Artemis C-terminal region facilitates V(D)J recombination through its interactions with DNA Ligase IV and DNA-PKcs

Cross-species analysis of the nuclease Artemis highlights its evolving function in domesticating RAG-like transposons and residues that are crucial for activity

The discovery of the ProtoRAG transposon in lancelets revealed that V(D)J recombination originates from the Recombination activating gene-like (RAGL) transposon. Analogous to the vertebrate RAG complex, the RAGL transposase nicks host flanking DNA and leads to the formation of hairpin ends. Here, we showed that the Artemis nuclease, which is capable of resolving DNA hairpin ends generated during V(D)J recombination, is also responsible for unraveling ProtoRAG-mediated DNA hairpin ends. Notably, like the RAGL transposon, Artemis originated from the eukaryotic common ancestor. By tracing the evolving function of Artemis from cephalochordates to vertebrates, we revealed the lineage specific allele polymorphism of lancelet Artemis and uncovered an increased activity on hairpin DNA opening in vertebrate Artemis. Additionally, the evolutionarily conserved LYCS motif in Artemis β6, which may be associated with disease, is demonstrated to be crucial for its function. Overall, this study highlights the evolving function of Artemis, identifies novel critical residues, and provides new insights into the evolution of RAG-mediated recombination and the clinical therapy of Artemis deficient disease.

Artemis and its role in cancer

Artemis is a key nuclease involved in the non-homologous end joining repair pathway upon DNA double-stranded breaks and during V(D)J recombination. It participates in various cellular processes and cooperates with various proteins involved in tumorigenesis. Its hereditary mutations lead to several pathological conditions, such as severe combined immunodeficiency with radiation sensitivity. Recent studies suggest that Artemis deregulation plays an important role in cancer and is associated with poorer oncologic outcomes and resistance to treatment including radiotherapy, chemotherapy and targeted therapeutics. Artemis emerges as an attractive candidate for cancer prognosis and treatment. Its role in modulating sensitivity to ionizing radiation and DNA-damaging agents makes it an appealing target for drug development. Various existing drugs and novel compounds have been described to inhibit Artemis activity. This review synthesizes the up-to-date information regarding Artemis function, its role in different malignancies and its clinical utility as a potential biomarker and therapeutic target in Oncology.

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.

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.

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 Ku70/80 heterodimer (Ku), XRCC4 in complex with DNA Ligase 4 (X4L4), and 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) have recently been 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 atomic 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 led 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.

The DNA binding domain and the C-terminal region of DNA Ligase IV specify its role in V(D)J recombination

DNA Ligase IV is responsible for the repair of DNA double-strand breaks (DSB), including DSBs that are generated during V(D)J recombination. Like other DNA ligases, Ligase IV contains a catalytic core with three subdomains-the DNA binding (DBD), the nucleotidyltransferase (NTD), and the oligonucleotide/oligosaccharide-fold subdomain (OBD). Ligase IV also has a unique C-terminal region that includes two BRCT domains, a nuclear localization signal sequence and a stretch of amino acid that participate in its interaction with XRCC4. Out of the three mammalian ligases, Ligase IV is the only ligase that participates in and is required for V(D)J recombination. Identification of the minimal domains within DNA Ligase IV that contribute to V(D)J recombination has remained unresolved. The interaction of the Ligase IV DNA binding domain with Artemis, and the interaction of its C-terminal region with XRCC4, suggest that both of these regions that also interact with the Ku70/80 heterodimer are important and might be sufficient for mediating participation of DNA Ligase IV in V(D)J recombination. This hypothesis was investigated by generating chimeric ligase proteins by swapping domains, and testing their ability to rescue V(D)J recombination in Ligase IV-deficient cells. We demonstrate that a fusion protein containing Ligase I NTD and OBDs flanked by DNA Ligase IV DBD and C-terminal region is sufficient to support V(D)J recombination. This chimeric protein, which we named Ligase 37, complemented formation of coding and signal joints. Coding joints generated with Ligase 37 were shorter than those observed with wild type DNA Ligase IV. The shorter length was due to increased nucleotide deletions and decreased nucleotide insertions. Additionally, overexpression of Ligase 37 in a mouse pro-B cell line supported a shift towards shorter coding joints. Our findings demonstrate that the ability of DNA Ligase IV to participate in V(D)J recombination is in large part mediated by its DBD and C-terminal region.

Dynamics of the Artemis and DNA-PKcs Complex in the Repair of Double-Strand Breaks

Pathologic chromosome breaks occur in human dividing cells ∼10 times per day, and physiologic breaks occur in each lymphoid cell many additional times per day. Nonhomologous DNA end joining (NHEJ) is the major pathway for the repair of all of these double-strand breaks (DSBs) during most of the cell cycle. Nearly all broken DNA ends require trimming before they can be suitable for joining by ligation. Artemis is the major nuclease for this purpose. Artemis is tightly regulated by one of the largest protein kinases, which tethers Artemis to its surface. This kinase is called DNA-dependent protein kinase catalytic subunit (or DNA-PKcs) because it is only active when it encounters a broken DNA end. With this activation, DNA-PKcs permits the Artemis catalytic domain to enter a large cavity in the center of DNA-PKcs. Given this remarkably tight supervision of Artemis by DNA-PKcs, it is an appropriate time to ask what we know about the Artemis:DNA-PKcs complex, as we integrate recent structural information with the biochemistry of the complex and how this relates to other NHEJ proteins and to V(D)J recombination in the immune system.

Structural analysis of the basal state of the Artemis:DNA-PKcs complex

Artemis nuclease and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) are key components in nonhomologous DNA end joining (NHEJ), the major repair mechanism for double-strand DNA breaks. Artemis activation by DNA-PKcs resolves hairpin DNA ends formed during V(D)J recombination. Artemis deficiency disrupts development of adaptive immunity and leads to radiosensitive T- B- severe combined immunodeficiency (RS-SCID). An activated state of Artemis in complex with DNA-PK was solved by cryo-EM recently, which showed Artemis bound to the DNA. Here, we report that the pre-activated form (basal state) of the Artemis:DNA-PKcs complex is stable on an agarose-acrylamide gel system, and suitable for cryo-EM structural analysis. Structures show that the Artemis catalytic domain is dynamically positioned externally to DNA-PKcs prior to ABCDE autophosphorylation and show how both the catalytic and regulatory domains of Artemis interact with the N-HEAT and FAT domains of DNA-PKcs. We define a mutually exclusive binding site for Artemis and XRCC4 on DNA-PKcs and show that an XRCC4 peptide disrupts the Artemis:DNA-PKcs complex. All of the findings are useful in explaining how a hypomorphic L3062R missense mutation of DNA-PKcs could lead to insufficient Artemis activation, hence RS-SCID. Our results provide various target site candidates to design disruptors for Artemis:DNA-PKcs complex formation.

Recent Advances in the Development of Non-PIKKs Targeting Small Molecule Inhibitors of DNA Double-Strand Break Repair

The vast majority of cancer patients receive DNA-damaging drugs or ionizing radiation (IR) during their course of treatment, yet the efficacy of these therapies is tempered by DNA repair and DNA damage response (DDR) pathways. Aberrations in DNA repair and the DDR are observed in many cancer subtypes and can promote de novo carcinogenesis, genomic instability, and ensuing resistance to current cancer therapy. Additionally, stalled or collapsed DNA replication forks present a unique challenge to the double-strand DNA break (DSB) repair system. Of the various inducible DNA lesions, DSBs are the most lethal and thus desirable in the setting of cancer treatment. In mammalian cells, DSBs are typically repaired by the error prone non-homologous end joining pathway (NHEJ) or the high-fidelity homology directed repair (HDR) pathway. Targeting DSB repair pathways using small molecular inhibitors offers a promising mechanism to synergize DNA-damaging drugs and IR while selective inhibition of the NHEJ pathway can induce synthetic lethality in HDR-deficient cancer subtypes. Selective inhibitors of the NHEJ pathway and alternative DSB-repair pathways may also see future use in precision genome editing to direct repair of resulting DSBs created by the HDR pathway. In this review, we highlight the recent advances in the development of inhibitors of the non-phosphatidylinositol 3-kinase-related kinases (non-PIKKs) members of the NHEJ, HDR and minor backup SSA and alt-NHEJ DSB-repair pathways. The inhibitors described within this review target the non-PIKKs mediators of DSB repair including Ku70/80, Artemis, DNA Ligase IV, XRCC4, MRN complex, RPA, RAD51, RAD52, ERCC1-XPF, helicases, and DNA polymerase θ. While the DDR PIKKs remain intensely pursued as therapeutic targets, small molecule inhibition of non-PIKKs represents an emerging opportunity in drug discovery that offers considerable potential to impact cancer treatment.

Physical ARTEMIS:DNA-PKcs interaction is necessary for V(D)J recombination

The nuclease ARTEMIS and the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) are involved in the repair of physiological and pathogenic DNA double strand breaks. Both proteins are indispensable for the hairpin-opening activity in V(D)J recombination and therefore essential for the adaptive immune response. ARTEMIS and DNA-PKcs interact, however experimental evidence for in vivo significance is missing. We demonstrate that mutations abolishing this protein-protein interaction affect nuclease function. In DNA-PKcs, mutation L3062R impairs the physical interaction with ARTEMIS and was previously identified as pathogenic variant, resulting in radiosensitive severe combined immunodeficiency. In ARTEMIS, specific mutations in two conserved regions affect interaction with DNA-PKcs. In combination they impair V(D)J recombination activity, independent of ARTEMIS autoinhibitory self-interaction between the ARTEMIS C-terminus and the N-terminal nuclease domain. We describe small fragments from both proteins, capable of interaction with the corresponding full-length partner proteins: In DNA-PKcs 42 amino acids out of FAT region 2 (PKcs3041-3082) can mediate interaction with ARTEMIS. In the nuclease we have defined 26 amino acids (ARM378-403) as minimal DNA-PKcs interacting fragment. The exact mapping of the ARTEMIS:DNA-PKcs interaction may pave the way for the design of specific inhibitors targeting the repair of DNA double strand breaks.

Structure and Function of SNM1 Family Nucleases

Three human nucleases, SNM1A, SNM1B/Apollo, and SNM1C/Artemis, belong to the SNM1 gene family. These nucleases are involved in various cellular functions, including homologous recombination, nonhomologous end-joining, cell cycle regulation, and telomere maintenance. These three proteins share a similar catalytic domain, which is characterized as a fused metallo-β-lactamase and a CPSF-Artemis-SNM1-PSO2 domain. SNM1A and SNM1B/Apollo are exonucleases, whereas SNM1C/Artemis is an endonuclease. This review contains a summary of recent research on SNM1's cellular and biochemical functions, as well as structural biology studies. In addition, protein structure prediction by the artificial intelligence program AlphaFold provides a different view of the proteins' non-catalytic domain features, which may be used in combination with current results from X-ray crystallography and cryo-EM to understand their mechanism more clearly.

Function and Molecular Mechanism of the DNA Damage Response in Immunity and Cancer Immunotherapy

The DNA damage response (DDR) is an organized network of multiple interwoven components evolved to repair damaged DNA and maintain genome fidelity. Conceptually the DDR includes damage sensors, transducer kinases, and effectors to maintain genomic stability and accurate transmission of genetic information. We have recently gained a substantially improved molecular and mechanistic understanding of how DDR components are interconnected to inflammatory and immune responses to stress. DDR shapes both innate and adaptive immune pathways: (i) in the context of innate immunity, DDR components mainly enhance cytosolic DNA sensing and its downstream STimulator of INterferon Genes (STING)-dependent signaling; (ii) in the context of adaptive immunity, the DDR is needed for the assembly and diversification of antigen receptor genes that is requisite for T and B lymphocyte development. Imbalances between DNA damage and repair impair tissue homeostasis and lead to replication and transcription stress, mutation accumulation, and even cell death. These impacts from DDR defects can then drive tumorigenesis, secretion of inflammatory cytokines, and aberrant immune responses. Yet, DDR deficiency or inhibition can also directly enhance innate immune responses. Furthermore, DDR defects plus the higher mutation load in tumor cells synergistically produce primarily tumor-specific neoantigens, which are powerfully targeted in cancer immunotherapy by employing immune checkpoint inhibitors to amplify immune responses. Thus, elucidating DDR-immune response interplay may provide critical connections for harnessing immunomodulatory effects plus targeted inhibition to improve efficacy of radiation and chemotherapies, of immune checkpoint blockade, and of combined therapeutic strategies.

Mechanistic Insights From Single-Molecule Studies of Repair of Double Strand Breaks

DNA double strand breaks (DSBs) are among some of the most deleterious forms of DNA damage. Left unrepaired, they are detrimental to genome stability, leading to high risk of cancer. Two major mechanisms are responsible for the repair of DSBs, homologous recombination (HR) and nonhomologous end joining (NHEJ). The complex nature of both pathways, involving a myriad of protein factors functioning in a highly coordinated manner at distinct stages of repair, lend themselves to detailed mechanistic studies using the latest single-molecule techniques. In avoiding ensemble averaging effects inherent to traditional biochemical or genetic methods, single-molecule studies have painted an increasingly detailed picture for every step of the DSB repair processes.

Structural and mechanistic insights into the Artemis endonuclease and strategies for its inhibition

Artemis (SNM1C/DCLRE1C) is an endonuclease that plays a key role in development of B- and T-lymphocytes and in dsDNA break repair by non-homologous end-joining (NHEJ). Artemis is phosphorylated by DNA-PKcs and acts to open DNA hairpin intermediates generated during V(D)J and class-switch recombination. Artemis deficiency leads to congenital radiosensitive severe acquired immune deficiency (RS-SCID). Artemis belongs to a superfamily of nucleases containing metallo-β-lactamase (MBL) and β-CASP (CPSF-Artemis-SNM1-Pso2) domains. We present crystal structures of the catalytic domain of wildtype and variant forms of Artemis, including one causing RS-SCID Omenn syndrome. The catalytic domain of the Artemis has similar endonuclease activity to the phosphorylated full-length protein. Our structures help explain the predominantly endonucleolytic activity of Artemis, which contrasts with the predominantly exonuclease activity of the closely related SNM1A and SNM1B MBL fold nucleases. The structures reveal a second metal binding site in its β-CASP domain unique to Artemis, which is amenable to inhibition by compounds including ebselen. By combining our structural data with that from a recently reported Artemis structure, we were able model the interaction of Artemis with DNA substrates. The structures, including one of Artemis with the cephalosporin ceftriaxone, will help enable the rational development of selective SNM1 nuclease inhibitors.

Stages, scaffolds and strings in the spatial organisation of non-homologous end joining: Insights from X-ray diffraction and Cryo-EM

Non-homologous end joining (NHEJ) is the preferred pathway for the repair of DNA double-strand breaks in humans. Here we describe three structural aspects of the repair pathway: stages, scaffolds and strings. We discuss the orchestration of DNA repair to guarantee robust and efficient NHEJ. We focus on structural studies over the past two decades, not only using X-ray diffraction, but also increasingly exploiting cryo-EM to investigate the macromolecular assemblies.

A Novel Non-Coding Variant in DCLRE1C Results in Deregulated Splicing and Induces SCID Through the Generation of a Truncated ARTEMIS Protein That Fails to Support V(D)J Recombination and DNA Damage Repair

Severe Combined Immune Deficiency (SCID) is a primary deficiency of the immune system in which opportunistic and recurring infections are often fatal during neonatal or infant life. SCID is caused by an increasing number of genetic defects that induce an abrogation of T lymphocyte development or function in which B and NK cells might be affected as well. Because of the increased availability and usage of next-generation sequencing (NGS), many novel variants in SCID genes are being identified and cause a heterogeneous disease spectrum. However, the molecular and functional implications of these new variants, of which some are non-coding, are often not characterized in detail. Using targeted NGS, we identified a novel homozygous c.465-1G>C splice acceptor site variant in the DCLRE1C gene in a T-B-NK+ SCID patient and fully characterized the molecular and functional impact. By performing a minigene splicing reporter assay, we revealed deregulated splicing of the DCLRE1C transcript since a cryptic splice acceptor in exon 7 was employed. This induced a frameshift and the generation of a p.Arg155Serfs*15 premature termination codon (PTC) within all DCLRE1C splice variants, resulting in the absence of full-length ARTEMIS protein. Consistently, a V(D)J recombination assay and a G0 micronucleus assay demonstrated the inability of the predicted mutant ARTEMIS protein to perform V(D)J recombination and DNA damage repair, respectively. Together, these experiments molecularly and functionally clarify how a newly identified c.465-1G>C variant in the DCLRE1C gene is responsible for inducing SCID. In a clinical context, this demonstrates how the experimental validation of new gene variants, that are identified by NGS, can facilitate the diagnosis of SCID which can be vital for implementing appropriate therapies.

Developing a Peptide That Inhibits DNA Repair by Blocking the Binding of Artemis and DNA Ligase IV to Enhance Tumor Radiosensitivity

PURPOSE

Artemis and DNA Ligase IV are 2 critical elements in the nonhomologous end joining pathway of DNA repair, acting as the nuclease and DNA ligase, respectively. Enhanced cellular radiosensitivity by inhibition of either protein contributes to a promising approach to develop molecular targeted radiosensitizers. The interaction between Artemis and DNA Ligase IV is required for the activation of Artemis as nuclease at 3'overhang DNA; thus, we aim to generate an inhibitory peptide targeting the interaction between Artemis and DNA Ligase IV for novel radiosensitizer development.

METHODS AND MATERIALS

We synthesized the peptide BAL, which consists of the interaction residues of Artemis to DNA Ligase IV. The radiosensitization effect of BAL was evaluated by colony formation assay. The effects of BAL on radiation-induced DNA repair were evaluated with Western blotting and immunofluorescence. The effects of BAL on cell proliferation, cell cycle arrest, and cell apoptosis were assessed via CCK-8 and flow cytometry assays. The potential synergistic effects of BAL and irradiation in vivo were investigated in a xenograft mouse model.

RESULTS

The generated peptide BAL blocking the interaction between Artemis and DNA Ligase IV significantly enhanced the radiosensitivity of GBC-SD and HeLa cell lines. BAL prolonged DNA repair after irradiation; BAL and irradiation showed synergistic effects on cell proliferation, cell cycle, and cell apoptosis, and these functions are all DNA Ligase IV-related. Finally, we confirmed the endogenous radiosensitization effect of BAL in a xenograft mouse model.

CONCLUSIONS

The inhibitory peptide BAL targeting the binding of Artemis and DNA Ligase IV successfully functions as a novel radiosensitizer that delays DNA repair and synergizes with irradiation to inhibit cell proliferation, induce cell cycle arrest, and promote cell apoptosis.

"An End to a Means": How DNA-End Structure Shapes the Double-Strand Break Repair Process

Endogenously-arising DNA double-strand breaks (DSBs) rarely harbor canonical 5′-phosphate, 3′-hydroxyl moieties at the ends, which are, regardless of the pathway used, ultimately required for their repair. Cells are therefore endowed with a wide variety of enzymes that can deal with these chemical and structural variations and guarantee the formation of ligatable termini. An important distinction is whether the ends are directly “unblocked” by specific enzymatic activities without affecting the integrity of the DNA molecule and its sequence, or whether they are “processed” by unspecific nucleases that remove nucleotides from the termini. DNA end structure and configuration, therefore, shape the repair process, its requirements, and, importantly, its final outcome. Thus, the molecular mechanisms that coordinate and integrate the cellular response to blocked DSBs, although still largely unexplored, can be particularly relevant for maintaining genome integrity and avoiding malignant transformation and cancer.

Structural evidence for an in trans base selection mechanism involving Loop1 in polymerase μ at an NHEJ double-strand break junction

Eukaryotic DNA polymerase (Pol) X family members such as Pol μ and terminal deoxynucleotidyl transferase (TdT) are important components for the nonhomologous DNA end-joining (NHEJ) pathway. TdT participates in a specialized version of NHEJ, V(D)J recombination. It has primarily nontemplated polymerase activity but can take instructions across strands from the downstream dsDNA, and both activities are highly dependent on a structural element called Loop1. However, it is unclear whether Pol μ follows the same mechanism, because the structure of its Loop1 is disordered in available structures. Here, we used a chimeric TdT harboring Loop1 of Pol μ that recapitulated the functional properties of Pol μ in ligation experiments. We solved three crystal structures of this TdT chimera bound to several DNA substrates at 1.96-2.55 Å resolutions, including a full DNA double-strand break (DSB) synapsis. We then modeled the full Pol μ sequence in the context of one these complexes. The atomic structure of an NHEJ junction with a Pol X construct that mimics Pol μ in a reconstituted system explained the distinctive properties of Pol μ compared with TdT. The structure suggested a mechanism of base selection relying on Loop1 and taking instructions via the in trans templating base independently of the primer strand. We conclude that our atomic-level structural observations represent a paradigm shift for the mechanism of base selection in the Pol X family of DNA polymerases.

The Emerging Role of Cohesin in the DNA Damage Response

Faithful transmission of genetic material is crucial for all organisms since changes in genetic information may result in genomic instability that causes developmental disorders and cancers. Thus, understanding the mechanisms that preserve genome integrity is of fundamental importance. Cohesin is a multiprotein complex whose canonical function is to hold sister chromatids together from S-phase until the onset of anaphase to ensure the equal division of chromosomes. However, recent research points to a crucial function of cohesin in the DNA damage response (DDR). In this review, we summarize recent advances in the understanding of cohesin function in DNA damage signaling and repair. First, we focus on cohesin architecture and molecular mechanisms that govern sister chromatid cohesion. Next, we briefly characterize the main DDR pathways. Finally, we describe mechanisms that determine cohesin accumulation at DNA damage sites and discuss possible roles of cohesin in DDR.

Impact of DNA repair and stability defects on cortical development

Maintenance of genome stability is a crucial cellular function for normal mammalian development and physiology. However, despite the general relevance of this process, genome stability alteration due to genetic or non-genetic conditions has a particularly profound impact on the developing cerebral cortex. In this review, we will analyze the main pathways involved in maintenance of genome stability, the consequences of their alterations with regard to central nervous system development, as well as the possible molecular and cellular basis of this specificity.

Nonhomologous DNA end-joining for repair of DNA double-strand breaks

Nonhomologous DNA end-joining (NHEJ) is the predominant double-strand break (DSB) repair pathway throughout the cell cycle and accounts for nearly all DSB repair outside of the S and G₂ phases. NHEJ relies on Ku to thread onto DNA termini and thereby improve the affinity of the NHEJ enzymatic components consisting of polymerases (Pol μ and Pol λ), a nuclease (the Artemis·DNA-PKcs complex), and a ligase (XLF·XRCC4·Lig4 complex). Each of the enzymatic components is distinctive for its versatility in acting on diverse incompatible DNA end configurations coupled with a flexibility in loading order, resulting in many possible junctional outcomes from one DSB. DNA ends can either be directly ligated or, if the ends are incompatible, processed until a ligatable configuration is achieved that is often stabilized by up to 4 bp of terminal microhomology. Processing of DNA ends results in nucleotide loss or addition, explaining why DSBs repaired by NHEJ are rarely restored to their original DNA sequence. Thus, NHEJ is a single pathway with multiple enzymes at its disposal to repair DSBs, resulting in a diversity of repair outcomes.

Structure and Dynamics of an Intrinsically Disordered Protein Region That Partially Folds upon Binding by Chemical-Exchange NMR

Many intrinsically disordered proteins (IDPs) and protein regions (IDRs) engage in transient, yet specific, interactions with a variety of protein partners. Often, if not always, interactions with a protein partner lead to partial folding of the IDR. Characterizing the conformational space of such complexes is challenging: in solution-state NMR, signals of the IDR in the interacting region become broad, weak, and often invisible, while X-ray crystallography only provides information on fully ordered regions. There is thus a need for a simple method to characterize both fully and partially ordered regions in the bound state of IDPs. Here, we introduce an approach based on monitoring chemical exchange by NMR to investigate the state of an IDR that folds upon binding through the observation of the free state of the protein. Structural constraints for the bound state are obtained from chemical shifts, and site-specific dynamics of the bound state are characterized by relaxation rates. The conformation of the interacting part of the IDR was determined and subsequently docked onto the structure of the folded partner. We apply the method to investigate the interaction between the disordered C-terminal region of Artemis and the DNA binding domain of Ligase IV. We show that we can accurately reproduce the structure of the core of the complex determined by X-ray crystallography and identify a broader interface. The method is widely applicable to the biophysical investigation of complexes of disordered proteins and folded proteins.

Effects of DNA end configuration on XRCC4-DNA ligase IV and its stimulation of Artemis activity

In humans, nonhomologous DNA end-joining (NHEJ) is the major pathway by which DNA double-strand breaks are repaired. Recognition of each broken DNA end by the DNA repair protein Ku is the first step in NHEJ, followed by the iterative binding of nucleases, DNA polymerases, and the XRCC4-DNA ligase IV (X4-LIV) complex in an order influenced by the configuration of the two DNA ends at the break site. The endonuclease Artemis improves joining efficiency by functioning in a complex with DNA-dependent protein kinase, catalytic subunit (DNA-PKcs) that carries out endonucleolytic cleavage of 5' and 3' overhangs. Previously, we observed that X4-LIV alone can stimulate Artemis activity on 3' overhangs, but this DNA-PKcs-independent endonuclease activity of Artemis awaited confirmation. Here, using in vitro nuclease and ligation assays, we find that stimulation of Artemis nuclease activity by X4-LIV and the efficiency of blunt-end ligation are determined by structural configurations at the DNA end. Specifically, X4-LIV stimulated Artemis to cut near the end of 3' overhangs without the involvement of other NHEJ proteins. Of note, this ligase complex is not able to stimulate Artemis activity at hairpins or at 5' overhangs. We also found that X4-LIV and DNA-PKcs interfere with one another with respect to stimulating Artemis activity at 3' overhangs, favoring the view that these NHEJ proteins are sequentially rather than concurrently recruited to DNA ends. These data suggest specific functional and positional relationships among these components that explain genetic and molecular features of NHEJ and V(D)J recombination within cells.

ATM, ATR, and DNA-PK: The Trinity at the Heart of the DNA Damage Response

In vertebrate cells, the DNA damage response is controlled by three related kinases: ATM, ATR, and DNA-PK. It has been 20 years since the cloning of ATR, the last of the three to be identified. During this time, our understanding of how these kinases regulate DNA repair and associated events has grown profoundly, although major questions remain unanswered. Here, we provide a historical perspective of their discovery and discuss their established functions in sensing and responding to genotoxic stress. We also highlight what is known regarding their structural similarities and common mechanisms of regulation, as well as emerging non-canonical roles and how our knowledge of ATM, ATR, and DNA-PK is being translated to benefit human health.

Non-homologous DNA end joining and alternative pathways to double-strand break repair

DNA double-strand breaks (DSBs) are the most dangerous type of DNA damage because they can result in the loss of large chromosomal regions. In all mammalian cells, DSBs that occur throughout the cell cycle are repaired predominantly by the non-homologous DNA end joining (NHEJ) pathway. Defects in NHEJ result in sensitivity to ionizing radiation and the ablation of lymphocytes. The NHEJ pathway utilizes proteins that recognize, resect, polymerize and ligate the DNA ends in a flexible manner. This flexibility permits NHEJ to function on a wide range of DNA-end configurations, with the resulting repaired DNA junctions often containing mutations. In this Review, we discuss the most recent findings regarding the relative involvement of the different NHEJ proteins in the repair of various DNA-end configurations. We also discuss the shunting of DNA-end repair to the auxiliary pathways of alternative end joining (a-EJ) or single-strand annealing (SSA) and the relevance of these different pathways to human disease.

Autoinhibition of the Nuclease ARTEMIS Is Mediated by a Physical Interaction between Its Catalytic and C-terminal Domains

The nuclease ARTEMIS is essential for the development of B and T lymphocytes. It is required for opening DNA hairpins generated during antigen receptor gene assembly from variable (V), diversity (D), and joining (J) subgenic elements (V(D)J recombination). As a member of the non-homologous end-joining pathway, it is also involved in repairing a subset of pathological DNA double strand breaks. Loss of ARTEMIS function therefore results in radiosensitive severe combined immunodeficiency (RS-SCID). The hairpin opening activity is dependent on the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), which can bind to and phosphorylate ARTEMIS. The ARTEMIS C terminus is dispensable for cellular V(D)J recombination and in vitro nuclease assays with C-terminally truncated ARTEMIS showing DNA-PKcs-independent hairpin opening activity. Therefore, it has been postulated that ARTEMIS is regulated via autoinhibition by its C terminus. To obtain evidence for the autoinhibition model, we performed co-immunoprecipitation experiments with combinations of ARTEMIS mutants. We show that an N-terminal fragment comprising the catalytic domain can interact both with itself and with a C-terminal fragment. Amino acid exchanges N456A+S457A+E458Q in the C terminus of full-length ARTEMIS resulted in unmasking of the N terminus and in increased ARTEMIS activity in cellular V(D)J recombination assays. Mutations in ARTEMIS-deficient patients impaired the interaction with the C terminus and also affected protein stability. The interaction between the N- and C-terminal domains was not DNA-PKcs-dependent, and phosphomimetic mutations in the C-terminal domain did not result in unmasking of the catalytic domain. Our experiments provide strong evidence that a physical interaction between the C-terminal and catalytic domains mediates ARTEMIS autoinhibition.

Different DNA End Configurations Dictate Which NHEJ Components Are Most Important for Joining Efficiency

The nonhomologous DNA end-joining (NHEJ) pathway is a key mechanism for repairing dsDNA breaks that occur often in eukaryotic cells. In the simplest model, these breaks are first recognized by Ku, which then interacts with other NHEJ proteins to improve their affinity at DNA ends. These include DNA-PKcs and Artemis for trimming the DNA ends; DNA polymerase μ and λ to add nucleotides; and the DNA ligase IV complex to ligate the ends with the additional factors, XRCC4 (X-ray repair cross-complementing protein 4), XLF (XRCC4-like factor/Cernunos), and PAXX (paralog of XRCC4 and XLF). In vivo studies have demonstrated the degrees of importance of these NHEJ proteins in the mechanism of repair of dsDNA breaks, but interpretations can be confounded by other cellular processes. In vitro studies with NHEJ proteins have been performed to evaluate the nucleolytic resection, polymerization, and ligation steps, but a complete system has been elusive. Here we have developed a NHEJ reconstitution system that includes the nuclease, polymerase, and ligase components to evaluate relative NHEJ efficiency and analyze ligated junctional sequences for various types of DNA ends, including blunt, 5' overhangs, and 3' overhangs. We find that different dsDNA end structures have differential dependence on these enzymatic components. The dependence of some end joining on only Ku and XRCC4·DNA ligase IV allows us to formulate a physical model that incorporates nuclease and polymerase components as needed.

Structural Basis for a New Templated Activity by Terminal Deoxynucleotidyl Transferase: Implications for V(D)J Recombination

Eukaryotic DNA polymerase of the polX family, such as pol μ and terminal deoxynucleotidyl transferase (TdT), are key components of the non-homologous end-joining or V(D)J recombination machinery, respectively. The established role of TdT is to add random nucleotides during V(D)J recombination. Here we show that TdT also has a templated-polymerase activity, similar to pol μ, in the presence of higher concentrations of a downstream DNA duplex, and performs a micro-homology single base-pair search to align the DNA synapsis. To understand the molecular basis of this alignment, we solve crystal structures of TdT with four DNA strands and study the influence of the 3' protruding end. Two mutations in TdT inspired by sequence alignments with pol μ further improve the templated activity. We propose that both templated and untemplated activities of TdT are needed to explain the distributions of lengths of N regions observed experimentally in T cell receptors and antibodies.

Non-homologous end joining: advances and frontiers

DNA double-strand breaks (DSBs) are the most serious form of DNA damage. In human cells, non-homologous end joining (NHEJ) is the major pathway for the repair of DSBs. Different types of DSBs result in different subsets of NHEJ repair strategies. These variations in NHEJ repair strategies depend on numerous elements, such as the flexible recruitment of NHEJ-related proteins, the complexity of the DSB ends, and the spatial- and temporal-ordered formation of the multi-protein complex. On the one hand, current studies of DNA DSBs repair focus on the repair pathway choices between homologous recombination and classic or alternative NHEJ. On the other hand, increasing researches have also deepened the significance and dug into the cross-links between the NHEJ pathway and the area of genome organization and aging. Although remarkable progress has been made in elucidating the underlying principles during the past decades, the detailed mechanism of action in response to different types of DSBs remains largely unknown and needs further evaluation in the future study.

A nonsense mutation of human XRCC4 is associated with adult-onset progressive encephalocardiomyopathy

We studied two monozygotic twins, born to first cousins, affected by a multisystem disease. At birth, they both presented with bilateral cryptorchidism and malformations. Since early adulthood, they developed a slowly progressive neurological syndrome, with cerebellar and pyramidal signs, cognitive impairment, and depression. Dilating cardiomyopathy is also present in both. By whole-exome sequencing, we found a homozygous nucleotide change in XRCC4 (c.673C>T), predicted to introduce a premature stop codon (p.R225*). XRCC4 transcript levels were profoundly reduced, and the protein was undetectable in patients' skin fibroblasts. XRCC4 plays an important role in non-homologous end joining of DNA double-strand breaks (DSB), a system that is involved in repairing DNA damage from, for example, ionizing radiations. Gamma-irradiated mutant cells demonstrated reduction, but not abolition, of DSB repair. In contrast with embryonic lethality of the Xrcc4 KO mouse, nonsense mutations in human XRCC4 have recently been associated with primordial dwarfism and, in our cases, with adult-onset neurological impairment, suggesting an important role for DNA repair in the brain. Surprisingly, neither immunodeficiency nor predisposition to malignancy was reported in these patients.

Ionizing radiation-induced DNA injury and damage detection in patients with breast cancer

Breast cancer is the most common malignancy in women. Radiotherapy is frequently used in patients with breast cancer, but some patients may be more susceptible to ionizing radiation, and increased exposure to radiation sources may be associated to radiation adverse events. This susceptibility may be related to deficiencies in DNA repair mechanisms that are activated after cell-radiation, which causes DNA damage, particularly DNA double strand breaks. Some of these genetic susceptibilities in DNA-repair mechanisms are implicated in the etiology of hereditary breast/ovarian cancer (pathologic mutations in the BRCA 1 and 2 genes), but other less penetrant variants in genes involved in sporadic breast cancer have been described. These same genetic susceptibilities may be involved in negative radiotherapeutic outcomes. For these reasons, it is necessary to implement methods for detecting patients who are susceptible to radiotherapy-related adverse events. This review discusses mechanisms of DNA damage and repair, genes related to these functions, and the diagnosis methods designed and under research for detection of breast cancer patients with increased radiosensitivity.

Functional analysis of naturally occurring DCLRE1C mutations and correlation with the clinical phenotype of ARTEMIS deficiency

BACKGROUND

The endonuclease ARTEMIS, which is encoded by the DCLRE1C gene, is a component of the nonhomologous end-joining pathway and participates in hairpin opening during the V(D)J recombination process and repair of a subset of DNA double-strand breaks. Patients with ARTEMIS deficiency usually present with severe combined immunodeficiency (SCID) and cellular radiosensitivity, but hypomorphic mutations can cause milder phenotypes (leaky SCID).

OBJECTIVE

We sought to correlate the functional effect of human DCLRE1C mutations on phenotypic presentation in patients with ARTEMIS deficiency.

METHODS

We studied the recombination and DNA repair activity of 41 human DCLRE1C mutations in Dclre1c(-/-) v-abl kinase-transformed pro-B cells retrovirally engineered with a construct that allows quantification of recombination activity by means of flow cytometry. For assessment of DNA repair efficacy, resolution of γH2AX accumulation was studied after ionizing radiation.

RESULTS

Low or absent activity was detected for mutations causing a typical SCID phenotype. Most of the patients with leaky SCID were compound heterozygous for 1 loss-of-function and 1 hypomorphic allele, with significant residual levels of recombination and DNA repair activity. Deletions disrupting the C-terminus result in truncated but partially functional proteins and are often associated with leaky SCID. Overexpression of hypomorphic mutants might improve the functional defect.

CONCLUSIONS

Correlation between the nature and location of DCLRE1C mutations, functional activity, and the clinical phenotype has been observed. Hypomorphic variants that have been reported in the general population can be disease causing if combined in trans with a loss-of-function allele. Therapeutic strategies aimed at inducing overexpression of hypomorphic alleles might be beneficial.

Structural basis for a novel mechanism of DNA bridging and alignment in eukaryotic DSB DNA repair

Eukaryotic DNA polymerase mu of the PolX family can promote the association of the two 3'-protruding ends of a DNA double-strand break (DSB) being repaired (DNA synapsis) even in the absence of the core non-homologous end-joining (NHEJ) machinery. Here, we show that terminal deoxynucleotidyltransferase (TdT), a closely related PolX involved in V(D)J recombination, has the same property. We solved its crystal structure with an annealed DNA synapsis containing one micro-homology (MH) base pair and one nascent base pair. This structure reveals how the N-terminal domain and Loop 1 of Tdt cooperate for bridging the two DNA ends, providing a templating base in trans and limiting the MH search region to only two base pairs. A network of ordered water molecules is proposed to assist the incorporation of any nucleotide independently of the in trans templating base. These data are consistent with a recent model that explains the statistics of sequences synthesized in vivo by Tdt based solely on this dinucleotide step. Site-directed mutagenesis and functional tests suggest that this structural model is also valid for Pol mu during NHEJ.

The DNA-dependent protein kinase: A multifunctional protein kinase with roles in DNA double strand break repair and mitosis

The DNA-dependent protein kinase (DNA-PK) is a serine/threonine protein kinase composed of a large catalytic subunit (DNA-PKcs) and the Ku70/80 heterodimer. Over the past two decades, significant progress has been made in elucidating the role of DNA-PK in non-homologous end joining (NHEJ), the major pathway for repair of ionizing radiation-induced DNA double strand breaks in human cells and recently, additional roles for DNA-PK have been reported. In this review, we will describe the biochemistry, structure and function of DNA-PK, its roles in DNA double strand break repair and its newly described roles in mitosis and other cellular processes.

Interaction between HIV-1 Tat and DNA-PKcs modulates HIV transcription and class switch recombination

HIV-1 tat targets a variety of host cell proteins to facilitate viral transcription and disrupts host cellular immunity by inducing lymphocyte apoptosis, but whether it influences humoral immunity remains unclear. Previously, our group demonstrated that tat depresses expression of DNA-PKcs, a critical component of the non-homologous end joining pathway (NHEJ) of DNA double-strand breaks repair, immunoglobulin class switch recombination (CSR) and V(D)J recombination, and sensitizes cells to ionizing radiation. In this study, we demonstrated that HIV-1 Tat down-regulates DNA-PKcs expression by directly binding to the core promoter sequence. In addition, Tat interacts with and activates the kinase activity of DNA-PKcs in a dose-dependent and DNA independent manner. Furthermore, Tat inhibits class switch recombination (CSR) at low concentrations (≤4 µg/ml) and stimulates CSR at high concentrations (≥8 µg/ml). On the other hand, low protein level and high kinase activity of DNA-PKcs promotes HIV-1 transcription, while high protein level and low kinase activity inhibit HIV-1 transcription. Co-immunoprecipitation results revealed that DNA-PKcs forms a large complex comprised of Cyclin T1, CDK9 and Tat via direct interacting with CDK9 and Tat but not Cyclin T1. Taken together, our results provide new clues that Tat regulates host humoral immunity via both transcriptional depression and kinase activation of DNA-PKcs. We also raise the possibility that inhibitors and interventions directed towards DNA-PKcs may inhibit HIV-1 transcription in AIDS patients.

The role of RecQ helicases in non-homologous end-joining

DNA double-strand breaks are highly toxic DNA lesions that cause genomic instability, if not efficiently repaired. RecQ helicases are a family of highly conserved proteins that maintain genomic stability through their important roles in several DNA repair pathways, including DNA double-strand break repair. Double-strand breaks can be repaired by homologous recombination (HR) using sister chromatids as templates to facilitate precise DNA repair, or by an HR-independent mechanism known as non-homologous end-joining (NHEJ) (error-prone). NHEJ is a non-templated DNA repair process, in which DNA termini are directly ligated. Canonical NHEJ requires DNA-PKcs and Ku70/80, while alternative NHEJ pathways are DNA-PKcs and Ku70/80 independent. This review discusses the role of RecQ helicases in NHEJ, alternative (or back-up) NHEJ (B-NHEJ) and microhomology-mediated end-joining (MMEJ) in V(D)J recombination, class switch recombination and telomere maintenance.

The fidelity of the ligation step determines how ends are resolved during nonhomologous end joining

Nonhomologous end joining (NHEJ) can effectively resolve chromosome breaks despite diverse end structures, but it is unclear how the steps employed for resolution are determined. We sought to address this question by analyzing cellular NHEJ of ends with systematically mispaired and damaged termini. We show NHEJ is uniquely proficient at bypassing subtle terminal mispairs and radiomimetic damage by direct ligation. Nevertheless, bypass ability varies widely, with increases in mispair severity gradually reducing bypass products from 85% to 6%. End-processing by nucleases and polymerases is increased to compensate, though paths with the fewest number of steps to generate a substrate suitable for ligation are favored. Thus, both the frequency and nature of end processing are tailored to meet the needs of the ligation step. We propose a model where the ligase organizes all steps during NHEJ within the stable paired-end complex to limit end processing and associated errors.

DNA Ligase IV regulates XRCC4 nuclear localization

DNA Ligase IV, along with its interacting partner XRCC4, are essential for repairing DNA double strand breaks by non-homologous end joining (NHEJ). Together, they complete the final ligation step resolving the DNA break. Ligase IV is regulated by XRCC4 and XLF. However, the mechanism(s) by which Ligase IV control the NHEJ reaction and other NHEJ factor(s) remains poorly characterized. Here, we show that a C-terminal region of Ligase IV (aa 620-800), which encompasses a NLS, the BRCT I, and the XRCC4 interacting region (XIR), is essential for nuclear localization of its co-factor XRCC4. In Ligase IV deficient cells, XRCC4 showed deregulated localization remaining in the cytosol even after induction of DNA double strand breaks. DNA Ligase IV was also required for efficient localization of XLF into the nucleus. Additionally, human fibroblasts that harbor hypomorphic mutations within the Ligase IV gene displayed decreased levels of XRCC4 protein, implicating that DNA Ligase IV is also regulating XRCC4 stability. Our results provide evidence for a role of DNA Ligase IV in controlling the cellular localization and protein levels of XRCC4.

Non-homologous end joining: emerging themes and unanswered questions

Non-homologous end joining (NHEJ) is the major pathway for the repair of ionizing radiation induced DNA double strand breaks in human cells. Here, we discuss current insights into the mechanism of NHEJ and the interplay between NHEJ and other pathways for repair of IR-induced DNA damage.

One ring to bring them all--the role of Ku in mammalian non-homologous end joining

The repair of DNA double strand breaks is essential for cell survival and several conserved pathways have evolved to ensure their rapid and efficient repair. The non-homologous end joining pathway is initiated when Ku binds to the DNA break site. Ku is an abundant nuclear heterodimer of Ku70 and Ku80 with a toroidal structure that allows the protein to slide over the broken DNA end and bind with high affinity. Once locked into placed, Ku acts as a tool-belt to recruit multiple interacting proteins, forming one or more non-homologous end joining complexes that act in a regulated manner to ensure efficient repair of DNA ends. Here we review the structure and functions of Ku and the proteins with which it interacts during non-homologous end joining.

The spatial organization of non-homologous end joining: from bridging to end joining

Non-homologous end joining (NHEJ) repairs DNA double-strand breaks generated by DNA damage and also those occurring in V(D)J recombination in immunoglobulin and T cell receptor production in the immune system. In NHEJ DNA-PKcs assembles with Ku heterodimer on the DNA ends at double-strand breaks, in order to bring the broken ends together and to assemble other proteins, including DNA ligase IV (LigIV), required for DNA repair. Here we focus on structural aspects of the interactions of LigIV with XRCC4, XLF, Artemis and DNA involved in the bridging and end-joining steps of NHEJ. We begin with a discussion of the role of XLF, which interacts with Ku and forms a hetero-filament with XRCC4; this likely forms a scaffold bridging the DNA ends. We then review the well-defined interaction of XRCC4 with LigIV, and discuss the possibility of this complex interrupting the filament formation, so positioning the ligase at the correct positions close to the broken ends. We also describe the interactions of LigIV with Artemis, the nuclease that prepares the ends for ligation and also interacts with DNA-PK. Lastly we review the likely affects of Mendelian mutations on these multiprotein assemblies and their impacts on the form of inherited disease.

Structural insights into NHEJ: building up an integrated picture of the dynamic DSB repair super complex, one component and interaction at a time

Non-homologous end joining (NHEJ) is the major pathway for repair of DNA double-strand breaks (DSBs) in human cells. NHEJ is also needed for V(D)J recombination and the development of T and B cells in vertebrate immune systems, and acts in both the generation and prevention of non-homologous chromosomal translocations, a hallmark of genomic instability and many human cancers. X-ray crystal structures, cryo-electron microscopy envelopes, and small angle X-ray scattering (SAXS) solution conformations and assemblies are defining most of the core protein components for NHEJ: Ku70/Ku80 heterodimer; the DNA dependent protein kinase catalytic subunit (DNA-PKcs); the structure-specific endonuclease Artemis along with polynucleotide kinase/phosphatase (PNKP), aprataxin and PNKP related protein (APLF); the scaffolding proteins XRCC4 and XLF (XRCC4-like factor); DNA polymerases, and DNA ligase IV (Lig IV). The dynamic assembly of multi-protein NHEJ complexes at DSBs is regulated in part by protein phosphorylation. The basic steps of NHEJ have been biochemically defined to require: (1) DSB detection by the Ku heterodimer with subsequent DNA-PKcs tethering to form the DNA-PKcs-Ku-DNA complex (termed DNA-PK), (2) lesion processing, and (3) DNA end ligation by Lig IV, which functions in complex with XRCC4 and XLF. The current integration of structures by combined methods is resolving puzzles regarding the mechanisms, coordination and regulation of these three basic steps. Overall, structural results suggest the NHEJ system forms a flexing scaffold with the DNA-PKcs HEAT repeats acting as compressible macromolecular springs suitable to store and release conformational energy to apply forces to regulate NHEJ complexes and the DNA substrate for DNA end protection, processing, and ligation.

Nonhomologous end joining: a good solution for bad ends

Double strand breaks pose unique problems for DNA repair, especially when broken ends possess complex structures that interfere with standard DNA transactions. Nonhomologous end joining can use multiple strategies to solve these problems. It further uses sophisticated means to ensure the strategy chosen provides the ideal balance of flexibility and accuracy.

Autoimmune hemolytic anemia in a teenager: a wolf in sheep's clothing

This case report describes a 14-year-old boy who presented to the emergency department with symptoms of severe anemia. He was diagnosed with autoimmune hemolytic anemia, and on further investigation, it was noted that he had no functioning T cells. Despite no antecedent history of severe infection, he was worked up for a severe combined immunodeficiency, and was found to have a compound hypomorphic mutation in an enzyme responsible for recombination of the B- and T-cell receptors. He was subsequently diagnosed with severe combined immunodeficiency, presenting with autoimmunity, and received a bone marrow transplant. As our knowledge of the immune system continues to expand, we are learning that dysregulation can occur in any one of the complex immune pathways, and may have a variety of clinical presentations. A high index of suspicion for immune defects should be maintained in cases of atypical or severe infections, autoimmunity or malignancy, particularly by the general practitioner, who is often the first to encounter these challenging patients.

Repair of double-strand breaks by end joining

Nonhomologous end joining (NHEJ) refers to a set of genome maintenance pathways in which two DNA double-strand break (DSB) ends are (re)joined by apposition, processing, and ligation without the use of extended homology to guide repair. Canonical NHEJ (c-NHEJ) is a well-defined pathway with clear roles in protecting the integrity of chromosomes when DSBs arise. Recent advances have revealed much about the identity, structure, and function of c-NHEJ proteins, but many questions exist regarding their concerted action in the context of chromatin. Alternative NHEJ (alt-NHEJ) refers to more recently described mechanism(s) that repair DSBs in less-efficient backup reactions. There is great interest in defining alt-NHEJ more precisely, including its regulation relative to c-NHEJ, in light of evidence that alt-NHEJ can execute chromosome rearrangements. Progress toward these goals is reviewed.

Structure of the catalytic region of DNA ligase IV in complex with an Artemis fragment sheds light on double-strand break repair

Nonhomologous end joining (NHEJ) is central to the repair of double-stranded DNA breaks throughout the cell cycle and plays roles in the development of the immune system. Although three-dimensional structures of most components of NHEJ have been defined, those of the catalytic region of DNA ligase IV (LigIV), a specialized DNA ligase known to work in NHEJ, and of Artemis have remained unresolved. Here, we report the crystal structure at 2.4 Å resolution of the catalytic region of LigIV (residues 1-609) in complex with an Artemis peptide. We describe interactions of the DNA-binding domain of LigIV with the continuous epitope of Artemis, which, together, form a three-helix bundle. A kink in the first helix of LigIV introduced by a conserved VPF motif gives rise to a hydrophobic pocket, which accommodates a conserved tryptophan from Artemis. We provide structural insights into features of LigIV among human DNA ligases.

Role of non-homologous end joining in V(D)J recombination

The pathway of V(D)J recombination was discovered almost three decades ago. Yet it continues to baffle scientists because of its inherent complexity and the multiple layers of regulation that are required to efficiently generate a diverse repertoire of T and B cells. The non-homologous end-joining (NHEJ) DNA repair pathway is an integral part of the V(D)J reaction, and its numerous players perform critical functions in generating this vast diversity, while ensuring genomic stability. In this review, we summarize the efforts of a number of laboratories including ours in providing the mechanisms of V(D)J regulation with a focus on the NHEJ pathway. This involves discovering new players, unraveling unknown roles for known components, and understanding how deregulation of these pathways contributes to generation of primary immunodeficiencies. A long-standing interest of our laboratory has been to elucidate various mechanisms that control RAG activity. Our recent work has focused on understanding the multiple protein-protein interactions and protein-DNA interactions during V(D)J recombination, which allow efficient and regulated generation of the antigen receptors. Exploring how deregulation of this process contributes to immunodeficiencies also continues to be an important area of research for our group.

Detection and repair of ionizing radiation-induced DNA double strand breaks: new developments in nonhomologous end joining

DNA damage can occur as a result of endogenous metabolic reactions and replication stress or from exogenous sources such as radiation therapy and chemotherapy. DNA double strand breaks are the most cytotoxic form of DNA damage, and defects in their repair can result in genome instability, a hallmark of cancer. The major pathway for the repair of ionizing radiation-induced DSBs in human cells is nonhomologous end joining. Here we review recent advances on the mechanism of nonhomologous end joining, as well as new findings on its component proteins and regulation.