Conformational changes in the D' domain of von Willebrand factor induced by CYS 25 and CYS 95 mutations lead to factor VIII binding defect and multimeric impairment

We report 2 new mutations identified in 3 patients and characterized by the markedly decreased affinity of von Willebrand factor (vWF) for factor VIII (FVIII). Patients 2 and 3, who have a typical type 2N phenotype, were found to be compound heterozygous for Arg91Gln and Cys25Tyr or Cys95Phe, respectively. Patient 1, who is the first cousin of patient 2, had an FVIII binding defect of vWF, low levels of vWF, and multimeric impairment. She was found to be compound heterozygous for the mutations Cys25Tyr and a stop codon (D93ter) in exon 4. Transient expression of recombinant vWF (rvWF) containing either Cys25Tyr or Cys95Phe mutations resulted in mutated rvWF with markedly reduced FVIII binding ability, multimeric structure impairment, and a significant decrease in the vWF expression level. Moreover, the use of anti-vWF monoclonal antibodies that inhibit the FVIII binding showed that these 2 mutations likely induce a conformational change in the D′ domain. These results show that the native conformation of the D′ domain of vWF is not only required for FVIII binding but also for normal multimerization and optimal secretion.