High Von Willebrand Factor Levels Research Articles

Thrombotic Manifestations in Patients with Vexas Syndrome

Introduction: VEXAS syndrome, caused by somatic mutations in ubiquitin-like modifier activating enzyme 1 (UBA1) gene, is a late-onset and progressive autoinflammatory disease. Main hematologic manifestations include macrocytic anemia, thrombocytopenia, and dyspoiesis. Concomitant diagnoses of myelodysplastic syndrome (MDS) and plasma cell dyscrasia (PCD) are common. Venous thromboembolism (VTE) and arterial thrombosis (AT) have been reported. We describe the incidence and characteristics of thrombosis in a large cohort of 86 patients with VEXAS syndrome, along with detailed coagulation profiles in a subgroup of patients. We also explore potential predictors of thrombosis, and effect on overall survival (OS). Methods: Two cohorts of patients with confirmed VEXAS were included. Thirty patients were evaluated at the National Institutes of Health (NIH) Clinical Center and 56 patient records reviewed from referring institutions. Logistic regression models were used to determine predictors of thrombosis. Cox proportional hazards models and Kaplan Meier curves were used for survival analysis. Results: The total population consisted of 86 patients. Eighty-five patients (99%) were male with a median age of 64 years (range, 40-84 years) at disease onset. Hematologic manifestations included macrocytic anemia in 63 (73%), MDS in 25 (29%) and PCD in 17 (20%) patients. Thrombosis occurred in 45 patients (52%); 41 (48%) had VTE and 11 (12%) had AT, with a median time to event from disease onset of 18 and 34 months, respectively; five patients (5%) had both VTE and AT. Among VTE, there were 26 (30%) deep venous thrombosis (DVT), 12 (14%) pulmonary embolism (PE), and 7 (8%) superficial thrombophlebitis. Twenty-two (54%) of VTE cases were unprovoked. Recurrent VTE was seen in 15 (17%) of patients, among which 9 (53%) occurred while receiving anticoagulation. Arterial events included myocardial infarction in 7 (8%) patients, stroke in 2 (2%) patients, and peripheral arterial occlusion in 2 (2%) patients. Recurrent AT was seen in 2 (2%) patients. Patients from the NIH cohort underwent extensive coagulation testing. Fourteen patients (46%) had a positive lupus anticoagulant (LA), which was persistently positive 12 weeks apart in 8 of 9 patients re-tested. Of these 8 patients, 6 had a VTE, 1 had a stroke and 1 did not have thrombotic manifestations. Thirteen patients (43%) with thrombotic events were negative for LA. Median levels of fibrinogen, D-dimer, Factor (F) VII, FIX, and especially von Willebrand factor (VWF) and FVIII, were high in 15 tested VEXAS patients, but there was no significant difference between those with and without thrombosis (Table 1). Of 10 patients with high VWF and 17 with high FVIII activity, 6 (60%) and 9 (53%) had a thrombotic event, respectively; these correlated with high levels of CRP and ESR. No patients were found to have abnormal antithrombin III activity, Factor V Leiden, or prothrombin gene mutations. No difference in age, UBA1 gene mutation, median levels of CRP and ESR, presence of MDS or PCD was found between patients with or without thrombosis; MDS, PCD and UBA1 gene mutation were also not predictive of thrombosis. Among inflammatory disease manifestations, cardiac involvement (myocarditis, pericarditis, or pericardial effusion) correlated with the presence of any thrombosis (p = 0.03). When considering VTE alone, cardiac manifestations strongly predicted PE and DVT (p = 0.004 and p = 0.005 respectively), while the presence of chondritis correlated with PE (p = 0.02). Presence of thrombosis did not impact OS (Figure 1). Conclusions: Patients with VEXAS syndrome are at high risk of thrombosis, mainly venous, which can be recurrent despite anticoagulation therapy. The underlying etiology remains unclear, but many VEXAS patients have positive LA, high VWF and FVIII levels, suggesting inflammation, endothelial activation and/or an immune process as pathophysiologic. Cardiac manifestations, including myocarditis, pericarditis, and pericardial effusion, significantly correlated with thrombosis in general, while chondritis correlated with presence of PE. Presence of thrombosis did not impact OS. Given the high incidence of thrombosis, VEXAS patients should be carefully risk assessed for VTE and considered for prophylactic anticoagulation, and indefinite treatment if thrombosis has occurred. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

The Intriguing Connections between von Willebrand Factor, ADAMTS13 and Cancer.

von Willebrand factor (VWF) is a complex and large protein that is cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13), and together they serve important roles in normal hemostasis. Malignancy can result in both a deficiency or excess of VWF, leading to aberrant hemostasis with either increased bleeding or thrombotic complications, as respectively seen with acquired von Willebrand syndrome and cancer-associated venous thromboembolism. There is emerging evidence to suggest VWF also plays a role in inflammation, angiogenesis and tumor biology, and it is likely that VWF promotes tumor metastasis. High VWF levels have been documented in a number of malignancies and in some cases correlate with more advanced disease and poor prognosis. Tumor cells can induce endothelial cells to release VWF and certain tumor cells have the capacity for de novo expression of VWF, leading to a proinflammatory microenvironment that is likely conducive to tumor progression, metastasis and micro-thrombosis. VWF can facilitate tumor cell adhesion to endothelial cells and aids with the recruitment of platelets into the tumor microenvironment, where tumor/platelet aggregates are able to form and facilitate hematogenous spread of cancer. As ADAMTS13 moderates VWF level and activity, it too is potentially involved in the pathophysiology of these events. VWF and ADAMTS13 have been explored as tumor biomarkers for the detection and prognostication of certain malignancies; however, the results are underdeveloped and so currently not utilized for clinical use. Further studies addressing the basic science mechanisms and real word epidemiology are required to better appreciate the intriguing connections between VWF, ADAMTS13 and malignancy. A better understanding of the role VWF and ADAMTS13 play in the promotion and inhibition of cancer and its metastasis will help direct further translational studies to aid with the development of novel cancer prognostic tools and treatment modalities.

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.

Influence of blood group and von Willebrand factor on population pharmacokinetics and dose individualization of recombinant factor VIII in Taiwanese patients with haemophilia A.

The large interpatient variability in the pharmacokinetic (PK) parameters of recombinant Factor VIII (rFVIII) observed in haemophilia A hinders efficient and cost-beneficial prophylactic regimen initiation. Identification of factors influencing the PK of rFVIII may shed more light on personalised treatment. This study aimed to develop a population PK model in the Taiwanese haemophilia A and evaluate the current national health insurance (NHI) reimbursement guidelines of Taiwan for haemophilia treatment. A population PK analysis was established based on 69 Taiwanese with moderate or severe haemophilia A. A nonlinear mixed-effects modelling (NONMEM® ) was used to estimate PK parameters and their variabilities. A Monte Carlo simulation was performed to evaluate different prophylactic regimens. A two-compartment model with first-order elimination best described the rFVIII data. Weight-based allometric scaling was related to clearance and central volume of distribution. Blood type and baseline von Willebrand factor (VWF) were significant covariates for clearance. For single dose simulations, a time achieving target level (>1IU/dL) was associated with increasing rFVIII dose and VWF level. The multiple dose simulations showed that>96.4% of patients with high VWF level (>200%) had predicted trough level>1IU/dL for all dosing regimens (15-40IU/kg, two to three times weekly). However, for twice weekly dosing, lower percentage (47.62-62.20%) of patients with blood group O and low VWF level (<50%) achieved a predicted trough level>1IU/dL. The population PK of rFVIII was successfully developed. Dose adjustment based on blood type and VWF level should be considered.

Uncomplicated Plasmodium vivax malaria: mapping the proteome from circulating platelets

BackgroundThrombocytopenia is frequent in Plasmodium vivax malaria but the role of platelets in pathogenesis is unknown. Our study explores the platelet (PLT) proteome from uncomplicated P. vivax patients, to fingerprint molecular pathways related to platelet function. Plasma levels of Platelet factor 4 (PF4/CXCL4) and Von Willebrand factor (VWf), as well as in vitro PLTs—P. vivax infected erythrocytes (Pv-IEs) interactions were also evaluated to explore the PLT response and effect on parasite development.MethodsA cohort of 48 patients and 25 healthy controls were enrolled. PLTs were purified from 5 patients and 5 healthy controls for Liquid Chromatography–Mass spectrometry (LC–MS/MS) analysis. Plasma levels of PF4/CXCL4 and VWf were measured in all participants. Additionally, P. vivax isolates (n = 10) were co-cultured with PLTs to measure PLT activation by PF4/CXCL4 and Pv-IE schizonts formation by light microscopy.ResultsThe proteome from uncomplicated P. vivax patients showed 26 out of 215 proteins significantly decreased. PF4/CXCL4 was significantly decreased followed by other proteins involved in platelet activation, cytoskeletal remodeling, and endothelial adhesion, including glycoprotein V that was significantly decreased in thrombocytopenic patients. In contrast, acute phase proteins, including SERPINs and Amyloid Serum A1 were increased. High levels of VWf in plasma from patients suggested endothelial activation while PF4/CXCL4 plasma levels were similar between patients and controls. Interestingly, high levels of PF4/CXCL4 were released from PLTs—Pv-IEs co-cultures while Pv-IEs schizont formation was inhibited.ConclusionsThe PLT proteome analyzed in this study suggests that PLTs actively respond to P. vivax infection. Altogether, our findings suggest important roles of PF4/CXCL4 during uncomplicated P. vivax infection through a possible intracellular localization. Our study shows that platelets are active responders to P. vivax infection, inhibiting intraerythrocytic parasite development. Future studies are needed to further investigate the molecular pathways of interaction between platelet proteins found in this study and host response, which could affect parasite control as well as disease progression.

Genetic Variation in ADAMTS13 is Related to VWF Levels, Atrial Fibrillation and Cerebral Ischemic Events.

ADAMTS13 cleaves von Willebrand factor (VWF) multimers into less active fragments. Both markers have been related to cardiovascular disease (CVD). We aimed to investigate the influence of ADAMTS13 single nucleotide polymorphisms (SNPs) on levels of ADAMTS13 and VWF, and CVD. The c.1342C>G, g.41635A>G and c.2699C>T polymorphisms were determined in patients with chronic coronary syndrome (n = 1000). VWF and ADAMTS13 were analyzed. Clinical endpoints after 2 years (n = 106) were unstable angina pectoris, myocardial infarction, non-hemorrhagic stroke and death. The SNPs did not affect ADAMTS13 levels. The 41635A-allele associated with higher VWF levels (P < .001). Patients with the 1342G-allele had significantly higher frequency of previous atrial fibrillation (n = 26, P = .016) and cerebral ischemic events (n = 47, P = .030). Heterozygous of the 1342CG variant experienced more clinical endpoints compared to homozygous (CC and GG) (P = .028). The association between the 41635A-allele and VWF indicates a role for this polymorphism in VWF regulation. ADAMTS13 has previously been linked to atrial fibrillation and ischemic stroke, and our findings suggest that the 1342G-allele may be of significance. The association between the 1342CG genotype and endpoints needs further investigations.Clinicaltrials.gov, ASCET, NCT00222261. https://clinicaltrials.gov/ct2/show/NCT00222261?term=NCT00222261&draw=2&rank=1.

Severe COVID‐19 is associated with endothelial activation and abnormal glycosylation of von Willebrand factor in patients undergoing hemodialysis

BackgroundA major clinical feature of severe coronavirus diease 2019 (COVID‐19) is microvascular thrombosis linked to endothelial cell activation. Consistent with this, a number of studies have shown that patients with severe COVID‐19 have highly elevated plasma levels of von Willebrand Factor (VWF) that may contribute to the prothrombotic phenotype. In the current study, we investigated the extent of endothelial activation in patients receiving hemodialysis who had either mild or severe COVID‐19. MethodsPlasma VWF, ADAMTS‐13, angiopoietin‐2 (Ang2), and syndecan‐1 levels were determined by ELISA. The sialic acid content of VWF was investigated using a modified ELISA to measure elderberry bark lectin, specific for sialic acid residues, binding to VWF. ResultsPatients receiving hemodialysis with severe COVID‐19 had significantly higher plasma levels of VWF and lower ADAMTS‐13. VWF levels peaked and were sustained during the first 10 days after positive confirmation of infection. While Ang2 trended toward being higher in severely ill patients, this did not reach significance; however, severely ill patients had significantly higher soluble syndecan‐1 levels, with high levels related to risk of death. Finally, higher VWF levels in severely ill patients were correlated with lower VWF sialic acid content. ConclusionsSevere COVID‐19 in patients undergoing hemodialysis is associated with both acute and sustained activation of the endothelium, leading to alteration of the VWF/ADAMTS‐13 axis. Lower VWF sialic acid content represents altered VWF processing and further confirms the disturbance caused to the endothelium in COVID‐19.

Efficacy, Safety and Pharmacokinetic Results of a Phase III, Open-Label, Multicenter Study with a Plasma-Derived Von Willebrand Factor (VWF)/Factor VIII (FVIII) Concentrate in Pediatric Patients

BackgroundPlasma-derived von Willebrand factor/factor VIII (pdVWF/FVIII; VONCENTO®, CSL Behring) is a high-concentration, low-volume, high-purity concentrate, with a high level of VWF high-molecular-weight multimers and a VWF/FVIII ratio of ~2.4:1.MethodsThis study (NCT01229007) investigated the pharmacokinetics (PK), efficacy and safety of pdVWF/FVIII in 35 previously treated (minimum 20 exposure days [EDs]) pediatric patients (<12 years) with severe hemophilia A. PK was evaluated with a single 50 IU FVIII/kg dose of pdVWF/FVIII. Efficacy and safety analyses were performed during on-demand treatment (n=17) or prophylaxis (n=18) for up to 100 EDs with a maximum study duration of 12 months.ResultsPK profiles were similar for patients aged <6 years and those aged 6–12 years, and, as expected, the youngest patients had an increased clearance. On-demand patients reported 320 non-surgical bleeding (NSB) events and received a median number of 29.0 infusions (median dose 34.2 IU FVIII/kg). Hemostatic efficacy was assessed by the investigator as excellent/good in all cases (24%/76%). The 18 patients in the prophylaxis arm experienced 173 NSB events (97 NSBs [56%] in three patients). Five patients (28%) had no NSB events. Overall, patients received a median number of 92 infusions (median dose 30.6 IU FVIII/kg). The majority of bleeds (92%) were successfully controlled with only one infusion. Hemostatic efficacy was assessed by the investigator as excellent (86%) or good (14%). Inhibitors occurred in three patients of which two were transient (low titer) and one persisted (high titer). These three patients had known risk factors for inhibitor development.ConclusionThis study demonstrated comparable PK profiles for pediatric patients aged <6 years and aged 6–12 years, and an excellent efficacy and safety profile in this population. The adverse events reported were mostly mild to moderate with inhibitor rates within the expected incidence range.

VWF/ADAMTS13 Imbalance, But Not Global Coagulation or Fibrinolysis, Is Associated With Outcome and Bleeding in Acute Liver Failure.

Background and AimsRecent studies of acute liver failure (ALF) in man and animals have suggested that rebalanced hemostasis occurs, with distinct hypercoagulable features clinically evidenced by a low risk of bleeding. Rodent models have shown a link between intrahepatic microthrombus formation and progression of ALF. We sought to confirm these earlier findings in a large series of patients with well‐characterized ALF from the Acute Liver Failure Study Group.Approach and ResultsCitrated plasma samples taken on admission from 676 patients with ALF or acute liver injury (international normalized ratio ≥2.0 without hepatic encephalopathy) were used to determine levels of von Willebrand factor (VWF), a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) activity, thrombomodulin‐modified thrombin generation, and clot lysis time (CLT) and compared with the levels in 40 healthy controls. Patients had 3‐fold increased VWF levels, 4‐fold decreased ADAMTS13 activity, similar thrombin generating capacity, and 2.4‐fold increased CLT, compared with controls. Increasing disease severity was associated with progressively more elevated VWF levels as well as hypofibrinolysis. Patients who died or underwent liver transplantation within 21 days of admission had higher VWF levels, lower ADAMTS13 activity, but similar thrombin generation and a similar proportion of patients with severe hypofibrinolysis, when compared with transplant‐free survivors. Likewise, patients with bleeding complications had higher VWF levels and lower ADAMTS13 activity compared to those without bleeding. Thrombin generation and CLT did not differ significantly between bleeding and nonbleeding patients.ConclusionsRebalanced hemostatic status was confirmed in a large cohort of patients with acute liver injury/ALF, demonstrating that VWF/ADAMTS13 imbalance is associated with poor outcome and bleeding. The association between VWF/ADAMTS13 imbalance and bleeding suggests that bleeding in ALF relates more to systemic inflammation than a primary coagulopathy.

ABO blood group and COVID-19: a review on behalf of the ISBT COVID-19 Working Group.

Growing evidence suggests that ABO blood group may play a role in the immunopathogenesis of SARS‐CoV‐2 infection, with group O individuals less likely to test positive and group A conferring a higher susceptibility to infection and propensity to severe disease. The level of evidence supporting an association between ABO type and SARS‐CoV‐2/COVID‐19 ranges from small observational studies, to genome‐wide‐association‐analyses and country‐level meta‐regression analyses. ABO blood group antigens are oligosaccharides expressed on red cells and other tissues (notably endothelium). There are several hypotheses to explain the differences in SARS‐CoV‐2 infection by ABO type. For example, anti‐A and/or anti‐B antibodies (e.g. present in group O individuals) could bind to corresponding antigens on the viral envelope and contribute to viral neutralization, thereby preventing target cell infection. The SARS‐CoV‐2 virus and SARS‐CoV spike (S) proteins may be bound by anti‐A isoagglutinins (e.g. present in group O and group B individuals), which may block interactions between virus and angiotensin‐converting‐enzyme‐2‐receptor, thereby preventing entry into lung epithelial cells. ABO type‐associated variations in angiotensin‐converting enzyme‐1 activity and levels of von Willebrand factor (VWF) and factor VIII could also influence adverse outcomes, notably in group A individuals who express high VWF levels. In conclusion, group O may be associated with a lower risk of SARS‐CoV‐2 infection and group A may be associated with a higher risk of SARS‐CoV‐2 infection along with severe disease. However, prospective and mechanistic studies are needed to verify several of the proposed associations. Based on the strength of available studies, there are insufficient data for guiding policy in this regard.

ADAMTS13 activity, high VWF and FVIII levels in the pathogenesis of deep vein thrombosis

BackgroundDeep vein thrombosis (DVT) is a common multi-factorial disease with a partially understood aetiology. Although the roles of high factor (F)VIII and von Willebrand factor (VWF) levels are recognized, that of ADAMTS13 is still unclear. AimTo assess the association between ADAMTS13 activity levels, VWF antigen (VWF:Ag) and FVIII coagulant activity (FVIII:C) levels and DVT. Materials and methods365 Italian DVT patients and 292 age- and sex-matched controls were considered. Plasma ADAMTS13 activity was measured using FRETS-VWF73 assay. VWF:Ag and FVIII:C were measured using immunoassay and one-stage clotting assay (ACL TOP analyzer), respectively. Quartile analyses were performed to evaluate the individual association between ADAMTS13 activity, VWF:Ag, FVIII:C and DVT. The combined effect of high VWF levels (> 4th quartile) and low ADAMTS13 levels (< 1st quartile) was evaluated using binary variables. All models were age- and sex-adjusted. Estimated risks were reported as Odds ratio (OR) with 95% confidence intervals (CI). ResultsADAMTS13 activity was lower in DVT patients (94% vs. 98% of controls). Patients with an ADAMTS13 activity <1st quartile (86%) showed a 1.6-fold increased risk of DVT (95%CI, 1.05–2.55). The combination of low ADAMTS13 activity and high VWF:Ag levels was associated with a 15–fold increased risk (95%CI, 7.80–33.80). VWF:Ag and FVIII:C were associated to DVT with a dose-response relationship. ConclusionsADAMTS13 activity < 86% was associated with a moderate risk of DVT. The co-presence of low ADAMTS13 activity and high VWF levels resulted in a strong synergistic effect on DVT risk. The association of VWF:Ag and FVIII:C with DVT was confirmed.

In vitro hypercoagulability and ongoing in vivo activation of coagulation and fibrinolysis in COVID‐19 patients on anticoagulation

BackgroundCOVID‐19 is associated with a substantial risk of venous thrombotic events, even in the presence of adequate thromboprophylactic therapy. ObjectivesWe aimed to better characterize the hypercoagulable state of COVID‐19 patients in patients receiving anticoagulant therapy. MethodsWe took plasma samples of 23 patients with COVID‐19 who were on prophylactic or intensified anticoagulant therapy. Twenty healthy volunteers were included to establish reference ranges. ResultsCOVID‐19 patients had a mildly prolonged prothrombin time, high von Willebrand factor levels and low ADAMTS13 activity. Most rotational thromboelastometry parameters were normal, with a hypercoagulable maximum clot firmness in part of the patients. Despite detectable anti‐activated factor X activity in the majority of patients, ex vivo thrombin generation was normal, and in vivo thrombin generation elevated as evidenced by elevated levels of thrombin‐antithrombin complexes and D‐dimers. Plasma levels of activated factor VII were lower in patients, and levels of the platelet activation marker soluble CD40 ligand were similar in patients and controls. Plasmin‐antiplasmin complex levels were also increased in patients despite an in vitro hypofibrinolytic profile. ConclusionsCOVID‐19 patients are characterized by normal in vitro thrombin generation and enhanced clot formation and decreased fibrinolytic potential despite the presence of heparin in the sample. Anticoagulated COVID‐19 patients have persistent in vivo activation of coagulation and fibrinolysis, but no evidence of excessive platelet activation. Ongoing activation of coagulation despite normal to intensified anticoagulant therapy indicates studies on alternative antithrombotic strategies are urgently required.

Blood Markers of Portal Hypertension Are Associated with Blood Loss and Transfusion Requirements during Orthotopic Liver Transplantation.

There is increasing evidence that portal hypertension plays a major role in bleeding risk during orthotopic liver transplantation (OLT). We investigated the association between preoperative blood levels of von Willebrand factor (VWF) and soluble CD163 (sCD163), which are established markers of portal hypertension, and blood loss and transfusion requirements during OLT. We measured levels of VWF and sCD163 in preoperative serum samples of 168 adult patients undergoing a primary OLT between 1998 and 2012. Preoperative levels of VWF and sCD163 correlated with the model of end-stage liver disease (MELD) score (r = 0.414, p < 0.001 and r = 0.382, p < 0.001, respectively). Patients with high VWF or sCD163 levels (VWF and sCD163 levels above the median) had a substantially increased risk of needing red blood cell transfusion compared with patients with low VWF or sCD163 levels (VWF and sCD163 levels below the median) (odds ratio 3.5 [95% confidence interval, CI 1.7-7.0] and 2.3 [95% CI 1.1-4.5], respectively). Blood loss was highest in patients with both high VWF or sCD163 levels and a high preoperative international normalized ratio. Elevated blood levels of markers of portal hypertension are associated with increased blood loss and transfusion requirements during OLT and support the notion that portal hypertension is an important contributor to perioperative blood loss.

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

AbstractDeep 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.

Covid-19: The Rollercoaster of Fibrin(Ogen), D-Dimer, Von Willebrand Factor, P-Selectin and Their Interactions with Endothelial Cells, Platelets and Erythrocytes.

Severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2), also known as coronavirus disease 2019 (COVID-19)-induced infection, is strongly associated with various coagulopathies that may result in either bleeding and thrombocytopenia or hypercoagulation and thrombosis. Thrombotic and bleeding or thrombotic pathologies are significant accompaniments to acute respiratory syndrome and lung complications in COVID-19. Thrombotic events and bleeding often occur in subjects with weak constitutions, multiple risk factors and comorbidities. Of particular interest are the various circulating inflammatory coagulation biomarkers involved directly in clotting, with specific focus on fibrin(ogen), D-dimer, P-selectin and von Willebrand Factor (VWF). Central to the activity of these biomarkers are their receptors and signalling pathways on endothelial cells, platelets and erythrocytes. In this review, we discuss vascular implications of COVID-19 and relate this to circulating biomarker, endothelial, erythrocyte and platelet dysfunction. During the progression of the disease, these markers may either be within healthy levels, upregulated or eventually depleted. Most significant is that patients need to be treated early in the disease progression, when high levels of VWF, P-selectin and fibrinogen are present, with normal or slightly increased levels of D-dimer (however, D-dimer levels will rapidly increase as the disease progresses). Progression to VWF and fibrinogen depletion with high D-dimer levels and even higher P-selectin levels, followed by the cytokine storm, will be indicative of a poor prognosis. We conclude by looking at point-of-care devices and methodologies in COVID-19 management and suggest that a personalized medicine approach should be considered in the treatment of patients.

Von Willebrand factor and ADAMTS13 impact on the outcome of Staphylococcus aureus sepsis

BackgroundPrevious clinical evidence correlates levels of von Willebrand factor (VWF) and its cleaving protease ADAMTS13 with outcome in septic patients. No previous studies addressed if VWF and ADAMTS13 affected the outcome of Staphylococcus aureus sepsis. ObjectivesWe studied the role of VWF and ADAMTS13 in S. aureus sepsis both in patients and in mice. MethodsVWF levels and ADAMTS13 activity levels were measured in plasma samples from 89 S. aureus bacteremia patients by chemiluminescent assays and were correlated with clinical sepsis outcome parameters. In wild‐type mice and mice deficient in VWF and ADAMTS13, we investigated the outcome of S. aureus sepsis and quantified bacterial clearance and organ microthrombi. ResultsIn patients with S. aureus bloodstream infections, high VWF levels and low ADAMTS13 activity levels correlated with disease severity and with parameters of inflammation and disseminated intravascular coagulation. In septic mice, VWF deficiency attenuated mortality, whereas ADAMTS13 deficiency increased mortality. Bacterial clearance was enhanced in VWF‐deficient mice. The differences in mortality for the studied genotypes were associated with differential loads of organ microthrombi in both liver and kidneys. ConclusionsIn conclusion, this study reports the consistent relation of VWF, ADAMTS13 and their ratio to disease severity in patients and mice with S. aureus sepsis. Targeting VWF multimers and/or the relative ADAMTS13 deficiency that occurs in sepsis should be explored as a potential new therapeutic target in S. aureus endovascular infections.

Release of endothelial activation markers in lungs of patients with malaria-associated acute respiratory distress syndrome

BackgroundMalaria-associated acute respiratory distress syndrome (MA-ARDS) is an understudied complication of malaria and is characterized by pulmonary inflammation and disruption of the alveolar-capillary membrane. Its pathogenesis remains poorly understood. Since endothelial activation plays an important role in other malarial complications, the expression of two endothelial activation markers, von Willebrand factor (VWF) and angiopoietin-2 (ANG-2), was investigated in the lungs of patients with MA-ARDS.MethodsPost-mortem lung sections of Plasmodium falciparum-infected patients without alveolar oedema (NA), P. falciparum-infected patients with alveolar oedema (MA-ARDS), and uninfected people who died accidentally with no pathological changes to the lungs (CON) were immunohistochemically stained for VWF and ANG-2, and were evaluated with semi-quantitative analysis.ResultsAlveolar oedematous VWF and ANG-2 and intravascular VWF staining were significantly increased in patients with MA-ARDS versus infected and uninfected control groups. The levels of VWF in the alveolar septa and endothelial lining of large blood vessels of patients with MA-ARDS was significantly decreased compared to controls. ANG-2 expression was increased in the alveolar septa of malaria patients without alveolar oedema versus control patients, while ANG-2+ leukocytes were increased in the alveoli in both infected patient groups.ConclusionsThis study documents a high level of VWF and ANG-2, two endothelial activation markers in the oedematous alveoli of post-mortem lung sections of Thai patients with MA-ARDS. Decreased detection of VWF in the endothelial lining of blood vessels, in parallel with an increased presence of intravascular VWF staining suggests marked endothelial activation and Weibel–Palade body release in the lungs of patients with MA-ARDS.

ADAMTS13, an Anti-thrombotic Protein: Evidence Outside of Thrombotic Thrombocytopenic Purpura

von Willebrand factor (VWF) is a large multimeric plasma glycoprotein that plays a crucial role in hemostasis and thrombosis. VWF recruits platelets at sites of vascular injury by acting as a molecular bridge between circulating platelets and the site of injured or activated blood vessels. Biosynthesis of VWF is restricted to endothelial cells and megakaryocytes. Endothelial VWF is constitutively secreted into plasma and subendothelium, or is stored as "ultra-large" (UL)-VWF multimers in endothelial Weibel-Palade bodies. VWF produced in megakaryocytes is packaged as UL-VWF in the a-granules of platelets. VWF stored in endothelial and platelet storage organelles is secreted in a regulated process in response to stimulation by secretagogues. Absence or dysfunction of VWF results in bleeding symptoms, as observed in patients with von Willebrand disease. An abnormally high activity of VWF can lead to thrombotic events. Interestingly, the activity of VWF is determined by the size of its multimers and UL-VWF can spontaneously form platelet aggregates. An important regulator of VWF size is the metalloprotease ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), which digests large thrombogenic VWF molecules into smaller, less reactive multimers via cleavage of the Y1605-M1606 bond in the VWF A2 domain. ADAMTS13 is mainly synthesized in the liver by hepatic stellate cells, but other sites of synthesis, including renal podocytes, tubular epithelial cell, platelets and endothelial cells, have also been described. ADAMTS13 is released as an active enzyme into the circulation with no physiological inhibitors. Reduced or absent ADAMTS13 activity causes the microangiopathic disorder thrombotic thrombocytopenic purpura (TTP), characterized by VWF and platelet-rich microthrombi that cause multiple organ failure and even death when left untreated. Besides its clear role in the pathophysiology of TTP, the anti-thrombotic and even anti-inflammatory properties of ADAMTS13 have also become apparent in various other thrombotic conditions. High VWF levels and low ADAMTS13 levels are associated with increased risk or even worse outcome of cardiovascular disease, including ischemic stroke and myocardial infarction. Preclinical studies in mouse models showed the beneficial effect of ADAMTS13 in both cerebral and myocardial ischemia/ reperfusion injury by decreasing both thrombosis and inflammation. In addition, ADAMTS13 was shown to also exert a direct thrombolytic effect on VWF-rich thrombi. In a mouse model of ischemic stroke, this thrombolytic activity resulted in efficient lysis of intracranial thrombi that were resistant to standard treatment with tissue plasminogen activator. Hence, ADAMTS13, as a therapeutic agent, could become an interesting avenue, not only to manage TTP, but also to treat other thrombotic complications. Disclosures De Meyer: Fonds voor Wetenschappelijk Onderzoek: Research Funding; KU Leuven: Employment, Research Funding; Ablynx: Consultancy, Research Funding; Cerenovus: Membership on an entity's Board of Directors or advisory committees; WhiteSwell: Consultancy.

Factor VIII Levels in Blood Group O and Non-O in Patients with Non-Severe Haemophilia a

Introduction ABO blood group affects levels of von Willebrand Factor (VWF) and Factor (F) VIII, as individuals with blood group non-O have higher levels of VWF and FVIII. To the best of our knowledge there is no data available regarding the association between blood group and FVIII in non-severe haemophilia. Aim We aimed to explore the influence of ABO blood group on VWF and FVIII levels in patients with non-severe haemophilia A (HA). Methods We recruited persons with non-severe HA (FVIII levels of 1-40%). Adult patients (age ≥ 18 years), with platelet count &gt; 105 /µl, without impaired renal and hepatic function, active cancer, surgery within the last 6 weeks, or overt infection within the last 2 weeks, who gave their informed consent were included. A blood sample was collected for laboratory analysis. The diagnosis of haemophilia was confirmed by molecular analysis of the FVIII gene in all patients. None of the patients had inhibitors against FVIII at the time of sample collection. The lowest FVIII level ever measured in each patient's history served as basis for the assessment of severity. Healthy male persons with a median age comparable to the patient group served as controls. We determined the following parameters: ABO blood group, FVIII, VWF activity (VWF:Act), and VWF antigen (VWF:Ag). FVIII was measured with a one-stage assay (Sysmex-CA7000 - Sysmex, Kobe, Japan, in multi-dilution mode with native FVIII-deficient plasma - Technoclone, Vienna, Austria and Aktin FS activator - Siemens, Marburg, Germany), VWF:Act (BC von Willebrand Reagent or Innovance VWF Ac - Siemens Healthcare, Marburg, Germany) and VWF:Ag levels with a latex agglutination assay (STA LIATEST VWF, Diagnostica Stago). Comparison of VWF and FVIII levels between blood group O and non-O was calculated with Mann-Whitney-U test, correlation between FVIII and VWF levels was calculated with Spearman's correlation, association of FVIII and VWF separated for blood group was calculated with univariable linear regression. Association of FVIII with VWF and blood group was calculated using multivariable linear regression. All calculations were performed with SPSS (IBM Version 25.0). Results A total of 89 persons with HA (71 with mild and 18 with moderate haemophilia) and 82 healthy controls were included. Patient characteristics are listed in Table 1. Median levels of VWF were significantly higher both in patients (p=0.002 for both VWF:Ag and VWF:Act) and healthy controls (p&lt;0.001 for both VWF:Ag and VWF:Act) with blood group non-O compared to individuals with blood group O. FVIII levels were not significantly different between blood group non-O and O in HA patients (15% vs. 14.1%, p=0.716) but significantly higher in healthy controls with blood group non-O versus O (150.0% vs. 109.5%, p&lt;0.001; Table 2). In HA there was no correlation between FVIII and VWF:Act (rho=0.180, p=0.095) or between FVIII and VWF:Ag (rho=0.028, p=0.795, Table 3). In univariable linear regression there was no significant association between VWF and FVIII in HA, neither in those with blood group non-O nor in those with blood group O (Figure 1). Also in multivariable linear regression there was no significant association between VWF (p=0.632) and blood group (p=0.929) with FVIII in patients with HA. In healthy controls, there was a strong positive correlation between FVIII and VWF:Act (rho=0.751, p&lt;0.001) and between FVIII and VWF:Ag (rho=0.790, p&lt;0.001) (Table 3). In the healthy controls with blood group non-O 1% elevation in the VWF:Ag was associated with 0.63% of elevation in the FVIII:Act and in blood group O 1% elevation in the VWF:Ag was associated with 0.66% of elevation in the FVIII:Act (Figure 2). Conclusions Neither the blood group nor the VWF had an influence on FVIII levels in non-severe HA patients. We conclude that the impact of the genetic mutation of the FVIII gene by far outweighs the influence of VWF levels and the blood group. Thus, for the diagnosis of HA and for determination of HA severity, the blood group needs not to be taken into account. Disclosures Pabinger: Roche: Honoraria; Sobi: Research Funding; Novo Nordisk: Research Funding; CSL Behring: Research Funding; Pfizer: Honoraria; Shire: Honoraria; Bayer: Honoraria; Sobi: Honoraria.

P13. Abstract Title: Deep Mutational Scan of VWF to Define Mutations that Enhance or Impair Secretion

<h3>Rationale</h3> Von Willebrand Factor (VWF) is a multimeric glycoprotein that recruits platelets to sites of blood vessel injury and serves as a molecular chaperone for coagulation factor VIII (fVIII). Plasma VWF levels are a known risk factor for various thrombotic disorders including VTE, myocardial infarction, and stroke. GWAS studies have identified genetic variants in VWF that contribute to increased and decreased circulating VWF antigen levels. However, it is currently not known why some variants result in high VWF levels while others cause low VWF levels in circulation. We hypothesize that a subset of VWF variants impact its biosynthesis and secretion. <h3>Objective</h3> To identify every possible mutation in VWF that either increase or decrease its biosynthesis and secretion from cells. <h3>Results</h3> We have developed a protein secretion assay in HEK293T-Rex cells based on VWF- eGFP. Mutations in VWF that affect biosynthesis and secretion can be identified based on cell fluorescence intensity using Fluorescence Activated Cell Sorting (FACS). Initial studies have confirmed that cells expressing VWF mutants with known secretion defects have increased intracellular eGFP intensity (eGFP MFI: V1822G=157.72±5.40, C2693Y=184.06±2.10), relative to cells expressing WT VWF (eGFP MFI: WT=69.22±1.87) (p<0.001). This validates that eGFP specific cell retention is a consequence of impaired VWF basal secretion. Mutations of VWF may additionally cause inappropriate VWF secretion in response to physiological stimuli, and exacerbate imbalanced VWF plasma levels under various environmental influences. Cells treated with stimulus show time-dependent stimulation of VWF secretion in cells expressing WT (83.7% MFI reduction, p<0.01) and the C2693Y VWF (46.7% MFI reduction, p<0.01), but minimal response in V1822G VWF expressing cells (8.85% MFI reduction, p<0.05). This suggests that VWF variants may exhibit sensitized/reduced responses to stimulated VWF secretion. <h3>Conclusions</h3> These data provide the proof-of-concept to perform an unbiased deep mutational scan of VWF, to identify every possible mutation that can increase/decrease biosynthesis and secretion. Results of this project will provide a detailed structure/function analysis of VWF in mammalian cells and identify variants in VWF that can impair or enhance its secretion.