Laboratory variability in the diagnosis of type 2 VWD variants

FRET Visualization of High Mechanosensation of von Willebrand Factor to Hydrodynamic Force

von Willebrand factor (vWF) is a large glycoprotein in the circulation system, which senses hydrodynamic force at vascular injuries and then recruits platelets in assembling clots. How vWF mechanosenses shear flow for molecular unfolding is an important topic. Here, a Förster resonance energy transfer (FRET) biosensor was developed to monitor vWF conformation change to hydrodynamic force. The vWF-based biosensor is anchored on the cell surface, in which the A2 domain is flanked with a FRET pair. With 293T cells seeded into microfluidic channels, 2.8 dyn/cm2 of shear force (i.e., 28 μN/cm2, or 264.1/s in shear rate) induced a remarkable FRET change (~60%) in 30 min. A gradient micro-shear below 2.8 dyn/cm2 demonstrated FRET responses positively related to flow magnitudes, with 0.14 dyn/cm2 (1.4 μN/cm2) inducing an obvious change (~16%). The FRET increases indicate closer positioning of A2's two terminals in vWF or the addition of a more parallel orientation of the FRET pair, supported with the high FRET of the A2-only-based biosensor, which probably resulted from flow-induced A2 dissociation from vWF intramolecular binding such as that in A1/A3 domains. Interestingly, gradient flow increases from 2.8 to 28 dyn/cm2 led to decreasing FRET changes, suggesting the second-level unfolding in the A2 domain. The LOCK-vWF biosensor with bridged A2 two terminals or an A2-only biosensor could not sense the shear, indicating a structure-flexible A2 and large vWF molecules that are important in the mechanosensation. In conclusion, the developed vWF-based biosensor demonstrated the high mechanosensation of vWF with two-level unfolding to shear force: the dissociation of the A2 domain from vWF intramolecular binding under a micro-shear, and then the unfolding of A2 in vWF under a higher shear; the FRET response to shear force at a very low scale may support the observed clot formation at microvascular wounds. This study provides new insights into the vWF's mechanosensitive feature for its physiological functions and implicated disorders.

Type 2M/2A von Willebrand disease: a shared phenotype between type 2M and 2A

ABSTRACT

Four variants have been continuously subjected to debate and received different von Willebrand disease (VWD) classifications: p.R1315L, p.R1315C, p.R1374H, and p.R1374C. We chose to comprehensively investigate these variants with full set of VWD tests, protein-modeling predictions and applying structural biology. Patients with p.R1315L, p.R1315C, p.R1374H, and p.R1374C were included. A group with type 2A and 2M was included to better understand similarities and differences. Patients were investigated for phenotypic assays and underlying disease mechanisms. We applied deep protein modeling predictions and structural biology to elucidate the causative effects of variants. Forty-three patients with these variants and 70 with 2A (n = 35) or 2M (n = 35) were studied. Patients with p.R1315L, p.R1374H, or p.R1374C showed a common phenotype between 2M and 2A using von Willebrand factor (VWF):GPIbR/VWF:Ag and VWF:CB/VWF:Ag ratios and VWF multimeric profile, whereas p.R1315C represented a type 2M phenotype. There was an overall reduced VWF synthesis or secretion in 2M and cases with p.R1315L, p.R1374H, and p.R1374C, but not in 2A. Reduced VWF survival was observed in most 2A (77%), 2M (80%), and all 40 cases with p.R1315L, p.R1374H, and p.R1374C. These were the only variants that fall at the interface between the A1-A2 domains. p.R1315L/C mutants induce more compactness and internal mobility, whereas p.R1374H/C display a more extended overall geometry. We propose a new classification of type 2M/2A for p.R1315L, p.R1374H, and p.R1374C because they share a common phenotype with 2M and 2A. Our structural analysis shows the unique location of these variants on the A1-A2 domains and their distinctive effect on VWF.

Von Willebrand Factor (VWF) multiplex activity assay differentiation of type 1 von Willebrand Disease (VWD) and variant VWD

INTRODUCTION

VWD diagnosis is challenging requiring multiple VWF activity tests using many individual assays. We have developed an ELISA-based VWF Multiplex Activity Assay (VWF-MAA) to address this concern; however, the ability of the VWF-MAA to discriminate between type 1 VWD, variant VWD, and normal subjects has not been evaluated.

AIM

To evaluate the VWF-MAA and its ability to differentiate between type 1 VWD, variant VWD and normal subjects in individuals undergoing an initial laboratory evaluation for bleeding.

METHODS

A total of 177 plasma samples from the Zimmerman Program: Comparative Effectiveness in the Diagnosis of VWD were evaluated from 11 centres across the US and Canada. The VWF-MAA was compared to Versiti Blood Research Institute (VBRI) and Local Center (LC) assigned VWD diagnosis.

RESULTS

Overall, 129/177 (72.9%) were correctly assigned as normal (non-VWD), type 1, or variant VWD compared to the VBRI assigned diagnosis. VWF-MAA assigned non-VWD accurately in 29/57 (50.9%) samples, and type 1 VWD accurately in 93/110 (84.6%) samples. Considering LC diagnosis where there was agreement with VWF-MAA and not VBRI diagnosis, type 1 VWD was accurate in 105/110 (95.5%) samples. Bland-Altman analysis demonstrated good correlation between laboratory methods. VWD, types 2A, 2B, 1C VWD were also assigned by the VWF-MAA.

CONCLUSIONS

We demonstrate that the VWF-MAA has utility in differentiating type 1 VWD, variant VWD and normal subjects in individuals undergoing an initial laboratory evaluation for bleeding.

A comparative study in patients with type 2 von Willebrand disease using 4 different platelet-dependent von Willebrand factor assays

Background

Several assays are now available to evaluate platelet-dependent von Willebrand factor (VWF) activity.

Objective

To report the results obtained using 4 different assays in patients with von Willebrand disease (VWD) carrying variants mainly in the A1 domain, which is critical for VWF binding to glycoprotein Ib (GPIb) and ristocetin.

Methods

We evaluated 4 different assays, 2 gain-of-function mutant GPIb binding (VWF:GPIbM) and 2 ristocetin cofactor (VWF:RCo) assays, in 76 patients with type 2 VWD. Patients and healthy controls were tested using VWF:GPIbM enzyme-linked immunosorbent assay (ELISA), VWF:GPIbM automated, VWF:RCo aggregometric, and VWF:RCo automated assays.

Results

There was a good correlation (Pearson's r>0.82) and agreement (Bland-Altman plots assessment) between the 4 assays, although several outliers existed among the type 2B without high-molecular-weight multimers (HMWM). The VWF activity/VWF:antigen ratios, calculated for each assay, were used to establish the percentage of a correct diagnosis of type 2 (ratio<0.60) in these patients: VWF:RCo aggregometric, 2A(100%), 2M(78%), 2M/2A(100%), 2B(68%); VWF:RCo automated, 2A(88%), 2M(89%), 2M/2A(100%), 2B(63%); VWF:GPIbM ELISA, 2A(96%), 2M(67%), 2M/2A(67%), 2B(0%); VWF:GPIbM automated, 2A(73%), 2M(44%), 2M/2A(75%), 2B(84%). In type 2B patients with HMWM, all assays gave a ratio ≥0.60.

Conclusion

The VWF:GPIbM-automated assay is the most effective to diagnose as type 2 the 2B variants, whereas the VWF:RCo assays are the most effective in detecting 2M and 2M/2A variants. The VWF:GPIbM ELISA greatly overestimates the activity of the type 2B patients lacking HMWM. In this study, the use of a VWF activity/VWF:antigen ratio cut-off of 0.70 halved the number of misdiagnosed patients.

Ristocetin dependent cofactor activity in von Willebrand disease diagnosis: Limitations of relying on a single measure

Background

Von Willebrand disease (VWD) is a common inherited bleeding disorder, however the diagnosis can be complicated by a subjective bleeding history and issues with some current von Willebrand factor (VWF) laboratory assays.

Objectives

In the Zimmerman Program, we sought to determine how often a type 1 diagnosis was based on a single low VWF ristocetin cofactor (VWF:RCo) level resulting from the common genetic variant p.D1472H or an isolated assay issue, if that low value was corroborated by the VWF glycoprotein-IbM (VWF:GPIbM) assay, and if retesting confirmed original levels.

Methods

New patients being evaluated for bleeding were consented. Analysis included VWF sequencing, bleeding scores, and comparisons of local VWF antigen (VWF:Ag) and VWF:RCo to central VWF:Ag and VWF:GPIbM.

Results

A total of 18% of VWD subjects had a low local VWF:RCo, but normal VWF:Ag and normal central testing including VWF:GPIbM. Seventy percent of the low VWF:RCo cohort had no pathogenic VWF variants; however, 33% carried p.D1472H. Low VWF:RCo subjects with follow-up local testing within 2 years showed those with p.D1472H continued to have low VWF:RCo and VWF:RCo/VWF:Ag ratio with normal VWF:GPIbM. Subjects without p.D1472H had an increase mean VWF:RCo, resulting in 59% with normal levels on repeat testing.

Conclusions

The diagnosis of VWD based on a single low VWF:RCo but normal VWF:Ag, was often attributed to p.D1472H or variability in VWF:RCo that was eliminated with VWF:GPIbM. Our study suggests that using VWF:RCo alone for diagnostic purposes may be insufficient while repeat VWF:RCo or VWF:GPIbM testing can be valuable in establishing a VWD diagnosis.

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.

Phenotypic and genetic characterizations of the Milan cohort of von Willebrand disease type 2

von Willebrand disease (VWD) type 2 is caused by qualitative abnormalities of von Willebrand factor (VWF). This study aimed to determine the genotypic and phenotypic characterizations of a large VWD type 2 cohort from Milan. We included 321 patients (54% female) within 148 unrelated families from 1995 to 2021. Patients were fully characterized using laboratory phenotypic tests, and the genotypic diagnosis was confirmed by target genetic analysis using Sanger sequencing. Patients were diagnosed with type 2A (n = 98; 48 families), 2B (n = 85; 38 families), 2M (n = 112; 50 families), or 2N (n = 26; 12 families). Eighty-two unique VWF variants, including 8 novel variants, were found. The potential pathogenic effect of novel variants was assessed by in silico analysis. Most patients were heterozygous for a single variant (n = 259; 81%), whereas 37 cases (11%) had 2 variants (4 homozygous, 9 in trans, and 24 in cis). Twenty-five patients (8%) had ≥3 variants, mainly as a result of gene conversions. Among the 82 distinct variants identified, 5 different types, including missense (n = 64), gene conversion (n = 10), synonymous (n = 1), deletion (n = 4), and splice (n = 3), were observed. The results from this large cohort showed that VWD type 2 is invariably due to variants that do not prevent the synthesis of the protein, and a vast majority of patients (88%) had missense variants. Given the complexity of type 2 diagnosis and the necessity of performing several phenotypic tests, genetic analysis for patients suspected of having type 2 is beneficial to establish the correct diagnosis.

Von Willebrand disease type 2M: Correlation between genotype and phenotype

BACKGROUND

An appropriate clinical diagnosis of von Willebrand disease (VWD) can be challenging because of a variable bleeding pattern and laboratory phenotype. Genotyping is a powerful diagnostic tool and may have an essential role in the diagnostic field of VWD.

OBJECTIVES

To unravel the clinical and laboratory heterogeneity of genetically confirmed VWD type 2M patients and to investigate their relationship.

METHODS

Patients with a confirmed VWD type 2M genetic variant in the A1 or A3 domain of von Willebrand factor (VWF) and normal or only slightly aberrant VWF multimers were selected from all subjects genotyped at the Radboud university medical center because of a high suspicion of VWD. Bleeding scores and laboratory results were analyzed.

RESULTS

Fifty patients had a clinically relevant genetic variant in the A1 domain. Median bleeding score was 5. Compared with the nationwide Willebrand in the Netherlands study type 2 cohort, bleeding after surgery or delivery was reported more frequently and mucocutaneous bleedings less frequently. Median VWF activity/VWF antigen (VWF:Act/VWF:Ag) ratio was 0.32, whereas VWF collagen binding activity/VWF antigen (VWF:CB/VWF:Ag) ratio was 0.80. Variants in the A3 domain were only found in two patients with low to normal VWF:Act/VWF:Ag ratios (0.45, 1.03) and low VWF:CB/VWF:Ag ratios (0.45, 0.63).

CONCLUSION

Genetically confirmed VWD type 2M patients have a relatively mild clinical phenotype, except for bleeding after surgery and delivery. Laboratory phenotype is variable and depends on the underlying genetic variant. Addition of genotyping to the current phenotypic characterization may improve diagnosis and classification of VWD.

2B von Willebrand disease diagnosis: Considerations reflecting on 2021 multisociety guidelines

The recent American Society of Hematology/ISTH/National Hemophilia Foundation/World Federation of Hemophilia 2021 guidelines on the diagnosis of von Willebrand disease (VWD) is an outstanding effort to unify the diagnosis of VWD. However, as mentioned in the guidelines, there are limitations due to the low certainty in the evidence identified for most questions. The panel encouraged critical review of the guidelines. Compared to other subtypes, there is considerable complexity with diagnosis of type 2B VWD, a type that results from a gain-of-function mutation in the VWF gene. Additionally, the discrimination from its phenocopy platelet-type VWD, representing a gain-of-function mutation in the GP1BA gene, is crucial as this determines treatment decisions. In this forum, we highlight the complexities of a type 2B VWD diagnosis; discuss important issues with respect to these complexities: genotype/phenotype/clinical correlations, challenges with platelet aggregation and ristocetin-induced platelet agglutination testing, platelet count, and thrombocytopathy; and, finally, suggest the consideration of some of these complexities in future iterations of the VWD guidelines.

Von Willebrand Disease: Current Status of Diagnosis and Management

Von Willebrand disease (VWD) is a common bleeding disorder, affecting male and female individuals equally, that often manifests in mucosal bleeding. VWD can be secondary to a quantitative (Type 1 and Type 3) or qualitative (Type 2) defects in Von Willebrand factor (VWF). Initial testing includes VWF antigen, as well as a platelet binding assay to differentiate between qualitative and quantitative defects. Further subtyping requires additional testing and is needed to ensure appropriate treatment. Desmopressin, antifibrinolytics, hormonal treatments for heavy menstrual bleeding, and VWF concentrates are commonly used in the treatment of VWD.

Resolving Differential Diagnostic Problems in von Willebrand Disease, in Fibrinogen Disorders, in Prekallikrein Deficiency and in Hereditary Hemorrhagic Telangiectasia by Next-Generation Sequencing

Diagnosis of rare bleeding disorders is challenging and there are several differential diagnostics issues. Next-generation sequencing (NGS) is a useful tool to overcome these problems. The aim of this study was to demonstrate the usefulness of molecular genetic investigations by summarizing the diagnostic work on cases with certain bleeding disorders. Here we report only those, in whom NGS was indicated due to uncertainty of diagnosis or if genetic confirmation of initial diagnosis was required. Based on clinical and/or laboratory suspicion of von Willebrand disease (vWD, n = 63), hypo-or dysfibrinogenemia (n = 27), hereditary hemorrhagic telangiectasia (HHT, n = 10) and unexplained activated partial thromboplastin time (APTT) prolongation (n = 1), NGS using Illumina platform was performed. Gene panel covered 14 genes (ACVRL1, ENG, MADH4, GDF2, RASA1, F5, F8, FGA, FGB, FGG, KLKB1, ADAMTS13, GP1BA and VWF) selected on the basis of laboratory results. We identified forty-seven mutations, n = 29 (6 novel) in vWD, n = 4 mutations leading to hemophilia A, n = 10 (2 novel) in fibrinogen disorders, n = 2 novel mutations in HHT phenotype and two mutations (1 novel) leading to prekallikrein deficiency. By reporting well-characterized cases using standardized, advanced laboratory methods we add new pieces of data to the continuously developing “bleeding disorders databases”, which are excellent supports for clinical patient management.