Clearance Of VWF Research Articles

Oligosaccharide structures of von Willebrand factor and their potential role in von Willebrand disease

Oligosaccharides make up approximately 20% of the mass of VWF and although their structures are well established, their functional role remains unclear. Modification of the VWF oligosaccharide structures has been shown to result in increased plasma clearance of the protein. A mutation which alters cell type-specific expression of the Galgt2 glycosyltransferase gene in the RIIIS/J mouse results in an autosomal dominant partial quantitative deficiency of VWF. Increased plasma clearance of VWF has been demonstrated in some individuals with a partial quantitative deficiency of the protein and it is possible that variation in VWF glycosylation may contribute towards this. ABH antigens occur within the oligosaccharide component of VWF and may account for the variation in plasma VWF:Ag levels observed between individuals of different ABO blood groups. The structures and functional roles of the oligosaccharide side chains of VWF and possible pathogenetic mechanisms by which they may contribute towards VWD are reviewed in this article.

Severe Thrombocytopenia, Type 2B von Willebrand Disease and Pregnancy

VON Willebrand’s Disease (vWD) is the most common inherited bleeding disorder in humans with an estimated incidence as high as 2–3% in the general population. Characterized by abnormal platelet interactions with the subendothelium or other platelets, the disease is caused by changes in the multimeric glycoprotein, von Willebrand’s Factor (vWF). Types 1 and 3 vWD are associated with relative or absolute quantitative defects in the protein, respectively. Type 1 vWD accounts for 70% of all cases and is likely to temporally improve in parturients as a result of an increase in vWF and factor VIII with pregnancy. Type 2 vWD has qualitative abnormalities and comprises 20–30% of all vWD diagnoses. A unique subtype, type 2B, accounts for less than 20% of all type 2 vWD and is characterized by an increased affinity of vWF for platelet glycoprotein Ib, resulting in spontaneous binding and clearance of both vWF and platelets. This feature allows type 2B, unlike most other vWD variants, to be exacerbated by pregnancy and to exhibit a poor or worsening response to desmopressin (DDAVP; Aventis, Bridgewater, NJ). We present the management of a parturient with type 2B vWD who had severe thrombocytopenia during the peripartum period.

3 Acquired von Willebrand disease

Acquired von Willebrand disease (AvWD) is an acquired bleeding disorder which may suddenly become manifest in individuals, usually in the absence of a personal or family history of bleedings and frequently in association with monoclonal gammopathies, lymphoproliferative, myeloproliferative and autoimmune disorders. In a minority of the cases AvWD may develop in association with drugs or solid tumours. Pathogenetic mechanisms involve autoantibodies directed against von Willebrand factor (vWF) resulting in a rapid clearance of vWF from the circulation and/or inactivation of plasma vWF; absorption or adsorption of plasma vWF to malignant cells; drug-induced or cell-mediated proteolysis of plasma vWF; acquired decrease in synthesis of vWF and/or release of vWF from storage sites; or precipitation of plasma vWF. Treatment options include--whenever possible--treatment of the underlying disorder or symptomatic treatment aimed at replacing the loss of vWF by either infusion of vWF-rich concentrates or administration of desmopressin (DDAVP). In selected cases with anti-vWF antibodies, administration of high-dose intravenous gammaglobulin, plasma exchange or extracorporeal immunoadsorption may be successful.

Von Willebrand antigen II (vWAgII) distinguishes acquired von Willebrand Syndrome (AvWS) due to decreased synthesis from that due to increased clearance of vWF. † 795

Acquired von Willebrand syndrome (AvWS) is bleeding disorder that develops de novo in an individual with a negative past personal and family history of bleeding in association with laboratory studies indicative of von Willebrand disease. This diagnosis can be difficult to distinguish from hereditary von Willebrand syndrome, especially in children. Potential mechanisms causing AvWS include decreased protein synthesis, increased proteolysis of mature von Willebrand factor or accelerated clearance of mature vWF(on an immunologic or adsorptive basis). VWF is synthesized in endothelial cells and megakaryocytes as a pre-propolypeptide that undergoes proteolytic cleavage forming the propolypeptide (vWAgII) and mature vWF. These two proteins are immunologically distinct. We hypothesized that since vWAgII has different functional and immunological characteristics from mature vWF, levels of vWAgII should be normal or increased in states of increased vWF clearance or proteolysis. In contrast, levels of vW AgII should parallel those of mature vWF in states of decreased protein synthesis. We studied 24 individuals, 3-79 years of age, referred for evaluation of AvWS. All 24 had reduced levels of vWF antigen, mean 17±3 u/dL. There was a striking difference in vWAgII levels for the 5 individuals with hypothyroidism when compared to all other individuals, mean 55±4 for hypothyroid subjects v. 120±13 u/dL for all other diagnoses (p=.0001). We had previously shown that the ratio of vWF antigen to vWAgII in normal plasma and in plasma from individuals with type 1 von Willebrand disease was unity. This ratio was markedly different between the groups with AvWS, mean vWF/vWAgII for hypothyroid subjects was 0.53±.11 v. 0.14±.04 for other diagnoses (p=.02). In support of the hypothesis that decreased vWF and vWAgII levels in hypothyroidism are secondary to decreased protein synthesis, 4 patients demonstrated normalization of both vWF and vWAgII following thyroid replacement. Although no subject studied had an inhibitor of vWF ristocetin cofactor activity, 3 euthyroid subjects who underwent DDAVP trials demonstrated accelerated clearance of vWF antigen. vWAgII successfully differentiates AvWS caused by decreased synthesis from AvWS due to increased clearance of vWF.

First report of combined factor VII Padua defect and von Willebrandʼs disease due to casual association of the two defects

We report a family with a combined factor VII Padua defect and von Willebrand's disease (vWd). The propositus is a 9-year-old child with a moderate bleeding tendency who appeared to be heterozygous for both factor VII Padua and type I vWd. The diagnosis of factor VII Padua was based on a normal factor VII antigen and factor VII activity which was low with rabbit brain thromboplastin but normal with ox brain thromboplastin. Type I vWd was diagnosed because of a concomitant decrease of von Willebrand factor antigen (vWf:Ag) and vWf ristocetin-cofactor activity (vWf:RCoF), associated with the presence of vWf multimers of all sizes in plasma and platelets. The parents were not consanguineous but came from the same isolated river Piave valley in North Eastern Italy where the factor VII Padua defect was first described. The father had the factor VII Padua defect but was clinically asymptomatic in accordance with the heterozygous state. The propositus's mother had type I vWd and was mildly symptomatic. The propositus' sisters, who were clinically asymptomatic, were both heterozygotes for factor VII Padua. The infusion of DDAVP normalized the factor VIII/vWf pattern in all patients. In the propositus, in contrast to the mother and normal subjects, showed a more rapid clearance both of vWf and factor VIII. The same pattern, albeit to a lesser degree, was also observed in the father.

First report of combined factor VII Padua defect and von Willebrandʼs disease due to casual association of the two defects

We report a family with a combined factor VII Padua defect and von Willebrand's disease (vWd). The propositus is a 9-year-old child with a moderate bleeding tendency who appeared to be heterozygous for both factor VII Padua and type I vWd. The diagnosis of factor VII Padua was based on a normal factor VII antigen and factor VII activity which was low with rabbit brain thromboplastin but normal with ox brain thromboplastin. Type I vWd was diagnosed because of a concomitant decrease of von Willebrand factor antigen (vWf:Ag) and vWf ristocetin-cofactor activity (vWf:RCoF), associated with the presence of vWf multimers of all sizes in plasma and platelets. The parents were not consanguineous but came from the same isolated river Piave valley in North Eastern Italy where the factor VII Padua defect was first described. The father had the factor VII Padua defect but was clinically asymptomatic in accordance with the heterozygous state. The propositus's mother had type I vWd and was mildly symptomatic. The propositus' sisters, who were clinically asymptomatic, were both heterozygotes for factor VII Padua. The infusion of DDAVP normalized the factor VIII/vWf pattern in all patients. In the propositus, in contrast to the mother and normal subjects, showed a more rapid clearance both of vWf and factor VIII. The same pattern, albeit to a lesser degree, was also observed in the father.

Von Willebrand factor bound to glycoprotein Ib is cleared from the platelet surface after platelet activation by thrombin

We recently reported that after activation of human platelets by thrombin, glycoprotein (GP) Ib-IX complexes are translocated to the surface-connected canalicular system (SCCS) (Blood 76:1503, 1990). As GPIb is a major receptor for von Willebrand factor (vWF) in platelet adhesion, we have now examined the consequences of thrombin activation on the organization of vWF bound to GPIb on the platelet surface. Studies were performed using monoclonal or polyclonal antibodies in either immunogold staining and electron microscopy (Au-EM) or in flow cytometry. When unstirred platelet-rich plasma was incubated with ristocetin, bound vWF was located by Au-EM as discrete masses regularly distributed over the cell surface. Platelets from a patient with Glanzmann's thrombasthenia, lacking GPIIb-IIIa complexes, gave a similar pattern, confirming that this represented binding to GPIb. That ristocetin was not precipitating vWF before their binding to the platelets was shown by the detection of similar masses on the surface of platelets of a patient with type IIB von Willebrand disease. Experiments were continued using washed normal platelets incubated in Tyrode-EDTA, the purpose of the EDTA being to limit the surface expression of endogenous vWF after platelet stimulation. Under these conditions, platelets were treated with ristocetin for 5 minutes at 37 degrees C in the presence of increasing amounts of purified vWF. This was followed by incubation with thrombin (0.5 U/mL) for periods of up to 10 minutes. Flow cytometry showed a time-dependent loss in the surface expression of vWF bound to GPIb and these changes were confirmed by Au-EM. In particular, immunogold staining performed on ultrathin sections showed that the bulk of the vWF was being cleared to internal membrane systems. Surface clearance of vWF during thrombin- induced platelet activation is a potential mechanism for regulating platelet adhesivity.

Von Willebrand Factor Bound to Glycoprotein Ib Is Cleared From the Platelet Surface After Platelet Activation by Thrombin

AbstractWe recently reported that after activation of human platelets by thrombin, glycoprotein (GP) Ib-IX complexes are translocated to the surface-connected canalicular system (SCCS) (Blood 76:1503, 1990). As GPIb is a major receptor for von Willebrand factor (vWF) in platelet adhesion, we have now examined the consequences of thrombin activation on the organization of vWF bound to GPIb on the platelet surface. Studies were performed using monoclonal or polyclonal antibodies in either immunogold staining and electron microscopy (Au-EM) or in flow cytometry. When unstirred platelet-rich plasma was incubated with ristocetin, bound vWF was located by Au-EM as discrete masses regularly distributed over the cell surface. Platelets from a patient with Glanz-mann’s thrombasthenia, lacking GPIIb-IIIa complexes, gave a similar pattern, confirming that this represented binding to GPIb. That ristocetin was not precipitating vWF before their binding to the platelets was shown by the detection of similar masses on the surface of platelets of a patient with type IIB von Willebrand disease. Experiments were continued using washed normal platelets incubated in Tyrode-EDTA, the purpose of the EDTA being to limit the surface expression of endogenous vWF after platelet stimulation. Under these conditions, platelets were treated with ristoce-tin for 5 minutes at 37°C in the presence of increasing amounts of purified vWF. This was followed by incubation with thrombin (0.5 U/mL) for periods of up to 10 minutes. Flow cytometry showed a time-dependent loss in the surface expression of vWF bound to GPIb and these changes were confirmed by Au-EM. In particular, immunogold staining performed on ultrathin sections showed that the bulk of the vWF was being cleared to internal membrane systems. Surface clearance of vWF during thrombin-induced platelet activation is a potential mechanism for regulating platelet adhesivity.