ATM orchestrates the DNA-damage response to counter toxic non-homologous end-joining at broken replication forks
Gabriel Balmus, Domenic Pilger, Julia Coates, Mukerrem Demir, Matylda Sczaniecka-Clift, Ana C. Barros, Michael Woods, Beiyuan Fu, Fengtang Yang, Elisabeth Chen, Matthias Ostermaier, Tatjana Stankovic, Hannes Ponstingl, Mareike Herzog, Kosuke Yusa, Francisco Munoz Martinez, Stephen T. Durant, Yaron Galanty, Petra Beli, David J. Adams, Allan Bradley, Emmanouil Metzakopian, Josep V. Forment & Stephen P. Jackson Nature communications 10 (87)
Mutations in the ATM tumor suppressor gene confer hypersensitivity to DNA-damaging chemotherapeutic agents. To explore genetic resistance mechanisms, we performed genome-wide CRISPR-Cas9 screens in cells treated with the DNA topoisomerase I inhibitor topotecan. Thus, we here establish that inactivating terminal components of the non-homologous end-joining (NHEJ) machinery or of the BRCA1-A complex specifically confer topotecan resistance to ATM-deficient cells. We show that hypersensitivity of ATM-mutant cells to topotecan or the poly-(ADP-ribose) polymerase (PARP) inhibitor olaparib reflects delayed engagement of homologous recombination at DNA-replication-fork associated single-ended double-strand breaks (DSBs), allowing some to be subject to toxic NHEJ. Preventing DSB ligation by NHEJ, or enhancing homologous recombination by BRCA1-A complex disruption, suppresses this toxicity, highlighting a crucial role for ATM in preventing toxic LIG4-mediated chromosome fusions. Notably, suppressor mutations in ATM-mutant backgrounds are different to those in BRCA1-mutant scenarios, suggesting new opportunities for patient stratification and additional therapeutic vulnerabilities for clinical exploitation.