Association of CDKAL1, IGF2BP2, CDKN2A/B, HHEX, SLC30A8, and KCNJ11 with susceptibility to type 2 diabetes in a Japanese population

Genetic variants in peroxisome proliferator-activated receptor-γ and retinoid X receptor-α gene and type 2 diabetes risk: a case-control study of a Chinese Han population

BACKGROUND

The serum levels of adiponectin are paradoxically decreased in obesity and may play important roles in the development of type 2 diabetes mellitus (T2DM). Potentially functional polymorphisms in the peroxisome proliferator-activated receptor-γ (PPAR-γ) and retinoid X receptor-α (RXR-α) genes may alter T2DM risks by increasing the human adiponectin promoter activity in cells. Therefore, we hypothesized that single nucleotide polymorphisms (SNPs) in PPAR-γ and RXR-α were associated with risk of T2DM. To test this hypothesis, three potentially functional SNPs of PPAR-γ and four of RXR-α with a minor allele frequency of ≥ 0.05 in the Chinese Han population were identified from the National Center for Biotechnology Information dbSNPs database to evaluate their association with T2DM.

METHODS

Polymerase chain reaction-restriction fragment length polymorphism was performed to test the genotypes in T2DM patients (n = 540) and normal controls (n = 604).

RESULTS

The variant genotypes rs2920502CC, rs3856806CT, rs3856806CT/TT, and rs4240711AG/GG were associated with T2DM. Furthermore, the prevalences of haplotype GTC and CTG in PPAR-γ and GTAC in RXR-α were less frequent in cases (17.1%, 2.6%, and 2.4%, respectively) than in controls (22.3%, 3.8%, and 6.6%, respectively), whereas GTGT in RXR-α was more frequent in cases (6.9%) than in controls (4.4%) (P < 0.05 for both two-sided χ(2) test and thousand times permutation tests). Patients with genotype CT/TT of rs3856806 and genotype AG/GG of rs4240711 had higher levels of serum adiponectin than those with the genotype CC and genotype AA (P = 0.026 and 0.021, respectively). Model X2 X5 X6 X7 (rs3856806, rs3132291, rs4240711, and rs4842194) was the best model with the highest test balanced accuracy (0.5764) (cross-validation consistency = 10/10) in the multifactor dimensionality reduction method.

CONCLUSIONS

The PPAR-γ and RXR-α gene variants associated with the development of T2DM in this study must be investigated in a larger population to reveal any potential effects on metabolism.

LOC387761 polymorphism is associated with type 2 diabetes in the Mexican population

Worldwide researchers have invested time, effort, and money during the last years to find new genes associated with diabetes susceptibility, such as LOC387761, HHEX, EXT2, and SLC30A8. The aim of the present study was to evaluate whether single-nucleotide polymorphisms (SNPs) of these genes are associated with type 2 diabetes (T2D) and metabolic traits in the Mexican population. We also assessed these SNPs in Mexican indigenous groups to identify a possible inherited susceptibility. Seven SNPs were analyzed in 789 Mexicans (234 control subjects, 455 type 2 diabetic patients, and 100 of indigenous origin), using the KASPar assay (KBioscience Company). Analysis of the data showed an association of the LOC387761 SNP rs7480010 with T2D (p = 0.019). The risk allele A of rs7480010 increased body mass index in diabetic patients (p = 0.01). In addition, there was no association between T2D and the SNPs of HHEX, EXT2, and SLC30A8. Our findings suggest that the SNP rs7480010 (LOC387761) can contribute to a failure in insulin secretion, thus increasing the susceptibility to T2D in Mexicans.

Functional loss of Cdkal1, a novel tRNA modification enzyme, causes the development of type 2 diabetes

A number of whole-genome association studies show the cdk5 regulatory associated protein 1-like 1 (cdkal1) gene to be one of the most reproducible risk genes in type 2 diabetes (T2D). Variations in the gene are associated with impaired insulin secretion but not insulin resistance or obesity. Although the physiological functions of Cdkal1 had been unclear, recent studies show that it is a tRNA modification enzyme, a mammalian methylthiotransferase that biosynthesizes 2-methylthio-N(6)-threonylcarbamoyladenosine (ms(2)t(6)A) at position 37 of tRNA(Lys)(UUU). The ms(2)t(6)A modification in tRNA(Lys)(UUU) is important for preventing the misreading of its cognate codons, especially when the rate of translation is relatively high. In both general and pancreatic β-cell-specific cdkal1-deficient mice, impaired mitochondrial ATP generation and first-phase insulin secretion are observed. Moreover, the β-cell-specific knockout mice show pancreatic islet hypertrophy and impaired blood glucose control. The mice are also hypersensitive to high-fat diet-induced ER stress. In this review, we provide an overview of the physiological functions of Cdkal1 and the molecular pathogenesis of T2D in patients carrying cdkal1 risk alleles.

Genetics of type 2 diabetes: the GWAS era and future perspectives [Review]

Genome-wide association studies (GWAS) have facilitated a substantial and rapid rise in the number of confirmed genetic susceptibility variants for type 2 diabetes (T2D). Approximately 40 variants have been identified so far, many of which were discovered through GWAS. This success has led to widespread hope that the findings will translate into improved clinical care for the increasing numbers of patients with diabetes. Potential areas or clinical translation include risk prediction and subsequent disease prevention, pharmacogenetics, and the development of novel therapeutics. However, the genetic loci so far identified account for only a small fraction (approximately 10%) of the overall heritable risk for T2D. Uncovering the missing heritability is essential to the progress of T2D genetic studies and to the translation of genetic information into clinical practice.

Combined effects of 19 common variations on type 2 diabetes in Chinese: results from two community-based studies

BACKGROUND

Many susceptible loci for type 2 diabetes mellitus (T2DM) have recently been identified from Caucasians through genome wide association studies (GWAS). We aimed to determine the association of 11 known loci with T2DM and impaired glucose regulation (IGR), individually and in combination, in Chinese.

METHODS/PRINCIPAL FINDINGS

Subjects were enrolled in: (1) a case-control study including 1825 subjects with T2DM, 1487 with IGR and 2200 with normal glucose regulation; and (2) a prospective cohort with 734 non-diabetic subjects at baseline. The latter was followed up for 3.5 years, in which 67 subjects developed T2DM. Nineteen single nucleotide polymorphisms (SNPs) were selected to replicate in both studies. We found that CDKAL1 (rs7756992), SLC30A8 (rs13266634, rs2466293), CDKN2A/2B (rs10811661) and KCNQ1 (rs2237892) were associated with T2DM with odds ratio from 1.21 to 1.35. In the prospective study, the fourth quartile of risk scores based on the combined effects of the risk alleles had 3.05 folds (95% CI, 1.31-7.12) higher risk for incident T2DM as compared with the first quartile, after adjustment for age, gender, body mass index and diabetes family history. This combined effect was confirmed in the case-control study after the same adjustments. The addition of the risk scores to the model of clinical risk factors modestly improved discrimination for T2DM by 1.6% in the case-control study and 2.9% in the prospective study.

CONCLUSIONS/SIGNIFICANCE

Our study provided further evidence for these GWAS derived SNPs as the genetic susceptible loci for T2DM in Chinese and extended this association to IGR.

The "thrifty" gene encoding Ahsg/Fetuin-A meets the insulin receptor: Insights into the mechanism of insulin resistance

Ahsg (fetuin-A) is a 55-59kDa phosphorylated glycoprotein synthesized in the adult predominantly by hepatocytes, from which it enters the circulation. When dysregulated, this glycoprotein operates to influence the clinical sequelae of insulin resistance-type 2 diabetes and cardiovascular disease. The pathological sequelae likely arise from two separable molecular "faces" of Ahsg-one acting at the level of the insulin receptor and a second face influencing ectopic biomineralization in the intima. A detailed understanding of these two functional faces of Ahsg is not yet clear for lack of structural studies. Ahsg has a physiological role in the biomineralization of bone, which when dysregulated can lead to ectopic calcification of soft tissues in the vasculature. Ahsg has a second physiological function in regulating how insulin signals through its receptor, a transmembrane tyrosine kinase. Dysregulation of this "face" of Ahsg results in morbid sequelae such as impaired glucose disposal and fatty liver. Ahsg binds to tandem fibronectin type 3 (Fn3) domains present in the 194 amino acid residue extracellular portion of the β-subunit of the insulin receptor, distant from the high-affinity pocket formed by two complementing α-subunits where insulin binds. Only two proteins are known to bind directly to the insulin receptor ectodomain - insulin and Ahsg - the former turns on the receptor's intrinsic tyrosine kinase (TK) activity, and the latter shuts it down. Recent X-ray crystallographic studies of the ectodomain of the insulin receptor now sharpen our understanding of the receptor's extracellular α-subunit and linked β-subunit. Ahsg genotype and its circulating level have been correlated with body morphometrics (obese versus lean and visceral adiposity) in epidemiological studies enrolling thousands of patients. Epidemiological studies from the clinic reveal high levels of circulating Ahsg in insulin resistance and diabetes. This review endeavors to explain how one protein can mediate diverse pathologies, but specifically addresses its metabolic "face" blunting insulin receptor activity, an action leading to insulin resistance.

Association of a cyclin-dependent kinase 5 regulatory subunit-associated protein 1-like 1 (CDKAL1) polymorphism with elevated hemoglobin A₁(c) levels and the prevalence of metabolic syndrome in Japanese men: interaction with dietary energy intake

Genome-wide association studies have identified the cyclin-dependent kinase 5 regulatory subunit-associated protein 1-like 1 (CDKAL1) gene as a novel risk factor for type 2 diabetes mellitus. Application of this genetic marker for prevention of type 2 diabetes and metabolic syndrome (MetS) in healthy populations has not yet been evaluated. The authors examined the effects of a CDKAL1 polymorphism (rs9465871) on metabolic phenotype and of gene-lifestyle (CDKAL1-energy intake) interaction on MetS in a cohort of apparently healthy Japanese men examined in 2003. The CC genotype of the CDKAL1 variant was associated with elevated glycosylated hemoglobin A₁(c) (HbA1c) levels. The prevalence of MetS was 25.6% for CC and 16.3% for TT + CT (odds ratio = 2.18, 95% confidence interval: 1.06, 4.48; P = 0.035). When dietary energy intake was accounted for, the variant's effect on HbA1c was observed in the highest energy-intake group (mean: CC, 5.6% (standard deviation, 1.7); TT + CT, 5.0% (standard deviation, 0.5); P = 0.025). In addition, the positive association between HbA1c and energy intake was stronger in subjects with the CC genotype than in subjects with TT + CT. These results suggest that the interaction between the CDKAL1 polymorphism and dietary energy intake influences the dysglycemic phenotype leading to MetS, possibly through impaired insulin secretion. The CDKAL1 polymorphism may be a marker for MetS in the Japanese population.

Variations in/nearby genes coding for JAZF1, TSPAN8/LGR5 and HHEX-IDE and risk of type 2 diabetes in Han Chinese

Several genetic loci (JAZF1, CDC123/CAMK1D, TSPAN8/LGR5, ADAMTS9, VEGFA and HHEX-IDE) were identified to be significantly related to the risk of type 2 diabetes and quantitative metabolic traits in European populations. Here, we aimed to evaluate the impacts of these novel loci on type 2 diabetes risk in a population-based case-control study of Han Chinese (1912 cases and 2041 controls). We genotyped 13 single-nucleotide polymorphisms (SNPs) in/near these genes and examined the differences in allele/genotype frequency between cases and controls. We found that both IDE rs11187007 and HHEX rs1111875 were associated with type 2 diabetes risk (for both variants: odds ratio (OR)=1.15, 95% confidence interval (CI) 1.04-1.28, P=0.009). In a meta-analysis where we pooled our data with the three previous studies conducted in East Asians, we found that the variants of JAZF1 rs864745 (1.09 (1.03-1.16); P=3.49 × 10(-3)) and TSPAN8/LGR5 rs7961581 (1.11(1.05-1.17); P=1.89 × 10(-4)) were significantly associated with type 2 diabetes risk. In addition, the meta-analysis (7207 cases and 8260 controls) also showed that HHEX rs1111875 did have effects on type 2 diabetes in Chinese population (OR=1.15(1.10-1.21); P=1.93 × 10(-8)). This large population-based study and meta-analysis further confirmed the modest effects of the JAZF1, TSPAN8/LGR5 and HHEX-IDE loci on type 2 diabetes in Chinese and other East Asians.

Variant rs9939609 in the FTO gene is associated with body mass index among Chinese children

BACKGROUND

Fat-mass and obesity-associated (FTO) gene is a gene located in chromosome region 16q12.2. Genetic variants in FTO are associated with the obesity phenotype in European and Hispanic populations. However, this association still remains controversial in Asian population. We aimed to test the association of FTO genetic variants with obesity and obesity-related metabolic traits among children living in Beijing, China.

METHODS

We genotyped FTO variants rs9939609 in 670 children (332 girls and 338 boys) aged 8-11 years living in Beijing, and analyzed its association with obesity and obesity-related metabolic traits. Overweight and obesity were defined by age- and sex-specific BMI reference for Chinese children. Obesity-related metabolic traits included fasting plasma glucose, lipid profiles, leptin, ghrelin, adiponectin and blood pressures.

RESULTS

The frequency of rs9939609 A allele was 12.2%, which was 21.9% for the heterozygote and 1.2% for the homozygote of the A allele. The obesity prevalence among the carriers of AA/AT genotypes was significantly higher than that among those with TT genotype (36.4% vs. 22.6%, P=0.004). Compared to the carrier of TT genotype, the likelihood of obesity was 1.79 (95% confidence interval (95% CI) 1.20-2.67, P=0.004) for the carrier of AA/AT genotype, after adjustment of sex, age and puberty stages. The BMI Z-score of children with AA/AT genotype were significantly higher than that of their counterparts with the TT genotype (1.1±0.1 vs. 0.8±0.1, P=0.02). The concentration of triglyceride was 1.03±0.52 mmol/L among TT carrier and 1.13±0.68 mmol/L among AA/AT carrier (P=0.045). While, the concentrations of adiponectin were 18.0±0.4 μg/ml among carriers of TT and 16.2±0.7 μg/ml among subjects with AA/AT genotype (P=0.03). The level of glucose marginally increased in the AA/AT genotype subjects (4.67±0.40 mmol/L vs. 4.60±0.35 mmol/L, P=0.08). The evidence of association was reduced after adjustment for BMI (P=0.38 for triglyceride, P=0.20 for adiponectin and glucose). There was weak evidence of association between rs9939609 and other obesity-related metabolic traits including total cholesterol (3.92±0.03 mmol/L vs. 4.02±0.05 mmol/L, P=0.10), insulin (2.69±1.77 ng/ml vs. 3.12±2.91 ng/ml, P=0.14), and insulin resistance (HOMA-IR 0.56±0.03 vs. 0.66±0.05, P=0.10).

CONCLUSIONS

Genetic variation in the FTO gene associates with obesity in Chinese children.

A genome-wide association study in the Japanese population identifies susceptibility loci for type 2 diabetes at UBE2E2 and C2CD4A-C2CD4B

We conducted a genome-wide association study of type 2 diabetes (T2D) using 459,359 SNPs in a Japanese population with a three-stage study design (stage 1, 4,470 cases and 3,071 controls; stage 2, 2,886 cases and 3,087 controls; stage 3, 3,622 cases and 2,356 controls). We identified new associations in UBE2E2 on chromosome 3 and in C2CD4A-C2CD4B on chromosome 15 at genome-wide significant levels (rs7612463 in UBE2E2, combined P = 2.27 × 10⁻⁹; rs7172432 in C2CD4A-C2CD4B, combined P = 3.66 × 10⁻⁹). The association of these two loci with T2D was replicated in other east Asian populations. In the European populations, the C2CD4A-C2CD4B locus was significantly associated with T2D, and a combined analysis of all populations gave P = 8.78 × 10⁻¹⁴, whereas the UBE2E2 locus did not show association to T2D. In conclusion, we identified two new loci at UBE2E2 and C2CD4A-C2CD4B associated with susceptibility to T2D.

Genetic bases for glaucoma

Glaucoma is the leading cause of visual impairment and blindness throughout the world. Primary open angle glaucoma (POAG; MIM 137760) is the main type of glaucoma in most populations, and more than 20 genetic loci for POAG have been reported. Only three causative genes have been identified in these loci, viz. myocilin (MYOC), optineurin (OPTN), and WD repeat domain 36 (WDR36). However, mutations in these genes account for only a small percentage of the patients with POAG. Some of these glaucoma cases have a Mendelian inheritance pattern, and a considerable fraction of the cases result from a large number of variants in several genes each contributing small effects. Glaucoma is considered to be a common disease such as diabetes mellitus, coronary disease, Crohn disease, and several( )common cancers. The main technological approaches used to identify the genes associated with glaucoma are the candidate gene approach, linkage analysis, case-control association study, and genome-wide association study. Association studies have found about 27 genes related to POAG, but the glaucoma-causing effects of these genes need to be investigated in more detail. The current trend is to use case-control association studies or genome-wide association studies to map the genes associated with glaucoma. Such studies are expected to greatly advance our understanding of the genetic basis of glaucoma, and to provide information on the effectiveness of glaucoma therapy. This review gives an overview on the genetic aspects of glaucoma.

Impact of common variants of PPARG, KCNJ11, TCF7L2, SLC30A8, HHEX, CDKN2A, IGF2BP2, and CDKAL1 on the risk of type 2 diabetes in 5,164 Indians

OBJECTIVE

Common variants in PPARG, KCNJ11, TCF7L2, SLC30A8, HHEX, CDKN2A, IGF2BP2, and CDKAL1 genes have been shown to be associated with type 2 diabetes in European populations by genome-wide association studies. We have studied the association of common variants in these eight genes with type 2 diabetes and related traits in Indians by combining the data from two independent case-control studies.

RESEARCH DESIGN AND METHODS

We genotyped eight single nucleotide polymorphisms (PPARG-rs1801282, KCNJ11-rs5219, TCF7L2-rs7903146, SLC30A8-rs13266634, HHEX-rs1111875, CDKN2A-rs10811661, IGF2BP2-rs4402960, and CDKAL1-rs10946398) in 5,164 unrelated Indians of Indo-European ethnicity, including 2,486 type 2 diabetic patients and 2,678 ethnically matched control subjects.

RESULTS

We confirmed the association of all eight loci with type 2 diabetes with odds ratio (OR) ranging from 1.18 to 1.89 (P = 1.6 x 10(-3) to 4.6 x 10(-34)). The strongest association with the highest effect size was observed for TCF7L2 (OR 1.89 [95% CI 1.71-2.09], P = 4.6 x 10(-34)). We also found significant association of PPARG and TCF7L2 with homeostasis model assessment of beta-cell function (P = 6.9 x 10(-8) and 3 x 10(-4), respectively), which looked consistent with recessive and under-dominant models, respectively.

CONCLUSIONS

Our study replicates the association of well-established common variants with type 2 diabetes in Indians and shows larger effect size for most of them than those reported in Europeans.

Replication study for the association between four Loci identified by a genome-wide association study on European American subjects with type 1 diabetes and susceptibility to diabetic nephropathy in Japanese subjects with type 2 diabetes

OBJECTIVE

Genetic factors are believed to contribute to the development and progression of diabetic nephropathy. Recently, a genome-wide association study for diabetic nephropathy revealed four novel candidate loci in European American subjects with type 1 diabetes. In this study, we determined the association of the four loci with diabetic nephropathy in Japanese subjects with type 2 diabetes.

RESEARCH DESIGN AND METHODS

We genotyped 11 singlenucleotide polymorphisms (SNPs) in four distinct loci (rs39059 and rs39075 in the CPVL/CHN2, rs1888747 and rs10868025 in FRMD3, rs739401 and rs451041 in CARS, and rs1041466, rs1411766, rs6492208, rs7989848, and rs9521445 in a chromosome 13q locus) in four independent Japanese populations.

RESULTS

Six SNPs were nominally associated with diabetic nephropathy in one of the four Japanese populations (P < 0.05; rs451041 in study 1; rs39059 and rs1888747 in study 3; rs1411766 in studies 1 and 4; and rs7989848 and rs9521445 in study 4); however, no significant association was observed for any SNP after correction for multiple testing errors in the individual populations. Nevertheless, a meta-analysis performed for the data obtained from all four populations revealed that one SNP (rs1411766) in chromosome 13q was significantly associated with diabetic nephropathy in the Japanese populations (nominal P = 0.004, corrected P = 0.04, odds ratio 1.26 [95% CI = 1.07-1.47]).

CONCLUSIONS

Our results suggest that the rs1411766 locus may be commonly involved in conferring susceptibility to diabetic nephropathy among subjects with type 1 or type 2 diabetes across different ethnic groups.

Acute cytokine-mediated downregulation of the zinc transporter ZnT8 alters pancreatic beta-cell function

Genetic studies suggest that Zn transporters such as ZnT8 play a role in insulin secretion by pancreatic beta-cells; however, little is known about the dynamic roles of Zn trafficking pathways on beta-cell physiology. To test the acute effects of the inflammatory cytokines interleukin 1 beta (IL1 beta) and tumor necrosis factor alpha (TNFalpha) on Zn homeostasis, the mRNA expression profile of Zn transporters of the ZnT and ZIP families was examined. Exposure of MIN6 cells or primary murine islets to IL1 beta or TNFalpha altered the mRNA expression profile of Zn transporters; most notable was decreased ZnT8 mRNA levels. siRNA-mediated gene knockdown was used to examine the effects of decreased ZnT8 expression in primary dispersed murine islet cells from C57/BL6 mice and MIN6 cells. ZnT8 knockdown in these murine islets led to reduced glucose stimulated insulin secretion without altering the total cellular insulin content or cell viability at normal or supraphysiological Zn concentrations. The labile Zn content determined by flow cytometry after loading with the Zn-specific sensor FluoZin-3 AM was decreased in MIN6 cells following ZnT8 knockdown or IL1 beta treatment. These results suggest that an acute decrease in ZnT8 levels impairs beta-cell function and Zn homeostasis, and may contribute to inflammatory cytokine-induced alterations in beta-cell function.

IGF2BP1, IGF2BP2 and IGF2BP3 genotype, haplotype and genetic model studies in metabolic syndrome traits and diabetes

OBJECTIVE

Genetic variation at the insulin-like binding protein 2 (IGF2BP2) gene has been associated with type 2 diabetes (T2D) by genome-wide association studies and by replication analyses. Our aim was to explore the underlying genetic model and mechanism of action, factors accounting for non-replications of the associations, and the effect of variation from pathway-related genes IGF2BP1 and IGF2BP3.

METHOD

We analysed here the association between T2D (and related traits) and rs4402960 and rs1470579 in IGF2BP2, and rs46522 and rs6949019 (marking IGF2BP1 and IGF2BP3 respectively) from the Age, Gene/Environment Susceptibility (AGES)-Reykjavik Study (N approximately 2500 aged 65-96 years). We undertook a retrospective analysis of the deviations from the multiplicative model in previous studies and the present study.

RESULTS

We replicated an association between rs4402960 and T2D status, and reported significant associations with anthropometric traits, fasting insulin, HOMA-IR and HOMA-%B. These associations were also observed for rs1470579, but not for the SNPs marking IGF2BP1 and IGF2BP3.

CONCLUSIONS

The lower fasting insulin levels and the impaired beta-cell function associated with IGF2BP2 SNPs are independent of obesity phenotypes. The action of these SNPs on T2D may result from an effect on beta-cell function. This could lead to lower insulin levels, the association with anthropometric traits being secondary. We discuss possible mechanisms of action relating IGF2BP2 with T2D traits. The occurrence of null alleles, the inclusion of T2D patients in analyses of metabolic syndrome risk traits and the genetic model, are possible factors accounting for non-replications of IGF2BP2 associations with T2D.

Meta-analysis added power to identify variants in FTO associated with type 2 diabetes and obesity in the Asian population

Several common variants in the intron 1 of FTO (fat mass and associated obesity) gene have been reliably associated with BMI and obesity in European populations. We analyzed two variants (rs9939609 and rs8050136) in 4,189 Chinese Han individuals and conducted a meta-analysis of published studies in Asian population to investigate whether these variants are associated with type 2 diabetes (T2D) and obesity in Asian population. In this study, both the minor allele A of rs9939609 and the minor allele A of rs805136 were associated with increased risk of T2D, independent of measures of BMI; the odds ratios (ORs) per copy of the risk allele were 1.19 for rs9939609 (95% confidence interval (CI), 1.04-1.37; P = 0.01) and 1.22 for rs8050136 (95% CI, 1.07-1.40; P = 0.004) after adjusting for age, sex, and BMI. Our results also showed association with risk of obesity (rs9939609: OR = 1.39 (95% CI 1.04-1.85), P = 0.02; rs8050136: OR = 1.45 (95% CI 1.09-1.93), P = 0.01) but no association with overweight. These results were consistent with the pooled results from our meta-analysis study (for diabetes, rs8050136, P = 1.3 x 10(-3); rs9939609, P = 9.8 x 10(-4); for obesity, rs8050136, P = 2.2 x 10(-7); rs9939609, P = 9.0 x 10(-9)). Our findings indicate that the two variants (rs9939609 and rs8050136) in the FTO gene contribute to obesity and T2D in the Asian populations.

Recent progress in the genetics of common obesity

The genetic contribution to interindividual variation in common obesity has been estimated at 40-70%. Yet, despite a relatively high heritability, the search for obesity susceptibility genes has been an arduous task. This paper reviews recent progress made in the obesity genetics field with an emphasis on established obesity susceptibility loci identified through candidate gene as well as genome-wide studies. For the last 15 years, candidate gene and genome-wide linkage studies have been the two main genetic epidemiological approaches to identify genetic loci for common traits, yet progress has been slow and success limited. Only recently have candidate gene studies started to succeed; by means of large-scale studies and meta-analyses at least five variants in four candidate genes have been found to be robustly associated with obesity-related traits. Genome-wide linkage studies, however, have so far not been able to pinpoint genetic loci for common obesity. The genome-wide association approach, which has become available in recent years, has dramatically changed the pace of gene discoveries for common disease, including obesity. Three waves of large-scale high-density genome-wide association studies have already discovered at least 15 previously unanticipated genetic loci incontrovertibly associated with body mass index and extreme obesity risk. Although the combined contribution of these loci to the variation in obesity risk at the population level is small and their predictive value is typically low, these recently discovered loci are set to improve fundamentally our insights into the pathophysiology of obesity.

A genome-wide association study identifies genetic variants in the CDKN2BAS locus associated with endometriosis in Japanese

Although the pathogenesis of endometriosis is not well understood, genetic factors have been considered to have critical roles in its etiology. Through a genome-wide association study and a replication study using a total of 1,907 Japanese individuals with endometriosis (cases) and 5,292 controls, we identified a significant association of endometriosis with rs10965235 (P = 5.57 x 10(-12), odds ratio = 1.44), which is located in CDKN2BAS on chromosome 9p21, encoding the cyclin-dependent kinase inhibitor 2B antisense RNA. By fine mapping, the SNP showing the strongest association was located in intron 16 of CDKN2BAS and was implicated in regulating the expression of p15, p16 and p14. A SNP, rs16826658, in the LD block including WNT4 on chromosome 1p36, which is considered to play an important role in the development of the female genital tract, revealed a possible association with endometriosis (P = 1.66 x 10(-6), odds ratio = 1.20). Our findings suggest that these regions are new susceptibility loci for endometriosis.

CDKAL1 and type 2 diabetes: a global meta-analysis

CDKAL1 (cyckin-dependent kinase 5 regulatory subunit-associated protein 1-like 1) has been shown to be associated with type 2 diabetes in various ethnic groups; however, contradictory results have been reported. We performed a comprehensive meta-analysis of 21 studies for rs7756992, 17 studies for rs7754840 and 10 studies for rs10946398 variants of the CDKAL1 gene to evaluate the effect of CDKAL1 on genetic susceptibility for type 2 diabetes. We found a significant association of rs7756992, rs7754840 and rs10946398 in CDKAL1 with type 2 diabetes (odds ratio (OR) = 1.15, 95% confidence interval (CI) = 1.07-1.23, P < 0.0001; OR = 1.14, 95%CI = 1.06-1.24, P = 0.001, and OR = 1.12, 95%CI = 1.07-1.18, P < 0.0001, respectively). We conclude that there are significant associations between CDKAL1 polymorphisms and type 2 diabetes, but these associations vary in different ethnic populations.

Risk of type 2 diabetes and obesity is differentially associated with variation in FTO in whites and African-Americans in the ARIC study

Single nucleotide polymorphisms (SNPs) in the fat mass and obesity associated (FTO) gene are associated with body mass index (BMI) in populations of European descent. The FTO rs9939609 variant, first detected in a genome-wide association study of diabetes, conferred an increased disease risk that was abolished after adjustment for BMI, suggesting that the association may be due to variation in adiposity. The relationship between diabetes, four previously identified FTO polymorphisms that span a 19.6-kb genomic region, and obesity was therefore evaluated in the biracial population-based Atherosclerosis Risk in Communities Study with the goal of further refining the association by comparing results between the two ethnic groups. The prevalence of diabetes and obesity (BMI ≥30 kg/m²) was established at baseline, and diabetes was determined by either self-report, a fasting glucose level ≥126 mg/dL, or non-fasting glucose ≥200 mg/dL. There were 1,004 diabetes cases and 10,038 non-cases in whites, and 670 cases and 2,780 non-cases in African-Americans. Differences in mean BMI were assessed by a general linear model, and multivariable logistic regression was used to predict the risk of diabetes and obesity. For white participants, the FTO rs9939609 A allele was associated with an increased risk of diabetes (odds ratio (OR)  = 1.19, p<0.001) and obesity (OR = 1.22, p<0.001) under an additive genetic model that was similar for all of the SNPs analyzed. In African-Americans, only the rs1421085 C allele was a determinant of obesity risk (OR = 1.17, p = 0.05), but was found to be protective against diabetes (OR = 0.79, p = 0.03). Adjustment for BMI did not eliminate any of the observed associations with diabetes. Significant statistical interaction between race and the FTO variants suggests that the effect on diabetes susceptibility may be context dependent.

Meta-analysis and functional effects of the SLC30A8 rs13266634 polymorphism on isolated human pancreatic islets

BACKGROUND

The C-allele of rs13266634 located in SLC30A8 (ZNT8) has been strongly associated with decreased insulin release and with type 2 diabetes (T2D) susceptibility in some but not all studies. To shed further light on this issue, we performed a meta-analysis of the association between rs13266634 and T2D in different ethnic groups and assessed the relationships between SLC30A8 genotypes and some properties of isolated human islets.

METHODS

From 32 original articles, a total of 77,234 control individuals and 44,945 subjects with T2D were studied in meta-analysis. To assess the relationships between SLC30A8 genotype and islet cell phenotype, insulin secretion in response to glucose, glucose plus arginine and glucose plus glibenclamide was determined in pancreatic islets isolated from 82 multiorgan donors genotyped for the rs13266634 polymorphism. Quantitative expression of SLC30A8, Insulin and Glucagon mRNA was also measured.

RESULTS

Overall, each SLC30A8 risk allele was associated with a 14% increased risk for T2D (P=2.78 x 10(-34)). The population risk of T2D attributable to this polymorphism was estimated at 9.5% in Europeans and 8.1% in East Asians. Basal and stimulated insulin secretion from human islets as well as islet expressions of SLC30A8, Insulin and Glucagon were not affected by the presence of the polymorphism. However, SLC30A8 expression was positively correlated with Insulin (r=0.75, P=6.43 x 10(-6)) and Glucagon (r: 0.70, P=4.89 x 10(-5)) levels.

CONCLUSIONS

The SLC30A8 rs13266634 polymorphism is among the most confirmed genetic markers of T2D in Europeans and East Asians. In isolated human islets, the risk C-allele does not affect ex-vivo insulin secretion and SLC30A8 expression, which is correlated with that of insulin and glucagon.

Impact of limited population diversity of genome-wide association studies

The article describes the limited population diversity of genome-wide association studies and its resulting impact on the development of commercial genetic tests with restricted applicability and usefulness to certain groups, potentially increasing existing disparities. To enable development of new clinical tools applicable to all groups, much more focus is needed to engage minority communities to enroll in genetics or genomic research studies and on investigators to reach out to underrepresented communities.

Combining genetic markers and clinical risk factors improves the risk assessment of impaired glucose metabolism

BACKGROUND

Although several candidate gene polymorphisms (SNPs) have been associated with increased risk of type 2 diabetes mellitus (T2DM), relatively few studies have assessed the ability of T2DM candidate genes to assess the risk of impaired fasting glucose (IFG), impaired glucose tolerance (IGT), and T2DM beyond the information provided by clinical risk factors.

OBJECTIVE

To test whether the inclusion of genetic markers in a regression model provides a better assessment of the risk of IFG, IGT, and T2DM than a model based only on non-genetic risk factors commonly assessed in clinical settings.

METHODS

Subjects (n = 485; 213 parents, 272 offspring) from the Quebec Family Study, not known to haveT2DM, were measured for several risk factors and underwent an oral glucose tolerance test. Thirty-eight SNPs in 25 susceptibility/ candidate genes previously reported to be associated with T2DM were genotyped. In order to identify risk factors associated with IFG/IGT/T2DM, two logistic regression models were tested: a full model (FM) including age, sex, body mass index (BMI), systolic and diastolic blood pressure, smoking status, and the 38 SNPs; and a reduced model (RM), in which the SNPs were dropped, which allowed us to test the null-hypothesis that the markers are not associated with the risk of IFG/IGT/T2DM. Performances of the models were compared by using a likelihood ratio test and the receiver-operating characteristic curves (ROC).The area under the curve (AUC) was calculated from the ROC curve.

RESULTS

The analyses showed that age (P < 0.0001), BMI (P < 0.0001), and six variants (IGF2BP2 rs4402960, P = 0.002; ADIPOQ+276 G>T, P = 0.004; UCP2Ala55Val, P = 0.01; CDKN2AI2B rs3731201, P = 0.02; rs495490, P = 0.02, and rsl 0811661, P = 0.03) were significantly associated with the risk of IFG/IGT/T2DM. Dropping genetic markers from the analysis significantly reduced the fit of the model to the data (chi-square = 38.98, P < 0.00001 contrasting RM to FM), suggesting that the genetic markers are significantly associated with the risk of IFG/IGT/T2DM. Furthermore, the AUC was higher for FM than for RM (0.85 (95% CI 0.81-0.89) versus 0.81 (95% CI 0.76-0.85), P = 0.004).

CONCLUSION

Our results suggest that combining genetic markers with traditional clinical risk factors has the potential to improve our ability to assess the risk of complex diseases such as T2DM.

A genome-wide association study identifies susceptibility variants for type 2 diabetes in Han Chinese

To investigate the underlying mechanisms of T2D pathogenesis, we looked for diabetes susceptibility genes that increase the risk of type 2 diabetes (T2D) in a Han Chinese population. A two-stage genome-wide association (GWA) study was conducted, in which 995 patients and 894 controls were genotyped using the Illumina HumanHap550-Duo BeadChip for the first genome scan stage. This was further replicated in 1,803 patients and 1,473 controls in stage 2. We found two loci not previously associated with diabetes susceptibility in and around the genes protein tyrosine phosphatase receptor type D (PTPRD) (P = 8.54×10⁻¹⁰; odds ratio [OR] = 1.57; 95% confidence interval [CI] = 1.36–1.82), and serine racemase (SRR) (P = 3.06×10⁻⁹; OR = 1.28; 95% CI = 1.18–1.39). We also confirmed that variants in KCNQ1 were associated with T2D risk, with the strongest signal at rs2237895 (P = 9.65×10⁻¹⁰; OR = 1.29, 95% CI = 1.19–1.40). By identifying two novel genetic susceptibility loci in a Han Chinese population and confirming the involvement of KCNQ1, which was previously reported to be associated with T2D in Japanese and European descent populations, our results may lead to a better understanding of differences in the molecular pathogenesis of T2D among various populations.

Type 2 diabetes (T2D) is a complex disease that involves many genes and environmental factors. Genome-wide and candidate-gene association studies have thus far identified at least 19 regions containing genes that may confer a risk for T2D. However, most of these studies were conducted with patients of European descent. We studied Chinese patients with T2D and identified two genes, PTPRD and SRR, that were not previously known to be involved in diabetes and are involved in biological pathways different from those implicated in T2D by previous association reports. PTPRD is a protein tyrosine phosphatase and may affect insulin signaling on its target cells. SRR encodes a serine racemase that synthesizes D-serine from L-serine. Both D-serine (coagonist) and the neurotransmitter glutamate bind to NMDA receptors and trigger excitatory neurotransmission in the brain. Glutamate signaling also regulates insulin and glucagon secretion in pancreatic islets. Thus, SRR and D-serine, in addition to regulating insulin and glucagon secretion, may play a role in the etiology of T2D. Our study suggests that, in different patient populations, different genes may confer risks for diabetes. Our findings may lead to a better understanding of the molecular pathogenesis of T2D.

A genetic variation in the fat mass- and obesity-associated gene is associated with obesity and newly diagnosed type 2 diabetes in a Chinese population

BACKGROUND

Recently, a genome-wide association study identified a strong association between the fat mass- and obesity-associated rs9939609 single nucleotide polymorphism (SNP) and the risk of obesity in European population. However, the results in Chinese population were reported to be contradictory. Therefore, our aim was to examine whether this SNP is associated with obesity and newly diagnosed type 2 diabetes (T2D) in Chinese population.

METHODS

We genotyped rs9939609 in 2587 subjects [obesity 243, overweight 976, normal weight 1368 or newly diagnosed T2D 877, impaired glucose regulation 305, normal glucose tolerance (NGT) 1405] using an allelic discrimination assay-by-design TaqMan method on ABI7900HT. We analysed associations of the rs9939609 SNP with obesity and newly diagnosed T2D through Logistic regression analysis.

RESULTS

In obesity case-control study, we found that the A allele was strongly associated with obesity and overweight. The odds ratios for the allele A versus T were 1.447 for Obesity versus normal weight (95% CI 1.104-1.896, p = 0.007) and 1.363 for Overweight versus normal weight (95% CI 1.149-1.617, p < 0.0001). In T2D case-control study, the odds ratios for the allele A versus T were 1.305 for T2D versus NGT (95% CI 1.097-1.552, p = 0.003) and 1.280 for combined T2D and impaired glucose regulation (IGR) versus NGT (95% CI 1.089-1.503, p = 0.003). The associations of the A allele with T2D and combined T2D and IGR remained significant with adjustment for age, sex and body mass index (BMI).

CONCLUSION

The fat mass- and obesity-associated gene rs9939609 SNP is strongly associated with risk of obesity and newly diagnosed T2D in the Chinese population.

Diabetes in Asia

Prevalence of type 2 diabetes has rapidly increased in native and migrant Asian populations. Diabetes develops at a younger age in Asian populations than in white populations, hence the morbidity and mortality associated with the disease and its complications are also common in young Asian people. The young age of these populations and the high rates of cardiovascular risk factors seen in Asian people substantially increase lifetime risk of cardiovascular disease. Several distinctive features are apparent in pathogenetic factors for diabetes and their thresholds in Asian populations. The economic burden due to diabetes at personal, societal, and national levels is huge. National strategies to raise public awareness about the disease and to improve standard of care and implementation of programmes for primary prevention are urgently needed.

Association of the common fat mass and obesity associated (FTO) gene polymorphism with obesity in a Japanese population

The association of the FTO gene polymorphism, rs9939609, with obesity was examined using the population of the Takahata study (n (M/F): 2,639 (1,168 / 1,470); age: 63.0 +/- 10.2 years), a Japanese community-based study. The effects of lifestyle-related factors, including nutritional intake and physical activities, on the association were also examined. Body mass index (BMI) was significantly associated with the FTO gene polymorphism (p<0.001). A case-control association study of the FTO gene polymorphism with obesity using multiple logistic regression analysis showed a significant association of the genotype AA (odds ratio, 1.53 [95% confidential interval, 1.04-2.24]) after adjustment for age and gender. Analysis to examine the differences in lifestyle-related factors among the genotype groups showed a significant difference in the energy expenditure for moderate to high-intensity physical activity (PA) (> or = 3.0 METs) (p=0.012) with a significant decrease toward the genotype AA (p=0.027). The effect of energy expenditure for moderate to high-intensity PA on the association of the polymorphism with obesity was then examined using study groups stratified based on the energy expenditure for moderate to high-intensity PA (Low-PA and High-PA). The BMI was significantly higher in the genotype AA in the Low-PA group (p=0.016) but not in the High-PA group (p=0.103). Furthermore, the genotype AA was significantly associated with obesity (odds ratio, 2.39 [95% confidential interval, 1.19-4.80]) in the Low-PA group but not in the High- PA group (p=0.650). The FTO gene, rs9939609, was associated with obesity, and the association was evident in subjects with low-PA, suggesting a PA-dependent association.

Ethnic difference in patients with type 2 diabetes mellitus in inter-East Asian populations: a systematic review and meta-analysis focusing on gene polymorphism

BACKGROUND

We previously reported that the fasting serum insulin level was significantly lower in Japanese patients than in Korean and Chinese patients, and showed evidence that a difference in the dietary component would be one of the most influential factors for the ethnic difference. However, it is well known that type 2 diabetes mellitus (T2DM) results from the interaction between genetic predispositions and environmental risk factors. Therefore, we investigated ethnic differences by focusing on gene polymorphism, possibly related to T2DM in Japanese, Korean, and Chinese subjects.

METHODS

Data sources included MEDLINE and EMBASE between January 2001 and October 2008. We conducted a search for articles containing minor allele frequency (MAF) in the gene polymorphisms of peroxisome proliferator-activated receptor-γ (PPARG), inward-rectifying potassium channel Kir6.2 (KCNJ11), Calpain 10 (CAPN10), and transcription factor 7-like 2 (TCF7L2). The pooled odds ratio was calculated by using a fixed-effects model with the Mantel-Haenszel method after confirming statistical evidence of homogeneity across the ethnicities using the Breslow-Day test.

RESULTS

The Breslow-Day test revealed that there were no statistically significant differences between ethnicities in pooled odds ratios for the gene polymorphisms in PPARG (P = 0.828), KCNJ11 (P = 0.194), CAPN10 (P = 0.090), and TCF7L2 (P = 0.376). Also, pooled odds ratios of each gene polymorphism in East Asians were 0.645 for PPARG (P = 0.000), 1.168 for KCNJ11 (P = 0.000), 0.967 for CAPN10 (P = 0.759), and 1.386 for TCF7L2 (P = 0.000).

CONCLUSION

The results of this study and our previous studies suggest that behavioral and environmental risk factors have a more significant impact on ethnic difference in East Asian patients with T2DM compared with genetic predispositions.

FTO polymorphisms are associated with obesity but not with diabetes in East Asian populations: a meta-analysis

OBJECTIVE

To clarify the contradictory findings in patients with obesity and type 2 diabetes by meta-analysis.

METHODS

PubMed and Embase were searched for articles published up to March 2009. All studies on the association of FTO polymorphisms with obesity and type 2 diabetes were included. Pooled odds ratio was calculated using the model of fixed or random effects. Sensitivity analysis was performed to evaluate the stability of meta-analytic results.

RESULTS

Meta-analysis suggested that rs9939609 A allele was more significantly associated with obesity risk than T allele (3 studies / 004 cases and 4544 control subjects): random effect odds ratio (OR)=1.28, 95%CI=1.05 and 1.55, P heterogeneity =0.05, I2=66.6%. Similar results were observed in rs8050136 polymorphism (3 studies/2404 cases and 5713 control subjects): fixed effect OR =1.25, 95%CI=1.13, 1.37, P heterogeneity=0.12, I2=51.9%. However, no significant association was found between genetics and risk of type 2 diabetes after control of potential confounders (at least for BMI) either for rs9939609 (fixed effect OR=1.05, 95% CI=0.97,1.13) or for rs8050136 polymorphism (fixed effect OR =1.07, 95%CI: 0.99, 1.16). Furthermore, the sensitivity analysis strengthened our confidence in validity of the association. Conclusion FTO polymorphisms are associated with obesity but not with type 2 diabetes in East Asian populations. Further large-scale studies are required to conclusively establish the association.

Association of the common rs9939609 variant of FTO gene with polycystic ovary syndrome in Chinese women

Variations in the fat mass and obesity-associated (FTO) gene have recently been associated with obesity and type 2 diabetes mellitus among different ethnic populations. Given that the phenotype of polycystic ovary syndrome (PCOS) overlaps with obesity and type 2 diabetes, we hypothesize that the common rs9939609 variant of FTO gene is related to PCOS susceptibility. We performed a case-control association study on 215 women with PCOS, using 227 healthy women as the control. We examined the association between rs9939609 variant and PCOS susceptibility, as well as between PCOS and obesity-related parameters in Chinese women. We observed significant differences in the allelic and genotypic distributions between PCOS patients and the control group. The A allele was significantly more frequent among PCOS patients than in the control population (15.1% vs. 9.9%; A allele vs. T allele, OR = 1.62, P = 0.019). The A allele carrier genotype (AA and AT) frequencies were also significantly greater in PCOS patients than in the controls (28% vs. 19%; AT and TT vs. TT genotype, OR = 1·61, P = 0.035). In logistic regression, the strength of this association was attenuated after adjustment for body mass index (BMI) (A allele vs. T allele, OR = 1.39, P = 0.286; AT and TT genotypes vs. TT genotype, OR = 1.40, P = 0.312). However, we did not find any significant associations of rs9939609 variant with obesity-related traits. In conclusions, the rs9939609 variant in the FTO gene is associated with PCOS susceptibility in the Chinese population, probably because of its effect on BMI.

Genetic variation in the hypothalamic pathways and its role on obesity

Over recent decades, the prevalence of obesity has increased dramatically worldwide. Although this epidemic is mainly attributable to modern (western) lifestyle, multiple twin and adoption studies indicate the significant role of genes in the individual's predisposition to becoming obese. As the hypothalamus plays a central role in controlling body weight, its regulatory circuits may represent a crucial system in the pathogenesis of the disorder. Genetic variations in genes in the hypothalamic pathways may therefore contribute to the susceptibility for obesity in humans and animals. We summarize current knowledge on the physiological role of the hypothalamus in body-weight regulation and review genetic studies on the hypothalamic candidate genes in relation to obesity. Together, data from functional and genetic studies as well as the new, common, obesity loci identified in genome-wide association scans support an important role for the hypothalamic genes in predisposing to obesity. However, findings are still inconclusive for many candidate genes. To improve our understanding of the genetic architecture of common obesity, we suggest that specific obesity phenotypes should be considered and different analytical approaches used. Such studies should consider multiple genes from the same physiological pathways, together with environmental risk factors.

Diabetes in Japan: a review of disease burden and approaches to treatment

In recent years there has been rapid growth in diabetes in Japan which now is one of the nations most affected by the worldwide diabetes epidemic. Diabetes has been identified as a healthcare priority by the Ministry of Health, Labour and Welfare (MHLW). Type 1 diabetes is rare in Japan, and type 2 diabetes predominates in both adults and children. The growth in diabetes is due to increases in the number of people with type 2 diabetes associated with increased longevity and lifestyle changes. Approximately 13.5% of the Japanese population now has either type 2 diabetes or impaired glucose tolerance. This high prevalence of type 2 diabetes is associated with a significant economic burden, with diabetes accounting for up to 6% of the total healthcare budget. The costs of diabetes are increased in patients with co-morbidities such as hypertension and hyperlipidaemia and in patients who develop complications, of which retinopathy has the highest cost. Costs increase with increasing number of complications. Current guidelines from the Japan Diabetes Society (JDS) recommend a target HbA(1c) of 6.5% for glycaemic control. This is achieved in approximately one third of patients with type 2 diabetes, and Japanese patients typically have lower HbA(1c) than patients in Western countries (e.g. US, UK). Japanese patients with type 2 diabetes have better adherence with diet and exercise recommendations than their peers in Western countries. Sulfonylureas have been the most widely prescribed first-line treatment for type 2 diabetes, although there is increasing use of combination therapy and of insulin.

Zinc and diabetes--clinical links and molecular mechanisms

Zinc is an essential trace element crucial for the function of more than 300 enzymes and it is important for cellular processes like cell division and apoptosis. Hence, the concentration of zinc in the human body is tightly regulated and disturbances of zinc homeostasis have been associated with several diseases including diabetes mellitus, a disease characterized by high blood glucose concentrations as a consequence of decreased secretion or action of insulin. Zinc supplementation of animals and humans has been shown to ameliorate glycemic control in type 1 and 2 diabetes, the two major forms of diabetes mellitus, but the underlying molecular mechanisms have only slowly been elucidated. Zinc seems to exert insulin-like effects by supporting the signal transduction of insulin and by reducing the production of cytokines, which lead to beta-cell death during the inflammatory process in the pancreas in the course of the disease. Furthermore, zinc might play a role in the development of diabetes, since genetic polymorphisms in the gene of zinc transporter 8 and in metallothionein (MT)-encoding genes could be demonstrated to be associated with type 2 diabetes mellitus. The fact that antibodies against this zinc transporter have been detected in type 1 diabetic patients offers new diagnostic possibilities. This article reviews the influence of zinc on the diabetic state including the molecular mechanisms, the role of the zinc transporter 8 and MT for diabetes development and the resulting diagnostic and therapeutic options.

Pathomechanisms of type 2 diabetes genes

Type 2 diabetes mellitus is a complex metabolic disease that is caused by insulin resistance and beta-cell dysfunction. Furthermore, type 2 diabetes has an evident genetic component and represents a polygenic disease. During the last decade, considerable progress was made in the identification of type 2 diabetes risk genes. This was crucially influenced by the development of affordable high-density single nucleotide polymorphism (SNP) arrays that prompted several successful genome-wide association scans in large case-control cohorts. Subsequent to the identification of type 2 diabetes risk SNPs, cohorts thoroughly phenotyped for prediabetic traits with elaborate in vivo methods allowed an initial characterization of the pathomechanisms of these SNPs. Although the underlying molecular mechanisms are still incompletely understood, a surprising result of these pathomechanistic investigations was that most of the risk SNPs affect beta-cell function. This favors a beta-cell-centric view on the genetics of type 2 diabetes. The aim of this review is to summarize the current knowledge about the type 2 diabetes risk genes and their variants' pathomechanisms.

Prognostic significance of FTO genotype in the development of obesity in Japanese: the J-SHIPP study

OBJECTIVE

Susceptibility of fat mass and obesity-associated (FTO) gene polymorphisms to obesity has been reported in various populations. Polymorphisms in the melanocortin 4 receptor (MC4R) gene were recently explored as another susceptible locus. However, prognostic significance of these genetic variations has not been fully elucidated. Here, we investigated the involvement of FTO rs9939609 and MC4R rs17782313 polymorphisms in the development of obesity. Association with type 2 diabetes mellitus (T2DM) was also investigated.

SUBJECTS

We analyzed 2806 community-dwelling middle-aged to elderly subjects (61+/-14 years). Clinical parameters were obtained from the subjects' personal health records, evaluated at their annual medical check-up.

RESULTS

FTO genotype was significantly associated with current body mass index (BMI; TT 23.2+/-3.2, TA 23.7+/-3.2, AA 24.4+/-3.2 kg m(-2), P=2.5 x 10(-6)) and frequency of obesity (26.6, 32.0, 43.0% respectively, P=2.0 x 10(-4)). Age- and sex-adjusted odds ratio for obesity was 1.30 (P=0.004) in TA and 2.07 (P=0.002) in AA genotype. During the 9.4 years comprising the follow-up period, 214 new cases of obesity were diagnosed among 1718 subjects whose retrospective data were available. A allele frequency of the FTO genotype was significantly higher in subjects who developed obesity (22.2, 15.8%, P=0.001), Age-, sex- and initial BMI-adjusted odds ratio for the development of obesity was 1.46 (95% confidence interval, 1.04-2.04) (P=0.031). However, association studies and meta-analysis of T2DM did not actively support the involvement of FTO genotype. No significant differences were observed between the MC4R genotype and BMI (P=0.015), and the frequency of obesity (P=0.284).

CONCLUSION

FTO genotype is an independent risk factor for future development of obesity.

Replication study for the association of new meta-analysis-derived risk loci with susceptibility to type 2 diabetes in 6,244 Japanese individuals

AIMS/HYPOTHESIS

Additional susceptibility loci for type 2 diabetes have been identified by a meta-analysis of genome-wide association studies (GWASs) in European populations. To examine further the roles of these new loci, we performed a replication study for the association of these single-nucleotide polymorphism (SNP) loci with the disease in three independent Japanese populations.

METHODS

We genotyped seven of the 11 SNPs that emerged in stage 2 of the meta-analysis for European GWASs (rs864745 in JAZF1, rs12779790 near CDC123/CAMK1D, rs7961581 near TSPAN8/LGR5, rs4607103 near ADAMTS9, rs10923931 in NOTCH2, rs1153188 near DCD and rs9472138 near VEGFA) for three independent Japanese populations (first set, 1,630 type 2 diabetes patients vs 1,064 controls; second set, 1,272 type 2 diabetes patients vs 856 controls; third set, 486 type 2 diabetes patients vs 936 controls) using a TaqMan assay. The association of the SNP loci in each population was analysed using a logistic regression analysis, adjusting for age, sex and BMI, and the data were evaluated by a meta-analysis.

RESULTS

A meta-analysis for the three case-control studies identified a nominal association of rs864745 in JAZF1 with type 2 diabetes (OR 1.148, 95% CI 1.034-1.275, p = 0.0098, corrected p = 0.069). The association of other loci did not reach statistically significant levels (nominal p > 0.05).

CONCLUSIONS/INTERPRETATION

From these results the contribution of these seven loci in conferring susceptibility to type 2 diabetes is considered minor in the Japanese population, if they are present.

Zn(2+)-transporter-8: a dual role in diabetes

Diabetes mellitus arises from defects in insulin secretion or action, or both. In pancreatic islets, insulin production is linked with zinc transport mediated by zinc transporter ZnT-8, a product of the SLC30A8 gene. Therefore, altered activity of ZnT-8 is expected to be associated with impaired glucose-induced insulin response and promote progression from glucose intolerance to diabetes. Recent findings do emerge with a role of SLC30A8 in diabetes. Genome-wide association scans for type 2 diabetes (T2D) susceptibility loci revealed and then replicated a highly significant association between the R allele of the R325W variant of SLC30A8 (marker rs13266634) and susceptibility to T2D in Caucasians. A role of ZnT-8 as a new major self-antigenic determinant in type 1 diabetes (T1D) was found. Marker rs13266634 was also shown to modulate anti-ZnT-8 self-antibody specificity in islet autoimmunity. Hence, these findings suggest for a dual role of SLC30A8 in diabetes, which is consisted in conferring genetic susceptibility to T2D and being a major islet self-antigen in T1D as well. Here we characterize an emerging role of ZnT-8 in diabetes and discuss potential mechanisms of its involvement in the etiology of both forms of diabetes.

Genetics factors contributing to type 2 diabetes across ethnicities

Type 2 diabetes mellitus (T2DM) is among the many common diseases with a strong genetic component, but until recently, the variants causing this disease remained largely undiscovered. With the ability to interrogate most of the variation in the genome, the number of genetic variants has grown from 2 to 19 genes, many with multiple variants. An additional three genes are associated primarily with fasting glucose rather than T2DM. Despite the plethora of new markers, the individual effect is uniformly small, and the cumulative effect explains little of the genetic risk for T2DM. Furthermore, the success is largely restricted to European populations. Despite success in mapping genes in Asian populations, success in United States minorities, particularly African Americans, has been limited. The genetic findings highlight the role of the beta cell in diabetes pathogenesis, but much remains to be discovered before genetic prediction and individualized medicine can become a reality for this disease.

Confirmation of multiple risk Loci and genetic impacts by a genome-wide association study of type 2 diabetes in the Japanese population

OBJECTIVE

To identify novel type 2 diabetes gene variants and confirm previously identified ones, a three-staged genome-wide association study was performed in the Japanese population.

RESEARCH DESIGN AND METHODS

In the stage 1 scan, we genotyped 519 case and 503 control subjects with 482,625 single nucleotide polymorphism (SNP) markers; in the stage 2 panel comprising 1,110 case subjects and 1,014 control subjects, we assessed 1,456 SNPs (P < 0.0025, stage 1); additionally to direct genotyping, 964 healthy control subjects formed the in silico control panel. Along with genome-wide exploration, we aimed to replicate the disease association of 17 SNPs from 16 candidate loci previously identified in Europeans. The associated and/or replicated loci (23 SNPs; P < 7 x 10(-5) for genome-wide exploration and P < 0.05 for replication) were examined in the stage 3 panel comprising 4,000 case subjects and 12,569 population-based samples, from which 4,889 nondiabetic control subjects were preselected. The 12,569 subjects were used for overall risk assessment in the general population.

RESULTS

Four loci-1 novel with suggestive evidence (PEPD on 19q13, P = 1.4 x 10(-5)) and three previously reported-were identified; the association of CDKAL1, CDKN2A/CDKN2B, and KCNQ1 were confirmed (P < 10(-19)). Moreover, significant associations were replicated in five other candidate loci: TCF7L2, IGF2BP2, SLC30A8, HHEX, and KCNJ11. There was substantial overlap of type 2 diabetes susceptibility genes between the two populations, whereas effect size and explained variance tended to be higher in the Japanese population.

CONCLUSIONS

The strength of association was more prominent in the Japanese population than in Europeans for more than half of the confirmed type 2 diabetes loci.

The E23K variation in the KCNJ11 gene is associated with type 2 diabetes in Chinese and East Asian population

The genes (ABCC8 and KCNJ11) have a key role in glucose-stimulated insulin secretion and thus have always been considered as excellent susceptibility candidates for involvement in type 2 diabetes. Common polymorphisms (KCNJ11 E23K and ABCC8 exon16-3t/c) in these genes have been reported to be associated with type 2 diabetes in various European-descent populations. However, there were inconsistent results in previous studies in East Asian populations and no large case-control studies have been carried out in the Chinese Han population. In this study, these two variants were genotyped in about 4000 Chinese by using TaqMan technology on an ABI7900 system. A meta-analysis was also used to assess the results of association between the two variants and type 2 diabetes in East Asian populations. Our investigation confirmed the association between the KCNJ11 E23K variant and type 2 diabetes under a recessive model (KK vs EK+EE) in the Chinese Han population (odds ratio (OR)=1.25, 95% confidence interval (95% CI) 1.04-1.50, P=0.017). The meta-analysis of East Asian populations also showed a strong significant association of the K allele with diabetes (OR=1.15, P=3 x 10(-9)), whereas the exon16-3t/c variant (rs1799854) in ABCC8 showed no significant association. Thus, the common E23K variant is considered as a strong candidate for type 2 diabetes susceptibility across different ethnicities.

Previously associated type 2 diabetes variants may interact with physical activity to modify the risk of impaired glucose regulation and type 2 diabetes: a study of 16,003 Swedish adults

OBJECTIVE

Recent advances in type 2 diabetes genetics have culminated in the discovery and confirmation of multiple risk variants. Two important and largely unanswered questions are whether this information can be used to identify individuals most susceptible to the adverse consequences of sedentary behavior and to predict their response to lifestyle intervention; such evidence would be mechanistically informative and provide a rationale for targeting genetically susceptible subgroups of the population.

RESEARCH DESIGN AND METHODS

Gene x physical activity interactions were assessed for 17 polymorphisms in a prospective population-based cohort of initially nondiabetic middle-aged adults. Outcomes were 1) impaired glucose regulation (IGR) versus normal glucose regulation determined with either fasting or 2-h plasma glucose concentrations (n = 16,003), 2) glucose intolerance (in mmol/l, n = 8,860), or 3) incident type 2 diabetes (n = 2,063 events).

RESULTS

Tests of gene x physical activity interactions on IGR risk for 3 of the 17 polymorphisms were nominally statistically significant:CDKN2A/B rs10811661 (P(interaction) = 0.015), HNF1B rs4430796 (P(interaction) = 0.026), and PPARG rs1801282 (P(interaction) = 0.04). Consistent interactions were observed for the CDKN2A/B (P(interaction) = 0.013) and HNF1B (P(interaction) = 0.0009) variants on 2-h glucose concentrations. Where type 2 diabetes was the outcome, only one statistically significant interaction effect was observed, and this was for the HNF1B rs4430796 variant (P(interaction) = 0.0004). The interaction effects for HNF1B on IGR risk and incident diabetes remained significant after correction for multiple testing (P(interaction) = 0.015 and 0.0068, respectively).

CONCLUSIONS

Our observations suggest that the genetic predisposition to hyperglycemia is partially dependent on a person's lifestyle.

Down-regulation of ZnT8 expression in INS-1 rat pancreatic beta cells reduces insulin content and glucose-inducible insulin secretion

The SLC30A8 gene codes for a pancreatic beta-cell-expressed zinc transporter, ZnT8. A polymorphism in the SLC30A8 gene is associated with susceptibility to type 2 diabetes, although the molecular mechanism through which this phenotype is manifest is incompletely understood. Such polymorphisms may exert their effect via impacting expression level of the gene product. We used an shRNA-mediated approach to reproducibly downregulate ZnT8 mRNA expression by >90% in the INS-1 pancreatic beta cell line. The ZnT8-downregulated cells exhibited diminished uptake of exogenous zinc, as determined using the zinc-sensitive reporter dye, zinquin. ZnT8-downregulated cells showed reduced insulin content and decreased insulin secretion (expressed as percent of total insulin content) in response to hyperglycemic stimulus, as determined by insulin immunoassay. ZnT8-depleted cells also showed fewer dense-core vesicles via electron microscopy. These data indicate that reduced ZnT8 expression in cultured pancreatic beta cells gives rise to a reduced insulin response to hyperglycemia. In addition, although we provide no direct evidence, these data suggest that an SLC30A8 expression-level polymorphism could affect insulin secretion and the glycemic response in vivo.

Admixture mapping of 15,280 African Americans identifies obesity susceptibility loci on chromosomes 5 and X

The prevalence of obesity (body mass index (BMI) > or =30 kg/m(2)) is higher in African Americans than in European Americans, even after adjustment for socioeconomic factors, suggesting that genetic factors may explain some of the difference. To identify genetic loci influencing BMI, we carried out a pooled analysis of genome-wide admixture mapping scans in 15,280 African Americans from 14 epidemiologic studies. Samples were genotyped at a median of 1,411 ancestry-informative markers. After adjusting for age, sex, and study, BMI was analyzed both as a dichotomized (top 20% versus bottom 20%) and a continuous trait. We found that a higher percentage of European ancestry was significantly correlated with lower BMI (rho = -0.042, P = 1.6x10(-7)). In the dichotomized analysis, we detected two loci on chromosome X as associated with increased African ancestry: the first at Xq25 (locus-specific LOD = 5.94; genome-wide score = 3.22; case-control Z = -3.94); and the second at Xq13.1 (locus-specific LOD = 2.22; case-control Z = -4.62). Quantitative analysis identified a third locus at 5q13.3 where higher BMI was highly significantly associated with greater European ancestry (locus-specific LOD = 6.27; genome-wide score = 3.46). Further mapping studies with dense sets of markers will be necessary to identify the alleles in these regions of chromosomes X and 5 that may be associated with variation in BMI.

Polycomb protein Ezh2 regulates pancreatic beta-cell Ink4a/Arf expression and regeneration in diabetes mellitus

Proliferation of pancreatic islet beta cells is an important mechanism for self-renewal and for adaptive islet expansion. Increased expression of the Ink4a/Arf locus, which encodes the cyclin-dependent kinase inhibitor p16(INK4a) and tumor suppressor p19(Arf), limits beta-cell regeneration in aging mice, but the basis of beta-cell Ink4a/Arf regulation is poorly understood. Here we show that Enhancer of zeste homolog 2 (Ezh2), a histone methyltransferase and component of a Polycomb group (PcG) protein complex, represses Ink4a/Arf in islet beta cells. Ezh2 levels decline in aging islet beta cells, and this attrition coincides with reduced histone H3 trimethylation at Ink4a/Arf, and increased levels of p16(INK4a) and p19(Arf). Conditional deletion of beta-cell Ezh2 in juvenile mice also reduced H3 trimethylation at the Ink4a/Arf locus, leading to precocious increases of p16(INK4a) and p19(Arf). These mutant mice had reduced beta-cell proliferation and mass, hypoinsulinemia, and mild diabetes, phenotypes rescued by germline deletion of Ink4a/Arf. beta-Cell destruction with streptozotocin in controls led to increased Ezh2 expression that accompanied adaptive beta-cell proliferation and re-establishment of beta-cell mass; in contrast, mutant mice treated similarly failed to regenerate beta cells, resulting in lethal diabetes. Our discovery of Ezh2-dependent beta-cell proliferation revealed unique epigenetic mechanisms underlying normal beta-cell expansion and beta-cell regenerative failure in diabetes pathogenesis.

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.

Analysis of FTO gene variants with measures of obesity and glucose homeostasis in the IRAS Family Study

Multiple studies have identified FTO gene variants associated with measures of adiposity in European-derived populations. The objective of the study was to determine whether FTO variants were associated with adiposity, including visceral and subcutaneous adipose tissue (VAT, SAT), and glucose homeostasis measures in the Insulin Resistance Atherosclerosis Family Study (IRASFS). A total of 27 SNPs in FTO intron 1, including SNPs prominent in the literature (rs9939609, rs8050136, rs1121980, rs17817449, rs1421085, and rs3751812), were genotyped in 1,424 Hispanic Americans and 604 African Americans. Multiple SNPs were associated with BMI and SAT (P values ranging from 0.001 to 0.033), and trending or associated with waist circumference (P values ranging from 0.008 to 0.099) in the Hispanic Americans. No association was observed with VAT, illustrating that FTO variants are associated with overall fat mass instead of specific fat depots. For the glucose homeostasis measures, variants were associated with fasting insulin but, consistent with other studies, after BMI adjustment, no evidence of association remained. The lack of association of FTO SNPs with insulin sensitivity is consistent with the lack of association with VAT, since these traits are strongly correlated. In the African Americans, only rs8050136 and rs9939609 were associated with BMI and WAIST (P values of 0.011 and 0.034), and associated or trending towards association with SAT (P values of 0.038 and 0.058). These results confirm that FTO variants are associated with adiposity measures, predisposing individuals to obesity by increasing overall fat mass in Hispanic Americans and to a lesser degree in African Americans.

FTO gene variation and measures of body mass in an African population

BACKGROUND

Variation in the fat mass and obesity associated (FTO) gene has been reproducibly associated with body mass index (BMI) and obesity in populations of White European origin. Data from Asians and African-Americans is less conclusive.

METHODS

We assessed the effect of 16 FTO polymorphisms on body mass in a large population of predominantly lean Gambians (N(max) 2208) participating in a long-term surveillance program providing contemporary and early-life anthropometric measurements.

RESULTS

Sixteen FTO tagSNPs screened here, including several associated with BMI in Europeans, were not associated with birth weight (BWT), early weight gain in 1-2 year olds, BMI in adults (> or = 18 y), or weight-for-height (WFH) z-score across all ages. No association was seen between genotype and WFH z-score or other measures of body mass. The confidence limits indicate that the effect size for WFH z-score never exceeded 0.17 units per allele copy for any SNP (excluding the three SNPs with allele < 15%). with much the lowest allele frequency. The confidence interval of the effect size for rs9939609 did not overlap that reported previously in Europeans.

CONCLUSION

To our knowledge this is the first study of FTO gene variation in a well-characterised African population. Our results suggest that FTO gene variation does not influence measures of body mass in Gambians living a traditional lifestyle, or has a smaller effect than that detected in Europeans. These findings are not directly comparable to results from previous studies in African-Americans due to differences in study design and analysis. It is also possible that any effect of FTO genotype on body mass is of limited relevance in a lean population where little excess food is available, compared to similar ethnic populations where food supply is plentiful.