Resistance to activated protein C, the FV:Q506 allele, and venous thrombosis

Update on platelet procoagulant mechanisms in health and in bleeding disorders

Platelet procoagulant mechanisms are emerging to be complex and important to achieving haemostasis. The mechanisms include the release of procoagulant molecules from platelet storage granules, and strong agonist-induced expression of procoagulant phospholipids on the outer platelet membrane for tenase and prothrombinase assembly. The release of dense granule polyphosphate is important to platelet procoagulant function as it promotes the activation of factors XII, XI and V, inhibits tissue factor pathway inhibitor and fibrinolysis, and strengthens fibrin clots. Platelet procoagulant function also involves the release of partially activated factor V from platelets. Scott syndrome has provided important insights on the mechanisms that regulate procoagulant phospholipids expression on the external platelet membrane, which require strong agonist stimulation that increase cystolic calcium levels, mitochondrial calcium uptake, the loss of flippase function and activation of the transmembrane scramblase protein anoctamin 6. There have been advances in the methods used to directly and indirectly assess platelet procoagulant function in health and disease. Assessments of thrombin generation with platelet rich plasma samples has provided new insights on how platelet procoagulant function is altered in inherited platelet disorders, and how platelets influence the bleeding phenotype of a number of severe coagulation factor deficiencies. Several therapies, including desmopressin and recombinant factor VIIa, improve thrombin generation by platelets. There is growing interest in targeting platelet procoagulant function for therapeutic benefit. This review highlights recent advances in our understanding of platelet-dependent procoagulant mechanisms in health and in bleeding disorders.

Venous Thromboembolism in Heterozygotes for Factor V Leiden—The Second-Hit Hypothesis: A Report of Two Patients and a Review of the English-Language Literature

Resistance to activated protein C (factor V Leiden, FV-R506Q) is the most prevalent inherited hypercoagulable state with a frequency of 4.5% in the American population, but 70% of heterozygotes do not experience venous thromboembolic disease. Heterozygosity for FV-R506Q will coexist in patients with deficiencies of protein S (26.1 %), protein C (14.9%), antithrombin III (15.1%), homocystinuria/ hyperhomocysteinemia (29.6%), pregnancy or postpartum (27.9%), oral contraceptives (27.5%), trauma and surgery (18.6%), and lupus anticoagulant and/or antiphospholipid antibodies (40%). The second-hit hypothesis proposes that heterozygotes for factor V Leiden mutation who do experience venous thrombosis will have a second hypercoagulable state either hereditary or acquired. Key Words: Thromboembolism—Factor V Leiden—Heterozygotes.

The Predictability of Factor V Leiden (FV:Q506) Gene Mutation via Clotting-Based Diagnosis of Activated Protein C Resistance

After the discovery of activated protein C resistance (APCR) due to factor V Leiden mutation and the causal relationship of the phenomenon with clinical thromboembolism, a wide variety of functional clotting-based assays were developed for testing of APCR in relation to the specific DNA-based analysis of FV:Q506 Leiden. The aim of this study is to assess a clotting-based APCR assay using procoagulant crotalidae snake venom with respect to the sensitivity, specificity, and predictability for the presence of the factor V Leiden mutation. APCR testing and factor V DNA analyses have been performed concurrently on 319 patient specimens. APCR values of the patients with homozygous factor V Leiden mutation (70.4±13.5 s) were significantly lower (p<0.001) in comparison to the subjects with the heterozygous mutation (87.6±13.4 s). The assay is highly sensitive (98.7%) and specific (91.9%) for the screening of factor V Leiden mutation. The sensitivity and specificity of the APCR testing reached to 100% below the cut-off value of 120 s among the patients with homozygous factor V Leiden mutation. Therefore, this method could help the desired effective optimal screening strategy for the laboratory search of hereditary thrombophilia focusing on the diagnosis of APCR due to FV:Q506.

Mechanisms Regulating Acquisition of Platelet-Derived Factor V/Va by Megakaryocytes

Factor Va serves as the nonenzymatic protein cofactor for the prothrombinase complex, which converts prothrombin to thrombin in the events leading to formation of a hemostatic plug. Several observations support the concept that platelet-derived factor V/Va is physically and functionally distinct and plays a more important role in thrombin generation at sites of vascular injury as compared to its plasma counterpart. Platelet-derived factor V/Va is generated following endocytosis of the plasma-derived molecule by the platelet precursor cells, megakaryocytes, via a two receptor system consisting of low density lipoprotein (LDL) receptor-related protein-1 (LRP-1) and an unidentified specific "binding site". More recently, it was suggested that a cell surface-expressed β-galactoside binding protein, galectin-8, was involved in factor V endocytosis. Endocytosed factor V is trafficked through the cell and retailored prior to its storage in α-granules. Given the essential role of platelet-derived factor Va in clot formation, understanding the cellular and molecular mechanisms that regulate how platelets acquire this molecule will be important for the treatment of excessive bleeding or clotting.

Thrombosis in the Neonatal Intensive Care Unit

Neonates have the highest risk for pathologic thrombosis among pediatric patients. A combination of genetic and acquired risk factors significantly contributes to this risk, with the most important risk factor being the use of central venous catheters. Proper imaging is critical for confirming the diagnosis. Despite a significant number of these events being life- and limb-threatening, there is limited evidence on what the appropriate management strategy should be. Evaluation and treatment of any neonate with a clinically significant thrombosis should occur at a tertiary referral center that has proper support.

Management of neonatal thrombosis

Neonates have one of the highest risks for thromboembolism among pediatric patients. This risk is attributable to a combination of multiple genetic and acquired risk factors. Despite a significant number of these events being either life threatening or limb threatening, there is limited evidence on appropriate management strategy. Most of what is recommended is based on uncontrolled studies, case series, or expert opinion. This review begins with a discussion of the neonatal hemostatic system, focusing on the common sites and imaging modalities for the detection of neonatal thrombosis. Perinatal and postnatal risk factors are presented and management options discussed.

A case of unilateral thalamic hemorrhagic infarction as a result of the vein of Galen and straight sinus thrombosis

Straight sinus vein thrombosis represents 15% of all diagnosed sinus vein thrombosis. Thrombotic occlusion of the deep cerebral venous system, the straight sinus, and the vein of Galen causes centrally located and usually bilateral thalamic infarcts. Unilateral thalamic venous infarction is extremely rare. The clinical and radiologic findings can be nonspecific and diagnostically challenging. We report a patient with this unusual condition and review the available literature.

The evaluation and management of neonatal coagulation disorders

Neonatal hemostatic abnormalities can present diagnostic and therapeutic challenges to the physician. Developmental deficiencies and/or increases of certain coagulation proteins, coupled with acquired or genetic risk factors, can result in a hemorrhagic or thromboembolic emergency. The timely diagnosis of a congenital hemorrhagic or thrombotic disorder can avoid significant long-term sequelae. However, due to the lack of randomized clinical trials addressing the management of neonatal coagulation disorders, treatment strategies are usually empiric and not evidence-based. In this chapter, we will review the neonatal hemostatic system and will discuss the most common types of hemorrhagic and thrombotic disorders. Congenital and acquired risk factors for hemorrhagic and thromboembolic disorders will be presented, as well as current treatment options. Finally, suggested evaluations for neonates with either hemorrhagic or thromboembolic problems will be reviewed.

Genetic prothrombotic mutations are common in neonates but are not associated with umbilical catheter-associated thrombosis

OBJECTIVE

To evaluate the prevalence of hereditary prothrombotic mutations, and their effect on the incidence and severity of umbilical arterial or venous catheter (UAC or UVC)-associated thrombosis.

STUDY DESIGN

All neonates with a UAC or UVC were studied prospectively for the presence, severity and timing of thrombosis with duplex Doppler ultrasound scan. Genetic testing for factor V Leiden (FVL), prothrombin mutation (PTm) and methylene-tetrahydrofolate reductase (MTHFR) mutations was performed using PCR and restriction fragment length polymorphism assays.

RESULT

Umbilical catheter (UC)-associated thrombosis developed in 16/53 (31%) neonates; 23% of UACs and 22% of UVCs were associated with thrombosis. The prevalence of a significant prothrombotic mutation was present in 10/51 (20%) of infants: FVL (8%), MTHFR667 homozygosity (10%), MTHFR1298 homozygosity (2%) and PTm (0%). There was no increase in the risk of UC-associated thrombus in patients carrying these prothrombotic mutations; our study had the power to detect a 2.5-fold increased risk of thrombosis for any of these significant mutations. In addition, MTHFR667 heterozygosity was found in 41% of infants and MTHFR1298 heterozygosity in 52% and also were not associated with increased risk of UC-associated thrombus. The risk of MTHFR double heterozygosity (db het) was 14%, the risk of a significant or db het was 17/51 (33%) and the risk of any mutation was 90%.

CONCLUSION

Prothrombotic genetic mutations are common in our Neonatal Intensive Care Unit population but do not appear to increase the risk of UC-associated thrombosis.

[Mechanisms of action of recombinant human activated Protein C]

Human activated protein C (APC) is a serineprotease and one of the most important physiological inhibitors of the coagulation system. Apart from anticoagulative effects, profibrinolytic and anti-inflammatory modes of action have been reported for APC. The administration of recombinant human activated protein C (rhAPC), drotrecogin alfa (activated), Xigris, to patients with severe sepsis and sepsis-induced multi-organ failure reduced mortality in large clinical trials. Anti-apoptotic and immunomodulatory effects of rhAPC have been examined in in vitro experiments and in experimental animal studies. Moreover, a reduction of endothelial cell permeability, enhanced endothelial cell survival as well as improvements of microcirculatory disorders have been proposed for rhAPC. The manifold mechanisms of action of APC may give reasons for its application in diseases other than sepsis, which are characterized by endothelial and microcirculatory dysfunction, e.g. acute pulmonary or renal failure, ischemic stroke, ischemia-reperfusion injury and acute pancreatitis. A better understanding of the anti-inflammatory, anti-apoptotic and immunomodulatory modes of action of APC could be relevant for dosing and mode of application and may lead to a broadening of the indication field for rhAPC.

Hypercoagulability, a serious problem in patients with ESRD on maintenance hemodialysis, and its correction after kidney transplantation

BACKGROUND

Recurrent vascular access thrombosis (VAT) resulting in failure to continue maintenance hemodialysis (HD) therapy is not an uncommon event. The cause of VAT in these circumstances remains uncertain. We describe results of our studies to identify changes in hemostatic balance in patients on maintenance HD therapy that probably contributed to a hypercoagulable state.

METHODS

We studied 82 patients with end-stage renal disease on maintenance HD therapy who underwent HD for 11 to 52 months (39.3 +/- 27.4 months). Forty-nine episodes of VAT occurred in 22 patients; a single episode occurred in 12 patients; and 2 or more episodes, in 10 patients. Blood coagulation studies, including assays of inhibitors and activated protein C (PC) resistance (APCR), were performed using standard techniques.

RESULTS

Investigations showed the presence of lupus anticoagulant (LA) in 5.6%, anticardiolipin antibody immunoglobulin G (IgG) in 3.9% and IgM in 5.3%, APCR in 20.5%, and deficiencies in protein S (PS), PC, and antithrombin III (ATIII) in 32.1%, 24.4%, and 19.2%, respectively. When parameters were compared between patients with and without VAT episodes, LA, PC, PS, and APCR levels were significantly abnormal in those who experienced VAT. Sixteen subjects with hypercoagulable states on HD therapy underwent renal transplantation and were evaluated 9.3 +/- 4.2 months posttransplantation. Deficiencies in PC (P = 0.014), PS (P = 0.001), ATIII (P = 0.017), and APCR (P = 0.0001) were completely corrected in all subjects.

CONCLUSION

Hypercoagulability is a risk factor for recurrent VAT in HD patients, and renal transplantation successfully corrects these abnormalities.

Liver transplantation for acute Budd-Chiari syndrome in identical twin sisters with Factor V leiden mutation

BACKGROUND

Budd-Chiari syndrome (BCS) is uncommon in the children. The cause of BCS comprises several diseases leading to thrombophilia. Activated protein C resistance as a result of a single gene mutation in factor V, the so called factor V Leiden (FVL), is the most common cause of thrombophilia.

METHODS

We report a simultaneous occurrence of BCS in identical twin sisters of 13 years of age with heterozygous FVL mutation.

RESULTS

One sister presented with acute BCS leading to fulminant hepatic failure. She underwent liver transplantation with subsequent normalization of activated protein C resistance. The other twin sister, who was diagnosed with extensive thromboses of the inferior vena cava, portal vein, and hepatic veins, was successfully managed by aggressive chemical and mechanical thrombolysis followed by therapeutic anticoagulation. Genomic DNA studies confirmed heterozygosity of FVL mutation in the sisters' father and older brother.

CONCLUSIONS

The exact cause of the BCS in children should be thoroughly and rapidly investigated, and, if necessary, immediate family members should also be tested for genetic defects in factor V or concomitant thrombophilia.

Cerebrovascular disease in Italy and Europe: it is necessary to prevent a 'pandemia'

In Italy and Europe, strokes are the third most common cause of death and resulting invalidity. In the ever-increasing 80-years-old-and-over population, strokes become more serious due to the clinical presentation during the acute phase and the ten times higher mortality, but also in relation to the twice as high resulting disability as for younger subjects. With the growing number of ailing and not-self-sufficient elderly, other resources will have to be relocated to this field of public health. Then, the dependence index and the ensuing equivalence based on estimates for the first decades of 2000 will create more difficulties in retrieving the funds for social policies. However, stroke prevention is possible both through correct behavioural habits and pharmacological means. Besides the well-known preventive effects of an adequate antihypertensive, antidiabetic and/or antiaggregant/anticoagulant therapy, there is increasing evidence of the effectiveness of statin therapy in stroke prevention. Subjects with a personal history of cerebrovascular events have an increased coronary risk and vice versa. The greatest part of the risk factors for the cerebrovascular disease coincides with those for cardiovascular disease, for which the correction of the former automatically involves a reduction in incidence of both pathologies. In this context, a statin's rational use can therefore represent an important tool for the combined prevention of the two pathologies. Finally, different hypotheses link the origin of Alzheimer's disease to that of progressive cerebrovascular dementia caused by cerebral microcirculation damage. The aim of this review is to resume the actual knowledge about the epidemiology of cerebrovascular disease in Italy and Europe, and about the means available to prevent this phenomenon.

Activated protein C resistance (FV(Leiden)) and thrombosis: factor V mutations causing hypercoagulable states

The integrity of the vascular system is of prime importance for survival. Therefore, several emergency and repair systems safeguard the circulatory system. Multiple processes jointly limit vascular damage and blood loss. In this article, the authors focus on the protein C anticoagulant pathway and the role of activated protein C resistance in thrombotic disease, and they discuss the involvement in thrombosis of mutations other than the Arg506Gln mutation in the gene encoding for factor V.

Detection of factor V leiden and prothrombin gene mutations in patients who died with thrombotic events

CONTEXT

Individuals with factor V or prothrombin gene mutations are at increased risk for thrombotic events. Furthermore, the risk of recurrent deep venous thrombosis in heterozygous carriers of both factor V Leiden and prothrombin gene mutations is high enough that some investigators suggest lifelong warfarin prophylaxis for these individuals, even with a single spontaneous thrombotic event.

OBJECTIVES

To assess the incidence of factor V Leiden and prothrombin gene mutations in an autopsy population and to determine if these tests can prove useful in identification of at-risk family members.

DESIGN

We analyzed factor V Leiden and prothrombin gene mutations in 45 patients who died with or of thrombotic events, using archival tissue and multiplex allele-specific polymerase chain reaction amplification. The wild-type factor V gene was amplified in all 45 patients, whereas the wild-type prothrombin gene was amplified in 29 patients.

RESULTS

Two patients (4.4%) who died with thrombotic events at the ages of 35 and 92 years were heterozygous for factor V gene mutation. Two additional patients (6.7%), who died with thrombotic events at the ages of 26 and 39 years, were heterozygous for prothrombin gene mutation. Patients homozygous for either factor V or prothrombin gene or simultaneously heterozygous for both genes were not detected in our study.

CONCLUSIONS

Our findings suggest that screening the relatives of elderly patients who die with thrombotic events would not be cost-effective because of the low incidence of these mutations in the autopsy population. However, because the incidence of these mutations appeared significantly more frequently among individuals who died at 39 years or younger, testing the relatives of this subset of patients may prove useful for detection of at-risk individuals who would benefit from preventive anticoagulation therapy.

Prevalence of preterminal pulmonary thromboembolism among patients on maintenance hemodialysis treatment before and after introduction of recombinant erythropoietin

The prevalence of pulmonary thromboembolism at autopsy was assessed in a retrospective study of a cohort of 185 patients undergoing maintenance hemodialysis treatment who died in the last decade. The overall frequency of thromboembolism was 12.43% in the dialysis population, which statistically was significantly less than in a control group of 8,051 nondialysis patients (21.77%; P = 0.0023). Moreover, pulmonary thromboembolism was less frequently fatal or contributing to death in the dialysis group than in the control group (P = 0.039). The prevalence of pulmonary thromboembolism in the dialysis group remained statistically unchanged over the 10-year period and was independent of a steady increase in the percentage of patients receiving recombinant erythropoietin therapy and the average hematocrit values. The occurrence of preterminal pulmonary thromboembolism was associated with a shorter period since onset of hemodialysis treatment and with infection as cause of death (P = 0. 031; P = 0.029, respectively). No statistically significant influence of the type of basic renal disease, type of dialysis anticoagulation, or dialysis access could be found. Our data suggest that, at least in the preterminal stage, the introduction of recombinant erythropoietin within the last decade had no substantial influence on the prevalence of pulmonary thromboembolism.

Secretable Human Platelet-Derived Factor V Originates From the Plasma Pool

Factor Va (FVa), derived from plasma or released from stimulated platelets, is the essential protein cofactor of the prothrombinase complex. Plasma-derived factor V (FV) is synthesized by the liver, whereas the source of the platelet-derived cofactor has not been unambiguously identified. Megakaryocytes, platelet precursors, are known to synthesize platelet proteins and to endocytose proteins from plasma (ie, fibrinogen) and then package these proteins into -granules. To determine which mechanism accounts for FV presence in platelets, two patients heterozygous for FVLeiden who underwent allogeneic transplantation from homozygous FV wild-type donors (bone marrow [BM] or liver) were studied. Patient JMW, whose skin biopsy specimen showed heterozygous FVLeiden, received a BM transplant from a wild-type homozygous FV donor as analyzed from posttransplant peripheral blood cells. Patient FW, whose native liver is heterozygous for FVLeiden, received a homozygous wild-type FV liver. Because each individual has two distinct genetic pools of factor V in liver and megakaryocytes, it was possible to determine whether secretable platelet-derived FV was normal or contained the FVLeiden mutation. Platelet-derived FVa released from thrombin-activated platelets from a normal individual, an individual heterozygous for the FVLeiden mutation, and the two patients was incubated with phospholipid vesicles and activated protein C (APC). Western blotting analyses using a monoclonal antibody that allows distinction between platelet-derived FVa and FVaLeiden subsequent to APC-catalyzed cleavage were then performed. Based on the accumulation of proteolytic fragments derived from APC-induced cleavage, analyses of platelet-derived FVa from JMW demonstrated both normal FVa and FVaLeiden consistent with a plasma-derived origin of the secretable platelet-derived FVa. Western blotting analyses of the APC-cleaved platelet-derived FVa from FW showed a wild-type phenotype, despite the presence of a FVLeiden allele in her megakaryocyte genome, also consistent with a plasma origin of her secretable platelet-derived FVa. Platelets do not appear to endocytose the plasma cofactor, because a 35-hour incubation of platelet-rich plasma with 125I-factor V showed no specific association/uptake of the radiolabeled ligand with the platelet pellet. Collectively, these results show for the first time that the majority of secretable platelet-derived factor V is endocytosed by megakaryocytes from plasma and is not exclusively synthesized by these cells, as previously believed.

© 1998 by The American Society of Hematology.

Secretable Human Platelet-Derived Factor V Originates From the Plasma Pool

Factor Va (FVa), derived from plasma or released from stimulated platelets, is the essential protein cofactor of the prothrombinase complex. Plasma-derived factor V (FV) is synthesized by the liver, whereas the source of the platelet-derived cofactor has not been unambiguously identified. Megakaryocytes, platelet precursors, are known to synthesize platelet proteins and to endocytose proteins from plasma (ie, fibrinogen) and then package these proteins into -granules. To determine which mechanism accounts for FV presence in platelets, two patients heterozygous for FVLeiden who underwent allogeneic transplantation from homozygous FV wild-type donors (bone marrow [BM] or liver) were studied. Patient JMW, whose skin biopsy specimen showed heterozygous FVLeiden, received a BM transplant from a wild-type homozygous FV donor as analyzed from posttransplant peripheral blood cells. Patient FW, whose native liver is heterozygous for FVLeiden, received a homozygous wild-type FV liver. Because each individual has two distinct genetic pools of factor V in liver and megakaryocytes, it was possible to determine whether secretable platelet-derived FV was normal or contained the FVLeiden mutation. Platelet-derived FVa released from thrombin-activated platelets from a normal individual, an individual heterozygous for the FVLeiden mutation, and the two patients was incubated with phospholipid vesicles and activated protein C (APC). Western blotting analyses using a monoclonal antibody that allows distinction between platelet-derived FVa and FVaLeiden subsequent to APC-catalyzed cleavage were then performed. Based on the accumulation of proteolytic fragments derived from APC-induced cleavage, analyses of platelet-derived FVa from JMW demonstrated both normal FVa and FVaLeiden consistent with a plasma-derived origin of the secretable platelet-derived FVa. Western blotting analyses of the APC-cleaved platelet-derived FVa from FW showed a wild-type phenotype, despite the presence of a FVLeiden allele in her megakaryocyte genome, also consistent with a plasma origin of her secretable platelet-derived FVa. Platelets do not appear to endocytose the plasma cofactor, because a 35-hour incubation of platelet-rich plasma with 125I-factor V showed no specific association/uptake of the radiolabeled ligand with the platelet pellet. Collectively, these results show for the first time that the majority of secretable platelet-derived factor V is endocytosed by megakaryocytes from plasma and is not exclusively synthesized by these cells, as previously believed.

© 1998 by The American Society of Hematology.

Evaluation of role of factor V Leiden mutation in fatal pulmonary thromboembolism

Association of a mutation in the coagulation factor V gene (FV Leiden) with deep vein thrombosis and pulmonary thromboembolism has been well documented in the literature, but no study has specifically screened cases of fatal pulmonary thromboembolism for the mutation. This study sought to determine whether FV Leiden plays a role in the pathogenesis of fatal pulmonary thromboembolism. We isolated DNA from archived paraffin-embedded tissues derived from 46 necropsy cases in which pulmonary thromboembolism was listed as the cause of death (n = 27) or was secondarily associated with death (n = 19). FV Leiden genotypes were determined by using polymerase chain reaction and MnlI digestion of amplified products. The Leiden mutation occurred in the heterozygous state in one (2.1%) of the necropsy specimens. The prevalence of the mutation was higher (8.7%) in gender- and ethnic-matched blood donor controls. The FV Leiden mutation is not independently associated with fatal pulmonary thromboembolism in the group of patients analyzed. The results suggest different etiologies for nonfatal, chronic deep vein thrombosis/pulmonary thromboembolism and fatal, acute pulmonary thromboembolism.

Platelet-Derived Factor Va/VaLeiden Cofactor Activities Are Sustained on the Surface of Activated Platelets Despite the Presence of Activated Protein C

We investigated the role of the thrombin-activated platelet in modulating the rate and extent of activated protein C (APC)-catalyzed inactivation of platelet-derived factor Va and factor VaLeiden. Platelet-derived factor Va and factor VaLeiden were inactivated by APC at near identical rates; however, complete inactivation of the cofactors was never achieved. Greater residual cofactor activity remained when using thrombin-activated platelets compared with that observed with synthetic phospholipid vesicles and platelet-derived microparticles, suggesting that thrombin-activated platelets protect the cofactors from APC-catalyzed inactivation. This apparent protection was not due to (1) an insufficient number of membrane binding sites for APC or factor Va; (2) the destruction of these sites; or (3) the presence of a platelet-associated APC inhibitor. Results from a plasma-based clotting assay (with or without APC) with platelets or PCPS vesicles added to induce clot formation indicated that, even in the presence of high concentrations of APC, platelets offered protection of the cofactor by delaying cleavage at Arg506. This resulted in incomplete proteolysis of the heavy chain, suggesting that platelets can also protect plasma-derived factor Va from APC-catalyzed inactivation. However, additional experiments indicated that the plasma-derived cofactor, bound to thrombin-activated platelets, was completely inactivated by APC, suggesting that the plasma and platelet-derived cofactor pools represent different substrates for APC. Collectively, these results indicate that platelets sustain procoagulant events by providing a membrane surface that delays cofactor inactivation and by releasing a cofactor molecule that displays an APC resistant phenotype. Thus, at sites of arterial injury, the factor VLeidenmutation may not as readily predict arterial thrombosis, because the normal and variant platelet-derived cofactors are equally resistant to APC at the activated platelet surface.

Molecular Genetics of Factor V Leiden: Genetic Origins and Modern DNA-Based Detection Strategies

It has been acknowledged that a prothrombotic tendency may result from a specific genetic defect. Resistance to activated protein C (APC) (factor V Leiden) is now recognized as the most prevalent cause of increased thrombogenicity, being found in 2% to 5% of the world population. The APC-resistant phenotype arises from a well-characterized transitional mutation, resulting in an arginine(R)-506-glutamine(Q) amino acid substitution. Much remains to be uncovered concerning the importance of this mutation and how it relates to other conditions on a broader basis. Relevant and accurate detection methods that quickly identify the genetic lesion will play a major part in this strategy. This article details recent advances in identifying the factor V Leiden mutation by modern molecular techniques.

Activated protein C resistance in patients with central retinal vein occlusion

AIM/BACKGROUND

A new defect in the anticoagulant system has recently been discovered--activated protein C resistance. The frequency of this disorder has been shown to be increased in young patients (< 50 years of age) with central retinal vein occlusion. This study was carried out to determine if there was any overrepresentation of activated protein C resistance in patients > 50 years of age with central retinal vein occlusion.

METHODS

Blood samples were obtained from 83 patients > 50 years of age and with a history of central retinal vein occlusion. The blood samples were analysed for activated protein C resistance with standard clinical laboratory methods.

RESULTS

In this material 11% of the patients were resistant to activated protein C. The normal incidence of activated protein C resistance in the same geographical area is 10-11%.

CONCLUSION

Activated protein C resistance does not seem to be a cause of central retinal vein occlusion in people older than 50 years.

Dural puncture and activated protein C resistance: risk factors for cerebral venous sinus thrombosis

OBJECTIVES

Dural puncture is regarded a safe procedure when contraindications are carefully excluded and has so far not been recognised as a risk factor for cerebral venous sinus thrombosis (CVST). Five patients are described with CVST after dural puncture in the presence of additional risk factors.

METHODS

In four out of five patients complete investigations for thrombophilia were performed at least one month after withdrawal of oral anticoagulation.

RESULTS

In three out of four patients tested, activated protein C (APC) resistance due to heterozygous coagulation factor V R506Q mutation (factor V Leiden) was found. One patient was using oral contraceptives as a circumstantial risk factor and three had had spinal anaesthesia for surgical procedures. Family history of venous thromboembolism was negative in all patients. Retrospective evaluation of 66 patients with CVST disclosed that dural puncture was the fourth most common risk factor (8%) possibly contributing to thrombosis.

CONCLUSION

Dural puncture may constitute an additional risk factor for CVST especially in patients with APC resistance or surgery. In such patients a thrombophilia screen is indicated.

Activated protein C resistance due to a common factor V gene mutation is a major risk factor for venous thrombosis

Inherited resistance to activated protein C (APC) was recently discovered to be a cause of familial thrombophilia and is now known to be the most common genetic risk factor for venous thrombosis. It is caused by a single point mutation in the gene for factor V, which predicts substitution or arginine (R) at position 506 with a glutamine (Q). Accordingly, the activated form of mutated factor V (FVa:Q506) is more slowly degraded by activated protein C than normal FVa (FVa:R506) is, resulting in hypercoagulability and a lifelong 5- to 10-fold increased risk of venous thrombosis. Previously known inherited hypercoagulable states, i.e. deficiencies of the anticoagulant proteins antithrombin III, protein S, and protein C, are found fewer than 10-15% of thrombosis patients in western countries, whereas inherited APC resistance is present in 20-60% of such patients. The FV mutation is common in populations of Caucasian origin, with prevalences ranging from 1-15%, whereas it is not found in certain other ethnic groups such as Japanese and Chinese. The high prevalence of APC resistance, in combination with the availability of simple laboratory tests, will have a profound influence on the development of therapeutic and prophylactic regimens for thrombosis and will, it is hoped, result in a decreased incidence of thromboembolic events.

Familial thrombophilia: clinical and molecular analysis of Swedish families with inherited resistance to activated protein C or protein S deficiency

This report describes the characterization of Swedish families with inherited resistance to activated protein C (APC resistance) and/or protein S deficiency, two genetic disorders associated with functional impairment of the protein C anticoagulant pathway. The APC resistance phenotype was linked to the factor V gene locus in a kindred with independent inheritance of APC resistance and protein S deficiency. A point mutation changing Arg506 to a Gln (FV:Q506) in the factor V gene was the cause of APC resistance. In studies of 50 families with hereditary APC resistance, the FV:Q506 mutation was identified in 94% (47/50) of the families, and the thrombotic risk was found to be dependent on the factor V genotype. Moreover, 18 families with hereditary deficiency of free protein S were investigated. Type I protein S deficiency (low free and total protein S) and type III deficiency (low free but normal total protein S) coexisted in 78% (14/18) of the families, suggesting the two types to be phenotypic variants of the same genetic disorder. Deficiency of free protein S was caused by equimolar relationship between protein S and beta-chain containing isoforms of C4BP. Though protein S deficiency was a strong risk factor for thrombosis, the FV:Q506 mutation was identified as an additional genetic risk factor in 39% of the families. Thus, familial thrombophilia is a multiple gene disorder. The thrombophilic tendency associated with APC resistance or protein S deficiency was related to increased levels of prothrombin fragment 1 + 2, reflecting increased activation of the common coagulation pathway.