ATM safeguards DNA replication at endogenous base lesions

Poly(ADP-ribose) polymerase inhibitors (PARPi) exploit homologous recombination deficiency (HRD) in BRCA1/2-mutated cancers to induce synthetic lethality 1–3 . PARPi also kill cancer cells lacking the DNA damage-responsive kinase ATM 4–6 , however, inconsistent evidence of HRD 7–9 and variable clinical responses 9–11 have obscured the underlying mechanism. Here we define how PARPi induce cytotoxicity in ATM-deficient cells and reveal a critical role for ATM in regulating DNA replication. In the absence of ATM, unrestrained PRIMPOL-dependent repriming at spontaneous oxidative base adducts generates discontinuous daughter strands containing DNA gaps that activate PARP. This defect is sustained by aberrant suppression of replication fork slowing – presumably via fork reversal – by the BRCA1-A complex, whose recruitment to stalled forks is normally counteracted by ATM. The resulting gaps require homologous recombination (HR) for post-replicative repair and underlie the synthetic lethal interaction with PARPi. Suppressing repriming, reducing oxidative stress, or blocking base excision repair alleviates these defects. Collectively, our findings reveal how spontaneous base damage cooperates with replication dysfunction to drive PARPi sensitivity and establish a paradigm of post-replicative repair addiction in ATM-deficient cells. Together, our findings define a mechanistic link between oxidative DNA damage and ATM-dependent replication control, illuminating how oxidative stress may exacerbate genome instability in Ataxia Telangiectasia.