Polymerase θ inhibition activates the cGAS-STING pathway and cooperates with immune checkpoint blockade in models of BRCA-deficient cancer.

Recently developed inhibitors of polymerase theta (POLθ) have demonstrated synthetic lethality in BRCA-deficient tumor models. To examine the contribution of the immune microenvironment to antitumor efficacy, we characterized the effects of POLθ inhibition in immunocompetent models of BRCA1-deficient triple-negative breast cancer (TNBC) or BRCA2-deficient pancreatic ductal adenocarcinoma (PDAC). We demonstrate that genetic POLQ depletion or pharmacological POLθ inhibition induces both innate and adaptive immune responses in these models. POLθ inhibition resulted in increased micronuclei, cGAS/STING pathway activation, type I interferon gene expression, CD8+ T cell infiltration and activation, local paracrine activation of dendritic cells and upregulation of PD-L1 expression. Depletion of CD8+ T cells compromised the efficacy of POLθ inhibition, whereas antitumor effects were augmented in combination with anti-PD-1 immunotherapy. Collectively, our findings demonstrate that POLθ inhibition induces immune responses in a cGAS/STING-dependent manner and provide a rationale for combining POLθ inhibition with immune checkpoint blockade for the treatment of HR-deficient cancers.

Targeting DNA Repair with Combined Inhibition of NHEJ and MMEJ Induces Synthetic Lethality in TP53-Mutant Cancers.

DNA repair pathway inhibitors are a new class of anticancer drugs that are advancing in clinical trials. Peposertib is an inhibitor of DNA-dependent protein kinase (DNA-PK), which is a key driver of nonhomologous end-joining (NHEJ). To identify regulators of response to peposertib, we performed a genome-wide CRISPR knockout screen and found that loss of POLQ (polymerase theta, POLθ) and other genes in the microhomology-mediated end-joining (MMEJ) pathway are key predictors of sensitivity to DNA-PK inhibition. Simultaneous disruption of two DNA repair pathways via combined treatment with peposertib plus a POLθ inhibitor novobiocin exhibited synergistic synthetic lethality resulting from accumulation of toxic levels of DNA double-strand break end resection. TP53-mutant tumor cells were resistant to peposertib but maintained elevated expression of POLQ and increased sensitivity to novobiocin. Consequently, the combination of peposertib plus novobiocin resulted in synthetic lethality in TP53-deficient tumor cell lines, organoid cultures, and patient-derived xenograft models. Thus, the combination of a targeted DNA-PK/NHEJ inhibitor with a targeted POLθ/MMEJ inhibitor may provide a rational treatment strategy for TP53-mutant solid tumors. SIGNIFICANCE: Combined inhibition of NHEJ and MMEJ using two nontoxic, targeted DNA repair inhibitors can effectively induce toxic DNA damage to treat TP53-deficient cancers.