Molecular Weight Von Willebrand Factor Multimers Research Articles

The Oxidative Modification of Von Willebrand Factor Is Associated with Thrombotic Angiopathies in Diabetes Mellitus

The thrombogenic activity of Von Willebrand factor (VWF) is proportional to its molecular size and inversely related to its proteolysis by ADAMTS-13. Oxidation of VWF severely impairs its proteolysis by the metalloprotease. This study was aimed at assessing in patients with type 1 and type 2 diabetes whether protein carbonyls, marker of oxidative stress, are associated with both the level and oxidation status of VWF as well as with micro- and macroangiopathic complications. Eighty-three diabetic patients (41 type 1 and 42 type 2 diabetic subjects) and their respective eighty-three healthy controls were studied after verifying the availability, through institutional databases, of clinical biochemistry records spanning at least 3 years. VWF and protein carbonyls were measured by immunoassays, whereas VWF multimers were studied by SDS-agarose gel electrophoresis. Type 2 diabetic subjects had higher levels of VWF antigen (VWF:ag), VWF activity (VWF:act) and plasma proteins’ carbonyls compared to both their controls and type 1 diabetic subjects. Moreover, high molecular weight VWF multimers and specific VWF-bound carbonyls were significantly increased in subjects with micro- and macro-angiopathic complications. In both type 1 and type 2 diabetic subjects, ADAMTS-13 activity was in the normal range. In a multivariable analysis, only VWF-bound carbonyls were significantly associated with any form of thrombotic angiopathy in the entire group of T1- and T2 diabetic patients. These data provide first evidence that not only high VWF levels but also its oxidation status and the presence of high molecular weight VWF multimers that are not observed in SDS-agarose gel electrophoresis of normal subjects are associated with thrombotic angiopathies in diabetes mellitus. These findings may help identify diabetic patients particularly at risk for these complications and elucidate a new pathophysiological mechanism of thrombotic angiopathies in this clinical setting.

Prospective Evaluation of a TTP Prediction Score in an Independent Cohort

AbstractAbstract 1093 Background:Idiopathic thrombotic thrombocytopenic purpura (TTP) is a rare, clinically-diagnosed disorder characterized by widespread intravascular platelet thrombosis. The pathophysiology involves acquired deficiency of ADAMTS13 (A disintegrin and metalloprotease with thrombospondin type 1 repeats), the enzyme responsible for cleavage of high molecular weight vonWillebrand factor multimers. Disease mortality is high, though prompt treatment with plasma exchange is generally effective. A readily available and highly reliable method of identifying ADAMTS13-deficient patients for appropriate plasma exchange is therefore of interest. Methods:Our initial study (Bentley et al, Transfusion 2010;50:1654–1664) involved the assessment of multiple clinical and laboratory variables in patients with clinically-suspected TTP for whom ADAMTS13 assay was performed. Five variables were found to be of significant predictive power. This enabled the development of a point-based scoring system to efficiently determine the likelihood of TTP and response to plasma exchange in a given patient. (seeTable 1). This current study involved a separate validation cohort of patients with clinically-suspected TTP who underwent ADAMTS13 testing within two large health care systems in Utah between 2009 and 2011. The previously derived score was applied to this cohort and its performance was analyzed. Additionally, the original and validation cohorts were combined to revisit the predictive power of individual variables, derive more parsimonious models, and refine diagnostic scores. Results:A total of 96 patients comprised the validation population. The percentage of TTP diagnoses in this group (10%) was identical to that in the initial cohort. Using an ADAMTS13 activity of <10% of normal, our original score correctly predicted or excluded TTP in all but one patient when data for all variables was available. The mis-predicted patient was a “false positive;” or, in other words, our score predicted TTP, when in fact this patient’s ADAMTS13 activity was not low, and the ultimate diagnosis was HUS. On closer review, this patient was a child whose creatininevalue, though within the normal range for an adult, was indicative of significant renal insufficiency in a pediatric patient. Our score did not inappropriately exclude the diagnosis of TTP in any patient with ADAMTS13 <10% of normal. However, in cases with one or more predictive variables missing, it performed less well.In combined analysis, four of the five originally derived predictive variables remained highly significant: platelets (p=0.004), reticulocytes (p=0.009), d-dimer (p=0.027), and creatinine (p=0.005); the fifth, indirect bilirubin, was not statistically significant as a single variable (p=0.128). However, in creating a parsimonious model using only two variables, the most predictive power comes from platelet counts and indirect bilirubin. ROC analysis of individual, combined, and parsimonious models was performed (Figure 1). Discussion:This work confirms the predictive power of a simple point-based score to exclude TTP in appropriately selected patients. It may enable clinicians to begin plasma exchange, or to pursue an alternative cause of thrombotic microangiopathyand subsequently to begin prompt and appropriate treatment.Table 1Clinical prediction score.*VariableAssigned pointsTotal Score by ModelProbability of TTPCreatinine >2.0 mg/dl−11.5<20 points0%Platelets >35 K/ul−3020–30 points40%D-dimer >4.0 μg/ml−10>30 points100%Reticulocyte >3%21Indirect Bilirubin >1.5 mg/dl20.5*The probability of TTP could be estimated by summing results of individual predictor variable point scores. [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Light Chain of Factor VIII Is Sufficient for Accelerating Cleavage of von Willebrand Factor by ADAMTS13 Metalloprotease

We previously demonstrated that coagulation factor VIII (FVIII) accelerates proteolytic cleavage of von Willebrand factor (VWF) by A disintegrin and metalloprotease with thrombospondin type 1 repeats (ADAMTS13) under fluid shear stress. In this study, the structural elements of FVIII required for the rate-enhancing effect and the biological relevance of this cofactor activity are determined using a murine model. An isolated light chain of human FVIII (hFVIII-LC) increases proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. The maximal rate-enhancing effect of hFVIII-LC is ∼8-fold, which is comparable with human full-length FVIII and B-domain deleted FVIII (hFVIII-BDD). The heavy chain (hFVIII-HC) and the light chain lacking the acidic (a3) region (hFVIII-LCΔa3) have no effect in accelerating VWF proteolysis by ADAMTS13 under the same conditions. Although recombinant hFVIII-HC and hFVIII-LCΔa3 do not detectably bind immobilized VWF, recombinant hFVIII-LC binds VWF with high affinity (K(D), ∼15 nM). Moreover, ultra-large VWF multimers accumulate in the plasma of fVIII(-/-) mice after hydrodynamic challenge but not in those reconstituted with either hFVIII-BDD or hFVIII-LC. These results suggest that the light chain of FVIII, which is not biologically active for clot formation, is sufficient for accelerating proteolytic cleavage of VWF by ADAMTS13 under fluid shear stress and (patho) physiological conditions. Our findings provide novel insight into the molecular mechanism of how FVIII regulates VWF homeostasis.

Detection of Continuous Flow Left Ventricular Assist Device -Associated Acquired Von Willebrand Factor (VWF) Abnormality by An Automated Immunoturbidimetric VWF Activity Assay

Abstract▪2273▪This icon denotes a clinically relevant abstract Background:Continuous flow left ventricular assist device (CF-LVAD) recipients have high tendency of post-surgical gastrointestinal (GI) bleeding. We previously described the loss of high molecular weight VWF multimers (HMWM), due to either an accelerated clearance or enhanced cleavage of the HMWM, in most CF-LVAD recipients (Ann Thorac Surg. 90:1263–9). However, besides the tedious plasma VWF multimer analysis, neither VWF ristocetin cofactor nor collagen binding activity assay provides sufficient sensitivity for detecting such an acquired VWF abnormality (AVWA). In this study, we examined a new automated latex particle-enhanced immunoturbidimetric VWF activity assay (VWF:Lx) for its ability of detecting CF-LVAD-AVWA. We also analyzed the VWF pro-peptide (VWF:pp) to explored the potential mechanism of CF-LVAD-AVWA. Design:As part of an on-going prospective multicenter study, pre- and post-CF-LVAD implantation (7, 30 days and 5–7 months) blood samples were collected from 15 LVAD recipients (median age 52 years; 10 male and 5 female; 2008∼2009). Plasma VWF antigen (VWF:Ag), VWF:Lx activity, VWF:pp/Ag ratios were measured; and plasma VWF multimer analyses were performed on all available plasma samples. Standard statistical analyses were employed. Result:Loss of VWF HMWM was observed in 3 patients prior to CF-LVAD implantation and virtually all patients after surgery. VWF:Ag, VWF:Lx or VWF:pp/Ag ratios of the pre- and post-implantation samples were not significantly different (P> 0.1). However, VWF:Lx/Ag ratios of the post-implantation samples were significantly decreased (P<0.05). At a cut off of 0.8, the VWF:Lx/Ag ratio has a 90% sensitivity and specificity for detecting AVWA. Conclusions:VWF:Lx/Ag ratio has excellent sensitivity and specificity in detecting CF-LVAD-AVWA, and its impact on predicting post-surgical bleeding tendency in CF-LVAD recipients is being investigated. The distinct CF-LVAD-AVWA is most likely caused by enhanced cleavage, rather than clearance, of the HMWM.Table 1:Laboratory Characteristics of VWF in Pre- and Post-CF-LVAD Implantation Patient Plasma Samples.Laboratory Results (Mean±Standard Deviation)Pre-implantation (n=15)Day 7 Post (n=13)Day 30 Post (n=13)5–7 month Post (n=4)VWF:Ag (IU/dL)318 ± 178340 ± 136263.8 ± 122.7224 ± 78VWF:Lx (U/dL)258 ± 145246 ± 87102.5 ± 64.6165 ± 44VWF:Lx/Ag Ratio0.82 ± 0.120.74 ± 0.080.75 ± 0.060.76 ± 0.12VWF:pp/Ag Ratio1.04 ± 0.361.00 ± 0.300.99 ± 0.231.02 ± 0.10Samples with HMWM Loss (Present/Total)3/1510/1312/134/4Agreement between abnormal VWF:Lx/Ag ratio (<0.8) and VWF Multimer Analysis13/1511/1312/134/4 Disclosures:No relevant conflicts of interest to declare.

Molecular cloning, in vitro expression and functional characterization of canine ADAMTS13

A disintegrin and metalloproteinase with thrombospondin type 1 motifs, number 13 (ADAMTS13) is a plasma zinc metalloprotease also known as von Willebrand factor (VWF)-cleaving protease. Deficiency of ADAMTS13 activity is known to cause thrombotic thrombocytopenic purpura (TTP) in humans. We isolated the canine ADAMTS13 cDNA, which encodes 1502 amino acids, and expressed the recombinant protein to evaluate VWF-cleaving ability. Although the propeptide domain was longer and the TSP1 repeat domain was shorter than those in other species, the overall structures were similar to human and mouse ADAMTS13. Recombinant canine ADAMTS13 cleaved the 250-kDa VWF monomer into two fragments of 150kDa and 120kDa. Furthermore, high molecular weight VWF multimers were abolished based on the activity of ADAMTS13. These results could facilitate research into hemostatic disorders such as TTP in dogs.

Treatment of von Willebrand disease with FVIII/VWF concentrates.

Treatment of von Willebrand disease with FVIII/VWF concentrates.

The pathophysiology of von Willebrand disease: therapeutic implications

von Willebrand disease (VWD) is a bleeding disorder characterized by quantitative or qualitative defects in von Willebrand factor (VWF), a multimeric glycoprotein that is essential for platelet-dependent primary hemostasis. High molecular-weight multimers of VWF circulate and bind to collagen and platelets to induce primary hemostasis. The activity of VWF and its eventual proteolytic degradation are dependent on shear stress, ensuring that, under normal conditions, VWF is active in a high shear stress environment only. Deficiency in VWF results in mucocutaneous bleeding, including epistaxis, menorrhagia, and excessive bleeding after trauma or surgery. Classification of VWD is based on the combined results of multiple laboratory tests related to VWF amount and activity as well as the relative amounts of large VWF multimers as determined by gel electrophoresis. Recently, specific mutations in the gene encoding VWF have been linked to characteristic multimer profiles and may aid in subtyping patients with VWD and predicting response to therapy. These genotype–phenotype correlations are improving our understanding of the pathophysiology of VWD and helping to provide a more accurate diagnosis and classification with important treatment-related implications.

Co-Trafficking of Coagulation Factor VIII with Von Willebrand Factor Alters the Macromolecular Structure Inside Secretory Weibel-Palade Bodies

AbstractAbstract 325The liver is generally recognized as the major site of coagulation factor (F)VIII synthesis. However, there is now increasing evidence that FVIII can also be synthesized in specific endothelial cells where it is stored with its natural carrier protein von Willebrand factor (VWF) in the Weibel-Palade bodies (WPBs). WPBs have a typical cigar-shaped appearance that most likely originates from the macromolecular organization of VWF multimers into tubules. The tubular storage of VWF is thought to be essential for orderly secretion of VWF strings during activation of endothelial cells. Recently we have shown that expression of FVIII with VWF changes the WPB morphology to spherical vesicles. This finding suggests alterations in the biochemical properties of stored VWF. We now studied in detail the effect of FVIII co-expression on the VWF molecule using a combination of innovative techniques, including correlative light-electron microscopy (CLEM), and live-cell fluorescence microscopy under flow conditions. Analysis of human blood outgrowth endothelial cells (BOECs) expressing human B-domain deleted FVIII-GFP by CLEM revealed that FVIII containing WPBs were electron-dense, spherical structures. These structures contained disorganized short VWF tubules, which was confirmed in 3D by electron tomography. Double immunogold labelling with VWF and GFP antibodies showed that the spherical FVIII containing structures were always positive for VWF. These observations imply that FVIII blocks the expansion of VWF tubules, possibly by binding to the N-terminal VWF domains. As the N-terminal domains are also implicated in the formation of multimers, we therefore investigated whether FVIII affects VWF multimer size. Indeed, multimer analysis showed that VWF secreted by FVIII-GFP transduced BOECs was multimerized to a lesser extent when compared to VWF secreted by non-transduced BOECs. The combined absence of high molecular weight (HMW) VWF multimers and long VWF tubules made us question whether these cells could still release ultra-large VWF (UL-VWF) strings. UL-VWF strings play a key role in bleeding arrest, as platelets adhere to the released VWF string which ultimately leads to the formation of a platelet plug. We examined the release of UL-VWF strings under shear stress from BOECs expressing FVIII-GFP employing live-cell confocal imaging. This technique allowed us to follow FVIII release during exocytosis of WPBs in real-time as well. When we stimulated FVIII-transduced BOECs with histamine, these cells were equally able to release VWF strings as non-transduced BOECs. Although spherical WPBs lacked long VWF tubules and did not secrete HMW multimers, released VWF strings were of similar length as strings secreted by non-transduced BOECs. Surprisingly, released VWF strings were completely covered with FVIII which remained attached to the strings throughout the whole experiment. Another remarkable observation was that platelet binding to the FVIII-covered VWF strings was almost completely absent. We hypothesize that FVIII either shields the A1 domain for platelet binding or causes a conformational change in the VWF strings that prevents platelets from binding to the strings. Our results demonstrate that FVIII co-trafficking with VWF has a major impact on properties of VWF as it reduces the degree of multimerization, shortens tubules and prevents platelets from adhering to strings. This leads us to the conclusion that the macromolecular structure of VWF is considerably altered when FVIII is present in WPBs. Disclosures:No relevant conflicts of interest to declare.

Recombinant Human Von Willebrand Factor (rhVWF): First-In-Human Study Evaluating Pharmacokinetics, Demonstrating Safety and Tolerability In Type 3 Von Willebrand Disease

AbstractAbstract 237Von Willebrand Disease (VWD) is an inherited rare bleeding disorder caused by a deficiency of von Willebrand factor (VWF). VWF is the largest soluble multimeric plasma glycoprotein, which facilitates platelet aggregation and stabilizes FVIII in the circulation. Patients with type 3 disease display severe hemorrhagic symptoms, mainly in mucosal tissues, muscle and joints. Replacement of VWF stabilizes endogenous FVIII to hemostatic levels within hours. Commercially available VWF/FVIII concentrates are plasma-derived (pd) and subject to limitations such as donor dependency, risk of blood-borne pathogen transmission, lack of high molecular weight VWF multimers, and variation in multimer composition. A novel recombinant human VWF (rhVWF) has been developed using a plasma-free method, which represents the largest protein ever produced using recombinant technology. Safety, tolerability and pharmacokinetics of the rhVWF combined at a fixed ratio with rFVIII were investigated in a Phase 1 multicenter, international clinical study in 31 patients with type 3 VWD and severe type 1 VWD. Four concentrations of rhVWF (2, 7.5, 20 and 50 IU VWF:RCo/kg) were administered in a dose-escalating manner in separate cohorts. rhVWF was well tolerated, and no thrombotic events, VWF inhibitors or other serious adverse reactions were observed. Pharmacokinetics of rhVWF/rhFVIII (50 IU VWF:RCo/kg and 38.5 IU FVIII/kg) compared with pdVWF/pdFVIII (50 IU VWF:RCo/kg and 21 IU/kg FVIII/kg) were evaluated in a sub-group of 8/31 patients using a randomized, crossover design (8-day minimum washout period). Interim data in 8 subjects show a higher degree of secondary FVIII activity with rhVWF/rhFVIII compared to pdVWF/pdFVIII (see Figure 1) that is not solely due the difference in the rhVVF:FVIII infusion ratio (1.3:1 rhVWF/rhFVIII vs. approximately 2:1 pdVWF/pdFVIII). The pharmacokinetics of the rhVWF:RCo and pdVWF:RCo were comparable and were also reflected in the VWF:Ag and collagen binding activity. Evidence is also provided for the in vivo cleavage of the ultra-high molecular weight multimers of rhVWF by endogenous ADAMTS13. In summary, interim data from the ongoing Phase 1 study, demonstrate that rhVWF is safe and well tolerated, has VWF:RCo pharmacokinetics that are comparable to pdVWF and enhances stabilization of endogenous FVIII. Multiple doses of rhVWF/rhFVIII would be expected to have beneficial effects in major surgery and severe mucosal bleeding events. These data would also support the treatment concept to administer rhVWF alone once a therapeutic baseline level of endogenous FVIII is achieved (after 1–2 doses). [Display omitted] Disclosures:Suiter:Baxter BioScience: Employment. Laffan:Baxter BioScience: Consultancy. Mannucci:Baxter BioScience: Consultancy. Kempton:Baxter BioScience: Consultancy. Romond:Baxter BioScience: Consultancy. Shapiro: Baxter BioSci- ence: Consultancy. Birschmann:Baxter BioScience: Consultancy. Gill:Baxter BioScience: Consultancy. Ragni:Baxter BioScience: Consultancy. Turecek:Baxter BioScience: Employment. Ewenstein:Baxter Bioscience: Employment. Baxter BioScience:Baxter BioScience: Employment.

Acquired Von Willebrand Syndrome In Mitral Valve Leak

AbstractAbstract 4657 BackgroundSeveral studies have shown that between 15% to 25% of patients with severe aortic stenosis present bleeding episodes that may be attributed to an acquired von Willebrand syndrome (AVWS). Until now, to our knowledge, no association of AVWS with mitral valve disfunction has been reported. Design and MethodsFour patients with mitral valve leak presented acquired abnormalities of von Willebrand factor (VWF) and a bleeding history. Two of them presented severe bleedings requiring blood transfusions. All of them were within an adequate range of oral anticoagulation. ResultsPrior to surgery, these patients presented an APTT prolonged and in two of them the closure time determined by the platelet function analyzer (PFA-100®) (with COL/ADP and COL/Epi) was prolonged also. Factor VIII procoagulant activity (FVIII:C), VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo) and VWF collagen binding (VWF:CB) were considerably elevated and the VWF multimers in plasma, showed a lower relative proportion of the high molecular weight VWF multimers (HMWM), to some extent similar to type 2A congenital von Willebrand disease (VWD). In two of them, the VWF:RCo/VWF:Ag or VWF: CB/VWF:Ag ratios were less than normal range (>0.7) while in the other two were normal. After surgery, FVIII:C, and VWF properties were extremely increased and the ratios VWF:RCo/VWF:Ag and VWF: CB/VWF:Ag > 0.7. After surgery, FVIII:C, VWF:Ag, VWF:RCo and VWF:CB increased considerably. The ratios were > 0.7. The PFA-100® (COL/ADP and COL/Epi) was corrected in the two patients who had it prolonged. The multimeric VWF profile were also corrected in all of them. ConclusionsThe present study describes acquired VWF qualitative alterations for the first time in mitral valve leak. When such alterations are important they may be associated or to contribute to a bleeding diathesis. This problem was reported previously in aortic valve stenosis in relationship with a suspected very high shear stress. This situation may be not usually present in mitral valve stenosis, but it seems that it must occur in the presence of mitral valve leak. Consequently, the AVWS should be taken into account in patients with mitral valve leak that present a bleeding diathesis, not explained by an excess of oral anticoagulation. ACKNOWLEDGEMENTSThis work was supported by the Fondo de Investigación Sanitaria, F.I.S. Carlos III, Ministerio de Sanidad, Spain (FIS PI# 07/0229), and Consellería de Innovación e Industria, Xunta de Galicia (INCITE08ENA916109ES). Disclosures:No relevant conflicts of interest to declare.

Association of Quantitative and Qualitative Abnormalities of Von Willebrand Factor and Risk of Death In Hemodialysis Patients

AbstractAbstract 2217In order to evaluate the possible role of abnormalities of von Willebrand factor in the hemostatic defects seen in indivdiuals with chronic kidney disease (CKD), a cohort study was performed evaluating pre- and post-dialysis levels of von Willebrand factor (VWF), VWF multimer profiles and levels of its cleaving protease, ADAMTS-13. There were 57 subjects (31 males, 26 females) enrolled with CKD with a mean age of 75 years (range 60 – 90). Subjects with known vascular disease were recruited; 49 (86%) had documented ischemic heart disease, 16 (29%) had cerebrovascular disease and 17 (31%) had peripheral vascular disease. A little over half had diabetes mellitus (30 subjects or 54%), 37 (67%) were on antiplatelet therapy and 7 (13%) were chronically anticoagulated with warfarin. Blood samples were drawn immediately pre- and again post-dialysis and all results were compared with a group of age-matched normal controls (Table 1). As has been previously reported, VWF antigen levels (VWF:Ag) and VWF functional activity as measured by the ristocetin cofactor assay (VWF:RCo) were higher in the pre-dialysis samples compared with controls, and both levels were increased even further following dialysis. Additionally, the percentage of high molecular weight VWF multimers (% HMWM) were significantly increased in the pre-dialysis samples compared with controls. This is a novel finding, and the level of % HMWM seen in the subjects is similar to what has been reported in individuals with Thrombotic Thrombocytopenic Purpura (TTP). This difference decreased following dialysis, potentially due to the effect of shear stress on VWF and the resultant proteolytic processing, however still remained significantly higher when compared with controls. ADAMTS-13 functional activity was lower in the subjects compared with controls, providing a possibly explanation for the increase in % HMWM. IL-6 levels are higher in subjects compared with controls. IL-6, which is an inflammatory cytokine known to be increased in patients with CKD, has been previously reported as a marker of inactivation of ADAMTS-13. Two years after enrollment, follow up of the subjects revealed that 22 had died, 17 from documented cardiovascular events. Higher VWF levels at the time of enrollment significantly correlated with risk of death (p=0.041) during the study period. The increase in % HMWM suggests that a “TTP-like phenotype” may also be playing a role. Taken together, these data suggest that both quantitative and qualitative abnormalities of VWF contribute to the risk of thrombotic death in chronic kidney disease.Table 1Controls n=30Pre-Dialysis n=56Post-Dialysis n=56P value (ANOVA)Mean VWF:Ag IU/mL (range)1.39 (0.55–2.72)2.26 (0.64–4.71)2.70 (0.90–5.28)<0.001Mean VWF:RCo IU/mL (range)1.00 (0.59–1.43)2.03 (0.53–4.22)2.16 (0.59–3.98)<0.001Mean HMWM (%) (range)12 (4–20)18 (6–30)16 (4–29)<0.001Mean ADAMTS-13 IU/mL (range)1.10 (0.60–1.51)0.84 (0.44–1.29)0.91 (0.49–1.45)0.001Mean IL-6 pg/mL (range)2.43 (0.10–10)19 (0.24–371)20 (0.81–286)<0.001 Disclosures:No relevant conflicts of interest to declare.

Prospective study of hemostatic alterations in children with acute lymphoblastic leukemia

In a group of newly diagnosed acute lymphocytic leukemia (ALL) children we evaluated a number of hemostatic and inflammatory markers at diagnosis and at different time points during chemotherapy for the remission induction to identify alterations in the plasma levels of prothrombotic markers before and during the course of chemotherapy. The following plasma markers were evaluated: thrombin-antithrombin complex (TAT), D-Dimer, plasminogen activator inhibitor 1 (PAI-1), antithrombin, fibrinogen, von Willebrand factor (VWF) antigen and high molecular weight VWF (HMW-VWF) multimers, P-selectin, tumor necrosis factor alpha (TNF-alpha), and interleukin 6 (IL-6). Plasma samples were collected at the following time points: at T0 (baseline) and T1 (+24 days of therapy), T2 (+36 days therapy), and T3 (+64 days therapy). The results show that, at diagnosis, ALL children presented with laboratory signs of increased thrombin generation and fibrin formation (i.e. high TAT and D-dimer levels), fibrinolysis inhibition (i.e. high PAI-1 level), endothelial activation (i.e., high HMW-VWF and soluble P-selectin levels) and inflammation (i.e. high TNF-alpha and IL-6 levels). After starting induction therapy, the thrombin generation markers and inflammatory cytokines significantly decreased. To the opposite, PAI-1 and P-selectin significantly increased, suggesting an insult by chemotherapy on the vascular endothelium. These effects were more evident during steroid administration. Symptomatic venous thromboembolism (VTE) episodes developed in two cases during induction therapy, which did not allow the evaluation of the predictive value for VTE of laboratory markers.

104 Latent obstruction elicits a rapid proteolysis of von Willebrand factor in patients with hypertrophic cardiomyopathy. A rest and exercise echocardiographic study

Latent obstruction in hypertrophic cardiomyopathy (HCM) have clinical and prognostic consequences. Baseline obstruction impairs hemostatic properties of von Willebrand factor (VWF) in HCM. We sought to assess the acute effect of exercise-induced obstruction on VWF in patients with latent obstruction, and the determinants of obstruction and VWF impairment. A comprehensive echocardiography was performed at rest and during exercise in 32 patients with HCM. Sixteen patients (44±16 years) with latent obstruction (baseline peak gradient < 30 mmHg and exercise peak gradient≥30 mmHg) were matched with 16 patients without obstruction. Maximal peak gradient was recorded; the type and duration of obstruction during exercise were characterized by an obstruction score. Blood was sampled before and after exercise. Baseline median [25-75 th percentiles] peak gradient was 8[6-11] mmHg, and rose up to 32[17-104] mmHg with exercise. Baseline predictors of exercise-induced obstruction (R_ of the model 0.66) were incomplete SAM (r=0.76, p<0.0001) and mitral S velocity (r=0.34, p=0.004). At rest, VWF function was modestly impaired in patients with latent obstruction. VWF-collagen binding activity to antigen ratio (VWF:CB/Ag) and the percentage of high molecular weight multimers of VWF (%HMWM) did not change after exercise in the non obstructive group but were lowered in patients with latent obstruction (both p=0.003). Incomplete SAM at rest was the strongest independent predictor of%HMWM drop (r=-0.70, p<0.0001). %HMWM after exercise tightly correlated with exercise peak gradient (r=-0.78, p<0.0001) and the persistence of obstruction during recovery (r=-0.67, p=0.005). Incomplete SAM and mitral S velocity at rest are the main predictors of latent obstruction in HCM. Latent obstruction elicits a rapid cleavage of the largest multimers of VWF which is related to the peak gradient but also to the persistence of obstruction during recovery.

Decreased ADAMTS13 Levels in Patients after Living Donor Liver Transplantation

Introduction Thrombotic microangiopathy (TMA) is a complication occurring after liver transplantation (LT), and an unusually large multimer (ULM) of Von Willebrand factor (VWF) and ADAMTS13 may play an important role in the onset of TMA during LT. Material and Methods Eight-one patients underwent living donor LT (LDLT). Seventeen of those patients had both severe thrombocytopenia and hemolytic anemia with fragmented red cells and were diagnosed as TMA- like syndrome (TMALS). Results and Conclusions A significant reduction of ADAMTS13 and an increase of VWF were observed in the patients with TMALS. The ADAMTS13 activity in patients after LDLT was significantly reduced from day 1 to day 21, and it was significantly low in those with TMALS at day 14 and 28. The VWF levels in patients with LDLT were significantly high, and the VWF/ADAMTS13 ratio was significantly increased in patients at 7, 14 and 28 days after LDLT, especially in patients with TMALS at day 14 and 28 after LDLT. High molecular weight multimers of VWF were observed to have increased in patients with LDLT, and the high molecular weight multimers of VWF were further increased in those with mild TMALS but they decreased in those with severe TMA. These findings suggest that ULM- VWF and ADAMTS13 might be associated with the onset of TMA after LT.

Autosomal dominant C1149R von Willebrand disease: phenotypic findings and their implications

Mutation C1149R in the von Willebrand factor (VWF) gene has been thought to cause autosomal dominant severe type 1 von Willebrand disease (VWD). Eight patients from three unrelated families with this mutation were included in the present study who had distinct VWF abnormalities, not described in earlier studies. The patients showed notably low levels of VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), VWF collagen binding (VWF:CB), and a reduced ristocetin-induced platelet aggregation (RIPA). VWF:RCo/VWF:Ag and VWF:CB/VWF:Ag ratios were lower than 0.7. At basal conditions, all the VWF multimers were decreased in plasma, with a clearly lower relative proportion of the high molecular weight VWF multimers (HMWM). In high-resolution agarose gels, a large decrease in the relative proportions of the satellite bands was seen. The patients had a brief good response to desmopressin (DDAVP) administration, but the released VWF half-life was shorter than normal, indicating an accelerated clearance of their VWF. Platelet VWF was abnormal. We conclude from the results obtained in these patients for plasma phenotypic data that this mutation should be classified as a VWD type 2A (IIE). DDAVP therapy may be somewhat helpful for this mutation, at least for mild to moderate bleeding. These data provide evidence that for VWD classification factors other than basal VWF, such as DDAVP response and platelet VWF, should be considered.

Von Willebrand factor/factor VIII concentrates in the treatment of von Willebrand disease

Therapy for von Willebrand disease (VWD) aims to restore the hemostatic function conferred by von Willebrand factor (VWF), which facilitates platelet adhesion and aggregation, and serves to increase potentially low coagulation factor VIII (FVIII) in plasma. In patients unresponsive to desmopressin (DDAVP), the preferred treatment is with plasma-derived VWF-containing FVIII concentrates. Only a few of the available VWF/FVIII concentrates have been licensed for use in VWD based on prospective studies. The efficacy of VWF/FVIII concentrates depends on the content and quality of VWF and FVIII. Several studies have demonstrated the variability of the VWF contents, as well as the differences in the VWF multimer patterns (including the high molecular weight VWF multimers that are most effective in restoring hemostasis), among these concentrates. Treating physicians should be aware of these disparities and the potential clinical implications for patients with different VWD subtypes. Dosing has traditionally been calculated based on the FVIII content of the products, although dosing based on VWF functional activity [e.g., VWF ristocetin cofactor activity (VWF:RCo)] addresses the primary protein deficiency in VWD patients. Several clinical studies have demonstrated the efficacy of concentrates dosed according to VWF:RCo. Dosing is generally consistent across VWD subtypes, although patients with severe phenotypes or undergoing major procedures may require more infusions or longer treatment duration. Other considerations for the use of VWF-containing concentrates include laboratory monitoring of efficacy and safety issues such as thrombosis risk and thromboprophylaxis.

Function And Activity Of Von Willebrand Factor Is Regulated By A Hierarchy Of Mechanical Forces

The von Willebrand factor (VWF) is a shear-flow sensitive multimeric protein. Under normal flow conditions VWF is in a globular state, it unfolds at high shear rates and is activated for adhesion at the blood vessel wall [Schneider et al. 2007 PNAS p7899]. The elongation of multimeric VWF results in a force pulling along the VWF length axis. Based on a model of the VWF A domain organization, we performed force probe molecular dynamics simulations. We reveal the basis of two force-sensing VWF functions, and test the results by experiments.Our results indicate a competition between VWF A2 domain and glycoprotein Ib (GPIb) for the same binding site of the VWF A1 domain. When the stretching force along VWF reaches a critical point, the A1 A2 interaction is lost. The domains remain connected by a linker that gives space for GPIb to bind to the A1 domain. We thus suggest a force-dependent platelet binding to VWF as mediated by GPIb, which is experimentally testable and represents an alternative mechanism to recently published studies [Chen et al. 2008 Biophys J p1303; Lou et al. 2008 PNAS p13847].We show how proteolysis of the VWF is activated under shear conditions. The specific proteolytic site is buried in the VWF A2 domain [Sutherland et al. 2004 J Mol Model p259]. At extreme forces as present in high molecular weight VWF multimers, the A2 domain C terminus unfolds until the ADAMTS13 cleavage site is uncovered. Introducing a disulfide bond by mutagensis prevents VWF cleavage. This explains the size regulation of VWF by ADAMTS13: larger multimers involve higher pulling forces and therefore higher unfolding rates under shear flow. Larger VWF is cleaved faster, preventing blood clots and thrombosis [in preparation].

Idiopathic Non-Cirrhotic Intrahepatic Portal Hypertension Is Associated with Sustained ADAMTS13 Deficiency.

Idiopathic non-cirrhotic portal hypertension (NCPH) is caused by obliterative venopathy affecting the terminal radicals of the hepatic portal vein. It has been associated with areas of low socio-economic development. We observed a long history of thrombocytopenia and association with enteric disorders in a Caucasian population presenting with NCPH and postulated that the underlying mechanism could be deficiency of ADAMTS13, leading to the circulation of ultra large molecular weight von Willebrand factor (VWF) multimers and consequent platelet aggregate formation, causing occlusion of hepatic portal venules. ADAMTS13 activity was measured in all patients by a collagen binding assay, while antigen and IgG antibody were measured using ELISA kits (American Diagnostica Inc), normal ranges: 66–126%, 485–1242ng/ml and <11AU/ml, respectively. Plasma ADAMTS13 activity and antigen levels were measured in 14 patients with NCPH and 25 control patients with miscellaneous liver diseases associated with portal hypertension and Model for End Stage Liver Disease (MELD) scores indicating similar disease severity, median 12 (IQR 10–14) and 11 (8–16), for NCPH cases and controls. The platelet count was reduced in 13/14 NCPH cases median 69 x109/L (IQR 40–97) and 123 (85–193) in cases and controls, but did not correlate with MELD score or ADAMTS13 activity. ADAMTS13 activity was very low in 13/14 NCPH cases (median 12% (IQR <5–20)) and significantly lower than controls (59% (4–84), p<0.0001). In 4 cases, serial samples over several months revealed consistently low ADAMTS13 levels. There was a corresponding reduction in ADAMTS13 antigen, median 238ng/ml (IQR 186–361) and 584 (442–831), although NCPH cases showed a small excess of antigen over activity (antigen:activity ratio: median 25.7 (IQR 11.3–32.6) and 9.4 (8.1–13.8), in cases and controls, p<0.001), suggesting that an abnormal molecular species could be present. An inhibitor was not detected, although 10 cases and 15 controls had IgG anti-ADAMTS13: median levels 14.8AU/ml (8.8–20.0) and 13.4 (9.0–22.6), but these IgG levels did not correlate with activity. Non-inhibitory IgG antibodies to ADAMTS13 have been previously observed in certain other clinical groups (eg. coronary syndromes, sepsis, etc.). In NCPH, there was no correlation between ADAMTS13 level and either VWF antigen or VWF collagen binding activity. Increased very high molecular weight VWF multimers were observed in 4/11 NCPH cases and 0/22 controls. CD31 positive platelet aggregates were observed in the hepatic vessels of 7/20 (35%) NCPH cases and 1/15 (7%) controls. Our results suggest that sustained deficiency of ADAMTS13 associated with thrombocytopenia is characteristic of NCPH. It differs from other hepatic pathologies, where the decrease in ADAMTS13 has been reported to correlate with the severity of liver disease. NCPH also differs from thrombotic thrombocytopenic purpura because there is a chronic disease process, there is an absence of obvious red cell fragments and the primary organ affected is the liver. Since the predominant plasma source of ADAMTS13 appears to be hepatic stellate cells, the portal microcirculation is most at risk from pathological interactions of VWF and platelets if enteric factors further deplete circulating levels of ADAMTS13. The cause of the decreased ADAMTS13 remains to be elucidated, but could be due to decreased expression and synthesis, immune depletion (IgM and IgA anti-ADAMTS13 have not been investigated), or consumption in the circulation.

Activation of Endothelial Cells by Cocaine.

Cocaine use is associated with cardiac ischemia, sudden death, and stroke in patients with few additional risk factors for cardiovascular disease. Despite the very short halflife of cocaine in circulation (less than 1 hr), the risk of these cardiovascular events remains elevated for up to one week after cocaine ingestion. Pathologic findings following cocaine-associated cardiac events include platelet-rich microthrombi and accelerated atherosclerosis. In addition, observational studies have identified increased levels of von Willebrand Factor (VWF) at steady state in plasma of chronic cocaine abusers, which increase following cocaine challenge. Plasma VWF is derived primarily from endothelial cells, with a smaller contribution from platelets. Thus, cocaine may activate platelets and/or endothelial cells contributing to tissue ischemia, in addition to its well-known effects of vasoconstriction and increased tissue metabolic demand. When first released, VWF is hyperadhesive and capable of spontaneously binding platelets. This form, termed ultralarge VWF (ULVWF) because some of the multimers are extremely large, is normally rapidly processed by the metalloprotease ADAMTS13 into the smaller, less reactive forms usually found in plasma. Failure of this process, due to congenital or acquired deficiency of ADAMTS13, leads to accumulation of ULVWF, and in its most severe form produces the devastating microthrombotic disease, thrombotic thrombocytopenic purpura (TTP). We hypothesized that cocaine could activate the endothelium to secrete ULVWF, and thus promote thrombosis and/or vaso-occlusion. To test this possibility, we exposed cultured human umbilical vein endothelial cells (HUVEC) to various concentrations of cocaine. We found that 0.5 mg/ml cocaine, a concentration comparable to peak blood levels detectable in cocaine abusers, induced secretion of ULVWF almost as efficiently as did 25 μM histamine, a potent ULVWF secretogogue. If platelets were then perfused over the endothelium in a parallel plate flow chamber at 2.5 dyne/cm2, they spontaneously formed beads-on-a-string structures on the attached ULVWF strands. We also evaluated the ability of cocaine metabolites to induce ULVWF release. Neither norcocaine nor ecoginine methyl ester (both minor metabolites) stimulated ULVWF release. However, benzoylecgonine, the major cocaine metabolite (detectable in plasma up to 6–8 days after cocaine use) potently induced ULVWF release, as detected by the formation of ULVWF strings and accumulation of higher molecular weight VWF multimers in the HUVEC supernatant on non-reducing agarose gels. These effects of cocaine are likely to contribute to cardiovascular syndromes associated with cocaine use, including the triggering of vasoocclusive crises in sickle cell anemia, and may explain the observed association of cocaine use with TTP. Furthermore, these results suggest that clinical management of cocaineinduced ischemia or vaso-occlusion may benefit from therapies aimed at disrupting ULVWF.

Diagnosis and treatment of von Willebrand disease: new perspectives and nuances

In 1926, Erik von Willebrand, a Finnish internist andacademic, evaluated and described a 5-year-old girlwith extreme bleeding and bruising due to what hadbeen designated the A˚landic haemorrhagic diseaseby inhabitants of this archipelago island in the Gulfof Bothnia. Hjordis, the propositus, was the ninth of12 children born to a pedigree in which four femalesiblings had already died before the age of 4 withuncontrolled haemorrhage and in which 23 of 66family members, predominantly females, had expe-rienced significant bleeding and bruising complica-tions [1]. In fact, Hjordis herself eventually died atthe age of 13 during her fourth menstrual cycle.Professor von Willebrand mistakenly concluded thatthis bleeding diathesis was an unusual form ofhaemophilia and decided to label the new diseaseas pseudo-haemophilia to differentiate it from thesex-linked recessive haemophilia A. Little did vonWillebrand realize that this disease, eventually tobecomeeponymous[vonWillebranddisease(VWD)],would become the most commonly diagnosed con-genital bleeding disorder, with a prevalence rangingbetween 1 per 10 000 individuals to 1.3% [2]. Type1 VWD is the most common subtype, representingup to 75–80% of all cases while the subtype 3 VWDoccurs in approximately 1 per million population inthe United States and Europe [2]. Determination ofthe exact prevalence of VWD is hindered somewhatby the heterogeneity of clinical and diagnosticlaboratory features. Even Professor von Willebrandappreciated the challenges of diagnosing VWD as hecollaborated with Professor Rudolf Ju¨gens at theBerlin University to examine patients blood sampleswith a newly invented kapilla¨rtrombometer appa-ratus [3]. Although they were technically mistakenwhen they attributed the bleeding manifestations ofVWD to a platelet defect, their observations wereremarkably prescient since it was not until early1970s that the existence of a specific von Willebrandfactor (VWF) glycoprotein separate from factor VIIIwas finally appreciated and demonstrated to supportplatelet adhesion to the subendothelial matrix ofdamaged blood vessels.Over the last 81 years since von Willebranddescribed the bleeding disorder, there have beennumerous attempts and approaches to refine thediagnosis of the disease so that appropriate andefficacious treatment can be delivered. The clinicalphenotype of VWD includes, in part, information onwhether bleeding is spontaneous or related to surgi-cal or physical trauma; family history and inheri-tance pattern; menstrual history in women; age ofonset of bleeding (to distinguish between acquired vs.inherited coagulation disorders); and sites of bleed-ing(mucocutaneousvs.visceral,intra-articular,intra-muscular, or soft tissue locations). Subsequentlaboratory testing is necessary to exclude or confirmthe diagnosis and to further classify the subtype ofVWD. This article will focus on selected vagariesassociated with the ability to utilize clinical pheno-typic information gathered through the history andphysical examination to predict the presence ofVWD. The clinical diagnosis of VWD must be based,however, on more than physician suspicion andintuition. Thus, the need for confirmatory testing inthe laboratory. The frailties of conventional labora-tory testing for diagnosing VWD and the use of anin vitro surrogate of the bleeding time will also be