Mechanisms of synthetic lethality between BRCA1/2 and 53BP1 deficiencies and DNA polymerase theta targeting

A synthetic lethal relationship exists between disruption of polymerase theta (Pol theta), and loss of either 53BP1 or homologous recombination (HR) proteins, including BRCA1; however, the mechanistic basis of these observations are unclear. Here we reveal two distinct mechanisms of Pol theta synthetic lethality, identifying dual influences of 1) whether Pol theta is lost or inhibited, and 2) the underlying susceptible genotype. Firstly, we find that the sensitivity of BRCA1/2- and 53BP1-deficient cells to Pol theta loss, and 53BP1-deficient cells to Pol theta inhibition (ART558) requires RAD52, and appropriate reduction of RAD52 can ameliorate these phenotypes. We show that in the absence of Pol theta, RAD52 accumulations suppress ssDNA gap-filling in G2/M and encourage MRE11 nuclease accumulation. In contrast, the survival of BRCA1-deficient cells treated with Pol theta inhibitor are not restored by RAD52 suppression, and ssDNA gap-filling is prevented by the chemically inhibited polymerase itself. These data define an additional role for Pol theta, reveal the mechanism underlying synthetic lethality between 53BP1, BRCA1/2 and Pol theta loss, and indicate genotype-dependent Pol theta inhibitor mechanisms. What underlies the synthetic lethality between BRCA1/2 or 53BP1 loss and Pol theta loss is unclear. Here, the authors show that RPA-RAD52-MRE11 drive lethality when Pol theta is absent in these cells. In BRCA1/2-deficient cells, sensitivity to Pol theta inhibition additionally depends on the inhibited protein.