Targeted disruption of the Rad51 gene leads to lethality in embryonic mice

Halenaquinone, a chemical compound that specifically inhibits the secondary DNA binding of RAD51

Mutations and single-nucleotide polymorphisms affecting RAD51 gene function have been identified in several tumors, suggesting that the inappropriate expression of RAD51 activity may cause tumorigenesis. RAD51 is an essential enzyme for the homologous recombinational repair (HRR) of DNA double-strand breaks. In the HRR pathway, RAD51 catalyzes the homologous pairing between single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), which is the central step of the HRR pathway. To identify a chemical compound that regulates the homologous-pairing activity of RAD51, in the present study, we screened crude extract fractions from marine sponges by the RAD51-mediated homologous-pairing assay. Halenaquinone was identified as an inhibitor of the RAD51 homologous-pairing activity. A surface plasmon resonance analysis indicated that halenaquinone directly bound to RAD51. Intriguingly, halenaquinone specifically inhibited dsDNA binding by RAD51 alone or the RAD51-ssDNA complex, but only weakly affected the RAD51-ssDNA binding. In vivo, halenaquinone significantly inhibited the retention of RAD51 at double-strand break sites. Therefore, halenaquinone is a novel type of RAD51 inhibitor that specifically inhibits the RAD51-dsDNA binding.

RAD51 and XRCC3 polymorphisms: impact on the risk and treatment outcomes of de novo inv(16) or t(16;16)/CBFβ-MYH11(+) acute myeloid leukemia

DNA double-strand break repair via homologous recombination (HR) is essential in maintaining genetic integrity, and may modulate susceptibility to the development of acute myeloid leukemia (AML) and influence outcomes of AML. This study was designed to evaluate the effects of polymorphisms in HR repair genes RAD51 and XRCC3 on the risk and treatment outcomes of inv(16)/t(16;16)/CBFβ-MYH11(+) AML. The distribution of polymorphisms in RAD51-G135C and XRCC3-Thr241Met were studied by PCR-RFLP analysis in 625 cases of de novo AML, including 105 cases with inv(16)/t(16;16)/CBFβ-MYH11, 806 family controls and 704 volunteer controls. It was found that the XRCC3-241Met variant significantly increased the risk of the development of the AML with inv(16)/t(16;16) as compared with both the volunteer control (OR=7.22; 95% CI, 4.37-11.91) and the family control (OR=7.99; 95% CI, 5.03-12.69). A retrospective study conducted in 103 inv(16)/t(16;16) AML patients. In multivariate analysis for the potential prognostic factors, the XRCC3-241Met variant significantly reduced disease-free survival (DFS) in complete remission (CR) achieved patients (HR=2.34, 95% CI, 1.32-4.16). These data indicate that the XRCC3-241Met variant may not be only a susceptibility factor to the AML with inv(16)/t(16;16), but also an independent poor-prognostic factor for this AML subtype.

Homologous recombination proteins are associated with centrosomes and are required for mitotic stability

In response to DNA damage, cells need robust repair mechanisms to complete the cell cycle successfully. Severe forms of DNA damage are repaired by homologous recombination (HR), in which the XRCC2 protein plays a vital role. Cells deficient in XRCC2 also show disruption of the centrosome, a key component of the mitotic apparatus. We find that this centrosome disruption is dynamic and when it occurs during mitosis it is linked directly to the onset of mitotic catastrophe in a significant fraction of the XRCC2-deficient cells. However, we also show for the first time that XRCC2 and other HR proteins, including the key recombinase RAD51, co-localize with the centrosome. Co-localization is maintained throughout the cell cycle, except when cells are finishing mitosis when RAD51 accumulates in the midbody between the separating cells. Taken together, these data suggest a tight functional linkage between the centrosome and HR proteins, potentially to coordinate the deployment of a DNA damage response at vulnerable phases of the cell cycle.

Cell cycle adaptations and maintenance of genomic integrity in embryonic stem cells and induced pluripotent stem cells

Pluripotent stem cells have the capability to undergo unlimited self-renewal and differentiation into all somatic cell types. They have acquired specific adjustments in the cell cycle structure that allow them to rapidly proliferate, including cell cycle independent expression of cell cycle regulators and lax G(1) to S phase transition. However, due to the developmental role of embryonic stem cells (ES) it is essential to maintain genomic integrity and prevent acquisition of mutations that would be transmitted to multiple cell lineages. Several modifications in DNA damage response of ES cells accommodate dynamic cycling and preservation of genetic information. The absence of a G(1)/S cell cycle arrest promotes apoptotic response of damaged cells before DNA changes can be fixed in the form of mutation during the S phase, while G(2)/M cell cycle arrest allows repair of damaged DNA following replication. Furthermore, ES cells express higher level of DNA repair proteins, and exhibit enhanced repair of multiple types of DNA damage. Similarly to ES cells, induced pluripotent stem (iPS) cells are poised to proliferate and exhibit lack of G(1)/S cell cycle arrest, extreme sensitivity to DNA damage, and high level of expression of DNA repair genes. The fundamental mechanisms by which the cell cycle regulates genomic integrity in ES cells and iPS cells are similar, though not identical.

Modulation of Rad51, ERCC1, and thymidine phosphorylase by emodin result in synergistic cytotoxic effect in combination with capecitabine

Thymidine phosphorylase (TP) is the rate-limiting enzyme for the activation of capecitabine (pro-drug of fluorouracil), and as a useful predictor of tumor response to capecitabine-based chemotherapy. Overexpression of Rad51 and ERCC1 induce resistance to chemotherapeutic agents. Emodin, one of the main bioactive anthraquinone derivatives in the roots and rhizomes of numerous plants, possesses potent antitumor effects. Accordingly, we aimed to explore the molecular mechanism of emodin enhances the capecitabine-induced cytotoxicity through controlling Rad51, ERCC1, and TP expression in human non-small cell lung cancer (NSCLC). The results show that capecitabine increases the phosphorylation of MKK1/2-ERK1/2 and protein levels of Rad51 and ERCC1 through enhancing the protein stability. Depletion of endogenous Rad51 or ERCC1 expression by specific small interfering RNA transfection significantly increases capecitabine-induced cell death and growth inhibition. Emodin enhances the capecitabine-induced cytotoxic effects through ERK1/2 inactivation and decreasing the Rad51 and ERCC1 protein levels induced by capecitabine. Enhancement of ERK1/2 signaling by constitutively active MKK1/2 (MKK1/2-CA) results in increasing Rad51 and ERCC1 protein levels and cell viability in NSCLC cell lines treated with emodin and capecitabine. Interestingly, emodin enhances TP mRNA and protein expression in capecitabine treated NSCLC cell lines, and depletion of the TP expression decreases the cytotoxic effects induced by capecitabine and emodin. We conclude that enhancing the cytotoxicity to capecitabine by emodin is mediated by down-regulation the expression of Rad51 and ERCC1 and up-regulation TP expression.

Rad52 inactivation is synthetically lethal with BRCA2 deficiency

Synthetic lethality is a powerful approach to study selective cell killing based on genotype. We show that loss of Rad52 function is synthetically lethal with breast cancer 2, early onset (BRCA2) deficiency, whereas there was no impact on cell growth and viability in BRCA2-complemented cells. The frequency of both spontaneous and double-strand break-induced homologous recombination and ionizing radiation-induced Rad51 foci decreased by 2-10 times when Rad52 was depleted in BRCA2-deficient cells, with little to no effect in BRCA2-complemented cells. The absence of both Rad52 and BRCA2 resulted in extensive chromosome aberrations, especially chromatid-type aberrations. Ionizing radiation-induced and S phase-associated Rad52-Rad51 foci form equally well in the presence or absence of BRCA2, indicating that Rad52 can respond to DNA double-strand breaks and replication stalling independently of BRCA2. Rad52 thus is an independent and alternative repair pathway of homologous recombination and a target for therapy in BRCA2-deficient cells.

Rad51 protects nascent DNA from Mre11-dependent degradation and promotes continuous DNA synthesis

The role of Rad51 in an unperturbed cell cycle has been difficult to dissect from its DNA repair function. Here, using electron microscopy (EM) to visualize replication intermediates (RIs) assembled in Xenopus laevis egg extract we show that Rad51 is required to prevent the accumulation of ssDNA gaps at replication forks and behind them. ssDNA gaps at forks arise from extended uncoupling of leading and lagging strand DNA synthesis. Instead, ssDNA gaps behind forks, which are exacerbated on damaged templates, result from Mre11 dependent degradation of newly synthesized DNA strands as they can be suppressed by inhibition of Mre11 nuclease activity. These findings reveal direct and unanticipated roles for Rad51 at replication forks demonstrating that Rad51 protects newly synthesised DNA from Mre11 dependent degradation and promotes continuous DNA synthesis.

Stereoselective synthesis of chiral IBR2 analogues

Two stereoselective routes were developed to synthesize optically pure IBR2 analogues 1-16. The first features addition of N-Boc-3-bromoindole 26 to the sulfinamide 25, providing a 1:1 ratio of the separable diasteroisomers 27 and 28 in good yield. In a straightforward fashion, the sulfinamides 27 and 28 were conveniently converted into the key amines 39 and 47 over 8 steps, respectively, from which a series of 3,4-dihydroisoquinolinyl IBR2 analogues 1-14 containing fluorinated and trifluoromethylated benzyl groups were prepared. Another route highlights the highly enantioselective addition of indole to the sulfonyl amide 50 with bifunctional aminothioureas 57 and 58 as catalysts. After the reaction conditions were optimized, the desired sulfonyl amides (R)-55 and (S)-55 were obtained in 99% ee and 98% ee, respectively. Acylation of (R)-55 and (S)-55 separately and subsequent allylation gave compounds 60 and 63, respectively, which were further subjected to RCM to furnish compounds 61 and 64 and, after removal of the Boc groups, the desired IBR2 analogues 15 and 16.

Essential roles of Jab1 in cell survival, spontaneous DNA damage and DNA repair

Jun activation domain-binding protein 1 (JAB1) is a multifunctional protein that participates in the control of cell proliferation and the stability of multiple proteins. JAB1 overexpression has been implicated in the pathogenesis of human cancer. JAB1 regulates several key proteins and thereby produces varied effects on cell cycle progression, genome stability and cell survival. However, the biological significance of JAB1 activity in these cellular signaling pathways is unclear. Therefore, we developed mice that were deficient in Jab1 and analyzed the null embryos and heterozygous cells. This disruption of Jab1 in mice resulted in early embryonic lethality due to accelerated apoptosis. Loss of Jab1 expression sensitized both mouse primary embryonic fibroblasts and osteosarcoma cells to γ-radiation-induced apoptosis, with an increase in spontaneous DNA damage and homologous recombination (HR) defects, both of which correlated with reduced levels of the DNA repair protein Rad51 and elevated levels of p53. Furthermore, the accumulated p53 directly binds to Rad51 promoter, inhibits its activity and represents a major mechanism underlying the HR repair defect in Jab1-deficient cells. These results indicate that Jab1 is essential for efficient DNA repair and mechanistically link Jab1 to the maintenance of genome integrity and to cell survival.

Mouse embryonic stem cells, but not somatic cells, predominantly use homologous recombination to repair double-strand DNA breaks

Embryonic stem (ES) cells give rise to all cell types of an organism. Since mutations at this embryonic stage would affect all cells and be detrimental to the overall health of an organism, robust mechanisms must exist to ensure that genomic integrity is maintained. To test this proposition, we compared the capacity of murine ES cells to repair DNA double-strand breaks with that of differentiated cells. Of the 2 major pathways that repair double-strand breaks, error-prone nonhomologous end joining (NHEJ) predominated in mouse embryonic fibroblasts, whereas the high fidelity homologous recombinational repair (HRR) predominated in ES cells. Microhomology-mediated end joining, an emerging repair pathway, persisted at low levels in all cell types examined. The levels of proteins involved in HRR and microhomology-mediated end joining were highly elevated in ES cells compared with mouse embryonic fibroblasts, whereas those for NHEJ were quite variable, with DNA Ligase IV expression low in ES cells. The half-life of DNA Ligase IV protein was also low in ES cells. Attempts to increase the abundance of DNA Ligase IV protein by overexpression or inhibition of its degradation, and thereby elevate NHEJ in ES cells, were unsuccessful. When ES cells were induced to differentiate, however, the level of DNA Ligase IV protein increased, as did the capacity to repair by NHEJ. The data suggest that preferential use of HRR rather than NHEJ may lend ES cells an additional layer of genomic protection and that the limited levels of DNA Ligase IV may account for the low level of NHEJ activity.

Focus on histone variant H2AX: to be or not to be

Phosphorylation of histone variant H2AX at serine 139, named gammaH2AX, has been widely used as a sensitive marker for DNA double-strand breaks (DSBs). gammaH2AX is required for the accumulation of many DNA damage response (DDR) proteins at DSBs. Thus it is believed to be the principal signaling protein involved in DDR and to play an important role in DNA repair. However, only mild defects in DNA damage signaling and DNA repair were observed in H2AX-deficient cells and animals. Such findings prompted us and others to explore H2AX-independent mechanisms in DNA damage response. Here, we will review recent advances in our understanding of H2AX-dependent and independent DNA damage signaling and repair pathways in mammalian cells.

Mitotic homologous recombination maintains genomic stability and suppresses tumorigenesis

Mitotic homologous recombination promotes genome stability through the precise repair of DNA double-strand breaks and other lesions that are encountered during normal cellular metabolism and from exogenous insults. As a result, homologous recombination repair is essential during proliferative stages in development and during somatic cell renewal in adults to protect against cell death and mutagenic outcomes from DNA damage. Mutations in mammalian genes encoding homologous recombination proteins, including BRCA1, BRCA2 and PALB2, are associated with developmental abnormalities and tumorigenesis. Recent advances have provided a clearer understanding of the connections between these proteins and of the key steps of homologous recombination and DNA strand exchange.

Homologous recombination in cancer development, treatment and development of drug resistance

Although DNA double-strand breaks (DSBs) are substrates for homologous recombination (HR) repair, it is becoming apparent that DNA lesions produced at replication forks, for instance by many anticancer drugs, are more significant substrates for HR repair. Cells defective in HR are hypersensitive to a wide variety of anticancer drugs, including those that do not produce DSBs. Several cancers have mutations in or epigenetically silenced HR genes, which explain the genetic instability that drives cancer development. There are an increasing number of reports suggesting that mutation or epigenetic silencing of HR genes explains the sensitivity of cancers to current chemotherapy treatments. Furthermore, there are also many examples of re-expression of HR genes in tumours to explain drug resistance. Emerging data suggest that there are several different subpathways of HR, which can compensate for each other. Unravelling the overlapping pathways in HR showed that BRCA1- and BRCA2-defective cells rely on the PARP protein for survival. This synthetic lethal interaction is now being exploited for selective treatment of BRCA1- and BRCA2-defective cancers with PARP inhibitors. Here, I discuss the diversity of HR and how it impacts on cancer with a particular focus on how HR can be exploited in future anticancer strategies.

Single-molecule imaging brings Rad51 nucleoprotein filaments into focus

The Rad51 protein is essential for DNA repair by homologous recombination. After DNA damage, Rad51 localizes to nuclear foci that represent sites of DNA repair in vivo. In vitro, Rad51 self-assembles on single- or double-stranded DNA to form a nucleoprotein filament. Recently, the merging of innovative single-molecule techniques with ensemble methods has provided unique insights into the dynamic nature of this filament and its cellular function. The assembly and disassembly of Rad51 nucleoprotein filaments is seen to be regulated by recombination accessory proteins. In this regard, the BRC repeats of the BRCA2 protein were shown to modulate the DNA binding selectivity of Rad51. Furthermore, single-molecule studies explained the need for a DNA translocase, Rad54 protein, in the disassembly of Rad51 double-stranded DNA filaments.

Rad54, the motor of homologous recombination

Homologous recombination (HR) performs crucial functions including DNA repair, segregation of homologous chromosomes, propagation of genetic diversity, and maintenance of telomeres. HR is responsible for the repair of DNA double-strand breaks and DNA interstrand cross-links. The process of HR is initiated at the site of DNA breaks and gaps and involves a search for homologous sequences promoted by Rad51 and auxiliary proteins followed by the subsequent invasion of broken DNA ends into the homologous duplex DNA that then serves as a template for repair. The invasion produces a cross-stranded structure, known as the Holliday junction. Here, we describe the properties of Rad54, an important and versatile HR protein that is evolutionarily conserved in eukaryotes. Rad54 is a motor protein that translocates along dsDNA and performs several important functions in HR. The current review focuses on the recently identified Rad54 activities which contribute to the late phase of HR, especially the branch migration of Holliday junctions.

RAD51 loss of function abolishes gene targeting and de-represses illegitimate integration in the moss Physcomitrella patens

Gene targeting (GT) is a major tool for basic and applied research during which the transforming DNA, which shares sequence homology with a chromosomal target, integrates at the corresponding locus by homologous recombination (HR). In eukaryotes, GT recruits enzymes from the HR-mediated double strand break repair pathway. Different mechanisms of HR have been described which depend on the Rad52 epistasis group of genes, but which specific mechanism is used by the cell for GT remains unclear. In Saccharomyces cerevisiae, the RAD52 protein is essential for GT, and the RAD51 protein plays a minor role. In filamentous fungi and animal cells, however, GT depends on RAD51 and is weakly affected by suppression of RAD52. Genetic evidence also indicates that the non-homologous end-joining pathway of DSB repair has a negative impact on GT efficiencies, but how the balance between these two pathways is controlled is poorly understood. Here, we have examined the role of RAD51 in the only plant that exhibits high GT frequencies, the model bryophyte Physcomitrella patens. Our results show that the two RAD51 proteins have partially redundant functions in the maintenance of genome integrity and resistance to ionizing radiation. Furthermore, we demonstrate that loss of function of the two RAD51 proteins completely abolishes GT and strongly increases illegitimate integration rates in this moss. These findings demonstrate for the first time in plant the critical role of RAD51 in controlling the balance between targeted and random integration events observed upon transgenesis, and confirm that P. patens is a particularly interesting tool for studying GT in higher eukaryotes.

MRE11-RAD50-NBS1 complex dictates DNA repair independent of H2AX

DNA double-strand breaks (DSBs) represent one of the most serious forms of DNA damage that can occur in the genome. Here, we show that the DSB-induced signaling cascade and homologous recombination (HR)-mediated DSB repair pathway can be genetically separated. We demonstrate that the MRE11-RAD50-NBS1 (MRN) complex acts to promote DNA end resection and the generation of single-stranded DNA, which is critically important for HR repair. These functions of the MRN complex can occur independently of the H2AX-mediated DNA damage signaling cascade, which promotes stable accumulation of other signaling and repair proteins such as 53BP1 and BRCA1 to sites of DNA damage. Nevertheless, mild defects in HR repair are observed in H2AX-deficient cells, suggesting that the H2AX-dependent DNA damage-signaling cascade assists DNA repair. We propose that the MRN complex is responsible for the initial recognition of DSBs and works together with both CtIP and the H2AX-dependent DNA damage-signaling cascade to facilitate repair by HR and regulate DNA damage checkpoints.

The ups and downs of DNA repair biomarkers for PARP inhibitor therapies

PARP inhibitors are emerging as a valuable new drug class in the treatment of cancer. Recent discoveries make a compelling case for the complexity of DNA repair biomarker evaluation and underscore the need to examine at multiple biomarkers in a relational manner. This review updates the current trends in DNA repair biomarker strategies in use for the PARP inhibitors and describes the impact of many DNA repair biomarkers on PARP inhibitor benefit in the cancer clinic.

Role of Rad51 down-regulation and extracellular signal-regulated kinases 1 and 2 inactivation in emodin and mitomycin C-induced synergistic cytotoxicity in human non-small-cell lung cancer cells

Emodin (1,3,8-trihydroxy-6-methyl-anthraquinone) is a natural anthraquinone derivative found in the roots and rhizomes of numerous plants. It is a tyrosine kinase inhibitor and has anticancer effects on lung cancer. Rad51 plays a central role in homologous recombination, and high levels of Rad51 expression are observed in chemo- or radioresistant carcinomas. Our previous studies have shown that the mitogen-activated protein kinase kinase (MKK) 1/2-extracellular signal-regulated kinase (ERK) 1/2 signal pathway maintains the expression of Rad51. Therefore, in this study, we hypothesized that emodin could enhance the effects of the antitumor antibiotic mitomycin C (MMC)-mediated cytotoxicity by decreasing the expression of Rad51 and the phosphorylation of ERK1/2. Exposure of the human non-small-cell lung cancer H1703 or A549 cell lines to emodin decreased the MMC-elicited phosphorylated ERK1/2 and Rad51 levels. Moreover, emodin significantly decreased the MMC-elicited Rad51 mRNA and protein levels by increasing the instability of Rad51 mRNA and protein. In emodin- and MMC-cotreated cells, ERK1/2 phosphorylation was enhanced by constitutively active MKK1/2 (MKK1/2-CA), thus increasing Rad51 protein levels and protein stability. The synergistic cytotoxic effects induced by emodin combined with MMC were remarkably decreased by MKK1-CA-mediated enhancement of ERK1/2 activation. Depletion of endogenous Rad51 expression by small interfering Rad51 RNA transfection significantly enhanced MMC-induced cell death and cell growth inhibition. In contrast, overexpression of Rad51 protects lung cancer cells from the synergistic cytotoxic effects induced by emodin and MMC. We conclude that suppression of Rad51 expression or a combination of emodin with chemotherapeutic agents may be considered as potential therapeutic modalities for lung cancer.

Influence of DNA repair RAD51 gene variants in overall survival of non-small cell lung cancer patients treated with first line chemotherapy

PURPOSE

Lung cancer continues to be the most frequent cancer with approximately one million people worldwide dying of this disease each year. Non-small-cell lung cancer (NSCLC) accounts for approximately 80% of all lung cancers. The RAD51 protein is the key protein for homologous recombination, an evolutionarily conserved mechanism for DNA damage repair and the generation of genetic diversity. We conducted this study in order to investigate the effect of the RAD51 G135C polymorphism in treatment response to combined platinum taxanes/gemcitabine first line chemotherapy in NSCLC patients.

METHODS

We analysed RAD51 G135C polymorphism in 243 NSCLC patients using PCR-RFLP methodology.

RESULTS

There were no statistically significant differences between the groups of NSCLC patients with the different genotypes regarding tumour stage (p = 0.232). Our results indicate that the mean survival rates were statistically different according to the patient's genotypes. The group of patients carrying the C allele presented a higher mean survival rate than the other patients (56.0 months vs. 41.7 months; p = 0.024). Moreover, regarding smoking history, our results demonstrate that overall survival time differed significantly according to the patient's genotypes in smoker and ex-smoker individuals (p = 0.034). No statistically significant differences were found in the genotype frequencies and overall survival rate among non-smoker NSCLC patients (p = 0.413).

CONCLUSIONS

This is the first study evaluating the effect of the RAD51 G135C polymorphism in NSCLC patient survival. Our results suggest that RAD51 genotypes could be useful molecular markers for predicting the clinical outcome of NSCLC patients.

NBS1 cooperates with homologous recombination to counteract chromosome breakage during replication

Nijmegen breakage syndrome (NBS) is characterized by genome instability and cancer predisposition. NBS patients contain a mutation in the NBS1 gene, which encodes the NBS1 component of the DNA double-strand break (DSB) response complex MRE11/RAD50/NBS1. To investigate the NBS phenotype in more detail, we combined the mouse mimic of the most common patient mutation (Nbs1(Delta B/DeltaB)) with a Rad54 null mutation, which diminishes homologous recombination. Double mutant cells were particularly sensitive to treatments that cause single strand breaks (SSBs), presumably because these SSBs can be converted into detrimental DSBs upon passage of a replication fork. The persistent presence of nuclear RAD51 foci and increased levels of chromatid type breaks in metaphase spreads indicated that replication-associated DSBs are repaired inefficiently in the double mutant cells. We conclude that Nbs1 and Rad54 function cooperatively, but in separate pathways to counteract this type of DNA damage and discuss mechanistic implications of these findings.

Roles of MKK1/2-ERK1/2 and phosphoinositide 3-kinase-AKT signaling pathways in erlotinib-induced Rad51 suppression and cytotoxicity in human non-small cell lung cancer cells

Erlotinib (Tarceva) is a selective epidermal growth factor receptor tyrosine kinase inhibitor in the treatment of human non-small cell lung cancer (NSCLC). In this study, we investigated the roles of ERK1/2 and AKT signaling pathways in regulating Rad51 expression and cytotoxic effects in different NSCLC cell lines treated with erlotinib. Erlotinib decreased cellular levels of phosphorylated ERK1/2, phosphorylated AKT, Rad51 protein, and mRNA in erlotinib-sensitive H1650, A549, and H1869 cells, leading to cell death via apoptosis, but these results were not seen in erlotinib-resistant H520 and H1703 cells. Erlotinib decreased Rad51 protein levels by enhancing Rad51 mRNA and protein instability. Enforced expression of constitutively active MKK1 or AKT vectors could restore Rad51 protein levels, which were inhibited by erlotinib, and decrease erlotinib-induced cytotoxicity. Knocking down endogenous Rad51 expression by si-Rad51 RNA transfection significantly enhanced erlotinib-induced cytotoxicity. In contrast, overexpression of Rad51 by transfection with Rad51 vector could protect the cells from cytotoxic effects induced by erlotinib. Blocking the activations of ERK1/2 and AKT by MKK1/2 inhibitor (U0126) and phosphoinositide 3-kinase inhibitor (wortmannin) suppressed the expression of Rad51 and enhanced the erlotinib-induced cell death in erlotinib-resistant cells. In conclusion, suppression of Rad51 may be a novel therapeutic modality in overcoming drug resistance of erlotinib in NSCLC.

Crystal structure of an archaeal Rad51 homologue in complex with a metatungstate inhibitor

Archaeal RadAs are close homologues of eukaryal Rad51s ( approximately 40% sequence identities). These recombinases promote a hallmark strand exchange process between homologous single-stranded and double-stranded DNA substrates. This DNA-repairing function also plays a key role in cancer cells' resistance to chemo- and radiotherapy. Inhibition of the strand exchange process may render cancer cells more susceptible to therapeutic treatment. We found that metatungstate is a potent inhibitor of RadA from Methanococcus voltae. The tungsten cluster binds RadA in the axial DNA-binding groove. This polyanionic species appears to inhibit RadA by locking the protein in its inactive conformation.

Repair of gaps opposite lesions by homologous recombination in mammalian cells

Damages in the DNA template inhibit the progression of replication, which may cause single-stranded gaps. Such situations can be tolerated by translesion DNA synthesis (TLS), or by homology-dependent repair (HDR), which is based on transfer or copying of the missing information from the replicated sister chromatid. Whereas it is well established that TLS plays an important role in DNA damage tolerance in mammalian cells, it is unknown whether HDR operates in this process. Using a newly developed plasmid-based assay that distinguishes between the three mechanisms of DNA damage tolerance, we found that mammalian cells can efficiently utilize HDR to repair DNA gaps opposite an abasic site or benzo[a]pyrene adduct. The majority of these events occurred by a physical strand transfer (homologous recombination repair; HRR), rather than a template switch mechanism. Furthermore, cells deficient in either the human RAD51 recombination protein or NBS1, but not Rad18, exhibited decreased gap repair through HDR, indicating a role for these proteins in DNA damage tolerance. To our knowledge, this is the first direct evidence of gap-lesion repair via HDR in mammalian cells, providing further molecular insight into the potential activity of HDR in overcoming replication obstacles and maintaining genome stability.

Replication fork reversal and the maintenance of genome stability

The progress of replication forks is often threatened in vivo, both by DNA damage and by proteins bound to the template. Blocked forks must somehow be restarted, and the original blockage cleared, in order to complete genome duplication, implying that blocked fork processing may be critical for genome stability. One possible pathway that might allow processing and restart of blocked forks, replication fork reversal, involves the unwinding of blocked forks to form four-stranded structures resembling Holliday junctions. This concept has gained increasing popularity recently based on the ability of such processing to explain many genetic observations, the detection of unwound fork structures in vivo and the identification of enzymes that have the capacity to catalyse fork regression in vitro. Here, we discuss the contexts in which fork regression might occur, the factors that may promote such a reaction and the possible roles of replication fork unwinding in normal DNA metabolism.

Genetic analysis of zinc-finger nuclease-induced gene targeting in Drosophila

Using zinc-finger nucleases (ZFNs) to cleave the chromosomal target, we have achieved high frequencies of gene targeting in the Drosophila germline. Both local mutagenesis through nonhomologous end joining (NHEJ) and gene replacement via homologous recombination (HR) are stimulated by target cleavage. In this study we investigated the mechanisms that underlie these processes, using materials for the rosy (ry) locus. The frequency of HR dropped significantly in flies homozygous for mutations in spnA (Rad51) or okr (Rad54), two components of the invasion-mediated synthesis-dependent strand annealing (SDSA) pathway. When single-strand annealing (SSA) was also blocked by the use of a circular donor DNA, HR was completely abolished. This indicates that the majority of HR proceeds via SDSA, with a minority mediated by SSA. In flies deficient in lig4 (DNA ligase IV), a component of the major NHEJ pathway, the proportion of HR products rose significantly. This indicates that most NHEJ products are produced in a lig4-dependent process. When both spnA and lig4 were mutated and a circular donor was provided, the frequency of ry mutations was still high and no HR products were recovered. The local mutations produced in these circumstances must have arisen through an alternative, lig4-independent end-joining mechanism. These results show what repair pathways operate on double-strand breaks in this gene targeting system. They also demonstrate that the outcome can be biased toward gene replacement by disabling the major NHEJ pathway and toward simple mutagenesis by interfering with the major HR process.

DIDS, a chemical compound that inhibits RAD51-mediated homologous pairing and strand exchange

RAD51, an essential eukaryotic DNA recombinase, promotes homologous pairing and strand exchange during homologous recombination and the recombinational repair of double strand breaks. Mutations that up- or down-regulate RAD51 gene expression have been identified in several tumors, suggesting that inappropriate expression of the RAD51 activity may cause tumorigenesis. To identify chemical compounds that affect the RAD51 activity, in the present study, we performed the RAD51-mediated strand exchange assay in the presence of 185 chemical compounds. We found that 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) efficiently inhibited the RAD51-mediated strand exchange. DIDS also inhibited the RAD51-mediated homologous pairing in the absence of RPA. A surface plasmon resonance analysis revealed that DIDS directly binds to RAD51. A gel mobility shift assay showed that DIDS significantly inhibited the DNA-binding activity of RAD51. Therefore, DIDS may bind near the DNA binding site(s) of RAD51 and compete with DNA for RAD51 binding.

Visualizing the disassembly of S. cerevisiae Rad51 nucleoprotein filaments

Rad51 is the core component of the eukaryotic homologous recombination machinery and assembles into elongated nucleoprotein filaments on DNA. We have used total internal reflection fluorescence microscopy and a DNA curtain assay to investigate the dynamics of individual Saccharomyces cerevisiae Rad51 nucleoprotein filaments. For these experiments the DNA molecules were end-labeled with single fluorescent semiconducting nanocrystals. The assembly and disassembly of the Rad51 nucleoprotein filaments were visualized by tracking the location of the labeled DNA end in real time. Using this approach, we have analyzed yeast Rad51 under a variety of different reaction conditions to assess parameters that impact the stability of the nucleoprotein filament. We show that Rad51 readily dissociates from DNA in the presence of ADP or in the absence of nucleotide cofactor, but that free ATP in solution confers a fivefold increase in the stability of the nucleoprotein filaments. We also probe how protein dissociation is coupled to ATP binding and hydrolysis by examining the effects of ATP concentration, and by the use of the nonhydrolyzable ATP analogue adenosine 5'-(beta, gamma-imido) triphosphate and ATPase active-site mutants. Finally, we demonstrate that the Rad51 gain-of-function mutant I345T dissociates from DNA with kinetics nearly identical to that of wild-type Rad51, but assembles 30% more rapidly. Together, these results provide a framework for studying the biochemical behaviors of S. cerevisiae Rad51 nucleoprotein filaments at the single-molecule level.

PLRG1 is an essential regulator of cell proliferation and apoptosis during vertebrate development and tissue homeostasis

PLRG1, an evolutionarily conserved component of the spliceosome, forms a complex with Pso4/SNEV/Prp19 and the cell division and cycle 5 homolog (CDC5L) that is involved in both pre-mRNA splicing and DNA repair. Here, we show that the inactivation of PLRG1 in mice results in embryonic lethality at 1.5 days postfertilization. Studies of heart- and neuron-specific PLRG1 knockout mice further reveal an essential role of PLRG1 in adult tissue homeostasis and the suppression of apoptosis. PLRG1-deficient mouse embryonic fibroblasts (MEFs) fail to progress through S phase upon serum stimulation and exhibit increased rates of apoptosis. PLRG1 deficiency causes enhanced p53 phosphorylation and stabilization in the presence of increased gamma-H2AX immunoreactivity as an indicator of an activated DNA damage response. p53 downregulation rescues lethality in both PLRG1-deficient MEFs and zebrafish in vivo, showing that apoptosis resulting from PLRG1 deficiency is p53 dependent. Moreover, the deletion of PLRG1 results in the relocation of its interaction partner CDC5L from the nucleus to the cytoplasm without general alterations in pre-mRNA splicing. Taken together, the results of this study identify PLRG1 as a critical nuclear regulator of p53-dependent cell cycle progression and apoptosis during both embryonic development and adult tissue homeostasis.

A mouse PRMT1 null allele defines an essential role for arginine methylation in genome maintenance and cell proliferation

Protein arginine methyltransferase 1 (PRMT1) is the major enzyme that generates monomethylarginine and asymmetrical dimethylarginine. We report here a conditional null allele of PRMT1 in mice and that the loss of PRMT1 expression leads to embryonic lethality. Using the Cre/lox-conditional system, we show that the loss of PRMT1 in mouse embryonic fibroblasts (MEFs) leads to the loss of arginine methylation of substrates harboring a glycine-arginine rich motif, including Sam68 and MRE11. The loss of PRMT1 in MEFs leads to spontaneous DNA damage, cell cycle progression delay, checkpoint defects, aneuploidy, and polyploidy. We show using a 4-hydroxytamoxifen-inducible Cre that the loss of PRMT1 in MEFs leads to a higher incidence of chromosome losses, gains, structural rearrangements, and polyploidy, as documented by spectral karyotyping. Using PRMT1 small interfering RNA in U2OS cells, we further show that PRMT1-deficient cells are hypersensitive to the DNA damaging agent etoposide and exhibit a defect in the recruitment of the homologous recombination RAD51 recombinase to DNA damage foci. Taken together, these data show that PRMT1 is required for genome integrity and cell proliferation. Our findings also suggest that arginine methylation by PRMT1 is a key posttranslational modification in the DNA damage response pathway in proliferating mammalian cells.

Mutation of the mouse Syce1 gene disrupts synapsis and suggests a link between synaptonemal complex structural components and DNA repair

In mammals, the synaptonemal complex is a structure required to complete crossover recombination. Although suggested by cytological work, in vivo links between the structural proteins of the synaptonemal complex and the proteins of the recombination process have not previously been made. The central element of the synaptonemal complex is traversed by DNA at sites of recombination and presents a logical place to look for interactions between these components. There are four known central element proteins, three of which have previously been mutated. Here, we complete the set by creating a null mutation in the Syce1 gene in mouse. The resulting disruption of synapsis in these animals has allowed us to demonstrate a biochemical interaction between the structural protein SYCE2 and the repair protein RAD51. In normal meiosis, this interaction may be responsible for promoting homologous synapsis from sites of recombination.

Rad51 overexpression rescues radiation resistance in BRCA2-defective cancer cells

Breast cancers with BRCA2 mutations exhibit DNA repair defects and are particularly sensitive to radiation. BRCA2 interacts with Rad51 in a complex manner involving internal BRC and C-terminal TR2 domains which play a key role in homologous recombination. BRCA2 expression also modulates Rad51 protein levels such that Rad51 protein is relatively decreased in BRCA2-defective cancer cells. This is mediated in part through BRCA2's capacity to protect Rad51 from caspase-3 proteolytic degradation. In order to distinguish between functional and expression related roles for BRCA2 we studied the results of Rad51 overexpression in mouse and human cells with inactivating BRCA2 mutations. The results show that overexpression of wild-type Rad51 partially rescues BRCA2 deficiency but that overexpression of a caspase-3 resistant Rad51 completely complements the BRCA2 defect in radiation responsiveness. These results indicate that Rad51 can compensate for some aspects of a BRCA2 gene defect and suggest that Rad51 expression levels may be an important modifier of the BRCA2 defective genotype.

Loss of Rad51c leads to embryonic lethality and modulation of Trp53-dependent tumorigenesis in mice

RecA/Rad51 protein family members (Rad51, Rad51b, Rad51c, Rad51d, Xrcc2, and Xrcc3) are essential for DNA repair by homologous recombination, and their role in cancers has been anticipated. Here we provide the first direct evidence for a tumor suppressor function for a member of the Rad51 family. We show that Rad51c deficiency leads to early embryonic lethality, which can be delayed on a Trp53-null background. To uncover the role of Rad51c in tumorigenesis, we have exploited the fact that Rad51c and Trp53 are both closely located on the mouse chromosome 11. We have generated double heterozygous (DH) mice carrying mutant alleles of both genes either on different (DH-trans) or on the same chromosome (DH-cis), the latter allowing for a deletion of wild-type alleles of both genes by loss of heterozygosity. DH-trans mice, in contrast to DH-cis, developed tumors with latency and spectrum similar to Trp53 heterozygous mice. Strikingly, Rad51c mutation in DH-cis mice promoted the development of tumors of specialized sebaceous glands and suppressed tumors characteristic of Trp53 mutation. In addition, DH-cis females developed tumors significantly earlier than any other group.

O6-methylguanine-induced cell death involves exonuclease 1 as well as DNA mismatch recognition in vivo

Alkylation-induced O(6)-methylguanine (O(6)MeG) DNA lesions can be mutagenic or cytotoxic if unrepaired by the O(6)MeG-DNA methyltransferase (Mgmt) protein. O(6)MeG pairs with T during DNA replication, and if the O(6)MeG:T mismatch persists, a G:C to A:T transition mutation is fixed at the next replication cycle. O(6)MeG:T mismatch detection by MutSalpha and MutLalpha leads to apoptotic cell death, but the mechanism by which this occurs has been elusive. To explore how mismatch repair mediates O(6)MeG-dependent apoptosis, we used an Mgmt-null mouse model combined with either the Msh6-null mutant (defective in mismatch recognition) or the Exo1-null mutant (impaired in the excision step of mismatch repair). Mouse embryonic fibroblasts and bone marrow cells derived from Mgmt-null mice were much more alkylation-sensitive than wild type, as expected. However, ablation of either Msh6 or Exo1 function rendered these Mgmt-null cells just as resistant to alkylation-induced cytotoxicity as wild-type cells. Rapidly proliferating tissues in Mgmt-null mice (bone marrow, thymus, and spleen) are extremely sensitive to apoptosis induced by O(6)MeG-producing agents. Here, we show that ablation of either Msh6 or Exo1 function in the Mgmt-null mouse renders these rapidly proliferating tissues alkylation-resistant. However, whereas the Msh6 defect confers total alkylation resistance, the Exo1 defect leads to a variable tissue-specific alkylation resistance phenotype. Our results indicate that Exo1 plays an important role in the induction of apoptosis by unrepaired O(6)MeGs.

Biochemical analysis of the human DMC1-I37N polymorphism

The DMC1 protein, a meiosis-specific DNA recombinase, promotes homologous pairing and strand exchange. The I37N single nucleotide polymorphism of the human DMC1 protein was reported as a result of human genome sequencing projects. In this study, we purified the human DMC1-I37N variant, as a recombinant protein. The DMC1 protein is known to require DNA for efficient ATP hydrolysis. By contrast, the DMC1-I37N variant efficiently hydrolyzed ATP in the absence of DNA. Like the conventional DMC1 protein, the DMC1-I37N variant promoted strand exchange, but it required a high Ca2+ concentration (4-8 mm), a condition that inactivates the strand-exchange activity of the conventional DMC1 protein. These biochemical differences between the DMC1 and DMC1-I37N proteins suggest that the DMC1-I37N polymorphism may be a source of improper meiotic recombination, causing meiotic defects in humans.

Mechanisms of dealing with DNA damage-induced replication problems

During every S phase, cells need to duplicate their genomes so that both daughter cells inherit complete copies of genetic information. Given the large size of mammalian genomes and the required precision of DNA replication, genome duplication requires highly fine-tuned corrective and quality control processes. A major threat to the accuracy and efficiency of DNA synthesis is the presence of DNA lesions, caused by both endogenous and exogenous damaging agents. Replicative DNA polymerases, which carry out the bulk of DNA synthesis, evolved to do their job extremely precisely and efficiently. However, they are unable to use damaged DNA as a template and, consequently, are stopped at most DNA lesions. Failure to restart such stalled replication forks can result in major chromosomal aberrations and lead to cell dysfunction or death. Therefore, a well-coordinated response to replication perturbation is essential for cell survival and fitness. Here we review how this response involves activating checkpoint signaling and the use of specialized pathways promoting replication restart. Checkpoint signaling adjusts cell cycle progression to the emergency situation and thus gives cells more time to deal with the damage. Replication restart is mediated by two pathways. Homologous recombination uses homologous DNA sequence to repair or bypass the lesion and is therefore mainly error free. Error-prone translesion synthesis employs specialized, low fidelity polymerases to bypass the damage.

Rad50 is dispensable for the maintenance and viability of postmitotic tissues

The majority of spontaneous chromosome breakage occurs during the process of DNA replication. Homologous recombination is the primary mechanism of repair of such damage, which probably accounts for the fact that it is essential for genome integrity and viability in mammalian cells. The Mre11 complex plays diverse roles in the maintenance of genomic integrity, influencing homologous recombination, checkpoint activation, and telomere maintenance. The complex is essential for cellular viability, but given its myriad influences on genomic integrity, the mechanistic basis for the nonviability of Mre11 complex-deficient cells has not been defined. In this study we generated mice carrying a conditional allele of Rad50 and examined the effects of Rad50 deficiency in proliferative and nonproliferative settings. Depletion of Rad50 in cultured cells caused extensive DNA damage and death within 3 to 5 days of Rad50 deletion. This was not associated with gross telomere dysfunction, suggesting that the telomeric functions of the Mre11 complex are not required for viability. Rad50 was also dispensable for the viability of quiescent liver and postmitotic Purkinje cells of the cerebellum. These findings support the idea that the essential functions of the Mre11 complex are associated with DNA replication and further suggest that homologous recombination is not essential in nondividing cells.

Restoration of CAPAN-1 cells with functional BRCA2 provides insight into the DNA repair activity of individuals who are heterozygous for BRCA2 mutations

Mutations in the BRCA2 gene are associated with inherited, early-onset breast cancer. CAPAN-1 cells have been useful for studying how BRCA2 mutations contribute to malignant transformation. They exhibit loss of heterozygosity (LOH), and the remaining copy of BRCA2 has a 6174delT mutation, which causes a premature C-terminal truncation that removes the domains for DNA repair and the nuclear localization signals. The DNA repair protein RAD51, which interacts with BRCA2, exhibits impaired nuclear translocation in CAPAN-1. It has been speculated that RAD51 may require BRCA2 for nuclear entry and that C-terminally truncated BRCA2 may retain RAD51 in the cytoplasm. This may cause heterozygous individuals to exhibit deficient DNA repair and cell viability comparable to individuals with LOH or biallelic BRCA2 mutations. We simulated a heterozygous condition by using stably transfected CAPAN-1 cells with wild-type BRCA2. Fusion of a nuclear localization signal to RAD51 did not increase its ability to independently enter the nuclei of CAPAN-1 cells. Furthermore, restoration of functional BRCA2 did not significantly improve DNA repair, nor did it reestablish cell viability in CAPAN-1 cells. The results imply that C-terminally truncated BRCA2 hinders RAD51 nuclear translocation, possibly contributing to genetic instabilities in homozygous as well as heterozygous individuals.

Homologous recombination properties of OsRad51, a recombinase from rice

cDNA corresponding to OsRad51 protein was isolated from cDNA library of rice flowers (Oryza sativa, Indica cultivar group) and cloned in to pET28a expression vector. The protein was over expressed in E. coli BL21 (DE3) and purified. Purified OsRad51 could bind single and double stranded DNA, however it showed higher affinity for single stranded DNA. Transmission Electron Microscopy (TEM) studies of OsRad51-DNA complexes showed that this protein formed ring like structures and bound DNA forming filaments. OsRad51 protein promoted renaturation of complementary single strands in to duplex DNA molecules and also showed ATPase activity, which was stimulated by single strand DNA. Fluorescence resonance energy transfer (FRET) assays revealed that OsRad51 promoted homology dependent renaturation as well as strand exchange reactions. Renaturation activity was ATP dependent; however strand exchange activity was ATP independent. This is the first report on in vitro characterization of Rad51 protein from crop plants.

Large-scale evaluation of candidate genes identifies associations between DNA repair and genomic maintenance and development of benzene hematotoxicity

Benzene is an established human hematotoxicant and leukemogen but its mechanism of action is unclear. To investigate the role of single-nucleotide polymorphisms (SNPs) on benzene-induced hematotoxicity, we analyzed 1395 SNPs in 411 genes using an Illumina GoldenGate assay in 250 benzene-exposed workers and 140 unexposed controls. Highly significant findings clustered in five genes (BLM, TP53, RAD51, WDR79 and WRN) that play a critical role in DNA repair and genomic maintenance, and these regions were then further investigated with tagSNPs. One or more SNPs in each gene were associated with highly significant 10-20% reductions (P values ranged from 0.0011 to 0.0002) in the white blood cell (WBC) count among benzene-exposed workers but not controls, with evidence for gene-environment interactions for SNPs in BLM, WRN and RAD51. Further, among workers exposed to benzene, the genotype-associated risk of having a WBC count <4000 cells/microl increased when using individuals with progressively higher WBC counts as the comparison group, with some odds ratios >8-fold. In vitro functional studies revealed that deletion of SGS1 in yeast, equivalent to lacking BLM and WRN function in humans, caused reduced cellular growth in the presence of the toxic benzene metabolite hydroquinone, and knockdown of WRN using specific short hairpin RNA increased susceptibility of human TK6 cells to hydroquinone toxicity. Our findings suggest that SNPs involved in DNA repair and genomic maintenance, with particular clustering in the homologous DNA recombination pathway, play an important role in benzene-induced hematotoxicity.

Mechanism of eukaryotic homologous recombination

Homologous recombination (HR) serves to eliminate deleterious lesions, such as double-stranded breaks and interstrand crosslinks, from chromosomes. HR is also critical for the preservation of replication forks, for telomere maintenance, and chromosome segregation in meiosis I. As such, HR is indispensable for the maintenance of genome integrity and the avoidance of cancers in humans. The HR reaction is mediated by a conserved class of enzymes termed recombinases. Two recombinases, Rad51 and Dmc1, catalyze the pairing and shuffling of homologous DNA sequences in eukaryotic cells via a filamentous intermediate on ssDNA called the presynaptic filament. The assembly of the presynaptic filament is a rate-limiting process that is enhanced by recombination mediators, such as the breast tumor suppressor BRCA2. HR accessory factors that facilitate other stages of the Rad51- and Dmc1-catalyzed homologous DNA pairing and strand exchange reaction have also been identified. Recent progress on elucidating the mechanisms of action of Rad51 and Dmc1 and their cohorts of ancillary factors is reviewed here.

DNA damage and homologous recombination signaling induced by thymidylate deprivation

DNA damage is accepted as a consequence of thymidylate deprivation induced by chemotherapeutic inhibitors of thymidylate synthase (TS), but the types of damage and signaling responses remain incompletely understood. Thymidylate deprivation increases dUTP and uracil in DNA, which is removed by base excision repair (BER). Because BER requires a synthesis step, strand break intermediates presumably accumulate. Thymidylate deprivation also induces cell cycle arrest during replication. Homologous recombination (HR) is a means of repairing persistent BER intermediates and collapsed replication forks. There are also intimate links between HR and S-phase checkpoint pathways. In this study, the goals were to determine the involvement of HR-associated proteins and DNA damage signaling responses to thymidylate deprivation. When RAD51, which is a central component of HR, was depleted by siRNA cells were sensitized to raltitrexed (RTX), which specifically inhibits TS. To our knowledge, this is the first demonstration in mammalian cells that depletion of RAD51 causes sensitivity to thymidylate deprivation. Activation of DNA damage signaling responses was examined following treatment with RTX. Phosphorylation of replication protein A (RPA2 subunit) and formation of damage-induced foci were strikingly evident following IC(50) doses of RTX. Induction was much more striking following RTX treatment than with hydroxyurea, which is commonly used to inhibit replication. RTX treatment also induced foci of RAD51, gamma-H2AX, phospho-Chk1, and phospho-NBS1, although the extent of co-localization with RPA2 foci varied. Collectively, the results suggest that HR and S-phase checkpoint signaling processes are invoked by thymidylate deprivation and influence cellular resistance to thymidylate deprivation.