Loss of CHTF18-RFC2/5 leads to replicative gaps and sensitivity to PARP inhibitors.

The use of PARP inhibitors (PARPi) has profoundly changed the treatment of BRCA1/BRCA2-mutated cancers. Despite this, acquired resistance to PARPi has become a major challenge in the clinic. Hence, a more detailed understanding of the mechanisms underlying PARPi sensitivity is crucially needed. Here, we show that loss of the alternative clamp loader complex CHTF18-RFC2/5 leads to a remarkable sensitization to PARPi. Loss of CHTF18 is not associated with defective RAD51 foci formation excluding a defect in homologous recombination. On the contrary, treatment with PARPi triggers replicative gap accumulation in CHTF18 knockout (KO) cells. By performing transient silencing experiments, we highlight PARP1-PARP2 trapping at replicative gaps as a major determinant of sensitivity to these compounds. Crucially, loss of 53BP1 does not rescue PARPi sensitivity in CHTF18 KO cells, outlining Polε and the CHTF18-RFC2/5 complex as potential novel targets for cancer therapeutics.