‘A mover and a shaker’: 53BP1 allows DNA doublestrand breaks a chance to dance and unite

The DNA damage response mediator protein, p53-binding protein 1 (53BP1), is dispensable for the repair of most DNA doublestrand breaks (DSBs) induced by ionising radiation. However, two recent studies have shown that 53BP1 is required for rejoining of distant DSB ends and that it promotes the movement of uncapped telomeres. These results are discussed in the light of recent findings, and a model for the role of 53BP1 in DSB rejoining is presented

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

ATM and Artemis promote homologous recombination of radiation-induced DNA double-strand breaks in G2

Homologous recombination (HR) and non‐homologous end joining (NHEJ) represent distinct pathways for repairing DNA double‐strand breaks (DSBs). Previous work implicated Artemis and ATM in an NHEJ‐dependent process, which repairs a defined subset of radiation‐induced DSBs in G1‐phase. Here, we show that in G2, as in G1, NHEJ represents the major DSB‐repair pathway whereas HR is only essential for repair of ∼15% of X‐ or γ‐ray‐induced DSBs. In addition to requiring the known HR proteins, Brca2, Rad51 and Rad54, repair of radiation‐induced DSBs by HR in G2 also involves Artemis and ATM suggesting that they promote NHEJ during G1 but HR during G2. The dependency for ATM for repair is relieved by depleting KAP‐1, providing evidence that HR in G2 repairs heterochromatin‐associated DSBs. Although not core HR proteins, ATM and Artemis are required for efficient formation of single‐stranded DNA and Rad51 foci at radiation‐induced DSBs in G2 with Artemis function requiring its endonuclease activity. We suggest that Artemis endonuclease removes lesions or secondary structures, which inhibit end resection and preclude the completion of HR or NHEJ

Oncogenetics of Lung Cancer Induced by Environmental Carcinogens

The molecular landscape of non-tobacco-induced primary lung tumors displays specific oncogenetic features. The etiology of these tumors has been largely associated with exposure to well-established environmental lung carcinogens such as radon, arsenic, and asbestos. Environmental carcinogens can induce specific genetic and epigenetic alterations in lung tissue, leading to aberrant function of lung cancer oncogenes and tumor suppressor genes. These molecular events result in the disruption of key cellular mechanisms, such as protection against oxidative stress and DNA damage-repair, which promotes tumor development and progression. This chapter provides a comprehensive discussion of the specific carcinogenic mechanisms associated with exposure to radon, arsenic, and asbestos. It also summarizes the main protein-coding and non-coding genes affected by exposure to these environmental agents, and the underlying molecular mechanisms promoting their deregulation in lung cancer. Finally, the chapter examines the anticipated challenges in personalized intervention strategies in non-tobacco-induced lung cancer

γH2AX foci analysis for monitoring DNA double-strand break repair: Strengths, limitations and optimization

DNA double-strand breaks (DSBs) represent an important radiation-induced lesion and impaired DSB repair provides the best available correlation with radiosensitivity. Physical techniques for monitoring DSB repair require high, non-physiological doses and cannot reliably detect subtle defects. One outcome from extensive research into the DNA damage response is the observation that H2AX, a variant form of the histone H2A, undergoes extensive phosphorylation at the DSB, creating γH2AX foci that can be visualised by immunofluorescence. There is a close correlation between γH2AX foci and DSB numbers and between the rate of foci loss and DSB repair, providing a sensitive assay to monitor DSB repair in individual cells using physiological doses. However, γH2AX formation can occur at single-stranded DNA regions which arise during replication or repair and thus does not solely correlate with DSB formation. Here, we present and discuss evidence that following exposure to ionising radiation, γH2AX foci analysis can provide a sensitive monitor of DSB formation and repair and describe techniques to optimise the analysis. We discuss the limitations and benefits of the technique, enabling the procedure to be optimally exploited but not misused

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

Opposing roles for 53BP1 during homologous recombination

Although DNA non-homologous end-joining repairs most DNA double-strand breaks (DSBs) in G2 phase, late repairing DSBs undergo resection and repair by homologous recombination (HR). Based on parallels to the situation in G1 cells, previous work has suggested that DSBs that undergo repair by HR predominantly localize to regions of heterochromatin (HC). By using H3K9me3 and H4K20me3 to identify HC regions, we substantiate and extend previous evidence, suggesting that HC-DSBs undergo repair by HR. Next, we examine roles for 53BP1 and BRCA1 in this process. Previous studies have shown that 53BP1 is pro-non-homologous end-joining and anti-HR. Surprisingly, we demonstrate that in G2 phase, 53BP1 is required for HR at HC-DSBs with its role being to promote phosphorylated KAP-1 foci formation. BRCA1, in contrast, is dispensable for pKAP-1 foci formation but relieves the barrier caused by 53BP1. As 53BP1 is retained at irradiation-induced foci during HR, we propose that BRCA1 promotes displacement but retention of 53BP1 to allow resection and any necessary HC modifications to complete HR. In contrast to this role for 53BP1 in HR in G2 phase, we show that it is dispensable for HR in S phase, where HC regions are likely relaxed during replication

XLF-Cernunnos promotes DNA ligase IV–XRCC4 re-adenylation following ligation

XLF-Cernunnos (XLF) is a component of the DNA ligase IV–XRCC4 (LX) complex, which functions during DNA non-homologous end joining (NHEJ). Here, we use biochemical and cellular approaches to probe the impact of XLF on LX activities. We show that XLF stimulates adenylation of LX complexes de-adenylated by pyrophosphate or following LX decharging during ligation. XLF enhances LX ligation activity in an ATP-independent and dependent manner. ATP-independent stimulation can be attributed to enhanced end-bridging. Whilst ATP alone fails to stimulate LX ligation activity, addition of XLF and ATP promotes ligation in a manner consistent with XLF-stimulated readenylation linked to ligation. We show that XLF is a weakly bound partner of the tightly associated LX complex and, unlike XRCC4, is dispensable for LX stability. 2BN cells, which have little, if any, residual XLF activity, show a 3-fold decreased ability to repair DNA double strand breaks covering a range of complexity. These findings strongly suggest that XLF is not essential for NHEJ but promotes LX adenylation and hence ligation. We propose a model in which XLF, by in situ recharging DNA ligase IV after the first ligation event, promotes double stranded ligation by a single LX complex