The PARG frontier: mechanisms of PAR turnover and opportunities in precision oncology

ADP-ribosylation is a versatile post-translational modification that governs fundamental processes, including DNA repair, transcription, and stress adaptation. Its homeostasis relies on the dynamic interplay between poly(ADP-ribose) polymerases (PARPs), which assemble mono- or poly-ADP-ribose (PAR) chains on target macromolecules, and ADP-ribosyl hydrolases, which dismantle them. Disruption of this balance leads to the accumulation of toxic PAR and cell death, revealing vulnerabilities that can be therapeutically exploited. PARP inhibitors (PARPis) have revolutionised the treatment of homologous recombination-deficient cancers via synthetic lethality. Yet, emerging resistance limits their long-term efficacy, underscoring the need for novel targets within ADP-ribose signalling. The poly(ADP-ribose) glycohydrolase (PARG), the principal enzyme involved in hydrolysing PAR, has emerged as a compelling candidate: its inhibition amplifies replication stress, drives mitotic catastrophe, and selectively kills cancer cells, particularly those reliant on PAR turnover for survival. Elevated PARG expression correlates with aggressive tumours and poor prognosis, positioning it as both a prognostic biomarker and therapeutic target. This review integrates recent structural and biochemical insights into PARG, highlighting the mechanisms of PAR reversal, regulatory control, and potential synthetic lethal interactions. We also discuss the discovery and development of selective PARG inhibitors, which promise to expand the therapeutic landscape, overcome PARPis resistance, and exploit vulnerabilities in replication-stressed cancers. By bridging mechanistic understanding with translational potential, targeting PARG represents a frontier in precision cancer therapy.