BCLAF1 restrains stress responses in hematopoietic stem cells to support expansion and repopulation [CUT&RUN]

Hematopoietic stem cells (HSC) rapidly expand during fetal development and after stress. Here, we identify BCLAF1 as a regulator of HSC repopulation activity with functions in propagation of fetal HSCs and hematopoietic reconstitution after stem cell transplantation. Using mice with hematopoietic-specific and inducible deletion of Bclaf1, we find that BCLAF1 promotes fetal HSC expansion but is dispensible for maintenance of adult HSCs at steady state. Loss of BCLAF1 in either fetal or adult HSCs significantly impairs their self-renewal and multi-lineage reconstitution activity after stem cell transplantation. Single-cell RNA sequencing of fetal hematopoietic progenitors reveals that loss of BCLAF1 reduces long-term HSCs and restrains expression of stress response genes. BCLAF1 associates with chromatin throughout the genome of fetal and adult hematopoietic cells, likely through indirect mechanisms, to regulate transcriptional programs. These results establish a novel function for the transcriptional regulator BCLAF1 in limiting stress responses in HSCs to preserve their expansion during embryogenesis and after stem cell transplant.