Fifth Åland Island conference on von Willebrand disease.

The fifth Åland Island meeting on von Willebrand disease (VWD) was held on the Åland Islands, Finland, from 22 to 24 September 2016-90 years after the first case of VWD was diagnosed in a patient from the Åland Islands in 1926. This meeting brought together experts in the field of VWD to share knowledge and expertise on current trends and challenges in VWD. Topics included the storage and release of von Willebrand factor (VWF), epidemiology and diagnostics in VWD, treatment of VWD, angiogenesis and VWF inhibitors.

State of the art: von Willebrand disease.

The State of the Art in von Willebrand disease (VWD) has been impacted not only by discoveries in the field of haemostasis, but also by changes in practice in other fields. The development of bleeding assessment tools has led to the clarification of bleeding symptoms and phenotype in VWD. New discoveries in the biology and genetics of von Willebrand factor (VWF) are challenging our existing diagnostics and classification(s). An improved understanding of reproductive physiology and the pathology of VWD along with changing obstetric, gynaecologic and haemostatic therapies necessitate an evolving response to the care of women with VWD. The survival of patients with autoimmune disease, malignancies and congenital heart disease along with increasing use of circulatory support devices and extracorporeal membrane oxygenation is increasing the prevalence of acquired von Willebrand syndrome. In each of these challenges, there are opportunities to improve the care of our patients with VWD.

The natural history of occult or angiodysplastic gastrointestinal bleeding in von Willebrand disease.

Recurrent gastrointestinal bleeding is one of the most challenging complications encountered in the management of patients with von Willebrand disease (VWD). The commonest cause is angiodysplasia, but often no cause is identified due to the difficulty in making the diagnosis. The optimal treatment to prevent recurrences remains unknown. We performed a retrospective study of VWD patients with occult or angiodysplastic bleeding within the setting of the von Willebrand Disease Prophylaxis Network (VWD PN) to describe diagnostic and treatment strategies. Centres participating in the VWD PN recruited subjects under their care with a history of congenital VWD and gastrointestinal (GI) bleeding due to angiodysplasia, or cases in which the cause was not identified despite investigation. Patients with acquired von Willebrand syndrome or those for whom the GI bleeding was due to another cause were excluded. Forty-eight patients from 18 centres in 10 countries were recruited. Seven individuals had a family history of GI bleeding and all VWD types except 2N were represented. Angiodysplasia was confirmed in 38%, with video capsule endoscopy and GI tract endoscopies being the most common methods of making the diagnosis. Recurrent GI bleeding in VWD is associated with significant morbidity and required hospital admission on up to 30 occasions. Patients were treated with multiple pharmacological agents with prophylactic von Willebrand factor concentrate being the most efficient in preventing recurrence of the GI bleeding. The diagnosis and treatment of recurrent GI bleeding in congenital VWD remains challenging and is associated with significant morbidity. Prophylactic treatment with von Willebrand factor concentrate was the most effective method of preventing recurrent bleeding but its efficacy remains to be confirmed in a prospective study.

Pregnancy complications and obstetric care in women with inherited bleeding disorders.

Women with inherited bleeding disorders (IBD) require the input of a multidisciplinary team to improve outcomes of pregnancy. The role of the haemophilia nurse within the multidisciplinary team is to provide educational and emotional support to the women and to facilitate and co-ordinate patient-centred care. Prenatal diagnosis in cases of haemophilia is an integral part of the management of early pregnancy with a recent drive towards non-invasive prenatal diagnostic techniques. There is a current lack of data on the risk of miscarriage and bleeding complications during pregnancy. A clear association has only been established in women with fibrinogen and factor XIII deficiency. In the affected neonate with severe bleeding disorders such as haemophilia, the risk of head bleeding is significant, and appropriate management of labour and delivery has an important impact on reducing the risk. Women with IBD are at risk of both primary and secondary postpartum haemorrhage. Appropriate risk assessment and advance planning for haemostatic cover can reduce the bleeding risk.

Third Åland islands conference on von Willebrand disease, 26-28 September 2012: meeting report.

The first meeting of international specialists in the field of von Willebrand disease (VWD) was held in the Åland islands in 1998 where Erik von Willebrand had first observed a bleeding disorder in some members of a family from Föglö and a summary of the meeting was published in 1999. The second meeting was held in 2010 and a report of the meeting was published in 2012. Topics covered included progress in understanding of VWD over the last 50 years; multimers; classification of VWD; pharmacokinetics and laboratory assays; genetics; treating the paediatric patient; prophylaxis; geriatrics; gene therapy and treatment guidelines. This third meeting held over 3 days covered the structure and function of von Willebrand factor (VWF); type 1 VWD, the most common form of the disease; a lifespan of pharmacokinetics in VWD; detecting inhibitors in VWD patients; and special challenges in understanding and treating the female VWD patient.

Efficacy and safety during formulation switch of a pasteurized VWF/FVIII concentrate: results from an Italian prospective observational study in patients with von Willebrand disease.

Von Willebrand disease (VWD) is an inherited bleeding disorder caused by the quantitative or qualitative deficiency of von Willebrand factor (VWF). Replacement therapy with plasma-derived VWF/factor VIII (FVIII) concentrates is required in patients unresponsive to desmopressin. To assess the efficacy, safety and ease of use of a new, volume-reduced (VR) formulation of VWF/FVIII concentrate Haemate(®) P in patients requiring treatment for bleeding or prophylaxis for recurrent bleeding or for invasive procedures. Pharmacoeconomic variables were also recorded. Data were analysed using descriptive statistics. This was a multicentre, prospective, observational study. Consecutively enrolled patients received Haemate(®) P VR according to their needs, and were followed for 24 months. Of the 121 patients enrolled, 25.6% had type 3 VWD and more than 40% had severe disease. All patients were followed for 2 years, for a total of 521 visits. On-demand treatment was given to 61.9% of patients, secondary long-term prophylaxis to 25.6% and prophylaxis for surgery, dental or invasive procedures to 45.5%. The response to treatment was rated as good to excellent in >93-99% of interventions. The new formulation was well tolerated by all patients with no report of drug-related adverse events. The switch to volume-reduced Haemate(®) P was easy to perform and infusion duration was decreased twofold compared with the previous formulation. Volume-reduced Haemate(®) P was at least as effective and well-tolerated as the previous formulation.

Prophylaxis in severe forms of von Willebrand's disease: results from the von Willebrand Disease Prophylaxis Network (VWD PN).

The bleeding patterns of severe von Willebrand's disease (VWD) adversely affect quality of life, and may be life threatening. There is a presumed role for prophylaxis with VWF-containing concentrates, but data are scarce. The von Willebrand Disease Prophylaxis Network (VWD PN) was formed to investigate the role of prophylaxis in clinically severe VWD that is not responsive to other treatment(s).Using a retrospective design, the effect of prophylaxis was studied. Availability of records to document, or reliably assess, the type and frequency of bleeding episodes prior to, and after, the initiation of prophylaxis was required. Annualized bleeding rates were calculated for the period prior to prophylaxis, during prophylaxis and by primary bleeding indication defined as the site accounting for more than half of all bleeding symptoms. The Wilcoxon signed-rank test of differences in the medians was used. Sixty-one subjects from 20 centres in 10 countries were enrolled. Data for 59 were used in the analysis. The median age at onset of prophylaxis was 22.4 years. Type 3 VWD accounted for the largest number (N = 34, 57.6%). Differences in bleeding rates within individuals during compared with before prophylaxis were significant for the total group (P < 0.0001), and for those with primary bleeding indications of epistaxis (P = 0.0005), joint bleeding (P = 0.002) and GI bleeding (P = 0.001). The effect of prophylaxis was similar among those age < 18 years and those ≥ 18. One person developed an inhibitor during treatment. We conclude that prophylactic treatment of VWD is efficacious.

Understanding FVIII/VWF complex--report from a symposium of XXIX WFH meeting 2010.

von Willebrand factor (VWF) has the capacity to form a complex with factor VIII (FVIII) which may modulate the immunogenicity of FVIII. It has been proposed that a significant fraction of recombinant FVIII (rFVIII) is unable to bind VWF. In an experimental model studied at the McMaster University in Canada, this VWF-unbound rFVIII fraction showed no coagulant function. Sulphation of FVIII tyrosine (Tyr) 1680 has been reported as essential for the interaction with VWF. In a study performed at the Grifols and CNS-CSIC in Spain, Tyr1680 sulphation was observed to be incomplete in rFVIII and complete in plasma-derived FVIII (pdFVIII). This could explain the incapability of some rFVIII molecules to bind VWF. Experience with immune tolerance induction (ITI) at the Bonn Haemophilia Centre indicates that only eradication of FVIII inhibitors allows safe haemostasis control and the option of prophylactic treatment. Various clinical trials were planned to evaluate the clinical role VWF-containing FVIII concentrates (FVIII/VWF). RES.I.ST (an acronym for REScue Immunotolerance STudy) is an international, prospective study aimed at assessing whether FVIII/VWF can induce ITI in high-risk haemophilia patients (RES.I.ST naïve) and whether patients who previously failed ITI with FVIII alone can be rescued with FVIII/VWF (RES.I.ST experienced). Enrolment started in November 2009. In the FAIReSt.Will (Fanhdi and Alphanate Italian Retrospective Study in Willebrand disease) study, 120 von Willebrand disease (VWD) patients treated with Fanhdi(®) or Alphanate(®) were retrospectively analysed. Efficacy was excellent and no side effects were reported. The ongoing PRO.Will study is a prospective, multicenter trial aimed at assessing the efficacy, safety and pharmacoeconomics of secondary long-term prophylaxis in patients with severe inherited VWD.

Management of bleeding disorders in adults.

Development of FVIII inhibitors is currently the most severe and challenging complication of haemophilia A treatment and represents a very large economic burden for a chronic disease. As a result, clinical research is making major efforts to optimize the therapeutic approaches for this condition. In this section we will review some important aspects of the management of haemophilia in adults, including an overview of bleeding in women with von Willebrand disease, an analysis of FVIII consumption in patients with severe haemophilia A, an update of the ongoing RES.I.ST study, long-term prophylaxis and experience from the Pro.Will study, current evidence relating to economic aspects of the treatment of haemophilic patients with inhibitors (based on the PROFIT study), and an overview of musculoskeletal complications in adults with severe bleeding disorders.

von Willebrand's disease: a report from a meeting in the Åland islands.

von Willebrand's disease (VWD) is probably the most common bleeding disorder, with some studies indicating that up to 1% of the population may have the condition. Over recent years interest in VWD has fallen compared to that of haemophilia, partly the result of focus on blood-borne diseases such as HIV and hepatitis. Now the time has come to revisit VWD, and in view of this some 60 international physicians with clinical and scientific interest in VWD met over 4 days in 2010 in the Åland islands to discuss state-of-the-art issues in the disease. The Åland islands are where Erik von Willebrand had first observed a bleeding disorder in a number of members of a family from Föglö, and 2010 was also the 140th anniversary of his birth. This report summarizes the main papers presented at the symposium; topics ranged from genetics and biochemistry through to classification of VWD, pharmacokinetics and laboratory assays used in the diagnosis of the disease, inhibitors, treatment guidelines in different age groups including the elderly who often have comorbid conditions that present challenges, and prophylaxis. Other topics included managing surgeries in patients with VWD and the role of FVIII in VWF replacement, a controversial subject.

Efficacy and safety of highly purified, doubly virus-inactivated VWF/FVIII concentrates in inherited von Willebrand's disease: results of an Italian cohort study on 120 patients characterized by bleeding severity score.

The efficacy of highly purified VWF/FVIII concentrates with standardized ristocetin cofactor content (VWF:RCo) has been already proven in patients with von Willebrand's disease (VWD). Aim of this retrospective study is to confirm efficacy and safety of two highly purified, doubly virus-inactivated VWF/FVIII concentrates in a large cohort of patients with VWD who were characterized at enrolment by bleeding severity score. Study drugs Alphanate or Fanhdi were given to 120 cases (51 males, 69 females, median age 50 years, range 6-83 years). Patients had VWD3 (10), VWD2A (19), VWD2B (25), VWD2M (10) and DDAVP-unresponsive VWD1 (56) and a median bleeding severity score of 8 (range 0-27). A total of 114 bleeding episodes in 55 cases and 131 surgical procedures in 85 cases could be analysed. Excellent-good clinical responses were seen in 97% of bleeding episodes and in 99% of surgical procedures. To prevent recurrent gastrointestinal (GI) bleeding, cerebral (CNS) haemorrhage, haemarthroses, urogenital or multisite bleeding in more severe patients, secondary prophylaxis was also carried out in 15 cases with VWD3 (3), VWD2A (3), VWD2B (2), VWD1 (7). A median dose of 42 IU VWF:RCo kg(-1) given every other day or twice a week over a median period of 334 days (range 24-799) prevented bleeding completely in 13 cases and reduced its incidence in the remaining two. These results confirm the efficacy and safety of the study concentrates, not only in the management of bleeding and surgery but also in secondary prophylaxis of severe VWD.

Von Willebrand factor promotes endothelial cell adhesion via an Arg-Gly-Asp-dependent mechanism.

Von Willebrand factor (vWF) is a constitutive and specific component of endothelial cell (EC) matrix. In this paper we show that, in vitro, vWF can induce EC adhesion and promote organization of microfilaments and adhesion plaques. In contrast, human vascular smooth muscle cells and MG63 osteosarcoma cells did not adhere and spread on vWF. Using antibodies to the beta chains of fibronectin (beta 1) and vitronectin (beta 3) receptors it was found that ECs adherent to vWF show clustering of both receptors. The beta 1 receptor antibodies are arranged along stress fibers at sites of extracellular matrix contact while the beta 3 receptor antibodies were sharply confined at adhesion plaques. ECs release and organize endogenous fibronectin early during adhesion to vWF. Upon blocking protein synthesis and secretion, ECs can equally adhere and spread on vWF but, while the beta 3 receptors are regularly organized, the beta 1 receptors remain diffuse. This suggests that the organization of the beta 1 receptors depend on the release of fibronectin and/or other matrix proteins operated by the same cell. Antibodies to the beta 3 receptors fully block EC adhesion to vWF and detach ECs seeded on this substratum. In contrast, antibodies to the beta 1 receptors are poorly active. Overall these results fit with an accessory role of beta 1 receptors and indicate a leading role for the beta 3 receptors in EC interaction with vWF. To identify the EC binding domain on vWF we used monoclonal antibodies produced against a peptide representing the residues Glu1737-Ser1750 of the mature vWF and thought to be important in mediating its binding to the platelet receptor glycoprotein IIb-IIIa. We found that the antibody that recognizes the residues 1,744-1,746, containing the Arg-Gly-Asp sequence, completely inhibit EC adhesion to vWF whereas a second antibody recognizing the adjacent residues 1,740-1,742 (Arg-Gly-Asp-free) is inactive. Both antibodies do not interfere with EC adhesion to vitronectin. This defines the molecular domain on vWF that is specifically recognized by ECs and reaffirms the direct role of the Arg-Gly-Asp sequence as the integrin receptor recognition site also in the vWF molecule.

Gynaecological and obstetrical problems in women with different bleeding disorders.

The aim of this case-control study was to analyse the prevalence of gynaecological, obstetrical and other more general bleeding symptoms in 114 women affected by various inherited bleeding disorders, who were compared with 114 apparently healthy women. Retrospective information were collected by means of two specific questionnaires, one on gynaecological and obstetrical bleeding symptoms, with special focus on the presence of menorrhagia as defined by a pictorial blood loss assessment chart (PBAC); and the other on general bleeding symptoms, whose severity was graded by means of the bleeding score (BS). Compared to normal women, the whole group of women with inherited bleeding disorders had a higher prevalence of excessive bleeding at menarche (25% vs. 5%, P < 0.0001) and menorrhagia (59% vs. 46%, P = 0.06). Affected women also had a higher frequency than controls of general bleeding symptoms that scored as severe by a BS > or = 12 (49% vs. 0%, P < 0.0001). In affected women, the BS increased according to the severity of the haemostasis defect. In conclusions, the BS and the PBAC are simple tools to evaluate the severity of general bleeding symptoms and menorrhagia in women with inherited bleeding disorders. These instruments may help to identify those women for whom a therapeutic intervention is warranted.

The use of desmopressin in von Willebrand disease: the experience of the first 30 years (1977-2007).

The aim of the treatment for von Willebrand disease (VWD) is to correct the dual defect of haemostasis, i.e. the abnormal platelet adhesion as a result of reduced and/or dysfunctional von Willebrand factor (VWF) and the abnormal coagulation expressed by low levels of factor VIII (FVIII). Correction of both deficiencies can be achieved by administering the synthetic peptide desmopressin (DDAVP) or, in cases unresponsive to this agent, the plasma concentrates containing VWF and FVIII (VWF/FVIII). DDAVP is the treatment of choice for type 1 VWD because it can induce release of normal VWF from cellular compartments, but the drug can be clinically useful also in other VWD types, including acquired von Willebrand syndrome (AVWS). A test dose of DDAVP at the time of diagnosis is recommended to establish the individual patterns of biological response and to predict clinical efficacy during bleeding and surgery. DDAVP is not effective in VWD type 3 and in severe forms of VWD 1 and 2. It can induce transient thrombocytopenia in patients with VWD type 2B. The results of several retrospective studies on the use of DDAVP in VWD management have been reported by many authors in different countries for the last 30 years. However, despite the widespread use of DDAVP in the treatment of VWD, there are only a few prospective clinical trials in a large number of cases on DDAVP efficacy and safety aimed at determining benefits and limits of this therapeutic approach. An investigator-driven observational prospective study on clinical efficacy of DDAVP in 200 patients with VWD types 1 and 2 has been recently organized: the effectiveness and safety of DDAVP will be evaluated prospectively for 24 months during bleeding episodes and minor or major surgeries in the VWD patients who were exposed to an infusion trial at enrollment.

Expression studies of missense mutations p.D141Y, p.C275S located in the propeptide of von Willebrand factor in patients with type 3 von Willebrand disease.

Missense mutations are not considered a common cause of type 3 von Willebrand's disease (VWD), the most severe defect of von Willebrand factor (VWF) characterized by undetectable levels of this protein in plasma and platelets. Nevertheless, several missense mutations have been identified in these patients. In this study, we report the cases of two Italian patients with type 3 VWD, both compound heterozygotes for different missense mutations and null alleles, p.D141Y/c.2016_2019del and p.C275S/p.W222X. We performed in vitro expression studies of the candidate missense mutations, both located in the D1 domain of VWF propeptide, to confirm their link with the disease and to understand the mechanisms of type 3 VWD responsible in these patients. Mutant and wild-type (WT) expression vectors were used for transient transfection and co-transfection studies in COS-7 cells. Single construct transfections of both missense mutations showed a strongly reduced but detectable secretion of recombinant (r)VWFs (approximately 15% of WT), with essentially only dimers being visualized on multimeric analysis. As expected, expression of a single construct of either mutation with the WT, showed mildly reduced secretion (approximately 40% of WT) and a full set of multimers. These expression studies indicate that the two amino acids D141 and C275 are key residues in the tertiary structure of the VWF propeptide. Their replacement with a tyrosine and a serine, respectively, might compromise propeptide folding, affecting both its intracellular survival and its capacity to mediate multimerization. Co-expression of hybrid rVWFs confirmed the recessive inheritance pattern of these missense mutations.

Treatment of classic Kaposi sarcoma with a nicotine dermal patch: a phase II clinical trial.

BACKGROUND: Kaposi sarcoma (KS), a malignancy of dermal endothelial cells that is caused by human herpesvirus 8 (HHV8) infection, is sensitive to perturbations of immunity. Nicotine might be effective against KS because of its immunologic and vascular effects and because smoking is associated with a low risk of KS. OBJECTIVE AND STUDY DESIGN: We conducted a masked, randomized phase 2 clinical trial of transdermal nicotine and placebo patches to assess the safety and efficacy of nicotine against classic KS (cKS). SUBJECTS AND METHODS: Three cKS lesions, predominantly nodules, in each of 24 non-smoking patients were randomly assigned to 15 weeks continuous treatment with nicotine patch (escalated to 7 mg), identical masked placebo patch or no patch. Changes in lesion area and elevation from baseline through six follow-up visits, by direct measurement and by two independent readers using digital photographs of the lesions, were compared using non-parametric and regression methods. Changes in longitudinal levels of HHV8 antibodies and DNA in blood cells were similarly assessed. RESULTS: There were no systemic or serious adverse events, and compliance was good. One patient resumed smoking and discontinued patches, and two patients withdrew at week 12 for unrelated indications. Six (29%) of the remaining 21 suspended use of patches to relieve local skin irritation; four of these six completed the trial at reduced dose. Treatment assignment was not associated with significant or consistent changes in cKS lesion area or elevation, HHV8 viral load or antibodies. CONCLUSION: Transdermal nicotine and placebo patches caused no serious toxicities but had no demonstrable effect on nodular cKS lesions or HHV8 levels.

Gene deletions correlate with the development of alloantibodies in von Willebrand disease.

Among all patients with von Willebrand disease (vWD), alloantibodies to von Willebrand factor (vWF) have been described only in severe vWD (type III). The relationship between the development of alloantibodies and the nature of the genetic lesion in vWD is not known. In hemophilia B, large deletions within the factor IX gene appear to correlate with the occurrence of alloantibodies, whereas in hemophilia A no such correlation is apparent. We have studied 19 patients with severe recessive vWD (type III) and 19 with autosomal dominant vWD (type I) by Southern blotting with probes encompassing the full 9 kilobases (kb) of the vWF cDNA. Two apparently unrelated patients were shown to have large deletions within the vWF gene. Both patients had severe vWD (type III) and were the only patients among those studied that had inhibitory alloantibodies to vWF. The extent of deletion was similar in both patients, corresponding to at least the 3'-7.4 kb of the vWF cDNA. The deletion in each patient was estimated to exceed 110 kb. In addition, the localization of the vWF gene to chromosome 12 was confirmed, and a homologous sequence on chromosome 22 was identified.

Type IIB von Willebrand factor with normal sialic acid content induces platelet aggregation in the absence of ristocetin. Role of platelet activation, fibrinogen, and two distinct membrane receptors.

Three preparations of purified von Willebrand factor (vWF), obtained from unrelated patients affected by type IIB von Willebrand disease, were found to have normal sialic acid content (between 129 and 170 nmol/mg of vWF, as compared with 158 +/- 17 nmol/mg in four normal preparations) and to induce platelet aggregation in the presence of physiologic levels of divalent cations and without addition of ristocetin. A monoclonal antibody that blocks the vWF binding domain of the platelet glycoprotein (GP)Ib caused complete inhibition of IIB vWF-induced aggregation. In contrast, a monoclonal antibody that blocks the receptor for adhesive proteins on the platelet GPIIb/IIIa complex failed to inhibit the initial response of platelets to high concentrations of IIB vWF. Moreover, IIB vWF caused agglutination of formalin-fixed platelets that was blocked only by the anti-GPIb antibody, suggesting that the binding of vWF to GPIb, even in the absence of ristocetin, results in platelet-platelet interaction that is followed by exposure of the GPIIb/IIIa receptors for adhesive proteins. Endogenous ADP, normally active platelet metabolism and fibrinogen binding to GPIIb/IIIa were necessary for maximal and irreversible platelet aggregation. In the absence of fibrinogen, however, aggregation was mediated by vWF binding to GPIIb/IIIa. A 52/48-kD tryptic fragment containing the GPIb binding domain of normal vWF completely blocked the aggregation induced by all three IIB vWF preparations. The present study defines in detail the mechanisms involved in IIB vWF-induced platelet aggregation. Moreover, it establishes that the GPIb binding domain of normal and IIB vWF are closely related and that desialylation is not required for the direct interaction of IIB vWF with GPIb.

Carbohydrate moiety of von Willebrand factor is not necessary for maintaining multimeric structure and ristocetin cofactor activity but protects from proteolytic degradation.

To better define the role of carbohydrate in the structure and ristocetin cofactor activity of von Willebrand factor, we have removed up to 83% of total hexose by sequential treatment of the molecule with endo-beta-N-acetyl-glucosaminidase F (endo F), neuraminidase, and beta-galactosidase. Endo F alone removed 69% of total hexose and D-galactose, and 71% of sialic acid. However, there was no discernible loss of large multimers and the ristocetin cofactor activity was decreased by only 11%. The reduced von Willebrand factor subunit migrated more rapidly in polyacrylamide gels containing SDS, consistent with a 10% decrease of molecular mass. All multimers of unreduced carbohydrate-modified von Willebrand factor migrated more rapidly in SDS-agarose, but the triplet pattern of individual multimers was unchanged. This alteration in multimer migration rate did not resemble alterations found so far in von Willebrand disease variants. Further treatment of von Willebrand factor with neuraminidase and beta-galactosidase reduced the D-galactose to 15% and ristocetin cofactor activity to 57%. A similar decrease in ristocetin cofactor activity was seen if von Willebrand factor was treated only with neuraminidase and beta-galactosidase. In contrast, treating von Willebrand factor with neuraminidase and beta-galactosidase in the presence of protease inhibitors (20 mM benzamidine, 20 U/ml aprotonin, 15 micrograms/ml leupeptin) resulted in a comparable removal of carbohydrate with no change in ristocetin cofactor activity. Moreover, the multimeric structure remained intact in spite of 80% removal of D-galactose. This suggested that carbohydrate was protecting von Willebrand factor against traces of one or more protease contaminants. Evidence in support of this hypothesis was obtained by exposing von Willebrand factor to plasmin after pretreatment with neuraminidase alone or with neuraminidase and beta-galactosidase. A loss of large multimers was observed from von Willebrand factor that had been pretreated with neuraminidase, but this was even greater if pretreatment was also with beta-galactosidase. In contrast, the multimeric structure of von Willebrand factor with intact carbohydrate was not affected by plasmin under similar conditions. These studies suggest that carbohydrate protects von Willebrand factor from disaggregation occurring secondarily to proteolytic attack but does not play a direct role in maintaining its multimeric structure or ristocetin cofactor activity.

Distinct abnormalities in the interaction of purified types IIA and IIB von Willebrand factor with the two platelet binding sites, glycoprotein complexes Ib-IX and IIb-IIIa.

We have studied the interaction of the congenitally abnormal type IIA and IIB von Willebrand factor (vWF) molecules, both lacking the larger multimeric forms, with the two vWF binding sites on platelets, the glycoprotein (GP) Ib-IX and GP IIb-IIIa complexes. Variant as well as normal (N) vWF were purified from plasma. Estimates for binding of subunit molecules per platelet at saturation (Bmax) and dissociation constant in moles/liter (Kd), respectively, were obtained from binding isotherms of 125I-labeled vWF, with the following results. In the presence of ristocetin (binding to GP Ib-IX): N, 25,693 and 0.5 x 10(-8); IIA, both parameters not measurable; IIB, 17,708 and 0.87 x 10(-8). After thrombin stimulation (binding to GP IIb-IIIa): N, 17,059 and 1.12 x 10(-8); IIA, 23,751 and 4.87 x 10(-8); IIB, 19,890 and 2.52 x 10(-8). Distinct experiments based on measuring the ability of the variant species (from the same patients and one additional IIB patient) to inhibit the binding of normal 125I-vWF to platelets gave results in agreement with those reported above. Other studies showed that only IIB vWF bound to platelets in the absence of any mediating substance (Kd = 5.21 x 10(-8) mol/liter and Bmax = 9,599 subunits per platelet) and induced aggregation at a concentration of 10 micrograms/ml (3.6 x 10(-8) M). Thus, IIB vWF binds to GP Ib-IX with high affinity and induces platelet aggregation, whether with or without ristocetin, in spite of the absence of larger multimers. In contrast, the binding of IIA vWF to GP Ib-IX occurs with very decreased affinity, and this defective function may result from specific structural abnormalities rather than just being a reflection of the absence of larger multimeric forms. Both IIA and IIB vWF exhibit decreased affinity for GP IIb-IIIa. In this case, the extent of the defect correlates with the absence of larger multimers.