FZR1 deficiency induces synthetic lethality with ATR inhibition by promoting premature cell cycle and mitotic catastrophe in acute lymphoblastic leukemia

Clinical options to treat refractory and relapsed childhood acute lymphoblastic leukemia (ALL) remain limited. As ALL cells could be dependent on the DNA damage response pathway to cope with relapse and clonal evolution, we hypothesized that ATR inhibitors (ATRi) could offer a novel therapeutic strategy for these aggressive ALLs. We found that the ATR checkpoint kinase is essential for ALL cell survival, and that ATR-related pathways are significantly upregulated in relapsed ALL patients. To identify patients most likely to benefit from ATR inhibition, we performed a focused CRISPR/Cas9 screen with a clinically available ATRi, Ceralasertib. This screen, coupled with functional characterization, identified the APC/C co-activator FZR1 as a critical determinant of Ceralasertib sensitivity and implicated a synthetic lethal interaction between FZR1 deficiency and ATR inhibition both in vitro and in vivo. Mechanistically, FZR1 deficiency drives cells into premature S-phase by upregulating the ubiquitin ligase SKP2 and degrading the CKIs p21 and p27. This G1/S checkpoint disruption, combined with ATRi-mediated failure of S and G2/M checkpoints, dramatically exacerbates DNA damage accumulation, ultimately triggering lethal mitotic catastrophe at metaphase. Furthermore, the pharmacological APC/C inhibitor proTAME effectively synergized with Ceralasertib in ALL cell lines and primary cells. Our study suggests FZR1 status as a potential predictive biomarker for ATRi against refractory and relapsed ALL, and reveals a novel synthetic lethal mechanism among cell cycle checkpoints leading to mitotic catastrophe in cancer.