Identification of a novel human RAD51 homolog, RAD51B

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.

Etiology and pathogenesis of uterine leiomyomas: a review

Uterine leiomyomas, or fibroids, represent a major public health problem. It is believed that these tumors develop in the majority of American women and become symptomatic in one-third of these women. They are the most frequent indication for hysterectomy in the United States. Although the initiator or initiators of fibroids are unknown, several predisposing factors have been identified, including age (late reproductive years), African-American ethnicity, nulliparity, and obesity. Nonrandom cytogenetic abnormalities have been found in about 40% of tumors examined. Estrogen and progesterone are recognized as promoters of tumor growth, and the potential role of environmental estrogens has only recently been explored. Growth factors with mitogenic activity, such as transforming growth factor- (subscript)3(/subscript), basic fibroblast growth factor, epidermal growth factor, and insulin-like growth factor-I, are elevated in fibroids and may be the effectors of estrogen and progesterone promotion. These data offer clues to the etiology and pathogenesis of this common condition, which we have analyzed and summarized in this review.

A novel testis-specific RAG2-like protein, Peas: its expression in pachytene spermatocyte cytoplasm and meiotic chromatin

We report a novel gene Peas that constitutes an overlapping gene complex in mammalian genome. We have cloned human and mouse Peas cDNAs (hPEAS/mPeas) and analyzed their tissue and stage-specific expressions. Peas protein contains six repeated kelch motifs, structurally similar to RAG2, a V(D)J recombination activator, and is evolutionarily conserved among mammals, birds, insects, and nematodes. Northern, RNA in situ hybridization and immunohistochemical analyses showed that mPeas is specifically transcribed in testis, particularly in pachytene spermatocytes in which it is localized to the cytoplasm and meiotic chromatin. It is suggested that Peas may be involved in meiotic recombination process.

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.

BCR/ABL regulates response to DNA damage: the role in resistance to genotoxic treatment and in genomic instability

BCR/ABL regulates cell proliferation, apoptosis, differentiation and adhesion. In addition, BCR/ABL can induce resistance to cytostatic drugs and irradiation by modulation of DNA repair mechanisms, cell cycle checkpoints and Bcl-2 protein family members. Upon DNA damage BCR/ABL not only enhances reparation of DNA lesions (e.g. homologous recombination repair), but also prolongs activation of cell cycle checkpoints (e.g. G2/M) providing more time for repair of otherwise lethal lesions. Moreover, by modification of anti-apoptotic members of the Bcl-2 family (e.g. upregulation of Bcl-x(L)) BCR/ABL provides a cytoplasmic 'umbrella' protecting mitochondria from the 'rain' of apoptotic signals coming from the damaged DNA in the nucleus, thus preventing release of cytochrome c and activation of caspases. The unrepaired and/or aberrantly repaired (but not lethal) DNA lesions resulting from spontaneous and/or drug-induced damage can accumulate in BCR/ABL-transformed cells leading to genomic instability and malignant progression of the disease. Inhibition of BCR/ABL kinase activity by STI571 (Gleevec, imatinib mesylate) reverses drug resistance and, in combination with standard chemotherapeutics can exert strong anti-leukemia effect.

Role of RAD52 epistasis group genes in homologous recombination and double-strand break repair

The process of homologous recombination is a major DNA repair pathway that operates on DNA double-strand breaks, and possibly other kinds of DNA lesions, to promote error-free repair. Central to the process of homologous recombination are the RAD52 group genes (RAD50, RAD51, RAD52, RAD54, RDH54/TID1, RAD55, RAD57, RAD59, MRE11, and XRS2), most of which were identified by their requirement for the repair of ionizing-radiation-induced DNA damage in Saccharomyces cerevisiae. The Rad52 group proteins are highly conserved among eukaryotes, and Rad51, Mre11, and Rad50 are also conserved in prokaryotes and archaea. Recent studies showing defects in homologous recombination and double-strand break repair in several human cancer-prone syndromes have emphasized the importance of this repair pathway in maintaining genome integrity. Although sensitivity to ionizing radiation is a universal feature of rad52 group mutants, the mutants show considerable heterogeneity in different assays for recombinational repair of double-strand breaks and spontaneous mitotic recombination. Herein, I provide an overview of recent biochemical and structural analyses of the Rad52 group proteins and discuss how this information can be incorporated into genetic studies of recombination.

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.

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.

Genomic integrity and the repair of double-strand DNA breaks

The induction of double-strand breaks (DSBs) in DNA by exposure to DNA damaging agents or as intermediates in normal cellular processes, creates a severe threat for the integrity of the genome. Unrepaired or incorrectly repaired DSBs lead to broken chromosomes and/or gross chromosomal rearrangements which are frequently associated with tumor formation in mammals. To maintain the integrity of the genome and to prevent the formation of chromosomal aberrations, several pathways exist in eukaryotes: homologous recombination (HR), non-homologous end joining (NHEJ) and single-strand annealing (SSA). These mechanisms are conserved in evolution, but the relative contribution depends on the organism, cell type and stage of the cell cycle. In yeast, DSBs are primarily repaired via HR while in higher eukaryotes, both HR and NHEJ are important. In mammals, defects in both HR or NHEJ lead to a predisposition to cancer and at the cellular level, the frequency of chromosomal aberrations is increased. This review summarizes our current knowledge about DSB-repair with emphasis on recent progress in understanding the precise biochemical activities of individual proteins involved.

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.

Role of BRCA2 in control of the RAD51 recombination and DNA repair protein

Individuals carrying BRCA2 mutations are predisposed to breast and ovarian cancers. Here, we show that BRCA2 plays a dual role in regulating the actions of RAD51, a protein essential for homologous recombination and DNA repair. First, interactions between RAD51 and the BRC3 or BRC4 regions of BRCA2 block nucleoprotein filament formation by RAD51. Alterations to the BRC3 region that mimic cancer-associated BRCA2 mutations fail to exhibit this effect. Second, transport of RAD51 to the nucleus is defective in cells carrying a cancer-associated BRCA2 truncation. Thus, BRCA2 regulates both the intracellular localization and DNA binding ability of RAD51. Loss of these controls following BRCA2 inactivation may be a key event leading to genomic instability and tumorigenesis.

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.

The Rad51 paralog Rad51B promotes homologous recombinational repair

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

Recombination factors of Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been an excellent genetic and biochemical model for our understanding of homologous recombination. Central to the process of homologous recombination are the products of the RAD52 epistasis group of genes, whose functions we now know include the nucleolytic processing of DNA double-stand breaks, the ability to conduct a DNA homology search, and the capacity to promote the exchange of genetic information between homologous regions on recombining chromosomes. It is also clear that the basic functions of the RAD52 group of genes have been highly conserved among eukaryotes. Disruption of this important process causes genomic instability, which can result in a number of unsavory consequences, including tumorigenesis and cell death.

Evidence for simultaneous protein interactions between human Rad51 paralogs

In yeast, the Rad51-related proteins include Rad55 and Rad57, which form a heterodimer that interacts with Rad51. Five human Rad51 paralogs have been identified (XRCC2, XRCC3, Rad51B/Rad51L1, Rad51C/Rad51L2, and Rad51D/Rad51L3), and each interacts with one or more of the others. Previously we reported that HsRad51 interacts with XRCC3, and Rad51C interacts with XRCC3, Rad51B, and HsRad51. Here we report that in the yeast two-hybrid system, Rad51D interacts with XRCC2 and Rad51C. No other interactions, including self-interactions, were found, indicating that the observed interactions are specific. The yeast Rad51 interacts with human Rad51 and XRCC3, suggesting Rad51 conservation since the human yeast divergence. Data from yeast three-hybrid experiments indicate that a number of the pairs of interactions between human Rad51 paralogs can occur simultaneously. For example, Rad51B expression enhances the binding of Rad51C to XRCC3 and to HsRad51D, and Rad51C expression allows the indirect interaction of Rad51B with Rad51D. Experiments using 6xHis-tagged proteins in the baculovirus system confirm several of our yeast results, including Rad51B interaction with Rad51D only when Rad51C is simultaneously expressed and Rad51C interaction with XRCC2 only when Rad51D is present. These results suggest that these proteins may participate in one complex or multiple smaller ones.

A recombination repair gene of Schizosaccharomyces pombe, rhp57, is a functional homolog of the Saccharomyces cerevisiae RAD57 gene and is phylogenetically related to the human XRCC3 gene

To identify Schizosaccharomyces pombe genes involved in recombination repair, we identified seven mutants that were hypersensitive to both methyl methanesulfonate (MMS) and gamma-rays and that contained mutations that caused synthetic lethality when combined with a rad2 mutation. One of the mutants was used to clone the corresponding gene from a genomic library by complementation of the MMS-sensitive phenotype. The gene obtained encodes a protein of 354 amino acids whose sequence is 32% identical to that of the Rad57 protein of Saccharomyces cerevisiae. An rhp57 (RAD57 homolog of S. pombe) deletion strain was more sensitive to MMS, UV, and gamma-rays than the wild-type strain and showed a reduction in the frequency of mitotic homologous recombination. The MMS sensitivity was more severe at lower temperature and was suppressed by the presence of a multicopy plasmid bearing the rhp51 gene. An rhp51 rhp57 double mutant was as sensitive to UV and gamma-rays as an rhp51 single mutant, indicating that rhp51 function is epistatic to that of rhp57. These characteristics of the rhp57 mutants are very similar to those of S. cerevisiae rad57 mutants. Phylogenetic analysis suggests that Rhp57 and Rad57 are evolutionarily closest to human Xrcc3 of the RecA/Rad51 family of proteins.

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

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

HsRec2/Rad51L1, a protein influencing cell cycle progression, has protein kinase activity

Human Rec2/Rad51L1 is a member of the Rad51 family of proteins. Although recombinase activity, typical of this family, could not be established, its overexpression in mammalian cells has been shown to cause a delay in G1. Moreover, since hsRec2/Rad51L1 has been found to be induced by both ionizing and UV irradiation, it is likely that hsRec2/Rad51L1 is elevated following any DNA damage and causes a G1 delay to allow time for DNA repair to occur. Limited homology with catalytic domains X and XI of protein kinase A suggested that kemptide, an artificial substrate containing one phosphorylatable residue, a serine, might serve as a substrate for hsRec2/Rad51L1. Here, we report that hsRec2/Rad51L1 can phosphorylate kemptide, as well as myelin basic protein, p53, cyclin E, and cdk2, but not a peptide substrate containing tyrosine only. The finding that hsRec2/Rad51L1 exhibits protein kinase activity is a first step toward identifying a mechanism whereby this protein affects the cell cycle.

Disruption of muREC2/RAD51L1 in mice results in early embryonic lethality which can Be partially rescued in a p53(-/-) background

muREC2/RAD51L1 is a radiation-inducible gene that regulates cell cycle progression. To elucidate the biological function of muREC2/RAD51L1, the gene was disrupted in embryonic stem cells by homologous recombination. Mice heterozygous for muREC2/RAD51L1 appear normal and fertile; however, no homozygous pups were born after interbreeding of heterozygous mice. Timed pregnancy studies showed that homozygous mutant embryos were severely retarded in growth as early as ca. 5 days gestation (E5.5) and were completely resorbed by E8.5. Mutant blastocyst outgrowth was also severely impaired in a double-knockout embryo, but embryonic development did progress further in a p53-null background. These results suggest that muREC2/RAD51L1 plays a role in cell proliferation and early embryonic development, perhaps through interaction with p53.

hREC2, a RAD51-like gene, is disrupted by t(12;14) (q15;q24.1) in a uterine leiomyoma

A balanced translocation between chromosomes 12 and 14 is commonly seen in uterine leiomyoma (UL). We have previously cloned and characterized a 2 Mb segment of human chromosomal subband 14q24.1, and have shown that the t(12;14)(q15;q24.1) breakpoints from several ULs map within this region. Exon trapping of DNA clones spanning one such breakpoint revealed coding sequences from hREC2, a gene that shows significant amino acid sequence identity to the double-strand break repair enzyme RAD51. We report here that this breakpoint is located within a 19 kb intron of the hREC2 gene and that the translocation results in the premature truncation of the major hREC2 transcript. Mapping and sequence analyses show that alternative transcripts of the hREC2 gene, including novel isoforms identified in testis and uterus, are not interrupted by the translocation.

A double-strand break repair component is essential for S phase completion in fission yeast cell cycling

Fission yeast rad22(+), a homologue of budding yeast RAD52, encodes a double-strand break repair component, which is dispensable for proliferation. We, however, have recently obtained a cell division cycle mutant with a temperature-sensitive allele of rad22(+), designated rad22-H6, which resulted from a point mutation in the conserved coding sequence leading to one amino acid alteration. We have subsequently isolated rad22(+) and its novel homologue rti1(+) as multicopy suppressors of this mutant. rti1(+) suppresses all the defects of cells lacking rad22(+). Mating type switch-inactive heterothallic cells lacking either rad22(+) or rti1(+) are viable, but those lacking both genes are inviable and arrest proliferation with a cell division cycle phenotype. At the nonpermissive temperature, a synchronous culture of rad22-H6 cells performs DNA synthesis without delay and arrests with chromosomes seemingly intact and replication completed and with a high level of tyrosine-phosphorylated Cdc2. However, rad22-H6 cells show a typical S phase arrest phenotype if combined with the rad1-1 checkpoint mutation. rad22(+) genetically interacts with rad11(+), which encodes the large subunit of replication protein A. Deletion of rad22(+)/rti1(+) or the presence of rad22-H6 mutation decreases the restriction temperature of rad11-A1 cells by 4-6 degrees C and leads to cell cycle arrest with chromosomes incompletely replicated. Thus, in fission yeast a double-strand break repair component is required for a certain step of chromosome replication unlinked to repair, partly via interacting with replication protein A.

Human POMp75 is identified as the pro-oncoprotein TLS/FUS: both POMp75 and POMp100 DNA homologous pairing activities are associated to cell proliferation

We have previously developed an assay to measure DNA homologous pairing activities in crude extracts: The POM blot. In mammalian nuclear extracts, we detected two major DNA homologous pairing activities: POMp100 and POMp75. Here, we present the purification and identification of POMp75 as the pro-oncoprotein TLS/FUS. Because of the pro-oncogene status of TLS/FUS, we studied in addition, the relationships between cell proliferation and POM activities. We show that transformation of human fibroblasts by SV40 large T antigen results in a strong increase of both POMpl00 and TLS/POMp75 activities. Although detectable levels of both POMp100 and TLS/POMp75 are observed in non-immortalized fibroblasts or lymphocytes, fibroblasts at mid confluence or lymphocytes stimulated by phytohaemaglutinin, show higher levels of POM activities. Moreover, induction of differentiation of mouse F9 line by retinoic acid leads to the inhibition of both POMp100 and TLS/POMp75 activities. Comparison of POM activity of TLS/FUS with the amount of TLS protein detected by Western blot, suggests that the POM activity could be regulated by post-translation modification. Taken together, these results indicate that POMp100 and TLS/POMp75 activities are present in normal cells but are connected to cell proliferation. Possible relationship between cell proliferation, response to DNA damage and DNA homologous pairing activity of the pro-oncoprotein TLS/FUS are discussed.

A surfeit of RAD51-like genes?

The process of homologous recombination appears enigmatic: it creates genetic diversity but also provides an important way of repairing DNA damage without errors. The recent cloning and functional analysis of eukaryotic recombination genes suggests that at least part of the intricate mechanism involved is conserved from bacteria to humans, but is also yielding some surprises.

Multiple alternative transcripts of the human homologue of the mouse TRAD/R51H3/RAD51D gene, a member of the rec A/RAD51 gene family

Yeast RAD51, a homologue of Escherichia coli recA, plays a crucial role in mitotic and/or meiotic recombination and in the repair of double-strand DNA breaks. We have identified unique multiple alternative transcripts of a human TRAD/R51H3/RAD51D gene, a member of the recA/RAD51 gene family. One of the transcripts encoded a 328-amino-acid protein with 83.0% overall amino acid identity and 98. 2% similarity with the mouse TRAD gene and had two nucleotide binding consensus sequences, motif A and motif B, conserved among members of this family. Other transcripts encoded truncated proteins with a partial N-terminal region of the orthologue or short proteins lacking internal sequences which contain nucleotide binding motifs. Northern blot analysis revealed that multiple transcripts of the human TRAD gene were expressed in various tissues and their distribution was not ubiquitous.

Repair of ionizing radiation damage in mammalian cells. Alternative pathways and their fidelity

Ionizing radiation causes a variety of types of damage to DNA in cells, requiring the concerted action of a number of DNA repair enzymes to restore genomic integrity. The DNA base-excision repair and DNA double-strand break repair pathways are particularly important. While single base damages are rapidly excised and repaired using the opposite (undamaged) strand as a template, the correct repair of DNA double-strand breaks may present more difficulties to cellular enzymes owing to the loss of template. In the last few years evidence in support of several enzymatic pathways for the repair of such double-stranded damage has been found. At present we may distinguish at least three pathways: homologous recombination repair, non-homologous (DNA-PK-dependent) end joining, and repeat-driven end joining. This paper focuses on evidence for the first and third of these pathways, and considers in particular their relative importance in mammalian cells and implications for the fidelity of repair.

Sequestration of mammalian Rad51-recombination protein into micronuclei

The mammalian Rad51 protein is involved in homologous recombination and in DNA damage repair. Its nuclear distribution after DNA damage is highly dynamic, and distinct foci of Rad51 protein, distributed throughout the nuclear volume, are induced within a few hours after gamma irradiation; these foci then coalesce into larger clusters. Rad51-positive cells do not undergo DNA replication. Rad51 foci colocalize with both replication protein A and sites of unscheduled DNA repair synthesis and may represent a nuclear domain for recombinational DNA repair. By 24 h postirradiation, most foci are sequestered into micronuclei or assembled into Rad51-coated DNA fibers. These micronuclei and DNA fibers display genome fragmentation typical of apoptotic cell death. Other repair proteins, such as Rad52 and Gadd45, are not eliminated from the nucleus. DNA double strand breaks in repair-deficient cells or induced by the clastogen etoposide are also accompanied by the sequestering of Rad51 protein before cell death. The spindle poison colcemid causes cell cycle arrest and Rad51-foci formation without directly damaging DNA. Collectively, these observations suggest that mammalian Rad51 protein associates with damaged DNA and/or with DNA that is temporarily or irreversibly unable to replicate and these foci may subsequently be eliminated from the nucleus.

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

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

Human DNA repair systems: an overview

DNA repair systems act to maintain genome integrity in the face of replication errors, environmental insults, and the cumulative effects of age. More than 70 human genes directly involved in the five major pathways of DNA repair have been described, including chromosomal location and cDNA sequence. However, a great deal of information as to the precise functions of these genes and their role in human health is still lacking. Hence, we summarize what is known about these genes and their contra part in bacterial, yeast, and rodent systems and discuss their involvement in human disease. While some associations are already well understood, it is clear that additional diseases will be found which are linked to DNA repair defects or deficiencies.

The role of homologous recombination processes in the repair of severe forms of DNA damage in mammalian cells

The role of homologous recombination processes in the repair of severe forms of DNA damage is reviewed, with particular attention to the functions of members of the recA/RAD51 family of genes. In the yeast Saccharomyces cerevisiae, several of the gene products involved in homologous recombination repair (HRR) have been studied in detail, and a picture is beginning to emerge of the repair mechanism for DNA double-strand breaks. Knowledge is fragmentary for other eukaryotic organisms and for other types of DNA damage. In mammalian cells, while it has been known for some years that HRR occurs, the relative importance of the process in repairing DNA damage is unknown and very few of the gene products involved have been identified. Very recently, a number of RAD51-like genes have been identified in mammals, either through cloning genes complementing cell lines sensitive to DNA-damaging agents (XRCC2, XRCC3), or through homology searches (RAD51L1, RAD51L2, RAD51L3). As yet the role of these genes and their possible functions are speculative, although the combination of sequence conservation and gene expression patterns suggest that they function in HRR pathways.

Double-strand break repair by interchromosomal recombination: suppression of chromosomal translocations

To directly determine whether recombinational repair of double-strand breaks (DSBs) can occur between heterologous chromosomes and lead to chromosomal rearrangements in mammalian cells, we employed an ES cell system to analyze recombination between repeats on heterologous chromosomes. We found that recombination is induced at least 1000-fold following the introduction of a DSB in one repeat. Most (98%) recombinants repaired the DSB by gene conversion in which a small amount of sequence information was transferred from the unbroken chromosome onto the broken chromosome. The remaining recombinants transferred a larger amount of information, but still no chromosomal aberrations were apparent. Thus, mammalian cells are capable of searching genome-wide for sequences that are suitable for DSB repair. The lack of crossover events that would have led to translocations supports a model in which recombination is coupled to replication.

Molecular mechanisms of DNA double strand break repair

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

Xrcc3 is required for assembly of Rad51 complexes in vivo

Rad51 is a member of a family of eukaryotic proteins related to the bacterial recombinational repair protein RecA. Rad51 protein localizes to multiple subnuclear foci in Chinese hamster ovary cells. Subnuclear Rad51 foci are induced by ionizing radiation or the DNA cross-linking agent cisplatin. Formation of these foci is likely to reflect assembly of a multimeric form of Rad51 that promotes DNA repair. Formation of damage-induced Rad51 foci does not occur in the Chinese hamster ovary cell line irs1SF, which is sensitive to DNA damaging agents. The Rad51 focus formation defect of irs1SF cells is corrected by a construct that encodes the repair protein Xrcc3. Xrcc3 is a human homolog of Rad51 previously isolated by virtue of its ability to correct the radiation sensitivity of irs1SF cells. Changes in the steady state level of Rad51 protein do not account for the irs1SF defect nor do they account for the appearance of foci following DNA damage. These results suggest that Xrcc3 is required for the assembly or stabilization of a multimeric form of Rad51 during DNA repair. Cell lines defective in two different components of DNA protein kinase formed Rad51 foci in response to damage, indicating DNA protein kinase is not required for damaged-induced mobilization of Rad51.

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

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

Isolation and characterization of RAD51C, a new human member of the RAD51 family of related genes

The yeast and human RAD51 genes encode strand-transfer proteins that are thought to be involved in both recombinational repair of DNA damage and meiotic recombination. In yeast, the Rad51 family of related proteins also includes Rad55, Rad57 and Dmc1. In mammalian cells, five genes in this family have been identified (HsRAD51, XRCC2, XRCC3, RAD51B/hREC2 and HsDMC1), and here we report the isolation of the sixth member, RAD51C. RAD51C was originally identified by a computer screen of the EST database. A full-length approximately 1.3 kb cDNA clone has been isolated that encodes a protein of 376 aa, having a 18-26% aa identity with other human Rad51 family members. RAD51C includes a previously mapped sequenced-tagged site location near the end of chromosome 17q. The RAD51C transcript is expressed in various human tissues, with highest level of expression in testis, followed by heart muscle, spleen and prostate. Yeast two-hybrid experiments indicate that the Rad51C protein binds to two other members of the Rad51 protein family (Xrcc3 and Rad51B) but not to itself. These findings suggest that Rad51C may function similarly to the yeast Rad55 or Rad57 proteins, rather than as a Rad51 functional homolog.