Targeting the replication stress response through synthetic lethal strategies in cancer medicine

Abstract

The replication stress response (RSR) involves a downstream kinase cascade comprising ataxia telangiectasia-mutated (ATM), ATM and rad3-related (ATR), checkpoint kinases 1 and 2 (CHK1/2), and WEE1-like protein kinase (WEE1), which cooperate to arrest the cell cycle, protect stalled forks, and allow time for replication fork repair. In the presence of elevated replicative stress, cancers are increasingly dependent on RSR to maintain genomic integrity. An increasing number of drug candidates targeting key RSR nodes, as monotherapy through synthetic lethality, or through rational combinations with immune checkpoint inhibitors and targeted therapies, are demonstrating promising efficacy in early phase trials. RSR targeting is also showing potential in reversing PARP inhibitor resistance, an important area of unmet clinical need. In this review, we introduce the concept of targeting the RSR, detail the current landscape of monotherapy and combination strategies, and discuss emerging therapeutic approaches, such as targeting Polθ.

Keywords: PARP inhibition resistance; combination therapy; replicative stress response; synthetic lethality.

Figure 1:. The Replication Stress Response

The replication stress response (RSR). DNA damage or stalled replication forks occurring due to endogenous or exogenous factors trigger the RSR and downstream kinase cascade. ATM and rad3-related (ATR) and ataxia telangiectasia-mutated (ATM) primarily phosphorylate checkpoint kinase 1 (CHK1) and 2 (CHK2), respectively, which suppress CDC25C/A, preventing smooth transition through cell cycle checkpoints. WEE1-like protein kinase (WEE1) negatively regulates CDK2/ Cyclin E and CDK1/Cyclin B and is itself activated by CHK1/2. These kinases represent key nodes for therapeutic targeting of the RSR. Created with Biorender.com. Abbreviations: ATRi, ATR inhibitor; CCL5, chemokine ligand 5; CXCL10, C-X-C motif chemokine ligand 10; cGAS, cyclic GMP-AMP synthase; G2, gap 1; G2, gap 2; IFN, interferon; IRF3, interferon regulatory factor 3; M, mitosis; S, synthesis; STING, stimulator of interferon genes; TBK1, tank binding kinase 1.

Figure 2:. RSR blockade stimulates innate anti-tumor immunity through the cGAS-STING pathway

Inhibition of components of the RSR ultimately leads to cell cycle checkpoint abrogation and inappropriate mitotic entry of cells with unresolved DNA damage. This culminates in mitotic catastrophe and resultant micronuclei formation. Cytosolic DNA fragments released bind to and activate the cytosolic DNA sensor cGAS, which binds to the adaptor STING. When activated, STING translocates into Golgi body compartments and activates TBK1, which phosphorylates IRF3. IRF3 enters the nucleus to facilitate transcription of Type I interferon, TNFα, and interleukins. Type I interferon upregulates chemokines CCL5 and CXCL10, which encourage effector T cell infiltration into the tumor. Created with Biorender.com. Abbreviations: ATM, ataxia telangiectasia-mutated; ATR, ATM and rad3-related; CCL5, chemokine ligand 5; CDC25, cell division cycle 25; CHK, checkpoint kinase; CXCL10, C-X-C motif chemokine ligand 10; DSB, double strand break; cGAS, cyclic GMP-AMP synthase; G1, gap 1; G2, gap 2; IRF3, interferon regulatory factor 3; M, mitosis; RSR, replication stress response; S, synthesis; SSB, single strand break; STING, stimulator of interferon genes; TBK1, tank binding kinase 1; WEE1, WEE1-like protein kinase.