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

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.

Visualisation of human rad52 protein and its complexes with hRad51 and DNA

The human Rad52 protein stimulates joint molecule formation by hRad51, a homologue of Escherichia coli RecA protein. Electron microscopic analysis of hRad52 shows that it self-associates to form ring structures with a diameter of approximately 10 nm. Each ring contains a hole at its centre. hRad52 binds to single and double-stranded DNA. In the ssDNA-hRad52 complexes, hRad52 was distributed along the length of the DNA, which exhibited a characteristic "beads on a string" appearance. At higher concentrations of hRad52, "super-rings" (approximately 30 nm) were observed and the ssDNA was collapsed upon itself. In contrast, in dsDNA-hRad52 complexes, some regions of the DNA remained protein-free while others, containing hRad52, interacted to form large protein-DNA networks. Saturating concentrations of hRad51 displaced hRad52 from ssDNA, whereas dsDNA-Rad52 complexes (networks) were more resistant to hRad51 invasion and nucleoprotein filament formation. When Rad52-Rad51-DNA complexes were probed with gold-conjugated hRad52 antibodies, the presence of globular hRad52 structures within the Rad51 nucleoprotein filament was observed. These data provide the first direct visualisation of protein-DNA complexes formed by the human Rad51 and Rad52 recombination/repair proteins.

Interaction of human rad51 recombination protein with single-stranded DNA binding protein, RPA

Replication protein A (RPA), a heterotrimeric single-stranded DNA binding protein, is required for recombination, and stimulates homologous pairing and DNA strand exchange promoted in vitro by human recombination protein HsRad51. Co-immunoprecipitation revealed that purified RPA interacts physically with HsRad51, as well as with HsDmc1, the homolog that is expressed specifically in meiosis. The interaction with HsRad51 was mediated by the 70 kDa subunit of RPA, and according to experiments with deletion mutants, this interaction required amino acid residues 169-326. In exponentially growing mammalian cells, 22% of nuclei showed foci of RPA protein and 1-2% showed foci of Rad51. After gamma-irradiation, the percentage of cells with RPA foci increased to approximately 50%, and those with Rad51 foci to 30%. All of the cells with foci of Rad51 had foci of RPA, and in those cells the two proteins co-localized in a high fraction of foci. The interactions of human RPA with Rad51, replication proteins and DNA are suited to the linking of recombination to replication.

The BRCA2 gene product functionally interacts with p53 and RAD51

Germ-line mutations in the human BRCA2 gene confer susceptibility to breast cancer. Efforts to elucidate its function have revealed a putative transcriptional activation domain and in vitro interaction with the DNA repair protein RAD51. Other studies have indicated that RAD51 physically associates with the p53 tumor suppressor protein. Here we show that the BRCA2 gene product is a 460-kDa nuclear phosphoprotein, which forms in vivo complexes with both p53 and RAD51. Moreover, exogenous BRCA2 expression in cancer cells inhibits p53's transcriptional activity, and RAD51 coexpression enhances BRCA2's inhibitory effects. These findings demonstrate that BRCA2 physically and functionally interacts with two key components of cell cycle control and DNA repair pathways. Thus, BRCA2 likely participates with p53 and RAD51 in maintaining genome integrity.

Cre expression in primary spermatocytes: a tool for genetic engineering of the germ line

Transgenic mice were generated expressing a testicular Cre recombinase driven by promoter sequences derived from the gene encoding Synaptonemal Complex Protein 1 (Sycp1), expressed at an early stage of the male meiosis (leptotene to zygotene). Recombination at target LoxP sites was examined during germinal differentiation in mice harboring Sycp1-Cre and a second transgene where LoxP sites flank either the beta geo coding region, the Pgk1 promoter, or a tk-neo cassette inserted into the Rxr alpha locus. The LoxP-flanked transgenes were stably maintained in the somatic tissues of the double transgenic animals, as well as in the progeny of the females. Mice born after mating the double-transgenic males with normal females showed extensive deletions of the LoxP-flanked sequences. When the males were hemizygous for the Sycp1-Cre transgene, the deletions were observed even in the fraction of the offspring which had not inherited the Cre gene, thus demonstrating that expression occurred in the male parent during spermatogenesis. The high efficiency of excision at the LoxP sites makes the Sycp1-Cre transgenic males suitable for evaluating the role of defined gene functions in the germinal differentiation process.

Targeted inactivation of mouse RAD52 reduces homologous recombination but not resistance to ionizing radiation

The RAD52 epistasis group is required for recombinational repair of double-strand breaks (DSBs) and shows strong evolutionary conservation. In Saccharomyces cerevisiae, RAD52 is one of the key members in this pathway. Strains with mutations in this gene show strong hypersensitivity to DNA-damaging agents and defects in recombination. Inactivation of the mouse homologue of RAD52 in embryonic stem (ES) cells resulted in a reduced frequency of homologous recombination. Unlike the yeast Scrad52 mutant, MmRAD52(-/-) ES cells were not hypersensitive to agents that induce DSBs. MmRAD52 null mutant mice showed no abnormalities in viability, fertility, and the immune system. These results show that, as in S. cerevisiae, MmRAD52 is involved in recombination, although the repair of DNA damage is not affected upon inactivation, indicating that MmRAD52 may be involved in certain types of DSB repair processes and not in others. The effect of inactivating MmRAD52 suggests the presence of genes functionally related to MmRAD52, which can partly compensate for the absence of MmRad52 protein.

Homologous recombination, but not DNA repair, is reduced in vertebrate cells deficient in RAD52

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

An eukaryotic RuvB-like protein (RUVBL1) essential for growth

A human protein (RUVBL1), consisting of 456 amino acids (50 kDa) and highly homologous to RuvB, was identified by using the 14-kDa subunit of replication protein A (hsRPA3) as bait in a yeast two-hybrid system. RuvB is a bacterial protein involved in genetic recombination that bears structural similarity to subunits of the RF-C clamp loader family of proteins. Fluorescence in situ hybridization analysis demonstrated that the RUVBL1 gene is located at 3q21, a region with frequent rearrangements in different types of leukemia and solid tumors. RUVBL1 co-immunoprecipitated with at least three other unidentified cellular proteins and was detected in the RNA polymerase II holoenzyme complex purified over multiple chromatographic steps. In addition, two yeast homologs, scRUVBL1 and scRUVBL2 with 70 and 42% identity to RUVBL1, respectively, were revealed by screening the complete Saccharomyces cerevisiae genome sequence. Yeast with a null mutation in scRUVBL1 was nonviable. Thus RUVBL1 is an eukaryotic member of the RuvB/clamp loader family of structurally related proteins from bacteria and eukaryotes that is essential for viability of yeast.

Functional characterization of BRCA1 and BRCA2: clues from their interacting proteins

The familial breast and ovarian cancer susceptibility genes, BRCA1 and BRCA2 have been the subject of extensive functional analysis studies since their cloning. Clues to their biological role in maintaining the genomic integrity were provided by studies that revealed their interaction with the recombination repair protein HsRad51. The first clue of an interaction between HsRad51 and BRCA1 came from the colocalization of the characteristic nuclear foci formed by these two proteins during S phase of the cell cycle. An interaction between murine Brca2 and MmRad51 was detected by the yeast two hybrid system. Utilizing the yeast two hybrid system and other techniques several other Brca1 and Brca2 interacting proteins have been identified like, BARD1, importin-alpha, BIPs, RNA polymerase II holoenzyme, BRAP2 etc. Recently, mutations suggesting a role as a tumor suppressor have been identified in the BARD1 gene in primary human tumors. The identification of molecules that interact with Brca1 and Brca2 has greatly enhanced our knowledge of how BRCA1 and BRCA2 may function as tumor suppressors.

BRCA1 and BRCA2 proteins: roles in health and disease

Between 5% and 10% of all breast cancer is hereditary, with patients having a strong family history of the disease. The remaining 90-95% of cases are classed as sporadic. Within the inherited group, 80-90% of cases are the result of germline mutations affecting two recently identified genes: BRCA1 and BRCA2. Since the sequencing of these genes, considerable research on the genetics of the mutation carriers has been performed, with less attention having been focused on the BRCA1 and BRCA2 proteins themselves. The structure and function of the protein products thus continues to hold mystery and might be the key to the full understanding of this complex disease.

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

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

Specific mismatch recognition in heteroduplex intermediates by p53 suggests a role in fidelity control of homologous recombination

We demonstrate that wild-type p53 inhibits homologous recombination. To analyze DNA substrate specificities in this process, we designed recombination experiments such that coinfection of simian virus 40 mutant pairs generated heteroduplexes with distinctly unpaired regions. DNA exchanges producing single C-T and A-G mismatches were inhibited four- to sixfold more effectively than DNA exchanges producing G-T and A-C single-base mispairings or unpaired regions of three base pairs comprising G-T/A-C mismatches. p53 bound specifically to three-stranded DNA substrates, mimicking early recombination intermediates. The KD values for the interactions of p53 with three-stranded substrates displaying differently paired and unpaired regions reflected the mismatch base specificities observed in recombination assays in a qualitative and quantitative manner. On the basis of these results, we would like to advance the hypothesis that p53, like classical mismatch repair factors, checks the fidelity of homologous recombination processes by specific mismatch recognition.

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

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

Stable interaction between the products of the BRCA1 and BRCA2 tumor suppressor genes in mitotic and meiotic cells

BRCA1 and BRCA2 account for most cases of familial, early onset breast and/or ovarian cancer and encode products that each interact with hRAD51. Results presented here show that BRCA1 and BRCA2 coexist in a biochemical complex and colocalize in subnuclear foci in somatic cells and on the axial elements of developing synaptonemal complexes. Like BRCA1 and RAD51, BRCA2 relocates to PCNA+ replication sites following exposure of S phase cells to hydroxyurea or UV irradiation. Thus, BRCA1 and BRCA2 participate, together, in a pathway(s) associated with the activation of double-strand break repair and/or homologous recombination. Dysfunction of this pathway may be a general phenomenon in the majority of cases of hereditary breast and/or ovarian cancer.

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.

Sequence analysis and expression of a novel mouse homolog of Escherichia coli recA gene

Escherichia coli recA and its yeast homologs RAD51 and DMC1 play crucial roles in mitotic and/or meiotic recombination and in repair of double-strand DNA breaks. We have identified a murine novel recA-like gene (MmTRAD). The predicted 329 amino acid protein showed significant homology to mouse Rec2, Rad51, Dmc1 (or Lim15) and E. coli RecA. Northern blot analysis revealed that MmTRAD was ubiquitously transcribed in various tissues.

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

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

Identification and characterization of the RAD51 gene from the ciliate Tetrahymena thermophila

The RAD51 gene is a eukaryotic homolog of rec A, a critical component in homologous recombination and DNA repair pathways in Escherichia coli . We have cloned the RAD51 homolog from Tetrahymena thermophila , a ciliated protozoan. Tetrahymena thermophila RAD51 encodes a 36.3 kDa protein whose amino acid sequence is highly similar to representative Rad51 homologs from other eukaryotic taxa. Recombinant Rad51 protein was purified to near homogeneity following overproduction in a bacterial expression system. The purified protein binds to both single- and double-stranded DNA, possesses a DNA-dependent ATPase activity and promotes intermolecular ligation of linearized plasmid DNA. While steady-state levels of Rad51 mRNA are low in normally growing cells, treatment with UV light resulted in a >100-fold increase in mRNA levels. This increase in mRNA was time dependent, but relatively independent of UV dose over a range of 1400-5200 J/m2. Western blot analysis confirmed that Rad51 protein levels increase upon UV irradiation. Exposure to the alkylating agent methyl methane sulfonate also resulted in substantially elevated Rad51 protein levels in treated cells, with pronounced localization in the macronucleus. These data are consistent with the hypothesis that ciliates such as T.thermophila utilize a Rad51-dependent pathway to repair damaged DNA.

Overexpression of Rad51 protein stimulates homologous recombination and increases resistance of mammalian cells to ionizing radiation

Rad51 proteins share both structural and functional homologies with the bacterial recombinase RecA. The human Rad51 (HsRad51) is able to catalyse strand exchange between homologous DNA molecules in vitro . However the biological functions of Rad51 in mammals are largely unknown. In order to address this question, we have cloned hamster Rad51 cDNA and overexpressed the corresponding protein in CHO cells. We found that 2-3-fold overexpression of the protein stimulated the homologous recombination between integrated genes by 20-fold indicating that Rad51 is a functional and key enzyme of an intrachromosomal recombination pathway. Cells overexpressing Rad51 were resistant to ionizing radiation when irradiated in late S/G2phase of the cell cycle. This suggests that Rad51 participate in the repair of double-strand breaks most likely by homologous recombination involving sister chromatids formed after the S phase.

Breast cancer and genetic instability: the molecules behind the scenes

Germline mutations in either the BRCA1 or the BRCA2 gene are responsible for the majority of hereditary breast cancers. The proposition that BRCA1 might play a role as a caretaker of the genome was first put forward by the demonstration that, in mitotic and meiotic cells, BRCA1 can interact with Rad51, which plays a major role in repair and/or recombination processes. From there, a fair body of observations have converged to support the concept that BRCA1 and BRCA2 play a role in monitoring and/or repairing DNA lesions. The relaxation of this monitoring caused by mutations of either of these two genes leaves unrepaired events, leading to the accumulation of mutations and ultimately to cancer. Understanding the precise biochemical function of BRCA1 and BRCA2 should provide a basis for early diagnosis and prevention in women carrying a predisposition to breast cancer.

Proteolytic cleavage of HsRad51 during apoptosis

The Rad51 gene of Saccharomyces cerevisiae is required for genetic recombination and recombinational repair of DNA strand breaks. In higher eukaryotes Rad51 is essential for embryonic development, and is involved in cell proliferation and DNA repair. Here we show that human Rad51 (HsRad51) is proteolytically cleaved during apoptosis in two T-lymphocyte cell lines, Jurkat and PFI-285. Apoptosis was induced by camptothecin or anti-Fas monoclonal antibody (anti-Fas mAb). HsRad51 was cleaved with similar kinetics as human poly(ADP-ribose) polymerase (HsPARP) after treatment with either agent. The time course of cleavage coincided with internucleosomal DNA fragmentation. The HsRad51 fragments observed in apoptotic cells were identical to those generated from in vitro translated (IVT) HsRad51 exposed to activated Jurkat S-100 extract in a cell-free system. In each case, cleavage of HsRad51 was abolished by acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO). However, cleavage of IVT HsRad51 could not be demonstrated using purified caspase-2, -3 or -6 to -10, and the identity of the responsible protease thus remains to be determined. In summary, we have shown that HsRad51 belongs to a group of repair proteins, including PARP and DNA-dependent protein kinase, which are specifically cleaved during the execution phase of apoptosis.

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.

In vitro and in vivo potentiation of radiosensitivity of malignant gliomas by antisense inhibition of the RAD51 gene

The mammalian RAD51 gene is a homologue of the yeast RAD51 and E. coli RecA genes, which are related to the repair of DNA double-strand breaks and are also involved in recombination repair and various SOS responses to DNA damage by gamma-irradiation and alkylating reagents. In this study, we investigated both in vitro and in vivo whether inhibition of the RAD51 gene by antisense oligonucleotides (ODNs) enhances the radiosensitivity of mouse malignant gliomas. A volume of 100 nM of RAD51 antisense ODNs inhibited the level of mRNA by more than 95% and reduced the protein expression by about 70%. Treatment of mouse 203G glioma cells with 100 nM of RAD51 antisense ODNs significantly enhanced the radiation-induced cell kill compared to control cells, and cells treated with sense or scrambled ODNs. When the glioma cells were implanted in the cisterna magna of mice followed by treatment with RAD51 antisense ODNs, the survival time of the mice was markedly prolonged compared to that of the untreated group (p < 0.001, logrank test). In addition, the combination of antisense ODNs and irradiation extended the survival time of the glioma-bearing mice much longer than could be achieved with radiation alone (p < 0.0001, logrank test). These results suggest that inhibition of RAD51 can be expected to serve as a novel potentiator for radiation therapy in malignant gliomas by inhibiting DNA double-strand break repair.

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

Genes from the recA/RAD51 family play essential roles in homologous recombination in all organisms. Using sequence homologies from eukaryotic members of this family we have identified fragments of two additional mammalian genes with homology to RAD51. Cloning the full-length cDNAs for both human and mouse genes showed that the sequences are highly conserved, and that the predicted proteins have characteristic features of this gene family. One of the novel genes (R51H2) occurs in two forms in human cDNA, differing extensively at the 3' end, probably due to an unusual form of alternative splicing. The new genes (R51H2 and R51H3) were mapped to human chromosomes 14q23-24 and 17q1.2, respectively. Expression studies showed that R51H2 is expressed at lower levels than R51H3 , but that expression of both genes occurs at elevated levels in the testis compared with other tissues. The combination of gene structure conservation and the transcript expression patterns suggests that these new members of the recA/RAD51 family may also function in homologous recombination-repair pathways.

Recombination-based mechanisms for somatic hypermutation

We review some experiments designed to test recombination-based mechanisms for somatic hypermutation in mice, particularly mechanisms involving templated mutation or gene conversion. As recombination and repair functions are highly conserved among prokaryotes and eukaryotes, pathways of mutation in microorganisms may prove relevant to the mechanism of somatic hypermutation. Escherichia coli initiates a recombination-based pathway of mutation in response to environmental stimuli, and this "adaptive" pathway of mutation has striking similarities with somatic hypermutation, as does a process of mutagenic repair that occurs at double-strand breaks in Saccharomyces cerevisiae. We present a model for recombination-based hypermutation of the immunoglobulin loci which could result in either templated or non-templated mutation.

Meiotic prophase arrest with failure of chromosome synapsis in mice deficient for Dmc1, a germline-specific RecA homolog

DMC1 is a meiosis-specific gene first discovered in yeast that encodes a protein with homology to RecA and may be component of recombination nodules. Yeast dmc1 mutants are defective in crossing over and synaptonemal complex (SC) formation, and arrest in late prophase of meiosis I. We have generated a null mutation in the Dmc1 gene in mice and show that homozygous mutant males and females are sterile with arrest of gametogenesis in the first meiotic prophase. Chromosomes in mutant spermatocytes fail to synapse, despite the formation of axial elements that are the precursor to the SC. The strong similarity of phenotypes in Dmc1-deficient mice and yeast suggests that meiotic mechanisms have been highly conserved through evolution.

The mouse RecA-like gene Dmc1 is required for homologous chromosome synapsis during meiosis

The mouse Dmc1 gene is an E. coli RecA homolog that is specifically expressed in meiosis. The DMC1 protein was detected in leptotene-to-zygotene spermatocytes, when homolog pairing likely initiates. Targeted gene disruption in the male mouse showed an arrest of meiosis of germ cells at the early zygotene stage, followed by apoptosis. In female mice lacking the Dmc1 gene, normal differentiation of oogenesis was aborted in embryos, and germ cells disappeared in the adult ovary. Meiotic chromosome analysis of Dmc1-deficient mouse spermatocytes revealed random spread of univalent axial elements without correct pairing between homologs. In rare cases, however, we observed complex pairing among nonhomologs. Thus, the mouse Dmc1 gene is required for homologous synapsis of chromosomes in meiosis.

Identification, characterization, and genetic mapping of Rad51d, a new mouse and human RAD51/RecA-related gene

Homologous DNA recombination occurs in all organisms and is important for repair of DNA damage during mitosis. One of the critical genes for DNA repair and meiotic recombination in yeast is RAD51, and homologs of RAD51 have been identified in several species, including mouse and human. Here we describe a new RAD51-related mammalian gene, named Rad51d, identified by searching the EST database with the yeast RAD55 and human RAD51B/REC2 genes. A full-length 1.5-kb mouse cDNA clone that encodes a predicted 329-amino-acid protein was isolated. Rad51d mRNA was present in every mouse tissue examined. Four different transcript sizes were detected, one of which was specific to testis. Human cDNA clones that predicted 71% amino acid identity to the mouse protein were also isolated. Interestingly, the sequences of these human clones and of RT-PCR-derived products provided evidence for alternative splicing. These mRNAs are predicted to encode proteins that are truncated relative to the mouse and lack the ATP-binding motif characteristic of RecA-related proteins. Using an interspecific backcross mapping panel, Rad51d was mapped to mouse Chromosome 11, 48.5 cM from the centromere. By radiation hybrid mapping, the human ortholog RAD51D was mapped to chromosome 17q11, which is a region syntenic to mouse Chromosome 11. Due to its expression pattern and sequence similarity to other RAD51 family members, it is likely that Rad51d is part of a complex of proteins required for DNA repair and meiotic recombination.

Gene disruption in mice: models of development and disease

Gene targeting technology in mice by homologous recombination has become an important method to generate loss-of-function of genes in a predetermined locus. Although the inactivation is limited to irreversible alteration of chromosomal DNA and a surprising variety of genes have given unexpected and disappointing results, modification of the basic technology now provides additional choices for a more specific and variety of manipulations of the mouse genome. This includes conditional cell-type specific gene targeting, knockin technique and the induction of the specific balanced chromosomal translocations. In the past decade this technique not only generated a wealth of knowledge concerning the roles of growth factors, oncogenes, hormone receptors and Hox genes but also helped to produce animal models for several human genetic disorders. In the future it may provide more powerful and necessary tools to dissect the psychiatric disorders, understanding the complex central nervous system and to correct the inherited disorders.

Regulation of Rad51 function by c-Abl in response to DNA damage

The Rad51 protein, a homolog of bacterial RecA, functions in DNA double-strand break repair and genetic recombination. Whereas Rad51 catalyzes ATP-dependent pairing and strand exchange between homologous DNA molecules, regulation of this function is unknown. The c-Abl tyrosine kinase is activated by ionizing radiation and certain other DNA-damaging agents. Here we demonstrate that c-Abl interacts constitutively with Rad51. We show that c-Abl phosphorylates Rad51 on Tyr-54 in vitro. The results also show that treatment of cells with ionizing radiation induces c-Abl-dependent phosphorylation of Rad51. Phosphorylation of Rad51 by c-Abl inhibits the binding of Rad51 to DNA and the function of Rad51 in ATP-dependent DNA strand exchange reactions. These findings represent the first demonstration that Rad51 is regulated by phosphorylation and support a functional role for c-Abl in regulating Rad51-dependent recombination in the response to DNA damage.

Synergistic actions of Rad51 and Rad52 in recombination and DNA repair

In the yeast Saccharomyces cerevisiae, mutations in the genes RAD51 or RAD52 result in severe defects in genetic recombination and the repair of double-strand DNA breaks. These genes, and others of the RAD52 epistasis group (RAD50, RAD54, RAD55, RAD57, RAD59, MRE11 and XRS2), were first identified by their sensitivity to X-rays. They were subsequently shown to be required for spontaneous and induced mitotic recombination, meiotic recombination, and mating-type switching. Human homologues of RAD50, RAD51, RAD52, RAD54 and MRE11 have been identified. Targeted disruption of the murine RAD51 gene results in an embryonic lethal phenotype, indicating that Rad51 protein is required during cell proliferation. Biochemical studies have shown that human RAD51 encodes a protein of relative molecular mass 36,966 (hRad51) that promotes ATP-dependent homologous pairing and DNA strand exchange. As a structural and functional homologue of the RecA protein from Escherichia coli, hRad51 is thought to play a central role in recombination. Yeast Rad51 has been shown to interact with Rad52 protein, as does the human homologue. Here we show that hRad52 stimulates homologous pairing by hRad51. The DNA-binding properties of hRad52 indicate that Rad52 is involved in an early stage of Rad51-mediated recombination.

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

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

Identification of a novel human RAD51 homolog, RAD51B

The highly conserved Saccharomyces cerevisiae RAD51 protein functions in both mitotic and meiotic homologous recombination and in double-strand break repair. Screening of the public cDNA sequence database for RAD51-like genes led to the identification of a partial sequence from a breast tissue library present in the I.M.A.G.E. (Integrated Molecular Analysis of Genes and their Expression) collection. An extended 1764-bp cDNA clone encoding an open reading frame of 350 amino acids was isolated. This clone showed significant amino acid identity with other human RAD51 homologs. The new homolog, named RAD51B, was mapped to human chromosome 14q23-q24.2 using a panel of human-hamster somatic cell hybrids and fluorescence in situ hybridization. Northern blot analysis demonstrated that RAD51B mRNA is widely expressed and most abundant in tissues active in recombination. Functions associated with known RAD51 homologs suggest a role for RAD51B in meiotic recombination and/or recombinational repair.

A novel nucleic acid-binding protein that interacts with human rad51 recombinase

Using the yeast two-hybrid system, we isolated a cDNA encoding a novel human protein, named Pir51, that strongly interacts with human Rad51 recombinase. Analysis in vitro confirmed the interaction between Rad51 and Pir51. Pir51 mRNA is expressed in a number of human organs, most notably in testis, thymus, colon and small intestine. The Pir51 gene locus was mapped to chromosome 12p13.1-13. 2 by fluorescence in situ hybridization. The Pir51 protein was expressed in Escherichia coli and purified to near homogeneity. Biochemical analysis shows that the Pir51 protein binds both single- and double-stranded DNA, and is capable of aggregating DNA. The protein also binds RNA. The Pir51 protein may represent a new member of the multiprotein complexes postulated to carry out homologous recombination and DNA repair in mammalian cells.

Elevated recombination in immortal human cells is mediated by HsRAD51 recombinase

Normal diploid cells have a limited replicative potential in culture, with progressively increasing interdivision time. Rarely, cell lines arise which can divide indefinitely; like tumor cells, such "immortal" lines display frequent chromosomal aberrations which may reflect high rates of recombination. Recombination frequencies within a plasmid substrate were 3.5-fold higher in nine immortal human cell lines than in six untransformed cell strains. Expression of HsRAD51, a human homolog of the yeast RAD51 and Escherichia coli recA recombinase genes, was 4.5-fold higher in immortal cell lines than in mortal cells. Stable transformation of human fibroblasts with simian virus 40 large T antigen prior to cell immortalization increased both chromosomal recombination and the level of HsRAD51 transcripts by two- to fivefold. T-antigen induction of recombination was efficiently blocked by introduction of HsRAD51 antisense (but not control) oligonucleotides spanning the initiation codon, implying that HsRAD51 expression mediates augmented recombination. Since p53 binds and inactivates HsRAD51, T-antigen-p53 association may block such inactivation and liberate HsRAD51. Upregulation of HsRAD51 transcripts in T-antigen-transformed and other immortal cells suggests that recombinase activation can also occur at the RNA level and may facilitate cell transformation to immortality.

Nuclear Rad51 foci induced by DNA damage are distinct from Rad51 foci associated with B cell activation and recombination

Lipopolysaccharide (LPS) is a B cell mitogen which can stimulate murine primary B cells to proliferate and carry out immunoglobulin heavy chain class switch recombination. LPS can also function as an endotoxin, which may cause DNA damage and apoptosis in certain types of cells. We have previously reported that LPS-activated primary murine B cells contain nuclear foci that stain brightly with anti-Rad51 antibodies (Li et al. (1996) Proc. Natl. Acad. Sci. USA 93, 10222-10227). We have now analyzed Rad51 nuclear foci induced in both primary and immortalized B cells by treatment with the DNA damaging agent, methyl methanesulfonate (MMS). We have found that, in LPS-cultured primary B cells, MMS treatment increases the fraction of cells containing Rad51 foci and induces formation of a very high number of foci per cell. The foci induced by MMS treatment are small, punctate, and numerous; in contrast, the foci induced by LPS activation are large, brightly staining, and relatively few in number. In LPS-cultured primary B cells, Rad51 relocalizes during the cell cycle, and large, brightly staining nuclear foci are present in only restricted stages of the cell cycle. Rad51 foci similar to those present in LPS-activated primary B cells are also observed in immortalized B cells lines cultured in the absence of LPS. These foci are unaltered in number or appearance by culture with LPS, but treatment of immortalized B cell lines with MMS induces foci which are small and punctate in staining, like those induced by MMS in primary B cells. These data show that distinctive Rad51 foci are induced by DNA damaging agents and cell activation and that the response to DNA damage may involve pathways distinct from those associated with B cell activation and switch recombination.

Gene knockout and transgenic technologies in risk assessment: the next generation

Transgenic and knockout mice have been proposed as substitutes for one of the standard 2-yr rodent assays. The advantages of using genetically engineered mouse models is that fewer mice are needed, the time to develop disease is greatly reduced, and the mice are predisposed to developing cancer by virtue of gain or loss of functions. The models currently being used have yielded a large amount of data and have proved to be informative for risk assessment; however, they are still far from ideal. In fact, they inherently do not reflect the complexity of mutation and carcinogenesis in humans. Recent advances in technology and the creation of new knockout mice may produce more useful and more sensitive models. This review covers two recent advances in technology--inducible and regulatable gene expression and targeted genetic modifications in the genome--that will allow us to make better models. I also discuss new gene deletion and transgenic mouse models and their potential impact on risk-assessment assays. These models are presented in the context of four basic components or events that occur in the multistep process leading to cancer: maintenance of gene expression patterns, genome stability and DNA repair, cell-cell communication and signaling, and cell-cycle regulation. Finally, surrogate markers and utility in risk assessment are also discussed. This review is meant to stimulate further discussion in the field and to generate excitement about working toward the next generation of risk-assessment models.

Recombinational DNA repair: the RecF and RecR proteins limit the extension of RecA filaments beyond single-strand DNA gaps

In the presence of both the RecF and RecR proteins, RecA filament extension from a single strand gap into adjoining duplex DNA is attenuated. RecR protein alone has no effect, and RecF protein alone has a reduced activity. The RecFR complexes bind randomly, primarily to the duplex regions of the DNA, and the extension of the RecA filament is halted at the first complex encountered. A very slow lengthening of RecA filaments observed in the presence of RecFR is virtually eliminated when RecF is replaced with an RecF mutant protein that does not hydrolyze ATP. These observations are incorporated into an expanded model for the functions of RecF, RecO, and RecR proteins in the early stages of postreplication DNA repair.

Interaction of human recombination proteins Rad51 and Rad54

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

Recombination activities of HsDmc1 protein, the meiotic human homolog of RecA protein

Meiosis-specific homologs of RecA protein have been identified in Saccharomyces cerevisiae and higher eukaryotes including mammals, but their enzymatic activities have not been described. We have purified the human protein HsDmc1 produced in Escherichia coli from a cloned copy of the cDNA. The recombinant enzyme had DNA-dependent ATPase activity with an estimated kcat of 1.5 min-1. DNase protection experiments with oligonucleotides as substrates indicated that HsDmc1 protein binds preferentially to single-stranded DNA with a stoichiometry of approximately one molecule of protein per three nucleotide residues. HsDmc1 protein catalyzed the formation of D-loops in superhelical DNA, as well as strand exchange between single-stranded and double-stranded oligonucleotides. The requirements for strand exchange catalyzed by HsDmc1 were similar to those of RecA protein, but exchange caused by HsDmc1 was not supported by ATPgammaS.

The Drosophila melanogaster RAD54 homolog, DmRAD54, is involved in the repair of radiation damage and recombination

The RAD54 gene of Saccharomyces cerevisiae plays a crucial role in recombinational repair of double-strand breaks in DNA. Here the isolation and functional characterization of the RAD54 homolog of the fruit fly Drosophila melanogaster, DmRAD54, are described. The putative Dmrad54 protein displays 46 to 57% identity to its homologs from yeast and mammals. DmRAD54 RNA was detected at all stages of fly development, but an increased level was observed in early embryos and ovarian tissue. To determine the function of DmRAD54, a null mutant was isolated by random mutagenesis. DmRADS4-deficient flies develop normally, but the females are sterile. Early development appears normal, but the eggs do not hatch, indicating an essential role for DmRAD54 in development. The larvae of mutant flies are highly sensitive to X rays and methyl methanesulfonate. Moreover, this mutant is defective in X-ray-induced mitotic recombination as measured by a somatic mutation and recombination test. These phenotypes are consistent with a defect in the repair of double-strand breaks and imply that the RAD54 gene is crucial in repair and recombination in a multicellular organism. The results also indicate that the recombinational repair pathway is functionally conserved in evolution.

Distribution of the Rad51 recombinase in human and mouse spermatocytes

In vitro, the human Rad51 protein (hRad51) promotes homologous pairing and strand exchange reactions suggestive of a key role in genetic recombination. To analyse its role in this process, polyclonal antibodies raised against hRad51 were used to study the distribution of Rad51 in human and mouse spermatocytes during meiosis I. In human spermatocytes, hRad51 was found to form discrete nuclear foci from early zygotene to late pachytene. The foci always co-localized with lateral element proteins, components of the synaptonemal complex (SC). During zygotene, the largest foci were present in regions undergoing synapsis, suggesting that Rad51 is a component of early recombination nodules. Pachytene nuclei showed a greatly reduced level of Rad51 labelling, with the exceptions of any asynapsed autosomes and XY segments, which were intensely labelled. The distribution of Rad51 in mouse spermatocytes was similar to that found in human spermatocytes, except that in this case Rad51 was detectable at leptotene. From these results, we conclude that the Rad51 protein has a role in the interhomologue interactions that occur during meiotic recombination. These interactions are spatially and temporally associated with synapsis during meiotic prophase I.

The human Rad51 protein: polarity of strand transfer and stimulation by hRP-A

The human Rad51 protein is homologous to the RecA protein and catalyses homologous pairing and strand transfer reactions in vitro. Using single-stranded circular and homologous linear duplex DNA, we show that hRad51 forms stable joint molecules by transfer of the 5' end of the complementary strand of the linear duplex to the ssDNA. The polarity of strand transfer is therefore 3' to 5', defined relative to the ssDNA on which hRad51 initiates filament formation. This polarity is opposite to that observed with RecA. Homologous pairing and strand transfer require stoichiometric amounts of hRad51, corresponding to one hRad51 monomer per three nucleotides of ssDNA. Joint molecules are not observed when the protein is present in limiting or excessive amounts. The human ssDNA binding-protein, hRP-A, stimulates hRad51-mediated reactions. Its effect is consistent with a role in the removal of secondary structures from ssDNA, thereby facilitating the formation of continuous Rad51 filaments.

Direct repeats of the herpes simplex virus a sequence promote nonconservative homologous recombination that is not dependent on XPF/ERCC4

We have examined mechanisms of recombination in mammalian cells infected with herpes simplex virus type 1 (HSV-1). Amplification of plasmids containing a viral origin of replication, oriS, in cells superinfected with HSV-1 revealed that linear DNA could be efficiently converted to templates for replication. Two distinct pathways were observed: imprecise end joining and nonconservative homologous recombination. We noted that direct repeats of the viral a sequence promoted efficient nonconservative homologous recombination in BHK cells as well as human repair-proficient 1BR.3N cells and xeroderma pigmentosum group F (XP-F) cells. The reaction gave rise to functional a sequences supporting the formation of defective viruses. It did not seem to proceed by single-strand annealing since it occurred in the absence of XPF/ERCC4, the mammalian homolog of the Rad1 endonuclease from Saccharomyces cerevisiae. In contrast, direct repeats of a 161-bp nonviral sequence did not take part in nonconservative homologous recombination in XP-F cells. Our results suggest that homologous recombination may be involved in the circularization of viral genomes. Furthermore, they demonstrate that amplification of recombination products supported by HSV-1 allows a direct examination of pathways for double-strand-break repair in human cells.

DNA repair: RAD alert

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

Purification of human Rad51 protein by selective spermidine precipitation

The human Rad51 protein is a structural homolog of Escherichia coli RecA. The exact role of human Rad51 within the cell is poorly understood but, like its bacterial and yeast homologs, hRad51 is believed to play a central role in homologous recombination. However, recent reports that transgenic mice lacking the RAD51 gene die early in development suggest an additional and essential function for mammalian Rad51 in cell proliferation or genome maintenance. In this paper we describe a simple and quick method for the purification of human Rad51 overproduced in E. coli. Dialysis of cell-free extracts against buffer containing low concentrations of spermidine result in the formation of hRad51 microcrystals as observed by light and electron microscopy. The crystals were easily redissolved in phosphate buffer and hRad51 was further purified to homogeneity using hydroxylapatite, affi-gel heparin and Q-sepharose chromatography. When purified by this method hRad51 is free of endo- and exonuclease activities and suitable for enzymological studies. Spermidine precipitation also provides a rapid method for the large scale purification of hRad51 suitable for physical analysis.