von Willebrand factor clearance - biological mechanisms and clinical significance

von Willebrand disease

von Willebrand disease (VWD) is the most common inherited bleeding disorder. The disorder is characterized by excessive mucocutaneous bleeding. The most common bleeding manifestations of this condition include nosebleeds, bruising, bleeding from minor wounds, menorrhagia or postpartum bleeding in women as well as bleeding after surgery. Other less frequent symptoms include gastrointestinal bleeding, haematomas or haemarthroses. VWD pathophysiology is complex and results from defects in von Willebrand factor (VWF) glycoprotein. Quantitative deficiencies are responsible for type 1 VWD with a partial decrease of VWF and type 3 with the complete absence of VWF. Qualitative abnormalities cause type 2 VWD, being further divided into types 2A, 2B, 2M and 2N. Although common, VWD is at risk of misdiagnosis, overdiagnosis and underdiagnosis owing to several factors, including complex diagnosis, variability of bleeding symptoms, presence of external variables (blood groups and other physiological modifiers such as exercise, thyroid hormones, oestrogens, and ageing), and lack of disease awareness among non-specialist health-care providers. Establishing the correct VWD diagnosis requires an array of specialized phenotypic assays and/or molecular genetic testing of the VWF gene. The management of bleeding includes increasing endogenous VWF levels with desmopressin or infusion of exogenous VWF concentrates (plasma-derived or recombinant). Fibrinolytic inhibitors, topical haemostatic agents and hormonal therapies are used as effective adjunctive measures.

R1205H (Vicenza) causes conformational changes in the von Willebrand factor D'D3 domains and enhances von Willebrand factor binding to clearance receptors LRP1 and SR-AI

BACKGROUND

von Willebrand factor (VWF)-R1205H variant (Vicenza) results in markedly enhanced VWF clearance in humans that has been shown to be largely macrophage-mediated. However, the biological mechanisms underlying this enhanced clearance remain poorly understood.

OBJECTIVES

This study aimed to investigate the roles of (i) specific VWF domains and (ii) different macrophage receptors in regulating enhanced VWF-R1205H clearance.

METHODS

In vivo clearance of full-length and truncated wild-type (WT)-VWF and VWF with R1205 substitutions was investigated in VWF-/- mice. Plate-binding assays were employed to characterize VWF binding to purified scavenger receptor class A member 1 (SR-AI), low-density lipoprotein receptor-related protein-1 (LRP1) cluster II or cluster IV receptors, and macrophage galactose-type lectin.

RESULTS

In full-length VWF missing the A1 domain, introduction of R1205H led to significantly enhanced clearance in VWF-/- mice compared with WT-VWF missing the A1 domain. Importantly, R1205H in a truncated VWF-D'D3 fragment also triggered increased clearance compared with WT-VWF-D'D3. Additional in vivo studies demonstrated that VWF-R1205K (which preserves the positive charge at 1205) exhibited normal clearance, whereas VWF-R1205E (which results in loss of the positive charge) caused significantly enhanced clearance, pinpointing the importance of the positive charge at VWF-R1205. In vitro plate-binding studies confirmed increased VWF-R1205H interaction with SR-AI compared with WT-VWF. Furthermore, significantly enhanced VWF-R1205H binding to LRP1 cluster IV (P < .001) and less marked enhanced binding to LRP1 cluster II (P = .034) was observed. In contrast, VWF-R1205H and WT-VWF demonstrated no difference in binding affinity to macrophage galactose-type lectin.

CONCLUSION

Disruption of the positive charge at amino acid R1205 causes conformational changes in the VWF-D'D3 domains and triggers enhanced LRP1-mediated and SR-AI-mediated clearance.

Clearance of VWF by hepatic macrophages is critical for the protective effect of ADAMTS13 in sickle cell anemia mice

ABSTRACT

Although it is caused by a single-nucleotide mutation in the β-globin gene, sickle cell anemia (SCA) is a systemic disease with complex, incompletely elucidated pathologies. The mononuclear phagocyte system plays critical roles in SCA pathophysiology. However, how heterogeneous populations of hepatic macrophages contribute to SCA remains unclear. Using a combination of single-cell RNA sequencing and spatial transcriptomics via multiplexed error-robust fluorescence in situ hybridization, we identified distinct macrophage populations with diversified origins and biological functions in SCA mouse liver. We previously found that administering the von Willebrand factor (VWF)-cleaving protease ADAMTS13 alleviated vaso-occlusive episode in mice with SCA. Here, we discovered that the ADAMTS13-cleaved VWF was cleared from the circulation by a Clec4f+Marcohigh macrophage subset in a desialylation-dependent manner in the liver. In addition, sickle erythrocytes were phagocytized predominantly by Clec4f+Marcohigh macrophages. Depletion of macrophages not only abolished the protective effect of ADAMTS13 but exacerbated vaso-occlusive episode in mice with SCA. Furthermore, promoting macrophage-mediated VWF clearance reduced vaso-occlusion in SCA mice. Our study demonstrates that hepatic macrophages are important in the pathogenesis of SCA, and efficient clearance of VWF by hepatic macrophages is critical for the protective effect of ADAMTS13 in SCA mice.

Biofilm formation initiating rotifer-specific biopolymer and its predicted components

The rotifer-specific biopolymer, namely Rotimer, is a recently discovered group of the biomolecule family. Rotimer has an active role in the biofilm formation initiated by rotifers (e.g., Euchlanis dilatata or Adineta vaga) or in the female-male sexual interaction of monogononts. To understand the Ca2+- and polarity-dependent formation of this multifunctional viscoelastic material, it is essential to explore its molecular composition. The investigation of the rotifer-enhanced biofilm and Rotimer-inductor conglomerate (RIC) formation yielded several protein candidates to predict the Rotimer-specific main components. The exudate of E. dilatata males was primarily applied from different biopolimer-containing samples (biofilm or RIC). The advantage of males over females lies in their degenerated digestive system and simple anatomy. Thus, their exudate is less contaminated with food and endosymbiont elements. The sequenced and annotated genome and transcriptome of this species opened the way for identifying Rotimer proteins by mass spectrometry. The predicted rotifer-biopolymer forming components are SCO-spondins and 14-3-3 protein. The characteristics of Rotimer are similar to Reissner's fiber, which is found in the central nervous system of vertebrates and is mainly formed from SCO-spondins. This molecular information serves as a starting point for its interdisciplinary investigation and application in biotechnology, biomedicine, or neurodegeneration-related drug development.

Unraveling coagulation factor-mediated cellular signaling

Blood coagulation is initiated in response to blood vessel injury or proinflammatory stimuli, which activate coagulation factors to coordinate complex biochemical and cellular responses necessary for clot formation. In addition to these critical physiologic functions, plasma protein factors activated during coagulation mediate a spectrum of signaling responses via receptor-binding interactions on different cell types. In this review, we describe examples and mechanisms of coagulation factor signaling. We detail the molecular basis for cell signaling mediated by coagulation factor proteases via the protease-activated receptor family, considering new insights into the role of protease-specific cleavage sites, cofactor and coreceptor interactions, and distinct signaling intermediate interactions in shaping protease-activated receptor signaling diversity. Moreover, we discuss examples of how injury-dependent conformational activation of other coagulation proteins, such as fibrin(ogen) and von Willebrand factor, decrypts their signaling potential, unlocking their capacity to contribute to aberrant proinflammatory signaling. Finally, we consider the role of coagulation factor signaling in disease development and the status of pharmacologic approaches to either attenuate or enhance coagulation factor signaling for therapeutic benefit, emphasizing new approaches to inhibit deleterious coagulation factor signaling without impacting hemostatic activity.

Von Willebrand factor processing in patients with advanced chronic liver disease and its relation to portal hypertension and clinical outcome

BACKGROUND AND AIMS

Endothelial dysfunction and portal hypertension (PH) are reflected by increased von Willebrand factor antigen (VWF-Ag) levels in advanced chronic liver disease (ACLD). This study investigated VWF release and cleavage and their association with PH and clinical outcomes.

METHODS

Levels of VWF-Ag, VWF-N (VWF-propeptide), and VWF-A (VWF processed by the main VWF-cleaving protease ADAMTS13) were assessed in 229 patients with clinically stable ACLD (hepatic venous pressure gradient [HVPG] ≥ 6 mmHg; absence of bacterial infections or acute decompensation) undergoing HVPG-measurement. Liver-healthy individuals served as controls (n = 24).

RESULTS

VWF-Ag and VWF-N were similarly accurate for the identification of clinically significant PH (CSPH; HVPG ≥ 10 mmHg) in compensated ACLD (AUROC: VWF-Ag 0.748; VWF-N 0.728). ADAMTS13 activity was similar between patients with ACLD and controls and did not correlate with PH and disease severity, whereas VWF cleavage decreased in patients with CSPH (i.e., VWF-Ag/-A-ratio increased). In vitro VWF activity strongly reflected VWF-Ag levels (Spearman's r = 0.874, p < 0.001), but decreased (vs. controls) in patients with CSPH when normalized to VWF-Ag levels (VWF-activity/-Ag-ratio). VWF-Act/-Ag ratio correlated negatively with ADAMTS13 activity (r =- 0.256, p < 0.001). ADAMTS13 activity was independently predictive for (i) portal vein thrombosis (PVT) and (ii) hepatic decompensation or liver-related death.

CONCLUSIONS

VWF-Ag levels and its propeptide are similarly suitable surrogates of PH in patients with compensated ACLD. ADAMTS13-Act was not linked to disease and PH severity, however, when normalized to VWF-Ag, both VWF cleavage and VWF activity were decreased in patients with CSPH, as compared to liver-healthy individuals. Low ADAMTS13-Act was associated with presumably more procoagulant VWF and adverse outcomes.

CLINICAL TRIAL NUMBER

NCT03267615.

VWF-ADAMTS13 axis dysfunction in children with sickle cell disease treated with hydroxycarbamide vs blood transfusion

Previous studies have reported elevated von Willebrand factor (VWF) levels in patients with sickle cell disease (SCD) and demonstrated a key role for the VWF-ADAMTS13 axis in the pathobiology of SCD vaso-occlusion. Although blood transfusion is the gold standard for stroke prevention in SCD, the biological mechanisms underpinning its improved efficacy compared with hydroxycarbamide are not fully understood. We hypothesized that the improved efficacy of blood transfusion might relate to differences in VWF-ADAMTS13 axis dysfunction. In total, 180 children with a confirmed diagnosis of SCD (hemoglobin SS) on hydroxycarbamide (n = 96) or blood transfusion (n = 84) were included. Despite disease-modifying treatment, plasma VWF and VWF propeptide were elevated in a significant proportion of children with SCD (33% and 47%, respectively). Crucially, all VWF parameters were significantly higher in the hydroxycarbamide compared with the blood transfusion cohort (P < .05). Additionally, increased levels of other Weibel-Palade body-stored proteins, including factor VIII (FVIII), angiopoietin-2, and osteoprotegerin were observed, indicated ongoing endothelial cell activation. Children treated with hydroxycarbamide also had higher FVIII activity and enhanced thrombin generation compared with those in the blood transfusion cohort (P < .001). Finally, hemolysis markers strongly correlated with VWF levels (P < .001) and were significantly reduced in the blood transfusion cohort (P < .001). Cumulatively, to our knowledge, our findings demonstrate for the first time that despite treatment, ongoing dysfunction of the VWF-ADAMTS13 axis is present in a significant subgroup of pediatric patients with SCD, especially those treated with hydroxycarbamide.

Enhanced VWF clearance in low VWF pathogenesis: limitations of the VWFpp/VWF:Ag ratio and clinical significance

Increased von Willebrand factor (VWF) clearance plays a key role in the pathogenesis of type 1 and type 2 von Willebrand disease (VWD). However, the pathological mechanisms involved in patients with mild to moderate reductions in plasma VWF:Ag (range, 30-50 IU/dL; low VWF) remain poorly understood. In this study, we investigated the hypothesis that enhanced VWF clearance may contribute to the pathobiology of low VWF. Patients with low VWF were recruited to the LoVIC study after ethics approval and receipt of informed consent. Desmopressin was administered IV in 75 patients, and blood samples were drawn at baseline and at the 1-hour and 4-hour time points. As defined by recent ASH/ISTH/NHF/WFH guidelines, 20% of our low-VWF cohort demonstrated significantly enhanced VWF clearance. Importantly, from a clinical perspective, this enhanced VWF clearance was seen after desmopressin infusion, but did not affect the steady-state VWF propeptide (VWFpp)-to-VWF antigen (VWF:Ag) ratio (VWFpp/VWF:Ag) in most cases. The discrepancy between the VWFpp/VWF:Ag ratio and desmopressin fall-off rates in patients with mild quantitative VWD may have reflected alteration in VWFpp clearance kinetics. Finally, bleeding scores were significantly lower in patients with low VWF with enhanced VWF clearance, compared with those in whom reduced VWF biosynthesis represented the principle pathogenic mechanism. This trial was registered at http://www.clinicaltrials.gov as #NCT03167320.

Low von Willebrand Disease: A Bleeding Disorder of Unknown Cause?

von Willebrand disease (VWD) represents the most common inherited bleeding disorder. The majority of VWD cases are characterized by partial quantitative reductions in plasma von Willebrand factor (VWF) levels. Management of patients with mild to moderate VWF reductions in the range of 30 to 50 IU/dL poses a common clinical challenge. Some of these low VWF patients present with significant bleeding problems. In particular, heavy menstrual bleeding and postpartum hemorrhage can cause significant morbidity. Conversely, however, many individuals with mild plasma VWF:Ag reductions do not have any bleeding sequelae. In contrast to type 1 VWD, most patients with low VWF do not have detectable pathogenic VWF sequence variants, and bleeding phenotype correlates poorly with residual VWF levels. These observations suggest that low VWF is a complex disorder caused by variants in other genes beyond VWF. With respect to low VWF pathobiology, recent studies have shown that reduced VWF biosynthesis within endothelial cells likely plays a key role. However, pathological enhanced VWF clearance from plasma has also been described in approximately 20% of low VWF cases. For low VWF patients who require hemostatic treatment prior to elective procedures, tranexamic acid and desmopressin have both been shown to be efficacious. In this article, we review the current state of the art regarding low VWF. In addition, we consider how low VWF represents an entity that appears to fall between type 1 VWD on the one hand and bleeding disorders of unknown cause on the other.

Heterogeneity in the half-life of factor VIII concentrate in patients with hemophilia A is due to variability in the clearance of endogenous von Willebrand factor

BACKGROUND

Previous studies have reported marked interindividual variation in factor VIII (FVIII) clearance in patients with hemophilia (PWH) and proposed a number of factors that influence this heterogeneity.

OBJECTIVES

To investigate the importance of the clearance rates of endogenous von Willebrand factor (VWF) compared with those of other FVIII half-life modifiers in adult PWH.

METHODS

The half-life of recombinant FVIII was determined in a cohort of 61 adult PWH. A range of reported modifiers of FVIII clearance was assessed (including plasma VWF:antigen and VWF propeptide levels; VWF-FVIII binding capacity; ABO blood group; and nonneutralizing anti-FVIII antibodies). The FVIII-binding region of the VWF gene was sequenced. Finally, the effects of variation in FVIII half-life on clinical phenotype were investigated.

RESULTS

We demonstrated that heterogeneity in the clearance of endogenous plasma VWF is a key determinant of variable FVIII half-life in PWH. Both ABO blood group and age significantly impact FVIII clearance. The effect of ABO blood group on FVIII half-life in PWH is modulated entirely through its effect on the clearance rates of endogenous VWF. In contrast, the age-related effect on FVIII clearance is, at least in part, VWF independent. In contrast to previous studies, no major effects of variation in VWF-FVIII binding affinity on FVIII clearance were observed. Although high-titer immunoglobulin G antibodies (≥1:80) were observed in 26% of PWH, these did not impact FVIII half-life. Importantly, the annual FVIII usage (IU/kg/y) was significantly (p = .0035) increased in patients with an FVIII half-life of <12 hours.

CONCLUSION

Our data demonstrate that heterogeneity in the half-life of FVIII concentrates in patients with hemophilia A is primarily attributable to variability in the clearance of endogenous VWF.

Genetic determinants of enhanced von Willebrand factor clearance from plasma

BACKGROUND

Enhanced von Willebrand factor (VWF) clearance from plasma is associated with von Willebrand disease (VWD). However, the genetic background of this disease mechanism is not well defined.

OBJECTIVE

To determine VWF variants that are associated with reduced VWF survival.

METHODS

Two hundred fifty-four patients with VWD (type 1 = 50 and type 2 = 204) were investigated, and the results were compared with 120 healthy controls. The patients were comprehensively characterized for phenotypic and genetic features. The ratio of VWF propeptide (VWFpp)/VWF antigen (VWFpp ratio) was used to establish in each patient the VWF clearance state.

RESULTS

Out of 92 variants associated with type 1 (7 were novel) and type 2 VWD, 19 had a VWFpp ratio ranging from 1.7 to 2.2, 24 had a VWFpp ratio between 2.3 and 2.9, and 24 variants had a ratio of ≥3. The VWFpp median ratio in healthy controls was 0.98 (0.55-1.6) so that a cut-off value of >1.6 was considered an indicator of accelerated VWF clearance from plasma. An enhanced VWF clearance was observed in 34% of type 1 cases, 100% of type 1 Vicenza cases, 81% of 2A cases, 77% of 2B cases, 88% of 2M cases, and 36% of 2N cases.

CONCLUSIONS

An accelerated VWF clearance was found in most patients with type 2A, 2B, and 2M VWD, with a lower proportion of type 1 and 2N. Sixty-seven different variants alone or in combination with other variants were associated with an increased VWFpp ratio. The variants with the highest VWFpp ratio were mostly located in the D3-A1 VWF domains.

α1-Adrenergic Stimulation Increases Platelet Adhesion to Endothelial Cells Mediated by TRPC6

Stimulation of a1-adrenergic nervous system is increased during systemic inflammation and other pathological conditions with the consequent adrenergic receptors (ARs) activation. It has been reported that a1-stimulation contributes to coagulation since a1-AR blockers inhibit coagulation and its organic consequences. Also, coagulation induced by a1-AR stimulation can be greatly decreased using a1-AR blockers. In health, endothelial cells (ECs) perform anticoagulant actions at cellular and molecular level. However, during inflammation, ECs turn dysfunctional promoting a procoagulant state. Endothelium-dependent coagulation progresses at cellular and molecular levels, promoting endothelial acquisition of procoagulant properties to potentiate coagulation by means of prothrombotic and antifibrinolytic proteins expression increase in ECs releasing them to circulation, the thrombus formation is strengthened. Calcium signaling is a main feature of coagulation. Inhibition of ion channels involved in Ca²⁺ entry severely decreases coagulation. The transient receptor potential canonical 6 (TRPC6) is a non-selective Ca²⁺-permeable ion channel. TRPC6 activity is induced by diacylglycerol, suggesting that is regulated by a1-ARs. Furthermore, a1-ARs stimulation elicits a TRPC-like current in rat mesenteric artery smooth muscle and mesangial cells. However, whether TRPC6 could promote an ECs-mediated platelet adhesion induced by a1-adrenergic stimulation is currently not known. Therefore, the aim of this study was to examine if the TRPC6 calcium channel mediates platelet adhesion induced by a1-adrenergic stimulation. Our results suggest that platelet adhesion to ECs is enhanced by the a1-adrenergic stimulation evoked by phenylephrine mediated by TRPC6 activity. We conclude that TRPC6 is a molecular determinant in platelet adhesion to ECs with implications in systemic inflammatory diseases treatment.

Successful and safe response to ibrutinib alone in treating relapsed Waldenström macrogobulinemia and related acquired von Willebrand syndrome: an option to consider

Ibrutinib, a first-class Bruton tyrosine kinase inhibitor, is known to be associated with adverse bleeding events and has been recently approved for the treatment of relapse Waldenström macroglobulinemia (WM). Here, we report the exhaustive clinical and biological follow-up of 2 patients treated by ibrutinib alone in the context of relapsed WM with an acquired von Willebrand syndrome (AVWS) complication. In two cases, ibrutinib has been shown to be quickly efficient and safe for treating both AVWS and its underlying condition the WM, without bleeding complications. Interestingly, ibrutinib treatment brings a rapid and extended over time normalization of von Willebrand factor clearance. These observations show that ibrutinib is a valuable therapeutic option in relapsed WM patients associated with AVWS and highlighting the need for further cohort studies with long-term follow-up of patients to confirm the efficacy and safety of a treatment by ibrutinib for WM patients with AVWS complication.

von Willebrand factor links primary hemostasis to innate immunity

The plasma multimeric glycoprotein von Willebrand factor (VWF) plays a critical role in primary hemostasis by tethering platelets to exposed collagen at sites of vascular injury. Recent studies have identified additional biological roles for VWF, and in particular suggest that VWF may play an important role in regulating inflammatory responses. However, the molecular mechanisms through which VWF exerts its immuno-modulatory effects remain poorly understood. In this study, we report that VWF binding to macrophages triggers downstream MAP kinase signaling, NF-κB activation and production of pro-inflammatory cytokines and chemokines. In addition, VWF binding also drives macrophage M1 polarization and shifts macrophage metabolism towards glycolysis in a p38-dependent manner. Cumulatively, our findings define an important biological role for VWF in modulating macrophage function, and thereby establish a novel link between primary hemostasis and innate immunity.

Dispatch and delivery at the ER-Golgi interface: how endothelial cells tune their hemostatic response

Von Willebrand factor (VWF) is a glycoprotein that is secreted into the circulation and controls bleeding by promoting adhesion and aggregation of blood platelets at sites of vascular injury. Substantial inter-individual variation in VWF plasma levels exists among the healthy population. Prior to secretion, VWF polymers are assembled and condensed into helical tubules, which are packaged into Weibel-Palade bodies (WPBs), a highly specialized post-Golgi storage compartment in vascular endothelial cells. In the inherited bleeding disorder Von Willebrand disease (VWD), mutations in the VWF gene can cause qualitative or quantitative defects, limiting protein function, secretion, or plasma survival. However, pathogenic VWF mutations cannot be found in all VWD cases. Although an increasing number of genetic modifiers have been identified, even more rare genetic variants that impact VWF plasma levels likely remain to be discovered. Here, we summarize recent evidence that modulation of the early secretory pathway has great impact on the biogenesis and release of WPBs. Based on these findings, we propose that rare, as yet unidentified quantitative trait loci influencing intracellular VWF transport contribute to highly variable VWF levels in the population. These may underlie the thrombotic complications linked to high VWF levels, as well as the bleeding tendency in individuals with low VWF levels.

Siglec-5 and Siglec-14 mediate the endocytosis of ADAMTS13

BACKGROUND

The plasma metalloprotease ADAMTS13 regulates the thrombotic activity of the von Willebrand factor (VWF). ADAMTS13 is highly glycosylated and its carbohydrate chains are capped with sialic acid (SA). Thus, ADAMTS13 may interact with carbohydrate- and/or SA-binding plasma membrane receptors that are involved in the clearance of various plasma proteins. We have investigated ADAMTS13 endocytosis via Siglecs, which were originally identified as SA-binding immunoreceptor family proteins expressed on leukocytes and are also known as endocytic receptors.

MATERIALS AND METHODS

Endocytic internalization of fluorescently labeled ADAMTS13 into HEK293 cells expressing Siglecs was examined via fluorescence microscopy. In vitro binding of ADAMTS13 to the extracellular region of Siglec-5 was examined. Plasma ADAMTS13 activity in human Siglec-5-expressing mice was measured.

RESULTS AND CONCLUSIONS

Siglec-5- and Siglec-14-expressing cells internalized not only full-length ADAMTS13 (FL) but also the truncated form (MDTCS) at least partly in an SA-independent manner. Replacement of the V-set domain of Siglec-14 with that of Siglec-3 abrogated the internalization of ADAMTS13. ADAMTS13 directly bound to the extracellular region of Siglec-5 in vitro. Expression of Siglec-5 in the mouse liver resulted in a significant decrease in plasma ADAMTS13 activity. These results suggest that Siglec-5 and Siglec-14, which have nearly identical ligand-binding domains, may contribute to the regulation of plasma ADAMTS13 levels as endocytic receptors for ADAMTS13.

The p.P1127S pathogenic variant lowers von Willebrand factor levels through higher affinity for the macrophagic scavenger receptor LRP1: Clinical phenotype and pathogenic mechanisms

BACKGROUND

The index case is a 21-year-old Italian woman with a mild hemorrhagic syndrome and von Willebrand factor antigen (VWF:Ag) = 34.3 U/dl, VWF recombinant glycoprotein Ib (VWF:GpIbR) = 32.8 U/dl, and factor VIII (FVIII) = 55.3 IU/dl.

AIMS

The aim of this study is to characterize from a genetic and biochemical standpoint this low VWF phenotype.

METHODS

Coagulation and biochemical methods were used to study the structural and functional pattern of VWF multimers in the index case's plasma. Recombinant wild-type and p.P1127S VWF variants were produced using human embryonic kidney (HEK)-293 cells. In addition, genetic screening was carried out to detect single nucleotide variants of some scavenger VWF/FVIII receptor genes such as CLEC4M, STAB2, and ASGR2.

RESULTS

Genetic investigation revealed that the index case inherited from her mother the heterozygous missense mutation c.3379C > T (VWF exon 25), causing the p.P1127S substitution in the VWF D'D3 domain. The index case was also homozygous for the scavenger receptor ASGR2 c.-95 CC-genotype. Desmopressin normalized the VWF level of the patient, although its clearance was faster (t1/2  = 6.7 h) than in normal subjects (t1/2  = 12 ± 0.7 h). FVIII-VWF interaction, A Disintegrin And Metalloprotease with ThromboSpondin type 1 motif-13 levels, ristocetin-induced-platelet-aggregation, and VWF multimeric pattern were normal. The p.P1127S variant was normally synthesized and secreted by HEK-293 cells, and molecular modeling predicts a conformational change showing higher affinity for the macrophagic scavenger receptor lipoprotein receptor-related protein 1 (LRP1), as also experimentally verified.

CONCLUSIONS

The p.P1127S variant may cause a low VWF phenotype, stemming from an increased VWF affinity for the scavenger receptor LRP1 and, consequently, an accelerated clearance of VWF.

Sialylation on O-linked glycans protects von Willebrand factor from macrophage galactose lectin-mediated clearance

Terminal sialylation determines the plasma half-life of von Willebrand factor (VWF). A role for macrophage galactose lectin (MGL) in regulating hyposialylated VWF clearance has recently been proposed. In this study, we showed that MGL influences physiological plasma VWF clearance. MGL inhibition was associated with a significantly extended mean residence time and 3-fold increase in endogenous plasma VWF antigen levels (P<0.05). Using a series of VWF truncations, we further demonstrated that the A1 domain of VWF is predominantly responsible for enabling the MGL interaction. Binding of both full-length and VWF-A1-A2-A3 to MGL was significantly enhanced in the presence of ristocetin (P<0.05), suggesting that the MGL-binding site in A1 is not fully accessible in globular VWF. Additional studies using different VWF glycoforms demonstrated that VWF O-linked glycans, clustered at either end of the A1 domain, play a key role in protecting VWF against MGLmediated clearance. Reduced sialylation has been associated with pathological, increased clearance of VWF in patients with von Willebrand disease. Herein, we demonstrate that specific loss of α2-3 linked sialylation from O-glycans results in markedly increased MGL-binding in vitro, and markedly enhanced MGL-mediated clearance of VWF in vivo. Our data further show that the asialoglycoprotein receptor (ASGPR) does not have a significant role in mediating the increased clearance of VWF following loss of O-sialylation. Conversely however, we observed that loss of N-linked sialylation from VWF drives enhanced circulatory clearance predominantly via the ASGPR. Collectively, our data support the hypothesis that in addition to regulating physiological VWF clearance, the MGL receptor works in tandem with ASGPR to modulate enhanced clearance of aberrantly sialylated VWF in the pathogenesis of von Willebrand disease.

Role of von Willebrand factor in venous thromboembolic disease

Objective

Evolving evidence of the shared risk factors and pathogenic mechanisms in arterial and venous thrombosis questions of the strict vascular dichotomy of arterial vs venous. The connection between arterial and venous thrombosis has been highlighted by common underlying inflammatory processes, a concept known as thromboinflammatory disease. Using this relationship, we can apply knowledge from arterial disease to better understand and potentially mitigate venous disease. A protein that has been extensively studied in atherothrombotic disease and inflammation is von Willebrand factor (VWF). Because many predisposing and provoking factors of venous thromboembolism (VTE) have been shown to directly modulate VWF levels, it is, perhaps, not surprising that VWF has been highlighted by several recent association studies of patients with VTE.

Methods

In the present narrative review, we investigated more deeply the effects of VWF in venous disease by synthesizing the data from clinical studies of deep vein thrombosis of the limbs, pulmonary embolism, portal and cerebral vein thrombosis, and the complications of thrombosis, including post-thrombotic syndrome, venous insufficiency, and chronic thromboembolic pulmonary hypertension. We have also discussed the findings from preclinical studies to highlight novel VWF biochemistry in thrombosis and therapeutics.

Results

Across the spectrum of venous thromboembolic disease, we consistently observed that elevated VWF levels conferred an increased risk of VTE and long-term venous complications. We have highlighted important findings from VWF molecular research and have proposed mechanisms by which VWF participates in venous disease. Emerging evidence from preclinical studies might reveal novel targets for thromboinflammatory disease, including specific VWF pathophysiology. Furthermore, we have highlighted the utility of measuring VWF to prognosticate and risk stratify for VTE and its complications.

Conclusions

As the prevalence of inflammatory processes, such as aging, obesity, and diabetes increases in our population, it is critical to understand the evolving role of VWF in venous disease to guide clinical decisions and therapeutics.

The von Willebrand factor - ADAMTS-13 axis in malaria

Cerebral malaria (CM) continues to be associated with major morbidity and mortality, particularly in children aged <5 years in sub-Saharan Africa. Although the biological mechanisms underpinning severe malaria pathophysiology remain incompletely understood, studies have shown that cytoadhesion of malaria-infected erythrocytes to endothelial cells (ECs) within the cerebral microvasculature represents a key step in this process. Furthermore, these studies have also highlighted that marked EC activation, with secretion of Weibel-Palade bodies (WPBs), occurs at a remarkably early stage following malaria infection. As a result, plasma levels of proteins normally stored within WPBs (including high-molecular-weight von Willebrand factor [VWF] multimers, VWF propeptide, and angiopoietin-2) are significantly elevated. In this review, we provide an overview of recent studies that have identified novel roles through which these secreted WPB glycoproteins may directly facilitate malaria pathogenesis through a number of different platelet-dependent and platelet-independent pathways. Collectively, these emerging insights suggest that hemostatic dysfunction, and in particular disruption of the normal VWF-ADAMTS-13 axis, may be of specific importance in triggering cerebral microangiopathy. Defining the molecular mechanisms involved may offer the opportunity to develop novel targeted therapeutic approaches, which are urgently needed as the mortality rate associated with CM remains in the order of 20%.

Multifaceted Pathomolecular Mechanism of a VWF Large Deletion Involved in the Pathogenesis of Severe VWD

The present study demonstrates the dominant-negative impact of an in-frame large deletion on VWF biosynthesis and biogenesis of the WPBs.

The malformed WPBs/altered trafficking of its inflammatory cargos cause distresses in endothelial cell signaling pathways and phenotype.

An in-frame heterozygous large deletion of exons 4 through 34 of the von Willebrand factor (VWF) gene was identified in a type 3 von Willebrand disease (VWD) index patient (IP), as the only VWF variant. The IP exhibited severe bleeding episodes despite prophylaxis treatment, with a short VWF half-life after infusion of VWF/factor VIII concentrates. Transcript analysis confirmed transcription of normal VWF messenger RNA besides an aberrant deleted transcript. The IP endothelial colony-forming cells (ECFCs) exhibited a defect in the VWF multimers and Weibel-Palade bodies (WPBs) biogenesis, although demonstrating normal VWF secretion compared with healthy cells. Immunostaining of IP-ECFCs revealed subcellular mislocalization of WPBs pro-inflammatory cargos angiopoietin-2 (Ang2, nuclear accumulation) and P-selectin. Besides, the RNA-sequencing (RNA-seq) analysis showed upregulation of pro-inflammatory and proangiogenic genes, P-selectin, interleukin 8 (IL-8), IL-6, and GROα, copackaged with VWF into WPBs. Further, whole-transcriptome RNA-seq and subsequent gene ontology (GO) enrichment analysis indicated the most enriched GO-biological process terms among the differentially expressed genes in IP-ECFCs were regulation of cell differentiation, cell adhesion, leukocyte adhesion to vascular endothelial, blood vessel morphogenesis, and angiogenesis, which resemble downstream signaling pathways associated with inflammatory stimuli and Ang2 priming. Accordingly, our functional experiments exhibited an increased endothelial cell adhesiveness and interruption in endothelial cell–cell junctions of the IP-ECFCs. In conclusion, the deleted VWF has a dominant-negative impact on multimer assembly and the biogenesis of WPBs, leading to altered trafficking of their pro-inflammatory cargos uniquely, which, in turn, causes changes in cellular signaling pathways, phenotype, and function of the endothelial cells.

The Asialoglycoprotein Receptor Minor Subunit Gene Contributes to Pharmacokinetics of Factor VIII Concentrates in Hemophilia A

BACKGROUND

 The asialoglycoprotein receptor (ASGPR) binds with high affinity factor VIII (FVIII) through its N-linked oligosaccharides. However, its contribution to the wide inter-individual variation of infused FVIII pharmacokinetics (PK) in hemophilia A (HA) is unknown.

OBJECTIVE

 To investigate the variability in FVIII PK outcomes in relation to genetic variation in the ASGR2, encoding the ASGPR2 subunit.

METHODS

 Thirty-two HA patients with FVIII:C ≤2 IU/dL underwent 66 single-dose FVIII PK studies. PK parameters were evaluated in relation to ASGR2 5' untranslated region (5'UTR) polymorphisms, which were investigated by recombinant and white blood cell reverse transcription-polymerase chain reaction approaches.

RESULTS

 The 5'UTR polymorphisms determine a frequent and conserved haplotype (HT1) in a regulatory region. The HT1 homozygotes may differ in the amounts of alternatively spliced mRNA transcripts and thus ASGPR2 isoforms. Compared with the other ASGR2 genotypes, the c.-95TT homozygotes (n = 9), showed threefold longer Alpha HL (3.60 hours, 95% confidence interval: 1.44-5.76, p = 0.006), and the c.-95TC heterozygotes (n = 17) showed 25% shorter mean residence time (MRT; 18.5 hours, 15.0-22.0, p = 0.038) and 32% shorter Beta HL (13.5 hours, 10.9-16.0, p = 0.016). These differences were confirmed in patients (n = 27) undergoing PK studies (n = 54) with full-length FVIII only. In different linear regression models, the contribution of the ASGR2 genotypes remained significant after adjustment by ABO genotypes and von Willebrand factor (VWF) antigen levels, and explained 14% (MRT), 15 to 18% (Beta HL), and 22% (Alpha HL) of parameter variability.

CONCLUSIONS

 Infused FVIII distribution was modulated by frequent ASGR2 genotypes, independently from and together with ABO and VWF antigen levels, which has potential implications for genetically tailored substitutive treatment in HA.

Von Willebrand Factor in Health and Disease

Abstract—

Von Willebrand factor (vWF), the key component of hemostasis, is synthesized in endothelial cells and megakaryocytes and released into the blood as high molecular weight multimeric glycoproteins weighing up to 20 million Daltons. Blood plasma metalloprotease ADAMTS13 cleaves ultra-large vWF multimers to smaller multimeric and oligomeric molecules. The vWF molecules attach to the sites of damage at the surface of arterioles and capillaries and unfold under conditions of shear stress. On the unfolded vWF molecule, the regions interacting with receptors on the platelet membrane are exposed. After binding to the vWF filaments, platelets are activated; platelets circulating in the vessels are additionally attached to them, leading to thrombus formation, blocking of microvessels, and cessation of bleeding. This review describes the history of the discovery of vWF, presents data on the mechanisms of vWF secretion and its structure, and characterizes the processes of vWF metabolism in the body under normal and pathological conditions.

The Biological Significance of von Willebrand Factor O-Linked Glycosylation

Glycosylation is a key posttranslational modification, known to occur on more than half of all secreted proteins in man. As such, the role of N- and O-linked glycan structures in modulating various aspects of protein biology is an area of much research. Given their prevalence, it is perhaps unsurprising that variations in glycan structures have been demonstrated to play critical roles in modulating protein function and have been implicated in the pathophysiology of human diseases. von Willebrand factor (VWF), a plasma glycoprotein that is essential for normal hemostasis, is heavily glycosylated, containing 13 N-linked and 10 O-linked glycans. Together, these carbohydrate chains account for 20% of VWF monomeric mass, and have been shown to modulate VWF structure, function, and half-life. In this review, we focus on the specific role played by O-linked glycans in modulating VWF biology. Specifically, VWF O-linked glycans have been shown to modulate tertiary protein structure, susceptibility to ADAMTS13 proteolysis, platelet tethering, and VWF circulatory half-life.

Low VWF: insights into pathogenesis, diagnosis, and clinical management

von Willebrand disease (VWD) constitutes the most common inherited human bleeding disorder. Partial quantitative von Willebrand factor (VWF) deficiency is responsible for the majority of VWD cases. International guidelines recommend that patients with mild to moderate reductions in plasma VWF antigen (VWF:Ag) levels (typically in the range of 30-50 IU/dL) should be diagnosed with low VWF. Over the past decade, a series of large cohort studies have provided significant insights into the biological mechanisms involved in type 1 VWD (plasma VWF:Ag levels <30 IU/dL). In striking contrast, however, the pathogenesis underpinning low VWF has remained poorly understood. Consequently, low VWF patients continue to present significant clinical challenges with respect to genetic counseling, diagnosis, and management. For example, there is limited information regarding the relationship between plasma VWF:Ag levels and bleeding phenotype in subjects with low VWF. In addition, it is not clear whether patients with low VWF need treatment. For those patients with low VWF in whom treatment is deemed necessary, the optimal choice of therapy remains unknown. However, a number of recent studies have provided important novel insights into these clinical conundrums and the molecular mechanisms responsible for the reduced levels observed in low VWF patients. These emerging clinical and scientific findings are considered in this review, with particular focus on pathogenesis, diagnosis, and clinical management of low VWF.

Perspective - The case for zero bleeds and drug bioequivalence in the treatment of congenital hemophilia A in 2021

Not all patients with severe hemophilia A (HA) respond optimally to a given dose of a given product. Within-individual variance in cross-over studies makes each patient unique in the response to each standard half-life (SHL) factor VIII (FVIII) product in pharmacokinetic (PK) terms. This hampers the prediction of efficacy when a SHL FVIII product is employed. PK data showing that half-lives of SHL rFVIII are unsatisfactory to achieve zero bleeding in individual HA patients provide the rationale for switching from SHL to extended half-life (EHL) products. However, not all subjects receiving prophylaxis with EHL products achieve zero bleeding, the most cogent objective of personalized prophylaxis. Known determinants of FVIII half-life (age, von Willebrand factor [VWF] levels, blood group) cumulatively account for one third of the total inter-individual variation in FVIII clearance in subjects with severe HA. Investigations into precision, and accuracy of laboratory measurement to be employed; newer pathways for the clearance of both free-FVIII and VWF-bound FVIII, and adequately powered studies on omics and phenotypic heterogeneity, are likely to provide additional information on the remaining two thirds of inter-individual variation in FVIII clearance in HA. Variability in the clinical response has also been documented in patients when FVIII activity is mimicked by fixed subcutaneous doses of the bispecific antibody emicizumab. National registries that collect PK data of available FVIII products and ad hoc information on the individual response to emicizumab should be encouraged, to establish newer standards of care and ease personalized clinical decisions to achieve zero bleeding.

Endothelial Dysfunction, Atherosclerosis, and Increase of von Willebrand Factor and Factor VIII: A Randomized Controlled Trial in Swine

It is well known that high von Willebrand factor (VWF) and factor VIII (FVIII) levels are associated with an increased risk of cardiovascular disease. It is still debated whether VWF and FVIII are biomarkers of endothelial dysfunction and atherosclerosis or whether they have a direct causative role. Therefore, we aimed to unravel the pathophysiological pathways of increased VWF and FVIII levels associated with cardiovascular risk factors. First, we performed a randomized controlled trial in 34 Göttingen miniswine. Diabetes mellitus (DM) was induced with streptozotocin and hypercholesterolemia (HC) via a high-fat diet in 18 swine (DM + HC), while 16 healthy swine served as controls. After 5 months of follow-up, FVIII activity (FVIII:C) was significantly higher in DM + HC swine (5.85 IU/mL [5.00-6.81]) compared with controls (4.57 [3.76-5.40], p = 0.010), whereas VWF antigen (VWF:Ag) was similar (respectively 0.34 IU/mL [0.28-0.39] vs. 0.34 [0.31-0.38], p = 0.644). DM + HC swine had no endothelial dysfunction or atherosclerosis during this short-term follow-up. Subsequently, we performed a long-term (15 months) longitudinal cohort study in 10 Landrace-Yorkshire swine, in five of which HC and in five combined DM + HC were induced. VWF:Ag was higher at 15 months compared with 9 months in HC (0.37 [0.32-0.42] vs. 0.27 [0.23-0.40], p = 0.042) and DM + HC (0.33 [0.32-0.37] vs. 0.25 [0.24-0.33], p = 0.042). Both long-term groups had endothelial dysfunction compared with controls and atherosclerosis after 15 months. In conclusion, short-term hyperglycemia and dyslipidemia increase FVIII, independent of VWF. Long-term DM and HC increase VWF via endothelial dysfunction and atherosclerosis. Therefore, VWF seems to be a biomarker for advanced cardiovascular disease.

VWF, Platelets and the Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with the persistent presence of antiphospholipid antibodies (aPLs). Laboratory criteria for the classification of APS include the detection of lupus anticoagulant (LAC), anti-cardiolipin (aCL) antibodies and anti-β2glycoprotein I (aβ2GPI) antibodies. Clinical criteria for the classification of thrombotic APS include venous and arterial thrombosis, along with microvascular thrombosis. Several aPLs, including LAC, aβ2GPI and anti-phosphatidylserine/prothrombin antibodies (aPS/PT) have been associated with arterial thrombosis. The Von Willebrand Factor (VWF) plays an important role in arterial thrombosis by mediating platelet adhesion and aggregation. Studies have shown that aPLs antibodies present in APS patients are able to increase the risk of arterial thrombosis by upregulating the plasma levels of active VWF and by promoting platelet activation. Inflammatory reactions induced by APS may also provide a suitable condition for arterial thrombosis, mostly ischemic stroke and myocardial infarction. The presence of other cardiovascular risk factors can enhance the effect of aPLs and increase the risk for thrombosis even more. These factors should therefore be taken into account when investigating APS-related arterial thrombosis. Nevertheless, the exact mechanism by which aPLs can cause thrombosis remains to be elucidated.

The relationship between ABO blood group, von Willebrand factor, and primary hemostasis

Numerous studies have reported significant associations between ABO blood group and risk of cardiovascular disease. These studies have consistently demonstrated that thrombotic risk is significantly reduced in individuals in blood group O. Nevertheless, the biological mechanisms through which ABO influences hemostasis have remained poorly understood. Exciting recent data have provided novel insights into how these ABO effects are modulated and have highlighted that ABO group significantly influences platelet plug formation at sites of vascular injury (primary hemostasis). In particular, ABO affects multiple aspects of von Willebrand factor (VWF) biology. In keeping with their reduced thrombotic risk, plasma VWF levels are ∼25% lower in healthy group O compared with healthy group non-O individuals. In addition, blood group O VWF demonstrates enhanced susceptibility to ADAMTS13 proteolysis. Finally, preliminary findings suggest that the interaction of group O VWF with platelets may also be reduced. Although the molecular mechanisms underlying these ABO effects on VWF have not been fully elucidated, it seems likely that they are mediated in large part by the ABO(H) carbohydrate structures that are carried on both the N- and O-linked glycans of VWF. Interestingly, ABO(H) determinants are also expressed on several different platelet surface glycoprotein receptors. Recent studies support the hypothesis that ABO group not only exerts major quantitative and qualitative effects on VWF, but also affect specific aspects of platelet function. Given the severe morbidity and the mortality associated with thrombotic disorders, defining the mechanisms underlying these ABO effects is not only of scientific interest, but also of direct clinical importance.

Management of elective procedures in low von Willebrand factor patients in the LoVIC study

BACKGROUND

Most individuals with mild to moderate reductions in plasma von Willebrand factor (VWF) levels do not demonstrate increased bleeding. However, some patients with plasma VWF levels of 30-50 IU/dl do have a significant bleeding phenotype. Management of these "low VWF" patients, who may have significant bleeding scores >10, around times of elective procedures continues to pose a common clinical challenge because of a lack of evidence.

OBJECTIVE

To investigate the safety and efficacy of different periprocedural management options for adult patients with low VWF.

METHODS

Treatment and outcomes were retrospectively reviewed for 160 invasive procedures performed in 60 patients with well characterized low VWF enrolled in the previously described Low Von Willebrand factor Ireland Cohort study.

RESULTS

We demonstrate that 1-desamino-8-D-arginine vasopressin is efficacious in preventing bleeding for both minor or major elective procedures in adult low VWF patients, even in those with significant bleeding histories. In addition, tranexamic acid alone is effective for low VWF patients undergoing nondental minor procedures. Importantly, age-related increases in plasma VWF:antigen levels above 50 IU/dl were not necessarily associated with complete correction of bleeding phenotype. Procedure-related bleeding complications were increased in low VWF patients who did not receive any hemostatic cover before their procedure.

CONCLUSION

Elective procedures in adult patients with low VWF should be managed in liaison with a comprehensive care tertiary referral center so that personalized treatment plans may be implemented before all minor or major elective procedures.

Toward Personalized Treatment for Patients with Low von Willebrand Factor and Quantitative von Willebrand Disease

The biological mechanisms involved in the pathogenesis of type 2 and type 3 von Willebrand disease (VWD) have been studied extensively. In contrast, although accounting for the majority of VWD cases, the pathobiology underlying partial quantitative VWD has remained somewhat elusive. However, important insights have been attained following several recent cohort studies that have investigated mechanisms in patients with type 1 VWD and low von Willebrand factor (VWF), respectively. These studies have demonstrated that reduced plasma VWF levels may result from either (1) decreased VWF biosynthesis and/or secretion in endothelial cells and (2) pathological increased VWF clearance. In addition, it has become clear that some patients with only mild to moderate reductions in plasma VWF levels in the 30 to 50 IU/dL range may have significant bleeding phenotypes. Importantly in these low VWF patients, bleeding risk fails to correlate with plasma VWF levels and inheritance is typically independent of the VWF gene. Although plasma VWF levels may increase to > 50 IU/dL with progressive aging or pregnancy in these subjects, emerging data suggest that this apparent normalization in VWF levels does not necessarily equate to a complete correction in bleeding phenotype in patients with partial quantitative VWD. In this review, these recent advances in our understanding of quantitative VWD pathogenesis are discussed. Furthermore, the translational implications of these emerging findings are considered, particularly with respect to designing personalized treatment plans for VWD patients undergoing elective procedures.

Increasing levels of von Willebrand factor and factor VIII with age in patients affected by von Willebrand disease

Essentials VWF and FVIII increase with age in patients affected by VWD. VWF and FVIII increase in type 1 and in low levels of VWF patients. VWF and FVIII do not increase in type 1 Vicenza. FVIII increases in type 2 VWD patients. ABSTRACT: Background Increasing levels of von Willebrand factor (VWF) and factor VIII (FVIII:C) was associated with age in type 1 von Willebrand disease (VWD). Objectives To evaluate VWF and FVIII:C increase with age in a large group of patients with VWD and low levels of VWF, in whom levels were repeatedly measured. Methods Clinical charts from all patients evaluated at the A. Bianchi Bonomi Center between 1970 and 2018 were reviewed and data on VWF and FVIII:C collected. Patients affected by type 3, severe type 1 and 2N VWD were excluded. The repeated measurements were evaluated by linear mixed-effects models. A linear association between age and VWF/FVIII:C was shown after the age of 40 years in the linear mixed models and analyzed by calculating the regression slope coefficient (β). Results A total of 617 patients were included in the study (314 type 2, 112 type 1, 181 low VWF levels), with a median age at first measurement of 28 years (interquartile range 14/42) and a mean follow-up of 16 years (standard deviation 11). VWF and FVIII:C increased with age in the whole group. The increase became linear after the age of 40 years (3.68 and 7.44 IU/dL per decade for VWF:activity and FVIII:C). In type 2, FVIII:C increased with age, whereas an increase of both VWF:activity and FVIII:C were shown in patients with type 1 VWD and low levels of VWF. Conclusions A differential increase of VWF and FVIII:C with age was shown among in different ages and types of VWD.

Von Willebrand factor propeptide in severe coronavirus disease 2019 (COVID-19): evidence of acute and sustained endothelial cell activation

Endothelial cell (EC) activation plays a key role in the pathogenesis of pulmonary microvascular occlusion, which is a hallmark of severe coronavirus disease 2019 (COVID-19). Consistent with EC activation, increased plasma von Willebrand factor antigen (VWF:Ag) levels have been reported in COVID-19. Importantly however, studies in other microangiopathies have shown that plasma VWF propeptide (VWFpp) is a more sensitive and specific measure of acute EC activation. In the present study, we further investigated the nature of EC activation in severe COVID-19. Markedly increased plasma VWF:Ag [median (interquatile range, IQR) 608·8 (531-830)iu/dl] and pro-coagulant factor VIII (FVIII) levels [median (IQR) 261·9 (170-315) iu/dl] were seen in patients with severe severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection. Sequential testing showed that these elevated VWF-FVIII complex levels remained high for up to 3 weeks. Similarly, plasma VWFpp levels were also markedly elevated [median (IQR) 324·6 (267-524) iu/dl]. Interestingly however, the VWFpp/VWF:Ag ratio was reduced, demonstrating that decreased VWF clearance contributes to the elevated plasma VWF:Ag levels in severe COVID-19. Importantly, plasma VWFpp levels also correlated with clinical severity indices including the Sequential Organ Failure Assessment (SOFA) score, Sepsis-Induced Coagulopathy (SIC) score and the ratio of arterial oxygen partial pressure to fractional inspired oxygen (P/F ratio). Collectively, these findings support the hypothesis that sustained fulminant EC activation is occurring in severe COVID-19, and further suggest that VWFpp may have a role as a biomarker in this setting.

Expresser phenotype determines ABO(H) blood group antigen loading on platelets and von Willebrand factor

ABO blood group is associated with cardiovascular disease, with significantly lower risk in blood group O individuals. ABO(H) blood group determinants are expressed on different glycoproteins on platelet surfaces. In addition, ABO(H) structures are also present on VWF glycans. These ABO(H) carbohydrates influence both platelet and VWF function. Previous studies have reported that approximately 5-10% of normal blood donors express abnormally high or low levels of A or B blood group antigens on their platelet surfaces (high expresser phenotype, HXP or low expresser phenotype, LXP respectively). In this study, the biological effects of the ABO Expresser phenotype were investigated. ABO(H) expression on platelets and plasma VWF was studied in a series of 541 healthy blood donors. Overall, 5.6% of our study cohort were classified as HXP, whilst 4.4% satisfied criteria for LXP. We demonstrate that genotype at the ABO blood group locus plays a critical role in modulating the platelet HXP phenotype. In particular, A1A1 genotype is a major determinant of ABO high-expresser trait. Our data further show that ABH loading on VWF is also affected by ABO expresser phenotype. Consequently, A antigen expression on VWF was significantly elevated in HXP individuals and moderately reduced in LXP subjects (P < 0.05). Collectively, these findings suggest that ABO expresser phenotype influences primary hemostasis though several different pathways. Further studies will be required to define whether inter-individual variations in ABO(H) expression on platelets and/or VWF (particularly HXP and LXP) impact upon risk for cardiovascular disease.

von Willebrand Factor Is a Critical Mediator of Deep Vein Thrombosis in a Mouse Model of Diet-Induced Obesity

OBJECTIVE

Obesity is characterized by chronic low-grade inflammation and consequentially a hypercoagulable state, associating with an increased incidence of venous thromboembolism. Increased VWF (von Willebrand factor) plasma concentration and procoagulant function are independent risk factors for venous thromboembolism and are elevated in obese patients. Here, we explore the pathobiological role of VWF in obesity-associated venous thrombosis using murine models. Approach and Results: We first showed that diet-induced obese mice have increased VWF plasma levels and FVIII (factor VIII) activity compared with littermate controls. Elevated VWF levels appeared to be because of both increased synthesis and impaired clearance. Diet-induced obesity-associated venous thrombosis was assessed using the inferior vena cava-stenosis model of deep vein thrombosis. Diet-induced obese mice developed larger venous thrombi that were rich in VWF, erythrocytes, and leukocytes. Administering a polyclonal anti-VWF antibody or an anti-VWF A1 domain nanobody was protective against obesity-mediated thrombogenicity. Delayed administration (3 hours post-inferior vena cava stenosis) similarly reduced thrombus weight in diet-induced obese mice.

CONCLUSIONS

This study demonstrates the critical role of VWF in the complex, thrombo-inflammatory state of obesity. It adds to the growing rationale for targeting VWF-specific interactions in thrombotic disease.

Whole-exome sequencing identifies rare variants in STAB2 associated with venous thromboembolic disease

Deep vein thrombosis and pulmonary embolism, collectively defined as venous thromboembolism (VTE), are the third leading cause of cardiovascular death in the United States. Common genetic variants conferring increased varying degrees of VTE risk have been identified by genome-wide association studies (GWAS). Rare mutations in the anticoagulant genes PROC, PROS1 and SERPINC1 result in perinatal lethal thrombosis in homozygotes and markedly increased VTE risk in heterozygotes. However, currently described VTE variants account for an insufficient portion of risk to be routinely used for clinical decision making. To identify new rare VTE risk variants, we performed a whole-exome study of 393 individuals with unprovoked VTE and 6114 controls. This study identified 4 genes harboring an excess number of rare damaging variants in patients with VTE: PROS1, STAB2, PROC, and SERPINC1. At STAB2, 7.8% of VTE cases and 2.4% of controls had a qualifying rare variant. In cell culture, VTE-associated variants of STAB2 had a reduced surface expression compared with reference STAB2. Common variants in STAB2 have been previously associated with plasma von Willebrand factor and coagulation factor VIII levels in GWAS, suggesting that haploinsufficiency of stabilin-2 may increase VTE risk through elevated levels of these procoagulants. In an independent cohort, we found higher von Willebrand factor levels and equivalent propeptide levels in individuals with rare STAB2 variants compared with controls. Taken together, this study demonstrates the utility of gene-based collapsing analyses to identify loci harboring an excess of rare variants with functional connections to a complex thrombotic disease.

The spectrum and severity of bleeding in adolescents with low von Willebrand factor-associated heavy menstrual bleeding

Low von Willebrand factor (VWF) in adults is associated with significant bleeding, most notably heavy menstrual bleeding (HMB) and postpartum hemorrhage, although this has not been characterized in adolescents. The objectives of this analysis of a multicenter single arm observational cohort study in adolescents with low VWF-associated HMB were to describe the bleeding phenotype, HMB severity, and related complications. Eligibility criteria included postmenarchal females <21 years of age with HMB (Pictorial Blood Assessment Chart [PBAC] score >100) and low VWF (2 values of VWF activity ≥30 and ≤50 IU/dL). Patients diagnosed with other bleeding disorders were ineligible. Clinical phenotype data, including PBAC and Bleeding Assessment Tool (BAT) scores, laboratory data, and HMB management/outcome details, were extracted. Patient demographics and clinical characteristics were summarized as medians with minimum/maximum values or frequencies with percentages. Groups were compared using a Wilcoxon rank-sum test or Fisher's exact test. A total of 113 patients met inclusion criteria, and 2 were excluded. Ninety four percent had a significant bleeding phenotype (BAT score >2), with predominantly mucocutaneous bleeding (32%-44%), postprocedural/surgical bleeding (15%), and severe HMB (BAT HMB domain score ≥2; 90%). Bleeding complications included iron deficiency (60%), anemia (21%), transfusion (12%), and hospitalization (10%). Desmopressin challenge response in subjects tested was good and sustained. Several (48%) required combined therapy for HMB (hormonal/hemostatic), and one third did not show improvement despite therapy. Our results suggest that adolescent females with low VWF have a significant bleeding phenotype and resultant complications warranting a focus on prompt diagnosis, appropriate therapy, and prevention of complications.

Biological mechanisms underlying inter-individual variation in factor VIII clearance in haemophilia

Previous studies have highlighted marked inter‐individual variations in factor VIII (FVIII) clearance between patients with haemophilia (PWH). The half‐life of infused FVIII has been reported to vary from as little as 5.3 hours in some adult PWH, up to as long as 28.8 hours in other individuals. These differences in clearance kinetics have been consistently observed using a number of different plasma‐derived and recombinant FVIII products. Furthermore, recent studies have demonstrated that half‐life for extended half‐life (EHL‐) FVIII products also demonstrates significant inter‐patient variation. Since time spent with FVIII trough levels <1% has been shown to be associated with increased bleeding risk in PWH on prophylaxis therapy, this variability in FVIII clearance clearly has major clinical significance. Recent studies have provided significant novel insights into the cellular basis underlying FVIII clearance pathways. In addition, accumulating data have shown that endogenous plasma VWF levels, ABO blood group and age, all play important roles in regulating FVIII half‐life in PWH. Indeed, multiple regression analysis suggests that together these factors account for approximately 34% of the total inter‐individual variation in FVIII clearance observed between subjects with severe haemophilia A. In this review, we consider these and other putative modulators of FVIII half‐life, and discuss the biological mechanisms through which these factors impact upon FVIII clearance in vivo.

Investigating the clearance of VWF A-domains using site-directed PEGylation and novel N-linked glycosylation

BACKGROUND

Previous studies have demonstrated that the A1A2A3 domains of von Willebrand factor (VWF) play a key role in regulating macrophage-mediated clearance in vivo. In particular, the A1-domain has been shown to modulate interaction with macrophage low-density lipoprotein receptor-related protein-1 (LRP1) clearance receptor. Furthermore, N-linked glycans within the A2-domain have been shown to protect VWF against premature LRP1-mediated clearance. Importantly, however, the specific regions within A1A2A3 that enable macrophage binding have not been defined.

OBJECTIVE AND METHODS

To address this, we utilized site-directed PEGylation and introduced novel targeted N-linked glycosylation within A1A2A3-VWF and subsequently examined VWF clearance.

RESULTS

Conjugation with a 40-kDa polyethylene glycol (PEG) moiety significantly extended the half-life of A1A2A3-VWF in VWF-/- mice in a site-specific manner. For example, PEGylation at specific sites within the A1-domain (S1286) and A3-domain (V1803, S1807) attenuated VWF clearance in vivo, compared to wild-type A1A2A3-VWF. Furthermore, PEGylation at these specific sites ablated binding to differentiated THP-1 macrophages and LRP1 cluster II and cluster IV in-vitro. Conversely, PEGylation at other positions (Q1353-A1-domain and M1545-A2-domain) had limited effects on VWF clearance or binding to LRP1.Novel N-linked glycan chains were introduced at N1803 and N1807 in the A3-domain. In contrast to PEGylation at these sites, no significant extension in half-life was observed with these N-glycan variants.

CONCLUSIONS

These novel data demonstrate that site specific PEGylation but not site specific N-glycosylation modifies LRP1-dependent uptake of the A1A2A3-VWF by macrophages. This suggests that PEGylation, within the A1- and A3-domains in particular, may be used to attenuate LRP1-mediated clearance of VWF.

New developments in von Willebrand disease

Von Willebrand disease (VWD) constitutes the most common inherited human bleeding disorder. It is associated with a mucocutaneous bleeding phenotype that can significantly impact upon quality of life. Despite its prevalence and associated morbidity, the diagnosis and subclassification of VWD continue to pose significant clinical challenges. This is in part attributable to the fact that plasma von Willebrand factor (VWF) levels vary over a wide range in the normal population, together with the multiple different physiological functions played by VWF in vivo. Over recent years, substantial progress has been achieved in elucidating the biological roles of VWF. Significant advances have also been made into defining the pathophysiological mechanisms underpinning both quantitative and qualitative VWD. In particular, several new laboratory assays have been developed that enable more precise assessment of specific aspects of VWF activity. In the present review, we discuss these recent developments in the field of VWD diagnosis, and consider how these advances can impact upon clinical diagnostic algorithms for use in routine clinical practice. In addition, we review some important recent advances pertaining to the various treatment options available for managing patients with VWD.

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.

The scavenger receptor SCARA5 is an endocytic receptor for von Willebrand factor expressed by littoral cells in the human spleen

BACKGROUND

Scavenger receptors play a significant role in clearing aged proteins from the plasma, including the large glycoprotein coagulation factors von Willebrand factor (VWF) and factor VIII (FVIII). A large genome-wide association study meta-analysis has identified genetic variants in the gene SCARA5, which encodes the class A scavenger receptor SCARA5, as being associated with plasma levels of VWF and FVIII.

OBJECTIVES

The ability of SCARA5 to regulate the clearance of VWF-FVIII was characterized.

METHODS

VWF-FVIII interactions with SCARA5 were evaluated by solid phase binding assays and in vitro cell based assays. The influence of SCARA5 deficiency on VWF:Ag and half-life was assessed in a murine model. The expression pattern of SCARA5 and its colocalization with VWF was evaluated in human tissues.

RESULTS

VWF and the VWF-FVIII complex bound to human recombinant SCARA5 in a dose- and calcium-dependent manner. SCARA5 expressing HEK 293T cells bound and internalized VWF and the VWF-FVIII complex into early endosomes. In vivo, SCARA5 deficiency had a modest influence on the half-life of human VWF. mRNA analysis and immunohistochemistry determined that human SCARA5 is expressed in kidney podocytes and the red pulp, white pulp, and marginal zone of the spleen. VWF was found to colocalize with SCARA5 expressed by littoral cells lining the red pulp of the human spleen.

CONCLUSIONS

SCARA5 is an adhesive and endocytic receptor for VWF. In human tissues, SCARA5 is expressed by kidney podocytes and splenic littoral endothelial cells. SCARA5 may have a modest influence on VWF clearance in humans.

von Willebrand factor sialylation-A critical regulator of biological function

Essentials Von Willebrand Factor (VWF) is extensively glycosylated with serial studies demonstrating that these carbohydrate determinants play critical roles in regulating multiple aspects of VWF biology. Terminal sialic acid residues, expressed on both the N- and O-linked glycans of VWF, regulate VWF functional activity, susceptibility to proteolysis and plasma clearance in vivo. Quantitative and qualitative variations in VWF sialylation have been reported in patients with von Willebrand Disease, as well as in a number of other physiological and pathological states. Further studies are warranted to define the molecular mechanisms through which N- and O-linked sialylation impacts upon the multiple biological activities of VWF. von Willebrand factor (VWF) undergoes complex post-translational modification prior to its secretion into the plasma. Consequently, VWF monomers contain complex N-glycan and O-glycan structures that, together, account for approximately 20% of the final monomeric mass. An increasing body of evidence has confirmed that these carbohydrate determinants play critical roles in regulating multiple aspects of VWF biology. In particular, studies have demonstrated that terminal ABO blood group has an important effect on plasma VWF levels. This effect is interesting, given that only 15% of the N-glycans and 1% of the O-glycans of VWF actually express terminal ABO(H) determinants. In contrast, the vast majority of the N-glycans and O-glycans on human VWF are capped by terminal negatively charged sialic acid residues. Recent data suggest that sialylation significantly regulates VWF functional activity, susceptibility to proteolysis, and clearance, through a number of independent pathways. These findings are of direct clinical relevence, in that quantitative and qualitative variations in VWF sialylation have been described in patients with VWD, as well as in patients with a number of other physiologic and pathologic conditions. Moreover, platelet-derived VWF is significantly hyposialylated as compared with plasma-derived VWF, whereas the recently licensed recombinant VWF therapeutic is hypersialylated. In this review, we examine the evidence supporting the hypothesis that VWF sialylation plays multiple biological roles. In addition, we consider data suggesting that quantitative and qualitative variations in VWF sialylation may play specific roles in the pathogenesis of VWD, and that sialic acid expression on VWF may also differ across a number of other physiologic and pathologic conditions.

Increased galactose expression and enhanced clearance in patients with low von Willebrand factor

Glycan determinants on von Willebrand factor (VWF) play critical roles in regulating its susceptibility to proteolysis and clearance. Abnormal glycosylation has been shown to cause von Willebrand disease (VWD) in a number of different mouse models. However, because of the significant technical challenges associated with accurate assessment of VWF glycan composition, the importance of carbohydrates in human VWD pathogenesis remains largely unexplored. To address this, we developed a novel lectin-binding panel to enable human VWF glycan characterization. This methodology was then used to study glycan expression in a cohort of 110 patients with low VWF compared with O blood group-matched healthy controls. Interestingly, significant interindividual heterogeneity in VWF glycan expression was seen in the healthy control population. This variation included terminal sialylation and ABO(H) blood group expression on VWF. Importantly, we also observed evidence of aberrant glycosylation in a subgroup of patients with low VWF. In particular, terminal α(2-6)-linked sialylation was reduced in patients with low VWF, with a secondary increase in galactose (Gal) exposure. Furthermore, an inverse correlation between Gal exposure and estimated VWF half-life was observed in those patients with enhanced VWF clearance. Together, these findings support the hypothesis that loss of terminal sialylation contributes to the pathophysiology underpinning low VWF in at least a subgroup of patients by promoting enhanced clearance. In addition, alterations in VWF carbohydrate expression are likely to contribute to quantitative and qualitative variations in VWF levels in the normal population. This trial was registered at www.clinicaltrials.gov as #NCT03167320.

Diagnostic challenges of inherited mild bleeding disorders: a bait for poorly explored clinical and basic research

The best-known inherited mild bleeding disorders (MBDs), i.e. type 1 von Willebrand disease (VWD), platelet function disorders (PFDs), and mild to moderate clotting factor deficiencies, are characterized clinically by mucocutaneous bleeding, and, although they are highly prevalent, still pose difficult diagnostic problems. These include establishing the pathological nature of bleeding, and the uncertainties surrounding the clinical relevance of laboratory results. Furthermore, the high frequency of bleeding symptoms in the normal population and the subjective appraisal of symptoms by patients or parents makes elucidating the pathological nature of bleeding difficult. Standardized bleeding assessment tools and semiquantitative bleeding scores (BSs) help to discriminate normal from abnormal bleeding. However, as most MBDs have similar bleeding patterns, for example, bleeding sites, frequency, and severity, BSs are of little help for diagnosing specific diseases. Global tests of primary hemostasis (bleeding time; PFA-100/200) lack sensitivity and, like BSs, are not disease-specific. Problems with the diagnosis of type 1 VWD and PFD include assay standardization, uncertain definition of von Willebrand factor cut-off levels, and the lack of universal diagnostic criteria for PFD. Regarding clotting factor deficiencies, the bleeding thresholds of some coagulation factors, such as factor VII and FXI, are highly variable, and may lead to misinterpretation of the clinical relevance of mild to moderate deficiencies. Remarkably, a large proportion of MBDs remain undiagnosed even after comprehensive and repeated laboratory testing. These are tentatively considered to represent bleeding of undefined cause, with clinical features indistinguishable from those of classical MBD; the pathogenesis of this is probably multifactorial, and unveiling these mechanisms should constitute a fertile source of translational research.

Blood, blood components, plasma, and plasma products

This review of 2018 publications identifies side effects of blood, blood components, and plasma products. In addition, albumin, blood transfusion (erythrocytes, granulocytes, and platelets), blood substitutes (hemoglobin-based oxygen carriers), plasma products (alpha1-antitrypsin, C1 esterase inhibitor concentrate, cryoprecipitate, and fresh frozen plasma), plasma substitutes (etherified starches, and gelatin), globulins (intravenous immunoglobulin, subcutaneous immunoglobulin, and anti-D immunoglobulin), coagulation proteins (factor I, factor II, factor VIIa, factor VIII, factor IX, prothrombin complex concentrate, antithrombin III, and von Willebrand factor/factor VIII concentrates), erythropoietin and derivatives, thrombopoietin and receptor agonists, transmission of infectious agents through blood donation, and stem cells are reviewed. This chapter informs the reader about newly recognized and published data in the blood product domain.