Johanna K. Wolford

The interleukin-6 (−174) G/C promoter polymorphism is associated with type-2 diabetes mellitus in Native Americans and Caucasians

Chronic low-grade activation of the immune system may play a role in the pathogenesis of type-2 diabetes mellitus (T2DM). Interleukin-6 (IL6), a powerful inducer of hepatic acute phase response, has been implicated in the etiology of insulin resistance and T2DM. Recently, an IL6 promoter polymorphism (G/C) at position −174 was found to be associated with measures of insulin sensitivity. Because we have previously found an association between high IL6 levels and insulin resistance in both Pima Indians — a population with high rates of insulin resistance and T2DM — and Caucasians, we aimed to assess whether the IL6 promoter polymorphism is associated with T2DM in these populations. We genotyped the IL6 (−174) G/C polymorphism using pyrosequencing in 463 Native Americans and by PCR-RFLP in 329 Spanish Caucasians. Among the Spanish Caucasian subjects, there was a significant difference in genotypic distribution between diabetic and non-diabetic subjects (P=0.028); the GG genotype was more common in diabetic (0.40) than in non-diabetic (0.29) subjects. The G allele was much more frequent in the Native American sample, and among a sample of 143 cases and 145 controls, the GG genotype was significantly more common in diabetic subjects (P=0.019). When this sample population was stratified according to ethnic heritage, all 211 subjects who were of full Pima Indian heritage had the GG genotype, whereas in the 77 American Indian subjects with non-Pima admixture, T2DM was associated with IL6 genotype (P=0.001). These findings are consistent with a role for genetic determinants of inflammation in the development of T2DM in both Native Americans and Caucasians.

High-throughput SNP detection by using DNA pooling and denaturing high performance liquid chromatography (DHPLC)

Abstract. One of the critical steps in the positional cloning of a complex disease gene involves association analysis between a phenotype and a set of densely spaced diallelic markers, typically single nucleotide repeats (SNPs), covering the region of interest. However, the effort and cost of detecting sufficient numbers of SNPs across relatively large physical distances represents a significant rate-limiting step. We have explored DNA pooling, in conjunction with denaturing high performance liquid chromatography (DHPLC), as a possible strategy for augmenting the efficiency, economy, and throughput of SNP detection. DHPLC is traditionally used to detect variants in polymerase chain reaction products containing both allelic forms of a polymorphism (e.g., heterozygotes or a 1:1 mix of both alleles) via heteroduplex separation and thereby requires separate analyses of multiple individual test samples. We have adapted this technology to identify variants in pooled DNA. To evaluate the utility and sensitivity of this approach, we constructed DNA pools comprised of 20 previously genotyped individuals with a frequency representation of 0%–50% for the variant allele. Mutation detection was performed by using temperature-modulated heteroduplex formation/DHPLC and dye-terminator sequencing. Using DHPLC, we could consistently detect SNPs at lower than 5% frequency, corresponding to the detection of one variant allele in a pool of 20 alleles. In contrast, fluorescent sequencing detected variants in the same pools only if the frequency of the less common allele was at least 10%. We conclude that DNA pooling of samples for DHPLC analysis is an effective way to increase throughput efficiency of SNP detection.

Identification of PVT1 as a Candidate Gene for End-Stage Renal Disease in Type 2 Diabetes Using a Pooling-Based Genome-Wide Single Nucleotide Polymorphism Association Study

To identify genetic variants contributing to end-stage renal disease (ESRD) in type 2 diabetes, we performed a genome-wide analysis of 115,352 single nucleotide polymorphisms (SNPs) in pools of 105 unrelated case subjects with ESRD and 102 unrelated control subjects who have had type 2 diabetes for ≥10 years without macroalbuminuria. Using a sliding window statistic of ranked SNPs, we identified a 200-kb region on 8q24 harboring three SNPs showing substantial differences in allelic frequency between case and control pools. These SNPs were genotyped in individuals comprising each pool, and strong evidence for association was found with rs2720709 (P = 0.000021; odds ratio 2.57 [95% CI 1.66–3.96]), which is located in the plasmacytoma variant translocation gene PVT1. We sequenced all exons, exon-intron boundaries, and the promoter of PVT1 and identified 47 variants, 11 of which represented nonredundant markers with minor allele frequency ≥0.05. We subsequently genotyped these 11 variants and an additional 87 SNPs identified through public databases in 319-kb flanking rs2720709 (∼1 SNP/3.5 kb); 23 markers were associated with ESRD at P < 0.01. The strongest evidence for association was found for rs2648875 (P = 0.0000018; 2.97 [1.90–4.65]), which maps to intron 8 of PVT1. Together, these results suggest that PVT1 may contribute to ESRD susceptibility in diabetes.

IL6 gene promoter polymorphisms and type 2 diabetes: joint analysis of individual participants' data from 21 studies

Several lines of evidence indicate a causal role of the cytokine interleukin (IL)-6 in the development of type 2 diabetes in humans. Two common polymorphisms in the promoter of the IL-6 encoding gene IL6, −174G>C (rs1800795) and −573G>C (rs1800796), have been investigated for association with type 2 diabetes in numerous studies but with results that have been largely equivocal. To clarify the relationship between the two IL6 variants and type 2 diabetes, we analyzed individual data on >20,000 participants from 21 published and unpublished studies. Collected data represent eight different countries, making this the largest association analysis for type 2 diabetes reported to date. The GC and CC genotypes of IL6 −174G>C were associated with a decreased risk of type 2 diabetes (odds ratio 0.91, P = 0.037), corresponding to a risk modification of nearly 9%. No evidence for association was found between IL6 −573G>C and type 2 diabetes. The observed association of the IL6 −174 C-allele with a reduced risk of type 2 diabetes provides further evidence for the hypothesis that immune mediators are causally related to type 2 diabetes; however, because the association is borderline significant, additional data are still needed to confirm this finding.

A C-reactive protein promoter polymorphism is associated with type 2 diabetes mellitus in Pima Indians

Linkage analysis has identified a susceptibility locus for type 2 diabetes mellitus (T2DM) on chromosome 1q21-q23 in several populations. Results from recent prospective studies indicate that increased levels of C-reactive protein (CRP), a marker of immune system activation, are predictive of diabetes, independent of adiposity. Because CRP is located on 1q21, we considered it a potential positional candidate gene for T2DM. We therefore evaluated CRP and the nearby serum amyloid P-component, APCS, which is structurally similar to CRP, as candidate diabetes susceptibility genes. Approximately 10.9kb of the CRP-APCS locus was screened for polymorphisms using denaturing high performance liquid chromatography and direct sequencing. We identified 27 informative polymorphisms, including 26 single nucleotide polymorphisms (SNPs) and 1 insertion/deletion, which were divided into 7 linkage disequilibrium clusters. We genotyped representative SNPs in approximately 1300 Pima samples and found a single variant in the CRP promoter (SNP 133552) that was associated with T2DM (P=0.014), as well as a common haplotype (CGCG) that was associated with both T2DM (P=0.029) and corrected insulin response, a surrogate measure of insulin secretion in non-diabetic subjects (P=0.050). Linkage analyses that adjusted for the effect of these polymorphisms indicated that they do not in themselves account for the observed linkage with T2DM on chromosome 1q. However, these findings suggest that variation within the CRP locus may play a role in diabetes susceptibility in Pima Indians.

Variants in the Plasmacytoma Variant translocation gene ( PVT1 ) are associated with end-stage renal disease attributed to type 1 diabetes

OBJECTIVE— End-stage renal disease (ESRD) attributed to diabetes is strongly dependent on genetic factors. We previously reported association between variants in the plasmacytoma variant translocation gene (PVT1) and ESRD attributed to type 2 diabetes in Pima Indians. The objective of this study was to evaluate the extent to which these variants mediate susceptibility in other populations. RESEARCH DESIGN AND METHODS— We genotyped 24 markers showing the strongest evidence for association in Pima Indians in unrelated Caucasians with type 1 diabetes from the Genetics of Kidneys in Diabetes (GoKinD) study. The study sample was comprised of 531 case subjects with ESRD and 564 control subjects with diabetes duration >20 years and a maximum urinary albumin-to-creatinine ratio <150 mg/g. RESULTS— Markers rs13447075 (odds ratio [OR] 1.47 [95% CI 1.14–1.89] per copy of A allele; P = 0.003) and rs2648862 (2.66 [1.19–5.92] per copy of C allele; P = 0.008) were strongly associated with ESRD in analyses adjusting for age2, age3, duration of diabetes, and smoking status. We further identified a common haplotype containing the C allele at rs10808565 and the A allele at rs13447075 that was associated with ESRD (P = 0.003). PVT1 gene expression yields several isoforms, and rs13447075 is located within the coding region of one of these transcript variants. We identified expression of this isoform in four major human kidney cell types, including mesangial, cortical epithelial, epithelial, and proximal tubule cells. CONCLUSIONS— These results are the first to provide confirmatory evidence supporting a role for PVT1 in mediating susceptibility to ESRD attributable to diabetes.

Meta-Analysis of Genome-Wide Linkage Studies of Quantitative Lipid Traits in Families Ascertained for Type 2 Diabetes

Dyslipidemia is a major risk factor for coronary heart disease, which is the predominant cause of mortality in individuals with type 2 diabetes. To date, nine linkage studies for quantitative lipid traits have been performed in families ascertained for type 2 diabetes, individually yielding linkage results that were largely nonoverlapping. Discrepancies in linkage findings are not uncommon and are typically due to limited sample size and heterogeneity. To address these issues and increase the power to detect linkage, we performed a meta-analysis of all published genome scans for quantitative lipid traits conducted in families ascertained for type 2 diabetes. Statistically significant evidence (i.e., P < 0.00043) for linkage was observed for total cholesterol on 7q32.3-q36.3 (152.43–182 cM; P = 0.00004), 19p13.3-p12 (6.57–38.05 cM; P = 0.00026), 19p12-q13.13 (38.05–69.53 cM; P = 0.00001), and 19q13.13-q13.43 (69.53–101.1 cM; P = 0.00033), as well as LDL on 19p13.3-p12 (P = 0.00041). Suggestive evidence (i.e., P < 0.00860) for linkage was also observed for LDL on 19p12-q13.13, triglycerides on 7p11-q21.11 (63.72–93.29 cM), triglyceride/HDL on 7p11-q21.11 and 19p12-q13.13, and LDL/HDL on 16q11.2-q24.3 (65.2–130.4 cM) and 19p12-q13.13. Linkage for lipid traits has been previously observed on both chromosomes 7 and 19 in several unrelated studies and, together with the results of this meta-analysis, provide compelling evidence that these regions harbor important determinants of lipid levels in individuals with type 2 diabetes.

Molecular characterization of the human PEA15 gene on 1q21–q22 and association with type 2 diabetes mellitus in Pima Indians ☆

The PEA15 gene encoding a protein kinase C substrate is widely expressed, and its overexpression may contribute to impairment of glucose uptake. PEA15 is located within a region on human 1q linked with type 2 diabetes in both Pima Indians and Caucasians. To assess the potential contribution of genetic alterations within this locus to disease susceptibility in the Pimas, we have investigated its genomic sequences. The PEA15 locus is composed of four exons spanning approximately 10.2kb of genomic DNA, flanked upstream by an potentially expressed Alu element, downstream by the H326 gene, and is located within 250kb of KCNJ9. We also sequenced over 2kb of the promoter region and identified various motifs analogous to known transcription factor binding sites. By analysis of 22 Pimas, including 13 diabetic subjects, we detected four single nucleotide polymorphisms (SNPs) in the non-coding regions of PEA15, including three frequent variants that were in allelic disequilibrium, and one variant found only in a single Pima. The three SNPs were not associated with type 2 diabetes mellitus in 50 affected and 50 control Pimas (p=0.12-0.17), and we conclude that mutations in this gene probably do not contribute significantly to disease susceptibility in this Native American tribe. However, knowledge of the genomic structure of PEA15 provides the basis for similar systematic examinations of this candidate locus in relation to type 2 diabetes and other metabolic disorders in other populations.

Genetic Basis of Type 2 Diabetes Mellitus Implications for Therapy

Type 2 diabetes mellitus represents a multifactorial, heterogeneous group of disorders, which result from defects in insulin secretion, insulin action, or both. The prevalence of type 2 diabetes has increased dramatically worldwide over the past several decades, a trend that has been heavily influenced by the relatively recent changes in diet and physical activity levels. There is also strong evidence supporting a genetic component to type 2 diabetes susceptibility and several genes underlying monogenic forms of diabetes have already been identified. However, common type 2 diabetes is likely to result from the contribution of many genes interacting with different environmental factors to produce wide variation in the clinical course of the disease. Not surprisingly, the etiologic complexity underlying type 2 diabetes has made identification of the contributing genes difficult.Current therapies in the management of type 2 diabetes include lifestyle intervention through diet modification and exercise, and oral or injected hypoglycemic agents; however, not all individuals with type 2 diabetes respond in the same way to these treatments. Because of variability in the clinical course of the disease and in the responsiveness to pharmacologic therapies, identification and characterization of the genetic variants underlying type 2 diabetes susceptibility will be important in the development of individualized treatment. Findings from linkage analyses, candidate gene studies, and animal models will be valuable in the identification of novel pathways involved in the regulation of glucose homeostasis, and will augment our understanding of the gene-gene and gene-environment interactions, which impact on type 2 diabetes etiology and pathogenesis. In addition, identification of genetic variants that determine differences in antidiabetic drug responsiveness will be useful in assessing a first-line pharmacologie therapy for diabetic patients.

Cloning, expression and genomic structure of human LMX1A, and variant screening in Pima Indians ☆

LIM-homeodomain containing protein LMX1A activates transcription of the insulin gene. The human LMX1A gene maps to 1q22-q23, a region identified as a putative type 2 diabetes mellitus (T2DM) locus in several different populations. We analyzed LMX1A as a positional and biological candidate gene for T2DM in the Pima Indians, in whom a linkage of T2DM to 1q21-q23 has been previously reported. In the present study, we describe the cloning, expression and genomic organization of the LMX1A gene, which is composed of 11 exons spanning approximately 151 kb. In addition to a transcript encoding the predicted full-length protein of 382 amino acids, we identified two truncated cDNA forms produced via additional transcription start sites and alternative splicing. We identified seven single nucleotide polymorphisms (SNPs) throughout the LMX1A locus and determined allele frequency distributions in 150 diabetic and 150 unaffected Pimas. We did not find evidence for association of any LMX1A SNPs with T2DM and conclude that LMX1A does not contribute significantly to T2DM etiology in Pima Indians.