53BP1-mediated DNA double strand break repair: insert bad pun here.

53BP1 is an established player in the cellular response to DNA damage and is a canonical component of ionizing-radiation induced foci--that cadre of proteins which assemble at DNA double strand breaks following radiation exposure and which are readily visualized by immunofluorescence microscopy. While its roles in p53 regulation and cell cycle checkpoint activation have been studied for some time, the impact of 53BP1 on DNA double strand break rejoining has only come to light in the past few years. Convincing evidence now exists for 53BP1 significantly affecting the outcome of DNA double strand break repair in several contexts, many of which hint to an important role in modulating chromatin structure surrounding the break site. Here, we highlight the known and emerging roles of 53BP1 in DNA double strand break repair, including the repair of lesions induced within heterochromatin, following telomere uncapping, in long-range V(D)J recombination, during immunoglobulin class switch recombination and its much debated role in regulating resection during homologous recombination.

Analysis of human syndromes with disordered chromatin reveals the impact of heterochromatin on the efficacy of ATM-dependent G2/M checkpoint arrest.

Heterochromatin (HC) poses a barrier to γH2AX focus expansion and DNA double-strand break (DSB) repair, the latter being relieved by ATM-dependent KAP-1 phosphorylation. Using high-resolution imaging, we show here that the HC superstructure markedly restricts ATM signaling to cell cycle checkpoint proteins. The impact of HC is greater than anticipated from the percentage of HC-DNA and, in distinction to DSB repair, ATM only partly overcomes the constraints posed by HC. Importantly, we examine ATM signaling in human syndromes with disordered HC. After depletion of MeCP2 and DNMT3B, proteins defective in the Rett and immunodeficiency with centromere instability and facial anomalies (ICF) syndromes, respectively, we demonstrate enhanced γH2AX signal expansion at HC-chromocenters in mouse NIH 3T3 cells, which have visible HC-chromocenters. Previous studies have shown that the G(2)/M checkpoint is inefficient requiring multiple DSBs to initiate arrest. MeCP2 and DNMT3B depletion leads to hypersensitive radiation-induced G(2)/M checkpoint arrest despite normal DSB repair. Cell lines from Rett, ICF, and Hutchinson-Guildford progeria syndrome patients similarly showed hyperactivated ATM signaling and hypersensitive and prolonged G(2)/M checkpoint arrest. Collectively, these findings reveal that heterochromatin contributes to the previously described inefficient G(2)/M checkpoint arrest and demonstrate how the signaling response can be uncoupled from DSB repair.

Role of ATM and the damage response mediator proteins 53BP1 and MDC1 in the maintenance of G(2)/M checkpoint arrest.

ATM-dependent initiation of the radiation-induced G(2)/M checkpoint arrest is well established. Recent results have shown that the majority of DNA double-strand breaks (DSBs) in G(2) phase are repaired by DNA nonhomologous end joining (NHEJ), while approximately 15% of DSBs are slowly repaired by homologous recombination. Here, we evaluate how the G(2)/M checkpoint is maintained in irradiated G(2) cells, in light of our current understanding of G(2) 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.

53BP1-dependent robust localized KAP-1 phosphorylation is essential for heterochromatic DNA double-strand break repair.

DNA double-strand breaks (DSBs) trigger ATM (ataxia telangiectasia mutated) signalling and elicit genomic rearrangements and chromosomal fragmentation if misrepaired or unrepaired. Although most DSB repair is ATM-independent, approximately 15% of ionizing radiation (IR)-induced breaks persist in the absence of ATM-signalling. 53BP1 (p53-binding protein 1) facilitates ATM-dependent DSB repair but is largely dispensable for ATM activation or checkpoint arrest. ATM promotes DSB repair within heterochromatin by phosphorylating KAP-1 (KRAB-associated protein 1, also known as TIF1beta, TRIM28 or KRIP-1; ref. 2). Here, we show that the ATM signalling mediator proteins MDC1, RNF8, RNF168 and 53BP1 are also required for heterochromatic DSB repair. Although KAP-1 phosphorylation is critical for 53BP1-mediated repair, overall phosphorylated KAP-1 (pKAP-1) levels are only modestly affected by 53BP1 loss. pKAP-1 is transiently pan-nuclear but also forms foci overlapping with gammaH2AX in heterochromatin. Cells that do not form 53BP1 foci, including human RIDDLE (radiosensitivity, immunodeficiency, dysmorphic features and learning difficulties) syndrome cells, fail to form pKAP-1 foci. 53BP1 amplifies Mre11-NBS1 accumulation at late-repairing DSBs, concentrating active ATM and leading to robust, localized pKAP-1. We propose that ionizing-radiation induced foci (IRIF) spatially concentrate ATM activity to promote localized alterations in regions of chromatin otherwise inhibitory to repair.

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.

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 < 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.

Organization of interphase microtubules in fission yeast analyzed by electron tomography.

Polarized cells, such as neuronal, epithelial, and fungal cells, all display a specialized organization of their microtubules (MTs). The interphase MT cytoskeleton of the rod-shaped fission yeast, Schizosaccharomyces pombe, has been extensively described by fluorescence microscopy. Here, we describe a large-scale, electron tomography investigation of S. pombe, including a 3D reconstruction of a complete eukaryotic cell volume at sufficient resolution to show both how many MTs there are in a bundle and their detailed architecture. Most cytoplasmic MTs are open at one end and capped at the other, providing evidence about their polarity. Electron-dense bridges between the MTs themselves and between MTs and the nuclear envelope were frequently observed. Finally, we have investigated structure/function relationships between MTs and both mitochondria and vesicles. Our analysis shows that electron tomography of well-preserved cells is ideally suited for describing fine ultrastructural details that were not visible with previous techniques.