A C-reactive protein promoter polymorphism is associated with type 2 diabetes mellitus in Pima Indians
Introduction
Linkage to type 2 diabetes mellitus (T2DM) has been demonstrated for chromosome 1q21–q23 in a number of populations including Pima Indians [1], Utah Caucasians [2], French Caucasians [3], British Caucasians [4], and Chinese [5]. To date, however, the specific polymorphism(s) responsible for the linkage findings have not been identified. The present study represents part of our ongoing effort to identify genetic determinants of T2DM susceptibility in the 1q21–q23 region in Pima Indians. These efforts include analysis of linkage disequilibrium between T2DM and single nucleotide polymorphisms (SNPs)1 spaced densely throughout the linked region, and analysis of variants in candidate genes mapping to the 1q21–q23 interval. We considered the C-reactive protein gene (CRP) one of the strongest, as yet unexamined, candidates mapping to this interval.
CRP is an acute-phase reactant, elevations of which are indicative of an inflammatory process. Several investigators have recently proposed an etiologic role for chronic inflammation in the development of insulin resistance [6] and T2DM [6], [7], [8]. In support of this hypothesis, T2DM has been associated with increased serum levels of acute-phase response markers, including C-reactive protein (CRP), serum amyloid P component (APCS), and sialic acid [9]. In addition, results from two separate prospective studies showed that higher baseline CRP levels predicted the development of T2DM, independent of BMI, insulin, and fasting concentrations of triglycerides and glucose [10], [11]. Together, these findings suggest a role for CRP as a risk factor for the development of T2DM and consequently implicated the gene as a potential candidate for T2DM susceptibility.
The aim of the present study was to assess the role of CRP as a possible candidate gene affecting T2DM susceptibility in Pima Indians. To this end, we screened through the entire CRP locus, including 2 kb from the coding sequence in the 5′ and 3′ directions, to identify allelic variants and test them for association with T2DM in Pimas. We also investigated the adjacent APCS gene, which is located within 124 kb of CRP. APCS is structurally similar to CRP and serum levels of APCS are also increased in subjects with T2DM [12]. However, the potential role played by APCS in diabetes susceptibility has remained largely unexamined.
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Research design and methods
Subjects. Subjects selected for genomic screening are participants in ongoing longitudinal studies of diabetes conducted among members of the Gila River Indian Community since 1965 [13]. Diabetes was diagnosed using WHO criteria as described previously [1]. This study was approved by the Institutional Board of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and the Tribal Council of the Gila River Indian Community. All subjects provided informed written consent
Results
Identification of SNPs in the CRP-APCS locus. We screened approximately 10.9 kb of sequence corresponding to the exons, introns, and 5′/3′ flanking sequences of the CRP and APCS genes, including the CRP pseudogene CRPP1, and identified 27 polymorphisms, which were divided into 7 clusters based on equal allelic frequencies and near-complete linkage disequilibrium (Fig. 1). For CRP, these included 6 SNPs in the 5′ flanking region, 1 SNP in the sole CRP intron, and 3 SNPs in the 3′ untranslated
Discussion
The present study represents a candidate gene analysis and provides evidence that variants in CRP are associated with T2DM in Pima Indians. We screened the CRP locus, including the neighboring APCS and CRPP1 genes, to identify polymorphisms and found one variant, SNP 133552, which was significantly associated with T2DM. Our findings are consistent with the hypothesis that CRP may play an etiologic role in the development of T2DM. Results from prospective studies showed that baseline CRP levels
Acknowledgements
We thank Timothy Monette and Dr. Farook Thameem for expert technical assistance and Sayuko Kobes for analytical assistance. We are also grateful for the generous support of the Gila River Indian Community.
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