The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity

The clinical success of PARP1/2 inhibitors prompts the expansion of their applicability beyond homologous recombination deficiency. Here, we demonstrate that the loss of the accessory subunits of DNA polymerase epsilon, POLE3 and POLE4, sensitizes cells to PARP inhibitors. We show that the sensitivity of POLE4 knockouts is not due to a compromised response to DNA damage or homologous recombination deficiency. Instead, POLE4 deletion generates replication stress with the accumulation of single-stranded DNA gaps upon PARP inhibitor treatment. In POLE4 knockouts, replication stress leads to elevated DNA-PK signaling revealing a role of POLE4 in regulating DNA-PK activation. Moreover, POLE4 knockouts show synergistic sensitivity to the co-inhibition of ATR and PARP. Finally, POLE4 loss enhances the sensitivity of BRCA1-deficient cells to PARP inhibitors and counteracts acquired resistance consecutive to restoration of homologous recombination. Altogether, our findings establish POLE4 as a promising target to improve PARP inhibitor driven therapies and hamper acquired PARP inhibitor resistance.