Heterozygous ITGA2B R995W mutation inducing constitutive activation of the αIIbβ3 receptor affects proplatelet formation and causes congenital macrothrombocytopenia

Congenital macrothrombocytopenia is a genetically heterogeneous group of rare disorders. alpha IIb beta 3 has not been implicated in these conditions. We identified a novel, conserved heterozygous ITGA2B R995W mutation in 4 unrelated families. The surface expression of platelet alpha IIb beta 3 was decreased to 50% to 70% of control. There was spontaneous PAC-1 and fibrinogen binding to resting platelets without CD62p expression. The activation state of alpha IIb beta 3 in 293T cells was higher for alpha IIb-W995 than for beta 3-H723 but was weaker than for beta 3-N562. FAK was spontaneously phosphorylated in alpha IIb-W995/beta 3-transfected 293T cells. These results indicate that alpha IIb-W995/beta 3 has a constitutive, activated conformation but does not induce platelet activation. alpha IIb-W995/beta 3-transfected CHO cells developed membrane ruffling and abnormal cytoplasmic protrusions. The increased size and decreased number of proplatelet tips in alpha IIb-W995/beta 3-transduced mouse fetal liver-derived megakaryocytes indicate defective proplatelet formation. We propose that activating mutations in ITGA2B and ITGB3 represent the etiology of a subset of congenital macrothrombocytopenias. (Blood. 2011;117(20):5479-5484)