Dimerization and multimerization defects of von Willebrand factor due to mutated cysteine residues

VWF-Gly2752Ser, a novel non-cysteine substitution variant in the CK domain, exhibits severe secretory impairment by hampering C-terminal dimer formation

BACKGROUND

Von Willebrand factor (VWF) is a multimeric glycoprotein that plays important roles in hemostasis and thrombosis. C-terminal interchain-disulfide bonds in the cystine knot (CK) domain are essential for VWF dimerization. Previous studies have reported that missense variants of cysteine in the CK domain disrupt the intrachain-disulfide bond and cause type 3 von Willebrand disease (VWD). However, type 3 VWD-associated noncysteine substitution variants in the CK domain have not been reported.

OBJECTIVE

To investigate the molecular mechanism of a novel non-cysteine variant in the CK domain, VWF c.8254 G>A (p.Gly2752Ser), which was identified in a patient with type 3 VWD as homozygous.

METHODS

Genetic analysis was performed by whole exome sequencing and Sanger sequencing. VWF multimer analysis was performed using SDS-agarose electrophoresis. VWF production and subcellular localization were analyzed using ex vivo endothelial colony forming cells (ECFCs) and an in vitro recombinant VWF (rVWF) expression system.

RESULTS

The patient was homozygous for VWF-Gly2752Ser. Plasma VWF enzyme-linked immunosorbent assay showed that the VWF antigen level of the patient was 1.2% compared with healthy subjects. A tiny amount of VWF was identified in the patient's ECFC. Multimer analysis revealed that the circulating VWF-Gly2752Ser presented only low molecular weight multimers. Subcellular localization analysis of VWF-Gly2752Ser-transfected cell lines showed that rVWF-Gly2752Ser was severely impaired in its ER-to-Golgi trafficking.

CONCLUSION

VWF-Gly2752Ser causes severe secretory impairment because of its dimerization failure. This is the first report of a VWF variant with a noncysteine substitution in the CK domain that causes type 3 VWD.

The disulfide bond Cys2724-Cys2774 in the C-terminal cystine knot domain of von Willebrand factor is critical for its dimerization and secretion

Background

Type 3 von Willebrand disease (VWD) exhibits severe hemorrhagic tendency with complicated pathogenesis. The C-terminal cystine knot (CTCK) domain plays an important role in the dimerization and secretion of von Willebrand factor (VWF). The CTCK domain has four intrachain disulfide bonds including Cys2724-Cys2774, Cys2739-Cys2788, Cys2750-Cys2804 and Cys2754-Cys2806, and the single cysteine mutation in Cys2739-Cys2788, Cys2750-Cys2804 and Cys2754-Cys2806 result in type 3 VWD, demonstrating the crucial role of these three disulfide bonds in VWF biosynthesis, however, the role of the remaining disulfide bond Cys2724-Cys2774 remains unclear.

Method and results

In this study, by the next-generation sequencing we found a missense mutation a c.8171G>A (C2724Y) in the CTCK domain of VWF allele in a patient family with type 3 VWD. In vitro, VWF C2724Y protein was expressed normally in HEK-293T cells but did not form a dimer or secrete into cell culture medium, suggesting that C2724 is critical for the VWF dimerization, and thus for VWF multimerization and secretion.

Conclusions

Our findings provide the first genetic evidence for the important role of Cys2724-Cys2774 in VWF biosynthesis and secretion. Therefore, all of the four intrachain disulfide bonds in CTCK monomer contribute to VWF dimerization and secretion.

Genotypes of European and Iranian patients with type 3 von Willebrand disease enrolled in 3WINTERS-IPS

Type 3 von Willebrand disease (VWD3) is a rare and severe bleeding disorder characterized by often undetectable von Willebrand factor (VWF) plasma levels, a recessive inheritance pattern, and heterogeneous genotype. The objective of this study was to identify the VWF defects in 265 European and Iranian patients with VWD3 enrolled in 3WINTERS-IPS (Type 3 Von Willebrand International Registries Inhibitor Prospective Study). All analyses were performed in centralized laboratories. The VWF genotype was studied in 231 patients with available DNA (121 [115 families] from Europe [EU], and 110 [91 families] from Iran [IR]). Among 206 unrelated patients, 134 were homozygous (EU/IR = 57/77) and 50 were compound heterozygous (EU/IR = 43/7) for VWF variants. In 22 patients, no or only one variant was found. A total of 154 different VWF variants (EU/IR = 101/58 [5 shared]) were identified among the 379 affected alleles (EU/IR = 210/169), of which 48 (EU/IR = 18/30) were novel. The variants p.Arg1659*, p.Arg1853*, p.Arg2535*, p.Cys275Ser, and delEx1_Ex5 were found in both European and Iranian VWD3 patients. Sixty variants were identified only in a single allele (EU/IR = 50/10), whereas 18 were recurrent (≥3 patients) within 144 affected alleles. Nine large deletions and one large insertion were found. Although most variants predicted null alleles, 21% of patients carried at least 1 missense variant. VWD3 genotype was more heterogeneous in the European population than in the Iranian population, with nearly twice as many different variants. A higher number of novel variants were found in the Iranian VWD3 patients.

Quantitative 3D microscopy highlights altered von Willebrand factor α-granule storage in patients with von Willebrand disease with distinct pathogenic mechanisms

BACKGROUND

Platelets play a key role in hemostasis through plug formation and secretion of their granule contents at sites of endothelial injury. Defects in von Willebrand factor (VWF), a platelet α-granule protein, are implicated in von Willebrand disease (VWD), and may lead to defective platelet adhesion and/or aggregation. Studying VWF quantity and subcellular localization may help us better understand the pathophysiology of VWD.

OBJECTIVE

Quantitative analysis of the platelet α-granule compartment and VWF storage in healthy individuals and VWD patients.

PATIENTS/METHODS

Structured illumination microscopy (SIM) was used to study VWF content and organization in platelets of healthy individuals and patients with VWD in combination with established techniques.

RESULTS

SIM capably quantified clear morphological and granular changes in platelets stimulated with proteinase-activated receptor 1 (PAR-1) activating peptide and revealed a large intra- and interdonor variability in VWF-positive object numbers within healthy resting platelets, similar to variation in secreted protein acidic and rich in cysteine (SPARC). We subsequently characterized VWD platelets to identify changes in the α-granule compartment of patients with different VWF defects, and were able to stratify two patients with type 3 VWD rising from different pathological mechanisms. We further analyzed VWF storage in α-granules of a patient with homozygous p.C1190R using electron microscopy and found discrepant VWF levels and different degrees of multimerization in platelets of patients with heterozygous p.C1190 in comparison to VWF in plasma.

CONCLUSIONS

Our findings highlight the utility of quantitative imaging approaches in assessing platelet granule content, which may help to better understand VWF storage in α-granules and to gain new insights in the etiology of VWD.

In vivo modulation of a dominant-negative variant in mouse models of von Willebrand disease type 2A

Essentials Treatment options for von Willebrand disease (VWD) patients are limited. The p.P1127_C1948delinsR deletion/variant is a useful model to study VWD in vitro and in vivo. Counteracting dominant-negative effects restores von Willebrand factor multimerization in mice. This is the first siRNA-based treatment applied to a mouse model of VWD-type 2A. ABSTRACT: Background Treatment options for patients suffering from von Willebrand disease (VWD) are limited. Von Willebrand factor (VWF) is a polymeric protein that undergoes regulated dimerization and subsequent multimerization during its biosynthesis. Numerous heterozygous variants within the VWF gene display a dominant-negative effect and result in severe VWD. Previous studies have suggested that preventing the assembly of wild-type and mutant heteropolymers using siRNAs may have beneficial effects on VWF phenotypes in vitro. Objectives To study heterozygous dominant-negative variants in vivo, we developed a mouse model of VWD-type 2A and tested two independent strategies to modulate its detrimental effect. Methods The p.P1127_C1948delinsR deletion/variant, causing defective VWF multimerization, was expressed in mice as a model of VWD-type 2A variant. Two corrective strategies were applied. For the first time in a mouse model of VWD, we applied siRNAs selectively inhibiting translation of the mutant transcripts and we combined the VWD-type 2A deletion with the Cys to Arg substitution at position 2773, which is known to prevent dimerization. Results The RNA silencing approach induced a modest but consistent improvement of the VWF multimer profile. However, due to incomplete efficiency, the dominant-negative effect of the original variant could not be completely prevented. In contrast, the DNA approach resulted in increased antigen levels and restoration of a normal multimer profile. Conclusions Our data showed that preventing the detrimental impact of dominant-negative VWF variants by independent molecular mechanisms has beneficial consequences in vivo, in mouse models of dominant VWD.

SLC44A2 deficient mice have a reduced response in stenosis but not in hypercoagulability driven venous thrombosis

BACKGROUND

Genome wide association studies (GWAS) identified SLC44A2 as a novel susceptibility gene for venous thrombosis (VT) and previous work established that SLC44A2 contributed to clot formation upon vascular injury.

OBJECTIVE

To further investigate the role of SLC44A2 in VT by utilizing SLC44A2 deficient mice (Slc44a2-/- ) in two representative disease models.

METHODS

Mice were included in a hypercoagulability model driven by siRNA-mediated hepatic gene silencing of anticoagulants Serpinc1 (antithrombin) and Proc (protein C) and a flow restriction (stenosis) model induced by partial ligation of the inferior vena cava.

RESULTS

In the hypercoagulability model, no effect in onset was observed in Slc44a2-/- animals; however, a drop in plasma fibrinogen and von Willebrand factor coinciding with an increase in blood neutrophils was recorded. In the neutrophil dependent stenosis model after 48 hours, Slc44a2-/- mice had significantly smaller thrombi both in length and weight with less platelet accumulation as a percentage of the total thrombus area. During the initiation of thrombosis at 6 hours post-stenosis, Slc44a2-/- mice also had smaller thrombi both in length and weight, with circulating platelets remaining elevated in Slc44a2-/- animals. Platelet activation and aggregation under both static- and venous and arterial shear conditions were normal for blood from Slc44a2-/- mice.

CONCLUSIONS

These studies corroborate the original GWAS findings and establish a contributing role for SLC44A2 during the initiation of VT, with indications that this may be related to platelet-neutrophil interaction. The precise mechanism however remains elusive and warrants further investigation.

Molecular Genetics of von Willebrand Disease in Korean Patients: Novel Variants and Limited Diagnostic Utility of Multiplex Ligation-Dependent Probe Amplification Analyses

BACKGROUND

von Willebrand disease (VWD), characterized by quantitative or qualitative defects of von Willebrand factor (VWF), is the most common inheritable bleeding disorder. Data regarding the genetic background of VWD in Korean patients is limited. To our knowledge, this is the first comprehensive molecular genetic investigation of Korean patients with VWD.

METHODS

Twenty-two unrelated patients with VWD were recruited from August 2014 to December 2017 (age range 28 months-64 years; male:female ratio 1.2:1). Fifteen patients had type 1, six had type 2, and one had type 3 VWD. Blood samples were collected for coagulation analyses and molecular genetic analyses from each patient. Direct sequencing of all exons, flanking intronic sequences, and the promoter of VWF was performed. In patients without sequence variants, multiplex ligation-dependent probe amplification (MLPA) was performed to detect dosage variants. We adapted the American College of Medical Genetics and Genomics guidelines for variant interpretation and considered variants of uncertain significance, likely pathogenic variants, and pathogenic variants as putative disease-causing variants.

RESULTS

VWF variants were identified in 15 patients (68%): 14 patients with a single heterozygous variant and one patient with two heterozygous variants. The variants consisted of 13 missense variants, one small insertion, and one splicing variant. Four variants were novel: p.S764Efs*16, p.C889R, p.C1130Y, and p.W2193C. MLPA analysis in seven patients without reportable variants revealed no dosage variants.

CONCLUSIONS

This study revealed the spectrum of VWF variants, including novel ones, and limited diagnostic utility of MLPA analyses in Korean patients with VWD.

Unraveling the effect of silent, intronic and missense mutations on VWF splicing: contribution of next generation sequencing in the study of mRNA

Large studies in von Willebrand disease patients, including Spanish and Portuguese registries, led to the identification of >250 different mutations. It is a challenge to determine the pathogenic effect of potential splice site mutations on VWF mRNA. This study aimed to elucidate the true effects of 18 mutations on VWF mRNA processing, investigate the contribution of next-generation sequencing to in vivo mRNA study in von Willebrand disease, and compare the findings with in silico prediction. RNA extracted from patient platelets and leukocytes was amplified by RT-PCR and sequenced using Sanger and next generation sequencing techniques. Eight mutations affected VWF splicing: c.1533+1G>A, c.5664+2T>C and c.546G>A (p.=) prompted exon skipping; c.3223-7_3236dup and c.7082-2A>G resulted in activation of cryptic sites; c.3379+1G>A and c.7437G>A) demonstrated both molecular pathogenic mechanisms simultaneously; and the p.Cys370Tyr missense mutation generated two aberrant transcripts. Of note, the complete effect of three mutations was provided by next generation sequencing alone because of low expression of the aberrant transcripts. In the remaining 10 mutations, no effect was elucidated in the experiments. However, the differential findings obtained in platelets and leukocytes provided substantial evidence that four of these would have an effect on VWF levels. In this first report using next generation sequencing technology to unravel the effects of VWF mutations on splicing, the technique yielded valuable information. Our data bring to light the importance of studying the effect of synonymous and missense mutations on VWF splicing to improve the current knowledge of the molecular mechanisms behind von Willebrand disease. clinicaltrials.gov identifier:02869074.

Characterization of hemostasis in mice lacking the novel thrombosis susceptibility gene Slc44a2

INTRODUCTION

Recent genome wide association studies (GWAS) identified a novel susceptibility locus for thrombosis, harbouring the SLC44A2 gene which encodes the Solute Carrier Family 44 Member 2 protein (SLC44A2). Thus far, SLC44A2 has not been studied in the context of thrombosis, and may be a unique contributor to thrombotic disease. Here we utilize mice lacking SLC44A2 (Slc44a2^-/-) to evaluate a possible role of SLC44A2 in hemostasis.

METHODS

Slc44a2^-/- mice were evaluated in key aspects of normal hemostasis including a challenge of vascular damage by applying laser induced injury to the cremaster muscle arteriole.

RESULTS

Slc44a2^-/- mice had comparable levels of thrombin generation and gene expression of coagulation related genes, as compared to littermate wild type controls. Lower levels of circulating plasma Von Willebrand factor (VWF) were measured in Slc44a2^-/- mice, while no difference in VWF multimerization or vascular localization was detected. Upon in vivo laser injury of the cremaster arterioles, we detected an impairment of clot formation for Slc44a2^-/- mice.

CONCLUSIONS

Although mice lacking SLC44A2 are normal for several hemostasis parameters, we do observe a reduction of plasma VWF levels and an altered response upon vascular damage, which suggests that SLC44A2 contributes to hemostasis upon injury. These findings are in line with the reported GWAS data and support further research on SLC44A2 in thrombosis.

Correction of a dominant-negative von Willebrand factor multimerization defect by small interfering RNA-mediated allele-specific inhibition of mutant von Willebrand factor

Essentials Substitution therapy for von Willebrand (VW) disease leaves mutant VW factor (VWF) unhindered. Presence of mutant VWF may negatively affect phenotypes despite treatment. Inhibition of VWF by allele-specific siRNAs targeting single-nucleotide polymorphisms is effective. Allele-specific inhibition of VWF p.Cys2773Ser improves multimerization.

SUMMARY

Background Treatment of the bleeding disorder von Willebrand disease (VWD) focuses on increasing von Willebrand factor (VWF) levels by administration of desmopressin or VWF-containing concentrates. Both therapies leave the production of mutant VWF unhindered, which may have additional consequences, such as thrombocytopenia in patients with VWD type 2B, competition between mutant and normal VWF for platelet receptors, and the potential development of intestinal angiodysplasia. Most cases of VWD are caused by dominant-negative mutations in VWF, and we hypothesize that diminishing expression of mutant VWF positively affects VWD phenotypes. Objectives To investigate allele-specific inhibition of VWF by applying small interfering RNAs (siRNAs) targeting common single-nucleotide polymorphisms (SNPs) in VWF. This approach allows allele-specific knockdown irrespective of the mutations causing VWD. Methods Four SNPs with a high predicted heterozygosity within VWF were selected, and siRNAs were designed against both alleles of the four SNPs. siRNA efficiency, allele specificity and siRNA-mediated phenotypic improvements were determined in VWF-expressing HEK293 cells. Results Twelve siRNAs were able to efficiently inhibit single VWF alleles in HEK293 cells that stably produce VWF. Transient cotransfections of these siRNAs with two VWF alleles resulted in a clear preference for the targeted allele over the untargeted allele for 11 siRNAs. We also demonstrated siRNA-mediated phenotypic improvement of the VWF multimerization pattern of the VWD type 2A mutation VWF p.Cys2773Ser. Conclusions Allele-specific siRNAs are able to distinguish VWF alleles on the basis of one nucleotide variation, and are able to improve a severe multimerization defect caused by VWF p.Cys2773Ser. This holds promise for the therapeutic application of allele-specific siRNAs in dominant-negative VWD.

Von Willebrand disease mutation spectrum and associated mutation mechanisms

Von Willebrand disease (VWD) is a bleeding disorder that is mainly caused by mutations in the multimeric protein von Willebrand factor (VWF). These mutations may lead to deficiencies in plasma VWF or dysfunctional VWF. VWF is a heterogeneous protein and over the past three decades, hundreds of VWF mutations have been identified. In this review we have organized all reported mutations, spanning a timeline from the late eighties until early 2017. This resulted in an overview of 750 unique mutations that are divided over the VWD types 1, 2A, 2B, 2M, 2N and 3. For many of these mutations the disease-causing effects have been characterized in vitro through expression studies, ex vivo by analysis of patient-derived endothelial cells, as well as in animal or (bio)physical models. Here we describe the mechanisms associated with the VWF mutations per VWD type.

Angiogenic characteristics of blood outgrowth endothelial cells from patients with von Willebrand disease

BACKGROUND

Endothelial von Willebrand factor (VWF) inhibits angiogenesis. Accordingly, blood outgrowth endothelial cells (BOECs) isolated from von Willebrand disease (VWD) patients showed enhanced in vitro angiogenesis when compared with healthy control BOECs. Characterization of the angiogenic response of VWD BOECs is limited and differences between the different types of VWD have not been investigated in detail.

OBJECTIVES

The aim of this study was to further explore the potential pathogenic effect of VWF mutations on angiogenesis.

METHODS

BOECs were isolated from four healthy individuals, 10 patients with VWD and one heterozygous carrier of a type 2N mutation. Cell migration and tube formation were measured.

RESULTS

Migration velocity and total tube formation were similar between VWD patients and controls in general. BOECs from the type 3 VWD patient and one type 2B patient showed increased migratory velocity and tube formation compared with BOECs from other patients and healthy controls. Directional migration was impaired in eight out of 10 VWD BOECs and the ability to form tubes was limited to early passage numbers, but not for BOECs from healthy controls.

CONCLUSION

BOECs can be a useful tool for ex vivo assessment of endothelial cell function in patients with different types of VWD, but possible limitations, such as early loss of angiogenic capacity, should be recognized. BOECs from most VWD patients consistently showed impairment in the directionality of migration. This is the first report on angiogenic properties of a type 3 VWD BOEC, which showed increased in vitro angiogenesis.

No evidence for a direct effect of von Willebrand factor's ABH blood group antigens on von Willebrand factor clearance

BACKGROUND

One of the major determinants of von Willebrand factor (VWF) plasma levels is ABO blood group status, and individuals with blood group O have ~ 25% lower plasma levels. The exact mechanism behind this relationship remains unknown, although effects on clearance have been postulated.

OBJECTIVES

To determine whether clearance of VWF is directly dependent on the presence of ABH antigens on VWF.

METHODS

Three type 3 von Willebrand disease (VWD) patients were infused with Haemate-P, and the relative loading of VWF with ABH antigens at different time points was measured. VWF-deficient mice were injected with purified plasma-derived human VWF obtained from donors with either blood group A, blood group B, or blood group O.

RESULTS

In mice, we found no difference in clearance rate between plasma-derived blood group A, blood group B and blood group O VWF. Faster clearance of the blood group O VWF present in Haemate-P infused in type 3 VWD patients would have resulted in a relative increase in the loading of VWF with A and B antigens over time. However, we observed a two-fold decrease in the loading with A and B antigens in two out of three patients, and stable loading in the third patient.

CONCLUSION

There is no direct effect of ABH antigens on VWF in VWF clearance. We demonstrate that, in a direct comparison within one individual, blood group O VWF is not cleared faster than blood group A or blood group B VWF. Clearance differences between blood group O and non-blood group O individuals may therefore be related to the blood group status of the individual rather than the ABH antigen loading on VWF itself.

Storage and secretion of naturally occurring von Willebrand factor A domain variants

Von Willebrand disease (VWD) is a bleeding disorder characterized by reduced plasma von Willebrand factor (VWF) levels or functionally abnormal VWF. Low VWF plasma levels in VWD patients are the result of mutations in the VWF gene that lead to decreased synthesis, impaired secretion, increased clearance or a combination thereof. However, expression studies of variants located in the A domains of VWF are limited. We therefore characterized the biosynthesis of VWF mutations, located in the VWF A1-A3 domains, that were found in families diagnosed with VWD. Human Embryonic Kidney 293 (HEK293) cells were transiently transfected with plasmids encoding full-length wild-type VWF or mutant VWF. Six mutations in the A1-A3 domains were expressed. We found that all mutants, except one, showed impaired formation of elongated pseudo-Weibel-Palade bodies (WPB). In addition, two mutations also showed reduced numbers of pseudo-WPB, even in the heterozygous state, and increased endoplasmic reticulum retention, which is in accordance with the impaired regulated secretion seen in patients. Regulated secretion upon stimulation of transfected cells reproduced the in vivo situation, indicating that HEK293 cells expressing VWF variants found in patients with VWD can be used to properly assess defects in regulated secretion.

Pseudohemophilia of Erik von Willebrand caused by homozygous one nucleotide deletion in exon 18 of the VW-factor gene

The original description of a novel severe bleeding disorder as “Hereditary Pseudohemophilia” by Erik von Willebrand can currently be labelled as von Willebrand disease (VWD) type 3. VWD type 3 is autosomal recessive caused by homozygous or double heterozygous null mutations in the von Willebrand factor (VWF) gene and typically characterized by prolonged bleeding time and APTT, FVIII: C levels below 2%, undetectable VWF: Ag, VWF: RCo and VWF: CB and absence of ristocetin induced platelet aggregation (RIPA). Autosomal recessive von Willebrand disease type 3 VWD with virtual complete VWF deficiency are homozygous or compound heterozygous for two null alleles (gene deletions, stop codons, frame shift mutations, splice site mutations, and absence of mRNA). Reports on severe recessive VWD compound heterozygous for a null allele and a missense mutation and homozygous or double heterozygous for missense mutations are associated with very low but measurable FVIII and VWF: Ag and should be reclassified as severe recessive type 1 VWD. Homozygous missense or compound missense/null mutations related to recessive severe type 1 VWD have been indentified in the VWF prosequence D1 and D2 domains, the D4, B1-3, C1-2 domains, and only a very few in the dimmerization site (D3 domain). The detection of even tiny amounts of VWF: Ag after desmopressin acetate (DDAVP) or in hidden sites like platelets allows the differentiation between patients with VWD type 3 and homozygous or double heterozygous recessive severe type 1. Carriers of a null allele related to VWD type 3 or a missense mutation related with severe recessive type 1 VWD may present with mild VWD with low penetrance of bleeding in particular when associated with blood group O. Heterozygous obligatory carriers (OC) of a null mutation or a missense mutation related to recessive VWD type 3 or severe type 1 both present with asymptomatic or mild VWD type 1 in particular when associated with blood group O. The response to DDAVP of OC of either a nonsense or a missense mutation appears to be abnormal and diagnostic with a 3-times higher response of FVIII: C as compared to VWF: Ag. In contrast, the responses to DDAVP of FVIII: C and VWF: Ag are equally good in individuals with low VWF levels related to blood group O and a normal VWF gene and protein (pseudo-VWD). These observations are completely in line with and extend the original observations of von Willebrand in a large family with VWD type 3 and asymptomatic or mild true type 1 VWD in OC.

Clearance of von Willebrand factor

Quantitative deficiencies in von Willebrand factor (VWF) are associated with abnormal hemostasis that can manifest in bleeding or thrombotic complications. Consequently, many studies have endeavored to elucidate the mechanisms underlying the regulation of VWF plasma levels. This review focuses on the role of VWF clearance pathways. A summary of recent developments are provided, including results from genetic studies, the relationship between glycosylation and VWF clearance, the contribution of increased VWF clearance to the pathogenesis of von Willebrand disease and the identification of VWF clearance receptors. These different studies converge in their conclusion that VWF clearance is a complex phenomenon that involves multiple mechanisms. Deciphering how such different mechanisms coordinate their role in this process is but one of the remaining challenges. Nevertheless, a better insight into the complex clearance pathways of VWF may help us to better understand the clinical implications of aberrant clearance in the pathogenesis of von Willebrand disease and perhaps other disorders as well as aid in developing alternative therapeutic approaches.

Analysis of the storage and secretion of von Willebrand factor in blood outgrowth endothelial cells derived from patients with von Willebrand disease

Patients with von Willebrand disease (VWD) are often heterozygous for a missense mutation in the von Willebrand factor (VWF) gene. Investigating the pathogenic features of VWF mutations in cells directly derived from patients has been challenging. Here, we have used blood outgrowth endothelial cells (BOECs) isolated from human peripheral blood to analyze the storage and secretion of VWF. BOECs showed full endothelial characteristics and responded to Weibel-Palade body (WPB) secretagogues except desmopressin. We examined BOECs derived from a single subject heterozygous for a type 2N mutation (p.Arg854Gln) and from 4 patients with type 1 VWD who were, respectively, heterozygous for p.Ser1285Pro, p.Leu1307Pro, p.Tyr1584Cys, and p.Cys2693Tyr. Compared with normal BOECs, BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr showed morphologically abnormal WPB and retention of VWF in the endoplasmic reticulum, whereas BOECs heterozygous for p.Arg854Gln or p.Tyr1584Cys showed normal WPB. The agonist-induced exocytosis of WPB from BOECs and formation of VWF strings on BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr, but not for p.Arg854Gln or p.Tyr1584Cys, were reduced. In conclusion, VWD phenotype can be recapitulated in BOECs, and thus BOECs provide a feasible bona fide cell model to study the pathogenic effects of VWF mutations.

Formation of platelet-binding von Willebrand factor strings on non-endothelial cells

BACKGROUND AND OBJECTIVE

Von Willebrand factor (VWF) forms strings on activated vascular endothelial cells that recruit platelets and initiate clot formation. Alterations in VWF strings may disturb hemostasis. This study was aimed at developing a flexible model system for structure-function studies of VWF strings.

METHODS

VWF strings were generated by inducing exocytosis of pseudo-Weibel-Palade bodies from VWF-transfected HEK293 cells, and the properties of these strings under static conditions and under flow were characterized.

RESULTS

Upon exocytosis, VWF unfurled into strings several hundred micrometers in length. These strings could form bundles and networks, and bound platelets under flow, resembling authentic endothelial VWF strings. Anchorage of the platelet-decorated VWF strings was independent of P-selectin and integrin α(V) β(3). Translocation of platelets along the strings, elongation and fragmentation of the strings frequently occurred under flow. Furthermore, VWF variants with the p.Tyr87Ser and p.Cys2773Ser mutations, which are defective in multimer assembly, did not give rise to VWF strings. Also, insertion of the green fluorescent protein into VWF inhibited string formation.

CONCLUSIONS

HEK293 cells provide a flexible and useful model system for the study of VWF string formation. Our results suggest that structural changes in VWF may modulate string formation and function, and contribute to hemostatic disorders.

Molecular characterization, recombinant protein expression, and mRNA analysis of type 3 von Willebrand disease: Studies of an Italian cohort of 10 patients

Type 3 von Willebrand disease (VWD3) is characterized by unmeasurable von Willebrand factor (VWF) levels in plasma and platelets and severe but variable hemorrhagic symptoms. To identify and characterize the causal mutations, we screened 10 Italian patients with VWD3 by several techniques including Multiplex Ligation-dependent Probe Amplification to identify large insertions and deletions, High Resolution Melting and PCR coupled with Sanger sequencing. Fourteen different mutations scattered throughout the VWF gene were identified, 10 of which were novel. As expected, most of these mutations caused null alleles: five were deletions (del exons 1-3, del exon 17, c.2157delA, c.2269delCT, and c.3940delG), three nonsense (p.Q1526X, p.E1549X, and p.C2448X) and three potential splice-site mutations (c.658-2A>G, c.7729+7C>T, and c.8155+1G>T). Three candidate missense mutations (p.C2184S, p.C2212R, and p.C2325S) were also identified. Missense mutations and the putative splice-site defects were confirmed to be disease related by in vitro expression studies and mRNA analysis. None of these patients have developed alloantibodies against VWF. This study extends our previous finding that most of the mutations that we identified in VWD3 patients arise independently and are scattered throughout the entire VWF gene.

Reduced von Willebrand factor secretion is associated with loss of Weibel-Palade body formation

BACKGROUND

von Willebrand disease (VWD) is caused by mutations in von Willebrand factor (VWF) that have different pathophysiologic effect in causing low plasma VWF levels. Type 1 VWD includes quantitative plasma VWF deficiency with normal VWF structure and function.

OBJECTIVES

We report three novel type 1 VWF mutations (A1716P, C2190Y and R2663C) located in different VWF domains that are associated with reduced secretion and reduced formation of elongated Weibel-Palade body (WPB)-like granules.

METHODS

Transient expression of recombinant mutant full-length VWF in 293 EBNA cells was performed and secretion, collagen binding and GpIb binding assessed in comparison with wild-type VWF. Expression was also examined in HEK293 cells that form WPB-like granules when transfected with wild-type VWF.

RESULTS

Laboratory results and multimer analysis of plasma VWF was compatible with type 1 VWD. Expression experiments demonstrated slightly reduced VWF synthesis and drastically impaired secretion upon homozygous expression. In HEK293 cells, homozygous expression of A1716P and C2190Y VWF variants failed to form elongated WPB-like granules, while R2663C was capable of WPB-like granules. Heterozygous expression of VWF variants had a negative impact on wild-type VWF with a reduction in elongated WPB-like granules in co-transfected cells.

CONCLUSIONS

Our results demonstrate that homozygous and heterozygous quantitative VWF deficiency caused by missense VWF mutations in different VWF domains can be associated with inability to form endothelial WPB-like granules.

Intersection of mechanisms of type 2A VWD through defects in VWF multimerization, secretion, ADAMTS-13 susceptibility, and regulated storage

Type 2A VWD is characterized by the absence of large VWF multimers and decreased platelet-binding function. Historically, type 2A variants are subdivided into group 1, which have impaired assembly and secretion of VWF multimers, or group 2, which have normal secretion of VWF multimers and increased ADAMTS13 proteolysis. Type 2A VWD patients recruited through the T. S. Zimmerman Program for the Molecular and Clinical Biology of VWD study were characterized phenotypically and potential mutations identified in the VWF D2, D3, A1, and A2 domains. We examined type 2A variants and their interaction with WT-VWF through expression studies. We assessed secretion/intracellular retention, multimerization, regulated storage, and ADAMTS13 proteolysis. Whereas some variants fit into the traditional group 1 or 2 categories, others did not fall clearly into either category. We determined that loss of Weibel-Palade body formation is associated with markedly reduced secretion. Mutations involving cysteines were likely to cause abnormalities in multimer structure but not necessarily secretion. When coexpressed with wild-type VWF, type 2A variants negatively affected one or more mechanisms important for normal VWF processing. Type 2A VWD appears to result from a complex intersection of mechanisms that include: (1) intracellular retention or degradation of VWF, (2) defective multimerization, (3) loss of regulated storage, and (4) increased proteolysis by ADAMTS13.

Biogenesis of Weibel-Palade bodies in von Willebrand's disease variants with impaired von Willebrand factor intrachain or interchain disulfide bond formation

BACKGROUND

Mutations of cysteine residues in von Willebrand factor are known to reduce the storage and secretion of this factor, thus leading to reduced antigen levels. However, one cysteine mutation, p.Cys2773Ser, has been found in patients with type 2A(IID) von Willebrand's disease who have normal plasma levels of von Willebrand factor. We hypothesize that disruption of either intra- or interchain disulfide bonds by cysteine mutations in von Willebrand factor has different effects on the biogenesis of Weibel-Palade bodies.

DESIGN AND METHODS

The effect of specific cysteine mutations that either disrupt intrachain (p.Cys1130Phe and p.Cys2671Tyr) or interchain (p.Cys2773Ser) disulfide bonds on storage and secretion of von Willebrand factor was studied by transient transfection of human embryonic kidney cell line 293. Upon expression of von Willebrand factor these cells formed endothelial Weibel-Palade body-like organelles called pseudo-Weibel-Palade bodies. Storage of von Willebrand factor was analyzed with both confocal immunofluorescence and electron microscopy. Regulated secretion of von Willebrand factor was induced by phorbol 12-myristate 13-acetate.

RESULTS

p.Cys1130Phe and p.Cys2671Tyr reduced the storage of von Willebrand factor into pseudo-Weibel-Palade bodies with notable retention of von Willebrand factor in the endoplasmic reticulum, whereas p.Cys2773Ser-von Willebrand factor was stored normally. As expected, wild-type von Willebrand factor formed proteinaceous tubules that were seen under electron microscopy as longitudinal striations in pseudo-Weibel-Palade bodies. p.Cys2773Ser caused severe defects in von Willebrand factor multimerization but the factor formed normal tubules. Furthermore, the basal and regulated secretion of von Willebrand factor was drastically impaired by p.Cys1130Phe and p.Cys2671Tyr, but not by p.Cys2773Ser.

CONCLUSIONS

We postulate that natural mutations of cysteines involved in the formation of interchain disulfide bonds do not affect either the storage in Weibel-Palade bodies or secretion of von Willebrand factor, whereas mutations of cysteines forming intrachain disulfide bonds lead to reduced von Willebrand factor storage and secretion because the von Willebrand factor is retained in the endoplasmic reticulum.

Intracellular storage and regulated secretion of von Willebrand factor in quantitative von Willebrand disease

Several missense mutations in the von Willebrand Factor (VWF) gene of von Willebrand disease (VWD) patients have been shown to cause impaired constitutive secretion and intracellular retention of VWF. However, the effects of those mutations on the intracellular storage in Weibel-Palade bodies (WPBs) of endothelial cells and regulated secretion of VWF remain unknown. We demonstrate, by expression of quantitative VWF mutants in HEK293 cells, that four missense mutations in the D3 and CK-domain of VWF diminished the storage in pseudo-WPBs, and led to retention of VWF within the endoplasmic reticulum (ER). Immunofluorescence and electron microscopy data showed that the pseudo-WPBs formed by missense mutant C1060Y are indistinguishable from those formed by normal VWF. C1149R, C2739Y, and C2754W formed relatively few pseudo-WPBs, which were often short and sometimes round rather than cigar-shaped. The regulated secretion of VWF was impaired slightly for C1060Y but severely for C1149R, C2739Y, and C2754W. Upon co-transfection with wild-type VWF, both intracellular storage and regulated secretion of all mutants were (partly) corrected. In conclusion, defects in the intracellular storage and regulated secretion of VWF following ER retention may be a common mechanism underlying VWD with a quantitative deficiency of VWF.

C1272F: a novel type 2A von Willebrand's disease mutation in A1 domain; its clinical significance

Most mutations identified in 2A VWD patients are localized in the A2 domain, although missense substitutions have also been recognized in the A1 domain. We describe a novel heterozygous missense mutation in the A1 domain of VWF gene responsible for type 2A phenotype. Analysis of the complete exon 28 was carried out in a patient and his mother with life-long histories of moderate to severe bleeding and laboratory data of type 2A VWD. The analysis of exon 28 of VWF gene showed a 3815 G → T transversion resulting in C1272F mutation. It is probably associated with a group I mechanism according to patients' clinical symptoms, and, in the case of the propositus, the lack of clinical response to treatment with desmopressin. The mutation was not found in 100 normal alleles. This substitution affected the normal S-S bound between C1272 and C1458, which is involved in A1 loop structure, altering the normal multimerization and function of VWF. The VWFpp/VWF:Ag ratio in the propositus and his mother was >3, suggesting a shortened survival of VWF. We believe it is important to report the complete clinical phenotype corresponding to the new mutation to increase the knowledge in the clinical field.

The dominant-negative von Willebrand factor gene deletion p.P1127_C1948delinsR: molecular mechanism and modulation

Understanding molecular mechanisms in the dominant inheritance of von Willebrand disease would improve our knowledge of pathophysiologic processes underlying its prevalence. Cellular models of severe type 2 von Willebrand disease, caused by a heterozygous deletion in the von Willebrand factor (VWF) gene, were produced to investigate the altered biosynthesis. Coexpression of the wild-type and in-frame deleted (p.P1127_C1948delinsR) VWF forms impaired protein secretion, high molecular weight multimer formation and function (VWF collagen-binding 1.9% ± 0.5% of wild-type), which mimicked the patient's phenotype. mRNA, protein, and cellular studies delineated the highly efficient dominant-negative mechanism, based on the key role of heterodimers as multimer terminators. The altered VWF, synthesized in large amounts with the correctly encoded "cysteine knot" domain, formed heterodimers and heterotetramers with wild-type VWF, in addition to deleted homodimers. Impaired multimerization was associated with reduced amounts of VWF in late endosomes. Correction of the dominant-negative effect was explored by siRNAs targeting the mRNA breakpoint, which selectively inhibited the in-frame deleted VWF expression. Although the small amount of the deleted protein synthesized after inhibition still exerted dominant, even though weakened, negative effects, the siRNA treatment restored secretion of large multimers with improved function (VWF collagen-binding 28.0% ± 3.3% of wild-type).

A cluster of mutations in the D3 domain of von Willebrand factor correlates with a distinct subgroup of von Willebrand disease: type 2A/IIE

Among the different phenotypes of von Willebrand disease (VWD) type 2A, we identified a particular subgroup with a high frequency of 29%, characterized by a relative decrease of large von Willebrand factor (VWF) multimers and decreased A Disintegrin And Metalloproteinase with ThromboSpondin type 1 motifs, member 13 (ADAMTS13)-mediated proteolysis previously described in a single family as VWD type IIE (VWD2A/IIE). Phenotype and genotype of 57 patients from 38 unrelated families displaying a particular multimer pattern resembling the original VWD2A/IIE were studied. Pathogenicity of candidate mutations was confirmed by expression studies and phenotypic characterization of recombinant mutants. Specific mutations were identified in all patients. Twenty-two different mutations, most of them affecting cysteine residues, 17 of them being novel, are clustering mainly in the VWF D3 domain and correlate with the VWD2A/IIE phenotype. An intracellular retention of most mutants and/or a defect of multimerization seem to be the main pathogenic molecular mechanisms. ADAMTS13 proteolysis of mutant VWF was not different from wild-type VWF in a static assay, suggesting that reduced in vivo proteolysis is not an intrinsic property of mutant VWF. Our study identified a distinct VWD subtype with a common molecular background which contributes significantly to the heterogeneous spectrum of VWD.

Laboratory testing for von Willebrand disease: toward a mechanism-based classification

The heterogeneity of von Willebrand disease reflects the varied roles of von Willebrand factor in coagulation. Significant challenges remain in the detection, classification, and determination of bleeding risk in disorders related to von Willebrand factor. A clearer understanding of the specific disease mechanisms is essential to the development of improved methods for prognosis and management in this and other conditions with abnormalities of the von Willebrand factor system.

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

BACKGROUND

Mutation C1149R in the von Willebrand factor (VWF) gene has been thought to cause autosomal dominant severe type 1 von Willebrand disease (VWD).

DESIGN AND METHODS

Eight patients from three unrelated families with this mutation were included in the present study who had distinct VWF abnormalities, not described in earlier studies.

RESULTS

The patients showed notably low levels of VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), VWF collagen binding (VWF:CB), and a reduced ristocetin-induced platelet aggregation (RIPA). VWF:RCo/VWF:Ag and VWF:CB/VWF:Ag ratios were lower than 0.7. At basal conditions, all the VWF multimers were decreased in plasma, with a clearly lower relative proportion of the high molecular weight VWF multimers (HMWM). In high-resolution agarose gels, a large decrease in the relative proportions of the satellite bands was seen. The patients had a brief good response to desmopressin (DDAVP) administration, but the released VWF half-life was shorter than normal, indicating an accelerated clearance of their VWF. Platelet VWF was abnormal.

CONCLUSIONS

We conclude from the results obtained in these patients for plasma phenotypic data that this mutation should be classified as a VWD type 2A (IIE). DDAVP therapy may be somewhat helpful for this mutation, at least for mild to moderate bleeding. These data provide evidence that for VWD classification factors other than basal VWF, such as DDAVP response and platelet VWF, should be considered.

Type 1 von Willebrand disease: application of emerging data to clinical practice

There has been much recent data published on type 1 von Willebrand disease (VWD) predominantly from three multi-centre cohort studies. These data have influenced a revision of the classification of type 1 VWD and have important implications for the management of this disorder. Patients with low von Willebrand factor (VWF) levels tend to have VWF mutations and VWD is transmitted predictably within families. In patients with VWF levels close to the lower end of the normal range, candidate mutations are found less often, ABO blood group is a more important factor and the disease has variable heritability within families. The importance of bleeding symptoms, in addition to VWF levels, in the diagnosis of type 1 VWD has been highlighted.

Expression studies of missense mutations p.D141Y, p.C275S located in the propeptide of von Willebrand factor in patients with type 3 von Willebrand disease

Missense mutations are not considered a common cause of type 3 von Willebrand's disease (VWD), the most severe defect of von Willebrand factor (VWF) characterized by undetectable levels of this protein in plasma and platelets. Nevertheless, several missense mutations have been identified in these patients. In this study, we report the cases of two Italian patients with type 3 VWD, both compound heterozygotes for different missense mutations and null alleles, p.D141Y/c.2016_2019del and p.C275S/p.W222X. We performed in vitro expression studies of the candidate missense mutations, both located in the D1 domain of VWF propeptide, to confirm their link with the disease and to understand the mechanisms of type 3 VWD responsible in these patients. Mutant and wild-type (WT) expression vectors were used for transient transfection and co-transfection studies in COS-7 cells. Single construct transfections of both missense mutations showed a strongly reduced but detectable secretion of recombinant (r)VWFs (approximately 15% of WT), with essentially only dimers being visualized on multimeric analysis. As expected, expression of a single construct of either mutation with the WT, showed mildly reduced secretion (approximately 40% of WT) and a full set of multimers. These expression studies indicate that the two amino acids D141 and C275 are key residues in the tertiary structure of the VWF propeptide. Their replacement with a tyrosine and a serine, respectively, might compromise propeptide folding, affecting both its intracellular survival and its capacity to mediate multimerization. Co-expression of hybrid rVWFs confirmed the recessive inheritance pattern of these missense mutations.

Molecular study of VWF gene from Mexican Mestizo patients with von Willebrand disease, and the finding of three new mutations

To investigate the origin of von Willebrand disease in Mexican Mestizo population, we analyzed exons 18, 19, 20, 28, 45, and 52 of the VWF gene from 34 Mexican Mestizo index cases, 28 of them affected but not related, using DNA amplification by polymerase chain reaction and direct sequencing. We found three novel mutations: E1447Q in one patient with type 1; P2781S in one patient with type 2M; and P812L in another type 1/2N patient. These mutations were not found in 100 normal alleles. Moreover, we found other mutations previously reported in the literature; one of them (G1609R) was the most frequent (6/28) in patients with VWD type 2A. This is the first molecular study in a Mexican group that has a particular mixture of Indigenous, Caucasian, and African genes.

Type 1 von Willebrand disease

Since its first description in 1926, the precise nature and indeed significance of von Willebrand factor (VWD) in the area of human bleeding has been unsure and often controversial. The recognition of VWD as a distinct entity in blood and the cloning of the von Willebrand factor (VWF) gene in the 1980s encouraged both phenotypic and genotypic studies, culminating in 1994 with the recognition, by the VWF subcommittee of the Scientific and Standardization Committee (SSC) of International Society of Thrombosis and Haemostasy (ISTH), of three types of VWD, characterized by severe plasma VWF deficiency (type 3), functionally deficient plasma VWF (type 2) and reduced (below normal) levels of plasma VWF, which is functionally essentially normal (type 1; 70% of all cases). Since then, whereas gene analysis has recognized VWF gene (VWF) mutations in most individuals with type 3 and type 2 disease, the latter mutations correlating well with recognized functional domains within the VWF protein, few mutations have been reported in cases with type 1 VWD. This led to speculation that other factors, particularly ABO blood group, may be primarily responsible for the majority of such patients, perhaps combined with a generic bleeding tendency throughout the normal population. Recent large studies in Europe and Canada have considerably clarified this situation, revealing that the majority of type 1 VWD is associated with mutations within VWF. The role of these mutations in the aetiology of the disease opens up new approaches to the study of the diagnosis and treatment of the condition. Conversely, the lack of a change in the VWF gene in many recruited families will lead to enhanced efforts to identify non-VWF gene causes both at the genetic and epigenetic level.

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.

Homozygous C2362F von Willebrand factor induces intracellular retention of mutant von Willebrand factor resulting in autosomal recessive severe von Willebrand disease

The missense mutation of cysteine 2362 to a phenylalanine in von Willebrand factor (VWF) has been detected in several Italian families with autosomal recessive, severe von Willebrand disease. We investigated how this amino acid change in VWF may lead to a predominantly quantitative defect. This mutation was studied in vitro by transient expression of the full-length mutant VWF-C2362F protein and in vivo by analysis of plasma VWF after infusion of 1-deamino-8-d-arginine vasopressin (DDAVP) in a patient homozygous for this mutation. Single transfections of pSVHVWF-C2362F and co-transfections of mutant and wild-type constructs resulted in 8% and 50% VWF antigen, respectively, in conditioned medium. These reduced levels are in accordance with observations in homozygous and heterozygous carriers of the mutation. In addition, VWF-C2362F was retained intracellularly. Similar results were obtained for C2362F and C2362A. After infusion of DDAVP in a homozygous patient, a twofold decrease in half-life of plasma VWF-C2362F was observed. This was not explained by increased susceptibility of recombinant VWF-C2362F to ADAMTS13. It was concluded that VWF-C2362F causes reduced VWF plasma levels due to impaired secretion and intracellular retention. Furthermore, it is the loss of cysteine 2362 rather than the introduction of the bulky amino acid side chain that causes these effects.

Correction of a murine model of von Willebrand disease by gene transfer

von Willebrand disease (VWD), the most common inherited bleeding disorder in the U.S. population, is caused by defects in the expression and processing of von Willebrand factor (VWF), a blood glycoprotein required for normal hemostasis that mediates the adhesion of platelets to sites of vascular damage by binding to specific platelet glycoproteins and to constituents of exposed connective tissue. To assess whether VWF deficiency can be corrected by gene transfer, a plasmid expressing the intact 8.4-kb murine VWF coding sequence, directed by the cyto-megalovirus immediate/early promoter/enhancer, was delivered through hydrodynamic tail vein injection into VWF knockout mice (VWF(-/-)) that exhibit defects in hemostasis, including highly prolonged bleeding time and spontaneous bleeding events, closely mimicking severe human VWD. VWF antigen levels in plasma from animals receiving VWF cDNA, but not control animals, revealed normalized levels of circulating VWF that persisted for at least 1 week after injection. Western blot analysis of plasma from animals receiving VWF cDNA, but not control animals, revealed high molecular-weight multimers with patterns similar to those observed in wild-type mice. Reverse transcription-polymerase chain reaction (RT-PCR) on RNA isolated from the livers of animals receiving VWF cDNA, but not control animals, demonstrated that VWF was expressed in the liver, and immunohistochemical analysis of the livers of treated VWF(-/-) mice revealed VWF-specific staining throughout the liver parenchyma but not in endothelial cells. Plasma from treated VWF(-/-) mice, but not control VWF(-/-) mice, supported the hypothesis that murine platelets aggregate in the presence of botrocetin. Although levels of circulating factor VIII in untreated VWF(-/-) mice were less than 10% those in wild-type mice, levels of factor VIII in VWF(-/-) animals treated with VWF cDNA, but not in control animals, were normalized to values in wild-type mice, indicating the restoration of factor VIII carrier function for VWF in treated mice that persisted for at least 1 week at higher doses of VWF cDNA. Most important, bleeding time was normalized by 48 hours after the delivery of VWF cDNA, but not by the control plasmid. These data suggest that with the use of gene transfer of VWF cDNA, VWF protein can be expressed, processed, and secreted in a physiologically active form; thus, it may be possible to correct VWD using gene transfer.

Mutations C1157F and C1234W of von Willebrand factor cause intracellular retention with defective multimerization and secretion

The D3 domain of von Willebrand factor (VWF) is involved in the multimerization process of the protein through the formation of disulfide bridges. We identified heterozygous substitutions, C1157F and C1234W, in the VWF D3 domain in two unrelated families with unclassified and type 2A von Willebrand disease, respectively. VWF was characterized by a low plasmatic level, an abnormal binding to platelet GPIb and a high capacity of secretion from endothelial cells following DDAVP infusion. Using site-directed mutagenesis and expression in mammalian cells, we have investigated the impact of these mutations upon the multimerization, secretion and storage of VWF. Using COS-7 cells both mutated recombinant VWF (rVWF) displayed only lower molecular weight multimers. Pulse-chase analysis and endoglycosidase H digestion experiments showed the intracellular retention of mutated rVWF in pre-Golgi compartments. Study of hybrid rVWF obtained with a constant amount of wild-type (WT) DNA and increasing proportions of mutated plasmids established that both substitutions reduced the release of WT VWF in a dose-dependent manner and failed to form high molecular weight multimers. Using transfected AtT-20 stable cell lines, we observed similar granular storage of the two mutants and WT rVWF. Our data suggest that cysteines 1157 and 1234 play a crucial role in the early step of the folding of the molecule required for a normal transport pathway, maturation and constitutive secretion. In contrast, their substitution does not prevent the storage and inducible secretion of VWF.

Cysteine-mutations in von Willebrand factor associated with increased clearance

BACKGROUND

von Willebrand disease (VWD) is a bleeding disorder caused by the decrease of functional von Willebrand factor (VWF). Low levels of VWF can result from decreased synthesis, impaired secretion, increased clearance or combinations thereof. Several mutations lead to impaired synthesis or secretion of VWF, however, little is known about the survival of VWF in the circulation.

OBJECTIVES

To evaluate the effect of several VWF mutations on VWF clearance.

PATIENTS/METHODS

The effect of three cysteine-mutations (C1130F, C1149R or C2671Y) on the in vivo survival of VWF was studied in patients carrying these mutations and in a VWF-deficient mice model.

RESULTS

In patients carrying these mutations, we observed increased propeptide/mature VWF ratios and rapid disappearance of VWF from the circulation after desmopressin treatment. Detailed analysis of in vivo clearance of recombinant VWF in a VWF-deficient mice model revealed a fourfold increased clearance rate of the mutants. The mutations C1130F, C1149R and C2671Y are each associated with reduced survival of VWF in the circulation. Detailed analysis of the recombinant mutant VWF demonstrated that increased clearance was not due to increased proteolysis by ADAMTS-13. We did not identify functional or structural characteristics that the mutant proteins have in common and could be associated with the phenomenon of increased clearance.

CONCLUSIONS

Cysteine-mutations in VWF may result in reduced in vivo survival. The observation that various mutations are associated with increased in vivo clearance may have major implications for the therapeutic strategies that rely on the rise of endogenous VWF after desmopressin administration.

von Willebrand factor: two sides of a coin

Everyone experiences minor bleeding and clotting, and many illnesses feature extremes of hemorrhage or thrombosis. Recent advances have illuminated the ways in which von Willebrand factor (VWF) contributes to both kinds of hemostatic emergency, whether mundane or life threatening, often through disturbances in VWF synthesis or catabolism. von Willebrand factor multimer assembly depends on the ability of the propeptide to promote disulfide bond formation in the Golgi, possibly by acting as a pH-sensitive oxidoreductase. Once secreted into the blood, multimers are subject to competing processes of clearance and of proteolysis by ADAMTS-13. Defects in the secretion or intravascular clearance of VWF can cause exceptionally severe forms of von Willebrand disease (VWD) type 1. Defects in the assembly of VWF multimers, or exaggerated proteolytic degradation by ADAMTS-13, can cause VWD type 2A and contribute to VWD type 2B. Conversely, defects in the feedback proteolysis of VWF by ADAMTS-13 can cause thrombotic thrombocytopenic purpura (TTP). The pathophysiologic importance of VWF is not limited to the dramatic phenotypes of VWD and TTP. In fact, VWF level also correlates with thrombosis risk and inversely with bleeding risk within the apparently healthy population. More research is needed to understand how VWF function is regulated, and to enable physicians to use this knowledge for the benefit of their patients.