Background Germline 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.

Aims To understand RUNX1 regulation of F13A1 expression in platelet/megakaryocyte and the mechanisms and consequences of decreased F13A1 in RHD.

Methods We 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.

Results Platelet 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.

Conclusions RUNX1 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 αIIbβ3 activation.

Scientific category – Platelets and thrombopoiesis

Essentials

RUNX1 regulates expression of FXIII-A chain (F13A1) in megakaryocytes (MK) and platelets.

Platelet and MK F13A1 expression and clot contraction are decreased in RUNX1 deficiency.

MK clot contraction, myosin phosphorylation and PAC1-binding are impaired in F13A1 deficiency.

Defective clot contraction in RHD arises from defects in multiple platelet-MK mechanisms.

The authors have declared no competing interest.

This study was supported by research funding from NIH (NHLBI) R01 HL137376 and R01 HL109568 to AKR NIH (NHLBI) R01 HL137207 and HL159006 to LEG NIH (NHLBI) R35 HL150698 to MP American Heart Association Transformational Project Award 20TPA35490278 to LEG RUNX1 Research Program-Alexs Lemonade Foundation grant to MP.

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IRB of Lewis Katz School od Medicine at Temple University gave ethical approval of this work.

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