Genotype-by-sex interaction on fasting insulin concentration: the HyperGEN study

Statistical Genetic Approaches to Investigate Genotype-by-Environment Interaction: Review and Novel Extension of Models

Statistical genetic models of genotype-by-environment (G×E) interaction can be divided into two general classes, one on G×E interaction in response to dichotomous environments (e.g., sex, disease-affection status, or presence/absence of an exposure) and the other in response to continuous environments (e.g., physical activity, nutritional measurements, or continuous socioeconomic measures). Here we develop a novel model to jointly account for dichotomous and continuous environments. We develop the model in terms of a joint genotype-by-sex (for the dichotomous environment) and genotype-by-social determinants of health (SDoH; for the continuous environment). Using this model, we show how a depression variable, as measured by the Beck Depression Inventory-II survey instrument, is not only underlain by genetic effects (as has been reported elsewhere) but is also significantly determined by joint G×Sex and G×SDoH interaction effects. This model has numerous applications leading to potentially transformative research on the genetic and environmental determinants underlying complex diseases.

Prediabetes is associated with genetic variations in the gene encoding the Kir6.2 subunit of the pancreatic ATP-sensitive potassium channel (KCNJ11): A case-control study in a Han Chinese youth population

BACKGROUND

The E23K variant of the potassium voltage-gated channel subfamily J member 11 (KCNJ11) gene has been reported to be associated with type 2 diabetes (T2D) in many populations. However, little is known about the role of E23K in the development of prediabetes in Chinese youth.

METHODS

To investigate the role of E23K in the development of prediabetes, 279 subjects with prediabetes and 240 normal controls (mean [± SD] age 18.1 ± 3.2 and 17.8 ± 4.3 years, respectively) were recruited to the study. Height, weight, and hip and waist circumferences were measured by trained physicians. Genotyping of KCNJ11 polymorphisms and clinical laboratory tests to determine cholesterol, triglyceride (TG), blood glucose, and insulin levels were performed.

RESULTS

The carrier rate of K23 allele-containing genotypes was higher for prediabetic than control subjects (P = 0.005). Logistic regression analyses revealed that higher body mass index percentiles (P = 0.013), lower insulin levels at 30 min during an oral glucose tolerance test (P = 0.001), a higher ratio of total cholesterol: high-density lipoprotein cholesterol (P = 0.001), and a K allele-containing genotype (P = 0.019) are independent risk factors for prediabetes in Chinese Han youth. Furthermore, K23 allele-containing genotypes were associated with impaired indices of insulin secretion and β-cell function in female youth with prediabetes. These effects were not seen in male youth with prediabetes.

CONCLUSIONS

The results confirm that the common E23K polymorphism of KCNJ11 carries a higher susceptibility to the development of prediabetes in the Chinese Han population. The results suggest that E23K may have a greater effect on the development of T2D in female Chinese youth.

Next-generation GLP-1 therapy: an introduction to liraglutide

Liraglutide, a once-daily human glucagon-like peptide-1 (GLP-1) analog, was approved by the US Food and Drug Administration in 2010 for the treatment of type 2 diabetes mellitus (T2DM). Glucagon-like peptide-1 enhances insulin secretion and inhibits glucagon in a glucose-dependent manner. The efficacy and safety of liraglutide were evaluated in 6 phase 3 trials in > 4000 patients in the Liraglutide Effect and Action in Diabetes (LEAD) program, in another trial in comparison with sitagliptin, and in another trial where basal insulin was added to liraglutide + metformin. At liraglutide doses of 1.2 mg or 1.8 mg once daily, significant mean reductions in glycated hemoglobin (HbA1c) (1%-1.6%) and fasting plasma glucose (15-43 mg/dL), as well as sustained weight loss (2-3 kg) and a low rate of hypoglycemia occurred. Mild and transient nausea, reported in 6% to 41% of patients, was the most frequent adverse event reported. Incretin-based therapies such as liraglutide provide an important expansion of options for the treatment of T2DM.

Genetic variation within the TRPM5 locus associates with prediabetic phenotypes in subjects at increased risk for type 2 diabetes

The functional knockout of the calcium-sensitive, nonselective cation channel TRPM5 alters glucose-induced insulin secretion and glucose tolerance. We hypothesized that genetic variation in the TRPM5 gene may contribute to prediabetic phenotypes, including pancreatic β-cell dysfunction. We genotyped 1798 white subjects at increased type 2 diabetes mellitus risk for 9 TRPM5 single nucleotide polymorphisms (namely, rs2301696, rs800344, rs800345, rs800347, rs800348, rs2074234, rs2301698, rs886277, and rs2301699) and also performed correlational analyses with metabolic traits. An oral glucose tolerance test (OGTT) was conducted on all subjects, and a subset (n = 525) additionally underwent a hyperinsulinemic-euglycemic clamp. The 9 chosen single nucleotide polymorphisms cover 100% of the common genetic variation (minor allele frequency ≥0.05) within the TRPM5 locus (D' = 1.0; r² ≥ 0.8). Rs800344, rs800345, and rs2301699 were significantly associated with area under the curve (AUC) glucose during the OGTT in the additive and dominant models after adjustment for sex, age, and body mass index (all Ps ≤ .0025). Furthermore, rs800344 was significantly associated with 2-hour glucose in the dominant model (P = .0009). After stratification for sex, rs2301699 was significantly associated with the ratio of AUC insulin 0 to 30 minutes to AUC glucose 0 to 30 minutes in women (P = .0097), but not in men (P = .3), in the dominant model. Female minor allele carriers of rs2301699 showed significantly lower glucagon-like peptide-1 levels at 30 minutes during the OGTT compared with major allele homozygotes (P = .0124), whereas in male subjects, no significant differences were found (P = .3). In our German population, the common TRPM5 variants are likely to be associated with prediabetic phenotypes; and this may in turn contribute to the development of type 2 diabetes mellitus.

Mapping quantitative traits in unselected families: algorithms and examples

Linkage analysis has been widely used to identify from family data genetic variants influencing quantitative traits. Common approaches have both strengths and limitations. Likelihood ratio tests typically computed in variance component analysis can accommodate large families but are highly sensitive to departure from normality assumptions. Regression-based approaches are more robust but their use has primarily been restricted to nuclear families. In this paper, we develop methods for mapping quantitative traits in moderately large pedigrees. Our methods are based on the score statistic, which in contrast to the likelihood ratio statistic can use nonparametric estimators of variability to achieve robustness of the false-positive rate against departures from the hypothesized phenotypic model. Because the score statistic is easier to calculate than the likelihood ratio statistic, our basic mapping methods utilize relatively simple computer code that performs statistical analysis on output from any program that computes estimates of identity by descent. This simplicity also permits development and evaluation of methods to deal with multivariate and ordinal phenotypes, and with gene-gene and gene-environment interaction. We demonstrate our methods on simulated data and on fasting insulin, a quantitative trait measured in the Framingham Heart Study.

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.

Diabetes-specific genetic effects on obesity traits in American Indian populations: the Strong Heart Family Study

Background

Body fat mass distribution and deposition are determined by multiple environmental and genetic factors. Obesity is associated with insulin resistance, hyperinsulinemia, and type 2 diabetes. We previously identified evidence for genotype-by-diabetes interaction on obesity traits in Strong Heart Family Study (SHFS) participants. To localize these genetic effects, we conducted genome-wide linkage scans of obesity traits in individuals with and without type 2 diabetes, and in the combined sample while modeling interaction with diabetes using maximum likelihood methods (SOLAR 2.1.4).

Methods

SHFS recruited American Indians from Arizona, North and South Dakota, and Oklahoma. Anthropometric measures and diabetes status were obtained during a clinic visit. Marker allele frequencies were derived using maximum likelihood methods estimated from all individuals and multipoint identity by descent sharing was estimated using Loki. We used variance component linkage analysis to localize quantitative trait loci (QTLs) influencing obesity traits. We tested for evidence of additive and QTL-specific genotype-by-diabetes interactions using the regions identified in the diabetes-stratified analyses.

Results

Among 245 diabetic and 704 non-diabetic American Indian individuals, we detected significant additive gene-by-diabetes interaction for weight and BMI (P < 0.02). In analysis accounting for QTL-specific interaction (P < 0.001), we detected a QTL for weight on chromosome 1 at 242 cM (LOD = 3.7). This chromosome region harbors the adiponectin receptor 1 gene, which has been previously associated with obesity.

Conclusion

These results suggest distinct genetic effects on body mass in individuals with diabetes compared to those without diabetes, and a possible role for one or more genes on chromosome 1 in the pathogenesis of obesity.

Sex-specific differences in chromosome-dependent regulation of vascular reactivity in female consomic rat strains from a SSxBN cross

High-throughput studies in the Medical College of Wisconsin Program for Genomic Applications (Physgen) were designed to link chromosomes with physiological function in consomic strains derived from a cross between Dahl salt-sensitive SS/JrHsdMcwi (SS) and Brown Norway normotensive BN/NHsdMcwi (BN) rats. The specific goal of the vascular protocol was to characterize the responses of aortic rings from these strains to vasoconstrictor and vasodilator stimuli (phenylephrine, acetylcholine, sodium nitroprusside, and bath hypoxia) to identify chromosomes that either increase or decrease vascular reactivity to these vasoactive stimuli. Because previous studies demonstrated sex-specific quantitative trait loci (QTLs) related to regulation of cardiovascular phenotypes in an F2 cross between the parental strains, males and females of each consomic strain were included in all experiments. As there were significant sex-specific differences in aortic sensitivity to vasoconstrictor and vasodilator stimuli compared with the parental SS strain, we report the results of the females separately from the males. There were also sex-specific differences in aortic ring sensitivity to these vasoactive stimuli in consomic strains that were fed a high-salt diet (4% NaCl) for 3 wk to evaluate salt-induced changes in vascular reactivity. Differences in genetic architecture could contribute to sex-specific differences in the development and expression of cardiovascular diseases via differential regulation and expression of genes. Our findings are the first to link physiological traits with specific chromosomes in female SS rats and support the idea that sex is an important environmental variable that plays a role in the expression and regulation of genes.

Preserving insulin secretion in Type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a complex disease characterized by insulin resistance and a progressive decline in β-cell function and mass. Current evidence suggests that β-cell dysfunction is present early in the course of the disease and that this dysfunction, rather than insulin resistance, is primarily responsible for the progression of T2DM. β-cell dysfunction can be accelerated by glucose toxicity, lipotoxicity, oxidative stress, chronic increases in inflammatory mediators and, potentially, the use of sulfonylureas. This review suggests that future efforts to limit the impact of T2DM must focus on strategies to preserve β-cell function. Several interventions have shown promise in this regard, including lifestyle modifications, thiazolidinediones, potassium channel openers, incretin mimetics, cytokine antagonists, bariatric surgery and dipeptidyl peptidase IV inhibitors, although therapeutic insulin remains the most robust and physiological approach.

Systematic, genome-wide, sex-specific linkage of cardiovascular traits in French Canadians

The sexual dimorphism of cardiovascular traits, as well as susceptibility to a variety of related diseases, has long been recognized, yet their sex-specific genomic determinants are largely unknown. We systematically assessed the sex-specific heritability and linkage of 539 hemodynamic, metabolic, anthropometric, and humoral traits in 120 French-Canadian families from the Saguenay-Lac-St-Jean region of Quebec, Canada. We performed multipoint linkage analysis using microsatellite markers followed by peak-wide linkage scan based on Affymetrix Human Mapping 50K Array Xba240 single nucleotide polymorphism genotypes in 3 settings, including the entire sample and then separately in men and women. Nearly one half of the traits were age and sex independent, one quarter were both age and sex dependent, and one eighth were exclusively age or sex dependent. Sex-specific phenotypes are most frequent in heart rate and blood pressure categories, whereas sex- and age-independent determinants are predominant among humoral and biochemical parameters. Twenty sex-specific loci passing multiple testing criteria were corroborated by 2-point single nucleotide polymorphism linkage. Several resting systolic blood pressure measurements showed significant genotype-by-sex interaction, eg, male-specific locus at chromosome 12 (male-female logarithm of odds difference: 4.16; interaction P=0.0002), which was undetectable in the entire population, even after adjustment for sex. Detailed interrogation of this locus revealed a 220-kb block overlapping parts of TAO-kinase 3 and SUDS3 genes. In summary, a large number of complex cardiovascular traits display significant sexual dimorphism, for which we have demonstrated genomic determinants at the haplotype level. Many of these would have been missed in a traditional, sex-adjusted setting.

Intake of whole grains, refined grains, and cereal fiber measured with 7-d diet records and associations with risk factors for chronic disease

BACKGROUND

Research studies examining foods are important, because they account for biological interactions that might otherwise be lost in the analysis of individual nutrients. Single-nutrient studies are also needed to explore the mechanisms by which foods may be protective.

OBJECTIVE

Our objective was to examine associations between whole grains, refined grains, and cereal fiber and chronic disease risk factors.

DESIGN

In a cross-sectional analysis of participants in the Baltimore Longitudinal Study of Aging, associations between dietary intakes and risk factors were examined with multivariate linear regression analysis. Dietary intakes were assessed with 7-d dietary records and quantified in g/d.

RESULTS

Compared with subjects in the lowest quintile (Q1) of whole-grain intake, subjects in the highest quintile (Q5) had lower body mass index (BMI; in kg/m(2); Q1: 25.5; Q5: 24.8; P for trend <0.0001) and weight (Q1: 75.0 kg; Q5: 72.4 kg; P for trend = 0.004) and smaller waist circumference (Q1: 87.4 cm; Q5: 85.0 cm; P for trend = 0.002). Whole grains were also inversely associated with total cholesterol (P for trend = 0.02), LDL cholesterol (P for trend = 0.04), and 2-h glucose (P for trend = 0.0006). Associations between cereal fiber and anthropometrics and plasma lipids were similar. In subgroup analyses, refined grains were positively associated with fasting insulin among women (P for trend = 0.002).

CONCLUSIONS

Similar associations of whole grains and cereal fiber with weight, BMI, waist circumference, plasma cholesterol, and 2-h glucose were observed, suggesting that cereal fiber and its constituents may in part mediate these relations. Refined grains were associated with fasting insulin among women but not men. Additional research should explore potential interaction effects with BMI, sex, age, and genes.

The Pro12Ala polymorphism of the PPAR-gamma2 gene affects associations of fish intake and marine n-3 fatty acids with glucose metabolism

BACKGROUND/OBJECTIVES

Data on associations between marine n-3 fatty acids and glucose metabolism are inconsistent. Therefore, we explored effects of the Pro12Ala polymorphism in peroxisome proliferator-activated receptor (PPAR)-gamma2 gene on associations of fish intake and dietary and plasma eicosapentaenoic and docosahexaenoic acid with glucose metabolism. The design comprises of the cross-sectional analysis.

SUBJECTS/METHODS

The Pro12Ala variant in the PPAR-gamma2 (PPARG) gene was genotyped in 571 non-diabetic relatives of subjects with type II diabetes. The dietary intake was measured by a 3-day food record, and the plasma cholesterol ester fatty acid composition was analysed with gas chromatography. Associations of dietary and plasma variables with insulin resistance and fasting and 2-h glucose and free fatty acid concentrations were analysed with multiple linear regression analysis.

RESULTS

In men, there was a significant interaction between PPARG polymorphism and plasma docosahexaenoic acid on fasting free fatty acid concentration (P=0.036), and genotype-stratified models showed an inverse association in Pro homozygotes only (P=0.028). In women, the proportion of plasma eicosapentaenoic acid was higher in Ala-allele carriers compared to Pro homozygotes (1.67 vs 1.44% respectively, P=0.006). A significant interaction between PPARG polymorphism and fish intake on 2-h glucose was found in women (P=0.021), and genotype-stratified models suggested an inverse association in Ala-allele carriers only (P=0.039).

CONCLUSIONS

The findings suggest that PPARG polymorphism might affect the plasma proportion of eicosapentaenoic acid and modulate the associations of fish intake and marine n-3 fatty acids with glucose metabolism and fasting free fatty acids.

Gene-environment interactions in the pathogenesis of type 2 diabetes and metabolism

PURPOSE OF REVIEW

Gene-environment interaction can be viewed as a departure from an otherwise expected additivity of genetic and environmental factors on a given outcome measure. Important genetic and environmental factors contribute to the pathogenesis of type 2 diabetes and intermediary traits, probably modulated by their complex interaction. This paper provides an update on the current literature investigating gene-environment interactions of type 2 diabetes and metabolic phenotypes, and discusses the future perspectives of this research.

RECENT FINDINGS

Recent advances in gene-environment interaction studies of metabolism have involved LIPC, APOA5 and PPARG variation, and nutrition and physical activity, of which the most consistently replicated observations have been obtained for APOA5. Also, intervention studies of the promising TCF7L2 type 2 diabetes gene and possible future strategies are discussed.

SUMMARY

Possibly as a result of the complexity of these multifactorial diseases, recent years have seen only limited success in unravelling significant gene-environment interactions, but important insights have been gained and they hold promise for implementation in lifestyle intervention strategies. We need to evolve to more complex, but realistic, scenarios involving several genes and environmental factors. Recent progress in statistical methods allowing for higher-order interactions may make this possible.