Increased tissue factor-initiated prothrombin activation as a result of the Arg506 --> Gln mutation in factor VLEIDEN

Knockdown and Knockout of Tissue Factor Pathway Inhibitor in Zebrafish

Tissue Factor Pathway Inhibitor (TFPI) is an anticoagulant that inhibits factor VIIa and Xa in the blood coagulation pathways. TFPI contains three Kunitz domains, K1, K2, and K3. K1 and K2 inhibit factor VIIa and Xa, respectively. However, the regulation of TFPI is poorly studied. Since zebrafish has become an alternate model to discover novel actors in hemostasis, we hypothesized that TFPI regulation could be studied using this model. As a first step, we confirmed the presence of tfpia in zebrafish using RT-PCR. We then performed piggyback knockdowns of tfpia and found increased coagulation activity in tfpia knockdown. We then created a deletion mutation in tfpia locus using CRISPR/Cas9 method. The tfpia homozygous deletion mutants showed increased coagulation activities similar to that found in tfpia knockdown. Taken together, our data suggest that tfpia is a negative regulator for zebrafish coagulation, and silencing it leads to thrombotic phenotype. Also, the zebrafish tfpia knockout model could be used for reversing this thrombotic phenotype to identify antithrombotic novel factors by the genome-wide piggyback knockdown method.

Platelet-primed interactions of coagulation and anticoagulation pathways in flow-dependent thrombus formation

In haemostasis and thrombosis, platelet, coagulation and anticoagulation pathways act together to produce fibrin-containing thrombi. We developed a microspot-based technique, in which we assessed platelet adhesion, platelet activation, thrombus structure and fibrin clot formation in real time using flowing whole blood. Microspots were made from distinct platelet-adhesive surfaces in the absence or presence of tissue factor, thrombomodulin or activated protein C. Kinetics of platelet activation, thrombus structure and fibrin formation were assessed by fluorescence microscopy. This work revealed: (1) a priming role of platelet adhesion in thrombus contraction and subsequent fibrin formation; (2) a surface-independent role of tissue factor, independent of the shear rate; (3) a mechanism of tissue factor-enhanced activation of the intrinsic coagulation pathway; (4) a local, suppressive role of the anticoagulant thrombomodulin/protein C pathway under flow. Multiparameter analysis using blood samples from patients with (anti)coagulation disorders indicated characteristic defects in thrombus formation, in cases of factor V, XI or XII deficiency; and in contrast, thrombogenic effects in patients with factor V-Leiden. Taken together, this integrative phenotyping approach of platelet–fibrin thrombus formation has revealed interaction mechanisms of platelet-primed key haemostatic pathways with alterations in patients with (anti)coagulation defects. It can help as an important functional add-on whole-blood phenotyping.

Spellbinding Effects of the Acidic COOH-Terminus of Factor Va Heavy Chain on Prothrombinase Activity and Function

Human factor Va (hfVa) is the important regulatory subunit of prothrombinase. Recent modeling data have suggested a critical role for amino acid Arg⁷⁰¹ of hfVa for human prothrombin (hPro) activation by prothrombinase. Furthermore, it has also been demonstrated that hfVa has a different effect than that of bovine fVa on prethrombin-1 activation by prothrombinase. The difference between the two cofactor molecules was also found within the Asn⁷⁰⁰-Arg⁷⁰¹ dipeptide in the human factor V (hfV) molecule, which is replaced by the Asp-Glu sequence in bfV. As a consequence, we produced a recombinant hfV (rhfV) molecule with the substitution ⁷⁰⁰NR⁷⁰¹→DE. rhfV^NR→DE together with the wild-type molecule (rhfV^WT) were expressed in COS7 cells, purified, and tested for their capability to function within prothrombinase. Kinetic studies showed that the K_d of rhfVa^NR→DE for human fXa as well as the k_cat and K_m of prothrombinase made with rhfVa^NR→DE for hPro activation were similar to the values obtained following hPro activation by prothrombinase made with rhfVa^WT. Remarkably, sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses of hPro activation time courses demonstrated that the rate of cleavage of hPro by prothrombinase reconstituted with rhfVa^NR→DE was significantly delayed with substantial accumulation of meizothrombin, and delayed thrombin generation, when compared to activation of hPro by prothrombinase made with rhfVa^WT. These unanticipated results provide significant insights on the role of the carboxyl-terminal end of the heavy chain of hfVa for hPro cleavage and activation by prothrombinase and show that residues ⁷⁰⁰NR⁷⁰¹ regulate at least in part the enzyme-substrate/product interaction during fibrin clot formation.

Reduced Prothrombinase Inhibition by Tissue Factor Pathway Inhibitor Contributes to the Factor V Leiden Hypercoagulable State

Activated factor V (FVa) and factor X (FXa) form prothrombinase, which converts prothrombin to thrombin. The α isoform of tissue factor (TF) pathway inhibitor (TFPI) dampens early procoagulant events, partly by interacting with FV. FV Leiden (FVL) is the most common genetic thrombophilia in Caucasians. Thrombosis risk is particularly elevated in women with FVL taking oral contraceptives, which produce acquired TFPIα deficiency. In mice, FVL combined with 50% reduction in TFPI causes severe thrombosis and perinatal lethality. However, a possible interaction between FVL and TFPIα has not been defined in humans. Here, we examined this interaction using samples from patients with FVL in thrombin generation and fibrin formation assays. In dilute TF- or FXa-initiated reactions, these studies exposed a TFPI-dependent activation threshold for coagulation initiation that was greatly reduced by FVL. The reduced threshold was progressively overcome with higher concentrations of TF or FXa. Plasma assays using anti-TFPI antibodies or a TFPI peptide that binds and inhibits FVa demonstrated that the decreased activation threshold resulted from reduced TFPIα inhibition of prothrombinase. In assays using purified proteins, TFPIα was a 1.7-fold weaker inhibitor of prothrombinase assembled with FVL than with FV. Thus, FVL reduces the threshold for initiating coagulation, and this threshold is further reduced in situations of low TFPIα concentration. Individuals with FVL are likely prone to thrombosis in response to weak procoagulant stimuli that would not initiate blood clot formation in individuals with FV.

The association of factor V Leiden with various clinical patterns of venous thromboembolism-the factor V Leiden paradox

BACKGROUND

Factor V Leiden (FVL) supposedly carries relatively higher risk of deep vein thrombosis (DVT), compared to the risk of pulmonary embolism (PE).

AIM

To prove this paradox in a group of patients with various clinical presentation of venous thromboembolism (VTE).

MATERIALS AND METHODS

We retrospectively evaluated clinical pattern of VTE in patients who had been referred to vascular clinic shortly after an acute VTE event. In FVL positive and FVL negative groups we compared the prevalence of isolated symptomatic DVT (proximal or distal) and symptomatic PE with/without DVT, and, moreover, asymptomatic DVT or PE.

RESULTS

Of 575 patients (mean age 57 years, 50.1% women), 120 were FVL positive and those had significantly higher prevalence of isolated symptomatic DVT, compared to symptomatic PE with/without DVT. Proximal DVT location was significantly more frequent in FVL carriers. The prevalence of asymptomatic PE did not differ between the two groups. The rate of asymptomatic DVT tended to be higher in FVL negative group. In a multivariate analysis, we confirmed FVL to be positively associated with isolated DVT presentation (odds ratio OR 1.757; 95% confidence interval (CI) 1.148-2.690). On the contrary, increasing age and unprovoked nature of VTE event carried a higher risk of symptomatic PE.

CONCLUSIONS

We confirmed FVL to be significantly associated with isolated symptomatic DVT despite higher prevalence of proximal DVT in FVL carriers. The fact of relatively lower risk of PE in FVL positive patients might have clinical implication. However, mechanisms of FVL paradox remain to be elucidated.

A HYBRID-SYSTEM MODEL OF THE COAGULATION CASCADE: SIMULATION, SENSITIVITY, AND VALIDATION

The process of human blood clotting involves a complex interaction of continuous-time/continuous-state processes and discrete-event/discrete-state phenomena, where the former comprise the various chemical rate equations and the latter comprise both threshold-limited behaviors and binary states (presence/absence of a chemical). Whereas previous blood-clotting models used only continuous dynamics and perforce addressed only portions of the coagulation cascade, we capture both continuous and discrete aspects by modeling it as a hybrid dynamical system. The model was implemented as a hybrid Petri net, a graphical modeling language that extends ordinary Petri nets to cover continuous quantities and continuous-time flows. The primary focus is simulation: (1) fidelity to the clinical data in terms of clotting-factor concentrations and elapsed time; (2) reproduction of known clotting pathologies; and (3) fine-grained predictions which may be used to refine clinical understanding of blood clotting. Next we examine sensitivity to rate-constant perturbation. Finally, we propose a method for titrating between reliance on the model and on prior clinical knowledge. For simplicity, we confine these last two analyses to a critical purely-continuous subsystem of the model.

Proteolysis of plasma-derived factor V following its endocytosis by megakaryocytes forms the platelet-derived factor V/Va pool

BACKGROUND

Central to appropriate thrombin formation at sites of vascular injury is the concerted assembly of plasma- and/or platelet-derived factor (F) Va and FXa on the activated platelet surface. While the plasma-derived procofactor, FV, must be proteolytically activated by α-thrombin to FVa to function in prothrombinase, the platelet molecule is released from α-granules in a partially activated state, obviating the need for proteolytic activation.

OBJECTIVES

The current study was performed to test the hypothesis that subsequent to its endocytosis by megakaryocytes, plasma-derived FV is proteolytically processed to form the platelet-derived pool.

METHODS & RESULTS

Subsequent to FV endocytosis, a time-dependent increase in FV proteolytic products was observed in megakaryocyte lysates by SDS-PAGE followed by phosphorimaging or western blotting. This cleavage was specific and resulted in the formation of products similar in size to FV/Va present in a platelet lysate as well as to the α-thrombin-activated FVa heavy chain and light chain, and their respective precursors. Other proteolytic products were unique to endocytosed FV. The product/precursor relationships of these fragments were defined using anti-FV heavy and light chain antibodies with defined epitopes. Activity measurements indicated that megakaryocyte-derived FV fragments exhibited substantial FVa cofactor activity that was comparable to platelet-derived FV/Va.

CONCLUSIONS

Taken together, these observations suggest that prior to its packaging in α-granules endocytosed FV undergoes proteolysis by one or more specific megakaryocyte protease(s) to form the partially activated platelet-derived pool.

Thrombomodulin-dependent effect of factor V Leiden mutation on the cross-linking of α2-plasmin inhibitor to fibrin and its consequences on fibrinolysis

INTRODUCTION

It has been shown that thrombomodulin (TM) considerably delays factor XIII (FXIII) activation and this effect is abrogated by Factor V Leiden (FV(Leiden)) mutation. The aim of the study was to explore the effect of TM on the cross-linking of α(2)-plasmin inhibitor (α(2)-PI) to fibrin in plasma samples of different FV genotypes and how this effect is related to the impaired fibrinolysis of FV(Leiden) carriers.

METHODS

In the plasma samples of fifteen individuals with different FV genotypes and in FV deficient plasma supplemented with wild type FV or FV(Leiden) coagulation was initiated by recombinant human tissue factor and phospholipids with or without recombinant human TM (rhTM). In the recovered clots the extent of α(2)-PI-fibrin cross-linking was evaluated by Western blotting and quantitative densitometry. The effect of rhTM on tissue plasminogen activator (tPA) induced clot lysis was measured by turbidimetric method.

RESULTS

rhTM significantly delayed the formation of α(2)-PI-fibrin α-chain heterodimers/oligomers in plasma samples containing wild type FV. This effect of rhTM was impaired in the presence of FV(Leiden). rhTM delayed tPA-induced clot lysis and this effect of rhTM was more pronounced in plasma containing FV(Leiden). When TAFIa was inhibited by potato carboxypeptidase inhibitor, rhTM accelerated clot lysis in the presence of wild type FV, which is explained by the delayed α(2)-PI-fibrin cross-linking. This effect of rhTM did not prevail in the presence of FV(Leiden).

CONCLUSION

FV(Leiden) abrogates the delaying effect of rhTM on α(2)-PI-fibrin cross-linking, which contributes to the impaired fibrinolysis observed in FV(Leiden) carriers.

Activated protein C inhibitor for correction of thrombin generation in hemophilia A blood and plasma1

BACKGROUND

Replacement therapy for hemophilic patient treatment is costly, because of the high price of pharmacologic products, and is not affordable for the majority of patients in developing countries.

OBJECTIVE

To generate and evaluate low molecular weight agents that could be useful for hemophilia treatment.

METHODS

Potential agents were generated by synthesizing specific inhibitors [6-(Lys-Lys-Thr-[homo]Arg)amino-2-(Lys[carbobenzoxy]-Lys[carbobenzoxy]-O-benzyl)naphthalenesulfonamide] (PNASN-1)] for activated protein C (APC) and tested in plasma and fresh blood from hemophilia A patients.

RESULTS

In the activated partial thromboplastin time-based APC resistance assay, PNASN-1 partially neutralized the effect of APC. In calibrated automated thrombography, PNASN-1 neutralized the effect of APC on thrombin generation in normal and congenital factor VIII-deficient plasma (FVIII:C < 1%). The addition of PNASN-1 to tissue factor-triggered (5 pm) contact pathway-inhibited fresh blood from 15 hemophilia A patients with various degrees of FVIII deficiency (FVIII:C < 1-51%) increased the maximum level of thrombin generated from 78 to 162 nm, which approached that observed in blood from a healthy individual (201 nm). PNASN-1 also caused a 47% increase in clot weight in hemophilia A blood.

CONCLUSIONS

Specific APC inhibitors compensate to a significant extent for FVIII deficiency, and could be used for hemophilia treatment.

Thrombomodulin-dependent effect of factor VLeiden mutation on factor XIII activation

INTRODUCTION

Factor V Leiden mutation (FV(Leiden)) is associated with impaired down-regulation of activated FV procoagulant activity and loss of FV anticoagulant function that result in an increased risk of venous thromboembolism. As the downstream effects of FV(Leiden) on clot formation and fibrinolyis have only partially been revealed, we investigated its effect on the activation of factor XIII (FXIII) and the cross-linking of fibrin.

METHODS

In the plasma samples of fifteen healthy individuals with known FV genotypes coagulation was initiated by recombinant human tissue factor and phospholipids with or without recombinant human thrombomodulin (rhTM). FV deficient plasma supplemented with purified wild type FV or FV(Leiden) were also investigated. Clots were recovered and analyzed by SDS-PAGE and quantitative densitometric evaluation of Western blots.

RESULTS

rhTM considerably delayed the activation of FXIII in the plasma from FV wild type individuals. This effect of rhTM was significantly impaired in the plasma from FV(Leiden) carriers. The results were confirmed in experiments with FV deficient plasma supplemented by FV prepared from wild type individuals or FV(Leiden) homozygotes. Fibrin γ-chain dimerization was also considerably delayed by rhTM in plasma samples from individuals without Leiden mutation, but not in plasma samples from FV(Leiden) heterozygotes or homozygotes. The difference between heterozygotes and homozygotes was not statistically significant.

CONCLUSION

The highly diminished delaying effect of TM on FXIII activation and on the cross-linking of fibrin in FV(Leiden) carriers might represent a novel mechanism contributing to the increased thrombosis risk of these individuals.

Risk factors for peripheral venous disease resemble those for venous thrombosis: the San Diego Population Study

BACKGROUND

Clinically silent deep vein thrombosis (DVT) is common and may cause chronic venous disease that resembles post-thrombotic syndrome.

OBJECTIVE

We evaluated whether peripheral venous disease in a general population shares risk factors with DVT.

METHODS

In an established cohort of 2404 men and women, the San Diego Population Study, peripheral venous disease was evaluated using physical examination, symptom assessment and venous ultrasound. We performed a case-control study including 308 cases in four hierarchical groups by severity and 346 controls without venous abnormalities, frequency matched to cases by 10-year age group, race and gender. Cases and controls had no prior history of venous thrombosis. Hemostatic risk factors were measured in cases and controls.

RESULTS

Accounting for age, obesity and family history of leg ulcer, odds ratios (ORs) of venous disease for elevated factor VIII, von Willebrand factor (VWF), D-dimer and for factor V Leiden were 1.4 (95% CI 0.9-2.1), 1.5 (CI 1.0-2.3), 1.7 (CI 1.1-2.8) and 1.1 (CI 0.5-2.4), respectively. These associations were larger for the two most severe case groups; ORs 2.0 (CI 1.0-3.8), 1.7 (CI 0.9-3.3), 2.7 (CI 1.2-6.1) and 2.3 (CI 0.8-7.1). Each hemostatic factor was also associated with severity of venous disease, for example elevated D-dimer was associated with a 2.2-fold increased odds of being in one higher severity case group. Prothrombin 20210A was not associated with venous disease.

CONCLUSIONS

DVT risk factors are associated with presence and severity of peripheral venous disease. Results support a hypothesis that peripheral venous disease may sometimes be post-thrombotic syndrome as a result of a previous unrecognized DVT.

Citrate anticoagulation and the dynamics of thrombin generation

BACKGROUND

Sodium citrate has been used as an anticoagulant to stabilize blood and blood products for over 100 years, presumably by sequestering Ca(++) ions in vitro. Anticoagulation of blood without chelation can be achieved by inhibition of the contact pathway by corn trypsin inhibitor (CTI).

OBJECTIVE

To evaluate the influence of citrate anticoagulation on the performance of blood, platelet-rich and platelet-poor plasma assays.

METHODS

Blood was anticoagulated in three ways: by collection into citrate, CTI and citrate with CTI. Plasma was prepared using each anticoagulation regimen. Functional analyses included calibrated automated thrombography, thromboelastography, plasma clotting, the synthetic coagulation proteome and platelet aggregation. Coagulation reactions were initiated with tissue factor-phospholipid and Ca(++) (when indicated).

RESULTS

In all cases, citrate anticoagulation resulted in reaction dynamics significantly altered relative to blood or plasma stabilized with CTI alone. Subsequent experiments showed that calcium citrate itself impairs coagulation dynamics.

CONCLUSION

Coagulation analyses using blood that has been exposed to citrate and recalcified do not yield reliable depictions of the natural dynamics of blood coagulation processes.

Polymorphisms of the tissue factor pathway inhibitor gene are associated with venous thromboembolism in the antiphospholipid syndrome and carriers of factor V Leiden

Polymorphisms within the tissue factor pathway inhibitor (TFPI) gene may determine TFPI expression and increase the risk of venous thromboembolism (VTE) in predisposed individuals. We tested this hypothesis by comparing TFPI activity and the frequency of common TFPI polymorphisms, -33T->C, -399C->T and -287T->C, in patients with antiphospholipid syndrome (APS) (n = 24) or factor V Leiden (n = 44) who had a history of VTE (n = 26), compared with those without VTE (n = 42) and also with normal control individuals (n = 56). TFPI activity was measured using a modified amidolytic assay and genotypes were determined by polymerase chain reaction and restriction fragment length polymorphism. We found that only APS patients with a history of venous thrombosis had TFPI activity levels significantly different from control individuals (1.77 +/- 0.60 vs 0.77 +/- 0.19 U/ml; P = 0.0001), and this was associated with inheritance of the TFPI -33C allele (1.70 +/- 0.72 U/ml for TC/CC genotypes vs 0.97 +/- 0.56 U/ml for TT; P = 0.01). Multivariate analysis of APS and factor V Leiden patients revealed that the greatest independent contributor to VTE was TFPI activity (adjusted odds ratio = 16.84; 95% confidence interval = 2.47-114.36, P = 0.004), while inheritance of either the TFPI -33C or -399T alleles each increased the odds of VTE by nearly 13 times (95% confidence interval = 2.39-69.91, P = 0.003; and 95% confidence interval = 2.25-71.23, P = 0.004, respectively). These results indicate that the TFPI -33T->C and -399C->T polymorphisms are significantly associated with venous thrombosis in the presence of other risk factors, especially APS, and may be clinically relevant in patients who are prone to hypercoagulability.

Peptidomimetic inhibitors for activated protein C: implications for hemophilia management

BACKGROUND

Several clinical studies and experiments with transgenic mice have suggested that the severity of the bleeding phenotype in hemophilic patients is substantially reduced in association with impaired inactivation of factor (F) Va by activated protein C (APC) in the presence of the FV Leiden mutation. Experiments using a synthetic coagulation proteome model showed that the presence of FV Leiden significantly increased thrombin generation in the absence of FVIII or FIX.

OBJECTIVE

To test the effect of APC inhibition on thrombin generation in hemophilia.

METHODS

Prothrombinase and a synthetic coagulation proteome model of tissue factor-triggered thrombin generation were used.

RESULTS

Peptide-based APC inhibitors, which mimic the P4-P4' residues surrounding the APC cleavage site at Arg306 of FVa, were synthesized. These compounds are specific and reversible inhibitors of APC, with Ki values as low as 1-2 microM; most have insignificant affinity for FXa or thrombin. The affinity for APC is dependent upon the location and character of the protecting groups. Representatives of this group of compounds inhibit FVa inactivation by APC and prolong FVa functional activity in the prothrombinase complex. When evaluated in a synthetic coagulation proteome model, one inhibitor partially compensated for the absence of FVIII.

CONCLUSIONS

Synthetic APC inhibitors may be useful as adjuvants for hemophilia treatment.

Identifying novel genetic determinants of hemostatic balance

Incomplete penetrance and variable expressivity confound the diagnosis and therapy of most inherited thrombotic and hemorrhagic disorders. For many of these diseases, some or most of this variability is determined by genetic modifiers distinct from the primary disease gene itself. Clues toward identifying such modifier genes may come from studying rare Mendelian disorders of hemostasis. Examples include identification of the cause of combined factor V and VIII deficiency as mutations in the ER Golgi intermediate compartment proteins LMAN1 and MCFD2. These proteins form a cargo receptor that facilitates the transport of factors V and VIII, and presumably other proteins, from the ER to the Golgi. A similar positional cloning approach identified ADAMTS-13 as the gene responsible for familial TTP. Along with the work of many other groups, these findings identified VWF proteolysis by ADAMTS-13 as a key regulatory pathway for hemostasis. Recent advances in mouse genetics also provide powerful tools for the identification of novel genes contributing to hemostatic balance. Genetic studies of inbred mouse lines with unusually high and unusually low plasma VWF levels identified polymorphic variation in the expression of a glycosyltransferase gene, Galgt2, as an important determinant of plasma VWF levels in the mouse. Ongoing studies in mice genetically engineered to carry the factor V Leiden mutation may similarly identify novel genes contributing to thrombosis risk in humans.

Does the genotype predict the phenotype? Evaluations of the hemostatic proteome

In this review, the complexity arising from the heterogeneity of the human hemostatic proteome is introduced and discussed with respect to impact on the diagnosis, prophylaxis and therapeutic interventions in thrombotic and hemorrhagic diseases. In the 'healthy' population, coagulation factor levels extend over a 2-4-fold range in concentration. In addition, the qualitative performance of these proteins is governed by many molecular events which are influenced both by genetic instructions which influence post-translational modification and by environmental processes that alter coagulation proteins during circulation. As a consequence, the stimulus-response coupling which follows tissue factor presentation to blood and the subsequent expression of thrombin activity is highly variable even in the 'normal' population. The consequences of this molecular heterogeneity and its potential influence on the diagnosis, prophylaxis and ultimate therapy of coagulation diseases are illustrated. It is the intention of the authors to be provocative; encouraging further investigations to understand the clinical significance of the heterogeneity of the human hemostatic proteome.

Factor V Leiden and perioperative risk

Factor V Leiden (FVL) is the most common known inherited cause of thrombophilia; it is present in approximately 5% of the Caucasian population. Although the risk of venous thrombosis associated with this polymorphism in various medical settings is well described, its effect on perioperative risk is only beginning to be explored. Specifically, there are few studies addressing the potential risks of FVL in the surgical population, in which both hemorrhagic and thrombotic complications convey substantial clinical and economic significance. There are speculations and unproven hypotheses regarding FVL in this population, and these therefore highlight the need to comprehensively address this issue. This review will describe the physiology of the FVL mutation, briefly clarify its risk in the nonsurgical setting, and assess current data regarding FVL in noncardiac and cardiac surgery. Finally, a summary of current clinical evidence and a plan for more detailed investigation of this potentially significant risk factor will be proposed.

The Factor V Activation Paradox

The prothrombinase complex consists of the protease factor Xa, Ca2+, and factor Va assembled on an anionic membrane. Factor Va functions both as a receptor for factor Xa and a positive effector of factor Xa catalytic efficiency and thus is key to efficient conversion of prothrombin to thrombin. The activation of the procofactor, factor V, to factor Va is an essential reaction that occurs early in the process of tissue factor-initiated blood coagulation; however, the catalytic sequence leading to formation of factor Va is a subject of disagreement. We have used biophysical and biochemical approaches to establish the second order rate constants and reaction pathways for the activation of phospholipid-bound human factor V by native and recombinant thrombin and meizothrombin, by mixtures of prothrombin activation products, and by factor Xa. We have also reassessed the activation of phospholipid-bound human prothrombin by factor Xa. Numerical simulations were performed incorporating the various pathways of factor V activation including the presence or absence of the pathway of factor V-independent prothrombin activation by factor Xa. Reaction pathways for factor V activation are similar for all thrombin forms. Empirical rate constants and the simulations are consistent with the following mechanism for factor Va formation. alpha-Thrombin, derived from factor Xa cleavage of phospholipid-bound prothrombin via the prethrombin 2 pathway, catalyzes the initial activation of factor V; generation of factor Va in a milieu already containing factor Xa enables prothrombinase formation with consequent meizothrombin formation; and meizothrombin functions as an amplifier of the process of factor V activation and thus has an important procoagulant role. Direct activation of factor V by factor Xa at physiologically relevant concentrations does not appear to be a significant contributor to factor Va formation.

The Contribution of Amino Acid Region Asp695-Tyr698 of Factor V to Procofactor Activation and Factor Va Function

There is strong evidence that a functionally important cluster of amino acids is located on the COOH-terminal portion of the heavy chain of factor Va, between amino acid residues 680 and 709. To ascertain the importance of this region for cofactor activity, we have synthesized five overlapping peptides representing this amino acid stretch (10 amino acids each, HC1-HC5) and tested them for inhibition of prothrombinase assembly and function. Two peptides, HC3 (spanning amino acid region 690-699) and HC4 (containing amino acid residues 695-704), were found to be potent inhibitors of prothrombinase activity with IC(50) values of approximately 12 and approximately 10 microm, respectively. The two peptides were unable to interfere with the binding of factor Va to active site fluorescently labeled Glu-Gly-Arg human factor Xa, and kinetic analyses showed that HC3 and HC4 are competitive inhibitors of prothrombinase with respect to prothrombin with K(i) values of approximately 6.3 and approximately 5.3 microm, respectively. These data suggest that the peptides inhibit prothrombinase because they interfere with the incorporation of prothrombin into prothrombinase. The shared amino acid motif between HC3 and HC4 is composed of Asp(695)-Tyr-Asp-Tyr-Gln(699) (DYDYQ). A pentapeptide with this sequence inhibited both prothrombinase function with an IC(50) of 1.6 microm (with a K(D) for prothrombin of 850 nm), and activation of factor V by thrombin. Peptides HC3, HC4, and DYDYQ were also found to interact with immobilized thrombin. A recombinant factor V molecule with the mutations Asp(695) --> Lys, Tyr(696) --> Phe, Asp(697) --> Lys, and Tyr(698) --> Phe (factor V(2K2F)) was partially resistant to activation by thrombin but could be readily activated by RVV-V activator (factor Va(RVV)(2K2F)) and factor Xa (factor Va(Xa)(2K2F)). Factor Va(RVV)(2K2F) and factor Va(Xa)(2K2F) had impaired cofactor activity within prothrombinase in a system using purified reagents. Our data demonstrate for the first time that amino acid sequence 695-698 of factor Va heavy chain is important for procofactor activation and is required for optimum prothrombinase function. These data provide functional evidence for an essential and productive contribution of factor Va to the activity of prothrombinase.

Amino acids Glu323, Tyr324, Glu330, and Val331 of factor Va heavy chain are essential for expression of cofactor activity

We have recently demonstrated that amino acid region 323-331 of factor Va heavy chain (9 amino acids, AP4') contains a binding site for factor Xa (Kalafatis, M., and Beck, D. O. (2002) Biochemistry 41, 12715-12728). To ascertain which amino acids within this region are important for the effector and receptor properties of the cofactor with respect to factor Xa, we have synthesized three overlapping peptides (5 amino acids each) spanning the amino acid region 323-331 and tested them for their effect on prothrombinase complex assembly and function. Peptide containing amino acids 323EYFIA327 alone was found to increase the catalytic efficiency of factor Xa but had no effect on the fluorescent anisotropy of active site-labeled factor Xa (human factor Xa labeled in the active site with Oregon Green 488; [OG488]-EGR-hXa). In contrast, peptide containing the sequence 327AAEEV331 was found to interact with [OG488]-EGR-hXa with half-maximal saturation reached at approximately 150 microm, but it was unable to produce a cofactor effect on factor Xa. Peptide 325FIAAE329 inhibited prothrombinase activity and was able to partially decrease the fluorescent anisotropy of [OG488]-EGR-hXa but could not increase the catalytic efficiency of factor Xa with respect to prothrombin. A control peptide with the sequence FFFIA did not increase the catalytic efficiency of factor Xa, whereas a peptide with the sequence AAEMI was impaired in its capability to interact with [OG488]-EGR-hXa. Two mutant recombinant factor Va molecules (Glu323 --> Phe/Tyr324 --> Phe, factor VaFF; Glu330 --> Met/Val331 --> Ile, factor VaMI) showed impaired cofactor activity when used at limiting cofactor concentration, whereas the quadruple mutant (Glu323 --> Phe/Tyr324 --> Phe and Glu330 --> Met/Val331 --> Ile, factor VaFF/MI) had no cofactor activity under similar experimental conditions. Our data demonstrate that amino acid residues Glu323, Tyr324, Glu330, and Val331 of factor Va heavy chain are critical for expression of factor Va cofactor activity.

What is all that thrombin for?

The hemostatic process initiated by the exposure of tissue factor to blood is a threshold limited reaction which occurs in two distinct phases. During an initiation phase, small amounts of factor (F)Xa, FIXa and thrombin are generated. The latter activates the procofactors FV and FVIII to the activated cofactors which together with their companion serine proteases form the intrinsic FX activator (FVIIIa-FIXa) and prothrombinase (FVa-FXa) which generate the bulk of FXa and thrombin during a propagation phase. The clotting process (fibrin formation) occurs at the inception of the propagation phase when only 5-10 nM thrombin has been produced. Consequently, the vast majority (greater than 95%) of thrombin is produced after clotting during the propagation phase of thrombin generation. The blood of individuals with either hemophilia A or hemophilia B has no ability to generate the intrinsic FXase, and hence is unable to support the propagation phase of the reaction. Since clot based assays conclude before the propagation phase they are not sensitive to hemophilia A and B. The inception and magnitude of the propagation phase of thrombin generation is influenced by genetic polymorphisms associated with thrombotic and hemorrhagic disease, by the natural abundance of pro- and anticoagulants in healthy individuals and by pharmacologic interventions which influence thrombotic pathology. Therefore, it is our suspicion that the performance of the entire process of thrombin generation from initiation through propagation and termination phases of the reaction are relevant with respect to both hemorrhagic and thrombotic pathology.

The dynamics of thrombin formation

The central event of the hemostatic process is the generation of thrombin through the tissue factor pathway. This is a highly regulated, dynamic process in which thrombin itself plays many roles, positively and negatively its production and destruction. The hemostatic process is essential to normal physiology and is also the Achilles heel of our aging population. The inappropriate generation of thrombin may lead to vascular occlusion with the consequence of myocardial infarction, stroke, pulmonary embolism, or venous thrombosis. In this review, we summarize our present views regarding the tissue factor pathway by which thrombin is generated and the roles played by extrinsic and intrinsic factor Xa generating complexes in hemostasis and the roles of the stoichiometric and dynamic inhibitors that regulate thrombin generation.

Isolation and characterization of an antifactor V antibody causing activated protein C resistance from a patient with severe thrombotic manifestations

A 44-year-old woman with a history of severe thrombotic manifestations presented with a markedly reduced activated protein C-sensitivity ratio (APC-SR). DNA sequencing of and around the regions encoding the APC cleavage sites in the factor Va molecule excluded the presence of the factor VLeiden mutation and of other known genetic mutations. No antiphospholipid antibodies were present in the patient's plasma and both prothrombin time and activated partial thromboplastin time were normal. The total immunoglobulin fraction was isolated from the patient's plasma and found to induce severe APC resistance when added to normal plasma and to factor V-deficient plasma supplemented with increasing concentrations of factor V. Immunoblotting and immunoprecipitation experiments with the total immunoglobulin fraction purified from the patient's plasma demonstrated that the antibody recognizes factor V, is polyclonal, and has conformational epitopes on the entire factor V molecule (heavy and light chains, and B region). Thus, the immunoglobulin fraction interferes with the anticoagulant pathway involving factor V. The inhibitor was isolated by sequential affinity chromatography on protein G-Sepharose and factor V-Sepharose. The isolated immunoglobulin fraction inhibited factor Va inactivation by APC because of impaired cleavage at Arg306 and Arg506 of the heavy chain of the cofactor. The isolated immunoglobulin fraction was also found to inhibit the cofactor effect of factor V for the inactivation of factor VIII by the APC/protein S complex. Our data provide for the first time the demonstration of an antifactor V antibody not related to the presence of antiphospholipid antibodies, which is responsible for thrombotic rather than hemorrhagic symptoms.

Lethal perinatal thrombosis in mice resulting from the interaction of tissue factor pathway inhibitor deficiency and factor V Leiden

BACKGROUND

Factor V Leiden (FVL) is a common genetic risk factor for thrombosis in humans. The incomplete penetrance of FVL suggests important contributions from other genetic or environmental modifying factors. Variation in the expression of tissue factor pathway inhibitor (TFPI) has also been proposed as a risk factor for venous thrombosis and has been shown to enhance the prothrombotic effect of FVL in vitro.

METHODS AND RESULTS

To examine the potential in vivo interaction between Tfpi and FvL, we analyzed crosses between mice carrying FvL and a deficiency of TFPI. The Fv(Q/Q),Tfpi(+/-) genotype was nearly completely fatal in the early perinatal period. Increased fibrin deposition was observed in multiple organs from the Fv(Q/Q),Tfpi(+/-) fetuses, suggesting disseminated thrombosis.

CONCLUSIONS

These observations demonstrate the prothrombotic effect of modest variations in the level of TFPI expression and suggest that TFPI could be an important genetic modifier for the thrombosis associated with FVL in humans.

A matrix-assisted laser desorption/ionization time-of-flight based method for screening the 1691G --> A mutation in the factor V gene

A point mutation, 1691 G --> A in the coagulation factor V gene results in an Arg506 --> Gln amino acid substitution in the factor V molecule. This mutation, defined as factor VLEIDEN, results in activated protein C (APC) resistance and is the most common genetic risk factor for familial thrombophilia. A new mini-sequencing method using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was developed for the screening of the 1691G --> A substitution in factor V. In this method, a fragment of genomic DNA containing the 1691st base is first amplified, followed by mini-sequencing in the presence of dGTP and ddATP, ddCTP, and ddTTP. In this manner, the primer is extended by one base from one allele and two bases from the other allele. The extended products are analyzed using MALDI-TOF mass spectrometry. The base at position 1691 is identified based on the number of nucleotides added. We have used this method to genotype 16 APC-resistant patients previously identified by conventional methods and 11 normal control samples. The genotypes of all samples were correctly identified. This method is accurate, fast, and potentially allows for simultaneous multiplex genotyping of a number of mutation sites associated with thrombophilia and clot formation.

Prevalence of factor V Leiden and prothrombin 20210 A variant in Bulgarian patients with pulmonary thromboembolism and deep venous thrombosis

Factor V Leiden mutation and prothrombin variant 20210 A are well-known risk factors for venous thrombosis (DVT). Recent papers have reported a lower prevalence of factor V Leiden in patients with pulmonary thromboembolism (PTE) than in patients with deep venous thrombosis. The aim of the present study was to compare the prevalence of factor V Leiden and the prothrombin 20210 G <-- A mutation in patients with DVT and in patients with PTE. We studied 128 consecutive patients (45 with DVT, 40 with PTE, and 43 with DVT and PTE) for factor V Leiden and prothrombin 20210 A. One hundred healthy persons matched by age and sex were used as controls. Factor V Leiden was present in five of the patients with PTE [12.5%; 95% confidence interval (CI), 1.5-23.5%; not significant], 15 of the patients with DVT (33.3%; 95% CI, 9.6-38.7%; P < 0.001), and 12 of the patients with DVT and PTE (27.9%; 95% CI, 4.8-33%; P = 0.001). Results for the prothrombin 20210 A mutation were as follows: four of 40 patients with PTE (10%; 95% CI, 0-13.3%; P = 0.46), nine of 45 (20%) of the patients with DVT (95% CI, 0.5-25.5%; P < 0.05) and eight of 43 with DVT and PTE were heterozygous (18.6%; 95% CI, 0-23.9%; P = 0.02). In conclusion, there is a significantly higher frequency of factor V Leiden among patients with DVT than in patients with PTE. However, there is no significant difference of factor V Leiden or 20210 A prothrombin mutation in patients with DVT than in patients with combined DVT/PTE, therefore patients with DVT, carriers of the mutations, do not appear to be at lower risk for pulmonary embolism.

Genetic engineering for xenotransplantation

Xenotransplantation is being pursued vigorously to solve the shortage of allogeneic donor organs. Experimental studies of the major xenoantigen (Gal) and of complement regulation enable model xenografts to survive hyperacute rejection. When the Gal antigen is removed or reduced and complement activation is controlled, the major barriers to xenograft survival include unregulated coagulation within the graft and cellular reactions involving macrophages, neutrophils, natural killer (NK) cells, and T lymphocytes. Unlike allografts, where specific immune responses are the sole barrier to graft survival, molecular differences between xenograft and recipient that affect normal receptor-ligand interactions (largely active at the cell surface and which may not be immunogenic), are also involved in xenograft failure. Transgenic strategies provide the best options to control antigen expression, complement activation, and coagulation. Although the Gal antigen can be eliminated by gene knockout in mice, that outcome has only become a possibility in pigs due to the recent cloning of pigs after nuclear transfer. Instead, the use of transgenic glycosyl transferase enzymes and glycosidases, which generate alternative terminal carbohydrates on glycolipids and glycoproteins, has reduced antigen in experimental models. As a result, novel strategies are being tested to seek the most effective solution. Transgenic pigs expressing human complement-regulating proteins (DAF/CD55, MCP/CD46, or CD59) have revealed that disordered regulation of the coagulation system requires attention. There will undoubtedly be other molecular incompatibilities that need addressing. Xenotransplantation, however, offers hope as a therapeutic solution and provides much information about homeostatic mechanisms.

Prevalence of factor V Leiden and prothrombin G20210A mutations in unselected patients with venous thromboembolism

We determined the prevalence of factor V Leiden and of prothrombin G20210A mutations in a cohort of unselected outpatients (n = 748) referred for suspected deep vein thrombosis (DVT) and/or pulmonary embolism (PE) and a pooled analysis of similar studies was also performed. Based on the clinical presentation, the prevalence of factor V Leiden was 15.7% in the 83 patients with DVT and 14.1% in the 99 patients with PE compared with 5.3% in patients without DVT and/or PE (control group). The prevalence of the prothrombin G20210A mutation did not differ among the three groups (3.9% for controls, 4. 8% for DVT and 3.9% for PE patients). We then divided the 99 patients with PE by separately analysing those with PE but without DVT (n = 57) and those with PE and DVT (n = 42). Compared with the control group, the prevalence of factor V Leiden was 10.5%, odds ratio (OR) 2.10 [95% confidence interval (95% CI) 0.68-5.45] in patients with primary PE and 19.1%, OR 4.20 (95% CI 1.54-10.30) in patients with DVT and PE. For the prothrombin G20210A mutation, no statistically significant differences were found between the control group and the three other groups. In conclusion, our data and the pooled analysis indicate that patients with primary PE are less often affected by the factor V Leiden mutation. No statistically significant differences were observed between patients and controls for the prothrombin G20210A mutation.

“Normal” Thrombin Generation

We have investigated the influence of alterations in plasma coagulation factor levels between 50% and 150% of their mean values for prothrombin, factor X, factor XI, factor IX, factor VII, factor VIII, factor V, protein C, protein S, antithrombin III (AT-III), and tissue factor pathway inhibitor (TFPI) as well as combinations of extremes, eg, 50% anticoagulants and 150% procoagulants or 50% procoagulants and 150% anticoagulants in a synthetic “plasma” system. The reaction systems were constructed in vitro using purified, natural, and recombinant proteins and synthetic phospholipid vesicles or platelets with the reactions initiated by recombinant tissue factor (TF)-factor VIIa complex (5 pmol/L). To investigate the influence of the protein C system, soluble thrombomodulin (Tm) was also added to the reaction mixture. For the most extreme situations in which the essential plasma procoagulants (prothrombin, and factors X, IX, V, and VIII) and the stoichiometric anticoagulants (AT-III and TFPI) were collectively and inversely altered by 50%, a 28-fold difference in the total available thrombin generated was observed. Variations of most of these proteins 50% above and below the “normal” range, with the remainder at 100%, had only modest influences on the peak and total levels of thrombin generated. The dominant factors influencing thrombin generation were prothrombin and AT-III. When these 2 components were held at 100% and all other plasma procoagulants were reduced to 50%, there was a 60% reduction in the available thrombin generated. No increase in the thrombin generated was observed when the 150% level of all plasma procoagulants other than prothrombin was evaluated. When only prothrombin was raised to 150%, and all other factors were maintained at 100%, the thrombin generated increased by 71% to 121%. When AT-III was at 50% and all other constituents were at 100%, thrombin production was increased by 104% to 196%. The additions of protein C and protein S over the 50% to 150% ranges with Tm at 0.1 nmol/L concentration had limited influence on thrombin generation. Individual variations in factors VII, XI, and X concentrations had little effect on the duration of the initiation phase, the peak thrombin level achieved, or the available thrombin generated. Paradoxically, increases in factor IX concentration to 150% led to lowered thrombin generation, while decreases to 50% led to enhanced thrombin generation, most likely a consequence of factor IX as a competitive substrate with factor X for factor VIIa-TF. Reductions in factor V or factor VIII concentration led to prolongations of the initiation phase, while the reduction of TFPI to 50% led to shortening of this phase. However, none of these alterations led to significant changes in the available thrombin generated. Based on these data, one might surmise that increases in prothrombin and reductions in AT-III, within the normal range, would be potential risk factors for thrombosis and that algorithms that combine normal factor levels may be required to develop predictive tests for thrombosis.

Phenotype and genotype expression in pseudohomozygous factor VLEIDEN : the need for phenotype analysis

The presence of a DNA mutation is frequently used to define a disease or a risk state. Because DNA typing has become easy and convenient in contrast to protein characterization, it is generally assumed that a mutation if present (or not) at the DNA level will be also present (or not) in the corresponding protein. However, discrepancies between phenotype and genotype can occur. A point mutation in the coagulation factor V gene (G1691-->A, resulting in an Arg506-->Gln amino acid substitution in the factor V molecule [factor VLEIDEN], leading to activated protein C resistance) is the most common genetic risk factor for familial thrombophilia. A pseudohomozygous factor VLEIDEN phenotype would occur if a heterozygous individual for factor VLEIDEN also did not express the "normal" (non-Leiden) factor V allele. However, to date, no data have been available to confirm the presence of only the factor VLEIDEN form in the plasma of these individuals. Platelet mRNA from 2 presumed pseudohomozygous patients and their family members was isolated, the amplified partial cDNAs were sequenced or restricted, and the allelic bands were quantified. Both patients were found to be heterozygous for the G1691-->A substitution at both the DNA and mRNA levels. The presence of either the normal or mutated form of factor V in the patients' plasmas was investigated using a monoclonal antibody to factor V that recognizes an epitope located between residues 307 and 506 of the factor Va heavy chain. No normal factor V could be detected in the plasmas of the 2 propositi. The present data demonstrate absence of a correlation between genotype at position 1691 (at the DNA and mRNA levels) and the corresponding phenotype data found in the plasmas of patients with pseudohomozygous factor VLEIDEN. Overall, these data suggest the existence of heterogeneous genetic "lesions," which interfere with factor V expression, processing, secretion, and/or stability. Because the presence of the factor VLEIDEN molecule in plasma is directly related to pathology, identification and quantification of the circulating forms of factor V in plasma may be required for the diagnosis of individuals with activated protein C resistance.

Evaluation of the initiation phase of blood coagulation using ultrasensitive assays for serine proteases

The initiation phase of enzyme generation in a reconstituted model of the tissue factor (TF) pathway to thrombin was evaluated. At 1.25 pM added TF, no thrombin generation was observed in the absence of factor V. The substitution of factor Va for factor V increased the rate of thrombin generation. Factor X activation during the initiation phase was not influenced by the absence of factor VIII or thrombin, leading to the conclusion that initially factor Xa is generated exclusively by the factor VIIa-TF complex. When thrombin was eliminated from the system, no contribution of the factor IXa-factor VIIIa complex to factor X activation was observed during the propagation phase. Similarly, factor V activation was also not observed in the absence of thrombin, indicating that thrombin is the only enzyme responsible for factor V and factor VIII activation. Only subnanomolar amounts of factor VII were activated when prothrombin activation was almost complete. In the absence of coagulation inhibitors, factor XI did not influence thrombin generation initiated by 1.25 pM factor VIIa-TF complex. The termination of factor XIa generation by added hirudin in the factor XI experiment indicates that factor XI activation occurs exclusively by thrombin.