A number sign (#) is used with this entry because of evidence that resistance and sensitivity to coumarin (warfarin) treatment can be influenced by variations in several genes, including CYP2A6 (122720), VKORC1 (608547), CYP2C9 (601130), and CYP4F2 (604426).
Coumarin sensitivity can also result from certain mutations in the factor IX gene (e.g., 300746.0103).Description
Warfarin is a widely prescribed anticoagulant for the prevention of thromboembolic diseases for subjects with deep vein thrombosis, atrial fibrillation, or mechanical heart valve replacement (Yuan et al., 2005). The dose requirement is highly variable, both interindividually and interethnically.
Variation in the VKORC1 gene is believed to be the most important individual predictor of warfarin dose, accounting for about 30% of the variance observed in dosing (Ross et al., 2010).Clinical Features
O'Reilly et al. (1964) described resistance to the hypoprothrombinemic effects of coumarin drugs in 7 persons in 3 generations of a family with no male-to-male transmission. They postulated that an autosomal gene is responsible for the synthesis of a clotting factor dependent on vitamin K and that in this family affected persons have an abnormal factor with decreased affinity for the coumarin drug or increased affinity for vitamin K. O'Reilly (1970) described a second kindred of which 18 members were shown to have relative resistance to oral anticoagulant drugs. Several instances of male-to-male transmission were observed. Of the various possible mechanisms for the relative resistance, all could be excluded except mutation in the vitamin K-anticoagulant receptor site. Positive evidence favoring the latter included the correction of hypoprothrombinemia by small amounts of exogenous vitamin K and the fact that the anticoagulant dose-response curves for the probands of the 2 families studied by O'Reilly and normal subjects are parallel. Pool et al. (1968) concluded that the resistance to warfarin is due to a decreased affinity of the receptor sites in the liver to coumarin anticoagulant drugs. This mendelian variation must be distinguished from the polygenic variation in coumarin responsiveness due to variations in metabolism of the drug.
Lewis et al. (1967) reported a single patient with warfarin resistance resulting from abnormally rapid clearance of the drug. Resistance to phenindione, a drug of different structure, was also demonstrated.
Alving et al. (1985) reported a black family in which the proposita and her daughter had relative resistance to the anticoagulant effects of warfarin. A diet deficient in vitamin K was accompanied by enhanced effects of warfarin. For vitamin K to participate in the carboxylation of factors II, VII, IX, and X, it must be in a reduced form. It becomes an epoxide as carboxylation occurs and is recycled to its reduced form by a vitamin K reductase (Whitlon et al., 1978). Warfarin has an inhibitory effect on the reductase. The genetic defect in warfarin resistance in both man and rat may result in an altered affinity of the enzyme for the drug.Mapping
Kohn and Pelz (2000) mapped the warfarin-resistance locus of the rat, Rw, and placed the ortholog on mouse chromosome 7 and 3 candidate human chromosomes, including 10q25.3-q26. The CYP2C9 gene maps to chromosome 10q24.Molecular Genetics
Rost et al. (2004) identified 4 different heterozygous mutations in the VKORC1 gene (608547.0002-608547.0005), encoding vitamin K epoxide reductase, in individuals with warfarin resistance.
In 45 patients, Shikata et al. (2004) analyzed mutations of 7 genes encoding vitamin K-dependent proteins and the CYP2C9 (601130) gene and investigated whether any contributed to the large interpatient variability in the warfarin dose-effect relationship. Multiple regression analysis revealed that warfarin sensitivity was independently associated with the -402G-A polymorphism of the factor VII gene (F7; 613878), the CAA repeat of the gamma-glutamyl carboxylase gene (GGCX; 137167), CYP2C9*3 (601130.0001), and the thr165-to-met (T165M) polymorphism of the factor II gene (F2; 176930).
In a genomewide analysis of 181 white individuals taking warfarin, Cooper et al. (2008) found that the most significant independent effect of variation in warfarin maintenance dose was conferred by SNPs in the VKORC1 gene (p = 6.2 x 10(-13)), with lesser associations with SNPs the CYP2C9 gene (p = 10(-4)). These associations were replicated in 2 populations, yielding combined p values of 4.7 x 10(-34) and 6.2 x 10(-12) for VKORC1 and CYP2C9, respectively. The warfarin dose variance explained by the 2 genes was estimated to be 25% and 9%, respectively. No significant associations with other SNPs were identified, and Cooper et al. (2008) concluded that the VKORC1 and CYP2C9 genes are the primary genetic determinants of stabilized warfarin dose and that common SNPs with large effects on warfarin dose are unlikely to be discovered outside of these 2 genes.
Among 273 African Americans and 302 European Americans undergoing warfarin therapy, Limdi et al. (2008) found that variation in the VKORC1 gene could explain 5% and 18%, respectively, of variability in warfarin dosage. An additive effect was observed when also accounting for polymorphisms in the CYP2C9 gene (8% and 30%, in African Americans and European Americans, respectively). Four common VKORC1 haplotypes were identified in European Americans, and 12 in African Americans, consistent with higher genomic sequence diversity in populations of African descent. African Americans had a lower frequency of the low-dose haplotype compared with European Americans (10.6% vs 35%, p less than 0.0001). The variability in dose explained by VKORC1 haplotype or haplotype groups was similar to that of a single informative polymorphism. Two SNPs in the VKORC1 gene (rs9934438) or (rs9923231) were best predictors of warfarin dose in both groups.
The International Warfarin Pharmacogenetics Consortium (2009) found that a pharmacogenetic dose algorithm for warfarin based on the genotype at VKORC1 and CYP2C9 accurately identified larger proportions of patients who required 21 mg of warfarin or less per week and those who required 49 mg or more per week to achieve the targeted international normalized ratio than did a clinical algorithm alone (49.4% vs 33.3%, p less than 0.001, among patients requiring 21 mg or less per week; and 24.8% vs 7.2%, p less than 0.001, among those requiring 49 mg or more per week). The authors concluded that the use of a pharmacogenetic algorithm for estimating the appropriate initial dose of warfarin produces recommendations that are significantly closer to the required stable therapeutic dose than those derived from a clinical algorithm or a fixed-dose approach. The greatest benefits were observed in the 46.2% of the population that required 21 mg or less of warfarin per week or 49 mg or more per week for therapeutic anticoagulation.
Using the Affymetrix drug-metabolizing enzymes and transporters (DMET) assay to screen SNPS from 144 genes involved in drug metabolism, Caldwell et al. (2008) found an association between a C-to-T transition (rs2108622) in the CYP4F2 gene (604426) and warfarin dose variance among individuals on warfarin therapy (p = 2.4 x 10(-7)). The findings were replicated in 2 additional cohorts, yielding a total of 1,051 individuals for all 3 cohorts. CC homozygotes required less warfarin, TT homozygotes required more warfarin, and CT heterozygotes required intermediate doses to achieve a therapeutic effect. The minor allele frequency in whites and Asians is approximately 30%, compared to 7% in blacks, predicting a lesser effect of this SNP in blacks as a population.
Borgiani et al. (2009) also reported an association between rs2108622 in the CYP4F2 gene and warfarin dose variance among 141 Italian individuals on warfarin therapy. TT homozygotes required 5.49 mg/day compared to 2.93 mg/day for CC homozygotes. Analysis of variance indicated that about 7% of mean weekly warfarin dose variance could be explained by CYP4F2 genotype. A linear regression model including CYP4F2, CYP2C9 and VKORC1 genetic variants, age, and weight, could explain 60.5% of the interindividual variability.
In a genomewide association study of 1,053 Swedish individuals, Takeuchi et al. (2009) found a significant association between warfarin dose and 2 main regions: SNPs clustering near the VKORC1 gene (p less than 10(-78)) and SNPs near CYP2C9 (p less than 10(-31)). Multiple regression analyses adjusting for known influences on warfarin dose (VKORC1, CYP2C9, age, gender) allowed further identification of an association with rs2108622 (p = 8.3 x 10(-10)) in the CYP4F2 gene. SNPs in and near the VKORC1 and CYP2C9 genes explained about 30% and 12%, of warfarin dose variance, respectively, whereas the SNP in the CYP4F2 gene explained about 1.5% of the dose variance.Population Genetics
In an editorial, Shurin and Nabel (2007) noted that evidence from various clinical and population studies suggested that patients of Asian, European, and African ancestry require, on average, lower, intermediate, and higher doses of warfarin, respectively. They suggested that additional studies involving larger numbers of patients of African and Asian descent were needed to confirm these associations (Takahashi et al., 2006).
Ross et al. (2010) reported frequency analysis of 4 SNPs affecting warfarin dosage in 963 individuals from 7 geographic regions worldwide and in 316 Canadians of European, East Asian, and South Asian ancestry. The SNPs analyzed included rs9923231 in the VKORC1 gene (608547.0006); rs1799853 and rs1057910, both in the CYP2C9 gene (601130.0002 and 601130.0001, respectively); and rs2108622 in the CYP4F2 gene (604426). The VKORC1 SNP showed the highest differentiation in allele frequency among different geographic regions.Animal Model
Hereditary resistance to warfarin in rats, which may be a comparable condition, is inherited as an autosomal dominant (Greaves and Ayres, 1967). A single gene difference in ability to 7-hydroxylate coumarin is known in mice (Wood and Conney, 1974). The gene locus, called Coh, is known to be on chromosome 7 of the mouse (see mouse gene map in fifth edition of Mendelian Inheritance in Man).
Lush and Andrews (1978) suggested that there may be 2 closely linked genes on mouse chromosome 7 determining cytochrome P-450 isozymes with different substrate specificities. Coumarin-7-hydroxylase in the mouse is encoded by a gene, symbolized Cyp2a-5 (Nebert et al., 1991), in the P450C2A family. (The CYP2A subfamily has many genes in which the orthologs in different species cannot be established for certain; therefore, each gene, as it becomes characterized, regardless of the species, is given the next available number (Nebert, 1994). When the human CYP2A3 gene was cloned, it was shown to encode IIA3, the enzyme for coumarin-7-hydroxylase (Yamano et al., 1990). Unfortunately, the CYP2A3 designation had already been taken for the rat gene and it was uncertain that the human gene was orthologous to the rat gene. Therefore, the human IIA3 gene product is encoded by a gene which is now designated CYP2A6 (Nebert, 1994).)
Lindberg et al. (1992) compared the Coh locus in mouse strains having high coumarin 7-hydroxylase activity (H) with those having low activity (L). The nucleotide sequences of cDNAs from the 2 strains differed by a single base, which resulted in an amino acid difference at position 117: val in P450coh(H) and ala in P450coh(L). They found evidence that a recent duplication in the ancestral mouse established the line of descent from the ancestral P450coh gene to the P450 enzyme with steroid 15-alpha-hydroxylase activity. During evolution, amino acid substitutions occurred selectively at positions that altered the enzyme's substrate specificity and increased its specific activity.