Analyzing the Opportunities to Target DNA Double-Strand Breaks Repair and Replicative Stress Responses to Improve Therapeutic Index of Colorectal Cancer

Abstract

Despite the ample improvements of CRC molecular landscape, the therapeutic options still rely on conventional chemotherapy-based regimens for early disease, and few targeted agents are recommended for clinical use in the metastatic setting. Moreover, the impact of cytotoxic, targeted agents, and immunotherapy combinations in the metastatic scenario is not fully satisfactory, especially the outcomes for patients who develop resistance to these treatments need to be improved. Here, we examine the opportunity to consider therapeutic agents targeting DNA repair and DNA replication stress response as strategies to exploit genetic or functional defects in the DNA damage response (DDR) pathways through synthetic lethal mechanisms, still not explored in CRC. These include the multiple actors involved in the repair of DNA double-strand breaks (DSBs) through homologous recombination (HR), classical non-homologous end joining (NHEJ), and microhomology-mediated end-joining (MMEJ), inhibitors of the base excision repair (BER) protein poly (ADP-ribose) polymerase (PARP), as well as inhibitors of the DNA damage kinases ataxia-telangiectasia and Rad3 related (ATR), CHK1, WEE1, and ataxia-telangiectasia mutated (ATM). We also review the biomarkers that guide the use of these agents, and current clinical trials with targeted DDR therapies.

Keywords: DNA double strand break repair; colorectal cancer; replication stress; target therapy.

Figure 1

Cell cycle-dependent response to DSB formation. Depending on the stage of cell cycle (A), the induction of DSB (B) will lead to distinct cellular ATM or ATR signaling responses and DNA repair pathways. While DNA-PK drives NHEJ repair without resection of DNA ends, the MRN-CtIP removes DNA-PK and HR can function and use sister chromatids as a template for DNA repair synthesis. (C) Upon some context, including HR defect, the MMEJ and SSA repair pathways can act on the resected single-stranded DNA. MMEJ can act as a backup for NHEJ in G1 phase.

Figure 2

Major double-strand break (DSB) repair pathways in human cells. DSBs in G1 of the cell cycle are primarily repaired by non-homologous end joining (NHEJ) (Top). This pathway is initiated by the Ku heterodimer, which recognizes broken DNA. NHEJ leads to repair with minimal alteration to the original sequence. DSBs in S/G2 phases are subjected to resection leading to stretches of single-stranded DNA. Resected DSBs are substrates for homologous recombination (HR) with a critical role of Rad51 leading to an accurate repair synthesis. When HR is defective (for example when BRCA genes are mutated), an alternative pathway named Pol theta-mediated end-joining (TMEJ) can act as a back-up repair pathway. Polθ promotes the synapsis of the opposing ends, identifies internal microhomologies, which can be annealed, and performs a repair DNA synthesis with poor processivity and frequent aborted synthesis, resulting in a high rate of mutations. Single-strand annealing (SSA) is a HR sub-pathway in mammalian cells with the essential role of RAD52 DNA-binding protein. SSA is an error-prone repair leading to large deletions between the homologous repeats.

Figure 3

Possible targets for DDR inhibitors (repair proteins in blue and checkpoint kinases in grey) in CRC chemotherapy with supporting evidence in cancer cell lines and animal models, or candidates “to be tested” (in red), with estimated sensitivity based on results obtained in cells with deficiency of the respective target. HRD—homologous recombination deficiency, IR—ionizing radiation, Topoi—topoisomerase inhibitor, MMS—microsatellite stable, MSI—microsatellite instable, RS—replication stress, dATR-Chk1—deficient ATR-Chk1 signaling, FOLFIRI—folinic acid/fluorouracil/irinotecan chemotherapy regimen.