Von Willebrand Disease Type 2B Research Articles

Identification of Diagnostic SNPs in Exon 28 of the Von Willebrand Factor Gene for Diagnosis of VWD Type 2B in Patients Referred to IBTO Coagulation Lab: In-silico Approach

Background and Aims: von Willebrand disease (VWD) is brought on by hereditary impairments in the von Willebrand factor (VWF). It has been determined that most of the molecular abnormalities are present in exon 28 of the VWF gene qualitative variations. In this regard, investigating mutations in exon 28 of the VWF coding gene can help identify the VWD type and can be used to manage patients by using appropriate strategies.
 Materials and Methods: 10 suspected VWD type 2B patients were investigated for the genotype of exon 28. Routine coagulation tests were performed for the patients. Molecular sequencing was also used for the evaluation of mutations in exon 28. Determining the tertiary structure of VWF can aid in understanding its functional residues and interactions. The nsSNVs’ pathogenicity was examined utilizing potent bioinformatics methods. Step-by-step testing of the high-risk mutations was done using SIFT, PolyPhen-2, I-Mutant 2.0, PHD-SNPg, and SNPs&GO. Then, the secondary structure, amino acid conservation, and feature of amino acids were investigated via Protparam, Cofactor, Interprosurf, ConSurf, NetSurfP-2.0, and HOPE.
 Results: N1231T, V1229G, V1316M, and P1266Q missense mutations in VWF were detected. 3D structure of VWF predicted and evaluated. Putative conserved domains were identified. P1266Q and V1316M amino acid substitutions are predicted as “Not tolerated substitution” damaging and Disease, while V1229G and N1231T amino acid substitutions are predicted as “tolerated substitution” benign and Neutral.
 Conclusion: V1316M and P1266Q amino acid substitutions were determined as high-risk mutations using powerful bioinformatics tools in VWD patients.

Efficacy of platelet-inspired hemostatic nanoparticles on bleeding in von Willebrand disease murine models

AbstractThe lack of innovation in von Willebrand disease (VWD) originates from many factors including the complexity and heterogeneity of the disease but also from a lack of recognition of the impact of the bleeding symptoms experienced by patients with VWD. Recently, a few research initiatives aiming to move past replacement therapies using plasma-derived or recombinant von Willebrand factor (VWF) concentrates have started to emerge. Here, we report an original approach using synthetic platelet (SP) nanoparticles for the treatment of VWD type 2B (VWD-2B) and severe VWD (type 3 VWD). SP are liposomal nanoparticles decorated with peptides enabling them to concomitantly bind to collagen, VWF, and activated platelets. In vitro, using various microfluidic assays, we show the efficacy of SPs to improve thrombus formation in VWF-deficient condition (with human platelets) or using blood from mice with VWD-2B and deficient VWF (VWF-KO, ie, type 3 VWD). In vivo, using a tail-clip assay, SP treatment reduced blood loss by 35% in mice with VWD-2B and 68% in mice with VWF-KO. Additional studies using nanoparticles decorated with various combinations of peptides demonstrated that the collagen-binding peptide, although not sufficient by itself, was crucial for SP efficacy in VWD-2B; whereas all 3 peptides appeared necessary for mice with VWF-KO. Clot imaging by immunofluorescence and scanning electron microscopy revealed that SP treatment of mice with VWF-KO led to a strong clot, similar to those obtained in wild-type mice. Altogether, our results show that SP could represent an attractive therapeutic alternative for VWD, especially considering their long half-life and stability.

A thrombopoietin receptor agonist to rescue an unusual platelet transfusion-induced reaction in a p.V1316M-associated von Willebrand disease type 2B patient.

This report describes the first case of splenic injury in a patient with p.V1316M-associated von Willebrand disease type 2B (VWD2B) with chronic thrombocytopenia, successfully treated with nonoperative management including von Willebrand factor (VWF) replacement therapy, and platelet transfusions relayed by a thrombopoietin receptor agonist (TPO-RA, Eltrombopag). Eltrombopag was initially introduced to rescue an unusual post-platelet-transfusion reaction exacerbating the thrombocytopenia. In-depth analysis of the dramatic platelet count drop and VWF measurements timeline ruled out an allo-immune reaction and supported an alternative hypothesis of a sudden platelet clearance as a consequence of stress-induced release of abnormal VWF. One year later, a second life-threatening bleeding episode required urgent surgery successfully managed with VWF replacement therapy and platelet transfusions. Eltrombopag was further introduced in the post-surgery period to allow bleeding-free and platelet-transfusion-free successful recovery. Treatment decisions are particularly challenging in patients with VWD2B, and this case highlights how such decisions can benefit from understanding the molecular origin of platelet count fluctuations observed in these patients. Here, we successfully used a new therapeutic approach combining VWF-replacement therapy and initial platelet-transfusion relayed by TPO-RA to optimize patient management.Plain language summaryA combination of von Willebrand factor replacement and thrombopoietin receptor agonist in thrombocytopenic patients with von Willebrand disease type 2B: a new therapy approach to optimize patient management?Therapeutic management of patients with von Willebrand disease type 2B are particularly challenging in case of severe thrombocytopenia.Treatment includes von Willebrands factor replacement therapy and iterative platelet transfusions.We describe the first case of splenic injury in a patient with p.V1316M-associated von Willebrand disease type 2B successfully treated with nonoperative management including von Willebrand factor replacement therapy and platelet transfusions relayed by a thrombopoietin receptor agonist.We showed that the unusual post-platelet-transfusion reaction associated with a dramatic platelet count drop was a consequence of stress-induced release of abnormal von Willebrand factor.The combination of von Willebrand factor replacement therapy and thrombopoietin receptor agonist may offer a new therapeutic approach to optimize patient management.

Von Willebrand disease type 2B caused by gene mutation in a newborn

患儿 女，8日龄，因“血小板减少5 d”于2020年4月收入北京儿童医院。血常规示血小板减少，外院予人免疫球蛋白治疗无效，血小板输注暂时有效，外周血及骨髓涂片可见巨大血小板，低浓度瑞斯托霉素诱导血小板聚集阳性，二代测序提示血管性血友病因子（vWF）基因c.3946G>A（p.V1316M）杂合突变，诊断为血管性血友病2B型。本例系自发性突变，我国尚无该突变位点报道。本病以预防出血为主要处理原则，患儿无活动性出血，病情平稳，未输注血制品（含成分血）。.

Effects of Pro1266Leu mutation on structure and function of glycoprotein Ib binding domain of von Willebrand factor.

Glycoprotein Ibα (GpIbα) binding ability of A1 domain of von Willebrand factor (vWF) facilitates platelet adhesion that plays a crucial role in maintaining hemostasis and thrombosis at the site of vascular damage. There are both "loss as well as gain of function" mutations observed in this domain. Naturally occurring "gain of function" mutations leave self-activating impacts on the A1 domain which turns the normal binding to characteristic constitutive binding with GPIbα. These "gain of function" mutations are associated with the von Willebrand disease type 2B. In recent years, studies focused on understanding the mechanism and conformational patterns attached to these phenomena have been conducted, but the conformational pathways leading to such binding patterns are poorly understood as of now. To obtain a microscopic picture of such events for the better understanding of pathways, we used molecular dynamics (MD) simulations along with principal component analysis and normal mode analysis to study the effects of Pro1266Leu (Pro503Leu in structural context) mutation on the structure and function of A1 domain of vWF. MD simulations have provided atomic-level details of intermolecular motions as a function of time to understand the dynamic behavior of A1 domain of vWF. Comparative analysis of the trajectories obtained from MD simulations of both the wild type and Pro503Leu mutant suggesting appreciable conformational changes in the structure of mutant which might provide a basis for assuming the "gain of function" effects of these mutations on the A1 domain of vWF, resulting in the constitutive binding with GpIbα.

The physical spacing between the von Willebrand factor D'D3 and A1 domains regulates platelet adhesion in vitro and in vivo

Background Previous exvivo studies using truncated VWF (von Willebrand factor) suggest that domain-level molecular architecture may control platelet-GpIbα binding function. Objective We determined if this is the case with multimeric VWF invivo. Methods Full-length human VWF ('hV') was modified with a 22-amino acid mucinous stretch at either the N-terminus of VWF-A1 to create 'hNV' or C-terminus to yield 'hCV'. This extends the physical distance between VWF-A1 and the adjacent domains by ~6nm. Similar mucin inserts were also introduced into a human-murine chimera ('h[mA1]V') where murine-A1 replaced human-A1 in hV. This yielded 'h[mA1]NV' and 'h[mA1]CV', with N- and C-terminal inserts. The constructs were tested exvivo and invivo. Results Mucin insertion at the N-terminus, but not C-terminus, in both types of constructs resulted in >50-fold increase in binding to immobilized GpIbα. N-terminal insertion also resulted in greater shear-induced platelet activation, more thrombus formation on collagen, enhanced platelet accumulation and slower platelet translocation on immobilized VWF in microfluidics assays. Hydrodynamic injection-based expression of h[mA1]NV, but not h[mA1]V or h[mA1]CV, in VWF-/- mice caused profound thrombocytopenia, reduced plasma VWF concentrations, lower multimer distribution, and incessant tail bleeding that is reminiscent of von Willebrand disease type 2B. Platelet plugs were noted in the portal veins and hepatic arteries. An anti-D'D3 mAb DD3.3 that displays enhanced binding to VWF containing the N-terminal mucin insert also exhibited increased binding to wild-type VWF under shear and upon ristocetin addition. Conclusion Conformation changes at the VWF D'D3-A1 interface may be a key regulator of thrombosis invivo. Structural features at the A1-A2 interface are likely of less significance.

Ristocetin-Induced Platelet Aggregation (RIPA) and RIPA Mixing Studies.

Ristocetin-induced platelet aggregation (RIPA) is used as an in vitro test to determine the presence and integrity of the platelet glycoprotein (GP) Ibα-V-IX complex and von Willebrand factor (VWF) interaction and is usually performed using platelet-rich plasma (PRP). Impairment in the response of VWF/GPIbα-V-IX is measured with reference to several established concentrations of ristocetin and may indicate defects in VWF or in GPIbα-V-IX function. RIPA-based mixing studies comprise an additional approach to testing this interaction to help define whether defects identified by RIPA lie in VWF or in GPIbα-V-IX. For example, the correction of an abnormal RIPA trace after mixing PRP with normal plasma and rechallenging with ristocetin at 1.0mg/mL suggests VWF function/quantity defect. RIPA mixing studies at lower doses of ristocetin (0.5mg/mL) are recommended for discrimination of von Willebrand disease type 2B (VWD2B) from the rarer platelet-type (PT) VWD and for the phenotypic laboratory diagnosis of VWD2B. The demonstration of a plasma factor capable of inducing platelet aggregation at such low doses of ristocetin represents the hallmark for the phenotypic laboratory diagnosis of VWD2B. Moreover, since both VWD2B and PT-VWD may present with thrombocytopenia, RIPA-based mixing studies are also useful in thrombocytopenic patients in whom RIPA testing is difficult to assess.

Phenotypic Parameters in Genotypically Selected Type 2B von Willebrand Disease Patients: A Large, Single-Center Experience Including a New Novel Mutation.

von Willebrand disease type 2B (VWD2B) expresses gain-of-function mutations that enhance binding of an individual's von Willebrand factor (VWF) to its platelet ligand, glycoprotein Ib (GPIb), and which are usually identified by increased ristocetin-induced platelet aggregation (RIPA). We describe here the phenotypic profile of 38 genotypically selected VWD2B-affected family members (AFMs) belonging to 19 unrelated families. Major bleeding was observed in 68.4% of AFMs (previous to their diagnosis and registered by lifetime interviews), with a total of 46 episodes (1.21/patient), and was found to be highly related to the individual bleeding score and presence of thrombocytopenia, but otherwise unrelated to other laboratory parameters. Excessive muco-cutaneous bleeding symptoms were often reported, the most frequent of which comprised menorrhagia, epistaxis, easy bruising, and bleeding after teeth extraction/in oral cavity. Eight unaffected family members were also studied. The prevalence of VWD2B within families was 0.826, and the penetrance of mutations was complete, making it mandatory to study entire family sets to complete diagnostic profiles. Seven heterozygous missense mutations were found, the most common being p.V1316M. In the p.R1308C group, 75% of the AFMs showed absence of RIPA at 0.5 mg/mL, 66.6% of whom had VWF:RCo < 10 IU/dL, and 50% of whom had VWF:CB < 10 IU/dL. In the p.S1310F group, none of the AFMs had VWF:RCo/VWF:Ag < 0.6 (RCo/Ag), but 100% had VWF:CB/VWF:Ag < 0.6/(CB/Ag). Patients with p.P1266L and p.R1304V were characterized as atypical VWD2B. Two de novo mutations were found in four AFMs belonging to two families. We also describe a novel mutation: p.Y1258C. Of our patients, 70.5% had O blood group. In conclusion, a normal RCo/Ag and a negative RIPA at 0.5 mg/mL do not necessarily rule out a diagnosis of VWD2B.

Use of a thrombopoietin receptor agonist in von Willebrand disease type 2B (p.V1316M) with severe thrombocytopenia and intracranial hemorrhage

We present here a 63-year old woman with a long history of immune thrombocytopenia. She was hospitalized for a traumatic intracranial hemorrhage with thrombocytopenia. Following inefficient treatment of four platelet transfusions, immunoglobulins, and corticosteroids, we initiated treatment with a thrombopoietin (TPO) receptor agonist (eltrombopag 25 mg/d) with a good efficacy. Her mother and sister also had chronic thrombocytopenia. Clinical history, hemostasis results, and gene analysis revealed von Willebrand disease (VWD) type 2B with the mutation (c.3946G>A; p.V1316M), which combines a von Willebrand factor defect with severe thrombocytopenia, as well as a thrombocytopathy. The efficacy of TPO receptor agonists appears to counterbalance, at least to some extent, the thrombocytopathy associated with this mutation. As such, the use of TPO receptor agonists could represent an alternative therapeutic approach in cases of VWD type 2B with severe thrombocytopenia.

Apoptotic Platelet Events Are Not Observed in Severe von Willebrand Disease-Type 2B Mutation p.V1316M.

Thrombocytopenia and increased platelet clearance observed in von Willebrand disease-type 2B (VWD-2B) may be explained by platelet apoptosis triggered by the constitutive binding of VWF to its receptor, glycoprotein Ib (GPIb). Apoptosis was assessed in platelets from two patients with a severe VWD-2B mutation VWF/p.V1316M and from mice transiently expressing VWF/p.V1316M. We now report that the VWD-2B mutation VWF/p.V1316M which binds spontaneously to its receptor GPIbα does not induce apoptosis. In 2 unrelated patients (P1 and P2) exhibiting different VWF plasma levels (70% and 36%, respectively, compared with normal pooled human plasma given as 100%), inner transmembrane depolarization of mitochondria, characteristic of apoptotic events was undetectable in platelets, whether washed or in whole blood. No or a moderate phosphatidyl serine (PS) exposure as measured by annexin-V staining was observed for P1 and P2, respectively. Expression of pro-apoptotic proteins Bak and Bax, and caspase-3 activity were similar to control platelets. In the VWD-2B mouse model expressing high levels of mVWF/p.V1316M (423%), similar to what is found in inflammatory pathologies, no significant difference was observed between mice expressing mVWF/WT and mVWF/p.V1316M. These results strongly argue against apoptosis as a mechanism for the thrombocytopenia of severe VWD-2B exhibiting the VWF/p.V1316M mutation.

Mechanisms and Therapeutic Modulation of the Bleeding Tendency in Genetically-Engineered Von Willebrand Disease Type 2B Mice

Von Willebrand disease type 2B (VWD2B) is characterized by an abnormal increased affinity of von Willebrand factor (VWF) for platelets and related to mutations in the A1 domain of VWF. In patients, the VWD2B-mutant p.V1316M is specifically associated with a severe phenotype including hemorrhagic tendency, loss of high molecular weight VWF multimers, thrombocytopenia and platelet dysfunction related to defective αIIbβ3 integrin activation (Casari et al. JCI 2013, 123:5071). Genetically-engineered mice carrying the p.V1316M mutation (V1316M+/+ mice) perfectly mimic the human phenotype. We used this murine model to investigate the relative contribution of platelet count and platelet dysfunction to the occurrence of bleeding tendency and explore new therapeutic options.Bleeding tendency was evaluated using a novel tail-vein transection bleeding model involving the specific transection of the lateral vein at standardized depth and diameter (0.7 and 2.3mm respectively). Time (TVT) and blood loss were measured in wild-type (WT) and V1316M+/+ mice at baseline and after platelet count alteration.Compared to WT mice (platelet count: 1019±154G/L, n=58), V1316M+/+ mice (platelet count: 455±139G/L, n=52) have a significant increase (p<0.01) in TVT (25±9min, n=12 vs 2±1min, n=14) and blood loss (448±273µl vs 34±19µl). Induced thrombocytopenia (after infusion of anti-GPIb antibody) in WT mice (platelet count: 347±93G/L, n=6), to a similar level of V1316M+/+ mice, did not induce a hemorrhagic tendency (TVT: 2±1min, blood loss: 14±13µl). An increased bleeding phenotype in WT mice was exclusively observed in case of severe thrombocytopenia (platelet count: 70±25G/L, TVT>30min, blood loss: 532±168ml, n=6). Conversely, romiplostim-treated V1316M+/+ mice with normalized platelet count (platelet count: 866±100G/L, n=8) did not improve their bleeding tendency (TVT: 21±8min, blood loss: 448±282ml). These results suggest the bleeding phenotype in V1316M+/+ mice is independent of their thrombocytopenia, and could be related to the p.V1316M-associated defect in platelet function in general, and αIIbβ3 integrin activation in particular.Based on the hemostatic efficacy of recombinant activated factor VII (rFVIIa) in Glanzmann thrombasthenia, we tested rFVIIa in thrombocytopenic V1316M+/+ mice.A single infusion of rFVIIa (3 mg/kg) induced a complete and immediate correction of the bleeding phenotype (TVT: 4±2min, blood loss: 69±25ml, n=6). Thrombin generation measured in platelet-rich plasma before and after treatment showed a significant reduction of the lag time (1.13min, n=12 vs 1.8min, n=8 respectively, p<0.01).In conclusion, genetically-engineered mice carrying the VWD2B p.V1316M mutation display a bleeding phenotype that is not explained by their reduced platelet count, but is possibly related to the impaired platelet function induced by this VWF mutant. The bleeding tendency was rapidly and completely controlled by using rFVIIa. Based on these promising data, it would be of interest to consider rFVIIa as a therapeutic option in VWD-2B patients characterized by a severe hemorrhagic phenotype. DisclosuresNo relevant conflicts of interest to declare.

Characterisation of the p.A1461D mutation causing von Willebrand disease type 2B with severe thrombocytopenia, circulating giant platelets, and defective α-granule secretion

Characterisation of the p.A1461D mutation causing von Willebrand disease type 2B with severe thrombocytopenia, circulating giant platelets, and defective α-granule secretion -

Binding Of VWF To Type IV Collagen: An Additional Collagen Binding Mechanism Beyond Types I, III, and VI Collagen?

Von Willebrand disease (VWD) is a common bleeding disorder characterized by defects in the interaction of von Willebrand factor (VWF) with its ligands factor VIII, platelet glycoprotein Ib, and/or collagen. Nearly all assessment of collagen binding is performed with types I or III collagen. Type VI collagen, which is formed as a tight triple helix, has also been reported to interact with VWF. In contrast, type IV collagen lacks this tight helical structure and is located in the basal lamina. Type IV collagen may therefore serve as another important ligand for VWF. We examined this interaction in samples from the Zimmerman Program for the Molecular and Clinical Biology of VWD, a large US study of subjects with all types of VWD.Subjects were enrolled based on a pre-existing diagnosis of VWD and compared to a cohort of healthy controls. VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), and type III collagen binding using 1 mcg/mL human placental type III collagen (VWF:CB3) were performed in the clinical laboratory at the BloodCenter of Wisconsin. Type IV and type VI collagen binding (VWF:CB4 and VWF:CB6) were performed in the research laboratory by ELISA using 1 mcg/mL human placental type IV or type VI collagen captured on amine binding plates. DNA sequencing was performed on all subjects to identify VWF sequence variations. Bleeding scores were calculated according to the scoring system used in the MCMDM-1VWD study.For this analysis, 251 healthy controls, 535 type 1, 49 type 2A, 16 type 2B, 18 type 2M, 11 type 2N, and 18 type 3 VWD were included. The mean VWF:CB4/VWF:Ag ratio for the healthy controls was 0.96, with a standard deviation of 0.24. This was similar to the mean VWF:CB6/VWF:Ag ratio (mean 0.95, p=NS), but slightly lower than the mean VWF:CB3/VWF:Ag ratio (mean 1.05, p<0.01). The mean VWF:CB4/VWF:Ag ratio was 0.87 for the type 1 VWD subjects. The mean VWF:CB4/VWF:Ag ratio was significantly lower in type 1 subjects as compared to healthy controls (p<0.01). This was attributed in part to the presence of several A1 domain sequence variations, including p.R1399H, p.F1369I, and p.L1382P. While the latter sequence variations were isolated findings in single subjects, p.R1399H was present in 1% of the healthy controls, 1% of the type 1, and 6% of the type 2M VWD subjects.Mean VWF:CB4/VWF:Ag ratios were low for both type 2A and type 2B VWD subjects, at 0.54 and 0.52 respectively. This was not significantly different from the VWF:CB3/VWF:Ag data (p=NS for both type 2A and 2B VWD), consistent with the loss of high molecular weight multimers in these subjects and the concomitant reduction in collagen binding. As expected, VWF:CB4/VWF:Ag ratios were normal in the type 2N subjects, with a mean of 1.04. The type 2M subjects, however, displayed a significant reduction in type IV collagen binding, with a mean VWF:CB4/VWF:Ag ratio of 0.79. One subject with an 11 amino acid deletion (from p.R1392 through p.Q1402) had absent binding to type IV collagen yet normal binding to type III collagen with a VWF:CB3/VWF:Ag ratio of 0.82. Another subject with p.I1425F and p.R1399H had a VWF:CB4/VWF:Ag ratio of 0.48, and VWF:CB3/VWF:Ag ratio of 1.10. Type IV collagen binding did not correlate with VWF:RCo (R squared less than 0.1). The type 2M subjects with a mean VWF:CB4/VWF:Ag ratio less than 0.7 had higher bleeding scores, with a mean bleeding score of 9 compared to a mean of 6 for type 2M subjects with a mean VWF:CB4/VWF:Ag ratio greater than 0.7. Not surprisingly, the subjects with type 3 VWD and undetectable VWF:Ag also had undetectable VWF:CB4.These data suggest that type IV collagen binding reflects an independent function of VWF apart from platelet binding, and distinct from binding to type III collagen. In our study, more variation was seen between type IV and type III collagen than between type IV and type VI collagen. Neither VWF:CB4 nor VWF:CB6 are currently tested as part of the clinical workup of VWD. Measurement of this function may assist in identification of variant VWD, particularly in the subgroup of patients with type 2M VWD. Disclosures:No relevant conflicts of interest to declare.

Gene Sequencing Aids in Differentiation of von Willebrand Disease Type 2B from Platelet‐Type vWD

An elderly female with a positive bleeding history (personal and family) and prior but undocumented diagnosis of von Willebrand disease (vWD) presented for surgery. Testing showed mild thrombocytopenia, decreased ristocetin cofactor activity to von Willebrand factor antigen ratio, and absent high‐molecular‐weight multimers, suggesting a qualitative (type 2) subtype. Low‐dose ristocetin‐induced platelet aggregation demonstrated abnormally increased aggregation, consistent with either type 2B vWD or platelet‐type vWD (PT‐vWD). Differentiation between these subtypes guides therapy. Optimal therapy for type 2B vWD consists of vWF replacement, while platelet transfusions are used to treat PT‐vWD. Traditional options for differentiation include vWF/platelet binding assays or specialized platelet aggregation studies. However, these subspecialized tests have limited availability and sample stability. Gene sequencing performed in our laboratory allowed for detection of 1 of 4 known mutations in GP1BA, (c.763A&gt;;G, p.Met255Val, using standard nomenclature) responsible for PT‐VWD. This finding confirmed her diagnosis and appropriate therapy and facilitates diagnosis for affected family members in this disorder with an autosomal dominant inheritance pattern. Gene sequencing is a useful tool to aid in diagnosis and subtyping of type 2 vWD. kristi.smock@aruplab.com

The Co-Influence of VWD Type 2B and Type 2M Mutations and Platelet GPIbα On the Susceptibility of VWF to ADAMTS13 Under Shear Stress

Abstract 1108 Introduction:Von Willebrand Factor (VWF) is one kind of vital proteins involved in thrombosis, which exists in the form of multimeric glycoprotein in plasma of human. The size of VWF multimer is down regulated by metalloproteinase ADAMTS13. This physiological process is modulated by multiple factors. It was reported that the binding of platelets or GPIbα recombinant fragment to VWF domain A1 may increase the cleavage by ADAMTS13 to recombinant A1A2A3 segment in the condition of static state. Due to the mutations, VWF synthesized by patients of von Willebrand disease (VWD) type 2B and type 2M may enhance and decrease the binding of platelet membrane glycoprotein Ib (GPIb), respectively. This study aims to observe the influence of 2B/2M VWD mutations and recombinant GPIbα active fragment on the susceptibility of VWF to ADAMTS13, in the condition of high shear stress. Materials and Methods:We constructed and expressed different forms of recombinant full-length human VWF (rhVWF) including three type 2B mutations (P1337L, H1268D, R1308C), three type 2M mutations (M740I, R1205H, D1302G) and wide-type (WT) in Hela cells. Recombinant human ADAMTS13 (rhADAMTS13) and GPIbα (H1-V289) were both from our own lab. The different types of rhVWF were characterized by multimeric analysis and ristocetin-induced platelet aggregation (RIPA). We use rhADAMTS13 to digest seven full-length rhVWF in the reaction Buffer (20 mM Tris-HCl, 150mM NaCl, pH7.4, 1% bovine serum albumin) in the absence and presence of GPIbα (H1-V289). To develop a more physiological assay under fluid shear stress, we assessed the proteolytic cleavage of rhVWF by rhADAMTS13 under constant vortexing (2,500 rpm) in a PCR tube mini-mixer that mimics arterial fluid shear stress (30 dynes/cm2). The 176kDa fragment resulting from the cleavage of rhVWF at the Y1605-M1606 bond by rhADAMTS13 was visualized by 5% SDS-PAGE under reducing conditions and Western blot analysis. Results:SDS-agarose gel electrophoresis showed that all rhVWF mutants were in the similar multimeric distribution to the rhVWF-WT. In the RIPA assay, type 2B rhVWF exhibited clearly enhanced RIPA responsiveness with ristocetin at 0.5 mg/ml (R1308C > P1337L > H1268D > WT), while type2M rhVWF showed decreased RIPA responsiveness with ristocetin at 1.0 mg/ml (WT > M740I > R1205H > D1302G). At physiological salt concentration (150nM NaCl), type 2B mutants exhibited more susceptibility to rhADAMTS13 than WT and type 2M mutants in the absence of GPIbα (H1-V289). In the presence of GPIbα (H1-V289), the higher susceptibility of mutants to rhADAMTS13 was detected, which decreased in the following order: type 2B > WT > type 2M, whereas no obvious enhancement happened to the two type 2M mutants (R1205H and D1302G). Conclusions:Our study further confirmed that platelet GPIbα binding to VWF A1 domain could increase the susceptibility to ADAMTS13 at physiological conditions under high stress. The three type 2B mutations are all localized in the A1 domain and these mutations alone may also increase the susceptibility to ADAMTS13. While type 2M mutations M740I, R1205H and D1302G are localized in the D2, D3 and A1 domain, respectively. It is concluded that type 2M mutations localized close to or in the A1 domain may exhibit significantly lower susceptibility to ADAMTS13 in the presence of GPIbα (H1-V289). It may partially give the explanation that some VWD type 2M may display larger than normal VWF multimers in plasma. Disclosures:No relevant conflicts of interest to declare.

Inhibition of Von Willebrand Factor (VWF) by ARC1779 Normalizes Von Willebrand Factor/FVIII Levels and Corrects Thrombocytopenia in VWD Type 2b Patients,

Abstract 3377 Introduction:Infusion of the anti-VWF aptamer ARC1779 effectively elevated platelet counts in some patients with congenital thrombocytopenic purpura, and blunted the platelet drop induced by desmopressin in patients with type 2b von Willebrand disease (VWD). Thus, we hypothesized that ARC1779 may increase VWF levels and correct thrombocytopenia by blocking the hyperactive VWF A1 domain in type 2b VWD. Methods:Thrombocytopenic patients suffering from type 2b VWD received intravenous infusions of ARC1779 (4 mcg/kg/min) for 72 hours in a prospective clinical trial. Patients were monitored using previsouly published assays (Thromb Haemost. 2010;104:563–70). Data are provided as median and the range and were tested for significance by repeated measures analysis of variance. Results:All patients had moderate thrombocytopenia, low levels of VWF:RCo, VWF:CBA, FVIII:C, and collagen/ADP closure times >300s (platelet function analyzer PFA-100). ARC1779 was well tolerated and safe even in a patient who suffered from active chronic gastrointestinal bleeding due to angiodysplasias before start of infusion. Plasma concentrations of ARC1779 increased to 76 mcg/mL (59–130) which suppressed free VWF A1 domains in the REAADS assay to a minimum of <3 to 5%. Consistent with the estimated in vivo half-life of VWF, VWF/FVIII levels increased as early as 12h after start of infusion, peaked near the end of infusion, and returned to baseline at follow-up. That was best exemplified by VWF:RCo levels which increased from 32% (26–45%) 10-fold to 367% (215–418%) 8h after end of infusion, and decreased again to 33% (20–40%) during follow up. Similarly VWF:CBA levels increased 16-fold (8 to 69-fold), VWF: Ag levels 5.2 fold (3 to 27-fold), and FVIII levels 4.3 fold (3 to 6-fold). Most importantly inhibition of hyperactive VWF rapidly increased platelet counts from 40/nL (38–58/nL) to a maximum of 146/nL (107–248/nL). This was observed although patients were suffering from chronic liver disease due to hepatitis C infection or associated immune thrombocytopenia, which could have blunted the platelet response. All changes were statistically significant (p<0.007). Conclusion:This clinical trial underscores the therapeutic efficacy of the VWF A1 antagonist aptamer ARC1779. ARC1779 significantly increases VWF/FVIII levels and most importantly improves or even corrects thrombocytopenia in VWD type 2b patients. This could facilitate (i) the collection of autologous platelets for platelet transfusions pre-operatively or (ii) interferon therapy in patients whose HCV therapy could not be initiated or had to be stopped due to thrombocytopenia. As angiodysplasias may be caused by defective or deficient VWF (Blood 2011;117:1071–80), it may also be tempting to further investigate the potential of chronic anti-VWF aptamer therapy to treat angiodysplasia. Disclosures:Knoebl:Archemix Corp.: Consultancy. Pabinger:Archemix Corp.: Consultancy. Gilbert:Archemix Corp.: previous employment. Schaub:Archemix Corp.: Consultancy, previous employment. Jilma:Archemix Corp.: Consultancy, Research Funding.

Abnormal VWF modifies megakaryocytopoiesis: studies of platelets and megakaryocyte cultures from patients with von Willebrand disease type 2B

Abstractvon Willebrand factor (VWF) is an essential mediator of platelet adhesion to the vessel wall, but little is known about its role in megakaryocytopoiesis. VWF and its platelet receptor, glycoprotein Ibα (GPIbα), are both expressed during megakaryocyte (MK) maturation. This study was designed to evaluate whether the enhanced VWF-GPIbα interactions typical of patients with von Willebrand disease type 2B (VWD2B) modify platelet production. Platelets from 9 patients with VWD2B with 7 different gain-of-function mutations were examined by electron microscopy (EM) and immunofluorescence labeling. For the patients with VWD2B, EM characteristically showed variable numbers of structurally abnormal giant platelets, sometimes in agglutinates. Cultures of MKs from controls performed with or without purified VWF confirmed a positive influence of VWF on platelet production with specific inhibition by an antibody blocking VWF binding to GPIbα. VWD2B MK cultures examined by EM showed a disorganized demarcation membrane system and abnormal granule distribution. They produced platelets with structural abnormalities typical of VWD2B. Confocal examination of MK revealed limited extension of pseudopods with few large proplatelets. These results confirm that megakaryocytopoiesis is modified by the enhanced VWF-GPIbα interactions. These data obtained for controls and patients with VWD2B suggest a novel regulatory role of VWF-GPIbα interactions in platelet production.

Platelet morphological changes in 2 patients with von Willebrand disease type 3 caused by large homozygous deletions of the von Willebrand factor gene

Platelet morphological changes in 2 patients with von Willebrand disease type 3 caused by large homozygous deletions of the von Willebrand factor gene

Dominant von Willebrand Disease Type 2M and 2U Are Variable Expressions of One Distinct Disease Entity Caused by Loss-of-Function Mutations in the A1 Domain of the von Willebrand Factor Gene

A complete set of laboratory investigations, including bleeding time, PFA-100 closure time, factor VIII coagulant activity (FVIII:C), von Willebrand factor (VWF) ristocetin cofactor activity (RCo), collagen binding (CB) and antigen concentration (Ag), ristocetin-induced platelet aggregation (RIPA) and multimeric analysis of VWF in low and medium SDS-agarose resolution gels, is warranted to diagnose and classify all variants of von Willebrand disease (VWD). VWD type 2M and 2U are typically characterized by decreased RIPA and a poor response of VWF:RCo to desmopressin (DDAVP), but normal VWF:CB and good responses of VWF:CB, VWF:Ag and FVIII:C to DDAVP. VWF multimeric analysis in patients with VWD 2M and 2U show relative decreases in large VWF multimers with less resolved triplet structure of each of the multimeric bands in low-, medium- or high-resolution gels. VWD type 2M or 2U are caused by a loss-of-function mutation in the A1 domain. The laboratory manifestations and molecular defects in the A1 domain causing VWD type 2M and 2U are clearly distinct from all variants of type 1 VWD and also from all other variants [VWD type 2A, 2B, 2E (IIE) and 2C (IIC)].

The Montreal platelet syndrome kindred has type 2B von Willebrand disease with the VWF V1316M mutation

AbstractMontreal platelet syndrome (MPS), hitherto described in only one kindred, is a hereditary thrombocytopenia associated with mucocutaneous bleeding, giant platelets, and spontaneous platelet aggregation in vitro. These are features shared with some forms of type 2B von Willebrand disease (VWD); however, the MPS kindred had not been investigated for VWD. We found that all affected MPS family members had borderline to normal von Willebrand factor antigen (VWF:Ag; 0.43-0.75 U/mL), discrepantly low ristocetin cofactor activity (VWF:RCo; 0.16-0.29 U/mL), and normal factor VIII coagulant activity (FVIII:C; 0.57-1.04 U/mL). Unaffected family members all had normal VWF:Ag, VWF:RCo, and FVIII:C levels. In addition, persons with MPS, but not unaffected family members, had loss of plasma (but not platelet) high molecular weight VWF multimers, and were heterozygous for the previously reported V1316M type 2B VWD mutation. Thus, in reevaluating this kindred, we determined that patients with MPS have type 2B VWD with the V1316M VWF mutation.