Fli1 Is Essential for the Maintenance of Hematopoietic Stem Cell Homeostasis and Function

The ETS family of transcription factors is known to play an essential role in hematopoietic and vascular development. One such factor that is widely expressed in all vascular beds and almost all hematopoietic lineages, extending from long term hematopoietic stem cells to terminally differentiated peripheral blood cells, is Fli1. Global deletion of Fli1 leads to embryonic lethality at E12.5 due to dramatic hemorrhaging caused by poor vascular integrity and platelet dysfunction. The role of Fli1 in adults has been explored in several different cells types ranging from megakaryocytes to B and T cells. It has also been implicated in transcriptional regulation of hematopoietic stem and progenitor cells through combinatorial analysis, however its exact contribution to stem cell maintenance and function remains unclear. We hypothesized that Fli1 plays a dual role in regulating both the seed and the soil - specifically hematopoietic stem cell (HSC) function in a cell autonomous manner as well as the niche required for nurturing these cells. In this study, we focused our attention on the cell autonomous function of Fli1. We found that global deletion of Fli1 leads to lethality as a result of complete peripheral blood failure in addition to aberrant vasculature. Using the cre-lox system and various transplantation strategies, we identify Fli1 as one of the critical regulators of adult hematopoietic stem cell function. Specific deletion of Fli1 in the hematopoietic compartment alone is sufficient to induce significant reduction in peripheral blood counts, accompanied by hemorrhage, resulting in lethality. On further inspection, Fli1^-/- HSCs are unable to expand ex vivo or engraft in a competitive setting. Additionally, Fli1 is essential for hematopoietic reconstitution post radiation and its deletion abolishes the ability of stem cells to reconstitute the bone marrow and contribute to peripheral blood lineages. ChIP-seq analysis demonstrates Fli1 occupancy in almost all essential hematopoietic gene enhancers and promoters. By correlating Fli1^-/- HSC RNA seq analysis with the ChIP occupancy data we can conclude that Fli1 is, in fact, one of the master regulators of the HSC transcriptional program essential to maintain the balance between self-renewal and differentiation. Taken together, our results indicate that Fli1 is required for maintenance of HSC homeostasis and function, making it one of the unique transcription factors to play a critical role in HSC function in adults as well as during development.