Control of replication stress and mitosis in colorectal cancer stem cells through the interplay of PARP1, MRE11 and RAD51.

Cancer stem cells (CSCs) are tumor subpopulations driving disease development, progression, relapse and therapy resistance, and their targeting ensures tumor eradication. CSCs display heterogeneous replication stress (RS), but the functionality/relevance of the RS response (RSR) centered on the ATR-CHK1 axis is debated. Here, we show that the RSR is efficient in primary CSCs from colorectal cancer (CRC-SCs), and describe unique roles for PARP1 and MRE11/RAD51. First, we demonstrated that PARP1 is upregulated in CRC-SCs resistant to several replication poisons and RSR inhibitors (RSRi). In these cells, PARP1 modulates replication fork speed resulting in low constitutive RS. Second, we showed that MRE11 and RAD51 cooperate in the genoprotection and mitosis execution of PARP1-upregulated CRC-SCs. These roles represent therapeutic vulnerabilities for CSCs. Indeed, PARP1i sensitized CRC-SCs to ATRi/CHK1i, inducing replication catastrophe, and prevented the development of resistance to CHK1i. Also, MRE11i + RAD51i selectively killed PARP1-upregulated CRC-SCs via mitotic catastrophe. These results provide the rationale for biomarker-driven clinical trials in CRC using distinct RSRi combinations.

The bromodomain and extra-terminal domain inhibitor JQ1 synergistically sensitizes human colorectal cancer cells to topoisomerase I inhibitors through repression of Mre11-mediated DNA repair pathway.

Camptothecin (CPT) and its derivatives, irinotecan and topotecan are specific topoisomerase I (Top1) inhibitors and potent anticancer drugs. Mechanistically, they induce DNA double-strand breaks (DSBs). Although CPT is an effective chemotherapeutic agent used in the management of advanced colorectal cancer, there exist associated side effects. Herein, we aimed to establish novel drug combinations that can effectively aid in managing the CPT-related side effects. Besides, bromodomain and extra-terminal domain (BET) inhibitors have proved as promising drugs that target epigenetic mechanisms in various cancers, they alter DNA repair processes, hence are a potential candidate for CPT synthetic lethality. A novel BET inhibitor JQ1 synergized with CPT, exerted antiproliferative effects. Through cell cycle analyses and apoptosis assays, we revealed that a combination of CPT and JQ1 induces subG1-phase arrest and enhances cell apoptosis. This combination increased the intensity of γ-H2AX staining, a specific marker of DSBs. Moreover, colorectal cancer cells highly expressing Top1 showed greater sensitivity to JQ1, which was lowered through the lentiviral shRNA-mediated knockdown of Top1. JQ1, combined with CPT, impeded the recruitment of the Mre11-mediated MRN complex. Finally, JQ1 enhanced the in vivo sensitivity of tumors to CPT without inducing toxicity. These results demonstrate that a combination of BET inhibitor with Top1 inhibitor is safe and exerts positive chemotherapeutic effects in colorectal cancer.