Replication Stress, DNA Damage, Inflammatory Cytokines and Innate Immune Response

Abstract

Complete and accurate DNA replication is essential to genome stability maintenance during cellular division. However, cells are routinely challenged by endogenous as well as exogenous agents that threaten DNA stability. DNA breaks and the activation of the DNA damage response (DDR) arising from endogenous replication stress have been observed at pre- or early stages of oncogenesis and senescence. Proper detection and signalling of DNA damage are essential for the autonomous cellular response in which the DDR regulates cell cycle progression and controls the repair machinery. In addition to this autonomous cellular response, replicative stress changes the cellular microenvironment, activating the innate immune response that enables the organism to protect itself against the proliferation of damaged cells. Thereby, the recent descriptions of the mechanisms of the pro-inflammatory response activation after replication stress, DNA damage and DDR defects constitute important conceptual novelties. Here, we review the links of replication, DNA damage and DDR defects to innate immunity activation by pro-inflammatory paracrine effects, highlighting the implications for human syndromes and immunotherapies.

Keywords: DNA damage response; DNA repair; cGAS-STING; inflammation; innate immunity; replicative stress.

Figure 1

cGAS-STING pathway activation by replicative stress. Two different pathways can trigger cGAS-STING activation by replicative stress: (1) the formation of micronuclei and (2) the accumulation of replication fork-derived DNA in the cytoplasm. These events will trigger the activation of cGAS, which produces cGAMP, activating the STimulator of INterferon Genes (STING) protein. Once activated, STING recruits TBK1 that is then able to phosphorylate its different targets that include Interferon Regulatory Factor 3 (IRF3) and different members of the NF-κB signalling pathway. This culminates in an upregulation of pro-inflammatory factors.

Figure 2

Replication fork protection and restart. (A). A replication fork can be broken, or arrested; when reaching an obstacle that arrested it, replication fork can also be reversed generating a so called “chicken foot” structure (right panel). Various actions may take place to protect the replication forks including: BRCA2 loading of RAD51 to protect the forks and/or BRCA2 stabilizing the RAD51 nucleofilament on the single-stranded DNA (ssDNA) regions, thereby preventing MRE11, CtIP, and EXO1-dependent resection (preventing degradation of the arrested forks). (B). The strand exchange activity of RAD51 allows then to restart arrested replication forks, using a homologous sequence as matrix, generally the sister chromatid, leading to sister chromatid exchange.

Figure 3

ATR-interacting protein-ataxia telangiectasia and Rad3 related (ATRIP-ATR) and ataxia telangiectasia-mutated (ATM) signalling. (A). Replication stress events (e.g., replication fork stalling), ssDNA gaps or resected DSB ends lead to the formation of ssDNA stretches. Replication Protein A (RPA) binds the ssDNA stretches and serve as a platform for ATRIP-ATR recruitment. Two modes of ATRIP-ATR activation have been described: in association with accessory proteins TOPBP1 or ETAA1 bind ATR triggering its autophosphorylation. Activated ATR, then, phosphorylate its downstream targets that include the protein kinase CHK1. (B). The MRN (MRE11-RAD50-NBS1) complex recognize the DSB. ATM is recruited to the DSB site and binds the MRN complex, which leads to ATM activation. ATM then phosphorylates its downstream targets, including CHK2 and p53 during the canonical DNA damage response (DDR) and/or H2AX and BRCA1 for chromatin remodelling and DNA repair.

Figure 4

The 2-step DSB repair pathway choice model [79,87,91]. After signalling of the DSB by ATM and MRN, the selection of the DSB repair process act in two successive steps: (1) competition between the canonical NHEJ (C-NHEJ) pathway (KU/DNA-PKcs/ligase 4-dependent) versus resection. Note that C-NHEJ is conservative for DSB repair (for review see [79,87]). The nuclease activity of MRE11 and CtIP favour ssDNA resection, which can then at the second alternative step (2) initiate the conservative homologous recombination (HR) versus non-conservative single-strand annealing (SSA) or alternative end-joining (A-EJ) pathways. Loading of RAD51 on resected ssDNA, by BRCA2, engages DSB repair toward HR. The RAD51 nucleoprotein filament invades the intact homologous duplex DNA, priming DNA synthesis and the intact DNA molecule is copied, creating a D-Loop.