N1421K mutation in the glycoprotein Ib binding domain impairs ristocetin- and botrocetin-mediated binding of von Willebrand factor to platelets

von Willebrand disease (VWD) is a common inheritable bleeding disorder caused by deficiency of von Willebrand Factor (VWF), which is involved in platelet adhesion and aggregation. We report a family consisting of three patients with VWD characterized by an apparently normal multimeric pattern, moderately decreased plasma factor VIII (FVIII) and VWF levels, and disproportionately low-plasma VWF:RCo levels. The patients were found to be heterozygous for the novel N1421K mutation, caused by a 4263C > G transversion in exon 28 of the VWF gene coding for the A1 domain. Botrocetin- and ristocetin-mediated binding of plasma VWF to GPIb were reduced in the patients. In vitro mutagenesis and expression in COS-7 cells confirmed the impairment of the mutant in botrocetin- and ristocetin-mediated VWF binding to GPIb. VWF collagen binding capacity was unaffected in plasma from the heterozygous individuals as well as in medium from transfected COS-7 cells. Our findings indicate that the N1421K substitution in the VWF affects the GPIb binding site or a recognition element by a conformational change of the A1 domain.