The increasing maturity of the von Willebrand factor collagen binding in von Willebrand disease diagnosis

VWF maturation and release are controlled by 2 regulators of Weibel-Palade body biogenesis: exocyst and BLOC-2

von Willebrand factor (VWF) is an essential hemostatic protein that is synthesized in endothelial cells and stored in Weibel-Palade bodies (WPBs). Understanding the mechanisms underlying WPB biogenesis and exocytosis could enable therapeutic modulation of endogenous VWF, yet optimal targets for modulating VWF release have not been established. Because biogenesis of lysosomal related organelle-2 (BLOC-2) functions in the biogenesis of platelet dense granules and melanosomes, which like WPBs are lysosome-related organelles, we hypothesized that BLOC-2-dependent endolysosomal trafficking is essential for WPB biogenesis and sought to identify BLOC-2-interacting proteins. Depletion of BLOC-2 caused misdirection of cargo-carrying transport tubules from endosomes, resulting in immature WPBs that lack endosomal input. Immunoprecipitation of BLOC-2 identified the exocyst complex as a binding partner. Depletion of the exocyst complex phenocopied BLOC-2 depletion, resulting in immature WPBs. Furthermore, releasates of immature WPBs from either BLOC-2 or exocyst-depleted endothelial cells lacked high-molecular weight (HMW) forms of VWF, demonstrating the importance of BLOC-2/exocyst-mediated endosomal input during VWF maturation. However, BLOC-2 and exocyst showed very different effects on VWF release. Although BLOC-2 depletion impaired exocytosis, exocyst depletion augmented WPB exocytosis, indicating that it acts as a clamp. Exposure of endothelial cells to a small molecule inhibitor of exocyst, Endosidin2, reversibly augmented secretion of mature WPBs containing HMW forms of VWF. These studies show that, although BLOC-2 and exocyst cooperate in WPB formation, only exocyst serves to clamp WPB release. Exocyst function in VWF maturation and release are separable, a feature that can be exploited to enhance VWF release.

The heparin-von Willebrand factor interaction and conventional tests of haemostasis - the challenges in predicting bleeding in cardiopulmonary bypass

Bleeding is a significant complication of cardiopulmonary bypass (CPB), despite routine anticoagulation monitoring. This is likely to be multifactorial. In this prospective, single-centre cohort study of 30 patients undergoing CPB surgery, our aim was to characterise the changes in von Willebrand factor (VWF) function, platelet interaction and the global coagulation changes during and after CPB surgery and to determine whether bleeding can be predicted. Samples were taken at six time points before, during and after CPB surgery. We observed a significant rise in VWF antigen (VWF:Ag) throughout surgery, which continued postoperatively. The absolute VWF collagen-binding assays (VWF:CB) and VWF ristocetin cofactor (VWF:RCo) rose significantly but the VWF:CB/VWF:Ag and VWF:Ag/VWF:RCo fell significantly (P = 0·0015 and P = 0·0143), suggesting loss of large multimers. We detected a non-significant trend to loss of VWF:RCo after heparinisation and a significant recovery after protamine reversal which could reflect a direct heparin effect. There was a significant increase in the R and K times with a fall in alpha angle and maximum amplitude after heparin administration, using heparinase-thromboelastography (TEG). The parameters both significantly improved following protamine (P = 0·007 and P = 0·0054). The activated clotting time (ACT) and heparin anti-Xa level correlated poorly; neither predicted clinically significant bleeding. None of these parameters had a relationship with intraoperative blood loss or requirement for blood product replacement.

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

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

Evaluation of a fully automated von Willebrand factor assay panel for the diagnosis of von Willebrand disease

INTRODUCTION

von Willebrand disease (VWD) diagnosis starts with first level tests: factor VIII coagulant activity, VWF antigen (VWF:Ag) and platelet-dependent VWF activity (VWF:RCo, VWF:Ab, VWF:GPIbR or VWF:GPIbM). The VWF collagen binding (VWF:CB) assay measures the binding capacity of von Willebrand factor (VWF) to collagen.

AIM

To assess, in previously diagnosed VWD patients, the performance of a fully automated chemiluminescent test panel including VWF:Ag, VWF:GPIbR and VWF:CB assays.

METHODS

The patients, historically evaluated using in-house VWF:Ag and VWF:CB assays and an automated latex enhanced immunoassay VWF:GPIbR method, were re-evaluated using the VWF test panel HemosIL AcuStar.

RESULTS

The VWF:GPIbR/VWF:Ag and VWF:CB/VWF:Ag obtained by means of AcuStar showed an overall good concordance with the corresponding data obtained at the time of the historical diagnosis. When discrepancies occurred, these were generally due to the lower VWF:CB/VWF:Ag obtained with AcuStar as compared with that obtained with the historical methods and this affected particularly the diagnosis of VWD type 2M. Together, the AcuStar VWF:GPIbR/VWF:Ag and VWF:CB/VWF:Ag were able to distinguish type 1 from types 2A, 2B and 2M, whereas no distinction was possible between type 2A and 2B.

CONCLUSION

The AcuStar panel offers a good performance in the differential diagnosis between VWD type 1 and 2A/2B patients. A high rate of coincidence with historical diagnosis was obtained for VWD types 3, 2A/2B and 1. Even though in some cases more tests (eg, RIPA/multimeric analysis) are needed to complete an accurate VWD classification, the AcuStar panel is considered a sensitive, rapid and reliable tool to diagnose VWD patients.

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

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