Compound heterozygosity of novel missense mutations in the gamma-glutamyl-carboxylase gene causes hereditary combined vitamin K-dependent coagulation factor deficiency

Hereditary combined vitamin K-dependent (VKD) coagulation factor deficiency is an autosomal recessive bleeding disorder associated with defects in either the gamma-carboxylase, which carboxylates VKD proteins to render them active, or the vitamin K epoxide reductase (VKORC1), which supplies the reduced vitamin K cofactor required for carboxylation. Such deficiencies are rare, and we report the fourth case resulting from mutations in the carboxylase gene, identified in a Tunisian girl who exhibited impaired function in hemostatic VKD factors that was not restored by vitamin K administration. Sequence analysis of the proposita did not identify any mutations in the VKORC1 gene but, remarkably, revealed 3 heterozygous mutations in the carboxylase gene that caused the substitutions Asp31Asn, Trp157Arg, and Thr591Lys. None of these mutations have previously been reported. Family analysis showed that Asp31Asn and Thr591Lys were coallelic and maternally transmitted while Trp157Arg was transmitted by the father, and a genomic screen of 100 healthy individuals ruled out frequent polymorphisms. Mutational analysis indicated wild-type activity for the Asp31Asn carboxylase. In contrast, the respective Trp157Arg and Thr591Lys activities were 8% and 0% that of wild-type carboxylase, and their compound heterozygosity can therefore account for functional VKD factor deficiency. The implications for carboxylase mechanism are discussed.

The influence of sequence variations in factor VII, gamma-glutamyl carboxylase and vitamin K epoxide reductase complex genes on warfarin dose requirement

The degree of interpatient variability in the warfarin dose required to achieve the desired anticoagulant response can only partly be explained by polymorphisms in the CYP2C9 gene, suggesting that additional genetic factors such as polymorphisms in genes involved in blood coagulation may influence warfarin dose requirement. In total, 165 Caucasian outpatients on stable maintenance warfarin treatment previously genotyped for CYP2C9 were analysed for common polymorphisms in FVII, GGCX and VKORC1 genes. The -402G > A polymorphism and a variable number of repeats in intron 7 of FVII gene did not significantly influence warfarin dose. The mean warfarin doses increased with the number of (CAA) repeats in the GGCX gene, but the differences were significant only in the CYP2C9*1/*1 subgroup of patients (p = 0.032). Common polymorphism (6484C > T) in intron 1 of the VKORC1 gene led to lower warfarin dose requirement; the means were 5.70 (95% C.I. 4.95-6.45), 3.49 (3.07-3.90) and 2.11 (1.80-2.42) mg/day for 6484 CC, CT and TT genotypes, respectively (p < 0.001). In contrast, 9041G > A polymorphism in 3'UTR of theVKORC1 gene led to higher warfarin dose requirement; the means were 3.09 (2.58- 3.60), 4.26 (3.69-4.82) and 5.86 (4.53-7.19) mg/day for 9041 GG, GA and AA genotypes, respectively (p < 0.001). With a regression model we explained 60.0% of variability in warfarin dose, which was due to gene polymorphisms (CYP2C9, VKORC1), age and body-surface-area. When aiming for individualised warfarin therapy, at least VKORC1 polymorphisms should be included in predictive genotyping besides CYP2C9.

Warfarin resistance in a French strain of rats

A warfarin-resistant strain and a warfarin-susceptible strain of wild rats (Rattus norvegicus) maintained in enclosures of the National Veterinary School of Lyon (France) were studied to determine the mechanism of the resistance to anticoagulant rodenticides. A low vitamin K epoxide reductase (VKOR) activity has been reported for many resistant rat strains. As recently suggested, mutations in the vitamin K epoxide reductase subunit 1 (VKORC1) gene are the genetic basis of anticoagulant resistance in wild populations of rats from various locations in Europe. Here we report, for our strain, one of the seven described mutations (Tyr139Phe) for VKORC1 in rats. In addition, a low expression of mRNA encoding VKORC1 gene is observed in resistant rats, which could explain their low VKOR activity. We calculated kinetic parameters of VKOR in the warfarin-resistant and warfarin-susceptible rats. The V(max) and the K(m) of the VKOR obtained in resistant rats were lowered by 57 and 77%, respectively, compared to those obtained in susceptible rats. As a consequence, the enzymatic efficiency (V(m)/K(m)) of the VKOR was similar between resistant and susceptible rats. This result could be a good explanation to the observation that no clinical signs of vitamin K deficiency was observed in the warfarin-resistant strain, while a low VKOR activity was found. VKOR activity in warfarin-resistant rats was poorly inhibited by warfarin (K(i) for warfarin is 29 microM and 0.72 microM for resistant and susceptible rats, respectively).

Polymorphisms in the VKORC1 gene are strongly associated with warfarin dosage requirements in patients receiving anticoagulation

BACKGROUND

Warfarin is a mainstay of therapy for conditions associated with an increased risk of thromboembolic events. However, the use of this common agent is fraught with complications and little is known regarding inter-individual variation in warfarin response.

OBJECTIVE

We tested for association between single nucleotide polymorphisms (SNPs) in VKORC1 and CYP2C9 and average weekly warfarin dose required to maintain patients at their desired anticoagulation target.

METHODS

The sample consisted of 93 European-American patients from anticoagulation clinics at the University of North Carolina at Chapel Hill. Data on mean weekly warfarin dose were collected over a mean treatment period of 20.6 months. ANCOVA models were used and haplotype analysis was performed.

RESULTS

Three of six VKORC1 SNPs were found to be very strongly associated with the average warfarin dose required to achieve the target international normalised ratio (INR; p<0.0001). The mean weekly dose by genotype ranged from approximately 27 to 47 mg. There was no evidence for an association between either of the two CYP2C9 polymorphisms studied, CYP2C9*2 and CYP2C9*3. CYP2C9*3 was significantly (p = 0.05) associated with average warfarin dosage after adjustment for VKORC1*1173.

CONCLUSIONS

These results are of considerable clinical interest and confirm recently published results regarding the role of these two genes in modifying warfarin metabolism and maintenance dosage. The consistent findings regarding the role of VKORC1 and CYP2C9 in warfarin metabolism and maintenance dosage represent a clinically useful proof of principal for the use of pharmacogenomic information in medicine and may lead to improved understanding of warfarin's actions.

The c.-1639G > A polymorphism of the VKORC1 gene is a major determinant of the response to acenocoumarol in anticoagulated patients

Much of the variability in the sensitivity to warfarin in anticoagulated patients is associated with the c.-1639G > A polymorphism of the vitamin K-epoxide reductase (VKORC1) gene. However, its association with the acenocoumarol dose in patients under anticoagulant therapy has not been studied. The c.-1639G > A genotype of VKORC1 was determined in 113 patients on stable anticoagulation requiring low (n = 42), medium (n = 42) or high (n = 21) acenocoumarol doses. To evaluate the association between acenocoumarol requirements and the c.-1639G > A variant, multivariate logistic regression models were fitted, adjusting for age, gender, and the c.430C > T and c.1075A > C variants of cytochrome P450 2C9 (CYP2C9). A total of 90.5% of the patients in the low acenocoumarol dose group carried the A allele of VKORC1:c.-1639G > A. The A allele independently increased the odds of requiring a low acenocoumarol dose [odds ratio (OR) 9.4; 95% confidence interval (CI) 1.9-46.4; P = 0.006], especially when the homozygous form was present (OR 44.2; 95% CI 5.5-354.6; P < 0.001). The A allele was less frequent in the high dose group showing an inverse association with the requirement for high doses (OR 0.04; 95% CI 0.01-0.22; P < 0.001). The A allele of the c.-1639G > A polymorphism of VKORC1 is therefore associated with a low-dose requirement for acenocoumarol in patients receiving anticoagulant therapy.

VKORC1 haplotypes are associated with arterial vascular diseases (stroke, coronary heart disease, and aortic dissection)

BACKGROUND

The haplotypes in the gene vitamin K epoxide reductase complex subunit 1 (VKORC1) have been found to affect warfarin dose response through effects on the formation of reduced-form vitamin K, a cofactor for gamma-carboxylation of vitamin K-dependent proteins, which is involved in the coagulation cascade and has a potential impact on atherosclerosis. We hypothesized that VKORC1-dependent effects on the coagulation cascade and atherosclerosis would contribute to susceptibility for vascular diseases.

METHODS AND RESULTS

To test the hypothesis, we studied the association of polymorphisms of VKORC1 with stroke (1811 patients), coronary heart disease (740 patients), and aortic dissection (253 patients) compared with matched controls (n=1811, 740, and 416, respectively). Five common noncoding single-nucleotide polymorphisms of VKORC1 were identified in a natural haplotype block with strong linkage disequilibrium (D'>0.9, r2>0.9), then single-nucleotide polymorphism (SNP) +2255 in the block was selected for the association study. We found that the presence of the C allele of the +2255 locus conferred almost twice the risk of vascular disease (odds ratio [OR] 1.95, 95% confidence interval [CI] .58 to 2.41, P<0.001 for stroke; OR 1.72, 95% CI 1.24 to 2.38, P<0.01 for coronary heart disease; and OR 1.90, 95% CI 1.04 to 3.48, P<0.05 for aortic dissection). We also observed that subjects with the CC and CT genotypes had lower levels of undercarboxylated osteocalcin (a regulator for the bone), probably vascular calcification, and lower levels of protein induced in vitamin K absence or antagonism II (PIVKA-II, a des-gamma-carboxy prothrombin) than those with TT genotypes.

CONCLUSIONS

The haplotype of VKORC1 may serve as a novel genetic marker for the risk of stroke, coronary heart disease, and aortic dissection.

Vitamin K epoxide reductase complex subunit 1 (VKORC1): the key protein of the vitamin K cycle

Vitamin K epoxide, a by-product of the carboxylation of blood coagulation factors, is reduced to vitamin K by an enzymatic system possessing vitamin K epoxide reductase (VKOR) activity. This system is the target of coumarin-derived drugs widely used in thrombosis therapy and prophylaxis. Recently, the key protein of the VKOR system has been identified. The human VKORC1 gene maps to chromosome 16 and consists of 3 exons encoding a 163-amino acid integral ER membrane protein with three or four predicted transmembrane alpha- helices. Expression of human VKORC1 in Spodoptera frugiperda (Sf9) cells and in Pichia pastoris results in enhanced VKOR activity over low endogenous constitutive levels. Sequence based search methods reveal that human VKORC1 belongs to a large family of homologous genes found in vertebrates, insects, plants, protists, archea, and bacteria. All orthologs share five completely conserved amino acids, including two cysteines found in a tetrapeptide motif presumably required for redox function. The recent discovery of the VKORC1 gene has initiated renewed interest in understanding VKOR activity. Analysis of VKORC1 protein structure and function will be crucial in understanding the VKOR catalytic mechanism, how anticoagulant drugs modulate VKOR activity, and the role of VKORC1 in downstream physiological and pathological pathways.

[Pharmacogenetics of warfarin]

There are significant differences among patients treated with warfarin in the dosage volumes necessary to reach an optimum therapeutic effect. Apart from the external influences (interactions with drugs and food), genetic predispositions play an important role. Polymorphysms of the P 450 2C9 cytochrome bear upon the speed ofbreaking down S-warfarin, polymorfysms VKORC1 bear on the volume and quality of epoxide reductase--an enzyme whose blockade is the crux of the mechanism how cumarin anticoagulants act. These two genes are responsible for at least 50% of the warfarin effect variability. Warfarin's effect is further determined by the genetic variants of gamma-carboxylase, prothrombin, factors VII and IX. In near future, further results of pharmacogenetic research and clinical studies can be expected. They study the impact of the findings in clinical practice.

Interethnic variability of warfarin maintenance requirement is explained by VKORC1 genotype in an Asian population

BACKGROUND

Chinese and Malay subjects have been reported to require less maintenance warfarin than Indians that could not be accounted for by cytochrome P450 (CYP) 2C9 variants. Vitamin K epoxide reductase complex 1 (VKORC1) is the target enzyme of warfarin, and VKORC1 intronic variants and haplotypes have recently been shown to influence VKORC1 activity and warfarin requirements.

METHODS

We sequenced the coding regions of CYP2C9 and VKORC1 and inferred VKORC1 haplotype from 10 intronic variants in 147 Chinese, 85 Malay, and 43 Indian patients receiving maintenance warfarin.

RESULTS

The mean weight-normalized warfarin dose was lower for Chinese and Malays than for Indians (0.058 +/- 0.025 mg/kg, 0.059 +/- 0.023 mg/kg, and 0.089 +/- 0.036 mg/kg, respectively; P < .001 for comparisons between Chinese and Malays with Indians). CYP2C9*2 and VKORC1 coding region variants were rare (<2%), whereas CYP2C9*3 associated with lower warfarin requirements was less common in Chinese and Malays (7% and 9%, respectively) than in Indians (18%) and could not account for their lower warfarin requirements. VKORC1 H1 and H7/H8/H9 haplotypes were associated with lower and higher warfarin requirements, respectively (0.050 +/- 0.019 mg/kg and 0.092 +/- 0.057 mg/kg, respectively; P < .001). VKORC1 H1 haplotype (requiring low warfarin doses) was common in Chinese (87%) and Malays (65%) but uncommon in Indians (12%), whereas H7, H8, and H9 haplotypes (requiring high warfarin doses) were rare in Chinese (9%), intermediate in Malays (30%), and common in Indians (82%). The interethnic difference in warfarin requirements became nonsignificant when adjusted for VKORC1 haplotype.

CONCLUSIONS

Interethnic difference in VKORC1 haplotypes accounts for the difference in warfarin requirements between Chinese, Malays, and Indians, providing interesting insights into genetic variation between ethnogeographically distinct Asian groups.

Influence of coagulation factor, vitamin K epoxide reductase complex subunit 1, and cytochrome P450 2C9 gene polymorphisms on warfarin dose requirements

INTRODUCTION

The primary objective of this study was to determine whether variability in warfarin dose requirements is determined by common polymorphisms in genes whose products are involved in the pharmacodynamics and pharmacokinetics of warfarin, namely, the coagulation factors, vitamin K epoxide reductase complex subunit 1 (VKORC1), and cytochrome P450 (CYP) 2C9.

METHODS

Patients (N = 350) receiving stable doses of warfarin at 3 consecutive visits were enrolled, and a deoxyribonucleic acid sample was collected. Samples were genotyped for polymorphisms in the factor II, factor VII, factor X, VKORC1, and CYP2C9 genes. A stepwise linear regression analysis was used to determine the independent effects of genetic and nongenetic factors on mean warfarin dose requirements.

RESULTS

Variables associated with lower warfarin dose requirements were VKORC1 3673 AA genotype (P < .0001), VKORC1 3673 GA genotype (P < .0001), 1 variant CYP2C9 allele (P < .0001), 2 variant CYP2C9 alleles (P = .0004), increasing age (P = .0005), concomitant CYP2C9 inhibitors (P = .0005), and goal international normalized ratio (P = .01). Variables associated with higher warfarin dose requirements were weight (P < .0001), current smoker status (P = .0009), mean international normalized ratio (P = .001), concomitant CYP2C9 inducers (P = .006), factor X insertion/deletion genotype (P = .01), factor X insertion/insertion genotype (P = .04), factor VII deletion/deletion genotype (P = .04), and calculated vitamin K intake (P = .05). The linear regression model explained 51.4% of the variability in warfarin dose requirements.

CONCLUSION

Polymorphisms in warfarin drug target and metabolizing enzyme genes, in addition to nongenetic factors, were important determinants of warfarin dose requirements.

Different contributions of polymorphisms in VKORC1 and CYP2C9 to intra- and inter-population differences in maintenance dose of warfarin in Japanese, Caucasians and African-Americans

OBJECTIVE

To investigate pharmacokinetic and pharmacodynamic factors associated with population differences in warfarin doses needed to achieve anticoagulation, in particular the possible involvement of genetic variability in vitamin K epoxide reductase (VKOR) and CYP2C9.

METHODS

Warfarin maintenance dose, unbound plasma S-warfarin concentration [Cu(S)] and INR were determined in 157 Caucasians, 172 Japanese, and 36 African-Americans stably anticoagulated patients. In a subset (n = 166), fully carboxylated plasma normal prothrombin levels (NPT) were also measured. Genotyping for seven CYP2C9 (CYP2C9*1 through 6 and *11) and seven VKORC1 variants were performed in 115 Caucasians and 64 Japanese patients and 66 healthy African-Americans. Multivariate analysis was performed to identify covariates associated with warfarin requirement.

RESULTS

The relationship between NPT and Cu(S) indicated Japanese are more susceptible to inhibition of NPT production by S-warfarin than the other two populations. VKORC1 1173 C > T had a greater frequency in Japanese (89.1%) than Caucasians (42.2%) and African-Americans (8.6%). CYP2C9 variants with reduced metabolizing ability were less frequent in Japanese compared to the other two populations. The median warfarin dose was significantly higher in Caucasians than Japanese patients (5.5 versus 3.5 mg/day), however, when matched for CYP2C9*1 homozygosity, no difference in dose was observed between VKORC1 genotype-matched groups. Furthermore, VKORC1 1173C > T and CYP2C9 (*2/*3/*11) genotypes, age and weight were identified as independent covariates contributing to interpatient variability in warfarin dosage.

CONCLUSIONS

Both VKORC1 and CYP2C9 polymorphisms contribute to inter-population difference in warfarin doses among the three populations, but their contribution to intra-population variability may differ within each population.

Contribution of CYP2C9 to variability in vitamin K antagonist metabolism

CYP2C9 is the third most important cytochrome P450 (CYP) in terms of number of drugs metabolised. A considerable amount of information on this isoform is now available with respect to its structural biology, the mechanisms by which it can be induced and the existence of a range of variant alleles, which are often functionally significant. CYP2C9 makes a very important contribution to metabolism of vitamin K antagonist anticoagulants, and is the main oxidising enzyme for S-warfarin and S-acenocoumarol as well as contributing to phenprocoumon metabolism. A large number of studies have now shown that CYP2C9 genotype predicts dose requirement for both warfarin and acenocoumarol, with a possible contribution for phenprocoumon. Patients with variant alleles are likely to require a lower dose and may be at risk of overcoagulation and resultant bleeding, especially during the induction phase of therapy. Although CYP2C9 genotype is clearly a predictor of vitamin K antagonist dose requirement, especially in Caucasian populations in whom variant alleles are common, a number of recent studies have shown that age, genotype for the gene encoding the target gene vitamin K epoxide reductase and concomitant drugs are equally important factors in determining dose. There is a need for prospective studies to assess the value of predicting dose requirement on the basis of all these factors, including the CYP2C9 genotype.

Association of VKORC1 and CYP2C9 polymorphisms with warfarin dose requirements in Japanese patients

Warfarin is the most commonly used oral anticoagulant for treatment of thromboembolism, but adjustment of the dose appropriate to each patient is not so easy because of the large inter-individual variation in dose requirement. We analyzed single nucleotide polymorphism (SNP) genotypes of the VKORC1 and CYP2C9 genes using DNA from 828 Japanese patients treated with warfarin, and investigated association between SNP genotype and warfarin-maintenance dose. Five SNPs in VKORC1, 5' flanking-1413A > G, intron 1-136T > C, intron 2+124C > G, intron 2+837T > C and exon 3 343G > A, were in absolute linkage disequilibrium, and showed a significant association with daily warfarin dose of these patients. The median warfarin dose of patients with homozygosity for the minor allele was 4.0 mg/day, which is significantly higher than those heterozygous for the minor allele (3.5 mg/day) or those homozygous for the major allele (2.5 mg/day; P = 5.1 x 10(-11) in the case of intron 1-136T > C SNP). We then genotyped the CYP2C9 gene for the Japanese common genetic variant, CYP2C9*3 and, based on the genotype of these two genes, classified patients into three categories, which we call "warfarin-responsive index." The median warfarin daily dose varied significantly in this classification according to the warfarin-responsive index (2.0 mg/day for index 0 group, 2.5 mg/day for index 1 group, and 3.5 mg/day for index 2 group; P = 4.4 x 10(-13)). Thus, analysis of the combination of VKORC1 and CYP2C9 genotypes should identify warfarin-sensitive patients who require a lower dose of drug, allowing personalized warfarin treatment.

Pharmacogenetics of Target Genes Across the Warfarin Pharmacological Pathway

Warfarin is a widely prescribed anticoagulant for thromboembolic disorders and exhibits wide inter-individual differences in its pharmacodynamic effects. Warfarin exerts its anticoagulant effect by inhibiting the enzymatic activity of vitamin K 2,3-epoxide reductase complex, subunit 1 (VKORC1) which regenerates reduced vitamin K as an essential cofactor for the post-translational gamma-carboxylation of glutamic acid residues on coagulation factors II, VII, IX and X, and the anticoagulant proteins C, S and Z. Recent studies have shown polymorphisms in genes involved in the uptake of vitamin K (apolipoprotein E [ApoE]), reduction of vitamin K 2,3-epoxide (VKORC1), metabolism of warfarin (cytochrome P450 2C9 [CYP2C9]), and gamma carboxylation (gamma-glutamyl carboxylase [GGCX]) to influence the pharmacokinetics and pharmacodynamics of warfarin in patients from different ethnic backgrounds, resulting in variable warfarin dose requirements. Understanding the causal relationship of these polygenic influences on warfarin dose requirements in patients of different ethnicity may be vital in reducing inter-patient variability and optimising anticoagulant therapy.

Heterogeneity of the coumarin anticoagulant targeted vitamin K epoxide reduction system. Study of kinetic parameters in susceptible and resistant mice (Mus musculus domesticus)

Vitamin K epoxide reductase (VKOR) activity in liver microsomes from a susceptible and a genetically warfarin-resistant strain of mice (Mus Musculus domesticus) was analyzed to determine the mechanism of resistance to this 4-hydroxycoumarin derivative. Kinetic parameters for VKOR were calculated for each strain by incubating liver microsomes with vitamin K epoxide +/- warfarin. In susceptible mice, an Eadie-Hofstee plot of the data was not linear and suggested the involvement of at least two different components. Apparent kinetic parameters were obtained by nonlinear regression using a Michaelis--Menten model, which takes into account two enzymatic components. Component A presents a high Km and a high Vm, and as a consequence only an enzymatic efficiency Vm/Km was obtained (0.0024 mL/min/mg). Estimated warfarin Ki was 0.17 microM. Component B presented an apparent Km of 12.73 microM, an apparent Vm of 0.32 nmol/min/mg, and an apparent Ki for warfarin of 6.0 microM. In resistant mice, the enzymatic efficiency corresponding to component A was highly decreased (0.0003-0.00066 mL/min/mg) while the Ki for warfarin was not modified. The apparent Vm of component B was poorly modified between susceptible and resistant mice. The apparent Km of component B observed in resistant mice was similar to the Km observed in susceptible mice. These modifications of the catalytic properties are associated with a single nucleotide polymorphism (T175G) in the VKOR-C1 gene, which corresponds to a Trp59Gly mutation in the protein.

Pharmacogenetics-Based Coumarin Therapy

To reduce the risk of hemorrhage, experts advocate prescribing the anticipated therapeutic dose to patients who are beginning coumarin therapy, but until now there was no accurate way to estimate that dose. Using pharmacogenetics-based coumarin therapy, clinicians can now estimate the therapeutic dose by genotyping their patients for single nucleotide polymorphisms (SNPs) that affect coumarin metabolism or sensitivity.

SNPs in the cytochrome P450 complex (CYP2C9) affect coumarin metabolism. Patients with either of two common variants, CYP2C9*2 or CYP2C9*3, metabolize coumarins slowly and are twice as likely to have a laboratory or clinical adverse event, unless their initial coumarin doses are reduced. SNPs in vitamin K epoxide reductase (VKORC1) correlate with coumarin sensitivity. Patients known to be homozygous for a common VKORC1 promoter polymorphism, −1639 G>A (also designated as VKOR 3673, haplotype A, or haplotype*2), should be started on lower coumarin doses than genotype GG patients. By providing an estimate of the therapeutic coumarin dose, pharmacogenetics-based therapy may improve the safety and effectiveness of coumarin therapy.

[Pharmacogenetics and interindividual variability in drug response: cytochrome P-450 2C9 and coumarin anticoagulants]

Pharmacogenetics, a discipline still in its infancy, is the study of genetically determined variations in how individuals respond to drugs. Mutations may affect drug metabolism, transmembrane transport into cells, or target receptors. Genetic polymorphisms affecting drug metabolism were the first to be identified. Genetic factors control the activity of phase I reactions involving cytochrome (CYP) P450 isoenzymes. Three CYP families catalyze drug metabolism in humans. Their genes have been identified and polymorphisms have been described in various populations, leading to either high activity ("extensive metabolizer" phenotype) or low activity ("poor metabolizer" phenotype). The CYP2C9 polymorphism illustrates the potential clinical importance of pharmacogenetics. This enzyme catalyzes the metabolism of the coumarinic oral anticoagulants acenocoumarol and warfarin. The homozygous mutant genotype CYP 2C9 *31*3, present in 0, 7% of Caucasians, leads to low enzyme activity and thus to the accumulation of these drugs in the body; this in turn increases the anticoagulant activity and induces a higher risk of bleeding. In three clinical studies of patients and healthy volunteers, we found that this CYP2C9 *3 mutant allele was responsible for 14% of the variability in the response to these drugs. Then, by studying the genetic polymorphism of the receptor site of oral anticoagulants--the vitamin K epoxide reductase multiprotein complex--we showed that a combination of the two genetic variants (CYP2C9 and the receptor site) was responsible for 50% of the variability. These data suggest that patients who have both genetic polymorphisms could be at an increased risk of bleeding during oral anticoagulant therapy.

[Pharmacogenetics of indirect anticoagulants: value of genotype for improvement of efficacy and safety of therapy]

The review is devoted to the contemporary state of the problem of pharmacogenetics of indirect anticoagulants. At present there are data about effects of allele variants of CYP2C9, VKORC1, APOE genes on efficacy and safety of therapy with indirect anticoagulants. Detection of these variants is a perspective way to individualization of therapy with indirect anticoagulants.

Site-directed mutagenesis of coumarin-type anticoagulant-sensitive VKORC1: evidence that highly conserved amino acids define structural requirements for enzymatic activity and inhibition by warfarin

Coumarin and homologous compounds are the most widely used anticoagulant drugs worldwide. They function as antagonists of vitamin K, an essential cofactor for the posttranslational gamma-glutamyl carboxylation of the so-called vitamin K-dependent proteins. As vitamin K hydroquinone is converted to vitamin K epoxide (VKO) in every carboxylation step, the epoxide has to be recycled to the reduced form by the vitamin K epoxide reductase complex (VKOR). Recently, a single coumarin-sensitive protein of the putative VKOR enzyme complex was identified in humans (vitamin K epoxide reductase complex subunit 1, VKORC1). Mutations in VKORC1 result in two different phenotypes: warfarin resistance (WR) and multiple coagulation factor deficiency type 2 (VKCFD2). Here,we report on the expression of site-directed VKORC1 mutants, addressing possible structural and functional roles of all seven cysteine residues (Cys16, Cys43, Cys51, Cys85, Cys96, Cys132, Cys135), the highly conserved residue Ser/Thr57, and Arg98, known to cause VKCFD2 in humans. Our results support the hypothesis that the C132-X-X-C135 motif in VKORC1 comprises part of the redox active site that catalyzes VKO reduction and also suggest a crucial role for the hydrophobic Thr-Tyr-Ala motif in coumarin binding. Furthermore, our results support the concept that different structural components of VKORC1 define the binding sites for vitamin K epoxide and coumarin.

VKORC1 haplotypes and their impact on the inter-individual and inter-ethnical variability of oral anticoagulation

In order to elucidate the role of VCORC1 sequence variants in warfarin sensitivity, we established a complete SNP map of the VKORC1 gene locus in 200 blood donors from Western Germany. Nearly all of the genetic variability of the VKORC1 gene in Europeans is reflected by three main haplotypes. Recently described polymorphisms associated with low warfarin dose requirement (dbSNP:rs9934438; dbSNP:rs17878363) were found in complete linkage disequilibrium with the VKORC1*2 haplotype. In two patient cohorts of European origin with either increased coumarin sensitivity (n= 14) or partial coumarin resistance (n=36) the VKORC1*2 frequency varied highly significant between the two groups and also when compared to 200 blood donor controls (coumarin sensitive 96%, coumarin resistant 7%, controls 42%) thus demonstrating a strong association between these two phenotypes and the VKORC1 haplotype (p = 1.6 x 10(-8) for coumarin sensitive and p = 1.9 x 10(-8) for coumarin resistant). Analysis of database derived VKORC1 genotypes of African Americans and Chinese revealed that haplotype frequencies in these populations differ significantly from the European sample (for VKORC1*2: Europeans 42%, Chinese 95%, African Americans 14%). These observations suggest VKORC1 as principal genetic modulator of the ethnic differences in warfarin response. Since hereditary pharmacodynamic (VKORC1) and pharmacokinetic (CYP2C9) factors account for up to 50% of the inter-individual variability of the warfarin response, these genetic markers may serve as clinically relevant predictors of warfarin dosing in future studies.

Vitamin K epoxide reductase significantly improves carboxylation in a cell line overexpressing factor X

Previously we reported that we could increase the fraction of carboxylated factor X by reducing the affinity of the propeptide for its binding site on human gamma glutamyl carboxylase. We attributed this to an increased turnover rate. However, even with the reduced affinity propeptide, when sufficient overproduction of factor X is achieved, there is still a significant fraction of uncarboxylated recombinant factor X. We report here that the factor X of such a cell line was only 52% carboxylated but that the fraction of carboxylated factor X could be increased to 92% by coexpressing the recently identified gene for vitamin K epoxide reductase. Because vitamin K is in excess in both the untransfected and vitamin K epoxide reductase (VKOR)–transfected cells, the simplest explanation for this result is that VKOR catalyzes both the reduction of vitamin K epoxide to vitamin K and the conversion of vitamin K to vitamin K hydroquinone. In addition to its mechanistic relevance, this observation has practical implications for overproducing recombinant vitamin K–dependent proteins for therapeutic use.

Vitamin k epoxide reductase: a protein involved in angiogenesis

Vitamin K epoxide reductase (VKOR) is a newly identified protein which has been reported to convert the epoxide of vitamin K back to vitamin K, a cofactor essential for the posttranslational gamma-carboxylation of several blood coagulation factors. We found that the gene is expressed ubiquitously including vascular endothelial cells, smooth muscle cells, fibroblasts and cardiomyocytes, and is overexpressed in 11 tumor tissues on microarray. Stable transfection of VKOR cDNA into tumor cell line A549 and H7402 did not promote the cell proliferation. These results promoted us to hypothesize that VKOR may also be involved in angiogenesis. To test this hypothesis, the expression of VKOR was studied in different vascular cells in developmental and pathologic heart tissues. The effects of overexpression and suppressing expression of VKOR on endothelial cell proliferation, migration, adhesion, and tubular network formation were explored. We found that VKOR expression in arteries was prominent in vascular endothelial cells and was high in the ventricular aneurysm tissue of human heart and human fetal heart. In vitro studies showed that overexpression of VKOR slightly but significantly stimulated human umbilical vein endothelial cell proliferation (by 120%), migration (by 118%), adhesion (by 117%), as well as tubular network formation. Antisense to VKOR gene inhibited the proliferation (by 67%), migration (by 64%), adhesion (by 50%), and tubular network formation. Our findings support the impact of VKOR in the process of angiogenesis; hence, the molecule may have a potential application in cardiovascular disease and cancer therapy.

Common VKORC1 and GGCX polymorphisms associated with warfarin dose

We report a novel combination of factors that explains almost 60% of variable response to warfarin. Warfarin is a widely used anticoagulant, which acts through interference with vitamin K epoxide reductase that is encoded by VKORC1. In the next step of the vitamin K cycle, gamma-glutamyl carboxylase encoded by GGCX uses reduced vitamin K to activate clotting factors. We genotyped 201 warfarin-treated patients for common polymorphisms in VKORC1 and GGCX. All the five VKORC1 single-nucleotide polymorphisms covary significantly with warfarin dose, and explain 29-30% of variance in dose. Thus, VKORC1 has a larger impact than cytochrome P450 2C9, which explains 12% of variance in dose. In addition, one GGCX SNP showed a small but significant effect on warfarin dose. Incorrect dosage, especially during the initial phase of treatment, carries a high risk of either severe bleeding or failure to prevent thromboembolism. Genotype-based dose predictions may in future enable personalised drug treatment from the start of warfarin therapy.

The impact of CYP2C9 and VKORC1 genetic polymorphism and patient characteristics upon warfarin dose requirements: proposal for a new dosing regimen

Current dosing algorithms do not account for genetic and environmental factors for warfarin dose determinations. This study investigated the contribution of age, CYP2C9 and VKORC1 genotype, and body size to warfarin-dose requirements. Studied were 297 patients with stable anticoagulation with a target international normalized ratio (INR) of 2.0 to 3.0. Genetic analyses for CYP2C9 (*2 and *3 alleles) and VKORC1 (-1639 polymorphism) were performed and venous INR and plasma R- and S-warfarin concentrations determined. The mean warfarin daily dose requirement was highest in CYP2C9 homozygous wild-type patients, compared with those with the variant *2 and *3 alleles (P &lt; .001) and highest in patients with the VKORC1 (position -1639) GG genotype compared with those with the GA genotype and the AA genotype (P &lt; .001). Mean warfarin daily dose requirements fell by 0.5 to 0.7 mg per decade between the ages of 20 to 90 years. Age, height, and CYP2C9 genotype significantly contributed to S-warfarin and total warfarin clearance, whereas only age and body size significantly contributed to R-warfarin clearance. The multivariate regression model including the variables of age, CYP2C9 and VKORC1 genotype, and height produced the best model for estimating warfarin dose (R2 = 55%). Based upon the data, a new warfarin dosing regimen has been developed. The validity of the dosing regimen was confirmed in a second cohort of patients on warfarin therapy.

A C1173T dimorphism in the VKORC1 gene determines coumarin sensitivity and bleeding risk

BACKGROUND

A C1173T polymorphism in intron 1 of the VKORC1 gene has been claimed to determine the interindividual variability in the response to vitamin K antagonist therapy (VKA), but it is unknown whether it also influences bleeding risk. We aimed to confirm the relationship between C1173T status and phenprocoumon or acenocoumarol use, and to examine the risk of severe bleeding for the various genotypes.

METHODS AND FINDINGS

We studied this in a case-control study of 110 patients who bled during VKA therapy and 220 control patients free of bleeding under the same therapy. To achieve the same target INR, CT genotype and TT genotype control patients required less phenprocoumon (CC genotype 2.9 mg/d [95% confidence interval (CI): 2.6-3.2], CT genotype 2.6 mg/d [95% CI: 2.1-3.1], TT genotype 1.4 mg/d [95 % CI: 1.1-1.7]) or acenocoumarol (CC genotype 3.2 mg/d [95% CI: 2.9-3.5], CT genotype 2.3 mg/d [95% CI: 2.1-2.5], TT genotype 1.7 mg/d [95% CI: 1.3-2.1]) than CC genotype control patients. Compared with CC genotype individuals, carriers of at least one T allele had an increased risk of bleeding in the phenprocoumon users (crude odds ratio = 2.6, 95% CI: 1.2-5.7), but not in acenocoumarol users (crude odds ratio = 1.2, 95% CI: 0.6-2.3).

CONCLUSION

These findings encourage taking further steps towards the evaluation of the use of VKORC1 genetic testing for bleeding prevention in individuals who receive VKA therapy.

Pharmacogenetics in the management of coumarin anticoagulant therapy: the way forward or an expensive diversion?

Greaves discusses a new study in PLoS Medicine that takes us closer to being able to adopt a pharmacogenetic approach to reduce bleeding risk from coumarin therapy.

Association of Vitamin K epoxide reductase complex 1 (VKORC1) variants with warfarin dose in a Hong Kong Chinese patient population

OBJECTIVE

To evaluate the association of VKORC1 genetic variants with warfarin dose requirements in a Hong Kong Chinese patient population.

METHODS

A retrospective study of Hong Kong Chinese patients chronically maintained on warfarin was conducted. Single nucleotide polymorphisms (SNPs) in VKORC1 and CYP2C9 were genotyped. Stable warfarin dose data were retrieved from patient medical records.

RESULTS

Sixty-nine patients were included in the analysis. VKORC1 haplotypes H1 (group A) and H7 (group B) were most common, accounting for 86% and 13% of all haplotypic variation in this cohort. Patients carrying at least one copy of a VKORC1 group B haplotype (n = 16) required a significantly higher stable warfarin dose (5.17+/-1.53 mg/day) than patients that were homozygous for group A haplotypes (n = 53; 2.93+/-1.22 mg; P < 0.001). In the VKORC1 A/A group, four patients (5.8%) were heterozygous for CYP2C9*3 and had a lower dose requirement (1.94+/-0.43 mg) than patients that exhibited the CYP2C9 *1/*1 genotype (3.01+/-1.23 mg), P = 0.004. In multivariate analysis, VKORC1 and CYP2C9 explained 31% and 7.9% of the variability in warfarin dose, respectively.

CONCLUSIONS

VKORC1 genotype is the dominant genetic influence on inter-individual variability in warfarin dose in Hong Kong Chinese. The lower mean dose of warfarin in Chinese, relative to Europeans, appears to be a reflection of their preponderance of the 'low-dose' VKORC1 H1/H1 (homozygous group A) genotype.

The vitamin K cycle

Post-translational modification of glutamate to gamma carboxyl glutamate is required for the activity of vitamin K-dependent proteins. Carboxylation is accomplished by the enzyme gamma glutamyl carboxylase (GGCX) which requires the propeptide-containing substrate and three co-substrates: reduced vitamin K, CO2, and O2. Most propeptides bind tightly to GGCX and all of the Glu residues that will be modified are modified during one binding event. Complete carboxylation is thus dependent upon the rate of carboxylation and the dissociation rate constant of the substrate from the GGCX enzyme. If the propeptide is released before carboxylation is complete, partially carboxylated vitamin K-dependent proteins are produced. The rate of carboxylation is mainly controlled by the level of reduced vitamin K available for the reactions while the dissociation rate constant is dependent upon both the propeptide and the Gla domain of the substrate. In addition, there are allosteric effects that increase the rate of dissociation of the fully carboxylated substrates. Carboxylation requires the abstraction of a proton from the 4-carbon of glutamate by reduced vitamin K and results in the conversion of vitamin K to vitamin K epoxide. The vitamin K epoxide must be recycled to vitamin K before it can be reused, a reaction catalyzed by the enzyme vitamin K epoxide reductase (VKOR). The gene for VKOR has recently been identified but the enzyme itself has not been purified to homogeneity. It appears, however, that most of the variability observed in patients response to warfarin may be attributed to variability in the VKOR gene.

Increased production of functional recombinant human clotting factor IX by baby hamster kidney cells engineered to overexpress VKORC1, the vitamin K 2,3-epoxide-reducing enzyme of the vitamin K cycle

Some recombinant vitamin K-dependent blood coagulation factors (factors VII, IX, and protein C) have become valuable pharmaceuticals in the treatment of bleeding complications and sepsis. Because of their vitamin K-dependent post-translational modification, their synthesis by eukaryotic cells is essential. The eukaryotic cell harbors a vitamin K-dependent gamma-carboxylation system that converts the proteins to gamma-carboxyglutamic acid-containing proteins. However, the system in eukaryotic cells has limited capacity, and cell lines overexpressing vitamin K-dependent clotting factors produce only a fraction of the recombinant proteins as fully gamma-carboxylated, physiologically competent proteins. In this work we have used recombinant human factor IX (r-hFIX)-producing baby hamster kidney (BHK) cells, engineered to stably overexpress various components of the gamma-carboxylation system of the cell, to determine whether increased production of functional r-hFIX can be accomplished. All BHK cell lines secreted r-hFIX into serum-free medium. Overexpression of gamma-carboxylase is shown to inhibit production of functional r-hFIX. On the other hand, cells overexpressing VKORC1, the reduced vitamin K cofactor-producing enzyme of the vitamin K-dependent gamma-carboxylation system, produced 2.9-fold more functional r-hFIX than control BHK cells. The data are consistent with the notion that VKORC1 is the rate-limiting step in the system and is a key regulatory protein in synthesis of active vitamin K-dependent proteins. The data suggest that overexpression of VKORC1 can be utilized for increased cellular production of recombinant vitamin K-dependent proteins.

Pharmacodynamic resistance to warfarin associated with a Val66Met substitution in vitamin K epoxide reductase complex subunit 1

The gene encoding vitamin K epoxide reductase complex subunit 1 (VKORC1), a component of the enzyme that is the therapeutic target site for warfarin, has recently been identified. In order to investigate the relationship betweenVKORC1 and warfarin dose response, we studied the VKORC1 gene (VKORC1) in patients with warfarin resistance. From a study group of 820 patients, we identified 4 individuals who required more than 25 mg of warfarin daily for therapeutic anticoagulation. Three of these had serum warfarin concentrations within the therapeutic range of 0.7-2.3 mg/l and showed wild-type VKORC1 sequence. The fourth warfarin resistant individual had consistently high (> or =5.7 mg/l) serum warfarin concentrations, yet had no clinically discernible cause for warfarin resistance. VKORC1 showed a heterozygous 196G-->A transition that predicted aVal66Met substitution in the VKORC1 polypeptide. This transition was also identified in 2 asymptomatic family members who had never received warfarin. These individuals had normal vitamin-K dependent coagulation factor activities and undetectable serum PIVKAII and vitamin K1 2,3 epoxide suggesting that their basal vitamin K epoxide reductase activity was not adversely affected by the VKORC1 Val66Met substitution. The association between a nucleotide transition in VKORC1 and pharmacodynamic warfarin resistance supports the hypothesis that VKORC1 is the site of action of warfarin and indicates thatVKORC1 sequence is an important determinant of the warfarin dose response.

Effect of VKORC1 haplotypes on transcriptional regulation and warfarin dose

BACKGROUND

The management of warfarin therapy is complicated by a wide variation among patients in drug response. Variants in the gene encoding vitamin K epoxide reductase complex 1 (VKORC1) may affect the response to warfarin.

METHODS

We conducted a retrospective study of European-American patients receiving long-term warfarin maintenance therapy. Multiple linear-regression analysis was used to determine the effect of VKORC1 haplotypes on the warfarin dose. We determined VKORC1 haplotype frequencies in African-American, European-American, and Asian-American populations and VKORC1 messenger RNA (mRNA) expression in human liver samples.

RESULTS

We identified 10 common noncoding VKORC1 single-nucleotide polymorphisms and inferred five major haplotypes. We identified a low-dose haplotype group (A) and a high-dose haplotype group (B). The mean (+/-SE) maintenance dose of warfarin differed significantly among the three haplotype group combinations, at 2.7+/-0.2 mg per day for A/A, 4.9+/-0.2 mg per day for A/B, and 6.2+/-0.3 mg per day for B/B (P<0.001). VKORC1 haplotype groups A and B explained approximately 25 percent of the variance in dose. Asian Americans had a higher proportion of group A haplotypes and African Americans a higher proportion of group B haplotypes. VKORC1 mRNA levels varied according to the haplotype combination.

CONCLUSIONS

VKORC1 haplotypes can be used to stratify patients into low-, intermediate-, and high-dose warfarin groups and may explain differences in dose requirements among patients of different ancestries. The molecular mechanism of this warfarin dose response appears to be regulated at the transcriptional level.

Common genetic variants of microsomal epoxide hydrolase affect warfarin dose requirements beyond the effect of cytochrome P450 2C9

BACKGROUND

Warfarin dose response is partially explained by the polymorphisms in the cytochrome P450 (CYP) 2C9 gene, affecting S -warfarin clearance, as well as by age and body weight. We examined the influence on warfarin dose requirements of candidate genes encoding microsomal epoxide hydrolase (mEH), as well as glutathione S -transferase A1 (GSTA1) components of vitamin K epoxide reductase and the gamma-glutamylcarboxylase (GGCX) gene.

METHODS

We studied the effects of CYP2C9, mEH, GSTA1, and GGCX genotypes on warfarin maintenance doses, accounting for age, weight, vitamin K plasma concentrations and concurrent medications, in 100 patients undergoing therapeutic anticoagulation.

RESULTS

Allele frequencies were 76.5%, 12.5%, and 11% for CYP2C9*1 , *2 , and *3 , respectively; 75% and 25% for mEH T 612 C; 75.8% and 24.2% for mEH A 691 G; 73.5% and 26.5% for GSTA1 T 631 G; and 70.5% and 29.5% for GGCX G 8762 A. Warfarin doses differed among the CYP2C9 ( 2C9*1 , 2C9*2 , and 2C9*3 ) genotype groups: 6.3 +/- 1.9 mg/d, 5.3 +/- 1.8 mg/d, and 3.8 +/- 1.7 mg/d, respectively (F = 4.83, P < .01). There were no differences in any of the other genotype groups. Among the 62 wild-type CYP2C9 patients, variant mEH T 612 C homozygotes required higher doses than heterozygotes and wild-type patients (7.5 +/- 2.9 mg/d, 6.5 +/- 4.2 mg/d, and 6.0 +/- 2.6 mg/d, respectively [F = 3.57, P = .03]). The odds ratio for requiring greater than 7 mg/d in variant mEH T 612 C patients versus wild-type patients was 3.14 (95% confidence interval, 1.47-6.67), accounting for CYP2C9.

CONCLUSIONS

Variant mEH T 612 C genotypes are associated with warfarin doses of greater than 50 mg/wk beyond the effect of CYP2C9.

Paracetamol (acetaminophen) warfarin interaction: NAPQI, the toxic metabolite of paracetamol, is an inhibitor of enzymes in the vitamin K cycle

Paracetamol (acetaminophen) is generally considered to be the analgesic of choice for patients undergoing oral anticoagulant therapy. Occasionally, however, interactions have been reported with therapeutic doses of the analgesic, e.g. if the drug is taken for a longer period of time. The mechanism of this interaction is not clearly understood. We investigated the effects of paracetamol and its toxic metabolite N-acetyl-para-benzoquinoneimine (NAPQI) on in vitro vitamin K-dependent gamma-carboxylase (VKD-carb) and vitamin K epoxide reductase (VKOR) activities. Paracetamol had no effect in either enzymatic reactions. NAPQI, on the other hand, appeared to interfere with VKD carb activity via two mechanisms; 1) oxidation of the cofactor vitamin K-hydroquinone, 2) inactivation of the enzyme. The inactivation, in micromolar ranges, is not reversible and may be the result of covalent binding of NAPQI with functional amino acids. NAPQI also inhibited VKOR, but at higher concentrations. Unexpectedly, N-acetylcysteine was found to inhibit VKOR activity at concentrations that are obtained during rescue therapy of paracetamol intoxication. We conclude that, the potentiation of the oral anticoagulant effect by paracetamol is likely to result from NAPQI-induced inhibition of enzymes of the vitamin K cycle, particularly VKD-carb.

A novel functional VKORC1 promoter polymorphism is associated with inter-individual and inter-ethnic differences in warfarin sensitivity

Warfarin, a commonly prescribed anticoagulant, exhibited large inter-individual and inter-ethnic differences in the dose required for its anticoagulation effect. Asian populations, including Chinese, require a much lower maintenance dose than Caucasians, for which the mechanisms still remain unknown. We determined DNA sequence variants in CYP2C9 and VKORC1 in 16 Chinese patients having warfarin sensitivity (< or = 1.5 mg/day, n = 11) or resistance (> or = 6.0 mg/day, n = 5), 104 randomly selected Chinese patients receiving warfarin, 95 normal Chinese controls and 92 normal Caucasians. We identified three CYP2C9 variants, CYP2C9*3, T299A and P382L, in four warfarin-sensitive patients. A novel VKORC1 promoter polymorphism (-1639 G > A) presented in the homozygous form (genotype AA) was found in all warfarin-sensitive patients. The resistant patients were either AG or GG. Among the 104 randomly selected Chinese patients receiving warfarin, AA genotype also had lower dose than the AG/GG genotype (P < 0.0001). Frequencies of AA, AG and GG genotypes were comparable in Chinese patients receiving warfarin (79.7, 17.6 and 2.7%) and normal Chinese controls (82, 18 and 0%), but differed significantly from Caucasians (14, 47 and 39%) (P < 0.0001). The promoter polymorphism abolished the E-box consensus sequences and dual luciferase assay revealed that VOKRC1 promoter with the G allele had a 44% increase of activity when compared with the A allele. The differences in allele frequencies of A/G allele and its levels of VKORC1 promoter activity may underscore the inter-individual differences in warfarin dosage as well as inter-ethnic differences between Chinese and Caucasians.

The genetic basis of resistance to anticoagulants in rodents

Anticoagulant compounds, i.e., derivatives of either 4-hydroxycoumarin (e.g., warfarin, bromadiolone) or indane-1,3-dione (e.g., diphacinone, chlorophacinone), have been in worldwide use as rodenticides for >50 years. These compounds inhibit blood coagulation by repression of the vitamin K reductase reaction (VKOR). Anticoagulant-resistant rodent populations have been reported from many countries and pose a considerable problem for pest control. Resistance is transmitted as an autosomal dominant trait although, until recently, the basic genetic mutation was unknown. Here, we report on the identification of eight different mutations in the VKORC1 gene in resistant laboratory strains of brown rats and house mice and in wild-caught brown rats from various locations in Europe with five of these mutations affecting only two amino acids (Tyr139Cys, Tyr139Ser, Tyr139Phe and Leu128Gln, Leu128Ser). By recombinant expression of VKORC1 constructs in HEK293 cells we demonstrate that mutations at Tyr139 confer resistance to warfarin at variable degrees while the other mutations, in addition, dramatically reduce VKOR activity. Our data strongly argue for at least seven independent mutation events in brown rats and two in mice. They suggest that mutations in VKORC1 are the genetic basis of anticoagulant resistance in wild populations of rodents, although the mutations alone do not explain all aspects of resistance that have been reported. We hypothesize that these mutations, apart from generating structural changes in the VKORC1 protein, may induce compensatory mechanisms to maintain blood clotting. Our findings provide the basis for a DNA-based field monitoring of anticoagulant resistance in rodents.

Design and synthesis of novel diphenacoum-derived, conformation-restricted vitamin K 2,3-epoxide reductase inhibitors

Two novel diphenacoum-derived analogues 5 and 6 are designed, synthesized and tested as potential vitamin K 2,3-epoxide reductase (VKOR) inhibitors. The inhibition studies indicated that 5 is a potent VKOR inhibitor, which confirmed that the replacement of the tetrahydronaphthalene on diphenacoum to a chroman functionality does not have a major impact on inhibition potency. The conformation-restricted compound 6 is a moderate inhibitor which may serve as a lead compound for further study of the mode of action of coumarin-type anticoagulants at the molecular level.

Membrane Topology Mapping of Vitamin K Epoxide Reductase by in Vitro Translation/Cotranslocation

Vitamin K epoxide reductase (VKOR) catalyzes the conversion of vitamin K 2,3-epoxide into vitamin K in the vitamin K redox cycle. Recently, the gene encoding the catalytic subunit of VKOR was identified as a 163-amino acid integral membrane protein. In this study we report the experimentally derived membrane topology of VKOR. Our results show that four hydrophobic regions predicted as the potential transmembrane domains in VKOR can individually insert across the endoplasmic reticulum membrane in vitro. However, in the intact enzyme there are only three transmembrane domains, residues 10-29, 101-123, and 127-149, and membrane-integration of residues 75-97 appears to be suppressed by the surrounding sequence. Results of N-linked glycosylation-tagged full-length VKOR shows that the N terminus of VKOR is located in the endoplasmic reticulum lumen, and the C terminus is located in the cytoplasm. Further evidence for this topological model of VKOR was obtained with freshly prepared intact microsomes from insect cells expressing HPC4-tagged full-length VKOR. In these experiments an HPC4 tag at the N terminus was protected from proteinase K digestion, whereas an HPC4 tag at the C terminus was susceptible. Altogether, our results suggest that VKOR is a type III membrane protein with three transmembrane domains, which agrees well with the prediction by the topology prediction program TMHMM.

Genes encoding vitamin-K epoxide reductase are present in Drosophila and trypanosomatid protists

Vitamin-K epoxide reductase is encoded by the VKORC1 gene in mammals and other vertebrates, which also have a paralog, VKORC1L1. Single homologs are present in basal deuterostome and insect genomes, including Drosophila, and three trypanosomatid protists. VKOR is therefore an ancient gene/protein that can be studied in the Drosophila model system.

Cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase (VKORC1) genotypes as determinants of acenocoumarol sensitivity

The aim of the study is to explore the contribution of genetic factors related either to drug metabolism (cytochrome P450 2C9) or to drug target (vitamin K epoxide reductase) to variability in the response to acenocoumarol among 222 healthy volunteers after a single oral dose. Associations between a pharmacodynamic index (reduction in factor VII activity and international normalized ratio [INR] change) and several genetic polymorphisms (VKORC1: -4931T>C, -4451C>A, -2659G>C, -1877A>G, -1639G>A, 497C>G, 1173C>T, and CYP2C9*3) were investigated using haplotype and univariate analyses. VKORC1 haplotypes were associated with the pharmacologic response, and this association can be explained only by the effect of the -1639G>A polymorphism (or alternatively by 1173C>T, which is in complete association with it). Indeed, it explains about one third of the variability of the pharmacologic response (37% of factor VII decrease and 30% of INR change). Moreover, the previously observed effect of the CYP2C9*3 allele is independent of the VKORC1 gene effect. These 2 polymorphisms account for up to 50% of the interindividual variability. The simple genotyping of 2 single-nucleotide polymorphisms (SNPs), VKORC1 -1639G>A or 1173C>T and the CYP2C9*3 polymorphisms, could thus predict a high risk of overdose before initiation of anticoagulation with acenocoumarol, and provide a safer and more individualized anticoagulant therapy.

Engineering of a Recombinant Vitamin K-dependent γ-Carboxylation System with Enhanced γ-Carboxyglutamic Acid Forming Capacity

The vitamin K-dependent gamma-carboxylation system in the endoplasmic reticulum membrane responsible for gamma-carboxyglutamic acid modification of vitamin K-dependent proteins includes gamma-carboxylase and vitamin K 2,3-epoxide reductase (VKOR). An understanding of the mechanism by which this system works at the molecular level has been hampered by the difficulty of identifying VKOR involved in warfarin sensitive reduction of vitamin K 2,3-epoxide to reduced vitamin K(1)H(2), the gamma-carboxylase cofactor. Identification and cloning of VKORC1, a proposed subunit of a larger VKOR enzyme complex, have provided opportunities for new experimental approaches aimed at understanding the vitamin K-dependent gamma-carboxylation system. In this work we have engineered stably transfected baby hamster kidney cells containing gamma-carboxylase and VKORC1 cDNA constructs, respectively, and stably double transfected cells with the gamma-carboxylase and the VKORC1 cDNA constructs in a bicistronic vector. All engineered cells showed increased activities of the enzymes encoded by the cDNAs. However increased activity of the gamma-carboxylation system, where VKOR provides the reduced vitamin K(1)H(2) cofactor, was measured only in cells transfected with VKORC1 and the double transfected cells. The results show that VKOR is the rate-limiting step in the gamma-carboxylation system and demonstrate successful engineering of cells containing a recombinant vitamin K-dependent gamma-carboxylation system with enhanced capacity for gamma-carboxyglutamic acid modification. The proposed thioredoxin-like (132)CXXC(135) redox center in VKORC1 was tested by expressing the VKORC1 mutants Cys(132)/Ser and Cys(135)/Ser in BHK cells. Both of the expressed mutant proteins were inactive supporting the existence of a CXXC redox center in VKOR.

A polymorphism in the VKORC1 gene is associated with an interindividual variability in the dose-anticoagulant effect of warfarin

Patients require different warfarin dosages to achieve the target therapeutic anticoagulation. The variability is largely genetically determined, and it can be only partly explained by genetic variability in the cytochrome CYP2C9 locus. In 147 patients followed from the start of anticoagulation with warfarin, we have investigated whether VKORC1 gene mutations have affected doses of drug prescribed to acquire the target anticoagulation intensity. Two synonymous mutations, 129C&gt;T at Cys43 and 3462C&gt;T at Leu120, and 2 missense mutations, Asp38Tyr and Arg151Gln, were identified. None of these mutations was found to affect the interindividual variability of warfarin prescribed. Finally, 2 common polymorphisms were found, 1173C&gt;T in the intron 1 and 3730G&gt;A transition in the 3′ untranslated region (UTR). Regardless of the presence of confounding variables, the mean adjusted dose required of warfarin was higher (6.2 mg) among patients with the VKORC1 1173CC genotype than those of patients carrying the CT (4.8 mg; P = .002) or the TT genotype (3.5 mg; P &lt; .001). In the present setting, VKORC1 and CYP2C9 genetic variants investigated accounted for about a third (r2, 0.353) of the interindividual variability. Genetic variants of the VKORC1 gene locus modulate the mean daily dose of drug prescribed to acquire the target anticoagulation intensity.

Comparative Pharmacokinetics of Vitamin K Antagonists

Vitamin K antagonists belong to the group of most frequently used drugs worldwide. They are used for long-term anticoagulation therapy, and exhibit their anticoagulant effect by inhibition of vitamin K epoxide reductase. Each drug exists in two different enantiomeric forms and is administered orally as a racemate. The use of vitamin K antagonists is complicated by a narrow therapeutic index and an unpredictable dose-response relationship, giving rise to frequent bleeding complications or insufficient anticoagulation. These large dose response variations are markedly influenced by pharmacokinetic aspects that are determined by genetic, environmental and possibly other yet unknown factors. Previous knowledge in this regard principally referred to warfarin. Cytochrome P450 (CYP) 2C9 has clearly been established as the predominant catalyst responsible for the metabolism of its more potent S-enantiomer. More recently, CYP2C9 has also been reported to catalyse the hydroxylation of phenprocoumon and acenocoumarol. However, the relative importance of CYP2C9 for the clearance of each anticoagulant substantially differs. Overall, the CYP2C9 isoenzyme appears to be most important for the clearance of warfarin, followed by acenocoumarol and, lastly, phenprocoumon. The less important role of CYP2C9 for the clearance of phenprocoumon is due to the involvement of CYP3A4 as an additional catalyst of phenprocoumon hydroxylation and significant excretion of unchanged drug in bile and urine, while the elimination of warfarin and acenocoumarol is almost completely by metabolism. Consequently, the effects of CYP2C9 polymorphisms on the pharmacokinetics and anticoagulant response are also least pronounced in the case of phenprocoumon; this drug seems preferable for therapeutic anticoagulation in poor metabolisers of CYP2C9. In addition to these vitamin K antagonists, oral thrombin inhibitors are currently under clinical development for the prevention and treatment of thromboembolism. Of these, ximelagatran has recently gained marketing authorisation in Europe. These novel drugs all feature some major advantages over traditional anticoagulants, including a wide therapeutic interval, the lack of anticoagulant effect monitoring and a low drug-drug interaction potential. However, they are also characterised by some pitfalls. Amendments of traditional anticoagulant therapy, including self-monitoring of international normalised ratio values or prospective genotyping for individual dose-tailoring may contribute to the continuous use of warfarin, phenprocoumon and acenocoumarol in the future.

Vitamin K epoxide reductase: homology, active site and catalytic mechanism

Vitamin K epoxide reductase (VKOR) recycles reduced vitamin K, which is used subsequently as a co-factor in the gamma-carboxylation of glutamic acid residues in blood coagulation enzymes. VKORC1, a subunit of the VKOR complex, has recently been shown to possess this activity. Here, we show that VKORC1 is a member of a large family of predicted enzymes that are present in vertebrates, Drosophila, plants, bacteria and archaea. Four cysteine residues and one residue, which is either serine or threonine, are identified as likely active-site residues. In some plant and bacterial homologues the VKORC1 homologous domain is fused with domains of the thioredoxin family of oxidoreductases. These might reduce disulfide bonds of VKORC1-like enzymes as a prerequisite for their catalytic activities.