The molecular defect in type IIB von Willebrand disease. Identification of four potential missense mutations within the putative GpIb binding domain

ASH ISTH NHF WFH 2021 guidelines on the diagnosis of von Willebrand disease

BACKGROUND

von Willebrand disease (VWD) is the most common inherited bleeding disorder known in humans. Accurate and timely diagnosis presents numerous challenges.

OBJECTIVE

These evidence-based guidelines of the American Society of Hematology (ASH), the International Society on Thrombosis and Haemostasis (ISTH), the National Hemophilia Foundation (NHF), and the World Federation of Hemophilia (WFH) are intended to support patients, clinicians, and other health care professionals in their decisions about VWD diagnosis.

METHODS

ASH, ISTH, NHF, and WFH established a multidisciplinary guideline panel that included 4 patient representatives and was balanced to minimize potential bias from conflicts of interest. The Outcomes and Implementation Research Unit at the University of Kansas Medical Center (KUMC) supported the guideline-development process, including performing or updating systematic evidence reviews up to 8 January 2020. The panel prioritized clinical questions and outcomes according to their importance for clinicians and patients. The panel used the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach, including GRADE Evidence-to-Decision frameworks, to assess evidence and make recommendations, which were subsequently subject to public comment.

RESULTS

The panel agreed on 11 recommendations.

CONCLUSIONS

Key recommendations of these guidelines include the role of bleeding-assessment tools in the assessment of patients suspected of VWD, diagnostic assays and laboratory cutoffs for type 1 and type 2 VWD, how to approach a type 1 VWD patient with normalized levels over time, and the role of genetic testing vs phenotypic assays for types 2B and 2N. Future critical research priorities are also identified.

Von Willebrand disease type 2B with a novel mutation in the VWF gene

We report a 38-year-old woman who presented with a subdural hematoma after minor facial trauma in a stressful situation. The laboratory data showed a subnormal platelet count (166×10⁹/L), VWF:RCo activity was 45% and VWF:Ag was 53% with a VWF:RCo/VWF Ag ratio of 0.79. Hemostasis results and gene analysis revealed von Willebrand disease (VWD) type 2B with normal multimers and a novel mutation c.4136 G>T (R1379L), which appears to be a novel mutation of VWD type 2B that is mainly diagnosed with hypersensitivity to ristocetin and an hyperfixation of platelet Willebrand to a recombinant Gp1b. SIMILAR CASES PUBLISHED: None.

Inherited Thrombocytopenia: Update on Genes and Genetic Variants Which may be Associated With Bleeding

Inherited thrombocytopenia (IT) is comprised of a group of hereditary disorders characterized by a reduced platelet count as the main feature, and often with abnormal platelet function, which can subsequently lead to impaired haemostasis. Inherited thrombocytopenia results from genetic mutations in genes implicated in megakaryocyte differentiation and/or platelet formation and clearance. The identification of the underlying causative gene of IT is challenging given the high degree of heterogeneity, but important due to the presence of various clinical presentations and prognosis, where some defects can lead to hematological malignancies. Traditional platelet function tests, clinical manifestations, and hematological parameters allow for an initial diagnosis. However, employing Next-Generation Sequencing (NGS), such as Whole Genome and Whole Exome Sequencing (WES) can be an efficient method for discovering causal genetic variants in both known and novel genes not previously implicated in IT. To date, 40 genes and their mutations have been implicated to cause many different forms of inherited thrombocytopenia. Nevertheless, despite this advancement in the diagnosis of IT, the molecular mechanism underlying IT in some patients remains unexplained. In this review, we will discuss the genetics of thrombocytopenia summarizing the recent advancement in investigation and diagnosis of IT using phenotypic approaches, high-throughput sequencing, targeted gene panels, and bioinformatics tools.

Defective collagen binding and increased bleeding in a murine model of von Willebrand disease affecting collagen IV binding

Essentials Defective binding to collagen IV has been seen in von Willebrand factor (VWF) A1 domain variants. We developed a murine model of defective VWF-collagen IV interactions with VWF variant p.R1399H. p.1399HH homozygous mice had decreased binding to collagen IV and increased bleeding times. p.1399HH homozygous mice had increased time to thrombosis and decreased platelet adhesion. SUMMARY: Background von Willebrand factor (VWF) binding to type IV collagen occurs via the VWF A1 domain, with p.R1399H being the most common VWF variant affecting this interaction. Objectives We generated a murine model of 1399H VWF to investigate its in vivo effects. Methods Mice expressing the murine 1399H variant were generated via gene targeting in embryonic stem cells. VWF antigen and VWF collagen binding were measured with ELISA. Tail bleeding time assays were performed by clipping a 3-mm segment. Ferric chloride-induced thrombosis was measured via ultrasound in the carotid artery. Platelet aggregation in response to collagens I and IV was measured. VWF-dependent platelet adhesion to collagen IV was measured under flow. Results Breeding of heterozygous p.R1399H and homozygous p.1399HH mice was observed to follow normal Mendelian ratios. No spontaneous bleeding was observed for any of the offspring. VWF expression was normal, but VWF binding to collagen IV was decreased in both heterozygous and homozygous offspring. Blood loss following tail resection was increased for p.1399HH mice, and occlusion times following ferric chloride-induced thrombosis were prolonged. Platelet aggregation was unaffected, but platelet adhesion to collagen IV under flow was diminished for p.1399HH mice. Conclusions These results show that a decrease in the ability of 1399H VWF to bind collagen IV under static conditions corresponds to a decrease in binding under flow conditions, an increased bleeding time, and a prolonged time to thrombosis. This study supports the potential for a bleeding phenotype in patients with aberrant VWF-collagen IV binding.

The Platelet Napoleon Complex-Small Cells, but Big Immune Regulatory Functions

Platelets have dual physiologic roles as both cellular mediators of thrombosis and immune modulatory cells. Historically, the thrombotic function of platelets has received significant research and clinical attention, but emerging research indicates that the immune regulatory roles of platelets may be just as important. We now know that in addition to their role in the acute thrombotic event at the time of myocardial infarction, platelets initiate and accelerate inflammatory processes that are part of the pathogenesis of atherosclerosis and myocardial infarction expansion. Furthermore, it is increasingly apparent from recent studies that platelets impact the pathogenesis of many vascular inflammatory processes such as autoimmune diseases, sepsis, viral infections, and growth and metastasis of many types of tumors. Therefore, we must consider platelets as immune cells that affect all phases of immune responses.

Novel insights into the clinical phenotype and pathophysiology underlying low VWF levels

Critical clinical questions remain unanswered regarding diagnosis and management of patients with low von Willebrand factor (VWF) levels (30-50 IU/dL). To address these questions, the Low VWF Ireland Cohort (LoVIC) study investigated 126 patients registered with low VWF levels. Despite marginally reduced plasma VWF levels, International Society of Thrombosis and Haemostasis Bleeding Assessment Tool (ISTH BAT) confirmed significant bleeding phenotypes in the majority of LoVIC patients. Importantly, bleeding tendency did not correlate with plasma VWF levels within the 30 to 50 IU/dL range. Furthermore, bleeding phenotypes could not be explained by concurrent hemostatic defects. Plasma factor VIII to VWF antigen (VWF:Ag) ratios were significantly increased in LoVIC patients compared with controls (P < .0001). In contrast, VWF propeptide to VWF:Ag ratios >3 were observed in only 6% of the LoVIC cohort. Furthermore, platelet-VWF collagen binding activity levels were both significantly reduced compared with controls (P < .05). In response to 1-desamino-8-D-arginine vasopressin (DDAVP), peak VWF:Ag levels exceeded 100 IU/dL in 88% of patients and was sustained >100 IU/dL after 4 hours in 72% of subjects. In conclusion, our novel data suggest that low VWF levels can be associated with significant bleeding and are predominantly due to reductions in VWF synthesis and/or constitutive secretion. Although enhanced VWF clearance may contribute to the pathophysiology in some individuals, the absolute reduction in VWF plasma half-life is usually mild and not sufficient to significantly impact upon the duration of DDAVP-induced VWF response. This trial was registered at www.clinicaltrials.gov as #NCT03167320.

Type 2B von Willebrand disease with or without large multimers: A distinction of the two sides of the disorder is long overdue

Most, but not all patients with type 2B von Willebrand disease (VWD)—which features gain-of-function mutations in the A1 domain of von Willebrand factor (VWF)—have no circulating large VWF multimers. Similarities and differences were analysed in 33 type 2B patients, 12 with a normal and 21 with an abnormal multimer pattern, to see whether they should be considered separately. The minimum aggregating dose of ristocetin was similarly reduced in both patient groups, and modulated by their underlying VWF mutations. Platelet VWF content was normal in all patients lacking in large multimers, but sometimes reduced in those with a normal multimer pattern. All the former patients and none of the latter had persistent or transient thrombocytopenia. A short VWF half-life (affecting plasma VWF levels) was seen in both groups, but more pronounced in patients without large multimers. Bleeding scores were also high in all patients, but more so in those without large multimers, apparently regardless of their platelet count. The marked phenotypic heterogeneity of type 2B VWD concerns not only patients’ VWF multimer pattern, but also their bleeding risk, and consequently their appropriate treatment too. Hence the need to clearly distinguish between type 2B VWD with normal or abnormal VWF multimers.

Management of type 2b von Willebrand disease in the neonatal period

Von Willebrand disease (VWD) is the most common inherited bleeding disorder, affecting one in 1,000 people. Type 2b VWD is a less common subtype caused by a gain-of-function mutation in von Willebrand factor (VWF) that leads to the formation of large, ineffective VWF-platelet multimers in circulation. This unique pathophysiology creates diagnostic and treatment dilemmas. There is limited information on the management of type 2b VWD in the neonatal period. This report describes the management of a neonate with type 2b VWD with an emphasis on the added benefit of concomitant platelet transfusion and factor replacement therapy over factor replacement therapy alone.

Inherited thrombocytopenia: novel insights into megakaryocyte maturation, proplatelet formation and platelet lifespan

The study of patients with inherited bleeding problems is a powerful approach in determining the function and regulation of important proteins in human platelets and their precursor, the megakaryocyte. The normal range of platelet counts in the bloodstream ranges from 150 000 to 400 000 platelets per microliter and is normally maintained within a narrow range for each individual. This requires a constant balance between thrombopoiesis, which is primarily controlled by the cytokine thrombopoietin (TPO), and platelet senescence and consumption. Thrombocytopenia can be defined as a platelet count of less than 150 000 per microliter and can be acquired or inherited. Heritable forms of thrombocytopenia are caused by mutations in genes involved in megakaryocyte differentiation, platelet production and platelet removal. In this review, we will discuss the main causative genes known for inherited thrombocytopenia and highlight their diverse functions and whether these give clues on the processes of platelet production, platelet function and platelet lifespan. Additionally, we will highlight the recent advances in novel genes identified for inherited thrombocytopenia and their suggested function.

The co-influence of VWD type 2B/2M mutations in the A1 domain and platelet GPIbα on the rate of cleavage to VWF by ADAMTS13

INTRODUCTION

In plasma, the size of the von Willebrand factor (VWF) multimer is down-regulated by ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13). The binding of platelets or glycoprotein (GP) Ibα recombinant fragment to VWF domain A1 may increase the cleavage by ADAMTS13 to VWF. Both type 2B and type 2M von Willebrand disease (VWD) result in bleeding disorders with the diathesis of increased and decreased binding affinity between GPIbα and VWF, respectively. However, the influence of 2B/2M VWD mutations in the A1 domain and GPIbα on cleavage by ADAMTS13 to VWF needs further study.

MATERIALS AND METHODS

Different types of full-length human recombinant VWF (rVWF) were expressed, including three type 2B mutations (P1337L, H1268D, and R1308C), one type 2M mutation (D1302G), and wild type (WT). The three characterized types of rVWF were digested by ADAMTS13 under static conditions or high-shear stress. The interaction of rVWF and ADAMTS13 was also tested by plate-binding assays.

RESULTS

Under static (natured) conditions or high-shear stress, type 2B mutants exhibited a higher susceptibility to ADAMTS13 than rVWF-WT, whereas type 2M mutant was normal. While under static (denatured) conditions or high-shear stress (with GPIbα fragment) rVWF-WT showed an even higher susceptibility to ADAMTS13 than the two type 2B mutants studied.

CONCLUSION

Type 2B mutations localized in the A1 domain could enhance the sensitivity to ADAMTS13-mediated proteolysis. When GPIbα participated, there was a dramatically increased proteolytic cleavage of VWF by ADAMTS13 to rVWF-WT, excluding some type 2B mutants.

Misfolding of vWF to pathologically disordered conformations impacts the severity of von Willebrand disease

The primary hemostatic von Willebrand factor (vWF) functions to sequester platelets from rheological blood flow and mediates their adhesion to damaged subendothelium at sites of vascular injury. We have surveyed the effect of 16 disease-causing mutations identified in patients diagnosed with the bleeding diathesis disorder, von Willebrand disease (vWD), on the structure and rheology of vWF A1 domain adhesiveness to the platelet GPIbα receptor. These mutations have a dynamic phenotypical range of bleeding from lack of platelet adhesion to severe thrombocytopenia. Using new rheological tools in combination with classical thermodynamic, biophysical, and spectroscopic metrics, we establish a high propensity of the A1 domain to misfold to pathological molten globule conformations that differentially alter the strength of platelet adhesion under shear flow. Rheodynamic analysis establishes a quantitative rank order between shear-rate-dependent platelet-translocation pause times that linearly correlate with clinically reported measures of patient platelet counts and the severity of thrombocytopenia. These results suggest that specific secondary structure elements remaining in these pathological conformations of the A1 domain regulate GPIbα binding and the strength of vWF-platelet interactions, which affects the vWD functional phenotype and the severity of thrombocytopenia.

The venomous cocktail of the vampire snail Colubraria reticulata (Mollusca, Gastropoda)

Background

Hematophagy arose independently multiple times during metazoan evolution, with several lineages of vampire animals particularly diversified in invertebrates. However, the biochemistry of hematophagy has been studied in a few species of direct medical interest and is still underdeveloped in most invertebrates, as in general is the study of venom toxins. In cone snails, leeches, arthropods and snakes, the strong target specificity of venom toxins uniquely aligns them to industrial and academic pursuits (pharmacological applications, pest control etc.) and provides a biochemical tool for studying biological activities including cell signalling and immunological response. Neogastropod snails (cones, oyster drills etc.) are carnivorous and include active predators, scavengers, grazers on sessile invertebrates and hematophagous parasites; most of them use venoms to efficiently feed. It has been hypothesized that trophic innovations were the main drivers of rapid radiation of Neogastropoda in the late Cretaceous.

We present here the first molecular characterization of the alimentary secretion of a non-conoidean neogastropod, Colubraria reticulata. Colubrariids successfully feed on the blood of fishes, throughout the secretion into the host of a complex mixture of anaesthetics and anticoagulants. We used a NGS RNA-Seq approach, integrated with differential expression analyses and custom searches for putative secreted feeding-related proteins, to describe in detail the salivary and mid-oesophageal transcriptomes of this Mediterranean vampire snail, with functional and evolutionary insights on major families of bioactive molecules.

Results

A remarkably low level of overlap was observed between the gene expression in the two target tissues, which also contained a high percentage of putatively secreted proteins when compared to the whole body. At least 12 families of feeding-related proteins were identified, including: 1) anaesthetics, such as ShK Toxin-containing proteins and turripeptides (ion-channel blockers), Cysteine-rich secretory proteins (CRISPs), Adenosine Deaminase (ADA); 2) inhibitors of primary haemostasis, such as novel vWFA domain-containing proteins, the Ectonucleotide pyrophosphatase/phosphodiesterase family member 5 (ENPP5) and the wasp Antigen-5; 3) anticoagulants, such as TFPI-like multiple Kunitz-type protease inhibitors, Peptidases S1 (PS1), CAP/ShKT domain-containing proteins, Astacin metalloproteases and Astacin/ShKT domain-containing proteins; 4) additional proteins, such the Angiotensin-Converting Enzyme (ACE: vasopressive) and the cytolytic Porins.

Conclusions

Colubraria feeding physiology seems to involve inhibitors of both primary and secondary haemostasis, anaesthetics, a vasoconstrictive enzyme to reduce feeding time and tissue-degrading proteins such as Porins and Astacins. The complexity of Colubraria venomous cocktail and the divergence from the arsenal of the few neogastropods studied to date (mostly conoideans) suggest that biochemical diversification of neogastropods might be largely underestimated and worth of extensive investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1648-4) contains supplementary material, which is available to authorized users.

An update on type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) accounts for fewer than 5% of all VWD patients. In this disease, mutations in the A1 domain result in increased von Willebrand factor (VWF) binding to platelet GPIbα receptors, causing increased platelet clearance and preferential loss of high molecular weight VWF multimers. Diagnosis is complicated because of significant clinical variations even among patients with identical mutations. Platelet transfusion often provides suboptimal results since transfused platelets may be aggregated by the patients' abnormal VWF. Desmopressin may cause a transient decrease in platelet count that could lead to an increased risk of bleeding. Replacement therapy with factor VIII/VWF concentrates is the most effective approach to prevention and treatment of bleeding in type 2B VWD.

Critical von Willebrand factor A1 domain residues influence type VI collagen binding

BACKGROUND

von Willebrand factor (VWF) binds to subendothelial collagen at sites of vascular injury. Laboratory testing for von Willebrand disease (VWD), however, does not always include collagen binding assays (VWF:CB) and standard VWF:CB assays use type I and/or type III collagen rather than type VI collagen.

OBJECTIVES

We report here on several mutations that exclusively alter binding to type VI collagen.

PATIENTS/METHODS

Healthy controls and index cases from the Zimmerman Program for the Molecular and Clinical Biology of VWD were analyzed for VWF antigen (VWF:Ag), VWF ristocetin cofactor activity and VWF:CB with types I, III and VI collagen. VWF gene sequencing was performed for all subjects.

RESULTS

Two healthy controls and one type 1 VWD subject were heterozygous for an A1 domain sequence variation, R1399H, and displayed a selective decreased binding to type VI collagen but not types I and III. Expression of recombinant 1399H VWF resulted in absent binding to type VI collagen. Two other VWF A1 domain mutations, S1387I and Q1402P, displayed diminished binding to type VI collagen. An 11 amino acid deletion in the A1 domain also abrogated binding to type VI collagen.

CONCLUSIONS

VWF:CB may be useful in diagnosis of VWD, as a decreased VWF:CB/VWF:Ag ratio may reflect specific loss of collagen binding ability. Mutations that exclusively affect type VI collagen binding may be associated with bleeding, yet missed by current VWF testing.

von Willebrand factor: the complex molecular genetics of a multidomain and multifunctional protein

von Willebrand disease (VWD), the most common inherited bleeding disorder in humans, is characterised by a prolonged bleeding time due to quantitative and/or functional deficits of von Willebrand factor (VWF), a huge multimeric protein. Given the large size and complexity of the protein, the many functions of VWF, for example, binding to collagen, to platelet GPIb, and to FVIII, the localisation of these binding sites in different VWF domains, as well as the dependence on a high molecular weight multimer structure for proper function, VWF is prone to quantitative and very heterogeneous structural and functional defects. Comprehensive clinical and laboratory phenotypic description of patients with VWD in correlation to the genotype has considerably increased our knowledge on this disorder and the physiology and pathophysiology of VWF. This article focuses on the phenotype/genotype relationship in VWD and the context of VWD types and subtypes with particular VWF domains.

A family having type 2B von Willebrand disease with an R1306W mutation: Severe thrombocytopenia leads to the normalization of high molecular weight multimers

In type 2B von Willebrand disease (2B VWD), abnormal von Willebrand factor (VWF) spontaneously binds to platelets. This leads to the clearance of the high molecular weight multimers (HMWM) of VWF and results in thrombocytopenia. Herein we report a family of 2B VWD with an R1306W mutation which caused thrombocytopenia with giant platelets. The most important finding in this study is dynamic changes in VWF values in association with platelet counts. When the proband (2 years of age) had severe thrombocytopenia, his HMWM were normal, however, hematological examination showed a low level of VWF and a lack of HMWM after platelet count recovered. His affected sister also exhibited similar phenomenona. These results suggest that the severe thrombocytopenia leads to decreased clearance of VWF HMWM and restoration of VWF HMWM in plasma. We must consider 2B VWD in the case of recurrent thrombocytopenia following infection or other stress condition.

von Willebrand disease type 2B must be always considered in the differential diagnosis of genetic thrombocytopenias with giant platelets

Type 2B von Willebrand's disease (VWD) is an inherited bleeding disorder characterized by spontaneous binding of large von Willebrand factor (VWF) multimers to platelets in vivo. This phenomenon induces the clearance of both large multimers and platelets, usually resulting in thrombocytopenia with slightly increased platelet size. We describe a newborn with a VWD type 2B due to the heterozygous missense mutation V1316M who presented the atypical feature of giant platelets in peripheral blood. Based on this observation and literature review, we suggest that the diagnosis of VWD 2B should be always considered in patients with chronic thrombocytopenia and giant platelets.

Clinical and molecular predictors of thrombocytopenia and risk of bleeding in patients with von Willebrand disease type 2B: a cohort study of 67 patients

Type 2B von Willebrand disease (VWD2B) is caused by an abnormal von Willebrand factor (VWF) with increased affinity for the platelet receptor glycoprotein Ib-alpha (GPIb-alpha) that may result in moderate to severe thrombocytopenia. We evaluated the prevalence and clinical and molecular predictors of thrombocytopenia in a cohort of 67 VWD2B patients from 38 unrelated families characterized by VWF mutations. Platelet count, mean platelet volume, and morphologic evaluations of blood smear were obtained at baseline and during physiologic (pregnancy) or pathologic (infections, surgeries) stress conditions. Thrombocytopenia was found in 20 patients (30%) at baseline and in 38 (57%) after stress conditions, whereas platelet counts were always normal in 16 patients (24%) from 5 families carrying the P1266L/Q or R1308L mutations. VWF in its GPIb-alpha-binding conformation (VWF-GPIb-alpha/BC) was higher than normal in all except the 16 cases without thrombocytopenia (values up to 6-fold higher than controls). The risk of bleeding was higher in patients with thrombocytopenia (adjusted hazard ratio = 4.57; 95% confidence interval, 1.17-17.90) and in those with the highest tertile of bleeding severity score (5.66; 95% confidence interval, 1.03-31.07). Prediction of possible thrombocytopenia in VWD2B by measuring VWF-GPIb-alpha/BC is important because a low platelet count is an independent risk factor for bleeding.

Update on the pathophysiology and classification of von Willebrand disease: a report of the Subcommittee on von Willebrand Factor

von Willebrand disease (VWD) is a bleeding disorder caused by inherited defects in the concentration, structure, or function of von Willebrand factor (VWF). VWD is classified into three primary categories. Type 1 includes partial quantitative deficiency, type 2 includes qualitative defects, and type 3 includes virtually complete deficiency of VWF. VWD type 2 is divided into four secondary categories. Type 2A includes variants with decreased platelet adhesion caused by selective deficiency of high-molecular-weight VWF multimers. Type 2B includes variants with increased affinity for platelet glycoprotein Ib. Type 2M includes variants with markedly defective platelet adhesion despite a relatively normal size distribution of VWF multimers. Type 2N includes variants with markedly decreased affinity for factor VIII. These six categories of VWD correlate with important clinical features and therapeutic requirements. Some VWF gene mutations, alone or in combination, have complex effects and give rise to mixed VWD phenotypes. Certain VWD types, especially type 1 and type 2A, encompass several pathophysiologic mechanisms that sometimes can be distinguished by appropriate laboratory studies. The clinical significance of this heterogeneity is under investigation, which may support further subdivision of VWD type 1 or type 2A in the future.

Impaired megakaryocytopoiesis in type 2B von Willebrand disease with severe thrombocytopenia

In type 2B von Willebrand disease, there is spontaneous binding of mutated von Willebrand factor (VWF) multimers to platelets. Here we report a family in which severe thrombocytopenia may also be linked to abnormal megakaryocytopoiesis. A heterozygous mutation in the VWF A1 domain gave a R1308P substitution in an interactive site for glycoprotein Ibalpha (GPIbalpha). Electron microscopy showed clusters of platelets in close contact. Binding of antibodies to the GPIbalpha N-terminal domain was decreased, whereas GPIX and GPV were normally detected. In Western blotting (WB), GPIbalpha, alphaIIb, and beta3 were normally present. Proteins involved in Ca(2+) homeostasis were analyzed by quantitating platelet mRNA or by WB. Plasma membrane Ca(2+) ATPase (PMCA)-4b and type III inositol trisphosphate receptor (InsP(3)-R3) were selectively increased. The presence of degradation products of polyadenosine diphosphate (ADP)-ribose polymerase protein (PARP) suggested ongoing caspase-3 activity. These were findings typical of immature normal megakaryocytes cultured from peripheral blood CD34(+) cells with TPO. Significantly, megakaryocytes from the patients in culture produced self-associated and interwoven proplatelets. Immunolocalization showed VWF not only associated with platelets, but already on the megakaryocyte surface and within internal channels. In this family, type 2B VWD is clearly associated with abnormal platelet production.

Genetic linkage and association analysis in type 1 von Willebrand disease: results from the Canadian type 1 VWD study

BACKGROUND

von Willebrand disease (VWD) is the most common bleeding disorder known in humans, with type 1 VWD representing the majority of cases. Unlike the other variant forms of VWD, type 1 disease represents a complex genetic trait, influenced by both genetic and environmental factors.

AIM

To evaluate the contribution of the von Willebrand factor (VWF) and ABO blood group loci to the type 1 VWD phenotype, and to assess the potential for locus heterogeneity in this condition, we have performed genetic linkage and association studies on a large, unselected type 1 VWD population.

METHOD

We initially collected samples from 194 Canadian type 1 VWD families for analysis. After the exclusion of families found to have either type 2 or type 3 VWD, and pedigrees with samples from single generations, linkage and association analysis was performed on 155 type 1 VWD families.

RESULTS AND CONCLUSION

The linkage study has shown a low heterogeneity LOD score of 2.13 with the proportion of families linked to the VWF gene estimated to be 0.41. Linkage was not detected to the ABO locus in this type 1 VWD population. In the family-based association test, significant association was found between the type 1 VWD phenotype, the quantitative traits, VWF:Ag, VWF:RCo, and FVIII:C and the ABO 'O' and 'A' alleles and the VWF codon 1584 variant. There was also weak association with the -1185 promoter polymorphism and VWF:Ag, VWF:RCo, and FVIII:C plasma levels. These studies provide further evidence to support the role for genetic loci other than VWF and ABO in the pathogenesis of type 1 VWD.

Gastrointestinal angiodysplasia in a patient with type 2 von Willebrand's disease and analysis of exon 28 of the von Willebrand factor gene

Although the association between gastrointestinal angiodysplasia and von Willebrand's disease has been suggested, molecular mechanisms involved in the formation of angiodysplasia in patients with von Willebrand's disease remained undetermined. We examined exon 28 of the von Willebrand factor gene in a patient with both von Willebrand's disease and recurrent bleeding from angiodysplasia in the duodenum as well as his father's, and found a point mutation, C 3916-->T (amino acid substitution; Arg 543-->Trp), in the A1 domain of the von Willebrand factor gene. This mutation was identical with a previously reported mutation in a patient with von Willebrand's disease complicated with gastrointestinal angiodysplasia.

A first Taiwanese Chinese family of type 2B von Willebrand disease with R1306W mutation

Clinical, laboratory and genetic defect of a Taiwanese family with type 2B von Willebrand disease (VWD) were studied. The proband was a 55-year-old woman who gave birth to two daughters and one son aged 30, 29 and 27, respectively. All had abnormal mucocutaneous bleedings since their childhood. In proband, PT, PTT and platelet count were normal; template bleeding time was 14 min; VIII:C was 51%, von Willebrand factor antigen (VWF:Ag), 42% and von Willerand factor ristocetin-cofactor (VWF:RCo, 15%); ristocetin-induced platelet aggregation (RIPA) at 0.3 and 0.6 mg/ml of ristocetin was 16% and 68%, respectively. The enhanced response to ristocetin was identified to be in plasma, not in platelet itself, by mixing studies. Analysis of von Willebrand factor (VWF) multimer of plasma but not of platelets showed absence of high-molecular weight (HMW) multimer. All three children had similar laboratory findings. Exon 28 of VWF gene was amplified using polymerase chain reaction (PCR) and sequenced. The proband and three children were all found to be heterozygous for C to T transition at nucleotide 3916 resulting in Arg 1306 Trp (R1306W) substitution. This mutation in the glycoprotein Ib (GPIb)-binding site has been found to increase the affinity of plasma VWF for platelets, and thus cause loss of HMW multimers and often thrombocytopenia. In conclusion, a first report of type 2B VWD in a Taiwanese Chinese family who show R1306W mutation in VWF gene was described.

Platelets: thrombotic thrombocytopenic purpura

Abnormalities of plasma von Willebrand factor (VWF) have been recognized to be associated with thrombotic thrombocytopenic purpura (TTP) for over 20 years. Patients with chronic, relapsing TTP have VWF multimers that are larger than normal, similar in size to those secreted by cultured endothelial cells. Recent observations have documented that a deficiency of a VWF-cleaving protease (termed ADAMTS13) may be responsible for the presence of these unusually large VWF multimers. Multiple mutations of the ADAMTS13 gene can result in ADAMTS13 deficiency and cause congenital TTP; autoantibodies neutralizing ADAMTS13 protease activity have been associated with acquired TTP. In Section I, Dr. Evan Sadler reviews the structure, biosynthesis, and function of the ADAMTS13 protease. He describes the mutations that have been identified in congenital TTP and describes the relationship of ADAMTS13 deficiency to the development of both congenital and acquired TTP. Dr. Sadler postulates that the development of TTP may be favored by conditions that combine increased VWF secretion, such as during the later stages of pregnancy, and decreased ADAMTS13 activity. In Section II, Dr. Bernhard Lämmle describes the assay methods for determining ADAMTS13 activity. Understanding the complexity of these methods is essential for understanding the difficulty of assay performance and the interpretation of assay data. Dr. Lämmle describes his extensive experience measuring ADAMTS13 activity in patients with TTP as well as patients with acute thrombocytopenia and severe illnesses not diagnosed as TTP. His data suggest that a severe deficiency of ADAMTS13 activity (< 5%) is a specific feature of TTP. However, he emphasizes that, although severe ADAMTS13 deficiency may be specific for TTP, it may not be sensitive enough to identify all patients who may be appropriately diagnosed as TTP and who may respond to plasma exchange treatment. In Section III, Dr. James George describes the evaluation and management of patients with clinically suspected TTP, as well as adults who may be described as having hemolytic-uremic syndrome (HUS). Dr. George presents a classification of TTP and HUS in children and adults. Appropriate evaluation and management are related to the clinical setting in which the diagnosis is considered. A clinical approach is described for patients in whom the diagnosis of TTP or HUS is considered (1) following bone marrow transplantation, (2) during pregnancy or the postpartum period, (3) in association with drugs which may cause TTP either by an acute immune-mediated toxicity or a dose-related toxicity, (4) following a prodrome of bloody diarrhea, (5) in patients with autoimmune disorders, and (6) in patients with no apparent associated condition who may be considered to have idiopathic TTP. Patients with idiopathic TTP appear to have the greatest frequency of ADAMTS13 deficiency and appear to be at greatest risk for a prolonged clinical course and subsequent relapse. Management with plasma exchange has a high risk of complications. Indications for additional immunosuppressive therapy are described.

Founder von Willebrand factor haplotype associated with type 1 von Willebrand disease

To date, no dominant mutation has been identified in a significant proportion of patients with type 1 von Willebrand disease (VWD). In this study, we examined 70 families as part of the Canadian Type 1 VWD Study. The entire VWF gene was sequenced for 1 index case, revealing 2 sequence variations: intron 30 (5312-19A>C) and exon 28 at Tyr1584Cys (4751A>G). The Tyr1584Cys variation was identified in 14.3% (10 of 70) of the families and was in phase with the 5312-19A>C variation in 7 (10.0%) families. Both variants were observed in 2 of 10 UK families with type 1 VWD, but neither variant was found in 200 and 100 healthy, unrelated persons, respectively. Mean von Willebrand factor antigen (VWF:Ag), VWF ristocetin cofactor (VWF:RCo), and factor VIII coagulant activity (FVIII:C) for the index cases in these families are 0.4 U/mL, 0.36 U/mL, and 0.54 U/mL, respectively, and VWF multimer patterns show no qualitative abnormalities. Aberrant VWF splicing was not observed in these patients, and both alleles of the VWF gene are expressed as RNA. Molecular dynamic simulation was performed on a homology model of the VWF-A2 domain containing the Tyr1584Cys mutation. This showed that no significant structural changes occur as a result of the substitution but that a new solvent-exposed reactive thiol group is apparent. Expression studies revealed that the Tyr1584Cys mutation results in increased intracellular retention of the VWF protein. We demonstrate that all the families with the Tyr1584Cys mutation share a common, evolved VWF haplotype, suggesting that this mutation is ancient. This is the first report of a mutation that segregates in a significant proportion of patients with type 1 VWD.

Molecular genetics of type 2 von Willebrand disease

Type 2 von Willebrand disease (VWD) is characterized by a wide heterogeneity of functional and structural defects. These abnormalities' cause either defective von Willebrand factor (VWF)-dependent platelet function in subtypes 2A, 2B, and 2M or defective VWF-factor VIII (FVIII) binding in subtype 2N. The diagnoses of types 2A, 2B, and 2M VWD may be guided by the observation of disproportionately low levels of ristocetin cofactor activity or collagen-binding capacity relative to VWF antigen. The abnormal platelet-dependent function is often associated with the absence of high molecular weight (HMW) multimers (type 2A, type 2B), but the HMW multimers may also be present (type 2M, some type 2B), and supranormal multimers may exist ("Vicenza" variant). The observation of a low FVIII-to-VWF:Ag ratio is a hallmark of type 2N VWD. in which the FVIII levels depend on the severity of the FVIII-binding defect. Today, the identification of mutations in particular domains of the pre-pro-VWF is helpful in classifying these variants and providing further insight into the structure-function relationship and the biosynthesis of VWF. Thus, mutations in the D2 domain, involved in the multimerization process, are found in patients with type 2A, formerly named IIC VWD. Mutations located in the D' domain or in the N terminus of the D3 domain define type 2N VWD. Mutations in the D3 domain characterize Vicenza and IIE patients. Mutations in the A1 domain may modify the binding of VWF multimers to platelets, either increasing (type 2B) or decreasing (type 2M, 2A/2M) the affinity of VWF for platelets. In type 2A VWD, molecular abnormalities identified in the A2 domain, which contains a specific proteolytic site, are associated with alterations in folding, impairing VWF secretion or increasing its susceptibility to proteolysis. Finally, a mutation localized in the carboxy-terminus CK domain, which is crucial for the dimerization of the VWF subunit, has been identified in a rare subtype 2A, formerly named IID.

Von Willebrand factor and von Willebrand disease

von Willebrand disease (vWD) is caused by quantitative and/or qualitative defects of the von Willebrand factor (vWF), a multimeric high molecular weight glycoprotein. Typically, it affects the primary hemostatic system, which results in a mucocutaneous bleeding tendency simulating a platelet function defect. The vWF promotes its function in two ways: (i) by initiating platelet adhesion to the injured vessel wall under conditions of high shear forces, and (ii) by its carrier function for factor VIII in plasma. Accumulating knowledge of the different clinical phenotypes and the pathophysiological basis of the disease translated into a classification that differentiated between quantitative and qualitative defects by means of quantitative and functional parameters, and by analyzing the electrophoretic pattern of vWF multimers. The advent of molecular techniques provided the opportunity for conducting genotype-phenotype studies which have recently helped, not only to elucidate or confirm important functions of vWF and its steps in post-translational processing, but also many disease causing defects. Acquired von Willebrand syndrome (avWS) has gained more attention during the recent years. An international registry was published and recommendation by the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis in 2000. It concluded that avWS, although not a frequent disease, is nevertheless probably underdiagnosed. This should be addressed in future prospective studies. The aim of treatment is the correction of the impaired hemostatic system of the patient, ideally including the defects of both primary and secondary hemostasis. Desmopressin is the treatment of choice in about 70% of patients, mostly with type 1, while the others merit treatment with concentrates containing vWF.

The molecular biology of von Willebrand disease

von Willebrand disease (VWD) is a common autosomally inherited bleeding disorder associated with mucosal or trauma-related bleeding in affected individuals. VWD results from either a quantitative or qualitative deficiency of von Willebrand factor (VWF)--a glycoprotein with essential roles in primary haemostasis and as a carrier of coagulation factor VIII (FVIII) in the circulation. In recent years the identification of mutations in the VWF gene in patients with VWD has improved our understanding of the structure and function of the VWF protein, and has illustrated the importance of specific regions of VWF for its interaction with other components of the vasculature. The underlying genetic lesions and associated molecular pathology have been identified in many cases of type 2A, type 2B, type 2M, type 2N and type 3 VWD. However in the most common variant, type 1 VWD, the causative molecular defect is unknown in the large majority of cases. In the absence of an understanding of the molecular pathology underlying type 1 VWD, precise diagnosis and classification of this common disorder remains problematic.

Type 2 von Willebrand disease causing defective von Willebrand factor-dependent platelet function

Type 2 von Willebrand disease causing defective von Willebrand factor-dependent platelet function comprises mainly subtypes 2A, 2B and 2M. The diagnosis of type 2 von Willebrand disease may be guided by the observation of a disproportionately low level of ristocetin cofactor activity or collagen-binding activity relative to the von Willebrand factor antigen level. The decreased platelet-dependent function is often associated with an absence of high molecular weight multimers (types 2A and 2B), but the high molecular weight multimers may also be present (type 2M and some type 2B), and supranormal multimers may exist (as in the Vicenza variant). Today, the identification of mutations in particular domains of the pre-provon Willebrand factor is helpful to classify these variants and to provide further insight into the structure-function relationship and the biosynthesis of von Willebrand factor. Thus, mutations in the D2 domain, involved in the multimerization process, are found in patients with type 2A, formerly named IIC von Willebrand disease. Mutations in the D3 domain characterize the Vicenza variant, or type IIE patients. Mutations in the A1 domain may modify the binding of von Willebrand factor multimers to platelets, either increasing (type 2B) or decreasing (types 2M and 2A/2M) the affinity of von Willebrand factor for platelets. In type 2A disease, molecular abnormalities identified in the A2 domain, which contains a specific proteolytic site, are associated with alterations in folding that impair the secretion of von Willebrand factor or increase its susceptibility to proteolysis. Finally, a mutation localized in the C terminus cysteine knot domain, which is crucial for the dimerization of von Willebrand factor subunit, has been identified in a rare subtype 2A, formerly named IID.

A molecular approach to the classification of von Willebrand disease

The marked heterogeneity of von Willebrand disease was already recognized by von Willebrand in 1926. The accumulating knowledge of the different clinical phenotypes and the pathophysiological basis of the disease was translated into a classification that differentiated between quantitative and qualitative defects by means of quantitative and functional parameters and by analysing the electrophoretic pattern of von Willebrand factor multimers. The increasing number of different von Willebrand disease phenotypes required a revision of the nomenclature at a time when only a few types of von Willebrand disease had already been analysed at the molecular level. Consequently, the molecular data played only a minor role in the revised classification. Given the pronounced, even intra-individual, variation in the manifestation of von Willebrand disease and the diagnostic difficulties caused by a non-standardized methodology, it is clear that biochemical methods alone are insufficient for a clear classification. The advent of molecular techniques provided the opportunity for genotype-phenotype studies that recently helped to elucidate or confirm not only the important functions of von Willebrand factor and the steps of its post-translational processing, but also many disease-causing defects. The reproducible correlation between certain phenotypes and particular mutations can now be used for a molecular approach towards a final classification of von Willebrand disease, equally useful for the clinician and for research requirements.

Platelet ultrastructural abnormalities in three patients with type 2B von Willebrand disease

Several reports have described the presence of giant platelets in patients with type 2B von Willebrand disease (VWD). We have now characterized the ultrastructural changes in platelets from three unrelated patients with type 2B VWD and different mutations within exon 28 of the von Willebrand factor (VWF) gene. Electron microscopy showed that each of these subjects had an increased proportion of large platelets when compared with those of a patient with type 2A VWD or control subjects. Immunogold labelling for VWF was performed. Large masses detected by anti-VWF antibody were seen not only on the platelet surface, but also inside the platelet surface-connected canalicular system (SCCS) when ultrathin sections were labelled. This suggested translocation of the abnormally bound VWF from the platelet surface. Labelling of the alpha-granules was eccentric as for normal platelets. Labelling for glycoprotein (GP) Ib was seen on the surface and within the SCCS, suggesting co-localization with the bound VWF. However, there was no evidence for VWF in endosomes or other endocytic vesicles. The presence of platelet-bound VWF was not accompanied by high levels of platelet activation, as detected by electron microscopy, or by using monoclonal antibodies against P-selectin or activation-dependent determinants on GP IIb-IIIa in flow cytometry. Intriguingly, platelet ultrastructure often resembled that seen in patients with congenital thrombocytopathies characteristic of giant platelet syndromes.

Type 2B von Willebrand's disease in thirteen individuals from five unrelated Australian families: phenotype and genotype correlations

Type 2B von Willebrand's disease (VWD) is due to a qualitative defect in von Willebrand factor (VWF) in which there is an increased affinity for the platelet glycoprotein Ib-IX-V receptor complex. Spontaneous binding of type 2B VWF to platelets and subsequent clearance from the plasma is thought to account for the characteristic phenotype of type 2B VWD. These gain-of-function mutations are due to single amino substitutions that are clustered within the functionally important A1 domain of VWF. We describe 13 individuals from five unrelated families in Australia with type 2B VWD, report their phenotypic abnormalities, and delineate their causative mutations. We confirm that the mutation Arg543Trp is also particularly common among families with type 2B VWD in Australia.

von Willebrand disease: recent advances in pathophysiology and treatment

This review focuses on new developments in the pathophysiology and treatment of von Willebrand disease (vWd). New aspects of the cell biology, gene control, and structure-function correlates of von Willebrand factor (vWf) are reviewed. vWd is more prevalent than previously recognized, affecting up to 1% of the population; this is particularly evident in women's health. Blood group is an important determinant of von Willebrand factor levels; individuals of blood group O tend to have lower plasma levels of vWf than those in other blood groups. Currently available blood tests of vWf quantity and function are discussed, in addition to newer tests undergoing validation. Treatment of classical vWd with desmopressin acetate and plasma derivatives is discussed, as is the potential for intravenous immunoglobulin and corticosteroids in acquired vWd. Special situations, such as the management of vWd in pregnancy, are also discussed.

Mutations in the ER-Golgi intermediate compartment protein ERGIC-53 cause combined deficiency of coagulation factors V and VIII

Combined deficiency of factors V and VIII is an autosomal recessive bleeding disorder resulting from alterations in an unknown gene on chromosome 18q, distinct from the factor V and factor VIII genes. ERGIC-53, a component of the ER-Golgi intermediate compartment, was mapped to a YAC and BAC contig containing the critical region for the combined factors V and VIII deficiency gene. DNA sequence analysis identified two different mutations, accounting for all affected individuals in nine families studied. Immunofluorescence and Western analysis of immortalized lymphocytes from patients homozygous for either of the two mutations demonstrate complete lack of expression of the mutated gene in these cells. These findings suggest that ERGIC-53 may function as a molecular chaperone for the transport from ER to Golgi of a specific subset of secreted proteins, including coagulation factors V and VIII.

Giant platelet disorder in a patient with type 2B von Willebrand's disease

While patients with type 2B von Willebrand's disease often exhibit thrombocytopenia, platelet morphology is typically normal. We describe a 44-year-old Jamaican man with thrombocytopenia and a history of bleeding, who had giant platelets on his peripheral blood film. Functional studies and von Willebrand factor gene sequencing showed him to have type 2B von Willebrand's disease with a heterozygous point mutation resulting in a V553M (V1316M in the new von Willebrand factor gene mutation nomenclature) amino acid substitution. Family studies showed one of his two sisters to have an ill-defined giant-platelet-syndrome with mild thrombocytopenia, but not von Willebrand's disease, indicating that the association of giant platelets and von Willebrand's disease in our patient was most likely coincidental. This report describes the rare concurrence of two uncommon disorders. It also demonstrates how the thrombocytopenia of type 2B von Willebrand's disease can be misdiagnosed as ITP, leading to unnecessary and potentially harmful therapeutic interventions.

Expression and Functional Characterization of an Abnormal Platelet Membrane Glycoprotein Ibα (Met239 → Val) Reported in Patients With Platelet-Type von Willebrand Disease

Platelet-type von Willebrand disease (vWD) is a congenital bleeding disorder characterized by heightened ristocetin-induced platelet aggregation caused by abnormally high affinity between the platelet membrane glycoprotein (GP) Ib/IX complex and von Willebrand factor (vWF ). Two distinct point mutations, Gly233 to Val and Met239 to Val, have been reported in GPIbα. We have constructed a recombinant GPIbα fragment containing the latter mutation, Met239 to Val (M239V) and characterized the mutant molecule using two methods, ie, interaction between soluble vWF and immobilized M239V and inhibition of platelet aggregation by purified soluble M239V. Spontaneous binding (ie, binding without any inducers) was observed between 125I-vWF and immobilized M239V but not between 125I-vWF and immobilized wild-type (WT) GPIbα. The addition of low concentrations of ristocetin (0.2 mg/mL) induced specific 125I-vWF binding to immobilized M239V, but not to WT GPIbα. At high concentrations of ristocetin (1.2 mg/mL), both WT GPIbα and M239V specifically bound to 125I-vWF. Thus, M239V reproduced the unique functional abnormality of the GPIb/IX complex in platelet-type vWD. Moreover, the purified soluble M239V inhibited platelet aggregation induced by low concentration of ristocetin (0.3 mg/mL) in platelet-rich plasma from a patient having Met239 to Val mutation, whereas purified WT did not. These results provide direct evidences that the reported point mutation is the responsible molecular basis of this disorder.

Expression and Functional Characterization of an Abnormal Platelet Membrane Glycoprotein Ibα (Met239 → Val) Reported in Patients With Platelet-Type von Willebrand Disease

Platelet-type von Willebrand disease (vWD) is a congenital bleeding disorder characterized by heightened ristocetin-induced platelet aggregation caused by abnormally high affinity between the platelet membrane glycoprotein (GP) Ib/IX complex and von Willebrand factor (vWF ). Two distinct point mutations, Gly233 to Val and Met239 to Val, have been reported in GPIbα. We have constructed a recombinant GPIbα fragment containing the latter mutation, Met239 to Val (M239V) and characterized the mutant molecule using two methods, ie, interaction between soluble vWF and immobilized M239V and inhibition of platelet aggregation by purified soluble M239V. Spontaneous binding (ie, binding without any inducers) was observed between 125I-vWF and immobilized M239V but not between 125I-vWF and immobilized wild-type (WT) GPIbα. The addition of low concentrations of ristocetin (0.2 mg/mL) induced specific 125I-vWF binding to immobilized M239V, but not to WT GPIbα. At high concentrations of ristocetin (1.2 mg/mL), both WT GPIbα and M239V specifically bound to 125I-vWF. Thus, M239V reproduced the unique functional abnormality of the GPIb/IX complex in platelet-type vWD. Moreover, the purified soluble M239V inhibited platelet aggregation induced by low concentration of ristocetin (0.3 mg/mL) in platelet-rich plasma from a patient having Met239 to Val mutation, whereas purified WT did not. These results provide direct evidences that the reported point mutation is the responsible molecular basis of this disorder.

Thrombocytopenia in pregnancy

Thrombocytopenia is a common finding in normal pregnancy. The introduction of routine automated complete blood counting has clearly documented this. In the past, these patients would go undetected and be spared unnecessary testing, procedures, or medications. This article reviews the common causes of thrombocytopenia and offers criteria on which the diagnosis of clinically significant thrombocytopenia can be made. In addition, we will discuss therapeutic approaches to manage patients with pathologic thrombocytopenia.

von Willebrand disease

Considerable progress has been made in characterizing the specific molecular defects responsible for the heterogeneous disorder known as von Willebrand disease (VWD). A large number of molecular defects have been identified and precise characterization may now be possible in the majority of type 2A, type 2B, type 2N, and potentially also type 3 VWD cases. However, the most common variant, type 1 VWD, still remains a major challenge. Continued progress in this area will improve our understanding of the pathogenesis of VWD and lead to more rapid and precise diagnosis and classification for this common disorder. The problems of incomplete VWD penetrance and poor diagnostic sensitivity and accuracy for the currently available clinical laboratory tests provide strong incentives for the development of DNA-based diagnostics. In addition, prenatal diagnosis is now possible either at the level of single point mutations (for some subtypes) or by RFLP analysis (assuming linkage to the von Willebrand factor [VWF] gene) and will probably be applied with increasing frequency for VWD type 3 (17, 133, 175). Understanding the molecular basis of VWD also has important implications for VWF structure and function and is helping to define critical binding domains within the VWF molecule. Insights gained from these studies may eventually lead to improved therapeutic approaches not only for VWD, but also for a variety of other genetic and acquired hemorrhagic and thrombotic disorders.

von Willebrand disease in children and adolescents

The term von Willebrand disease includes many bleeding disorders caused by abnormalities of vWF. Frequent or severe bleeding may be indicative of vWD or other bleeding conditions. Primary care practitioners need to be familiar with vWD and evaluate possibly affected individuals with appropriate laboratory studies. Patients with vWD should be educated about their disorder and preventive measures to limit its effect. Medications are available that can treat or prevent bleeding complications for most patients with vWD. Intervention with blood products is occasionally necessary.