CRISPR screens identify POLB as a synthetic lethal enhancer of PARP inhibition exclusively in BRCA-mutated tumors.

Poly (ADP-ribose) polymerases inhibitors (PARPi) are an approved class of anticancer therapeutics that inhibit the activities of PARP1/2 and produce synthetic lethality in BRCA1/2 mutated cancers due to the absence of a functional homologous recombination (HR)-dependent DNA repair pathway. Although PARP inhibitors have led to successful clinical outcomes, two-thirds of patients develop acquired resistance, limiting long-term utility as maintenance therapy. Motivated by this clinical need, we utilized a CRISPR target discovery screening platform to identify POLB as a gene that acts selectively and synergistically with PARPi in BRCA1/2 mutated cancers and found that POLB knockout along with PARPi treatment enhanced loss of viability in BRCA1/2 mutant and BRCA2 null cells, but not in isogenic BRCA1/2 wildtype cells. Overexpression of either POLB wildtype or catalytically inactive mutants confirmed that perturbation of both the polymerase and lyase catalytic activities of POLB are required for synergistic PARP-BRCA synthetic lethality. Mechanistically, POLB knockout was associated with an increase in single- and double-strand DNA breaks, cell cycle arrest, and apoptosis when in combination with PARP inhibition. The translational nature of this interaction was further examined using murine xenograft models of BRCA1 mutant and BRCA2 null cell lines, wherein the combination of POLB knockout and niraparib led to profound tumor regression and prevented tumor regrowth even after cessation of treatment. Together, these results suggest that POLB is a synergistic enhancer of the synthetic lethal interaction between PARP and BRCA and support POLB as a promising therapeutic target for improving antitumor responses to PARPi in HR-deficient cancers.