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The heterochromatic barrier to DNA double strand break repair: how to get the entry visa.

Goodarzi AA, Jeggo PA - Int J Mol Sci (2012)

Bottom Line: Over recent decades, a deep understanding of pathways that repair DNA double strand breaks (DSB) has been gained from biochemical, structural, biophysical and cellular studies.Chromatin is broadly divided into open, transcriptionally active, euchromatin (EC) and highly compacted, transcriptionally inert, heterochromatin (HC), although these represent extremes of a spectrum.Moreover, DSBs within HC (HC-DSBs) are rapidly relocalized to the EC-HC interface.

View Article: PubMed Central - PubMed

Affiliation: Southern Alberta Cancer Research Institute, Departments of Biochemistry & Molecular Biology and Oncology, University of Calgary, Calgary, Alberta, T2N 4N1, Canada.

ABSTRACT
Over recent decades, a deep understanding of pathways that repair DNA double strand breaks (DSB) has been gained from biochemical, structural, biophysical and cellular studies. DNA non-homologous end-joining (NHEJ) and homologous recombination (HR) represent the two major DSB repair pathways, and both processes are now well understood. Recent work has demonstrated that the chromatin environment at a DSB significantly impacts upon DSB repair and that, moreover, dramatic modifications arise in the chromatin surrounding a DSB. Chromatin is broadly divided into open, transcriptionally active, euchromatin (EC) and highly compacted, transcriptionally inert, heterochromatin (HC), although these represent extremes of a spectrum. The HC superstructure restricts both DSB repair and damage response signaling. Moreover, DSBs within HC (HC-DSBs) are rapidly relocalized to the EC-HC interface. The damage response protein kinase, ataxia telangiectasia mutated (ATM), is required for HC-DSB repair but is dispensable for the relocalization of HC-DSBs. It has been proposed that ATM signaling enhances HC relaxation in the DSB vicinity and that this is a prerequisite for HC-DSB repair. Hence, ATM is essential for repair of HC-DSBs. Here, we discuss how HC impacts upon the response to DSBs and how ATM overcomes the barrier that HC poses to repair.

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Related in: MedlinePlus

The influence of heterochromatin on ATM signaling and cell cycle checkpoint arrest. (A, 1–3) At early times after DSB formation, a significant difference in the magnitude of IRIF formation and consequential ATM signaling is observed based on the relative proximity of the DSB to heterochromatic centers, with the expansion of IRIF forming at DSBs located near or within heterochromatin BEING constrained. (A, 4) Where defective heterochromatin is encountered, as a result of germ-line mutation or siRNA-mediated knockdown of heterochromatic building factors, IRIF expansion is unconstrained and a greater degree of ATM signaling is produced relative to normal. (A, 5) In the absence of 53BP1 or the dense, localized phosphorylation of KAP-1, IRIF penetrance into heterochromatin is severely hindered causing a reduction in the amount of ATM signaling. (B) Cell cycle checkpoint arrest is triggered by a threshold number of DSBs in normal cells, due to a defined amount of ATM signaling being produced per DSB within the cell. The magnitude of ATM signaling per DSB is “set” by the natural balance between euchromatin and heterochromatin. Where defective heterochromatin is present, DSBs within or bordering on heterochromatic centers signal to a greater extent than they would in normal cells, since IRIF expansion and ATM signaling are no longer constrained. Hence, the number of DSBs required to achieve the minimum level of ATM signaling needed to trigger checkpoint arrest is lowered and these cells display hypersensitive checkpoint initiation and prolonged maintenance. Note that the figures display events at early times post irradiation. At later times, IRIF expansion occurs into euchromatin efficiently so that the overall size of the foci from DSBs within euchromatin versus heterochromatin are similar although most of the IRIF occurs within the euchromatin region. At later times, IRIF at DSBs within defective heterochromatin are larger than those within euchromatin suggesting that factors may be recruited to euchromatin to restrict their expansion.
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f3-ijms-13-11844: The influence of heterochromatin on ATM signaling and cell cycle checkpoint arrest. (A, 1–3) At early times after DSB formation, a significant difference in the magnitude of IRIF formation and consequential ATM signaling is observed based on the relative proximity of the DSB to heterochromatic centers, with the expansion of IRIF forming at DSBs located near or within heterochromatin BEING constrained. (A, 4) Where defective heterochromatin is encountered, as a result of germ-line mutation or siRNA-mediated knockdown of heterochromatic building factors, IRIF expansion is unconstrained and a greater degree of ATM signaling is produced relative to normal. (A, 5) In the absence of 53BP1 or the dense, localized phosphorylation of KAP-1, IRIF penetrance into heterochromatin is severely hindered causing a reduction in the amount of ATM signaling. (B) Cell cycle checkpoint arrest is triggered by a threshold number of DSBs in normal cells, due to a defined amount of ATM signaling being produced per DSB within the cell. The magnitude of ATM signaling per DSB is “set” by the natural balance between euchromatin and heterochromatin. Where defective heterochromatin is present, DSBs within or bordering on heterochromatic centers signal to a greater extent than they would in normal cells, since IRIF expansion and ATM signaling are no longer constrained. Hence, the number of DSBs required to achieve the minimum level of ATM signaling needed to trigger checkpoint arrest is lowered and these cells display hypersensitive checkpoint initiation and prolonged maintenance. Note that the figures display events at early times post irradiation. At later times, IRIF expansion occurs into euchromatin efficiently so that the overall size of the foci from DSBs within euchromatin versus heterochromatin are similar although most of the IRIF occurs within the euchromatin region. At later times, IRIF at DSBs within defective heterochromatin are larger than those within euchromatin suggesting that factors may be recruited to euchromatin to restrict their expansion.

Mentions: Cell cycle checkpoint arrest represents a significant consequence of the DDR. Such checkpoints include the G1/S, intra-S and G2/M checkpoint. The G2/M checkpoint is interesting in this context since a range of studies have shown that low doses of ionizing radiation fail to efficiently activate checkpoint arrest—i.e., there appears to be a defined threshold of DDR signaling required to activate checkpoint arrest [57,58]. Similarly, checkpoint arrest is not maintained until the completion of DSB repair. These findings prompted us to ask if loss of HC factors in human disorders or siRNA-mediated depletion of factors required to sustain the HC superstructure might impact upon the sensitivity of G2/M checkpoint arrest. The striking finding was that G2/M checkpoint arrest was activated following exposure to lower doses in Rett Syndrome cells compared to control cells and arrest was maintained for a prolonged period of time. Since DSB repair was normal in Rett Syndrome cells, this suggests that checkpoint arrest was activated and maintained by lower DSB numbers compared to control cell lines (depicted in Figure 3) [33]. A similar finding was observed with ICFa syndrome cells. Significantly, ICFa syndrome is characterized by premature ageing raising the possibility that hyper-activation of senescence could arise from enhanced signaling from uncapped telomeres.


The heterochromatic barrier to DNA double strand break repair: how to get the entry visa.

Goodarzi AA, Jeggo PA - Int J Mol Sci (2012)

The influence of heterochromatin on ATM signaling and cell cycle checkpoint arrest. (A, 1–3) At early times after DSB formation, a significant difference in the magnitude of IRIF formation and consequential ATM signaling is observed based on the relative proximity of the DSB to heterochromatic centers, with the expansion of IRIF forming at DSBs located near or within heterochromatin BEING constrained. (A, 4) Where defective heterochromatin is encountered, as a result of germ-line mutation or siRNA-mediated knockdown of heterochromatic building factors, IRIF expansion is unconstrained and a greater degree of ATM signaling is produced relative to normal. (A, 5) In the absence of 53BP1 or the dense, localized phosphorylation of KAP-1, IRIF penetrance into heterochromatin is severely hindered causing a reduction in the amount of ATM signaling. (B) Cell cycle checkpoint arrest is triggered by a threshold number of DSBs in normal cells, due to a defined amount of ATM signaling being produced per DSB within the cell. The magnitude of ATM signaling per DSB is “set” by the natural balance between euchromatin and heterochromatin. Where defective heterochromatin is present, DSBs within or bordering on heterochromatic centers signal to a greater extent than they would in normal cells, since IRIF expansion and ATM signaling are no longer constrained. Hence, the number of DSBs required to achieve the minimum level of ATM signaling needed to trigger checkpoint arrest is lowered and these cells display hypersensitive checkpoint initiation and prolonged maintenance. Note that the figures display events at early times post irradiation. At later times, IRIF expansion occurs into euchromatin efficiently so that the overall size of the foci from DSBs within euchromatin versus heterochromatin are similar although most of the IRIF occurs within the euchromatin region. At later times, IRIF at DSBs within defective heterochromatin are larger than those within euchromatin suggesting that factors may be recruited to euchromatin to restrict their expansion.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3472778&req=5

f3-ijms-13-11844: The influence of heterochromatin on ATM signaling and cell cycle checkpoint arrest. (A, 1–3) At early times after DSB formation, a significant difference in the magnitude of IRIF formation and consequential ATM signaling is observed based on the relative proximity of the DSB to heterochromatic centers, with the expansion of IRIF forming at DSBs located near or within heterochromatin BEING constrained. (A, 4) Where defective heterochromatin is encountered, as a result of germ-line mutation or siRNA-mediated knockdown of heterochromatic building factors, IRIF expansion is unconstrained and a greater degree of ATM signaling is produced relative to normal. (A, 5) In the absence of 53BP1 or the dense, localized phosphorylation of KAP-1, IRIF penetrance into heterochromatin is severely hindered causing a reduction in the amount of ATM signaling. (B) Cell cycle checkpoint arrest is triggered by a threshold number of DSBs in normal cells, due to a defined amount of ATM signaling being produced per DSB within the cell. The magnitude of ATM signaling per DSB is “set” by the natural balance between euchromatin and heterochromatin. Where defective heterochromatin is present, DSBs within or bordering on heterochromatic centers signal to a greater extent than they would in normal cells, since IRIF expansion and ATM signaling are no longer constrained. Hence, the number of DSBs required to achieve the minimum level of ATM signaling needed to trigger checkpoint arrest is lowered and these cells display hypersensitive checkpoint initiation and prolonged maintenance. Note that the figures display events at early times post irradiation. At later times, IRIF expansion occurs into euchromatin efficiently so that the overall size of the foci from DSBs within euchromatin versus heterochromatin are similar although most of the IRIF occurs within the euchromatin region. At later times, IRIF at DSBs within defective heterochromatin are larger than those within euchromatin suggesting that factors may be recruited to euchromatin to restrict their expansion.
Mentions: Cell cycle checkpoint arrest represents a significant consequence of the DDR. Such checkpoints include the G1/S, intra-S and G2/M checkpoint. The G2/M checkpoint is interesting in this context since a range of studies have shown that low doses of ionizing radiation fail to efficiently activate checkpoint arrest—i.e., there appears to be a defined threshold of DDR signaling required to activate checkpoint arrest [57,58]. Similarly, checkpoint arrest is not maintained until the completion of DSB repair. These findings prompted us to ask if loss of HC factors in human disorders or siRNA-mediated depletion of factors required to sustain the HC superstructure might impact upon the sensitivity of G2/M checkpoint arrest. The striking finding was that G2/M checkpoint arrest was activated following exposure to lower doses in Rett Syndrome cells compared to control cells and arrest was maintained for a prolonged period of time. Since DSB repair was normal in Rett Syndrome cells, this suggests that checkpoint arrest was activated and maintained by lower DSB numbers compared to control cell lines (depicted in Figure 3) [33]. A similar finding was observed with ICFa syndrome cells. Significantly, ICFa syndrome is characterized by premature ageing raising the possibility that hyper-activation of senescence could arise from enhanced signaling from uncapped telomeres.

Bottom Line: Over recent decades, a deep understanding of pathways that repair DNA double strand breaks (DSB) has been gained from biochemical, structural, biophysical and cellular studies.Chromatin is broadly divided into open, transcriptionally active, euchromatin (EC) and highly compacted, transcriptionally inert, heterochromatin (HC), although these represent extremes of a spectrum.Moreover, DSBs within HC (HC-DSBs) are rapidly relocalized to the EC-HC interface.

View Article: PubMed Central - PubMed

Affiliation: Southern Alberta Cancer Research Institute, Departments of Biochemistry & Molecular Biology and Oncology, University of Calgary, Calgary, Alberta, T2N 4N1, Canada.

ABSTRACT
Over recent decades, a deep understanding of pathways that repair DNA double strand breaks (DSB) has been gained from biochemical, structural, biophysical and cellular studies. DNA non-homologous end-joining (NHEJ) and homologous recombination (HR) represent the two major DSB repair pathways, and both processes are now well understood. Recent work has demonstrated that the chromatin environment at a DSB significantly impacts upon DSB repair and that, moreover, dramatic modifications arise in the chromatin surrounding a DSB. Chromatin is broadly divided into open, transcriptionally active, euchromatin (EC) and highly compacted, transcriptionally inert, heterochromatin (HC), although these represent extremes of a spectrum. The HC superstructure restricts both DSB repair and damage response signaling. Moreover, DSBs within HC (HC-DSBs) are rapidly relocalized to the EC-HC interface. The damage response protein kinase, ataxia telangiectasia mutated (ATM), is required for HC-DSB repair but is dispensable for the relocalization of HC-DSBs. It has been proposed that ATM signaling enhances HC relaxation in the DSB vicinity and that this is a prerequisite for HC-DSB repair. Hence, ATM is essential for repair of HC-DSBs. Here, we discuss how HC impacts upon the response to DSBs and how ATM overcomes the barrier that HC poses to repair.

Show MeSH
Related in: MedlinePlus