DNA fork remodeling proteins, Zranb3 and Smarcal1, are uniquely essential for aging hematopoiesis.

Over a lifetime, hematopoietic stem and progenitor cells (HSPCs) are forced to repeatedly proliferate to maintain hematopoiesis, increasing their susceptibility to DNA damaging replication stress. However, the proteins that mitigate this stress, protect HSPC replication, and prevent aging-driven dysregulation are unknown. We report two evolutionarily conserved, ubiquitously expressed chromatin remodeling enzymes with similar DNA replication fork reversal biochemical functions, Zranb3 and Smarcal1, have surprisingly specialized roles in distinct HSPC populations. While both proteins actively mitigate replication stress and prevent DNA damage and breaks during lifelong hematopoiesis, the loss of either resulted in distinct biochemical and biological consequences. Notably, defective long-term HSC function, revealed with bone marrow transplantation, caused hematopoiesis abnormalities in young mice lacking Zranb3. Aging significantly worsened these hematopoiesis defects in Zranb3-deficient mice, including accelerating the onset of myeloid-biased hematopoietic dysregulation to early in life. Such Zranb3-deficient HSPC abnormalities with age were driven by accumulated DNA damage and replication stress. Conversely, Smarcal1 loss primarily negatively affected progenitor cell functions that were exacerbated with aging, resulting in a lymphoid bias. Simultaneous loss of both Zranb3 and Smarcal1 compounded HSPC defects. Additionally, HSPC DNA replication fork dynamics had unanticipated HSPC type and age plasticity that depended on the stress and Zranb3 and/or Smarcal1. Our data reveal both Zranb3 and Smarcal1 have essential HSPC cell intrinsic functions in lifelong hematopoiesis that protect HSPCs from replication stress and DNA damage in unexpected, unique ways.