Type 2N von Willebrand Disease: Rapid Genetic Diagnosis of G2811A (R854Q), C2696T (R816W), T2701A (H817Q) and G2823T (C858F) – Detection of a Novel Candidate Type 2N Mutation: C2810T (R854W)

Thermodynamic analysis of the interaction of factor VIII with von Willebrand factor

Factor VIII (FVIII) is a glycoprotein that plays an important role in the intrinsic pathway of coagulation. In circulation, FVIII is protected upon binding to von Willebrand factor (VWF), a chaperone molecule that regulates its half-life, distribution, and activity. Despite the biological significance of this interaction, its molecular mechanisms are not fully characterized. We determined the equilibrium and activation thermodynamics of the interaction between FVIII and VWF. The equilibrium affinity determined by surface plasmon resonance was temperature-dependent with a value of 0.8 nM at 35 °C. The FVIII-VWF interaction was characterized by very fast association (8.56 × 10(6) M(-1) s(-1)) and fast dissociation (6.89 × 10(-3) s(-1)) rates. Both the equilibrium association and association rate constants, but not the dissociation rate constant, were dependent on temperature. Binding of FVIII to VWF was characterized by favorable changes in the equilibrium and activation entropy (TΔS° = 89.4 kJ/mol, and -TΔS(++) = -8.9 kJ/mol) and unfavorable changes in the equilibrium and activation enthalpy (ΔH° = 39.1 kJ/mol, and ΔH(++) = 44.1 kJ/mol), yielding a negative change in the equilibrium Gibbs energy. Binding of FVIII to VWF in solid-phase assays demonstrated a high sensitivity to acidic pH and a sensitivity to ionic strength. Our data indicate that the interaction between FVIII and VWF is mediated mainly by electrostatic forces, and that it is not accompanied by entropic constraints, suggesting the absence of conformational adaptation but the presence of rigid "pre-optimized" binding surfaces.

Therapeutic consequences for misdiagnosis of type 2N von Willebrand disease

Patients presenting with a low FVIII:C and with normal VWF levels are usually presumed to have hemophilia (males) or be carriers for hemophilia (females). Some of these patients may instead have VWD:2N. Such patients if misdiagnosed are likely to suffer from insufficiently treated bleeds. We report 2 males and 1 female who presented with a low FVIII:C (1-21%) and minimally reduced/normal VWF and were assumed to have, or be a carrier for, hemophilia A. Eventually all were found to have VWD:2N. Prior to the correct diagnosis the males had been treated with rFVIII with poor responses and ultimately adverse clinical consequences.

Validation of the first commercial ELISA for type 2N von Willebrand's disease diagnosis

Type 2N von Willebrand's disease (VWD) is characterized by a factor VIII (FVIII) deficiency and a low FVIII/VWF ratio related to a markedly decreased affinity of von Willebrand factor (VWF) to FVIII. Type 2N VWD is diagnosed using assays allowing the measurement of plasma VWF capacity to bind FVIII (VWF:FVIIIB). These assays, crucial in order to distinguish type 2N VWD patients from mild haemophiliacs A and haemophilia A carriers, remain exclusively homemade and limited to laboratories possessing a high level of expertise in VWD. We evaluated the first commercial ELISA (Asserachrom® VWF:FVIIIB; Stago) comparated to a reference method in a multicentric study involving 205 subjects: 60 healthy volunteers, 37 haemophiliacs A, 17 haemophilia A carriers, 37 patients with type 2N VWD, 9 heterozygous carriers for a 2N mutation and 45 patients with miscellaneous other types of VWD (all previously characterized). A diluted plasma sample adjusted to 10 IU dL(-1) of VWF:Ag was incubated with a rabbit antihuman VWF polyclonal antibody. After removing the endogenous FVIII, recombinant FVIII (rFVIII) was added and bound rFVIII was quantified using a peroxydase-conjugated mouse antihuman FVIII monoclonal antibody. The intra-assay and inter-assay reproducibility was satisfactory. In all subgroups, both methods were well correlated. All type 2N VWD patients exhibited a markedly decreased VWF:FVIIIB (lower than 15%) and all heterozygous 2N carriers had a moderately decreased VWF:FVIIIB (between 30% and 65%). All controls (healthy subjects, haemophiliacs A and haemophilia A carriers) had a normal VWF:FVIIIB (higher than 80%) except one healthy volunteer and three haemophiliacs who exhibited a moderately decreased VWF:FVIIIB suggesting a heterozygous status for a 2N mutation. In conclusion, the Asserachrom® VWF:FVIIIB is easy to perform, standardized and accurate for type 2N VWD diagnosis with a 100% sensitivity and specificity.

Homozygous type 2N R854W von Willebrand factor is poorly secreted and causes a severe von Willebrand disease phenotype

BACKGROUND

von Willebrand disease (VWD) type Normandy (VWD 2N) is caused by mutations at the factor (F)VIII-binding site of von Willebrand factor (VWF), located in the D'and D3 domains on the N-terminus of mature VWF. The R854Q mutation is the most frequent cause of this phenotype.

OBJECTIVES

We report the characterization of a homozygous VWD 2N mutation, R854W, detected in a patient with a severe VWD phenotype.

METHODS

The plasma VWF phenotype was studied, transient expression of recombinant mutant full-length VWF in 293 EBNA cells was performed, and the results were compared with those obtained with wild-type (WT) VWF. Furthermore, expression was also examined in HEK293 cells, which form Weibel-Palade body-like granules when transfected with WT VWF.

RESULTS

The multimer analysis of plasma VWF showed the lack of the typical triplet structure, with the presence of the central band only, and a relative decrease in the high molecular mass multimers. Homozygous expression of recombinant R854W VWF resulted in normal amounts of cellular VWF, but with a severe reduction in secretion into the medium. Severe reductions in FVIII binding to R854W VWF, glycoprotein Ib binding activity and collagen binding of secreted W854 VWF was observed, and reproduced the phenotypic parameters of plasma VWF. In HEK293 cells, homozygous R854W VWF failed to form Weibel-Palade body-like granules.

CONCLUSIONS

Our results demonstrate that a homozygous R854W mutation in the D' domain of VWF induces impaired secretion and activity of the protein, thereby explaining the severe phenotype of the patient.

Diagnosis and incidence of inherited von Willebrand disease

Von Willebrand disease is the most commonly inherited bleeding disorder, caused by the inheritance of a quantitative or qualitative abnormality of von Willebrand factor. Clinical manifestations of this disorder are diverse, and traditional diagnostic tools vary in sensitivity, specificity, and overall usefulness. However, as more accurate diagnostic testing is developed and implemented, determination of the disease's incidence and prevalence will improve, allowing the identification and treatment of patients who suffer from this disorder.

The molecular biology of von Willebrand disease

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

High concentrations of coagulation factor VIII and thrombosis: Is the factor VIII-binding domain of von Willebrand factor implicated?

The possibility that high factor VIII (FVIII) levels in thrombosis patients is principally explained by a gain of function in the FVIII-binding domain of von Willebrand factor (VWF), arising from amino acid substitution(s) or polymorphism(s), was investigated. Exons 18-24 of the VWF gene were sequenced in 13 thrombosis patients with high FVIII (> 1.50 IU/ml). No novel mutations were found. Four known polymorphisms were detected: G2615A and C2635T (Ex18), G2805A (Ex20) and G3130A (Ex22). Their frequencies showed no significant differences in a thrombosis vs. control cohort. The data suggest that amino acid substitutions/polymorphisms in the VWF-FVIII-binding domain are not the principal explanation for high FVIII in thrombosis patients.

Type 2N von Willebrand disease: clinical manifestations, pathophysiology, laboratory diagnosis and molecular biology

Type 2N von Willebrand disease encompasses all patients with factor VIII deficiency caused by a markedly decreased affinity of von Willebrand factor for factor VIII. It is recessively inherited and clinically similar to mild haemophilia. The differential biological diagnosis is of major importance for providing the optimal treatment and relevant genetic counselling. This accurate diagnosis is based on an evaluation of the factor VIII-binding capacity of plasma von Willebrand factor. Furthermore, molecular biology techniques allow the identification of missense mutations in the von Willebrand factor gene. All of these induce the substitution of amino acid residues located in the N terminal part of the mature von Willebrand factor molecule, which contains the factor VIII binding site. Most of them induce a classical type 2N von Willebrand disease phenotype with factor VIII deficiency but a normal level and multimeric pattern of von Willebrand factor.