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.     

BCR/ABL Recruits p53 Tumor Suppressor Protein to Induce Drug Resistance

Tumors expressing the ABL oncoproteins (BCR/ABL, TEL/ABL, v-ABL) can avoidapoptosis triggered by DNA damaging agents. The tumor suppressor protein p53 is animportant activator of apoptosis in normal cells; conversely its functional loss may causedrug resistance. The ABL oncoprotein - p53 paradigm represents the relationship between anoncogenic tyrosine kinase and a tumor suppressor gene. Here we show that BCR/ABLoncoproteins employ p53 to induce resistance to DNA damage in myeloid leukemia cells.Cells transformed by the ABL oncoproteins displayed accumulation of p53 upon DNAdamage. In contrast, only a modest increase of p53 expression followed by activation ofcaspase-3 were detected in normal cells expressing endogenous c-ABL. Phosphatidylinositol-3 kinase-like protein kinases (ATR and also ATM) -dependent phosphorylation of p53-Ser15residue was associated with the accumulation of p53, and stimulation of p21Waf-1 andGADD45, resulting in G2/M delay in BCR/ABL cells after genotoxic treatment. Inhibition ofp53 by siRNA or by the temperature-sensitive mutation reduced G2/M accumulation anddrug resistance of BCR/ABL cells. In conclusion, accumulation of the p53 proteincontributed to prolonged G2/M checkpoint activation and drug resistance in myeloid cellsexpressing the BCR/ABL oncoproteins.     

Shu proteins promote the formation of homologous recombination intermediates that are processed by Sgs1-Rmi1-Top3

CSM2, PSY3, SHU1, and SHU2 (collectively referred to as the SHU genes) were identified in Saccharomyces cerevisiae as four genes in the same epistasis group that suppress various sgs1 and top3 mutant phenotypes when mutated. Although the SHU genes have been implicated in homologous recombination repair (HRR), their precise role(s) within this pathway remains poorly understood. Here, we have identified a specific role for the Shu proteins in a Rad51/Rad54-dependent HRR pathway(s) to repair MMS-induced lesions during S-phase. We show that, although mutation of RAD51 or RAD54 prevented the formation of MMS-induced HRR intermediates (X-molecules) arising during replication in sgs1 cells, mutation of SHU genes attenuated the level of these structures. Similar findings were also observed in shu1 cells in which Rmi1 or Top3 function was impaired. We propose a model in which the Shu proteins act in HRR to promote the formation of HRR intermediates that are processed by the Sgs1-Rmi1-Top3 complex.     

Phosphatidylinositol 3-kinase p85{alpha} subunit-dependent interaction with BCR/ABL-related fusion tyrosine kinases: molecular mechanisms and biological consequences

The p85alpha subunit of phosphatidylinositol 3-kinase (PI-3k) forms a complex with a protein network associated with oncogenic fusion tyrosine kinases (FTKs) such as BCR/ABL, TEL/ABL, TEL/JAK2, TEL/PDGFbetaR, and NPM/ALK, resulting in constitutive activation of the p110 catalytic subunit of PI-3k. Introduction of point mutations in the N-terminal and C-terminal SH2 domain and SH3 domain of p85alpha, which disrupt their ability to bind phosphotyrosine and proline-rich motifs, respectively, abrogated their interaction with the BCR/ABL protein network. The p85alpha mutant protein (p85mut) bearing these mutations was unable to interact with BCR/ABL and other FTKs, while its binding to the p110alpha catalytic subunit of PI-3k was intact. In addition, binding of Shc, c-Cbl, and Gab2, but not Crk-L, to p85mut was abrogated. p85mut diminished BCR/ABL-dependent activation of PI-3k and Akt kinase, the downstream effector of PI-3k. This effect was associated with the inhibition of BCR/ABL-dependent growth of the hematopoietic cell line and murine bone marrow cells. Interestingly, the addition of interleukin-3 (IL-3) rescued BCR/ABL-transformed cells from the inhibitory effect of p85mut. SCID mice injected with BCR/ABL-positive hematopoietic cells expressing p85mut survived longer than the animals inoculated with BCR/ABL-transformed counterparts. In conclusion, we have identified the domains of p85alpha responsible for the interaction with the FTK protein network and transduction of leukemogenic signaling.     

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.     

Caffeine could not efficiently sensitize homologous recombination repair-deficient cells to ionizing radiation-induced killing

Caffeine inhibits ATM and ATR, two important checkpoint regulators, abolishes ionizing radiation-induced checkpoint response, and radiosensitizes cells. Radiation-induced DNA double-strand breaks (DSBs) are repaired by two major processes, homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). It remains unclear which repair process, HRR or NHEJ, is affected when the checkpoint responses are abolished by caffeine. In this study we observed the effect of caffeine on gene-targeted DT40 chicken lymphoblast cells. We show that caffeine efficiently abolishes S- and G(2)-phase checkpoint responses after irradiation in all cell lines tested and greatly radiosensitizes wild-type and ATM(-/-) cells, the partially checkpoint-deficient cells. However, caffeine has a much smaller radiosensitizing effect on RAD54(-/-) cells and has no effect on RAD51-deficient cells. RAD51 and RAD54 are the important factors for HRR. Our results indicate that the checkpoint responses abolished by caffeine (S and G(2)) mainly affect HRR, which results in cell radiosensitization.     

Inhibition of ERβ induces resistance to cisplatin by enhancing Rad51-mediated DNA repair in human medulloblastoma cell lines

Cisplatin is one of the most widely used and effective anticancer drugs against solid tumors including cerebellar tumor of the childhood, Medulloblastoma. However, cancer cells often develop resistance to cisplatin, which limits therapeutic effectiveness of this otherwise effective genotoxic drug. In this study, we demonstrate that human medulloblastoma cell lines develop acute resistance to cisplatin in the presence of estrogen receptor (ER) antagonist, ICI182,780. This unexpected finding involves a switch from the G2/M to G1 checkpoint accompanied by decrease in ATM/Chk2 and increase in ATR/Chk1 phosphorylation. We have previously reported that ERβ, which is highly expressed in medulloblastomas, translocates insulin receptor substrate 1 (IRS-1) to the nucleus, and that nuclear IRS-1 binds to Rad51 and attenuates homologous recombination directed DNA repair (HRR). Here, we demonstrate that in the presence of ICI182,780, cisplatin-treated medulloblastoma cells show recruitment of Rad51 to the sites of damaged DNA and increase in HRR activity. This enhanced DNA repair during the S phase preserved also clonogenic potential of medulloblastoma cells treated with cisplatin. In conclusion, inhibition of ERβ considered as a supplemental anticancer therapy, has been found to interfere with cisplatin-induced cytotoxicity in human medulloblastoma cell lines.     

The RAD51 family member, RAD51L3, is a DNA-stimulated ATPase that forms a complex with XRCC2

The Rad51 protein in eukaryotic cells is a structural and functional homolog of Escherichia coli RecA with a role in DNA repair and genetic recombination. Several proteins showing sequence similarity to Rad51 have previously been identified in both yeast and human cells. In Saccharomyces cerevisiae, two of these proteins, Rad55p and Rad57p, form a heterodimer that can stimulate Rad51-mediated DNA strand exchange. Here, we report the purification of one of the representatives of the RAD51 family in human cells. We demonstrate that the purified RAD51L3 protein possesses single-stranded DNA binding activity and DNA-stimulated ATPase activity, consistent with the presence of "Walker box" motifs in the deduced RAD51L3 sequence. We have identified a protein complex in human cells containing RAD51L3 and a second RAD51 family member, XRCC2. By using purified proteins, we demonstrate that the interaction between RAD51L3 and XRCC2 is direct. Given the requirements for XRCC2 in genetic recombination and protection against DNA-damaging agents, we suggest that the complex of RAD51L3 and XRCC2 is likely to be important for these functions in human cells.     

RAD51B plays an essential role during somatic and meiotic recombination in Physcomitrella

The eukaryotic RecA homologue Rad51 is a key factor in homologous recombination and recombinational repair. Rad51-like proteins have been identified in yeast (Rad55, Rad57 and Dmc1), plants and vertebrates (RAD51B, RAD51C, RAD51D, XRCC2, XRCC3 and DMC1). RAD51 and DMC1 are the strand-exchange proteins forming a nucleofilament for strand invasion, however, the function of the paralogues in the process of homologous recombination is less clear. In yeast the two Rad51 paralogues, Rad55 and Rad57, have been shown to be involved in somatic and meiotic HR and they are essential to the formation of the Rad51/DNA nucleofilament counterbalancing the anti-recombinase activity of the SRS2 helicase. Here, we examined the role of RAD51B in the model bryophyte Physcomitrella patens. Mutant analysis shows that RAD51B is essential for the maintenance of genome integrity, for resistance to DNA damaging agents and for gene targeting. Furthermore, we set up methods to investigate meiosis in Physcomitrella and we demonstrate that the RAD51B protein is essential for meiotic homologous recombination. Finally, we show that all these functions are independent of the SRS2 anti-recombinase protein, which is in striking contrast to what is found in budding yeast where the RAD51 paralogues are fully dependent on the SRS2 anti-recombinase function.     

Repair of radiation induced DNA double strand breaks by backup NHEJ is enhanced in G2

In higher eukaryotes DNA double strand breaks (DSBs) are repaired by homologous recombination (HRR) or non-homologous end joining (NHEJ). In addition to the DNA-PK dependent pathway of NHEJ (D-NHEJ), cells employ a backup pathway (B-NHEJ) utilizing Ligase III and PARP-1. The cell cycle dependence and coordination of these pathways is being actively investigated. We examine DSB repair in unperturbed G1 and G2 phase cells using mouse embryo fibroblast (MEF) mutants defective in D-NHEJ and/or HRR. WT and Rad54(-/-) MEFs repair DSBs with similar efficiency in G1 and G2 phase. LIG4(-/-), DNA-PKcs(-/-), and Ku70(-/-) MEFs show more pronounced repair defects in G1 than in G2. LIG4(-/-)/Rad54(-/-) MEFs repair DSBs as efficiently as LIG4(-/-) MEFs suggesting that the increased repair efficiency in G2 relies on enhanced function of B-NHEJ rather than HRR. In vivo and in vitro plasmid end joining assays confirm an enhanced function of B-NHEJ in G2. The results show a new and potentially important cell cycle regulation of B-NHEJ and generate a framework to investigate the mechanistic basis of HRR contribution to DSB repair.     

Telomere maintenance requires the RAD51D recombination/repair protein

The five RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3) are required in mammalian cells for normal levels of genetic recombination and resistance to DNA-damaging agents. We report here that RAD51D is also involved in telomere maintenance. Using immunofluorescence labeling, electron microscopy, and chromatin immunoprecipitation assays, RAD51D was shown to localize to the telomeres of both meiotic and somatic cells. Telomerase-positive Rad51d(-/-) Trp53(-/-) primary mouse embryonic fibroblasts (MEFs) exhibited telomeric DNA repeat shortening compared to Trp53(-/-) or wild-type MEFs. Moreover, elevated levels of chromosomal aberrations were detected, including telomeric end-to-end fusions, a signature of telomere dysfunction. Inhibition of RAD51D synthesis in telomerase-negative immortalized human cells by siRNA also resulted in telomere erosion and chromosome fusion. We conclude that RAD51D plays a dual cellular role in both the repair of DNA double-strand breaks and telomere protection against attrition and fusion.     

Functional conservation of the yeast and Arabidopsis RAD54-like genes

The Saccharomyces cerevisiae RAD54 gene has critical roles in DNA double-strand break repair, homologous recombination, and gene targeting. Previous results show that the yeast gene enhances gene targeting when expressed in Arabidopsis thaliana. In this work we address the trans-species compatibility of Rad54 functions. We show that overexpression of yeast RAD54 in Arabidopsis enhances DNA damage resistance severalfold. Thus, the yeast gene is active in the Arabidopsis homologous-recombination repair system. Moreover, we have identified an A. thaliana ortholog of yeast RAD54, named AtRAD54. This gene, with close sequence similarity to RAD54, complements methylmethane sulfonate (MMS) sensitivity but not UV sensitivity or gene targeting defects of rad54Delta mutant yeast cells. Overexpression of AtRAD54 in Arabidopsis leads to enhanced resistance to DNA damage. This gene's assignment as a RAD54 ortholog is further supported by the interaction of AtRad54 with AtRad51 and the interactions between alien proteins (i.e., yeast Rad54 with AtRAD51 and yeast Rad51 with AtRad54) in a yeast two-hybrid experiment. These interactions hint at the molecular nature of this interkingdom complementation, although the stronger effect of the yeast Rad54 in plants than AtRad54 in yeast might be explained by an ability of the Rad54 protein to act alone, independently of its interaction with Rad51.     

The role of RAD6 in recombinational repair,checkpoints and meiosis via histone modification

The Rad6 ubiquitin-conjugating enzyme in Saccharomyces cerevisiae is known to interact with three separate ubiquitin ligase proteins (Ubr1, Rad18, and Bre1) specific to different targets. The Rad6/Rad18 complex is central to translesion synthesis and the family of DNA transactions known as post-replication repair (PRR). A less well-known aspect of Rad6-mediated DNA repair, however, involves its function with Bre1 in mono-ubiquitinating the histone H2B residue lysine 123. Here, we review how this ubiquitination impacts histone H3 methylation, and how this in turn impacts the DNA damage response. In S. cerevisiae this pathway is required for checkpoint activation in G1, and contributes to DNA repair via the homologous recombination pathway (HRR) in G2 cells. Thus, RAD6 clearly plays a role in HRR in addition to its central role in PRR. We also summarize what is known about related repair pathways in other eukaryotes, including mammals. Recent literature emphasizes the role of methylated histones in S. cerevisiae, Schizosaccharomyces pombe and mammals in attracting the related DNA damage checkpoint proteins Rad9, Crb2 and 53BP1, respectively, to chromatin at the sites of DNA double-strand breaks. However, the specific histone modification pathways involved diverge in these different eukaryotes.     

RAD51-dependent break-induced replication differs in kinetics and checkpoint responses from RAD51-mediated gene conversion

Diploid Saccharomyces cells experiencing a double-strand break (DSB) on one homologous chromosome repair the break by RAD51-mediated gene conversion >98% of the time. However, when extensive homologous sequences are restricted to one side of the DSB, repair can occur by both RAD51-dependent and RAD51-independent break-induced replication (BIR) mechanisms. Here we characterize the kinetics and checkpoint dependence of RAD51-dependent BIR when the DSB is created within a chromosome. Gene conversion products appear within 2 h, and there is little, if any, induction of the DNA damage checkpoint; however, RAD51-dependent BIR occurs with a further delay of 2 to 4 h and cells arrest in response to the G(2)/M DNA damage checkpoint. RAD51-dependent BIR does not require special facilitating sequences that are required for a less efficient RAD51-independent process. RAD51-dependent BIR occurs efficiently in G(2)-arrested cells. Once repair is initiated, the rate of repair replication during BIR is comparable to that of normal DNA replication, as copying of >100 kb is completed less than 30 min after repair DNA synthesis is detected close to the DSB.     

Homologous Recombination, but Not DNA Repair, Is Reduced in Vertebrate Cells Deficient in RAD52

Rad52 plays a pivotal role in double-strand break (DSB) repair and genetic recombination in Saccharomyces cerevisiae, where mutation of this gene leads to extreme X-ray sensitivity and defective recombination. Yeast Rad51 and Rad52 interact, as do their human homologues, which stimulates Rad51-mediated DNA strand exchange in vitro, suggesting that Rad51 and Rad52 act cooperatively. To define the role of Rad52 in vertebrates, we generated RAD52^−/− mutants of the chicken B-cell line DT40. Surprisingly, RAD52^−/− cells were not hypersensitive to DNA damages induced by γ-irradiation, methyl methanesulfonate, or cis-platinum(II)diammine dichloride (cisplatin). Intrachromosomal recombination, measured by immunoglobulin gene conversion, and radiation-induced Rad51 nuclear focus formation, which is a putative intermediate step during recombinational repair, occurred as frequently in RAD52^−/− cells as in wild-type cells. Targeted integration frequencies, however, were consistently reduced in RAD52^−/− cells, showing a clear role for Rad52 in genetic recombination. These findings reveal striking differences between S. cerevisiae and vertebrates in the functions of RAD51 and RAD52.     

Recovery of deficient homologous recombination in Brca2-depleted mouse cells by wild-type Rad51 expression

The BRCA2 tumor suppressor is important in maintaining genomic stability. BRCA2 is proposed to control the availability, cellular localization and DNA binding activity of the central homologous recombination protein, RAD51, with loss of BRCA2 resulting in defective homologous recombination. Nevertheless, the roles of BRCA2 in regulating RAD51 and how other proteins implicated in RAD51 regulation, such as RAD52 and RAD54 function relative to BRCA2 is not known. In this study, we tested whether defective homologous recombination in Brca2-depleted mouse hybridoma cells could be rectified by expression of mouse Rad51 or the Rad51-interacting mouse proteins, Rad52 and Rad54. In the Brca2-depleted cells, defective homologous recombination can be restored by over-expression of wild-type mouse Rad51, but not mouse Rad52 or Rad54. Correction of the homologous recombination defect requires Rad51 ATPase activity. A sizeable fraction ( approximately 50%) of over-expressed wild-type Rad51 is nuclear localized. The restoration of homologous recombination in the presence of a low (i.e., non-functional) level of Brca2 by wild-type Rad51 over-expression is unexpected. We suggest that Rad51 may access the nuclear compartment in a Brca2-independent manner and when Rad51 is over-expressed, the normal requirement for Brca2 control over Rad51 function in homologous recombination is dispensable. Our studies support loss of Rad51 function as a critical underlying factor in the homologous recombination defect in the Brca2-depleted cells.