Structure and function of von Willebrand factor: relationship to von Willebrand's disease

Successful use of lenalidomide to treat refractory acquired von Willebrand disease associated with monoclonal gammopathy

Acquired von Willebrand syndrome (AVWS) is a rare hematologic disorder characterized by quantitative or qualitative defects of von Willebrand factor (vWF), a protein crucial for normal hemostasis. AVWS has been described in association with several pathologic entities with varied mechanisms. Among these, lymphoproliferative disorders are the most common, with monoclonal gammopathy of undetermined significance (MGUS) being the most frequently reported. AVWS in this setting is commonly associated with the development of bleeding that is clinically challenging to manage due to accelerated clearance of vWF, limiting the utility of many conventional treatment modalities such as DDAVP or vWF/FVIII. We report a case of a 43-year-old male who was sent to our institution for new-onset easy bruising and laboratories concerning for von Willebrand disease (vWD). Further diagnostic workup revealed evidence of an IgG monoclonal gammopathy and findings suggestive of vWF inhibition. Ultimately, he was found to have monoclonal gammopathy of clinical significance (MGCS)-associated AVWS refractory to conventional treatment but responsive to lenalidomide and dexamethasone. This case suggests that lenalidomide may be suitable for patients with AVWS secondary to MGCS.

The Fibrinogen Antigenic Turbidimetric Assay (FIATA): The X2x Test—The Corrected Chi-Square Comparison Against the Control-mean

Vancomycin precipitates fibrinogen. The turbidity induced by this vancomycin-fibrinogen interaction is used to establish a simple standardized antigenic assay for plasmatic fibrinogen, the FIATA. 1 mM vancomycin or 2 mM chloramine-T inactivates 50% of fibrinogen in human plasma. In contrast to chloramine-T, vancomycin does not react in NaJ-based photometric assay for chloramines, vancomycin does not inactivate the singlet oxygen-sensible antithrombin III, and the vancomycin action against fibrinogen is not changed in spite of the presence of the 1O2 quenchers methionine or ascorbic acid. The FIATA is performed as follows: to 25 μL plasma 50 μL PBS are added and the absorbance (A) at 405 nm is read. Then 50 μL FIATA-reagent, consisting of 4.4 mM vancomycin in PBS, are added. After 2 minutes (RT) δA is determined and standardized against a plasma pool of 100% of norm (2.8 g/L) fibrinogen. The FIATA is nearly linear up to a fibrinogen concentration of about 150% of norm (4.2 g/L), resulting in a δA of about 600 mA. The lower detection limit is 4% of norm (0.1 g/L). The intra-assay and interassay CV values are < 4%. The normal range of FIATA is 100% ± 20% ([ILLEGIBLE] ± 1 SD). In = 321 or 344 unselected patient plasmas the FIATA ([ILLEGIBLE] = 130%; SD = 52% or 43%) correlated with the functional fibrinogen assays a) modified Clauss-Method ([ILLEGIBLE] = 4.1 g/L; SD = 1.7 g/L) with r = 0.755 and b) FIFTA ([ILLEGIBLE] = 124%; SD = 40%) with r = 0.813. The vancomycin/fibrinogen interaction (binding of about 16 molecules of vancomycin/molecule of fibrinogen) can be used to purify fibrinogen out of plasma. Vancomycin also clouds dysfunctional fibrinogen (fibrinogen in presence of EDTA or chloramine-T) or soluble fibrin. Vancomycin-reacted fibrinogen stimulates tissue-type plasminogen activator (t-PA) up to about 20-fold. The experimental data are analyzed by a new significance test: the twofold Yates-corrected chi-square comparison against the mean value of the control-collective, called the [ILLEGIBLE] Test. The P < .05 barrier calculated with the [ILLEGIBLE] significance - Test is equivalent to that calculated with the Fisher’s Exact Test. The FIATA might be considered an interesting screening test for inactive fibrinogen forms or soluble fibrin, as eg in disseminated intravascular coagulation. Fibrinogen precipitation by vancomycin within the blood vessel might explain why vancomycin has to be infused slowly (< 10 mg/min) to prevent nephrotoxicity. The FIATA is of such a simplicity that the determination of fibrinogen antigen in plasma can be performed anywhere—even outside a hospital—within seconds. Thus, the presented FIATA might contribute to extrahospital testing of patients for assessing their risk for myocardial or cerebral ischemia/infarction.

ABO blood groups in relation to breast carcinoma incidence and associated prognostic factors in Moroccan women

The association between blood groups ABO and different types of diseases was established in several previous studies. Our aim was to seek the possible association between the ABO blood group and breast cancer-associated prognostic factors. The Chi-squared analytic test was used to compare phenotypic ABO distribution among Moroccan blood donors and 442 cases of women suffering from breast carcinoma with archived files in Maternity Ward of University Hospital C.H.U Ibn Rochd between 2008 and 2011. High incidence of breast carcinoma was observed in blood type B patients (p < 0.05). Blood type B was associated with breast carcinomas overexpressing human epidermal growth factor receptor HER2 (p < 0.05) and high risk of cancer at age over 70 years (p < 0.001). Blood type A was associated with high risk of cancer among women younger than 35 years old. Blood type A and AB were associated with high incidence of lymph node metastasis (p < 0.05). Multivariate analysis has shown correlation between O blood type and estrogen receptor-positive tumor. Patients with blood group A, B, and AB were more likely to develop aggressive breast carcinoma. Further follow-up studies are necessary to clarify the role of ABH antigens in the progression of breast carcinoma.

Von Willebrand Factor and Factor VIII levels in Egyptian children with newly diagnosed acute lymphoblastic leukemia in relation to peripheral blast cells and steroid therapy

BACKGROUND

Endothelial dysfunction has been reported in children with acute lymphoblastic leukemia (ALL). The aim of this study is to assess Von Willebrand Factor antigen (VWF antigen) and Factor VIII (FVIII) in newly diagnosed ALL patients, in relation to peripheral blast (PB) cells, steroid therapy, and any prognostic potential.

PROCEDURE

VWF antigen and FVIII were assessed initially (D0) and at day 8 (D8) steroid therapy for 32 newly diagnosed ALL patients with and without peripheral blood blast cells.

RESULTS

At diagnosis, patients with PBs had a significantly higher levels of VWF antigen (102.7 ± 22.9% vs. 56.9 ± 8%, P<0.001) and FVIII (93.4 ± 15.9% vs. 6 62.6 ± 18.1%, P<0.001) than those without. Following steroid therapy, both factors decreased in those with PBs, whereas an increase above baseline was observed in those without PBs. Furthermore, there was a significant positive correlation between PBs and both VWF antigen (P<0.001) and FVIII levels (P=0.002). High-risk patients were comparable with standard-risk group in mean values of VWF antigen (P=0.234) and FVIII (P=0.891) at diagnosis. After 12 months from diagnosis, all patients without PB achieved and maintained complete remission. Those with initial PB reported relapse (12.5%) or death (4.2%) during follow-up.

CONCLUSIONS

Markers of endothelial dysfunction namely VWF and FVIII were related to circulating blast cells and steroids therapy through lysis of lymphoblasts results in reduction of both factors, with risk of thrombosis during induction with marked disintegration of malignant cells.

Evaluation of von Willebrand factor and von Willebrand factor propeptide in models of vascular endothelial cell activation, perturbation, and/or injury

Pharmacologically, vasoactive agents targeting endothelial and/or smooth muscle cells (SMC) are known to cause acute drug-induced vascular injury (DIVI) and the resulting pathology is due to endothelial cell (EC) perturbation, activation, and/or injury. Alteration in EC structure and/or function may be a critical event in vascular injury and, therefore, evaluation of the circulatory kinetic profile and secretory pattern of EC-specific proteins such as VWF and VWFpp could serve as acute vascular injury biomarkers. In rat and dog models of DIVI, this profile was determined using pharmacologically diverse agents associated with functional stimulation/perturbation (DDAVP), pathological activation (lipopolysaccharide [LPS]/endotoxin), and structural damage (fenoldopam [FD], dopamine [DA], and potassium channel opener (PCO) ZD6169). In rats, FD caused moderate DIVI and time-related increase in plasma VWF levels ∼33% while in control rats VWF increased ∼5%. In dogs, VWF levels transiently increased ∼30% when there was morphologic evidence of DIVI by DA or ZD6169. However, in dogs, VWFpp increased >60-fold (LPS) and >6-fold (DDAVP), respectively. This was in comparison to smaller dynamic 1.38-fold (LPS) and 0.54-fold (DDAVP) increases seen in plasma VWF. Furthermore, DA was associated with a dose-dependent increase in plasma VWFpp. In summary, VWF and VWFpp can discriminate between physiological and pathological perturbation, activation, and injury to ECs.

A conformation-sensitive monoclonal antibody against the A2 domain of von Willebrand factor reduces its proteolysis by ADAMTS13

The size of von Willebrand factor (VWF), controlled by ADAMTS13-dependent proteolysis, is associated with its hemostatic activity. Many factors regulate ADAMTS13-dependent VWF proteolysis through their interaction with VWF. These include coagulation factor VIII, platelet glycoprotein 1bα, and heparin sulfate, which accelerate the cleavage of VWF. Conversely, thrombospondin-1 decreases the rate of VWF proteolysis by ADAMTS13 by competing with ADAMTS13 for the A3 domain of VWF. To investigate whether murine monoclonal antibodies (mAbs) against human VWF affect the susceptibility of VWF to proteolysis by ADAMTS13 in vitro, eight mAbs to different domains of human VWF were used to evaluate the effects on VWF cleavage by ADAMTS13 under fluid shear stress and static/denaturing conditions. Additionally, the epitope of anti-VWF mAb (SZ34) was mapped using recombinant proteins in combination with enzyme-linked immunosorbent assay and Western blot analysis. The results indicate that mAb SZ34 inhibited proteolytic cleavage of VWF by ADAMTS13 in a concentration-dependent manner under fluid shear stress, but not under static/denaturing conditions. The binding epitope of SZ34 mAb is located between A1555 and G1595 in the central A2 domain of VWF. These data show that an anti-VWF mAb against the VWF-A2 domain (A1555-G1595) reduces the proteolytic cleavage of VWF by ADAMTS13 under shear stress, suggesting the role of this region in interaction with ADAMTS13.

Prevalence of Von Willebrand Disease in Dogs from São Paulo State, Brazil

The aims of the current study were to assess the prevalence of von Willebrand disease (vWD) in dogs from the region of Botucatu, São Paulo State, Brazil, and to evaluate laboratory tests to diagnose this disease. The study included 350 dogs of various ages, different breeds, and both sexes. Dogs included in the study had no historical or clinical evidence of abnormal bleeding. von Willebrand factor antigen (vWF: Ag), buccal mucosal bleeding time, activated partial thromboplastin time, and factor VIII activity were evaluated in their ability to diagnose vWD. The prevalence of vWD in dogs was 1.43% in the Botucatu region of Brazil. Determination of vWF: Ag was the best laboratory test to diagnose vWD.

Plasma von Willebrand factor level as a prognostic indicator of patients with metastatic colorectal carcinoma

AIM: To evaluate the correlations of plasma von Willebrand factor (vWF) level with the distant metastasis and prognosis of patients with colorectal cancer.

METHODS: A total of 86 patients with histologically confirmed metastatic colorectal cancers receiving treatment at Taipei Veterans General Hospital were enrolled. All patients had measurable metastatic lesions and life expectancies of more than 3 mo. Plasma vWF levels were measured by immuno-turbidimetric assay and compared with results from 40 non-metastatic colorectal cancer patients and 22 healthy controls. Patients with metastatic colorectal cancer were divided into two groups according to serum vWF levels and the differences between these two groups were analyzed using χ² test. Data on age, gender, performance status, location of primary tumor, extent of metastasis, site of metastases, histological differentiation, serum CEA and plasma vWF levels were analyzed to determine association with survival. Survival curves were constructed by Kaplan-Meier product limit method and the data was analyzed using log-rank test on a microcomputer. Multivariate analysis using the Cox’s proportional hazards regression model was then performed to determine the independent prognostic indicators among all of the possible variables.

RESULTS: Colorectal cancer patients were identified as having significantly higher plasma vWF concentrations than healthy controls (P<0.05). Moreover, higher vWF plasma levels were associated with advanced tumor stage (P<0.05) and the presence of multiple metastases (P = 0.014). Patients with lower vWF plasma levels (≤160%) survived significantly longer than those with a higher plasma vWF level (log-rank test, P = 0.0043). By multivariate analysis, plasma vWF levels (P<0.001), the extent of metastasis (P = 0.012), and the performance status (P = 0.014) were identified as independent prognostic factors.

CONCLUSION: Our data indicates that high plasma vWF concentrations correlate with advanced diseases and significantly poor prognosis of patients with metastatic colorectal carcinoma. It may serve as a potential biological marker of disease progression in these patients.

Cell type-specific regulation of von Willebrand factor expression by the E4BP4 transcriptional repressor

Mechanisms of tissue-restricted patterns of von Willebrand factor (VWF) expression involve activators and repressors that limit expression to endothelial cells and megakaryocytes. The relative transcriptional activity of the proximal VWF promoter was assessed in VWF-producing and -nonproducing cells, and promoter activity was highest in endothelial cells followed by megakaryocytes. Only basal VWF promoter activity was seen in nonendothelial cells. Here we identify a negative response element located at nucleotides (nts) +96/+105 and demonstrate, using chromatin immunoprecipitation (ChIP) analysis, that in vivo this sequence interacts with the E4BP4 transcriptional repressor. Differences in size and relative abundance of nuclear E4BP4 were observed. In HepG2 cells, low levels of larger forms of E4BP4 are present that directly interact with the negative response element. In VWF-expressing cells, high levels of smaller forms predominate with no evidence of direct DNA binding. However, in endothelial cells, mutation of the VWF E4BP4 binding motif not only restores but also further elevates VWF promoter activity, suggesting that E4BP4 may be part of a coordinated binding complex. These observations implicate this binding motif in repressing both activated and basal levels of VWF transcription by different cell type-specific mechanisms, and support the hypothesis that E4BP4 sequesters negative regulators of transcription, thereby enhancing activated gene expression.

A novel type 2A (Group II) von Willebrand disease mutation (L1503Q) associated with loss of the highest molecular weight von Willebrand factor multimers

Type 2A von Willebrand disease (VWD) is characterized by decreased platelet-dependent function of von Willebrand factor (VWF) associated with an absence of high-molecular-weight multimers. In this study, sequence analysis of the VWF gene from a Type 2A VWD patient showed a novel, heterozygous T-->A transversion at nucleotide 4510, resulting in the non-conservative substitution of L1503Q in the mature VWF subunit. This substitution, which was not found in 55 unrelated normal individuals, was reproduced by in vitro site directed mutagenesis of a full-length VWF cDNA and was subsequently expressed in COS-7 cells. The corresponding recombinant mutant VWF protein was partially retained in COS-7 cells yet the full spectrum of multimers was observed, suggesting that the absence of the highest molecular weight multimers results from increased proteolysis. The recombinant mutant VWF protein was digested with the ADAMTS13 protease from VWF-depleted plasma and the aberrant VWF multimer pattern was observed. These results suggest that the L1503Q substitution induces a conformational change in the VWF protein, which increases the protein's susceptibility to proteolysis. A three-dimensional model of the A2 domain demonstrates that the L1503Q mutation and the physiological proteolytic cleavage site for ADAMTS13 (Y(1605)-M(1606)) are localized close together in two adjacent parallel beta-sheets. The mutation L1503Q does not significantly disrupt the conformation of the protein; thus the subtle loss of multimers in this patient may be due to altered interactions with the ADAMTS13 protease.

A new flow cytometric method for measurement of von Willebrand factor activity

BACKGROUND

Patients with von Willebrand's disease may have normal levels of von Willebrand factor (VWF) antigen. It is therefore important to measure not only the antigen concentration but also the VWF activity. The most widely used method for measurement of VWF activity is the ristocetin cofactor assay (VWF:RCo), which is still crucial for the laboratory diagnosis of von Willebrand's disease (VWD). However, VWF:RCo has low precision, poor inter-laboratory reproducibility and requires an aggregometer. Many routine laboratories are not equipped with aggregometers but have flow cytometers instead.

METHODS

In this study a simple, precise and rapid flow cytometric assay was developed for the determination of von Willebrand factor activity, utilizing formalin-fixed platelets, fluorescein isothiocyanate-conjugated chicken anti-VWF antibodies (Fab-fragments) and phycoerythrine-conjugated anti-GPIIb/IIIa antibodies.

RESULTS

In samples from healthy controls and from patients with von Willebrand disease type 1, the flow cytometry assay showed good correlation with the VWF:RCo assay (r2 = 0.69) and the VWF antigen assays (r2 = 0.83), which was better than the correlation between the VWF:RCo assay and VWF antigen assays (r2 = 0.72). The flow cytometry method had good within-assay and total precision, C.V. 4.2%, and C.V. 7.5%, at a mean concentration of 0.40 IU/mL, respectively. Results obtained with the flow cytometric method on samples from two patients with von Willebrand disease 2B were lower than those obtained with the antigen method in accordance with the diagnosis.

CONCLUSION

The accuracy and precision of the von Willebrand activity assay may be improved if a flow cytometer is utilized for measurement of the impact of ristocetin on binding of VWF to formalin-fixed platelets instead of measuring agglutination utilizing an aggregometer. In addition, our flow cytometric method assay enables measurement of von Willebrand factor activity at many more hospitals than was previously possible with the traditional ristocetin cofactor platelet aggregometry assay, and this trend is likely to increase in the future when routine hematological instruments are equipped with built-in flow cytometers.

Factor VIII activity in canine von Willebrand disease

This study was conducted to determine the relationship between factor VIII (FVIII) activity and von Willebrand factor antigen (vWf:Ag) concentration in canine von Willebrand Disease (vWD). In addition, the clinical utility of measuring FVIII activity in vWD was assessed. This was performed by the concurrent analysis of both FVIII activity and vWf:Ag concentration in three breeds of dogs, namely Dobermans (n=183), Scottish Terriers (n=169), and Labrador Retrievers (n=146). In the three breeds tested, linear regression analysis illustrated a positive relationship between FVIII activity and vWf:Ag concentration. This was reaffirmed in the Doberman and Scottish Terrier breeds, in which dogs with vWf:Ag concentrations < 50 CU/dL ("carriers") had lower median FVIII activities than dogs with vWf:Ag concentrations > 70 CU/dL ("normals"). The determination of various FVIII "cut-off" values was a poor test to separate Dobermans with and without clinical signs of hemorrhage attributable to vWD. In addition, the occurrence of hemorrhage in Dobermans with vWf:Ag concentrations < 50 CU/dL was not influenced by the FVIII activity. Various tests were performed to determine if the measurement of FVIII activity aided in the identification of "carriers" of the vWD gene in the Doberman and Scottish Terrier breeds. These included the use of optimal FVIII "cut-off" values for each breed and a FVIII "cut-off" value of 55 CU/dL; FVIII/vWf:Ag ratios and FVIII/vWf:Ag ratio "cut-off" values; and linear regression analysis of vWf:Ag concentration against FVIII activity. Of all these tests, only the determination of FVIII/vWf:Ag ratios appeared to have promise for "carrier" detection. The data in the present study indicated that routine FVIII assessment in vWD is not warranted; however, measurement of FVIII activity may be of use in confirming the "carrier" status of vWD.

Labelling of rat endothelial cells with antibodies to vWF, RECA-1, PECAM-1, ICAM-1, OX-43 and ZO-1

Labelling with endothelium specific monoclonal antibodies, von Willebrand Factor (vWF), rat endothelial cell antigen-1 (RECA-1), platelet-endothelial cell adhesion molecule-1 (PECAM-1), intercellular adhesion molecule-1 (ICAM-1), OX-43 and zonula occludentes-1 (ZO-1), was investigated in cryostat sections of vessels from rats of different ages using a confocal microscope. The results showed that labelling of the vWF was positive in endothelial cells from adult, fetal and different ages of embryonic rat. Labelling with RECA-1 was weakly positive in adult rat aorta and lung endothelial cells but not in embryonic yolk sac endothelial cells. Labelling using PECAM-1, ICAM-1 and OX-43 was negative in both adult and embryonic endothelial cells. ZO-1 showed positive but very weak reactivity in embryonic yolk sac endothelial cells. The expression of vWF on vessels from adult and 19.5-day fetal tissues was strongly positive. However, the expression of vWF in embryonic endothelial cells was dependent on the gestational age. While the 11.5-day yolk sac vessels stained weakly, staining gradually increased in 13.5-, 15.5- and 17.5-day-old yolk sac vessels. The results suggest that vWF is a reliable endothelial cell marker in rat vascular endothelial cells, including both fetal and embryonic stages.

von Willebrand factor antigen levels in plasma of patients with malignant breast disease

von Willebrand factor (vWF) is a protein that mediates platelet adherence to the subendothelium during primary hemostasis. High plasma vWF concentrations have been reported in patients with various types of cancer, such as head and neck, laryngeal and prostatic cancer, probably representing an acute phase reactant. In the present study we determined the plasma levels of vWF antigen (vWF:Ag) by quantitative immunoelectrophoresis in 128 female patients with breast cancer as well as in 47 women with benign breast disease and in 27 healthy female controls. The levels of vWF:Ag were 170.7 +/- 78 U/dl in patients with cancer, 148.4 +/- 59 U/dl in patients with benign disease and 130.6 +/- 45 U/dl in controls (P<0.005). We also detected a significant increase in the levels of vWF:Ag (P<0.0001) in patients with advanced stages of the disease (stage IV = 263.3 +/- 113 U/dl, stage IIIB = 194.0 +/- 44 U/dl) as compared to those with earlier stages of the disease (stage I = 155.3 +/- 65 U/dl, stage IIA = 146.9 +/- 75 U/dl). In conclusion, vWF levels were increased in plasma of patients with malignant breast disease, and these levels correlated with tumor progression.

Recombinant von Willebrand factor: potential therapeutic use

Human von Willebrand factor (vWF) produced by recombinant technology offers a new perspective in treatment of von Willebrand disease (vWD). Several limitations connected with plasma-derived vWF concentrates, such as proteolytic degradation during the manufacture process, variation in multimer composition, lack of high molecular weight multimers, and donor dependence, can be overcome by rec-vWF. Recombinant vWF (rec-vWF) is produced by continuous fermentation of transformed mammalian cells. Biotechnological processes have been developed to isolated rec-vWF fractions with low, medium, and high degrees of multimerization. Structural analysis of rec-vWF demonstrated that it undergoes post-translational modifications comparable with plasma-derived vWF, such as multimerization, pro-peptide processing, and glycosylation. Functional analysis showed that rec-vWF exhibited activities comparable with plasma-derived vWF, such as platelet binding, platelet aggregation, collagen binding, and coagulation factor VIII (FVIII) binding. Collagen binding and platelet aggregation activity increased with the increasing multimer size of rec-vWF. Infusion of rec-vWF in antibody-induced vWF-deficient mice resulted in a significant decrease in bleeding. Infusion of rec-vWF in vWF-deficient dogs and pigs with severe vWD caused an increase in the endodenous FVIII level. Stabilization of FVIII in vivo was mediated both by high and low molecular weight rec-vWF molecules. Apparently, rec-vWF resisted proteolytic degradation in the circulation and no satellite bands were formed. Functional analysis in vitro and in vivo demonstrated the therapeutic potentials of rec-vWF, correction of vWF level, and stabilization of FVIII in plasma.

Type III von Willebrand's disease in Dutch kooiker dogs

Type III von Willebrand's disease (vWD) was diagnosed in 38 Dutch kooiker dogs. Ten male and 9 female probands had been referred independently of each other to the Utrecht University Clinic for Companion Animals because of a moderate to severe bleeding tendency. Screening of 717 Dutch kooiker dogs, including 356 puppies, detected vWD in another 19 dogs. Diagnosis was based on non-detectable amounts (< 1.6%) of von Willebrand factor antigen (vWF:Ag) in plasma by ELISA. Capillary bleeding time (CBT) was prolonged (> 10 min) and polybrene cofactor activity (vWF:PbCo) was not detectable in 11 dogs tested. No distinguishable protein bands were detected by multimer analysis. As in Scottish terriers with type III vWD, factor VIII clotting activity (FVIII:C) in affected Dutch kooiker dogs was decreased but considerably less than in humans with type III vWD. A recessive mode of inheritance was indicated by the normal or subnormal but measurable amounts of vWF:Ag in the plasma of eight pairs of parents of affected dogs. The F1 offspring resulting from the experimental mating of two affected dogs consisted of three affected males and four affected females. In 39 obligatory carriers vWF:Ag ranged from 30% to 114% with median and mean vWF values of 64% and 64.2%, respectively, and was subnormal (< 50%) in only 9 animals.

Type 2M von Willebrand Disease: F606I and I662F Mutations in the Glycoprotein Ib Binding Domain Selectively Impair Ristocetin- but not Botrocetin-Mediated Binding of von Willebrand Factor to Platelets

von Willebrand disease (vWD) is a common, autosomally inherited, bleeding disorder caused by quantitative and/or qualitative deficiency of von Willebrand factor (vWF). We describe two families with a variant form of vWD where affected members of both families have borderline or low vWF antigen levels, normal vWF multimer patterns, disproportionately low ristocetin cofactor activity, and significant bleeding symptoms. Whereas ristocetin-induced binding of plasma vWF from affected members of both families to fixed platelets was reduced, botrocetin-induced platelet binding was normal. The sequencing of genomic DNA identified unique missense mutations in each family in the vWF exon 28. In Family A, a missense mutation at nucleotide 4105T → A resulted in a Phe606Ile amino acid substitution (F606I) and in Family B, a missense mutation at nucleotide 4273A → T resulted in an Ile662Phe amino acid substitution (I662F). Both mutations are within the large disulfide loop between Cys509 and Cys695 in the A1 domain that mediates vWF interaction with platelet glycoprotein Ib. Expression of recombinant vWF containing either F606I or I662F mutations resulted in mutant recombinant vWF with decreased ristocetin-induced platelet binding, but normal multimer structure, botrocetin-induced platelet binding, collagen binding, and binding to the conformation-sensitive monoclonal antibody, AvW-3. Both mutations are phenotypically distinct from the previously reported variant type 2MMilwaukee-1 because of the presence of normal botrocetin-induced platelet binding, collagen binding, and AvW-3 binding, as well as the greater frequency and intensity of clinical bleeding. When the reported type 2M mutations are mapped on the predicted three-dimensional structure of the A1 loop of vWF, the mutations cluster in one region that is distinct from the region in which the type 2B mutations cluster.

Type 2M von Willebrand Disease: F606I and I662F Mutations in the Glycoprotein Ib Binding Domain Selectively Impair Ristocetin- but not Botrocetin-Mediated Binding of von Willebrand Factor to Platelets

von Willebrand disease (vWD) is a common, autosomally inherited, bleeding disorder caused by quantitative and/or qualitative deficiency of von Willebrand factor (vWF). We describe two families with a variant form of vWD where affected members of both families have borderline or low vWF antigen levels, normal vWF multimer patterns, disproportionately low ristocetin cofactor activity, and significant bleeding symptoms. Whereas ristocetin-induced binding of plasma vWF from affected members of both families to fixed platelets was reduced, botrocetin-induced platelet binding was normal. The sequencing of genomic DNA identified unique missense mutations in each family in the vWF exon 28. In Family A, a missense mutation at nucleotide 4105T → A resulted in a Phe606Ile amino acid substitution (F606I) and in Family B, a missense mutation at nucleotide 4273A → T resulted in an Ile662Phe amino acid substitution (I662F). Both mutations are within the large disulfide loop between Cys509 and Cys695 in the A1 domain that mediates vWF interaction with platelet glycoprotein Ib. Expression of recombinant vWF containing either F606I or I662F mutations resulted in mutant recombinant vWF with decreased ristocetin-induced platelet binding, but normal multimer structure, botrocetin-induced platelet binding, collagen binding, and binding to the conformation-sensitive monoclonal antibody, AvW-3. Both mutations are phenotypically distinct from the previously reported variant type 2MMilwaukee-1 because of the presence of normal botrocetin-induced platelet binding, collagen binding, and AvW-3 binding, as well as the greater frequency and intensity of clinical bleeding. When the reported type 2M mutations are mapped on the predicted three-dimensional structure of the A1 loop of vWF, the mutations cluster in one region that is distinct from the region in which the type 2B mutations cluster.

von Willebrand's disease in Scottish Terriers in Australia

Over a 5-year period (1988-92), von Willebrand factor antigen (vWf:Ag) assays were performed on plasma samples from 207 Scottish Terriers. Based on these tests, 47 dogs (23%) had vWf:Ag concentrations < 50 canine units (CU)/dL and were classified as heterozygous carriers of the von Willebrand's disease (vWD) gene, while 9 (4%) had concentrations below the sensitivity of the assays and were classified as homozygous. There was thus an overall prevalence of 27% for the vWD gene in the Scottish Terriers tested. The homozygous dogs (median age 0.6 years at diagnosis) consisted of 7 males and 2 females. Eight of these had haemorrhage attributable to the disease, mostly spontaneous and from the oral mucosa. Other signs included haemorrhage induced by trauma or surgery, easy bruising and epistaxis. Many haemorrhagic episodes were severe enough to warrant therapeutic intervention and there was a single fatality. Pedigree analysis, possible in 7 of the dogs, revealed that each was the progeny of a mating between dogs with vWf:Ag concentrations < 50 CU/dL, which supported an autosomal recessive mode of inheritance. A single heterozygous carrier suffered haemorrhage after surgery that, in contrast to the homozygotes, was mild and did not require therapy. The data indicate that vWD is a significant problem in Scottish Terriers in Australia. Accordingly, we recommend that steps be taken to reduce the prevalence of the disease and thereby the number of clinically affected dogs, such as the establishment of a national testing scheme to determine the vWD status of all breeding dogs.

New variant of type II von Willebrand's disease with structural abnormality of plasma von Willebrand factor in a patient with very mild bleeding history

A new variant of von Willebrand's disease has been discovered in 2 members of a Macedonian family of 6. The proband, an 8-year-old boy, showed a prolonged bleeding episode on 1 occasion. Ristocetin-induced platelet aggregation and bleeding time were normal. In plasma, ristocetin cofactor activity (RCo) and von Willebrand factor (vWf) antigen were reduced to the same clearly low level. The determination of vWf antigen of platelets resulted in borderline values, while RCo was clearly reduced. Low- and intermediate-resolution agarose gel electrophoresis showed absence of the largest multimers in plasma vWf, and slight reduction in platelet vWf. High-resolution gels revealed abnormal multimeric structure only in plasma vWf. The smaller multimers could be resolved in a broad central band flanked by 4 fainter satellite bands; however, satellite bands close to the central band were more intense, and more distant ones were fainter, compared to normal plasma. The central band of the fastest-moving multimer was markedly intensified, and the mobility of the whole quintuplet was slightly reduced. Heredity seems to be autosomal-dominant. No mutation was found in exon 28 of the vWf gene. Because there was only 1 mild bleeding episode in the family, this structural variation seems to have only little clinical consequence. We conclude that this vWf abnormality is different from those observed in other type II variants previously described. Based on the revised classification by the International Society on Thrombosis and Haemostasis, we proposed designation type 2A-Bern for this new subtype.

A monoclonal antibody (B724) to von Willebrand factor recognizing an epitope within the A1 disulphide loop (Cys509-Cys695) discriminates between type 2A and type 2B von Willebrand disease

Monoclonal antibody (MoAb) B724 to von Willebrand factor (vWF) completely inhibits its interaction with heparin, sulphatides and botrocetin and consequently botrocetin-induced binding of vWF to platelets. MoAb B724 has no effect on the binding of vWF to collagen or to ristocetin-treated platelets nor on vWF-dependent platelet aggregation induced with ristocetin and asialo-vWF-mediated platelet aggregation. MoAb B724 preferentially recognizes a conformation of native vWF, in solution, or immobilized through a coated antibody. It exhibits a markedly lower affinity for vWF immobilized onto collagen or plastic surfaces. Using proteolytic fragments of vWF, B724 epitope was localized within the 512-673 sequence of the A1 disulphide loop of vWF, MoAb B724 was used as second antibody in a two-site ELISA to test a series of patients with type 1, 2A, 2B and 2N vWD or haemophilia A and recombinant wild type or mutated vWFs. Results were compared with those obtained by control ELISAs performed using polyclonal antibodies. Using MoAb B724, strikingly lower levels of vWFAg were observed in plasma from most patients with type 2B vWD, and in seven out of the eight rvWF mutated close to or within the A1 disulphide loop. Therefore MoAb B724, which interferes with this loop involved in the function of vWF, appears to be a useful tool for rapid screening of conformational changes in this region.

Proteolytic processing of von Willebrand factor subunit: heterogeneity in type-IIA von Willebrand disease

Type IIA von Willebrand disease (vWD) is a heterogeneous disorder for which two different pathogenetic mechanisms have been proposed: increased proteolytic susceptibility of von Willebrand factor (vWF), and/or interference of its post-translational processing. Subunit analysis of vWF in type-IIA vWD has revealed an increased relative proportion of the 176- and 140-kDa subunit-derived fragments, suggesting an augmented fragmentation of vWF, even in the resting state. We analyzed the subunit pattern of vWF in plasma from five previously described patients with type-IIA vWD. All of them showed the above-mentioned pattern. In addition, the presence of a new band with an apparent molecular mass of 200 kDa, not described in normal individuals or in patients with vWD, was repeatedly observed in one of these patients. This patient also exhibited an abnormal vWF multimeric structure in platelets and in plasma, before and after desmopressin administration, when the blood was collected either in the presence or in the absence of proteinase inhibitors. We believe that an abnormal primary structure of vWF could be responsible for this abnormal proteolytic fragmentation pattern, as well as for the abnormal multimerization of vWF. Moreover, an abnormal susceptibility to proteolysis appears to be present, as suggested by the increase in the relative proportion of the 176-kDa fragment observed in the same patient. Future sequencing studies and genetic analysis may clarify whether there are one or two different defects related to the vWF of that patient. Our results indicate that the subunit analysis of vWF may reveal additional defects present in type-IIA vWD that may help our understanding of the pathogenesis of such disease.

Quantitative defect of glycoprotein Ib in severe cirrhotic patients

A decrease of platelet agglutination induced by ristocetin has been described in cirrhotic patients. In order to investigate the relationship of such phenomenon with a putative defect on platelet membrane glycoprotein Ib, we have studied the quantitative and functional status of Gp Ib in eleven severe alcoholic cirrhotic patients, and the ability of their formalin-fixed platelets to agglutinate in the presence of normal plasma plus ristocetin. Interestingly, we found a significant decrease of immunoreactive GP Ib molecules (9,978 +/- 1,534 vs. 17,064 +/- 404 molecules per platelet) and ristocetin-dependent binding of vWF (9,113 +/- 1,338 vs. 13,992 +/- 1,968 molecules per platelet) (P < 0.01) in comparison to the levels found in a control group of healthy subjects. Immunoblotting analysis of platelet lysates confirmed the reduction of GP Ib level in cirrhotic patients, but showed no modification on the precipitation pattern of this glycoprotein. The ristocetin-induced platelet agglutination (light transmission %) was also significantly lower in patients with cirrhosis (48 +/- 7.6 vs. 92 +/- 3.6, P < 0.01), and correlated with the binding of normal vWF (r = 0.863, P = 0.0013). Patients with bleeding times longer than 7 min and/or clinical history of bleeding episodes showed the lowest values of platelet agglutination and GP Ib level. In conclusion, our present results indicate that liver cirrhosis is associated with a relevant decrease of functional GP Ib molecules on the platelet surface. This reduction might be related to the prolongation of bleeding time and to the bleeding diathesis observed in cirrhotic patients.

Multimeric analysis of von Willebrand factor by vertical sodium dodecyl sulphate agarose gel electrophoresis, vacuum blotting technology and sensitive visualization by alkaline phosphatase anti-alkaline phosphatase complex

To detect von Willebrand factor multimers in plasma samples and factor VIII concentrates, a vertical discontinuous SDS electrophoresis was developed. A vacuum blotting system allowed to improve the transfer to the nitrocellulose membrane. The visualization of the separated multimers was sensitized by applying an alkaline phosphatase anti-alkaline phosphatase staining technique. The reported method clearly shows structural abnormalities of von Willebrand factor and deficiency of high multimers, the vacuum transfer is efficient and the sensitivity of the staining system is very high.

Functional analysis of a type IIB von Willebrand disease missense mutation: increased binding of large von Willebrand factor multimers to platelets

Type IIB von Willebrand disease is an autosomal dominant bleeding disorder characterized by the selective loss of high molecular weight von Willebrand factor (vWF) multimers in plasma, presumably due to their abnormally increased reactivity with platelets. We and others have recently identified a panel of missense mutations clustered in the platelet glycoprotein Ib binding domain of vWF from patients with type IIB von Willebrand disease. We now report functional analysis of one of the most frequent type IIB missense mutations, Arg-543----Trp (vWF R543W). vWF from a human umbilical vein endothelial cell culture heterozygous for the vWF R543W mutation showed markedly increased binding of large vWF multimers to platelets in the presence of a low dose of ristocetin compared to vWF from a normal control culture. Recombinant vWF containing the vWF R543W mutation expressed in COS-7 cells also demonstrated increased binding of large vWF multimers. Mixed multimers obtained by cotransfection of mutant and wild-type cDNAs showed partial dominance of the vWF R543W mutation. Thus these data demonstrate that the vWF R543W mutation alone is sufficient to confer increased binding of large vWF multimers to platelets in a dominant fashion and that no other factors relating to vWF posttranslational processing or secretion in endothelial cells are required for this effect.