Novel cysteine substitution p.(Cys1084Tyr) causes variable expressivity of qualitative and quantitative VWF defects

Von Willebrand factor (VWF) is an extremely cysteine-rich multimeric protein that is essential for maintaining normal hemostasis. The cysteine residues of VWF monomers form intra- and inter-molecular disulfide bonds that regulate its structural conformation, multimer distribution and ultimately its hemostatic activity. In this study we investigated and characterized the molecular and pathogenic mechanisms through which a novel cysteine variant p.(Cys1084Tyr) causes an unusual, mixed phenotype form of von Willebrand disease (VWD). Phenotypic data including bleeding scores, laboratory values, VWF multimer distribution and desmopressin response kinetics were investigated in 5 members (2 parents and 3 daughters) of a consanguineous family. VWF synthesis and secretion were also assessed in a heterologous expression system and in a transient transgenic mouse model. Heterozygosity for p.(Cys1084Tyr) was associated with variable expressivity of qualitative VWF defects. Heterozygous individuals had reduced VWF:GPIbM (<0.40IU/mL) and VWF:CB (<0.35IU/mL) as well as relative reductions in high-molecular weight multimers, consistent with type 2A VWD. In addition to these qualitative defects, homozygous individuals also displayed reduced FVIII:C/VWF:Ag leading to very low FVIII levels (0.03-0.1IU/mL) as well as reduced VWF:Ag (<0.40IU/mL) and VWF:GPIbM (<0.30IU/ml). Accelerated VWF clearance and impaired VWF secretion contributed to the fully expressed homozygous phenotype with impaired secretion arising due to disordered disulfide connectivity.

Rare Occurrence of Inhibitors in Von Willebrand Disease: A Case Report

Introduction: Type 3 Von Willebrand Disease (VWD) is the least common but the most severe form of a disease, with a prevalence of about 0. 5 to 1 per million in Western countries. The prevalence of type 3 VWD in the developing countries, with a high degree of consanguinity, is about 6 per million. Moreover, due to underdiagnosis of the milder cases, the prevalence of type 3 VWD is about 50% of the cases. Rarely, some patients develop the Von Willebrand Factor (VWF) inhibitors, which may subsequently develop severe anaphylactic reactions on further exposure to the VWF containing factor replacement therapy. The prevalence of inhibitor development in patients with type 3 VWD has been shown to be in the range of 5.8 to 9.5%. In the absence of a gold standard assay for the quantitation of VWF inhibitors, a correct diagnosis and management of these patients are often challenging.Objectives: The objective of this study is to standardize the Bethesda assay for the VWF inhibitors and to estimate the VWD inhibitor titer in two cases of congenital type 3 VWD, which developed the VWF inhibitors.Results and Conclusions: We could successfully standardize the Bethesda assay for the quantitation of VWF inhibitors in two patients with congenital type 3 VWD with inhibitors.

A novel mouse model of type 2N VWD was developed by CRISPR/Cas9 gene editing and recapitulates human type 2N VWD

Type 2N von Willebrand disease is caused by mutations in the factor VIII (FVIII) binding site of von Willibrand factor (VWF), resulting in dysfunctional VWF with defective binding capacity for FVIII. Here we developed a novel type 2N mouse model using CRISPR/Cas9 technology. In homozygous VWF2N/2N mice, plasma VWF levels were normal (1167±257 mU/ml) but the VWF was completely incapable of binding FVIII, resulting in 53±23 mU/ml of plasma FVIII levels that were similar to those in VWF deficient (VWF-/-) mice. When wild-type human or mouse VWF was infused into VWF2N/2N mice, endogenous plasma FVIII was restored, peaking at 4-6 hours post-infusion, demonstrating that FVIII expressed in VWF2N mice is viable, but short-lived unprotected in plasma due to dysfunctional 2N-VWF. The whole blood clotting time and thrombin generation were impaired in VWF2N/2N but not in VWF-/- mice. The bleeding time and blood loss in VWF2N/2N mice were similar to wild-type mice in the lateral tail vein or ventral artery injury model. However, VWF2N/2N, but not VWF-/- mice, lost a significant amount of blood during the primary bleeding phase after a tail tip amputation injury model, indicating that there are other alternative pathway(s) that can at least partially restore hemostasis when VWF is absent. In summary, we have developed a novel mouse model by gene editing with both the pathophysiology and clinical phenotype found in severe type 2N patients. This unique model can be used to investigate the biological properties of VWF/FVIII association in hemostasis and beyond.

Risk and Management of Intracerebral Hemorrhage in Patients with Bleeding Disorders

Intracerebral hemorrhage (ICH) is the most dreaded complication, and the main cause of death, in patients with congenital bleeding disorders. ICH can occur in all congenital bleeding disorders, ranging from mild, like some platelet function disorders, to severe disorders such as hemophilia A, which can cause catastrophic hemorrhage. While extremely rare in mild bleeding disorders, ICH is common in severe coagulation factor (F) XIII deficiency. ICH can be spontaneous or trauma-related. Spontaneous ICH occurs more often in adults, while trauma-related ICH is more prevalent in children. Risk factors that can affect the occurrence of ICH include the type of bleeding disorder and its severity, genotype and genetic polymorphisms, type of delivery, and sports and other activities. Patients with hemophilia A; afibrinogenemia; FXIII, FX, and FVII deficiencies; and type 3 von Willebrand disease are more susceptible than those with mild platelet function disorders, FV, FXI, combined FV–FVIII deficiencies, and type 1 von Willebrand disease. Generally, the more severe the disorder, the more likely the occurrence of ICH. Contact sports and activities can provoke ICH, while safe and noncontact sports present more benefit than danger. An important risk factor is stressful delivery, whether it is prolonged or by vacuum extraction. These should be avoided in patients with congenital bleeding disorders. Familiarity with all risk factors of ICH can help prevent occurrence of this diathesis and reduce related morbidity and mortality.