Inactivation of p53 results in high rates of homologous recombination

Mitotic death: a mechanism of survival? A review

Mitotic death is a delayed response of p53 mutant tumours that are resistant to genotoxic damage. Questions surround why this response is so delayed and how its mechanisms serve a survival function. After uncoupling apoptosis from G1 and S phase arrests and adapting these checkpoints, p53 mutated tumour cells arrive at the G2 compartment where decisions regarding survival and death are made. Missed or insufficient DNA repair in G1 and S phases after severe genotoxic damage results in cells arriving in G2 with an accumulation of point mutations and chromosome breaks. Double strand breaks can be repaired by homologous recombination during G2 arrest. However, cells with excessive chromosome lesions either directly bypass the G2/M checkpoint, starting endocycles from G2 arrest, or are subsequently detected by the spindle checkpoint and present with the features of mitotic death. These complex features include apoptosis from metaphase and mitosis restitution, the latter of which can also facilitate transient endocycles, producing endopolyploid cells. The ability of cells to initiate endocycles during G2 arrest and mitosis restitution most likely reflects their similar molecular environments, with down-regulated mitosis promoting factor activity. Resulting endocycling cells have the ability to repair damaged DNA, and although mostly reproductively dead, in some cases give rise to mitotic progeny. We conclude that the features of mitotic death do not simply represent aberrations of dying cells but are indicative of a switch to amitotic modes of cell survival that may provide additional mechanisms of genotoxic resistance.

Prospects of chimeric RNA-DNA oligonucleotides in gene therapy

A strategy called targeted gene repair was developed to facilitate the process of gene therapy using a chimeric RNA-DNA oligonucleotide. Experiments demonstrated the feasibility of using the chimeric oligonucleotide to introduce point conversion in genes in vitro and in vivo. However, barriers exist in the low and/or inconstant frequency of gene repair. To overcome this difficulty, three main aspects should be considered. One is designing a more effective structure of the oligonucleotide. Trials have included lengthening the homologous region, displacing the mismatch on the chimeric strand and inventing a novel thioate-modified single-stranded DNA, which was demonstrated to be more active than the primary chimera in cell-free extracts. The second aspect is optimizing the delivery system. Producing synthetic carriers for efficient and specific transfection is demanding, especially for treatment in vivo where targeting is difficult. The third and most important aspect lies in the elucidation of the mechanism of the strategy. Investigation of the mechanism of strand exchange between the oligonucleotide molecule and double-stranded DNA in prokaryotes may greatly help to understand the mechanism of gene repair in eukaryotes. The development of this strategy holds great potential for the treatment of genetic defects and other purposes.

Homologous recombination in extrachromosomal plasmid substrates is not suppressed by p53

We and others reported previously that the tumor suppressor p53 down-regulates spontaneous homologous recombination in chromosomally integrating plasmid substrates, but how p53 affects homology-dependent repair of DNA double-strand breaks has not been established. Furthermore, it has been hypothesized that p53 may suppress homologous recombination by direct interaction with recombination intermediates, but it is not known whether p53 directly acts on extrachromosomal plasmid substrates. In the present study, we asked whether p53 can suppress extrachromosomal spontaneous and double-strand break-induced homologous recombination. A plasmid shuttle assay was employed utilizing episomally replicating substrates, which carried mutated tandem repeats of a CAT reporter gene. Spontaneous homologous recombination and homology-dependent repair of double-strand breaks induced by the I-SceI nuclease led to reconstitution of the reporter. Extrachromosomal homologous recombination was found to proceed independently of the p53 status of isogenic mouse fibroblast lines, contrasting the p53-mediated suppression of chromosomal recombination. The lack of p53 effect applied not only to the dominating single-strand annealing pathway, which is Rad51-independent, but also to Rad51-dependent gene conversion events. Comparison of homologous and non-homologous recombination frequencies revealed similar contributions to the repair of I-SceI-induced breaks irrespective of p53 status. Our results are consistent with a model in which the regulation of homologous recombination by p53 is restricted to the highly ordered chromosomal chromatin structure. These data may serve as a cautionary note for future investigations using solely extrachromosomal model systems to address DNA repair in intact cells.

When X-ray-inducible proteins meet DNA double strand break repair

Cellular responses to ionizing radiation (IR) include (a) activation of signal transduction enzymes; (b) stimulation of DNA repair, most notably DNA double strand break (DSB) repair by homologous or nonhomologous recombinatorial pathways; (c) activation of transcription factors and subsequent IR-inducible transcript and protein changes; (d) cell cycle checkpoint delays in G(1), S, and G(2) required for repair or for programmed cell death of severely damaged cells; (e) activation of zymogens needed for programmed cell death (although IR is a poor inducer of such responses in epithelial cells); and (f) stimulation of IR-inducible proteins that may mediate bystander effects influencing signal transduction, DNA repair, angiogenesis, the immune response, late responses to IR, and possibly adaptive survival responses. The overall response to IR depends on the cell's inherent genetic background, as well as its ability to biochemically and genetically respond to IR-induced damage. To improve the anti-tumor efficacy of IR, our knowledge of these pleiotropic responses must improve. The most important process for the survival of a tumor cell following IR is the repair of DNA double strand breaks (DSBs). Using yeast two-hybrid analyses along with other molecular and cellular biology techniques, we cloned transcripts/proteins that are involved in, or presumably affect, nonhomologous DNA double strand break end-joining (NHEJ) repair mediated by the DNA-PK complex. Using Ku70 as bait, we isolated a number of Ku-binding proteins (KUBs). We identified the first X-ray-inducible transcript/protein (xip8, Clusterin (CLU)) that associates with DNA-PK. A nuclear form of CLU (nCLU) prevented DNA-PK-mediated end joining, and stimulated cell death in response to IR or when overexpressed in the absence of IR. Structure-function analyses using molecular and cellular (including green fluorescence-tagged protein trafficking) biology techniques showed that nCLU appears to be an inactive protein residing in the cytoplasm of epithelial cells. Following IR injury, nCLU levels increase and an as yet undefined posttranslational modification appears to alter the protein, exposing nuclear localization sequences (NLSs) and coiled-coil domains. The modified protein translocates to the nucleus and triggers cell death, presumably through its interaction specifically with Ku70. Understanding nCLU responses, as well as the functions of the KUBs, will be important for understanding DSB repair. Knowledge of DSB repair may be used to improve the antitumor efficacy of IR, as well as other chemotherapeutic agents.

A novel role for the Bcl-2 protein family: specific suppression of the RAD51 recombination pathway

The oncogenic role of Bcl-2 is generally attributed to its protective effect against apoptosis. Here, we show a novel role for Bcl-2: the specific inhibition of the conservative RAD51 recombination pathway. Bcl-2 or Bcl-X(L) overexpression inhibits UV-C-, gamma-ray- or mutant p53-induced homologous recombination (HR). Moreover, Bcl-2 recombination inhibition is independent of the role of p53 in G1 arrest. At an acute double-strand break in the recombination substrate, Bcl-2 specifically inhibits RAD51-dependent gene conversion without affecting non-conservative recombination. Bcl-2 consistently thwarts recombination stimulated by RAD51 overexpression and alters Rad51 protein by post-translation modification. Moreover, a mutant (G145A)Bcl-2, which is defective in Bax interaction and in apoptosis repression, also inhibits recombination, showing that the death and recombination repression functions of Bcl-2 are separable. Inhibition of error-free repair pathways by Bcl-2 results in elevated frequencies of mutagenesis. The Bcl-2 gene therefore combines two separable cancer-prone phenotypes: apoptosis repression and a genetic instability/mutator phenotype. This dual phenotype could represent a mammalian version of the bacterial SOS repair system.

Manipulating the mammalian genome by homologous recombination

Gene targeting in mammalian cells has proven invaluable in biotechnology, in studies of gene structure and function, and in understanding chromosome dynamics. It also offers a potential tool for gene-therapeutic applications. Two limitations constrain the current technology: the low rate of homologous recombination in mammalian cells and the high rate of random (nontargeted) integration of the vector DNA. Here we consider possible ways to overcome these limitations within the framework of our present understanding of recombination mechanisms and machinery. Several studies suggest that transient alteration of the levels of recombination proteins, by overexpression or interference with expression, may be able to increase homologous recombination or decrease random integration, and we present a list of candidate genes. We consider potentially beneficial modifications to the vector DNA and discuss the effects of methods of DNA delivery on targeting efficiency. Finally, we present work showing that gene-specific DNA damage can stimulate local homologous recombination, and we discuss recent results with two general methodologies--chimeric nucleases and triplex-forming oligonucleotides--for stimulating recombination in cells.

Deficiency of human BRCA2 leads to impaired homologous recombination but maintains normal nonhomologous end joining

Carriers of BRCA2 germline mutations are at high risk to develop early-onset breast cancer. The underlying mechanisms of how BRCA2 inactivation predisposes to malignant transformation have not been established. Here, we provide direct functional evidence that human BRCA2 promotes homologous recombination (HR), which comprises one major pathway of DNA double-strand break repair. We found that up-regulated HR after transfection of wild-type (wt) BRCA2 into a human tumor line with mutant BRCA2 was linked to increased radioresistance. In addition, BRCA2-mediated enhancement of HR depended on the interaction with Rad51. In contrast to the tumor suppressor BRCA1, which is involved in multiple DNA repair pathways, BRCA2 status had no impact on the other principal double-strand break repair pathway, nonhomologous end joining. Thus, there exists a specific regulation of HR by BRCA2, which may function to maintain genomic integrity and suppress tumor development in proliferating cells.

Loss of mismatch repair activity in simian virus 40 large T antigen-immortalized BPH-1 human prostatic epithelial cell line

Simian virus 40 large T antigen (SVLTAg) has been used to immortalize cells; however, the mechanism leading to immortalization is still unclear. We hypothesize that DNA mismatch repair (MMR) activity is important during SVLTAg-induced immortalization. To test this hypothesis, we used the SVLTAg-immortalized cell line BPH-1 derived from human benign prostate epithelial cells to analyze MMR activity and the expression of MMR genes (hMLH1, hPMS1, hPMS2, hMSH2, hMSH3, and hMSH6). The results demonstrated that BPH-1 cells were deficient in repairing G:T, A:C, and G:G mispairs in bacteriophage M13mp2. Reverse-transcription polymerase chain reaction experiments indicated MMR genes (hMSH3, hMSH6, and hPMS1) were expressed at a low level in BPH-1 cells. In contrast, all six MMR genes were expressed in human benign prostate hyperplasia tissues. Downregulation of hMSH3, hMSH6, and hPMS1 genes is not a result of the hypermethylation mechanism because demethylation with 5-aza-2'-deoxycytidine did not restore expression of these genes. Although the hMLH1 gene is expressed in BPH-1 cells, western blotting and exon analyses demonstrated that hMLH1 was mutated and/or deleted in BPH-1 cells.

Gene targeting in livestock: a preview

Until recently genetically modified livestock could only be generated by pronuclear injection. The discovery that animals can be cloned by nuclear transfer from cultured somatic cells means that it will now be possible to achieve gene targeting in these species. We discuss current developments in NT, the prospects and technical challenges for introducing targeted changes into the germline by this route, and the types of application for which this new technology will be used.

Homologous recombination as a mechanism of carcinogenesis

Cancer develops when cells no longer follow their normal pattern of controlled growth. In the absence or disregard of such regulation, resulting from changes in their genetic makeup, these errant cells acquire a growth advantage, expanding into pre-cancerous clones. Over the last decade many studies have revealed the relevance of genomic mutation in this process, be it by misreplication, environmental damage or a deficiency in repairing endogenous and exogenous damage. Here we discuss homologous recombination as another mechanism that can result in loss of heterozygosity or genetic rearrangements. Some of these genetic alterations may play a primary role in carcinogenesis, but they are more likely to be involved in secondary and subsequent steps of carcinogenesis by which recessive oncogenic mutations are revealed. Patients whose cells display an increased frequency of recombination also have an elevated frequency of cancer, further supporting the link between recombination and carcinogenesis. In addition, homologous recombination is induced by a wide variety of carcinogens, many of which are classically considered to be efficiently repaired by other repair pathways. Overall, homologous recombination is a process that has been widely overlooked but may be more central to the process of carcinogenesis than previously described.

Telomere dysfunction alters the chemotherapeutic profile of transformed cells

Telomerase inhibition has been touted as a novel cancer-selective therapeutic goal based on the observation of high telomerase levels in most cancers and the importance of telomere maintenance in long-term cellular growth and survival. Here, the impact of telomere dysfunction on chemotherapeutic responses was assessed in normal and neoplastic cells derived from telomerase RNA null (mTERC(-/-)) mice. Telomere dysfunction, rather than telomerase per se, was found to be the principal determinant governing chemosensitivity specifically to agents that induced double-stranded DNA breaks (DSB). Enhanced chemosensitivity in telomere dysfunctional cells was linked to therapy-induced fragmentation and multichromosomal fusions, whereas telomerase reconstitution restored genomic integrity and chemoresistance. Loss of p53 function muted the cytotoxic effects of DSB-inducing agents in cells with telomere dysfunction. Together, these results point to the combined use of DSB-inducing agents and telomere maintenance inhibition as an effective anticancer therapeutic approach particularly in cells with intact p53-dependent checkpoint responses.

Evaluation of the genotoxicity of the phytoestrogen, coumestrol, in AHH-1 TK(+/-) human lymphoblastoid cells

Coumestrol, a phytoestrogen found in high levels in alfalfa and red clover, is of concern since endocrine disorders have been observed in farm animals exposed to high levels of phytoestrogens. Previous studies found that coumestrol was an effective inducer of DNA strand breaks, micronuclei, and mutations in the Hypoxanthine phosphoribosyl transferase (HPRT) gene of Chinese hamster ovary cells. In the experiments presented here, we extended the previous studies to examine the effect of coumestrol exposure on AHH-1 TK(+/-) human lymphoblastoid cells. Micronuclei were induced with the highest frequency occurring at day 2 after exposure. Flow cytometric analysis of annexin V-FITC-7-aminoactinomycin D stained cells indicated that the primary pathway of cell death was by apoptosis. Mutations were induced in the Thymidine Kinase (TK) gene and were due primarily to the induction of clones with the slow-growth phenotype. Subsequent molecular analysis revealed the loss of exon 4 in the coumestrol-induced clones, indicative of loss-of heterozygosity and consistent with a proposed inhibition of topoisomerase-II activity as a mechanism of action for coumestrol. Taken together, these results suggest that coumestrol exhibits both mutagenic and clastogenic properties in cultured human lymphoblastoid cells.

A novel host cell reactivation assay to assess homologous recombination capacity in human cancer cell lines

Repair of DNA double-strand breaks (DSB) is essential for cell viability and genome stability. Homologous recombination repair plays an important role in DSB repair and impairment of this repair mechanism may lead to loss of genomic integrity, which is one of the hallmarks of cancer. Recent research has shown that the tumor suppressor genes p53 and BRCA1 and -2 are involved in the proper control of homologous recombination, suggesting a role of this type of repair in human cancer. We developed a novel assay based on recombination between two Green Fluorescent Protein (GFP) sequences in transiently transfected plasmid DNA. The plasmid construct contains an intact, emission-shifted, "blue" variant of GFP (BFP), with a 300 nucleotide stretch of homology to a nonfunctional copy of GFP. In the absence of homologous recombination only BFP is present, but homologous recombination can create a functional GFP. The homologous regions in the plasmid were constructed in both the direct and the inverted orientation of transcription to detect possible differences in the recombination mechanisms involved. A panel of human tumor cell lines was chosen on the basis of genetic background and chromosome integrity and tested for homologous recombination using this assay. The panel included cell lines with varying levels of karyotypic abnormalities, isogenic cell lines with normal and mutant p53, isogenic cell lines with or without DNA mismatch repair, BRCA1 and -2 mutant cell lines, and the lymphoma cell line DT40. With this assay, the observed differences between cell lines with the lowest and highest levels of recombination were about 100-fold. Increased levels of recombination were associated with mutant p53, whereas a low level of recombination was found in the BRCA1 mutant cell line. In the cell line HT1080TG, a mutagenized derivative of HT1080 with two mutant alleles of p53, high levels of recombination were found with the direct orientation but not with the inverted orientation plasmid. No difference in recombination was detected between two isogenic cell lines that only differed in DNA mismatch repair capability. We conclude that this assay can detect differences in homologous recombination capacity in cultured cell lines and that these differences follow the patterns that would be expected from the different genotypes of these cell lines. Future application in normal cells may be useful to identify genetic determinants controlling genomic integrity or to detect differences in DNA repair capacity in individuals.

Recombination during early herpes simplex virus type 1 infection is mediated by cellular proteins

Homologous recombination was examined in cells infected with herpes simplex virus type I. Circular and linear DNA with directly repeated sequences was introduced as recombination substrates into cells. Recombination was measured either by origin-dependent amplification of recombination products or by recombination-dependent expression of luciferase from a disrupted gene. Homologous recombination in baby hamster kidney cells converted linear DNA to circular templates for DNA replication and luciferase expression in the complete absence of virus. The products of homologous recombination were efficiently amplified by the viral replication apparatus. The efficiency of recombination was dependent on the structure of the substrate as well as the cell type. Linear DNA with the direct repeats at internal positions failed to recombine in Balb/c 3T3 cells and induced p53-dependent apoptosis. In contrast, linear DNA with directly repeated sequences precisely at the ends recombined and replicated in 3T3 cells. Homologous recombination in baby hamster kidney cells did not depend on the position of the repeated sequences. We conclude that homologous recombination is independent of viral gene functions and that it is likely to be carried out by cellular proteins. We suggest that homologous recombination between directly repeated sequences in the linear herpes simplex virus type 1 chromosome may help to avoid p53-dependent apoptosis and to promote viral DNA replication.

Human DNA repair genes

DNA repair systems are essential for the maintenance of genome integrity. Consequently, the disregulation of repair genes can be expected to be associated with significant, detrimental health effects, which can include an increased prevalence of birth defects, an enhancement of cancer risk, and an accelerated rate of aging. Although original insights into DNA repair and the genes responsible were largely derived from studies in bacteria and yeast, well over 125 genes directly involved in DNA repair have now been identified in humans, and their cDNA sequence established. These genes function in a diverse set of pathways that involve the recognition and removal of DNA lesions, tolerance to DNA damage, and protection from errors of incorporation made during DNA replication or DNA repair. Additional genes indirectly affect DNA repair, by regulating the cell cycle, ostensibly to provide an opportunity for repair or to direct the cell to apoptosis. For about 70 of the DNA repair genes listed in Table I, both the genomic DNA sequence and the cDNA sequence and chromosomal location have been elucidated. In 45 cases single-nucleotide polymorphisms have been identified and, in some cases, genetic variants have been associated with specific disorders. With the accelerating rate of gene discovery, the number of identified DNA repair genes and sequence variants is quickly rising. This report tabulates the current status of what is known about these genes. The report is limited to genes whose function is directly related to DNA repair.

Loss of heterozygosity is related to p53 mutations and smoking in lung cancer

Carcinogenesis results from an accumulation of several genetic alterations. Mutations in the p53 gene are frequent and occur at an early stage of lung carcinogenesis. Loss of multiple chromosomal regions is another genetic alteration frequently found in lung tumours. We have examined the association between p53 mutations, loss of heterozygosity (LOH) at frequently deleted loci in lung cancer, and tobacco exposure in 165 tumours from non-small cell lung cancer (NSCLC) patients. A highly significant association between p53 mutations and deletions on 3p, 5q, 9p, 11p and 17p was found. There was also a significant correlation between deletions at these loci. 86% of the tumours with concordant deletion in the 4 most involved loci (3p21, 5q11-13, 9p21 and 17p13) had p53 mutations as compared to only 8% of the tumours without deletions at the corresponding loci (P< 0.0001). Data were also examined in relation to smoking status of the patients and histology of the tumours. The frequency of deletions was significantly higher among smokers as compared to non-smokers. This difference was significant for the 3p21.3 (hMLH1 locus), 3p14.2 (FHIT locus), 5q11-13 (hMSH3 locus) and 9p21 (D9S157 locus). Tumours with deletions at the hMLH1 locus had higher levels of hydrophobic DNA adducts. Deletions were more common in squamous cell carcinomas than in adenocarcinomas. Covariate analysis revealed that histological type and p53 mutations were significant and independent parameters for predicting LOH status at several loci. In the pathogenesis of NSCLC exposure to tobacco carcinogens in addition to clonal selection may be the driving force in these alterations.

Genetic Stabilization by p53 Involves Growth Regulatory and Repair Pathways

p53 performs a plethora of activities, which are directed towards the maintenance of the genomic integrity and constitute its universal role as a tumor suppressor. 1000 to 10000 latent p53 molecules are permanently available in order to monitor DNA exchange processes in mitotically growing cells. After the introduction of major DNA injuries the levels of posttranslationally modified p53 proteins rise, which in turn transcriptionally signal transient cell cycle arrest or apoptotic cell death, depending on the extent of damage. Taken together, p53 inhibits the manifestation of genomic instabilities at different control levels both during naturally occurring metabolic processes and in response to genotoxic treatments.

Bcr-Abl protein tyrosine kinase activity induces a loss of p53 protein that mediates a delay in myeloid differentiation

Chronic myeloid leukaemia is a haemopoietic stem cell disorder, the hallmark of which is the expression of the Bcr-Abl Protein Tyrosine Kinase (PTK). We have previously reported that activation of a temperature sensitive Bcr-Abl PTK in the multipotent haemopoietic cell line FDCP-Mix for short periods resulted in subtle changes including, a transient suppression of apoptosis and no inhibition of differentiation. In contrast, activation of the Bcr-Abl PTK for 12 weeks results in cells that display a delay in differentiation at the early granulocyte stage. Flow cytometric analysis also indicates that the expression of cell surface differentiation markers and nuclear morphology are uncoupled. Furthermore, a significant number of the mature neutrophils display abnormal morphological features. Prolonged exposure to Bcr-Abl PTK results in interleukin-3 independent growth and decreased p53 protein levels. FDCP-Mix cells expressing a dominant negative p53 and p53null FDCP-Mix cells demonstrate that the reduction in p53 is causally related to the delay in development. Returning the cells to the restrictive temperature restores the p53 protein levels, the growth factor dependence and largely relieves the effects on development. We conclude that prolonged Bcr-Abl PTK activity within multipotent cells results in a reduction of p53 that drives a delayed and abnormal differentiation.

Targeted single-base correction by RNA-DNA oligonucleotides

An oligonucleotide composed of a contiguous stretch of RNA and DNA residues has been developed to facilitate correction of single-base mutations of episomal and chromosomal targets in mammalian cells. We demonstrated that an RNA-DNA oligonucleotide (RDO) induced heritable correction of a point mutation in the tyrosinase gene at the level of genomic sequence, protein, and phenotype of albino mouse melanocytes and albino mouse skin. Such RDOs might hold promise as a therapeutic method for the treatment of skin diseases. However, the general application of RDO technology has been hampered by the absence of a standardized system to measure the gene conversion in a particular cell type in a rapid and reproducible manner. For this purpose, we established an in vitro system in which nuclear extracts from mammalian cells showed RDO-mediated gene correction of a shuttle vector containing a point mutation in the E. coli beta-galactosidase gene. This sensitive and convenient assay has been utilized to optimize the design of RDOs and to compare frequencies of gene conversion among different cell types. The general application of the RDO for site-specific gene correction or mutation would benefit from such mechanistic studies.

A gain of function p53 mutant promotes both genomic instability and cell survival in a novel p53-null mammary epithelial cell model

Approximately 40% of human breast cancers contain alterations in the tumor suppressor p53. The p53 172R-H gain-of-function mutant (equivalent to the common 175R-H human breast cancer mutant) has been shown to promote aneuploidy and tumorigenesis in the mammary gland in transgenic mice and may affect genomic stability in part by causing centrosome abnormalities. The precise mechanism of action of these gain-of-function mutants is not well understood, and has been studied primarily in fibroblast cell lines. A novel p53-null mouse mammary epithelial cell line developed from p53-null mice has been used in adenovirus-mediated transient transfection experiments to study the properties of this p53 mutant. Marked centrosome amplification and an increased frequency of aberrant mitoses were observed within 72 h of introduction of p53 172R-H. However, few cells with aberrant centrosome numbers were observed in cells stably expressing the p53 172R-H mutant. Furthermore, stable expression of this p53 mutant reduced both basal and DNA damage-induced apoptosis. This result may be mediated in part through abrogation of p73 function. The p53 172R-H mutant, therefore, appears to influence tumorigenesis at the molecular level in two distinct ways: promoting the development of aneuploidy in cells while also altering their apoptotic response after DNA damage.

p53 C-terminal interaction with DNA ends and gaps has opposing effect on specific DNA binding by the core

In addition to binding DNA in a sequence-specific manner, the p53 tumour suppressor protein can interact with damaged DNA. In order to understand which structural features in DNA the C-teminal domain recognises we have studied the interaction of p53 protein with different types of DNA oligonucleotides imitating damaged DNA. Here we show that one unpaired nucleotide within double-stranded (ds)DNA is sufficient for recognition by the p53 C-terminus, either as a protruding end or as an internal gap in dsDNA. C-terminal interaction with DNA ends facilitated core domain binding to DNA, whereas interaction with gaps prevented core domain-DNA complexing, implying that p53 might adopt distinct conformations upon binding to different DNA lesions. These observations suggest that both single-strand and double-strand breaks can serve as a target for p53 C-terminal recognition in vivo and indicate that p53 might recruit different repair factors to the sites of damaged DNA depending on the type of lesion.

Role of heteroduplex joints in the functional interactions between human Rad51 and wild-type p53

Our previous work (Dudenhöffer et al., 1999) unveiled a link between the capacity of p53 to regulate homologous recombination processes and to specifically bind to heteroduplex junction DNAs. Here, we show that p53 participates in ternary complex formation after preassembly of nucleoproteins, consisting of the human recombinase hRad51 and junction DNA. The cancer-related mutant p53(273H), which is defective in inhibiting recombination processes, displays a reduced capacity to associate with hRad51-DNA complexes, even under conditions which support DNA-binding. This suggests that hRad51-p53 contacts play a role in targeting p53 to heteroduplex joints and indicates an involvement in recombination immediately following hRad51-mediated strand transfer. To study the initial phase of strand exchange, when heteroduplex joints arise, we applied oligonucleotide based strand transfer assays. We observed that hRad51 stimulates exonucleolytic DNA degradation by p53, when it generates strand transfer intermediates. In agreement with this observation, artificial 3-stranded junction DNAs, designed to mimic nascent recombination intermediates, were found to represent preferred exonuclease substrates, especially when comprising a mismatch within the heteroduplex part. From our data, we propose a model according to which, p53-dependent correction of DNA exchange events is triggered by high-affinity binding to joint molecules and by stabilizing contacts with hRad51 oligomers. Oncogene (2000) 19, 4500 - 4512.

Requirement of wild-type p53 protein for maintenance of chromosomal integrity

Chromosomal double-strand breaks (DSBs) occurring in mammalian cells can initiate genomic instability, and their misrepairs result in chromosomal deletion, amplification, and translocation, common findings in human tumors. The tumor-suppressor protein p53 is involved in maintaining genomic stability. In this study, we demonstrate that the deficiency of wild-type p53 protein may allow unrepaired DSBs to initiate chromosomal instability. The human lymphoblastoid cell line TK6-E6 was established by transfection with human papilloma virus 16 (HPV16) E6 cDNA into parental TK6 cells via a retroviral vector. Abrogation of p53 function by E6 resulted in an increase in the spontaneous mutation frequencies at the heterozygous thymidine kinase (TK) locus but not at the hemizygous hypoxanthine phosphoribosyl transferase (HPRT) locus. Almost all TK-deficient mutants from TK6-E6 cells exhibited loss of heterozygosity (LOH) with the hemizygous TK allele. LOH analysis with microsatellite loci spanning the long arm of chromosome 17, which harbors the TK locus, showed that LOH extended over half of 17q toward the terminal end. Cytogenetic analysis of LOH mutants by chromosome painting indicated a mosaic of chromosomal aberrations involving chromosome 17, in which partial chromosome deletions, amplifications, and multiple translocations appeared heterogeneously in a single mutant. We speculate that spontaneous DSBs trigger the breakage-fusion bridge cycle leading to such multiple chromosome aberrations. In contrast, no chromosomal alterations were observed in TK-deficient mutants from TK6-20C cells expressing wild-type p53. In wild-type p53 cells, spontaneous DSBs appear to be promptly repaired through recombination between homologous chromosomes. These results support a model in which p53 protein contributes to the maintenance of genomic integrity through recombinational repair.

Chromosome instability contributes to loss of heterozygosity in mice lacking p53

The p53 tumor suppressor protein participates in multiple cellular processes including cell cycle checkpoints and programmed cell death. In cell lines, loss of p53 function is associated with increased genetic instability including aneuploidy, gene amplification, and point mutation. Although similar genetic instability often accompanies the progression of malignancy in tumors, its role in tumor initiation in normal cells is not clear. To study whether or not loss of p53 leads to genetic instability in normal cells in vivo, we have examined mechanisms of loss of heterozygosity (LOH) at the Aprt (adenine phosphoribsyltransferase) and flanking loci in normal fibroblasts and T lymphocytes of p53-deficient mice. Somatic cell variants that arose in vivo as a consequence of genetic or epigenetic alterations abolishing Aprt function were selected and expanded in vitro by virtue of their resistance to 2,6-diaminopurine (DAP). We observed that p53 null mice produced about three times as many DAP-resistant fibroblast colonies than wild-type mice, but the frequency of DAP-resistant T lymphocyte colonies was not significantly changed. Mitotic recombination, but not point mutation, partly accounted for the increase in the frequency of DAP-resistant fibroblasts. Most significantly, chromosome loss/duplication and interstitial deletion, which were extremely rare events in the wild-type mice, represented a significant proportion of LOH events in both fibroblasts and T lymphocytes of p53 null mice. Also, increased interstitial deletion was observed in fibroblasts of p53 heterozygous mice. These data suggest that increased genetic variation, including chromosome instability, starts at the initiation stage of tumorigenesis when functional p53 is absent or reduced.

Homologous and nonhomologous recombination resulting in deletion: effects of p53 status, microhomology, and repetitive DNA length and orientation

Repetitive DNA elements frequently are precursors to chromosomal deletions in prokaryotes and lower eukaryotes. However, little is known about the relationship between repeated sequences and deletion formation in mammalian cells. We have created a novel integrated plasmid-based recombination assay to investigate repeated sequence instability in human cells. In a control cell line, the presence of direct or inverted repeats did not appreciably influence the very low deletion frequencies (2 x 10(-7) to 9 x 10(-7)) in the region containing the repeat. Similar to what has been observed in lower eukaryotes, the majority of deletions resulted from the loss of the largest direct repeat present in the system along with the intervening sequence. Interestingly, in closely related cell lines that possess a mutant p53 gene, deletion frequencies in the control and direct-repeat plasmids were 40 to 300 times higher than in their wild-type counterparts. However, mutant p53 cells did not preferentially utilize the largest available homology in the formation of the deletion. Surprisingly, inverted repeats were approximately 10,000 times more unstable in all mutant p53 cells than in wild-type cells. Finally, several deletion junctions were marked by the addition of novel bases that were homologous to one of the preexisting DNA ends. Contrary to our expectations, only 6% of deletions in all cell lines could be classified as arising from nonhomologous recombination.

p53 splice acceptor site mutation and increased HsRAD51 protein expression in Bloom's syndrome GM1492 fibroblasts

GM1492 human diploid skin fibroblasts derived from a patient with Bloom's syndrome (BS), lack detectable p53 mRNA and protein as shown by Northern and Western blotting, and express an increased RecA-like activity. Here we demonstrate that the p53 gene is grossly intact in GM1492 cells according to Southern blotting. DNA sequencing did not reveal any mutations in the promoter region of p53. A highly sensitive RT-PCR produced a p53 cDNA fragment that was shorter than expected. DNA sequence analysis of p53 cDNA showed that exon 6 was missing, explaining the shorter PCR product. Furthermore, sequencing of genomic DNA revealed a base substitution at the nucleotide preceding the AG splice acceptor site of intron 5. The omission of exon 6 creates a frameshift at the junction of exons 5 and 7, and a premature stop codon in exon 7. The aberrant transcript is predicted to encode a truncated p53 protein containing 189 amino acid residues. Moreover, Western blotting demonstrated elevated HsRAD51 protein levels in GM1492 cells. The lack of sufficient levels of wild-type p53 and increased levels of HsRad51 protein may contribute to the elevated RecA-like activity in the GM1492 fibroblasts.

Recombination in telomere-length maintenance

Although telomerase is the major mechanism for telomere elongation in most cells, telomerase-independent mechanisms of telomere maintenance can allow cell survival. Yeast cells that lack telomerase maintain telomere length through a form of recombination known as gene conversion. Understanding the role that telomeric recombination might play in mammalian cells has important implications for cancer therapeutics.

Regulation of double-strand break-induced mammalian homologous recombination by UBL1, a RAD51-interacting protein

Mammalian RAD51 protein plays essential roles in DNA homologous recombination, DNA repair and cell proliferation. RAD51 activities are regulated by its associated proteins. It was previously reported that a ubiquitin-like protein, UBL1, associates with RAD51 in the yeast two-hybrid system. One function of UBL1 is to covalently conjugate with target proteins and thus modify their function. In the present study we found that non-conjugated UBL1 forms a complex with RAD51 and RAD52 proteins in human cells. Overexpression of UBL1 down-regulates DNA double-strand break-induced homologous recombination in CHO cells and reduces cellular resistance to ionizing radiation in HT1080 cells. With or without overexpressed UBL1, most homologous recombination products arise by gene conversion. However, overexpression of UBL1 reduces the fraction of bidirectional gene conversion tracts. Overexpression of a mutant UBL1 that is incapable of being conjugated retains the ability to inhibit homologous recombination. These results suggest a regulatory role for UBL1 in homologous recombination.

Dissociation of p53-mediated suppression of homologous recombination from G1/S cell cycle checkpoint control

The tumor suppressor p53 is considered as the guardian of the genome which is activated following genotoxic stress. In many cell types, p53 mediates G1 cell cycle arrest as the predominant cellular response. Inactivation of wild-type p53 leads to loss of G1/S checkpoint control and to genomic instability, including increased spontaneous homologous recombination (HR). To determine whether regulation of the G1/S checkpoint is required for suppression of HR, we assessed recombination events using a plasmid substrate that stably integrated into the genome of p53-null mouse fibroblasts. Exogenous expression of a temperature-sensitive p53 protein (Ala135 to Val), which had lost trans-activation function and could not regulate G1/S transition when in mutant conformation, reduced HR rates to the same extent as wild-type p53. Furthermore, a p53 construct with an alternatively-spliced carboxy terminus also retained this ability in the absence of both activities, G1/S control and non-sequence specific DNA binding as mediated by the carboxy terminus. Our data dissociate regulation of HR by p53 from its role as a cell cycle checkpoint protein. The results support a model which extends p53's role as a guardian of the genome to include transactivation-independent regulatory functions in DNA repair, replication and recombination.

p53 status in multiple human urothelial cancers: assessment for clonality by the yeast p53 functional assay in combination with p53 immunohistochemistry

Multifocal synchronous or metachronous tumor development is a common observation in human urothelial cancer cases. However, the underlying mechanism has remained obscure. We have employed a new tool to investigate the p53 gene status, the yeast p53 functional assay, in combination with immunohistochemistry in a total of 50 tumor samples from 32 cases with urothelial cancers, including 8 with multiple synchronous tumor development and 2 demonstrating metachronous tumors. p53 mutations were found in 13 cases (9 with missense mutations, 3 with deletion, 1 with splicing mutation) by the yeast p53 functional assay. p53 protein overexpression was seen in all 9 cases with missense mutations, but in only one of the 4 cases with nonsense mutations. Two tumors without p53 mutation also showed positive p53 immunoreactivity. Overall, p53 abnormalities including mutations and/or protein overexpression were found in 15 (47%) cases. p53 abnormalities were significantly more frequent in non-papillary and in high grade tumors. Loss of the wild type allele in addition to a p53 mutation was suggested in 8 of the 15 (53%) cases. All 4 cases with mutations in multiple synchronous tumors had identical p53 mutations in the separate urothelial cancers, strongly suggestive of monoclonality. The one case with multiple metachronous tumors, in contrast, was characterized by variation in the p53 status, indicative of different clonal origins. In conclusion, combined assessment for p53 status as used here (yeast p53 functional assay plus immunohistochemistry) may provide insights into the molecular mechanisms of urothelial carcinogenesis.

Identification and characterization of genes whose expressions are altered in rat 6 fibroblasts transformed by mutant p53(val135)

The wild-type tumor suppressor gene p53 is known as a transcription factor in activating or suppressing target genes that encode proteins in regulating genome stability, DNA damage, cell arrest, and apoptosis. However, the role of mutant p53 in the process of cell transformation is still unclear. Our recent work indicated that overexpression of mutant p53(val135) induced high incidence of spontaneous transformation in prolonged cultures of Rat 6 fibroblasts. In order to identify genes related to neoplastic transformation induced by the mutant p53, the p53(val135)-overexpressor R6#13-8 and its derived spontaneously transformed cell line T2 were analyzed by mRNA differential display. In a systematic screening with 80 primer sets of RT-PCR reactions, three genes were found to be differentially expressed between R6#13-8 and T2 cells. Two genes, identified as homologues of the growth factor inducible immediate-early gene Cyr61 and the human nonmuscle myosin heavy chain-B, were down-regulated in T2 cells. Interestingly, both genes were also suppressed in Rat 6 cells transformed by c-H-ras and v-myc, but not by v-src genes. The third gene is a homologue of the frizzled related protein, a gene family that acts, in some cases, as an antagonist to the Wnt signaling pathway. It is intriguing that the rat homologue of the frizzled related protein was only expressed in p53(val135)-overexpressing cells, but not in the parental Rat 6 cells. However, the same gene was also highly expressed in ras-transformed Rat 6 cells, and moderately expressed in v-src-transformed Rat 6 cells. This is the first study in which the association of mutant p53 to these three genes is revealed. Our current report may provide new clues to the role of mutant p53 in the process of cell transformation.

Maintenance of genomic integrity by p53: complementary roles for activated and non-activated p53

In this review we describe the multiple functions of p53 in response to DNA damage, with an emphasis on p53's role in DNA repair. We summarize data demonstrating that p53, through its various biochemical activities and via its ability to interact with components of the repair and recombination machinery, actively participates in various processes of DNA repair and DNA recombination. An important aspect in evaluating p53 functions arises from the finding that the p53 core domain harbors two mutually exclusive biochemical activities, sequence-specific DNA binding, required for its transactivation function, and 3'->5' exonuclease activity, possibly involved in various aspects of DNA repair. As modifications of p53 that lead to activation of its sequence-specific DNA-binding activity result in inactivation of its 3'-> 5' exonuclease activity, we propose that p53 exerts its functions as a 'guardian of the genome' at various levels: in its non-induced state, p53 should not be regarded as a non-functional protein, but might be actively involved in prevention and repair of endogenous DNA damage, for example via its exonuclease activity. Upon induction through exogenous DNA damage, p53 will exert its well-documented functions as a superior response element in various types of cellular stress. The dual role model for p53 in maintaining genomic integrity significantly enhances p53's possibilities as a guardian of the genome.

A sequence-specific gene correction by an RNA-DNA oligonucleotide in mammalian cells characterized by transfection and nuclear extract using a lacZ shuttle system

The variability in gene conversion frequency by an RNA-DNA oligonucleotide (RDO) prompted us to develop a system as a means of measuring the conversion frequency rapidly and reproducibly. A shuttle vector was constructed to measure the frequency of targeted gene correction by RDO of the E. coli beta-galactosidase gene containing a single point mutation (G --> A), that resulted in inactivation of enzymatic activity. An RDO corrected the point mutation and restored the enzymatic activity, approximately 1%, determined by a histochemical staining in mammalian cells and by a color selection (blue or white) of bacteria transformed with Hirt DNA. In addition, we established an in vitro system capable of gene correction using nuclear extracts. CHO-K1 nuclear extracts corrected the point mutation approximately 0.1%, determined by bacterial transformation. Using the in vitro reaction, frequency of gene conversion in different cell types was measured. The embryonic fibroblasts from p53-/- mouse showed higher gene correction than that of the isogenic p53+/+ cells. Nuclear extracts from DT40 cells, which have a higher homologous recombination rate than any other mammalian cells exhibited 0.1-0.6% of gene correction. These results indicated that recombination may be rate-limiting in gene conversion by RDO in cells with competent mismatch repair activities. Utilizing transfection and in vitro reaction, we demonstrated that such a shuttle system might be useful in comparing the frequency of targeting among different cell types and to investigate the mechanism of gene conversion by RDO.

Involvement of p53 in X-ray induced intrachromosomal recombination in mice

The tumor suppressor gene Trp53 (also known as p53) is the most frequently mutated gene in human cancers. p53 is induced in response to DNA damage and effects a G(1) cell cycle arrest. It is believed that p53 plays a key role in maintaining genomic integrity following exposure to DNA-damaging agents. We determined the frequency of spontaneous and DNA damage-induced homologous intrachromosomal recombination in p53-deficient mouse embryos. Homologous intrachromosomal recombination events resulting in deletions at the pink eyed unstable (p(un)) locus result in reversion to the p gene. Reversions occurring in embryonic premelanocytes give rise to black spots on the gray fur of the offspring. Pregnant C57BL/6J p(un)/p(un) p53(+/-) mice were exposed to X-rays (1 Gy) or administered benzo¿apyrene (B¿aP; 30 or 150 mg/kg i.p.) 10 days after conception. Frequencies of spontaneous p(un) reversions in p53(-/-) and p53(+/-) animals were not significantly different compared with their wild-type littermates. X-ray treatment increased the recombination frequency in wild-type and p53(+/-), but surprisingly not in p53(-/-) offspring. In contrast, B¿aP treatment caused a dose-dependent increase in p(un) reversion frequencies in all three genotypes. Western blot analysis of embryos indicated that p53 protein levels increased approximately 3-fold following X-ray treatment, while B¿aP had no effect on p53 expression. These results are in agreement with the proposal that p53 is involved in the DNA damage response following X-ray exposure and suggest that X-ray-induced double-strand breaks are processed differently in p53(-/-) animals.

Fractional allele loss indicates distinct genetic populations in the development of squamous cell carcinoma of the head and neck (SCCHN)

Loss of heterozygosity (LOH) had been widely used to assess genetic instability in tumours and a high LOH on chromosome arms 3p, 9p and 17p has been considered to be a common event in squamous cell carcinoma of the head and neck (SCCHN). We have investigated LOH in 52 SCCHN using a range of microsatellite markers. LOH was observed in 69% of individuals on 17p using seven markers, in 64% of individuals on 3p using 17 markers and in 61% of individuals on 9p using 11 markers. Fractional allele loss (FAL) has been calculated for each tumour (FAL is the number of chromosomal arms showing LOH divided by the number of informative chromosomal arms) and a median FAL value of 0.25 was obtained in the 52 SCCHN studied. The LOH data were examined on the basis of FAL scores: low FAL (LFAL), 0.00-0.19; medium FAL (MFAL), 0.20-0.32; high FAL (HFAL), 0.33-0.88. HFAL tumours demonstrated a significantly higher LOH on chromosome arms 3p, 9p and 17p, with 94% LOH on 3p, 94% on 9p and 100% on 17p compared with LFAL tumours. Six of the 16 patients in the LFAL group were found to have no LOH on 3p, 9p or 17p and of these four had LOH at other sites, on chromosomes 2p25-p24, 5q21-22, 7pter-p22, 8q13-q22.1, 11q23.3, 13q32, 17q, 18p11.21, 18q21.31 and 19q12-q13.1. These results indicate that LFAL patients form a subset of SCCHN tumours with distinct molecular initiating events which may represent a discrete genetic population.

Dissociation of the recombination control and the sequence-specific transactivation function of P53

Recently, we described a new biological function of p53 in inhibiting recombination processes when encountering mismatches in heteroduplexes (Dudenhöffer et al., 1998). Here, we characterized protein domains of p53 participating in this process by in vitro analysis of mutated p53 proteins, and by applying our SV40-based assay system on monkey cells, which express different p53 variants. We present evidence that both binding of artificial recombination intermediates and p53-dependent recombination control require an intact p53 core and the oligomerization domain, strongly suggesting that the recognition of DNA undergoing recombination represents an essential step of this genomic surveillance mechanism. Further analyses indicated a role of the C-terminus in negatively regulating recombination control, an effect which can be neutralized by concurrent mismatch recognition. p53 lacking the oligomerization domain totally lost its ability to suppress homologous recombination. The cancer-related mutant p53(273H) was also significantly defective in this function, although we observed only twofold reductions in the corresponding transactivation activities on p53-response elements in episomal constructs. HDM2, an inhibitor of p53's transcriptional and growth regulatory activities, interfered with the inhibition of DNA exchange processes by p53 only weakly. Thus, functions of p53 in recombination control can be structurally dissociated from p53-dependent transcriptional transactivation.

Is TP53 dysfunction required for BRCA1-associated carcinogenesis?

The identification of the breast/ovarian susceptibility genes, BRCA1 and BRCA2 was an important advancement in the field of breast and ovarian cancer research. About 40-50% of site specific hereditary breast cancers and up to 80% of hereditary breast-ovarian cancers result from mutations in the BRCA1 gene. Although BRCA1 mediates multiple functions in the cell, including a role in DNA damage repair and gene transcription, the role of BRCA1 has not completely been elucidated yet. It has been suggested that mutational inactivation of TP53 may be required for BRCA1-associated tumorigenesis. Several studies have shown that TP53 is more frequently inactivated in BRCA1-associated tumors than in sporadic breast or ovarian cancer. Up to 90% of BRCA1-associated tumors harbor either a TP53 mutation and/or TP53 protein accumulation. The remaining tumors may well have other alterations affecting the cell cycle checkpoint. Loss of this checkpoint may be obligatory for BRCA1-tumorigenesis. In this review, we discuss recent advances in BRCA1-research and stress the pivotal role TP53 may play in BRCA1-associated carcinogenesis.

p53 is a rate-limiting factor in the repair of higher-order DNA structure

The product of the p53 tumor suppressor gene has been implicated in safeguarding genomic stability by transactivating genes involved in cell cycle arrest, repair of DNA damage or induction of apoptosis. Several properties of p53 suggest that it might be directly involved in DNA repair processes. Eukaryotic DNA is highly organized in supercoiled loops anchored to the nuclear matrix. This organization is very important for cell function and survival, suggesting that repair of DNA damage must include both, the integrity of the double helix and the complex DNA topology. In this work, we studied the kinetics and efficiency of higher-order DNA structure repair in cells with normal and reduced levels of p53, and present evidence suggesting that p53 may be involved in the stabilization and/or repair of higher-order DNA structure.

Cell division transforms mutagenic lesions into deletion-recombinagenic lesions in yeast cells

Cell proliferation has been recognized as an important factor in human and experimental carcinogenesis. Point mutations as well as larger chromosomal rearrangements are involved in the initiation of cancer. In this paper we compared the relative potencies of radiation and chemical carcinogens for inducing point mutations vs. deletions in cell cycle arrested with dividing cells of Saccharomyces cerevisiae. Point mutation substrates and deletion (DEL) recombination substrates were constructed with the genes CDC28 and TUB2 that are required for cell cycle progression through G1 and G2, respectively. The carcinogens ionizing radiation, UV, MMS, EMS and 4-NQO induced point mutations in G1 and in G2 arrested as well as in dividing cells. UV, MMS, EMS and 4-NQO caused very weak if any increases in DEL recombination in G1 or G2 arrested cells, but large increases in dividing cells. When cells treated with carcinogen either in G1 or G2 were allowed to progress through the cell cycle, a time-dependent increase in DEL recombination was seen. Ionizing radiation and the site-specific endonuclease I-SceI, which both directly create double-strand breaks, induced DEL recombination in G1 as well as in G2 arrested cells. In conclusion, UV-, MMS-, EMS- and 4-NQO-induced DNA damage was converted during DNA replication to a lesion capable of inducing DEL recombination which is probably a DNA strand break. Thus, cell proliferation is not necessary to turn DNA alkylation or UV damage into a mutagenic lesion but to convert the damage into a lesion that induces DNA deletions. These results are discussed with respect to mechanisms of carcinogenesis.

The tumor suppressor p53 can reduce stable transfection in the presence of irradiation

The tumor suppressor p53 is believed to play an essential role in maintaining genome stability. Although it is currently unknown how p53 is involved in this important biological safeguard, several previous publications indicate that p53 can help to maintain genome integrity through the recombination-mediated DNA repair process. The integration of linearized plasmid DNA into the host chromosome utilizes the same repair process, and the frequency can be measured by clonogenic assays in which cells that were stably transfected by plasmid integration can be scored by their colony-forming abilities. To gain insight into whether p53 has a direct role in plasmid integration into the host chromosome, we determined the frequency of stable transfection with CHO cells expressing either wild-type or mutant p53 in the presence and absence of irradiation. We found that low-dose irradiation ( approximately 50 to 100 cGy) increased stable transfection frequencies in CHO cells regardless of their p53 status. However, the increase of transfection frequency was significantly lower in CHO cells expressing wild-type p53. Our data thus suggest that wild-type p53 can suppress plasmid DNA integration into the host genome. This p53 function may play a direct and significant role in maintaining genome stability.

Requirement for p53 in ionizing-radiation-inhibition of double-strand-break rejoining by human lymphoblasts

Ionizing radiation (IR) triggers apoptosis, cell-cycle arrest, and DNA-repair induction in mammalian cells. These responses are mediated by proteins, including p53, which are activated or induced by IR. To determine the role of p53 in double-strand break (DSB) repair following irradiation of mammalian cells, we compared the abilities of unirradiated and irradiated TK6 human lymphoblast line and its derivatives TK6-E6-20C and TK6-E6-5E to repair restriction-enzyme-linearized shuttle pZ189 and the luciferase-reporter plasmid pGL3-control. TK6-E6-20C expresses wild-type p53 like the parental TK6 line, while TK6-E6-5E is p53 null. DSB-rejoining capacity was determined from the ratio of viable progenies arising from DSB-containing plasmids (linDNA) to the number of viable progenies from undamaged, supercoiled plasmids (scDNA). The ratio from the p53wt hosts was two- to three-fold higher than that from the p53null host, using either pZ189 or pGL3-control plasmid. After exposure of both hosts to 0.5 Gy gamma-radiation, DSB-rejoining capacity of p53null increased two-fold compared to unirradiated null controls, if transfection occurred immediately after irradiation. In contrast, the DSB-rejoining capacity of p53wt was unaffected by irradiation. If transfection was delayed for 2 h following irradiation, however, DSB-rejoining declined in both p53wt and p53null hosts. Irradiation also altered DSB-rejoining fidelity, measured from the mutation frequencies, among progenies of pZ189 linDNA. But, unlike rejoining capacity, changes in DSB-rejoining fidelity were similar in p53wt and p53null hosts. Changes in cell-cycle distribution in p53wt and p53null hosts were also similar following irradiation. These findings show that IR increases DSB-rejoining capacity in mammalian cells without functional p53, suggesting that p53 participates in suppressing DSB-rejoining following exposure of mammalian cells to IR.

Mutant p53 proteins stimulate spontaneous and radiation-induced intrachromosomal homologous recombination independently of the alteration of the transactivation activity and of the G1 checkpoint

We report here a systematic analysis of the effects of different p53 mutations on both spontaneous and radiation-stimulated homologous recombination in mouse L cells. In order to monitor different recombination pathways, we used both direct and inverted repeat recombination substrates. In each line bearing one of these substrates, we expressed p53 proteins mutated at positions: 175, 248 or 273. p53 mutations leading to an increased spontaneous recombination rate also stimulate radiation-induced recombination. The effect on recombination may be partially related to the conformation of the p53 protein. Moreover, p53 mutations act on recombination between direct repeats as well as between inverted repeats indicating that strand invasion mechanisms are stimulated. Although all of the p53 mutations affect the p53 transactivation activity measured on the WAF1 and MDM2 gene promoters, no correlation between the transactivation activity and the extent of homologous recombination can be drawn. Finally, some p53 mutations do not affect the G1 arrest after radiation but stimulate radiation-induced recombination. These results show that the role of p53 on transactivation and G1 cell cycle checkpoint is separable from its involvement in homologous recombination. A direct participation of p53 in the recombination mechanism itself is discussed.

The ability of p53 to activate downstream genes p21(WAF1/cip1) and MDM2, and cell cycle arrest following DNA damage is delayed and attenuated in scid cells deficient in the DNA-dependent protein kinase

scid mouse embryonic fibroblasts are deficient in DNA-dependent protein kinase activity due to a mutation in the C-terminal domain of the catalytic subunit (DNA-PKcs). When exposed to ionizing radiation, the increase in levels of p53 was the same as in normal mouse embryonic fibroblasts. However, the rise in p21(WAF1/cip1) and mdm2 was found to be delayed and attenuated, which correlated in time with delayed onset of G1/S arrest by flow cytometric analysis. The p53-dependent G1 checkpoint was not eliminated: inactivation of p53 by the E6 protein in scid cells resulted in the complete loss of detectable G1/S arrest after DNA damage. Immunofluorescence analysis of normal cells revealed p53 to be localized predominantly within the cytoplasm prior to irradiation and then translocate to the nucleus after irradiation. In contrast, scid cells show abnormal accumulation of p53 in the nucleus independent of irradiation, which was confirmed by immunoblot analysis of nuclear lysates. Taken together, these data suggest that loss of DNA-PK activity appears to attenuate the kinetics of p53 to activate downstream genes, implying that DNA-PK plays a role in post-translational modification of p53, without affecting the increase in levels of p53 in response to DNA damage.

Analysis of intrachromosomal homologous recombination in mammalian cell, using tandem repeat sequences

In all the organisms, homologous recombination (HR) is involved in fundamental processes such as genome diversification and DNA repair. Several strategies can be devised to measure homologous recombination in mammalian cells. We present here the interest of using intrachromosomal tandem repeat sequences to measure HR in mammalian cells and we discuss the differences with the ectopic plasmids recombination. The present review focuses on the molecular mechanisms of HR between tandem repeats in mammalian cells. The possibility to use two different orientations of tandem repeats (direct or inverted repeats) in parallel constitutes also an advantage. While inverted repeats measure only events arising by strand exchange (gene conversion and crossing over), direct repeats monitor strand exchange events and also non-conservative processes such as single strand annealing or replication slippage. In yeast, these processes depend on different pathways, most of them also existing in mammalian cells. These data permit to devise substrates adapted to specific questions about HR in mammalian cells. The effect of substrate structures (heterologies, insertions/deletions, GT repeats, transcription) and consequences of DNA double strand breaks induced by ionizing radiation or endonuclease (especially the rare-cutting endonuclease ISce-I) on HR are discussed. Finally, transgenic mouse models using tandem repeats are briefly presented.

Recombination and its roles in DNA repair, cellular immortalization and cancer

Genetic recombination is the creation of new gene combinations in a cell or gamete, which differ from those of progenitor cells or parental gametes. In eukaryotes, recombination may occur at mitosis or meiosis. Mitotic recombination plays an indispensable role in DNA repair, which presumably directed its early evolution; the multiplicity of recombination genes and pathways may be best understood in this context, although they have acquired important additional functions in generating diversity, both somatically (increasing the immune repertoire) and in germ line (facilitating evolution). Chromosomal homologous recombination and HsRad51 recombinase expression are increased in both immortal and preimmortal transformed cells, and may favor the occurrence of multiple oncogenic mutations. Tumorigenesis in vivo is frequently associated with karyotypic instability, locus-specific gene rearrangements, and loss of heterozygosity at tumor suppressor loci - all of which can be recombinationally mediated. Genetic defects which increase the rate of somatic mutation (several of which feature elevated recombination) are associated with early incidence and high risk for a variety of cancers. Moreover, carcinogenic agents appear to quite consistently stimulate homologous recombination. If cells with high recombination arise, either spontaneously or in response to "recombinogens," and predispose to the development of cancer, what selective advantage could favor these cells prior to the occurrence of growth-promoting mutations? We propose that the augmentation of telomere-telomere recombination may provide just such an advantage, to hyper-recombinant cells within a population of telomerase-negative cells nearing their replicative (Hayflick) limit, by extending telomeres in some progeny cells and thus allowing their continued proliferation.

p53 and the CNS: tumors and developmental abnormalities

This article reviews the recent molecular and clinical studies that characterize the role of p53 in pathologies of the central nervous system, p53 has many important biological functions, notably, maintenance of DNA stability and regulation of apoptosis. These features are essential to avoid cellular transformation and ensure normal brain development. Lack of p53 function in the brain results in tumor formation in the astrocytic and lymphoid lineages and in severe neurodevelopmental diseases, such as exencephaly.

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

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

Genetic instabilities in human cancers

Whether and how human tumours are genetically unstable has been debated for decades. There is now evidence that most cancers may indeed be genetically unstable, but that the instability exists at two distinct levels. In a small subset of tumours, the instability is observed at the nucleotide level and results in base substitutions or deletions or insertions of a few nucleotides. In most other cancers, the instability is observed at the chromosome level, resulting in losses and gains of whole chromosomes or large portions thereof. Recognition and comparison of these instabilities are leading to new insights into tumour pathogenesis.