Abstract 2379: Elaborating the mechanism of PARP synthetic lethality in ATM loss

Abstract PARP inhibitors have transformed the treatment for ovarian cancer, which is known to have high incidence of homologous recombination (HR) deficiency through the loss of BRCA1 or BRCA2 genes. 30-35% of ovarian cancer patients have defects in BRCA1/2 genes caused by germline, somatic or epigenetic alterations. PARP inhibitors induce cell death in HR-deficient cells through the mechanism of synthetic lethality where cancer cells cannot tolerate the loss of both SSB and DSB repair machinery. Although genetic or epigenetic alterations in other pathways such as Fanconi anemia, or in non-authentic homologous recombination repair (HRR) genes such as PTEN, IDH, and CDK12 have been reported to contribute to HR deficiency, mechanistic understanding of HR loss remains to be determined. Another DNA damage repair (DDR) protein, ATM, a kinase that phosphorylates and activates major DNA damage checkpoints, i.e. CHK2 & BRCA1, has also being increasingly recognized recently as a synthetic lethal partner with PARP inhibitors in multiple tumor indications such as prostate, lung and colorectal cancers. ATM functions in both cell cycle arrest and DNA repair in response to DSBs, however how cells respond to inhibition of PARP in the absence of ATM is not yet fully understood. In this study, we demonstrated that loss of ATM function, i.e. via generating an ATM KO by CRISPR/Cas9 in two genetic backgrounds, i.e. in DLD-1 and HeLa cell lines, induced HR-deficiency indicated by the impairment of Rad51-foci formation and increased phosphorylation of histone H2AX in ATM KO (-/-) cell lines. Furthermore DLD-1 ATM KO (-/-) cell line demonstrated increased sensitivity to niraparib (i.e. 45-fold difference in sensitivity compared to the DLD-1 parental, HR-proficient cell line) in the 14day colony formation assay. In comparison, BRCA2 (-/-) KO in DLD-1 gave a 125-fold increase compared to parental cell line. In DLD-1 ATM KO (-/-) cell line, we have also detected that loss of ATM was associated with low BRCA1 and BRCA2 protein expression suggestive of downstream effect on impairment of HR-mediated DNA checkpoint signaling in ATM loss. Thus, PARPi synthetic lethality phenotype observed in tumors with ATM loss could be regulated through the combination effect coming from the loss of both of ATM & BRCA1/2 and the subsequent effect on both HR gene functions. Our findings highlight that ATM has a critical role in DNA repair for the synergism with PARP inhibitors. Citation Format: Asli Muvaffak, Kevin G. Coleman. Elaborating the mechanism of PARP synthetic lethality in ATM loss [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2379.