Search

Poly (ADP-ribose) glycohydrolase (PARG) is a dePARylating enzyme which promotes DNA repair by removal of poly (ADP-ribose) (PAR) from PARylated proteins. Loss or inhibition of PARG results in replication stress and sensitizes cancer cells to DNA-damaging agents. PARG inhibitors are now undergoing clinical development for patients having tumors with homologous recombination deficiency (HRD), such as cancer patients with germline or somatic BRCA1/2 -mutations. PARP inhibitors kill BRCA-deficient cancer cells by increasing single-stranded DNA gaps (ssGAPs) during replication. Here, we report that, like PARP inhibitor (PARPi), PARG inhibitor (PARGi) treatment also causes an accumulation of ssGAPs in sensitive cells. PARGi exposure increased accumulation of S-phase-specific PAR, a marker for Okazaki fragment processing (OFP) defects on lagging strands and induced ssGAPs, in sensitive cells but not in resistant cells. PARGi also caused accumulation of PAR at the replication forks and at the ssDNA sites in sensitive cells. Additionally, PARGi exhibited monotherapy activity in specific HR-deficient, as well as HR-proficient, patient-derived, or patient-derived xenograft (PDX)-derived organoids of ovarian cancer, and drug sensitivity directly correlated with the accumulation of ssGAPs. Taken together, PARGi treatment results in toxic accumulation of PAR at replication forks resulting in ssGAPs due to OFP defects during replication. Regardless of the BRCA/ HRD - status, the induction of ssGAPs in preclinical models of ovarian cancer cells correlates with PARGi sensitivity. Patient-derived organoids (PDOs) may be a useful model system for testing PARGi sensitivity and functional biomarkers., Competing Interests: Competing interests statement:A.D.D. reports consulting for AbbVie, Deerfield Management Company L.P., Impact Therapeutics, PrimeFour Therapeutics, Schrödinger Inc., and Servier Bio-Innovation LLC; is an Advisory Board member for Covant Therapeutics, and Impact Therapeutics; stockholder in Impact Therapeutics, and PrimeFour Therapeutics; and reports receiving commercial research grants from Bristol Myers Squibb, EMD Serono, Moderna, and Tango Therapeutics. G.I.S. reports grant support from Merck KGaA/EMD-Serono, Tango Therapeutics, Bristol Myers Squibb, and Merck & Co., all related to DNA repair inhibitors, as well as from Eli Lilly and Pfizer. He has served on advisory boards for Merck KGaA/EMD-Serono, Circle Pharmaceuticals, Schrödinger, Janssen and Xinthera. He holds patents entitled, “Dosage regimen for sapacitabine and seliciclib,” also issued to Cyclacel Pharmaceuticals, and “Compositions and methods for predicting response and resistance to CDK4/6 inhibition.” J. Liu reports personal fees from AstraZeneca, Bristol Myers Squibb, Clovis Oncology, Daiichi Sankyo, Eisai, Genentech/Roche, GlaxoSmithKline, Regeneron Therapeutics, Revolution Medicine, Zentalis Pharmaceuticals, and Deciphera Pharmaceuticals outside the submitted work and institutional funding for clinical trials from 2X Oncology, Aravive, Arch Oncology, AstraZeneca, Bristol Myers Squibb, Clovis Oncology, GlaxoSmithKline, Impact Therapeutics, Pfizer, Regeneron, Seagen, SystImmune, Vigeo Therapeutics, and Zentalis Pharmaceuticals.

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., (© 2023. The Author(s).)

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., (©2022 American Association for Cancer Research.)