Do prothrombotic factors influence clinical phenotype of severe haemophilia? A review of the literature

Genotypes, phenotypes and whole genome sequence: Approaches from the My Life Our Future haemophilia project

INTRODUCTION

Information from the genes encoding factor VIII (F8) and IX (F9) is used in reproductive planning and to inform inhibitor formation, bleeding severity and response to therapies. Advances in technology and our understanding of the human genome now allows more comprehensive methods to study genomic variation and its impact on haemophilia.

AIMS

The My Life Our Future (MLOF) programme was begun in 2012 to provide genetic analysis and to expand research in haemophilia through a research repository.

METHODS

MLOF enrolled haemophilia A and B patients followed at haemophilia treatment centers in the U.S., including, since 2015, known and potential genetic carriers. Initial F8 and F9 DNA analysis was performed utilizing a next generation sequencing approach which allowed simultaneous detection of F8 inversions and other variants. Candidate variants were confirmed using a second method and multiplex ligation-dependent probe amplification was used to detect structural variants.

RESULTS

The initial phase of MLOF completed enrollment in December 2017 with 11,356 patients, genetic carriers, and potential carriers enrolled. In the 9453 subjects in whom analysis is complete, 687 unique previously unreported variants were found. Simultaneous sequencing of the F8 and F9 genes resulted in identification of non-deleterious variants previously reported as causative in haemophilia. DNA from 5141 MLOF subjects has undergone whole genome sequencing through the NHLBI TOPMed programme of the U.S. NIH.

CONCLUSION

MLOF has provided genetic information for patients and their families to help inform clinical care and has established a repository of data and biospecimens to further advance haemophilia research.

Novel therapies and current clinical progress in hemophilia A

The evolution of hemophilia treatment and care is a fascinating one but has been fraught with many challenges at every turn. Over the last 50 years or so patients with hemophilia and providers have witnessed great advances in the treatment of this disease. With these advances, there has been a dramatic decrease in the mortality and morbidity associated with hemophilia. Even with the remarkable advancements in treatment, however, new and old challenges continue to plague the hemophilia community. The cost of factor replacement and the frequency of infusions, especially in patients with severe hemophilia on prophylaxis, remains a significant challenge for this population. Other challenges include obtaining reliable venous access, especially in younger patients, and the development of neutralizing alloantibodies (inhibitors). The development of extended half-life products, a bispecific antibody which mimics the coagulation function of factor VIII (FVIII) and inhibition of anticoagulation proteins such as antithrombin with antibodies, aptamers or RNA interference technology have offered novel therapeutic approaches to overcome some of these existing challenges. Additionally, ongoing gene therapy research offers a way to possibly cure hemophilia. These novel treatment tools in conjunction with the establishment of an increasing number of comprehensive hemophilia centers and worldwide advocacy efforts have continued to push the progress of hemophilia care to new frontiers. This review highlights and summarizes these novel therapeutic approaches and the current clinical progress of hemophilia A.

Practical considerations in choosing a factor VIII prophylaxis regimen: Role of clinical phenotype and trough levels

Current therapy for haemophilia A is guided by severity of the disease, which in turn is best reflected in patients' endogenous factor VIII activity levels. For patients with severe haemophilia (particularly children), prophylaxis with continuous routine factor replacement has become standard of care in developed countries and is gradually becoming the standard of care in developing countries. The question arises then: what is an appropriate prophylaxis regimen to prevent bleeding events and arthropathy, while also maximizing patient quality of life and taking into consideration the costs of prophylaxis? Should all patients be treated with one standard, fixed prophylaxis regimen, or should prophylaxis be individualised for each patient? If so, what factors need to be considered in choosing the appropriate dose and frequency of factor administration? If prophylaxis is tailored to the individual patient, then patient-related factors (bleeding phenotype, activity profiles, age, joint status) and product-specific factors (half-life of the replacement factor in the individual patient) will determine the choice of regimen, whether it be a fixed-regimen prophylaxis or prophylaxis that is tailored to patient activity and bleeding risk. Regardless of the choice of prophylaxis regimen, for any regimen to be effective, adherence to therapy is key to optimising outcomes.

Clot stability as a determinant of effective factor VIII replacement in hemophilia A

Background

Factor VIII (FVIII) replacement is standard of care for patients with hemophilia A (HemA); however, patient response does not always correlate with FVIII levels. We hypothesize this may be in part due to the physical properties of clots and contributions of fibrin, platelets, and erythrocytes, which may be important for hemostasis.

Objective

To understand how FVIII contributes to effective hemostasis in terms of clot structure and mechanical properties.

Patients/Methods

In vitro HemA clots in human plasma or whole blood were analyzed using turbidity waveform analysis, confocal microscopy, and rheometry with or without added FVIII. In vivo clots from saphenous vein puncture in wild-type and HemA mice with varying FVIII levels were examined using scanning electron microscopy.

Results

FVIII profoundly affected HemA clot structure and physical properties; added FVIII converted the open and porous fibrin meshwork and low stiffness of HemA clots to a highly branched and dense meshwork with higher stiffness. Platelets and erythrocytes incorporated into clots modulated clot properties. The clots formed in the mouse saphenous vein model contained variable amounts of compressed erythrocytes (polyhedrocytes), fibrin, and platelets depending on the levels of FVIII, correlating with bleeding times. FVIII effects on clot characteristics were dose-dependent and reached a maximum at ~25% FVIII, such that HemA clots formed with this level of FVIII resembled clots from unaffected controls.

Conclusions

Effective clot formation can be achieved in HemA by replacement therapy, which alters the architecture of the fibrin network and associated cells, thus increasing clot stiffness and decreasing clot permeability.

Modifiers of clinical phenotype in severe congenital hemophilia

Patients with inherited hemophilia A and B usually exhibit a bleeding tendency of a severity proportional to the degree of plasmatic deficiency of the coagulant activity of factor VIII (FVIII:C) and factor IX (FIX:C). Although patients with severe hemophilia (i.e., with FVIII:C and FIX:C levels <1IU/dL) are generally those with the most severe bleeding phenotype, it is common experience that a variable proportion of them experiences a milder bleeding tendency. In this review, we summarize the current knowledge on the possible mechanisms at the basis of the phenotypic heterogeneity of severe hemophilia, focusing in particular on the role of FVIII/FIX gene mutations and thrombophilic polymorphisms. Finally, the possible therapeutic implications of such modifiers will be analyzed.

Impact of Prothrombotic Risk Factors in a Cohort of Egyptian Hemophilia A Patients

INTRODUCTION

Hemophilias are a group of related bleeding disorders that show an X-linked pattern of inheritance. The clinical phenotype of severe hemophilia may vary markedly among patients as a result of many factors, including genetic prothrombotic risk factors.

OBJECTIVES

Our objective was to study the incidence of the most common prothrombotic risk factors for additive effects among Egyptian patients with hemophilia A and their impact on clinical phenotype; annual bleeding frequency and severity of hemophilic arthropathy, as well as the effect of a single variation in these patients.

METHODS

This study was carried out in 100 patients with hemophilia A. Genotyping for factor V Leiden (FVL) G1691A, prothrombin G20210A, MTHFR C677T, and A1298C mutations was conducted using a real time-polymerase chain reaction (RT-PCR) assay.

RESULTS

Our study revealed mutations in hemophilia patients as follows: prothrombin G20210A (3 %), FVL (14 %), MTHFR C677T (42 %), and A1298C (59 %). Despite a lack of statistical significance when each gene was analysed separately, heterozygosity of prothrombin G20210A or FVL was always associated with either a mild or moderate, but never a severe, clinical presentation. The lowest bleeding frequency (less than once per month) was identified among patients with two heterozygous variants irrespective of the involved genes. In addition, the incidence of hemarthrosis was significantly higher among patients with a wild genotype of the prothrombin gene and FVL, and the average number of affected joints was significantly higher among patients with wild-type prothrombin and FVL genes than among heterozygous patients.

CONCLUSION

These prothrombotic mutations have a cumulative effect in amelioration of the severity of bleeding in hemophiliacs. The most prominent effect is that of prothrombin G20210A and FVL, while MTHFR C677A and A1298C gene mutations are less conclusive.

Factors that influence the bleeding phenotype in severe hemophilic patients

Hemophilia A and B are rare, X-linked bleeding disorders resulting from a partial or total deficiency of functionally active coagulation factor VIII or factor IX, respectively. Endogenous factor levels have traditionally been used to characterize the severity of the disorder, with severe hemophilia considered as circulating levels of factor less than 1% of normal. Identifying patients with severe hemophilia is essential to effective treatment, since these patients are at highest risk of spontaneous life or limb-threatening bleeding and disability resulting from repeated joint bleeding and are most likely to benefit from prophylaxis. However, there is variability in bleeding tendency, even among patients with severe hemophilia. This article will review potential modifiers of hemophilia-associated bleeding other than endogenous factor activity, which may influence bleeding tendencies and complications in hemophilic patients considered to have severe hemophilia. These potential modifiers include physiologic factors, such as elements of the hemostatic system; pathophysiologic factors, such as hemophilic arthropathy, associated inflammation, and angiogenesis; and others, such as seasonal variation, body weight, and physical activity.

Arterial and venous thrombosis in patients with von Willebrand’s disease: A critical review of the literature

All patients with von Willebrand's disease (vWD) who showed an arterial or venous thrombosis and were reported in the literature have been evaluated. 11 patients had arterial thrombosis while 19 had venous thrombosis for a total of 30 cases. 9 out the 11 cases with arterial thrombosis had myocardial infarction. Two had cerebral thrombosis. Associated risk factors for arterial thrombosis were available only for three patients who showed, respectively, smoking and dyslipidemia (2 cases) and smoking and intravenous desmopressin infusion (1 case). The majority of patients with venous thrombosis showed DVT with or without PE. Four patients presented with apparently isolated PE. In two instances thrombosis occurred in unusual sites (central retinal vein and portal vein, respectively). Several associated risk factors were present, mainly: infusion of FVIII or FVIII + vWF concentrates in 7 cases; surgery in 8 cases, pregnancy in 1, desmopressin infusion in 1, variable coagulation defects or polymorphisms in 5. More than one of these associated conditions were present in a few patients. The majority of vWD patients who showed thrombotic phenomena were type I patient, but in 6 cases were also type 3. The type of defect was not reported in 6 patients. As a conclusion of this review it seems safe to assume that both arterial and venous thrombosis appear rare in vWD. This is confirmed by the fact that arterial or venous thrombosis appears slightly more frequent in hemophilia A and B.

Thrombotic complications in von Willebrand disease

Thrombotic events occur rarely in patients with von Willebrand disease (VWD). In this review we analyze the data reported in the literature, selected through a PubMed search, on thrombotic complications in VWD patients. On the basis of this analysis, we conclude that thrombotic complications in VWD patients often have a multi-factorial pathogenesis resulting from a complex interaction between acquired (coagulation factor replacement, surgery, cardiovascular risk factors) and/or inherited (thrombophilic gene mutations) prothrombotic risk factors.

Recent advances in the development of coagulation factors and procoagulants for the treatment of hemophilia

Hemophilia is a family of rare bleeding disorders. The two primary types, hemophilia A and hemophilia B, are caused by recessive X-chromosome linked mutations that result in deficiency of coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Clinically, hemophilia is manifested by spontaneous bleeding, particularly into the joints (haemarthrosis) and soft tissue, and excessive bleeding following trauma or surgery. The total overall number of hemophilia patients worldwide is approximately 400,000, however only about 100,000 of these individuals are treated. The first treatment of hemophilia was initiated when it was determined that the clotting deficiency could be corrected by a plasma fraction taken from normal blood. The discovery of factor VIII enrichment by cryoprecipitation of plasma opened a new era of therapy which eventually led to the production of factor concentrates and the subsequent development of highly purified forms of plasma factors. The most significant improvements have been the availability of recombinant forms of factors VIII and IX. Unfortunately, recombinant factors still retain some of the limitations of plasma concentrates. These limitations include development of antibody responses in patients and the relatively short half-life of the molecules requiring frequent injection to maintain effective concentration. Treatment beyond replacement of native factors has been tried. They include the development of modified factor VIII and IX molecules with improved potency, stability and circulating half-life and enhancement of a prothrombotic responses and/or stabilization of coagulation factors via inhibition of key negative regulatory pathways. These approaches will be reviewed in this commentary.

Study of 18 functional hemostatic polymorphisms in mucocutaneous bleeding disorders

Hereditary disorders of primary hemostasis, characterized by mucocutaneous bleeding (MCB), are highly prevalent in children. Few cases are clearly monogenic, but the overwhelming majority are classified as mild bleeding disorders, with wide clinical and laboratory heterogeneity suggestive of complex polygenic diseases. In this framework, and by homology with venous thrombosis, some functional polymorphisms affecting the hemostatic system should be considered. We evaluated the role of 18 common hemostatic polymorphisms on the occurrence and severity of MCB in a case-control study including 269 patients and 286 matched controls consecutively recruited. FV Leiden was associated with milder bleeding severity, assessed by a standardized bleeding score (p = 0.013). Multivariate analysis revealed that three additional polymorphisms protected against MCB (F13 Leu34, OR = 0.66; 95% CI, 0.47-0.94; p = 0.024; VKORC1 1173T, OR = 0.59; 95% CI, 0.40-0.87; p = 0.009; and non-O blood group alleles, OR = 0.59; 95% CI, 0.41-0.86; p = 0.006). When combined, these polymorphisms showed an additive protection (OR = 0.24; 95% CI, 0.11-0.52), supporting the polygenic nature of MCB. Our data suggest that some common polymorphisms affecting hemostasis-related genes could protect from bleeding.

Polymorphism in factor VII gene modifies phenotype of severe haemophilia

The basis for 10-15% of patients with severe haemophilia having clinically mild disease is not fully understood. We hypothesized that polymorphisms in various coagulant factors may affect frequency of bleeding while functionally significant polymorphisms in inflammatory and immunoregulatory genes may also contribute to variations in the extent of joint damage. These variables were studied in patients with severe haemophilia, who were categorized as 'mild' (<5 bleeds in the preceding year, <10 World Federation of Haemophilia clinical and <10 Pettersson scores, n = 14) or 'severe' (all others, n = 100). A total of 53 parameters were studied in each individual for their association with the clinical severity. Age, F8:c activity and the incidence of thrombotic markers were comparable between the groups while the median number of bleeds, number of affected joints, clinical, radiological and functional joint scores (P < or = 0.001) and life-time clotting factor use (P < or = 0.007) were different. Patients with severe molecular defects had a 4.1-fold increased risk for a severe phenotype (95% CI: 1.18-14.42, P = 0.026) compared with other mutations. Of the polymorphisms studied, the FVII353Q (RR = 3.5, 95% CI: 1.04-12.05, P = 0.044) allele was associated with a severe phenotype. This data shows that apart from the F8/F9 genotype, functional polymorphisms in FVII gene affect the phenotype of patients with severe haemophilia.

Low plasma levels of tissue factor pathway inhibitor in patients with congenital factor V deficiency

Severe factor V (FV) deficiency is associated with mild to severe bleeding diathesis, but many patients with FV levels lower than 1% bleed less than anticipated. We used calibrated automated thrombography to screen patients with severe FV deficiency for protective procoagulant defects. Thrombin generation in FV-deficient plasma was only measurable at high tissue factor concentrations. Upon reconstitution of FV-deficient plasma with purified FV, thrombin generation increased steeply with FV concentration, reaching a plateau at approximately 10% FV. FV-deficient plasma reconstituted with 100% FV generated severalfold more thrombin than normal plasma, especially at low tissue factor concentrations (1.36 pM) or in the presence of activated protein C, suggesting reduced tissue factor pathway inhibitor (TFPI) levels in FV-deficient plasma. Plasma TFPI antigen and activity levels were indeed lower (P < .001) in FV-deficient patients (n = 11; 4.0 +/- 1.0 ng/mL free TFPI) than in controls (n = 20; 11.5 +/- 4.8 ng/mL), while persons with partial FV deficiency had inter-mediate levels (n = 16; 7.9 +/- 2.5 ng/mL). FV immunodepletion experiments in normal plasma and surface plasmon resonance analysis provided evidence for the existence of a FV/TFPI complex, possibly affecting TFPI stability/clearance in vivo. Low TFPI levels decreased the FV requirement for minimal thrombin generation in FV-deficient plasma to less than 1% and might therefore protect FV-deficient patients from severe bleeding.

The Impact of Prothrombotic Mutations on Factor Consumption in Adult Patients with Severe Hemophilia

About 10% of patients with severe hemophilia exhibit a milder clinical phenotype with less frequent bleeds. Among many other factors, coinheritance of prothrombotic mutations have been proposed to act as modulators of clinical severity in severe hemophilia. We conducted a study to evaluate the impact of 3 prothrombotic mutations (factor V Leiden, factor II, and methylenetetrahydrofolate reductase mutations) on clinical phenotype of patients with severe hemophilia in our institution. For this purpose we compared the average annual factor concentrate consumption between carriers and noncarriers of prothrombotic mutations. A total of 38 hemophilia A and B patients with factor levels less than 1 were recruited between October 2006 and October 2007. Prothrombotic mutations were detected in venous blood using polymerase chain reaction amplification technique. Eighteen patients (47%) carried no prothrombotic mutations. The remaining 20 patients (53%) were found to be carriers of either 1 or 2 mutations. Median age in both carrier and the non-carrier groups was 27 years. None of the patients in either group gave a history of thromboembolic event. Median annual factor concentrate consumptions in carriers and noncarriers were 610 ± 530 units/kg and 770 ± 670 units/kg, respectively ( P = .203). Our results demonstrated no significant difference in annual factor concentrate consumption between carriers and noncarriers of prothrombotic mutations. Considering that average annual factor consumption is a surrogate indicator of clinical phenotype, we concluded that coinheritance of prothrombotic mutations was not associated with occurrence of different clinical phenotypes in severe hemophilia.

The hemostatic balance revisited through the lessons of mankind evolution

Under physiologic conditions, a hemostatic balance is achieved through the effects of natural procoagulant and anticoagulant factors which, in equilibrium with each other, provide hemostasis at the sites of vascular injury. Abnormalities of these hemostasis factors can result in a tendency toward hemorrhagic or thrombotic events. In this review the influence of inherited prothrombotic risk factors--especially the more frequent factor V Leiden and prothrombin gene mutations--on normal and abnormal hemostasis is analyzed from an evolutionary point of view. The effect of inherited bleeding disorders on the development of thrombotic or atherosclerotic processes is also discussed.

Phenotypic heterogeneity in severe hemophilia

Large heterogeneity in bleeding pattern and arthropathy is observed among patients with severe hemophilia. Studies have reported a large variability in bleeding pattern among patients with severe hemophilia. Of special interest are some 10% of the patients with severe hemophilia who only rarely bleed and don't need prophylactic therapy. Prothrombotic risk factors seem to influence phenotype but they can account for only a small part of the heterogeneity. Half-lives for factor VIII (FVIII) range between 7 and 20 h; a significantly shorter half-life has been reported in patients with blood group O and a low von Willebrand antigen level. In addition, thrombin generation tests have been used to differentiate between mild and more severe phenotypes. As the advanced forms of these tests also measure the effects of platelets, it has been argued that they are more sensitive to differentiate phenotypes. We conclude that the origin of the large heterogeneity of phenotypes in severe hemophilia is multifactorial. As they produce no FVIII, patients with severe hemophilia and an intron 22 inversion are ideal candidates to study further bleeding variability. Until other parameters have been identified, the heterogeneity of the clinical phenotype may best be predicted by the first onset of the clinical features. At the moment, age at first joint bleed seems to be the most reliable factor to differentiate between phenotypes.

Genetics of coagulation: considerations for cardiac surgery

Genetic variants in the coagulation system have been known since antiquity. Today, because of modern improvements in diagnosis and medical management, the clinician is likely to encounter a spectrum of coagulation factor deficiencies and identified polymorphic variants in the surgical population. Because perioperative hemorrhagic and thrombotic complications are potentially serious, it is important to understand the role that these defects and variants may play in predicting risk and optimizing patient management. The implications of coagulation genetics on the perioperative management of the cardiac surgery patient are reviewed.

[Relevance of early paediatric care for boys with severe haemophilia]

Major molecular and genetic findings over the last decades and subsequent applications for diagnosis and therapeutic concerns have dramatically improved the evolution of severe haemophilia in countries with high economic resources. Another major factor of progress consisted in the setting of comprehensive care centres coordinating the care at a regional level. The early involvement of paediatricians for this rare and potentially serious chronic disease, which may be symptomatic from birth, is relevant in this context. Indeed, the early diagnosis of severe haemophilia and the involvement of expert caregivers in a multidisciplinary approach, are essential to make the acceptance of the disease easier. The diagnosis announcement should go together with a therapeutic project, which is nowadays based on long-term prophylaxis. Awaiting for likely curative treatments in the future, such as gene therapy, early implementation of prophylaxis and observance of this gold standard treatment during all the period of growth are critical to prevent the haemophilic arthropathy, to favour the future social and work-related integration and overall to improve the quality of life. The occurrence of an inhibitor represents the major residual complication of replacement therapy, especially for young children with severe haemophilia A. Even though new therapeutic resources brought substantial improvements for inhibitor patients, a better understanding of risk factors is a key issue since more accurate replacement regimen might induce tolerance during the first exposures and subsequently might prevent this complication. Prophylaxis and inhibitors that represent major concerns in paediatric care of severe haemophilia are included as main research objectives for the national registry " FranceCoag Network".

Non-catheter associated venous thrombosis in hemophilia A and B. A critical review of all reported cases

All reported cases of non-catheter induced venous thrombosis in patients with hemophilia A or B have been carefully evaluated. A total of 27 cases were reported,12 patients with hemophilia A and 15 patients with hemophilia B. The age of patients varied between 9 and 67 years. There were 10 cases of deep vein thrombosis, 8 patients with pulmonary embolism accompanied or not by deep vein thrombosis, 5 cases of superficial vein thrombosis. In addition, there were 3 cases of thrombosis in unusual sites (1 retinal central vein thrombosis and 2 portal vein thrombosis). Finally, in one case, venous thrombosis was multiple. There was a fatality in a hemophilia B patient with pulmonary embolism. The most frequent risk or triggering factor in hemophilia A was the administration of Feiba or rFVIIa concentrates in patients with inhibitors. Surgery together with Prothrombin Complex concentrates was the most frequent cause in hemophilia B patients. Congenital associated prothrombotic risk factors were present in two patients. No or very few therapeutic procedures were initiated in these patients but for a suspension or reduction of concentrates infusion. In a few instances low molecular weight heparin was given for a few days. The frequent association of venous thrombosis with infusion of concentrates indicates the need for a careful evaluation of patients about to receive such therapy.

High prevalence of inherited prothrombotic risk factors in 134 consecutive patients with von Willebrand disease

We screened 134 consecutive patients with von Willebrand disease (VWD) (106 type 1, 26 type 2, and 2 type 3 VWD) for the most important inherited prothrombotic risk factors. One hundred eight patients (80.6%) were positive for at least one of the prothrombotic risk factors screened for. A high prevalence of prothrombin G20210A (10.5%) and factor V Leiden (11.9%) mutations was found with allele frequencies of 5.2 and 6%, respectively. Three carriers of multiple prothrombotic gene mutations experienced a thrombotic event. Our study suggests that the recent evidence of an association between inherited thrombotic and hemorrhagic disorders also holds true in VWD patients.

Von Willebrand factor and thrombosis

There is increasing evidence that von Willebrand factor (VWF), an adhesive multimeric protein that has an important function in primary hemostasis and as a carrier of factor VIII, has a pivotal role in thrombogenesis. In fact, while the presence in plasma of unusually large VWF multimers due to a congenital or acquired deficiency of a VWF-cleaving metalloprotease has been implicated in the pathogenesis of thrombotic thrombocytopenic purpura (TTP), high plasma levels of VWF have been associated with a slightly increased risk of arterial thrombosis. With regard to the association between VWF and venous thrombosis, clear conclusions cannot yet be drawn from the conflicting published data. Patients with von Willebrand disease, an inherited hemorrhagic disorder, may also paradoxically experience thrombotic events as a result of interactions among multiple prothrombotic risk factors. After a description of the structure and physiology of VWF, all these aspects are discussed in the present review.

Thrombin generation and phenotypic correlation in haemophilia A

The clinical phenotype of patients with haemophilia A (HA) often differs between individuals with the same factor VIII (FVIII) gene defect (e.g. within the same family) or the same coagulant activity of FVIII (FVIII:C). We proposed that because the thrombin generation assay in platelet-poor plasma of HA patients provides more information [peak thrombin concentration, endogenous thrombin potential (ETP), rate of thrombin generation and lag-time] than a clot-based FVIII assay it might provide insight into these differences. We therefore investigated the relation between the results of the thrombin generation assay and the clinical severity in nine families with HA (23 patients with different phenotypes). We also examined the contribution of prothrombotic risk factors: (FV Leiden G1691A and prothrombin G20210A), the coagulant activity of FVIII and tissue factor (5'UTR) polymorphisms. Our data detect marked differences between individuals but these did not correlate with the reported clinical phenotype. These differences were also reflected in a marked difference in response to the therapeutic amounts of FVIII. This might account for differences in amounts of treatment consumption. Reduced peak and possibly rate of thrombin generation, rather than FVIII:C or ETP appear to represent the critical defects in FVIII-deficient plasma. We suggest that the analysis of parameters in thrombin generation is a useful tool to detect bleeding tendency in HA but not to predict the modulation of the haemorrhagic tendency in patients within families. However the presence of the other factors such as vessel wall components, protein C and platelets might need to be incorporated into this system.

Variability in clinical phenotype of severe haemophilia: the role of the first joint bleed

To quantify variation in clinical phenotype of severe haemophilia we performed a single centre cohort study among 171 severe haemophilia patients. Age at first joint bleed, treatment requirement (i.e. annual clotting factor use), annual bleeding frequency and arthropathy were documented. Because treatment strategies intensified during follow-up, patients were stratified in two age groups: patients born 1968-1985 (n = 91), or 1985-2002 (n = 80). A total of 2166 patient-years of follow-up were available (median 12.0 years per patient). Age at first joint bleed ranged from 0.2 to 5.8 years. Patients who had their first joint bleed later needed less treatment and developed less arthropathy. In patients born 1968-1985 during both on-demand and prophylactic treatment, the 75th percentile of annual joint bleed frequency was consistently four times as high as the 25th percentile. In both age groups variation in annual clotting factor use between 25th and 75th percentiles was 1.4-1.5 times for prophylaxis and 3.8 times for on-demand treatment. To conclude, the onset of joint bleeding is inversely related with treatment requirement and arthropathy and may serve as an indicator of clinical phenotype. Thus, providing a starting point for aetiological research and individualization of treatment.

Can long-term prophylaxis for severe haemophilia be stopped in adulthood? Results from Denmark and the Netherlands

Prophylaxis is the treatment of choice for children with severe haemophilia. As prophylaxis is especially important during the period of growth, the need for continued prophylaxis in adulthood should be considered. The aim of this study was to describe the incidence and outcome of stopping prophylaxis in patients with severe haemophilia who were offered prophylaxis during childhood. All patients with severe haemophilia (factor VIII/IX <0.01 IU/ml), born 1970-80, treated in two Danish and one Dutch treatment centre were studied. Data on discontinuation of prophylaxis, treatment, joint bleed frequency, clinical scores and radiological scores were collected. Eighty patients were studied. Median follow-up was 19 years (range 7-29). A total of 35% of patients discontinued prophylaxis at a median age of 21.5 years [interquartile range (IQR) 18.4-24.4], experiencing only three joint bleeds per year (IQR 1.4-8.7). Median clinical scores were similar in patients who discontinued prophylaxis [4 points (IQR 0-6)] and those who continued [3 points (IQR 1-6)], as were median Pettersson scores at 13 (IQR 1-24) vs. 13 points (IQR 5-23) respectively. In conclusion one-third of young adults with severe haemophilia on a prophylactic regimen discontinued prophylaxis in early adulthood, while maintaining a low joint bleed frequency and similar arthropathy after 4 years.

Haemophilia A: from mutation analysis to new therapies

Haemophilia is caused by hundreds of different mutations and manifests itself in clinical conditions of varying severity. Despite being inherited in monogenic form, the clinical features of haemophilia can be influenced by other genetic factors, thereby confounding the boundary between monogenic and multifactorial disease. Unlike sufferers of other genetic diseases, haemophiliacs can be treated successfully by intravenous substitution of coagulation factors. Haemophilia is also the most attractive model for developing gene-therapy protocols, as the normal life expectancy of haemophiliacs allows the side effects of gene therapy, as well as its efficiency, to be monitored over long periods.

Does activated protein C-resistant factor V contribute to thrombin generation in hemophilic plasma?

In this study we assessed the role of factor V (FV) inactivation in hemophilic plasma with particular reference to the activated protein C (APC)-resistant variants FV-R506Q (FV Leiden) and FV-R306T (FV Cambridge). Purified recombinant full-length FV carrying these single substitutions and FV-R306T/R506Q were used in thrombin generation experiments. Plasma was first immunodepleted of FV, and subsequently of factors VIII, IX, or combinations thereof. Thrombin generation was initiated by low concentrations of recombinant tissue factor. Recombinant soluble thrombomodulin (TM) was used to trigger the APC system. Surprisingly, TM concentrations that reduced thrombin generation in normal plasma by no more than 50% virtually abolished thrombin formation in plasma deficient in the factor VIII/IX complex. This was already apparent at TM levels as low as 0.1 nmol L(-1). By varying the concentrations of purified (activated) protein C to plasma that was additionally depleted of protein C, we confirmed that impaired thrombin generation indeed was the result of the action of APC. In contrast, this did not occur when FV-depleted plasma had been reconstituted with FV-R306T/R506Q. Addition of FV-R306T or FV-R506Q partially reduced prothrombin activation, demonstrating the involvement of both APC cleavage sites. FV inactivation also occurred on the surface of human microvascular endothelial cells. Apparently, these cells express sufficient TM to down-regulate thrombin production via the APC pathway. We further conclude that in hemophilic plasma this pathway can induce a secondary defect because of premature FV inactivation. It therefore seems conceivable that APC-resistant FV has the potential of alleviating hemophilic bleeding.