Isolation of novel human and mouse genes of the recA/RAD51 recombination-repair gene family

Molecular cloning and expression analysis of rad51 gene associated with gametogenesis in Chinese soft-shell turtle (Pelodiscus sinensis)

Rad51 is a recA-like recombinase that plays a crucial role in repairing DNA double-strand breaks through homologous recombination during mitosis and meiosis in mammals and other organisms. However, its role in reptiles remains largely unclear. In this study, we aimed to investigate the physiological role of the rad51 gene in reptiles, particularly in Pelodiscus sinensis. Firstly, the cDNA of rad51 gene was cloned and analyzed in P. sinensis. The cloned cDNA contained an open reading frame (ORF) of 1020 bp and encodeed a peptide of 339 amino acids. The multiple alignments and phylogenetic tree analysis of Rad51 showed that P. sinensis shares the high identity with Chelonia mydas (97.95%) and Mus musculus (95.89%). Secondly, reverse transcription-polymerase chain reaction (RT-PCR) and real-time quantitative polymerase chain reaction (RT-qPCR) analysis showed that rad51 mRNA was highly expressed in both ovary and testis, while being weak in the somatic tissues examined in this study. Furthermore, chemical in situ hybridization (CISH) was performed to examine the expression profile of rad51 mRNA in germ cells at different stages. In the testis, rad51 mRNA expression was found to be stronger in the germ cells at early stages, specifically in spermatogonia and spermatocytes, but it was undetectable in spermatids. In the ovary, rad51 mRNA exhibited a uniform distribution in the cytoplasm of oocytes at early stages. The signal intensity of rad51 mRNA was highest in primary oocytes and gradually declined during oogenesis as the oocytes developed. These results suggest that rad51 plays a vital role in the development of germ cells, particularly during the early stages of gametogenesis in P. sinensis. The dynamic expression pattern of rad51 mRNA provides insights into the mechanisms underlying germ cell development and differentiation into gametes in turtles, even in reptiles.

New Facets of DNA Double Strand Break Repair: Radiation Dose as Key Determinant of HR versus c-NHEJ Engagement

Radiation therapy is an essential component of present-day cancer management, utilizing ionizing radiation (IR) of different modalities to mitigate cancer progression. IR functions by generating ionizations in cells that induce a plethora of DNA lesions. The most detrimental among them are the DNA double strand breaks (DSBs). In the course of evolution, cells of higher eukaryotes have evolved four major DSB repair pathways: classical non-homologous end joining (c-NHEJ), homologous recombination (HR), alternative end-joining (alt-EJ), and single strand annealing (SSA). These mechanistically distinct repair pathways have different cell cycle- and homology-dependencies but, surprisingly, they operate with widely different fidelity and kinetics and therefore contribute unequally to cell survival and genome maintenance. It is therefore reasonable to anticipate tight regulation and coordination in the engagement of these DSB repair pathway to achieve the maximum possible genomic stability. Here, we provide a state-of-the-art review of the accumulated knowledge on the molecular mechanisms underpinning these repair pathways, with emphasis on c-NHEJ and HR. We discuss factors and processes that have recently come to the fore. We outline mechanisms steering DSB repair pathway choice throughout the cell cycle, and highlight the critical role of DNA end resection in this process. Most importantly, however, we point out the strong preference for HR at low DSB loads, and thus low IR doses, for cells irradiated in the G2-phase of the cell cycle. We further explore the molecular underpinnings of transitions from high fidelity to low fidelity error-prone repair pathways and analyze the coordination and consequences of this transition on cell viability and genomic stability. Finally, we elaborate on how these advances may help in the development of improved cancer treatment protocols in radiation therapy.

DNA Damage Clustering after Ionizing Radiation and Consequences in the Processing of Chromatin Breaks

Charged-particle radiotherapy (CPRT) utilizing low and high linear energy transfer (low-/high-LET) ionizing radiation (IR) is a promising cancer treatment modality having unique physical energy deposition properties. CPRT enables focused delivery of a desired dose to the tumor, thus achieving a better tumor control and reduced normal tissue toxicity. It increases the overall radiation tolerance and the chances of survival for the patient. Further improvements in CPRT are expected from a better understanding of the mechanisms governing the biological effects of IR and their dependence on LET. There is increasing evidence that high-LET IR induces more complex and even clustered DNA double-strand breaks (DSBs) that are extremely consequential to cellular homeostasis, and which represent a considerable threat to genomic integrity. However, from the perspective of cancer management, the same DSB characteristics underpin the expected therapeutic benefit and are central to the rationale guiding current efforts for increased implementation of heavy ions (HI) in radiotherapy. Here, we review the specific cellular DNA damage responses (DDR) elicited by high-LET IR and compare them to those of low-LET IR. We emphasize differences in the forms of DSBs induced and their impact on DDR. Moreover, we analyze how the distinct initial forms of DSBs modulate the interplay between DSB repair pathways through the activation of DNA end resection. We postulate that at complex DSBs and DSB clusters, increased DNA end resection orchestrates an increased engagement of resection-dependent repair pathways. Furthermore, we summarize evidence that after exposure to high-LET IR, error-prone processes outcompete high fidelity homologous recombination (HR) through mechanisms that remain to be elucidated. Finally, we review the high-LET dependence of specific DDR-related post-translational modifications and the induction of apoptosis in cancer cells. We believe that in-depth characterization of the biological effects that are specific to high-LET IR will help to establish predictive and prognostic signatures for use in future individualized therapeutic strategies, and will enhance the prospects for the development of effective countermeasures for improved radiation protection during space travel.

RAD54 is essential for RAD51-mediated repair of meiotic DSB in Arabidopsis

An essential component of the homologous recombination machinery in eukaryotes, the RAD54 protein is a member of the SWI2/SNF2 family of helicases with dsDNA-dependent ATPase, DNA translocase, DNA supercoiling and chromatin remodelling activities. It is a motor protein that translocates along dsDNA and performs multiple functions in homologous recombination. In particular, RAD54 is an essential cofactor for regulating RAD51 activity. It stabilizes the RAD51 nucleofilament, remodels nucleosomes, and stimulates the homology search and strand invasion activities of RAD51. Accordingly, deletion of RAD54 has dramatic consequences on DNA damage repair in mitotic cells. In contrast, its role in meiotic recombination is less clear. RAD54 is essential for meiotic recombination in Drosophila and C. elegans, but plays minor roles in yeast and mammals. We present here characterization of the roles of RAD54 in meiotic recombination in the model plant Arabidopsis thaliana. Absence of RAD54 has no detectable effect on meiotic recombination in otherwise wild-type plants but RAD54 becomes essential for meiotic DSB repair in absence of DMC1. In Arabidopsis, dmc1 mutants have an achiasmate meiosis, in which RAD51 repairs meiotic DSBs. Lack of RAD54 leads to meiotic chromosomal fragmentation in absence of DMC1. The action of RAD54 in meiotic RAD51 activity is thus mainly downstream of the role of RAD51 in supporting the activity of DMC1. Equivalent analyses show no effect on meiosis of combining dmc1 with the mutants of the RAD51-mediators RAD51B, RAD51D and XRCC2. RAD54 is thus required for repair of meiotic DSBs by RAD51 and the absence of meiotic phenotype in rad54 plants is a consequence of RAD51 playing a RAD54-independent supporting role to DMC1 in meiotic recombination.

Homologous recombination is a universal pathway which repairs broken DNA molecules through the use of homologous DNA templates. It is both essential for maintenance of genome stability and for the generation of genetic diversity through sexual reproduction. A central step of the homologous recombination process is the search for and invasion of a homologous, intact DNA sequence that will be used as template. This key step is catalysed by the RAD51 recombinase in somatic cells and RAD51 and DMC1 in meiotic cells, assisted by a number of associated factors. Among these, the chromatin-remodelling protein RAD54 is a required cofactor for RAD51 in mitotic cells. Understanding of its role during meiotic recombination however remains elusive. We show here that RAD54 is required for repair of meiotic double strand breaks by RAD51 in the plant Arabidopsis thaliana, and this function is downstream of the meiotic role of RAD51 in supporting the activity of DMC1. These results provide new insights into the regulation of the central step of homologous recombination in plants and very probably also other multicellular eukaryotes.

Criteria of the German Consortium for Hereditary Breast and Ovarian Cancer for the Classification of Germline Sequence Variants in Risk Genes for Hereditary Breast and Ovarian Cancer

More than ten years ago, the German Consortium for Hereditary Breast and Ovarian Cancer (GC-HBOC) set up a panel of experts (VUS Task Force) which was tasked with reviewing the classifications of genetic variants reported by individual centres of the GC-HBOC to the central database in Leipzig and reclassifying them, where necessary, based on the most recent data. When it evaluates variants, the VUS Task Force must arrive at a consensus. The resulting classifications are recorded in a central database where they serve as a basis for ensuring the consistent evaluation of previously known and newly identified variants in the different centres of the GC-HBOC. The standardised VUS evaluation by the VUS Task Force is a key element of the recall system which has also been set up by the GC-HBOC. The system will be used to pass on information to families monitored and managed by GC-HBOC centres in the event that previously classified variants are reclassified based on new information. The evaluation algorithm of the VUS Task Force was compiled using internationally established assessment methods (IARC, ACMG, ENIGMA) and is presented here together with the underlying evaluation criteria used to arrive at the classification decision using a flow chart. In addition, the characteristics and special features of specific individual risk genes associated with breast and/or ovarian cancer are discussed in separate subsections. The URLs of relevant databases have also been included together with extensive literature references to provide additional information and cover the scope and dynamism of the current state of knowledge on the evaluation of genetic variants. In future, if criteria are updated based on new information, the update will be published on the website of the GC-HBOC ( https://www.konsortium-familiaerer-brustkrebs.de/ ).

Genetic and Small Molecule Disruption of the AID/RAD51 Axis Similarly Protects Nonobese Diabetic Mice from Type 1 Diabetes through Expansion of Regulatory B Lymphocytes

B lymphocytes play a key role in type 1 diabetes (T1D) development by serving as a subset of APCs preferentially supporting the expansion of autoreactive pathogenic T cells. As a result of their pathogenic importance, B lymphocyte-targeted therapies have received considerable interest as potential T1D interventions. Unfortunately, the B lymphocyte-directed T1D interventions tested to date failed to halt β cell demise. IgG autoantibodies marking humans at future risk for T1D indicate that B lymphocytes producing them have undergone the affinity-maturation processes of class switch recombination and, possibly, somatic hypermutation. This study found that CRISPR/Cas9-mediated ablation of the activation-induced cytidine deaminase gene required for class switch recombination/somatic hypermutation induction inhibits T1D development in the NOD mouse model. The activation-induced cytidine deaminase protein induces genome-wide DNA breaks that, if not repaired through RAD51-mediated homologous recombination, result in B lymphocyte death. Treatment with the RAD51 inhibitor 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid also strongly inhibited T1D development in NOD mice. The genetic and small molecule-targeting approaches expanded CD73⁺ B lymphocytes that exert regulatory activity suppressing diabetogenic T cell responses. Hence, an initial CRISPR/Cas9-mediated genetic modification approach has identified the AID/RAD51 axis as a target for a potentially clinically translatable pharmacological approach that can block T1D development by converting B lymphocytes to a disease-inhibitory CD73⁺ regulatory state.

RAD51B Activity and Cell Cycle Regulation in Response to DNA Damage in Breast Cancer Cell Lines

Common genetic variants mapping to two distinct regions of RAD51B, a paralog of RAD51, have been associated with breast cancer risk in genome-wide association studies (GWAS). RAD51B is a plausible candidate gene because of its established role in the homologous recombination (HR) process. How germline genetic variation in RAD51B confers susceptibility to breast cancer is not well understood. Here, we investigate the molecular function of RAD51B in breast cancer cell lines by knocking down RAD51B expression by small interfering RNA and treating cells with DNA-damaging agents, namely cisplatin, hydroxyurea, or methyl-methanesulfonate. Our results show that RAD51B-depleted breast cancer cells have increased sensitivity to DNA damage, reduced efficiency of HR, and altered cell cycle checkpoint responses. The influence of RAD51B on the cell cycle checkpoint is independent of its role in HR and further studies are required to determine whether these functions can explain the RAD51B breast cancer susceptibility alleles.

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.

The HsRAD51B-HsRAD51C stabilizes the HsRAD51 nucleoprotein filament

There are six human RAD51 related proteins (HsRAD51 paralogs), HsRAD51B, HsRAD51C, HsRAD51D, HsXRCC2, HsXRCC3 and HsDMC1, that appear to enhance HsRAD51 mediated homologous recombinational (HR) repair of DNA double strand breaks (DSBs). Here we model the structures of HsRAD51, HsRAD51B and HsRAD51C and show similar domain orientations within a hypothetical nucleoprotein filament (NPF). We then demonstrate that HsRAD51B-HsRAD51C heterodimer forms stable complex on ssDNA and partially stabilizes the HsRAD51 NPF against the anti-recombinogenic activity of BLM. Moreover, HsRAD51B-HsRAD51C stimulates HsRAD51 mediated D-loop formation in the presence of RPA. However, HsRAD51B-HsRAD51C does not facilitate HsRAD51 nucleation on a RPA coated ssDNA. These results suggest that the HsRAD51B-HsRAD51C complex plays a role in stabilizing the HsRAD51 NPF during the presynaptic phase of HR, which appears downstream of BRCA2-mediated HsRAD51 NPF formation.

Effects of XRCC2 and RAD51B mutations on somatic and meiotic recombination in Arabidopsis thaliana

Homologous recombination is key to the maintenance of genome integrity and the creation of genetic diversity. At the mechanistic level, recombination involves the invasion of a homologous DNA template by broken DNA ends, repair of the break and exchange of genetic information between the two DNA molecules. Invasion of the template in eukaryotic cells is catalysed by the RAD51 and DMC1 recombinases, assisted by a number of accessory proteins, including the RAD51 paralogues. Eukaryotic genomes encode a variable number of RAD51 paralogues, ranging from two in yeast to five in animals and plants. The RAD51 paralogues form at least two distinct protein complexes, believed to play roles in the assembly and stabilization of the RAD51-DNA nucleofilament. Somatic recombination assays and immunocytology confirm that the three 'non-meiotic' paralogues of Arabidopsis, RAD51B, RAD51D and XRCC2, are involved in somatic homologous recombination, and that they are not required for the formation of radioinduced RAD51 foci. Given the presence of all five proteins in meiotic cells, the apparent absence of a meiotic role for RAD51B, RAD51D and XRCC2 is surprising, and perhaps simply the result of a more subtle meiotic phenotype in the mutants. Analysis of meiotic recombination confirms this, showing that the absence of XRCC2, and to a lesser extent RAD51B, but not RAD51D, increases rates of meiotic crossing over. The roles of RAD51B and XRCC2 in recombination are thus not limited to mitotic cells.

Chromatin modifications and the DNA damage response to ionizing radiation

In order to survive, cells have evolved highly effective repair mechanisms to deal with the potentially lethal DNA damage produced by exposure to endogenous as well as exogenous agents. Ionizing radiation exposure induces highly lethal DNA damage, especially DNA double-strand breaks (DSBs), that is sensed by the cellular machinery and then subsequently repaired by either of two different DSB repair mechanisms: (1) non-homologous end joining, which re-ligates the broken ends of the DNA and (2) homologous recombination, that employs an undamaged identical DNA sequence as a template, to maintain the fidelity of DNA repair. Repair of DSBs must occur within the natural context of the cellular DNA which, along with specific proteins, is organized to form chromatin, the overall structure of which can impede DNA damage site access by repair proteins. The chromatin complex is a dynamic structure and is known to change as required for ongoing cellular processes such as gene transcription or DNA replication. Similarly, during the process of DNA damage sensing and repair, chromatin needs to undergo several changes in order to facilitate accessibility of the repair machinery. Cells utilize several factors to modify the chromatin in order to locally open up the structure to reveal the underlying DNA sequence but post-translational modification of the histone components is one of the primary mechanisms. In this review, we will summarize chromatin modifications by the respective chromatin modifying factors that occur during the DNA damage response.

MOF and histone H4 acetylation at lysine 16 are critical for DNA damage response and double-strand break repair

The human MOF gene encodes a protein that specifically acetylates histone H4 at lysine 16 (H4K16ac). Here we show that reduced levels of H4K16ac correlate with a defective DNA damage response (DDR) and double-strand break (DSB) repair to ionizing radiation (IR). The defect, however, is not due to altered expression of proteins involved in DDR. Abrogation of IR-induced DDR by MOF depletion is inhibited by blocking H4K16ac deacetylation. MOF was found to be associated with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), a protein involved in nonhomologous end-joining (NHEJ) repair. ATM-dependent IR-induced phosphorylation of DNA-PKcs was also abrogated in MOF-depleted cells. Our data indicate that MOF depletion greatly decreased DNA double-strand break repair by both NHEJ and homologous recombination (HR). In addition, MOF activity was associated with general chromatin upon DNA damage and colocalized with the synaptonemal complex in male meiocytes. We propose that MOF, through H4K16ac (histone code), has a critical role at multiple stages in the cellular DNA damage response and DSB repair.

The importance of XRCC2 in RAD51-related DNA damage repair

The repair of DNA damage by homologous recombination (HR) is a key pathway for the maintenance of genetic stability in mammalian cells, especially during and following DNA replication. The central HR protein is RAD51, which ensures high fidelity DNA repair by facilitating strand exchange between damaged and undamaged homologous DNA segments. Several RAD51-like proteins, including XRCC2, appear to help with this process, but their roles are not well understood. Here we show that XRCC2 is highly conserved and that most substantial truncations of the protein destroy its ability to function. XRCC2 and its partner protein RAD51L3 are found to interact with RAD51 in the 2-hybrid system, and XRCC2 is shown to be important but not essential for the accumulation of RAD51 at the sites of DNA damage. We visualize the localization of XRCC2 protein at the same sites of DNA damage for the first time using specialized irradiation conditions. Our data indicate that an important function of XRCC2 is to enhance the activity of RAD51, so that the loss of XRCC2 results in a severe delay in the early response of RAD51 to DNA damage.

Plant DNA recombinases: a long way to go

DNA homologous recombination is fundamental process by which two homologous DNA molecules exchange the genetic information for the generation of genetic diversity and maintain the genomic integrity. DNA recombinases, a special group of proteins bind to single stranded DNA (ssDNA) nonspecifically and search the double stranded DNA (dsDNA) molecule for a stretch of DNA that is homologous with the bound ssDNA. Recombinase A (RecA) has been well characterized at genetic, biochemical, as well as structural level from prokaryotes. Two homologues of RecA called Rad51 and Dmc1 have been detected in yeast and higher eukaryotes and are known to mediate the homologous recombination in eukaryotes. The biochemistry and mechanism of action of recombinase is important in understanding the process of homologous recombination. Even though considerable progress has been made in yeast and human recombinases, understanding of the plant recombination and recombinases is at nascent stage. Since crop plants are subjected to different breeding techniques, it is important to know the homologous recombination process. This paper focuses on the properties of eukaryotes recombinases and recent developments in the field of plant recombinases Dmc1 and Rad51.

Recombination activator function of the novel RAD51- and RAD51B-binding protein, human EVL

The RAD51 protein is a central player in homologous recombinational repair. The RAD51B protein is one of five RAD51 paralogs that function in the homologous recombinational repair pathway in higher eukaryotes. In the present study, we found that the human EVL (Ena/Vasp-like) protein, which is suggested to be involved in actin-remodeling processes, unexpectedly binds to the RAD51 and RAD51B proteins and stimulates the RAD51-mediated homologous pairing and strand exchange. The EVL knockdown cells impaired RAD51 assembly onto damaged DNA after ionizing radiation or mitomycin C treatment. The EVL protein alone promotes single-stranded DNA annealing, and the recombination activities of the EVL protein are further enhanced by the RAD51B protein. The expression of the EVL protein is not ubiquitous, but it is significantly expressed in breast cancer-derived MCF7 cells. These results suggest that the EVL protein is a novel recombination factor that may be required for repairing specific DNA lesions, and that may cause tumor malignancy by its inappropriate expression.

The RAD51 gene family in bread wheat is highly conserved across eukaryotes, with RAD51A upregulated during early meiosis

The RADiation sensitive protein 51 (RAD51) recombinase is a eukaryotic homologue of the bacterial Recombinase A (RecA). It is required for homologous recombination of DNA during meiosis where it plays a role in processes such as homology searching and strand invasion. RAD51 is well conserved in eukaryotes with as many as four paralogues identified in vertebrates and some higher plants. Here we report the isolation and preliminary characterisation of four RAD51 gene family members in hexaploid (bread) wheat (Triticum aestivum L.). RAD51A1, RAD51A2 and RAD51D were located on chromosome group 7, and RAD51C was on chromosome group 2. Q-PCR gene expression profiling revealed that RAD51A1 was upregulated during meiosis with lower expression levels seen in mitotic tissue, and bioinformatics analysis demonstrated the evolutionary linkages of this gene family to other eukaryotic RAD51 sequences. Western blot analysis of heterologously expressed RAD51 from bread wheat has shown that it is detectable using anti-human RAD51 antibodies and that molecular modelling of the same protein revealed structural conservation when compared with yeast, human, Arabidopsis and maize RAD51A orthologues. This report has widened the knowledge base of this important protein family in plants, and highlighted the high level of structural conservation among RAD51 proteins from various species.

Regulation of alternative splicing of pre-mRNAs by stresses

A substantial fraction (approximately 30%) of plant genes is alternatively spliced, but how alternative splicing is regulated remains unknown. Many plant genes undergo alternative splicing in response to a variety of stresses. Large-scale computational analyses and experimental approaches focused on select genes are beginning to reveal that alternative splicing constitutes an integral part of gene regulation in stress responses. Based on the studies discussed in this chapter, it appears that alternative splicing generates transcriptome/proteome complexity that is likely to be important for stress adaptation. However, the signaling pathways that relay stress conditions to splicing machinery and if and how the alternative spliced products confer adaptive advantages to plants are poorly understood.

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.

Repression of mutagenesis by Rad51D-mediated homologous recombination

Homologous recombinational repair (HRR) restores chromatid breaks arising during DNA replication and prevents chromosomal rearrangements that can occur from the misrepair of such breaks. In vertebrates, five Rad51 paralogs are identified that contribute in a nonessential but critical manner to HRR proficiency. We constructed and characterized a knockout of the paralog Rad51D in widely studied CHO cells. The rad51d mutant (clone 51D1) displays sensitivity to a diverse spectrum of induced DNA damage including γ-rays, ultraviolet (UV)-C radiation, and methyl methanesulfonate (MMS), indicating the broad relevance of HRR to genotoxicity. Spontaneous chromatid breaks/gaps and isochromatid breaks are elevated 3- to 12-fold, but the chromosome number distribution remains unchanged. Most importantly, 51D1 cells exhibit a 12-fold-increased rate of hprt mutation, as well as 4- to 10-fold increased rates of gene amplification at the dhfr and CAD loci, respectively. Xrcc3 irs1SF cells from the same parental CHO line show similarly elevated mutagenesis at these three loci. Collectively, these results confirm the a priori expectation that HRR acts in an error-free manner to repress three classes of genetic alterations (chromosomal aberrations, loss of gene function and increased gene expression), all of which are associated with carcinogenesis.

Cellular localization of human Rad51C and regulation of ubiquitin-mediated proteolysis of Rad51

Rad51-catalyzed homologous recombination is an important pathway for repair of DNA double strand breaks and maintenance of genome integrity in vertebrate cells. Five proteins referred to as Rad51 paralogs promote Rad51 activity and are proposed to act at various, and in some cases, multiple stages in the recombination pathway. Imaging studies of native Rad51 have revealed its cellular response to DNA damage, yet visualization of the paralog proteins has met with limited success. In this study, we are able to detect endogenous Rad51C and Xrcc3 in human cells. In an effort to determine how Rad51, Rad51C, and Xrcc3 influence the pattern of localization of each other over the time course of DNA damage and repair, we have made the unexpected observation that Rad51 degradation via the ubiquitin-mediated proteasome pathway occurs as a natural part of recombinational DNA repair. Additionally, we find that Rad51C plays an important role in regulating this process. This article contains supplementary material, which may be viewed at the Journal of Cellular Biochemistry website at http://www.interscience.wiley.com/jpages/0730-2312/suppmat/index.html.

Extensive chromosomal instability in Rad51d-deficient mouse cells

Homologous recombination is a double-strand break repair pathway required for resistance to DNA damage and maintaining genomic integrity. In mitotically dividing vertebrate cells, the primary proteins involved in homologous recombination repair are RAD51 and the five RAD51 paralogs, RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3. In the absence of Rad51d, human and mouse cells fail to proliferate, and mice defective for Rad51d die before birth, likely as a result of genomic instability and p53 activation. Here, we report that a p53 deletion is sufficient to extend the life span of Rad51d-deficient embryos by up to 6 days and rescue the cell lethal phenotype. The Rad51d-/- Trp53-/- mouse embryo-derived fibroblasts were sensitive to DNA-damaging agents, particularly interstrand cross-links, and exhibited extensive chromosome instability including aneuploidy, chromosome fragments, deletions, and complex rearrangements. Additionally, loss of Rad51d resulted in increased centrosome fragmentation and reduced levels of radiation-induced RAD51-focus formation. Spontaneous frequencies of sister chromatid exchange were not affected by the absence of Rad51d, but sister chromatid exchange frequencies did fail to be induced upon challenge with the DNA cross-linking agent mitomycin C. These findings support a crucial role for mammalian RAD51D in normal development, recombination, and maintaining mammalian genome stability.

Arabidopsis Rad51B is important for double-strand DNA breaks repair in somatic cells

Rad51 paralogs belong to the Rad52 epistasis group of proteins and are involved in homologous recombination (HR), especially the assembly and stabilization of Rad51, which is a homolog of RecA in eukaryotes. We previously cloned and characterized two RAD51 paralogous genes in Arabidopsis, named AtRAD51C and AtXRCC3, which are considered the counterparts of human RAD51C and XRCC3, respectively. Here we describe the identification of RAD51B homologue in Arabidopsis, AtRAD51B. We found a higher expression of AtRAD51B in flower buds and roots. Expression of AtRAD51B was induced by genotoxic stresses such as ionizing irradiation and treatment with a cross-linking reagent, cisplatin. Yeast two-hybrid analysis showed that AtRad51B interacted with AtRad51C. We also found and characterized T-DNA insertion mutant lines. The mutant lines were devoid of AtRAD51B expression, viable and fertile. The mutants were moderately sensitive to gamma-ray and hypersensitive to cisplatin. Our results suggest that AtRAD51B gene product is involved in the repair of double-strand DNA breaks (DSBs) via HR.

Differing requirements for the Arabidopsis Rad51 paralogs in meiosis and DNA repair

In addition to the recombinase Rad51, vertebrates have five paralogs of Rad51, all members of the Rad51-dependent recombination pathway. These paralogs form two complexes (Rad51C/Xrcc3 and Rad51B/C/D/Xrcc2), which play roles in somatic recombination, DNA repair and chromosome stability. However, little is known of their possible involvement in meiosis, due to the inviability of the corresponding knockout mice. We have recently reported that the Arabidopsis homolog of one of these Rad51 paralogs (AtXrcc3) is involved in DNA repair and meiotic recombination and present here Arabidopsis lines carrying mutations in three other Rad51 paralogs (AtRad51B, AtRad51C and AtXrcc2). Disruption of any one of these paralogs confers hypersensitivity to the DNA cross-linking agent Mitomycin C, but not to gamma-irradiation. Moreover, the atrad51c-1 mutant is the only one of these to show meiotic defects similar to those of the atxrcc3 mutant, and thus only the Rad51C/Xrcc3 complex is required to achieve meiosis. These results support conservation of functions of the Rad51 paralogs between vertebrates and plants and differing requirements for the Rad51 paralogs in meiosis and DNA repair.

Genomic structure and multiple alternative transcripts of the mouse TRAD/RAD51L3/RAD51D gene, a member of the recA/RAD51 gene family

The RecA/RAD51 family plays a central role in DNA recombinational repair. The targeted disruption of mouse RAD51L3/TRAD is lethal during embryogenesis, suggesting that this protein is essential for development. Recently, we reported multiple alternative splice variants of human RAD51L3/TRAD transcripts. In this study, we have identified multiple mouse transcript variants. Complete sequence analysis of the genomic and cDNA clones has confirmed that the exon-intron structures obey the GT/AG splicing rule, and that the multiplicity of the transcripts is due to alternative splicing. In addition, we have determined the transcription initiation site by rapid amplification of cDNA 5'-end (5'-RACE). These results show that the mouse gene structure is very similar to that of humans.

Human Rad51C deficiency destabilizes XRCC3, impairs recombination, and radiosensitizes S/G2-phase cells

The highly conserved Rad51 protein plays an essential role in repairing DNA damage through homologous recombination. In vertebrates, five Rad51 paralogs (Rad51B, Rad51C, Rad51D, XRCC2, and XRCC3) are expressed in mitotically growing cells and are thought to play mediating roles in homologous recombination, although their precise functions remain unclear. Among the five paralogs, Rad51C was found to be a central component present in two complexes, Rad51C-XRCC3 and Rad51B-Rad51C-Rad51D-XRCC2. We have shown previously that the human Rad51C protein exhibits three biochemical activities, including DNA binding, ATPase, and DNA duplex separation. Here we report the use of RNA interference to deplete expression of Rad51C protein in human HT1080 and HeLa cells. In HT1080 cells, depletion of Rad51C by small interfering RNA caused a significant reduction of frequency in homologous recombination. The level of XRCC3 protein was also sharply reduced in Rad51C-depleted HeLa cells, suggesting that XRCC3 is dependent for its stability upon heterodimerization with Rad51C. In addition, Rad51C-depleted HeLa cells showed hypersensitivity to the DNA-cross-linking agent mitomycin C and moderately increased sensitivity to ionizing radiation. Importantly, the radiosensitivity of Rad51C-deficient HeLa cells was evident in S and G(2)/M phases of the cell cycle but not in G(1) phase. Together, these results provide direct cellular evidence for the function of human Rad51C in homologous recombinational repair.

DNA double-strand break repair by homologous recombination

DNA double-strand breaks (DSB) are presumed to be the most deleterious DNA lesions as they disrupt both DNA strands. Homologous recombination (HR), single-strand annealing, and non-homologous end-joining are considered to be the pathways for repairing DSB. In this review, we focus on DSB repair by HR. The proteins involved in this process as well as the interactions among them are summarized and characterized. The main emphasis is on eukaryotic cells, particularly the budding yeast Saccharomyces cerevisiae and mammals. Only the RAD52 epistasis group proteins are included.

Preferential binding to branched DNA strands and strand-annealing activity of the human Rad51B, Rad51C, Rad51D and Xrcc2 protein complex

The Rad51B, Rad51C, Rad51D and Xrcc2 proteins are Rad51 paralogs, and form a complex (BCDX2 complex) in mammalian cells. Mutant cells defective in any one of the Rad51-paralog genes exhibit spontaneous genomic instability and extreme sensitivity to DNA-damaging agents, due to inefficient recombinational repair. Therefore, the Rad51 paralogs play important roles in the maintenance of genomic integrity through recombinational repair. In the present study, we examined the DNA-binding preference of the human BCDX2 complex. Competitive DNA-binding assays using seven types of DNA substrates, single-stranded DNA (ssDNA), double-stranded DNA, 5'- and 3'-tailed duplexes, nicked duplex DNA, Y-shaped DNA and a synthetic Holliday junction, revealed that the BCDX2 complex preferentially bound to the two DNA substrates with branched structures (the Y-shaped DNA and the synthetic Holliday junction). Furthermore, the BCDX2 complex catalyzed the strand-annealing reaction between a long linear ssDNA (1.2 kb in length) and its complementary circular ssDNA. These properties of the BCDX2 complex may be important for its roles in the maintenance of chromosomal integrity.

Domain mapping of the Rad51 paralog protein complexes

The five human Rad51 paralogs are suggested to play an important role in the maintenance of genome stability through their function in DNA double-strand break repair. These proteins have been found to form two distinct complexes in vivo, Rad51B-Rad51C-Rad51D-Xrcc2 (BCDX2) and Rad51C-Xrcc3 (CX3). Based on the recent Pyrococcus furiosus Rad51 structure, we have used homology modeling to design deletion mutants of the Rad51 paralogs. The models of the human Rad51B, Rad51C, Xrcc3 and murine Rad51D (mRad51D) proteins reveal distinct N-terminal and C-terminal domains connected by a linker region. Using yeast two-hybrid and co-immunoprecipitation techniques, we have demonstrated that a fragment of Rad51B containing amino acid residues 1-75 interacts with the C-terminus and linker of Rad51C, residues 79-376, and this region of Rad51C also interacts with mRad51D and Xrcc3. We have also determined that the N-terminal domain of mRad51D, residues 4-77, binds to Xrcc2 while the C-terminal domain of mRad51D, residues 77-328, binds Rad51C. By this, we have identified the binding domains of the BCDX2 and CX3 complexes to further characterize the interaction of these proteins and propose a scheme for the three-dimensional architecture of the BCDX2 and CX3 paralog complexes.

Functional interaction between the Bloom's syndrome helicase and the RAD51 paralog, RAD51L3 (RAD51D)

Bloom's syndrome (BS) is a genetic disorder associated with short stature, fertility defects, and a predisposition to the development of cancer. BS cells are characterized by genomic instability; in particular, a high rate of reciprocal exchanges between sister-chromatids and homologous chromosomes. The BS gene product, BLM, is a helicase belonging to the highly conserved RecQ family. BLM is known to form a complex with the RAD51 recombinase, and to act upon DNA intermediates that form during homologous recombination, including D-loops and Holliday junctions. Here, we show that BLM also makes a direct physical association with the RAD51L3 protein (also known as RAD51D), a so-called RAD51 paralog that shows limited sequence similarity to RAD51 itself. This interaction is mediated through the N-terminal domain of BLM. To analyze functional interactions between BLM and RAD51L3, we have purified a heteromeric complex comprising RAD51L3 and a second RAD51 paralog, XRCC2. We show that the RAD51L3-XRCC2 complex stimulates BLM to disrupt synthetic 4-way junctions that model the Holliday junction. We also show that a truncated form of BLM, which retains helicase activity but is unable to bind RAD51L3, is not stimulated by the RAD51L3-XRCC2 complex. Our data indicate that the activity of BLM is modulated through an interaction with the RAD51L3-XRCC2 complex, and that this stimulatory effect on BLM is dependent upon a direct physical association between the BLM and RAD51L3 proteins. We propose that BLM co-operates with RAD51 paralogs during the late stages of homologous recombination processes that serve to restore productive DNA replication at sites of damaged or stalled replication forks.

Region and amino acid residues required for Rad51C binding in the human Xrcc3 protein

The Xrcc3 protein, which is required for the homologous recombinational repair of damaged DNA, forms a complex with the Rad51C protein in human cells. Mutations in either the Xrcc3 or Rad51C gene cause extreme sensitivity to DNA-damaging agents and generate the genomic instability frequently found in tumors. In the present study, we found that the Xrcc3 segment containing amino acid residues 63-346, Xrcc3(63-346), is the Rad51C-binding region. Biochemical analyses revealed that Xrcc3(63-346) forms a complex with Rad51C, and the Xrcc3(63-346)- Rad51C complex possesses ssDNA and dsDNA binding abilities comparable to those of the full-length Xrcc3-Rad51C complex. Based on the structure of RecA, which is thought to be the ancestor of Xrcc3, six Xrcc3 point mutants were designed. Two-hybrid and biochemical analyses of the Xrcc3 point mutants revealed that Tyr139 and Phe249 are essential amino acid residues for Rad51C binding. Superposition of the Xrcc3 Tyr139 and Phe249 residues on the RecA structure suggested that Tyr139 may function to ensure proper folding and Phe249 may be important to constitute the Rad51C-binding interface in Xrcc3.

The mammalian XRCC genes: their roles in DNA repair and genetic stability

Analysis of the XRCC genes has played an important part in understanding mammalian DNA repair processes, especially those involved in double-strand break (DSB) repair. Most of these genes were identified through their ability to correct DNA damage hypersensitivity in rodent cell lines, and they represent components of several different repair pathways including base-excision repair, non-homologous end joining, and homologous recombination. We document the phenotypic effects of mutation of the XRCC genes, and the current state of our knowledge of their functions. In addition to their continuing importance in discovering mechanisms of DNA repair, analysis of the XRCC genes is making a substantial contribution to the understanding of specific human disorders, including cancer.

Holliday junction binding activity of the human Rad51B protein

The human Rad51B protein is involved in the recombinational repair of damaged DNA. Chromosomal rearrangements of the Rad51B gene have been found in uterine leiomyoma patients, suggesting that the Rad51B gene suppresses tumorigenesis. In the present study, we found that the purified Rad51B protein bound to single-stranded DNA and double-stranded DNA in the presence of ATP and either Mg(2+) or Mn(2+) and hydrolyzed ATP in a DNA-dependent manner. When the synthetic Holliday junction was present along with the half-cruciform and double-stranded oligonucleotides, the Rad51B protein only bound to the synthetic Holliday junction, which mimics a key intermediate in homologous recombination. In contrast, the human Rad51 protein bound to all three DNA substrates with no obvious preference. Therefore, the Rad51B protein may have a specific function in Holliday junction processing in the homologous recombinational repair pathway in humans.

Complex formation by the human Rad51B and Rad51C DNA repair proteins and their activities in vitro

The human Rad51 protein is essential for DNA repair by homologous recombination. In addition to Rad51 protein, five paralogs have been identified: Rad51B/Rad51L1, Rad51C/Rad51L2, Rad51D/Rad51L3, XRCC2, and XRCC3. To further characterize a subset of these proteins, recombinant Rad51, Rad51B-(His)(6), and Rad51C proteins were individually expressed employing the baculovirus system, and each was purified from Sf9 insect cells. Evidence from nickel-nitrilotriacetic acid pull-down experiments demonstrates a highly stable Rad51B.Rad51C heterodimer, which interacts weakly with Rad51. Rad51B and Rad51C proteins were found to bind single- and double-stranded DNA and to preferentially bind 3'-end-tailed double-stranded DNA. The ability to bind DNA was elevated with mixed Rad51 and Rad51C, as well as with mixed Rad51B and Rad51C, compared with that of the individual protein. In addition, both Rad51B and Rad51C exhibit DNA-stimulated ATPase activity. Rad51C displays an ATP-independent apparent DNA strand exchange activity, whereas Rad51B shows no such activity; this apparent strand exchange ability results actually from a duplex DNA destabilization capability of Rad51C. By analogy to the yeast Rad55 and Rad57, our results suggest that Rad51B and Rad51C function through interactions with the human Rad51 recombinase and play a crucial role in the homologous recombinational repair pathway.

Molecular cloning and characterization of RAD51-like genes from Arabidopsis thaliana

Homologous recombination is an essential process for the maintenance and variability of the genome. In eukaryotes, the Rad52 epistasis group proteins serve the main role for meiotic recombination and/or homologous recombinational repair. Rad51-like proteins, such as Rad55 and Rad57 in yeast, play a role in assembly or stabilization of multimeric Rad51 that promotes homologous pairing and strand exchange reactions. We cloned two RAD51-like genes named AtXRCC3 and AtRAD51C from Arabidopsis thaliana. Both AtXRCC3 and AtRAD51C expressed two alternatively spliced transcripts, and AtRAD51C produced two different sizes of isoforms, a long (AtRAD51Calpha) and a short one (AtRAD51Cbeta). The predicted protein sequences of these genes showed characteristic features of the RecA/Rad51 family; especially the amino acids around the ATP-binding motifs were well conserved. The transcripts of AtXRCC3 and AtRAD51C were detected in various tissues, with the highest level of expression in flower buds. Expression of both genes was induced by gamma-ray irradiation. The results of yeast two-hybrid assays suggested that Arabidopsis Rad51 family proteins form a complex, which could participate in meiotic recombination and/or homologous recombinational repair.

Role of mammalian RAD51L2 (RAD51C) in recombination and genetic stability

The highly conserved RAD51 protein has a central role in homologous recombination. Five novel RAD51-like genes have been identified in mammalian cells, but little is known about their functions. A DNA damage-sensitive hamster cell line, irs3, was found to have a mutation in the RAD51L2 gene and an undetectable level of RAD51L2 protein. Resistance of irs3 to DNA-damaging agents was significantly increased by expression of the human RAD51L2 gene, but not by other RAD51-like genes or RAD51 itself. Consistent with a role for RAD51L2 in homologous recombination, irs3 cells show a reduction in sister chromatid exchange, an increase in isochromatid breaks, and a decrease in damage-dependent RAD51 focus formation compared with wild type cells. As recently demonstrated for human cells, we show that RAD51L2 forms part of two separate complexes of hamster RAD51-like proteins. Strikingly, neither complex of RAD51-like proteins is formed in irs3 cells. Our results demonstrate that RAD51L2 has a key role in mammalian RAD51-dependent processes, contingent on the formation of protein complexes involved in homologous recombination repair.

Mammalian Rad51C contributes to DNA cross-link resistance, sister chromatid cohesion and genomic stability

The eukaryotic Rad51 protein is a structural and functional homolog of Escherichia coli RecA with a role in DNA repair and genetic recombination. Five paralogs of Rad51 have been identified in vertebrates, Rad51B, Rad51C, Rad51D, Xrcc2 and Xrcc3, which are also implicated in recombination and genome stability. Here, we identify a mammalian cell mutant in Rad51C. We show that the Chinese hamster cell mutant, CL-V4B, has a defect in Rad51C. Sequencing of the hamster Rad51C cDNA revealed a 132 bp deletion corresponding to an alternatively spliced transcript with lack of exon 5. CL-V4B was hypersensitive to the interstrand cross-linking agents mitomycin C (MMC) and cisplatinum, the alkylating agent methyl methanesulfonate and the topoisomerase I inhibitor campthotecin and showed impaired Rad51 foci formation in response to DNA damage. The defect in Rad51C also resulted in an increase of spontaneous and MMC-induced chromosomal aberrations as well as a lack of induction of sister chromatid exchanges. However, centrosome formation was not affected. Intriguingly, a reduced level of sister chromatid cohesion was found in CL-V4B cells. These results reveal a role for Rad51C that is unique among the Rad51 paralogs.

Homologous pairing and ring and filament structure formation activities of the human Xrcc2*Rad51D complex

The Xrcc2 and Rad51D/Rad51L3 proteins, which belong to the Rad51 paralogs, are required for homologous recombinational repair (HRR) in vertebrates. The Xrcc2 and Rad51D/Rad51L3 genes, whose products interact with each other, have essential roles in ensuring normal embryonic development. In the present study, we coexpressed the human Xrcc2 and Rad51D/Rad51L3 proteins (Xrcc2 and Rad51D, respectively) in Escherichia coli, and purified the Xrcc2*Rad51D complex to homogeneity. The Xrcc2 small middle dotRad51D complex catalyzed homologous pairing between single-stranded and double-stranded DNA, similar to the function of the Xrcc3*Rad51C complex, which is another complex of the Rad51 paralogs. An electron microscopic analysis showed that Xrcc2*Rad51D formed a multimeric ring structure in the absence of DNA. In the presence of ssDNA, Xrcc2*Rad51D formed a filamentous structure, which is commonly observed among the human homologous pairing proteins, Rad51, Rad52, and Xrcc3*Rad51C.

RAD51C interacts with RAD51B and is central to a larger protein complex in vivo exclusive of RAD51

RAD51B and RAD51C are two of five known paralogs of the human RAD51 protein that are thought to function in both homologous recombination and DNA double-strand break repair. This work describes the in vitro and in vivo identification of the RAD51B/RAD51C heterocomplex. The RAD51B/RAD51C heterocomplex was isolated and purified by immunoaffinity chromatography from insect cells co-expressing the recombinant proteins. Moreover, co-immunoprecipitation of the RAD51B and RAD51C proteins from HeLa, MCF10A, and MCF7 cells strongly suggests the existence of an endogenous RAD51B/RAD51C heterocomplex. We extended these observations to examine the interaction between the RAD51B/RAD51C complex and the other RAD51 paralogs. Immunoprecipitation using protein-specific antibodies showed that RAD51C is central to a single large protein complex and/or several smaller complexes with RAD51B, RAD51D, XRCC2, and XRCC3. However, our experiments showed no evidence for the inclusion of RAD51 within these complexes. Further analysis is required to elucidate the function of the RAD51B/RAD51C heterocomplex and its association with the other RAD51 paralogs in the processes of homologous recombination and DNA double-strand break repair.

The organization of Physcomitrella patensRAD51 genes is unique among eukaryotic organisms

Genetic recombination pathways and genes are well studied, but relatively little is known in plants, especially in lower plants. To study the recombination apparatus of a lower land plant, a recombination gene well characterized particularly in yeast, mouse, and man, the RAD51 gene, was isolated from the moss Physcomitrella patens and characterized. Two highly homologous RAD51 genes were found to be present. Duplicated RAD51 genes have been found thus far exclusively in eukaryotes with duplicated genomes. Therefore the presence of two highly homologous genes suggests a recent genome duplication event in the ancestry of Physcomitrella. Comparison of the protein sequences to Rad51 proteins from other organisms showed that both RAD51 genes originated within the group of plant Rad51 proteins. However, the two proteins form a separate clade in a phylogenetic tree of plant Rad51 proteins. In contrast to RAD51 genes from other multicellular eukaryotes, the Physcomitrella genes are not interrupted by introns. Because introns are a common feature of Physcomitrella genes, the lack of introns in the RAD51 genes is unusual and may indicate the presence of an unusual recombination apparatus in this organism. The presence of duplicated intronless RAD51 genes is unique among eukaryotes. Studies of further members of this lineage are needed to determine whether this feature may be typical of lower plants.

Involvement of Rad51C in two distinct protein complexes of Rad51 paralogs in human cells

Genetic studies in rodent and chicken mutant cell lines have suggested that Rad51 paralogs (XRCC2, XRCC3, Rad51B/Rad51L1, Rad51C/Rad51L2 and Rad51D/Rad51L3) play important roles in homologous recombinational repair of DNA double-strand breaks and in maintaining chromosome stability. Previous studies using yeast two- and three-hybrid systems have shown interactions among these proteins, but it is not clear whether these interactions occur simultaneously or sequentially in vivo. By utilizing immunoprecipitation with extracts of human cells expressing epitope-tagged Rad51 paralogs, we demonstrate that XRCC2 and Rad51D, while stably interacting with each other, co-precipitate with Rad51C but not with XRCC3. In contrast, Rad51C is pulled down with XRCC3, whereas XRCC2 and Rad51D are not. In addition, Rad51B could be pulled down with Rad51C and Rad51D, but not with XRCC3. These results suggest that Rad51C is involved in two distinct in vivo complexes: Rad51B-Rad51C-Rad51D-XRCC2 and Rad51C-XRCC3. In addition, we demonstrate that Rad51 co-precipitates with XRCC3 but not with XRCC2 or Rad51D, suggesting that Rad51 can be present in an XRCC3-Rad51C-Rad51 complex. These complexes may act as functional units and serve accessory roles for Rad51 in the presynapsis stage of homologous recombinational repair.

Interactions involving the Rad51 paralogs Rad51C and XRCC3 in human cells

Homologous recombinational repair of DNA double-strand breaks and crosslinks in human cells is likely to require Rad51 and the five Rad51 paralogs (XRCC2, XRCC3, Rad51B/Rad51L1, Rad51C/Rad51L2 and Rad51D/Rad51L3), as has been shown in chicken and rodent cells. Previously, we reported on the interactions among these proteins using baculovirus and two- and three-hybrid yeast systems. To test for interactions involving XRCC3 and Rad51C, stable human cell lines have been isolated that express (His)6-tagged versions of XRCC3 or Rad51C. Ni2+-binding experiments demonstrate that XRCC3 and Rad51C interact in human cells. In addition, we find that Rad51C, but not XRCC3, interacts directly or indirectly with Rad51B, Rad51D and XRCC2. These results argue that there are at least two complexes of Rad51 paralogs in human cells (Rad51C-XRCC3 and Rad51B-Rad51C-Rad51D-XRCC2), both containing Rad51C. Moreover, Rad51 is not found in these complexes. X-ray treatment did not alter either the level of any Rad51 paralog or the observed interactions between paralogs. However, the endogenous level of Rad51C is moderately elevated in the XRCC3-overexpressing cell line, suggesting that dimerization between these proteins might help stabilize Rad51C.

Identification and purification of two distinct complexes containing the five RAD51 paralogs

Cells defective in any of the RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3) are sensitive to DNA cross-linking agents and to ionizing radiation. Because the paralogs are required for the assembly of DNA damage-induced RAD51 foci, and mutant cell lines are defective in homologous recombination and show genomic instability, their defect is thought to be caused by an inability to promote efficient recombinational repair. Here, we show that the five paralogs exist in two distinct complexes in human cells: one contains RAD51B, RAD51C, RAD51D, and XRCC2 (defined as BCDX2), whereas the other consists of RAD51C with XRCC3. Both protein complexes have been purified to homogeneity and their biochemical properties investigated. BCDX2 binds single-stranded DNA and single-stranded gaps in duplex DNA, in accord with the proposal that the paralogs play an early (pre-RAD51) role in recombinational repair. Moreover, BCDX2 complex binds specifically to nicks in duplex DNA. We suggest that the extreme sensitivity of paralog-defective cell lines to cross-linking agents is owing to defects in the processing of incised cross links and the consequential failure to initiate recombinational repair at these sites.

Rad52 partially substitutes for the Rad51 paralog XRCC3 in maintaining chromosomal integrity in vertebrate cells

Yeast Rad52 DNA-repair mutants exhibit pronounced radiation sensitivity and a defect in homologous re combination (HR), whereas vertebrate cells lacking Rad52 exhibit a nearly normal phenotype. Bio chemical studies show that both yeast Rad52 and Rad55-57 (Rad51 paralogs) stimulate DNA-strand exchange mediated by Rad51. These findings raise the possibility that Rad51 paralogs may compensate for lack of Rad52 in vertebrate cells, explaining the absence of prominent phenotypes for Rad52-deficient cells. To test this hypothesis, using chicken DT40 cells, we generated conditional mutants deficient in both RAD52 and XRCC3, which is one of the five vertebrate RAD51 paralogs. Surprisingly, the rad52 xrcc3 double-mutant cells were non-viable and exhibited extensive chromosomal breaks, whereas rad52 and xrcc3 single mutants grew well. Our data reveal an overlapping (but non-reciprocal) role for Rad52 and XRCC3 in repairing DNA double-strand breaks. The present study shows that Rad52 can play an important role in HR repair by partially substituting for a Rad51 paralog.

Repair of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

Complex formation by the human RAD51C and XRCC3 recombination repair proteins

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

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

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

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.