Following the discovery in 1998 of γ-H2AX, the first histone modification induced by DNA damage, interest in the changes to chromatin induced by DNA damage has exploded, and a vast amount of ...information has been generated. However, there has been a discrepancy between our rapidly advancing knowledge of how chromatin responds to DNA damage and the understanding of why cells mobilize large segments of chromatin to protect the genome against destabilizing effects posed by tiny DNA lesions. Recent research has provided insights into these issues and suggests that chromatin responses induced by DNA damage are not simply the accumulation of 'nuclear foci' but are mechanisms required to guard genome integrity.
ATR, activated by replication stress, protects replication forks locally and suppresses origin firing globally. Here, we show that these functions of ATR are mechanistically coupled. Although ...initially stable, stalled forks in ATR-deficient cells undergo nucleus-wide breakage after unscheduled origin firing generates an excess of single-stranded DNA that exhausts the nuclear pool of RPA. Partial reduction of RPA accelerated fork breakage, and forced elevation of RPA was sufficient to delay such “replication catastrophe” even in the absence of ATR activity. Conversely, unscheduled origin firing induced breakage of stalled forks even in cells with active ATR. Thus, ATR-mediated suppression of dormant origins shields active forks against irreversible breakage via preventing exhaustion of nuclear RPA. This study elucidates how replicating genomes avoid destabilizing DNA damage. Because cancer cells commonly feature intrinsically high replication stress, this study also provides a molecular rationale for their hypersensitivity to ATR inhibitors.
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•RPA is rate limiting for shielding replication forks against breakage•ATR signaling prevents RPA exhaustion by restraining origin firing•Exhaustion of RPA leads to pan-nuclear “replication catastrophe”•RPA exhaustion explains the hypersensitivity of cancer cells to ATR inhibitors
ATR-mediated suppression of dormant origins limits the generation of single-stranded DNA and prevents the exhaustion of the nuclear RPA reservoir, therefore protecting cells against “replication catastrophe.”
To safeguard genome integrity in response to DNA double-strand breaks (DSBs), mammalian cells mobilize the neighbouring chromatin to shield DNA ends against excessive resection that could undermine ...repair fidelity and cause damage to healthy chromosomes
. This form of genome surveillance is orchestrated by 53BP1, whose accumulation at DSBs triggers sequential recruitment of RIF1 and the shieldin-CST-POLα complex
. How this pathway reflects and influences the three-dimensional nuclear architecture is not known. Here we use super-resolution microscopy to show that 53BP1 and RIF1 form an autonomous functional module that stabilizes three-dimensional chromatin topology at sites of DNA breakage. This process is initiated by accumulation of 53BP1 at regions of compact chromatin that colocalize with topologically associating domain (TAD) sequences, followed by recruitment of RIF1 to the boundaries between such domains. The alternating distribution of 53BP1 and RIF1 stabilizes several neighbouring TAD-sized structures at a single DBS site into an ordered, circular arrangement. Depletion of 53BP1 or RIF1 (but not shieldin) disrupts this arrangement and leads to decompaction of DSB-flanking chromatin, reduction in interchromatin space, aberrant spreading of DNA repair proteins, and hyper-resection of DNA ends. Similar topological distortions are triggered by depletion of cohesin, which suggests that the maintenance of chromatin structure after DNA breakage involves basic mechanisms that shape three-dimensional nuclear organization. As topological stabilization of DSB-flanking chromatin is independent of DNA repair, we propose that, besides providing a structural scaffold to protect DNA ends against aberrant processing, 53BP1 and RIF1 safeguard epigenetic integrity at loci that are disrupted by DNA breakage.
Failure to complete DNA replication is a stochastic by-product of genome doubling in almost every cell cycle. During mitosis, under-replicated DNA (UR-DNA) is converted into DNA lesions, which are ...inherited by daughter cells and sequestered in 53BP1 nuclear bodies (53BP1-NBs). The fate of such cells remains unknown. Here, we show that the formation of 53BP1-NBs interrupts the chain of iterative damage intrinsically embedded in UR-DNA. Unlike clastogen-induced 53BP1 foci that are repaired throughout interphase, 53BP1-NBs restrain replication of the embedded genomic loci until late S phase, thus enabling the dedicated RAD52-mediated repair of UR-DNA lesions. The absence or malfunction of 53BP1-NBs causes premature replication of the affected loci, accompanied by genotoxic RAD51-mediated recombination. Thus, through adjusting replication timing and repair pathway choice at under-replicated loci, 53BP1-NBs enable the completion of genome duplication of inherited UR-DNA and prevent the conversion of stochastic under-replications into genome instability.
Accumulation of repair proteins on damaged chromosomes is required to restore genomic integrity. However, the mechanisms of protein retention at the most destructive chromosomal lesions, the DNA ...double-strand breaks (DSBs), are poorly understood. We show that RNF8, a RING-finger ubiquitin ligase, rapidly assembles at DSBs via interaction of its FHA domain with the phosphorylated adaptor protein MDC1. This is accompanied by an increase in DSB-associated ubiquitylations and followed by accumulation of 53BP1 and BRCA1 repair proteins. Knockdown of RNF8 or disruption of its FHA or RING domains impaired DSB-associated ubiquitylation and inhibited retention of 53BP1 and BRCA1 at the DSB sites. In addition, we show that RNF8 can ubiquitylate histone H2A and H2AX, and that its depletion sensitizes cells to ionizing radiation. These data suggest that MDC1-mediated and RNF8-executed histone ubiquitylation protects genome integrity by licensing the DSB-flanking chromatin to concentrate repair factors near the DNA lesions.
Human cancers are characterized by the presence of oncogene-induced DNA replication stress (DRS), making them dependent on repair pathways such as break-induced replication (BIR) for damaged DNA ...replication forks. To better understand BIR, we performed a targeted siRNA screen for genes whose depletion inhibited G1 to S phase progression when oncogenic cyclin E was overexpressed. RAD52, a gene dispensable for normal development in mice, was among the top hits. In cells in which fork collapse was induced by oncogenes or chemicals, the Rad52 protein localized to DRS foci. Depletion of Rad52 by siRNA or knockout of the gene by CRISPR/Cas9 compromised restart of collapsed forks and led to DNA damage in cells experiencing DRS. Furthermore, in cancer-prone, heterozygous APC mutant mice, homozygous deletion of the Rad52 gene suppressed tumor growth and prolonged lifespan. We therefore propose that mammalian RAD52 facilitates repair of collapsed DNA replication forks in cancer cells.
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•Mammalian RAD52 is involved in the oncogene-induced DNA replication stress response•Mammalian RAD52 functions in the repair of collapsed DNA replication forks•Rad52 deficiency prolongs the lifespan of Apcmin/+ mice
RAD52 is dispensable for life in mammals but is required for double-stranded break (DSB) repair in yeast. Sotiriou et al. now show that mammalian RAD52 plays an important role in response to DNA replication stress by mediating the repair of collapsed DNA replication forks.
DNA double-strand breaks (DSBs) not only interrupt the genetic information, but also disrupt the chromatin structure, and both impairments require repair mechanisms to ensure genome integrity. We ...showed previously that RNF8-mediated chromatin ubiquitylation protects genome integrity by promoting the accumulation of repair factors at DSBs. Here, we provide evidence that, while RNF8 is necessary to trigger the DSB-associated ubiquitylations, it is not sufficient to sustain conjugated ubiquitin in this compartment. We identified RNF168 as a novel chromatin-associated ubiquitin ligase with an ability to bind ubiquitin. We show that RNF168 interacts with ubiquitylated H2A, assembles at DSBs in an RNF8-dependent manner, and, by targeting H2A and H2AX, amplifies local concentration of lysine 63-linked ubiquitin conjugates to the threshold required for retention of 53BP1 and BRCA1. Thus, RNF168 defines a new pathway involving sequential ubiquitylations on damaged chromosomes and uncovers a functional cooperation between E3 ligases in genome maintenance.
Protein recruitment to DNA double-strand breaks (DSBs) relies on ubiquitylation of the surrounding chromatin by the RING finger ubiquitin ligases RNF8 and RNF168. Flux through this pathway is opposed ...by several deubiquitylating enzymes (DUBs), including OTUB1 and USP3. By analyzing the effect of individually overexpressing the majority of human DUBs on RNF8/RNF168-mediated 53BP1 retention at DSB sites, we found that USP44 and USP29 powerfully inhibited this response at the level of RNF168 accrual. Both USP44 and USP29 promoted efficient deubiquitylation of histone H2A, but unlike USP44, USP29 displayed nonspecific reactivity toward ubiquitylated substrates. Moreover, USP44 but not other H2A DUBs was recruited to RNF168-generated ubiquitylation products at DSB sites. Individual depletion of these DUBs only mildly enhanced accumulation of ubiquitin conjugates and 53BP1 at DSBs, suggesting considerable functional redundancy among cellular DUBs that restrict ubiquitin-dependent protein assembly at DSBs. Our findings implicate USP44 in negative regulation of the RNF8/RNF168 pathway and illustrate the usefulness of DUB overexpression screens for identification of antagonizers of ubiquitin-dependent cellular responses.
Background: The RNF8/RNF168 E3 ligase cascade promotes ubiquitin-dependent protein assembly at DNA double-strand breaks (DSBs).
Results: An overexpression screen identified new deubiquitylating enzymes (DUBs) opposing the RNF8/RNF168 pathway in human cells.
Conclusion: USP44 counteracts RNF168-dependent ubiquitylation of proteins at DSB sites, including histone H2A.
Significance: Identification of antagonizers of the RNF8/RNF168 pathway is crucial for understanding how cells regulate DSB repair.
Histone ubiquitylation is a prominent response to DNA double-strand breaks (DSBs), but how these modifications are confined to DNA lesions is not understood. Here, we show that TRIP12 and UBR5, two ...HECT domain ubiquitin E3 ligases, control accumulation of RNF168, a rate-limiting component of a pathway that ubiquitylates histones after DNA breakage. We find that RNF168 can be saturated by increasing amounts of DSBs. Depletion of TRIP12 and UBR5 allows accumulation of RNF168 to supraphysiological levels, followed by massive spreading of ubiquitin conjugates and hyperaccumulation of ubiquitin-regulated genome caretakers such as 53BP1 and BRCA1. Thus, regulatory and proteolytic ubiquitylations are wired in a self-limiting circuit that promotes histone ubiquitylation near the DNA lesions but at the same time counteracts its excessive spreading to undamaged chromosomes. We provide evidence that this mechanism is vital for the homeostasis of ubiquitin-controlled events after DNA breakage and can be subverted during tumorigenesis.
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► Histone ubiquitylation can be saturated by increasing amounts of DNA breaks ► RNF168 abundance determines the dynamic range for chromatin ubiquitylation ► TRIP12 and UBR5 suppress chromatin ubiquitylation by degrading RNF168 ► Hyperaccumulation of RNF168 alters DNA repair and can be found in human cancers
A self-limiting circuit of ubiquitin-ligases restricts the ubiquitylation of histones to the chromatin immediately surrounding a DNA double-strand break. The mechanism is important for coordinating proper repair and avoiding excessive transcriptional silencing.
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