Dysmegakaryopoiesis of FPD/AML pedigrees with constitutional RUNX1 mutations is linked to myosin II deregulated expression

Abstract

FPD/AML is a familial platelet disorder characterized by platelet defects, predisposition to acute myelogenous leukemia (AML) and germ-line heterozygous RUNX1 alterations. Here we studied the in vitro megakaryopoiesis of three FPD/AML pedigrees. A 60-80% decrease in the output of megakaryocytes (MK) from CD34+ was observed. MK ploidy level was low and mature MK displayed a major defect in proplatelet formation. To explain these defects, we focused on myosin II expression as RUNX1 has been shown to regulate MYL9 and MYH10 in an inverse way. In FPD/AML MK, expression of MYL9 and MYH9 was decreased while MYH10 expression was increased and the MYH10 protein was still present in the cytoplasm of mature MK. Myosin II activity inhibition by blebbistatin rescued the ploidy defect of FPD/AML MK. Finally, we demonstrate that MYH9 is a direct target of RUNX1 by chromatin immunoprecipitation and luciferase assays and we identified new RUNX1 binding sites in the MYL9 promoter region. Together, these results demonstrate that the defects in megakaryopoiesis observed in FPD/AML are, in part, related to a deregulation of myosin IIA and IIB expression leading to both a defect in ploidization and proplatelet formation.