The Runx-PU.1 pathway preserves normal and AML/ETO9a leukemic stem cells

Runx transcription factors contribute to hematopoiesis and are frequently implicated in hematologic malignancies. All three Runx isoforms are expressed at the earliest stages of hematopoiesis; however their function in hematopoietic stem cells (HSCs) is not fully elucidated. Here, we show that Runx factors are essential in HSCs by driving the expression of the hematopoietic transcription factor PU.1. Mechanistically, using a knock-in mouse model in which all three Runx binding sites in the -14kb enhancer of PU.1 are disrupted we observed failure to form chromosomal interactions between the PU.1 enhancer and its proximal promoter. Consequently, decreased PU.1 levels resulted in diminished long-term HSC function through HSC-exhaustion, which could be rescued by reintroducing a PU.1 transgene. Similarly, in a mouse model of AML/ETO9a leukemia, disrupting the Runx binding sites resulted in decreased PU.1 levels. Leukemia onset was delayed and limiting dilution transplantation experiments demonstrated functional loss of leukemia initiating cells. This is surprising, since low PU.1 levels have been considered a hallmark of AML/ETO leukemia, as indicated in mouse models and as shown here in leukemic patient samples. Our data demonstrate that Runx-dependent PU.1 chromatin interaction and transcription of PU.1 are essential for both normal and leukemia stem cells.