A novel heterozygous ITGB3 p.T720del inducing spontaneous activation of integrin αIIbß3 in autosomal dominant macrothrombocytopenia with aggregation dysfunction.

ABSTRACT

We identified a novel heterozygous ITGB3 p.T720del mutation in a pedigree with macrothrombocytopenia exhibiting aggregation dysfunction. Platelet aggregation induced by ADP and collagen was significantly reduced, while ristocetin aggregation was normal. Integrin αIIbß3 was partially activated in a resting status, but platelet expression of αIIbß3 was downregulated. Functional analysis using a cell line showed spontaneous phosphorylation of FAK in αIIb/ß3 (p.T720del)-transfected 293T cells in suspension conditions. Abnormal cytoplasmic protrusions, membrane ruffling, and cytoplasmic localization of αIIbß3 were observed in αIIb/ß3 (p.T720del)-transfected CHO cells. Such morphological changes were reversed by treatment with an FAK inhibitor. These findings imply spontaneous, but partial, activation of αIIbß3 followed by phosphorylation of FAK as the initial mechanism of abnormal thrombopoiesis. Internalization and decreased surface expression of αIIbß3 would contribute to aggregation dysfunction. We reviewed the literature of congenital macrothrombocytopenia associated with heterozygous ITGA2B or ITGB3 mutations. Reported mutations were highly clustered at the membrane proximal region of αIIbß3, which affected the critical interaction between αIIb R995 and ß3 D723, resulting in a constitutionally active form of the αIIbß3 complex. Macrothrombocytopenia caused by a heterozygous activating mutation of ITGA2B or ITGB3 at the membrane proximal region forms a distinct entity of rare congenital thrombocytopenia.