DNA ligase IV functions in DNA nonhomologous end-joining and V(D)J recombination. Four patients with features including immunodeficiency and developmental and growth delay were found to have mutations in the gene encoding DNA ligase IV (LIG4). Their clinical phenotype closely resembles the DNA damage response disorder, Nijmegen breakage syndrome (NBS). Some of the mutations identified in the patients directly disrupt the ligase domain while others impair the interaction between DNA ligase IV and Xrcc-4. Cell lines from the patients show pronounced radiosensitivity. Unlike NBS cell lines, they show normal cell cycle checkpoint responses but impaired DNA double-strand break rejoining. An unexpected V(D)J recombination phenotype is observed involving a small decrease in rejoining frequency coupled with elevated imprecision at signal junctions.

In eukaryotic cells, double-strand breaks (DSBs) in DNA are generally repaired by the pathway of homologous recombination or by DNA nonhomologous end joining (NHEJ). Both pathways have been highly conserved throughout eukaryotic evolution, but no equivalent NHEJ system has been identified in prokaryotes. The NHEJ pathway requires a DNA end-binding component called Ku. We have identified bacterial Ku homologs and show that these proteins retain the biochemical characteristics of the eukaryotic Ku heterodimer. Furthermore, we show that bacterial Ku specifically recruits DNA ligase to DNA ends and stimulates DNA ligation. Loss of these proteins leads to hypersensitivity to ionizing radiation in Bacillus subtilis. These data provide evidence that many bacteria possess a DNA DSB repair apparatus that shares many features with the NHEJ system of eukarya and suggest that this DNA repair pathway arose before the prokaryotic and eukaryotic lineages diverged.

The major mechanism for the repair of DNA double-strand breaks (DSBs) in mammalian cells is non-homologous end-joining (NHEJ), a process that involves the DNA-dependent protein kinase [1] [2], XRCC4 and DNA ligase IV [3] [4] [5] [6]. Rodent cells and mice defective in these components are radiation-sensitive and defective in V(D)J-recombination, showing that NHEJ also functions to rejoin DSBs introduced during lymphocyte development [7] [8]. 180BR is a radiosensitive cell line defective in DSB repair, which was derived from a leukaemia patient who was highly sensitive to radiotherapy [9] [10] [11]. We have identified a mutation within a highly conserved motif encompassing the active site in DNA ligase IV from 180BR cells. The mutated protein is severely compromised in its ability to form a stable enzyme-adenylate complex, although residual activity can be detected at high ATP concentrations. Our results characterize the first patient with a defect in an NHEJ component and suggest that a significant defect in NHEJ that leads to pronounced radiosensitivity is compatible with normal human viability and does not cause any major immune dysfunction. The defect, however, may confer a predisposition to leukaemia.

Upon DNA damage, p53-binding protein 1 (53BP1) relocalizes to sites of DNA double-strand breaks and forms discrete nuclear foci, suggesting its role in DNA damage responses. We show that 53BP1 changed its localization from the detergent soluble to insoluble fraction after treatment of cells with x-ray, but not with ultraviolet or hydroxyurea. Either DNase or phosphatase treatment of the insoluble fraction released 53BP1 into the soluble fraction, showing that 53BP1 binds to chromatin in a phosphorylation-dependent manner after X-irradiation of cells. 53BP1 was retained at discrete nuclear foci in X-irradiated cells even after detergent extraction of cells, showing that the chromatin binding of 53BP1 occurs at sites of DNA double-strand breaks. The minimal domain for focus formation was identified by immunofluorescence staining of cells ectopically expressed with 53BP1 deletion mutants. This domain consisted of conserved Tudor and Myb motifs. The Tudor plus Myb domain possessed chromatin binding activity in vivo and bound directly to both double-stranded and singlestranded DNA in vitro. This domain also stimulated endjoining by DNA ligase IV/Xrcc4, but not by T4 DNA ligase in vitro. We conclude that 53BP1 has the potential to participate directly in the repair of DNA double-strand breaks.

Graves' disease (GD) is a common autoimmune disease (AID) that shares many of its susceptibility loci with other AIDs. The thyroid stimulating hormone receptor (TSHR) represents the primary autoantigen in GD, in which autoantibodies bind to the receptor and mimic its ligand, thyroid stimulating hormone, causing the characteristic clinical phenotype. Although early studies investigating the TSHR and GD proved inconclusive, more recently we provided convincing evidence for association of the TSHR region with disease. In the current study, we investigated a combined panel of 98 SNPs, including 70 tag SNPs, across an extended 800 kb region of the TSHR to refine association in a cohort of 768 GD subjects and 768 matched controls. In total, 28 SNPs revealed association with GD (P < 0.05), with strongest SNP associations at rs179247 (chi(2) = 32.45, P = 8.90 x 10(-8), OR = 1.53, 95% CI = 1.32-1.78) and rs12101255 (chi(2) = 30.91, P = 1.95 x 10(-7), OR = 1.55, 95% CI = 1.33-1.81), both located in intron 1 of the TSHR. Association of the most associated SNP, rs179247, was replicated in 303 GD families (P = 7.8 x 10(-4)). In addition, we provide preliminary evidence that the disease-associated genotypes of rs179247 (AA) and rs12101255 (TT) show reduced mRNA expression ratios of flTSHR relative to two alternate TSHR mRNA splice variants.

DNA nonhomologous end-joining (NHEJ) is

DNA ligase IV functions in DNA non-homologous endjoining, in V(D)J recombination, and during brain development. We previously reported a homozygous mutation (R278H) in DNA ligase IV in a developmentally normal leukemia patient who overresponded to radiotherapy. The impact of this hypomorphic mutation has been evaluated using cellular, biochemical, and structural approaches. Structural modeling using T7 DNA ligase predicts that the activity and conformational stability of the protein is likely to be impaired. We show that wild type DNA ligase IV-Xrcc4 is an efficient double-stranded ligase with distinct optimal requirements for adenylate complex formation versus rejoining. The mutation impairs the formation of an adenylate complex as well as reducing the rejoining activity. Additionally, it imparts temperature-sensitive activity to the protein consistent with the predictions of the structural modeling. At the cellular level, the mutation confers a unique V(D)J recombination phenotype affecting the fidelity of signal joint formation with little effect on the frequency of the reaction. These findings suggest that hypomorphic mutations in ligase IV may allow normal development but confer marked radiosensitivity.

Non-homologous end-joining is a major pathway of DNA double-strand break repair in mammalian cells, deficiency in which confers radiosensitivity and immune deficiency at the whole organism level. A core protein complex comprising the Ku70/80 heterodimer together with a complex between DNA ligase IV and XRCC4 is conserved throughout eukaryotes and assembles at double-strand breaks to mediate ligation of broken DNA ends. In Saccharomyces cerevisiae an additional NHEJ protein, Nej1p, physically interacts with the ligase IV complex and is required in vivo for ligation of DNA double-strand breaks. Recent studies with cells derived from radiosensitive and immune-deficient patients have identified the human protein, XLF (also named Cernunnos), as a crucial NHEJ protein. Here we show that XLF and Nej1p are members of the same protein superfamily and that this family has members in diverse eukaryotes. Indeed, we show that a member of this family encoded by a previously uncharacterized open-reading frame in the Schizosaccharomyces pombe genome is required for NHEJ in this organism. Furthermore, our data reveal that XLF family proteins can bind to DNA and directly interact with the ligase IV-XRCC4 complex to promote DSB ligation. We therefore conclude that XLF family proteins interact with the ligase IV-XRCC4 complex to constitute the evolutionarily conserved enzymatic core of the NHEJ machinery.The repair of DNA double-strand breaks (DSBs) 3 is crucial for the maintenance of genomic integrity (1, 2). In mammalian cells, non-homologous end-joining (NHEJ) is the predominant pathway for DSB repair, particularly in the G 0 and G 1 phases of the cell cycle, and is mediated by the Ku70/Ku80 heterodimer, the DNA-dependent protein kinase catalytic subunit (DNAPKcs), XRCC4 and DNA ligase IV (3, 4). It is believed that during NHEJ, newly created DSB ends are bound by the Ku heterodimer (5, 6) and subsequently, DNA-PKcs is recruited upon inward translocation of Ku70/Ku80 (7-9). The Ku-DNA-PKcs complex is thought to stabilize the DNA ends and prevent extensive end resection (10), and most likely also plays an important role in aligning the DNA termini (11-14). The DNAPKcs-Ku complex then promotes the recruitment of a preformed complex between XRCC4 and DNA ligase IV (ligase IV), which mediates the final ligation steps of the NHEJ process (8, 15).XRCC4 is a ϳ36-kDa protein that consists of a globular N-terminal domain followed by a protruding coiled-coil arm (16,17). It has been well established that XRCC4 forms homodimers, as well as higher order multimers (16 -19), and that an active ligase IV complex is most likely composed of one XRCC4 homodimer bound to one ligase IV molecule (17, 18). Significantly, XRCC4 not only promotes ligase IV activity but is also required for ligase protein stability (20,21). In addition to the above core set of NHEJ proteins, a number of other factors have been identified that are required for the efficient repair of DSBs by NHEJ. These factors, most prominently Artemis (22, 23), polynucleotide kinase ...