Heterochromatic DNA Double Strand Break Repair

Eukaryotic chromatin is segregated into highly condensed heterochromatin and comparably relaxed euchromatin. Although heterochromatic gene expression is either transiently or permanently impeded, the integrity of heterochromatic DNA is critical for cell survival as it contributes to the regulation of nuclear architecture, gene expression, ribosome biogenesis, chromosome stability and mitosis. Formed by a plethora of proteins, structurally complex heterochromatin is generally inaccessible to DNA processing enzymes, including those repair factors required to rejoin DNA double strand breaks (DSBs). To be repaired, heterochromatic lesions require the Ataxia Telangiectasia Mutated (ATM) pathway to transiently modify heterochromatic factors surrounding the DSB, relaxing its structure and thereby allowing DNA non-homologous end-joining (NHEJ) to function. Cells deficient for ATM or proteins involved in its signalling cascade repair euchromatic DSBs normally but are unable to resolve lesions within heterochromatin. Depletion of key heterochromatic proteins, including the KAP-1 transcriptional co-repressor, Heterochromatin Protein 1 (HP1) or histone deacetylases 1&2 (HDAC1&2), relieves the requirement for ATM signalling in DSB repair. Importantly, KAP-1 is a highly dose dependent, transient and specific substrate of ATM and the manipulation of KAP-1 phosphorylation regulates heterochromatic DSB repair. We propose that KAP-1 is a critical heterochromatic factor that undergoes specific modifications following DSB formation to promote repair in a manner that allows localised and transient chromatin relaxation but precludes widespread dismantling of the heterochromatic superstructure

The Maintenance of ATM Dependent G2/M Checkpoint Arrest Following Exposure to Ionizing Radiation

The G2/M checkpoint is important in preventing cells with unrepaired DNA double strand breaks (DSBs) entering mitosis, an event which is likely to result in genomic instability. We recently reported that checkpoint arrest is maintained until close to completion of DSB repair and that the duration of checkpoint arrest depends on the dose and DSB repair capacity rather than lasting for a fixed period of time. ATM leads to phosphorylation of Chk1/2 in G2 phase following exposure to ionizing radiation. These transducer kinases can phosphorylate and inhibit Cdc25 activity, which is the phosphatase regulating mitotic entry. In this study we dissect three processes that contribute to the maintenance of checkpoint arrest in irradiated G2 phase cells. First, the ATR-Chk1 pathway contributes to maintaining checkpoint arrest, although it is dispensable for the initial activation of checkpoint arrest. Second, ongoing ATM to Chk2 signalling from unrepaired DSBs contributes to checkpoint arrest. This process plays a greater role in a repair defective background. Finally, slow decay of the initially activated Chk2 also contributes to the maintenance of checkpoint arrest. 53BP1 and MDC1 defective cells show an initial checkpoint defect after low doses but are proficient in initial activation of arrest after high doses. After higher radiation doses, however, 53BP1-/- and MDC1-/- MEFs fail to maintain checkpoint arrest. Furthermore 53BP1-/- and MDC1-/- MEFs display elevated mitotic breakage even after high doses. We show that the defect in the maintenance of checkpoint arrest conferred by 53BP1 and MDC1 deficiency substantially enhances chromosome breakage

Investigating the role of 53BP1 in DNA double strand break repair and checkpoint signalling

DNA double strand breaks arise endogenously in the cell as a result of routine activities such as metabolism and also as a result of exposure to exogenous agents such as ionising radiation. Repair of DNA DSBs is coordinated by two major repair pathways in mammalian cells; non-homologous end joining and homologous recombination. Nonhomologous end joining is the dominant repair pathway during G1 and G2 phase of the mammalian cell cycle. The core non-homologous end joining factors Ku, DNA-PKcs, XLF, DNA Ligase IV and XRCC4 are essential for efficient and accurate rejoining of double strand breaks. In 2004, Riballo et al. discovered that the ATM protein kinase and the Artemis endonuclease were important for repair of a fraction of DNA double strand breaks in G1 phase cells.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The impact of heterochromatin on DSB repair. Biochem Soc Trans

Abstract DNA NHEJ (non-homologous end-joining) is the major DNA DSB (double-strand break) repair pathway in mammalian cells. Although NHEJ-defective cell lines show marked DSB-repair defects, cells defective in ATM (ataxia telangiectasia mutated) repair most DSBs normally. Thus NHEJ functions independently of ATM signalling. However, ∼15 % of radiation-induced DSBs are repaired with slow kinetics and require ATM and the nuclease Artemis. DSBs persisting in the presence of an ATM inhibitor, ATMi, localize to heterochromatin, suggesting that ATM is required for repairing DSBs arising within or close to heterochromatin. Consistent with this, we show that siRNA (small interfering RNA) of key heterochromatic proteins, including KAP-1 [KRAB (Krüppel-associated box) domain-associated protein 1], HP1 (heterochromatin protein 1) and HDAC (histone deacetylase) 1/2, relieves the requirement for ATM for DSB repair. Furthermore, ATMi addition to cell lines with genetic alterations that have an impact on heterochromatin, including Suv39H1/2 (suppressor of variegation 3-9 homologue 1/2)-knockout, ICFa (immunodeficiency, centromeric region instability, facial anomalies syndrome type a) and Hutchinson-Guilford progeria cell lines, fails to have an impact on DSB repair. KAP-1 is a highly dose-dependent, transient and ATM-specific substrate, and mutation of the ATM phosphorylation site on KAP-1 influences DSB repair. Collectively, the findings show that ATM functions to overcome the barrier to DSB repair posed by heterochromatin. However, even in the presence of ATM, γ -H2AX (phosphorylated histone H2AX) foci form on the periphery rather than within heterochromatic centres. Finally, we show that KAP-1's association with heterochromatin is diminished as cells progress through mitosis. We propose that KAP-1 is a critical heterochromatic factor that undergoes specific modifications to promote DSB repair and mitotic progression in a manner that allows localized and transient chromatin relaxation, but precludes significant dismantling of the heterochromatic superstructure

The impact of heterochromatin on DSB repair

DNA NHEJ (non-homologous end-joining) is the major DNA DSB (double-strand break) repair pathway in mammalian cells. Although NHEJ-defective cell lines show marked DSB-repair defects, cells defective in ATM (ataxia telangiectasia mutated) repair most DSBs normally. Thus NHEJ functions independently of ATM signalling. However, approximately 15% of radiation-induced DSBs are repaired with slow kinetics and require ATM and the nuclease Artemis. DSBs persisting in the presence of an ATM inhibitor, ATMi, localize to heterochromatin, suggesting that ATM is required for repairing DSBs arising within or close to heterochromatin. Consistent with this, we show that siRNA (small interfering RNA) of key heterochromatic proteins, including KAP-1 [KRAB (Krüppel-associated box) domain-associated protein 1], HP1 (heterochromatin protein 1) and HDAC (histone deacetylase) 1/2, relieves the requirement for ATM for DSB repair. Furthermore, ATMi addition to cell lines with genetic alterations that have an impact on heterochromatin, including Suv39H1/2 (suppressor of variegation 3-9 homologue 1/2)-knockout, ICFa (immunodeficiency, centromeric region instability, facial anomalies syndrome type a) and Hutchinson-Guilford progeria cell lines, fails to have an impact on DSB repair. KAP-1 is a highly dose-dependent, transient and ATM-specific substrate, and mutation of the ATM phosphorylation site on KAP-1 influences DSB repair. Collectively, the findings show that ATM functions to overcome the barrier to DSB repair posed by heterochromatin. However, even in the presence of ATM, gamma-H2AX (phosphorylated histone H2AX) foci form on the periphery rather than within heterochromatic centres. Finally, we show that KAP-1's association with heterochromatin is diminished as cells progress through mitosis. We propose that KAP-1 is a critical heterochromatic factor that undergoes specific modifications to promote DSB repair and mitotic progression in a manner that allows localized and transient chromatin relaxation, but precludes significant dismantling of the heterochromatic superstructure

Role of ATM and the Damage Response Mediator Proteins 53BP1 and MDC1 in the Maintenance of G2/M Checkpoint Arrest▿ †

ATM-dependent initiation of the radiation-induced G2/M checkpoint arrest is well established. Recent results have shown that the majority of DNA double-strand breaks (DSBs) in G2 phase are repaired by DNA nonhomologous end joining (NHEJ), while ∼15% of DSBs are slowly repaired by homologous recombination. Here, we evaluate how the G2/M checkpoint is maintained in irradiated G2 cells, in light of our current understanding of G2 phase DSB repair. We show that ATM-dependent resection at a subset of DSBs leads to ATR-dependent Chk1 activation. ATR-Seckel syndrome cells, which fail to efficiently activate Chk1, and small interfering RNA (siRNA) Chk1-treated cells show premature mitotic entry. Thus, Chk1 significantly contributes to maintaining checkpoint arrest. Second, sustained ATM signaling to Chk2 contributes, particularly when NHEJ is impaired by XLF deficiency. We also show that cells lacking the mediator proteins 53BP1 and MDC1 initially arrest following radiation doses greater than 3 Gy but are subsequently released prematurely. Thus, 53BP1−/− and MDC1−/− cells manifest a checkpoint defect at high doses. This failure to maintain arrest is due to diminished Chk1 activation and a decreased ability to sustain ATM-Chk2 signaling. The combined repair and checkpoint defects conferred by 53BP1 and MDC1 deficiency act synergistically to enhance chromosome breakage

The Maintenance of ATM Dependent G2/M Checkpoint Arrest Following Exposure to Ionizing Radiation

The G2/M checkpoint is important in preventing cells with unrepaired DNA double strand breaks (DSBs) entering mitosis, an event which is likely to result in genomic instability. We recently reported that checkpoint arrest is maintained until close to completion of DSB repair and that the duration of checkpoint arrest depends on the dose and DSB repair capacity rather than lasting for a fixed period of time. ATM leads to phosphorylation of Chk1/2 in G2 phase following exposure to ionizing radiation. These transducer kinases can phosphorylate and inhibit Cdc25 activity, which is the phosphatase regulating mitotic entry. In this study we dissect three processes that contribute to the maintenance of checkpoint arrest in irradiated G2 phase cells. First, the ATR-Chk1 pathway contributes to maintaining checkpoint arrest, although it is dispensable for the initial activation of checkpoint arrest. Second, ongoing ATM to Chk2 signalling from unrepaired DSBs contributes to checkpoint arrest. This process plays a greater role in a repair defective background. Finally, slow decay of the initially activated Chk2 also contributes to the maintenance of checkpoint arrest. 53BP1 and MDC1 defective cells show an initial checkpoint defect after low doses but are proficient in initial activation of arrest after high doses. After higher radiation doses, however, 53BP1-/- and MDC1-/- MEFs fail to maintain checkpoint arrest. Furthermore 53BP1-/- and MDC1-/- MEFs display elevated mitotic breakage even after high doses. We show that the defect in the maintenance of checkpoint arrest conferred by 53BP1 and MDC1 deficiency substantially enhances chromosome breakage

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

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 ≤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