ATM signaling facilitates repair of DNA double-strand breaks associated with heterochromatin.

Genome Damage and Stability Centre, University of Sussex, East Sussex BN1 9RQ, UK.
Molecular cell (Impact Factor: 14.46). 07/2008; 31(2):167-77. DOI: 10.1016/j.molcel.2008.05.017
Source: PubMed

ABSTRACT Ataxia Telangiectasia Mutated (ATM) signaling is essential for the repair of a subset of DNA double-strand breaks (DSBs); however, its precise role is unclear. Here, we show that < or =25% of DSBs require ATM signaling for repair, and this percentage correlates with increased chromatin but not damage complexity. Importantly, we demonstrate that heterochromatic DSBs are generally repaired more slowly than euchromatic DSBs, and ATM signaling is specifically required for DSB repair within heterochromatin. Significantly, knockdown of the transcriptional repressor KAP-1, an ATM substrate, or the heterochromatin-building factors HP1 or HDAC1/2 alleviates the requirement for ATM in DSB repair. We propose that ATM signaling temporarily perturbs heterochromatin via KAP-1, which is critical for DSB repair/processing within otherwise compacted/inflexible chromatin. In support of this, ATM signaling alters KAP-1 affinity for chromatin enriched for heterochromatic factors. These data suggest that the importance of ATM signaling for DSB repair increases as the heterochromatic component of a genome expands.

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    ABSTRACT: Background: DNA methylation and repair of double-strand breaks (DSBs) are crucial for maintaining genomic integrity. In cancer, global hypomethylation leads to genomic instability. Nonetheless, the underlying mechanism(s) has not been identified. Although DSBs can be produced by numerous agents, they also occur spontaneously as endogenous DSBs (EDSBs). Interestingly, methylation levels of EDSBs are higher than at the genomic level. Objectives and methods: I propose a hypothesis as to how EDSBs are hypermethylated and suggest how this hypothesis may connect with the underlying mechanism by which global hypomethylation leads to genomic instability. Results: EDSBs are hypermethylated. EDSB processing is distinct and depends on DNA methylation status. Methylation of EDSBs exists in the genome prior to DNA breaks. There are significant levels of EDSBs in both replicating and non-replicating cells. However, hypermethylation of EDSBs is replication-independent. Methylated DNA is often associated with heterochromatin and radiation-induced DSB repair may be different depending on chromatin status. There are reports of a unique radiation-induced heterochromatin DNA-repair pathway in nonreplicating cells. A DSB-related histone modification, γ-H2AX, serine-139 phosphorylated form of histone H2AX, is formed less preferentially in heterochromatin after ionizing radiation. Moreover, radiation-induced heterochromatin DSB repair was also shown to be repaired slowly and is Ataxia Telangiectasia Mutated (ATM)-dependent. Therefore, if EDSB repair is similar to radiation-induced DSB repair, methylated EDSB repair may also be dependent on a more precise ATM-dependent non-homologous end-joining repair. Conclusion: The higher methylation level of EDSBs may be due to methylation-dependent EDSB repair. This repair may be slower and thereby possess better precision. Consequently, the increase in the spontaneous mutation rate in the hypomethylated cancer genome may be due to the fact that unmethylated EDSB repair is more error prone. Genome-wide loss of DNA methylation is a common epigenetic event in cancer (1-3). The biological role of DNA methylation is believed to be important in controlling gene expression and maintaining genomic integrity (4). Several reports indicate that global hypomethylation can lead to genomic instability (5-9). However, the mechanism linking these two phenomena is still unknown. Recently, we reported mechanical evidence of endogenous DNA double strand breaks (EDSBs) and their relationship to DNA methylation (10). This review is an attempt to integrate current knowledge to show that EDSB processing in cells may be one of the underlying mechanisms by which global hypomethylation leads to genomic instability.
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    ABSTRACT: We show here that mitochondria-targeted antioxidants composed of plastoquinone conjugated through hydrocarbon linker with cationic rhodamine 19 (SkQR1) protected against nuclear DNA damage induced by gamma radiation in K562 erythroleukemia cells. We also demonstrate that SkQR1 prevented the early (1 h postirradiation) accumulation of phosphorylated histone H2AX (γ-H2AX) an indicator of DNA double-strand break formation, as well as the radiation-induced increase in chromosomal aberrations. These data suggested that nuclear DNA damage induced by gamma radiation may be mediated by mitochondrial reactive oxygen species (ROS) production. We show that SkQR1 suppressed delayed accumulation of ROS 32 h after irradiation probably by inhibiting mitochondrial ROS-induced ROS release mechanisms. This suggests that mitochondria-targeted antioxidants may protect cells from the late consequences of radiation exposure related to delayed oxidative stress. We have previously reported that SkQRl is the substrate of multidrug resistance pump P-glycoproten (Pgp 170) and selectively protects Pgp 170-negative cells against oxidative stress. In line with this finding, we demonstrate here that SkQR1 did not protect Pgp170-positive K562 subline against DNA damage induced by gamma radiation. The selective radioprotection of normal Pgp 170-negative cells by mitochondria-targeted antioxidants could be a promising strategy to increase the efficiency of radiotherapy for multidrug-resistant tumors. © 2015 by Radiation Research Society.
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    ABSTRACT: The DNA damage response (DDR) occurs in the context of chromatin, and architectural features of chromatin have been implicated in DNA damage signaling and repair. Whereas a role of chromatin decondensation in the DDR is well established, we show here that chromatin condensation is integral to DDR signaling. We find that, in response to DNA damage chromatin regions transiently expand before undergoing extensive compaction. Using a protein-chromatin-tethering system to create defined chromatin domains, we show that interference with chromatin condensation results in failure to fully activate DDR. Conversely, forced induction of local chromatin condensation promotes ataxia telangiectasia mutated (ATM)- and ATR-dependent activation of upstream DDR signaling in a break-independent manner. Whereas persistent chromatin compaction enhanced upstream DDR signaling from irradiation-induced breaks, it reduced recovery and survival after damage. Our results demonstrate that chromatin condensation is sufficient for activation of DDR signaling and is an integral part of physiological DDR signaling. Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.
    Cell reports. 11/2014;

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