Augmenting Radiation Sensitivity by Targeting PAR-Dependent Replication Fork Vulnerability in IDH- Mutant Glioma

Mutations in isocitrate dehydrogenase 1 ( IDH1 ) drive the early stages of gliomagenesis while simultaneously imposing replication stress that creates targetable vulnerabilities. Using both in vitro and in vivo models, we show that inhibition of poly(ADP-ribose) glycohydrolase (PARG) induces a poly(ADP-ribose) (PAR)-dependent augmentation of radiosensitivity in IDH1 -mutant glioma cells. Metabolic repletion of NAD ⁺ fails to rescue this effect, indicating that the vulnerability cannot be explained solely by NAD ⁺ depletion. Instead, PARG inhibition profoundly alters replication fork progression and S-phase kinetics in IDH1 -mutant cells. Mechanistically, ionizing radiation preferentially activates replication fork-associated damage response proteins DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and X-ray repair cross-complementing protein 1 (XRCC1) in IDH1 -mutant cells, a response partially reversed by pharmacologic inhibition of mutant IDH1 . Importantly, pharmacologic inhibition of DNA-PKcs with AZD7648 during irradiation disrupts fork-associated repair signaling and markedly enhances cytotoxicity in IDH1 -mutant glioma models. Together, these findings identify a PAR-dependent replication fork vulnerability that can be therapeutically exploited to selectively enhance radiosensitivity in IDH1 -mutant gliomas.