Targeting RNA Polymerase I transcription synergises with TOP1 inhibition in potentiating the DNA damage response in high-grade serous ovarian cancer

Abstract

Limited effective therapeutic options are available for patients with recurrent highgrade serous carcinoma (HGSC), the most common histological subtype accounting for the majority of ovarian cancer deaths. We have shown efficacy in poly-ADP ribose polymerase (PARP) inhibitor-resistant HGSC for the RNA Polymerase I (Pol I) transcription inhibitor CX-5461 through its ability to activate a nucleolar-associated DNA damage response (DDR). Here, we screen the protein-coding genome to identify potential targets whose inhibition enhances the efficacy of CX-5461. We identify a network of cooperating inhibitory interactions, including components of homologous recombination (HR) DNA repair and DNA topoisomerase 1 (TOP1). We highlight that CX-5461 combined with topotecan, a TOP1 inhibitor used as salvage therapy in HGSC, induces robust cell cycle arrest and cell death in a panel of HRproficient HGSC cell lines. The combination potentiates a nucleolar-associated DDR via recruitment of phosphorylated replication protein A (RPA) and ataxia telangiectasia and Rad3 related protein (ATR). CX-5461 plus low-dose topotecan cooperate to potently inhibit xenograft tumour growth, indicating the potential for this strategy to improve salvage therapeutic regimens to treat HGSC.

rDNA Chromatin Activity Status as a Biomarker of Sensitivity to the RNA Polymerase I Transcription Inhibitor CX-5461

“…The strong association of higher proportions of active rDNA with sensitivity to growth inhibition by CX-5461 is consistent with CX-5461's mode of action in triggering defects associated with open chromatin and replication stress at the rDNA (Quin et al, 2016;Yan et al, 2019;Sanij et al, 2020), including potentially acting as a TOP 2 poison (Bruno et al, 2020) selectively at the rDNA and/or across the genome. We have demonstrated that CX-5461 activates nucleolar ATM and ATR leading to activation of cell cycle checkpoints and global replication-mediated DNA damage (Quin et al, 2016;Yan et al, 2019;Sanij et al, 2020). Our data therefore suggests that cells with a higher ratio of active rDNA are more sensitive to CX-5461-mediated nucleolar DDR and activation of cell cycle checkpoints, with faster proliferating cells being more responsive to cell cycle arrest.…”

“…We have previously shown that CX-5461 activates nucleolar DDR by inducing chromatin defects and replication stress at the rDNA ( Quin et al, 2016 ; Yan et al, 2019 ; Sanij et al, 2020 ). Therefore, we examined whether the number of active rDNA repeats and/or the proportion of active to inactive rDNA repeats are determining factors for CX-5461 sensitivity.…”

“…Pol I transcription is a therapeutic target for small anti-cancer drugs ( Bywater et al, 2012 ; Hein et al, 2013 ; Peltonen et al, 2014 ). The first-in-class Pol I transcription inhibitor, CX-5461 is a promising cancer therapy as a single agent and in combination therapy in pre-clinical models of lymphoma, acute myeloid leukemia, prostate and ovarian cancer ( Bywater et al, 2012 ; Devlin et al, 2016 ; Rebello et al, 2016 ; Hein et al, 2017 ; Yan et al, 2019 ; Sanij et al, 2020 ). Recently, the sensitivity profile of CX-5461 was shown to closely resemble a topoisomerase II (TOP2) poison ( Olivieri et al, 2019 ; Bruno et al, 2020 ).…”

The chemotherapeutic CX-5461 primarily targets TOP2B and exhibits selective activity in high-risk neuroblastoma

“…6A-6B), all genes converging on DNA damage response. Based on our mechanistic work showing the DNA damaging effects of CX-5461 and previous observations of their efficacy in other cancers 43, 44 we chose to investigate drugs targeting these genes as potential combinations with CX-5461. Thus, we screened CHP-134, IMR-5, BE(2)-M17, and KELLY neuroblastoma cell lines with the combination of CX-5461 and AZD-1390 (ATM inhibitor), AZD-6738 (ATR inhibitor), and SN-38 (the active metabolite of irinotecan; TOP1 inhibitor).…”