A Cys374Tyr homozygous mutation of platelet glycoprotein IIIa (β3) in a Chinese patient with Glanzmann's thrombasthenia

Abstract

A 20-year-old woman from a consanguineous family in the Human Province of the People's Republic of China was diagnosed as having Glanzmann's thrombasthenia based on (1) nearly a lifelong history of epistaxis, gum bleeding, petechiae, and purpura; (2) severe menorrhagia resulting in anemia and need for whole-blood transfusion; (3) normal coagulation assays; (4) prolonged bleeding time; (5) absent clot retraction; (6) decreased glass bead retention; (7) absent platelet aggregation in response to adenine diphosphate, epinephrine, and collagen; and (8) normal initial slope of platelet aggregation in response to ristocetin, but with a diminished maximal extent. The patient's platelets had a decreased level of platelet fibrinogen, but the deficiency was not as severe as in other Glanzmann's thrombasthenia patients. As judged monoclonal antibody binding studies, surface glycoprotein (GP) IIb/IIIa (α(IIb)β3) expression was less than 15% of normal and α(v)β3 vitronectin receptor expression was 15% to 19% of normal, suggesting that the defect was in GPIIIa (β3). Immunoblotting of platelet lysates demonstrated decreased levels of GPIIb (~30% to 35% of normal) and GPIIIa(~10% of normal), and the GPIIb had undergone normal maturational processing into GPIIb heavy and light chains. Sequence analysis of the patient's GPIIIa RNA identified a G to A mutation at nucleotide 1219, predicting a Cys to Tyr substitution at residue 374. The patient's parents, who are first cousins, are asymptomatic and have only minor reductions in platelet aggregation. Direct sequencing of polymerase chain reaction- amplified cDNA and GPIIIa exon VIII indicated that the patient is homozygous and her parents are heterozygous for the mutation. Transient transfection studies in Chinese hamster ovary cells indicated that the mutation results in an 85% to 90% reduction in GPIIb/IIIa surface expression, but these cells retain the ability to mediate adhesion to immobilized fibrinogen. The relative preservation of platelet fibrinogen despite the very low level of platelet surface GPIIb/IIIa expression in this patient raises some interesting questions regarding the mechanism of fibrinogen uptake end the pathophysiology of Glanzmann's thrombasthenia.