Type IIB mutation His-505-->Asp implicates a new segment in the control of von Willebrand factor binding to platelet glycoprotein Ib.

Type IIB von Willebrand disease is characterized by increased affinity of mutant von Willebrand factor (vWF) for platelet glycoprotein Ib. Eight different missense mutations that cause this phenotype have been reported within the disulfide loop defined by Cys-509 and Cys-695 of the mature vWF subunit; this disulfide loop is required for normal binding of vWF to platelet glycoprotein Ib. A new mutation was identified in a patient with type IIB von Willebrand disease (vWD) characterized by a lifelong bleeding disorder, mild thrombocytopenia, normal levels of factor VIII, vWF antigen, and ristocetin cofactor activity but increased ristocetin-induced platelet agglutination at low concentrations of ristocetin. Exon 28 of the patient's vWF gene was amplified, cloned, and sequenced. At nucleotide 3802 (numbering the cDNA from translation initiation), a C to G transversion was identified, which changes the encoded amino acid sequence from His-505 to Asp. The corresponding mutant recombinant vWF was expressed in transiently transfected COS cells. Relative to wild type vWF, the mutant vWF exhibited markedly increased binding to platelets at low concentrations of ristocetin, confirming the association between the His-505-->Asp substitution and the type IIB vWD phenotype. The His-505-->Asp mutation lies outside the disulfide loop affected by other type IIB vWD mutations and implicates a new segment of vWF in the regulation of platelet glycoprotein Ib binding.