Factor V Leiden: association with venous thromboembolism in pregnancy and screening issues

Disturbances of the natural balance between procoagulant and anticoagulant mechanisms can result in bleeding or thrombotic tendencies. Factor V, on activation by thrombin to factor Va, forms an essential component of the prothrombinase complex, in which it demonstrates its cofactor activity for factor Xa. Down-regulation of factor Va by activated protein C (APC) occurs through cleavage of specific peptide bonds in the heavy chain of the molecule. Factor V Leiden (FV Leiden) is a mutation of factor V that renders factor Va resistant to APC, due to loss of one of these cleavage sites. This mutation predisposes the patient to thrombosis. Prevalence of FV Leiden varies; however, heterozygosity for the FV Leiden mutation is recognised as the most common heritable thrombophilic defect in Caucasian populations. The association this inherited thrombophilia has with venous thromboembolism (VTE) is well established. Pregnancy is notably an acquired hypercoagulable state, due in part to physiological changes that occur in the coagulation system. This seems to have potential for interaction with FV Leiden to cause adverse experiences. A role has been suggested for FV Leiden in VTE events during pregnancy. At present only selected women are screened for FV Leiden. Pregnant women with a history of VTE or with a family history of the mutation are investigated. Whether or not the introduction of a routine screening plan for this mutation is justified remains a matter for debate.

Influence of Factor V Leiden on susceptibility to and outcome from critical illness: a genetic association study

INTRODUCTION

Disturbance of the pro-coagulatant and anti-coagulant balance is associated with a poor outcome from critical illness. The objective of this study is to determine whether the Factor V Leiden (FVL) mutation is associated with susceptibility to or death from critical illness.

METHODS

A genetic association study involving four case cohorts comprising two Gram negative sepsis, one invasive pneumococcal disease and one intensive care unit cohort with a total of 1,249 patients. Controls were derived from a population-based cohort study (N = 8,147). DNA from patients and controls was genotyped for the FVL mutation.

RESULTS

When all patients were investigated together no significant difference in the frequency of FVL mutation was observed compared with controls (odds ratio (OR), 1.03; 95% confidence interval (CI), 0.83 to 1.29). However, when stratified among patients admitted to intensive care (N = 237), susceptibility and the likelihood of long-term death was influenced by the FVL mutation. In adjusted logistic regression analysis, FVL carriers had an increased risk of ICU admission compared to non-carriers (OR 1.62; 95% CI, 1.08 to 2.42). In adjusted Cox regression analysis, FVL carriers were at increased risk of long-term death compared to non-carriers (relative risk 1.78; 95% CI, 1.13 to 2.81). FVL carrier status did not predict either susceptibility to or outcome from Gram negative, Escherichia coli or Streptococcus pneumoniae sepsis.

CONCLUSIONS

Overall, the FVL mutation did not appear to increase the risk of admission due to severe invasive infections. Nevertheless, in the subgroup of patients admitted to intensive care an increased risk and a poorer long-term outcome for individuals with critical illness were observed for FVL mutation carriers.

Hirudin treatment for multiple thromboses in a preterm infant with inherited thrombophilia

Thromboembolic events in preterm infants constitute a serious problem in neonatal intensive care. In most cases, treatment with low-molecular-weight heparin offers a sufficient therapy of thrombotic events. We report the case of a severely sick male preterm infant with a heterozygous factor V Leiden mutation and protein C deficiency. The infant developed multiple thromboses despite adequate anticoagulation with enoxaparin and was in a life-threatening situation. Treatment with hirudin prevented the occurrence of new thromboses without causing bleeding complications. After 2 weeks hirudin was discontinued and low-molecular-weight heparin therapy was started again. A successive recanalization of the vast majority of affected vessels was observed within the following 6 months. Despite some minor neurologic sequelae and a slight delay in neuro-motor development, the 2.5-year-old boy is in a healthy condition. This case demonstrates that hirudin can be an effective alternative anticoagulant in neonates and infants refractory to heparin treatment. Efficacy and safety issues of hirudin treatment, however, need to be evaluated in randomized trials.

Effect of phosphatidylcholine, phosphatidylethanolamine and lysophosphatidylcholine on the activated factor X-prothrombin system

Membrane phospholipids are essential in blood coagulation reactions. The importance of negatively changed phosphatidylserine has been shown. The roles of other phospholipids in the blood coagulation system, however, are not clear. This study examined the effects of phosphatidylcholine on the blood coagulation system using liposomes containing varying concentrations of phosphatidylcholine in the presence of phosphatidylserine at a constant concentration. In addition, with phosphatidylserine and phosphatidylcholine at constant concentrations, the effects of phosphatidylethanolamine and lysophosphatidylcholine on the blood coagulation system were examined. Using an in vitro reconstructed system of the activated factor X-prothrombin system, blood coagulation was measured by the rate of thrombin formation after the addition of liposome preparations. The results showed suppression of the system by phosphatidylcholine and phosphatidylethanolamine and acceleration by lysophosphatidylcholine. The results of the present study suggest that the cell membrane, the 'location' of blood coagulation, is one of the regulatory factors, and that changes in phosphatidylcholine content and phospholipid composition of the cell membrane regulate the coagulation reaction.

Effect of phosphatidylcholine, phosphatidylethanolamine and lysophosphatidylcholine on the protein C/protein S anticoagulation system

Phosphatidylserine is known to significantly accelerate the blood coagulation reaction. In a previous communication submitted for publication, we demonstrated that phosphatidylcholine, phosphatidylethanolamine and lysophosphatidylcholine showed effects on the blood coagulation reaction using the factor Xa-prothrombin reaction system, and discuss a new function of membrane phospholipids. The present study examined the role of phospholipids in the blood coagulation regulatory reaction (anticoagulation system), by studying the effects of phospholipids on the protein C/protein S reaction. We have established quantitative methods for measuring activated protein C activity and protein S activity, and used them to measure their activity after the addition of liposomes with different phospholipid compositions. We found that phosphatidylcholine inhibited activated protein C and protein S activities in a dose-dependent manner, as in the factor Xa-prothrombin reaction system. On the other hand, phosphatidylethanolamine and lysophosphatidylcholine showed no effect on activated protein C activity. Phosphatidylethanolamine inhibited and lysophosphatidylcholine accelerated coagulation activity in the factor Xa-prothrombin system, but such effects were not observed in the protein C/protein S reaction system. The coagulation and anticoagulation reactions are exquisitely balanced by thrombin, with a role both as a procoagulant and anticoagulant. Therefore, it is understandable that phosphatidylethanolamine and lysophosphatidylcholine show different effects in the factor Xa-prothrombin and protein C/protein S reaction systems. It appears that coagulation and anticoagulation reactions are co-ordinated and controlled by changes in phospholipid composition of the cellular membrane where the coagulation reaction takes place.

Effects of VLDL and Remnant Particles on Platelets

There is a considerable body of evidence supporting an association between hypertriglyceridaemia, a hypercoagulable state and atherothrombosis. A disorder of triglyceride metabolism is a key feature of the metabolic syndrome that increases risk of both ischaemic heart disease and type 2 diabetes approximately 3-fold. An increasing prevalence of obesity and metabolic syndrome is likely to contribute markedly to the prevalent ischaemic heart in the foreseeable future, and therefore it is crucial to understand mechanisms linking hypertriglyceridaemia and a hypercoagulable state. Activation of platelets and the coagulation cascade are intertwined. VLDL and remnant lipoprotein concentrations are often increased with the metabolic syndrome. These lipoproteins have the capacity to activate platelets and the coagulation pathway, and to support the assembly of the prothrombinase complex. VLDL also upregulates expression of the plasminogen activator inhibitor-1 gene and plasminogen activator inhibitor-1 antigen and activity, a process accompanied by platelet aggregation and clot formation. The surface membrane of activated platelets also supports the assembly and activity of the prothrombinase complex, resulting in further thrombin generation and amplification of the coagulation cascade. Fibrinolysis is also less efficient when thrombin is generated. Thrombin induces thrombin activatable fibrinolysis inhibitor. Thrombin activatable fibrinolysis inhibitor is a carboxypeptidase that cleaves the carboxylic lysine residues on fibrin, thereby abolishing the critical binding site for tPA-plasminogen decreasing plasmin formation. Thus the evidence is supportive of dysregulated coagulation, and impaired fibrinolysis with a predisposition to atherothrombosis, in conditions such as the metabolic syndrome, in which there are increased concentrations of VLDL and remnant lipoproteins. The purpose of this review is to describe the current evidence supporting a procoagulant state induced by VLDL and remnant lipoproteins. The role of these lipoprotein classes in (1) platelet activation; (2) the intrinsic coagulation cascade, and (3) clot formation and fibrinolysis is discussed.

The clot thickens

Vascular sclerosis has been linked to many risk factors, including smoking, family history, low birth weight, and hypertension. In interesting studies, Goforth et al. show an increased rate of mutations in thrombophilic molecules in patients with vascular sclerosis in renal biopsies, suggesting yet another mechanism.

Factor V Leiden thrombophilia: presentation of three patients and a literature review

Hypercoagulable states represent a condition with multiple etiologies in which an interplay of acquired and congenital coagulation defects contribute to abnormal clotting. Several of the thrombophilic disorders are relatively prevalent; one person may have multiple defects, leading to thrombosis without obvious external stimuli. Factor V Leiden thrombophilia is the most common inherited form of thrombophilia and it is occasionally associated with the anomalous prothrombin G20210A mutations. We presents three puertorrican middle-aged females diagnosed with Factor V Leiden after debuting with abnormal clotting events. They shared the common traits of being female, having been born in Puerto Rico, and having parents of European descent. The first two cases presented with deep venous thrombosis of lower and upper extremities, and both had the association of Factor V Leiden and prothrombin G20210A mutation. The third case presented with a cerebrovascular accident, evidencing arterial thrombosis, after receiving estrogen replacement therapy. This report adds Factor V Leiden as a cause of hypercoagulable states in puertorrican patients.

Inherited defects of coagulation factor V: the hemorrhagic side

Coagulation factor V (FV) is the protein cofactor required in vivo for the rapid generation of thrombin catalyzed by the prothrombinase complex. It also represents a central regulator in the early phases of blood clot formation, as it contributes to the anticoagulant pathway by participating in the downregulation of factor VIII activity. Conversion of precursor FV to either a procoagulant or anticoagulant cofactor depends on the local concentration of procoagulant and anticoagulant enzymes, so that FV may be regarded as a daring tight-rope walker gently balancing opposite forces. Given this dual role, genetic defects in the FV gene may result in opposite phenotypes (hemorrhagic or thrombotic). Besides a concise description on the structural, procoagulant and anticoagulant properties of FV, this review will focus on bleeding disorders associated with altered levels of this molecule. Particular attention will be paid to the mutational spectrum of type I FV deficiency, which is characterized by a remarkable genetic heterogeneity and by an uneven distribution of mutations throughout the FV gene.

Contribution of the -455G/A polymorphism at beta-fibrinogen gene and of the Leiden mutation to hemorheological parameters in ischemic stroke patients

The concentration of plasma fibrinogen (FIB) is an important factor in the coagulation cascade and also in the determination of blood and plasma viscosity depending on both genetic and acquired factors. The -455G/A polymorphism of the beta-FIB gene is connected to the plasma concentration of FIB but the effect of Leiden mutation on hemorheological parameters is unclear. The two genetic polymorphisms were studied by polymerase chain reaction in healthy subjects and ischemic stroke cohort and the effects on the concentration of plasma FIB, whole blood and plasma viscosity of patients as well. A total of 278 ischemic stroke patients and 173 control subjects were enrolled. Marcro-rheological parameters as plasma FIB concentration, whole blood viscosity (90 sec(-1) shear rate) and plasma viscosity have been measured also in the subgroup of young (age < 50 years) and in a subgroup of non-smoker patients.

RESULTS

No significant difference was found in the prevalency of H1/H2 genotype between controls and cases in pooled stroke group OR 0.95 (95% CI: 0.47-1.27), however H2/H2 genotype frequency was increased in young subgroup of patients (OR: 1.66 95% CI: 0.52-5.25). Plasma FIB concentration was increased both in the total cohort (p < 0.05) and in the non-smoker subgroup (p < 0.03) of patients carried H2/H2 as compared to H1/H1 genotype and the prevalence was increased in the group of patients having plasma FIB concentration > 4 g/l (p < 0.05). The whole blood viscosity was elevated in the H2/H2 group as compared to the group carrying wild type (p < 0.03). A tendency of increased plasma viscosity in the group of patients with H2/H2 genotype as compared to wild type was found (p = 0.07). Leiden mutation prevalence showed an increased risk OR: 1.67 (95% CI: 0.75-3.70) in the young patients group as compared to controls. In patients who have had the highest plasma viscosity, higher frequency of Leiden mutation was detected as compared to wild type, in total group (p = 0.01), in young patients (p = 0.03) and in subgroup of non-smoker patients (p = 0.05).

CONCLUSIONS

Our findings support the notion that the homozigous variant of beta-FIB gene can raise both plasma FIB concentration and whole blood viscosity. Leiden mutation connected to the elevation of plasma viscosity could demonstrate a new pathway of increased thrombophylic potential in ischemic stroke patients.

Thrombogenic risk factors for atherothrombosis

Thrombosis superimposed on a disrupted plaque is the proximate event that triggers most acute ischemic syndromes and episodes of sudden cardiac death. A significant number of acute ischemic events occur in individuals without traditional atherosclerosis-related risk factors. In an attempt to pinpoint additional risk factors, researchers are examining the thrombotic cascade and the cellular components, plasma proteins, and endothelium-derived mediators, as well as their genetic polymorphisms, that may affect this system. This article enumerates a number of potential hemostatic risk factors and discusses the evidence linking them to atherothrombotic events.

Expression of the normal factor V allele modulates the APC resistance phenotype in heterozygous carriers of the factor V Leiden mutation

BACKGROUND

Functional defects of the protein C pathway, detectable in plasma as activated protein C (APC) resistance, are a prevalent risk factor for venous thrombosis. The factor V (FV) Leiden mutation causes APC resistance by interfering with the APC-mediated inactivation of both FVa and FVIIIa. Co-inheritance of FV Leiden and quantitative FV deficiency on different alleles, a rare condition known as pseudo-homozygous APC resistance, is associated with pronounced APC resistance and 50% reduced FV levels, because of non-expression of the non-Leiden FV allele.

OBJECTIVES

The role of normal FV in modulating the APC resistance phenotype in carriers of FV Leiden was investigated in patients with pseudo-homozygous APC resistance and in model systems.

PATIENTS/METHODS

Four functional plasma assays probing both components of APC resistance (susceptibility of FVa to APC and cofactor activity of FV in FVIIIa inactivation) were employed to compare seven clinically and genetically characterized FV Leiden pseudo-homozygotes to 30 relatives with different FV genotypes (including 12 FV Leiden heterozygotes and seven carriers of FV deficiency) and to 32 unrelated FV Leiden homozygotes.

RESULTS AND CONCLUSIONS

All assays consistently indicated that FV Leiden pseudo-homozygotes are significantly more APC-resistant than heterozygotes and indistinguishable from homozygotes. Thrombin generation measurements in FV-deficient plasma reconstituted with purified normal FV and FV Leiden confirmed these observations and showed that the expression of the normal FV allele is an important modulator of APC resistance in FV Leiden heterozygotes. These findings provide an explanation for the higher thrombotic risk of FV Leiden pseudo-homozygotes when compared with heterozygotes.

Factor V Leiden improves in vivo hemostasis in murine hemophilia models

The role of factor V Leiden (FVL) as a modifier of the severe hemophilia phenotype is still unclear. We used mice with hemophilia A or B crossed with FVL to elucidate in vivo parameters of hemostasis. Real-time thrombus formation in the microcirculation was monitored by deposition of labeled platelets upon laser-induced endothelial injury using widefield microscopy in living animals. No thrombi formed in hemophilic A or B mice following vascular injuries. However, hemophilic mice, either heterozygous or homozygous for FVL, formed clots at all injured sites. Injection of purified activated FV into hemophilic A or B mice could mimic the in vivo effect of FVL. In contrast to these responses to a laser injury in a microvascular bed, FVL did not provide sustained hemostasis following damage of large vessels in a ferric chloride carotid artery injury model, despite of the improvement of clotting times and high circulating thrombin levels. Together these data provide evidence that FVL has the ability to improve the hemophilia A or B phenotype, but this effect is principally evident at the microcirculation level following a particular vascular injury. Our observations may partly explain the heterogeneous clinical evidence of the beneficial role of FVL in hemophilia.

The role of factor V Leiden in maternal health and the outcome of pregnancy

There is growing evidence that women with thrombophilia are at increased risk of pregnancy related venous thromboembolism and of adverse pregnancy outcome including pregnancy loss, pre-eclampsia, intrauterine growth retardation and placental abruption. The factor V Leiden mutation is a heritable thrombophilia present in 5-8% of Caucasian populations. In its heterozygous form it is associated with a 4-to 8-fold increase in thrombotic risk. Homozygous inheritance, however, confers around an 80-fold increase in relative risk of thrombosis. The relationship between factor V Leiden and adverse pregnancy outcome has been studied in the recent literature, however the size of the estimated risks varies between individual studies due to heterogeneity of study design and small sample size in many cases. The management of women with factor V Leiden in pregnancy with low molecular weight heparin has been shown to be both safe and effective in preventing venous thromboembolism and improving pregnancy loss. Large scale, randomised controlled studies are required to confirm these findings. Selective screening for factor V Leiden based on prior venous thromboembolism has been shown to be marginally more cost-effective than universal screening in pregnancy and a recent consensus statement has recommended screening for thrombophilia based on a strong personal or family history of venous thromboembolism. There is now some evidence that placental problems may be associated with factor V Leiden in the fetus. There has also been an observed association between maternal factor V Leiden and fetal or neonatal stroke. These areas require further study and at present there is no evidence-based approach to investigation, prevention or management.

Matched Case-Control Study on Factor V Leiden and the Prothrombin G20210A Mutation in Patients With Ischemic Stroke/Transient Ischemic Attack Up to the Age of 60 Years

Background and Purpose— The role of the factor V Leiden mutation ( FVL ) and the G20210A mutation of the prothrombin ( factor II [FII] ) gene for arterial thrombosis is not clear.

Methods— We investigated the prevalence of these mutations in 468 patients with an acute stroke or transient ischemic attack (TIA) before the age of 60 years and in a healthy control population individually matched for age and gender. We also analyzed interactions between the mutations, gender, standard vascular risk factors, and stroke risk.

Results— The prevalence of the FVL did not differ significantly between patients and control subjects. However, we found a significant interaction between the FVL , smoking, and risk of stroke in women: female smokers without FVL had a somewhat increased risk of stroke of 2.6 (95% CI, 1.5 to 4.6; P =0.001) compared with nonsmoking noncarriers of the FVL . Stroke risk was markedly higher in female smokers who had the FVL (OR, 8.8; 95% CI, 2.0 to 38.0; P =0.004) after multivariate adjustment. No such interaction was observed in men. In contrast, the frequency of the FII G20210A mutation was significantly higher in male patients compared with controls (6% versus 1%; adjusted OR, 6.1; 95% CI, 1.3 to 28.3; P =0.021). In females, the prevalence of the mutation was 3% in both groups. We found no significant interactions of the FII G20210A mutation with other vascular risk factors and stroke risk.

Conclusions— Our data indicate a highly increased risk of ischemic cerebrovascular events in women up to 60 years who smoke and have FVL . We also found evidence for an increased risk of stroke/TIA in men who have the FII G20210A mutation but not in women in this age group.

Hematologic ramifications of patent foramen ovale--role of hypercoagulable state

This article discusses the ramifications of a hypercoagulable state in patients with a patent foramen ovale.

Snake venom prothrombin activators similar to blood coagulation factor Xa

Activation of prothrombin to mature thrombin in vivo occurs by the proteolytic action of the prothrombinase complex consisting of serine proteinase factor Xa, and cofactors that include factor Va, Ca(2+) ions and phospholipids. Several exogenous prothrombin activators are found in snake venom. Among these, Group C prothrombin activators resemble the factor Xa-factor Va complex, while Group D activators are structurally and functionally similar to factor Xa. This review provides a detailed description of current knowledge on Group D prothrombin activators and highlights the importance of studying this family of proteins in enhancing our understanding of structure-function relationships in the mammalian prothrombinase complex.

Coagulation factor V: a plethora of anticoagulant molecules

PURPOSE OF REVIEW

Thrombin is necessary for survival and is produced after activation of prothrombin by prothrombinase at the site of a vascular injury. While the enzyme component of prothrombinase alone, factor Xa, bound to a membrane surface can activate prothrombin, incorporation of the cofactor molecule, factor Va, into prothrombinase results in a five orders of magnitude increase in the catalytic efficiency of factor Xa that provides the physiologic pathway for thrombin generation. While the kinetic constants and the identity of peptide bonds cleaved in prothrombin to generate alpha-thrombin have been long established, the peptidyl portions of the factor Va molecule responsible for its interactions with factor Xa, prothrombin, and the lipid surface are still the subject of intense investigation. In this review, we summarize the current state of knowledge with respect to the interactions of the factor Va molecule with the various components of prothrombinase.

RECENT FINDINGS

Binding sites for factor Xa have been identified on both the heavy and light chains of factor Va. Two amino acid regions that interact with factor Xa have been delineated on the heavy chain of the cofactor. It has also been demonstrated that the carboxyl-terminal portion of the heavy chain of factor Va contains hirudin-like motifs and appears to be responsible for the interaction of factor Va with prothrombin. This region of the molecule is important for procofactor activation by thrombin as well as cofactor function. Finally, the membrane-binding site of factor Va is contributed by several elements of the light chain and involves both electrostatic and hydrophobic interactions.

SUMMARY

The absence or dysfunction of factor Va leads to hemorrhagic diseases while prolonged existence of the active cofactor species is associated with thrombosis. Thus, modulation of the incorporation of factor Va into prothrombinase in vivo by using synthetic peptides that have the potential to impair factor Va binding to any of the components of prothrombinase, will allow for control of the rate of thrombin generation at the site of vascular damage. As a consequence, a systematic definition of the regions of factor Va governing its incorporation within prothrombinase will provide the scaffold for the synthesis of potent anticoagulant molecules that could modulate thrombin formation and suppress excessive clotting in thrombotic individuals.

Factor V Leiden: a disorder of factor V anticoagulant function

PURPOSE OF REVIEW

Activated protein C (APC) resistance, which is often associated with the factor V R506Q (FV Leiden) mutation, is a common risk factor for venous thrombosis. Study of the mechanism of APC resistance has revealed that coagulation FV stimulates the APC-catalysed inactivation of FVIIIa, and that this anticoagulant function of FV is impaired in FV Leiden. The present review covers the discovery, the physiological significance and the structural requirements of the APC-cofactor activity of FV.

RECENT FINDINGS

Recent in vitro and in vivo experiments indicate that the anticoagulant activity of FV is physiologically relevant and that FV plays a major role in the maintenance of the haemostatic balance. Quantitative and functional defects of the APC-cofactor activity of FV lead to increased thrombin generation and are associated with a prothrombotic state. Although the structural requirements for the expression of the APC-cofactor activity of FV are now beginning to be unravelled, the underlying molecular mechanism remains elusive.

SUMMARY

The APC-cofactor activity of FV and its impairment in FV Leiden can explain the different thrombosis risks associated with heterozygosity, homozygosity and pseudo-homozygosity for FV Leiden. Elucidation of the molecular mechanism of the anticoagulant function of factor V may provide novel targets for the design of antithrombotic drugs.

The response to activated protein C after cardiopulmonary bypass: impact of factor V leiden

Activated protein C (aPC) resistance is a recognized hypercoagulable phenotype that is associated with increased risk for thrombosis in multiple clinical settings. Factor V Leiden (FVL) represents a specific inherited cause of aPC resistance, but the perioperative thrombotic risk of FVL is unclear. In this investigation, we sought to quantify whether cardiopulmonary bypass produces alterations in aPC resistance in FVL carriers and noncarrier controls, testing the hypothesis that FVL is associated with a relatively hypercoagulable postoperative state. Two-hundred-five adult cardiac surgery patients were prospectively enrolled into a genetic registry whose purpose was to study the impact of genetic variables on clinical outcomes. For this study, 8 subjects heterozygous for FVL were identified (group L), as well as 2 control groups: group MC, matched controls, 18 matched subjects without FVL; and group UC, unmatched controls, 11 consecutive subjects without FVL. Plasma was sampled at the beginning of surgery, 10 min after protamine administration, and on postoperative day 1, and assayed for resistance to aPC (normal aPC ratio is >2.0). Both MC and UC groups exhibited normal aPC ratio at baseline (2.40 and 2.36, respectively), which increased significantly (to 2.76 and 2.75, P = 0.007 and 0.021, respectively) on postoperative day 1, indicating increased postoperative sensitivity to aPC. Conversely, group L subjects exhibited aPC resistance at baseline (aPC ratio 1.80), and did not change significantly postoperatively (P = 0.867). Patients without FVL therefore show laboratory evidence consistent with relative protection from postoperative thrombosis, whereas FVL carriers do not. These findings provide mechanistic support for previous speculations of increased postoperative thrombotic risk associated with FVL.

Identification of the MMRN1 Binding Region within the C2 Domain of Human Factor V

In platelets, coagulation cofactor V is stored in complex with multimerin 1 in alpha-granules for activation-induced release during clot formation. The molecular nature of multimerin 1 factor V binding has not been determined, although multimerin 1 is known to interact with the factor V light chain. We investigated the region in factor V important for multimerin 1 binding using modified enzyme-linked immunoassays and recombinant factor V constructs. Factor V constructs lacking the C2 region or entire light chain had impaired and absent multimerin 1 binding, respectively, whereas the B domain deleted construct had modestly reduced binding. Analyses of point mutated constructs indicated that the multimerin 1 binding site in the C2 domain of factor V partially overlaps the phosphatidylserine binding site and that the factor V B domain enhances multimerin 1 binding. Multimerin 1 did not inhibit factor V phosphatidylserine binding, and it bound to phosphatidylserine independently of factor V. There was a reduction in factor V in complex with multimerin 1 after activation, and thrombin cleavage significantly reduced factor V binding to multimerin 1. In molar excess, multimerin 1 minimally reduced factor V procoagulant activity in prothrombinase assays and only if it was added before factor V activation. The dissociation of factor V-multimerin 1 complexes following factor V activation suggests a role for multimerin 1 in delivering and localizing factor V onto platelets prior to prothrombinase assembly.

Factor V Leiden and risk of ischemic stroke in nonvalvular atrial fibrillation: the AnTicoagulation and Risk Factors in Atrial Fibrillation (ATRIA) Study

BACKGROUND

Atrial fibrillation is a major cause of cardioembolic stroke. Since atrial and venous pressures are similar, genetic variants that promote venous thromboembolism may increase the risk of atrial thrombi and subsequent stroke in atrial fibrillation.

METHODS

We conducted a nested case-control study of the association between the presence of factor V Leiden polymorphism and incident ischemic stroke within a prospective cohort of 13,559 adult patients with diagnosed nonvalvular atrial fibrillation between July 1, 1996 and December 31, 1997. Incident cases with ischemic strokes were identified through August 31, 1999 and matching stroke-free controls were enrolled.

RESULTS

One hundred thirty-seven case patients with incident stroke and 214 controls were enrolled. Cases were older, more likely to be women, and more likely to have a prior stroke, heart failure, hypertension, diabetes, and coronary disease. The factor V Leiden polymorphism was present in 5.8% of cases and 3.7% of controls (P = 0.36). Among non-anticoagulated patients, 7/96 (7.3%) case patients and 3/81 (3.6%) control subjects were heterozygous for factor V Leiden (Odds Ratio 2.1 [95% CI: 0.5-8.4]). Adjustment for known stroke risk factors did not significantly change the observed association in non-anticoagulated patients (adjusted OR 1.9 [0.5-8.0]).

CONCLUSIONS

Within a large nested case-control sample of patients with atrial fibrillation, factor V Leiden was not significantly associated with risk of stroke. However, given the suggestive nature of our findings, further study in even larger numbers of patients is needed to clarify the impact of factor V Leiden on stroke risk in atrial fibrillation.

Coagulation factor V

The coagulation cascade involves sequential enzymatic activations of serine protease zymogens that converge on the generation of thrombin. Factor V (FV) takes part in this process as a component of the prothrombinase complex. Besides its role as procoagulant factor, it is also involved in the physiologic anticoagulant pathway, by participating in the inactivation of activated factor VIII (FVIIIa). Given the dual role of FV, genetic defects in FV gene may result in opposite hemorrhagic or thrombotic phenotypes. This review focuses on the structure, function (procoagulant and anticoagulant), regulation (activation and inactivation) of FV as well as on the genetic defects associated with mutations in the FV gene.

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 contribution of inherited and acquired thrombophilic defects, alone or combined with antiphospholipid antibodies, to venous and arterial thromboembolism in patients with systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is associated with an increased risk of venous (VTE) and arterial thromboembolism (ATE). Lupus anticoagulant (LA) and anticardiolipin antibodies (ACAs) are established risk factors. We assessed the contribution of deficiencies of antithrombin, protein C, total protein S, factor V Leiden, the prothrombin G20210A mutation and APC resistance, either alone or in various combinations with LA and/or ACAs, to the thrombotic risk in a cohort of 144 consecutive patients with SLE. Median follow-up was 12.7 years. VTE had occurred in 10% and ATE in 11% of patients. LA,ACAs, factor V Leiden, and the prothrombin mutation were identified as risk factors for VTE. Annual incidences of VTE were 2.01 (0.74-4.37) in patients with one of these disorders and 3.05 (0.63-8.93) in patients with 2 disorders. The risk of VTE was 20- and 30-fold higher, respectively, compared with the normal population. In contrast with LA and ACAs, thrombophilic disorders did not influence the risk of ATE. In conclusion, factor V Leiden and the prothrombin mutation contribute to the risk of VTE in patients with SLE, and potentiate this risk when one of these thrombophilic defects are combined with LA and/or ACAs.

Functional characterization of factor V-Ile359Thr: a novel mutation associated with thrombosis

A missense mutation, FV-Ile359Thr (FV Liverpool), associated with thrombosis has recently been described. This mutation creates an additional potential N-linked glycosylation site (Asn-X-Ser/Thr) in factor V (FV) at Asn357 that could interfere with secretion and/or protein interactions. To investigate the molecular pathology of FV-Ile359Thr, the mutation was created by site-directed mutagenesis and expressed together with other mutations that could help explain the phenotype (FV-Arg306Gln/Ile359Thr/Arg679Gln, FV-Ile359Thr/Arg506Gln/Arg679Gln, and FV-Asn357Gln/Ile359Thr). The FV-Ile359Thr was secreted normally and had full procoagulant activity. Western blot analysis showed that FV-Ile359Thr migrated more slowly, while the FV-Asn357Gln/Ile359Thr was indistinguishable from FV-wild type (FV-WT), indicating that FV-Ile359Thr was expressed with an additional carbohydrate chain. Activated protein C (APC)-mediated inactivation in an FVa degradation assay showed that the Ile359Thr mutation significantly reduced the cleavage at Arg306 both in the presence and absence of protein S, whereas the cleavage at Arg506 was unaffected. When tested in an FVIIIa degradation assay, the FV-Ile359Thr variant exhibited equally low APC cofactor activity as FV Leiden (FVArg506Gln). In conclusion, the Ile359Thr mutation appears to affect anticoagulation by 2 mechanisms, impeding the APC-mediated down-regulation of the FVa molecule and additionally being a poor APC cofactor for the down-regulation of FVIIIa. These findings explain the association of the FV-Ile359Thr mutation with thrombosis.

Functional Properties of Recombinant Factor V Mutated in a Potential Calcium-Binding Site

Activated coagulation factor V (FVa) is a cofactor of activated factor X (FXa) in prothrombin activation. FVa is composed of a light chain (LC) and a heavy chain (HC) that are noncovalently associated in a calcium-dependent manner. We constructed a recombinant FV Asp111Asn/Asp112Asn mutant (rFV-NN) to abolish calcium binding to a potential calcium-binding site in FVa in order to study the specific role of these residues in the expression of FVa activity. Whereas thrombin-activated recombinant FV wild type (rFV-wt) presented with stable FVa activity, incubation of rFV-NN with thrombin resulted in a temporary increase in FVa activity, which was rapidly lost upon prolonged incubation. Loss of FVa activity was most likely due to dissociation of HC and LC since, upon chromatography of rFVa-NN on a SP-Sepharose column, the HC did not bind significantly to the resin whereas the LC bound and could be eluted at high ionic strength. In contrast, rFVa-wt adhered to the column, and both the HC and LC coeluted at high ionic strength. In the presence of phospholipid vesicles, the loss of rFVa-NN activity was partially prevented by FXa, active site inhibited FXa, and prothombin in a dose-dependent manner. We conclude that the introduced amino acid substitutions result in a loss of the high-affinity (calcium-dependent) interaction of the HC and LC of FVa. We propose that the introduced substitutions disrupt the calcium-binding site in FV, thereby yielding a FV molecule that rapidly loses activity following thrombin-catalyzed activation most likely via dissociation of the HC and LC.

Increased hemostasis potential persists in women with previous thromboembolism with or without APC resistance

BACKGROUND

Activated thrombin generation and depressed fibrinolysis due to the presence of activated protein C (APC) resistance with or without factor (F)V Leiden mutation are associated with development of deep venous thrombosis (DVT).

OBJECTIVE

A better understanding of the mechanism behind the risk of recurrence of DVT, using our new, recently developed assay of overall hemostasis potential (OHP).

PATIENTS AND METHODS

Levels of OHP, as well as APC resistance and FV Leiden mutation, were determined in 88 women (cases) who had previously experienced DVT in connection with pregnancy, and in 25 young healthy individuals (controls). Clotting time and clot lysis time were also investigated.

RESULTS

OHP levels in the patients were increased compared with the controls. In the cases with APC resistance and the Leiden mutation this imbalance in hemostasis potential was more severe than in those without. The group with the more severe imbalance had shorter clotting times and longer clot lysis times.

CONCLUSIONS

A procoagulant state perseveres in patients with a history of pregnancy-related DVT, even after the symptomatic phase is over. The mechanisms behind such an imbalance in overall hemostasis are enhanced thrombin generation and depressed fibrinolysis. These findings may underscore the need for thromboprophylaxis to prevent recurrence of thromboembolism in risk situations.

Survival advantage associated with heterozygous factor V Leiden mutation in patients with severe sepsis and in mouse endotoxemia

Sepsis is associated with systemic inflammation, coagulopathy, and disrupted protein C (PC) pathway function. The effect of prothrombotic polymorphism, factor V Leiden (Arg506Gln; FV Leiden), was examined in a large clinical trial (PROWESS) of severe sepsis and a mouse endotoxemia model. In PROWESS, 4.1% (n = 65) of patients were heterozygous FV Leiden (VL+/-) carriers. The 28-day mortality was lower in VL+/- (13.9%) than in non-FV Leiden (VL-/-; 27.9%) patients (P =.013). The mortality benefit of recombinant human activated PC (rhAPC) treatment was similar in VL+/- (placebo, 15.6%; rhAPC,12.1%) and VL-/- patients (placebo, 31.0%; rhAPC, 24.7%; interaction P =.981). VL+/- status did not appear to influence baseline biomarkers of coagulopathy and inflammation or disease severity, with the exception that vasopressor usage was less in VL+/- patients (46.2% versus 63.0%; P =.009). In a median lethal dose (40 mg/kg) endotoxin mouse model, VL+/- mice had lower mortality than wild-type mice (19% versus 57%; P =.008), whereas the mortality of homozygous (VL+/+) mice was almost identical to that of wild-type mice (65% versus 57%; P =.76). The findings suggest that FV Leiden constitutes a rare example of a balanced gene polymorphism that maintains the FV Leiden mutation in the general gene pool due to a survival advantage of VL+/- in severe sepsis.

The murine platelet and plasma factor V pools are biosynthetically distinct and sufficient for minimal hemostasis

Coagulation factor V (FV) is a central regulator of the coagulation cascade. Circulating FV is found in plasma and within platelet alpha granules. The specific functions of these distinct FV pools are uncertain. We now report the generation of transgenic mice with FV gene expression restricted to either the liver or megakaryocyte/platelet lineage using bacterial artificial chromosome (BAC) constructs. Six of 6 independent albumin BAC transgenes rescue the neonatal lethal hemorrhage of FV deficiency. Rescued mice all exhibit liver-specific Fv expression at levels ranging from 6% to 46% of the endogenous Fv gene, with no detectable FV activity within the platelet pool. One of the 3 Pf4 BAC transgenes available for analysis also rescues the lethal FV null phenotype, with FV activity restricted to only the platelet pool (approximately 3% of the wild-type FV level). FV-null mice rescued by either the albumin or Pf4 BAC exhibit nearly normal tail bleeding times. These results demonstrate that Fv expression in either the platelet or plasma FV pool is sufficient for basal hemostasis. In addition, these findings indicate that the murine platelet and plasma FV pools are biosynthetically distinct, in contrast to a previous report demonstrating a plasma origin for platelet FV in humans.

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.

Factor V Leiden and the risk of stillbirth in a German population

An association between the factor V Leiden variant and an increased risk of pregnancy loss has been reported. Most previous studies were performed with clinically recruited patients and controls. This approach may cause selection bias. The present analysis was performed with the aim to investigate the association between the factor V Leiden mutation and the risk of stillbirth in a population-based sample. The Study of Health in Pomerania (SHIP) is a survey that was carried out in North East Germany. A random sample from the population aged 20 to 79 years was taken. The total SHIP population comprised 4,310 participants. The presence of the factor V Leiden variant was determined by PCR and Mnl I digestion. The presence of the factor V Leiden variant was neither associated with the number of pregnancies nor with the number of children per women. Data from 1,768 females who had at least one pregnancy with known outcome was available for the present analysis. Seventy-three women (4.1%) reported at least one stillbirth. Women with and without the factor V Leiden mutation did not differ with respect to the number of women with at least one stillbirth (OR for factor V Leiden variant 1.57; 95%-CI 0.76 - 3.25). Furthermore, the number of women with two or more stillbirths, the number of stillbirths per affected woman and the number of stillbirths per number of pregnancies per woman was similar between both genotype groups. In conclusion, there is no association between the factor V Leiden mutation and the risk of stillbirth in a representative population sample.

Intracranial sinus thrombosis in a patient with Crohn disease and factor V Leiden mutation

There is a well-known risk of thrombosis in patients with inflammatory bowel disease. Documented cases of intracranial sinus thrombosis in this setting are rare. We present the case of a 30-year-old man with Crohn disease who spontaneously developed a superior sagittal sinus thrombosis and bifrontal infarcts that resulted in death. The patient was heterozygous for factor V Leiden mutation. The literature was searched to assess the frequency of cerebral venous infarcts in inflammatory bowel disease and the role that factor V Leiden plays in thrombosis in such patients.

Thrombotic and fibrinolytic alterations in the aseptic necrosis of femoral head

Recent reports seem to support the role of the thrombophilia and decreased fibrinolysis in the aetiopathogenesis of aseptic necrosis of bone. In the present study, haemostatic disturbances were analysed in adults (n = 49) and patients in childhood (Perthes disease) (n = 47) with aseptic necrosis of the femoral head. Fibrinolytic parameters (in vitro clot lysis, plasminogen, plasmatic plasminogen activator inhibitor-1 activity, D-dimer) along with lipoprotein (a) [Lp(a)] and fibrinogen were measured. von Willebrand factor, platelet activation and some thrombophilic factors (activated protein C resistance and factor V Leiden mutation, protein C, protein S activity) were also determined. Impaired fibrinolysis, an increased Lp(a) level along with slow clot lysis and increased platelet activation were found in adult cases. We detected five cases of factor V Leiden mutations (one heterozygotic and four homozygotic) among patients with Perthes disease. The clinical course of the heterozygous case was similar to the usual form of Perthes disease. The most severe form of Perthes disease has been observed in homozygous factor V Leiden mutation cases. The mutation of factor V Leiden per se probably does not induce the development of aseptic necrosis of bone tissue in childhood, but it does play a role in its acceleration. Homozygous factor V Leiden mutation definitely runs a more severe course. On the other hand, in adult cases, the disturbances of haemostasis, impaired fibrinolysis, elevated Lp(a) level, increased platelet activation and slight elevation of fibrinogen might have clinical relevance. Further studies should focus on proving the role of the haemostatic alterations in the pathogenesis of severe forms of aseptic bone necrosis. The use of antithrombotic drugs in order to slow the process of aseptic necrosis also has to be addressed in future surveys.

The discovery of factor V: a tricky clotting factor

The paper tells the story of how FV was discovered in 1943 and installed into the Morawitz theory, a dogma that had reigned the clotting field since 1905 without serious challenges. It is a witness to the fact, many times experienced throughout scientific history, that seminal achievements may arise from serendipity under awkward conditions. Under the worst of circumstances, only a brilliant mind, scientific curiosity and devotion, could the challenge Owren met in Mary's bleeding problem, lead to such a pivotal result. On top of establishing a new clotting factor, his work spurred an unprecedented activity in the field. The thorny road to the new factor's place and role in the clotting mechanism is depicted in some detail. But the factor turned out to be more capricious than its role in coagulation seemed to indicate. Thus, in recent times it has become clear that its platelet counterpart plays an additionally important role in hemostasis as a whole. The two polymorphisms of clinical importance discovered in platelet FV lead to a bleeding disorder, whereas one in plasma FV leads to a rather frequent venous thromboembolic state. Further surprises might therefore be expected from this chameleon of a factor - reflecting the increasingly appreciated tendency that one biological compound appears in different roles.

The in vitro analysis of the coagulation mechanism of activated factor VII using thrombelastogram

We investigated the effects of addition of recombinant activated coagulation factor VII (rFVIIa) to coagulation factor-deficient plasma and whole blood, using thrombelastograms (TEGs). The addition of rFVIIa to factor II- or X-deficient plasma did not correct hemostatic parameters, whereas it produced partial responses in factor V-, VIII- or IX-deficient plasma and good responses in factor VII-, XI- or XII-deficient plasma. Furthermore, the addition of rFVIIa and platelets (30-100 x 10(3)/ micro l) to platelet-poor plasma produced marked corrections, producing TEGs similar to those of platelet-rich plasma. These results indicate that factors II and X are essential for the hemostatic effects of rFVIIa, and that factors V and VIII promote these effects. We believe that TEGs are, at present, one of the most useful tools for evaluating in vitro hemostatic effects of rFVIIa.

Identification of a large deletion and three novel mutations in exon 13 of the factor V gene in a Spanish family with normal factor V coagulant and anticoagulant properties

As part of the GAIT (genetic analysis of idiopathic thrombophilia) project, we analyzed polymorphisms in the factor V (FV) gene to assess their role as genetic determinants of normal phenotypic variation of hemostasis-related traits in a Spanish population. During the analysis of exon 13 polymorphisms, we detected an abnormal PCR-amplified fragment in some members of the GAIT19 family. Direct sequence analysis revealed a deletion of 108 bp in eight out of 20 individuals in this family. This deletion removes exactly 36 amino acids from the B domain of FV; thus it does not alter the reading frame of the sequence. Among the deleted amino acids there is the 4070A>G polymorphism (H1299R), which could affect the level or function of FV. In addition, in the same family we identified three novel DNA variants (L1257I, Q1317Q and T1327T) in exon 13 of the F5 gene. Despite these variants, we did not detect any differences either in the coagulant or anticoagulant traits, or in the plasma protein levels involved in the blood coagulation cascade, between the carriers compared with their non-carrier relatives. From these results, we can conclude that the mutant allele is expressed and the resultant protein is functional. Moreover, it is unlikely that the 4070A>G polymorphism, within the deletion, and the novel DNA variants alter the functional properties of the mature FV protein. Further analyses of this naturally occurring mutation and the novel DNA variants should yield useful information for the understanding of the function of the B domain of FV.

Factor V Leiden, hormone replacement therapy, and risk of venous thromboembolic events in women with coronary disease

Oral contraceptive use in women with factor V Leiden is associated with increased rates of venous thromboembolic events (VTEs). However, the effects of hormone replacement therapy (HRT) in postmenopausal women with factor V Leiden are not known. A nested case-control study was conducted among women with established coronary disease enrolled in 2 randomized clinical trials of HRT, the Heart and Estrogen/Progestin Replacement Study (HERS) and the Estrogen Replacement and Atherosclerosis (ERA) trial. The Leiden mutation was present in 8 (16.7%) of 48 cases with VTE compared with only 7 (6.3%) of 112 controls (odds ratio [OR](Leiden) 3.3, 95% CI 1.1 to 9.8; P=0.03). In women without the factor V Leiden mutation, risk associated with HRT use was significantly increased (OR(HRT) 3.7, 95% CI 1.4 to 9.4; P<0.01). On the other hand, in women with the factor V Leiden mutation, the estimated risk associated with HRT was increased nearly 6-fold, although the CIs were wide and included unity (OR(HRT) 5.7, 95% CI 0.6 to 53.9; P=0.13). The OR for women with the Leiden mutation who were also assigned to HRT compared with wild-type women assigned to placebo was 14.1 (95% CI 2.7 to 72.4, P=0.0015). In women with the factor V Leiden mutation who were treated with HRT, the estimated absolute incidence of VTE was 15.4 of 1000 per year compared with 2.0 of 1000 per year in women without the mutation who were taking a placebo (P=0.0015). On the basis of these data, in women with coronary disease, the estimated number needed to screen for factor V Leiden to avoid an HRT-associated VTE during 5 years of treatment is 376. If factor V Leiden genotyping becomes less expensive, it could be cost effective to screen for the presence of the mutation before instituting HRT in women with coronary disease.

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.

The activated protein C (APC)-resistant phenotype of APC cleavage site mutants of recombinant factor V in a reconstituted plasma model

Recently, new missense mutations in the activated protein C (APC) cleavage sites of human factor V (FV) distinct from the R506Q (FV Leiden) mutation have been reported. These mutations affect the APC cleavage site at arginine (Arg) 306 in the heavy chain of activated FV. Whether these mutations result in APC resistance and are associated with a risk of thrombosis is not clear. The main objective of the present study was to identify the APC-resistant phenotype of FV molecules with different mutations in APC cleavage sites. To study this, recombinant FV mutants were reconstituted in FV-deficient plasma, after which normalized APC-sensitivity ratios (n-APC-SRs) were measured in activated partial thromboplastin time-based and Russell's Viper Venom time-based APC-resistance tests. The mutations introduced in FV were R306G, R306T, R506Q, R679A and combinations of these mutations. Based on the APC-sensitivity ratios, we conclude that the naturally occurring mutations at Arg306 (i.e. FV HongKong and FV Cambridge) result in a mildly reduced sensitivity for APC (n-APC-SR, 0.74-0.87), whereas much lower values (n-APC-SR, 0.41-0.51) are obtained for the mutation at Arg506 (FV Leiden). No effect on the n-APC-SR was observed for the recombinant FV mutant containing the single Ala679 mutation. Because reduced sensitivity for APC, not due to FV Leiden, is a risk factor for venous thrombosis, these data suggest that mutations at Arg306 might be associated with a mild risk of venous thrombosis.