Protein arginine deiminase 4 inactivates tissue factor pathway inhibitor-alpha by enzymatic modification of functional arginine residues

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an important regulator of coagulation and a link between inflammation and thrombosis. During thrombotic events, TFPI is proteolytically inactivated by neutrophil elastase while bound to neutrophil extracellular traps (NETs). Protein arginine deiminase 4 (PAD4) catalyzes the conversion of arginine to citrulline and is crucial for NET formation.

OBJECTIVES

Here, we show that PAD4 inactivates full-length TFPIα by citrullination of its functional arginines.

METHODS

Citrullination of TFPIα and of TFPI-constructs by PAD4 was studied using western blotting and mass spectrometry. Binding of TFPIα to PAD4 was investigated using a solid-phase assay. Functional consequences were investigated by factor Xa inhibition and thrombin generation assays.

RESULTS

Nanomolar PAD4 amounts eliminated factor Xa inhibition by TFPIα. A citrullinated mutant Kunitz 2 domain did not inhibit factor Xa. Citrullination of TFPIα was found to be time- and concentration-dependent. Immunoprecipitation of citrullinated proteins from whole blood after neutrophil activation suggested the presence of TFPIα. Negatively charged phospholipids inhibited citrullination and truncated variants K1K2 and TFPI 1-161, and the isolated K2 domain were less efficiently citrullinated by PAD4. TFPIα bound to PAD4 with nanomolar affinity and involved the basic C-terminus. Thrombin generation in TFPI-deficient plasma demonstrated reduced anticoagulant activity of citrullinated TFPI. Mass spectrometry demonstrated citrullination of surface-exposed arginine residues in TFPIα after incubation with PAD4.

CONCLUSION

Full-length TFPIα is sensitive to citrullination by PAD4, which causes loss of factor Xa inhibition. This process may play a role in the increased thrombosis risk associated with inflammation.

Resistance to activated protein C and impaired TFPI activity in women with previous hormone-induced venous thromboembolism

INTRODUCTION

Hormonal contraception is a well-known risk factor for venous thromboembolism (VTE). APC resistance and impaired functions of protein S and TFPI are thought to play an important role in the pathogenesis of hormone-related VTE. It is unknown, whether women, who develop VTE during hormonal contraception possess a vulnerability in these pathways, making them susceptible to thrombosis.

MATERIALS AND METHODS

Plasma samples were obtained from 57 premenopausal women in average 15.3 years after hormone-associated VTE and from 31 healthy controls. Thrombin generation at high tissue factor (TF) in the absence and in the presence of activated protein C (APC) and at low TF without and with inhibiting anti-protein S- and anti-TFPI-antibodies was measured via calibrated automated thrombography.

RESULTS

Women with previous hormone-related thrombosis had higher thrombin generation at low TF, higher APC resistance, protein S- and TFPI ratios, differences: 219.9 nM IIa.min (95%CI:90.4 to 349.3); 1.88 (95%CI:0.71 to 3.05); 0.13 (95%CI:0.01 to 0.26) and 0.19 (95%CI:0.08 to 0.30), respectively. Thrombin generation at high TF without APC did not differ between the groups. Smoking decreased thrombin generation at low TF by -222.6 nM IIa.min (95%CI: -381.1 to -64.1), the APC sensitivity ratio by -2.20 (95%CI: -3.63 to -0.77) and the TFPI ratio by -0.16 (95%CI: -0.29 to -0.03), but did not influence thrombin generation at high TF.

DISCUSSION

We demonstrated impairment of the protein S/TFPI system and increased APC resistance in women with previous hormone-induced VTE. Smoking decreased thrombin generation at assay conditions, dependent on the function of the TFPI system.

Severe thrombophilia in a factor V-deficient patient homozygous for the Ala2086Asp mutation (FV Besançon)

BACKGROUND

Coagulation factor V (FV), present in plasma and platelets, has both pro- and anticoagulant functions.

OBJECTIVE

We investigated an FV-deficient patient (FV:C 3%, FV:Ag 4%) paradoxically presenting with recurrent venous thrombosis (11 events) instead of bleeding.

METHODS/RESULTS

Thrombophilia screening revealed only heterozygosity for the F2 20210G>A mutation. Although thrombin generation in the patient's platelet-poor plasma was suggestive of a hypocoagulable state, thrombin generation in the patient's platelet-rich plasma (PRP) was higher than in control PRP and extremely resistant to activated protein C (APC). This was partially attributable to the complete abolition of the APC-cofactor activity of FV and a marked reduction of plasma tissue factor pathway inhibitor antigen and activity. The patient was homozygous for a novel missense mutation (Ala2086Asp, FVBesançon ) that favors a "closed conformation" of the C2 domain, predicting impaired binding of FV(a) to phospholipids. Recombinant FVBesançon was hardly secreted, indicating that this mutation is responsible for the patient's FV deficiency. Model system experiments performed using highly diluted plasma as a source of FV showed that, compared with normal FVa, FVaBesançon has slightly (≤1.5-fold) unfavorable kinetic parameters (Km , Vmax ) of prothrombin activation, but also a lower rate of APC-catalyzed inactivation in the presence of protein S.

CONCLUSIONS

FVBesançon induces a hypercoagulable state via quantitative (markedly decreased FV level) and qualitative (phospholipid-binding defect) effects that affect anticoagulant pathways (anticoagulant activities of FV, FVa inactivation, tissue factor pathway inhibitor α level) more strongly than the prothrombinase activity of FVa. A possible specific role of platelet FV cannot be excluded.

Effect of blood loss during caesarean section on coagulation parameters

INTRODUCTION

Venous thrombosis is the leading cause of pregnancy-related maternal morbidity and mortality. The thrombosis risk is increased by caesarean section and blood loss, though underlying mechanisms of these prothrombotic changes remain unknown.

MATERIALS AND METHODS

This prospective study recruited 50 pregnant women at term undergoing elective caesarean section at University Hospital Magdeburg, Germany. Blood loss during surgery was correlated with the changes in total protein S, full length TFPI (TFPI_fl), prothrombin, the endogenous thrombin potential (ETP) and resistance to activated protein C (APCsr) determined via calibrated automated thrombography.

RESULTS

Mean blood loss was 506 ml (95%CI: 456 to 557 ml). Total protein S was 0.63 (95%CI: 0.60 to 0.67) U/ml preoperatively, decreased by 14.8% after caesarean section and almost normalised five days later. TFPI_fl was 0.47 (95%CI: 0.41 to 0.53) U/ml before, remained unchanged immediately after and increased by 11.5% five days after surgery. Prothormbin was 1.10 (95%CI: 1.03 to 1.16) U/ml preoperatively, reduced by 10.4% immediately after and increased again five days after caesarean section, exceeding the preoperative values by 4.4% (-0.7 to 9.6). The ETP decreased by 3.9%, whereas the APCsr increased by 37.0% immediately after caesarean section. The changes in total protein S, prothrombin, thrombin generation and APC resistance showed a trend to be more pronounced in the subgroups with higher blood loss.

DISCUSSION

Moderate blood loss during caesarean section hardly reduces thrombin generation but aggravates pregnancy-induced APC resistance and combined deficiency of TFPI and protein S, which can account for the increased thrombosis risk in early puerperium.

New functional assays to selectively quantify the activated protein C- and tissue factor pathway inhibitor-cofactor activities of protein S in plasma

Essentials Protein S is a cofactor of activated protein C (APC) and tissue factor pathway inhibitor (TFPI). There are no assays to quantify separate APC and TFPI cofactor activities of protein S in plasma. We developed assays to measure the APC- and TFPI-cofactor activities of protein S in plasma. The assays were sensitive to protein S deficiency, and not affected by the Factor V Leiden mutation.

SUMMARY

Background Protein S plays an important role in the down-regulation of coagulation as cofactor for activated protein C (APC) and tissue factor pathway inhibitor (TFPI). Aim To develop functional assays to quantify the APC- and TFPI-cofactor activities of protein S in plasma. Methods APC- and TFPI-cofactor activities of protein S in plasma were measured using calibrated automated thrombography in protein S-depleted plasma supplemented with a small amount of sample plasma either in the presence of anti-TFPI antibodies and APC (APC-cofactor activity) or at excess full-length TFPI without APC (TFPI-cofactor activity). Total and free protein S levels in plasma were measured by ELISAs. Results Average APC-cofactor activities of protein S were 113%, 108% and 89% in plasma from normal individuals (n = 15), FV Leiden heterozygotes (n = 14) and FV Leiden homozygotes (n = 7), respectively, whereas the average APC-cofactor activity of protein S in plasma from heterozygous protein S-deficient individuals (n = 21) was significantly lower (55%). Similar trends were observed for the TFPI-cofactor activity of protein S, with averages of 109%, 115% and 124% in plasma from individuals with normal protein S levels and different FV Leiden genotypes, and 64% in plasma from protein S-deficient patients. APC-cofactor activities of protein S correlated significantly with free and total protein S antigen levels, whereas TFPI-cofactor activities correlated less with protein S antigen levels. Conclusion We have developed functional protein S assays that measure both the APC- and TFPI-cofactor activities of protein S in plasma, which are hardly if at all affected by the FV Leiden mutation.

Tissue factor-independent inhibition of thrombin generation by tissue factor pathway inhibitor-α

BACKGROUND

Tissue factor pathway inhibitor-α (TFPIα) inhibits factor Xa by forming a binary TFPI-FXa complex in a reaction that is stimulated by protein S. TF-FVIIa forms a quaternary complex with TFPIα and FXa, which shuts off the initiation of coagulation via the extrinsic pathway.

AIM

To investigate whether direct inhibition of FXa by TFPIα independently of TF plays a role in downregulating coagulation.

METHODS

Inhibition of FXa by TFPIα in plasma was determined by measuring thrombin generation triggered with FXa, the FX activator from Russell's viper venom (RVV-X), FXIa, or FIXa. TF-independent anticoagulant activities of TFPIα and its cofactor, protein S, were quantified: (i) after neutralization of TFPIα and protein S with anti-TFPI or anti-protein S antibodies; and (ii) in TFPI-depleted or protein S-depleted plasmas supplemented with varying amounts of TFPIα or protein S.

RESULTS

Both anti-TFPI and anti-protein S antibodies enhanced thrombin generation in plasma triggered with RVV-X, FXa, FIXa, or FXIa. Anti-TFPI and anti-protein S antibodies decreased the lag time and increased the peak height of thrombin generation to the same extent, indicating that inhibition of FXa by TFPIα requires the presence of protein S. TFPIα and protein S titrations in TFPI-depleted or protein S-depleted plasma in which thrombin formation was initiated with triggers other than TF also revealed TF-independent anticoagulant activity of TFPIα, which was completely dependent on the presence of protein S.

CONCLUSION

Direct inhibition of FXa by TFPIα contributes to the downregulation of coagulation.

Small peptides blocking inhibition of factor Xa and tissue factor-factor VIIa by tissue factor pathway inhibitor (TFPI)

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor that inhibits activated factor X (FXa) via a slow-tight binding mechanism and tissue factor-activated FVII (TF-FVIIa) via formation of a quaternary FXa-TFPI-TF-FVIIa complex. Inhibition of TFPI enhances coagulation in hemophilia models. Using a library approach, we selected and subsequently optimized peptides that bind TFPI and block its anticoagulant activity. One peptide (termed compound 3), bound with high affinity to the Kunitz-1 (K1) domain of TFPI (Kd ∼1 nM). We solved the crystal structure of this peptide in complex with the K1 of TFPI at 2.55-Å resolution. The structure of compound 3 can be segmented into a N-terminal anchor; an Ω-shaped loop; an intermediate segment; a tight glycine-loop; and a C-terminal α-helix that is anchored to K1 at its reactive center loop and two-stranded β-sheet. The contact surface has an overall hydrophobic character with some charged hot spots. In a model system, compound 3 blocked FXa inhibition by TFPI (EC50 = 11 nM) and inhibition of TF-FVIIa-catalyzed FX activation by TFPI (EC50 = 2 nM). The peptide prevented transition from the loose to the tight FXa-TFPI complex, but did not affect formation of the loose FXa-TFPI complex. The K1 domain of TFPI binds and inhibits FVIIa and the K2 domain similarly inhibits FXa. Because compound 3 binds to K1, our data show that K1 is not only important for FVIIa inhibition but also for FXa inhibition, i.e. for the transition of the loose to the tight FXa-TFPI complex. This mode of action translates into normalization of coagulation of hemophilia plasmas. Compound 3 thus bears potential to prevent bleeding in hemophilia patients.

Role of protein S and tissue factor pathway inhibitor in the development of activated protein C resistance early in pregnancy in women with a history of preeclampsia

Pregnancy increases the risk of venous thromboembolism. Particularly in early pregnancy, the thrombosis risk can be attributed to the changes in coagulation. Elevated thrombin generation and resistance to activated protein C (APC) are likely to contribute to the increased thrombosis risk during pregnancy. We studied changes and the determinants of thrombin generation and APC resistance in the first 16 weeks of gestation in women with history of preeclampsia. Additionally, we investigated the influence of pregnancy-induced haemodilution on the coagulation system. We measured thrombin generation, APC resistance and plasma levels of prothrombin, factor V, factor X, protein S and tissue factor pathway inhibitor (TFPI) in 30 non-pregnant and 21 pregnant women at 8, 12 and 16 weeks of gestation. All participants shared a history of a hypertensive complication in the preceding pregnancy. Thrombin generation and APC resistance were higher at eight weeks of pregnancy than in the non-pregnant state, and progressively increased between eight and 16 weeks of gestation. Changes in the TFPI and protein S levels accounted for ~70% of pregnancy-induced APC resistance. Interestingly, a significant correlation (slope 2.23; 95%CI: 1.56 to 2.91; r= 0.58) was observed between protein Stotal or protein Sfree levels and haematocrit. In conclusion, pregnancy induces a decrease of TFPIfree and protein Sfree levels that attenuates the function of the TFPI and protein C systems and results in elevated thrombin generation and increased APC resistance. Besides, our data suggest that pregnancy-dependent haemodilution may contribute to the decreased peripheral protein S levels.

Pregnancy-associated changes in the hemostatic system in wild-type and factor V Leiden mice

BACKGROUND

Pregnancy, oral contraceptive (OC)use and hormone replacement therapy (HRT) are established risk factors for venous thrombosis. Acquired resistance to activated protein C (APC) has been proposed to contribute to the increased thrombosis risk. Mouse models are often used for preclinical testing of newly developed hormone preparations. However, it is not known whether hormone-induced APC resistance is also observed in laboratory animals.

OBJECTIVES

To investigate whether hormonal changes modulate APC resistance in mice, we used pregnant mice as a model of hormone-induced APC resistance. The effect of pregnancy on APC resistance was studied in wild-type and factor (F)V Leiden mice.

METHODS

APC resistance was determined in mouse plasma using a thrombin generation-based APC resistance test. APC resistance determinants,i.e. prothrombin, FV, FX, antithrombin and protein S levels,and of tissue factor pathway inhibitor (TFPI) activity were evaluated in plasma from non-pregnant and pregnant mice.

RESULTS

In contrast to humans, pregnancy induced a decrease in APC resistance in wild-type and in FV Leiden mice.Pregnant mice had higher levels of prothrombin, FV, FX,protein S and TFPI activity as compared with non-pregnant mice.

CONCLUSIONS

Pregnancy causes a decrease in APC resistance in mice, which can be explained by the elevation of protein S levels and increased TFPI activity in plasma. Our findings show species specificity in the effects of pregnancy on the major determinants of the protein C system and suggest that protein S and TFPI play an important role in the development of pregnancy-induced APC resistance in humans.

Effect of oral contraceptives on thrombin generation measured via calibrated automated thrombography

In a study population consisting of healthy men (n = 8), women not using oral contraceptives (OC) (n = 28) and women using different kinds of OC (n = 187) we used calibrated automated thrombography (CAT) in the absence and presence of added activated protein C (APC) to compare parameters that can be obtained from thrombin generation curves, i.e. lag time, time to peak, peak height and endogenous thrombin potential (ETP). Both with and without APC, plasmas of OC users exhibited the shortest lag time and time to peak, and the highest peak height and ETP. In the absence of APC none of these parameters differed between users of OC containing different progestogens. In contrast, in the presence of APC shorter lag times and time to peak, and higher peak height and ETP were observed in plasma of users of gestodene-, desogestrel-, drospirenone- and cyproterone acetate-containing OC than in plasma of users of levonorgestrel- containing OC. The ETP determined in the absence of APC (ETP(-APC)) had no predictive value for the APCsr (r = 0.11; slope 0.9 x 10(-3); 95% CI: -0.1 x 10(-3) to 2.0 x 10(-3)) whereas the ETP measured in the presence of APC (ETP+APC) showed an excellent correlation with the APCsr (r = 0.95; slope 6.6 x 10(-3); 95% CI: 6.3 x 10(-3) to 6.9 x 10(-3)) indicating that the APCsr is entirely determined by the ETP+APC. In conclusion, OC use increases thrombin generation, but differential effects of second and third generation OCs on the protein C system likely determine the differences in the risk of venous thrombosis between these kinds of OC.

Differential effects of high prothrombin levels on thrombin generation depending on the cause of the hyperprothrombinemia

BACKGROUND

Hyperprothrombinemia, resulting from the prothrombin G20210A mutation or other causes, is associated with activated protein C (APC) resistance and increased thrombosis risk. When high prothrombin levels are a result of increased hepatic biosynthesis, these effects may be counteracted by concomitantly increased levels of the anticoagulant factors (particularly protein S). Differently, in prothrombin G20210A carriers only prothrombin levels are elevated.

OBJECTIVE

To investigate whether prothrombin G20210A carriers have a more severe hypercoagulable state than non-carriers with comparable prothrombin levels.

PATIENTS/METHODS

Coagulation factor levels, thrombin generation (Calibrated Automated Thrombogram in the presence and absence of APC) and APC resistance were measured in normal (n = 132), heterozygous (n = 167) and homozygous (n = 3) individuals.

RESULTS

Prothrombin levels, thrombin generation and APC resistance were higher in carriers of the prothrombin G20210A mutation (especially those who had experienced venous thrombosis) than in non-carriers, whereas protein S and antithrombin levels were similar among genotype groups. Because individuals with high prothrombin levels in the absence of the prothrombin G20210A mutation tend to have all liver-synthesized factors elevated, carriers of the mutation had lower protein S and antithrombin levels than non-carriers with equally high prothrombin levels. Accordingly, they also generated more thrombin and showed a tendency toward higher APC resistance. Analogous effects, but less pronounced, were observed in homozygotes for the prothrombin A19911G polymorphism, which also upregulates prothrombin levels.

CONCLUSIONS

Individuals with hyperprothrombinemia as a result of prothrombin gene mutations generate more thrombin and tend to be more APC-resistant than individuals with comparable prothrombin levels because of other causes.

Less Effect of Intranasal Than Oral Hormone Therapy on Factors Associated With Venous Thrombosis Risk in Healthy Postmenopausal Women

Objective— To compare the effects of intranasal and oral administration of 17β-estradiol (E 2 ) and norethisterone(acetate) [NET(A)] in healthy postmenopausal women on activated protein C (APC) resistance and other hemostatic parameters associated with venous thrombosis.

Methods and Results— In this 2-center, randomized, double-blind, 1-year trial, 90 postmenopausal women (56.6±4.7 years of age) received daily either an intranasal spray with 175 μg/275 μg E 2 /NET (n=47) or 1 mg/0.5 mg oral E 2 /NETA (n=43). Normalized APC sensitivity ratios (nAPCsr) were determined with a thrombin generation-based APC resistance test. After 1 year, the increase in nAPCsr was smaller in the intranasal than in the oral group: 11% (95% CI, 1% to 22%) versus 53% (95% CI, 37% to 72%). Overall, the decrease in antithrombin and increase in prothrombin fragment 1+2 (F1+2) were smaller and the decrease in free protein S larger in the intranasal compared with the oral group after 1 year. In both groups, the decreases in protein C and prothrombin, and the increase in d -dimer were similar.

Conclusion— Compared with oral E 2 /NETA therapy, intranasal administration of E 2 /NET had less effect on APC resistance and on a number of other parameters associated with venous thrombosis. This observation suggests the possibility of a lower venous thrombosis risk for intranasal E 2 /NET compared with oral therapy.

Effect of raloxifene on activated protein C (APC) resistance in postmenopausal women and on APC resistance and homocysteine levels in elderly men

Raloxifene, a selective estrogen receptor modulator, like hormonal replacement therapy increases the risk of venous thromboembolism in postmenopausal women. A possible explanation for the increased thrombotic risk could be an increase in acquired resistance to activated protein C (APC). In two randomized, placebo-controlled, double-blind studies we determined the effect of raloxifene on the normalized APC sensitivity ratios (nAPCsr). The nAPCsr were determined with the thrombin generation-based APC resistance test. In the first study 83 postmenopausal women (age, 51.1 +/- 2.7 years) randomly received daily 0.625 mg conjugated equine estrogen and 2.5 mg medroxyprogesterone acetate (n=17), 60 mg raloxifene (n=23), 150 mg raloxifene (n=20) or placebo (n=23) for 24 months. At baseline and after 6, 12 and 24 months the nAPCsr were measured. In the second study 30 elderly men (age, 64.4 +/- 2.4 years) randomly received 120 mg raloxifene (n=15) or placebo (n=15) for 3 months. At baseline and after 3 months the nAPCsr and fasting homocysteine levels were measured. In postmenopausal women conjugated equine estrogen/medroxyprogesterone acetate significantly increased the nAPCsr from 1.26 +/- 0.82 to 2.87 +/- 0.86 at 24 months (P <0.0005 compared with placebo). Raloxifene had no significant effect on nAPCsr compared with placebo in both women and men. The results did not change after excluding carriers of factor V Leiden. Also fasting homocysteine levels were not affected by raloxifene in the aging men. It is concluded that raloxifene, in contrast to combined hormonal replacement therapy, does not increase APC resistance.

Venous thrombosis and changes of hemostatic variables during cross-sex hormone treatment in transsexual people

The incidence of venous thrombosis associated with estrogen treatment in male-to-female (M-->F) transsexuals is considerably higher with administration of oral ethinyl estradiol (EE) than with transdermal (td) 17-beta-estradiol (E(2)). To find an explanation for the different thrombotic risks of oral EE and td E(2) use, we compared the effects of treatment of M-->F transsexuals with cyproterone acetate (CPA) only, and with CPA in combination with td E(2), oral EE, or oral E(2) on a number of hemostatic variables [activated protein C (APC) resistance and plasma levels of protein S, protein C, and prothombin], all of which are documented risk factors for venous thrombosis. APC resistance was determined by quantification of the effect of APC on the amount of thrombin generated during tissue factor-initiated coagulation; plasma levels of total and free protein S were determined by standard ELISA; and levels of prothrombin and protein C were determined with functional assays after complete activation of the zymogens with specific snake venom proteases. CPA-only, td-E(2)+CPA, or oral-E(2)+CPA treatment produced rather small effects on hemostatic variables, whereas oral EE treatment resulted in a large increase in APC resistance from 1.2 +/- 0.8 to 4.1 +/- 1 (P < 0.001), a moderate increase in plasma protein C (9%; P = 0.012), and a large decrease in both total and free plasma protein S (30%; P < 0.005). The large differential effect of oral EE and oral E(2) indicates that the prothrombotic effect of EE is due to its molecular structure rather than to a first-pass liver effect (which they share). Moreover, these differences may explain why M-->F transsexuals treated with oral EE are exposed to a higher thrombotic risk than transsexuals treated with td E(2). Testosterone administration to female-to-male transsexuals had an antithrombotic effect.

Endogenous factor V synthesis in megakaryocytes contributes negligibly to the platelet factor V pool

BACKGROUND AND OBJECTIVES

Coagulation factor V (FV) is distributed between two pools: 80% circulates in plasma and 20% is stored in platelets. The aim of the study was to determine the origin of platelet FV.

DESIGN AND METHODS

We investigated a FV Leiden heterozygous patient who had received an allogeneic bone marrow transplant from a normal donor. The patient had been referred to our laboratory for his marked activated protein C (APC) resistance in the apparent absence of FV Leiden. Analysis of the DNA from a buccal swab showed that the patient was indeed a heterozygous carrier of FV Leiden. The difference in FV genotype between the hepatocytes (heterozygous FV Leiden) and the blood cells (homozygous normal) of the patient provided a good model to investigate the origin of platelet FV. Platelets were isolated from the patient and the bone marrow donor and activated with thrombin and ionomycin to release and activate FV. APC was then added and the inactivation of platelet FVa was followed over time with a highly sensitive prothrombinase-based assay.

RESULTS

While the donor's platelet FVa showed a normal inactivation time course, the patient's platelet FVa was considerably resistant to APC. The kinetic pattern of APC-catalyzed inactivation of the patient's platelet FVa was indistinguishable from that of plasma FVa from a FV Leiden heterozygote.

INTERPRETATION AND CONCLUSIONS

These data indicate that platelet FV is derived from plasma and that endogenous FV synthesis by megakaryocytes contributes negligibly to the platelet FV pool.

Activated protein C resistance determined with a thrombin generation-based test predicts for venous thrombosis in men and women

Activated protein C (APC) resistance, determined with a thrombin-generation-based APC resistance test, may explain risk differences of venous thrombosis in users of second- and third-generation oral contraceptives (OC). To clinically validate this test, we analysed the Leiden thrombophilia case-control study (474 patients with a first episode of deep vein thrombosis and 474 age- and sex-matched control subjects). Data for men and women were analysed separately. As hormonal status in women is known to strongly influence the APC sensitivity ratio (APCsr), additional strata (OC use and menopausal state) were defined. The APCsr was higher in all patients than in control subjects. Odds ratios (OR), using the 90th percentile of all control subjects (APCsr > 4.5) as cut-off, were: 7.5 [95% confidence interval (CI) 1.6-33.8] for men, 3.0 (95% CI 1.0-8.8) for premenopausal women not using OC, 4.8 (95% CI 1.6-14.7) for premenopausal women using OC and 4.7 (95% CI 1.4-15.6) for postmenopausal women. After excluding the carriers of factor V Leiden, the OR became infinite for men (no control had an APCsr > 4.5), 1.4 (95% CI 0.2-8.2) for premenopausal women not using OC, 3.4 (95% CI 1.1-10.8) for premenopausal women using OC and 3.6 (95% CI 0.6-20.5) for postmenopausal women. A high APCsr, determined with the thrombin-generation-based APC resistance test, predicts venous thrombotic risk, in populations with and without factor V Leiden. In addition, acquired APC resistance resulting from OC use predicts an increased risk for venous thrombosis independent of factor V Leiden.

Effects of hereditary and acquired risk factors of venous thrombosis on a thrombin generation-based APC resistance test

BACKGROUND

Several hereditary and acquired risk factors for venous thromboembolism (VTE) are associated with impaired down-regulation of thrombin formation via the protein C pathway. To identify individuals at risk, functional tests are needed that estimate the risk to develop venous thrombosis.

METHOD

We determined the effects of hereditary and acquired risk factors of venous thrombosis on an APC resistance test that quantifies the influence of APC on the time integral of thrombin formation (the endogenous thrombin potential, ETP) initiated in plasma via the extrinsic coagulation pathway. APC sensitivity ratios (APCsr) were determined in plasma from carriers of factor V(Leiden) (n = 56) or prothrombin G20210A (n = 18), of individuals deficient in antithrombin (n = 9), protein C (n = 7) or protein S (n = 14) and of women exposed to acquired risk factors such as hormone replacement therapy (n = 49), oral contraceptive use (n = 126) or pregnancy (n = 35). We also analysed combinations of risk factors (n = 60).

RESULTS

The thrombin generation-based APC resistance test was sensitive for the factor V(Leiden) and prothrombin G20210A mutation, to protein S deficiency, hormone replacement therapy, oral contraceptive use and pregnancy. The assay was not influenced by antithrombin or protein C deficiency. The presence of more than one risk factor of venous thrombosis resulted in more pronounced APC resistance. The APCsr of individuals with a single or combined risk factors of VTE correlated well with reported risk increases.

INTERPRETATION

The thrombin generation-based APC resistance test identifies individuals at risk for venous thrombosis due to acquired risk factors and/or hereditary thrombophilic disorders that affect the protein C pathway.