A monoclonal antibody to von Willebrand factor (vWF) inhibits factor VIII binding. Localization of its antigenic determinant to a nonadecapeptide at the amino terminus of the mature vWF polypeptide

Phage display broadly identifies inhibitor-reactive regions in von Willebrand factor

BACKGROUND

Correction of von Willebrand factor (VWF) deficiency with replacement products containing VWF can lead to the development of anti-VWF alloantibodies (i.e., VWF inhibitors) in patients with severe von Willebrand disease (VWD).

OBJECTIVE

Locate inhibitor-reactive regions within VWF using phage display.

METHODS

We screened a phage library displaying random, overlapping fragments covering the full-length VWF protein sequence for binding to a commercial anti-VWF antibody or to immunoglobulins from three type 3 VWD patients who developed VWF inhibitors in response to treatment with plasma-derived VWF. Immunoreactive phage clones were identified and quantified by next-generation DNA sequencing (NGS).

RESULTS

Next-generation DNA sequencing markedly increased the number of phages analyzed for locating immunoreactive regions within VWF following a single round of selection and identified regions not recognized in previous reports using standard phage display methods. Extending this approach to characterize VWF inhibitors from three type 3 VWD patients (including two siblings homozygous for the same VWF gene deletion) revealed patterns of immunoreactivity distinct from the commercial antibody and between unrelated patients, though with notable areas of overlap. Alloantibody reactivity against the VWF propeptide is consistent with incomplete removal of the propeptide from plasma-derived VWF replacement products.

CONCLUSION

These results demonstrate the utility of phage display and NGS to characterize diverse anti-VWF antibody reactivities.

Conformation-dependent blockage of activated VWF improves outcomes of traumatic brain injury in mice

Traumatic brain injury-induced coagulopathy (TBI-IC) causes life-threatening secondary intracranial bleeding. Its pathogenesis differs mechanistically from that of coagulopathy arising from extracranial injuries and hemorrhagic shock, but it remains poorly understood. We report results of a study designed to test the hypothesis that von Willebrand factor (VWF) released during acute TBI is intrinsically hyperadhesive because its platelet-binding A1-domain is exposed and contributes to TBI-induced vascular leakage and consumptive coagulopathy. This hyperadhesive VWF can be selectively blocked by a VWF A2-domain protein to prevent TBI-IC and to improve neurological function with a minimal risk of bleeding. We demonstrated that A2 given through intraperitoneal injection or IV infusion reduced TBI-induced death by >50% and significantly improved the neurological function of C57BL/6J male mice subjected to severe lateral fluid percussion injury. A2 protected the endothelium from extracellular vesicle-induced injury, reducing TBI-induced platelet activation and microvesiculation, and preventing a TBI-induced hypercoagulable state. A2 achieved this therapeutic efficacy by specifically blocking the A1 domain exposed on the hyperadhesive VWF released during acute TBI. These results suggest that VWF plays a causal role in the development of TBI-IC and is a therapeutic target for this life-threatening complication of TBI.

Physiological Roles of the von Willebrand Factor-Factor VIII Interaction

Von Willebrand factor (VWF) and coagulation factor VIII (FVIII) circulate as a complex in plasma and have a major role in the hemostatic system. VWF has a dual role in hemostasis. It promotes platelet adhesion by anchoring the platelets to the subendothelial matrix of damaged vessels and it protects FVIII from proteolytic degradation. Moreover, VWF is an acute phase protein that has multiple roles in vascular inflammation and is massively secreted from Weibel-Palade bodies upon endothelial cell activation. Activated FVIII on the other hand, together with coagulation factor IX forms the tenase complex, an essential feature of the propagation phase of coagulation on the surface of activated platelets. VWF deficiency, either quantitative or qualitative, results in von Willebrand disease (VWD), the most common bleeding disorder. The deficiency of FVIII is responsible for Hemophilia A, an X-linked bleeding disorder. Here, we provide an overview on the role of the VWF-FVIII interaction in vascular physiology.

Mice deficient in the anti-haemophilic coagulation factor VIII show increased von Willebrand factor plasma levels

Von Willebrand factor (VWF) is the carrier protein of the anti-haemophilic Factor VIII (FVIII) in plasma. It has been reported that the infusion of FVIII concentrate in haemophilia A patients results in lowered VWF plasma levels. However, the impact of F8-deficiency on VWF plasma levels in F8^-/y mice is unresolved. In order to avoid confounding variables, we back-crossed F8-deficient mice onto a pure C57BL/6J background and analysed VWF plasma concentrations relative to C57BL/6J WT (F8^+/y) littermate controls. F8^-/y mice showed strongly elevated VWF plasma concentrations and signs of hepatic inflammation, as indicated by increased TNF-α, CD45, and TLR4 transcripts and by elevated macrophage counts in the liver. Furthermore, immunohistochemistry showed that expression of VWF antigen was significantly enhanced in the hepatic endothelium of F8^-/y mice, most likely resulting from increased macrophage recruitment. There were no signs of liver damage, as judged by glutamate-pyruvate-transaminase (GPT) and glutamate-oxalacetate-transaminase (GOT) in the plasma and no signs of systemic inflammation, as white blood cell subsets were unchanged. As expected, impaired haemostasis was reflected by joint bleeding, prolonged in vitro clotting time and decreased platelet-dependent thrombin generation. Our results point towards a novel role of FVIII, synthesized by the liver endothelium, in the control of hepatic low-grade inflammation and VWF plasma levels.

Acquired von Willebrand syndrome associated with left ventricular assist device

Left ventricular assist devices (LVAD) provide cardiac support for patients with end-stage heart disease as either bridge or destination therapy, and have significantly improved the survival of these patients. Whereas earlier models were designed to mimic the human heart by producing a pulsatile flow in parallel with the patient's heart, newer devices, which are smaller and more durable, provide continuous blood flow along an axial path using an internal rotor in the blood. However, device-related hemostatic complications remain common and have negatively affected patients' recovery and quality of life. In most patients, the von Willebrand factor (VWF) rapidly loses large multimers and binds poorly to platelets and subendothelial collagen upon LVAD implantation, leading to the term acquired von Willebrand syndrome (AVWS). These changes in VWF structure and adhesive activity recover quickly upon LVAD explantation and are not observed in patients with heart transplant. The VWF defects are believed to be caused by excessive cleavage of large VWF multimers by the metalloprotease ADAMTS-13 in an LVAD-driven circulation. However, evidence that this mechanism could be the primary cause for the loss of large VWF multimers and LVAD-associated bleeding remains circumstantial. This review discusses changes in VWF reactivity found in patients on LVAD support. It specifically focuses on impacts of LVAD-related mechanical stress on VWF structural stability and adhesive reactivity in exploring multiple causes of AVWS and LVAD-associated hemostatic complications.

A von Willebrand factor fragment containing the D'D3 domains is sufficient to stabilize coagulation factor VIII in mice

Plasma factor VIII (FVIII) and von Willebrand factor (VWF) circulate together as a complex. We identify VWF fragments sufficient for FVIII stabilization in vivo and show that hepatic expression of the VWF D'D3 domains (S764-P1247), either as a monomer or a dimer, is sufficient to raise FVIII levels in Vwf(-/-) mice from a baseline of ∼5% to 10%, to ∼50% to 100%. These results demonstrate that a fragment containing only ∼20% of the VWF sequence is sufficient to support FVIII stability in vivo. Expression of the VWF D'D3 fragment fused at its C terminus to the Fc segment of immunoglobulin G1 results in markedly enhanced survival in the circulation (t1/2 > 7 days), concomitant with elevated plasma FVIII levels (>25% at 7 days) in Vwf(-/-) mice. Although the VWF D'D3-Fc chimera also exhibits markedly prolonged survival when transfused into FVIII-deficient mice, the cotransfused FVIII is rapidly cleared. Kinetic binding studies show that VWF propeptide processing of VWF D'D3 fragments is required for optimal FVIII affinity. The reduced affinity of VWF D'D3 and VWF D'D3-Fc for FVIII suggests that the shortened FVIII survival in FVIII-deficient mice transfused with FVIII and VWF D'D3/D'D3-Fc is due to ineffective competition of these fragments with endogenous VWF for FVIII binding.

Visualization of a protein-protein interaction at a single-molecule level by atomic force microscopy

Atomic force microscopy is unmatched in terms of high-resolution imaging under ambient conditions. Over the years, substantial progress has been made using this technique to improve our understanding of biological systems on the nanometer scale, such as visualization of single biomolecules. For monitoring also the interaction between biomolecules, in situ high-speed imaging is making enormous progress. Here, we describe an alternative ex situ imaging method where identical molecules are recorded before and after reaction with a binding partner. Relocation of the identical molecules on the mica surface was thereby achieved by using a nanoscale scratch as marker. The method was successfully applied to study the complex formation between von Willebrand factor (VWF) and factor VIII (FVIII), two essential haemostatic components of human blood. FVIII binding was discernible by an appearance of globular domains appended to the N-terminal large globular domains of VWF. The specificity of the approach could be demonstrated by incubating VWF with FVIII in the presence of a high salt buffer which inhibits the interaction between these two proteins. The results obtained indicate that proteins can maintain their reactivity for subsequent interactions with other molecules when gently immobilized on a solid substrate and subjected to intermittent drying steps. The technique described opens up a new analytical perspective for studying protein-protein interactions as it circumvents some of the obstacles encountered by in situ imaging and other ex situ techniques.

Influence of single nucleotide polymorphisms in factor VIII and von Willebrand factor genes on plasma factor VIII activity: the ARIC Study

Factor VIII (FVIII) functions as a cofactor for factor IXa in the contact coagulation pathway and circulates in a protective complex with von Willebrand factor (VWF). Plasma FVIII activity is strongly influenced by environmental and genetic factors through VWF-dependent and -independent mechanisms. Single nucleotide polymorphisms (SNPs) of the coding and promoter sequence in the FVIII gene have been extensively studied for effects on FVIII synthesis, secretion, and activity, but impacts of non-disease-causing intronic SNPs remain largely unknown. We analyzed FVIII SNPs and FVIII activity in 10,434 healthy Americans of European (EA) or African (AA) descent in the Atherosclerosis Risk in Communities (ARIC) study. Among covariates, age, race, diabetes, and ABO contributed 2.2%, 3.5%, 4%, and 10.7% to FVIII intersubject variation, respectively. Four intronic FVIII SNPs associated with FVIII activity and 8 with FVIII-VWF ratio in a sex- and race-dependent manner. The FVIII haplotypes AT and GCTTTT also associated with FVIII activity. Seven VWF SNPs were associated with FVIII activity in EA subjects, but no FVIII SNPs were associated with VWF Ag. These data demonstrate that intronic SNPs could directly or indirectly influence intersubject variation of FVIII activity. Further investigation may reveal novel mechanisms of regulating FVIII expression and activity.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

What a polyclonal antibody sees in von Willebrand factor

INTRODUCTION

Von Willebrand factor (VWF) plays a critical role in hemostasis by carrying factor VIII (FVIII) and binding to specific ligands on the surface of blood platelets and within the blood vessel wall.

MATERIALS AND METHODS

We constructed a gene-specific phage display library containing small, random VWF fragments. Using this library, we mapped the repertoire of immune epitopes recognized by a commonly used commercial rabbit antihuman VWF polyclonal antibody.

RESULTS

A total of eight discrete epitopes within the VWF protein were identified, including two dominant epitopes that account for 74% of immuno selected VWF fragments. Comparison with previously mapped epitopes for mouse monoclonal antibodies reveals four overlapping regions that may identify common antigenic determinants. The distribution of these epitopes was not readily predicted from primary amino acid sequence divergence among these mammalian species or standard algorithms for the prediction of antigenicity, hydrophobicity, or surface probability.

CONCLUSION

Taken together with previous monoclonal antibody epitope mapping studies, our results suggest that a limited number of exposed domains on the surface of the human VWF protein may be the primary determinants of immunogenicity.

New approach of the treatment of von Willebrand’s disease during pregnancy

AIM

The aim of the study was to analyze the effectiveness of the application of DDAVP (desmopressin) and Hemate P with cryoprecipitate pre- and postpartum in patients with von Willebrand disease.

METHODS

We monitored 32 patients with von Willebrand disease during the study period 1993-2003. DDAVP was applied in the 36th/37th week of gestation and cryoprecipitate and fresh frozen plasma were applied 1 day before and 3 days after delivery. DDAVP treatment continued for 4 weeks. Factor VIII (Hemate P) at the day of delivery

RESULTS

No complications occurred in the studied population.

CONCLUSION

Precipitation of DDAVP, Hemate P, and cryoprecipitate may help in the treatment of pregnant women with von Willebrand disease.

The molecular biology of von Willebrand disease

von Willebrand disease (VWD) is a common autosomally inherited bleeding disorder associated with mucosal or trauma-related bleeding in affected individuals. VWD results from either a quantitative or qualitative deficiency of von Willebrand factor (VWF)--a glycoprotein with essential roles in primary haemostasis and as a carrier of coagulation factor VIII (FVIII) in the circulation. In recent years the identification of mutations in the VWF gene in patients with VWD has improved our understanding of the structure and function of the VWF protein, and has illustrated the importance of specific regions of VWF for its interaction with other components of the vasculature. The underlying genetic lesions and associated molecular pathology have been identified in many cases of type 2A, type 2B, type 2M, type 2N and type 3 VWD. However in the most common variant, type 1 VWD, the causative molecular defect is unknown in the large majority of cases. In the absence of an understanding of the molecular pathology underlying type 1 VWD, precise diagnosis and classification of this common disorder remains problematic.

Structure of von Willebrand factor and its function in platelet adhesion and thrombus formation

The adhesive protein von Willebrand factor mediates the initiation and progression of thrombus formation at sites of vascular injury. von Willebrand factor is synthesized in endothelial cells and megakaryocytes as a very large polymer composed of identical subunits. In the plasma, it appears as a series of multimers of regularly decreasing molecular mass, from several thousand to 500 kDa. The size of circulating von Willebrand factor multimers is controlled by proteolytic cleavage carried out by a specific protease. The biological functions of von Willebrand factor are exerted through specific domains that interact with extracellular matrix components and cell membrane receptors to promote the initial tethering and adhesion of platelets to subendothelial surfaces, as well as platelet aggregation. Moreover, von Willebrand factor binds the procoagulant co-enzyme, factor VIII, contributing to its stability and, indirectly, to its function in the generation of fibrin. This chapter presents a review of current knowledge on the structure, biosynthesis and functions of von Willebrand factor.

Conformational changes in the D′ domain of von Willebrand factor induced by CYS 25 and CYS 95 mutations lead to factor VIII binding defect and multimeric impairment

We report 2 new mutations identified in 3 patients and characterized by the markedly decreased affinity of von Willebrand factor (vWF) for factor VIII (FVIII). Patients 2 and 3, who have a typical type 2N phenotype, were found to be compound heterozygous for Arg91Gln and Cys25Tyr or Cys95Phe, respectively. Patient 1, who is the first cousin of patient 2, had an FVIII binding defect of vWF, low levels of vWF, and multimeric impairment. She was found to be compound heterozygous for the mutations Cys25Tyr and a stop codon (D93ter) in exon 4. Transient expression of recombinant vWF (rvWF) containing either Cys25Tyr or Cys95Phe mutations resulted in mutated rvWF with markedly reduced FVIII binding ability, multimeric structure impairment, and a significant decrease in the vWF expression level. Moreover, the use of anti-vWF monoclonal antibodies that inhibit the FVIII binding showed that these 2 mutations likely induce a conformational change in the D′ domain. These results show that the native conformation of the D′ domain of vWF is not only required for FVIII binding but also for normal multimerization and optimal secretion.

Conformational changes in the D′ domain of von Willebrand factor induced by CYS 25 and CYS 95 mutations lead to factor VIII binding defect and multimeric impairment

We report 2 new mutations identified in 3 patients and characterized by the markedly decreased affinity of von Willebrand factor (vWF) for factor VIII (FVIII). Patients 2 and 3, who have a typical type 2N phenotype, were found to be compound heterozygous for Arg91Gln and Cys25Tyr or Cys95Phe, respectively. Patient 1, who is the first cousin of patient 2, had an FVIII binding defect of vWF, low levels of vWF, and multimeric impairment. She was found to be compound heterozygous for the mutations Cys25Tyr and a stop codon (D93ter) in exon 4. Transient expression of recombinant vWF (rvWF) containing either Cys25Tyr or Cys95Phe mutations resulted in mutated rvWF with markedly reduced FVIII binding ability, multimeric structure impairment, and a significant decrease in the vWF expression level. Moreover, the use of anti-vWF monoclonal antibodies that inhibit the FVIII binding showed that these 2 mutations likely induce a conformational change in the D′ domain. These results show that the native conformation of the D′ domain of vWF is not only required for FVIII binding but also for normal multimerization and optimal secretion.

Two novel anti-von Willebrand factor monoclonal antibodies

Von Willebrand Factor is a multimer produced by endothelial cells and megakaryocytes, being stored in intracellular organelles, such as the Weibel-Palade bodies and alpha-granules in endothelial cells and platelets, respectively. This molecule acts as a carrier protein for factor VIIIc, involved in the intrinsic pathway of blood coagulation maintaining its stability in circulation. Von Willebrand Factor also plays an important role in platelet aggregation and adhesion to injured vessel wall. It interacts with platelets through two distinct glycoproteins, GPIb and GPIIb/IIIa. We raised two monoclonal antibodies, ECA-3 and ECA-4, against human umbilical vascular endothelial cells that recognize and immunoprecipitate von Willebrand Factor. Interestingly, ECA-4 monoclonal antibody is able to completely inhibit platelet agglutination induced by ristocetin, suggesting that it binds to von Willebrand Factor close to platelet GPIb binding site. The use of monoclonal antibodies to identify von Willebrand Factor binding regions to factor VIII or platelets has been reported by others. In pulmonary hypertension, abnormalities have been detected on the multimeric structure of the molecule as well as on its proteolytic fragments, by using monoclonal antibodies. Moreover, monoclonal antibodies raised against specific regions of von Willebrand Factor molecule may allow studies of functional abnormalities of this protein in inherited and acquired disorders like subtypes of von Willebrand's disease.

Effect of von Willebrand Factor and its proteolytic fragments on kinetics of interaction between the light and heavy chains of human factor VIII

It was previously shown that vWF increases the rate of divalent cation-mediated fVIII reconstitution from isolated light chain (LCh) and heavy chain (HCh) subunits. We examined the effect of vWF on kinetic parameters for interaction between LCh and HCh in the presence of Ca2+ and Mn2+ ions, the most effective mediators of fVIII reconstitution from isolated subunits, and determined the minimal structural portion of vWF able to enhance fVIII formation. We found that affinity (Kd) for LCh/HCh binding mediated by Ca2+ and Mn2+ was 91 and 34.9 nM in the absence of vWF and 15.5 and 5.6 nM in its presence. This decrease of Kd resulted from a sixfold increase of the association rate constant (k(on)) for this interaction. The value of the dissociation rate constant (k(off)) for LCh/HCh complex was lower in the presence of Mn2+ (k(off) 4.6x 10(-6) s(-1)) than Ca2+ (k(off) 8.4 x 10(-6) s(-1)) but in both cases vWF had no effect on k(off). This indicates that at physiological concentration of 1 nM the rate of fVIII inactivation via dissociation to subunits would be entirely determined by the k(off) value, and it should not depend on the presence of vWF. Indeed, our experiments demonstrated that vWF did not have any effect on the rate of fVIII inactivation resulting from its dissociation to subunits at the physiological concentrations of the fVIII and vWF proteins. We identified the minimal portion of the vWF molecule, able to enhance reconstitution of fVIII from isolated subunits. Only vWF large proteolytic N-terminal homodimeric fragment SPIII (vWF residues 1-1365), but not small monomeric N-terminal fragment SPIII-T4 (1-272), both of which are known to contain a major fVIII binding site, was able to support reconstitution of fVIII activity from isolated LCh and HCh subunits in the presence of Mn2+ or Ca2+. The effect of SPIII on the LCh/HCh association was similar to that of vWF, because both proteins identically increased of the value of k(on) and did not alter the k(off) value.

Collagen-bound von Willebrand factor has reduced affinity for factor VIII

von Willebrand factor (vWf) is a multimeric adhesive glycoprotein that serves as a carrier for factor VIII in plasma. Although each vWf subunit displays a high affinity binding site for factor VIII in vitro, in plasma, only 2% of the vWf sites for factor VIII are occupied. We investigated whether interaction of plasma proteins with vWf or adhesion of vWf to collagen may alter the affinity or availability of factor VIII-binding sites on vWf. When vWf was immobilized on agarose-linked monoclonal antibody, factor VIII bound to vWf with high affinity, and neither the affinity nor binding site availability was influenced by the presence of 50% plasma. Therefore, plasma proteins do not alter the affinity or availability of factor VIII-binding sites. In contrast, when vWf was immobilized on agarose-linked collagen, its affinity for factor VIII was reduced 4-fold, with KD increasing from 0.9 to 3.8 nM. However, one factor VIII-binding site remained available on each vWf subunit. A comparable reduction in affinity for factor VIII was observed when vWf was a constituent of the subendothelial cell matrix and when it was bound to purified type VI collagen. In parallel with the decreased affinity for factor VIII, collagen-bound vWf displayed a 6-fold lower affinity for monoclonal antibody W5-6A, with an epitope composed of residues 78-96 within the factor VIII-binding motif of vWf. We conclude that collagen induces a conformational change within the factor VIII-binding motif of vWf that lowers the affinity for factor VIII.

von Willebrand Factor Elevates Plasma Factor VIII Without Induction of Factor VIII Messenger RNA in the Liver

Factor VIII and von Willebrand factor (vWF) circulate in the plasma as a noncovalent protein complex. Circulating levels of factor VIII are coordinately regulated with circulating levels of vWF in which the ratio is maintained at 1 molecule of factor VIII for 50 to 100 vWF subunits. Infusion of vWF into vWF-deficient animal models and human patients yields a secondary increase in circulating levels of factor VIII. We have studied the mechanism of the secondary rise in factor VIII in a porcine model of vWF deficiency. On infusion of vWF into a vWF-deficient pig there was an approximately fivefold increase in circulating factor VIII activity. Liver biopsies were taken pre- and post-vWF infusion for isolation of total messenger RNA (mRNA). Factor VIII–specific mRNA was measured by an RNAse protection assay. The results showed no difference in the liver-specific factor VIII mRNA on vWF infusion. These results indicate that the secondary rise in factor VIII levels in response to exogenous vWF infusion is not dependent on increased steady-state levels of factor VIII mRNA in the liver.

von Willebrand Factor Elevates Plasma Factor VIII Without Induction of Factor VIII Messenger RNA in the Liver

Factor VIII and von Willebrand factor (vWF) circulate in the plasma as a noncovalent protein complex. Circulating levels of factor VIII are coordinately regulated with circulating levels of vWF in which the ratio is maintained at 1 molecule of factor VIII for 50 to 100 vWF subunits. Infusion of vWF into vWF-deficient animal models and human patients yields a secondary increase in circulating levels of factor VIII. We have studied the mechanism of the secondary rise in factor VIII in a porcine model of vWF deficiency. On infusion of vWF into a vWF-deficient pig there was an approximately fivefold increase in circulating factor VIII activity. Liver biopsies were taken pre- and post-vWF infusion for isolation of total messenger RNA (mRNA). Factor VIII–specific mRNA was measured by an RNAse protection assay. The results showed no difference in the liver-specific factor VIII mRNA on vWF infusion. These results indicate that the secondary rise in factor VIII levels in response to exogenous vWF infusion is not dependent on increased steady-state levels of factor VIII mRNA in the liver.

A Novel Mutation in the D3 Domain of von Willebrand Factor Markedly Decreases Its Ability to Bind Factor VIII and Affects Its Multimerization

In type 2N von Willebrand disease (vWD), von Willebrand factor (vWF) is characterized by normal multimeric pattern, normal platelet-dependent function, but a markedly decreased affinity for factor VIII (FVIII). In this report, we describe the case of a vWD patient who has an abnormal vWF multimers distribution associated with a markedly decreased vWF ability to bind FVIII. Sequencing analysis of patient’s vWF gene showed, at heterozygous state, a G→A transition resulting in the substitution of Asn for Asp at position 116 of the mature vWF subunit and a C→T transition, changing the codon for Arg 896 into a stop codon. His sister who has a subnormal vWF level, but a normal FVIII/vWF interaction, was found to be heterozygous for the Arg896ter mutation only. Recombinant vWF (rvWF) containing the candidate (Asn116) missense mutation was expressed in COS-7 cells. The expression level of Asn116rvWF was significantly decreased compared with wild-type rvWF. The multimeric pattern of Asn116rvWF was greatly impaired as shown by the decrease in high molecular weight forms. The FVIII binding ability of Asn116rvWF was dramatically decreased. These data show that the Asp116Asn substitution is the cause of both the defective FVIII/vWF interaction and the impaired multimeric pattern observed in the patient’s vWF. The monoclonal antibody 31H3 against D’ domain of vWF (epitope aa 66-76) that partially inhibits the FVIII binding and recognizes only nonreduced vWF, showed a decreased ability to bind Asn116rvWF when used as capture-antibody in enzyme-linked immunosorbent assay (ELISA). This result suggests that a potential conformation change in the D’ domain is induced by the Asp116Asn substitution, which is localized in the D3 domain.

A Novel Mutation in the D3 Domain of von Willebrand Factor Markedly Decreases Its Ability to Bind Factor VIII and Affects Its Multimerization

In type 2N von Willebrand disease (vWD), von Willebrand factor (vWF) is characterized by normal multimeric pattern, normal platelet-dependent function, but a markedly decreased affinity for factor VIII (FVIII). In this report, we describe the case of a vWD patient who has an abnormal vWF multimers distribution associated with a markedly decreased vWF ability to bind FVIII. Sequencing analysis of patient’s vWF gene showed, at heterozygous state, a G→A transition resulting in the substitution of Asn for Asp at position 116 of the mature vWF subunit and a C→T transition, changing the codon for Arg 896 into a stop codon. His sister who has a subnormal vWF level, but a normal FVIII/vWF interaction, was found to be heterozygous for the Arg896ter mutation only. Recombinant vWF (rvWF) containing the candidate (Asn116) missense mutation was expressed in COS-7 cells. The expression level of Asn116rvWF was significantly decreased compared with wild-type rvWF. The multimeric pattern of Asn116rvWF was greatly impaired as shown by the decrease in high molecular weight forms. The FVIII binding ability of Asn116rvWF was dramatically decreased. These data show that the Asp116Asn substitution is the cause of both the defective FVIII/vWF interaction and the impaired multimeric pattern observed in the patient’s vWF. The monoclonal antibody 31H3 against D’ domain of vWF (epitope aa 66-76) that partially inhibits the FVIII binding and recognizes only nonreduced vWF, showed a decreased ability to bind Asn116rvWF when used as capture-antibody in enzyme-linked immunosorbent assay (ELISA). This result suggests that a potential conformation change in the D’ domain is induced by the Asp116Asn substitution, which is localized in the D3 domain.

Peptide affinity chromatography of human clotting factor VIII. Screening of the vWF-binding domain

The region of von Willebrand factor, which is involved in the complex formation with factor VIII, was used to generate a panel of octapeptides. A peptide ladder was generated from the von Willebrand factor region aa40 to aa100 and was synthesized on cellulose membranes by spot technology. Four peptides with affinity for factor VIII were identified by incubation with plasma derived factor VIII and recombinant factor VIII. The peptides denoted as 010 (LCPPGMVRHE), 011 (RCPCFHQGK), 014 (CFHQGKEYA) and 015 (RDRKWNCTDHVC) were further characterized by real-time interaction analysis and small scale affinity chromatography. Biotinylated peptides were used for blotting assays. These experiments showed that the peptides are directed against the light chain of FVIII. We consider these peptides as valuable tools for in situ labeling and also as ligands suitable for affinity chromatography.

Binding of Factor VIII to von Willebrand Factor Is Enabled by Cleavage of the von Willebrand Factor Propeptide and Enhanced by Formation of Disulfide-Linked Multimers

von Willebrand factor (vWF) is a multimeric adhesive glycoprotein with one factor VIII binding site/subunit. Prior reports suggest that posttranslational modifications of vWF, including formation of N-terminal intersubunit disulfide bonds and subsequent cleavage of the propeptide, influence availability and/or affinity of factor VIII binding sites. We found that deletion of the vWF propeptide produced a dimeric vWF molecule lacking N-terminal intersubunit disulfide bonds. This molecule bound fluorescein-labeled factor VIII with sixfold lower affinity than multimeric vWF in an equilibrium flow cytometry assay (approximate KDs, 5 nmol/L v 0.9 nmol/L). Coexpression of propeptide-deleted vWF with the vWF propeptide in trans yielded multimeric vWF that displayed increased affinity for factor VIII. Insertion of an alanine residue at the N-terminus of the mature vWF subunit destroyed binding to factor VIII, indicating that the native mature N-terminus is required for factor VIII binding. The requirement for vWF propeptide cleavage was shown by (1) a point mutation of the vWF propeptide cleavage site yielding pro-vWF that was defective in factor VIII binding and (2) correlation between efficiency of intracellular propeptide cleavage and factor VIII binding. Furthermore, in a cell-free system, addition of the propeptide-cleaving enzyme PACE/furin enabled factor VIII binding in parallel with propeptide cleavage. Our results indicate that high-affinity factor VIII binding sites are located on N-terminal disulfide-linked vWF subunits from which the propeptide has been cleaved.

Binding of Factor VIII to von Willebrand Factor Is Enabled by Cleavage of the von Willebrand Factor Propeptide and Enhanced by Formation of Disulfide-Linked Multimers

von Willebrand factor (vWF) is a multimeric adhesive glycoprotein with one factor VIII binding site/subunit. Prior reports suggest that posttranslational modifications of vWF, including formation of N-terminal intersubunit disulfide bonds and subsequent cleavage of the propeptide, influence availability and/or affinity of factor VIII binding sites. We found that deletion of the vWF propeptide produced a dimeric vWF molecule lacking N-terminal intersubunit disulfide bonds. This molecule bound fluorescein-labeled factor VIII with sixfold lower affinity than multimeric vWF in an equilibrium flow cytometry assay (approximate KDs, 5 nmol/L v 0.9 nmol/L). Coexpression of propeptide-deleted vWF with the vWF propeptide in trans yielded multimeric vWF that displayed increased affinity for factor VIII. Insertion of an alanine residue at the N-terminus of the mature vWF subunit destroyed binding to factor VIII, indicating that the native mature N-terminus is required for factor VIII binding. The requirement for vWF propeptide cleavage was shown by (1) a point mutation of the vWF propeptide cleavage site yielding pro-vWF that was defective in factor VIII binding and (2) correlation between efficiency of intracellular propeptide cleavage and factor VIII binding. Furthermore, in a cell-free system, addition of the propeptide-cleaving enzyme PACE/furin enabled factor VIII binding in parallel with propeptide cleavage. Our results indicate that high-affinity factor VIII binding sites are located on N-terminal disulfide-linked vWF subunits from which the propeptide has been cleaved.

Can we improve on nature? "Super molecules" of factor VIII

Treatment of haemophilia A requires frequent infusion of plasma- or recombinant-derived factor VIII. This regimen is limited due to the high cost and inconvenient access to peripheral veins. In addition, patients frequently develop inhibitory antibodies that limit available therapeutic regimens. Two major advances in factor VIII research over the past 15 years were the ability to isolate homogeneous preparations of factor VIII and the isolation of the factor VIII gene that provided for a detailed biochemical and structural characterization of the factor VIII molecule. With an increased understanding of the requirements for factor VIII function, studies have attempted to produce improved factor VIII molecules for replacement therapy. These findings have produced forms of factor VIII that are more efficiently produced, that are less immunogenic, and that have higher specific activity. The future will see the engineering of novel factor VIII molecules with increased therapeutic efficiency while minimizing inhibitor antibody development. In addition, there are now structural models of factor VIII available that should in the future direct development of novel peptidomimetics that may eventually overcome the requirement for replacement therapy with factor VIII protein.

von Willebrand disease

Considerable progress has been made in characterizing the specific molecular defects responsible for the heterogeneous disorder known as von Willebrand disease (VWD). A large number of molecular defects have been identified and precise characterization may now be possible in the majority of type 2A, type 2B, type 2N, and potentially also type 3 VWD cases. However, the most common variant, type 1 VWD, still remains a major challenge. Continued progress in this area will improve our understanding of the pathogenesis of VWD and lead to more rapid and precise diagnosis and classification for this common disorder. The problems of incomplete VWD penetrance and poor diagnostic sensitivity and accuracy for the currently available clinical laboratory tests provide strong incentives for the development of DNA-based diagnostics. In addition, prenatal diagnosis is now possible either at the level of single point mutations (for some subtypes) or by RFLP analysis (assuming linkage to the von Willebrand factor [VWF] gene) and will probably be applied with increasing frequency for VWD type 3 (17, 133, 175). Understanding the molecular basis of VWD also has important implications for VWF structure and function and is helping to define critical binding domains within the VWF molecule. Insights gained from these studies may eventually lead to improved therapeutic approaches not only for VWD, but also for a variety of other genetic and acquired hemorrhagic and thrombotic disorders.

Identification of a His54Gln substitution in von Willebrand factor from a patient with defective binding of factor VIII

A patient with type 2N ("Normandy" variant) von Willebrand's disease is described. Her von Willebrand factor level was borderline low, while her factor VIII was markedly decreased to 7%. Her plasma von Willebrand factor demonstrated a decreased ability to complex with factor VIII in vitro, binding less than 10% when compared to normal plasma von Willebrand factor. The factor VIII released into the circulation after the patient received DDAVP had a shortened survival in vivo. Nucleotide sequence analysis revealed a T-to-A transition at nucleotide 2451 on both alleles. This transition results in a substitution of Gln for His at amino acid 54 in the mature subunit of von Willebrand factor.

A mechanism for inhibition of factor VIII binding to phospholipid by von Willebrand factor

von Willebrand factor (vWf) acts as a carrier for blood coagulation factor VIII (fVIII) in the circulation. The amino-terminal 272 residues of mature vWf contain a high affinity fVIII binding site. Upon thrombin activation, fVIII is released from vWf, thereby allowing its binding to phospholipid which is required for its procoagulant activity. Although phospholipid and vWf compete for fVIII binding, it was previously suggested that their binding sites are not closely juxtaposed within the fVIII protein because only amino-terminal vWf proteolytic fragments larger than SPIII-T4 (1-272) were able to block the binding of fVIII to phospholipid. We have demonstrated, however, that SPIII-T4 is able to inhibit fVIII binding to phosphatidylserine (PS) in a dose-dependent fashion, but only at concentrations higher than those used in previous experiments. Our demonstration that the Kd values for vWf and SPIII-T4 for fVIII are 0.52 nM and 48 nM, respectively, explain this discrepancy. Inhibition (> 95%) of SPIII-T4 binding to fVIII by a purified recombinant fVIII C2 domain polypeptide demonstrated that SPIII-T4 binds directly to C2, as we had previously shown for vWf. The similarity of the C2 binding sites for vWf and SPIII-T4 was further confirmed by the identical inhibitory effects of synthetic peptides and monoclonal antibodies (mAbs) on vWf-fVIII or SPIII-T4 fVIII binding. In both cases, binding was inhibited by synthetic peptide 2303-2332, containing a PS binding site, and by mAb NMC-VIII/5 Fab' (epitope within C2 residues 2170-2327). We propose that vWf, via residues 1-272, and PS compete for fVIII binding because they recognize overlapping sites within fVIII C2 domain residues 2303-2332.

Fine epitope mapping of monoclonal antibodies to the NH2-terminal part of von Willebrand factor (vWF) by using recombinant and synthetic peptides: interest for the localization of the factor VIII binding domain

Two different approaches were used in order to define the epitope of three monoclonal antibodies (MoAbs) against the NH2-terminal part of the mature subunit of von Willebrand factor (vWF) which contains its factor VIII (FVIII) binding site. First, a vWF cDNA fragment library using the bacteriophage lambda gt11 expression vector was screened with radiolabelled MoAbs. The epitope of each MoAb was defined, following sequence analysis, by the overlapping DNA sequence of immunoreactive clones. MoAb 32B12, a potent inhibitor of FVIII/vWF interaction, binds within the Glu35-Ile81 sequence of vWF subunit. MoAb 14A12, a non-inhibitory antibody, recognizes a sequence within Thr141-Val220. MoAb 31H3, a partial inhibitory antibody, gives no positive clone. In the second method, a panel of 24 synthetic pentadecapeptides corresponding to the first NH2-terminal 105 amino acid residues was used to block the binding of inhibitor MoAbs to immobilized vWF in an ELISA system. The localization of MoAb 32B12 epitope was confirmed and restricted to the Met51-Ala60 sequence. The MoAb 31H3 binding to vWF is inhibited by two synthetic peptides with the overlapping sequence Cys66-Gly76. All these data confirm that the FVIII binding site of vWF is not limited to the binding area (Thr78-Thr96) of the previously described MoAbs inhibiting FVIII/vWF interaction but is composed of several key sequences.

Direct demonstration of insulin-like growth factor-I-induced nitric oxide production by endothelial cells

Several lines of evidence indicate that insulin-like growth factor-I (IGF-I) is a potent mediator of vasodilation. To elucidate the mechanism and site of action of IGF-I, we performed continuous monitoring of nitric oxide (NO) release from endothelial cells using a highly-sensitive amperometric NO-sensor. Two types of cultured cells were used: human umbilical vein endothelial cells and immortalized rat renal interlobar artery endothelial cells. In separate experiments, [Ca2+]i changes in response to IGF-I were measured spectrofluorometrically in fura-2-loaded cells. Stimulation with IGF-I resulted in a rapid, dose-dependent increase in [NO] as detected by the NO-probe positioned 1 mm above the monolayers, followed by a sustained elevation lasting for at least five minutes. The effect of IGF-I was significantly suppressed by pretreatment with anti-IGF-I antibody, suggesting that it was specific for IGF-I. NG-nitro-L-arginine methyl ester, an inhibitor of NO synthesis, significantly blunted responses to IGF-I, but dexamethasone preincubation did not reduce the IGF-I-induced release of NO. These results indicate that the observed IGF-I-induced release of NO is a result of activation of the constitutive, rather than the inducible type of NO synthase in endothelial cells. Genistein, a tyrosine kinase inhibitor, resulted in a profound suppression of the IGF-I-induced release of NO. IGF-I did not affect [Ca2+]i in either type of cells. Therefore, IGF-I-induced NO production by both types of endothelial cells is mediated via a tyrosine kinase-dependent mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Autosomal recessive transmission of hemophilia A due to a von Willebrand factor mutation

The differential diagnosis of the genetic bleeding disorders, hemophilia A and von Willebrand disease, is occasionally confounded by the close molecular relationship of coagulation factor VIII and von Willebrand factor (vWF). This report describes the autosomal inheritance of a hemophilia A phenotype due to a mutation of vWF that results in defective factor VIII binding. The proband was a female patient with low levels of factor VIII activity. Polymerase chain reaction (PCR) amplification and DNA sequencing were employed to examine exons encoding the putative factor VIII binding domain of vWF. The patient was found to be homozygous for a single point mutation causing a Thr-->Met substitution at amino acid position 28 in the mature vWF subunit. The phenotypic expression of the mutation was determined to be recessive because heterozygous family members were clinically unaffected. Recombinant vWF containing the observed amino acid substitution was expressed in COS-1 cells. The mutant vWF was processed and secreted normally, and was functionally equivalent to wild-type vWF in its ability to bind to platelets. However, the mutant failed to bind factor VIII, demonstrating that the mutation was functionally related to the observed hemophilia phenotype. The family we describe demonstrates the recessive inheritance of a recently recognized class of genetic bleeding disorders, we call "autosomal hemophilia." We conclude that vWF mutation may be an under recognized cause of hemophilia, especially in cases where the inheritance pattern is not consistent with X-linked transmission.

The thrombin receptor extracellular domain contains sites crucial for peptide ligand-induced activation

A thrombin receptor (TR) demonstrating a unique activation mechanism has recently been isolated from a megakaryocytic (Dami) cell line. To further study determinants of peptide ligand-mediated activation phenomenon, we have isolated, cloned, and stably expressed the identical receptor from a human umbilical vein endothelial cell (HUVEC) library. Chinese hamster ovary (CHO) cells expressing a functional TR (CHO-TR), platelets, and HUVECs were then used to specifically characterize alpha-thrombin- and peptide ligand-induced activation responses using two different antibodies: anti-TR34-52 directed against a 20-amino acid peptide spanning the thrombin cleavage site, and anti-TR1-160 generated against the NH2-terminal 160 amino acids of the TR expressed as a chimeric protein in Escherichia coli. Activation-dependent responses to both alpha-thrombin (10 nM) and peptide ligand (20 microM) were studied using fura 2-loaded cells and microspectrofluorimetry. Whereas preincubation of CHO-TR with anti-TR34-52 abolished only alpha-thrombin-induced [Ca2+]i transients, preincubation with anti-TR1-160 abrogated both alpha-thrombin- and peptide ligand-induced responses. This latter inhibitory effect was dose dependent and similar for both agonists, with an EC50 of approximately 90 micrograms/ml. Anti-TR1-160 similarly abolished peptide ligand-induced [Ca2+]i transients in platelets and HUVECs, whereas qualitatively different responses characterized by delayed but sustained elevations in [Ca2+]i transients were evident using alpha-thrombin. Platelet aggregation to low concentrations of both ligands was nearly abolished by anti-TR1-160, although some shape change remained; anti-TR34-52 only inhibited alpha-thrombin-induced aggregation. These data establish that a critical recognition sequence for peptide ligand-mediated receptor activation is contained on the NH2-terminal portion of the receptor, upstream from the first transmembrane domain. Furthermore, alpha-thrombin-induced activation of HUVECs and platelets may be partially mediated by an alternative mechanism(s) or receptor(s).

Epitope mapping by cDNA expression of a monoclonal antibody which inhibits the binding of von Willebrand factor to platelet glycoprotein IIb/IIIa

In order to study the structure-function relationship of von Willebrand Factor (vWF), we have located the epitope of a well-characterized monoclonal antibody (MAb) to vWF (MAb 9). This MAb reacts with the C-terminal portion of the vWF subunit, SPII fragment [amino acids (aa) 1366-2050], which includes an Arg-Gly-Asp (RGD) sequence at positions 1744-1746, and totally inhibits vWF and SPII binding to platelet membrane glycoprotein IIb/IIIa (GPIIb/IIIa). A recombinant DNA library was constructed by cloning small (250-500 nucleotides) vWF cDNA fragments into the lambda gt11 vector and these inserts were expressed as fusion proteins with beta-galactosidase. Immunological screening of the library with 125I-MAb 9 identified three immunoreactive clones. vWF inserts were amplified by the PCR and their sequences demonstrated overlapping nucleotides from positions 7630 to 7855 of vWF cDNA, coding for aa residues 1698-1773 of the mature subunit, indicating that this is the epitope of MAb 9. vWF-beta-galactosidase fusion protein reacted with 125I-MAb 9 by Western blotting. In a solid-phase radioimmunoassay, the purified fusion proteins decreased the binding of vWF to 125I-MAb 9 by 50%, and this inhibition was dose-dependent between 3.5 and 120 nM. Therefore the epitope of MAb 9 is located within aa 1698-1773 of the vWF subunit, which includes the RGD sequence implicated in the binding of adhesive proteins of GPIIb/IIIa.

A patient with von Willebrand's disease characterized by a compound heterozygosity for a substitution of Arg854 by Gln in the putative factor-VIII-binding domain of von Willebrand factor (vWF) on one allele and very low levels of mRNA from the second vWF allele

We describe a patient with a lifelong bleeding disorder previously classified as von Willebrand's disease (vWD) type I. The factor VIII (FVIII) level in this patient was disproportionately low and we showed that this was due to a decreased factor VIII binding capacity of her vWF. To characterize the molecular defect in this type of vWD, a cDNA-dependent polymerase chain reaction (PCR) amplification was performed using platelet RNA as a template. Direct sequencing of the amplified fragment, which encodes for the FVIII-binding domain, showed a single nucleotide change in exon 20 at codon 854, resulting in the substitution of CAG glutamine (Gln) for CGG arginine (Arg). At the level of the cDNA only the mutated sequence was found, whereas at genomic DNA level the patient was heterozygous for this mutation. This patient is therefore a compound heterozygote for a point mutation resulting in a FVIII-binding defect and a vWF allele with low transcript levels.

Evidence that a secondary binding and protecting site for factor VIII on von Willebrand factor is highly unlikely

A binding domain for Factor VIII (F.VIII) has been previously identified on the N-terminal portion of human von Willebrand Factor (vWF) subunit [amino acids (AA) 1-272]. In order to characterize other possible structures of vWF involved in its capacity to bind and to protect F.VIII against human activated protein C (APC), we used a series of purified vWF fragments overlapping the whole sequence of the subunit. Among those were fragments SpIII (dimer; AA 1-1365), SpII (dimer; AA 1366-2050) and SpI (monomer; AA 911-1365) generated by Staphylococcus aureus V8 proteinase, a P34 species (monomer; AA 1-272) obtained with plasmin, a monomeric 39/34 kDa dispase fragment (AA 480-718) and a tetrameric III-T2 fragment (AA 273-511/674-728) produced from SpIII by trypsin. Three other fragments without precise extremities were located using selected monoclonal antibodies to vWF. Two C-terminal fragments of 270 and 260 kDa, overlapping SpI and SpII, were respectively generated from vWF with trypsin and protease 1 from Crotalus atrox venom. An N-terminal 120 kDa fragment, overlapping P34 and 39/34 kDa fragments, was produced by protease 1. Our results show that vWF bound to F.VIII and protected it from degradation by APC in a dose-dependent way. Among the C-terminal and central vWF fragments (SpII, tryptic 270 kDa, 260 kDa, SpI, 39/34 kDa and III-T2), none had the capacity to bind or to protect F.VIII, even at high concentrations. The three N-terminal fragments (SpIII, 120 kDa and P34) bound to F.VIII in a dose-dependent and saturable fashion. SpIII and the 120 kDa fragment had the capacity to protect F.VIII in a dose-dependent way. In contrast, the P34 species did not significantly protect F.VIII, even when using high concentrations of the fragment. In conclusion, the N-terminal end of vWF subunit (AA 1-272) plays a crucial role in binding to F.VIII, but requires additional structures of the 120 kDa fragment to protect it against APC. In addition, the presence of a secondary binding and/or protecting domain on other portions of the vWF subunit (potentially destroyed during the proteolysis of vWF) is highly unlikely.

Identification of two point mutations in the von Willebrand factor gene of three families with the 'Normandy' variant of von Willebrand disease

Plasma von Willebrand factor (vWf) is a multi-domain multimerized glycoprotein which has a dual role in haemostasis: it promotes platelet adhesion to subendothelium and is the carrier of blood coagulation factor VIII (FVIII). We previously characterized a functional defect of vWf, limited to its ability to bind FVIII, in two families whose affected members have the same phenotype that mimics mild haemophilia A and was tentatively named von Willebrand's disease (vWD) 'Normandy'. A homozygous point mutation C----T converting Thr 28 to Met in mature vWf subunit was identified in one of these patients who was born of third-cousin parents. In the present studies we report two unrelated new cases of vWD 'Normandy' and characterize, using the analysis of the vWf gene intron 40 region containing a variable number of tandem repeats, the recessive inheritance of the disease in two affected families without known consanguinity. Exons 18-24 of the vWf gene encoding for the first 311 amino acids of mature vWf subunit were amplified by the polymerase chain reaction method and sequenced. Two new missense mutations, both corresponding to a C----T transition and predicting respectively an Arg 53----Trp and an Arg 91----Gln substitution, were characterized. The three patients from family 1 were homozygous for the first-mentioned mutation while the patient from family 3 was homozygous for the second. The patient from family 2 was found a compound heterozygote for the two mutations. None of the two point mutations reported, both destroying a MspI restriction site, could be detected in DNA from 50 normal controls screened by restriction endonuclease analysis. Our data show that different mutations may be found in patients with the 'Normandy' phenotype. The mutations characterized so far are all localized on the N-terminal region of mature vWf subunit, within or near the major FVIII binding domain, and some of them occur within the epitope of monoclonal antibodies inhibiting the vWf/FVIII interaction. These observations suggest a causal relationship between these mutations and the vWD 'Normandy' phenotype.

Expression of von Willebrand factor "Normandy": an autosomal mutation that mimics hemophilia A

von Willebrand disease Normandy (vWD Normandy) is a recently described phenotype in which a mutant von Willebrand factor (vWF) appears structurally and functionally normal except that it does not bind to blood coagulation factor VIII. This interaction is required for normal survival of factor VIII in the circulation; consequently, vWD Normandy can present as apparent hemophilia A but with autosomal recessive rather than X chromosome-linked inheritance. A vWF missense mutation, Thr28----Met, was identified in the propositus in or near the factor VIII binding site. The corresponding mutant recombinant vWF(T28M) formed normal multimers and had normal ristocetin cofactor activity. However, vWF(T28M) exhibited the same defect in factor VIII binding as natural vWF Normandy, confirming that this mutation causes the vWD Normandy phenotype. The distinction between hemophilia A and vWD Normandy is clinically important and should be considered in families affected by apparent mild hemophilia A that fail to show strict X chromosome-linked inheritance and, particularly, in potential female carriers with low factor VIII levels attributed to extreme lyonization.

The association of factor VIII with von Willebrand factor

Factor VIII (FVIII) and von Willebrand factor (vWF) are plasma glycoproteins that circulate as a tightly associated complex. Because they tend to copurify during procedures designed to isolate the biologic activities associated with them, their identity as distinct entities became unequivocally established only during the past 10 years. Improved procedures for the isolation of FVIII, the deduction of the amino acid sequences of FVIII and vWF by using molecular cloning techniques and by direct sequencing, and the use of a variety of biophysical and immunochemical techniques have enhanced the understanding of the FVIII-vWF association. Each subunit of multimeric vWF potentially can bind a single heterodimeric FVIII molecule, although in vivo most of these binding sites are empty. The binding of FVIII to vWF is primarily, if not exclusively, mediated by the light chain of FVIII to the amino-terminal region of the vWF subunit. Cleavage of a fragment from the amino-terminal region of the FVIII light chain by thrombin results in rapid dissociation of the FVIII-vWF complex, a process that apparently is necessary for development of procoagulant activity. Whether this cleavage is needed for the activation of FVIII in the absence of vWF is controversial. The extracellular association of FVIII with vWF may be necessary for efficient secretion of FVIII from its cell of origin. The thermodynamics, kinetics, and nature of the molecular contacts involved in the interaction have not been studied. The association of FVIII with vWF prolongs the lifetime of FVIII in plasma. Whether the FVIII-vWF interaction has other functional roles, such as restricting the location of procoagulant activity, remains unknown.

The pro-polypeptide of von Willebrand factor is required for the formation of a functional factor VIII-binding site on mature von Willebrand factor

We have established that a recombinant von Willebrand Factor (vWF) mutant (vWFdelpro) that lacks the propolypeptide, in contrast with mature wild-type vWF, with which it is identical in terms of primary amino acid sequence, is not able to form a complex with Factor VIII. Wild-type vWF (flvWF) and vWFdelpro were expressed in AtT-20 cells. Under the culture conditions employed, completely processed multimerized flvWF and dimeric vWFdelpro were secreted into the medium. FlvWF and vWFdelpro were compared for their Factor VIII-binding properties in two distinct assay systems. In a direct binding assay, purified human Factor VIII was shown to bind to flvWF that had been immobilized on the surface of microtitre wells by using an anti-vWF monoclonal antibody. In contrast, Factor VIII did not bind to immobilized vWFdelpro. In a competition assay, fluid-phase flvWF appeared to inhibit efficiently the binding of Factor VIII to immobilized vWF isolated from plasma, whereas vWFdelpro did not influence Factor VIII binding. From these observations, it is argued that the pro-polypeptide serves an essential role in the post-translational processes that lead to the expression of a functional Factor VIII-binding site on the mature vWF subunit.