Homologous recombination induced by replication inhibition, is stimulated by expression of mutant p53

When PIP2 Meets p53: Nuclear Phosphoinositide Signaling in the DNA Damage Response

The mechanisms that maintain genome stability are critical for preventing tumor progression. In the past decades, many strategies were developed for cancer treatment to disrupt the DNA repair machinery or alter repair pathway selection. Evidence indicates that alterations in nuclear phosphoinositide lipids occur rapidly in response to genotoxic stresses. This implies that nuclear phosphoinositides are an upstream element involved in DNA damage signaling. Phosphoinositides constitute a new signaling interface for DNA repair pathway selection and hence a new opportunity for developing cancer treatment strategies. However, our understanding of the underlying mechanisms by which nuclear phosphoinositides regulate DNA damage repair, and particularly the dynamics of those processes, is rather limited. This is partly because there are a limited number of techniques that can monitor changes in the location and/or abundance of nuclear phosphoinositide lipids in real time and in live cells. This review summarizes our current knowledge regarding the roles of nuclear phosphoinositides in DNA damage response with an emphasis on the dynamics of these processes. Based upon recent findings, there is a novel model for p53’s role with nuclear phosphoinositides in DNA damage response that provides new targets for synthetic lethality of tumors.

Functional relationship between p53 and RUNX proteins

RUNX genes belong to a three-membered family of transcription factors, which are well established as master regulators of development. Of them, aberrations in RUNX3 expression are frequently observed in human malignancies primarily due to epigenetic silencing, which is often overlooked. At the G1 phase of the cell cycle, RUNX3 regulates the restriction (R)-point, a mechanism that decides cell cycle entry. Deregulation at the R-point or loss of RUNX3 results in premature entry into S phase, leading to a proliferative advantage. Inactivation of Runx1 and Runx2 induce immortalization of mouse embryo fibroblast. As a consequence, RUNX loss induces pre-cancerous lesions independent of oncogene activation. p53 is the most extensively studied tumour suppressor. p53 plays an important role to prevent tumour progression but not tumour initiation. Therefore, upon oncogene activation, early inactivation of RUNX genes and subsequent mutation of p53 appear to result in tumour initiation and progression. Recently, transcription-independent DNA repairing roles of RUNX3 and p53 are emerging. Being evolutionarily old genes, it appears that the primordial function of p53 is to protect genome integrity, a function that likely extends to the RUNX gene as well. In this review, we examine the mechanism and sequence of actions of these tumour suppressors in detail.

An LTR retrotransposon-derived lncRNA interacts with RNF169 to promote homologous recombination

LTR retrotransposons are abundant repetitive elements in the human genome, but their functions remain poorly understood. Here, we report the function and regulatory mechanism of an ERV-9 LTR retrotransposon-derived lncRNA called p53-regulated lncRNA for homologous recombination (HR) repair 1 (PRLH1) in human cells. PRLH1 is highly expressed in p53-mutated hepatocellular carcinoma (HCC) samples and promotes cell proliferation in p53-mutated HCC cells, and its transcription is promoted by NF-Y and suppressed by p53. Mechanistically, PRLH1 specifically binds to an uncharacterized domain of RNF169 through two GCUUCA boxes in its 5' terminal region to form a DNA repair complex that supplants 53BP1 at double-strand break (DSB) sites and then promotes the initiation of HR repair. Notably, PRLH1 is essential for the stabilization of RNF169, acting as an RNA platform to recruit and assemble HR protein factors. This study characterizes PRLH1 as a novel HR-promoting factor and provides new insights into the function and mechanism of LTR retrotransposon-derived lncRNAs.

Human RAD52 protein regulates homologous recombination and checkpoint function in BRCA2 deficient cells

Cancer cells exhibit HR defects, increased proliferation and checkpoint aberrations. Tumour suppressor proteins, BRCA2 and p53 counteract such aberrant proliferation by checkpoint regulation. Intriguingly, chemo-resistant cancer cells, exhibiting mutated BRCA2 and p53 protein survive even with increased DNA damage accumulation. Such cancer cells show upregulation of RAD52 tumour suppressor protein implying that RAD52 might be providing survival advantage to cancer cells. To understand this paradoxical condition of a tumour suppressor protein facilitating cancer cell survival, in the current study, we investigate the role of RAD52 overexpression in BRCA2 deficient cells. We provide evidence that RAD52 protein alleviates HR inhibition imposed by p53 in BRCA2 deficient cells. In addition, we study the role of RAD52 protein during short replication stress in BRCA2 deficient cells. BRCA2 deficient cells exhibit excessive origin firing and checkpoint evasion in the presence of prevailing DNA damage. Interestingly, overexpression of RAD52 rescues the excessive origin firing and checkpoint defects observed in BRCA2 deficient cells, indicating RAD52 protein compensates for the loss of BRCA2 function. We show that RAD52 protein, just as BRCA2, interacts with pCHK1 checkpoint protein and helps maintain the checkpoint control in BRCA2 deficient cells during DNA damage response.

p53 orchestrates DNA replication restart homeostasis by suppressing mutagenic RAD52 and POLθ pathways

Classically, p53 tumor suppressor acts in transcription, apoptosis, and cell cycle arrest. Yet, replication-mediated genomic instability is integral to oncogenesis, and p53 mutations promote tumor progression and drug-resistance. By delineating human and murine separation-of-function p53 alleles, we find that p53 null and gain-of-function (GOF) mutations exhibit defects in restart of stalled or damaged DNA replication forks that drive genomic instability, which isgenetically separable from transcription activation. By assaying protein-DNA fork interactions in single cells, we unveil a p53-MLL3-enabled recruitment of MRE11 DNA replication restart nuclease. Importantly, p53 defects or depletion unexpectedly allow mutagenic RAD52 and POLθ pathways to hijack stalled forks, which we find reflected in p53 defective breast-cancer patient COSMIC mutational signatures. These data uncover p53 as a keystone regulator of replication homeostasis within a DNA restart network. Mechanistically, this has important implications for development of resistance in cancer therapy. Combined, these results define an unexpected role for p53-mediated suppression of replication genome instability.

Involvement of p53 in the repair of DNA double strand breaks: multifaceted Roles of p53 in homologous recombination repair (HRR) and non-homologous end joining (NHEJ)

p53 is a tumor suppressor protein that prevents oncogenic transformation and maintains genomic stability by blocking proliferation of cells harboring unrepaired or misrepaired DNA. A wide range of genotoxic stresses such as DNA damaging anti-cancer drugs and ionizing radiation promote nuclear accumulation of p53 and trigger its ability to activate or repress a number of downstream target genes involved in various signaling pathways. This cascade leads to the activation of multiple cell cycle checkpoints and subsequent cell cycle arrest, allowing the cells to either repair the DNA or undergo apoptosis, depending on the intensity of DNA damage. In addition, p53 has many transcription-independent functions, including modulatory roles in DNA repair and recombination. This chapter will focus on the role of p53 in regulating or influencing the repair of DNA double-strand breaks that mainly includes homologous recombination repair (HRR) and non-homologous end joining (NHEJ). Through this discussion, we will try to establish that p53 acts as an important linchpin between upstream DNA damage signaling cues and downstream cellular events that include repair, recombination, and apoptosis.

p53 suppresses BRCA2-stimulated ATPase and strand exchange functions of human RAD51

Although homologous recombination (HR) is an important pathway for DNA repair, it can also be a cause for deleterious genomic rearrangements leading to carcinogenesis. Therefore, cells have evolved elaborate mechanisms to regulate HR, positively as well as negatively. Among many molecular components that regulate HR are tumour suppressors p53, a negative regulator and breast cancer early-onset (BRCA)2, a positive regulator. Both the players not only interact with each other but also directly interact with human RAD51 (hRAD51), the key recombinase in HR. Here, for the first time we studied HR regulation by the combined action of p53 and BRCA2, in vitro. While BRC4 peptide inhibits ATP hydrolysis by hRAD51, BRCA2(BRC1-8) stimulates DNA-independent and double-stranded DNA-dependent ATPase several fold and only marginally single-stranded DNA-dependent ATPase. Pull down assays demonstrated the occurrence of complex comprising of all three proteins and DNA, where p53 tends to compete out hRAD51 and BRCA2(BRC1-8), leading to not only the decline in ATP hydrolysis but also the strand exchange function of hRAD51 that was stimulated by BRCA2(BRC1-8). Our findings suggest a rigorous p53-mediated regulation on hRAD51 functions in HR even in the presence of BRCA2.

Chromosome instability and deregulated proliferation: an unavoidable duo

The concept that aneuploidy is a characteristic of malignant cells has long been known; however, the idea that aneuploidy is an active contributor to tumorigenesis, as opposed to being an associated phenotype, is more recent in its evolution. At the same time, we are seeing the emergence of novel roles for tumor suppressor genes and oncogenes in genome stability. These include the adenomatous polyposis coli gene (APC), p53, the retinoblastoma susceptibility gene (RB1), and Ras. Originally, many of these genes were thought to be tumor suppressive or oncogenic solely because of their role in proliferative control. Because of the frequency with which they are disrupted in cancer, chromosome instability caused by their dysfunction may be more central to tumorigenesis than previously thought. Therefore, this review will highlight how the proper function of cell cycle regulatory genes contributes to the maintenance of genome stability, and how their mutation in cancer obligatorily connects proliferation and chromosome instability.

p53 modulates homologous recombination at I-SceI-induced double-strand breaks through cell-cycle regulation

Inhibition of homologous recombination (HR) is believed to be a transactivation-independent function of p53 that protects from genetic instability. Misrepair by HR can lead to genetic alterations such as translocations, duplications, insertions and loss of heterozygosity, which all bear the risk of driving oncogenic transformation. Regulation of HR by wild-type p53 (wtp53) should prevent these genomic rearrangements. Mutation of p53 is a frequent event during carcinogenesis. In particular, dominant-negative mutants inhibiting wtp53 expressed from the unperturbed allel can drive oncogenic transformation by disrupting the p53-dependent anticancer barrier. Here, we asked whether the hot spot mutants R175H and R273H relax HR control in p53-proficient cells. Utilizing an I-SceI-based reporter assay, we observed a moderate (1.5 × ) stimulation of HR upon expression of the mutant proteins in p53-proficient CV-1, but not in p53-deficient H1299 cells. Importantly, the stimulatory effect was exactly paralleled by an increase in the number of HR competent S- and G2-phase cells, which can well explain the enhanced recombination frequencies. Furthermore, the impact on HR exerted by the transactivation domain double-mutant L22Q/W23S and mutant R273P, both of which were reported to regulate HR independently of G1-arrest execution, is also exactly mirrored by cell-cycle behavior. These results are in contrast to previous concepts stating that the transactivation-independent impact of p53 on HR is a general phenomenon valid for replication-associated and also for directly induced double-strand break. Our data strongly suggest that the latter is largely mediated by cell-cycle regulation, a classical transactivation-dependent function of p53.

Uncoupling of RAD51 focus formation and cell survival after replication fork stalling in RAD51D null CHO cells

In vertebrate cells, the five RAD51 paralogs (XRCC2/3 and RAD51B/C/D) enhance the efficiency of homologous recombination repair (HRR). Stalling and breakage of DNA replication forks is a common event, especially in the large genomes of higher eukaryotes. When cells are exposed to agents that arrest DNA replication, such as hydroxyurea or aphidicolin, fork breakage can lead to chromosomal aberrations and cell killing. We assessed the contribution of the HRR protein RAD51D in resistance to killing by replication-associated DSBs. In response to hydroxyurea, the isogenic rad51d null CHO mutant fails to show any indication of HRR initiation, as assessed by induction RAD51 foci, as expected. Surprisingly, these cells have normal resistance to killing by replication inhibition from either hydroxyurea or aphidicolin, but show the expected sensitivity to camptothecin, which also generates replication-dependent DSBs. In contrast, we confirm that the V79 xrcc2 mutant does show increased sensitivity to hydroxyurea under some conditions, which was correlated to its attenuated RAD51 focus response. In response to the PARP1 inhibitor KU58684, rad51d cells, like other HRR mutants, show exquisite sensitivity (>1000-fold), which is also associated with defective RAD51 focus formation. Thus, rad51d cells are broadly deficient in RAD51 focus formation in response to various agents, but this defect is not invariably associated with increased sensitivity. Our results indicate that RAD51 paralogs do not contribute equally to cellular resistance of inhibitors of DNAreplication, and that the RAD51 foci associated with replication inhibition may not be a reliable indicator of cellular resistance to such agents.

ATR-p53 restricts homologous recombination in response to replicative stress but does not limit DNA interstrand crosslink repair in lung cancer cells

Homologous recombination (HR) is required for the restart of collapsed DNA replication forks and error-free repair of DNA double-strand breaks (DSB). However, unscheduled or hyperactive HR may lead to genomic instability and promote cancer development. The cellular factors that restrict HR processes in mammalian cells are only beginning to be elucidated. The tumor suppressor p53 has been implicated in the suppression of HR though it has remained unclear why p53, as the guardian of the genome, would impair an error-free repair process. Here, we show for the first time that p53 downregulates foci formation of the RAD51 recombinase in response to replicative stress in H1299 lung cancer cells in a manner that is independent of its role as a transcription factor. We find that this downregulation of HR is not only completely dependent on the binding site of p53 with replication protein A but also the ATR/ATM serine 15 phosphorylation site. Genetic analysis suggests that ATR but not ATM kinase modulates p53's function in HR. The suppression of HR by p53 can be bypassed under experimental conditions that cause DSB either directly or indirectly, in line with p53's role as a guardian of the genome. As a result, transactivation-inactive p53 does not compromise the resistance of H1299 cells to the interstrand crosslinking agent mitomycin C. Altogether, our data support a model in which p53 plays an anti-recombinogenic role in the ATR-dependent mammalian replication checkpoint but does not impair a cell's ability to use HR for the removal of DSB induced by cytotoxic agents.

Oligo/polynucleotide-based gene modification: strategies and therapeutic potential

Oligonucleotide- and polynucleotide-based gene modification strategies were developed as an alternative to transgene-based and classical gene targeting-based gene therapy approaches for treatment of genetic disorders. Unlike the transgene-based strategies, oligo/polynucleotide gene targeting approaches maintain gene integrity and the relationship between the protein coding and gene-specific regulatory sequences. Oligo/polynucleotide-based gene modification also has several advantages over classical vector-based homologous recombination approaches. These include essentially complete homology to the target sequence and the potential to rapidly engineer patient-specific oligo/polynucleotide gene modification reagents. Several oligo/polynucleotide-based approaches have been shown to successfully mediate sequence-specific modification of genomic DNA in mammalian cells. The strategies involve the use of polynucleotide small DNA fragments, triplex-forming oligonucleotides, and single-stranded oligodeoxynucleotides to mediate homologous exchange. The primary focus of this review will be on the mechanistic aspects of the small fragment homologous replacement, triplex-forming oligonucleotide-mediated, and single-stranded oligodeoxynucleotide-mediated gene modification strategies as it relates to their therapeutic potential.

Analysis of p53 dependent damage response in sperm-irradiated mouse embryos

Ionizing radiation activates a series of DNA damage response, cell cycle checkpoints to arrest cells at G1/S, S and G2/M, DNA repair, and apoptosis. The DNA damage response is thought to be the major determinant of cellular radiosensitivity and thought to operate in all higher eukaryotic cells. However, the radiosensitivity is known to differ considerably during ontogeny of mammals and early embryos of mouse for example are much more sensitive to radiation than adults. We have focused on the radiation-induced damage response during pre-implantation stage of mouse embryo. Our study demonstrates a hierarchy of damage responses to assure the genomic integrity in early embryonic development. In the sperm-irradiated zygotes, p53 dependent S-phase checkpoint functions to suppress erroneous replication of damaged DNA. The transcription-dependent function is not required and the DNA-binging domain of the protein is essential for this p53 dependent S-phase checkpoint. p21 mediated cleavage arrest comes next during early embryogenesis to prevent delayed chromosome damage at morula/ blastocyst stages. Apoptosis operates even later only in the cells of ICM at the blastocyst stage to eliminate deleterious cells. Thus, early development of sperm-irradiated embryos is protected at least by three mechanisms regulated by p53 and by p21.

Absence of p53 enhances growth defects and etoposide sensitivity of human cells lacking the Bloom syndrome helicase BLM

The Bloom syndrome helicase BLM and the tumor-suppressor protein p53 play important roles in preserving genome integrity. Here, we knock out the genes for BLM and p53 in a human pre-B-cell line, Nalm-6. We show that p53 plays an important role in cell proliferation, but not apoptosis, when BLM is absent. Intriguingly, despite the apoptotic function of p53, BLM(/)TP53(/) cells were more sensitive than either single mutant to etoposide, an anticancer agent that poisons DNA topoisomerase II. Our results suggest a direct, BLM-independent role for p53 in etoposide-induced, topoisomerase II-mediated DNA damage in human cells.

Rad51 overexpression contributes to chemoresistance in human soft tissue sarcoma cells: a role for p53/activator protein 2 transcriptional regulation

We investigated whether Rad51 overexpression plays a role in soft tissue sarcoma (STS) chemoresistance as well as the regulatory mechanisms underlying its expression. The studies reported here show that Rad51 protein is overexpressed in a large panel of human STS specimens. Human STS cell lines showed increased Rad51 protein expression, as was also observed in nude rat STS xenografts. STS cells treated with doxorubicin exhibited up-regulation of Rad51 protein while arrested in the S-G(2) phase of the cell cycle. Treatment with anti-Rad51 small interfering RNA decreased Rad51 protein expression and increased chemosensitivity to doxorubicin. Because we previously showed that reintroduction of wild-type p53 (wtp53) into STS cells harboring a p53 mutation led to increased doxorubicin chemosensitivity, we hypothesized that p53 participates in regulating Rad51 expression in STS. Reintroduction of wtp53 into STS cell lines resulted in decreased Rad51 protein and mRNA expression. Using luciferase reporter assays, we showed that reconstitution of wtp53 function decreased Rad51 promoter activity. Deletion constructs identified a specific Rad51 promoter region containing a p53-responsive element but no p53 consensus binding site. Electrophoretic mobility shift assays verified activator protein 2 (AP2) binding to this region and increased AP2 binding to the promoter in the presence of wtp53. Mutating this AP2 binding site eliminated the wtp53 repressive effect. Furthermore, AP2 knockdown resulted in increased Rad51 expression. In light of the importance of Rad51 in modulating STS chemoresistance, these findings point to a potential novel strategy for molecular-based treatments that may be of relevance to patients burdened by STS.

p53 in recombination and repair

Convergent studies demonstrated that p53 regulates homologous recombination (HR) independently of its classic tumour-suppressor functions in transcriptionally transactivating cellular target genes that are implicated in growth control and apoptosis. In this review, we summarise the analyses of the involvement of p53 in spontaneous and double-strand break (DSB)-triggered HR and in alternative DSB repair routes. Molecular characterisation indicated that p53 controls the fidelity of Rad51-dependent HR and represses aberrant processing of replication forks after stalling at unrepaired DNA lesions. These findings established a genome stabilising role of p53 in counteracting error-prone DSB repair. However, recent work has also unveiled a stimulatory role for p53 in topoisomerase I-induced recombinative repair events that may have implications for a gain-of-function phenotype of cancer-related p53 mutants. Additional evidence will be discussed which suggests that p53 and/or p53-regulated gene products also contribute to nucleotide excision, base excision, and mismatch repair.

Reduction of gene repair by selenomethionine with the use of single-stranded oligonucleotides

Background

The repair of single base mutations in mammalian genes can be directed by single-stranded oligonucleotides in a process known as targeted gene repair. The mechanism of this reaction is currently being elucidated but likely involves a pairing step in which the oligonucleotide align in homologous register with its target sequence and a correction step in which the mutant base is replaced by endogenous repair pathways. This process is regulated by the activity of various factors and proteins that either elevate or depress the frequency at which gene repair takes place.

Results

In this report, we find that addition of selenomethionine reduces gene repair frequency in a dose-dependent fashion. A correlation between gene repair and altered cell cycle progression is observed. We also find that selenium induces expression of Ref-1 which, in turn, modifies the activity of p53 during the cell cycle.

Conclusion

We can conclude from the results that the suppression of gene repair by introduction of selenomethionine occurs through a p53-associated pathway. This result indicates that the successful application of gene repair for treatment of inherited disorders may be hampered by indirect activation of endogenous suppressor functions.

p53 modulates homologous recombination by transcriptional regulation of the RAD51 gene

DNA repair by homologous recombination is involved in maintaining genome stability. Previous data report that wild-type p53 suppresses homologous recombination and physically interacts with Rad51. Here, we show the in vivo binding of wild-type p53 to a p53 response element in the promoter of Rad51 and the downregulation of Rad51 messenger RNA and protein by wild-type p53, favoured by DNA damage. Moreover, wild-type p53 inhibits Rad51 foci formation in response to double-strand breaks, whereas p53 contact mutant R280K fails to repress Rad51 mRNA and protein expression and Rad51 foci formation. We propose that transcriptional repression of Rad51 by p53 participates in regulating homologous recombination, and impaired Rad51 repression by p53 mutants may contribute to malignant transformation.

Linking DNA damage to medulloblastoma tumorigenesis in patched heterozygous knockout mice

Hemizygous Ptc1 mice have many features of Gorlin syndrome, including predisposition to medulloblastoma development. Ionizing radiation synergize with Ptc1 mutation to induce medulloblastoma only in neonatally exposed mice. To explore the mechanisms underlying age-dependent susceptibility, we irradiated Ptc(neo67/+) mice at postnatal day 1 (P1) or 10 (P10). We observed a dramatic difference in medulloblastoma incidence, which ranged from 81% in the cerebellum irradiated at P1 to 3% in the cerebellum irradiated at P10. A striking difference was also detected in the frequency of cerebellar preneoplastic lesions (100 versus 14%). Our data also show significantly lower induction of apoptosis in the cerebellum of medulloblastoma-susceptible (P1) compared to -resistant (P10) mice, strongly suggesting that medulloblastoma formation in Ptc1 mutants may be associated with resistance to radiation-induced cell killing. Furthermore, in marked contrast with P10 mice, cerebellum at P1 displays substantially increased activation of the cell survival-promoting Akt/Pkb protein, and markedly decreased p53 levels in response to radiation-induced genotoxic stress. Overall, these results show that developing cerebellar granule neuron precursors' (CGNPs) radiosensitivity to radiation-induced cell death increases with progressing development and inversely correlates with their ability to neoplastically transform.

Poly(ADP-RIBOSE) polymerase-1 (Parp-1) antagonizes topoisomerase I-dependent recombination stimulation by P53

PARP-1 interacts with and poly(ADP-ribosyl)ates p53 and topoisomerase I, which both participate in DNA recombination. Previously, we showed that PARP-1 downregulates homology-directed double-strand break (DSB) repair. We also discovered that, despite the well-established role of p53 as a global suppressor of error-prone recombination, p53 enhances homologous recombination (HR) at the RARα breakpoint cluster region (bcr) comprising topoisomerase I recognition sites. Using an SV40-based assay and isogenic cell lines differing in the p53 and PARP-1 status we demonstrate that PARP-1 counteracts HR enhancement by p53, although DNA replication was largely unaffected. When the same DNA element was integrated in an episomal recombination plasmid, both p53 and PARP-1 exerted anti-recombinogenic rather than stimulatory activities. Strikingly, with DNA substrates integrated into cellular chromosomes, enhancement of HR by p53 and antagonistic PARP-1 action was seen, very similar to the HR of viral minichromosomes. siRNA-mediated knockdown revealed the essential role of topoisomerase I in this regulatory mechanism. However, after I-SceI-meganuclease-mediated cleavage of the chromosomally integrated substrate, no topoisomerase I-dependent effects by p53 and PARP-1 were observed. Our data further indicate that PARP-1, probably through topoisomerase I interactions rather than poly(ADP-ribosyl)ation, prevents p53 from stimulating spontaneous HR on chromosomes via topoisomerase I activity.

p53 Monitors Replication Fork Regression by Binding to “Chickenfoot” Intermediates

The tumor suppressor protein, p53, utilizes multiple mechanisms to ensure faithful transmission of the genome including regulation of DNA replication, repair, and recombination. Monitoring these pathways may involve direct binding of p53 to the DNA intermediates of these processes. In this study, we generated templates resembling stalled replication forks and utilized electron microscopy to examine p53 interactions with these substrates. Our results show that p53 bound with high affinity to the junction of stalled forks, whereas two cancer-derived p53 mutants showed weak binding. Additionally, some of the templates were rearranged to form "chickenfoot" structures in the presence of p53. These were mostly formed due to p53 trapping intermediates of spontaneous fork regression; however, in a small population, the protein appeared to be promoting their formation. Collectively, these results demonstrate the importance of sequence-independent binding in p53-mediated maintenance of genomic integrity.

Differences in the association of p53 phosphorylated on serine 15 and key enzymes of homologous recombination

Phosphorylation of p53 on serine 15 by ATM or ATR is a frequent modification and initiates a cascade of post-translational modifications. To identify possible mechanisms that modulate p53 functions in recombination surveillance, we compared the nuclear localization of p53 phosphorylated on serine 15 (p53pSer15) and the key enzymes of homologous recombination (HR) after replication fork stalling. We demonstrate an almost mutually exclusive subcompartmentalization with Rad52, while p53pSer15 was colocalizing with 40-60% of the Rad51 and Mre11 foci. Therefore, possible sites of p53pSer15-dependent regulation seem to be sites of Rad51- rather than Rad52-dependent HR processes. Remarkably, the association of p53pSer15 with repair complexes containing Rad51 or Mre11 was transient, because less than 20% of the Rad51 and Mre11 foci overlapped with p53pSer15 after 6 h. When we examined colocalization and co-immunoprecipitation of p53pSer15 and the RecQ helicase BLM with recombination surveillance and proapoptotic functions, we observed colocalization within a fraction of approximately 70% of the BLM foci and stable physical interactions until 6 h after replication arrest. Our data suggest that p53pSer15 plays a dual role in the functional interactions with early complexes of Rad51-dependent recombination and with BLM-associated surveillance and signalling complexes within distinct nuclear subcompartments.

Genetic interactions between RAD51 and its paralogues for centrosome fragmentation and ploidy control, independently of the sensitivity to genotoxic stresses

We evaluate here whether RAD51 and its paralogues XRCC2 and XRCC3 act via a common pathway for sensitivity to genotoxic stress, centrosome fragmentation and chromosome stability. We expressed the RAD51 dominant-negative SMRAD51 in irs1 and irs1SF cells, defective for XRCC2 and XRCC3, respectively, and in their corresponding wild-type cells (V79 and AA8, respectively). V79-SMRAD51 cells are sensitive to mitomycin C (MMC), but SMRAD51 did not further sensitize irs1 cells to MMC, showing that SMRAD51 and XRCC2 act on the same pathway for resistance to MMC. However, in contrast to irs1 and irs1SF cells, SMRAD51-V79 and SMRAD51-AA8 cells are not sensitive to gamma-rays or UV-C. Despite these differences in sensitivity, SMRAD51-expressing cells and xrcc2- or xrcc3-defective cells show similar increased levels of centrosome fragmentation. This spontaneous centrosome fragmentation is resistant to caffeine, suggesting that ATM and ATR are not involved. Consistent with centrosome fragmentation, increased aneuploidy was measured in irs1 and SMRAD51-expressing cells. Expression of SMRAD51 in irs1 or irs1SF cells did not increase further the frequency of multipolar cells. Thus, RAD51, XRCC2 and XRCC3 act in the same pathway for centrosome fragmentation, independently of the sensitivity to exogenous genotoxic stresses and of the ATM/ATR pathway.

Wild-type p53 stimulates homologous recombination upon sequence-specific binding to the ribosomal gene cluster repeat

p53 plays a central role in the maintenance of the genome integrity, both as a gatekeeper and a caretaker. Sequence-specific recognition of DNA is underlying the ability of p53 to transcriptionally transactivate target genes during checkpoint control and to regulate DNA replication at the TGCCT repeat from the ribosomal gene cluster (RGC). In contrast, suppression of recombination by p53 has been observed with nonconsensus DNA sequences. In this study, we discovered that wild-type p53 stimulates homologous recombination adjacent to the RGC repeat, whereas downregulation is seen with a mutated version thereof and with a microsatellite repeat sequence. Analysis of the causes possibly underlying the enhancement of homologous recombination revealed that p53 binding to the RGC element delays DNA synthesis. This was demonstrated after integration of the corresponding DNA fragments into our Simian virus 40-based model system, which was used to study recombination on replicating minichromosomes. Differently, with plasmid-based substrates, p53 did not stimulate recombination at the RGC sequence. Thus, in combination with our previous findings, p53 may promote homologous recombination by two separate mechanisms involving either molecular interactions with topoisomerase I or/and by specific binding to certain genomic regions, thereby causing replication fork stalling and recombination.

Alternative recombination pathways in UV-irradiated XP variant cells

XP variant (XP-V) cells lack the damage-specific polymerase eta and exhibit prolonged replication arrest after UV irradiation due to impaired bypass of UV photoproducts. To analyse the outcome of the arrested replication forks, homologous recombination (HR, Rad51 events) and fork breakage (Rad50 events) were assayed by immunofluorescent detection of foci-positive cells. Within 1 h of irradiation, XP-V cells showed more Rad51-positive cells than normal cells, while neither cell type showed an increase in Rad50 foci. Beyond 1 h, the frequency of Rad51-positive cells reached similar levels in both cell types, then declined at higher UV doses. At these later times, Rad50-positive cells increased with dose and to a greater extent in XP-V cells. Few cells were simultaneously positive for both sets of foci, suggesting a mutually exclusive recruitment of recombination proteins, or that these pathways operate at different stages during S phase. Analysis of cells containing a vector of tandemly arranged enhanced green fluorescent protein genes also showed that UV-induced HR was higher in XP-V cells. These results suggest that cells make an early commitment to HR, and that at later times a subset of arrested forks degrade into double-strand breaks, two alternative pathways that are greater in XP-V cells.

[Direct role of p53 on homologous recombination]

The tumor suppressor gene p53, which is the most frequently mutated gene in human tumors, controls cell cycle checkpoint and apoptosis via the transactivation of the transcription of a collection of genes. These activities avoid proliferation of cell bearing alteration of genetic material. However, like a two-edged sword, p53 can also directly participate to genome stability maintenance by repressing homologous recombination (HR), independently of the transactivation activity. This parallel activity allows to limit the deleterious consequences on an excess of HR. Beside genetic interactions, p53 protein physically interacts with both HR proteins and HR intermediates (heteroduplex and Holliday junctions). The core domain of p53 is required for interaction with Rad51 at an early step and the carboxy-terminal domain of p53 is involved in the interaction with Rad54 and HR intermediates, at a late step. We discuss here the putative consequences of this parallel activity of p53 on genome stability, speciation and tumor protection.

The interaction of p53 with replication protein A mediates suppression of homologous recombination

The tumor suppressor protein p53 is emerging as a central regulator of homologous recombination (HR) processes and DNA replication. P53 may downregulate HR through multiple mechanisms including the reported associations with the Rad51 and Rad54 recombinases, and the BLM and WRN helicases. Here, we investigated whether the interaction of p53 with human replication protein A (RPA) is necessary for the regulation of HR. By employing a plasmid-based HR assay in p53-null H1299 lung carcinoma cells, we studied the HR-suppressing properties of a panel of p53 mutants, which varied in their ability to interact with RPA. Both wild-type p53 and a transactivation-deficient p53 mutant (L22Q/W23S) suppressed HR and prevented RPA binding to ssDNA in vitro and in vivo. Conversely, p53 mutations that specifically disrupt the RPA-binding domain, while not compromising p53 transactivation function (D48H/D49H and W53S/F54S), did not affect HR. Suppression of HR was also not seen with missense mutations in the p53 core domain (His175 and His273), which retained the ability to interact with RPA, suggesting that the disruption of additional binding interactions of p53, for example, with Rad51 or recombination intermediates, also impacts on HR. We hypothesize that sequestration of RPA by p53 at the sites of recombination is one means by which p53 can inhibit HR processes. Our data support and extend the previously formulated 'dual model' of p53's role as guardian of the genome.

Resistance to the antibiotic Zeocin by stable expression of the Sh ble gene does not fully suppress Zeocin-induced DNA cleavage in human cells

Zeocin is a member of the bleomycin/phleomycin family of antibiotics, known to bind and cleave DNA. We established human SK-OV-3 cells that stably express the Zeocin resistance gene (Sh ble) using an ecdysone-inducible mammalian expression system. Surprisingly, our results demonstrated that Zeocin, added in the culture medium to maintain the expression of the ecdysone receptor, was responsible for the formation of DNA strand breaks in the recombinant cells. This suggests that the Zeocin is not completely detoxified and is still able to cleave DNA, despite the stable expression of the Sh ble gene in the recombinant clones. Our study indicates that one needs to be very cautious in the interpretation of data involving stable cell lines selected with Zeocin.

p53: traffic cop at the crossroads of DNA repair and recombination

p53 mutants that lack DNA-binding activities, and therefore, transcriptional activities, are among the most common mutations in human cancer. Recently, a new role for p53 has come to light, as the tumour suppressor also functions in DNA repair and recombination. In cooperation with its function in transcription, the transcription-independent roles of p53 contribute to the control and efficiency of DNA repair and recombination.

Generation of loss of heterozygosity and its dependency on p53 status in human lymphoblastoid cells

Loss of heterozygosity (LOH) is a critical event in the development of human cancers. LOH is thought to result from either a large deletion or recombination between homologous alleles during repair of DNA double-strand breaks (DSBs). These types of genetic alterations produce mutations in the TK gene mutation assay, which detects a wide mutational spectrum, ranging from point mutations to LOH-type mutations. TK6, a human lymphoblastoid cell line, is heterozygous for the thymidine kinase (TK) gene and has a wild-type p53 gene. The related cell lines, TK6-E6 and WTK-1, which are p53-deficient and p53-mutant (Ile237), respectively, are also heterozygous for the TK gene and LOH-type mutation can be detected in these cells. Therefore, comparative studies of TK mutation frequency and spectrum with these cell lines are useful for elucidating the role of p53 in generating LOH and maintaining genomic stability in human cells. We demonstrate here that LOH and its associated genomic instability strongly depend on the p53 status in these cells. TK6-E6 and WTK-1 are defective in the G1/S checkpoint and in apoptosis. Unrepaired DSBs that escape from the checkpoint can potentially initiate genomic instability after DNA replication, resulting in LOH and a variety of chromosome changes. Moreover, genomic instability is enhanced in WTK-1 cells. It is likely that the mutant p53 protein in WTK-1 cells increases LOH in a dominant-negative manner due to its abnormal recombination capacity. We discuss the mutator phenotype and genomic instability associated with p53 inactivation with the goal of elucidating the mechanisms of mutation and DNA repair in untargeted mutagenesis.

Transcription-independent suppression of DNA synthesis by p53 in sperm-irradiated mouse zygotes

Cell cycle arrest in response to DNA damage is important for the maintenance of genomic integrity in higher eukaryotes. We have previously reported the novel p53-dependent S-phase checkpoint operating in mouse zygotes fertilized with irradiated sperm. In the present study, we analysed the detail of the p53 function required for this S-phase checkpoint in mouse zygotes. The results indicate that ATM kinase is likely to be indispensable for the p53-dependent S-phase checkpoint since the suppression was abrogated by inhibitors such as caffeine and wortmannin. However, ATM phosphorylation site mutant proteins were still capable of suppressing DNA synthesis when microinjected into sperm-irradiated zygotes lacking the functional p53, suggesting that the target of the phosphorylation is not p53. In addition, the suppression was not affected by alpha-amanitin, and p53 protein mutated at the transcriptional activation domain was also functional in the suppression of DNA synthesis. However, p53 proteins mutated at the DNA-binding domain were devoid of the suppressing activity. Taken together, the transcription-independent function of p53 associated with the DNA-binding domain is involved in the S-phase checkpoint in collaboration with yet another unidentified target protein(s).

Gene repair in mammalian cells is stimulated by the elongation of S phase and transient stalling of replication forks

The repair of point mutations directed by modified single-stranded DNA oligonucleotides is dependent on the activity of proteins involved in homologous recombination (HR). As a consequence, factors that stimulate homologous recombination, such as double strand breaks, can impact the frequency with which repair occurs. Here, we report that the stalling of replication forks can also activate the gene repair pathway and lead to an enhanced level of nucleotide exchange. The mammalian cell line, DLD-1, containing an integrated mutant eGFP gene, was used as an assay system to explore how replication fork activity affects the overall repair reaction. The addition of 2',3'-dideoxycytidine (ddC), a nucleoside analog that retards the rate of elongation and effectively stalls the replication fork, results in a lengthened S phase and an increased number of gene repair events. This stimulation was reversed when caffeine was added to the reaction at concentrations that block the homologous recombination pathway. In contrast, the nucleoside analog, 1-beta-D-arabinofuranosylcytosine which stops replication in these cells, failed to stimulate the gene repair reaction to any appreciable degree until the block is released and active replication resumes. Furthermore, overexpression of wild-type p53 which is known to bind transiently to stalled replication forks blocked the stimulatory effect of ddC. Overexpression of mutant p53 genes, deficient in the capacity to bind DNA, however, did not inhibit the reaction. Our results indicate that an expansion of S phase and a transient stalling of replication forks can increase the frequency of targeted gene repair.

Discriminatory suppression of homologous recombination by p53

Homologous recombination (HR) is used in vertebrate somatic cells for essential, RAD51-dependent, repair of DNA double-strand-breaks (DSBs), but inappropriate HR can cause genome instability. A transcriptional transactivation-independent role for p53 in suppressing HR has been established, but is not detected in all HR assays. To address the basis of such exceptions, and the possibility that suppression by p53 may be discriminatory, we have conducted a controlled comparison of the effects of p53 depletion on three different kinds of HR. We show that, within the same cells, p53 depletion promotes both intra-chromosomal HR (ICHR) and extra-chromosomal HR (ECHR), but not homologous DNA integration (gene targeting; GT). This conclusion holds true for both spontaneous and DSB-induced ICHR and GT. We show further that non-conservative ICHR is more susceptible than conservative ICHR to inhibition by p53. These results provide strong evidence that p53 can discriminate between different forms of HR and, despite the fact that GT is used experimentally for gene disruption, is consistent with the possibility that p53 preferentially suppresses genome-destabilizing forms of HR. While the mechanism of suppression by p53 remains unclear, our data suggest that it is independent of mismatch repair and of changes in RAD51 protein levels.

Enhanced oligonucleotide-directed gene targeting in mammalian cells following treatment with DNA damaging agents

Targeted gene repair, a form of oligonucleotide-directed mutagenesis, employs end-modified single-stranded DNA oligonucleotides to mediate single-base changes in chromosomal DNA. In this work, we use a specific 72-mer to direct the repair of a mutated eGFP gene stably integrated in the genome of DLD-1 cells. Corrected cells express eGFP that can be identified and quantitated by FACS. The repair of this mutant gene is dependent on the presence of a specifically designed oligonucleotide and the frequency with which the mutation is reversed is affected by the induction of DNA damage. We used hydroxyurea, VP16 (etoposide), and thymidine to modulate the rate of DNA replication through the stalling of the replication forks or the introduction of lesions. Addition of hydroxyurea or VP16 before the electroporation of the oligonucleotide, results in an accumulation of double-strand breaks (DSB) whose repair is facilitated by either nonhomologous end joining (NHEJ) or homologous recombination (HR). The addition of thymidine results in DNA damage within replication forks, damage that is repaired through the process of homologous recombination. Our data suggest that gene repair activity is elevated when DNA damage induces or activates the homologous recombination pathway.

Camptothecin enhances the frequency of oligonucleotide-directed gene repair in mammalian cells by inducing DNA damage and activating homologous recombination

Camptothecin (CPT) is an anticancer drug that promotes DNA breakage at replication forks and the formation of lesions that activate the processes of homologous recombination (HR) and nonhomologous end joining. We have taken advantage of the CPT-induced damage response by coupling it to gene repair directed by synthetic oligonucleotides, a process in which a mutant base pair is converted into a wild-type one. Here, we show that pretreating DLD-1 cells with CPT leads to a significant stimulation in the frequency of correction of an integrated mutant enhanced green fluorescent protein gene. The stimulation is dose-dependent and coincident with the formation of double-strand DNA breaks. Caffeine, but not vanillin, blocks the enhancement of gene repair suggesting that, in this system, HR is the pathway most responsible for elevating the frequency of correction. The involvement of HR is further proven by studies in which wortmannin was seen to inhibit gene repair at high concentrations but not at lower levels that are known to inhibit DNA-PK activity. Taken together, our results suggest that DNA damage induced by CPT activates a cellular response that stimulates gene repair in mammalian cells.

Structured DNA promotes phosphorylation of p53 by DNA-dependent protein kinase at serine 9 and threonine 18

Phosphorylation at multiple sites within the N-terminus of p53 promotes its dissociation from hdm2/mdm2 and stimulates its transcriptional regulatory potential. The large phosphoinositide 3-kinase-like kinases ataxia telangiectasia mutated gene product and the ataxia telangectasia and RAD-3-related kinase promote phosphorylation of human p53 at Ser15 and Ser20, and are required for the activation of p53 following DNA damage. DNA-dependent protein kinase (DNA-PK) is another large phosphoinositide 3-kinase-like kinase with the potential to phosphorylate p53 at Ser15, and has been proposed to enhance phosphorylation of these sites in vivo. Moreover, recent studies support a role for DNA-PK in the regulation of p53-mediated apoptosis. We have shown previously that colocalization of p53 and DNA-PK to structured single-stranded DNA dramatically enhances the potential for p53 phosphorylation by DNA-PK. We report here the identification of p53 phosphorylation at two novel sites for DNA-PK, Thr18 and Ser9. Colocalization of p53 and DNA-PK on structured DNA was required for efficient phosphorylation of p53 at multiple sites, while specific recognition of Ser9 and Thr18 appeared to be dependent upon additional determinants of p53 beyond the N-terminal 65 amino acids. Our results suggest a role for DNA-PK in the modulation of p53 activity resultant from the convergence of p53 and DNA-PK on structured DNA.

p53 protects from replication-associated DNA double-strand breaks in mammalian cells

Genetic instability caused by mutations in the p53 gene is generally thought to be due to a loss of the DNA damage response that controls checkpoint functions and apoptosis. Cells with mutant p53 exhibit high levels of homologous recombination (HR). This could be an indirect consequence of the loss of DNA damage response or p53 could have a direct role in HR. Here, we report that p53-/- mouse embryonic fibroblasts (MEFs) exhibit higher levels of the RAD51 protein and increased level of spontaneous RAD51 foci Agents that stall replication forks, for example, hydroxyurea (HU), potently induce HR repair and RAD51 foci. To test if the increase in RAD51 foci in p53-/- MEFs was due to an increased level of damage during replication, we measured the formation of DNA double-strand breaks (DSBs) in p53+/+ and p53-/- MEFs following treatments with HU. We found that HU induced DSBs only in p53-/- MEFs, indicating that p53 is involved in a pathway to protect stalled replication forks from being collapsed into a substrate for HR. Also, p53 is upregulated in response to agents that inhibit DNA replication, which supports our hypothesis. Finally, we observed that the DSBs produced in p53-/- MEFs did not result in a permanent arrest of replication and that they were repaired. Altogether, we suggest that the effect of p53 on HR and RAD51 levels and foci can be explained by the idea that p53 suppresses formation of recombinogenic lesions.

p53 Inhibits Strand Exchange and Replication Fork Regression Promoted by Human Rad51

We explore the effects of p53 on strand exchange as well as regression of stalled replication forks promoted by human Rad51. We have found that p53 specifically inhibits strand exchange mediated by human Rad51, but not by Escherichia coli RecA. In addition, we provide in vitro evidence that human Rad51 can promote regression of a stalled replication fork, and p53 also inhibits this fork regression. Furthermore, we show that two cancer-related p53 mutant proteins cannot inhibit strand exchange and fork regression catalyzed by human Rad51. The results establish a direct functional link between p53 and human Rad51, and reveal that one of p53's functions in genome stabilization may be to prevent detrimental genome rearrangements promoted by human Rad51. Thus, the results support the hypothesis that p53 contributes to genome stability by a transcription-independent modulation of homologous recombination.

Homologous recombination and cell cycle checkpoints: Rad51 in tumour progression and therapy resistance

We provide an overview of the functional interrelationship between genes and proteins related to DNA repair by homologous recombination and cell cycle regulation in relation to the progression and therapy resistance of human tumours. To ensure the high-fidelity transmission of genetic information from one generation to the next, cells have evolved mechanisms to monitor genome integrity. Upon DNA damage, cells initiate complex response pathways including cell cycle arrest, activation of genes and gene products involved in DNA repair, and under some circumstances, the triggering of programmed cell death. Deregulation of this co-ordinated response leads to genetic instability and is fundamental to the aetiology of human cancer. Homologous recombination involved in DNA repair is induced by environmental damage as well as misreplication during the normal cell cycle. However, when not regulated properly, it can result in the loss of heterozygocity or genetic rearrangements, central to the process of carcinogenesis. The central step of homologous recombination is the DNA strand exchange reaction catalysed by the eukaryotic Rad51 protein. Here, we describe the recent progress in our understanding of how Rad51 is involved in the signalling and repair of DNA damage and how tumour suppressors, such as p53, ATM, BRCA1, BRCA2, BLM and FANCD2 are linked to Rad51-dependent pathways. An increased knowledge of the role of Rad51 in DNA repair by homologous recombination and its effects on cell cycle progression, tumour development and tumour resistance may provide opportunities for identifying improved diagnostic markers and developing more effective treatments for cancer.

Lentivector-Mediated Transfer of Bmi-1 and Telomerase in Muscle Satellite Cells Yields a Duchenne Myoblast Cell Line with Long-Term Genotypic and Phenotypic Stability

Conditionally immortalized human cells are valuable substrates for basic biologic studies, as well as for the production of specific proteins and for the creation of bioartificial organs. We previously demonstrated that the lentivector-mediated transduction of immortalizing genes into human primary cells is an efficient method for obtaining such cell lines. Here, we used human muscle satellite cells as model targets to examine the impact of the transduced genes on the genotypic and phenotypic characteristics of the immortalized cells. The most commonly used immortalizing gene, the SV40 large T antigen (T-Ag), was extremely efficient at inducing the continuous growth of primary myoblasts, but the resulting cells rapidly accumulated major chromosomal aberrations and exhibited profound phenotypic changes. In contrast, the constitutive expression of telomerase and Bmi-1 in satellite cells from a control individual and from a patient suffering from Duchenne's muscular dystrophy yielded cell lines that remained diploid and conserved their growth factor dependence for proliferation. However, despite the absence of detectable cytogenetic abnormalities, clones derived from satellite cells of a control individual exhibited a differentiation block in vitro. In contrast, a Duchenne-derived cell line exhibited all the phenotypic characteristics of its primary parent, including an ability to differentiate fully into myotubes when placed in proper culture conditions. This cell line should constitute a useful reagent for a wide range of studies aimed at this disease.

Abnormal kinetics of induction of UV-stimulated recombination in human DNA repair disorders

Recombination can result in genetic instability, and thus constitutes an important factor in the carcinogenic conversion of mammalian cells. Here we describe the occurrence of UV-stimulated recombination called enhanced recombination (EREC), measured with the use of Herpes Simplex Viruses type 1 mutants. In normal diploid human cells, EREC is induced by UV-C, mitomycin C and ENU, but not by X-ray or MMS. The kinetics of induction of EREC is similar to that of other SOS-like responses such as enhanced reactivation (ER) and enhanced mutagenesis (EM). In contrast to the latter responses, EREC is induced to higher levels and persists for longer periods in DNA repair deficient fibroblasts derived from xeroderma pigmentosum (XP), Cockayne syndrome (CS) and Trichothiodystrophy (TTD) patients. This observation indicates that EREC is a distinct SOS-like response. Apparently, the presence of unrepaired DNA lesions in the host genome is a strongly inducing signal for EREC. On the other hand, in cells derived from patients suffering from Bloom, Werner or Rothmund-Thomson syndrome (RTS) the EREC response is absent. These data indicate that determining EREC is a useful assay to investigate diploid human fibroblasts for abnormalities in UV-stimulated recombination.

Mutations of TP53 induce loss of DNA methylation and amplification of the TROP1 gene

p53 and DNA methylation play key roles in the maintenance of genome stability. In this work, we demonstrate that the two mechanisms are linked and that p53 plays a role in the maintenance of the DNA methylation levels. The loss of p53 was shown to induce loss of DNA methylation in the TROP1 gene, a human cancer-expressed locus that undergoes amplification when hypomethylated. This demethylation was reverted by the reintroduction of a wild-type TP53 (wtTP53) in the TP53-null cells. Using a gene-amplification assay in vivo, we demonstrate that the loss of p53 leads to a demethylation-dependent TROP1 gene amplification. The induction of gene amplification was reverted by the expression of a wtTP53 gene or by in vitro methylation of the transfected DNA with the Sss I DNA methylase. Taken together, these findings demonstrate that the inactivation of TP53 induces loss of DNA methylation and DNA methylation-dependent gene amplification.

BLM helicase-dependent transport of p53 to sites of stalled DNA replication forks modulates homologous recombination

Diverse functions, including DNA replication, recombination and repair, occur during S phase of the eukaryotic cell cycle. It has been proposed that p53 and BLM help regulate these functions. We show that p53 and BLM accumulated after hydroxyurea (HU) treatment, and physically associated and co-localized with each other and with RAD51 at sites of stalled DNA replication forks. HU-induced relocalization of BLM to RAD51 foci was p53 independent. However, BLM was required for efficient localization of either wild-type or mutated (Ser15Ala) p53 to these foci and for physical association of p53 with RAD51. Loss of BLM and p53 function synergistically enhanced homologous recombination frequency, indicating that they mediated the process by complementary pathways. Loss of p53 further enhanced the rate of spontaneous sister chromatid exchange (SCE) in Bloom syndrome (BS) cells, but not in their BLM-corrected counterpart, indicating that involvement of p53 in regulating spontaneous SCE is BLM dependent. These results indicate that p53 and BLM functionally interact during resolution of stalled DNA replication forks and provide insight into the mechanism of genomic fidelity maintenance by these nuclear proteins.

Activities of wildtype and mutant p53 in suppression of homologous recombination as measured by a retroviral vector system

DNA repair of double strand breaks, interstrand DNA cross-links, and other types of DNA damage utilizes the processes of homologous recombination and non-homologous end joining to repair the damage. Aberrant homologous recombination is likely to be responsible for a significant fraction of chromosomal deletions, duplications, and translocations that are observed in cancer cells. To facilitate measurement of homologous recombination frequencies in normal cells, mutant cells, and cancer cells, we have developed a high titer retroviral vector containing tandem repeats of mutant versions of a GFP-Zeocin resistance fusion gene and an intact neomycin resistance marker. Recombination between the tandem repeats regenerates a functional GFP-Zeo(R) marker that can be easily scored. This retroviral vector was used to assess homologous recombination frequencies in human cancer cells and rodent fibroblasts with differing dosages of wild type or mutant p53. Absence of wild type p53 stimulated spontaneous and ionizing radiation-induced homologous recombination, confirming previous studies. Moreover, p53(+/-) mouse fibroblasts show elevated levels of homologous recombination compared to their p53(+/+) counterparts following retroviral vector infection, indicating that p53 is haploinsufficient for suppression of homologous recombination. Transfection of vector-containing p53 null Saos-2 cells with various human cancer-associated p53 mutants revealed that these altered p53 proteins retain some recombination suppression function despite being totally inactive for transcriptional transactivation. The retroviral vector utilized in these studies may be useful in performing recombination assays on a wide array of cell types, including those not readily transfected by normal vectors.

Wild-type p53 inhibits replication-associated homologous recombination

In mammalian cells homologous recombination is stimulated, when the replication fork stalls at DNA breaks or unrepaired lesions. The tumor suppressor p53 downregulates homologous recombination independently of its transcriptional transactivation function and has been linked to enzymes of DNA recombination and replication. To study recombination with respect to replication, we utilized a SV40 virus based assay, to follow the synchronous events after primate cell infection. gamma-ray treatment at different times after viral entry unveiled an increase of interchromosomal exchange frequencies, when the damage was introduced during DNA synthesis. Elevated recombination frequencies were fully suppressed by p53. With respect to the downregulation of spontaneous recombination, we noticed a requirement for active p53 molecules, when replication started. After a transient treatment with replication inhibitors, we observed inhibition of the drug induced recombination by p53, particularly for the elongation inhibitor aphidicolin. Consequently, we propose that p53 is a surveillance factor of homologous recombination at replication forks, when they stall as a consequence of endogenous or of exogenously introduced damage.

SV40 large T-antigen disturbs the formation of nuclear DNA-repair foci containing MRE11

The accumulation of DNA repair proteins at the sites of DNA damage can be visualized in mutagenized cells at the single cell level as discrete nuclear foci by immunofluorescent staining. Formation of nuclear foci in irradiated human fibroblasts, as detected by antibodies directed against the DNA repair protein MRE11, is significantly disturbed by the presence of the viral oncogene, SV40 large T-antigen. The attenuation of foci formation was found in both T-antigen immortalized cells and in cells transiently expressing T-antigen, indicating that it is not attributable to secondary mutations but to T-antigen expression itself. ATM-mediated nibrin phosphorylation was not altered, thus the disturbance of MRE11 foci formation by T-antigen is independent of this event. The decrease in MRE11 foci was particularly pronounced in T-antigen immortalized cells from the Fanconi anaemia complementation group FA-D2. FA-D2 cells produce essentially no MRE11 DNA repair foci after ionizing irradiation and have a significantly increased cellular radiosensitivity at low radiation doses. The gene mutated in FA-D2 cells, FANCD2, codes for a protein which also locates to nuclear foci and may, therefore, be involved in MRE11 foci formation, at least in T-antigen immortalized cells. This finding possibly links Fanconi anaemia proteins to the frequently reported increased sensitivity of Fanconi anaemia cells to transformation by SV40. From a practical stand point these findings are particularly relevant to the many studies on DNA repair which exploit the advantages of SV40 immortalized cell lines. The interference of T-antigen with DNA repair processes, as demonstrated here, should be borne in mind when interpreting such studies.

Association of p53 and MSH2 with recombinative repair complexes during S phase

Our previous recombination and biochemical analyses have led to the hypothesis that the tumor suppressor p53 monitors homologous recombination, a function which was previously attributed to the mismatch repair protein MSH2. Here, we show that a certain fraction of p53 is concentrated within discrete nuclear foci of cells synchronized in G1 phase, a pattern which becomes even more pronounced in S phase, especially after gamma-ray treatment. p53 foci show some colocalization with MSH2 within distinct foci during G1 phase, while dots formed by BRCA1 display an independent localization pattern. In S phase nuclei, p53 foci almost completely colocalize with MSH2 foci and associate with the recombination surveillance factor BRCA1 in irradiated S phase cells. These p53 and MSH2 foci also show significant overlaps with foci of the recombination enzymes Rad50 and Rad51, which for the first time unveiled recombination-related functions of p53 in replicating cells. During S phase, p53 and MSH2 are maximally active in binding to early recombination intermediates, and coexist within the same nuclear DNA-protein complexes. Our data suggest that p53 is linked similarly to homologous recombination as MSH2 and provide further evidence for the new concept of a dual role of p53 in the regulation of growth and repair.