Bclaf1 promotes hematopoietic stem cell repopulating capacity and self-renewal

Abstract Hematopoietic stem cells (HSC) balance differentiation and self-renewal to promote maintenance of mature blood cell lineages. The mechanisms that regulate HSC expansion during fetal hematopoietic development and adult HSC maintenance remain significant knowledge gaps. BCLAF1 (Bcl2-associated factor 1), a transcriptional regulator, is highly expressed in hematopoietic cells. We recently defined its function as a modulator of ETS-family transcription factor activity in developing B cells. Our objective in this study is to determine the molecular mechanism of BCLAF1 in regulation of fetal and adult HSCs. We used mouse models with selective deletion of Bclaf1 in hematopoietic cells to test whether BCLAF1 regulates HSC development and function. We find that loss of Bclaf1 results in significantly reduced numbers of fetal HSCs. The reduced HSC numbers persist in adult mice; but there is no additional decline or development of bone marrow failure with aging. In competitive transplant experiments, Bclaf1-deficient fetal and adult HSCs have defective repopulation and self-renewal capacity resulting in reduced reconstitution of all hematopoietic populations. Bclaf1-deficient HSCs do not have abnormalities in emergence, proliferation, cell death, or homing activity. Using single cell RNA-sequencing of fetal HSCs, we find that loss of BCLAF1 results in reduction of a subpopulation of HSCs and upregulation of the early response genes Jun, Fos, and Egr1. Collectively, these findings suggest BCLAF1 restrains activity of immediate early genes to preserve HSC self-renewal capacity and promote HSC expansion during both fetal development and repopulation of hematopoiesis after HSC transplant. Supported by grants from Children's Discovery Institute, St. Louis Children's Hospital Foundation, Siteman Cancer Center, and NIH (T32 AI007163).