Warfarin (Coumadin (R)) has been a long-standing target of research because it is both difficult to determine the correct dose and has serious adverse effects. Currently, algorithms using polymorphisms in the CYP2C9 and VKORC1 genes have been developed to predict the correct maintenance dose of warfarin in Caucasians and Asians. However these algorithms, which include known non-genetic variables such as age, weight and concomitant medications, are much less predictive in African Americans. Discovery of which SNPs affect dose in African Americans and the mechanism underlying their effect remain a gap in the current knowledge. We hypothesize that these studies will identify clinically relevant SNPs that affect warfarin dose in African Americans.
The aims of this proposal are to provide validated evidence for novel genetic variation that affects warfarin dose in African Americans, to investigate the effect these SNPs have on the gene regulation, and to show the effect of these SNPs on both the pharmacokinetics of warfarin and its drug target Vitamin K epoxide reductase (VKOR). In pursuit of these goals, we have determined and genotyped the relevant haplotype tagging SNPs (htSNPs) in CYP2C9 and VKORC1 in African Americans, using comparative genomics and putative transcriptional binding sites prediction. By looking at htSNPs we can genotype just one SNP in the haplotype and capture the variation in linkage disequilibrium (LD) with that SNP. Tests for association in the discovery cohort have revealed interesting novel variation in both genes. However, validation of these findings is needed. To further evaluate regulation, we used a genome-wide bioinformatics tool (SCAN) to identify expression Quantitative Trait Loci (eQTLs), a method that has never been used to identify novel SNPs associated with warfarin dosing. To this end, we plan on genotyping the most highly associated SNPs in two independent African American anticoagulation cohorts. Those that replicate will then be assayed for function in a series of in vitro assays. We will then investigate the direct clinical effect of these SNPs on drug metabolism and the target protein by conducting a pharmacokinetic study evaluating the effect of CYP2C9 SNPs on S- to R-warfarin plasma concentration ratio and a pharmacodynamic study evaluating the effect of VKORC1 SNPs on a surrogate marker of VKOR protein function, Prothrombin induced by vitamin K absence or antagonism II (PIVKA-II). This proposed research is both timely and necessary to fill gaps in the current knowledge and to affect real translation of pharmacogenetics into clinical practice.

Public Health Relevance

The proposed studies will help identify genetic mutations that affect warfarin dose in African Americans, an under-studied population. The discovery of these novel mutations may lead to better dosing of this medication and less side-effects. These studies may also uncover new ways in which genes important to warfarin dosing may be regulated (i.e., produce more or less protein).

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Exploratory/Developmental Grants (R21)
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Clinical and Integrative Cardiovascular Sciences Study Section (CICS)
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Link, Rebecca P
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University of Chicago
Internal Medicine/Medicine
Schools of Medicine
United States
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Hernandez, W; Gamazon, E R; Aquino-Michaels, K et al. (2017) Integrated analysis of genetic variation and gene expression reveals novel variant for increased warfarin dose requirement in African Americans. J Thromb Haemost 15:735-743
Daneshjou, Roxana; Cavallari, Larisa H; Weeke, Peter E et al. (2016) Population-specific single-nucleotide polymorphism confers increased risk of venous thromboembolism in African Americans. Mol Genet Genomic Med 4:513-20
Hernandez, Wenndy; Gamazon, Eric R; Smithberger, Erin et al. (2016) Novel genetic predictors of venous thromboembolism risk in African Americans. Blood 127:1923-9
Kubo, K; Ohara, M; Tachikawa, M et al. (2016) Population differences in S-warfarin pharmacokinetics among African Americans, Asians and whites: their influence on pharmacogenetic dosing algorithms. Pharmacogenomics J :
Hernandez, Wenndy; Aquino-Michaels, Keston; Drozda, Katarzyna et al. (2015) Novel single nucleotide polymorphism in CYP2C9 is associated with changes in warfarin clearance and CYP2C9 expression levels in African Americans. Transl Res 165:651-7
Nagai, Rui; Ohara, Minami; Cavallari, Larisa H et al. (2015) Factors influencing pharmacokinetics of warfarin in African-Americans: implications for pharmacogenetic dosing algorithms. Pharmacogenomics 16:217-25
Hernandez, W; Gamazon, E R; Aquino-Michaels, K et al. (2014) Ethnicity-specific pharmacogenetics: the case of warfarin in African Americans. Pharmacogenomics J 14:223-8
Cavallari, Larisa H; Vaynshteyn, David; Freeman, Kimberly M et al. (2013) CYP2C9 promoter region single-nucleotide polymorphisms linked to the R150H polymorphism are functional suggesting their role in CYP2C9*8-mediated effects. Pharmacogenet Genomics 23:228-31
Perera, Minoli A; Cavallari, Larisa H; Limdi, Nita A et al. (2013) Genetic variants associated with warfarin dose in African-American individuals: a genome-wide association study. Lancet 382:790-6
Cavallari, Larisa H; Perera, Minoli A (2012) The future of warfarin pharmacogenetics in under-represented minority groups. Future Cardiol 8:563-76

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