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.