Targeting DNA Replication Stress for Cancer Therapy

Abstract

The human cellular genome is under constant stress from extrinsic and intrinsic factors, which can lead to DNA damage and defective replication. In normal cells, DNA damage response (DDR) mediated by various checkpoints will either activate the DNA repair system or induce cellular apoptosis/senescence, therefore maintaining overall genomic integrity. Cancer cells, however, due to constitutive growth signaling and defective DDR, may exhibit "replication stress" -a phenomenon unique to cancer cells that is described as the perturbation of error-free DNA replication and slow-down of DNA synthesis. Although replication stress has been proven to induce genomic instability and tumorigenesis, recent studies have counterintuitively shown that enhancing replicative stress through further loosening of the remaining checkpoints in cancer cells to induce their catastrophic failure of proliferation may provide an alternative therapeutic approach. In this review, we discuss the rationale to enhance replicative stress in cancer cells, past approaches using traditional radiation and chemotherapy, and emerging approaches targeting the signaling cascades induced by DNA damage. We also summarize current clinical trials exploring these strategies and propose future research directions including the use of combination therapies, and the identification of potential new targets and biomarkers to track and predict treatment responses to targeting DNA replication stress.

Keywords: DNA replication stress; cancer; targeted therapy.

Figure 1

Rationale for enhancing replication stress to kill cancer cells. DNA replicative stress can be induced by various factors including ROS, insufficient dNTP, oncogene activation or the loss/inactivation of tumor suppressors, etc. At the low to mild level, the replicative stress predominantly induces genomic instability, therefore facilitates tumorigenesis and cancer progression. However, when the replicative stress is enhanced to a high level through further loss of checkpoints, cancer cells may enter the mitotic phase with incomplete or uncorrected DNA replication, which eventually leads to cell death through mitotic catastrophe. Therefore, enhancing replicative stress can be a novel approach to kill cancer cells. TSG: Tumor suppressing gene.

Figure 2

Illustration of various approaches to target replication stress for cancer treatment. For simplicity, only the approaches that have been discussed in the text are illustrated here. As indicated, with the progression from DNA replication fork stalling to folk collapse and eventually premature entry of mitotic phase, there is accompanying enhancement of DNA replication stress. Shown here are different approaches exploited to enhance this stress level. While chemotherapeutic agents use different approaches to induce fork stalling (e.g., nucleotide disincorporation, DNA crosslinks and topoisomerase—DNA complex), radiation (XRT) induces direct DNA damage. These genetic errors activate ATR-Chk1 signaling which prevents further fork stalling. Therefore, inhibitors of either ATR or Chk1 may enhance replicative stress. Since both PARP and MELK prevent the progression to fork collapse, their corresponding inhibitors may also augment the level of replicative stress. Because the ubiquitin ligase substrate CDT1 causes DNA to replicate more than once and its activity is inhibited by neddylation, the NAE inhibitor can also be used to achieve this purpose. Finally, WEE1 inhibitor activates CDK1/2, therefore facilitating premature entry to S phase. The final consequence of all these approaches is cell death through mitotic catastrophe that is induced by the enhancement of replicative stress.

Figure 3

Current and potential targeting strategies in association with the cell cycle. Shown here are two major checkpoints, G1/S and G2/M. The G2/M checkpoint is crucial to induce cell cycle arrest and allow the cell to repair DNA defects before it enters M phase. Both checkpoints, however, can be inhibited by CDKs which facilitate cell cycle progression regardless of the DNA defects. Theoretically, if the cell is allowed to enter mitosis without DNA damage properly fixed, cell death could happen due to mitotic catastrophe. Therefore, in the setting of replicative stress, approaches facilitating cell cycle progression through G2-M, such as activating CDKs or reducing the inhibitory effect on CDKs could have a cell killing effect. DNA damage will result in both DSBs and ssDNA/SSBs, with the latter as the major cause of replicative stress. ssDNA/SSBs activate ATR-Chk1 signaling, which in turn activates tumor suppressors p53 and p21 and inhibits CDKs. ssDNA/SSBs may also activate PARP which in turn enhances Chk1 activity. Wee1 can directly inhibit CDKs. Therefore, inhibitors of PARP, ATR, Chk1 and Wee1 will all activate CDKs and allow the cell to enter mitosis despite the presence of unrepaired DNA. If SSBs are not fixed, they may become secondary DSBs, therefore targeting DSB-induced DNA repair mechanisms could also be a promising strategy. DSBs activate the tumor suppressor BRCA as well as ATM-Chk2 signaling and DNA-PK. Chk2 also activates p53. Therefore, it is not surprising that inhibition of ATR-Chk1 could be particularly useful for p53 deficient tumors, and PARP inhibition for patients with BRCA mutation. (DSBs: Double strand breaks; SSBs: Single strand breaks; DNA-PK: DNA-dependent protein kinase; CDK: Cyclin-dependent kinase).