Abnormal binding of factor VIII is linked with the substitution of glutamine for arginine 91 in von Willebrand factor in a variant form of von Willebrand disease.

von Willebrand factor (vWf) is a multimeric plasma glycoprotein that functions in hemostasis as the initiator of platelet adhesion to damaged blood vessels and as the carrier of Factor VIII (FVIII). Montgomery et al. (Montgomery, R.R., Hathaway, W.E., Johnson, J., Jacobsen, L., and Muntean, W. (1982) Blood 60, 201-207) reported a variant of von Willebrand disease characterized by the abnormal interaction between FVIII and a defective vWf. To identify the molecular basis of this abnormal interaction, we isolated platelet RNA from members of one of the affected families and determined the nucleotide sequence of the FVIII-binding domain encoded by the vWf mRNA. A single G to A transition at nucleotide 2561 was linked with disease expression and results in the substitution of Gln for Arg91 in mature vWf. A restriction fragment containing this mutation was introduced into a full-length vWf expression vector, and both wild type and mutant vWf were expressed in COS-7 cells. In a solid-phase binding assay, expressed vWf was captured with anti-vWf monoclonal antibody AVW1 and then incubated with 6.25-400 milliunits of recombinant FVIII. After washing, vWf-bound FVIII activity was determined with a chromogenic assay. Mutant vWf showed reduced binding of FVIII compared with wild type, suggesting that the substitution of Gln for Arg91 is the likely basis for the abnormal vWf/FVIII interaction in this von Willebrand disease variant.