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

VKORC1-dependent pharmacokinetics of intravenous and oral phylloquinone (vitamin K1) mixed micelles formulation

Objective

The pharmacokinetics of phylloquinone (vitamin K1) were evaluated in healthy human adult volunteers (15 male and 15 female) following oral and intravenous administration of a mixed micelles formulation (Konakion® MM 2 mg) in an open label study design. The subjects were allocated to one of three genotype-specific groups (n = 10 in each group) in terms of VKORC1 promoter polymorphism c.-1639 G > A to explore the relationship between genotype and pharmacokinetic parameters.

Methods

Blood samples were collected for up to 24 h after administration. Phylloquinone serum levels were determined by reversed phase HPLC with fluorometric detection after post-column zinc reduction. Pharmacokinetic evaluation was performed using non-compartmental analysis.

Results

Pharmacokinetic analysis of serum phylloquinone concentration versus time profiles revealed significant differences in the main pharmacokinetic parameters between groups. Upon oral administration, VKORC1 AG carriers showed 41 % higher mean bioavailability (p = 0.01) compared with homozygous AA individuals. Furthermore, AG subjects exhibited 30 % (p = 0.042) and 36 % (p = 0.021) higher mean AUC compared with GG and AA respectively. Terminal half-life was 32 % and 27 % longer for AG carriers in comparison to GG (p = 0.004) and AA (p = 0.015) genotypes respectively.

Conclusion

Pharmacokinetic differences indicated significant inter-individual variance of vitamin K fate in the human body. The influence of the VKORC1 promoter polymorphism c.-1639 G > A on the pharmacokinetic properties of phylloquinone could be demonstrated in humans. To gain deeper insight in other potential genetic determinants of systemic vitamin K exposure, further correlation of the phenotype–genotype relationship of different players in vitamin K turnover has to be gained.

Predictive value of the vitamin K epoxide reductase complex subunit 1 G-1639A and C1173T single nucleotide polymorphisms in retinal vein occlusion

BACKGROUND

  To determine if vitamin K epoxide reductase complex subunit 1 gene polymorphisms have an effect on the risk of having a retinal vein occlusion.

DESIGN

  Case-control study.

PARTICIPANTS

  The study population consisted of 68 patients who were newly diagnosed with retinal vein occlusion and 66 sex-matched controls.

METHODS

  Genomic DNA was extracted from peripheral leukocytes from ethylenediamine tetra-acetic acid-anticoagulated blood. Genotyping of the vitamin K epoxide reductase complex subunit 1 G-1639A (rs 9923231) and C1173T (rs 9934438) single nucleotide polymorphisms was performed using real-time polymerase chain reaction and commercially available kits.

MAIN OUTCOME MEASURES

  A full ophthalmological evaluation was performed in each subject, and all subjects were screened for hypertension, hypercholesterolaemia and diabetes. The genotypes of the vitamin K epoxide reductase complex subunit 1 single nucleotide polymorphisms were determined.

RESULTS

  The vitamin K epoxide reductase complex subunit 1 GG and CC genotypes were more frequent (41% vs. 21%; P = 0.021), and the combined GA/AA and CT/CC genotypes were less frequent in patients with retinal vein occlusion than in control subjects. After adjusting for hypertension, age, plasma fibrinogen levels and prevalence of diabetes and hypercholesterolaemia, the GG and CC genotypes were found to be an independent predictor of retinal vein occlusion (B = 2.28; odds ratio = 9.79; P = 0.003; 95% confidence interval: 2.22-43.24).

CONCLUSION

  It was found that subjects with the vitamin K epoxide reductase complex subunit 1 GG and CC genotypes had a higher risk of retinal vein occlusion.

Vascular calcification: the price to pay for anticoagulation therapy with vitamin K-antagonists

Vitamin K-antagonists (VKA) are the most widely used anti-thrombotic drugs with substantial efficacy in reducing risk of arterial and venous thrombosis. Several lines of evidence indicate, however, that VKA inhibit not only post-translational activation of vitamin K-dependent coagulation factors but also synthesis of functional extra-hepatic vitamin K-dependent proteins thereby eliciting undesired side-effects. Vascular calcification is one of the recently revealed side-effects of VKA. Vascular calcification is an actively regulated process involving vascular cells and a number of vitamin K-dependent proteins. Mechanistic understanding of vascular calcification is essential to improve VKA-based treatments of both thrombotic disorders and atherosclerosis. This review addresses vitamin K-cycle and vitamin K-dependent processes of vascular calcification that are affected by VKA. We conclude that there is a growing need for better understanding of the effects of anticoagulants on vascular calcification and atherosclerosis.

The protein disulfide isomerase family: key players in health and disease

SIGNIFICANCE

Protein disulfide isomerase (PDI) and its homologs have essential roles in the oxidative folding and chaperone-mediated quality control of proteins in the secretory pathway. In this review, the importance of PDI in health and disease will be examined, using examples from the fields of lipid homeostasis, hemostasis, infectious disease, cancer, neurodegeneration, and infertility.

RECENT ADVANCES

Recent structural studies, coupled with cell biological, biochemical, and clinical approaches, have demonstrated that PDI family proteins are involved in a wide range of physiological and disease processes.

CRITICAL ISSUES

Critical issues in the field include understanding how and why a PDI family member is involved in a given disease, and defining the physiological client specificity of the various PDI proteins when they are expressed in different tissues.

FUTURE DIRECTIONS

Future directions are likely to include the development of new and more specific reagents to study and manipulate PDI family function. The development of conditional mouse models in concert with clinical data will help us to understand the in vivo function of the different PDIs at the organism level. Taken together with advances in structural biology and biochemical studies, this should help us to further understand and modify PDIs' functional interactions.

Human herpesvirus 8 viral interleukin-6 interacts with splice variant 2 of vitamin K epoxide reductase complex subunit 1

Viral interleukin-6 (vIL-6) specified by human herpesvirus 8 is, unlike its cellular counterpart, secreted very inefficiently and can signal via vIL-6(2):gp130(2) signaling complexes from the endoplasmic reticulum (ER) compartment. Intracellular, autocrine activities of vIL-6 are important for proproliferative and prosurvival activities of the viral cytokine in latently infected primary effusion lymphoma (PEL) cells. However, the molecular determinants of vIL-6 ER localization and function are unclear. Using yeast two-hybrid analysis, we identified the database-documented but uncharacterized splice variant of vitamin K epoxide reductase complex subunit 1 (VKORC1), termed VKORC1 variant 2 (VKORC1v2), as a potential interaction partner of vIL-6. In transfected cells, epitope-tagged VKORC1v2 was found to localize to the ER, to adopt a single-transmembrane (TM) topology placing the C tail in the ER lumen, and to bind vIL-6 via these sequences. Deletion mutagenesis and coprecipitation assays mapped the vIL-6-binding domain (vBD) of VKORC1v2 to TM-proximal residues 31 to 39. However, while sufficient to confer vIL-6 binding to a heterologous protein, vBD was unable to induce vIL-6 secretion when fused to (secreted) hIL-6, suggesting a VKORC1v2-independent mechanism of vIL-6 ER retention. In functional assays, overexpression of ER-directed vBD led to suppression of PEL cell proliferation and viability, effects also mediated by VKORC1v2 depletion and, as reported previously, by vIL-6 suppression. The growth-inhibitory and proapoptotic effects of VKORC1v2 depletion could be rescued by transduced wild-type VKORC1v2 but not by a vIL-6-refractory vBD-altered variant, indicating the functional relevance of the vIL-6-VKORC1v2 interaction. Notably, gp130 signaling was unaffected by VKORC1v2 or vBD overexpression or by VKORC1v2 depletion, suggesting an alternative pathway of vIL-6 activity via VKORC1v2. Combined, our data identify a novel and functionally significant interaction partner of vIL-6 that could potentially be targeted for therapeutic benefit.

Biochemical characterization of spontaneous mutants of rat VKORC1 involved in the resistance to antivitamin K anticoagulants

Antivitamin K anticoagulants have been commonly used to control rodent pest all over the world for more than 50 years. These compounds target blood coagulation by inhibiting the vitamin K epoxide reductase (VKORC1), which catalyzes the reduction of vitamin K 2,3-epoxide to vitamin K. Resistance to anticoagulants has been reported in wild rat populations from different countries. From these populations, several mutations of the rVkorc1 gene have been reported. In this study, rat VKORC1 and its most frequent mutants L120Q-, L128Q-, Y139C-, Y139S- and Y139F-VKORC1 were expressed as membrane-bound proteins in Pichia pastoris and characterized by the determination of kinetic and inhibition parameters. The recombinant rVKORC1 showed similar properties than those of the native proteins expressed in the rat liver microsomes, validating the expression system as a good model to study the consequences of VKORC1 mutations. The determination of the inhibition parameters towards various antivitamin K anticoagulants demonstrated that mutations at Leu-120, Leu-128 and Tyr-139 confer the resistance to the first generation AVKs observed in wild rat populations.

How proteins form disulfide bonds

The identification of protein disulfide isomerase, almost 50 years ago, opened the way to the study of oxidative protein folding. Oxidative protein folding refers to the composite process by which a protein recovers both its native structure and its native disulfide bonds. Pathways that form disulfide bonds have now been unraveled in the bacterial periplasm (disulfide bond protein A [DsbA], DsbB, DsbC, DsbG, and DsbD), the endoplasmic reticulum (protein disulfide isomerase and Ero1), and the mitochondrial intermembrane space (Mia40 and Erv1). This review summarizes the current knowledge on disulfide bond formation in both prokaryotes and eukaryotes and highlights the major problems that remain to be solved.

Human vitamin K 2,3-epoxide reductase complex subunit 1-like 1 (VKORC1L1) mediates vitamin K-dependent intracellular antioxidant function

Human vitamin K 2,3-epoxide reductase complex subunit 1-like 1 (VKORC1L1), expressed in HEK 293T cells and localized exclusively to membranes of the endoplasmic reticulum, was found to support both vitamin K 2,3-epoxide reductase (VKOR) and vitamin K reductase enzymatic activities. Michaelis-Menten kinetic parameters for dithiothreitol-driven VKOR activity were: K(m) (μM) = 4.15 (vitamin K(1) epoxide) and 11.24 (vitamin K(2) epoxide); V(max) (nmol·mg(-1)·hr(-1)) = 2.57 (vitamin K(1) epoxide) and 13.46 (vitamin K(2) epoxide). Oxidative stress induced by H(2)O(2) applied to cultured cells up-regulated VKORC1L1 expression and VKOR activity. Cell viability under conditions of no induced oxidative stress was increased by the presence of vitamins K(1) and K(2) but not ubinquinone-10 and was specifically dependent on VKORC1L1 expression. Intracellular reactive oxygen species levels in cells treated with 2,3-dimethoxy-1,4-naphthoquinone were mitigated in a VKORC1L1 expression-dependent manner. Intracellular oxidative damage to membrane intrinsic proteins was inversely dependent on VKORC1L1 expression and the presence of vitamin K(1). Taken together, our results suggest that VKORC1L1 is responsible for driving vitamin K-mediated intracellular antioxidation pathways critical to cell survival.

Statistical Optimization of Pharmacogenomics Association Studies: Key Considerations from Study Design to Analysis

Research in human genetics and genetic epidemiology has grown significantly over the previous decade, particularly in the field of pharmacogenomics. Pharmacogenomics presents an opportunity for rapid translation of associated genetic polymorphisms into diagnostic measures or tests to guide therapy as part of a move towards personalized medicine. Expansion in genotyping technology has cleared the way for widespread use of whole-genome genotyping in the effort to identify novel biology and new genetic markers associated with pharmacokinetic and pharmacodynamic endpoints. With new technology and methodology regularly becoming available for use in genetic studies, a discussion on the application of such tools becomes necessary. In particular, quality control criteria have evolved with the use of GWAS as we have come to understand potential systematic errors which can be introduced into the data during genotyping. There have been several replicated pharmacogenomic associations, some of which have moved to the clinic to enact change in treatment decisions. These examples of translation illustrate the strength of evidence necessary to successfully and effectively translate a genetic discovery. In this review, the design of pharmacogenomic association studies is examined with the goal of optimizing the impact and utility of this research. Issues of ascertainment, genotyping, quality control, analysis and interpretation are considered.

Thirteen novel VKORC1 mutations associated with oral anticoagulant resistance: insights into improved patient diagnosis and treatment

BACKGROUND

Vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) is the molecular target of oral anticoagulants. Mutations in VKORC1 cause partial or total coumarin resistance.

OBJECTIVES

To identify new VKORC1 oral anticoagulant (OAC) resistance (OACR) mutations and compare the severity of patient phenotypes across different mutations and prescribed OAC drugs.

PATIENTS/METHODS

Six hundred and twenty-six individuals exhibiting partial or complete coumarin resistance were analyzed by VKORC1 gene sequencing and CYP2C9 haplotyping.

RESULTS

We identified 13 patients, each with a different, novel human VKORC1 heterozygous mutation associated with an OACR phenotype. These mutations result in amino acid substitutions: Ala26→Thr, His28→Gln, Asp36→Gly, Ser52→Trp, Ser56→Phe, Trp59→Leu, Trp59→Cys, Val66→Gly, Gly71→Ala, Asn77→Ser, Asn77→Tyr, Ile123→Asn, and Tyr139→His. Ten additional patients each had one of three previously reported VKORC1 mutations (Val29→Leu, Asp36→Tyr, and Val66→Met). Genotyping of frequent VKORC1 and CYP2C9 polymorphisms in these patients revealed a predominant association with combined non-VKORC1*2 and wild-type CYP2C9 haplotypes. Additionally, data for OAC dosage and the associated measured International Normalized Ratio (INR) demonstrate that OAC therapy is often discontinued by physicians, although stable therapeutic INR levels may be reached at higher OAC dosages. Bioinformatic analysis of VKORC1 homologous protein sequences indicated that most mutations cluster into protein sequence segments predicted to be localized in the lumenal loop or at the endoplasmic reticulum membrane-lumen interface.

CONCLUSIONS

OACR mutations of VKORC1 predispose afflicted patients to high OAC dosage requirements, for which stable, therapeutic INRs can sometimes be attained.

Prediction of phenprocoumon maintenance dose and phenprocoumon plasma concentration by genetic and non-genetic parameters

Purpose

The anticoagulation response to vitamin K antagonists is characterised by high inter-individual variability. The impact of single nucleotide polymorphisms (SNPs) in several genes of enzymes involved in the vitamin K cycle on phenprocoumon dose variability and phenprocoumon plasma concentrations is still under investigation.

Methods

We assessed the influence of VKORC1 c.-1639G>A, CYP2C9*2, CYP2C9*3, CYP4F2 c.1297G>A, CALU c.*4A>G, EPHX1 c.337T>C, GGCX c.214+597G>A, F7 c.-402G>A, F7 c.-401G>T, PROC c.-228C>T and PROC c.-215G>A along with clinical and demographic parameters on steady-state phenprocoumon therapy in 75 patients. A prediction model was developed for total phenprocoumon plasma concentrations and daily phenprocoumon doses required for therapeutic anticoagulation.

Results

The VKORC1 c.-1639 genotype was the main predictor of the phenprocoumon daily dose (adjusted R² = 37.6%) and the total phenprocoumon concentration (adjusted R² = 38.3%). CYP2C9 affected the phenprocoumon concentration, but not the dose requirements. SNPs in the other genes of the vitamin K cycle, concomitant medication, nicotine use and alcohol consumption did not predict phenprocoumon concentrations and phenprocoumon dose requirements in a multiple linear regression model. Phenprocoumon concentrations were predicted by VKORC1 c.-1639, CYP2C9 genotype, age and BMI. The final prediction model for the daily phenprocoumon dose requirements comprised VKORC1 c.-1639 genotype, age and height accounting for 48.6% of the inter-individual variability.

Conclusions

A rough prediction of phenprocoumon maintenance doses can be achieved by a limited set of parameters (VKORC1, age, height). The investigated SNPs in CYP4F2, CALU, EPHX1, GGCX, F7, and PROC did not improve the predictive value of a pharmacogenetic-based dosing equation for phenprocoumon.

Use of biological knowledge to inform the analysis of gene-gene interactions involved in modulating virologic failure with efavirenz-containing treatment regimens in ART-naïve ACTG clinical trials participants

Personalized medicine is a high priority for the future of health care. The idea of tailoring an individual's wellness plan to their unique genetic code is one which we hope to realize through the use of pharmacogenomics. There have been examples of tremendous success in pharmacogenomic associations however there are many such examples in which only a small proportion of trait variance has been explained by the genetic variation. Although the increased use of GWAS could help explain more of this variation, it is likely that a significant proportion of the genetic architecture of these pharmacogenomic traits are due to complex genetic effects such as epistasis, also known as gene-gene interactions, as well as gene-drug interactions. In this study, we utilize the Biofilter software package to look for candidate epistasis contributing to risk for virologic failure with efavirenz-containing antiretroviral therapy (ART) regimens in treatment-naïve participants of AIDS Clinical Trials Group (ACTG) randomized clinical trials. A total of 904 individuals from three ACTG trials with data on efavirenz treatment are analyzed after race-stratification into white, black, and Hispanic ethnic groups. Biofilter was run considering 245 candidate ADME (absorption, distribution, metabolism, and excretion) genes and using database knowledge of gene and protein interaction networks to produce approximately 2 million SNP-SNP interaction models within each ethnic group. These models were evaluated within the PLATO software package using pair wise logistic regression models. Although no interaction model remained significant after correction for multiple comparisons, an interaction between SNPs in the TAP1 and ABCC9 genes was one of the top models before correction. The TAP1 protein is responsible for intracellular transport of antigen to MHC class I molecules, while ABCC9 codes for a transporter which is part of the subfamily of ABC transporters associated with multi-drug resistance. This study demonstrates the utility of the Biofilter method to prioritize the search for gene-gene interactions in large-scale genomic datasets, although replication in a larger cohort is required to confirm the validity of this particular TAP1-ABCC9 finding.

New adjuvants to enhance anticoagulant activity of warfarin

New adjuvants of warfarin anticoagulant activity have been developed. These compounds, which are 1,4-methano-1,2,3,4-tetrahydroanthracene-9,10-diol derivatives, act synergistically with warfarin to potentiate its anticoagulant effect. None of the compounds tested is an effective oral anticoagulant in the absence of warfarin.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

Pharmakogenetik der oralen Antikoagulation mit Cumarinen

Die Klonierung des VKORC1-Gens hat maßgeblich zu einem besseren Verständnis des Vitamin-K-Zyklus beigetragen. Das VKORC1-Protein konnte als der molekulare Zielort (Target) der Cumarine identifiziert werden. Mutationen und SNP innerhalb der translatierten und nichttranslatierten Regionen des VKORC1-Gens verursachen eine partielle bis totale Cumarinresistenz oder -sensitivität. Die Verfügbarkeit einer molekulargenetischen Diagnostik (VKORC1, CYP2C9) und einer Laboranalytik mittels HPLC (zur Bestimmung des Cumarin-, Vitamin-K- und Vitamin-K-Epoxid-Spiegels) ist hilfreich in der Detektion hereditärer und erworbener Einflussgrößen der Cumarintherapie und könnte zukünftig für eine individualisierte, risikoärmere orale Antikoagulationstherapie zum Einsatz kommen.

Regulatory polymorphism in vitamin K epoxide reductase complex subunit 1 (VKORC1) affects gene expression and warfarin dose requirement

Warfarin dose requirements have been associated with 2 main haplotypes in VKORC1, but the responsible polymorphisms remain unknown. To search for regulatory polymorphisms, we measured allelic mRNA expression of VKORC1 in human liver, heart, and B lymphocytes. The observed 2-fold allelic mRNA expression imbalance narrowed possible candidate SNPs to -1639G>A and 1173C<T. This genotype effect was observed selectively in the liver but not in heart or lymphocytes. In vitro expression of VKORC1 gene constructs, including coding and promoter regions, failed to reveal any genotype effect on transcription and mRNA processing. Chromatin immunoprecipitation with antibodies against acetyl-histone3 and K4-trimethyl-histone3 revealed preferential association of the promoter -1639 G allele with active chromatin, consistent with enhanced mRNA expression. The minor -1639 A allele generates a suppressor E-box binding site, apparently regulating gene expression by a mechanism undetectable with reporter gene assays. A clinical association study demonstrated that promoter SNP -1639G>A, and the tightly linked intron1 SNP 1173C>T, predict warfarin dose more accurately than intron 2 SNP 1542G>C in blacks. Increased warfarin dose requirement in blacks was accounted for by lower frequency of the -1639 A allele. Therefore, -1639G>A is a suitable biomarker for warfarin dosing across ethnic populations.

Vitamin K epoxide reductase complex subunit 1 (VKORC1) polymorphism and aortic calcification: the Rotterdam Study

OBJECTIVE

Besides effects on hemostasis, vitamin K-dependent proteins play a role in bone mineralization and arterial calcification. We investigated the association between the VKORC1 1173C>T polymorphism and calcification of the aortic far wall in a large population-based cohort.

METHODS AND RESULTS

Aortic calcification was diagnosed by radiographic detection of calcified deposits in the abdominal aorta. In all cohort members for whom DNA was available, the C1173T SNP of VKORC1 (rs9934438) was determined. With multivariable logistic regression analysis the association between this polymorphism and the risk of aortic calcification was calculated, adjusted for potential confounders. The T allele frequency of the VKORC1 1173C>T polymorphism was 38.8%. 1185 (37.2%) persons were homozygous CC, 1529 (48,0%) were heterozygous CT and 473 (14.8%) were homozygous TT. Persons with at least one T-allele had a statistically significant 19% (95% CI 2 to 40%) risk increase of calcification of the aortic far wall compared to CC homozygous persons, adjusted for age and gender.

CONCLUSIONS

The T-allele of the VKORC1 1173C>T polymorphism was associated with a significantly higher risk of aortic calcification in Whites.

Structure and function of vitamin K epoxide reductase

Vitamin K epoxide reductase (VKOR) is an integral membrane protein that catalyzes the reduction of vitamin K 2,3-epoxide and vitamin K to vitamin K hydroquinone, a cofactor required for the gamma-glutamyl carboxylation reaction. VKOR is highly sensitive to inhibition by warfarin, the most commonly prescribed oral anticoagulant. Warfarin inhibition of VKOR decreases the concentration of reduced vitamin K, which reduces the rate of vitamin K-dependent carboxylation and leads to under-carboxylated, inactive vitamin K-dependent proteins. It is proposed that an active site disulfide needs to be reduced for the enzyme to be active. VKOR uses two sulfhydryl groups for the catalytic reaction and these two sulfhydryl groups are oxidized back to a disulfide bond during each catalytic cycle. The recent identification of the gene encoding VKOR allows us to study its structure and function relationship at the molecular level. The membrane topology model shows that VKOR spans the endoplasmic reticulum membrane three times with its amino-terminus residing in the lumen and the carboxyl-terminus residing in the cytoplasm. Both the active site (cysteines 132 and 135) and the proposed warfarin binding site (tyrosine 139) reside in the third transmembrane helix. VKOR is made at high levels in insect cells and is relatively easily purified. This should allow the determination of its three-dimensional structure. A detailed mechanism has been published and the purified enzyme should allow the testing of this mechanism. A major unanswered question is the physiological reductant of VKOR.