Linkage of type 2 diabetes mellitus and of age at onset to a genetic location on chromosome 10q in Mexican Americans

Genetic influences on type 2 diabetes and metabolic syndrome related quantitative traits in Mauritius

Epidemiological studies report a high prevalence of type 2 diabetes and metabolic syndrome in the island nation of Mauritius. The Mauritius Family Study was initiated to examine heritable factors that contribute to these high rates of prevalence and consists of 400 individuals in 24 large extended multigenerational pedigrees. Anthropometric and biochemical measurements relating to the metabolic syndrome were undertaken in addition to family and lifestyle based information for each individual. Variance components methods were used to determine the heritability of the type 2 diabetes and metabolic syndrome related quantitative traits. The cohort was made up of 218 females (55%) and 182 males with 22% diagnosed with type 2 diabetes and a further 30% having impaired glucose tolerance or impaired fasting glucose. Notably BMI was not significantly increased in those with type 2 diabetes (P= .12), however a significant increase in waist circumference was observed in these groups (P= .02). The heritable proportion of trait variance was substantial and greater than values previously published for hip circumference, LDL and total cholesterol, diastolic and systolic blood pressure and serum creatinine. Height, weight and BMI heritabilities were all in the upper range of those previously reported. The phenotypic characteristics of the Mauritius family cohort are similar to those previously reported in the Mauritian population with a high observed prevalence rate of type 2 diabetes. A high heritability for key type 2 diabetes and metabolic syndrome related phenotypes (range 0.23 to 0.68), suggest the cohort will have utility in identifying genes that influence these quantitative traits.

Tissue-specific alternative splicing of TCF7L2

Common variants in the transcription factor 7-like 2 (TCF7L2) gene have been identified as the strongest genetic risk factors for type 2 diabetes (T2D). However, the mechanisms by which these non-coding variants increase risk for T2D are not well-established. We used 13 expression assays to survey mRNA expression of multiple TCF7L2 splicing forms in up to 380 samples from eight types of human tissue (pancreas, pancreatic islets, colon, liver, monocytes, skeletal muscle, subcutaneous adipose tissue and lymphoblastoid cell lines) and observed a tissue-specific pattern of alternative splicing. We tested whether the expression of TCF7L2 splicing forms was associated with single nucleotide polymorphisms (SNPs), rs7903146 and rs12255372, located within introns 3 and 4 of the gene and most strongly associated with T2D. Expression of two splicing forms was lower in pancreatic islets with increasing counts of T2D-associated alleles of the SNPs: a ubiquitous splicing form (P = 0.018 for rs7903146 and P = 0.020 for rs12255372) and a splicing form found in pancreatic islets, pancreas and colon but not in other tissues tested here (P = 0.009 for rs12255372 and P = 0.053 for rs7903146). Expression of this form in glucose-stimulated pancreatic islets correlated with expression of proinsulin (r(2) = 0.84-0.90, P < 0.00063). In summary, we identified a tissue-specific pattern of alternative splicing of TCF7L2. After adjustment for multiple tests, no association between expression of TCF7L2 in eight types of human tissue samples and T2D-associated genetic variants remained significant. Alternative splicing of TCF7L2 in pancreatic islets warrants future studies. GenBank Accession Numbers: FJ010164-FJ010174.

Distinct genetic regulation of progression of diabetes and renal disease in the Goto-Kakizaki rat

Goto-Kakizaki (GK) rats develop early-onset type 2 diabetes (T2D) symptoms, with signs of diabetic nephropathy becoming apparent with aging. To determine whether T2D and renal disease share similar genetic architecture, we ran a quantitative trait locus (QTL) analysis in the F2 progeny of a GK x Brown Norway (BN) rat cross. Further, to determine whether genetic components change over time, we ran the QTL analysis on phenotypes collected longitudinally, at 3, 6, 9 and 12 mo, from the same animals. We confirmed three chromosomal regions that are linked to early diabetes phenotypes (chromosomes 1, 5, and 10) and a single region involved in the late progression of the disorder (chromosome 4). A single region was identified for the onset of the renal phenotype proteinuria (chromosome 5). This region overlaps the diabetic QTL, although it is not certain whether similar genes are involved in both phenotypes. A second QTL linked to the progression of the renal phenotype was found on chromosome 7. Linkage for triglyceride and cholesterol levels were also identified (chromosomes 7 and 8, respectively). These results demonstrate that, in general, different genetic components control diabetic and renal phenotypes in a diabetic nephropathy model. Furthermore, these results demonstrate that, over time, different genetic components are involved in progression of disease from those that were involved in disease onset. This observation would suggest that clinical studies collecting participants over a wide age distribution may be diluting genetic effects and reducing power to detect true effects.

A genomics study of type 2 diabetes mellitus in U.S. Air Force personnel

The military community is at high risk for type 2 diabetes (T2D), especially as it relates to military beneficiaries, although preventive measures can be implemented to reduce disease onset. This study evaluates the prevalence of risk-associated single nucleotide polymorphisms in patients diagnosed with T2D within active duty, retired military, and military-dependent populations on Lackland Air Force Base compared to nondiabetic controls. Results will be used as a basis of comparison to analyze risk-conferring genotypes in the young, healthy active duty population to generate the prevalence of T2D risk-associated factors in our current and future war fighters. Identifying genetic markers of T2D prior to abnormal glucose control and insulin resistance may ultimately adjust future risk through early detection, healthy lifestyle modifications, and disease management programs.

Genome-wide linkage scan for genes influencing plasma triglyceride levels in the Veterans Administration Genetic Epidemiology Study

OBJECTIVE

Elevated plasma triglyceride concentration is a component of the insulin resistance syndrome and is commonly associated with type 2 diabetes, obesity, and coronary heart disease. The goal of our study was to perform a genome-wide linkage scan to identify genetic regions that influence variation in plasma triglyceride levels in families that are enriched with individuals with type 2 diabetes.

RESEARCH DESIGN AND METHODS

We used phenotypic and genotypic data from 1,026 individuals distributed across 294 Mexican-American families, who were ascertained for type 2 diabetes, from the Veterans Administration Genetic Epidemiology Study (VAGES). Plasma triglyceride values were transformed, and a variance-components technique was used to conduct multipoint linkage analysis.

RESULTS

After adjusting for the significant effects of sex and BMI, heritability for plasma triglycerides was estimated as 46 +/- 7% (P < 0.0001). Multipoint linkage analysis yielded the strongest evidence for linkage of plasma triglycerides near marker D12S391 on chromosome 12p (logarithm of odds [LOD] = 2.4). Our linkage signal on chromosome 12p provides independent replication of a similar finding in another Mexican-American sample from the San Antonio Family Diabetes Study (SAFDS). Combined multipoint linkage analysis of the VAGES and SAFDS data yielded significant evidence for linkage of plasma triglycerides to a genetic location between markers GATA49D12 and D12S391 on 12p (LOD = 3.8, empirical P value = 2.0 x 10(-5)). This region on 12p harbors the gene-encoding adiponectin receptor 2 (AdipoR2), where we previously have shown that multiple single nucleotide polymorphisms are associated with plasma triglyceride concentrations in the SAFDS. In the present study, we provided suggestive evidence in favor of association for rs929434 with triglyceride concentrations in the VAGES.

CONCLUSIONS

Collectively, these results provide strong evidence for a major locus on chromosome 12p that influences plasma triglyceride levels in Mexican Americans.

Role of Wnt signaling in the development of type 2 diabetes

Type 2 diabetes is characterized by insulin resistance, insulin deficiency, and hyperglycemia. Susceptibility to type 2 diabetes has been linked to Wnt signaling, which plays an important role in intestinal tumorigenesis. Carriers of variants of the transcription factor 7-like 2 gene, an important component of the Wnt pathway, are at enhanced risk for developing type 2 diabetes. The modulation of proglucagon expression by Wnt activity may partially explain the link between Wnt signaling and diabetes, and one of the transcriptional and processing products of the proglucagon gene, the glucagon-like peptide-1 (GLP-1), exhibits a wide variety of antidiabetogenic activities. GLP-1 stimulates Wnt signaling in pancreatic beta cells, enhancing cell proliferation; thus, positive feedback between GLP-1 and Wnt signaling may result in increased proliferation, and suppressed apoptosis, of pancreatic cells. Since beta-cell protection is a potential treatment for type 2 diabetes, stimulation of Wnt activity may represent a valid therapeutic approach.

Impact of TCF7L2 rs7903146 on insulin secretion and action in young and elderly Danish twins

OBJECTIVE

We investigated the regulation and metabolic effects of TCF7L2 gene expression in human sc fat and skeletal muscle and the impact of the TCF7L2, rs7903146, T-allele on gene expression and measures of glucose metabolism including insulin secretion and peripheral and hepatic insulin action.

RESEARCH DESIGN AND METHODS

The rs7903146 was genotyped in 1) a population-based sample of 587 twins (55-64 yr) with glucose tolerance ranging from normal to type 2 diabetes and 2) a population of 196 nondiabetic young (22-31 yr) and elderly (57-66 yr) twins. All subjects underwent oral glucose tolerance tests, and population 2 was additionally examined with iv glucose tolerance tests and hyperinsulinemic, euglycemic clamps.

RESULTS

Elderly T-allele carriers had decreased plasma insulin responses and lower disposition index, whereas insulinogenic index was similar between genotype groups. Elderly nondiabetic T-allele carriers had increased peripheral insulin sensitivity (P = 0.03). Young T-allele carriers had impaired hepatic insulin sensitivity (P = 0.04) independent of plasma insulin levels. TCF7L2 gene expression in skeletal muscle and adipose tissue was not explained by genotype, sex, aerobic capacity, birth, or adult anthropometry and was not associated with in vivo glucose metabolism.

CONCLUSIONS

The rs7903146 T-allele associates with hepatic insulin resistance and diminished glucose-stimulated plasma insulin secretion. Our study does not provide evidence of a role of TCF7L2 gene expression in sc fat tissue and muscle tissue in the regulation of glucose homeostasis. This suggests that the primary defect of rs7903146 T-allele carriers is impairment of insulin secretion rather than a defect in insulin action in peripheral tissues.

CHOP T/C and C/T haplotypes contribute to early-onset type 2 diabetes in Italians

Type 2 diabetes (T2D) is characterized by impaired insulin secretion, insulin insensitivity and decreased beta-cell mass. Multiple genes contribute to T2D. The chromosome 12q13.1 region is in linkage to T2D in different populations, including our Italian dataset. CHOP is a candidate gene for the linkage, as it is located in the chromosome 12q13.1 region, and may contribute to T2D by increasing beta-cell apoptosis susceptibility and by impairing insulin sensitivity. Our goal was to identify any potential CHOP gene variants contributing to T2D in our Italian early-onset T2D families, which show linkage to the CHOP region. We directly sequenced the CHOP gene in 28 Italian probands of the linked T2D families and in 115 control subjects. We performed genotype and haplotype association tests with T2D of the identified single nucleotide polymorphisms (SNPs). We performed model-free and parametric association haplotype tests with T2D. We identified three SNPs [5'UTR-c.279T > C, 5'UTR-c.120A > G and + nt30C > T (F10F)] in CHOP. These SNPs are in complete linkage disequilibrium. The genotype association test showed an association trend with T2D of TT (F10F) and AG (-c.120A > G). The haplotype association test provided significant results for the haplotypes T/C (frequency = 0.33) and C/T (frequency = 0.01) (at 5'UTR-c.279T > C and + nt30C > T, respectively) under non-parametric analysis (P-value = 0.0000), recessive model (P-value = 0.0000) and additive model (P-value = 0.0014). Our data show that CHOP described haplotypes T/C and C/T, as an additive and as a homozygous variant, contribute significantly to T2D in our Italian early-onset group. We conclude that the CHOP T/C and C/T haplotype contributes to our T2D linkage signal on chromosome 12q13.1.

Evaluation of the association between retinal binding protein 4 polymorphisms and type 2 diabetes in Chinese by DHPLC

Serum retinal binding protein 4 (RBP4) was recently described as a new liver- and adipocyte-derived signal that may contribute to Type 2 diabetes mellitus (T2DM) and insulin resistance. The aim of this study was to test whether the RBP4 gene could be used as a genetic marker to predict the development of T2DM amongst the Chinese population of Han. For this study, a normal control group of 115 healthy subjects and an experimental group of 107 patients with T2DM were examined. A combined method of denaturing high-performance liquid chromatography (DHPLC) and sequencing was applied to the detection of the RBP4 gene variants. Two SNPs, rs17484721 and rs36035572, were analyzed. Phenotypes and biochemical indicators related to the metabolism of glucose and lipid were measured. We found that there are significant differences between the control group and the patients group in terms of their respective distributions of genotype and allele frequency. The TG levels of the TT and II genotype was significantly higher than that of the TC + CC and ID + DD, respectively, in both patient group and control group. These findings suggest that the variations in the RBP4 gene may be associated with T2DM and serum triglyeride levels in the Han Chinese.

Human QTL linkage mapping

Human quantitative trait locus (QTL) linkage mapping, although based on classical statistical genetic methods that have been around for many years, has been employed for genome-wide screening for only the last 10-15 years. In this time, there have been many success stories, ranging from QTLs that have been replicated in independent studies to those for which one or more genes underlying the linkage peak have been identified to a few with specific functional variants that have been confirmed in in vitro laboratory assays. Despite these successes, there is a general perception that linkage approaches do not work for complex traits, possibly because many human QTL linkage studies have been limited in sample size and have not employed the family configurations that maximize the power to detect linkage. We predict that human QTL linkage studies will continue to be productive for the next several years, particularly in combination with RNA expression level traits that are showing evidence of regulatory QTLs of large effect sizes and in combination with high-density genome-wide SNP panels. These SNP panels are being used to identify QTLs previously localized by linkage and linkage results are being used to place informative priors on genome-wide association studies.

The Wnt signaling pathway effector TCF7L2 and type 2 diabetes mellitus

Since the relationship between TCF7L2 (also known as TCF-4) polymorphisms and type 2 diabetes mellitus was identified in 2006, extensive genome-wide association examinations in different ethnic groups have further confirmed this relationship. As a component of the bipartite transcription factor beta-catenin/TCF, TCF7L2 is important in conveying Wnt signaling during embryonic development and in regulating gene expression during adulthood. Although we still do not know mechanistically how the polymorphisms within the intron regions of TCF7L2 affect the risk of type 2 diabetes, this transcriptional regulator was shown to be involved in stimulating the proliferation of pancreatic beta-cells and the production of the incretin hormone glucagon-like peptide-1 in intestinal endocrine L cells. In this review, we introduce background knowledge of TCF7L2 as a component of the Wnt signaling pathway, summarize recent findings demonstrating the association between TCF7L2 polymorphisms and the risk of type 2 diabetes, outline experimental evidence of the potential function of TCF7L2 in pancreatic and intestinal endocrine cells, and present our perspective views.

The G/G genotype of a single nucleotide polymorphism at -1066 of c-Jun N-terminal kinase 1 gene (MAPK8) does not affect type 2 diabetes susceptibility despite the specific binding of AP2alpha

OBJECTIVE

The c-Jun N-terminal kinase 1 (JNK1, mitogen-activated kinase 8; MAPK8) phosphorylates insulin receptor substrate-1 (IRS-1) at serine 307, which induces insulin resistance. MAPK8 activity is increased in obese insulin-resistant mice, whereas mapk8 (-/-) mice show decreased adiposity and improved insulin sensitivity. The aim of this study was to determine the relationship between single nucleotide polymorphisms (SNPs) of MAPK8 and type 2 diabetes (T2DM).

DESIGN, PATIENTS AND MEASUREMENTS

Approximately 2 kb of 5' flanking and the coding regions were initially sequenced in 24 Japanese T2DM subjects. Identified SNPs were genotyped in 204 T2DM cases and 201 nondiabetic controls. The function of promoter SNP-1066 (g.-1066G > A, rs10857561) was analysed by electrophoretic mobility shift assay (EMSA) and luciferase assay. SNP-1066 was further genotyped in a total of 498 cases and 407 controls, and in 2075 subjects in the general population.

RESULTS

In 204 cases and 201 controls, 11 identified SNPs were not associated with T2DM. These SNPs were in the same linkage disequilibrium (LD) block. The tag SNP-1066 was not associated with T2DM in a total of 498 cases and 407 controls with the power > 80% when the relative risk is > 1.31. Functionally, transcription factor AP2alpha specifically recognized G but not A at -1066. MAPK8 promoter activity was unchanged between G and A. In 2075 subjects, neither body mass index (BMI), fasting plasma glucose (FPG), homeostasis model assessment insulin resistance index (HOMA-IR), nor beta cell function index (HOMA-beta) was associated with SNP-1066.

CONCLUSIONS

The G/G genotype of MAPK8 SNP-1066 did not affect T2DM susceptibility despite specific binding of AP2alpha.

Effect of acute physiological hyperinsulinemia on gene expression in human skeletal muscle in vivo

This study was undertaken to test the hypothesis that short-term exposure (4 h) to physiological hyperinsulinemia in normal, healthy subjects without a family history of diabetes would induce a low grade inflammatory response independently of glycemic status. Twelve normal glucose tolerant subjects received a 4-h euglycemic hyperinsulinemic clamp with biopsies of the vastus lateralis muscle. Microarray analysis identified 121 probe sets that were significantly altered in response to physiological hyperinsulinemia while maintaining euglycemia. In normal, healthy human subjects insulin increased the mRNAs of a number of inflammatory genes (CCL2, CXCL2 and THBD) and transcription factors (ATF3, BHLHB2, HES1, KLF10, JUNB, FOS, and FOSB). A number of other genes were upregulated in response to insulin, including RRAD, MT, and SGK. CITED2, a known coactivator of PPARalpha, was significantly downregulated. SGK and CITED2 are located at chromosome 6q23, where we previously detected strong linkage to fasting plasma insulin concentrations. We independently validated the mRNA expression changes in an additional five subjects and closely paralleled the results observed in the original 12 subjects. A saline infusion in healthy, normal glucose-tolerant subjects without family history of diabetes demonstrated that the genes altered during the euglycemic hyperinsulinemic clamp were due to hyperinsulinemia and were unrelated to the biopsy procedure per se. The results of the present study demonstrate that insulin acutely regulates the levels of mRNAs involved in inflammation and transcription and identifies several candidate genes, including HES1 and BHLHB2, for further investigation.

Mutations in the beta1 adrenergic receptor gene and massive obesity in Japanese

Catecholamines strongly promote lipolysis and thermogenesis, and play a central role in the regulation of body fat content. The beta1 adrenergic receptor (BAR-1) is a major mediator of catecholamine-induced lipolysis and thermogenesis. To explore whether mutations in the BAR-1 gene contribute to morbid obesity in Japanese, we scanned for mutations in the coding sequence of the gene in 50 morbid obese [body mass index (BMI)>==35.0kg/m(2); 99.7th percentile] Japanese subjects. Direct DNA sequencing was performed following polymerase chain reaction (PCR) amplification. Two common polymorphisms, Gly49Arg and Arg389Ser, were detected in these subjects. The frequencies of these polymorphisms, as determined by PCR-restriction fragment length polymorphism (RFLP) analysis, showed no significant difference between 180 severely obese subjects (BMI>==30.0kg/m(2); 97th percentile) and 132 control (BMI<25.0kg/m(2)) subjects. This study represents the first investigations of genetic variations of BAR-1 in relationship to morbid obesity and suggests mutations in the BAR-1 coding sequence is not likely a major cause of morbid obesity at least in Japanese.

Association of TCF7L2 gene polymorphisms with reduced acute insulin response in Hispanic Americans

CONTEXT

Genetic variation at the transcription factor 7-like 2 locus has been linked to type 2 diabetes in predominantly European-derived populations. The biological basis of these associations remains to be determined.

OBJECTIVE

The objective of this study was to evaluate previously associated variants for association with measures of glucose homeostasis in Hispanic-Americans and African-Americans and determine the biological mechanism(s) through which these variants exert their effect.

DESIGN

This study was the Insulin Resistance Atherosclerosis Family Study (IRAS-FS).

SETTING

The IRAS-FS is a community-based study of Hispanic-Americans (San Antonio, TX, and San Luis Valley, CO) and African-Americans (Los Angeles, CA).

PARTICIPANTS

A total of 1040 Hispanic-American and 500 African-American individuals from the IRAS-FS formed the basis of this study. MAIN OUTCOMES MEASURES(S): The primary glucose homeostasis phenotypes of interest in this study were derived from the frequently sampled iv glucose tolerance test and include insulin sensitivity, acute insulin response, and disposition index.

RESULTS

In Hispanic-Americans, significant evidence of association was observed between single-nucleotide polymorphisms rs7903146 and rs112255372 with reduced insulin secretion as measured by acute insulin response and adjusted for the degree of insulin sensitivity (P = 0.032 and 0.036, respectively). Other quantitative measures, e.g. insulin sensitivity or disposition index, were not associated with the single nucleotide polymorphisms examined. In African-Americans there was no evidence of association observed.

CONCLUSIONS

These results suggest that transcription factor 7-like 2 variants could play a role in the pathogenesis of type 2 diabetes in the Hispanic-American population through a mechanism involving insulin secretion.

Effect of RBP4 gene variants on circulating RBP4 concentration and type 2 diabetes in a Chinese population

AIMS

Retinol binding protein 4 (RBP4) is a newly discovered adipokine, which plays a role in insulin resistance and obesity. The aim of this study was to determine the relationship between genetic variants of the RBP4 gene, circulating RBP4 concentrations and phenotypes related to glucose and lipid metabolism in the Chinese population.

METHODS

We sequenced exons and the putative promoter region to identify single nucleotide polymorphisms (SNPs) in the RBP4 gene in 32 Chinese subjects. Additional SNPs were selected from a public database to increase marker density. Taking account of the pairwise linkage disequilibrium and minor allele frequencies, a subset of SNPs was further genotyped in 255 Type 2 diabetic patients and 372 normal control subjects. Circulating RBP4 concentrations and phenotypes related to glucose and lipid metabolism were measured.

RESULTS

Ten SNPs were identified and five were further genotyped in the full sample. No individual SNP was significantly associated with Type 2 diabetes, but a rare haplotype CAA formed by +5388 C>T, +8201 T>A and +8204 T>A was more frequent in diabetic patients (P = 0.0343, empirical P = 0.0659 on 10 000 permutations). In both groups, non-coding SNPs were associated with circulating RBP4 concentrations (P < 0.05). In the normal control subjects, the SNP +5388 C>T was associated with serum C-peptide levels both fasting and 2 h after an oral glucose tolerance test (P = 0.0162 and P = 0.0075, respectively).

CONCLUSION

Our findings suggest that genetic variants in the RBP4 gene may be associated with circulating RBP4 concentration and phenotypes related to glucose metabolism.

Genetic contributions to preterm birth: implications from epidemiological and genetic association studies

Infants born before term (<37 weeks) have an increased risk of neonatal mortality as well as other health problems. The increasing rate of preterm birth in recent decades, despite improvements in health care, creates an impetus to better understand and prevent this disorder. Preterm birth likely depends on a number of interacting factors, including genetic, epigenetic, and environmental risk factors. Genetic studies may identify markers, which more accurately predict preterm birth than currently known risk factors, or novel proteins and/or pathways involved in the disorder. This review summarizes epidemiological and genetic studies to date, emphasizing the complexity of genetic influences on birth timing. While several candidate genes have been reportedly associated with the disorder, inconsistency across studies has been problematic. More systematic and unbiased genetic approaches are needed for future studies to examine the genetic etiology of human birth timing thoroughly.

Contemporary model-free methods for linkage analysis

Model-free linkage methods are based on identifying regions of the genome in which patterns of allele sharing among family members correspond to patterns of phenotype correlation among family members. Two general classes of model-free linkage methods are discussed in this chapter, relative pair methods designed primarily for analysis of discrete traits and variance component methods designed primarily for analysis of quantitative traits. These methods have been used to identify numerous genes influencing complex human phenotypes and remain viable approaches to gene localization in the twenty-first century.

Electrophoretic techniques applied to the detection and analysis of the human microsatellite DG10s478

Short nucleotide repetitions (STRs) are commonly used as genetic markers; thus their detection and analysis constitutes a very important tool for the mapping of genetic diseases, as well as for gathering information about genetic polymorphisms at the population level. STRs can be detected with agarose- or acrylamide-based electrophoretic techniques, followed by visualization of the DNA sample with ethidium bromide, silver nitrate, or fluorophore labeling. In this work, we analyzed genomic DNA from five individuals affected with type II diabetes mellitus (T2DM) and five controls (unaffected individuals) in order to know the most precise and reproducible technique for the analysis of the existing polymorphism in the STR DG10S478 of the TCF7L2 gene. The combination of PCR with labeling of the products with the CY5 fluorophore, followed by detection on an ALFexpress sequencer, offered the required resolution to detect the variability in this STR, based solely on size analysis. Our methodology offers similar accuracy and reproducibility at lower costs than existing methods based on the sequencing of PCR products, and is a faster alternative when applied to genotyping studies.

Effects of genetic variation in the human retinol binding protein-4 gene (RBP4) on insulin resistance and fat depot-specific mRNA expression

OBJECTIVE

Serum retinol binding protein 4 (RBP4) is a new liver- and adipocyte-derived signal that may contribute to insulin resistance. Therefore, the RBP4 gene represents a plausible candidate gene involved in susceptibility to type 2 diabetes.

RESEARCH DESIGN AND METHODS

In this study, the RBP4 gene was sequenced in DNA samples from 48 nonrelated Caucasian subjects. Five novel and three known single nucleotide polymorphisms (SNPs) were identified. Furthermore, five recently reported SNPs were genotyped in 90 subjects. Six SNPs, representative of their linkage disequilibrium groups, were then genotyped in 934 diabetic and 716 nondiabetic subjects.

RESULTS

A haplotype of six common SNPs (A-G-G-T-G-C) was significantly increased in 934 case subjects with type 2 diabetes compared with 537 healthy control subjects with normal glucose tolerance (P = 0.02; odds ratio 1.37 [95% CI 1.05-1.79]). Furthermore, in the cohort of 716 nondiabetic Caucasian subjects, carriers of the A-G-G-T-G-C haplotype had significantly higher mean fasting plasma insulin and 2-h plasma glucose than subjects without the haplotype. Two single SNPs (rs10882283 and rs10882273) were also associated with BMI, waist-to-hip ratio, and fasting plasma insulin, and several SNPs were associated with circulating free fatty acids (all adjusted P < 0.05). In addition, subjects carrying a previously reported diabetes-associated haplotype had significantly higher mRNA levels in visceral adipose tissue (adjusted P < 0.05) in a subgroup of nondiabetic subjects (n = 170) with measurements of RBP4 mRNA expression in visceral and subcutaneous fat depots.

CONCLUSIONS

Our data indicate a role of RBP4 genetic variation in susceptibility to type 2 diabetes and insulin resistance, possibly through an effect on RBP4 expression.

Genome-wide search for susceptibility genes to type 2 diabetes in West Africans: potential role of C-peptide

C-peptide is a substance that the pancreas releases into the circulation in equimolar amounts to insulin and has demonstrated important physiological effects which relate to the vascular field, in particular the microcirculation. For this analysis, we included 321 full and 36 half sibling pairs affected with type 2 diabetes (T2D) from West Africa. A genome-wide panel of 390 tri-nucleotide and tetra-nucleotide repeats with an average distance of 8.9 cM was performed on a total of 691 persons. Variance components based on multipoint linkage approach as implemented in SOLAR were performed for log C-peptide. Significant linkage evidences were observed on 10q23 at D10S2327 with a LOD score of 4.04 (nominal p-value=0.000008, empirical p-value=0.0004); and on 4p15 at D4S2632 with a LOD score of 3.48 (nominal p-value=0.000031, empirical p-value=0.0013). Other suggestive evidence of linkage were observed on 15q14 at D15S659 with a LOD score 2.41 (nominal p-value=0.000435, empirical p-value=0.0068), and on 18p11 near D18S976 with a LOD score 2.18 (nominal p-value=0.000771 and empirical p-value=0.0094). Interestingly, five positional candidate genes for diabetes and related complications are located in our linkage region (the pituitary adenylate cyclase activating polypeptide (PACAP in 18p11); the peroxisome proliferator-activated receptor gamma coactivator 1 (PPARGC1 in 4p15); PTEN, PPP1R5, and IDE located in 10q23. In conclusion, we identified four major genetic loci (10q23, 4p15, 15q14, and 18p11) influencing C-peptide concentration in West Africans with T2D.

Genotype by diabetes interaction effects on the detection of linkage of glomerular filtration rate to a region on chromosome 2q in Mexican Americans

OBJECTIVE

Glomerular filtration rate (GFR) is used to assess the progression of renal disease. We performed linkage analysis to localize genes that influence GFR using estimated GFR data from the San Antonio Family Diabetes/Gallbladder Study. We also examined the effect of genotype by diabetes interaction (G x DM) on the detection of linkage to address whether genetic effects on GFR differ in diabetic and nondiabetic subjects.

RESEARCH DESIGN AND METHODS

GFR (N = 453) was estimated using the recently recalculated Cockcroft-Gault (GFR-CGc) and the simplified Modification of Diet in Renal Disease (GFR-4VMDRD) formulae. Both estimates of GFR exhibited significant heritabilities, but only GFR-CGc showed significant G x DM interaction. We therefore performed multipoint linkage analyses on both GFR measures using models that did not include G x DM interaction effects (Model 1) and that included G x DM interaction effects (Model 2, in the case of GFR-CGc).

RESULTS

The strongest evidence for linkage (Model 1) of both GFR-CGc (logarithm of odds [LOD] 2.9) and GFR-4VMDRD (LOD 2.6) occurred between markers D9S922 and D9S1120 on chromosome 9q. However, using Model 2, the strongest evidence for linkage of GFR-CGc on chromosome 2q was found near marker D2S427 (corrected LOD score [LOD(C)] 3.3) compared with the LOD score of 2.7 based on Model 1. Potential linkages (LOD or LOD(C) >or=1.2) were found only for GFR-CGc on chromosomes 3p, 3q, 4p, 8q, 11q, and 14q.

CONCLUSIONS

We found a major locus on chromosome 2q that differentially influences GFR in diabetic and nondiabetic environments in the Mexican-American population.

Association of genetic variants of insulin degrading enzyme with metabolic features in women with polycystic ovary syndrome

OBJECTIVE

To evaluate the influence of the four single nucleotide polymorphisms of the insulin-degrading enzyme (IDE) gene on metabolic features in women with polycystic ovary syndrome (PCOS) in a Chinese population.

DESIGN

Prospective, case-control study.

SETTING

University-based hospital.

PATIENT(S)

Three hundred fifteen patients with PCOS and 327 healthy controls.

INTERVENTION(S)

Peripheral venous puncture, ultrasonography, oral glucose tolerance test (OGTT).

MAIN OUTCOME MEASURE(S)

Genotype analysis of four single nucleotide polymorphisms in the IDE gene, hormonal and metabolic phenotypes.

RESULT(S)

No significant differences in genotypes of these polymorphisms were found between PCOS patients and healthy controls. But the frequency of the C allele of rs2209972 was significantly higher in the PCOS group than that in the control group. The single nucleotide polymorphisms rs4646953, rs1887922, and rs1544210 had no impact on clinical and biochemical characteristics of women with PCOS. There were significant differences in body mass index (BMI) and insulin level in the rs2209972 genotype of women with PCOS. The women with PCOS with the CC genotype of rs2209972 had statistically significantly higher fasting insulin level and homeostasis model assessment for insulin resistance than the women with PCOS with the TT genotype.

CONCLUSION(S)

The single nucleotide polymorphism rs2209972 in the human IDE gene is associated with metabolic features of PCOS women in a Chinese population.

Retinol-binding protein 4 as a plasma biomarker of renal dysfunction and cardiovascular disease in type 2 diabetes

OBJECTIVE

The retinol-binding protein 4 (RBP4) has been linked to the insulin resistance state in obesity and type 2 diabetes in animal studies. Data in humans are controversial and their relationship with organ damage in diabetic patients is lacking. We studied the association of plasma RBP4 with organ complications in type 2 diabetic patients.

SETTING

Sant Joan University Hospital, Reus, Spain.

SUBJECTS

165 nonsmoker type 2 diabetic subjects according to American Diabetes Association criteria, aged 36-79 years, without proteinuria or severely decreased glomerular filtration rates (MDRD-GFR <30 mL min(-1) 1.73 m(-2)), were included in the study.

MAIN OUTCOME MEASURE

Plasma RBP4 concentrations were the primary outcome variable. Statistics were performed in relation to clinical and subclinical arteriosclerosis, renal function parameters and biochemical data.

RESULTS

Plasma RBP4 concentrations were positively correlated with serum creatinine levels (r = 0.322, P < 0.001) and inversely correlated with MDRD-GFR (r = -0.468, P = 0.009). Patients with moderately renal dysfunction (MDRD-GFR <60 mL min(-1) 1.73 m(-2)) had higher plasma RBP4 concentrations than those with normal to mildly decreased GFR (55.3 +/- 24.6 vs. 40.8 +/- 15.4, P <0.001). Patients in the top quartile of RBP4 concentrations had an increased adjusted odds ratio for moderately renal dysfunction compared with lower quartiles (4.68; 95% CI: 1.52-14.36, P = 0.007). The presence of microalbuminuria was not associated with RBP4. Plasma RBP4 concentrations were higher in those subjects with previous clinical arteriosclerosis than in event-free subjects (48.8 +/- 24.2 vs. 40.6 +/- 13.9, P = 0.045). The presence of retinopathy or polyneuropathy did not differ across RBP4 quartiles.

CONCLUSIONS

Plasma RBP4 concentration might be a biomarker of nephropathy and cardiovascular disease in type 2 diabetic subjects.

MGEA5-14 polymorphism and type 2 diabetes in Mexico City

A family-based study has recently reported that a variant located in intron 10 of the gene MGEA5 increases susceptibility to Type 2 Diabetes (T2D). We evaluated the distribution of this SNP in a sample of T2D patients (N = 271) and controls (N = 244) from Mexico City. The frequency of the T allele was higher in the cases (2.6%) than in the controls (1.8%). After adjusting for age, sex, BMI, education, and individual ancestry the odds ratio was 1.60 but the 95% confidence interval was wide and overlapped 1 (0.52-4.86, P-value : 0.404). In order to characterize the distribution of the MGEA5-14 polymorphism in the relevant parental populations, we genotyped this variant in European (and European Americans), West African, and Native American samples. The T-allele was present at a frequency of 2.3% in Spain, 4.2% in European Americans, and 13% in Western Africans, but was absent in two Native American samples from Mexico and Peru. Given the low frequency of the T-allele, further studies using large sample sizes will be required to confirm the role of this variant in T2D.

SORCS1: a novel human type 2 diabetes susceptibility gene suggested by the mouse

OBJECTIVE

A small number of susceptibility genes for human type 2 diabetes have been identified by candidate gene analysis or positional cloning. Genes found to influence diabetes or related traits in mice are likely to be susceptibility genes in humans. SorCS1 is the gene identified as responsible for the mouse chromosome 19 T2dm2 quantitative trait locus for fasting insulin levels, acting via impaired insulin secretion and increased islet disruption in obese females. Genes that impair compensatory insulin secretion in response to obesity-induced insulin resistance may be particularly relevant to human diabetes. Thus, we sought to determine whether variation in the human SORCS1 gene was associated with diabetes-related traits.

RESEARCH DESIGN AND METHODS

We assessed the contribution of variation in SORCS1 to human insulin-related traits in two distinct Mexican-American cohorts. One cohort (the Mexican-American Coronary Artery Disease [MACAD] cohort) consisted of nondiabetic individuals, allowing assessment of genetic association with subclinical intermediate insulin-related traits; the second cohort (the San Antonio Family Diabetes Study [SAFADS]) contained individuals with diabetes, allowing association analyses with overt disease.

RESULTS

We first found association of SORCS1 single nucleotide polymorphisms and haplotypes with fasting insulin levels and insulin secretion in the MACAD cohort. Similar to our results in the mice, the genetic association was strongest in overweight women. We then observed association with diabetes risk and age at diagnosis in women of the SAFADS cohort.

CONCLUSIONS

Identification of SORCS1 as a novel gene affecting insulin secretion and diabetes risk is likely to provide important insight into the biology of obesity-induced type 2 diabetes.

Association of TCF7L2 polymorphisms with type 2 diabetes in Mexico City

Polymorphisms within the transcription factor 7-like 2 gene (TCF7L2) have been associated with type 2 diabetes (T2D) in several recent studies. We characterized three of these polymorphisms (rs12255372, rs7903146 and the microsatellite DG10S478) in an admixed sample of 286 patients with T2D and 275 controls from Mexico City. We also analyzed three samples representative of the relevant parental populations: Native Americans from the state of Guerrero (Mexico), Spanish from Valencia and Nigerians (Bini from the Edo region). In order to minimize potential confounding because of the presence of population stratification in the sample, we evaluated the association of the three TCF7L2 polymorphisms with T2D by using the program admixmap to fit a logistic regression model incorporating individual ancestry, sex, age, body mass index and education. The markers rs12255372, rs7903146 and DG10S478 are in tight disequilibrium in the Mexican sample. We observed a significant association between the single-nucleotide polymorphism (SNP) rs12255372 and the microsatellite DG10S478 with T2D in the Mexican sample [rs12255372, odds ratio (OR) = 1.78, p = 0.017; DG10S478, OR = 1.62, p = 0.041]. The SNP rs7903146 shows similar trends, but its association with T2D is not as strong (OR = 1.39, p = 0.152). Analysis of the parental samples, as well as other available data, indicates that there are substantial population frequency differences for these polymorphisms: The frequencies of the T2D risk factors are more than 20% higher in European and West African populations than in East Asian and Native American populations.

Genetic association of single nucleotide polymorphisms in endonuclease G-like 1 gene with type 2 diabetes in a Japanese population

AIMS/HYPOTHESIS

In order to identify type 2 diabetes disease susceptibility gene(s) in a Japanese population, we applied a region-wide case-control association test to the 20.4 Mb region between D3S1293 and D3S2319 on chromosome 3p24.3-22.1, supported by linkage to type 2 diabetes and its related traits in Japanese and multiple populations.

MATERIALS AND METHODS

We performed a two-stage association test using 1,762 Japanese persons with 485 gene-centric, evenly spaced, common single nucleotide polymorphism (SNP) markers with minor allele frequency >0.1. For mouse studies, total RNA was extracted from various organs of BKS.Cg-+Lepr(db)/+Lepr(db) and control mice, and from MIN6, NIH3T3 and C2C12 cell lines.

RESULTS

We detected a landmark SNP375 (A/G) (rs2051211, p = 0.000046, odds ratio = 1.33, 95% CI 1.16-1.53) in intron 5 of the endonuclease G-like 1 (ENDOGL1) gene. Systematic dense SNPs approach identified a susceptibility linkage disequilibrium (LD) block of 116.5 kb by |D'|, an LD units map and a critical region of 2.1 kb by r (2) in ENDOGL1. A haplotype-based association test showed that an at-risk haplotype is associated with disease status (p = 0.00001). The expression of ENDOGL1 was rather ubiquitous with relatively abundant expression in the brain and also in a pancreatic islet beta cell line. Mouse Endogl1 expression increased in pancreatic islets of hyperglycaemic BKS.Cg-+Lepr(db)/+Lepr(db) mice compared with that in control mice.

CONCLUSIONS/INTERPRETATION

Based on the population genetics, fine mapping of LD block and haplotype analysis, we conclude that ENDOGL1 is a candidate disease-susceptibility gene for type 2 diabetes in a Japanese population. Further analysis in a larger sample size is required to substantiate this conclusion.

Replication study for the association of TCF7L2 with susceptibility to type 2 diabetes in a Japanese population

AIMS/HYPOTHESIS

The transcription factor 7-like 2 gene (TCF7L2) has been shown to be strongly associated with an increased risk of type 2 diabetes in white populations. To further investigate the involvement of TCF7L2 in conferring susceptibility to type 2 diabetes, we examined the association of TCF7L2 polymorphisms with type 2 diabetes in a Japanese population.

SUBJECTS AND METHODS

We analysed four SNPs (rs12255372, rs7903146, rs7901695 and rs11196205) and one tetranucleotide repeat polymorphism (DG10S478) in 1,630 Japanese subjects with type 2 diabetes and 1,064 control subjects.

RESULTS

All investigated polymorphisms were significantly associated with type 2 diabetes, and rs12255372 showed the strongest association (T vs G, chi2 = 9.20, p = 0.0024, odds ratio = 1.70, 95% CI = 1.20-2.41), although the frequency of the risk allele in our population was much lower than that in white populations. The microsatellite polymorphism showed an almost complete linkage disequilibrium to rs1255372 when the alleles with longer repeats (+8, +12) were considered as minor alleles and showed an association with type 2 diabetes (chi2 = 5.34, p = 0.021, odds ratio = 1.50, 95% CI = 1.06-2.12).

CONCLUSIONS/INTERPRETATION

These results indicate that TCF7L2 might be a strong candidate for conferring susceptibility to type 2 diabetes across different ethnicities.

Association of neuropeptide Y receptor Y5 polymorphisms with dyslipidemia in Mexican Americans

We examined the genetic association of neuropeptide Y receptor Y5 (NPY5R) single nucleotide polymorphisms (SNPs) with measures of the insulin resistance (metabolic) syndrome. We genotyped 10 NPY5R SNPs in 439 Mexican American individuals (age=43.3+/-17.3 years and BMI=30.0+/-6.7 kg/m2) distributed across 27 pedigrees from the San Antonio Family Diabetes Study and performed association analyses using the measured genotype approach as implemented in Sequential Oligogenic Linkage Analysis Routines (SOLAR). Minor alleles for five (rs11100493, rs12501691, P1, rs11100494, rs12512687) of the NPY5R SNPs were found to be significantly (p<0.05) associated with fasting plasma triglyceride concentrations and decreased high-density lipoprotein concentrations. In addition, the minor allele for SNP P2 was significantly associated (p=0.031) with a decreased homeostasis model assessment of beta-cell function (HOMA-%beta). Linkage disequilibrium between SNP pairs indicated one haplotype block of five SNPs (rs11100493, rs12501691, P1, rs11100494, rs12512687) that were highly correlated (r2>0.98). These preliminary results provide evidence for association of SNPs in the NPY5R gene with dyslipidemia (elevated triglyceride concentrations and reduced high-density lipoprotein levels) in our Mexican American population.

Retinol binding protein 4 as a candidate gene for type 2 diabetes and prediabetic intermediate traits

Serum retinol binding protein 4 (RBP4) was recently described as a new adipokine that reduced peripheral and hepatic insulin sensitivity and increased hepatic gluconeogenesis. The RBP4 gene maps to 10q23-24, near a region linked to T2DM in Caucasian and Mexican American populations. Hence, sequence variants that alter RBP4 expression or function could increase T2DM susceptibility and reduce insulin sensitivity. We screened the 6 exons, flanking intronic sequence, and 5' and 3' flanking sequences in 48 Caucasian and 48 African American subjects. We identified 21 SNPs, of which 8 were unique to the African American population. Additional public database SNPs were chosen for regions not screened. We selected SNPs for typing based on frequency, linkage disequilibrium, and location in a putative functional or conserved region. We typed 10 SNPs in 191 Caucasians with T2DM and a family history of T2DM, and 188 euglycemic controls with no family history of diabetes. We similarly typed 14 variants in 182 controls and 353 diabetic individuals of African American ancestry. No single variant was associated with type 2 diabetes in either population (p>0.15 in African Americans, p>0.09 in Caucasians), but a haplotype of 8 common SNPs in Caucasians was significantly increased in type 2 diabetics compared with controls (0.137 vs. 0.076, p=0.008). Furthermore, SNPs -804 and +9476 were associated with reduced insulin secretion, (p=0.01 and 0.001, respectively), and SNP +390 with reduced insulin sensitivity (p=0.0005) in Caucasians. Our data suggest that noncoding SNPs may increase diabetes susceptibility in Caucasians and may contribute to insulin resistance and reduced insulin secretion.

A genome-wide association study identifies novel risk loci for type 2 diabetes

Type 2 diabetes mellitus results from the interaction of environmental factors with a combination of genetic variants, most of which were hitherto unknown. A systematic search for these variants was recently made possible by the development of high-density arrays that permit the genotyping of hundreds of thousands of polymorphisms. We tested 392,935 single-nucleotide polymorphisms in a French case-control cohort. Markers with the most significant difference in genotype frequencies between cases of type 2 diabetes and controls were fast-tracked for testing in a second cohort. This identified four loci containing variants that confer type 2 diabetes risk, in addition to confirming the known association with the TCF7L2 gene. These loci include a non-synonymous polymorphism in the zinc transporter SLC30A8, which is expressed exclusively in insulin-producing beta-cells, and two linkage disequilibrium blocks that contain genes potentially involved in beta-cell development or function (IDE-KIF11-HHEX and EXT2-ALX4). These associations explain a substantial portion of disease risk and constitute proof of principle for the genome-wide approach to the elucidation of complex genetic traits.

P2 promoter variants of the hepatocyte nuclear factor 4alpha gene are associated with type 2 diabetes in Mexican Americans

Common and rare variants of the hepatocyte nuclear factor 4alpha (HNF4A) gene have been associated with type 2 diabetes and related traits in several populations suggesting the involvement of this transcription factor in diabetes pathogenesis. Single nucleotide polymorphisms (SNPs) within a large haplotype block surrounding the alternate P2 promoter, located approximately 45 kb upstream from the coding region, have been investigated in several populations of varying ethnicity with inconsistent results. Additionally, SNPs located within the P1 promoter and coding region have also been inconsistently associated with type 2 diabetes. Characterization of variation across this gene region in Mexican-American populations has not been reported. We therefore examined polymorphisms across the HNF4A gene in a cohort of Mexican-American pedigrees and assessed their association with type 2 diabetes. We observed evidence for association of SNPs in the P2 promoter region with type 2 diabetes (P = 0.003) and its age at diagnosis (P = 0.003). The risk allele frequency (53%) was intermediate to that reported in Caucasian populations (20-27%) and Pima Indians (83%). No other SNPs were associated with either trait. These results support the possibility that a variant in the P2 promoter region of HNF4A, or variants in linkage disequilibrium within this region, contributes to susceptibility to type 2 diabetes in many ethnic populations including Mexican Americans.

The genetic landscape of type 2 diabetes in mice

Inbred mouse strains provide genetic diversity comparable to that of the human population. Like humans, mice have a wide range of diabetes-related phenotypes. The inbred mouse strains differ in the response of their critical physiological functions, such as insulin sensitivity, insulin secretion, beta-cell proliferation and survival, and fuel partitioning, to diet and obesity. Most of the critical genes underlying these differences have not been identified, although many loci have been mapped. The dramatic improvements in genomic and bioinformatics resources are accelerating the pace of gene discovery. This review describes how mouse genetics can be used to discover diabetes-related genes, summarizes how the mouse strains differ in their diabetes-related phenotypes, and describes several examples of how loci identified in the mouse may directly relate to human diabetes.

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

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

Meta-analysis of 23 type 2 diabetes linkage studies from the International Type 2 Diabetes Linkage Analysis Consortium

BACKGROUND

The International Type 2 Diabetes Linkage Analysis Consortium was formed to localize type 2 diabetes predisposing variants based on 23 autosomal linkage scans.

METHODS

We carried out meta-analysis using the genome scan meta-analysis (GSMA) method which divides the genome into bins of approximately 30 cM, ranks the best linkage results in each bin for each sample, and then sums the ranks across samples. We repeated the meta-analysis using 2 cM bins, and/or replacing bin ranks with measures of linkage evidence: bin maximum LOD score or bin minimum p value for bins with p value <0.05 (truncated p value). We also carried out computer simulations to assess the empirical type I error rates of these meta-analysis methods.

RESULTS

Our analyses provided modest evidence for type 2 diabetes-predisposing variants on chromosomes 4, 10, and 14 (using LOD scores or truncated p values), or chromosome 10 and 16 (using ranks). Our simulation results suggested that uneven marker density across studies results in substantial variation in empirical type I error rates for all meta-analysis methods, but that 2 cM bins and scores that make more explicit use of linkage evidence, especially the truncated p values, reduce this problem.

CONCLUSION

We identified regions modestly linked with type 2 diabetes by summarizing results from 23 autosomal genome scans.

Effect of rosiglitazone on visfatin and retinol-binding protein-4 plasma concentrations in HIV-positive patients

Thiazolidinediones (TZD) may improve insulin resistance in patients with diabetes and HIV. The novel adipocytokines visfatin and retinol-binding protein-4 (RBP-4) have been proposed to influence the development of impaired glucose tolerance. The impact of TZD on these cytokines is yet unknown. In this randomized, double-blind, placebo-controlled parallel group study, 37 lean HIV-positive subjects aged 19-50 years were treated with 8 mg/day rosiglitazone (n=20) or placebo (n=17) for 6 months. Insulin sensitivity was estimated from the homeostasis model assessment (HOMA) index. Fasting visfatin, RBP-4, leptin, and adiponectin plasma concentrations were analyzed by immunoassays. Rosiglitazone had no effect on impaired insulin sensitivity, but increased median plasma visfatin from 6.2 ng/ml (95% CI: 5.9; 6.5) to 13.7 ng/ml (12.6; 19.1) (P<0.001) and adiponectin from 3.2 ng/ml (2.2; 4.0) to 4.0 ng/ml (3.3; 8.5; P<0.001). RBP-4 was lowered from 21.0 ng/ml (19.6; 23.1) to 16.3 ng/ml (15.2; 17.0; P<0.001), and leptin concentrations were unchanged. Adipocytokine concentrations were stable in subjects receiving placebo, where a deterioration in insulin sensitivity was detectable (P<0.05). Changes in visfatin and RBP-4 were correlated in subjects receiving rosiglitazone (r=-0.64, P<0.01) but not placebo (r=0.12, P=0.15). TZD treatment affects circulating adipocytokine concentrations in subjects with HIV. Reductions in RBP-4 and increases in visfatin may contribute to the pharmacodynamic action of TZD on glucose homeostasis. Quantification of adipocytokines might be useful to assess TZD treatment effectiveness in insulin-resistant subjects with HIV.

A strategy to search for common obesity and type 2 diabetes genes

Worldwide, the incidence of type 2 diabetes is rising rapidly, mainly because of the increase in the incidence of obesity, which is an important risk factor for this condition. Both obesity and type 2 diabetes are complex genetic traits but they also share some nongenetic risk factors. Hence, it is tempting to speculate that the susceptibility to type 2 diabetes and obesity might also partly be due to shared genes. By comparing all of the published genome scans for type 2 diabetes and obesity, five overlapping chromosomal regions for both diseases (encompassing 612 candidate genes) have been identified. By analysing these five susceptibility loci for type 2 diabetes and obesity, using six freely available bioinformatics tools for disease gene identification, 27 functional candidate genes have been pinpointed that are involved in eating behaviour, metabolism and inflammation. These genes might reveal a molecular link between the two disorders.

Epistasis between type 2 diabetes susceptibility Loci on chromosomes 1q21-25 and 10q23-26 in northern Europeans

Characterisation of the interactions between susceptibility loci (epistasis) is central to a full understanding of the genetic aetiology and the molecular pathology of complex diseases. We have examined, in British and French pedigrees, evidence for epistasis between the type 2 diabetes susceptibility loci on chromosomes 1q21-25 and 10q23-26 using two complementary linkage-based approaches. Joint two-locus linkage analysis of 1q and 10q in British pedigrees provided significant evidence for interaction (P < or = 0.003) when comparing a general epistasis model with multiplicative or additive-effects-only models. Conditional linkage analysis (which models epistasis as a deviation from multiplicativity only) confirmed these findings, with significant LOD score increases at the 1q (P = 0.0002) and 10q (P = 0.0023) loci. These analyses provided sizeable reductions in the 1-LOD support intervals for both loci. Analyses of the British and French pedigrees together yielded comparable, but not enhanced, findings, with significant (P < or = 0.003) evidence for epistasis in joint two-locus linkage analysis, and during conditional linkage analysis significant increases in linkage evidence at the 1q (P = 0.0002) and 10q (P = 0.0036) loci. Our findings of epistasis nevertheless substantiate the evidence for genuine genetic effects at both loci, facilitate endeavours to fine-map these loci in population samples, and support further examination of this interaction at the nucleotide level by providing a robust prior hypothesis.

Separately inherited defects in insulin exocytosis and beta-cell glucose metabolism contribute to type 2 diabetes

The effects of genetic variation on molecular functions predisposing to type 2 diabetes are still largely unknown. Here, in a specifically designed diabetes model, we couple separate gene loci to mechanisms of beta-cell pathology. Niddm1i is a major glucose-controlling 16-Mb region in the diabetic GK rat that causes defective insulin secretion and corresponds to loci in humans and mice associated with type 2 diabetes. Generation of a series of congenic rat strains harboring different parts of GK-derived Niddm1i enabled fine mapping of this locus. Congenic strains carrying the GK genotype distally in Niddm1i displayed reduced insulin secretion in response to both glucose and high potassium, as well as decreased single-cell exocytosis. By contrast, a strain carrying the GK genotype proximally in Niddm1i exhibited both intact insulin release in response to high potassium and intact single-cell exocytosis, but insulin secretion was suppressed when stimulated by glucose. Islets from this strain also failed to respond to glucose by increasing the cellular ATP-to-ADP ratio. Changes in beta-cell mass did not contribute to the secretory defects. We conclude that the failure of insulin secretion in type 2 diabetes includes distinct functional defects in glucose metabolism and insulin exocytosis of the beta-cell and that their genetic fundaments are encoded by different loci within Niddm1i.

Transcription factor TCF7L2 genetic study in the French population: expression in human beta-cells and adipose tissue and strong association with type 2 diabetes

Recently, the transcription factor 7-like 2 (TCF7L2) gene has been associated with type 2 diabetes in subjects of European origin in the DeCode study. We genotyped the two most associated variants (rs7903146 and rs12255372) in 2,367 French type 2 diabetic subjects and in 2,499 control subjects. Both the T-allele of rs7903146 and the T-allele of rs12255372 significantly increase type 2 diabetes risk with an allelic odds ratio (OR) of 1.69 (95% CI 1.55-1.83) (P = 6.0 x 10(-35)) and 1.60 (1.47-1.74) (P = 7.6 x 10(-28)), respectively. In nonobese type 2 diabetic subjects (BMI <30 kg/m2, n = 1,346), the ORs increased to 1.89 (1.72-2.09) (P = 2.1 x 10(-38)) and 1.79 (1.62-1.97) (P = 5.7 x 10(-31)), respectively. The rs7903146 T at-risk allele associates with decreased BMI and earlier age at diagnosis in the type 2 diabetic subjects (P = 8.0 x 10(-3) and P = 3.8 x 10(-4), respectively), which is supported by quantitative family-based association tests. TCF7L2 is expressed in most human tissues, including mature pancreatic beta-cells, with the exception of the skeletal muscle. In the subcutaneous and omental fat from obese type 2 diabetic subjects, TCF7L2 expression significantly decreased compared with obese normoglycemic individuals. During rat fetal beta-cell differentiation, TCF7L2 expression pattern mimics the key marker NGN3 (neurogenin 3), suggesting a role in islet development. These data provide evidence that TCF7L2 is a major determinant of type 2 diabetes risk in European populations and suggests that this transcription factor plays a key role in glucose homeostasis.

Identification of a regulatory SNP in the retinol binding protein 4 gene associated with type 2 diabetes in Mongolia

Increased levels of retinol binding protein 4 (RBP4) in serum is associated with insulin resistance. To examine this further, the genomic region of RBP4 was genetically surveyed in Mongolian people, who as a group are suffering from a recent rapid increase in diabetes. The RBP4 gene was screened by DHPLC system, and the PCR fragments which showed heteroduplex peaks in multiple samples were followed by direct sequencing to identify common polymorphisms in 48 Mongolian diabetic samples. Identified single nucleotide polymorphisms (SNPs) were genotyped in 511 control and 281 type 2 diabetes samples. The functions of SNPs in the regulatory region were assessed by reporter gene assay and electrophoretic mobility shift assay. Possible association between functional SNPs and serum RBP4 levels or metabolic parameters was statistically assessed. Nine SNPs were identified in the RBP4 gene. A case-control study revealed that the rare alleles of four SNPs were associated with increased risk of diabetes, even after Bonferroni correction (-803, G > A, P = 0.0054; +5169, C > T, P = 0.0025; +6969, G > C, P = 0.0015; +7542, T > del, P = 0.0015). The -803 G > A SNP influenced the transcription efficiency in a hepatocarcinoma cell line as well as the binding efficiency of hepatocyte nuclear factor 1 alpha to the motif. In addition, the -803 A allele was associated with increased serum RBP4 levels in diabetic patients. We have identified a functional SNP in the RBP4 gene associated with type 2 diabetes in Mongolian people.

Variant of transcription factor 7-like 2 (TCF7L2) gene and the risk of type 2 diabetes in large cohorts of U.S. women and men

Emerging evidence indicates that variation in the transcription factor 7-like 2 (TCF7L2) gene may play a role in the pathogenesis of type 2 diabetes. In a prospective, nested, case-control study (n = 3,520) within the Nurses' Health Study (687 type 2 diabetic case and 1,051 control subjects) and the Health Professionals Follow-up Study (886 case and 896 control subjects), we examined the association of a common variant of the TCF7L2 gene (rs12255372 [T/G]) with type 2 diabetes risk among Caucasians. Frequencies of the T-allele were significantly higher among case than control subjects; each copy of the T-allele was associated with a 1.32-fold (P = 0.0002) and 1.53-fold (P < 0.0001) increased type 2 diabetes risk in women and men, respectively. The odds ratios (95% CI) associated with homozygous carriers of the T-allele were 1.86 (1.30-2.67) and 2.15 (1.48-3.13) in women and men, respectively. Population-attributable risks for diabetes associated with the T-allele were 14.8 and 22.3% for women and men, respectively. In a meta-analysis of 3,347 case and 3,947 control subjects, each copy of the T-allele was associated with a 1.48-fold increased risk (P < 10(-16)). Our findings confirm that the TCF7L2 gene represents an important locus for predicting inherited susceptibility to type 2 diabetes.

Polymorphisms in the transcription factor 7-like 2 (TCF7L2) gene are associated with type 2 diabetes in the Amish: replication and evidence for a role in both insulin secretion and insulin resistance

Transcription factor 7-like 2 (TCF7L2) regulates genes involved in cell proliferation and differentiation. The TCF7L2 gene is located on chromosome 10q25 in a region of replicated linkage to type 2 diabetes. Recently, a microsatellite marker in intron 3 (DG10S478) and five correlated single nucleotide polymorphisms (SNPs) were identified in Icelandic individuals that showed strong association with type 2 diabetes, which was replicated in Danish and European-American cohorts. We genotyped four of the SNPs (rs7901695, rs7903146, rs11196205, and rs12255372) in Amish subjects with type 2 diabetes (n = 137), impaired glucose tolerance (IGT; n = 139), and normal glucose tolerance (NGT; n = 342). We compared genotype frequencies in subjects with type 2 diabetes with those with NGT and found marginal association for rs7901695 (P = 0.05; odds ratio [OR] 1.51); comparison between NGT control subjects and the combined type 2 diabetes/IGT case group showed strong association with rs7901695 and rs7903146 (P = 0.008-0.01; OR 1.53-1.57) and marginal association with rs11196205 and rs12255372 (P = 0.07 and P = 0.04, respectively). In an expanded set of 698 Amish subjects without diabetes, we found no association with insulin and glucose levels during a 3-h oral glucose tolerance test. We also genotyped these SNPs in nondiabetic, non-Amish subjects (n = 48), in whom intravenous glucose tolerance tests were performed, and found an association between rs7901695 and rs7903146 and insulin sensitivity (P = 0.003 and P = 0.005, respectively) and disposition index (P = 0.04 and P = 0.007, respectively). These data provide replicating evidence that variants in TCF7L2 increase the risk for type 2 diabetes and novel evidence that the variants likely influence both insulin secretion and insulin sensitivity.

High-resolution quantitative trait locus analysis reveals multiple diabetes susceptibility loci mapped to intervals<800 kb in the species-conserved Niddm1i of the GK rat

Niddm1i, a 16-Mb locus within the major diabetes QTL in the diabetic GK rat, causes impaired glucose tolerance in the congenic NIDDM1I strain. Niddm1i is homologous to both human and mouse regions linked with type 2 diabetes susceptibility. We employed multiple QTL analyses of congenic F2 progeny selected for one recombination event within Niddm1i combined with characterization of subcongenic strains. Fine mapping located one hyperglycemia locus within 700 kb (Niddm1i4, P=5x10(-6)). Two adjacent loci were also detected, and the GK allele at Niddm1i2 (500 kb) showed a glucose-raising effect, whereas it had a glucose-lowering effect at Niddm1i3 (400 kb). Most proximally, Niddm1i1 (800 kb) affecting body weight was identified. Experimental data from subcongenics supported the four loci. Sorcs1, one of the two known diabetes susceptibility genes in the region, resides within Niddm1i3, while Tcf7l2 maps outside all four loci. Multiple-marker QTL analysis incorporating the effect of cosegregating QTL as cofactors together with genetically selected progeny can remarkably enhance resolution of QTL. The data demonstrate that the species-conserved Niddm1i is a composite of at least four QTL affecting type 2 diabetes susceptibility and that two adjacent QTL (Niddm1i2GK and Niddm1i3GK) act in opposite directions.

Linkage and Association Studies of the Susceptibility Genes for Type 2 Diabetes

Type 2 diabetes mellitus (T2DM) is a complex disease characterized by hyperglycemia, insulin resistance, and impaired insulin secretion. T2DM is under strong genetic control. Identification and characterization of genes involved in determining T2DM will contribute to a greater understanding of the pathogenesis of T2DM, and ultimately might lead to the development of better diagnosis, prevention and treatment strategies. Efforts to identify T2DM susceptibility genes have focused on candidate gene approach (association studies) and genome-wide scans (linkage analyses). In this article, we review the current status for mapping and identification of genes for T2DM, with a focus on some promising regions (or genes) and future prospects.