Loss of the DNA Fork Remodeling Protein Smarcal1 Impairs the Replication Stress Response in Proliferating Hematopoietic Cells

Rapidly proliferating hematopoietic cells are particularly prone to threats to genomic integrity through replication stress. The response to replication stress involves complex molecular machinery to prevent replication fork collapse, avoid accumulation of DNA damage, and ensure the completion of DNA synthesis. Smarcal1, a DNA remodeling enzyme, has been linked to the replication stress response through stabilization, repair, and restart of stalled DNA replication forks. In humans, complete loss of function of Smarcal1 leads to the pleiotropic disorder Schimke immuno-osseous dysplasia (SIOD), which is characterized, in part, by a severe lymphoid immunodeficiency that leaves patients susceptible to opportunistic infections. Currently, the mechanisms driving the clinical immunodeficient phenotype of SIOD and the in vivo functions of Smarcal1 in hematopoietic cells remain largely unexplored. We evaluated the contribution of Smarcal1 to normal and stressed/emergency hematopoiesis, using a Smarcal1 knockout mouse model and hematopoietic cell replication stresses such as low-dose radiation, 5-fluorouracil, bone marrow transplantation, and oncogene- driven proliferation. We determined that, while loss of Smarcal1 does not affect hematopoietic stem and progenitor cells (HSPCs) or mature hematopoietic cell populations at a normal "resting" state, Smarcal1 is required in these cells undergoing proliferative stress. Following DNA replication stress, we detected defects in multiple Smarcal1-deficient hematopoietic cell populations. We also observed that loss of Smarcal1 led to significantly increased levels of replication stress, DNA damage, and apoptosis in proliferating cells. Through bone marrow transplantation, we determined that the deficiencies observed were not due to a defect in the bone marrow environment or an inability of the HSPCs to home to the bone marrow. Thus, our results reveal that Smarcal1 is critical to maintaining hematopoietic cell survival and function under conditions of DNA replication stress. Our data also suggest that the immunodeficiency of patients with Smarcal1 mutations may be due to an inability of hematopoietic cells, especially HSPCs, to properly respond to replication stress and proliferate. Since wild-type bone marrow is able to fully reconstitute and function in a Smarcal1-deficient environment, bone marrow transplantation may be an effective therapy for immunodeficient SIOD patients.