Reduced X-ray resistance and homologous recombination frequencies in a RAD54-/- mutant of the chicken DT40 cell line

Homologous DNA recombination in vertebrate cells

The RAD52 epistasis group genes are involved in homologous DNA recombination, and their primary structures are conserved from yeast to humans. Although biochemical studies have suggested that the fundamental mechanism of homologous DNA recombination is conserved from yeast to mammals, recent studies of vertebrate cells deficient in genes of the RAD52 epistasis group reveal that the role of each protein is not necessarily the same as that of the corresponding yeast gene product. This review addresses the roles and mechanisms of homologous recombination-mediated repair with a special emphasis on differences between yeast and vertebrate cells.

Rad54 protein stimulates the postsynaptic phase of Rad51 protein-mediated DNA strand exchange

Rad54 and Rad51 are important proteins for the repair of double-stranded DNA breaks by homologous recombination in eukaryotes. As previously shown, Rad51 protein forms nucleoprotein filaments on single-stranded DNA, and Rad54 protein directly interacts with such filaments to enhance synapsis, the homologous pairing with a double-stranded DNA partner. Here we demonstrate that Saccharomyces cerevisiae Rad54 protein has an additional role in the postsynaptic phase of DNA strand exchange by stimulating heteroduplex DNA extension of established joint molecules in Rad51/Rpa-mediated DNA strand exchange. This function depended on the ATPase activity of Rad54 protein and on specific protein:protein interactions between the yeast Rad54 and Rad51 proteins.

Manipulating the mammalian genome by homologous recombination

Gene targeting in mammalian cells has proven invaluable in biotechnology, in studies of gene structure and function, and in understanding chromosome dynamics. It also offers a potential tool for gene-therapeutic applications. Two limitations constrain the current technology: the low rate of homologous recombination in mammalian cells and the high rate of random (nontargeted) integration of the vector DNA. Here we consider possible ways to overcome these limitations within the framework of our present understanding of recombination mechanisms and machinery. Several studies suggest that transient alteration of the levels of recombination proteins, by overexpression or interference with expression, may be able to increase homologous recombination or decrease random integration, and we present a list of candidate genes. We consider potentially beneficial modifications to the vector DNA and discuss the effects of methods of DNA delivery on targeting efficiency. Finally, we present work showing that gene-specific DNA damage can stimulate local homologous recombination, and we discuss recent results with two general methodologies--chimeric nucleases and triplex-forming oligonucleotides--for stimulating recombination in cells.

Complex formation by the human RAD51C and XRCC3 recombination repair proteins

In vertebrates, the RAD51 protein is required for genetic recombination, DNA repair, and cellular proliferation. Five paralogs of RAD51, known as RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3, have been identified and also shown to be required for recombination and genome stability. At the present time, however, very little is known about their biochemical properties or precise biological functions. As a first step toward understanding the roles of the RAD51 paralogs in recombination, the human RAD51C and XRCC3 proteins were overexpressed and purified from baculovirus-infected insect cells. The two proteins copurify as a complex, a property that reflects their endogenous association observed in HeLa cells. Purified RAD51C--XRCC3 complex binds single-stranded, but not duplex DNA, to form protein--DNA networks that have been visualized by electron microscopy.

Requirement for repair of DNA double-strand breaks by homologous recombination in split-dose recovery

Utsumi, H., Tano, K., Takata, M., Takeda, S. and Elkind, M. M. Requirement for Repair of DNA Double-Strand Breaks by Homologous Recombination in Split-Dose Recovery. Radiat. Res. 155, 680-686 (2001). Split-dose recovery has been observed under a variety of experimental conditions in many cell systems and is believed to be the result of the repair of sublethal damage. It is considered to be one of the most widespread and important cellular responses in clinical radiotherapy. To study the molecular mechanism(s) of this repair, we analyzed the knockout mutants KU70-/-, RAD54-/-, and KU70-/-/RAD54-/- of the chicken B-cell line, DT40. RAD54 participates in the recombinational repair of DNA double-strand breaks (DSBs), while members of the KU family of proteins are involved in nonhomologous end joining. Split-dose recovery was observed in the parent DT40 and the KU70-/- cells. Moreover, the split-dose survival enhancement had all of the characteristics demonstrated earlier for the repair of sublethal damage, e.g., the reappearance of the shoulder on the survival curve with dose fractionation; cyclic fluctuation in cell survival at 37 degrees C; repair and no cyclic fluctuation at 25 degrees C. These results strongly suggest that repair of sublethal damage is due to DSB repair mediated by homologous recombination, and that these DNA DSBs constitute sublethal damage.

Efficient rejoining of radiation-induced DNA double-strand breaks in vertebrate cells deficient in genes of the RAD52 epistasis group

Rejoining of ionizing radiation (IR) induced DNA DSBs usually follows biphasic kinetics with a fast (t(50): 5-30 min) component attributed to DNA-PK-dependent non-homologous endjoining (NHEJ) and a slow (t(50): 1-20 h), as of yet uncharacterized, component. To examine whether homologous recombination (HR) contributes to DNA DSB rejoining, a systematic genetic study was undertaken using the hyper-recombinogenic DT40 chicken cell line and a series of mutants defective in HR. We show that DT40 cells rejoin IR-induced DNA DSBs with half times of 13 min and 4.5 h and contributions by the fast (78%) and the slow (22%) components similar to those of other vertebrate cells with 1000-fold lower levels of HR. We also show that deletion of RAD51B, RAD52 and RAD54 leaves unchanged the rejoining half times and the contribution of the slow component, as does also a conditional knock out mutant of RAD51. A significant reduction (to 37%) in the contribution of the fast component is observed in Ku70(-/-) DT40 cells, but the slow component, operating with a half time of 18.4 h, is still able to rejoin the majority (63%) of DSBs. A double mutant Ku70(-/-)/RAD54(-/-) shows similar half times to Ku70(-/-) cells. Thus, variations in HR by several orders of magnitude leave unchanged the kinetics of rejoining of DNA DSBs, and fail to modify the contribution of the slow component in a way compatible with a dependence on HR. We propose that, in contrast to yeast, cells of vertebrates are 'hard-wired' in the utilization of NHEJ as the main pathway for rejoining of IR-induced DNA DSBs and speculate that the contribution of homologous recombination repair (HRR) is at a stage after the initial rejoining.

Chromosome instability and defective recombinational repair in knockout mutants of the five Rad51 paralogs

The Rad51 protein, a eukaryotic homologue of Escherichia coli RecA, plays a central role in both mitotic and meiotic homologous DNA recombination (HR) in Saccharomyces cerevisiae and is essential for the proliferation of vertebrate cells. Five vertebrate genes, RAD51B, -C, and -D and XRCC2 and -3, are implicated in HR on the basis of their sequence similarity to Rad51 (Rad51 paralogs). We generated mutants deficient in each of these proteins in the chicken B-lymphocyte DT40 cell line and report here the comparison of four new mutants and their complemented derivatives with our previously reported rad51b mutant. The Rad51 paralog mutations all impair HR, as measured by targeted integration and sister chromatid exchange. Remarkably, the mutant cell lines all exhibit very similar phenotypes: spontaneous chromosomal aberrations, high sensitivity to killing by cross-linking agents (mitomycin C and cisplatin), mild sensitivity to gamma rays, and significantly attenuated Rad51 focus formation during recombinational repair after exposure to gamma rays. Moreover, all mutants show partial correction of resistance to DNA damage by overexpression of human Rad51. We conclude that the Rad51 paralogs participate in repair as a functional unit that facilitates the action of Rad51 in HR.

Bloom helicase is involved in DNA surveillance in early S phase in vertebrate cells

Bloom syndrome (BS) is a recessive human genetic disorder characterized by short stature, immunodeficiency and an elevated risk of malignancy. The gene mutated in BS, BLM, encodes a RecQ-type DNA helicase. BS cells have mutator phenotypes such as hyper-recombination, chromosome instability and an increased frequency of sister chromatid exchange (SCE). To define the primary role of BLM, we generated BLM(-/-) mutants of the chicken B-cell line DT40. In addition to characteristics of BLM(-/-) cells reported previously by the other group, they are hypersensitive to genotoxic agents such as etoposide, bleomycin and 4-nitroquinoline-1-oxide and irradiation with the short wave length of UV (UVC) light, whereas they exhibit normal sensitivity to X-ray irradiation and hydroxyurea. UVC irradiation to BLM(-/-) cells during G(1) to early S phase caused chromosomal instability such as chromatid breaks and chromosomal quadriradials, leading to eventual cell death. These results suggest that BLM is involved in surveillance of base abnormalities in genomic DNA that may be encountered by replication forks in early S phase. Such surveillance would maintain genomic stability in vertebrate cells, resulting in the prevention of cellular tumorigenesis.

The chicken B cell line DT40: a novel tool for gene disruption experiments

The use of the chicken DT40 B cell line is increasing in popularity due to the ease with which it can be manipulated genetically. It offers a targeted to random DNA integration ratio of more than 1:2, by far exceeding that of any mammalian cell line. The facility with which knockout cell lines can be generated, combined with a short generation time, makes the DT40 cell line attractive for phenotype analysis of single and multiple gene disruptions. Advantage has been taken of this to investigate such diverse fields as B cell antigen receptor (BCR) signaling, cell cycle regulation, gene conversion and apoptosis. In this review, we give a historical introduction and a practical guide to the use of the DT40 cell line, along with an overview of the main topics being researched using the DT40 cell line as a model system. These topics include B cell-specific subjects such as B cell signaling and Ig rearrangement, and subjects common to all cell types such as apoptosis, histones, mRNA modification, chromosomal maintenance and DNA repair. Attention is in each case brought to peculiarities of the DT40 cell line that are of relevance for the subject. Novel applications of the cell line, e.g., as a vector for gene targeting of human chromosomes, are also discussed in this review.

Spatial distribution of selected genetic loci in nuclei of human leukemia cells after irradiation

Fluorescence in situ hybridization (FISH) combined with high-resolution cytometry was used to determine the topographic characteristics of the centromeric heterochromatin (of the chromosomes 6, 8, 9, 17) and the tumor suppressor gene TP53 (which is located on chromosome 17) in cells of the human leukemia cell lines ML-1 and U937. Analysis was performed on cells that were either untreated or irradiated with gamma rays and incubated for different intervals after exposure. Compared to untreated cells, homologous centromeres and the TP53 genes were found closer to each other and also closer to the nuclear center 2 h after irradiation. The spatial relationship between genetic elements returned to that of the unirradiated controls during the next 2-3 h. Statistical evaluation of our experimental results shows that homologous centromeres and the homologous genes are positioned closer to each other 2 h after irradiation because they are localized closer to the center of the nucleus (probably due to more pronounced decondensation of the chromatin related to repair). This radial movement of genetic loci, however, is not connected with repair of DSBs by processes involving homologous recombination, because the angular distribution of homologous sequences remains random after irradiation.

Characterization of RAD52 homologs in the fission yeast Schizosaccharomyces pombe

The RAD52 gene of Saccharomyces cerevisiae is essential for repair of DNA double-strand breaks (DSBs) by homologous recombination. Inactivation of this gene confers hypersensitivity to DSB-inducing agents and defects in most forms of recombination. The rad22+ gene in Schizosaccharomyces pombe (here referred to as rad22A+) has been characterized as a homolog of RAD52 in fission yeast. Here, we report the identification of a second RAD52 homolog in Schizosaccharomyces pombe, called rad22B+. The amino acid sequences of Rad22A and Rad22B show significant conservation (38% identity). Deletion mutants of respectively, rad22A and rad22B, show different phenotypes with respect to sensitivity to X-rays and the ability to perform homologous recombination as measured by the integration of plasmid DNA. Inactivation of rad22A+ leads to a severe sensitivity to X-rays and a strong decrease in recombination (13-fold), while the rad22B mutation does not result in a decrease in homologous recombination or a change in radiation sensitivity. In a rad22A-rad22B double mutant the radiation sensitivity is further enhanced in comparison with the rad22A single mutant. Overexpression of the rad22B+ gene results in partial suppression of the DNA repair defects of the rad22A mutant strain. Meiotic recombination and spore viability are only slightly affected in either single mutant, but outgrowth of viable spores is almost 31-fold reduced in the rad22A-rad22B double mutant. The results obtained imply a crucial role for rad22A+ in repair and recombination in vegetative cells just like RAD52 in S. cerevisiae. The rad22B+ gene presumably has an auxiliary role in the repair of DSBs. The drastic reduced spore viability in the double mutant suggests that meiosis in S. pombe is dependent on the presence of either rad22A+ or rad22B+.

A novel chicken membrane-associated complement regulatory protein: molecular cloning and functional characterization

A cDNA encoding a membrane-associated complement (C) regulatory protein was identified here for the first time in an oviparous vertebrate, chicken. This protein, named Cremp, possessed five short consensus repeats (SCRs) and one SCR-like domain followed by a transmembrane domain and a cytoplasmic tail. SCR1/SCR2 of Cremp were 43.6% identical with SCR2/SCR3 of human decay-accelerating factor (CD55), and SCR3/SCR4 were 45.3% identical with those of human membrane cofactor protein (CD46). Cremp is likely to be an ancestral hybrid protein of human decay-accelerating factor and membrane cofactor protein rather than a homolog of rodent C receptor 1-related protein y, which structurally resembles human CR1 (CD35). Chinese hamster ovary cells transfected with Cremp were efficiently protected from chicken C but not from human or rabbit C in both classical and alternative pathways. Thus, chicken Cremp is a membrane C regulator for cell protection against homologous C. Cremp mRNA was seen as a doublet comprised of a faint band of 2.2 kb and a thick band of 3.0 kb on RNA blotting analysis. An Ab against chicken Cremp recognized a single band of 46.8 kDa on immunoblotting. mRNA and protein of Cremp were ubiquitously expressed in all chicken organs tested. Minute amounts of dimer were present in some tissues. Surface expression of Cremp was confirmed by flow cytometry and immunofluorescence analysis. These results suggested that even in nonmammals a C regulatory membrane protein with ubiquitous tissue distribution should be a prerequisite for protection of host cells from homologous C attack.

A Large Database of Chicken Bursal ESTs as a Resource for the Analysis of Vertebrate Gene Function

Chicken B cells create their immunoglobulin repertoire within the Bursa of Fabricius by gene conversion. The high homologous recombination activity is shared by the bursal B-cell-derived DT40 cell line, which integrates transfected DNA constructs at high rates into its endogenous loci. Targeted integration in DT40 is used frequently to analyze the function of genes by gene disruption. In this paper, we describe a large database of >7000 expressed sequence tags (ESTs) from bursal lymphocytes that should be a valuable resource for the identification of gene disruption targets in DT40. ESTs of interest can be recognized easily by online BLAST or keyword searches. Because the database reflects the gene expression profile of bursal lymphocytes, it provides valuable hints as to which genes might be involved in B-cell–specific processes related to immunoglobulin repertoire formation, signal transduction, transcription, and apoptosis. This large collection of chicken ESTs will also be useful for gene expression studies and comparative gene mapping within the chicken genome project. Details of the bursal EST sequencing project and access to database search forms can be found on the DT40 web site (http://genetics.hpi.uni-hamburg.de/dt40.html).

[The sequence data described in this paper have been submitted to the GenBank data library under accession nos. AJ392050–AJ399459.]

A large database of chicken bursal ESTs as a resource for the analysis of vertebrate gene function

Chicken B cells create their immunoglobulin repertoire within the Bursa of Fabricius by gene conversion. The high homologous recombination activity is shared by the bursal B-cell-derived DT40 cell line, which integrates transfected DNA constructs at high rates into its endogenous loci. Targeted integration in DT40 is used frequently to analyze the function of genes by gene disruption. In this paper, we describe a large database of >7000 expressed sequence tags (ESTs) from bursal lymphocytes that should be a valuable resource for the identification of gene disruption targets in DT40. ESTs of interest can be recognized easily by online or keyword searches. Because the database reflects the gene expression profile of bursal lymphocytes, it provides valuable hints as to which genes might be involved in B-cell-specific processes related to immunoglobulin repertoire formation, signal transduction, transcription, and apoptosis. This large collection of chicken ESTs will also be useful for gene expression studies and comparative gene mapping within the chicken genome project. Details of the bursal EST sequencing project and access to database search forms can be found on the DT40 web site (http://genetics.hpi.uni-hamburg.de/dt40.html).

Recombination between homologous chromosomes does not play a dominant role in the formation of radiation-induced chromosomal aberrations

PURPOSE

In mammalian cells, the relevance of homologous recombination in radiation-induced double-strand break (DSB) repair is not yet well understood. In the present work, the role of recombination between homologous chromosomes and homology-directed repair of DSB were studied, using X-ray-induced chromosomal aberrations as an end-point.

MATERIALS AND METHODS

Human-hamster hybrid cells containing one or two copies of human chromosome 8 were used. If recombination between homologous chromosomes plays a dominant role in DSB repair, it is expected that X-irradiation of cells with two copies of chromosome 8 would result in a lower frequency of aberrations involving this chromosome compared with cells with only one copy of chromosome 8. The aberrations involving human chromosome 8 were detected by fluorescence in situ hybridization (FISH). Furthermore, a comparison between the hamster cell line XR-C1 (defective in non-homologous repair), CHO-9 (the wild-type cells) and the cell line XR-C1#8 (in which the defect of XR-C1 is complemented by human chromosome 8) was made to determine, indirectly, the involvement of homology-directed recombination in DSB repair.

RESULTS

The observed frequencies of aberrations per human chromosome 8 were not significantly different between cells containing one or two copies of this chromosome. The frequency of chromatid-type aberrations was doubled in XR-C1 cells compared with CHO-9 and XR-C1#8 cells.

CONCLUSIONS

In hamster cells, recombination between homologous chromosomes appears not to have a major role in the formation of radiation-induced chromosomal aberrations, while nonhomologous repair seems to be important in both the G and G2 phases of the cell cycle.

The Rad51 paralog Rad51B promotes homologous recombinational repair

The highly conserved Saccharomyces cerevisiae Rad51 protein plays a central role in both mitotic and meiotic homologous DNA recombination. Seven members of the Rad51 family have been identified in vertebrate cells, including Rad51, Dmc1, and five Rad51-related proteins referred to as Rad51 paralogs, which share 20 to 30% sequence identity with Rad51. In chicken B lymphocyte DT40 cells, we generated a mutant with RAD51B/RAD51L1, a member of the Rad51 family, knocked out. RAD51B(-/-) cells are viable, although spontaneous chromosomal aberrations kill about 20% of the cells in each cell cycle. Rad51B deficiency impairs homologous recombinational repair (HRR), as measured by targeted integration, sister chromatid exchange, and intragenic recombination at the immunoglobulin locus. RAD51B(-/-) cells are quite sensitive to the cross-linking agents cisplatin and mitomycin C and mildly sensitive to gamma-rays. The formation of damage-induced Rad51 nuclear foci is much reduced in RAD51B(-/-) cells, suggesting that Rad51B promotes the assembly of Rad51 nucleoprotein filaments during HRR. These findings show that Rad51B is important for repairing various types of DNA lesions and maintaining chromosome integrity.

Rad54 protein is targeted to pairing loci by the Rad51 nucleoprotein filament

Rad51 and Rad54 proteins are important for the repair of double-stranded DNA (dsDNA) breaks by homologous recombination in eukaryotes. Rad51 assembles on single-stranded DNA (ssDNA) to form a helical nucleoprotein filament that performs homologous pairing with dsDNA; Rad54 stimulates this pairing substantially. Here, we demonstrate that Rad54 acts in concert with the mature Rad51-ssDNA filament. Enhancement of DNA pairing by Rad54 is greatest at an equimolar ratio relative to Rad51 within the filament. Reciprocally, the Rad51-ssDNA filament enhances both the dsDNA-dependent ATPase and the dsDNA unwinding activities of Rad54. We conclude that Rad54 participates in the DNA homology search as a component of the Rad51-nucleoprotein filament and that the filament delivers Rad54 to the dsDNA pairing locus, thereby linking the unwinding of potential target DNA with the homology search process.

A novel human rad54 homologue, Rad54B, associates with Rad51

Members of the SNF2/SWI2 family, characterized with sequence motifs similar to those found in DNA and RNA helicases, play roles in various aspects of cellular fundamental processes such as transcriptional regulation, chromosome stability, nucleotide excision repair, and recombination. We have isolated a novel member of the human SNF2/SWI2 family, RAD54B, which is highly homologous to mammalian RAD54. The RAD54 gene is a member of the RAD52 epistasis group which is involved in the recombinational repair of DNA damage. Here we demonstrate that human Rad54B (hRad54B), like human Rad54 (hRad54), associates with human Rad51 (hRad51). Both hRad54B and hRad54 associate with hRad51 through their NH(2)-terminal domains, but there are differences in their ways of association with hRad51. In contrast to Rad54, whose association with Rad51 is induced by ionizing radiation, Rad54B associates with Rad51 constitutively in immunoprecipitation experiments. Also, the failure to detect the interaction between hRad54B and hRad51 in the yeast two-hybrid assay suggests that their interaction, unlike that between hRad54 and hRad51, may be indirect. Immunofluorescence microscopy revealed that hRad54B formed nuclear foci that colocalized with hRad51, hRad54, and BRCA1. These findings suggest that Rad54B may be functionally distinct from Rad54, although it may play an active role in recombination processes in concert with other members of the RAD52 epistasis group.

Genetic analysis of homologous DNA recombination in vertebrate somatic cells

The maintenance of genomic stability and the ability to repair induced DNA damage in vertebrate cells require homologues of the yeast RAD52 epistasis group genes. The homologous recombination carried out by the products of these genes is essential and appears to be closely linked to DNA replication. Defects in recombination and associated activities are implicated in human cancer. This review summarises recent biochemical and genetic findings on the roles played by the vertebrate RAD52 group gene products in recombination. We describe the phenotypic analysis of genetically engineered mammalian and chicken mutants of homologous recombination genes.

Possible association of BLM in decreasing DNA double strand breaks during DNA replication

Bloom's syndrome (BS) is a rare genetic disorder and the cells from BS patients show genomic instability and an increased level of sister chromatid exchange (SCE). We generated BLM(-/-) and BLM(-/-)/RAD54(-/-) DT40 cells from the chicken B-lymphocyte line DT40. The BLM(-/-) DT40 cells showed higher sensitivity to methyl methanesulfonate and elevated levels of SCE as expected. The targeted integration frequency was also increased remarkably in BLM(-/-) cells. The SCE frequency increase in BLM(-/-) cells was considerably reduced and the enhanced targeted integration observed in BLM(-/-) cells was almost completely abolished in BLM(-/-)/RAD54(-/-) cells, indicating that a large portion of the SCE in BLM(-/-) cells occurs via homologous recombination, and homologous recombination events increase with the defect of BLM function. The BLM(-/-)/RAD54(-/-) cells showed a slow growth phenotype and an increased incidence of chromosome-type breaks/gaps while each single mutant showed relatively small numbers of chromosome-type breaks/gaps.

XRCC3 is required for efficient repair of chromosome breaks by homologous recombination

XRCC3 was originally identified as a human gene able to complement the DNA damage sensitivity, chromosomal instability and impaired growth of the mutant hamster cell line irs1SF. More recently, it has been cloned, sequenced and found to bear sequence homology to the highly conserved eukaryotic repair and recombination gene RAD51. The phenotype of irs1SF and the identification of XRCC3 as a member of the RAD51 gene family have suggested a role for XRCC3 in repair of DNA damage by homologous recombination. Homologous recombinational repair (HRR) of a specifically induced chromosomal double-strand break (DSB) was assayed in irs1SF cells with and without transient complementation by human XRCC3. Complementation with XRCC3 increased the frequencies of repair by 34- to 260-fold. The results confirm a role for XRCC3 in HRR of DNA DSB, and the importance of this repair pathway for the maintenance of chromosomal integrity in mammalian cells.

The breast cancer susceptibility gene, BRCA2: at the crossroads between DNA replication and recombination?

The identification and cloning of the familial breast cancer susceptibility gene, BRCA2, has excited much interest in its biological functions. Here, evidence is reviewed that the protein encoded by BRCA2 has an essential role in DNA repair through its association with mRad51, a mammalian homologue of bacterial and yeast proteins involved in homologous recombination. A model is proposed that the critical requirement for BRCA2 in cell division and the maintenance of chromosome stability stems from its participation in recombinational processes essential for DNA replication.

The controlling role of ATM in homologous recombinational repair of DNA damage

The human genetic disorder ataxia telangiectasia (A-T), caused by mutation in the ATM gene, is characterized by chromosomal instability, radiosensitivity and defective cell cycle checkpoint activation. DNA double-strand breaks (dsbs) persist in A-T cells after irradiation, but the underlying defect is unclear. To investigate ATM's interactions with dsb repair pathways, we disrupted ATM along with other genes involved in the principal, complementary dsb repair pathways of homologous recombination (HR) or non-homologous end-joining (NHEJ) in chicken DT40 cells. ATM(-/-) cells show altered kinetics of radiation-induced Rad51 and Rad54 focus formation. Ku70-deficient (NHEJ(-)) ATM(-/-) chicken DT40 cells show radiosensitivity and high radiation-induced chromosomal aberration frequencies, while Rad54-defective (HR(-)) ATM(-/-) cells show only slightly elevated aberration levels after irradiation, placing ATM and HR on the same pathway. These results reveal that ATM defects impair HR-mediated dsb repair and may link cell cycle checkpoints to HR activation.

Targeted integration of a dominant neo(R) marker into a 2- to 3-Mb human minichromosome and transfer between cells

The physical and genetic characterization of a stable human minichromosome in a Chinese hamster hybrid cell is described. The minichromosome is 2-3 Mb in size, is linear, and contains a complementing SDHC gene. It is derived from a human chromosome 1, including the centromere, some pericentric heterochromatin from 1q12, and 1-2 Mb of 1q21. Genomic DNA surrounding the SDHC gene was used to construct a targeting vector with a selectable drug resistance marker (neo(R)); the marker was then successfully integrated into the minichromosome. With the new selectable marker, the 8.2.3 minichromosome could be transferred into mouse LMTK(-) and 3T3 TK(-) cells.

Disruption of ATM in p53-null cells causes multiple functional abnormalities in cellular response to ionizing radiation

ATM is a member of the large phosphatidylinositol-3 kinase family and plays an important role in cellular response to DNA damage. To further define the physiological roles of ATM at the cellular level, we created an isogenic set of stable cell lines differing only in their ATM status from the chicken B cell line DT40 by targeted integration. These stable DT40 cell lines, as most of transformed chicken cell lines, do not express p53. However, ATM-/- DT40 cells displayed retarded cellular proliferation, defective G2/M checkpoint control and radio-resistant DNA synthesis. Furthermore, ATM-/- DT40 cells were sensitive to ionizing radiation and showed highly elevated frequencies of both spontaneous and radiation-induced chromosomal aberrations. In addition, a slight but significant reduction in targeted integration frequency was observed in ATM-/- DT40 cells. These results suggest that ATM has multiple p53-independent functions in cell cycle checkpoint control and in maintenance of chromosomal DNA. These ATM deficient DT40 clones therefore provide a useful model system for analysing p53-independent ATM functions.

Yeast Rad54 promotes Rad51-dependent homologous DNA pairing via ATP hydrolysis-driven change in DNA double helix conformation

Saccharomyces cerevisiae RAD54 gene functions in the formation of heteroduplex DNA, a key intermediate in recombination processes. Rad54 is monomeric in solution, but forms a dimer/oligomer on DNA. Rad54 dimer/oligomer alters the conformation of the DNA double helix in an ATP-dependent manner, as revealed by a change in the DNA linking number in a topoisomerase I-linked reaction. DNA conformational alteration does not occur in the presence of non-hydrolyzable ATP analogues, nor when mutant rad54 proteins defective in ATP hydrolysis replace Rad54. Accordingly, the Rad54 ATPase activity is shown to be required for biological function in vivo and for promoting Rad51-mediated homologous DNA pairing in vitro. Taken together, the results are consistent with a model in which a Rad54 dimer/oligomer promotes nascent heteroduplex joint formation via a specific interaction with Rad51 protein and an ability to transiently unwind duplex DNA.

The essential functions of human Rad51 are independent of ATP hydrolysis

Genetic recombination and the repair of double-strand DNA breaks in Saccharomyces cerevisiae require Rad51, a homologue of the Escherichia coli RecA protein. In vitro, Rad51 binds DNA to form an extended nucleoprotein filament and catalyzes the ATP-dependent exchange of DNA between molecules with homologous sequences. Vertebrate Rad51 is essential for cell proliferation. Using site-directed mutagenesis of highly conserved residues of human Rad51 (hRad51) and gene targeting of the RAD51 locus in chicken DT40 cells, we examined the importance of Rad51's highly conserved ATP-binding domain. Mutant hRad51 incapable of ATP hydrolysis (hRad51K-133R) binds DNA less efficiently than the wild type but catalyzes strand exchange between homologous DNAs. hRad51 does not need to hydrolyze ATP to allow vertebrate cell proliferation, form nuclear foci, or repair radiation-induced DNA damage. However, cells expressing hRad51K-133R show greatly reduced targeted integration frequencies. These findings show that ATP hydrolysis is involved in DNA binding by hRad51 and suggest that the extent of DNA complexed with hRad51 in nucleoprotein influences the efficiency of recombination.

Analysis of the RAD54 gene on chromosome 1p as a potential tumor-suppressor gene in parathyroid adenomas

Parathyroid adenomas causing primary hyperparathyroidism (pHPT) frequently exhibit allelic loss of DNA markers on the short arm of chromosome 1, indicating the presence of one or more tumor-suppressor genes on 1p. Since the development of pHPT is enhanced in individuals exposed to ionizing radiation to the neck, it could be anticipated that genes involved in DNA repair and recombination may be special targets for mutation in parathyroid tumorigenesis, whether irradiation-associated or not. RAD54 is a member of a family of genes involved in such functions, and RAD54 knockout mice show increased sensitivity to ionizing radiation. The localization of the RAD54 gene to 1p32 has therefore elevated it to a most compelling candidate parathyroid tumor-suppressor gene. Twelve parathyroid adenomas demonstrating allelic loss at chromosome 1p were selected from 55 parathyroid adenomas previously analyzed for loss of heterozygosity using polymorphic microsatellite markers. All 18 exons of the RAD54 gene were fully analyzed by automated sequencing for detection of point mutations or micro-deletions in each parathyroid adenoma. No mutational aberrations were detected in the RAD54 gene, strongly suggesting that complete somatic inactivation of RAD54 is infrequently, if ever, associated with the development of parathyroid adenomas. Whether genes controlling DNA repair and recombination are involved in parathyroid neoplasia remains to be determined.

Mammalian XRCC2 promotes the repair of DNA double-strand breaks by homologous recombination

The repair of DNA double-strand breaks is essential for cells to maintain their genomic integrity. Two major mechanisms are responsible for repairing these breaks in mammalian cells, non-homologous end-joining (NHEJ) and homologous recombination (HR): the importance of the former in mammalian cells is well established, whereas the role of the latter is just emerging. Homologous recombination is presumably promoted by an evolutionarily conserved group of genes termed the Rad52 epistasis group. An essential component of the HR pathway is the strand-exchange protein, known as RecA in bacteria or Rad51 in yeast. Several mammalian genes have been implicated in repair by homologous recombination on the basis of their sequence homology to yeast Rad51: one of these is human XRCC2. Here we show that XRCC2 is essential for the efficient repair of DNA double-strand breaks by homologous recombination between sister chromatids. We find that hamster cells deficient in XRCC2 show more than a 100-fold decrease in HR induced by double-strand breaks compared with the parental cell line. This defect is corrected to almost wild-type levels by transient transfection with a plasmid expressing XRCC2. The repair defect in XRCC2 mutant cells appears to be restricted to recombinational repair because NHEJ is normal. We conclude that XRCC2 is involved in the repair of DNA double-strand breaks by homologous recombination.

The Drosophila melanogaster DmRAD54 gene plays a crucial role in double-strand break repair after P-element excision and acts synergistically with Ku70 in the repair of X-ray damage

The RAD54 gene has an essential role in the repair of double-strand breaks (DSBs) via homologous recombination in yeast as well as in higher eukaryotes. A Drosophila melanogaster strain deficient in the RAD54 homolog DmRAD54 is characterized by increased X-ray and methyl methanesulfonate (MMS) sensitivity. In addition, DmRAD54 is involved in the repair of DNA interstrand cross-links, as is shown here. However, whereas X-ray-induced loss-of-heterozygosity (LOH) events were completely absent in DmRAD54(-/-) flies, treatment with cross-linking agents or MMS resulted in only a slight reduction in LOH events in comparison with those in wild-type flies. To investigate the relative contributions of recombinational repair and nonhomologous end joining in DSB repair, a DmRad54(-/-)/DmKu70(-/-) double mutant was generated. Compared with both single mutants, a strong synergistic increase in X-ray sensitivity was observed in the double mutant. No similar increase in sensitivity was seen after treatment with MMS. Apparently, the two DSB repair pathways overlap much less in the repair of MMS-induced lesions than in that of X-ray-induced lesions. Excision of P transposable elements in Drosophila involves the formation of site-specific DSBs. In the absence of the DmRAD54 gene product, no male flies could be recovered after the excision of a single P element and the survival of females was reduced to 10% compared to that of wild-type flies. P-element excision involves the formation of two DSBs which have identical 3' overhangs of 17 nucleotides. The crucial role of homologous recombination in the repair of these DSBs may be related to the very specific nature of the breaks.

Chicken histone deacetylase-2 controls the amount of the IgM H-chain at the steps of both transcription of its gene and alternative processing of its pre-mRNA in the DT40 cell line

Histone deacetylases (HDACs) are involved in the deacetylation of core histones, which is an important event in transcription regulation in eukaryotes through alterations in the chromatin structure. We cloned cDNAs and genomic DNAs encoding two chicken HDACs (chHDAC-1 and -2), which are preferentially localized in nuclei. Treatment with trichostatin A reduced the HDAC activities in immunoprecipitates obtained with anti-chHDAC-1 and -2 antisera. Using gene targeting techniques, we generated homozygous DT40 mutants, DeltachHDAC-1 and -2, devoid of two alleles of the chHDAC-1 and -2 genes, respectively. The protein patterns on two-dimensional PAGE definitely changed for DeltachHDAC-2, and the amounts of the IgM H- and L-chains increased in it. Of the two IgM H-chain forms, the secreted form mu(s) increased in DeltachHDAC-2, but the membrane-bound form mu(m) decreased. The IgM H-chain gene was transcribed more in DeltachHDAC-2 than in DT40 cells. In the mutant, the alternative processing of IgM H-chain pre-mRNA preferentially occurred, resulting in an increase in the amount of mu(s) mRNA, whereas the stability of the two types of mRNA, mu(s) and mu(m), was unchanged. In DT40 cells, treatment with trichostatin A increased both the amounts of IgM H-chain mRNAs and the switch from mu(m) to mu(s) mRNAs. Based on these results, we propose a model for a role of chHDAC-2 in both the transcription and alternative processing steps, resulting in control of the amount of the mu(s) IgM H-chain in the DT40 cell line.

Repair of endonuclease-induced double-strand breaks in Saccharomyces cerevisiae: essential role for genes associated with nonhomologous end-joining

Repair of double-strand breaks (DSBs) in chromosomal DNA by nonhomologous end-joining (NHEJ) is not well characterized in the yeast Saccharomyces cerevisiae. Here we demonstrate that several genes associated with NHEJ perform essential functions in the repair of endonuclease-induced DSBs in vivo. Galactose-induced expression of EcoRI endonuclease in rad50, mre11, or xrs2 mutants, which are deficient in plasmid DSB end-joining and some forms of recombination, resulted in G2 arrest and rapid cell killing. Endonuclease synthesis also produced moderate cell killing in sir4 strains. In contrast, EcoRI caused prolonged cell-cycle arrest of recombination-defective rad51, rad52, rad54, rad55, and rad57 mutants, but cells remained viable. Cell-cycle progression was inhibited in excision repair-defective rad1 mutants, but not in rad2 cells, indicating a role for Rad1 processing of the DSB ends. Phenotypic responses of additional mutants, including exo1, srs2, rad5, and rdh54 strains, suggest roles in recombinational repair, but not in NHEJ. Interestingly, the rapid cell killing in haploid rad50 and mre11 strains was largely eliminated in diploids, suggesting that the cohesive-ended DSBs could be efficiently repaired by homologous recombination throughout the cell cycle in the diploid mutants. These results demonstrate essential but separable roles for NHEJ pathway genes in the repair of chromosomal DSBs that are structurally similar to those occurring during cellular development.

Prospects for the precise engineering of plant genomes by homologous recombination

The targeting of chromosomal genes via homologous recombination (HR) is an essential tool of reverse genetics as applied for the functional assay of genes within complex genomes. However, in higher plants, foreign DNA integrates almost exclusively at random, non-homologous sites. A variety of environmental parameters known to influence levels of HR do not increase targeting frequencies when combined in gene-targeting experiments. The identification of cellular factors that may control the level of chromosomal HR in plant somatic cells is required. Plant genes encoding proteins similar to those involved in HR in other organisms can be found in the expanding sequence databases. Evidence for evolutionary conservation should help to decipher mechanisms of plant HR and possibly detect limiting factors. At present, however, only one genetic locus influencing levels of chromosomal recombination in plants has been well defined. Here we summarise current knowledge of HR and the status of gene targeting (GT) in plants, focusing on genetic approaches to molecular factors regulating HR levels.

Sister chromatid exchanges are mediated by homologous recombination in vertebrate cells

Sister chromatid exchange (SCE) frequency is a commonly used index of chromosomal stability in response to environmental or genetic mutagens. However, the mechanism generating cytologically detectable SCEs and, therefore, their prognostic value for chromosomal stability in mitotic cells remain unclear. We examined the role of the highly conserved homologous recombination (HR) pathway in SCE by measuring SCE levels in HR-defective vertebrate cells. Spontaneous and mitomycin C-induced SCE levels were significantly reduced for chicken DT40 B cells lacking the key HR genes RAD51 and RAD54 but not for nonhomologous DNA end-joining (NHEJ)-defective KU70(-/-) cells. As measured by targeted integration efficiency, reconstitution of HR activity by expression of a human RAD51 transgene restored SCE levels to normal, confirming that HR is the mechanism responsible for SCE. Our findings show that HR uses the nascent sister chromatid to repair potentially lethal DNA lesions accompanying replication, which might explain the lethality or tumorigenic potential associated with defects in HR or HR-associated proteins.

Gene targeting is enhanced in human cells overexpressing hRAD51

The ideal therapy for single gene disorders would be repair of the mutated disease genes. Homologous recombination is one of several cellular mechanisms for the repair of DNA damage. Recombination between exogenous DNA and homologous chromosomal loci (gene targeting) can be used to repair an endogenous gene, but the low efficiency of this process is a serious barrier to its therapeutic potential. Recent progress in the isolation and characterisation of mammalian genes and proteins involved in DNA recombination has raised the possibility that the cellular biochemistry of recombination can be manipulated to improve the efficiency of gene targeting. As an initial test of this approach, we have overexpressed the gene encoding hRAD51, a protein with homologous DNA pairing and strand exchange activities, in human cells and measured its effect on gene targeting. We report a two- to three-fold increase in gene targeting, and enhanced resistance to ionising radiation in hRAD51-overexpressing cells with no obvious detrimental effects. These observations provide valuable genetic evidence for the involvement of hRAD51 in both gene targeting and DNA repair in human cells. Our data also establish overexpression of recombination genes as a viable approach to improving gene targeting efficiencies.

DNA recombination: the replication connection

Chromosomal double-strand breaks (DSBs) arise after exposure to ionizing radiation or enzymatic cleavage, but especially during the process of DNA replication itself. Homologous recombination plays a critical role in repair of such DSBs. There has been significant progress in our understanding of two processes that occur in DSB repair: gene conversion and recombination-dependent DNA replication. Recent evidence suggests that gene conversion and break-induced replication are related processes that both begin with the establishment of a replication fork in which both leading- and lagging-strand synthesis occur. There has also been much progress in characterization of the biochemical roles of recombination proteins that are highly conserved from yeast to humans.

Mutations of a novel human RAD54 homologue, RAD54B, in primary cancer

Association of breast tumor susceptibility gene products BRCA1 and BRCA2 with the RAD51 recombination protein suggested that cancer could arise through defects in recombination. The identification of NBS1, responsible for Nijmegen breakage syndrome, from the MRE11/RAD50 recombination protein complex also supports this hypothesis. However, our mutation analysis revealed that known members of the RAD52 epistasis group are rarely mutated in human primary cancer. Here we describe the isolation of a novel member of the SNF2 superfamily, characterized with sequence motifs similar to those in DNA and RNA helicases. The gene, designated RAD54B, is significantly homologous to the RAD54 recombination gene. The expression of RAD54B was high in testis and spleen, which are active in meiotic and mitotic recombination. These findings suggest that RAD54B may play an active role in recombination processes in concert with other members of the RAD52 epistasis group. RAD54B maps to human chromosome 8q21.3-q22 in a region associated with cancer-related chromosomal abnormalities. Homozygous mutations at highly conserved positions of RAD54B were observed in human primary lymphoma and colon cancer. These findings suggest that some cancers arise through alterations of the RAD54B function.

Mutations in the RAD54 recombination gene in primary cancers

Association of a recombinational repair protein RAD51 with tumor suppressors BRCA1 and BRCA2 suggests that defects in homologous recombination are responsible for tumor formation. Also recent findings that a protein associated with the MRE11/RAD50 repair complex is mutated in Nijmegen breakage syndrome characterized by increased cancer incidence and ionizing radiation sensitivity strongly support this idea. However, the direct roles of BRCA proteins and the protein responsible for NBS in recombinational repair are not clear though they are associated with the recombinational repair complexes. Since RAD51 forms a complex with other members of the RAD52 epistasis group and with BRCA proteins, it is reasonable to ask if alterations of members of the RAD52 epistasis group lead to tumor development. Here we describe missense mutations at functional regions of RAD54 and the absence of the wild-type RAD54 expression resulting from aberrant splicing in primary cancers. Since RAD54 is a recombinational protein associated with RAD51, this is the first genetic evidence that cancer arises from a defect in repair processes involving homologous recombination.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Sister chromatid-based DNA repair is mediated by RAD54, not by DMC1 or TID1

In the mitotic cell cycle of the yeast Saccharomyces cerevisiae, the sister chromatid is preferred over the homologous chromosome (non-sister chromatid) as a substrate for DNA double-strand break repair. However, no genes have yet been shown to be preferentially involved in sister chromatid-mediated repair. We developed a novel method to identify genes that are required for repair by the sister chromatid, using a haploid strain that can embark on meiosis. We show that the recombinational repair gene RAD54 is required primarily for sister chromatid-based repair, whereas TID1, a yeast RAD54 homologue, and the meiotic gene DMC1, are dispensable for this type of repair. Our observations suggest that the sister chromatid repair pathway, which involves RAD54, and the homologous chromosome repair pathway, which involves DMC1, can substitute for one another under some circumstances. Deletion of RAD54 in S.cerevisiae results in a phenotype similar to that found in mammalian cells, namely impaired DNA repair and reduced recombination during mitotic growth, with no apparent effect on meiosis. The principal role of RAD54 in sister chromatid-based repair may also be shared by mammalian and yeast cells.

Search for mutations of the hRAD54 gene in sporadic meningiomas with deletion at 1p32

The hRAD54 gene is related to a family of genes involved in DNA recombination and repair and encodes a protein with DNA helicase activity. hRAD54 has been mapped to 1p32, a region frequently involved in deletions in a variety of tumor types, including atypical and anaplastic meningiomas. To determine whether alterations of hRAD54 are a common event in meningeal tumors, by means of polymerase chain reaction-single-stranded conformation analysis we examined 29 tumor samples characterized by 1p deletions for hRAD54 mutations. Although 18 tumors displayed allelic loss at the gene region (1p32) as determined by microsatellite marker analysis, the sole coding-sequence alteration detected corresponded to a T-->C transition, with no amino-acid change. The genotype distribution was 10.34% TT, 44.8% TC, and 44.8% CC, whereas in the normal controls it was 3.77% TT, 13.2% TC, and 83.01% CC, and most meningiomas with 1 p32 deletion retained allele C. Another polymorphism due to a T-->C change was evidenced at nt 3008, in the 3' untranslated region. This change was evidenced in all cases we sequenced. These results appear to exclude the involvement of the hRAD54 gene in the pathogenesis of the nontypical meningiomas, although a detrimental effect of the hRAD54 polymorphisms cannot be ruled out.

Use of gene knockouts in cultured cells to study apoptosis

The avian DT40 cell system represents a novel method to generate loss of function mutations in vertebrate cells. These chicken B lymphoma cells undergo homologous recombination at very high frequencies and can thus be used to "knock out" genes believed to function in apoptotic processes. The knockout cells can then be used to determine how the cell death process is modulated after induction of apoptosis and to order components in cell death pathways. The system can be further modified, using tetracycline-responsive promoters, to allow expression of wild-type cDNAs to rescue "knockout cells" if the gene of interest is essential. Alternatively, cDNA expression constructs containing mutations or deletions in the cDNA encoding the absent protein can be used to delineate functional domains. cDNA expression libraries or known proteins believed to function downstream of the target in a signal transduction pathway could also be transfected into the knockout cell line, and the resultant cells could be assayed for complementation and/or rescue of the apoptotic alteration/defect. Finally, the system has recently been adapted to allow disruption of human genes in DT40/human hybrid cell lines thereby potentially extending this system for use in studying human genes.

Gene targeting and somatic cell genetics--a rebirth or a coming of age?

A number of recent advances have significantly facilitated gene targeting in somatic cells. Gene targeting can now be performed with the same ease and efficiency in somatic cells as in murine embryonic stem cells. Rigorous genetic analyses can therefore be applied for the first time to the large number of excellent human cell culture systems. These tools will be important in areas where rodent models do not adequately represent human biology.

Mitogen-activated protein kinase kinase 2 activation is essential for progression through the G2/M checkpoint arrest in cells exposed to ionizing radiation

An increasing body of evidence suggests that mitogen-induced activation of the RAF/ERK signaling pathway is functionally separate from the stress-induced activation of the SEK/JNK/p38 signaling pathway. In general, stress stimuli strongly activate the p38s and the JNKs while only weakly activating ERK1 and ERK2. However, a number of independent groups have now shown that the RAF/ERK signaling pathway is strongly activated by ionizing radiation. In this work, we examine this paradox. We show that both mitogen-activated protein (MAP) kinase kinase 1 (MEK1) and MAP kinase kinase 2 (MEK2) are activated by ionizing radiation. Blockage of this activation through the use of dominant negative MEK2 increases sensitivity of the cell to ionizing radiation and decreases the ability of a cell to recover from the G2/M cell cycle checkpoint arrest. Blocking MEK2 activation does not affect double-strand DNA break repair, however. Although MEK1 is activated to a lesser extent by ionizing radiation, expression of a dominant negative MEK1 does not affect radiation sensitivity of the cell, the G2/M checkpoint of the cell, or double-strand break repair. Because ionizing radiation leads to a different cell cycle arrest (G2/M arrest) than that typically seen with other stress stimuli, and because we have shown that MEK2 can affect G2/M checkpoint kinetics, these results provide an explanation for the observation that the MEKs can be strongly activated by ionizing radiation and only weakly activated by other stressful stimuli.

A Role for RAD51 in the Generation of Immunoglobulin Gene Diversity in Rabbits

Ig VDJ genes in rabbit somatically diversify by both hyperpointmutation and gene conversion. To elucidate the mechanism of gene conversion of IgH genes, we cloned a rabbit homologue of RAD51, a gene involved in gene conversion in Saccharomyces cerevisiae (yeast), and tested whether it could complement a yeast rad51 mutant deficient in recombination repair. We found that rabbit RAD51 partially complemented the defect in switching mating types by gene conversion as well as in DNA double-strand break repair after γ-irradiation. Further, by Western blot analysis, we found that levels of Rad51 were higher in appendix-derived B lymphocytes of 6-wk-old rabbits, a time at which IgH genes diversify by somatic gene conversion. We suggest that Rad51 is involved in somatic gene conversion of rabbit Ig genes.

The contribution of homologous recombination in preserving genome integrity in mammalian cells

Although it is clear that mammalian somatic cells possess the enzymatic machinery to perform homologous recombination of DNA molecules, the importance of this process in mitigating DNA damage has been uncertain. An initial genetic framework for studying homologous recombinational repair (HRR) has come from identifying relevant genes by homology or by their ability to correct mutants whose phenotypes are suggestive of recombinational defects. While yeast has been an invaluable guide, higher eukaryotes diverge in the details and complexity of HRR. For eliminating DSBs, HRR and end-joining pathways share the burden, with HRR contributing critically during S and G2 phases. It is likely that the removal of interstrand cross-links is absolutely dependent on efficient HRR, as suggested by the extraordinary sensitivity of the ercc1, xpf/ercc4, xrcc2, and xrcc3 mutants to cross-linking chemicals. Similarly, chromosome stability in untreated cells requires intact HRR, which may eliminate DSBs arising during DNA replication and thereby prevent chromosome aberrations. Complex regulation of HRR by cell cycle checkpoint and surveillance functions is suggested not only by direct interactions between human Rad51 and p53, c-Abl, and BRCA2, but also by very high recombination rates in p53-deficient cells.

The nuclease activity of Mre11 is required for meiosis but not for mating type switching, end joining, or telomere maintenance

The Saccharomyces cerevisiae MRE11 gene is required for the repair of ionizing radiation-induced DNA damage and for the initiation of meiotic recombination. Sequence analysis has revealed homology between Mre11 and SbcD, the catalytic subunit of an Escherichia coli enzyme with endo- and exonuclease activity, SbcCD. In this study, the purified Mre11 protein was found to have single-stranded endonuclease activity. This activity was absent from mutant proteins containing single amino acid substitutions in either one of two sequence motifs that are shared by Mre11 and SbcD. Mutants with allele mre11-D56N or mre11-H125N were partially sensitive to ionizing radiation but lacked the other mitotic phenotypes of poor vegetative growth, hyperrecombination, defective nonhomologous end joining, and shortened telomeres that are characteristic of the mre11 null mutant. Diploids homozygous for the mre11-H125N mutation failed to sporulate and accumulated unresected double-strand breaks (DSB) during meiosis. We propose that in mitotic cells DSBs can be processed by other nucleases that are partially redundant with Mre11, but these activities are unable to process Spo11-bound DSBs in meiotic cells.

Molecular mechanisms of DNA double strand break repair

DNA double-strand breaks (DSBs) are major threats to the genomic integrity of cells. If not taken care of properly, they can cause chromosome fragmentation, loss and translocation, possibly resulting in carcinogenesis. Upon DSB formation, cell-cycle checkpoints are triggered and multiple DSB repair pathways can be activated. Recent research on the Nijmegen breakage syndrome, which predisposes patients to cancer, suggests a direct link between activation of cell-cycle checkpoints and DSB repair. Furthermore, the biochemical activities of proteins involved in the two major DSB repair pathways, homologous recombination and DNA end-joining, are now beginning to emerge. This review discusses these new findings and their implications for the mechanisms of DSB repair.