Disruption of mouse RAD54 reduces ionizing radiation resistance and homologous recombination

Homologous recombination and non-homologous end-joining pathways of DNA double-strand break repair have overlapping roles in the maintenance of chromosomal integrity in vertebrate cells

Eukaryotic cells repair DNA double-strand breaks (DSBs) by at least two pathways, homologous recombination (HR) and non-homologous end-joining (NHEJ). Rad54 participates in the first recombinational repair pathway while Ku proteins are involved in NHEJ. To investigate the distinctive as well as redundant roles of these two repair pathways, we analyzed the mutants RAD54(-/-), KU70(-/-) and RAD54(-/-)/KU70(-/-), generated from the chicken B-cell line DT40. We found that the NHEJ pathway plays a dominant role in repairing gamma-radiation-induced DSBs during G1-early S phase while recombinational repair is preferentially used in late S-G2 phase. RAD54(-/-)/KU70(-/-) cells were profoundly more sensitive to gamma-rays than either single mutant, indicating that the two repair pathways are complementary. Spontaneous chromosomal aberrations and cell death were observed in both RAD54(-/-) and RAD54(-/-)/KU70(-/-) cells, with RAD54(-/-)/KU70(-/-) cells exhibiting significantly higher levels of chromosomal aberrations than RAD54(-/-) cells. These observations provide the first genetic evidence that both repair pathways play a role in maintaining chromosomal DNA during the cell cycle.

okra and spindle-B encode components of the RAD52 DNA repair pathway and affect meiosis and patterning in Drosophila oogenesis

okra (okr), spindle-B (spnB), and spindle-D (spnD) are three members of a group of female sterile loci that produce defects in oocyte and egg morphology, including variable dorsal-ventral defects in the eggshell and embryo, anterior-posterior defects in the follicle cell epithelium and in the oocyte, and abnormalities in oocyte nuclear morphology. Many of these phenotypes reflect defects in grk-Egfr signaling processes, and can be accounted for by a failure to accumulate wild-type levels of Gurken and Fs(1)K10. We have cloned okr and spnB, and show that okr encodes the Drosophila homolog of the yeast DNA-repair protein Rad54, and spnB encodes a Rad51-like protein related to the meiosis-specific DMC1 gene. In functional tests of their role in DNA repair, we find that okr behaves like its yeast homolog in that it is required in both mitotic and meiotic cells. In contrast, spnB and spnD appear to be required only in meiosis. The fact that genes involved in meiotic DNA metabolism have specific effects on oocyte patterning implies that the progression of the meiotic cell cycle is coordinated with the regulation of certain developmental events during oogenesis.

Radiation induced chromosome aberrations: some biophysical considerations

The implications of recent results using FISH chromosome painting and soft X-ray exposures for the mechanisms of chromosome aberration formation are discussed. It is concluded that the evidence in favour of exchange aberrations arising from one radiation induced chromosome break has increased to the point where a 'change in paradigm' from the older breakage-reunion hypothesis needs to be taken seriously into account. A potential role for recombinational repair of DNA double strand breaks, as known in yeast, in the formation of aberrations in mammalian cells is presented and the relationship between DNA repair studies and radiation cytology is emphasized.

DNA breakage and repair

For many years it has been evident that mammalian cells differ dramatically from yeast and rejoin the majority of their DNA DSBs by a nonhomologous mechanism, recently termed NHEJ. In the last few years a number of genes and proteins have been identified that operate in the pathway providing insights into the mechanism. These proteins include the three components of DNA-PK, DNA ligase IV, and XRCC4. In yeast Sir2, -3, and -4 proteins are also involved in the process and therefore are likely to play a role in higher organisms. Studies with yeast suggest that NHEJ is an error-free mechanism. Although the process is far from understood, it is likely that the DNA-PK complex or Ku alone acts in a complex with the Sir proteins possibly protecting the ends and preventing random rejoining. Further work is required to establish the details of this mechanism and to determine whether this represents an accurate rejoining process for a complex break induced by ionizing radiation. It will be intriguing to discover how the cell achieves efficient and accurate rejoining without the use of homology. Interactions between the components of DNA-PK and other proteins playing a central role in damage response mechanisms are beginning to emerge. Interestingly, there is evidence that DNA repair and damage response mechanisms overlap in lower organisms. The overlapping defects of the yeast Ku mutants, tell mutants, and AT cell lines in telomere maintenance further suggest overlapping functions or interacting mechanisms. A challenge for the future will be to establish how these different damage response mechanisms overlap and interact.

Role of the human RAD51 protein in homologous recombination and double-stranded-break repair

Eukaryotic cells possess several mechanisms for repairing double-stranded breaks in DNA. One mechanism involves genetic recombination with an intact sister duplex. The recent identification of the RAD51 protein, a eukaryotic homologue of Escherichia coli RecA, represents a landmark discovery in our understanding of the key reactions in this repair pathway. RAD51 is similar to RecA, both biochemically and structurally: it promotes homologous pairing and strand exchange within a regular nucleoprotein filament. The isolation of yeast and human RecA homologues shows that homologous recombination and recombinational repair have been conserved throughout evolution. The goal is now to identify other factors involved in recombinational repair and to define their roles in this essential process.

Ku70 is required for late B cell development and immunoglobulin heavy chain class switching

Immunoglobulin (Ig) heavy chain (HC) class switch recombination (CSR) is a late B cell process that involves intrachromosomal DNA rearrangement. Ku70 and Ku80 form a DNA end-binding complex required for DNA double strand break repair and V(D)J recombination. Ku70(-/-) (K70T) mice, like recombination activating gene (RAG)-1- or RAG-2-deficient (R1T or R2T) mice, have impaired B and T cell development at an early progenitor stage, which is thought to result at least in part from defective V(D)J recombination (Gu, Y., K.J. Seidl, G.A. Rathbun, C. Zhu, J.P. Manis, N. van der Stoep, L. Davidson, H.L. Cheng, J.M. Sekiguchi, K. Frank, et al. 1997. Immunity. 7:653-665; Ouyang, H., A. Nussenzweig, A. Kurimasa, V.C. Soares, X. Li, C. Cordon-Cardo, W. Li, N. Cheong, M. Nussenzweig, G. Iliakis, et al. 1997. J. Exp. Med. 186:921-929). Therefore, to examine the potential role of Ku70 in CSR, we generated K70T mice that carry a germline Ig HC locus in which the JH region was replaced with a functionally rearranged VH(D)JH and Ig lambda light chain transgene (referred to as K70T/HL mice). Previously, we have shown that B cells from R1T or R2T mice carrying these rearranged Ig genes (R1T/HL or R2T/HL mice) can undergo CSR to IgG isotypes (Lansford, R., J. Manis, E. Sonoda, K. Rajewsky, and F. Alt. 1998. Int. Immunol. 10:325-332). K70T/HL mice had significant numbers of peripheral surface IgM+ B cells, which generated serum IgM levels similar to those of R2T/HL mice. However, in contrast to R2T/HL mice, K70T/HL mice had no detectable serum IgG isotypes. In vitro culture of K70T/HL B cells with agents that induce CSR in normal or R2T/HL B cells did lead to the induction of germline CH transcripts, indicating that initial signaling pathways for CSR were intact in K70T/HL cells. However, treatment with such agents did not lead to detectable CSR by K70T/HL B cells, and instead, led to cell death within 72 h. We conclude that Ku70 is required for the generation of B cells that have undergone Ig HC class switching. Potential roles for Ku70 in the CSR process are discussed.

Hypermutation of immunoglobulin genes in memory B cells of DNA repair-deficient mice

To investigate the possible involvement of DNA repair in the process of somatic hypermutation of rearranged immunoglobulin variable (V) region genes, we have analyzed the occurrence, frequency, distribution, and pattern of mutations in rearranged Vlambda1 light chain genes from naive and memory B cells in DNA repair-deficient mutant mouse strains. Hypermutation was found unaffected in mice carrying mutations in either of the following DNA repair genes: xeroderma pigmentosum complementation group (XP)A and XPD, Cockayne syndrome complementation group B (CSB), mutS homologue 2 (MSH2), radiation sensitivity 54 (RAD54), poly (ADP-ribose) polymerase (PARP), and 3-alkyladenine DNA-glycosylase (AAG). These results indicate that both subpathways of nucleotide excision repair, global genome repair, and transcription-coupled repair are not required for somatic hypermutation. This appears also to be true for mismatch repair, RAD54-dependent double-strand-break repair, and AAG-mediated base excision repair.

Transgenic assays for mutations and cancer: current status and future perspectives

Transgenic mouse modelling has proved to be a powerful approach to explore the various steps involved in spontaneous and induced carcinogenesis. Some of the multitude of models currently available have the potential to become a substitute for the expensive, long-term rodent bioassay to predict carcinogenicity of environmental compounds. Here, we review the progress in the development and use of transgenic mouse models specifically for the purpose of carcinogenicity and mutagenicity testing.

Catalysis of homologous DNA pairing by yeast Rad51 and Rad54 proteins

The Saccharomyces cerevisiae RAD51 and RAD54 genes are both required for the occurrence of homologous recombination and for the repair of double-stranded DNA breaks. Previous studies have indicated that Rad51 protein, together with the single-stranded DNA-binding factor replication protein A (RPA), can promote the formation of heteroduplex DNA, which is a key intermediate in homologous recombination. Here we report the purification of the Rad54 protein to near homogeneity and the biochemical testing of its molecular function. We find that Rad54 protein possesses a double-stranded DNA-dependent ATPase activity, and that it interacts with the Rad51 protein. Addition of Rad54 protein to reactions containing Rad51 strongly stimulates the rate of pairing between homologous single-stranded and double-stranded DNA molecules. We conclude that Rad54 acts to overcome kinetic impediments that would limit homologous DNA pairing between recombining chromosomes in vivo.

XRCC2 and XRCC3, new human Rad51-family members, promote chromosome stability and protect against DNA cross-links and other damages

The phenotypically similar hamster mutants irs1 and irs1SF exhibit high spontaneous chromosome instability and broad-spectrum mutagen sensitivity, including extreme sensitivity to DNA cross-linking agents. The human XRCC2 and XRCC3 genes, which functionally complement irs1 and irs1SF, respectively, were previously mapped in somatic cell hybrids. Characterization of these genes and sequence alignments reveal that XRCC2 and XRCC3 are members of an emerging family of Rad51-related proteins that likely participate in homologous recombination to maintain chromosome stability and repair DNA damage. XRCC3 is shown to interact directly with HsRad51, and like Rad55 and Rad57 in yeast, may cooperate with HsRad51 during recombinational repair. Analysis of the XRCC2 mutation in irs1 implies that XRCC2's function is not essential for viability in cultured hamster cells.

Somatic diversification of IgH genes in rabbit

Rabbits have helped elucidate one of the major immunologic puzzles, namely the genetic control of antibody diversity. The primary IgH antibody repertoire in rabbits is dominated by B cells that use the same germline VH-gene segment in VDJ gene rearrangements. The VDJ genes of essentially all B lymphocytes undergo somatic diversification within the first few weeks of the rabbit's life. Such diversification occurs both by a somatic gene conversion-like mechanism as well as by somatic hyperpointmutation. The diversification that occurs early in ontogeny takes place in gut-associated lymphoid tissues and potentially depends on external factors such as microbial antigens. Few, if any, new B lymphocytes develop in adult rabbits and we discuss how the antibody repertoire is maintained throughout life. Finally, we discuss the molecular mechanism of somatic gene conversion of Ig genes, including the possibility that this involves the use of RAD51, an enzyme required for gene conversion-mediated mating type switch in yeast.

Rad51-deficient vertebrate cells accumulate chromosomal breaks prior to cell death

Yeast rad51 mutants are viable, but extremely sensitive to gamma-rays due to defective repair of double-strand breaks. In contrast, disruption of the murine RAD51 homologue is lethal, indicating an essential role of Rad51 in vertebrate cells. We generated clones of the chicken B lymphocyte line DT40 carrying a human RAD51 transgene under the control of a repressible promoter and subsequently disrupted the endogenous RAD51 loci. Upon inhibition of the RAD51 transgene, Rad51- cells accumulated in the G2/M phase of the cell cycle before dying. Chromosome analysis revealed that most metaphase-arrested Rad51- cells carried isochromatid-type breaks. In conclusion, Rad51 fulfils an essential role in the repair of spontaneously occurring chromosome breaks in proliferating cells of higher eukaryotes.

Interaction of human recombination proteins Rad51 and Rad54

The cDNA for human protein HsRad54, which is a structural homolog of Saccharomyces cerevisiae recombination/repair protein Rad54, was cloned and expressed in Escherichia coli. As demonstrated by analysis in vitro and in vivo, HsRad54 protein interacts with human Rad51 recombinase. The interaction is mediated by the N-terminal domain of HsRad54 protein, which interacts with both free and DNA-bound HsRad51 protein.

Mammalian Rad51 protein: a RecA homologue with pleiotropic functions

During the last years, homologues of E coli RecA have been cloned in numerous species including man. These Rad51 proteins share sequence as well as functional homologies with the bacterial protein. Human Rad51 (HsRad51) is able to catalyze strand exchange in vitro between homologous DNAs, but with a lower efficiency compared to that of RecA. This suggests the requirement of additional factors. A very interesting feature of Rad51 is its essential role in mouse which could mean that it has gained an essential function in cell growth. The interaction of HsRad51 with several tumor suppressor genes namely p53, BRCA1 and BRCA2 implies possible role(s) of this protein in tumorigenesis. Thus, the continued study of Rad51 should bring important insights not only into homologous recombination mechanisms but also into cell proliferation regulation.

The X-linked immunodeficiency defect in the mouse is corrected by expression of human Bruton's tyrosine kinase from a yeast artificial chromosome transgene

Mutations in the gene for Bruton's tyrosine kinase result in the B cell differentiation defects X-linked agammaglobulinemia in man and X-linked immunodeficiency in mice. Here we describe the generation of two yeast artificial chromosome (YAC)-transgenic mouse strains in which high-level expression of human Btk is provided by endogenous regulatory cis-acting elements that are present on a 340-kb transgene, Yc340-hBtk. The expression pattern of the transgenic human Btk was found to parallel that of the endogenous murine gene. When the Yc340-hBtk-transgenic mice were mated onto a Btk-deficient background, the xid B cell defects were fully corrected: conventional and CD5+ B-1 B cells were present in normal numbers, serum IgM and IgG3 levels as well as responses to T cell-independent type II antigens were in the normal ranges. In vivo competition experiments in Btk+/- female mice demonstrated that in the conventional B cell population the Yc340-hBtk transgene could fully compensate the absence of expression of endogenous murine Btk. We conclude that in the YAC-transgenic mice Btk is appropriately expressed in the context of native regulatory sequences.

DNA repair: RAD alert

Mammalian homologues of two important yeast genes involved in DNA double-strand break repair and recombination, RAD51 and RAD54, have been isolated. Knock-out mutations of the genes in mice reveal both reassuring similarities to, and surprising differences from, the analogous mutant phenotypes in yeast.

Conditional gene targeted deletion by Cre recombinase demonstrates the requirement for the double-strand break repair Mre11 protein in murine embryonic stem cells

Repair of DNA damage resulting in double-strand breaks (DSBs) is controlled by gene products executing homologous recombination or end-joining pathways. The MRE11 gene has previously been implicated in DSB repair in the yeast Saccharomyces cerevisiae . Here we have developed a methodology to study the roles of the murine Mre11 homolog in pluripotent embryonic stem cells. Using a gene targeting approach, a triple LoxP site cassette was inserted into a region of MRE11 genomic DNA flanking conserved phosphodiesterase motifs. The addition of Cre recombinase activity promotes deletions of three types that can be scored. We find that deletion at phosphodiesterase motif III encoded in the N-terminus of Mre11 is acheived in the presence of a wild-type MRE11 allele. However, when the wild-type MRE11 allele is inactivated by gene targeted insertion of a neo marker, only Cre recombination events that allow expression of wild-type Mre11 protein are observed. Therefore, Mre11 is required for normal cell proliferation. This methodology introduces a means to study important regions of essential genes in cell culture models.

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

rad54 mutants of the yeast Saccharomyces cerevisiae are extremely X-ray sensitive and have decreased mitotic recombination frequencies because of a defect in double-strand break repair. A RAD54 homolog was disrupted in the chicken B cell line DT40, which undergoes immunoglobulin gene conversion and exhibits unusually high ratios of targeted to random integration after DNA transfection. Homozygous RAD54-/- mutant clones were highly X-ray sensitive compared to wildtype cells. The rate of immunoglobulin gene conversion was 6- to 8-fold reduced, and the frequency of targeted integration was at least two orders of magnitude decreased in the mutant clones. Reexpression of the RAD54 cDNA restored radiation resistance and targeted integration activity. The reported phenotype provides the first genetic evidence of a link between double-strand break repair and homologous recombination in vertebrate cells.