Knock‐in mice bearing constitutively active αIIb(R990W) mutation develop macrothrombocytopenia with severe platelet dysfunction

BACKGROUND: To date, several mutations that induce constitutive activation of integrin alphaIIbbeta3 have been identified in congenital macrothrombocytopenia. Of these, alphaIIb(R995W) is the most prevalent mutation observed in Japanese patients with alphaIIbbeta3-related congenital macrothrombocytopenia. OBJECTIVE AND METHODS: The present study aimed to explore the effects of constitutive activation of the alphaIIb(R995W) mutation on platelet production, morphology, and function. We generated alphaIIb(R990W) knock-in (KI) mice corresponding to human alphaIIb(R995W). RESULTS: Platelet counts of heterozygous (hetero) and homozygous (homo) KI mice were decreased by ~10% and ~25% relative to those of wild-type (WT) mice, respectively, with increase in platelet size. Decrease in absolute reticulated platelet numbers in steady state, delayed recovery from thrombocytopenia induced by anti-platelet antibody and impaired response to exogenous thrombopoietin administration suggested impaired platelet production in KI mice. WT and KI mice showed no significant differences in the number of megakaryocytes and ploidy of megakaryocytes, whereas proplatelet formation was significantly impaired in homo mice. We observed a slight but significant reduction in platelet lifespan in homo mice. The homo mice showed dramatic reduction in alphaIIbbeta3 expression in platelets, which was accompanied by severe in vivo and in vitro platelet dysfunction. CONCLUSION: The alphaIIb(R990W) KI mice developed macrothrombocytopenia, which was primarily attributed to impaired proplatelet formation. In addition, homo KI mice showed marked downregulation in alphaIIbbeta3 expression in platelets with severe impaired platelet function, similar to Glanzmann thrombasthenia.