Non redundant functions of ATM and DNAPKcs in response to clean DNA Double Strand Breaks.
ABSTRACT: DNA Double Strand Breaks (DSBs) are harmful lesions that require rapid detection and repair in order to avoid toxic genomic rearrangements. DSBs elicit the so called DNA Damage Response (DDR), largely relying on ataxia telangiectasia mutated (ATM) and DNA Protein Kinase (DNAPK), two members of the PI3K-like kinase family, whose respective functions during the sequential steps of the DDR remains controversial. Using the DIvA cell line, expressing the AsiSI restriction enzyme, we have investigated the role of ATM and DNAPK in several aspects of the DDR upon induction of multiple clean DSBs throughout the human genome. High resolution mapping revealed that they are activated and spread in cis on a confined region surrounding all DSBs, independently of the pathway used for repair. However, a thorough analysis of repair kinetics, H2AX domain establishment and H2AX foci structure and dynamics revealed non overlapping functions for the two kinases once recruited at DSBs. Our results suggest that ATM is not solely acting on chromatin marks but also on chromatin organisation in order to ensure repair accuracy and survival.
Project description:Chromatin undergoes major remodeling around DNA double strand breaks (DSB) to promote repair and DNA damage response (DDR) activation. We recently reported a high resolution map of gammaH2AX around multiple breaks on the human genome, using a new cell-based DSB inducible system. In an attempt to further characterize the chromatin landscape induced around DSBs, we now report the profile of SMC3, a subunit from the cohesin complex, previously characterized as required for repair by homologous recombination. We found that the recruitment of cohesin is moderate and restricted to the immediate vicinity of DSBs. In addition, we show that the cohesin complex, which was also recently proposed to be a key player in chromosome organisation and chromatin looping, controls gammaH2AX distribution within domains. Indeed, as we reported for transcription, cohesin binding antagonizes gammaH2AX spreading. Remarkably, depletion of cohesin leads to an increase of gammaH2AX at cohesin-bound genes (revelead by gammaH2AX mapping in upon SCC1 siRNA), associated with a decrease in their expression level after DSB induction. Thus our study identifies a novel role for the cohesin complex in protecting the genes located in gammaH2AX domains from both gammaH2AX spreading and transcriptional shut-down after DSB induction.
Project description:Chromatin acts as a key regulator of DNA related processes such as DNA damage repair. While ChIP-chip is a powerful technique to provide high-resolution maps of protein-genome interactions, its use to study DNA Double Strand Break (DSB) repair has been hindered by the limitations of the available damage induction methods. We have developed a human cell line that permits induction of multiple DSBs randomly distributed and unambiguously positioned within the genome. Using this system, we have generated the first genome-wide mapping of gammaH2AX around DSBs. We found that all DSBs trigger large gammaH2AX domains, which extend from the DSB in a bidirectional, discontinuous and not necessarily symmetrical manner. Strikingly we uncovered that, within domains, gammaH2AX distribution is highly influenced by gene transcription since parallel mapping of RNA Polymerase II and strand specific expression revealed that ?H2AX does not propagate on active genes. In addition, we demonstrate that transcription is accurately maintained within gammaH2AX domains, indicating that mechanisms may exist to protect genes transcription from gammaH2AX spreading and from the chromatin rearrangements induced by DSBs.
Project description:The DNA damage response (DDR) is an extensive signaling network that is robustly mobilized by DNA double-strand breaks (DSBs). The primary transducer of the DSB response is the protein kinase, ataxia-telangiectasia, mutated (ATM). Here, we establish nuclear poly(A)-binding protein 1 (PABPN1) as a novel target of ATM and a crucial player in the DSB response. PABPN1 usually functions in regulation of RNA processing and stability. We establish that PABPN1 is recruited to the DDR as a critical regulator of DSB repair. A portion of PABPN1 relocalizes to DSB sites and is phosphorylated on Ser95 in an ATM-dependent manner. PABPN1 depletion sensitizes cells to DSBinducing agents and prolongs the DSB-induced G2/M cell-cycle arrest, and DSB repair is hampered by PABPN1 depletion or elimination of its phosphorylation site. PABPN1 is required for optimal DSB repair via both nonhomologous end-joining (NHEJ) and homologous recombination repair (HRR), and specifically is essential for efficient DNAend resection, an initial, key step in HRR. Using mass spectrometry analysis, we capture DNA damage-induced interactions of phospho-PABPN1, including well-established DDR players as well as other RNA metabolizing proteins. Our results uncover a novel ATM-dependent axis in the rapidly growing interface between RNA metabolism and the DDR.
Project description:Cells have developed effective mechanisms, namely homologous recombination (HR) and non-homologous end-joining (NHEJ), to repair DNA double-strand breaks (DSBs), which are considered to be the most deleterious type of damage that can challenge genome integrity. While these pathways coexist to repair DSBs, the mechanisms by which one of these pathways is chosen to repair a particular DSB remain unclear. Here, we show that the chromatin context in which a break occurs participates in this choice and that transcriptionnaly active chromatin channels repair to HR. By using a human cell line expressing a restriction enzyme fused to the ligand binding domain of the oestrogen receptor (AsiSI-ER)2,3, together with a genome wide chromatin immunoprecipitation-sequencing (ChIP-seq) approach, we establish that distinct DSBs induced across the genome are not necessarily repaired by the same pathway. Indeed, we identify an HR-prone subset of DSBs that recruit the HR protein RAD51, undergo resection, and rely on RAD51 for efficient repair. These DSBs are located in actively transcribed genes, and repair at such DSBs can be switched to RAD51-independent repair pathway upon transcriptional inhibition. Moreover, we show that HR is targeted to transcribed loci thanks to the elongation-associated H3K36me3 histone mark. Indeed depletion of HYPB, the main H3K36 tri methyltransferase severally impedes the use of HR at those DSBs. Our study, thereby demonstrates a clear role for chromatin in DSB repair pathway choice in human cells.
Project description:We have investigated whether components of Polycomb repressive complex 1 (PRC1) are recruited to double-strand breaks (DSBs) generated by inducible expression of the AsiSI restriction enzyme in normal human fibroblasts. Using chromatin immunoprecipitation and deep sequencing (ChIP-seq), which detects PRC1 proteins at common sites throughout the genome, we did not find evidence for recruitment of PRC1 components to AsiSI-induced DSBs. In contrast, the S2056 phosphorylated form of DNA-PKcs and other DNA repair proteins were detected at a subset of AsiSI sites that are predominantly at the 5’ ends of transcriptionally active genes. Our data question the idea that PcG protein recruitment provides a link between DSB repairs and transcriptional repression. Single chromatin preparations from treated and untreated cells (+/- OHT), immunoprecipitated with two antibodies (pDNA-PKcs and MEL18) were sequenced as technical replicates, along with the corresponding input DNAs
Project description:Damage to genomic DNA, especially as DNA double strand breaks (DSB), elicits prompt activation of DNA damage response (DDR) which arrest cell-cycle either G1/S or G2/M to avoid entering S and M phase with DNA damage. In mammalian organs cells are in both proliferating and quiescent states. Quiescent cells are already arrested in G0, therefore there may be fundamental difference in DDR between proliferating and quiescent cells. To address these differences we studied recruitment of DSB repair factors and resolution of DNA lesions induced at site-specific DSBs occurring at different cell cycle phases, i.e. in asynchronously proliferating, G0, and G1 arrested cells. Strikingly, DSBs occurring in G0 quiescent cells are irreparable with a sustained activation of p53-pathway. Conversely, reentry of G0-damaged cells into cell cycle progression, show a delayed clearance of recruited DNA repair factors bound at DSBs, indicating an inefficient repair when compared to DSBs induced in asynchronously proliferating or G1 cells. Moreover, we found that initial recognition of DSBs and assembly of DSB factors is largely similar at different cell cycle phases. Our study thereby demonstrates the crucial role of cell cycle phases in repair and resolution of DSBs. Overall design: Examination of the resolution of DNA damage-associated marker gH2AX in growing and G0 arrested MCF10a cells.
Project description:Maintenance of genomic stability depends on the DNA damage response (DDR), a biological barrier in early stages of cancer development. Failure of this response results in genomic instability and high predisposition toward lymphoma, as seen in patients with ataxia-telangiectasia mutated (ATM) dysfunction. ATM activates multiple cell cycle checkpoints and DNA repair following DNA damage, but its influence on posttranscriptional gene expression has not been examined on a global level. We show that ionizing radiation (IR) modulates the dynamic association of the RNA-binding protein HuR with target mRNAs in an ATM-dependent manner, potentially coordinating the genotoxic response as an RNA operon. Pharmacologic ATM inhibition and use of ATM-null cells revealed a critical role for ATM in this process. Numerous mRNAs encoding cancer-related proteins were differentially associated with HuR depending on the functional state of ATM, in turn affecting expression of encoded proteins. The findings presented here reveal a previously unidentified role of ATM in controlling gene expression post-transcriptionally. Dysregulation of this DDR RNA operon is likely relevant to lymphoma development in ataxia-telangiectasia individuals. These novel RNA regulatory modules and genetic networks provide critical insight into the function of ATM in oncogenesis. B-lymphocyte cell lines GM02184 (wild type, ATM +/+) and GM03332 (AT, ATM -/-) were either untreated or exposed to 1 Gy of IR. 6 h later cells were harvested and used for immunoprecipitation (IP) in the presence of HuR antibody (Santa Cruz Biotech.). RNA from IP material was extracted and used for microarray analysis.
Project description:Translocations, that occur when two DNA Double Strand breaks (DSBs) are abnormally rejoined, represent highly deleterious genome rearrangements favoring cancer apparition and progression. However, the mechanisms that drive their formation are yet poorly deciphered. One prerequisite for translocation is the juxtaposition of two distant DSBs, an event that would be favored if DSB cluster, i.e. are brought together in close spatial proximity within the nucleus. Whether DSB cluster in higher eukaryotes has been subjected to a strong controversy over the past decade, due to conflicting results obtained using microscopy based methods1-9. Here we used for the first time a high throughput chromosome conformation capture assay (Capture Hi-C10) to investigate DSB clustering. We unambiguously found that DSBs do cluster in human nuclei but only when induced in transcriptionally active genes. Clustering of damaged genes mainly occur during the G1 cell cycle phase and coincide with delayed repair. Moreover DSB clustering depends on the MRN complex, as well as the Formin 2 (FMN2) nuclear actin organizer and the LINC (LInker of Nuclear and Cytoplasmic skeleton) complex, suggesting that active mechanisms promote DSB clustering. This work reveals that when damaged, active genes exhibit a very peculiar behavior compared to the rest of the genome, being mostly left unrepaired and clustered in G1 while being repaired by homologous recombination in post-replicative cells.
Project description:Maintenance of genomic stability depends on the DNA damage response (DDR), a biological barrier in early stages of cancer development. Failure of this response results in genomic instability and high predisposition toward lymphoma, as seen in patients with ataxia-telangiectasia mutated (ATM) dysfunction. ATM activates multiple cell cycle checkpoints and DNA repair following DNA damage, but its influence on posttranscriptional gene expression has not been examined on a global level. We show that ionizing radiation (IR) modulates the dynamic association of the RNA-binding protein HuR with target mRNAs in an ATM-dependent manner, potentially coordinating the genotoxic response as an RNA operon. Pharmacologic ATM inhibition and use of ATM-null cells revealed a critical role for ATM in this process. Numerous mRNAs encoding cancer-related proteins were differentially associated with HuR depending on the functional state of ATM, in turn affecting expression of encoded proteins. The findings presented here reveal a previously unidentified role of ATM in controlling gene expression post-transcriptionally. Dysregulation of this DDR RNA operon is likely relevant to lymphoma development in ataxia-telangiectasia individuals. These novel RNA regulatory modules and genetic networks provide critical insight into the function of ATM in oncogenesis. Overall design: B-lymphocyte cell lines GM02184 (wild type, ATM +/+) and GM03332 (AT, ATM -/-) were either untreated or exposed to 1 Gy of IR. 6 h later cells were harvested and used for immunoprecipitation (IP) in the presence of HuR antibody (Santa Cruz Biotech.). RNA from IP material was extracted and used for microarray analysis.
Project description:Recent observations show that the single-cell response of p53 to ionizing radiation (IR) is “digital” in that it is the number of oscillations rather than the amplitude of p53 that shows dependence on the radiation dose. We present a model of this phenomenon. In our model, double-strand break (DSB) sites induced by IR interact with a limiting pool of DNA repair proteins, forming DSB–protein complexes at DNA damage foci. The persisting complexes are sensed by ataxia telangiectasia mutated (ATM), a protein kinase that activates p53 once it is phosphorylated by DNA damage. The ATM-sensing module switches on or off the downstream p53 oscillator, consisting of a feedback loop formed by p53 and its negative regulator, Mdm2. In agreement with experiments, our simulations show that by assuming stochasticity in the initial number of DSBs and the DNA repair process, p53 and Mdm2 exhibit a coordinated oscillatory dynamics upon IR stimulation in single cells, with a stochastic number of oscillations whose mean increases with IR dose. The damped oscillations previously observed in cell populations can be explained as the aggregate behavior of single cell