Common DNA Damage Response Factors Required for Cellular Resistance to Inhibitors for the Ataxia Telangiectasia and Rad3-Related Checkpoint Kinase in Hematopoietic Cells.

Targeting checkpoints is one of the most promising strategies in cancer chemotherapy. Leukemia, in particular, is expected to yield high therapeutic efficacy due to its high replication stress. However, the DNA damage response factors involved in the vulnerability to checkpoint inhibitors of these hematopoietic cancers remain elusive. In this study, we reveal common factors required for cellular resistance to ATR inhibition in hematopoietic cancer cells. We explored the DNA damage response pathways contributing to cellular tolerance to three types of ATR inhibitors using an isogenic DNA repair factor mutant collection derived from the chicken lymphoma cell line, DT40. We first demonstrated significant ATR inhibition activity of the recently developed Torin2 analogous compounds, SPK67 and SPK98, under stressed replication conditions. We then compared cellular sensitivity patterns of the known ATR inhibitor, VE-821, and the potential ATR inhibitors, SPK67 and SPK98, in 24 types of mutants deficient in genome maintenance systems and found that RAD17/-, FEN1-/-, and POLB-/- cells exhibited hypersensitivity to all these drugs. Consistently, these mutant cells exhibited increased chromosome instability upon treatment with VE-821, SPK67, and SPK98, resulting in apoptosis. These results suggest that Rad17, Fen1, and Polymerase β play roles in responding to DNA damage caused by these drugs. However, ATR inhibition did not result in cell-cycle arrest, Chk1 phosphorylation, or increased γH2AX levels. These results suggest that, although ATR inhibition causes DNA damage, impaired checkpoint function suppresses the appropriate activation of DNA damage signaling pathways, thereby leading to cell death. This study is the first to demonstrate the importance of Rad17, Fen1, and Polymerase β in cellular tolerance to ATR inhibition in hematopoietic cells.