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CURATOR
A pinboard by
Azza Abdelaal

predocctoral fellow, Blood research institute

PINBOARD SUMMARY

Von Willebrand Disease (VWD) is the most common inherited bleeding disorder. Von Willebrand factor’s (VWF) complex intracellular processing may result in variable VWF expression which requires exploring the underlying mechanism related to individual variants. The mechanisms underlying type 1 VWD are not fully understood, although reduced secretion and increased clearance have been implicated. We aimed to characterize novel sequence variants (SV) identified in the VWF gene in type 1 VWD patients recruited through the Zimmerman Program in order to (1) define the underlying mechanism, (2) explore if mutations in a particular domain are mechanistically similar, and (3) correlate expression study data to patient phenotype.

Homozygous and heterozygous expression in mammalian cells was used to study the effect of SV on VWF secretion, intracellular retention, function, and multimerization. 19 Novel SV identified throughout the entire VWF protein were introduced into VWF-mycHis plasmid vector. Variant VWF cDNA was transfected homozygously and cotransfected heterozygously 1:1 with wild type (WT) VWF pCIneo into Human Embryonic Kidney cells (HEK293T). Supernatants (sup) and cell lysates from 3 separate transfections were collected and analyzed by ELISA for VWF:Ag, VWF binding to platelet glycoprotein 1b (VWF:GPIbM), VWF binding to collagen type III (VWF:CB). VWF multimer structure is analyzed by SDS-agarose electrophoresis and western blotting with differential staining of the VWF secreted from variant and WT alleles.

Results section are not shown here.

In summary, 52.6% of the 19 SV theoretically are not causative of the VWD phenotype observed in the patients. These subjects had normal VWF propeptide/VWF:Ag suggesting reduced plasma survival is unlikely to be the causative mechanism. The VWD mechanism in these subjects remains undefined. 15.7% of the SV behaved like Type 2 VWD SV with loss of HMW multimers. We identified 31.6% of the 19 VWF SV as likely to cause the type 1 VWD phenotype with reduced secretion. This study highlights the reliability of The VWF:CB/VWF:Ag ratio < 0.7 as an indicator of multimer structure. Also it reveals if any domain-specific correlation of VWF secretion or multimer abnormality was observed. In addition we demonstrate the consistent/ incongruity between the patients and the in vitro expression phenotypes.

8 ITEMS PINNED

Phenotype and genotype of a cohort of families historically diagnosed with type 1 von Willebrand disease in the European study, Molecular and Clinical Markers for the Diagnosis and Management of Type 1 von Willebrand Disease (MCMDM-1VWD).

Abstract: Type 1 von Willebrand disease (VWD) is characterized by a personal and family history of bleeding coincident with reduced levels of normal plasma von Willebrand factor (VWF). The molecular basis of the disorder is poorly understood. The aims of this study were to determine phenotype and genotype and their relationship in patients historically diagnosed with type 1 VWD. Families were recruited in 9 European countries based on previous type 1 VWD diagnosis. Bleeding symptoms were recorded, plasma phenotype analyzed, and VWF mutation analysis performed in all index cases (ICs). Phenotypic and molecular analysis stratified patients into those with or without phenotypes suggestive of qualitative VWF defects (abnormal multimers) and with or without mutations. A total of 105 of 150 ICs (70%) had mutations identified. A subgroup with abnormal multimers (38% of ICs, 57 of 150) showed a high prevalence of VWF gene mutations (95% of ICs, 54 of 57), whereas in those with qualitatively normal VWF, fewer mutations were identified (55% of ICs, 51 of 93). About one third of the type 1 VWD cases recruited could be reconsidered as type 2. The remaining group could be considered "true" type 1 VWD, although mutations were found in only 55%.

Pub.: 21 Sep '06, Pinned: 27 Oct '17

Linkage analysis in families diagnosed with type 1 von Willebrand disease in the European study, molecular and clinical markers for the diagnosis and management of type 1 VWD.

Abstract: von Willebrand disease (VWD) type 1 is a congenital bleeding disorder caused by genetic defects in the von Willebrand factor (VWF) gene and characterized by a reduction of structurally normal VWF. The diagnosis of type 1 VWD is difficult because of clinical and laboratory variability. Furthermore, inconsistency of linkage between type 1 VWD and the VWF locus has been reported.To estimate the proportion of type 1 VWD that is linked to the VWF gene.Type 1 VWD families and healthy control individuals were recruited. An extensive questionnaire on bleeding symptoms was completed and phenotypic tests were performed. Linkage between VWF gene haplotypes and the diagnosis of type 1 VWD, the plasma levels of VWF and the severity of bleeding symptoms was analyzed.Segregation analysis in 143 families diagnosed with type 1 VWD fitted a model of autosomal dominant inheritance. Linkage analysis under heterogeneity resulted in a summed lod score of 23.2 with an estimated proportion of linkage of 0.70. After exclusion of families with abnormal multimer patterns the linkage proportion was 0.46. LOD scores and linkage proportions were higher in families with more severe phenotypes and with phenotypes suggestive of qualitative VWF defects. About 40% of the total variation of VWF antigen could be attributed to the VWF gene.We conclude that the diagnosis of type 1 VWD is linked to the VWF gene in about 70% of families, however after exclusion of qualitative defects this is about 50%.

Pub.: 26 Apr '06, Pinned: 27 Oct '17

Impact of plasma von Willebrand factor levels in the diagnosis of type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1VWD).

Abstract: Presence of bleeding symptoms, inheritance and reduced von Willebrand factor (VWF) contribute to the diagnosis of type 1 von Willebrand disease (VWD). However, quantitative analysis of the importance of VWF antigen (VWF:Ag) and ristocetin cofactor activity (VWF:RCo) levels in the diagnosis is lacking.To evaluate the relative contribution of VWF measurement to the diagnosis of VWD.From the MCMDM-1VWD study cohort, 204 subjects (considered as affected by VWD based on the enrolling Center diagnoses and the presence of linkage with the VWF locus) were compared with 1155 normal individuals. Sensitivity, specificity and diagnostic positive likelihood ratios (LR) of VWF:Ag and VWF:RCo were computed.ABO blood group was the variable most influencing VWF levels, but adjustment of the lower reference limit for the ABO group did not improve sensitivity and specificity of VWF:Ag or VWF:RCo. The lower reference limit (2.5th percentile) was 47 IU dL(-1) for both VWF:Ag and VWF:RCo and showed similar diagnostic performance [receiver-operator curve area: 0.962 and 0.961 for VWF:Ag and VWF:RCo, respectively; P = 0.81]. The probability of VWD was markedly increased only for values below 40 IU dL(-1) (positive LR: 95.1 for VWF:Ag), whereas intermediate values (40 to 60 IU dL(-1)) of VWF only marginally indicated the probability of VWD.Although the conventional 2.5 lower percentile has good sensitivity and specificity, only VWF:Ag or VWF:RCo values below 40 IU dL(-1) appear to significantly indicate the likelihood of type 1 VWD. The LR profile of VWF level could be used in a diagnostic algorithm.

Pub.: 06 Apr '07, Pinned: 27 Oct '17

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

Abstract: 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.

Pub.: 08 Aug '06, Pinned: 27 Oct '17

Detailed von Willebrand factor multimer analysis in patients with von Willebrand disease in the European study, molecular and clinical markers for the diagnosis and management of type 1 von Willebrand disease (MCMDM-1VWD).

Abstract: Type 1 von Willebrand disease (VWD) is a congenital bleeding disorder characterized by a partial quantitative deficiency of plasma von Willebrand factor (VWF) in the absence of structural and/or functional VWF defects. Accurate assessment of the quantity and quality of plasma VWF is difficult but is a prerequisite for correct classification.To evaluate the proportion of misclassification of patients historically diagnosed with type 1 VWD using detailed analysis of the VWF multimer structure.Previously diagnosed type 1 VWD families and healthy controls were recruited by 12 expert centers in nine European countries. Phenotypic characterization comprised plasma VWF parameters and multimer analysis using low- and intermediate-resolution gels combined with an optimized visualization system. VWF genotyping was performed in all index cases (ICs).Abnormal multimers were present in 57 out of 150 ICs; however, only 29 out of these 57 (51%) had VWF ristocetin cofactor to antigen ratio below 0.7. In most cases multimer abnormalities were subtle, and only two cases had a significant loss of the largest multimers.Of the cases previously diagnosed as type 1 VWD, 38% showed abnormal multimers. Depending on the classification criteria used, 22 out of these 57 cases (15% of the total cohort) may be reclassified as type 2, emphasizing the requirement for multimer analysis compared with a mere ratio of VWF functional parameters and VWF:Ag. This is further supported by the finding that even slightly aberrant multimers are highly predictive for the presence of VWF mutations.

Pub.: 05 Mar '08, Pinned: 27 Oct '17

Expression of 14 von Willebrand factor mutations identified in patients with type 1 von Willebrand disease from the MCMDM-1VWD study.

Abstract: Candidate von Willebrand factor (VWF) mutations were identified in 70% of index cases in the European study 'Molecular and Clinical Markers for the Diagnosis and Management of type 1 von Willebrand Disease'. The majority of these were missense mutations.To assess whether 14 representative missense mutations are the cause of the phenotype observed in the patients and to examine their mode of pathogenicity.Transfection experiments were performed with full-length wild-type or mutant VWF cDNA for these 14 missense mutations. VWF antigen levels were measured, and VWF multimer analysis was performed on secreted and intracellular VWF.For seven of the missense mutations (G160W, N166I, L2207P, C2257S, C2304Y, G2441C, and C2477Y), we found marked intracellular retention and impaired secretion of VWF, major loss of high molecular weight multimers in transfections of mutant constructs alone, and virtually normal multimers in cotransfections with wild-type VWF, establishing the pathogenicity of these mutations. Four of the mutations (R2287W, R2464C, G2518S, and Q2520P) were established as being very probably causative, on the basis of a mild reduction in the secreted VWF or on characteristic faster-running multimeric bands. For three candidate changes (G19R, P2063S, and R2313H), the transfection results were indistinguishable from wild-type recombinant VWF and we could not prove these changes to be pathogenic. Other mechanisms not explored using this in vitro expression system may be responsible for pathogenicity.The pathogenic nature of 11 of 14 candidate missense mutations identified in patients with type 1 VWD was confirmed. Intracellular retention of mutant VWF is the predominant responsible mechanism.

Pub.: 02 Jul '09, Pinned: 27 Oct '17

Variable content of von Willebrand factor mutant monomer drives the phenotypic variability in a family with von Willebrand disease.

Abstract: Von Willebrand disease (VWD) is an inherited bleeding disorder characterized by incomplete penetrance and variable expressivity. We evaluated a 24-member pedigree with VWD type 2 caused by a T>G mutation at position 3911 that predicts a methionine to arginine (M1304R) change in the platelet-binding A1 domain of von Willebrand factor (VWF). This mutation manifests as an autosomal-dominant trait, with clinical and biochemical phenotypic variability among affected individuals, including differences in bleeding tendency and VWF quantity, activity, and multimer pattern. Sequencing of all VWF coding regions in 3 affected individuals did not identify additional mutations. When expressed in heterologous cells, M1304R was secreted in lower quantities, failed to drive formation of storage granules, and was defective in multimerization and platelet binding. When cotransfected in equal quantities with the wild-type complementary DNA, the mutant complementary DNA depressed VWF secretion, although multimerization was only mildly affected. A llama nanobody (AU/VWFa-11) that detects the mutant A1 domain demonstrated highly variable binding to VWF from different affected members, indicating that the VWF contained different percentages of mutant monomers in different individuals. Thus, the observed variability in VWD phenotypes could in part be determined by the extent of mutant monomer incorporation in the final multimer structure of plasma VWF.

Pub.: 29 May '15, Pinned: 27 Oct '17