Type 2N von Willebrand disease: clinical manifestations, pathophysiology, laboratory diagnosis and molecular biology

Clinical relevance of genetic variants in the von Willebrand factor according to in-silico methods

Clinical interpretation of genetic variants in the context of the patient's phenotype is a time-consuming and costly process. In-silico analysis using in-silico prediction tools, and molecular modeling have been developed to predict the influence of genetic variants on the quality and/or quantity of the resulting translated protein, and in this way, to alert clinicians of disease likelihood in the absence of previous evidence. Our objectives were to evaluate the success rate of the in-silico analysis in predicting the disease-causing variants as pathogenic and the single-nucleotide variants as neutral, and to establish the reliability of in-silico analysis for determining pathogenicity or neutrality of von Willebrand factor gene-associated genetic variants. Using in-silico analysis, we studied pathogenicity in 31 disease-causing variants, and neutrality in 61 single-nucleotide variants from patients previously diagnosed as type 2 von Willebrand disease. Disease-causing variants and non-synonymous single-nucleotide variants were explored by in-silico tools that analyze the amino acidic sequence. Intronic and synonymous single-nucleotide variants were analyzed by in-silico methods that evaluate the nucleotidic sequence. We found a consistent agreement between predictions achieved by in-silico prediction tools and molecular modeling, both for defining the pathogenicity of disease-causing variants and the neutrality of single-nucleotide variants. Based on our results, the in-silico analysis would help to define the pathogenicity or neutrality in novel genetic variants observed in patients with clinical and laboratory phenotypes suggestive of von Willebrand disease.

Insights into the Molecular Genetic of Hemophilia A and Hemophilia B: The Relevance of Genetic Testing in Routine Clinical Practice

Hemophilia A and hemophilia B are rare congenital, recessive X-linked disorders caused by lack or deficiency of clotting factor VIII (FVIII) or IX (FIX), respectively. The severity of the disease depends on the reduction of coagulation FVIII or FIX activity levels, which is determined by the type of the pathogenic variants in the genes encoding the two factors (F8 and F9, respectively). Molecular genetic analysis is widely applied in inherited bleeding disorders. The outcome of genetic analysis allows genetic counseling of affected families and helps find a link between the genotype and the phenotype. Genetic analysis in hemophilia has tremendously improved in the last decades. Many new techniques and modifications as well as analysis softwares became available, which made the genetic analysis and interpretation of the data faster and more accurate. Advances in genetic variant detection strategies facilitate identification of the causal variants in up to 97% of patients. In this review, we discuss the milestones in genetic analysis of hemophilia and highlight the importance of identification of the causative genetic variants for genetic counseling and particularly for the interpretation of the clinical presentation of hemophilia patients.

A novel mouse model of type 2N VWD was developed by CRISPR/Cas9 gene editing and recapitulates human type 2N VWD

A novel type 2N VWD mouse model was established in which VWF is incapable of binding FVIII but is otherwise fully functional.

VWF^2N/2N mice exhibited a severe bleeding phenotype after tail tip amputation but not in lateral tail vein or ventral artery injury models.

Type 2N von Willebrand disease is caused by mutations in the factor VIII (FVIII) binding site of von Willebrand factor (VWF), resulting in dysfunctional VWF with defective binding capacity for FVIII. We developed a novel type 2N mouse model using CRISPR/Cas9 technology. In homozygous VWF^2N/2N mice, plasma VWF levels were normal (1167 ± 257 mU/mL), but the VWF was completely incapable of binding FVIII, resulting in 53 ± 23 mU/mL of plasma FVIII levels that were similar to those in VWF-deficient (VWF^−/−) mice. When wild-type human or mouse VWF was infused into VWF^2N/2N mice, endogenous plasma FVIII was restored, peaking at 4 to 6 hours post-infusion, demonstrating that FVIII expressed in VWF^2N mice is viable but short-lived unprotected in plasma due to dysfunctional 2N VWF. The whole blood clotting time and thrombin generation were impaired in VWF^2N/2N but not in VWF^−/− mice. Bleeding time and blood loss in VWF^2N/2N mice were similar to wild-type mice in the lateral tail vein or ventral artery injury model. However, VWF^2N/2N mice, but not VWF^−/− mice, lost a significant amount of blood during the primary bleeding phase after a tail tip amputation injury model, indicating that alternative pathways can at least partially restore hemostasis when VWF is absent. In summary, we have developed a novel mouse model by gene editing with both the pathophysiology and clinical phenotype found in severe type 2N patients. This unique model can be used to investigate the biological properties of VWF/FVIII association in hemostasis and beyond.

Emicizumab improves thrombus formation of type 2A von willebrand disease under high shear condition

INTRODUCTION

Type 2A von Willebrand disease (VWD) is common in type-2 group caused by qualitative deficiency of von Willebrand factor (VWF). Emicizumab is a bispecific antibody that mimics activated factor VIII (FVIIIa) cofactor function, and emicizumab prophylaxis substantially reduces bleeding in patients with haemophilia A. It is unknown whether emicizumab affects thrombus formation in type 2A VWD characterized by not only low FVIII levels but also the impaired platelet adhesion and aggregation.

AIM

To examine the coagulant potential of emicizumab in type 2A VWD.

PATIENTS/METHODS

Perfusion chamber experiments combined with immunostaining were performed using whole blood from 5 patients with type 2A VWD under high shear condition (2500 s^-1 ).

RESULTS

The addition of FVIII to type 2A VWD whole blood did not augment thrombus formation, whilst supplementation with VWF or FVIII/VWF enhanced. FVIII appeared to contribute to thrombus height rather than surface coverage. The addition of emicizumab enhanced thrombus formation in type 2A VWD compared with FVIII, but this potency was less than the presence of VWF. The effect on thrombus formation mediated by emicizumab appeared to be more rapid than that by FVIII for non-requirement of activation step of FVIII, whilst that by FVIII showed more impact on thrombus formation at the late phase.

CONCLUSION

Emicizumab-induced enhancing effects of thrombus formation, independent on VWF, may be useful as an alternative therapy for type 2A VWD patients. These results supported a critical role for the FVIII-VWF complex facilitating thrombus formation under high shear.

Are Iranian patients with von Willebrand disease type 2N properly differentiated from hemophilia A and do they receive appropriate treatment?

: The defect function of the von Willebrand factor (VWF) in carrying factor VIII (FVIII) leads to von Willebrand disease type 2N (VWD 2N) which could be easily misdiagnosed as hemophilia A. Differentiating of VWD 2N from hemophilia A is crucial for patient treatment and genetic counseling. As a retrospective study, we aimed to evaluate the current diagnostic work-up of Iranian patients with mild/moderate deficiency of FVIII levels and the possibility of misdiagnosis of VWD 2N as hemophilia A. All patients who referred to the reference coagulation laboratory at the Iranian Blood Transfusion Organization in a 10-months period for bleeding diathesis work-up with the request of FVIII activity level were included. Clinical and laboratory phenotypes including International Society on Thrombosis and Hemostasis - Bleeding Assessment Tool, FVIII activity, VWF antigen, VWF ristocetin cofactor, and FVIII binding capacity of VWF were assessed on suspected cases for VWD 2N. In total, the results of 896 patients for investigation of VWD 2N were evaluated and five new patients were identified within unrelated families with abnormal VWF:FVIIIB levels. Four were heterozygous for VWD 2N and one homozygous whom all were misdiagnosed as hemophilia A and underwent inappropriate treatments. The median bleeding score of the VWD 2N population was nine (4-13). In Iran, probably a significant number of VWD 2N patients are misdiagnosed as hemophilia A due to insufficient test panel for subtyping of von Willebrand disease. This study also emphasized the need for inclusion of the VWF:FVIIIB in suspected hemophilia A to achieve an optimal treatment strategy.

von Willebrand disease: Diagnosis and treatment, treatment of women, and genomic approach to diagnosis

von Willebrand disease (VWD) is the most common inherited bleeding disorder. VWD is caused by deficiencies in von Willebrand factor (VWF), a critical adhesive haemostatic protein. This review provides an overview of VWD diagnosis and treatment, special considerations in treating women with VWD, and current genomic approaches to VWD. For diagnosis and treatment in VWD, an accurate diagnosis is critical to providing effective treatments, determining appropriate laboratory monitoring and for counselling the patient and family. Diagnosis of VWD begins with the clinical assessment for the bleeding phenotype, which is usually characterized by mucocutaneous and provoked bleeding. The diagnosis of VWD is then made by laboratory investigation. Multiple assays are used to assess VWF levels and functions. The mainstays of VWD treatment are tailored by VWD type and symptoms, and can include antifibrinolytic treatment, desmopressin and VWF replacement treatment. Women with VWD are also at risk for excessive uterine bleeding, such as with menses and childbirth. In addition to standard VWD treatments, heavy menstrual bleeding can be treated with hormones. Interdisciplinary management of childbirth and prophylaxis in the postpartum period are needed to reduce the risk of postpartum haemorrhage. Genomic approaches to VWD can inform VWD diagnosis, treatment, test assay selection, reproductive planning and family counselling. Most VWD patients have an identifiable VWF gene DNA variant. Next-generation sequencing is rapidly being adopted to provide more comprehensive VWF sequence information for patients with known or suspected VWD.

Updated overview on von Willebrand disease: focus on the interest of genotyping

Introduction: Von Willebrand disease (VWD) is the most common inherited bleeding disorder, characterized by a quantitative or qualitative defect of von Willebrand factor (VWF), a multimeric glycoprotein crucial for primary hemostasis and coagulation. VWD pathophysiology is heterogeneous as it includes several types and subtypes which therapeutic management is different. The mainstays of VWD treatment are desmopressin and replacement therapy based on both plasma-derived concentrates and a recently developed recombinant VWF. VWD definitive diagnosis is achieved by a battery of phenotypic biologic assays and genotyping is currently performed mostly for research.Areas covered: This narrative review will firstly present a general overview on VWD epidemiology, pathophysiology, classification, clinics, phenotypic biologic diagnosis, and treatment. Secondly, a focus on VWD genotyping will be presented with specific emphasis on the evolution of its technical aspects, its applications for research dedicated to a better understanding of VWD pathophysiology and epidemiology and its interest in both a faster diagnosis and an optimal treatment of VWD.Expert opinion: Based on analysis of the literature, it can be concluded that the fast evolution of genetic techniques together with the development of innovating treatments may significantly change diagnostic flow charts for VWD and their use for specific and personalized treatment.

D' domain region Arg782-Cys799 of von Willebrand factor contributes to factor VIII binding

In the complex with von Willebrand factor (VWF) factor VIII (FVIII) is protected from rapid clearance from circulation. Although it has been established that the FVIII binding site resides in the N-terminal D'-D3 domains of VWF, detailed information about the amino acid regions that contribute to FVIII binding is still lacking. In the present study, hydrogen-deuterium exchange mass spectrometry was employed to gain insight into the FVIII binding region on VWF. To this end, time-dependent deuterium incorporation was assessed in D'-D3 and the FVIII-D'-D3 complex. Data showed reduced deuterium incorporation in the D' region Arg782-Cys799 in the FVIII-D'-D3 complex compared to D'-D3. This implies that this region interacts with FVIII. Site-directed mutagenesis of the six charged amino acids in Arg782-Cys799 into alanine residues followed by surface plasmon resonance analysis and solid phase binding studies revealed that replacement of Asp796 affected FVIII binding. A marked decrease in FVIII binding was observed for the D'-D3 Glu787Ala variant. The same was observed for D'-D3 variants in which Asp796 and Glu787 were replaced by Asn796 and Gln787. Site-directed mutagenesis of Leu786, which together with Glu787 and Cys789 forms a short helical region in the crystal structure of D'-D3, also had a marked impact on FVIII binding. The combined results show that the amino acid region Arg782-Cys799 is part of a FVIII binding surface. Our study provides new insight into FVIII-VWF complex formation and defects therein that may be associated with bleeding caused by markedly reduced levels of FVIII.

The endothelial lectin clearance receptor CLEC4M binds and internalizes factor VIII in a VWF-dependent and independent manner

Essentials CLEC4M is an endocytic receptor for factor FVIII. CLEC4M interacts with FVIII in a VWF-dependent and independent manner. CLEC4M binds to mannose-containing glycans on FVIII. CLEC4M internalization of FVIII involves clathrin coated pits. SUMMARY: Background von Willebrand factor (VWF) and factor VIII (FVIII) circulate in the plasma as a non-covalent complex, and the majority of FVIII is likely to be cleared by VWF-dependent pathways. Clearance of VWF-free FVIII is rapid and underlies the pathological basis of some quantitative FVIII deficiencies. The receptor pathways that regulate the clearance of VWF-bound and VWF-free FVIII are incompletely uncharacterized. The human liver-expressed endothelial lectin CLEC4M has been previously characterized as a clearance receptor for VWF, although its influence on FVIII is unknown. Objective The interaction between FVIII and CLEC4M was characterized in the presence or absence of VWF. Methods FVIII interactions with CLEC4M were evaluated by in vitro cell-based and solid phase binding assays. Interactions between FVIII and CLEC4M or liver sinusoidal endothelial cells were evaluated in vivo by immunohistochemistry. Results CLEC4M-expressing HEK 293 cells bound and internalized recombinant and plasma-derived FVIII through VWF-dependent and independent mechanisms. CLEC4M binding to recombinant FVIII was dependent on mannose-exposed N-linked glycans. CLEC4M mediated FVIII internalization via a clathrin-coated pit-dependent mechanism, resulting in transport of FVIII from early and late endosomes for catabolism by lysosomes. In vivo hepatic expression of CLEC4M after hydrodynamic liver transfer was associated with a decrease in plasma levels of endogenous murine FVIII:C in normal mice, whereas infused recombinant human FVIII was associated with sinusoidal endothelial cells in the presence or absence of VWF. Conclusions These findings suggest that CLEC4M is a novel clearance receptor that interacts with mannose-exposed glycans on FVIII in the presence or absence of VWF.

von Willebrand Disease: Diagnostic Strategies and Treatment Options

von Willebrand disease (VWD) is one of the most common inherited bleeding disorders. Since its first description in 1926, the diagnosis and management of VWD has significantly improved due to increasing scientific knowledge of the genetics and biology of von Willebrand factor (VWF). This article reviews the molecular structure and function of VWF as well as the clinical symptoms, laboratory-based diagnostic workup, and classification schema for VWD. It highlights current treatment options and state-of-the art research in VWF and VWD.

Application of a molecular diagnostic algorithm for haemophilia A and B using next-generation sequencing of entire F8, F9 and VWF genes

Currently, molecular diagnosis of haemophilia A and B (HA and HB) highlights the excess risk-inhibitor development associated with specific mutations, and enables carrier testing of female relatives and prenatal or preimplantation genetic diagnosis. Molecular testing for HA also helps distinguish it from von Willebrand disease (VWD). Next-generation sequencing (NGS) allows simultaneous investigation of several complete genes, even though they may span very extensive regions. This study aimed to evaluate the usefulness of a molecular algorithm employing an NGS approach for sequencing the complete F8, F9 and VWF genes. The proposed algorithm includes the detection of inversions of introns 1 and 22, an NGS custom panel (the entire F8, F9 and VWF genes), and multiplex ligation-dependent probe amplification (MLPA) analysis. A total of 102 samples (97 FVIII- and FIX-deficient patients, and five female carriers) were studied. IVS-22 screening identified 11 out of 20 severe HA patients and one female carrier. IVS-1 analysis did not reveal any alterations. The NGS approach gave positive results in 88 cases, allowing the differential diagnosis of mild/moderate HA and VWD in eight cases. MLPA confirmed one large exon deletion. Only one case did have no pathogenic variants. The proposed algorithm had an overall success rate of 99 %. In conclusion, our evaluation demonstrates that this algorithm can reliably identify pathogenic variants and diagnose patients with HA, HB or VWD.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Type 2N von Willebrand disease: Characterization and diagnostic difficulties

INTRODUCTION

An abnormal factor VIII (FVIII) binding capacity of von Willebrand factor (VWF) identifies type 2N von Willebrand disease (VWD). Type 2N VWD patients are identified by means of the VWF FVIII binding (VWF:FVIIIB) assay, and especially their VWF:FVIIIB/VWF:Ag ratio (VWF:FVIIIB ratio).

AIM

We report on our 15-year experience of diagnosing type 2N VWD.

METHODS

We have performed 2178 VWF:FVIIIB assays in bleeders and normal subjects.

RESULTS

von Willebrand factor (VWF):FVIIIB was reduced in 682, but only 60 had low VWF:FVIIIB ratios (<0.74). Among nine patients who had a VWF:FVIIIB ratio below 0.3, four had normal VWF levels and were homozygotes for the p.R854Q mutation; the other five had low VWF levels due to a quantitative VWF mutation combined with p.R854Q. The VWF:FVIIIB ratio ranged between 0.3 and 0.73 in 51 subjects; 34 of them were heterozygotes for the p.R854Q mutation, while one carried the p.R760C. The heterozygotes for type 2N included subjects with or without bleeding symptoms, the former with significantly lower mean VWF levels than the latter. Among the 116 normal subjects tested, six were heterozygotes for the p.R854Q mutation (all asymptomatic).

CONCLUSIONS

The prevalence of type 2N in our VWD cohort was 2.5%, and 5.2% of the general population in Northeast Italy was found heterozygous for the p.R854Q mutation. It might be difficult to reveal a type 2N defect using routine tests alone, especially when it is combined with a quantitative VWF mutation. Accordingly, we always recommend VWF:FVIIIB assay in the diagnostic workup of VWD.

To serve and protect: The modulatory role of von Willebrand factor on factor VIII immunogenicity

Hemophilia A is a bleeding disorder characterized by the absence or dysfunction of blood coagulation factor VIII (FVIII). Patients are treated with regular infusions of FVIII concentrate. In response to treatment, approximately 30% of patients with severe hemophilia A develop inhibitory antibodies targeting FVIII. Both patient and treatment related risk factors for inhibitor development have been described. Multiple studies comparing the immunogenicity of recombinant and plasma-derived FVIII have yielded conflicting results. The randomized controlled SIPPET (Survey of Inhibitors in Plasma-Product Exposed Toddlers) trial demonstrated an increased risk of inhibitor development of recombinant FVIII when compared to von Willebrand factor (VWF)-containing plasma-derived FVIII. Presently, it is unclear which mechanism underlies the reduced immunogenicity of plasma-derived FVIII. In this review we address the potential role of VWF on FVIII immunogenicity and we discuss how VWF affects the immune recognition, processing and presentation of FVIII. We also briefly discuss the potential impact of glycan-composition on FVIII immunogenicity. It is well established that VWF shields the uptake of FVIII by antigen presenting cells. We have recently shown that VWF binds to the surface of dendritic cells. Here, we present a novel model in which surface bound FVIII-VWF complexes regulate the internalization of FVIII. Binding of FVIII to VWF is critically dependent on sulfation of Tyr1699 (HVGS numbering) in the light chain of FVIII. Incomplete sulfation of Tyr1699 has been suggested to occur in several recombinant FVIII products resulting in a loss of VWF binding. We hypothesize that this results in alternative pathways of FVIII internalization by antigen presenting cells which are not regulated by VWF. This hypothetical mechanism may explain the reduced immunogenicity of VWF containing plasma-derived FVIII concentrates as found in the SIPPET study.

Pitfalls in the diagnosis of hemophilia severity: What to do?

Measurements of factor VIII coagulation activity (FVIII:C) may vary and result in misclassification of hemophilia A with delay in initiation of prophylactic treatment. We describe two young brothers who were diagnosed as moderate hemophilia patients and therefore not prophylactically treated with factor VIII concentrate despite frequent bleeding events. These findings emphasize the importance of (i) multiple measurements of FVIII:C by certified laboratories, (ii) adjustment of treatment when test results do not correspond to clinical symptoms, (iii) relevance of additional DNA mutation analysis in patients with hemophilia A, and (iv) treatment in centers with expertise.

A Laboratory Phenotype/Genotype Correlation of 1167 French Patients From 670 Families With von Willebrand Disease: A New Epidemiologic Picture

von Willebrand disease (VWD) is a genetic bleeding disease due to a defect of von Willebrand factor (VWF), a glycoprotein crucial for platelet adhesion to the subendothelium after vascular injury. VWD include quantitative defects of VWF, either partial (type 1 with VWF levels <50 IU/dL) or virtually total (type 3 with undetectable VWF levels) and also qualitative defects of VWF (type 2 variants with discrepant antigenic and functional VWF levels). The most bleeding forms of VWD usually do not concern type 1 patients with the mildest VWF defects (VWF levels between 30 and 50 IU/dL). The French reference center for VWD performed a laboratory phenotypic and genotypic analysis in 1167 VWD patients (670 families) selected by their basic biologic phenotype: type 3, type 2, and type 1 with VWF levels <30 IU/dL. In these patients indeed, to achieve an accurate diagnosis of VWD type and subtype is crucial for the management (treatment and genetic counseling). A phenotype/genotype correlation was present in 99.3% of cases; 323 distinct VWF sequence variations (58% of novel) were identified (missense 67% versus truncating 33%). The distribution of VWD types was: 25% of type 1, 8% of type 3, 66% of type 2 (2A: 18%, 2B: 17%, 2M: 19%, 2N: 12%), and 1% of undetermined type. Type 1 VWD was related either to a defective synthesis/secretion or to an accelerated clearance of VWF. In type 3 VWD, bi-allelic mutations of VWF were found in almost all patients. In type 2A, the most frequent mechanism was a hyper-proteolysis of VWF. Type 2B showed 85% of patients with deleterious mutations (distinct from type 2B New York). Type 2M was linked to a defective binding of VWF to platelet glycoprotein Ib or to collagen. Type 2N VWD included almost half type 2N/3. This biologic study emphasizes the complex mechanisms for both quantitative and qualitative VWF defects in VWD. In addition, this study provides a new epidemiologic picture of the most bleeding forms of VWD in which qualitative defects are predominant.

Current controversies in the diagnosis and management of von Willebrand disease

Von Willebrand disease (VWD) is the most common inherited bleeding disorder in the world. The spectrum of VWD spans quantitative and qualitative deficiencies of von Willebrand factor (VWF), a platelet adhesive protein. It manifests primarily as mucocutaneous bleeding, but severely affected patients may suffer soft tissue bleeding and hemarthroses. There is disagreement in the multiple guidelines published regarding diagnosis, especially of type 1 VWD, which also remains the most opaque with respect to molecular characterization. Treatment with desmopressin (DDAVP) is most effective in type 1 VWD, but regimens are not standardized. It is not clear which type 2 VWD patients with qualitative deficiencies can be treated with DDAVP and which ones should receive VWF concentrates. No guidelines stipulate which patients might benefit from prophylactic VWF infusions and how they should be dosed. These are some current controversies in VWD that are discussed in this review.

How I treat type 2 variant forms of von Willebrand disease

Type 2 von Willebrand disease (VWD) includes a wide range of qualitative abnormalities of von Willebrand factor structure and function resulting in a variable bleeding tendency. According to the current classification, 4 different subtypes can be identified, each with distinctive phenotypic and therapeutic characteristics. Current available laboratory methods allow a straightforward approach to VWD subtyping, and although the precise molecular characterization remains complex, it is not required for appropriate treatment of the vast majority of cases. Desmopressin can be useful only in a few type 2 cases compared with patients with actual quantitative deficiency (type 1), most often in variants with a nearly normal multimeric pattern (type 2M). However, since no laboratory test accurately predicts response to desmopressin, a trial test should always be performed in all type 2 VWD patients, with the exception of type 2B ones. Replacement therapy with plasma-derived von Willebrand factor-factor VIII concentrates represents the safe mainstay of treatment of all patients, particularly those not responding to desmopressin or requiring a sustained hemostatic correction because of major surgery or bleeding. A significant patient bleeding history correlates with increased bleeding risk and should be considered in tailoring the optimal antihemorrhagic prophylaxis in the individual patient.

FVIII stabilization: VWF D'D3 will do

In this issue of Blood, Yee et al1 have demonstrated that expression or infusion of a truncated von Willebrand factor (VWF) fragment containing the factor VIII (FVIII)-binding D′D3 region of VWF is sufficient to stabilize endogenous FVIII levels in VWF-deficient mice. In the absence of the carrier function of VWF, FVIII is susceptible to rapid proteolysis and clearance resulting in markedly reduced plasma levels of FVIII that contribute to a bleeding diathesis.

von Willebrand disease: advances in pathogenetic understanding, diagnosis, and therapy

von Willebrand disease (VWD) is the most common autosomally inherited bleeding disorder. The disease represents a range of quantitative and qualitative pathologies of the adhesive glycoprotein von Willebrand factor (VWF). The pathogenic mechanisms responsible for the type 2 qualitative variants of VWF are now well characterized, with most mutations representing missense substitutions influencing VWF multimer structure and interactions with platelet GPIbα and collagen and with factor VIII. The molecular pathology of type 3 VWD has been similarly well characterized, with an array of different mutation types producing either a null phenotype or the production of VWF that is not secreted. In contrast, the pathogenetic mechanisms responsible for type 1 VWD remain only partially resolved. In the hemostasis laboratory, the measurement of VWF:Ag and VWF:RCo are key components in the diagnostic algorithm for VWD, although the introduction of direct GPIbα-binding assays may become the functional assay of choice. Molecular genetic testing can provide additional benefit, but its utility is currently limited to type 2 and 3 VWD. The treatment of bleeding in VWD involves the use of desmopressin and plasma-derived VWF concentrates and a variety of adjunctive agents. Finally, a new recombinant VWF concentrate has just completed clinical trial evaluation and has demonstrated excellent hemostatic efficacy and safety.

von Willebrand disease: advances in pathogenetic understanding, diagnosis, and therapy

von Willebrand disease (VWD) is the most common autosomally inherited bleeding disorder. The disease represents a range of quantitative and qualitative pathologies of the adhesive glycoprotein von Willebrand factor (VWF). The pathogenic mechanisms responsible for the type 2 qualitative variants of VWF are now well characterized, with most mutations representing missense substitutions influencing VWF multimer structure and interactions with platelet GPIbα and collagen and with factor VIII. The molecular pathology of type 3 VWD has been similarly well characterized, with an array of different mutation types producing either a null phenotype or the production of VWF that is not secreted. In contrast, the pathogenetic mechanisms responsible for type 1 VWD remain only partially resolved. In the hemostasis laboratory, the measurement of VWF:Ag and VWF:RCo are key components in the diagnostic algorithm for VWD, although the introduction of direct GPIbα-binding assays may become the functional assay of choice. Molecular genetic testing can provide additional benefit, but its utility is currently limited to type 2 and 3 VWD. The treatment of bleeding in VWD involves the use of desmopressin and plasma-derived VWF concentrates and a variety of adjunctive agents. Finally, a new recombinant VWF concentrate has just completed clinical trial evaluation and has demonstrated excellent hemostatic efficacy and safety.

Advances in the diagnosis and management of von Willebrand disease

von Willebrand disease (VWD), the most common inherited bleeding disorder, is caused by a quantitative (type 1 and 3) or qualitative (type 2) defect of von Willebrand factor (VWF). In this review, the present knowledge regarding the diagnosis and the management of VWD is briefly analyzed.

Comparison of a new chemiluminescent immunoassay for von Willebrand factor activity with the ristocetin cofactor-induced platelet agglutination method

Measuring von Willebrand factor (VWF) activity is essential for the diagnosis of von Willebrand disease (VWD). The VWF activity is usually assessed based on measurement of the ristocetin cofactor (VWF:RCo). However, that test is technically challenging and has high intra- and inter-assay variabilities. A new automated chemiluminescent immunoassay VWF activity has recently become commercially available (HemosIL AcuStar von Willebrand Factor Ristocetin Cofactor Activity). The main objective of this study was to evaluate this new method and to compare it with the VWF:RCo assay as the reference method. We studied 91 samples, 18 healthy volunteers samples and 73 samples from patients (VWF:RCo level <50 IU dL(-1) ): 29 type 1 VWD, 13 type 2A, 5 type 2B, 5 type 2M, 3 type 2N, 5 type 3, 4 type 3 under treatment, 5 type 3 carriers and 4 samples with other pathologies. The HemosIL AcuStar VWF:RCo assay was 96% sensitive and 100% specific for detecting VWF abnormalities. The good analytical performance, and the sensitivity and specificity of HemosIL AcuStar VWF:RCo to detect VWF deficiency renders it a suitable method for VWD screening.

The molecular characterization of von Willebrand disease: good in parts

Since the cloning of the gene that encodes von Willebrand factor (VWF), 27 years ago, significant progress has been made in our understanding of the molecular basis of the most common inherited bleeding disorder, von Willebrand disease (VWD). The molecular pathology of this condition represents a range of genetic mechanisms, some of which are now very well characterized, and others that are still under investigation. In general, our knowledge of the molecular basis of type 2 and 3 VWD is now well advanced, and in some instances this information is being used to enhance clinical management. In contrast, our understanding of the molecular pathogenesis of the most common form of VWD, type 1 disease, is still at an early stage, with preliminary evidence that this phenotype involves a complex interplay between environmental factors and the influence of genetic variability both within and outside of the VWF locus.

The molecular genetics of von Willebrand disease

Quantitative and/or qualitative deficiency of von Willebrand factor (vWF) is associated with the most common inherited bleeding disease von Willebrand disease (vWD). vWD is a complex disease with clinical and genetic heterogeneity. Incomplete penetrance and variable expression due to genetic and environmental factors contribute to its complexity. vWD also has a complex molecular pathogenesis. Some vWF gene mutations are associated with the affected vWF biosynthesis and multimerization, whereas others are associated with increased clearance and functional impairment. Moreover, in addition to a particular mutation, type O blood may result in the more severe phenotype. The present review aimed to provide a summary of the current literature on the molecular genetics of vWD.

Conflict of interest:None declared.

A common ancestor more than 10,000 years old for patients with R854Q-related type 2N von Willebrand's disease in Italy

The impaired capacity of von Willebrand factor to carry factor VIII is identified as type 2N von Willebrand's disease. R854Q is the most common type 2N mutation, and almost the only one identified in Italy. This aim of this study was to ascertain whether R854Q mutations in a cohort of Italian patients with type 2N von Willebrand's disease originated from a single event or recurrent events. Thirteen unrelated Italian families were investigated, analyzing the von Willebrand factor gene haplotype associated with the R854Q mutation. A common haplotype emerged in all the families, extending from single nucleotide polymorphisms rs2166902 to rs216293 over 48.2 kb and including five intragenic markers. This haplotype is infrequent in the healthy Italian population (17% versus 100%, P<0.0001) and each genetic marker within the said haplotype is similarly rare. These data strongly suggest a founder effect, with a single R854Q mutation event being the cause of the type 2N von Willebrand's disease in our cohort of patients. Using DMLE+ software and the mathematical model of Bengtsson and Thomson, it was estimated that the R854Q mutation occurred from 10,000 to 40,000 years ago, which is consistent with the short dimension of the haplotype shared by our patients. Together with the fact that the R854Q mutation seems to be limited to Caucasian populations, these findings suggest that a single mutational event took place after human populations moved from Africa towards Europe.

Therapeutic consequences for misdiagnosis of type 2N von Willebrand disease

Patients presenting with a low FVIII:C and with normal VWF levels are usually presumed to have hemophilia (males) or be carriers for hemophilia (females). Some of these patients may instead have VWD:2N. Such patients if misdiagnosed are likely to suffer from insufficiently treated bleeds. We report 2 males and 1 female who presented with a low FVIII:C (1-21%) and minimally reduced/normal VWF and were assumed to have, or be a carrier for, hemophilia A. Eventually all were found to have VWD:2N. Prior to the correct diagnosis the males had been treated with rFVIII with poor responses and ultimately adverse clinical consequences.

von Willebrand disease

von Willebrand disease is a common inherited bleeding disorder characterized by excessive mucocutaneous bleeding. Characteristic bleeding symptoms include epistaxis, easy bruising, oral cavity bleeding, menorrhagia, bleeding after dental extraction, surgery, and/or childbirth, and in severe cases, bleeding into joints and soft tissues. There are three subtypes: types 1 and 3 represent quantitative variants and type 2 is a group of four qualitative variants: (1) type 2A-characterized by defective von Willebrand factor-dependent platelet adhesion because of decreased high-molecular-weight von Willebrand factor multimers, (2) type 2B-caused by pathologically increased von Willebrand factor-platelet interactions, (3) type 2M-caused by decreased von Willebrand factor-platelet interactions not based on the loss of high-molecular-weight multimers, and (4) type 2N-characterized by reduced binding of von Willebrand factor to factor VIII. The diagnosis of von Willebrand disease requires specialized assays of von Willebrand factor and/or molecular genetic testing of von Willebrand factor. Severe bleeding episodes can be prevented or controlled with intravenous infusions of virally inactivated plasma-derived clotting factor concentrates containing both von Willebrand factor and factor VIII. Depending on the von Willebrand disease type, mild bleeding episodes usually respond to intravenous or subcutaneous treatment with desmopressin, a vasopressin analog. Other treatments that can reduce symptoms include fibrinolytic inhibitors and hormones for menorrhagia.

Genetic variation associated with plasma von Willebrand factor levels and the risk of incident venous thrombosis

In a recent genome-wide association study, variants in 8 genes were associated with VWF level, a risk factor for venous thrombosis (VT). In an independent, population-based, case-control study of incident VT, we tested hypotheses that variants in these genes would be associated with risk. Cases were 656 women who experienced an incident VT, and controls comprised 710 women without a history of VT. DNA was obtained from whole blood. Logistic regression was used to test associations between incident VT and single nucleotide polymorphisms (SNPs) in 7 genes not previously shown to be associated with VT. Associations with P < .05 were candidates for replication in an independent case-control study of VT in both sexes. Two of the 7 SNPs tested yielded P < .05: rs1039084 (P = .005) in STXBP5, a novel candidate gene for VT, and rs1063856 (P = .04) in VWF, a gene whose protein level is associated with VT risk. Association results for the remaining 5 variants in SCARA5, STAB2, STX2, TC2N, and CLEC4M were not significant. Both STXBP5 and VWF findings were replicated successfully. Variation in genes associated with VWF levels in the genome-wide association study was found to be independently associated with incident VT.

Validation of the first commercial ELISA for type 2N von Willebrand's disease diagnosis

Type 2N von Willebrand's disease (VWD) is characterized by a factor VIII (FVIII) deficiency and a low FVIII/VWF ratio related to a markedly decreased affinity of von Willebrand factor (VWF) to FVIII. Type 2N VWD is diagnosed using assays allowing the measurement of plasma VWF capacity to bind FVIII (VWF:FVIIIB). These assays, crucial in order to distinguish type 2N VWD patients from mild haemophiliacs A and haemophilia A carriers, remain exclusively homemade and limited to laboratories possessing a high level of expertise in VWD. We evaluated the first commercial ELISA (Asserachrom® VWF:FVIIIB; Stago) comparated to a reference method in a multicentric study involving 205 subjects: 60 healthy volunteers, 37 haemophiliacs A, 17 haemophilia A carriers, 37 patients with type 2N VWD, 9 heterozygous carriers for a 2N mutation and 45 patients with miscellaneous other types of VWD (all previously characterized). A diluted plasma sample adjusted to 10 IU dL(-1) of VWF:Ag was incubated with a rabbit antihuman VWF polyclonal antibody. After removing the endogenous FVIII, recombinant FVIII (rFVIII) was added and bound rFVIII was quantified using a peroxydase-conjugated mouse antihuman FVIII monoclonal antibody. The intra-assay and inter-assay reproducibility was satisfactory. In all subgroups, both methods were well correlated. All type 2N VWD patients exhibited a markedly decreased VWF:FVIIIB (lower than 15%) and all heterozygous 2N carriers had a moderately decreased VWF:FVIIIB (between 30% and 65%). All controls (healthy subjects, haemophiliacs A and haemophilia A carriers) had a normal VWF:FVIIIB (higher than 80%) except one healthy volunteer and three haemophiliacs who exhibited a moderately decreased VWF:FVIIIB suggesting a heterozygous status for a 2N mutation. In conclusion, the Asserachrom® VWF:FVIIIB is easy to perform, standardized and accurate for type 2N VWD diagnosis with a 100% sensitivity and specificity.

Homozygous type 2N R854W von Willebrand factor is poorly secreted and causes a severe von Willebrand disease phenotype

BACKGROUND

von Willebrand disease (VWD) type Normandy (VWD 2N) is caused by mutations at the factor (F)VIII-binding site of von Willebrand factor (VWF), located in the D'and D3 domains on the N-terminus of mature VWF. The R854Q mutation is the most frequent cause of this phenotype.

OBJECTIVES

We report the characterization of a homozygous VWD 2N mutation, R854W, detected in a patient with a severe VWD phenotype.

METHODS

The plasma VWF phenotype was studied, transient expression of recombinant mutant full-length VWF in 293 EBNA cells was performed, and the results were compared with those obtained with wild-type (WT) VWF. Furthermore, expression was also examined in HEK293 cells, which form Weibel-Palade body-like granules when transfected with WT VWF.

RESULTS

The multimer analysis of plasma VWF showed the lack of the typical triplet structure, with the presence of the central band only, and a relative decrease in the high molecular mass multimers. Homozygous expression of recombinant R854W VWF resulted in normal amounts of cellular VWF, but with a severe reduction in secretion into the medium. Severe reductions in FVIII binding to R854W VWF, glycoprotein Ib binding activity and collagen binding of secreted W854 VWF was observed, and reproduced the phenotypic parameters of plasma VWF. In HEK293 cells, homozygous R854W VWF failed to form Weibel-Palade body-like granules.

CONCLUSIONS

Our results demonstrate that a homozygous R854W mutation in the D' domain of VWF induces impaired secretion and activity of the protein, thereby explaining the severe phenotype of the patient.

Novel associations of multiple genetic loci with plasma levels of factor VII, factor VIII, and von Willebrand factor: The CHARGE (Cohorts for Heart and Aging Research in Genome Epidemiology) Consortium

BACKGROUND

Plasma levels of coagulation factors VII (FVII), VIII (FVIII), and von Willebrand factor (vWF) influence risk of hemorrhage and thrombosis. We conducted genome-wide association studies to identify new loci associated with plasma levels.

METHODS AND RESULTS

The setting of the study included 5 community-based studies for discovery comprising 23 608 European-ancestry participants: Atherosclerosis Risk In Communities Study, Cardiovascular Health Study, British 1958 Birth Cohort, Framingham Heart Study, and Rotterdam Study. All subjects had genome-wide single-nucleotide polymorphism (SNP) scans and at least 1 phenotype measured: FVII activity/antigen, FVIII activity, and vWF antigen. Each study used its genotype data to impute to HapMap SNPs and independently conducted association analyses of hemostasis measures using an additive genetic model. Study findings were combined by meta-analysis. Replication was conducted in 7604 participants not in the discovery cohort. For FVII, 305 SNPs exceeded the genome-wide significance threshold of 5.0x10(-8) and comprised 5 loci on 5 chromosomes: 2p23 (smallest P value 6.2x10(-24)), 4q25 (3.6x10(-12)), 11q12 (2.0x10(-10)), 13q34 (9.0x10(-259)), and 20q11.2 (5.7x10(-37)). Loci were within or near genes, including 4 new candidate genes and F7 (13q34). For vWF, 400 SNPs exceeded the threshold and marked 8 loci on 6 chromosomes: 6q24 (1.2x10(-22)), 8p21 (1.3x10(-16)), 9q34 (<5.0x10(-324)), 12p13 (1.7x10(-32)), 12q23 (7.3x10(-10)), 12q24.3 (3.8x10(-11)), 14q32 (2.3x10(-10)), and 19p13.2 (1.3x10(-9)). All loci were within genes, including 6 new candidate genes, as well as ABO (9q34) and VWF (12p13). For FVIII, 5 loci were identified and overlapped vWF findings. Nine of the 10 new findings were replicated.

CONCLUSIONS

New genetic associations were discovered outside previously known biological pathways and may point to novel prevention and treatment targets of hemostasis disorders.

Clinical and laboratory versus molecular markers for a correct classification of von Willebrand disease

von Willebrand disease (VWD) is the most common inherited bleeding disorder. In this perspective article, Drs. Federici and Canciani review how correlating the clinical, laboratory and genetic features of von Willebrand disease has led to improved understanding of its pathophysiology. They show how this has rationalized classification, which in turn can be used to improve treatment of these patients.

Optimizing treatment of von Willebrand disease by using phenotypic and molecular data

The wide clinical spectrum of von Willebrand disease (VWD), its complex pathophysiology and its classification into distinct quantitative (type 1 or type 3) and qualitative (type 2) types with further subtle distinctions have prevented most clinicians from establishing a straightforward approach to diagnosing and treating this inherited bleeding disorder. The results of studies involving large cohorts of patients with a wide range of bleeding manifestations and variable von Willebrand factor (VWF) reduction have recently become available. These data have allowed the proposal of minimal criteria for a clinically useful diagnosis and for differentiating patients with mild VWD from subjects with borderline or only slightly reduced VWF levels who will not benefit from a specific diagnosis. These criteria are based on measurement of VWF ristocetin cofactor (VWF:RCo), VWF antigen (VWF:Ag), factor VIII and a standardized bleeding score (BS). Demonstration of the inheritance of the disorder could help to classify patients for whom insufficient hemostatic challenges may produce a falsely reassuring BS (like in children). Using this approach, mild VWD appears to be mostly composed of type 1 cases. Complemented by the results of desmopressin trial infusion, these parameters form the basis for a clinically oriented classification of all forms of VWD and may be useful for selecting the best treatment according to the severity of the disease. Although few molecular data have revealed practical utility, there is no doubt that the clarification of the molecular pathophysiology of VWD has allowed the unification of this complex disorder into a simple conceptual framework. This framework underlies the proposed utilization of simple phenotypic markers for optimizing treatments in individual patients.

The Role Of von Willebrand Factor In Hemorrhagic And Thrombotic Disorders

von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion as well as platelet aggregation at sites of vascular injury and acts as a carrier of factor VIII. Although acquired or inherited VWF deficiency is associated with a bleeding tendency, there is increasing evidence that VWF has a pivotal role in thrombogenesis. In fact, while the presence in the 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, high plasma levels of VWF have been associated with an increased risk of both arterial and venous thrombosis. The role of VWF in normal and pathological hemostasis is discussed in this review, and important advances in the pathophysiology, diagnosis, and treatment of VWF-associated disorders are also described.