Summary The ATR replication checkpoint ensures that stalled forks remain stable when replisome movement is impeded. Using an improved iPOND protocol combined with SILAC mass spectrometry, we characterized human replisome dynamics in response to fork stalling. Our data provide a quantitative picture of the replisome and replication stress response proteomes in 32 experimental conditions. Importantly, rather than stabilize the replisome, the checkpoint prevents two distinct types of fork collapse. Unsupervised hierarchical clustering of protein abundance on nascent DNA is sufficient to identify protein complexes and place newly identified replisome-associated proteins into functional pathways. As an example, we demonstrate that ZNF644 complexes with the G9a/GLP methyltransferase at replication forks and is needed to prevent replication-associated DNA damage. Our data reveal how the replication checkpoint preserves genome integrity, provide insights into the mechanism of action of ATR inhibitors, and will be a useful resource for replication, DNA repair, and chromatin investigators.
Graphical AbstractHighlights d HMCES senses abasic sites in ssDNA and forms a covalent DNA-protein crosslink d HMCES shields the abasic site from TLS polymerases and endonucleases d HMCES is a suicide enzyme and the DPC is ubiquitylated and degraded d HMCES is conserved in all domains of life and loss results in genetic instability In BriefAn enzyme conserved from bacteria to eukaryotes can covalently bind to abasic sites in single-stranded DNA to shield them and prevent genomic instability.
Abasic (AP) sites are one of the most common DNA lesions that block replicative polymerases. HMCES recognizes and processes these lesions in the context of single-stranded DNA (ssDNA). A HMCES DNA-protein crosslink (DPC) intermediate is thought to shield the AP site from endonucleases and error-prone polymerases. The highly evolutionarily conserved SRAP domain of HMCES and its Escherichia coli ortholog YedK mediate lesion recognition. Here we discover the basis of AP site protection by SRAP domains from a crystal structure of the YedK DPC. YedK forms a stable thiazolidine linkage between a ring-opened AP site and the α-amino and sulfhydryl substituents of its N-terminal cysteine residue. The thiazolidine linkage explains the remarkable stability of the HMCES DPC, its resistance to strand cleavage, and the proteolysis requirement for resolution. Furthermore, its structure reveals that HMCES has specificity for AP sites in ssDNA at junctions found when replicative polymerases encounter the AP lesion.
Production of concatemeric DNA is an essential step during HSV infection, as the packaging machinery must recognize longer-than-unit-length concatemers; however, the mechanism by which they are formed is poorly understood. Although it has been proposed that the viral genome circularizes and rolling circle replication leads to the formation of concatemers, several lines of evidence suggest that HSV DNA replication involves recombination-dependent replication reminiscent of bacteriophages λ and T4. Similar to λ, HSV-1 encodes a 5′-to-3′ exonuclease (UL12) and a single strand annealing protein [SSAP (ICP8)] that interact with each other and can perform strand exchange in vitro. By analogy with λ phage, HSV may utilize viral and/or cellular recombination proteins during DNA replication. At least four double strand break repair pathways are present in eukaryotic cells, and HSV-1 is known to manipulate several components of these pathways. Chromosomally integrated reporter assays were used to measure the repair of double strand breaks in HSV-infected cells. Single strand annealing (SSA) was increased in HSV-infected cells, while homologous recombination (HR), non-homologous end joining (NHEJ) and alternative non-homologous end joining (A-NHEJ) were decreased. The increase in SSA was abolished when cells were infected with a viral mutant lacking UL12. Moreover, expression of UL12 alone caused an increase in SSA, which was completely eliminated when a UL12 mutant lacking exonuclease activity was expressed. UL12-mediated stimulation of SSA was decreased in cells lacking the cellular SSAP, Rad52, and could be restored by coexpressing the viral SSAP, ICP8, indicating that an SSAP is also required. These results demonstrate that UL12 can specifically stimulate SSA and that either ICP8 or Rad52 can function as an SSAP. We suggest that SSA is the homology-mediated repair pathway utilized during HSV infection.
The DNA damage response kinase ATR and its effector kinase CHEK1 are required for cancer cells to survive oncogene-induced replication stress. ATR inhibitors exhibit synthetic lethal interactions with deficiencies in the DNA damage response enzymes ATM and XRCC1 and with overexpression of the cell cycle kinase Cyclin E. Here we report a systematic screen to identify synthetic lethal interactions with ATR-pathway targeted drugs, rationalized by their predicted therapeutic utility in the oncology clinic. We found that reduced function in the ATR pathway itself provided the strongest synthetic lethal interaction. In addition, we found that loss of the structure specific-endonuclease ERCC1-XPF (ERCC4) is synthetic lethal with ATR pathway inhibitors. ERCC1-deficient cells exhibited elevated levels of DNA damage, which was increased further by ATR inhibition. When treated with ATR or CHEK1 inhibitors, ERCC1-deficient cells arrested in S phase and failed to complete cell cycle transit even after drug removal. Notably, triple-negative breast cancer cells and non-small cell lung cancer cells depleted of ERCC1 exhibited increased sensitivity to ATR-pathway targeted drugs. Overall, we concluded that ATR pathway-targeted drugs may offer particular utility in cancers with reduced ATR pathway function or reduced levels of ERCC4 activity.
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