Diabetes 53:3337-3341, 2004 © 2004 by the American Diabetes Association, Inc.
Polymorphisms in Both Promoters of Hepatocyte Nuclear Factor 4-
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| ABSTRACT |
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(HNF4A) is a transcription factor located on chromosome 20q13 that regulates expression of genes involved in glucose metabolism and homeostasis. Recently, two groups independently identified single nucleotide polymorphism (SNPs) in an alternate upstream promoter (P2) of HNF4A that were associated with type 2 diabetes in Ashkenazi Jews and Finns. We genotyped haplotype-tagging SNPs (htSNPs) across the two promoter regions and the coding region of HNF4A in individuals with type 2 diabetes (n = 137), impaired glucose tolerance (IGT) (n = 139), and normal glucose tolerance (n = 342) from the Amish Family Diabetes Study (AFDS) to test for association with type 2 diabetes. In the P1 promoter region, we observed a significant association between the A allele of rs2425640 and type 2 diabetes (odds ratio [OR] 1.60, P = 0.03). Furthermore, the mean age of type 2 diabetes onset was, on average, 5.1 years earlier in those with the AA or GA genotype at SNP rs2425640 than in those with the GG genotype (57.8 vs. 62.9 years, P = 0.011). In the P2 promoter, the htSNP rs1884614 showed borderline association with both type 2 diabetes (OR 1.40, P = 0.09) and the combined type 2 diabetes/IGT trait (1.35, P = 0.07). In an expanded set of 698 nondiabetic AFDS subjects, we found association between rs1884614 and glucose area under the curve during an oral glucose tolerance test (additive model, P = 0.022; dominant model, P = 0.010). The results of this study provide evidence that variants in both the P1 and P2 promoters of HNF4A increase risk for typical type 2 diabetes.
(HNF4A) is a transcription factor that is expressed in several tissues, including liver and pancreas, where it regulates expression of genes involved in gluconeogenesis and glucose-stimulated insulin secretion, respectively (14). Relatively rare mutations in HNF4A have been identified that cause maturity-onset diabetes of the young type 1 (rev. in 5), a dominantly inherited, early-onset form of type 2 diabetes characterized by impaired glucose-induced insulin secretion due to pancreatic ß-cell dysfunction (69). HNF4A expression patterns are complex as a result of alternative splicing and transcription from two different promoters, the proximal P1 promoter and the P2 promoter, which lies
45 kb upstream of the P1 promoter (1013).
The 12 coding exons of HNF4A span
29 kb on chromosome 20q13, a region of overlapping linkage to type 2 diabetes in several Caucasian (1419) and Asian (20,21) populations. Recently, through fine-mapping efforts in this region of chromosome 20q, two groups concurrently identified single nucleotide polymorphisms (SNPs) in the P2 and P1 promoter regions and coding exons of HNF4A that are associated with type 2 diabetes in the Ashkenazi Jews (22) and Finns (FUSION 1 [Finland-United States Investigation of NIDDM Genetics 1]) (23). Silander et al. (23) identified 10 SNPs across a 64-kb region spanning the P2 and P1 promoter regions and exons 13 of HNF4A that were associated with type 2 diabetes in the FUSION 1 population. In the Ashkenazi Jewish cohort, the SNPs closer to the P1 promoter and coding exons were not associated with type 2 diabetes (22); however, four SNPs spanning a
10-kb region encompassing the P2 promoter were associated with type 2 diabetes (rs4810424, rs1884613, rs1884614, and rs2144908). These four SNPs are located in a 177-kb region of strong linkage disequilibrium (LD), including >140 kb upstream of the P2 promoter (23). A second haplotype block was observed within the HNF4A coding region, while LD tended to decay across the
45-kb gap separating HNF4A from P2 (22,23). In addition to the observed association with type 2 diabetes, these P2 SNPs appeared to explain a significant portion of the linkage to chromosome 20q12-q13 observed in both the Ashkenazi Jews and Finns. The replicating evidence presented by these studies suggests that SNPs near the P2 promoter of HNF4A increase susceptibility to type 2 diabetes.
Although no evidence for linkage to type 2 diabetes was detected on chromosome 20q12-q13 in our genome-wide scan (average marker density = 9.7 cM) in the Old Order Amish (logarithm of odds = 0.00 between markers D20S107 and D20S119, which are
6 cM apart) (24), we tested whether SNPs in HNF4A and its promoters were associated with type 2 diabetes in the Amish. We selected six haplotype-tagging SNPs (htSNPs) spanning the P2 and P1 promoters and the HNF4A coding region from the LD blocks defined in the Ashkenazi Jews (22) and Finns (23). Given that the Amish are a young founder population, we hypothesized that haplotype blocks would be as large as or larger than those in the other populations, thus allowing us to capture most or all of the variation across the gene with these SNPs. These SNPs were genotyped in 618 individuals enrolled in the Amish Family Diabetes Study (AFDS), which included 137 subjects with type 2 diabetes, 139 individuals with impaired glucose tolerance (IGT), and 342 control subjects with normal glucose tolerance (NGT). The NGT control subjects selected were
38 years of age in order to increase the probability of their capacity for diabetes resistance. Table 1 summarizes the allele frequencies in individuals with type 2 diabetes, IGT, and NGT and the results of genotypic association analysis for each SNP. All SNPs conformed to Hardy-Weinberg expectations. For rs2425640, one of the SNPs located in the P1 promoter region, the frequency of the A allele was significantly higher in the type 2 diabetic group than in the control group in the Amish (genotypic odds ratio [OR] 1.60, P = 0.030). Furthermore, the mean age of diabetes onset was 58.0 years in subjects with the AA genotype for SNP rs2425640, 57.8 years in those with the GA genotype, and 62.9 years in those with the GG genotype. The mean age of type 2 diabetes onset was, on average, 5.1 years earlier in those with the AA or GA genotype at SNP rs2425640 than in those with the GG genotype (57.8 vs. 62.9 years, P = 0.011).
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Table 2 shows the pairwise LD (|D'| and r2) among the genotyped SNPs in the Amish. The haplotype block structure in the Amish appears very similar to that reported in Finns and Ashkenazi Jews, suggesting that the SNP density we chose is adequate for the detection of most of the common variation in HNF4A. As shown in the Ashkenazi Jews and the Finns, the SNP representing the P2 haplotype block that was genotyped in the Amish (rs1884614) was clearly not in LD with rs2425640 in the P1 promoter or with the other HNF4A SNPs.
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| RESEARCH DESIGN AND METHODS |
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Genotyping.
Genotyping was completed using the Orchid/Beckman SNPstream Ultra High Throughput genotyping platform. This genotyping method is described in detail elsewhere (26). Briefly, the protocol involved PCR amplification of target sequences surrounding the SNPs to be assayed in panels of 12-plex reactions. Following enzymatic purification, the PCR products were subjected to single-base primer extension with fluorescent-labeled dye terminators. Each extension primer contained a unique 20-nucleotide tail at its 5' end whose sequence was designed to hybridize to its complementary probe immobilized in a mini-array within each well of a 384-well SNP-IT plate (Beckman Coulter, Fullerton, CA). The microarray plate was imaged by the SNPscope reader (Beckman Coulter). The two-color system allowed the detection of the SNP by comparing signals from the two fluorescent dyes. The image signals were then transferred to genotyping software that translated the images of the arrays into genotype calls. The error rate based upon blind replicates for the SNPs examined in the present study was 01.2%.
Statistical analysis.
Before analysis, genotypes were checked for Mendelian consistency using the pedigree information and inconsistencies (<.5% of genotypes) were resolved or removed before analysis. Allele frequencies were calculated for each SNP by gene counting, and observed genotypes were tested for fit to the expectations of Hardy-Weinberg using the
2 test. Pairwise LD was computed between the SNPs using the two most commonly used statistics |D'| and r2, and haplotypes were inferred for each individual using an expectation maximization algorithm implemented in the ZAPLO software program (27).
We evaluated the association between SNP genotype and disease status (type 2 diabetes versus NGT and type 2 diabetes/IGT versus NGT) using a variance component approach, in which we modeled the probability that the subject was a case or control subject, as a function of the individuals age, sex, and genotype, conditional on the correlations in phenotype among relative pairs. For the primary analysis, we considered an additive genetic model in which the genotype was coded as 0, 1, or 2, depending on whether the subject was homozygous for the minor allele (genotype = 2), heterozygous (genotype = 1), or homozygous for the major allele (genotype = 0). Statistical testing was accomplished using the likelihood ratio test, in which we compared the likelihood of the data under a model in which the genotype effect was estimated against the likelihood of a nested model in which the genotype effect was constrained to be zero. Secondary analyses were carried out under the dominant and recessive genetic models by imposing appropriate constraints on the genotypic effects. Parameter estimates (i.e., ß coefficients) were obtained by maximum likelihood and ORs by taking the inverse log of the ß coefficient. The OR for the additive model was scaled to reflect the odds that a case was homozygous for the minor allele versus the odds that the case was homozygous for the major allele. The variance components analysis was carried out using the SOLAR software program (28).
Finally, mean levels of glucose (fasting and glucose area under the curve during a 3-h OGTT) and insulin (fasting and insulin area under the curve during a 3-h OGTT) were estimated according to HNF4A genotypes in an expanded set of nondiabetic AFDS subjects (n = 698). To account for the relatedness among family members, the measured genotype approach was used (29), in which we estimated the likelihood of specific genetic models given the pedigree structure. Parameter estimates were obtained by maximum likelihood methods, and the significance of association was tested by likelihood ratio tests. Within each model, we simultaneously estimated the effects of age and sex. Insulin values were transformed by their natural logarithms (ln) to reduce skewness. Quantitative trait analyses were conducted using the SOLAR program (28).
| ACKNOWLEDGMENTS |
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We gratefully acknowledge our Amish liaisons and field workers and the extraordinary cooperation and support of the Amish community, without whom these studies would not be possible.
Address correspondence and reprint requests to Alan R. Shuldiner, MD, Division of Endocrinology, Diabetes and Nutrition, University of Maryland School of Medicine, 660 West Redwood St., Room 494, Baltimore, MD 21201. E-mail: ashuldin{at}medicine.umaryland.edu
Received for publication June 16, 2004 and accepted in revised form August 27, 2004
Abbreviations: AFDS, Amish Family Diabetes Study; HNF, hepatocyte nuclear factor; htSNP, haplotype-tagging single nucleotide polymorphism; IGT, impaired glucose tolerance; LD, linkage disequilibrium; NGT, normal glucose tolerance; OGTT, oral glucose tolerance test; SNP, single nucleotide polymorphism
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gene predicts susceptibility to type 2 diabetes.
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