Available online at www.sciencedirect.com ScienceDirect SAMHD1 and the innate immune response to cytosolic DNA during DNA replication Flavie Coquel, Christoph Neumayer, Yea-Lih Lin and Philippe Pasero Cytosolic DNA of endogenous or exogenous origin is sensed by double-stranded DNA (dsDNA) in a sequence-indepen- the cGAS-STING pathway to activate innate immune dent manner and produces cyclic-GMP-AMP. This sec- responses. Besides microbial DNA, this pathway detects self- ond messenger binds STING and induces TBK1 activa- DNA in the cytoplasm of damaged or abnormal cells and plays tion, IRF3 phosphorylation and induction of type I IFNs a central role in antitumor immunity. The mechanism by which and other cytokine genes [1,2]. cytosolic DNA accumulates under genotoxic stress conditions is currently unclear, but recent studies on factors mutated in the Besides pathogens, the innate immune system also Aicardi-Goutie` res syndrome cells, such as SAMHD1, RNase responds to tissue damage by sensing damage-associated H2 and TREX1, are shedding new light on this key process. In molecular patterns, including cytosolic DNA of endoge- particular, these studies indicate that the rupture of micronuclei nous origin. The cGAS-STING pathway is activated by and the release of ssDNA fragments during the processing of cytosolic DNA accumulating after ionizing radiation [3] stalled replication forks and chromosome breaks represent or exposure to a variety of chemotherapeutic agents potent inducers of the cGAS-STING pathway. targeting DNA replication forks [4–6]. DNA sensing pathways act as a double-edged sword in cancer devel- Address opment in a context-dependent manner. On the one Institut de Ge´ ne´ tique Humaine, CNRS, Universite´ de Montpellier, Equipe hand, the cGAS-STING pathway contributes to the Labellise´ e Ligue contre le Cancer, Montpellier France elimination of abnormal cells by different means, includ- ing induction of senescence and senescence-associated Corresponding authors: Lin, Yea-Lih ([email protected]), Pasero, secretory phenotype (SASP) [7 ,8 ,9 ], the upregulation Philippe ([email protected]) of immunoreceptor ligands of natural killer cells [5,6,10] and the stimulation of adaptive immune responses Current Opinion in Immunology 2019, 56:24–30 [11,12]. On the other hand, the chronic induction of This review comes from a themed issue on Innate immunity DNA sensing pathways by genomic instability promotes metastasis in cancer cells in a STING-dependent man- Edited by Nicolas Manel and James Di Santo ner [13 ]. For a complete overview see the Issue and the Editorial Available online 5th October 2018 The cGAS-STING pathway is not only triggered by https://doi.org/10.1016/j.coi.2018.09.017 genotoxic agents, but also by endogenous sources of 0952-7915/ã 2018 Elsevier Ltd. All rights reserved. DNA damage, such as telomere shortening and onco- gene-induced DNA damage [14,15,16 ]. This pathway is frequently suppressed in human cancers, enabling malig- nant cells to escape immune surveillance [15,17]. This is reminiscent of the inactivation of p53 and other DNA damage response (DDR) factors in response to oncogene- Introduction induced DNA damage [18]. Since innate immunity and The innate immune system is the first line of host defense DDR pathways share multiple components [19], it is against a broad range of pathogens, such as viruses, likely that these processes cooperate to prevent tumori- bacteria, fungi and parasites. It acts by directly sensing genesis [20 ]. However, the interplay between DDR pathogens-associated molecular patterns through a set of factors and the cGAS-STING pathway remains poorly pattern recognition receptors that stimulate downstream characterized at the molecular level. Here, we discuss signaling cascades leading to the production of type I recent findings on three genes frequently mutated in interferons (IFNs) and other pro-inflammatory cytokines. human interferonopathies that shed new light on the This system operates on the premise that no free DNA mechanism by which DNA damage activates the inter- should be present in the cytosol and therefore recognizes feron response. cytosolic DNA as non-self [1]. This DNA is recognized by a variety of nucleic acids sensors that converge on STING (STimulation of Interferon Genes) to transactivate innate Insights from the Aicardi-Goutie` res syndrome immune response genes [2]. A key player in this process is The cGAS-STING pathway is constitutively activated in the cyclic GMP-AMP synthase (cGAS), which binds a rare Mendelian disorder called the Aicardi-Goutie`res Current Opinion in Immunology 2019, 56:24–30 www.sciencedirect.com SAMHD1 links replication stress to inflammation Coquel et al. 25 syndrome (AGS) and in other related interferonopathies, unrepaired DSBs during mitosis, as recently reported which are characterized by a chronic upregulation of type in gamma-irradiated cells [33 ]. I IFNs [21,22]. AGS is caused by germline mutations in genes encoding factors involved in nucleic acids metabo- SAMHD1 (sterile alpha motif and HD-domain containing lism, such as TREX1, RNase H2 and SAMHD1. protein 1) is a restriction factor for HIV-1 infection and a deoxynucleoside triphosphate (dNTP) hydrolase that TREX1 (Three Prime Repair Exonuclease 1) is a major depletes intracellular dNTP pools in non-cycling cells 0 0 3 -5 exonuclease that is anchored to the outer face of the [34,35]. SAMHD1 is also recruited to DNA repair foci in nuclear membrane and degrades both single-stranded and response to DNA damage [36,37 ] and plays a key role in double-stranded DNA (ssDNA and dsDNA), although it DSB repair by promoting the resection of DNA ends preferentially binds ssDNA [4,23,24]. Trex1À/À mouse [37 ,38 ]. SAMHD1 is mutated in 17% of AGS patients embryonic fibroblasts (MEFs) accumulate cytosolic DNA [39] and Samhd1 À/À mice show a chronic induction of type of endogenous origin, including endogenous retroele- I IFNs in a cGAS-dependent and STING-dependent ments, and show a chronic activation of type I IFNs manner [40]. Together, these data suggest that SAMHD1 [4,25]. Importantly, TREX1 regulates radiotherapy- plays a unique function at the interface between DNA induced tumor immunogenicity by controlling the repair and inflammation through the regulation of dNTP amount of IFN-stimulatory DNA present in cancer cells pools and/or the processing of chromosome breaks. [26 ,27 ] and prevents SASP [9 ]. SAMHD1 and the replication stress response RNase H2 plays a central role in the removal of ribonu- DNA replication is a complex process depending on the cleotides misincorporated into DNA during DNA repli- coordinated activation of thousands of replication origins cation and in the resolution of co-transcriptional RNA: and the unconstrained progression of replication forks, DNA hybrids [28]. RNase H2-deficient cells display through the action of DNA polymerases, helicases and spontaneous replication stress and accumulate DSBs in accessory proteins forming the replisome [41]. This pro- a Top1-dependent manner [29,30]. Moreover, recent cess is frequently challenged by events of endogenous or evidence indicate that RNase H2-deficient cells accumu- exogenous origin, collectively referred to as DNA repli- late micronuclei, which eventually become permeable to cation stress, leading to replication fork slowing or stalling cGAS and trigger IFN production [31 ,32 ]. These (Figure 1). Stalled forks are detected by the ATR check- micronuclei could result from the persistence of point kinase, which binds RPA-coated ssDNA and Figure 1 SAMHD1 dNTPs dNMPs CycA-CDK PP2A Replisome P STOP MRE11-SAMHD1 RPA-coated ssDNA Activation of the DNA-damage checkpoint, fork restart Current Opinion in Immunology Dual role of SAMHD1 during DNA replication. In G1, SAMHD1 forms a tetramer and acts as a dNTP hydrolase. This function is important to maintain balanced dNTP pools. Upon entry into S-phase, the phosphorylation of SAMHD1 by CycA-CDK on T592 activates a novel role of SAMHD1 in the stimulation of the exonuclease activity of MRE11. This function is important for the resection of nascent DNA strands (orange) on both strands of stalled replication forks. This resection activity generates ssDNA gaps that are covered by the ssDNA-binding complex RPA and that are important for the activation of the DNA damage checkpoint and for replication resumption. www.sciencedirect.com Current Opinion in Immunology 2019, 56:24–30 26 Innate immunity activates the effector kinase CHK1 to prevent premature accumulation of cytosolic ssDNA and the activation of entry into mitosis and promote replication resumption innate immune signaling [24,26 ,53]. Since SAMHD1 [30,42]. promotes resection at both DSBs and stalled forks, an attractive possibility could be that SAMHD1-deficient Depleted or imbalanced dNTP pools represent a com- cells activate the cGAS-STING pathway because of the mon source of replication stress, caused by genotoxic aberrant processing of stalled forks. This view is sup- drugs or by mutations affecting dNTP metabolism [30]. ported by the fact that, in the absence of SAMHD1, Since SAMHD1 plays a major role in this process, nascent DNA is displaced by the RECQ1 DNA helicase, efforts have been made to monitor the impact of leading to the formation of a flap that can be cleaved by SAMHD1 mutations on DNA replication. SAMHD1- the endonuclease activity of MRE11 [38 ]. The result- depleted cells show deregulated dNTP pools, impaired ing ssDNA fragments would eventually accumulate in replication fork progression and increased mutagenesis the cytosol and activate the cGAS-STING pathway [38 ,43,44]. However, the regulation of dNTP pools is (Figure 2). This model is consistent with the fact that not the only function of SAMHD1 during the S-phase the depletion of RECQ1 with siRNAs or the inhibition of the cell cycle. Indeed, it has been recently reported of MRE11 with Mirin prevents the accumulation of that SAMHD1 interacts physically with the MRE11 cytosolic ssDNA in SAMHD1-deficient cells [38 ].