Comparison of type I, type III and type VI collagen binding assays in diagnosis of von Willebrand disease

Fundamental considerations for designing endothelialized in vitro models of thrombosis

Endothelialized in vitro models for cardiovascular disease have contributed greatly to our current understanding of the complex molecular mechanisms underlying thrombosis. To further elucidate these mechanisms, it is important to consider which fundamental aspects to incorporate into an in vitro model. In this review, we will focus on the design of in vitro endothelialized models of thrombosis. Expanding our understanding of the relation and interplay between the different pathways involved will rely in part on complex models that incorporate endothelial cells, blood, the extracellular matrix, and flow. Importantly, the use of tissue-specific endothelial cells will help in understanding the heterogeneity in thrombotic responses between different vascular beds. The dynamic and complex responses of endothelial cells to different shear rates underlines the importance of incorporating appropriate shear in in vitro models. Alterations in vascular extracellular matrix composition, availability of bioactive molecules, and gradients in concentration and composition of these molecules can all regulate the function of both endothelial cells and perivascular cells. Factors modulating these elements in in vitro models should therefore be considered carefully depending on the research question at hand. As the complexity of in vitro models increases, so can the variability. A bottom-up approach to designing such models will remain an important tool for researchers studying thrombosis. As new techniques are continuously being developed and new pathways are brought to light, research question-dependent considerations will have to be made regarding what aspects of thrombosis to include in in vitro models.

The Role of the von Willebrand Factor Collagen-Binding Assay (VWF:CB) in the Diagnosis and Treatment of von Willebrand Disease (VWD) and Way Beyond: A Comprehensive 36-Year History

The von Willebrand factor (VWF) collagen binding (VWF:CB) assay was first reported for use in von Willebrand diagnostics in 1986, by Brown and Bosak. Since then, the VWF:CB has continued to be used to help diagnose von Willebrand disease (VWD) (correctly) and also to help assign the correct subtype, as well as to assist in the monitoring of VWD therapy, especially desmopressin (DDAVP). However, it is important to recognize that the specific value of any VWF:CB is predicated on the use of an optimized VWF:CB, and that not all VWF:CB assays are so optimized. There are some good commercial assays available, but there are also some "not-so-good" commercial assays available, and these may continue to give the VWF:CB "a bad reputation." In addition to VWD diagnosis and management, the VWF:CB found purpose in a variety of other applications, from assessing ADAMTS13 activity, to investigation into acquired von Willebrand syndrome (especially as associated with use of mechanical circulatory support or cardiac assist devices), to assessment of VWF activity in disease states in where an excess of high-molecular-weight VWF may accumulate, and lead to increased (micro)thrombosis risk (e.g., coronavirus disease 2019, thrombotic thrombocytopenic purpura). The VWF:CB turns 37 in 2023. This review is a celebration of the utility of the VWF:CB over this nearly 40-year history.

High levels of von Willebrand factor with reduced specific activities in hospitalized patients with or without COVID-19

The COVID-19 pandemic is often accompanied by severe respiratory illness and thrombotic complications. Von Willebrand Factor (VWF) levels are highly elevated in this condition. However, limited data are available on the qualitative activity of VWF in COVID-19. We measured plasma VWF levels quantitatively (VWF antigen) and qualitatively (ristocetin-induced platelet agglutination, glycoprotein IbM (GPIbM) binding, and collagen binding). Consistent with prior reports, VWF antigen levels were significantly elevated in hospitalized patients with or without COVID-19. The GPIbM and collagen binding activity-to-antigen ratios were significantly reduced, consistent with qualitative changes in VWF in COVID-19. Of note, critically ill hospitalized patients without COVID-19 had similar reductions in VWF activity-to-antigen ratios as patients with COVID-19. Our data suggest that qualitative changes in VWF in COVID-19 may not be specific to COVID-19. Future studies are warranted to determine the mechanisms responsible for qualitative changes in VWF in COVID-19 and other critical illnesses.• VWF levels were increased in COVID-19 compared to healthy controls.• VWF activity-to-antigen ratios were decreased in COVID-19 compared to healthy controls.• There were no differences in VWF activity-to-antigen ratios between hospitalized patients with or without COVID-19.• These findings are consistent with qualitative changes in VWF in systemic inflammation which are not specific to COVID-19.• Future studies are needed to define possible roles of changes in conformation or multimer length in the qualitative changes in VWF in systemic inflammation.

Tip-end fusion of a rod-shaped secretory organelle

Weibel–Palade bodies (WPB) are elongated, rod-like secretory organelles unique to endothelial cells that store the pro-coagulant von-Willebrand factor (VWF) and undergo regulated exocytosis upon stimulation with Ca2+- or cAMP-raising agonists. We show here that WPB preferentially initiate fusion with the plasma membrane at their tips and identify synaptotagmin-like protein 2-a (Slp2-a) as a positive regulator of VWF secretion most likely mediating this topological selectivity. Following secretagogue stimulation, Slp2-a accumulates at one WPB tip before fusion occurs at this site. Depletion of Slp2-a reduces Ca2+-dependent secretion of highly multimeric VWF and interferes with the formation of actin rings at WPB–plasma membrane fusion sites that support the expulsion of the VWF multimers and most likely require a tip-end fusion topology. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] binding via the C2A domain of Slp2-a is required for accumulation of Slp2-a at the tip ends of fusing WPB, suggesting that Slp2-a mediates polar exocytosis by initiating contacts between WPB tips and plasma membrane PI(4,5)P2.

Tissue dynamics of von Willebrand factor characterized by a novel fluorescent protein-von Willebrand factor chimera

BACKGROUND

Tissue dynamics of von Willebrand factor (VWF) that are vital to its biological function have not been fully characterized.

OBJECTIVE

To develop a new fluorescent protein--VWF chimera (FP-VWF) that has similar hematologic function to wild-type VWF and use it to monitor the tissue dynamics of VWF distribution.

METHODS

Genotyping, platelet counting, tail bleeding time assay, agarose gels, western blot, platelet aggregation, proteolytic analysis, and ELISA were applied in characterizing the function of FP-VWF; fluorescence spectrometer and confocal fluorescence microscope were used to monitor the plasma and tissue distribution of FP-VWF.

RESULTS

The transgenic mice that carry the FP-VWF retain hematologic activity of VWF with plasma levels of FP-VWF reduced by 50% and there are reduced high molecular weight FP-VWF multimers compared to the wild-type mice. The GPIb-binding and ADAMTS-13 (A Disintegrin and Metalloprotease with ThrombSpondin type 1 motif, member 13) proteolytic efficiency of FP-VWF are similar to wild-type VWF. The tissue distribution of FP-VWF was probed directly through its intrinsic fluorescence at normal or stimulated status, which indicated that the medicine-stimulated endogenous FP-VWF seems primarily released from the aorta and cleared in the spleen. Similar results were observed in non-fluorescent mice through a standard immunofluorescence approach. The fluorescence signals of FP-VWF were also similar to the standard dye-based approach in detecting the FeCl3 -induced blood clotting in vivo.

CONCLUSIONS

Together, these results suggest that this novel FP-VWF chimera is valuable in probing the tissue dynamics of VWF in quite a few biological and pharmaceutical applications.

Navigating the Myriad of von Willebrand Factor Assays

von Willebrand factor (VWF) represents a large and complex adhesive plasma protein whose main function is to provide a bridge between blood platelets and damaged endothelium, and thus facilitate primary hemostasis. VWF also binds to FVIII, preventing early proteolysis, and delivers this cargo to sites of vascular injury, thereby promoting clot formation and secondary hemostasis. An absence, deficiency, or defect in VWF can lead to a bleeding diathesis called von Willebrand disease (VWD), considered the most common inherited bleeding disorder. Contemporary laboratory assays used in VWD diagnosis/exclusion comprise a myriad of assays that identify the quantity (level) of VWF, as well as the multitude of VWF activities. These may use the following test abbreviations: VWF:Ag, VWF:RCo, VWF:CB, VWF:GPIbR, VWF:GPIbM, VWF:FVIIB, VWF:Ab. The current review explains what these assays are, as well as their place in VWD diagnostics.

Cleavage by MMP-13 renders VWF unable to bind to collagen but increases its platelet reactivity

BACKGROUND

Atherosclerotic plaque rupture and subsequent thrombosis underpin thrombotic syndromes. Under inflammatory conditions in the unstable plaque, perturbed endothelial cells secrete von Willebrand Factor (VWF) which, via its interaction with GpIbα, enables platelet rolling across and adherence to the damaged endothelium. Following plaque rupture, VWF and platelets are exposed to subendothelial collagen, which supports stable platelet adhesion, activation, and aggregation. Plaque-derived matrix metalloproteinase (MMP)-13 is also released into the surrounding lumen where it may interact with VWF, collagen, and platelets.

OBJECTIVES

We sought to discover whether MMP-13 can cleave VWF and whether this might regulate its interaction with both collagen and platelets.

METHODS

We have used platelet adhesion assays and whole blood flow experiments to assess the effects of VWF cleavage by MMP-13 on platelet adhesion and thrombus formation.

RESULTS

Unlike the shear-dependent cleavage of VWF by a disintegrin and metalloprotease with thrombospondin motif member 13 (ADAMTS13), MMP-13 is able to cleave VWF under static conditions. Following cleavage by MMP-13, immobilized VWF cannot bind to collagen but interacts more strongly with platelets, supporting slower platelet rolling in whole blood under shear. Compared with intact VWF, the interaction of cleaved VWF with platelets results in greater GpIbα upregulation and P-selectin expression, and the thrombi formed on cleaved VWF-collagen co-coatings are larger and more contractile than platelet aggregates on intact VWF-collagen co-coatings or on collagen alone.

CONCLUSIONS

Our data suggest a VWF-mediated role for MMP-13 in the recruitment of platelets to the site of vascular injury and may provide new insights into the association of MMP-13 in atherothrombotic and stroke pathologies.

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.

Disease progression and defects in primary hemostasis as major cause of bleeding in multiple myeloma

BACKGROUND AND OBJECTIVES

In patients with multiple myeloma (MM), unexpected bleeding complications remain a major issue. Since routine coagulation parameters are often inconspicuous, diagnosis and treatment of the underlying coagulation disorders are challenging.

PATIENTS AND METHODS

In our single-center observational study, we analyzed 164 patients with MM for coagulation disorders and bleeding complications.

RESULTS

Prolonged closure times (CTs), measured by PFA-100, were the most common, abnormal coagulation test, found in 66% of bleeding patients vs 5% in non-bleeding, followed by qualitative defects of von Willebrand factor (VWF:CB/VWF:Ag ratios), found in 34% vs 1% in the non-bleeding group. Increased serum free light chains (SFLC) and SFLC ratios were significantly associated with prolonged CTs and acquired von Willebrand syndrome (AVWS). Prolonged CTs and AVWS were associated with disease progression, determined by dynamics of SFLC ratios (P < .001), serum creatinine level (P = .013), Beta-2 microglobulin (P = .03), LDH (P = .016), and bone marrow infiltration (P < .001). Of note, response to myeloma therapy was frequently correlated with normalization of coagulation parameters.

CONCLUSIONS

Bleeding complications in MM are predominantly caused by defects in primary hemostasis and associated with disease progression. In a peri-interventional workup, determination of CTs and VWF:CB/VWF:Ag ratios are of significant importance to assess bleeding risk.

von Willebrand Disease: Diagnostic Strategies and Treatment Options

von Willebrand disease (VWD) is one of the most common inherited bleeding disorders. Since its first description in 1926, the diagnosis and management of VWD has significantly improved due to increasing scientific knowledge of the genetics and biology of von Willebrand factor (VWF). This article reviews the molecular structure and function of VWF as well as the clinical symptoms, laboratory-based diagnostic workup, and classification schema for VWD. It highlights current treatment options and state-of-the art research in VWF and VWD.

Evaluation of a microfluidic flow assay to screen for von Willebrand disease and low von Willebrand factor levels

Essentials von Willebrand factor (VWF) function is shear stress dependent. Platelet accumulation in a microfluidic assay correlates with VWF levels. The microfluidic assay discriminates type 1 von Willebrand disease from healthy controls. The microfluidic flow assay detects responses to therapeutic intervention (DDAVP).

SUMMARY

Background von Willebrand disease (VWD) is a mucocutaneous bleeding disorder with a reported prevalence of 1 in 10 000. von Willebrand factor (VWF) function and platelet adhesion are regulated by hemodynamic forces that are not integrated into most current clinical assays. Objective We evaluated whether a custom microfluidic flow assay (MFA) can screen for deficiencies in VWF in patients presenting with mucocutaneous bleeding. Methods Whole blood from individuals with mucocutaneous bleeding was assayed in a custom MFA. Results Thirty-two patients with type 1 VWD (10/32) or reported mucocutaneous bleeding were enrolled. The platelet adhesion velocity (r = 0.5978 for 750 s-1 and 0.6895 for 1500 s-1 ) and the maximum platelet surface area coverage (r = 0.5719 for 750 s-1 and 0.6633 for 1500 s-1 ) in the MFA correlated with VWF levels. Furthermore, the platelet adhesion velocity at 750 s-1 (type 1 VWD, mean 0.0009761, 95% confidence interval [CI] 0.0003404-0.001612; control, mean 0.003587, 95% CI 0.002455-0.004719) and at 1500 s-1 (type 1 VWD, mean 0.0003585, 95% CI 0.00003914-0.0006778; control, mean 0.003132, 95% CI 0.001565-0.004699) differentiated type 1 VWD from controls. Maximum platelet surface area coverage at 750 s-1 (type 1 VWD, mean 0.1831, 95% CI 0.03816-0.3281; control, mean 0.6755, 95% CI 0.471-0.88) and at 1500 s-1 (type 1 VWD, mean 0.07873, 95% CI 0.01689-0.1406; control, mean 0.6432, 95% CI 0.3607-0.9257) also differentiated type 1 VWD from controls. We also observed an improvement in platelet accumulation after 1-desamino-8-d-arginine vasopressin (DDAVP) treatment at 1500 s-1 (pre-DDAVP, mean 0.4784, 95% CI 0.1777-0.7791; post-DDAVP, mean 0.8444, 95% CI 0.7162-0.9726). Conclusions These data suggest that this approach can be used as a screening tool for VWD.

Combined effects of two mutations in von Willebrand disease 2M phenotype

BACKGROUND

Type 2M von Willebrand disease (VWD2M) is usually characterized by VWF:RCo/VWF:Ag<0.6 and normal multimeric profile; desmopressin (DDAVP) challenge test commonly shows poor response of VWF:RCo.

OBJECTIVE

We describe the bleeding tendency and the laboratory phenotype in a patient carrying two heterozygous mutations affecting VWF-A1 domain and VWF-A2 domain.

SUBJECTS/METHODS

A 12-year-old patient (O blood group) with severe hemorrhagic tendency was phenotypically and genotypically analyzed; his parents were also studied.

RESULTS

The proband showed decrease FVIII:C, VWF:RCo/VWF:Ag, and VWF:CB6/VWF:Ag ratios, but normal platelet count, VWF:CB1/VWF:Ag ratio, VWFpp and multimeric pattern, suggesting a VWD2M phenotype. The DDAVP challenge test, compared to controls (VWD2M patients with mutations in VWF-A1 domain), showed lower increase of FVIII:C and VWF:Ag than in heterozygous, but very similar to homozygous control. Two mutations were found in heterozygous and trans presentation: p.Pro1648fs*45 and a novel missense mutation, p.Arg1426Cys. The mother was p.Arg1426Cys heterozygous carrier, with few clinical symptoms. The father was asymptomatic, with no mutations. The p.Pro1648fs*45 was considered an apparent de novo mutation; proband's AS-PCR revealed mosaicism in the paternal allele. According to the predicted models, p.Arg1426Cys would not be affecting the binding of GPIbα to A1 domain, whereas p.Pro1648fs*45 seems to modify the folding of A2 domain, and in this way, it would affect the binding to GPIbα and type VI collagen. We believe that the combination of these two heterozygous mutations, in a child with O blood group, could result in a defective phenotype enhancer.

Entrapment of Autologous von Willebrand Factor on Polystyrene/Poly(methyl methacrylate) Demixed Surfaces

Human platelets play a vital role in haemostasis, pathological bleeding and thrombosis. The haemostatic mechanism is concerned with the control of bleeding from injured blood vessels, whereby platelets interact with the damaged inner vessel wall to form a clot (thrombus) at the site of injury. This adhesion of platelets and their subsequent aggregation is dependent on the presence of the blood protein von Willebrand Factor (vWF). It is proposed here that the entrapment of vWF on a substrate surface offers the opportunity to assess an individual’s platelet function in a clinical diagnostic context. Spin coating from demixed solutions of polystyrene (PS) and poly(methyl methacrylate) (PMMA) onto glass slides has been shown previously to support platelet adhesion but the mechanism by which this interaction occurs, including the role of vWF, is not fully understood. In this work, we report a study of the interaction of platelets in whole blood with surfaces produced by spin coating from a solution of a weight/weight mixture of a 25% PS and 75% PMMA (25PS/75PMMA) in chloroform in the context of the properties required for their use as a Dynamic Platelet Function Assay (DPFA) substrate. Atomic Force Microscopy (AFM) indicates the presence of topographical features on the polymer demixed surfaces in the sub-micron to nanometer range. X-ray Photoelectron Spectroscopy (XPS) analysis confirms that the uppermost surface chemistry of the coatings is solely that of PMMA. The deliberate addition of various amounts of 50 μm diameter PS microspheres to the 25PS/75PMMA system has been shown to maintain the PMMA chemistry, but to significantly change the surface topography and to subsequently effect the scale of the resultant platelet interactions. By blocking specific platelet binding sites, it has been shown that their interaction with these surfaces is a consequence of the entrapment and build-up of vWF from the same whole blood sample.

How much do we really know about von Willebrand disease?

PURPOSE OF REVIEW

In the last nine decades, large advances have been made toward the characterization of the pathogenic basis and clinical management of von Willebrand disease (VWD), the most prevalent inherited bleeding disorder. Pathological variations at the von Willebrand factor (VWF) locus present as a range of both quantitative and qualitative abnormalities that make up the complex clinical spectrum of VWD. This review describes the current understanding of the pathobiological basis of VWD.

RECENT FINDINGS

The molecular basis of type 2 (qualitative abnormalities) and type 3 VWD (total quantitative deficiency) have been well characterized in recent decades. However, knowledge of type 1 VWD (partial quantitative deficiency) remains incomplete because of the allelic and locus heterogeneity of this trait, and is complicated by genetic variability at the VWF gene, interactions between the VWF gene and the environment, and the involvement of external modifying loci. Recent genome wide association studies and linkage analyses have sought to identify additional genes that modify the type 1 VWD phenotype.

SUMMARY

Understanding the pathogenic basis of VWD will facilitate the development of novel treatment regimens for this disorder, and improve the ability to provide complementary molecular diagnostics for type 1 VWD.

The Von Willebrand Factor A1-Collagen III Interaction Is Independent of Conformation and Type 2 Von Willebrand Disease Phenotype

The blood von Willebrand factor (VWF) mediates platelet adhesion to injured vessels by sequestering platelets from blood flow and depositing them to collagen and other exposed subendothelial matrix proteins. This process of capturing platelets to facilitate formation of platelet plugs occurs through transient interactions with platelet glycoprotein Ibα via the VWF A1 domain which also binds collagen. Using a conformationally diverse collection of natively folded and mutation-induced misfolded von Willebrand disease (VWD) variants, we test a recently proposed affinity up-regulation hypothesis which states that collagen binding changes the conformation of the A1 domain to a high-affinity GPIbα binding competent state. With surface plasmon resonance (SPR), we present this diversified collection to collagen and quantify the kinetics of association and dissociation to ascertain the conformational selectivity of collagen. With analytical rheology, we quantify real-time platelet pause times and translocation velocities across a Cu²⁺ HisTag-chelated and collagen-bound A1 single domain and A1A2A3 tridomain fragment of VWF under shear stress in an ex vivo shear flow microfluidic chamber. In contrast to expected hypothetical outcomes, collagen has limited conformational selectivity for binding A1. A1-collagen binding is independent of gain- or loss-of-function phenotype and under shear stress, platelet translocation pause times on collagen-bound A1A2A3 are either normal or shorter depending on whether A1 is concertedly bound with the A3 domain to collagen. With respect to A1, collagen has an inhibitory role that provides an explanation for the lack of thrombosis in patients with gain-of-function VWD.

The functions of the A1A2A3 domains in von Willebrand factor include multimerin 1 binding

Multimerin 1 (MMRN1) is a massive, homopolymeric protein that is stored in platelets and endothelial cells for activation-induced release. In vitro, MMRN1 binds to the outer surfaces of activated platelets and endothelial cells, the extracellular matrix (including collagen) and von Willebrand factor (VWF) to support platelet adhesive functions. VWF associates with MMRN1 at high shear, not static conditions, suggesting that shear exposes cryptic sites within VWF that support MMRN1 binding. Modified ELISA and surface plasmon resonance were used to study the structural features of VWF that support MMRN1 binding, and determine the affinities for VWF-MMRN1 binding. High shear microfluidic platelet adhesion assays determined the functional consequences for VWF-MMRN1 binding. VWF binding to MMRN1 was enhanced by shear exposure and ristocetin, and required VWF A1A2A3 region, specifically the A1 and A3 domains. VWF A1A2A3 bound to MMRN1 with a physiologically relevant binding affinity (KD: 2.0 ± 0.4 nM), whereas the individual VWF A1 (KD: 39.3 ± 7.7 nM) and A3 domains (KD: 229 ± 114 nM) bound to MMRN1 with lower affinities. VWF A1A2A3 was also sufficient to support the adhesion of resting platelets to MMRN1 at high shear, by a mechanism dependent on VWF-GPIbα binding. Our study provides new information on the molecular basis of MMRN1 binding to VWF, and its role in supporting platelet adhesion at high shear. We propose that at sites of vessel injury, MMRN1 that is released following activation of platelets and endothelial cells, binds to VWF A1A2A3 region to support platelet adhesion at arterial shear rates.

Rapid discrimination of the phenotypic variants of von Willebrand disease

Approximately 20% to 25% of patients with von Willebrand disease (VWD) have a qualitative defect of the von Willebrand factor (VWF) protein activities. Variant VWD typically is classified as type 1C, 2A, 2B, 2M, or 2N depending on the VWF activity defect. Traditionally, diagnosis has relied on multiple clinical laboratory assays to assign VWD phenotype. We developed an enzyme-linked immunosorbent assay (ELISA) to measure the various activities of VWF on a single plate and evaluated 160 patient samples enrolled in the Zimmerman Program for the Molecular and Clinical Biology of von Willebrand Disease with type 2 VWD. Using linear discriminate analysis (LDA), this assay was able to identify type 1C, 2A, 2B, 2M, or 2N VWD with an overall accuracy of 92.5% in the patient study cohort. LDA jackknife analysis, a statistical resampling technique, identified variant VWD with an overall accuracy of 88.1%, which predicts the assay's performance in the general population. In addition, this assay demonstrated correlation with traditional clinical laboratory VWF assays. The VWF multiplex activity assay may be useful as a same-day screening assay when considering the diagnosis of variant VWD in an individual patient.

Crucial role for the VWF A1 domain in binding to type IV collagen

Von Willebrand factor (VWF) contains binding sites for platelets and for vascular collagens to facilitate clot formation at sites of injury. Although previous work has shown that VWF can bind type IV collagen (collagen 4), little characterization of this interaction has been performed. We examined the binding of VWF to collagen 4 in vitro and extended this characterization to a murine model of defective VWF-collagen 4 interactions. The interactions of VWF and collagen 4 were further studied using plasma samples from a large study of both healthy controls and subjects with different types of von Willebrand disease (VWD). Our results show that collagen 4 appears to bind VWF exclusively via the VWF A1 domain, and that specific sequence variations identified through VWF patient samples and through site-directed mutagenesis in the VWF A1 domain can decrease or abrogate this interaction. In addition, VWF-dependent platelet binding to collagen 4 under flow conditions requires an intact VWF A1 domain. We observed that decreased binding to collagen 4 was associated with select VWF A1 domain sequence variations in type 1 and type 2M VWD. This suggests an additional mechanism through which VWF variants may alter hemostasis.

How I treat type 2 variant forms of von Willebrand disease

Type 2 von Willebrand disease (VWD) includes a wide range of qualitative abnormalities of von Willebrand factor structure and function resulting in a variable bleeding tendency. According to the current classification, 4 different subtypes can be identified, each with distinctive phenotypic and therapeutic characteristics. Current available laboratory methods allow a straightforward approach to VWD subtyping, and although the precise molecular characterization remains complex, it is not required for appropriate treatment of the vast majority of cases. Desmopressin can be useful only in a few type 2 cases compared with patients with actual quantitative deficiency (type 1), most often in variants with a nearly normal multimeric pattern (type 2M). However, since no laboratory test accurately predicts response to desmopressin, a trial test should always be performed in all type 2 VWD patients, with the exception of type 2B ones. Replacement therapy with plasma-derived von Willebrand factor-factor VIII concentrates represents the safe mainstay of treatment of all patients, particularly those not responding to desmopressin or requiring a sustained hemostatic correction because of major surgery or bleeding. A significant patient bleeding history correlates with increased bleeding risk and should be considered in tailoring the optimal antihemorrhagic prophylaxis in the individual patient.

Analysis of the role of von Willebrand factor, platelet glycoprotein VI-, and α2β1-mediated collagen binding in thrombus formation

Rare missense mutations in the von Willebrand factor (VWF) A3 domain that disrupt collagen binding have been found in patients with a mild bleeding phenotype. However, the analysis of these aberrant VWF-collagen interactions has been limited. Here, we have developed mouse models of collagen-binding mutants and analyzed the function of the A3 domain using comprehensive in vitro and in vivo approaches. Five loss-of-function (p.S1731T, p.W1745C, p.S1783A, p.H1786D, A3 deletion) and 1 gain-of-function (p.L1757A) variants were generated in the mouse VWF complementary DNA. The results of these various assays were consistent, although the magnitude of the effects were different: the gain-of-function (p.L1757A) variant showed consistent enhanced collagen binding whereas the loss-of-function mutants showed variable degrees of functional deficit. We further analyzed the impact of direct platelet-collagen binding by blocking glycoprotein VI (GPVI) and integrin α2β1 in our ferric chloride murine thrombosis model. The inhibition of GPVI demonstrated a comparable functional defect in thrombosis formation to the VWF(-/-) mice whereas α2β1 inhibition demonstrated a milder bleeding phenotype. Furthermore, a delayed and markedly reduced thrombogenic response was still evident in VWF(-/-), GPVI, and α2β1 blocked animals, suggesting that alternative primary hemostatic mechanisms can partially rescue the bleeding phenotype associated with these defects.

Perils, problems, and progress in laboratory diagnosis of von Willebrand disease

Diagnosis of von Willebrand disease (VWD) merits consideration of personal and family history of bleeding symptoms along with confirmatory laboratory testing. As the latter yields quantifiable results, overreliance on a laboratory diagnosis may occur. However, existing tests for VWD contain potential sources for error. Both intrinsic and extrinsic factors affecting these assays can contribute to either falsely normal or falsely abnormal results. This article will discuss the present available assays as well as new developments in diagnostic testing. A clear understanding of the limitations of VWD testing is helpful for ensuring the correct diagnosis of affected patients.

von Willebrand disease: advances in pathogenetic understanding, diagnosis, and therapy

von Willebrand disease (VWD) is the most common autosomally inherited bleeding disorder. The disease represents a range of quantitative and qualitative pathologies of the adhesive glycoprotein von Willebrand factor (VWF). The pathogenic mechanisms responsible for the type 2 qualitative variants of VWF are now well characterized, with most mutations representing missense substitutions influencing VWF multimer structure and interactions with platelet GPIbα and collagen and with factor VIII. The molecular pathology of type 3 VWD has been similarly well characterized, with an array of different mutation types producing either a null phenotype or the production of VWF that is not secreted. In contrast, the pathogenetic mechanisms responsible for type 1 VWD remain only partially resolved. In the hemostasis laboratory, the measurement of VWF:Ag and VWF:RCo are key components in the diagnostic algorithm for VWD, although the introduction of direct GPIbα-binding assays may become the functional assay of choice. Molecular genetic testing can provide additional benefit, but its utility is currently limited to type 2 and 3 VWD. The treatment of bleeding in VWD involves the use of desmopressin and plasma-derived VWF concentrates and a variety of adjunctive agents. Finally, a new recombinant VWF concentrate has just completed clinical trial evaluation and has demonstrated excellent hemostatic efficacy and safety.

von Willebrand disease: advances in pathogenetic understanding, diagnosis, and therapy

von Willebrand disease (VWD) is the most common autosomally inherited bleeding disorder. The disease represents a range of quantitative and qualitative pathologies of the adhesive glycoprotein von Willebrand factor (VWF). The pathogenic mechanisms responsible for the type 2 qualitative variants of VWF are now well characterized, with most mutations representing missense substitutions influencing VWF multimer structure and interactions with platelet GPIbα and collagen and with factor VIII. The molecular pathology of type 3 VWD has been similarly well characterized, with an array of different mutation types producing either a null phenotype or the production of VWF that is not secreted. In contrast, the pathogenetic mechanisms responsible for type 1 VWD remain only partially resolved. In the hemostasis laboratory, the measurement of VWF:Ag and VWF:RCo are key components in the diagnostic algorithm for VWD, although the introduction of direct GPIbα-binding assays may become the functional assay of choice. Molecular genetic testing can provide additional benefit, but its utility is currently limited to type 2 and 3 VWD. The treatment of bleeding in VWD involves the use of desmopressin and plasma-derived VWF concentrates and a variety of adjunctive agents. Finally, a new recombinant VWF concentrate has just completed clinical trial evaluation and has demonstrated excellent hemostatic efficacy and safety.

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.

Translational medicine advances in von Willebrand disease

Following the recognition of von Willebrand disease (VWD) in 1926 and the cloning of the gene for von Willebrand factor (VWF) in 1985, significant advances have been made in our fundamental knowledge of both the disease and the protein. Some of this new knowledge has also begun to impact the clinical management of VWD. First, the progressive increase in our understanding of the molecular genetic basis of VWD has resulted in rational applications of molecular testing to complement the current range of phenotypic tests for VWD. These molecular genetic strategies are most effectively directed at the prenatal diagnosis of type 3 VWD and confirmatory testing for types 2B and 2N disease. In contrast, the use of molecular testing to clarify the diagnosis of type 1 VWD is of marginal benefit, at best. In terms of VWD therapies, a new recombinant VWF concentrate has recently completed successful clinical trials and is now awaiting more widespread application. There have even been some preclinical successes with VWF gene transfer although the clinical rationale for this therapeutic strategy needs careful consideration. Much more remains to be learnt about the biology of VWF and further translational advances for the enhancement of VWD care will inevitably be realized.

No increase in bleeding identified in type 1 VWD subjects with D1472H sequence variation

The diagnosis of von Willebrand disease (VWD) is complicated by issues with current laboratory testing, particularly the ristocetin cofactor activity assay (VWF:RCo). We have recently reported a sequence variation in the von Willebrand factor (VWF) A1 domain, p.D1472H (D1472H), associated with a decrease in the VWF:RCo/VWF antigen (VWF:Ag) ratio but not associated with bleeding in healthy control subjects. This report expands the previous study to include subjects with symptoms leading to the diagnosis of type 1 VWD. Type 1 VWD subjects with D1472H had a significant decrease in the VWF:RCo/VWF:Ag ratio compared with those without D1472H, similar to the findings in the healthy control population. No increase in bleeding score was observed, however, for VWD subjects with D1472H compared with those without D1472H. These results suggest that the presence of the D1472H sequence variation is not associated with a significant increase in bleeding symptoms, even in type 1 VWD subjects.

Variability in platelet- and collagen-binding defects in type 2M von Willebrand disease

Type 2M von Willebrand disease (VWD) includes qualitative defects in von Willebrand factor (VWF) function, with normal multimer distribution but a defect in VWF activity with respect to platelet or collagen binding. We characterized novel VWF gene mutations found in type 2M VWD subjects enrolled in the Zimmerman Program for the Molecular and Clinical Biology of VWD. Subjects were enrolled based on a pre-existing diagnosis of type 2M VWD. Testing included full-length gene sequencing, VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), VWF collagen binding and multimer distribution. Recombinant VWF variants were synthesized using site-directed mutagenesis and expressed in HEK293T cells. Platelet binding was measured by flow cytometry with fixed platelets and ELISA with recombinant glycoprotein Ibα (GPIbα). Four novel VWF A1 domain mutations were found in individuals with type 2M VWD: S1358N, S1387I, S1394F and Q1402P. All subjects had a history of bleeding, VWF:RCo < 40 IU dL(-1) , VWF:RCo/VWF:Ag ratios <0.6 and normal multimer distribution. No defect in expression, secretion, or multimerization was found for any of the mutations. All showed decreased binding to intact platelets, and decreased or absent binding to a mutant GPIbα construct with spontaneous VWF binding. 1387I had decreased binding to all collagen types tested. 1402P had reduced binding exclusively to type VI collagen. Type 2M VWD is a heterogeneous category comprised of both collagen- and platelet-binding defects. Understanding the precise defect for each mutation may ultimately lead to better diagnosis and treatment.

The molecular characterization of von Willebrand disease: good in parts

Since the cloning of the gene that encodes von Willebrand factor (VWF), 27 years ago, significant progress has been made in our understanding of the molecular basis of the most common inherited bleeding disorder, von Willebrand disease (VWD). The molecular pathology of this condition represents a range of genetic mechanisms, some of which are now very well characterized, and others that are still under investigation. In general, our knowledge of the molecular basis of type 2 and 3 VWD is now well advanced, and in some instances this information is being used to enhance clinical management. In contrast, our understanding of the molecular pathogenesis of the most common form of VWD, type 1 disease, is still at an early stage, with preliminary evidence that this phenotype involves a complex interplay between environmental factors and the influence of genetic variability both within and outside of the VWF locus.

Making a diagnosis of VWD

A clear understanding of the molecular basis of VWD can guide the choice and interpretation of appropriate diagnostic tests. This review briefly describes the lifecycle and molecular interactions of VWF and how they lead to the current clinical classification. It also includes a brief discussion of the differential diagnosis and general workup of mucocutaneous bleeding, a review of the various VWD subtypes, and pertinent laboratory assays for each, including genetic tests. Finally, common testing pitfalls and diagnostic dilemmas are covered, including the challenge created by the overlap of borderline low VWF levels and mild bleeding.

Making a diagnosis of VWD

A clear understanding of the molecular basis of VWD can guide the choice and interpretation of appropriate diagnostic tests. This review briefly describes the lifecycle and molecular interactions of VWF and how they lead to the current clinical classification. It also includes a brief discussion of the differential diagnosis and general workup of mucocutaneous bleeding, a review of the various VWD subtypes, and pertinent laboratory assays for each, including genetic tests. Finally, common testing pitfalls and diagnostic dilemmas are covered, including the challenge created by the overlap of borderline low VWF levels and mild bleeding.