PARG inhibition in ATM-deficient prostate cancer: from mechanistic discovery to therapeutic potential

BACKGROUND: ATM deficiency is frequently observed in castration-resistant prostate cancer (CRPC). However, effective therapeutic vulnerabilities associated with this genetic alteration remain poorly defined. This study aimed to identify synthetic lethal strategies that selectively target ATM-deficient prostate cancer cells. METHODS: An unbiased small-molecule compound screening was performed to identify agents exhibiting selective cytotoxicity in ATM-deficient prostate cancer cells. Candidate vulnerabilities were validated across multiple prostate cancer cell lines with genetic depletion or restoration of ATM. Mechanistic studies were conducted using molecular and biochemical assays to assess DNA damage, replication stress, and PARylation dynamics. In vivo efficacy was evaluated using ATM-deficient xenograft tumor models. RESULTS: ATM-deficient prostate cancer cells exhibited marked sensitivity to pharmacological inhibition of poly (ADP-ribose) glycohydrolase (PARG) using with PDD00017273, an effect that was consistently observed across multiple cell lines and partially restored by ATM re-expression. Mechanistically, PARG inhibition induced persistent PARylation in ATM-deficient cells, not via canonical DNA double-strand break signaling, but via misincorporated ribonucleotides processed by topoisomerase 1 during DNA replication. This replication-associated PARylation resulted in severe replication stress, checkpoint activation, and accumulation of DNA double-strand breaks, ultimately leading to cell death. This cytotoxic mechanism is distinct from classical PAR-dependent cell death pathways, including NAD⁺ depletion and parthanatos. In vivo, PARG inhibition significantly suppressed the growth of ATM-deficient xenograft tumors cells. CONCLUSIONS: This study identifies PARG inhibition as a previously unrecognized synthetic lethal vulnerability in ATM-deficient prostate cancer. These findings establish a mechanistic link between ATM loss, aberrant ribonucleotide processing, and replication-associated PARylation, supporting the clinical development of PARG inhibitors as a precision therapeutic strategy for ATM-deficient prostate cancer and potentially other malignancies harboring ATM deficiency.