The importance of DNAPKcs for blunt DNA end joining is magnified when XLF is weakened

Multivalent interactions of the disordered regions of XLF and XRCC4 foster robust cellular NHEJ and drive the formation of ligation-boosting condensates in vitro

In mammalian cells, DNA double-strand breaks are predominantly repaired by non-homologous end joining (NHEJ). During repair, the Ku70-Ku80 heterodimer (Ku), X-ray repair cross complementing 4 (XRCC4) in complex with DNA ligase 4 (X4L4) and XRCC4-like factor (XLF) form a flexible scaffold that holds the broken DNA ends together. Insights into the architectural organization of the NHEJ scaffold and its regulation by the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) were recently obtained by single-particle cryo-electron microscopy analysis. However, several regions, especially the C-terminal regions (CTRs) of the XRCC4 and XLF scaffolding proteins, have largely remained unresolved in experimental structures, which hampers the understanding of their functions. Here we used magnetic resonance techniques and biochemical assays to comprehensively characterize the interactions and dynamics of the XRCC4 and XLF CTRs at residue resolution. We show that the CTRs of XRCC4 and XLF are intrinsically disordered and form a network of multivalent heterotypic and homotypic interactions that promotes robust cellular NHEJ activity. Importantly, we demonstrate that the multivalent interactions of these CTRs lead to the formation of XLF and X4L4 condensates in vitro, which can recruit relevant effectors and critically stimulate DNA end ligation. Our work highlights the role of disordered regions in the mechanism and dynamics of NHEJ and lays the groundwork for the investigation of NHEJ protein disorder and its associated condensates inside cells with implications in cancer biology, immunology and the development of genome-editing strategies.

Identification of the main barriers to Ku accumulation in chromatin

Repair of DNA double-strand breaks by the non-homologous end-joining pathway is initiated by the binding of Ku to DNA ends. Multiple Ku proteins load onto linear DNAs in vitro. However, in cells, Ku loading is limited to ∼1-2 molecules per DNA end. The mechanisms enforcing this limit are currently unclear. Here, we show that the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), but not its protein kinase activity, is required to prevent excessive Ku entry into chromatin. Ku accumulation is further restricted by two mechanisms: a neddylation/FBXL12-dependent process that actively removes loaded Ku molecules throughout the cell cycle and a CtIP/ATM-dependent mechanism that operates in S phase. Finally, we demonstrate that the misregulation of Ku loading leads to impaired transcription in the vicinity of DNA ends. Together, our data shed light on the multiple mechanisms operating to prevent Ku from invading chromatin and interfering with other DNA transactions.

Distinct functions of PAXX and MRI during chromosomal end joining

A key step of Canonical Nonhomologous End Joining (C-NHEJ) is synapsis of DNA double strand break (DSB) ends for ligation. The DNA-PKcs dimer mediates synapsis in a long-range complex with DSB ends remaining apart, whereas the XLF homodimer can mediate synapsis in both long-range and short-range complexes. Recent structural studies found the PAXX homodimer may also facilitate synapsis in long-range complexes with DNA-PKcs via its interactions with Ku70. Thus, we examined the influence of PAXX in C-NHEJ of chromosomal DSBs, which we compared to another Ku-binding factor, MRI. Using EJ of blunt DSBs with Cas9 reporters as a readout for C-NHEJ, we found that PAXX and/or MRI are dispensable. However, when combined with disruption of DNA-PKcs, particularly with DNA-PKcs kinase inhibition, PAXX becomes important for blunt DSB EJ. In contrast, while DNA-PKcs is also important to suppress short deletion mutations with microhomology, this effect is not magnified with PAXX loss. MRI loss had no effect combined with DNA-PKcs disruption, but becomes important for blunt DSB EJ when combined with disruption of XLF, as is PAXX. Finally, XLF loss causes an increase in larger deletions compared to DNA-PKcs inhibition, which is magnified with combined loss of MRI. Altogether, we suggest that PAXX promotes DSB end synapsis during C-NHEJ in a manner that is partially redundant with DNA-PKcs and XLF, whereas MRI appears to be mainly important in the context of XLF disruption.

Exploring factors influencing choice of DNA double-strand break repair pathways

DNA double-strand breaks (DSBs) represent one of the most severe threats to genomic integrity, demanding intricate repair mechanisms within eukaryotic cells. A diverse array of factors orchestrates the complex choreography of DSB signaling and repair, encompassing repair pathways, such as non-homologous end-joining, homologous recombination, and polymerase-θ-mediated end-joining. This review looks into the intricate decision-making processes guiding eukaryotic cells towards a particular repair pathway, particularly emphasizing the processing of two-ended DSBs. Furthermore, we elucidate the transformative role of Cas9, a site-specific endonuclease, in revolutionizing our comprehension of DNA DSB repair dynamics. Additionally, we explore the burgeoning potential of Cas9's remarkable ability to induce sequence-specific DSBs, offering a promising avenue for precise targeting of tumor cells. Through this comprehensive exploration, we unravel the intricate molecular mechanisms of cellular responses to DSBs, shedding light on both fundamental repair processes and cutting-edge therapeutic strategies.

Non-homologous end joining shapes the genomic rearrangement landscape of chromothripsis from mitotic errors

Mitotic errors generate micronuclei entrapping mis-segregated chromosomes, which are susceptible to catastrophic fragmentation through chromothripsis. The reassembly of fragmented chromosomes by error-prone DNA double-strand break (DSB) repair generates diverse genomic rearrangements associated with human diseases. How specific repair pathways recognize and process these lesions remains poorly understood. Here we use CRISPR/Cas9 to systematically inactivate distinct DSB repair pathways and interrogate the rearrangement landscape of fragmented chromosomes. Deletion of canonical non-homologous end joining (NHEJ) components substantially reduces complex rearrangements and shifts the rearrangement landscape toward simple alterations without the characteristic patterns of chromothripsis. Following reincorporation into the nucleus, fragmented chromosomes localize within sub-nuclear micronuclei bodies (MN bodies) and undergo ligation by NHEJ within a single cell cycle. In the absence of NHEJ, chromosome fragments are rarely engaged by alternative end-joining or recombination-based mechanisms, resulting in delayed repair kinetics, persistent 53BP1-labeled MN bodies, and cell cycle arrest. Thus, we provide evidence supporting NHEJ as the exclusive DSB repair pathway generating complex rearrangements from mitotic errors.

Identification of the main barriers to Ku accumulation in chromatin

Repair of DNA double strand breaks by the non-homologous end-joining pathway is initiated by the binding of Ku to DNA ends. Given its high affinity for ends, multiple Ku proteins load onto linear DNAs in vitro. However, in cells, Ku loading is limited to ~1-2 molecules per DNA end. The mechanisms enforcing this limit are currently unknown. Here we show that the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), but not its protein kinase activity, is required to prevent excessive Ku entry into chromatin. Ku accumulation is further restricted by two mechanisms: a neddylation/FBXL12-dependent process which actively removes loaded Ku molecules throughout the cell cycle and a CtIP/ATM-dependent mechanism which operates in S-phase. Finally, we demonstrate that the misregulation of Ku loading leads to impaired transcription in the vicinity of DNA ends. Together our data shed light on the multiple layers of coordinated mechanisms operating to prevent Ku from invading chromatin and interfering with other DNA transactions.

Cold snapshots of DNA repair: Cryo-EM structures of DNA-PKcs and NHEJ machinery

The proteins and protein assemblies involved in DNA repair have been the focus of a multitude of structural studies for the past few decades. Historically, the structures of these protein complexes have been resolved by X-ray crystallography. However, more recently with the advancements in cryo-electron microscopy (cryo-EM) ranging from optimising the methodology for sample preparation to the development of improved electron detectors, the focus has shifted from X-ray crystallography to cryo-EM. This methodological transition has allowed for the structural determination of larger, more complex protein assemblies involved in DNA repair pathways and has subsequently led to a deeper understanding of the mechanisms utilised by these fascinating molecular machines. Here, we review some of the key structural advancements that have been gained in the study of non-homologous end joining (NHEJ) by the use of cryo-EM, with a focus on assemblies composed of DNA-PKcs and Ku70/80 (Ku) and the various methodologies utilised to obtain these structures.

DNA polymerases in precise and predictable CRISPR/Cas9-mediated chromosomal rearrangements

BACKGROUND

Recent studies have shown that, owning to its cohesive cleavage, Cas9-mediated CRISPR gene editing outcomes at junctions of chromosomal rearrangements or DNA-fragment editing are precise and predictable; however, the underlying mechanisms are poorly understood due to lack of suitable assay system and analysis tool.

RESULTS

Here we developed a customized computer program to take account of staggered or cohesive Cas9 cleavage and to rapidly process large volumes of junctional sequencing reads from chromosomal rearrangements or DNA-fragment editing, including DNA-fragment inversions, duplications, and deletions. We also established a sensitive assay system using HPRT1 and DCK as reporters for cell growth during DNA-fragment editing by Cas9 with dual sgRNAs and found prominent large resections or long deletions at junctions of chromosomal rearrangements. In addition, we found that knockdown of PolQ (encoding Polθ polymerase), which has a prominent role in theta-mediated end joining (TMEJ) or microhomology-mediated end joining (MMEJ), results in increased large resections but decreased small deletions. We also found that the mechanisms for generating small deletions of 1bp and >1bp during DNA-fragment editing are different with regard to their opposite dependencies on Polθ and Polλ (encoded by the PolL gene). Specifically, Polθ suppresses 1bp deletions but promotes >1bp deletions, whereas Polλ promotes 1bp deletions but suppresses >1bp deletions. Finally, we found that Polλ is the main DNA polymerase responsible for fill-in of the 5' overhangs of staggered Cas9 cleavage ends.

CONCLUSIONS

These findings contribute to our understanding of the molecular mechanisms of CRISPR/Cas9-mediated DNA-fragment editing and have important implications for controllable, precise, and predictable gene editing.

Structure and mechanism in non-homologous end joining

DNA double-stranded breaks (DSBs) are a particularly challenging form of DNA damage to repair because the damaged DNA must not only undergo the chemical reactions responsible for returning it to its original state, but, additionally, the two free ends can become physically separated in the nucleus and must be bridged prior to repair. In nonhomologous end joining (NHEJ), one of the major pathways of DSB repair, repair is carried out by a number of repair factors capable of binding to and directly joining DNA ends. It has been unclear how these processes are carried out at a molecular level, owing in part to the lack of structural evidence describing the coordination of the NHEJ factors with each other and a DNA substrate. Advances in cryo-Electron Microscopy (cryo-EM), allowing for the structural characterization of large protein complexes that would be intractable using other techniques, have led to the visualization several key steps of the NHEJ process, which support a model of sequential assembly of repair factors at the DSB, followed by end-bridging mediated by protein-protein complexes and transition to full synapsis. Here we examine the structural evidence for these models, devoting particular attention to recent work identifying a new NHEJ intermediate state and incorporating new NHEJ factors into the general mechanism. We also discuss the evolving understanding of end-bridging mechanisms in NHEJ and DNA-PKcs's role in mediating DSB repair.

Holding it together: DNA end synapsis during non-homologous end joining

DNA double strand breaks (DSBs) are common lesions whose misrepair are drivers of oncogenic transformations. The non-homologous end joining (NHEJ) pathway repairs the majority of these breaks in vertebrates by directly ligating DNA ends back together. Upon formation of a DSB, a multiprotein complex is assembled on DNA ends which tethers them together within a synaptic complex. Synapsis is a critical step of the NHEJ pathway as loss of synapsis can result in mispairing of DNA ends and chromosome translocations. As DNA ends are commonly incompatible for ligation, the NHEJ machinery must also process ends to enable rejoining. This review describes how recent progress in single-molecule approaches and cryo-EM have advanced our molecular understanding of DNA end synapsis during NHEJ and how synapsis is coordinated with end processing to determine the fidelity of repair.

Non-homologous end joining shapes the genomic rearrangement landscape of chromothripsis from mitotic errors

Errors in mitosis can generate micronuclei that entrap mis-segregated chromosomes, which are susceptible to catastrophic fragmentation through a process termed chromothripsis. The reassembly of fragmented chromosomes by error-prone DNA double-strand break (DSB) repair generates a spectrum of simple and complex genomic rearrangements that are associated with human cancers and disorders. How specific DSB repair pathways recognize and process these lesions remains poorly understood. Here we used CRISPR/Cas9 to systematically inactivate distinct DSB processing or repair pathways and interrogated the rearrangement landscape of fragmented chromosomes from micronuclei. Deletion of canonical non-homologous end joining (NHEJ) components, including DNA-PKcs, LIG4, and XLF, substantially reduced the formation of complex rearrangements and shifted the rearrangement landscape toward simple alterations without the characteristic patterns of cancer-associated chromothripsis. Following reincorporation into the nucleus, fragmented chromosomes localize within micronuclei bodies (MN bodies) and undergo successful ligation by NHEJ within a single cell cycle. In the absence of NHEJ, chromosome fragments were rarely engaged by polymerase theta-mediated alternative end-joining or recombination-based mechanisms, resulting in delayed repair kinetics and persistent 53BP1-labeled MN bodies in the interphase nucleus. Prolonged DNA damage signaling from unrepaired fragments ultimately triggered cell cycle arrest. Thus, we provide evidence supporting NHEJ as the exclusive DSB repair pathway generating complex rearrangements following chromothripsis from mitotic errors.

The flexible and iterative steps within the NHEJ pathway

Cellular and biochemical studies of nonhomologous DNA end joining (NHEJ) have long established that nuclease and polymerase action are necessary for the repair of a very large fraction of naturally-arising double-strand breaks (DSBs). This conclusion is derived from NHEJ studies ranging from yeast to humans and all genetically-tractable model organisms. Biochemical models derived from recent real-time and structural studies have yet to incorporate physical space or timing for DNA end processing. In real-time single molecule FRET (smFRET) studies, we analyzed NHEJ synapsis of DNA ends in a defined biochemical system. We described a Flexible Synapsis (FS) state in which the DNA ends were in proximity via only Ku and XRCC4:DNA ligase 4 (X4L4), and in an orientation that would not yet permit ligation until base pairing between one or more nucleotides of microhomology (MH) occurred, thereby allowing an in-line Close Synapsis (CS) state. If no MH was achievable, then XLF was critical for ligation. Neither FS or CS required DNA-PKcs, unless Artemis activation was necessary to permit local resection and subsequent base pairing between the two DNA ends being joined. Here we conjecture on possible 3D configurations for this FS state, which would spatially accommodate the nuclease and polymerase processing steps in an iterative manner. The FS model permits repeated attempts at ligation of at least one strand at the DSB after each round of nuclease or polymerase action. In addition to activation of Artemis, other possible roles for DNA-PKcs are discussed.

APLF and long non-coding RNA NIHCOLE promote stable DNA synapsis in non-homologous end joining

The synapsis of DNA ends is a critical step for the repair of double-strand breaks by non-homologous end joining (NHEJ). This is performed by a multicomponent protein complex assembled around Ku70-Ku80 heterodimers and regulated by accessory factors, including long non-coding RNAs, through poorly understood mechanisms. Here, we use magnetic tweezers to investigate the contributions of core NHEJ proteins and APLF and lncRNA NIHCOLE to DNA synapsis. APLF stabilizes DNA end bridging and, together with Ku70-Ku80, establishes a minimal complex that supports DNA synapsis for several minutes under piconewton forces. We find the C-terminal acidic region of APLF to be critical for bridging. NIHCOLE increases the dwell time of the synapses by Ku70-Ku80 and APLF. This effect is further enhanced by a small and structured RNA domain within NIHCOLE. We propose a model where Ku70-Ku80 can simultaneously bind DNA, APLF, and structured RNAs to promote the stable joining of DNA ends.

To indel or not to indel: Factors influencing mutagenesis during chromosomal break end joining

Chromosomal DNA double-strand breaks (DSBs) are the effective lesion of radiotherapy and other clastogenic cancer therapeutics, and are also the initiating event of many approaches to gene editing. Ligation of the DSBs by end joining (EJ) pathways can restore the broken chromosome, but the repair junctions can have insertion/deletion (indel) mutations. The indel patterns resulting from DSB EJ are likely defined by the initial structure of the DNA ends, how the ends are processed and synapsed prior to ligation, and the factors that mediate the ligation step. In this review, we describe key factors that influence these steps of DSB EJ in mammalian cells, which is significant both for understanding mutagenesis resulting from clastogenic cancer therapeutics, and for developing approaches to manipulating gene editing outcomes.