The synthetic lethality of targeting cell cycle checkpoints and PARPs in cancer treatment

Abstract

Continuous cell division is a hallmark of cancer, and the underlying mechanism is tumor genomics instability. Cell cycle checkpoints are critical for enabling an orderly cell cycle and maintaining genome stability during cell division. Based on their distinct functions in cell cycle control, cell cycle checkpoints are classified into two groups: DNA damage checkpoints and DNA replication stress checkpoints. The DNA damage checkpoints (ATM-CHK2-p53) primarily monitor genetic errors and arrest cell cycle progression to facilitate DNA repair. Unfortunately, genes involved in DNA damage checkpoints are frequently mutated in human malignancies. In contrast, genes associated with DNA replication stress checkpoints (ATR-CHK1-WEE1) are rarely mutated in tumors, and cancer cells are highly dependent on these genes to prevent replication catastrophe and secure genome integrity. At present, poly (ADP-ribose) polymerase inhibitors (PARPi) operate through "synthetic lethality" mechanism with mutant DNA repair pathways genes in cancer cells. However, an increasing number of patients are acquiring PARP inhibitor resistance after prolonged treatment. Recent work suggests that a combination therapy of targeting cell cycle checkpoints and PARPs act synergistically to increase the number of DNA errors, compromise the DNA repair machinery, and disrupt the cell cycle, thereby increasing the death rate of cancer cells with DNA repair deficiency or PARP inhibitor resistance. We highlight a combinational strategy involving PARP inhibitors and inhibition of two major cell cycle checkpoint pathways, ATM-CHK2-TP53 and ATR-CHK1-WEE1. The biological functions, resistance mechanisms against PARP inhibitors, advances in preclinical research, and clinical trials are also reviewed.

Keywords: Cancer; Cell cycle checkpoint; Drug resistance; PARP inhibitors; Synthetic lethality; Targeted therapy.

Fig. 1

Causes of the DNA damage response and replication stress response. The progression of replication forks faces frequent obstacles, such as RS due to various factors, including a reduced dNTPs pool, b repetitive sequence-composed DNA fragile sites, c TRCs and associated R-loop formation. Additionally, DNA undergoes constantly intrinsic and extrinsic assaults leading to the DDR. Intrinsic assaults include disabled DNA repair, nucleotide stochastic errors, and intracellular metabolite (e.g. ROS) activity, while extrinsic assaults include UV, IR, and anticancer drugs. Several bulky DNA adducts, such as DNA‒protein adducts, DNA intrastrand cross-links and DNA interstrand cross-links, are common causes of DDR and RS. The numbers on the right side represent: 1. DNA‒protein adducts, 2. bulky DNA adducts, 3. DNA intrastrand cross-links, 4. DNA interstrand crosslinks, 5. Base deletion, 6. DNA mismatches, 7. Base insertion, 8. Abasic sites, 9. Single-strand DNA breaks, 10. Double-strand DNA breaks. dNTPs Deoxyribonucleoside triphosphates; TRCs Transcription–replication conflicts; ROS Reactive oxygen species; UV Ultraviolet; IR Ionizing radiation; NER Nucleotide excision repair; FA pathway Fanconi anemia pathway; MMR Mismatch repair; BER Base excision repair; SSBR Single-strand break repair; HR Homologous recombination; NHEJ Nonhomologous end joining

Fig. 2

Multiple roles of two cell cycle checkpoint pathways (ATM/CHK2/TP53 and ATR/CHK1/WEE1) and PARP1 in the DNA damage response (DDR) and replication stress (RS) response. a ATM is stimulated by a few activators or via autophosphorylation through a feedback loop. ATM activates CHK2, which subsequently phosphorylates TP53 and CDC25A, leading to inhibition of CDK2/cyclin E and CDK1/cyclin B and impairment of G1/S phase transition. b Major trigger for ATR is stalled replication fork or resected DSB. The activation of ATR requires RPA-coated ssDNA, ETAA1, and TopBP1. Then, ATR mediates CHK1 phosphorylation, suppressing CDC25A and CDC25B and stimulating WEE1 activity. Alternatively, WEE1 inhibits CDC25C by phosphorylating CDC25C. The inactivation of the CDC family causes CDK2 and CDK1 incompetence, resulting in G2/M phase arrest. c PARP 1 senses single-strand DNA breaks and catalyzes other prominent members and itself through PARPylation, participating in BER, SSBR, and NER. Finally, poly(ADP-ribose) glycohydrolase (PARG) and PARylation itself release PARP1 from SSB sites. Similarly, PARP1 senses DSBs and recruits ATM, MRE11 and BRCA proteins to repair DNA strands via HR or facilitates MMEJ repair pathway activation in the absence of BRCA. d PARP1 plays key roles in fork reversal, fork protection and fork restart during the repair of stalled replication forks. PARP1 Poly(ADP-ribose) polymerase 1; DDR DNA damage response; RS Replication stress; BER Base excision repair; SSBR Single-strand break repair; NER Nucleotide excision repair; PARG Poly (ADP-ribose) glycohydrolase; SSBs Single-strand breaks; DSBs DNA double-strand breaks; ATM Ataxia–telangiectasia mutant; MRE11 Meiotic recombination 11 homolog 1; BRCA Breast cancer susceptibility gene; HR Homologous recombination; MMEJ Microhomology-mediated end joining

Fig. 3

Potential mechanisms of PARP inhibitor resistance. In a clinical setting, the mechanisms of resistance to PARP inhibitors (PARPi) present heterogeneity and mainly consist of a restoration of homologous recombination (HR) ability; b protection of replication fork stability; c reduction in PARP1 trapping; d mutations in cell cycle components; e efflux of the PGP-mediated drug pump; f interference in the microRNA (miRNA) environment

Fig. 4

Approaches for building PARP-inhibitor-resistant cancer cell lines. In preclinical trials, the construction of PARP inhibitor-resistant cancer cell lines was performed primarily by exploiting the following three mechanisms: a preexisting intrinsic PARP inhibitor resistance; b acquired PARP inhibitor resistance under extended and constant PARP inhibitor exposure; c de novo PARP inhibitor resistance via knockout or knockdown of key genes related to PARP inhibitor resistance. PARPi-resistance PARP inhibitor resistance; MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; CCK-8 Cell counting Kit-8; CFA Colony formation assay

Fig. 5

Preclinical and clinical trials targeting cell cycle checkpoints and PARP inhibitors (PARPi). The different colors of the blocks represent various cancer tissues, labeled with details. On the left side, the number in each block indicates the number of cell lines treated with the indicated combination. Each block on the right indicates a clinical trial, and the trial phase is indicated with Roman letters. ATRi ATR inhibitors; CHK1i CHK1 inhibitors; WEE1i WEE1 inhibitors; △ATM ATM alterations; △CHK2 CHK2 alterations; △TP53 TP53 alterations