Christine Powers

A Visfatin Promoter Polymorphism Is Associated with Low-Grade Inflammation and Type 2 Diabetes

Visfatin (also known as pre‐B cell colony‐enhancing factor, or PBEF) is a pro‐inflammatory adipokine expressed predominantly in visceral fat. We investigated whether polymorphisms at the visfatin/PBEF locus influence the risk of type 2 diabetes (T2D). Linkage disequilibrium analysis of 52 single nucleotide polymorphisms spanning the entire gene (34.7 kb) plus 20.5 kb of the upstream region and 25.5 kb of the downstream region revealed a single haplotype block that could be tagged by seven single nucleotide polymorphisms. These seven tags were typed in a group of T2D patients (n = 814) and a group of non‐diabetic controls (n = 320) of white origin. A significant association was observed at −948C>A, with minor allele frequencies of 0.157 in T2D cases and 0.119 in non‐diabetic controls (p = 0.021). In a non‐diabetic population (n = 630), the same −948 allele that conferred increased risk of T2D was significantly associated with higher plasma levels of fibrinogen and C‐reactive protein (p = 0.0022 and 0.0038, respectively). However, no significant associations were observed with BMI, waist circumference, serum glucose levels, or fasting insulin levels. Our findings suggest that the visfatin/PBEF gene may play a role in determining T2D susceptibility, possibly by modulating chronic, low‐grade inflammatory responses.

The TRIB3 Q84R polymorphism and risk of early-onset type 2 diabetes

CONTEXT The prevalence of type 2 diabetes (T2D), particularly among young adults, has been rising steadily during the past 2 decades. T2D, especially in its early-onset subtype, is under genetic control. TRIB3 inhibits insulin-stimulated Akt phosphorylation and subsequent insulin action. A TRIB3 gain-of-function polymorphism, Q84R (rs2295490), impairs insulin signaling. OBJECTIVE The objective of the study was to verify the association of TRIB3 Q84R with: 1) T2D, either subtyped or not according to age at diagnosis (early-onset, <45 yr, or >or= 45 yr); 2) insulin secretion and sensitivity in nondiabetic individuals; or 3) in vitro insulin secretion from isolated human islets. DESIGN Four different case-control samples comprising a total of 5,469 whites were examined. Insulinogenic and insulin sensitivity indexes and their interplay (disposition index) were assessed in 645 nondiabetic individuals at oral glucose tolerance test, glucose (16.7 mmol/liter)-induced in vitro insulin secretion was assessed in islets isolated from 54 nondiabetic donors. RESULTS In the whole sample, the R84 variant was nominally associated with T2D (odds ratio 1.17, 95% confidence interval 1.00-1.36, P = 0.04). When stratifying according to age of diabetes onset, R84 carriers had an increased risk of early-onset T2D (odds ratio 1.32, 95% confidence interval 1.10-1.58, P = 0.002). Among 645 nondiabetic subjects, R84 carriers had higher glucose levels (P = 0.005) and lower insulinogenic (P = 0.03) and disposition index (P = 0.02) during the oral glucose tolerance test. R84 islets were more likely to display relatively low glucose-stimulated insulin release (P = 0.04). CONCLUSIONS The TRIB3 R84 variant is associated with early-onset T2D in whites. Alteration in the insulin secretion/insulin sensitivity interplay appears to underlie this association.

The ENPP1 Q121 Variant Predicts Major Cardiovascular Events in High-Risk Individuals: Evidence for Interaction With Obesity in Diabetic Patients

OBJECTIVE Insulin resistance (IR) and cardiovascular disease may share a common genetic background. We investigated the role of IR-associated ENPP1 K121Q polymorphism (rs1044498) on cardiovascular disease in high-risk individuals. RESEARCH DESIGN AND METHODS A prospective study (average follow-up, 37 months) was conducted for major cardiovascular events (myocardial infarction [MI], stroke, cardiovascular death) from the Gargano Heart Study (GHS; n = 330 with type 2 diabetes and coronary artery disease), the Tor Vergata Atherosclerosis Study (TVAS; n = 141 who had MI), and the Cardiovascular Risk Extended Evaluation in Dialysis (CREED) database (n = 266 with end-stage renal disease). Age at MI was investigated in cross-sectional studies of 339 type 2 diabetic patients (n = 169 from Italy, n = 170 from the U.S.). RESULTS Incidence of cardiovascular events per 100 person--years was 4.2 in GHS, 10.8 in TVAS, and 11.7 in CREED. Hazard ratios (HRs) for KQ+QQ versus individuals carrying the K121/K121 genotype (KK) individuals were 1.47 (95% CI 0.80–2.70) in GHS, 2.31 (95% CI 1.22–4.34) in TVAS, and 1.36 (95% CI 0.88–2.10) in CREED, and 1.56 (95% CI 1.15–2.12) in the three cohorts combined. In the 395 diabetic patients, the Q121 variant predicted cardiovascular events among obese but not among nonobese individuals (HR 5.94 vs. 0.62, P = 0.003 for interaction). A similar synergism was observed in cross-sectional studies, with age at MI being 3 years younger in Q121 carriers than in KK homozygotes among obese but not among nonobese patients (P = 0.035 for interaction). CONCLUSIONS The ENPP1 K121Q polymorphism is an independent predictor of major cardiovascular events in high-risk individuals. In type 2 diabetes, this effect is exacerbated by obesity. Future larger studies are needed to confirm our finding.

IRS1 G972R polymorphism and type 2 diabetes: a paradigm for the difficult ascertainment of the contribution to disease susceptibility of 'low-frequency-low-risk' variants.

Aims/hypothesisThe aim of the study was to determine the association between IRS1 G972R polymorphism and type 2 diabetes; published data concerning this association have been conflicting. To obtain further insight into this topic, we performed a meta-analysis of all available case–control studies.MethodsWe performed a meta-analysis of 32 studies (12,076 cases and 11,285 controls).ResultsThe relatively infrequent R972 variant was not significantly associated with type 2 diabetes (OR 1.09, 95% CI 0.96–1.23, p = 0.184 under a dominant model). Some evidence of heterogeneity was observed across studies (p = 0.1). In the 14 studies (9,713 individuals) in which the mean age at type 2 diabetes diagnosis was available, this variable explained 52% of the heterogeneity (p = 0.03). When these studies were subdivided into tertiles of mean age at diagnosis, the OR for diabetes was 1.48 (95% CI 1.17–1.87), 1.22 (95% CI 0.97–1.53) and 0.88 (95% CI 0.68–1.13) in the youngest, intermediate and oldest tertile, respectively (p = 0.0022 for trend of ORs).Conclusions/interpretationOur findings illustrate the difficulties of ascertaining the contribution of ‘low-frequency–low-risk’ variants to type 2 diabetes susceptibility. In the specific context of the R972 variant, ~200,000 study individuals would be needed to have 80% power to identify a 9% increase in diabetes risk at a genome-wide significance level. Under these circumstances, a strategy aimed at improving outcome definition and decreasing its heterogeneity may critically enhance our ability to detect genetic effects, thereby decreasing the required sample size. Our data suggest that focusing on early-onset diabetes, which is characterised by a stronger genetic background, may be part of such a strategy.

The type 2 diabetes and insulin-resistance locus near IRS1 is a determinant of HDL cholesterol and triglycerides levels among diabetic subjects

OBJECTIVE SNP rs2943641 near the insulin receptor substrate 1 (IRS1) gene has been found to be associated with type 2 diabetes (T2D) and insulin-resistance in genome-wide association studies. We investigated whether this SNP is associated with cardiovascular risk factors and coronary artery disease (CAD) among diabetic individuals. METHODS SNP rs2943641 was typed in 2133 White T2D subjects and tested for association with BMI, serum HDL cholesterol and triglycerides, hypertension history, and CAD risk. RESULTS HDL cholesterol decreased by 1mg/dl (p = 0.004) and serum triglycerides increased by 6 mg/dl (p = 0.016) for each copy of the insulin-resistance allele. Despite these effects, no association was found with increased CAD risk (OR = 1.00, 95% CI 0.88-1.13). CONCLUSIONS The insulin-resistance and T2D locus near the IRS1 gene is a determinant of lower HDL cholesterol among T2D subjects. However, this effect is small and does not translate into a detectable increase in CAD risk in this population.

A polymorphism at the IL6ST (gp130) locus is associated with traits of the metabolic syndrome.

The interleukin 6 signal transducer (IL6ST, also known as gp130) is a ubiquitously expressed intermediate of the interleukin‐6 signaling pathway. We investigated whether genetic variability at the IL6ST locus is involved in the modulation of metabolic traits and the etiology of the metabolic syndrome (MS). Four haplotype‐tagging single nucleotide polymorphisms were typed in two populations of non‐diabetic subjects, one from Northern Italy (Padua (PD), n = 630), the other from Southern Italy (San Giovanni Rotondo (SGR), n = 553). In the PD population, a nominally significant association was observed between fasting glucose and rs715180 (P = 0.02), rs3729960 (P = 0.02), and rs10940495 (P = 0.05), between homeostasis model assessment index (HOMAIR) and rs715180 (P = 0.04), and between triglycerides and rs3729960 (P = 0.03). In the SGR population, high‐density lipoprotein (HDL) levels were associated with rs715180 (P = 0.01), systolic blood pressure and waist circumference with rs3729960 (P = 0.005 and 0.02, respectively). The frequency of rs715180 minor allele carriers progressively decreased from individuals with no MS components to those with three or more components (P for trend = 0.006 in the two populations combined). Compared to major allele homozygotes, minor allele carriers had 40% lower odds of having at least one MS component (Odds ratio = 0.6, 95% confidence interval 0.4–0.8, P = 0.005). These findings point to IL6ST variants as possible determinants of impaired glucose metabolism and other abnormalities of MS.

Examination of PPP1R3B as a candidate gene for the type 2 diabetes and MODY loci on chromosome 8p23

The product of the PPP1R3B gene (GL) is the regulatory subunit of PP1 ‐ a serine/threonine phosphatase involved in the modulation of glycogen synthesis in the liver and skeletal muscle. The PPP1R3B gene is located on chromosome 8p23 in a region that has been linked with type 2 diabetes and maturity‐onset diabetes of the young (MODY). We examined whether sequence variants at the PPP1R3B locus are responsible for the linkage with diabetes observed at this location. RT‐PCR analysis revealed the existence of two alternative promoters. These and the two exons of this gene were sequenced in the probands of 13 Joslin families showing the strongest evidence of linkage at 8p23. A total of 20 variants were observed: two in the 5′ flanking region, one in the intron (9 bp 5′ of exon 2), and 17 in the 3′ UTR. The intronic variant generated a new acceptor splice site, resulting in an alternative splice variant with a longer 5′ UTR. However, neither this nor other variants segregated with diabetes in the 13 ‘linked’ families. Furthermore, allele frequencies were similar in 90 family probands from the Joslin Study and 347 unrelated controls. Thus, genetic variability in the PPP1R3B gene does not appear to contribute to diabetes in our mostly Caucasian families. However, a role cannot be excluded in other populations such as the Japanese, among whom linkage to diabetes is also observed at 8p23 and a non‐synonymous mutation has been detected in the PPP1R3B gene.

Mutations in the SLC30A8 gene are not a major cause of MODY or other forms of early-onset, autosomal dominant type 2 diabetes

Maturity onset diabetes of the young or MODY is a relatively rare type of familial diabetescharacterized by a young age of onset and an autosomal dominant mode of inheritance. Theavailability of large families with multiple affected members has facilitated genetic studies ofthis syndrome, which have led to the identification of six distinct MODY genes [1]. Severalreports suggest that other MODY genes exist in addition to those identified to date. In England,about 15% of MODY cases do not seem to result from mutations in known MODY genes[2]. The proportion of unaccounted MODY is even higher if one uses a broader definition ofthe disease, one that takes into account the fact that MODY is often diagnosed after thetraditional age limit of 25 years [1].The recent identification of common polymorphisms contributing to multifactorial forms oftype 2 diabetes [ 3–6] raises the hypothesis that more severe, but less common mutations in thesame genes may be responsible for unaccounted MODY cases. Proving this hypothesis wouldprovide further support for a role of these genes in diabetes, and would offer useful monogenicmodels to study the mechanisms through which dysfunction of these genes causehyperglycemia. This would be analogous to the case of other genes such as

GRB10 gene and type 2 diabetes in Whites

GRB10 encodes for an inhibitor of insulin receptor signaling [1] and is therefore a candidate for type 2 diabetes (T2D). In a preliminary study, the minor allele (MA) of GRB10 rs4947710 was associated with a reduced T2D risk in Whites from Italy, whereas a trend in the opposite direction, albeit not significant, was observed in Whites from the US [2], making the overall observation uncertain. A different GRB10 single nucleotide polymorphism (SNP) (rs2237457) has been recently associated with T2D among Amish but not in other populations [3], suggesting the possibility of allelic heterogeneity. To further investigate the role of GRB10 variability in modulating susceptibility to T2D in Whites, we genotyped rs4947710 and rs2237457 in a total of 3,433 diabetic cases (1899 males/1534 females; age=61.4±9.2 yrs; BMI=31.5±6.1 Kg/m2) and 2,660 non-diabetic controls (1153 males/1507 females; age=45.5±16.0 yrs; BMI=27.8±6.1 Kg/m2) of European origin included in the “GENetics of T2D in Italy and United States (GENIUS) Consortium”. The clinical characteristics of these subjects have been previously reported in details [4]. All samples, which included those from the previous smaller study (2), were genotyped by TaqMan allelic discrimination assay. The average agreement rate of duplicate samples was >99%. Failure rate of genotyping was <3.0%. Both rs4947710 (MA frequency=0.073) and rs2247457 (MA frequency=0.39) were in Hardy Weinberg equilibrium in controls and cases. Since no genetic heterogeneity across the four centers of recruitment was evident (p values for heterogeneity = 0.11 and 0.16 for rs4947710 and rs2247457, respectively), data were analyzed after pooling the four data sets together and adjusting for center of recruitment, age and sex. No significant association with T2D was observed with either rs4947710 (allelic OR, 95% CI=1.11, 0.92-1.33, adjusted p=0.30) or rs2247457 (OR, 95%CI=0.98, 0.89-1.09, adjusted p=0.73). These results are in agreement with those from the DIAGRAM data set [5], which did not show a significant association between T2D and either rs4947710 (OR, 95% CI =1.08, 0.99-1.17; p=0.1) or rs2237457 (OR, 95% CI=1.06, 0.99-1.13; p=0.1). In conclusion, GRB10 rs2237457 and rs4947710 do not seem to play a significant role in modulating susceptibility for T2D in individuals of European ancestry.