Identification of a novel point mutation in platelet glycoprotein Ibα, Gly to Ser at residue 233, in a Japanese family with platelet-type von Willebrand disease

Abstract

Interaction between platelet glycoprotein (GP)Ibalpha and von Willebrand factor (VWF) has critical roles in both physiological hemostasis and thrombosis. Platelet-type von Willebrand disease (plt-VWD) is a congenital bleeding disorder characterized by gain-of-function mutations of GPIbalpha. To date, two mutations in GPIbalpha, G233V and M239V, have been reported in four unrelated families with plt-VWD. The present study aimed to determine whether G233S of GPIbalpha, a new mutation observed in plt-VWD patients, causes the plt-VWD phenotype and to examine whether conversions to other residues at this position affect VWF binding. The propositus was a 3-year-old Japanese male. He displayed bleeding symptoms and moderate thrombocytopenia. His brother was similarly affected. Platelets from both patients were analyzed by ristocetin- or shear-induced platelet aggregation. DNA sequencing was performed to analyze the GPIbalpha sequence. We examined the 125I-labeled VWF binding using a series of recombinant GPIbalpha fragments with different residues at position 233 (G233S, G233A, G233K, and G233D) together with naturally occurring mutations previously reported in patients (G233V and M239V). Platelet function analysis indicated that platelets from both patients had a typical plt-VWD phenotype. DNA sequencing analysis showed a heterozygous mutation of Gly to Ser at residue 233 of GPIbalpha in both patients. The 125I-labeled VWF binding to mutant compared with the wild type displayed three patterns, gain-of-function (G233S, G233V, and M239V), equivalent function (G233A), and loss-of-function (G233K and G233D). The G233S is a molecular basis of plt-VWD, and residue 233 plays critical roles in regulating VWF binding.