Formation of higher-order nuclear Rad51 structures is functionally linked to p21 expression and protection from DNA damage-induced apoptosis

p21CDKN1A allows the repair of replication-mediated DNA double-strand breaks induced by topoisomerase I and is inactivated by the checkpoint kinase inhibitor 7-hydroxystaurosporine

This study provides evidence for the importance of p21(CDKN1A) for the repair of replication-mediated DNA double-strand breaks (DSBs) induced by topoisomerase I. We report that defects of p21(CDKN1A) and p53 enhance camptothecin-induced histone H2AX phosphorylation (gammaH2AX), a marker for DNA DSBs. In human colon carcinoma HCT116 cells with wild-type (wt) p53, gammaH2AX reverses after camptothecin removal. By contrast, gammaH2AX increases after camptothecin removal in HCT116 cells deficient for p53 (p53-/-) or p21(CDKN1A) (p21-/-) as the cells reach the late-S and G2 phases. Since p21-/- cells exhibit similar S-phase arrest as wt cells in response to camptothecin and aphidicolin does not abrogate the enhanced gammaH2AX formation in p21-/- cells, we conclude that enhanced gammaH2AX formation in p21-/- cells is not due to re-replication. The cell cycle checkpoint abrogator and Chk1/Chk2 inhibitor 7-hydroxystaurosporine (UCN-01) also increases camptothecin-induced gammaH2AX formation and inhibits camptothecin-induced p21(CDKN1A) upregulation in HCT116 wt cells. TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling) assays demonstrate that gammaH2AX formation in late S and G2 cells following CPT treatment corresponds to DNA breaks. However, these breaks are not related to apoptotic DNA fragmentation. We propose that p21(CDKN1A) prevents the collapse of replication forks damaged by stabilized topoisomerase I cleavage complexes.

DNA damage responses at low radiation doses

Increased cell killing after exposure to low acute doses of X rays (0-0.5 Gy) has been demonstrated in cells of a number of human tumor cell lines. The mechanisms underlying this effect have been assumed to be related to a threshold dose above which DNA repair efficiency or fidelity increases. We have used cells of two radioresistant human tumor cell lines, one that shows increased sensitivity to low radiation doses (T98G) and one that does not (U373), to investigate the DNA damage response at low doses in detail and to establish whether there is a discontinuous dose response or threshold in activation of any important mediators of this response. In the two cell lines studied, we found a sensitive, linear dose response in early signaling and transduction pathways between doses of 0.1 and 2 Gy with no evidence of a threshold dose. We demonstrate that ATM-dependent signaling events to downstream targets including TP53, CHK1 and CHK2 occur after doses as low as 0.2 Gy and that these events promote an effective damage response. Using chemical inhibition of specific DNA repair enzymes, we show that inhibition of DNA-PK-dependent end joining has relatively little effect at low (<1 Gy) doses in hyper-radiosensitive cells and that at these doses the influence of RAD51-mediated repair events may increase, based on high levels of RAD51/BRCA2 repair foci. These data do not support a threshold model for activation of DNA repair in hyper-radiosensitive cells but do suggest that the balance of repair enzyme activity may change at low doses.

The human Rothmund-Thomson syndrome gene product, RECQL4, localizes to distinct nuclear foci that coincide with proteins involved in the maintenance of genome stability

Rothmund-Thomson syndrome (RTS) is a human genetic disorder characterized by genome instability, cancer susceptibility and premature aging. The gene defective in a subset of RTS cases, RECQL4, encodes a member of the RecQ family of DNA helicases. To better define the function of the RECQL4 protein, we have determined its subcellular localization. We have raised antibodies against the N- and C-terminal parts of RECQL4 and could show that in various human cells endogenous RECQL4 forms discrete nuclear foci that colocalize with promyelotic leukaemia protein (PML). The number of foci and their colocalization with PML does not significantly change after induction of different types of DNA damages. Silencing of RECQL4 expression by siRNA causes a significant reduction in RECQL4 nuclear foci formation. Furthermore, we demonstrate that RECQL4 foci coincide with foci formed by human Rad51 and regions of single-stranded DNA after induction of DNA double-strand breaks. In agreement with this, we also show that RECQL4 and Rad51 form a complex in human cells. Our findings suggest a role for RECQL4 in the repair of DNA double-strand breaks by homologous recombination and shed new light onto RECQL4's function in human cells.

Targeting of Rad51-dependent homologous recombination: implications for the radiation sensitivity of human lung cancer cell lines

The aim of the present work was to study the role of Rad51-dependent homologous recombination in the radiation response of non-small-cell lung cancer (NSCLC) cell lines. A dose- and time-dependent increase in the formation of Rad51 and γ-H2AX foci with a maximum at about 4 and 1 h after irradiation, followed by a decrease, has been found. The relative fraction of cells with persisting Rad51 foci was 20–30% in radioresistant and 60–80% in radiosensitive cell lines. In comparison, a higher fraction of residual Dsb was evident in cell lines with nonfunctional p53. Transfection with As-Rad51 significantly downregulates radiation-induced formation of Rad51 foci and increases apoptosis, but did not influence the rejoining of DNA double-strand breaks. Interestingly, wortmannin, a well-known inhibitor of nonhomologous end-joining, also inhibits Rad51 foci formation. In general, there was no correlation between the clonogenic survival at 2 Gy and the percentage of initial Rad51 or γ-H2AX foci after ionising radiation (IR). The most reliable predictive factor for radiosensitivity of NSCLC cell lines was the relative fraction of Rad51 foci remaining at 24 h after IR. Although most of the Rad51 foci are co-localised with γ-H2AX foci, no correlation of the relative fraction of persisting γ-H2AX foci and SF2 is evident.

Spontaneous homologous recombination is decreased in Rad51C-deficient hamster cells

The Chinese hamster cell mutant, CL-V4B that is mutated in the Rad51 paralog gene, Rad51C (RAD51L2), has been described to exhibit increased sensitivity to DNA cross-linking agents, genomic instability, and an impaired Rad51 foci formation in response to DNA damage. To directly examine an effect of the Rad51C protein on homologous recombination (HR) in mammalian cells, we compared the frequencies and rates of spontaneous HR in CL-V4B cells and in parental wildtype V79B cells, using a recombination reporter plasmid in host cell reactivation assays. Our results demonstrate that HR is reduced but not abolished in the CL-V4B mutant. We thus, provide direct evidence for a role of mammalian Rad51C in HR processes. The reduced HR events described here help to explain the deficient phenotypes observed in Rad51C mutants and support an accessory role of Rad51C in Rad51-mediated recombination.

RAD51, genomic stability, and tumorigenesis

Genomic instability is characteristic of malignant cells, and a strong correlation exists between abnormal karyotype and tumorigenicity. Increased expression of the homologous recombination and DNA repair protein Rad51 has been reported in immortalized cell lines and multiple primary tumor cell types which could alter recombination pathways to contribute to the chromosomal rearrangements found in these cells. In addition, Rad51 participates in a complex network of interactions that includes DNA damage sensors, tumor suppressors, and cell cycle and apoptotic regulators, and mutation of many of these proteins have also been associated with tumor initiation or progression. Insights into the connection between disregulated Rad51 and malignant phenotype indicate that Rad51 is a potential target for new anti-cancer regimens including those that use siRNA technology.

Defective DNA Strand Break Repair after DNA Damage in Prostate Cancer Cells

Together with cell cycle checkpoint control, DNA repair plays a pivotal role in protecting the genome from endogenous and exogenous DNA damage. Although increased genetic instability has been associated with prostate cancer progression, the relative role of DNA double-strand break repair in malignant versus normal prostate epithelial cells is not known. In this study, we determined the RNA and protein expression of a series of DNA double-strand break repair genes in both normal (PrEC-epithelial and PrSC-stromal) and malignant (LNCaP, DU-145, and PC-3) prostate cultures. Expression of genes downstream of ATM after ionizing radiation-induced DNA damage reflected the p53 status of the cell lines. In the malignant prostate cell lines, mRNA and protein levels of the Rad51, Xrcc3, Rad52, and Rad54 genes involved in homologous recombination were elevated approximately 2- to 5-fold in comparison to normal PrEC cells. The XRCC1, DNA polymerase-beta and -delta proteins were also elevated. There were no consistent differences in gene expression relating to the nonhomologous end-joining pathway. Despite increased expression of DNA repair genes, malignant prostate cancer cells had defective repair of DNA breaks, alkali-labile sites, and oxidative base damage. Furthermore, after ionizing radiation and mitomycin C treatment, chromosomal aberration assays confirmed that malignant prostate cells had defective DNA repair. This discordance between expression and function of DNA repair genes in malignant prostate cancer cells supports the hypothesis that prostate tumor progression may reflect aberrant DNA repair. Our findings support the development of novel treatment strategies designed to reinstate normal DNA repair in prostate cancer cells.

A novel apoptosis pathway activated by the carboxyl terminus of p21

Delivery of biologically active peptides into cells may help elucidate intracellular signal transduction pathways, identify additional in vivo functions, and develop new therapeutics. Although p21 was first identified as a major regulator of cell cycle progression, it is now clear that p21 subserves multiple functions. The amino terminus of p21 interacts with cyclins and cyclin-dependent kinases, while the carboxyl terminus interacts with proliferating cell nuclear antigen (PCNA), growth arrest and DNA damage-inducible gene 45 (GADD45), calmodulin, SET, and CCAAT/enhancer binding protein-alpha (C/EBP-alpha). A chimeric peptide, p21-IRS, consisting of the carboxyl terminal domain of p21 conjugated to a pentapeptide (RYIRS) rapidly enters lymphoid cells and activates apoptosis. In the present study, we investigate the molecular events involved in p21-activated apoptosis. Comparison of p21-IRS with other known proapoptotic agents demonstrates that p21-IRS activates a novel apoptotic pathway: mitochondria are central to the process, but caspases and a decrease in Deltapsi(m) are not involved. Targeting the p21 peptide to specific cell populations may allow development of novel therapies to eliminate aberrant cells in human diseases.

After double-strand break induction by UV-A, homologous recombination and nonhomologous end joining cooperate at the same DSB if both systems are available

After induction of DNA double-strand breaks (DSB) two repair systems, the error-prone 'nonhomologous end joining' (NHEJ) and the more accurate 'homologous recombination repair' (HRR) can compete for the same individual DSB site. In the human keratinocyte cell line, HaCaT, we have tested the spatial co-localisation and the temporal sequence of events. We used UV-A (365 nm) as a damaging agent, which can be applied in clearly defined doses and can lead to rare DSBs via propagation of clustered single-strand breaks (SSBs). DNA fragmentation and repair was measured by the Comet assay and persisting DSBs were quantified by the micronucleus assay. Direct DSB detection was performed by immunohistochemical labelling of gamma-H2AX, a phosphorylated histone that is assumed to form one foci per DSB. Intra- and inter-pathway interactions were quantified by co-localisation, FRET imaging and by co-immunoprecipitation (Co-IP) of XRCC4, DNA-PK and Ku70 as representatives of NHEJ, Rad51 and Rad52 for HRR and gamma-H2AX, Mre11 and Rad50 as representatives of both pathways. In G2 cells, where both systems are available, the temporal sequence after irradiation is: (1) gamma-H2AX (2) Mre11 (3) DNA-PK Rad51 (4) XRCC4. That is, the first two proteins involved in both pathways 'label' the damaged site and initiate repair, followed by the NHEJ, which is temporally overlapping with HRR activity. Taking all these observations together we suggest that a cell tries to repair DSBs with a combination of both HRR and NHEJ, if available.

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.

Caffeine inhibits homology-directed repair of I-SceI-induced DNA double-strand breaks

We recently reported that two Chinese hamster mutants deficient in the RAD51 paralogs XRCC2 and XRCC3 show reduced radiosensitization after treatment with caffeine, thus implicating homology-directed repair (HDR) of DNA double-strand breaks (DSBs) in the mechanism of caffeine radiosensitization. Here, we investigate directly the effect of caffeine on HDR initiated by DSBs induced by a rare cutting endonuclease (I-SceI) into one of two direct DNA repeats. The results demonstrate a strong inhibition by caffeine of HDR in wild-type cells, and a substantial reduction of this effect in HDR-deficient XRCC3 mutant cells. Inhibition of HDR and cell radiosensitization to killing shows similar dependence on caffeine concentration suggesting a cause-effect relationship between these effects. UCN-01, a kinase inhibitor that effectively abrogates checkpoint activation in irradiated cells, has only a small effect on HDR, indicating that similar to radiosensitization, inhibition of checkpoint signaling is not sufficient for HDR inhibition. Recombination events occurring during treatment with caffeine are characterized by rearrangements reminiscent to those previously reported for the XRCC3 mutant, and immunofluorescence microscopy demonstrates significantly reduced formation of IR-specific RAD51 foci after caffeine treatment. In summary, our results identify inhibition of HDR as a significant contributor to caffeine radiosensitization.

Rad51 overexpression promotes alternative double-strand break repair pathways and genome instability

Genomic instability is characteristic of tumor cells, and a strong correlation exists between abnormal karyotype and tumorigenicity. Increased expression of the homologous recombination and DNA repair protein Rad51 has been reported in immortalized and tumor cells, which could alter recombination pathways to contribute to the chromosomal rearrangements found in these cells. We used a genetic system to examine the potential for multiple double-strand breaks to lead to genome rearrangements in the presence of increased Rad51 expression. Analysis of repair revealed a novel class of products consistent with crossing over, involving gene conversion associated with an exchange of flanking markers leading to chromosomal translocations. Increased Rad51 also promoted aneuploidy and multiple chromosomal rearrangements. These data provide a link between elevated Rad51 protein levels, genome instability, and tumor progression.

MLL 5 protein forms intranuclear foci, and overexpression inhibits cell cycle progression

MLL5 is a mammalian trithorax group (trx-G) gene identified within chromosome band 7q22, a frequently deleted element found in cytogenetic aberrations of acute myeloid malignancies. MLL5 cDNA was linked with the FLAG and V5 tags at the N and C terminus, respectively, and transfected into 293T cells. Immunofluoresence staining of the expressed tagged MLL5 protein showed localization to the nucleus and exclusion from nucleoli, and no surface staining was detected. Both ectopically introduced and endogenous MLL5 protein displayed a speckled nuclear distribution. By using a series of MLL5-truncated mutants fused with enhanced GFP, a domain (residues 945-1,156) required for foci accumulation was identified, and regions containing functional nuclear localization signals were mapped. Ectopic overexpression of GFP-MLL5 induced cell cycle arrest in G(1) phase. This inhibition of cell cycle progression was indicated by delayed progression into nocodazole-induced mitotic arrest and was confirmed by a lack of BrdUrd incorporation. These findings suggest that MLL5 forms intranuclear protein complexes that may play an important role in chromatin remodeling and cellular growth suppression.

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.

Endopolyploid cells produced after severe genotoxic damage have the potential to repair DNA double strand breaks

p53 mutant tumour cells respond to genotoxic insults by bypassing G1 arrest and halting in G2. Following release from G2 arrest they undergo mitotic catastrophe, whereby mitotic cycling is suppressed, delayed apoptosis begins and endopolyploid cells are produced. The ability of these endopolyploid cells to participate in the restitution process is controversial. To facilitate recovery, these endopolyploid cells must repair the extensive DNA damage induced. DNA damage and its resolution were studied by observing the kinetics of gamma-H2AX foci formation and by comet assay analysis. Subsequently, the kinetics and distribution of Rad51 foci were studied as a measure of homologous recombination. Here we present evidence of the resolution of DNA damage in endopolyploid cells through a decrease of tail moment by comet assay and in the number of cells expressing gamma-H2AX foci. Rad51 foci expression reached a maximum in endopolyploid cells on days 5-6 after irradiation, when delayed apoptosis was maximal, indicating that cells were being selected for survival at this time. Furthermore, the proportion of Annexin-V-positive polyploid cells decreased as they continued ongoing rounds of DNA replication, suggesting endoreduplication is involved in selecting cells resistant to apoptosis. Our findings suggest that after severe genotoxic insult endopolyploid cells have a transient survival advantage that may contribute to radioresistance of tumours that undergo mitotic catastrophe.

RAD51 is involved in repair of damage associated with DNA replication in mammalian cells

The RAD51 protein, a eukaryotic homologue of the Escherichia coli RecA protein, plays an important role in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) in mammalian cells. Recent findings suggest that HR may be important in repair following replication arrest in mammalian cells. Here, we have investigated the role of RAD51 in the repair of different types of damage induced during DNA replication with etoposide, hydroxyurea or thymidine. We show that etoposide induces DSBs at newly replicated DNA more frequently than gamma-rays, and that these DSBs are different from those induced by hydroxyurea. No DSB was found following treatment with thymidine. Although these compounds appear to induce different DNA lesions during DNA replication, we show that a cell line overexpressing RAD51 is resistant to all of them, indicating that RAD51 is involved in repair of a wide range of DNA lesions during DNA replication. We observe fewer etoposide-induced DSBs in RAD51-overexpressing cells and that HR repair of etoposide-induced DSBs is faster. Finally, we show that induced long-tract HR in the hprt gene is suppressed in RAD51-overexpressing cells, although global HR appears not to be suppressed. This suggests that overexpression of RAD51 prevents long-tract HR occurring during DNA replication. We discuss our results in light of recent models suggested for HR at stalled replication forks.

Overexpression of cyclin E does not influence homologous recombination in Chinese hamster cells

Overexpressed cyclin E in tumours is a prognosticator for poor patient outcome. Cells that overexpress cyclin E have been shown to be impaired in S-phase progression and exhibit genetic instability that may drive this subset of cancers. However, the origin for genetic instability caused by cyclin E overexpression is unknown. Homologous recombination plays an important role in S-phase progression and is also regulated by the same proteins that regulate cyclin E-associated kinase activity, i.e., p53 and p21. To test the hypothesis that overexpressed cyclin E causes genetic instability through homologous recombination, we investigated the effect of cyclin E overexpression on homologous recombination in the hprt gene in a Chinese hamster cell line. Although cyclin E overexpression shortened the G1 phase in the cell cycle as expected, we could see no change in neither spontaneous nor etoposide-induced recombination. Also, overexpression of cyclin E did not affect the repair of DNA double-strand breaks and failed to potentiate the cytotoxic effects of etoposide. Our data suggest that genetic instability caused by overexpression of cyclin E is not mediated by aberrant homologous recombination.