A noncatalytic function of the ligation complex during nonhomologous end joining

Nonhomologous end joining is the primary deoxyribonucleic acid (DNA) double-strand break repair pathway in multicellular eukaryotes. To initiate repair, Ku binds DNA ends and recruits the DNA-dependent protein kinase (DNA-PK) catalytic subunit (DNA-PKcs) forming the holoenzyme. Early end synapsis is associated with kinase autophosphorylation. The XRCC4 (X4)–DNA Ligase IV (LIG4) complex (X4LIG4) executes the final ligation promoted by Cernunnos (Cer)–X4-like factor (XLF). In this paper, using a cell-free system that recapitulates end synapsis and DNA-PKcs autophosphorylation, we found a defect in both activities in human cell extracts lacking LIG4. LIG4 also stimulated the DNA-PKcs autophosphorylation in a reconstitution assay with purified components. We additionally uncovered a kinase autophosphorylation defect in LIG4-defective cells that was corrected by ectopic expression of catalytically dead LIG4. Finally, our data support a contribution of Cer-XLF to this unexpected early role of the ligation complex in end joining. We propose that productive end joining occurs by early formation of a supramolecular entity containing both DNA-PK and X4LIG4–Cer-XLF complexes on DNA ends.     

The Greek Goddess,Artemis, reveals the secrets of her cleavage

A recent paper in Cell by Ma et al. [Cell 8 (2002) 781] showed that the protein Artemis cleaves a hairpin intermediate during V(D)J recombination. Peter O'Neill and Penny Jeggo discuss this finding in the light of evidence that Artemis also functions to repair radiation damage [Cell 105 (2001) 177]. The findings suggest that Artemis may function in double strand break repair by "tidying up" double strand ends with associated base damage. The development of genetic diversity during immune development, therefore, seems to exploit damage response mechanisms that function to maintain genetic stability in other cell lineages.     

Interaction of the Ku heterodimer with the DNA ligase IV/Xrcc4 complex and its regulation by DNA-PK

DNA non-homologous end-joining (NHEJ) is a major mechanism for repairing DNA double-stranded (ds) breaks in mammalian cells. Here, we characterize the interaction between two key components of the NHEJ machinery, the Ku heterodimer and the DNA ligase IV/Xrcc4 complex. Our results demonstrate that Ku interacts with DNA ligase IV via its tandem BRCT domain and that this interaction is enhanced in the presence of Xrcc4 and dsDNA. Moreover, residues 644-748 of DNA ligase IV encompassing the first BRCT motif are necessary for binding. We show that Ku needs to be in its heterodimeric form to bind DNA ligase IV and that the C-terminal tail of Ku80, which mediates binding to DNA-PKcs, is dispensable for DNA ligase IV recognition. Although the interaction between Ku and DNA ligase IV/Xrcc4 occurs in the absence of DNA-PKcs, the presence of the catalytic subunit of DNA-PK kinase enhances complex formation. Previous studies have shown that DNA-PK kinase activity causes disassembly of DNA-PKcs from Ku at the DNA end. Here, we show that DNA-PK kinase activity also results in disassembly of the Ku/DNA ligase IV/Xrcc4 complex. Collectively, our findings provide novel information on the protein-protein interactions that regulate NHEJ in cells.     

Chromosome breakage after G2 checkpoint release

DNA double-strand break (DSB) repair and checkpoint control represent distinct mechanisms to reduce chromosomal instability. Ataxia telangiectasia (A-T) cells have checkpoint arrest and DSB repair defects. We examine the efficiency and interplay of ATM's G2 checkpoint and repair functions. Artemis cells manifest a repair defect identical and epistatic to A-T but show proficient checkpoint responses. Only a few G2 cells enter mitosis within 4 h after irradiation with 1 Gy but manifest multiple chromosome breaks. Most checkpoint-proficient cells arrest at the G2/M checkpoint, with the length of arrest being dependent on the repair capacity. Strikingly, cells released from checkpoint arrest display one to two chromosome breaks. This represents a major contribution to chromosome breakage. The presence of chromosome breaks in cells released from checkpoint arrest suggests that release occurs before the completion of DSB repair. Strikingly, we show that checkpoint release occurs at a point when approximately three to four premature chromosome condensation breaks and approximately 20 gammaH2AX foci remain.     

Radiation-induced chromosome damage in X-ray-sensitive mutants (xrs) of the Chinese hamster ovary cell line

The frequency of both spontaneous and X-ray- (95 rad) induced cytogenetical aberrations has been determined for 2 X-ray-sensitive strains (xrs-6 and xrs-7) of the Chinese hamster ovary cell line, and their wild-type parent (CHO-K1). Increased levels of spontaneous aberrations were not a general feature of the xrs strains, although xrs-7 did show a 2-fold increase in chromatid gaps. Unsynchronied populations of xrs cells, estimated to have been irradiated in late S and G2, showed a 3-5-fold increase in chromatid gaps, breaks and exchanges compared to CHO-K1. The irradiation of synchronised populations of xrs-7 and CHO-K1 in G1 demonstrated a 3-5-fold increase in chromosome breaks, gaps and exchanges in xrs-7. In addition xrs-7 displayed a large increase in chromatid-type aberrations, particularly triradials. These X-ray-sensitive strains have previously been shown to have a defect in double-strand break rejoining (Kemp et al., 1984), and an increased number of double-strand breaks (DBSs) remain in their DNA after irradiation compared to wild-type cells. The increased number of DSBs remaining in these strains 20 min after irradiation, correlates well with the increase in chromosome breaks.     

Azacytidine-induced reactivation of a DNA repair gene in Chinese hamster ovary cells.

Six X-ray-sensitive (xrs) strains of the CHO-K1 cell line were shown to revert at a very high frequency after treatment with 5-azacytidine. This suggested that there was a methylated xrs+ gene in these strains which was structurally intact, but not expressed. The xrs strains did not complement one another, and the locus was autosomally located. In view of the frequency of their isolation and their somewhat different phenotypes, we propose that the xrs strains are mutants derived from an active wild-type gene. However, there is in addition a methylated silent gene present in the genome. Azacytidine treatment reactivated this gene. We present a model for the functional hemizygosity of mammalian cell lines, which is based on the inactivation of genes by de novo hypermethylation. In contrast to results with xrs strains, other repair-defective lines were found not to be reverted by azacytidine.     

X-ray sensitive mutants of Chinese hamster ovary cells defective in double-strand break rejoining

Six CHO mutants have previously been described as being sensitive to ionizing radiation and bleomycin treatment, with little or no cross sensitivity to UV-radiation (Jeggo and Kemp, 1983). Their ability to rejoin single- and double-strand breaks has been examined here. Using two techniques, gradient sedimentation and alkaline elution, no difference could be observed between wild-type and mutant strains in the initial number of single-strand breaks induced, the rate of rejoining, or the final level of single-strand breaks rejoined. Thus, a major inability to rejoin single-strand breaks is not the basis for sensitivity in these mutants. In contrast, all 6 mutants showed a decreased ability to rejoin the double-strand breaks induced by gamma-irradiation as measured by neutral elution. Rejoining of half of the breaks occurred in 37 min in wild-type cells and reached a maximum level of 72% after 2 h. All the mutants showed a decreased rate of rejoining, and the final level was 17% of that observed in the wild-type in the most defective mutant, and ranged from 35 to 69% in the other 5 mutants. These are the first mammalian cell mutants to be described with a defect in double-strand break rejoining.     

Isolation and characterization of Escherichia coli K-12 mutants unable to induce the adaptive response to simple alkylating agents.

When Esherichia coli cells are exposed to a low level of simple alkylating agents, they induce the adaptive response which renders them more resistant to the killing and the mutagenic effects of the same or other alkylating agents. This paper describes the isolation of one strain that was deficient in mutagenic adaptation and five that were deficient in both mutagenic and killing adaptation, confirming previous suggestions that killing and mutagenic adaptation are, at least to some extent, separable. These six strains have been called Ada mutants. They were more sensitive to the killing and mutagenic effects of N-methy-N'-nitro-N-nitrosoguanidine (MNNG) than the unadapted Ada+ parent. Thus, the adaptation pathway is responsible for circumventing some alkylation-induced damage even in cells that are preinduced. The increase in mutation frequency seen in Ada cells treated with MNNG was the same whether the cells were lexA+ or lexA, showing that the extra mutations found in Ada- strains do not depend upon the SOS pathway. Ada strains accumulated more O6-methyl guanine lesions than the Ada+ parent on prolonged exposure to MNNG, and this supports the idea that O6-methyl guanine is the most important lesion for MNNG-induced mutagenesis. The ada mutations have been shown to map in the 47 to 53-min region of the E. coli chromosome.     

Membrane lipids and ethanol tolerance in the mouse. The influence of dietary fatty acid composition

Mice of the TO Swiss strain received diets containing different amounts of saturated or unsaturated fat throughout life. These diets produced characteristic changes in cardiac phospholipid fatty acid composition, but produced no significant differences in fatty acid composition of phospholipids from a crude membrane fraction of brain. When littermates of these animals were exposed to ethanol vapour in an inhalation chamber it was observed that mice which had received a diet high in saturated fat lost the righting reflex at an estimated concentration of ethanol in blood higher than that required for mice receiving a control diet, or a diet rich in polyunsaturated fat. Analysis of the brain membrane fraction from those animals which had received ethanol revealed that mice receiving the highly saturated fat diet now had a significantly greater proportion of saturated fatty acids in brain membrane phospholipids. These results are discussed in relation to the hypothesis that brain membrane lipid composition may influence the behavioural response to ethanol.