Transcription Factor RUNX1 Regulates Coagulation Factor XIII-A (F13A1): Decreased Platelet-Megakaryocyte F13A1 Expression and Clot Contraction in RUNX1 Haplodeficiency

BackgroundGermline RUNX1 haplodeficiency (RHD) is associated with thrombocytopenia, platelet dysfunction and predisposition to myeloid malignancies. Platelet expression profiling of a RHD patient showed decreased F13A1, encoding for the A subunit of factor XIII, a transglutaminase that cross-links fibrin and induces clot stabilization. FXIII-A is synthesized by hematopoietic cells, megakaryocytes and monocytes. AimsTo understand RUNX1 regulation of F13A1 expression in platelet/megakaryocyte and the mechanisms and consequences of decreased F13A1 in RHD. MethodsWe performed studies in platelets, HEL cells and human CD34+ cell-derived megakaryocytes including on clot contraction in cells following small inhibitor (si)RNA knockdown (KD) of RUNX1 or F13A1. ResultsPlatelet F13A1 mRNA and protein were decreased in our index patient and in two siblings from an unrelated family with RHD. Platelet-driven clot contraction was decreased in the patient and affected daughter. Promoter studies in HEL cells showed that RUNX1 regulates F13A1 transcription; RUNX1 overexpression increased and (si)RNA RUNX1 KD reduced F13A1 promoter activity and protein. Following RUNX1 or F13A1 KD clot contraction by HEL cells was decreased as were FXIII-A surface expression, myosin light chain phosphorylation and PAC1 binding upon activation. F13A1 expression and clot contraction were impaired on RUNX1 downregulation in human megakaryocytes. ConclusionsRUNX1 regulates platelet-megakaryocyte F13A1 expression, which is decreased in RHD, reflecting regulation of a coagulation protein by a hematopoietic transcription factor. Platelet and megakaryocyte clot contraction is decreased in RHD, related to multiple impaired mechanisms including F13A1 expression, myosin phosphorylation and IIb{beta}3 activation. Scientific category - Platelets and thrombopoiesis EssentialsO_LIRUNX1 regulates expression of FXIII-A chain (F13A1) in megakaryocytes (MK) and platelets. C_LIO_LIPlatelet and MK F13A1 expression and clot contraction are decreased in RUNX1 deficiency. C_LIO_LIMK clot contraction, myosin phosphorylation and PAC1-binding are impaired in F13A1 deficiency. C_LIO_LIDefective clot contraction in RHD arises from defects in multiple platelet-MK mechanisms. C_LI