Runx1 Promotes Murine Erythroid Progenitor Proliferation and Inhibits Differentiation by Preventing Pu.1 downregulation

Abstract

Pu.1 is an ETS family transcription factor (TF) that plays critical roles in erythroid progenitors by promoting proliferation and blocking terminal differentiation. However, the mechanisms controlling expression and downregulation of Pu.1 during early erythropoiesis have not been defined. In this study, we identify the actions of Runx1 and Pu.1 itself at the Pu.1 gene Upstream Regulatory Element (URE) as major regulators of Pu.1 expression in Burst-Forming Unit erythrocytes (BFUe). During early erythropoiesis, Runx1 and Pu.1 levels decline and chromatin accessibility at the URE is lost. Ectopic expression of Runx1 or Pu.1, both of which bind the URE, prevents Pu.1 downregulation and blocks terminal erythroid differentiation, resulting in extensive ex vivo proliferation and immortalization of erythroid progenitors. dCas9 mediated occlusion of Runx1 binding sites within the URE leads to decreased Pu.1 expression and ectopic expression of Runx1 in BFUe lacking a URE fails to block terminal erythroid differentiation. Thus, Runx1, acting at the URE, and Pu.1 itself, directly regulate Pu.1 levels in erythroid cells and loss of both factors is critical for Pu.1 downregulation during terminal differentiation. The molecular mechanism of URE inactivation in erythroid cells through loss of TF binding represents a distinct pattern of Pu.1 regulation from those described in other hematopoietic cell types such as T-cells which downregulate Pu.1 through active repression. The importance of downregulation of Runx1 and Pu.1 in erythropoiesis is further supported by genome-wide analyses showing that their DNA-binding motifs are highly over-represented in regions that lose chromatin accessibility during early erythroid development.