria. Bivariate correlations showed associations between HOMA-ir and waist circumference, body mass index (BMI), acanthosis nigricans, and the ATP III and IDF definitions for metabolic syndrome. Plasminogen activator inhibitor-1 was significantly correlated with waist circumference, BMI, fasting glucose, HOMA-ir, and ATP III. Both HOMA-ir and PAI-1 were significantly and negatively correlated with high-density lipoprotein cholesterol. High-sensitivity CRP was significantly correlated with BMI and 2-hour postglucose.

CONCLUSION

Both dysfibrinolysis (PAI-1 levels) and insulin resistance (HOMA-ir), when individually regressed on the ATP III definition of metabolic syndrome, explained 32% and 29% of the respective variance. The addition of HOMA-ir measurement may significantly improve early recognition of cardiometabolic risk among reproductive-age African American women who have not yet met the criteria for the ATP III or IDF definitions of metabolic syndrome. Likewise, acanthosis nigricans is potentially a clinically significant screening tool when used to determine early recognition of insulin resistance and/or cardiometabolic risk among this population. African American women's risk for cardiovascular disease is likely underestimated based on the sole use of ATP III criteria for diagnosis of metabolic syndrome. Clinicians should consider a broader definition of risk than that contained within ATP III. Inclusion of biomarkers of inflammation and dysfibrinolysis, along with measures of insulin resistance, may add to early detection of cardiometabolic risk and ultimate reduction in cardiovascular health disparities among African American women.

Agreement among type 2 diabetes linkage studies but a poor correlation with results from genome-wide association studies

AIMS/HYPOTHESIS

Little of the genetic basis for type 2 diabetes has been explained, despite numerous genetic linkage studies and the discovery of multiple genes in genome-wide association (GWA) studies. To begin to resolve the genetic component of this disease, we searched for sites at which genetic results had been corroborated in different studies, in the expectation that replication among studies should direct us to the genomic locations of causative genes with more confidence than the results of individual studies.

METHODS

We have mapped the physical location of results from 83 linkage reports (for type 2 diabetes and diabetes precursor quantitative traits [QTs, e.g. plasma insulin levels]) and recent large GWA reports (for type 2 diabetes) onto the same human genome sequence to identify replicated results in diabetes genetic 'hot spots'.

RESULTS

Genetic linkage has been found at least ten times at 18 different locations, and at least five times in 56 locations. All replication clusters contained study populations from more than one ethnic background and most contained results for both diabetes and QTs. There is no close relationship between the GWA results and linkage clusters, and the nine best replication clusters have no nearby GWA result.

CONCLUSIONS/INTERPRETATION

Many of the genes for type 2 diabetes remain unidentified. This analysis identifies the broad location of yet to be identified genes on 6q, 1q, 18p, 2q, 20q, 17pq, 8p, 19q and 9q. The discrepancy between the linkage and GWA studies may be explained by the presence of multiple, uncommon, mildly deleterious polymorphisms scattered throughout the regulatory and coding regions of genes for type 2 diabetes.

Variation in IGF2BP2 interacts with adiposity to alter insulin sensitivity in Mexican Americans

Genome-wide association studies showed variation in insulin-like growth factor-2 binding protein 2 (IGF2BP2) to be associated with type 2 diabetes mellitus (T2DM). We examined a 20-kb region of IGF2BP2 for association with T2DM-related quantitative traits in Mexican American families of a proband with gestational diabetes mellitus (GDM) from the BetaGene study. We genotyped 14 single-nucleotide polymorphisms (SNPs) in 717 individuals from 146 families phenotyped by oral glucose tolerance test (OGTT), intravenous glucose tolerance tests (IVGTTs) with minimal model analysis, and dual-energy X-ray absorptiometry scan for percent body fat. Three SNPs and one SNP combination that captured the majority of the variation in the region were tested for association with T2DM-related quantitative traits using a variance components framework. After correction for multiple testing, rs11705701 showed association with percent body fat (P(ACT) = 0.041) with body fat decreasing approximately 1.5-2% per copy of the A allele. We next tested whether the interaction between rs11705701 and body fat was associated with T2DM-relative quantitative traits. rs11705701 was significantly associated with insulin sensitivity (Bonferroni P = 0.028) and marginally associated with OGTT 2-h insulin (Bonferroni P = 0.066) and disposition index (DI) (Bonferroni P = 0.072). We conclude that rs11705701 in IGF2BP2 is associated with body fat and this effect on body fat influences insulin resistance which may contribute to T2DM risk.

[Metabolic syndrome, its phenotypes, and insulin resistance by HOMA-IR]

The diagnosis of the metabolic syndrome (MS) according to the National Cholesterol Education Program Adult Treatment Panel III does not reflect necessarily the presence of insulin resistance (IR), a potential therapeutical target for type 2 diabetes and cardiovascular disease prevention. Based on previous prevalence data, a cross-sectional study was conducted to determine the HOMA-IR relationship to the MS and some associated abnormalities. HOMA-IR > was higher in individuals with the MS (2.8+/-1.6 vs. 1.8+/-1.4) (p < 0.001). HOMA-IR >or= 2.5 allied good specificity and sensitivity levels for the association of MS and IR. Hyperglycemia, hypertrigliceridemia, and abdominal obesity, the MS components best related to IR, were statistically associated with HOMA-IR > 2.5, but not hypertension neither low HDL-c. The demonstration that some of MS phenotypes or associated abnormalities were more predictive for IR could point out to the possibility of the use of the index as a marker of the presence of IR associated to MS.