Effects of HsRad51 overexpression on cell proliferation, cell cycle progression, and apoptosis

HROB Is Implicated in DNA Replication

DNA replication represents a series of precisely regulated events performed by a complex protein machinery that guarantees accurate duplication of the genetic information. Since DNA replication is permanently faced by a variety of exogenous and endogenous stressors, DNA damage response, repair and replication must be closely coordinated to maintain genomic integrity. HROB has been identified recently as a binding partner and activator of the Mcm8/9 helicase involved in DNA interstrand crosslink (ICL) repair. We identified HROB independently as a nuclear protein whose expression is co-regulated with various DNA replication factors. Accordingly, the HROB protein level showed a maximum in S phase and a downregulation in quiescence. Structural prediction and homology searches revealed that HROB is a largely intrinsically disordered protein bearing a helix-rich region and a canonical oligonucleotide/oligosaccharide-binding-fold motif that originated early in eukaryotic evolution. Employing a flow cytometry Förster resonance energy transfer (FRET) assay, we detected associations between HROB and proteins of the DNA replication machinery. Moreover, ectopic expression of HROB protein led to an almost complete shutdown of DNA replication. The available data imply a function for HROB during DNA replication across barriers such as ICLs.

Human AAA+ ATPase FIGNL1 suppresses RAD51-mediated ultra-fine bridge formation

RAD51 filament is crucial for the homology-dependent repair of DNA double-strand breaks and stalled DNA replication fork protection. Positive and negative regulators control RAD51 filament assembly and disassembly. RAD51 is vital for genome integrity but excessive accumulation of RAD51 on chromatin causes genome instability and growth defects. However, the detailed mechanism underlying RAD51 disassembly by negative regulators and the physiological consequence of abnormal RAD51 persistence remain largely unknown. Here, we report the role of the human AAA+ ATPase FIGNL1 in suppressing a novel type of RAD51-mediated genome instability. FIGNL1 knockout human cells were defective in RAD51 dissociation after replication fork restart and accumulated ultra-fine chromosome bridges (UFBs), whose formation depends on RAD51 rather than replication fork stalling. FIGNL1 suppresses homologous recombination intermediate-like UFBs generated between sister chromatids at genomic loci with repeated sequences such as telomeres and centromeres. These data suggest that RAD51 persistence per se induces the formation of unresolved linkage between sister chromatids resulting in catastrophic genome instability. FIGNL1 facilitates post-replicative disassembly of RAD51 filament to suppress abnormal recombination intermediates and UFBs. These findings implicate FIGNL1 as a key factor required for active RAD51 removal after processing of stalled replication forks, which is essential to maintain genome stability.

RAD51 Inhibition Shows Antitumor Activity in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC), the major type of liver cancer, causes a high annual mortality worldwide. RAD51 is the critical recombinase responsible for homologous recombination (HR) repair in DNA damage. In this study, we identified that RAD51 was upregulated in HCC and that RAD51 silencing or inhibition reduced the proliferation, migration, and invasion of HCC cells and enhanced cell apoptosis and DNA damage. HCC cells with the combinatorial treatments of RAD51 siRNA or inhibitor and sorafenib demonstrated a synergistic effect in inhibiting HCC cell proliferation, migration, and invasion, as well as inducing cell apoptosis and DNA damage. Single RAD51 silencing or sorafenib reduced RAD51 protein expression and weakened HR efficiency, and their combination almost eliminated RAD51 protein expression and inhibited HR efficiency further. An in vivo tumor model confirmed the RAD51 inhibitor's antitumor activity and synergistic antitumor activity with sorafenib in HCC. RNA-Seq and gene set enrichment analysis (GSEA) in RAD51-inactivated Huh7 cells indicated that RAD51 knockdown upregulated cell apoptosis and G1/S DNA damage checkpoint pathways while downregulating mitotic spindle and homologous recombination pathways. Our findings suggest that RAD51 inhibition exhibits antitumor activities in HCC and synergizes with sorafenib. Targeting RAD51 may provide a novel therapeutic approach in HCC.

Expression of RAD51 and Its Clinical Impact in Oral Squamous Cell Carcinoma

Purpose

To examine the expression of RAD51 in oral squamous cell carcinoma (OSCC) and analyze its connection with pathological grade, clinical stage, and lymphatic metastasis potential.

Methods

For this study, 74 OSCC samples, 15 normal mucosa tissues, and 11 normal skin tissue samples were collected. RAD51 expression was investigated using immunohistochemistry. A follow-up visit was used to assess the prognosis of each patient. We compared RAD51 expression in oral mucosa epithelial cells (OMECs), keratinocytes, and tongue squamous cell carcinoma cells (TSCCs) by Western blot analysis.

Results

RAD51 expression was higher in tumor cells than in normal mucosal tissues. In addition, RAD51 expression was associated with higher tumor differentiation (P < 0.05). Also, RAD51 expression was higher (P < 0.05). Also, RAD51 expression was higher (P < 0.05). Also, RAD51 expression was higher (

Conclusion

A strong positive correlation was found between RAD51 expression and the degree of malignancy in OSCC patients, suggesting that RAD51 could be an excellent prognostic indicator for OSCC patients.

Rad51 paralogs and the risk of unselected breast cancer: A case-control study

A case-control study was conducted in which we evaluated the association between genetic variability of DNA repair proteins belonging to the Rad51 family and breast cancer (BrC) risk. In the study, 132 female BrC cases and 189 healthy control females were genotyped for a total of 14 common single nucleotide polymorphisms (SNPs) within Rad51 and Xrcc3. Moreover, our previously reported Rad51C genetic data were involved to explore the nonlinear interactions among SNPs within the three genes and effect of such interactions on BrC risk. The rare rs5030789 genotype (-4601AA) in Rad51 was found to significantly decrease the BrC risk (OR = 0.5, 95% CI: 0.3-1.0, p<0.05). An interaction between this SNP, rs2619679 and rs2928140 (both in Rad51), was found to result in a two three-locus genotypes -4719AA/-4601AA/2972CG and -4719AT/-4601GA/2972CC, both of which were found to increase the risk of BrC (OR = 8.4, 95% CI: 1.8-38.6, p<0.0001), instead. Furthermore, rare Rad51 rs1801320 (135CC) and heterozygous Xrcc3 rs3212057 (10343GA) genotypes were found to respectively increase (OR = 10.6, 95% CI: 1.9-198, p<0.02) and decrease (OR = 0.0, 95% CI: 0.0-NA, p<0.05) the risk of BrC. Associations between these SNPs and BrC risk were further supported by outcomes of employed machine learning analyses. In Xrcc3, the 4541A/9685A haplotype was found to be significantly associated with reduced BrC risk (OR = 0.5; 95% CI: 0.3-0.9; p<0.05). Concluding, our study indicates a complex role of SNPs within Rad51 (especially rs5030789) and Xrcc3 in BrC, although their significance with respect to the disease needs to be further clarified.

Determination of the number of RAD51 molecules in different human cell lines

Knowledge about precise numbers of specific molecules is necessary for understanding and verification of biological pathways. The RAD51 protein is central in the repair of DNA double-strand breaks (DSBs) by homologous recombination repair and understanding its role in cellular pathways is crucial to design mechanistic DNA repair models. Here, we determined the number of RAD51 molecules in several human cell lines including primary fibroblasts. We showed that between 20000 to 100000 of RAD51 molecules are available per human cell that theoretically can be used for simultaneously loading at least 7 DSBs. Interestingly, the amount of RAD51 molecules does not significantly change after the induction of DNA damage using bleomycin or γ-irradiation in cells but an accumulation of RAD51 on the chromatin occurs. Furthermore, we generated an EGFP-RAD51 fusion under the control of HSV thymidine kinase promoter sequences yielding moderate protein expression levels comparable to endogenously expressed RAD51. Initial characterizations suggest that these low levels of ectopically expressed RAD51 are compatible with cell cycle progression of human cells. Hence, we provide parameters for the quantitative understanding and modeling of RAD51-involving processes.

A Novel Cell-Penetrating Antibody Fragment Inhibits the DNA Repair Protein RAD51

DNA damaging chemotherapies are successful in cancer therapy, however, the damage can be reversed by DNA repair mechanisms that may be up-regulated in cancer cells. We hypothesized that inhibiting RAD51, a protein involved in homologous recombination DNA repair, would block DNA repair and restore the effectiveness of DNA damaging chemotherapy. We used phage-display to generate a novel synthetic antibody fragment that bound human RAD51 with high affinity (KD = 8.1 nM) and inhibited RAD51 ssDNA binding in vitro. As RAD51 is an intracellular target, we created a corresponding intrabody fragment that caused a strong growth inhibitory phenotype on human cells in culture. We then used a novel cell-penetrating peptide "iPTD" fusion to generate a therapeutically relevant antibody fragment that effectively entered living cells and enhanced the cell-killing effect of a DNA alkylating agent. The iPTD may be similarly useful as a cell-penetrating peptide for other antibody fragments and open the door to numerous intracellular targets previously off-limits in living cells.

PARP inhibitors affect growth, survival and radiation susceptibility of human alveolar and embryonal rhabdomyosarcoma cell lines

PURPOSE

PARP inhibitors (PARPi) are used in a wide range of human solid tumours but a limited evidence is reported in rhabdomyosarcoma (RMS), the most frequent childhood soft-tissue sarcoma. The cellular and molecular effects of Olaparib, a specific PARP1/2 inhibitor, and AZD2461, a newly synthesized PARP1/2/3 inhibitor, were assessed in alveolar and embryonal RMS cells both as single-agent and in combination with ionizing radiation (IR).

METHODS

Cell viability was monitored by trypan blue exclusion dye assays. Cell cycle progression and apoptosis were measured by flow cytometry, and alterations of specific molecular markers were investigated by, Real Time PCR, Western blotting and immunofluorescence experiments. Irradiations were carried out at a dose rate of 2 Gy (190 UM/min) or 4 Gy (380 UM/min). Radiosensitivity was assessed by using clonogenic assays.

RESULTS

Olaparib and AZD2461 dose-dependently reduced growth of both RH30 and RD cells by arresting growth at G2/M phase and by modulating the expression, activation and subcellular localization of specific cell cycle regulators. Downregulation of phospho-AKT levels and accumulation of γH2AX, a specific marker of DNA damage, were significantly and persistently induced by Olaparib and AZD2461 exposure, this leading to apoptosis-related cell death. Both PARPi significantly enhanced the effects of IR by accumulating DNA damage, increasing G2 arrest and drastically reducing the clonogenic capacity of RMS-cotreated cells.

CONCLUSIONS

This study suggests that the combined exposure to PARPi and IR might display a role in the treatment of RMS tumours compared with single-agent exposure, since stronger cytotoxic effects are induced, and compensatory survival mechanisms are prevented.

A Lamin-Binding Ligand Inhibits Homologous Recombination Repair of DNA Double-Strand Breaks

Nuclear lamins are type V intermediate filament proteins. Lamins, including LA, LB1, LB2, and LC, are the major protein components forming the nuclear lamina to support the mechanical stability of the mammalian cell nucleus. Increasing evidence has shown that LA participates in homologous recombination (HR) repair of DNA double-strand breaks (DSBs) . However, the mechanisms underlying this process are incompletely understood. We recently identified the first lamin-binding ligand 1 (LBL1) that directly binds LA and inhibited cancer cell growth. We provided here further mechanistic investigations of LBL1 and revealed that LA interacts with the HR recombinase Rad51 to protect Rad51 from degradation. LBL1 inhibits LA-Rad51 interaction leading to accelerated proteasome-mediated degradation of Rad51, culminating in inhibition of HR repair of DSBs. These results uncover a novel post-translational regulation of Rad51 by LA and suggest that targeting the LA-Rad51 axis may represent a promising strategy to develop cancer therapeutics.

Upregulation of RAD51 expression is associated with progression of thyroid carcinoma

AIMS

RAD51 participates in homologous recombination repair (HRR) of double-stranded DNA breaks (DSBs) which may cause genomic instability and cancer. The aim of this study was to investigate RAD51 gene expression at transcriptional and translational levels to measure mRNA and protein level and to correlate its relationship with proliferation marker, Ki67 in thyroid cancer patients. This study also explored correlation of these genes with different clinicopathological parameters of the study cohort by Spearman's rank correlation coefficient.

METHODS

Quantitative real time polymerase chain reaction (qRT-PCR) and immunohistochemistry were used to detect mRNA transcript levels and protein expression of RAD51 and Ki67 in 102 cases of thyroid cancer tissues and equal number of uninvolved healthy thyroid tissue controls.

RESULTS

Data showed that expression for both RAD51 and Ki67 was significantly increased in thyroid cancer (p<0.001). High RAD51 and Ki67 expression was associated with later stages, poor tissue differentiation, large tumor size, positive lymph node metastasis and distant metastasis. The correlation analysis demonstrated a strong positive correlation (r=0.461) between RAD51 and Ki67 on mRNA level and on protein level (r=0.866). Strong correlation was observed between clinicopathological characteristics and selected molecules.

CONCLUSION

The present study concluded that upregulation of RAD51 and overexpression of Ki67 may be associated with the progression of thyroid cancer.

Prognostic and predictive value of loss of nuclear RAD51 immunoreactivity in resected non-small cell lung cancer patients

OBJECTIVES

In response to DNA damage, recombination proteins are relocalized into sub-nuclear complexes that are microscopically detected as RAD51-containing nuclear foci. We aimed for assessing the prognostic and predictive value of loss of nuclear RAD51 immunoreactivity ('RAD51 loss') in 2 independent stage I to III non-small cell lung cancer (NSCLC) patient cohorts undergoing surgical resection and eventual perioperative chemo-/radiotherapy (CT/RT).

MATERIALS AND METHODS

The discovery set included 69 evaluable patients (19 adenocarcinomas, ADC, 50 squamous cell carcinomas, SCC) from Palacky University Hospital, 45/69 (65.2%) with additional platinum-based CT. The replication set entailed 845 evaluable patients (446 ADC, 399 SCC) from University Hospital Zurich, 308/845 (36.5%) with platinum based CT or RT. RAD51 loss was defined as ≤20% of tumor cell nuclei having any nuclear RAD51 expression. We assessed the prognostic value of RAD51 loss in all patients and its predictive value in patients receiving CT/RT.

RESULTS

RAD51 loss was observed in 40/69 (58.0%) and 439/845 (51.9%) evaluable tumors in the discovery and replication set, respectively (p=0.34). It was more frequent in ADC compared to SCC (57.2% vs 47.4%, p=0.003). RAD51 loss was significantly associated with worse OS in both the discovery (adjusted HR=2.39, p=0.039) and replication set (adjusted HR=1.31, p=0.008). The unfavourable prognostic effect of RAD51 loss seen in the overall population was not observed in patients receiving perioperative CT (adjusted HR=1.07, p=0.73) or perioperative RT (adjusted HR=1.05, p=0.82).

CONCLUSION

RAD51 loss has an unfavourable prognostic impact in NSCLC patients undergoing curative surgical resection, but it may have a favourable predictive value in the subgroup of patients receiving perioperative platinum-based CT or RT, most likely as a consequence of deficient DNA repair.

Recent Developments Using Small Molecules to Target RAD51: How to Best Modulate RAD51 for Anticancer Therapy?

Homologous recombination (HR) is an evolutionarily conserved DNA repair process. Overexpression of the key HR protein RAD51 is a common feature of malignant cells. RAD51 plays two distinct genome-stabilizing roles, including HR-mediated repair of double-strand breaks (DSBs) and the promotion of replication fork stability during replication stress. Because upregulation of RAD51 in cancer cells can promote tumor resistance to DNA-damaging oncologic therapies, we and others have worked to develop cancer therapeutics that target various aspects of RAD51 protein function. Herein, we provide an overview of recent developments in this field, together with our perspectives on the challenges associated with these evolving anticancer strategies.

Ewing sarcoma: The clinical relevance of the insulin-like growth factor 1 and the poly-ADP-ribose-polymerase pathway

BACKGROUND

In the last three decades the outcome for patients with localised Ewing sarcoma (ES) has improved significantly since the introduction of multimodality primary treatment. However, for patients with (extra-) pulmonary metastatic and/or non-resectable relapsed disease the outcome remains poor and new treatment options are urgently needed. Currently the insulin-like growth factor 1 receptor (IGF-1R) pathway and the poly-ADP(adenosinediphosphate)-ribose-polymerase (PARP) pathway are being investigated for potential targeted therapies. IGF-1R: The IGF-1R pathway is known to be deregulated by the EWSR1-FLI1 translocation which makes it a potential target for therapy. Clinical trials have been reported in which only ES patients were treated with an IGF-1R inhibitor, either as single agent or in combination. In total 291 ES patients were included in these trials, in which two (0.7%) complete responses, 32 (11%) partial responses of which some durable, and 61 (21%) stable diseases were observed. PARP: In the presence of a PARP inhibitor DNA strand breaks cannot be efficiently repaired, leading to cell death. The first phase II trial with ES patients was recently published and showed no clinical responses, which may have been due to the drug being non-effective as a single agent.

DISCUSSION

The IGF-1R pathway is an interesting target for ES and should be explored further, as biomarkers to select patients that might benefit from treatment are lacking. PARP inhibitors as single agent have so far failed to show improvement in outcome. Future directions include dual insulin receptor/IGF-1R blockade with linsitinib as well as chemotherapy-PARP combinations. Both therapeutic strategies are currently being explored.

Association between RAD 51 rs1801320 and susceptibility to glioblastoma

Glioblastoma is the most common and aggressive malignant primary brain tumor. Despite decades of research and the advent of new therapies, patients with glioblastoma continue to have a very poor prognosis. Radiation therapy has a major role as adjuvant treatment for glioblastoma following surgical resection. Many studies have shown that polymorphisms of genes involved in pathways of DNA repair may affect the sensitivity of the cells to treatment. Although the role of these polymorphisms has been investigated in relation to response to radiotherapy, their role as predisposing factors to glioblastoma has not been clarified yet. In the present study, we evaluated the association between polymorphisms in DNA repair genes, namely: XRCC1 rs25487, XRCC3 rs861539 and RAD51 rs1801320, with the susceptibility to develop glioblastoma. Eighty-five glioblastoma patients and 70 matched controls were recruited for this study. Data from the 1000 Genomes Project (98 Tuscans) were also downloaded and used for the association analysis. Subjects carrying RAD51 rs1801320 GC genotype showed an increased risk of glioblastoma (GC vs GG, χ(2) = 10.75; OR 3.0087; p = 0.0010). The C allele was also significantly associated to glioblastoma (χ(2) = 8.66; OR 2.5674; p = 0.0032). Moreover, RAD51 rs1801320 C allele increased the risk to develop glioblastoma also when combined to XRCC1 rs25487 G allele and XRCC3 rs861539 C allele (χ(2) = 6.558; p = 0.0053).

TODRA, a lncRNA at the RAD51 Locus, Is Oppositely Regulated to RAD51, and Enhances RAD51-Dependent DSB (Double Strand Break) Repair

Expression of RAD51, a crucial player in homologous recombination (HR) and DNA double-strand break (DSB) repair, is dysregulated in human tumors, and can contribute to genomic instability and tumor progression. To further understand RAD51 regulation we functionally characterized a long non-coding (lnc) RNA, dubbed TODRA (Transcribed in the Opposite Direction of RAD51), transcribed 69bp upstream to RAD51, in the opposite direction. We demonstrate that TODRA is an expressed transcript and that the RAD51 promoter region is bidirectional, supporting TODRA expression (7-fold higher than RAD51 in this assay, p = 0.003). TODRA overexpression in HeLa cells induced expression of TPIP, a member of the TPTE family which includes PTEN. Similar to PTEN, we found that TPIP co-activates E2F1 induction of RAD51. Analysis of E2F1's effect on the bidirectional promoter showed that E2F1 binding to the same site that promotes RAD51 expression, results in downregulation of TODRA. Moreover, TODRA overexpression induces HR in a RAD51-dependent DSB repair assay, and increases formation of DNA damage-induced RAD51-positive foci. Importantly, gene expression in breast tumors supports our finding that E2F1 oppositely regulates RAD51 and TODRA: increased RAD51 expression, which is associated with an aggressive tumor phenotype (e.g. negative correlation with positive ER (r = -0.22, p = 0.02) and positive PR status (r = -0.27, p<0.001); positive correlation with ki67 status (r = 0.36, p = 0.005) and HER2 amplification (r = 0.41, p = 0.001)), correlates as expected with lower TODRA and higher E2F1 expression. However, although E2F1 induction resulted in TPIP downregulation in cell lines, we find that TPIP expression in tumors is not reduced despite higher E2F1 expression, perhaps contributing to increased RAD51 expression. Our results identify TPIP as a novel E2F1 co-activator, suggest a similar role for other TPTEs, and indicate that the TODRA lncRNA affects RAD51 dysregulation and RAD51-dependent DSB repair in malignancy. Importantly, gene expression in breast tumors supports our finding that E2F1 oppositely regulates RAD51 and TODRA: increased RAD51 expression, which is associated with an aggressive tumor phenotype (e.g. negative correlation with positive ER (r = -0.22, p = 0.02) and positive PR status (r = -0.27, p<0.001); positive correlation with ki67 status (r = 0.36, p = 0.005) and HER2 amplification (r = 0.41, p = 0.001)), correlates as expected with lower TODRA and higher E2F1 expression. However, although E2F1 induction resulted in TPIP downregulation in cell lines, we find that TPIP expression in tumors is not reduced despite higher E2F1 expression, perhaps contributing to increased RAD51 expression. Our results identify TPIP as a novel E2F1 co-activator, suggest a similar role for other TPTEs, and indicate that the TODRA lncRNA affects RAD51 dysregulation and RAD51-dependent DSB repair in malignancy.

RAD51 G135C genetic polymorphism and their potential role in gastric cancer induced by Helicobacter pylori infection in Bhutan

In order to evaluate the role of the RAD51 G135C genetic polymorphism on the risk of gastric cancer induced by Helicobacter pylori infection, we determined allele frequency and genotype distribution of this polymorphism in Bhutan--a population documented with high prevalence of gastric cancer and extremely high prevalence of H. pylori infection. The status of RAD51 G135C was examined by restriction fragment length polymorphism analysis of PCR amplified fragments and sequencing. Histological scores were evaluated according to the updated Sydney system. G135C carriers showed significantly higher scores for intestinal metaplasia in the antrum than G135G carriers [mean (median) 0·33 (0) vs. 0·08 (0), P = 0·008]. Higher scores for intestinal metaplasia of G135C carriers compared to those of G135G carriers were also observed in H. pylori-positive patients [0·3 (0) vs. 0·1 (0), P = 0·002] and H. pylori-positive patients with gastritis [0·4 (0) vs. 0·1 (0), P = 0·002] but were not found in H. pylori-negative patients. Our findings revealed that a combination of H. pylori infection and RAD51 G135C genotype of the host showed an increasing score for intestinal metaplasia. Therefore, RAD51 G135C might be the important predictor for gastric cancer of H. pylori-infected patients.

Synthesis, molecular modeling, and biological evaluation of novel RAD51 inhibitors

RAD51 recombinase plays a critical role for cancer cell proliferation and survival. Targeting RAD51 is therefore an attractive strategy for treating difficult-to-treat cancers, e.g. triple negative breast cancers which are often resistant to existing therapeutics. To this end, we have designed, synthesized and evaluated a panel of new RAD51 inhibitors, denoted IBR compounds. Among these compounds, we have identified a novel small molecule RAD51 inhibitor, IBR120, which exhibited a 4.8-fold improved growth inhibition activity in triple negative human breast cancer cell line MBA-MD-468. IBR120 also inhibited the proliferation of a broad spectrum of other cancer cell types. Approximately 10-fold difference between the IC50 values in normal and cancer cells were observed. Moreover, IBR120 was capable of disrupting RAD51 multimerization, impairing homologous recombination repair, and inducing apoptotic cell death. Therefore, these novel RAD51 inhibitors may serve as potential candidates for the development of pharmaceutical strategies against difficult-to-treat cancers.

RAD54 family translocases counter genotoxic effects of RAD51 in human tumor cells

The RAD54 family DNA translocases have several biochemical activities. One activity, demonstrated previously for the budding yeast translocases, is ATPase-dependent disruption of RAD51-dsDNA binding. This activity is thought to promote dissociation of RAD51 from heteroduplex DNA following strand exchange during homologous recombination. In addition, previous experiments in budding yeast have shown that the same activity of Rad54 removes Rad51 from undamaged sites on chromosomes; mutants lacking Rad54 accumulate nonrepair-associated complexes that can block growth and lead to chromosome loss. Here, we show that human RAD54 also promotes the dissociation of RAD51 from dsDNA and not ssDNA. We also show that translocase depletion in tumor cell lines leads to the accumulation of RAD51 on chromosomes, forming complexes that are not associated with markers of DNA damage. We further show that combined depletion of RAD54L and RAD54B and/or artificial induction of RAD51 overexpression blocks replication and promotes chromosome segregation defects. These results support a model in which RAD54L and RAD54B counteract genome-destabilizing effects of direct binding of RAD51 to dsDNA in human tumor cells. Thus, in addition to having genome-stabilizing DNA repair activity, human RAD51 has genome-destabilizing activity when expressed at high levels, as is the case in many human tumors.

DNA repair pathway gene expression score correlates with repair proficiency and tumor sensitivity to chemotherapy

Mutagenesis is a hallmark of malignancy, and many oncologic treatments function by generating additional DNA damage. Therefore, DNA damage repair is centrally important in both carcinogenesis and cancer treatment. Homologous recombination (HR) and nonhomologous end joining are alternative pathways of double-strand DNA break repair. We developed a method to quantify the efficiency of DNA repair pathways in the context of cancer therapy. The recombination proficiency score (RPS) is based on the expression levels for four genes involved in DNA repair pathway preference (Rif1, PARI, RAD51, and Ku80), such that high expression of these genes yields a low RPS. Carcinoma cells with low RPS exhibit HR suppression and frequent DNA copy number alterations, which are characteristic of error-prone repair processes that arise in HR-deficient backgrounds. The RPS system was clinically validated in patients with breast or non-small cell lung carcinomas (NSCLCs). Tumors with low RPS were associated with greater mutagenesis, adverse clinical features, and inferior patient survival rates, suggesting that HR suppression contributes to the genomic instability that fuels malignant progression. This adverse prognosis associated with low RPS was diminished if NSCLC patients received adjuvant chemotherapy, suggesting that HR suppression and associated sensitivity to platinum-based drugs counteract the adverse prognosis associated with low RPS. Therefore, RPS may help oncologists select which therapies will be effective for individual patients, thereby enabling more personalized care.

Transient expression of RAD51 in the late G2-phase is required for cell cycle progression in synchronous Physarum cells

The homologous recombination factor RAD51 is highly conserved. This criterion enabled us to identify a RAD51 ortholog in Physarum polycephalum. We found that the Physarum protein presents a high homology to the human protein and cross-reacted with antibodies directed against the human RAD51. Taking advantage of the natural synchrony of millions of nuclei within a single cell of Physarum, we investigated the fluctuation of the amount of the PpRAD51 throughout the cell cycle. Our results showed that in the late G2-phase, RAD51 was transiently expressed in a large quantity. Furthermore, knocking-down RAD51 in the G2-phase abolished this transient expression before mitosis and affected cell cycle progression. These results support the idea that RAD51 plays a role in the progression of the cell cycle in the late G2-phase.

The RAD51-stimulatory compound RS-1 can exploit the RAD51 overexpression that exists in cancer cells and tumors

RAD51 is the central protein that catalyzes DNA repair via homologous recombination, a process that ensures genomic stability. RAD51 protein is commonly expressed at high levels in cancer cells relative to their noncancerous precursors. High levels of RAD51 expression can lead to the formation of genotoxic RAD51 protein complexes on undamaged chromatin. We developed a therapeutic approach that exploits this potentially toxic feature of malignancy, using compounds that stimulate the DNA-binding activity of RAD51 to promote cancer cell death. A panel of immortalized cell lines was challenged with the RAD51-stimulatory compound RS-1. Resistance to RS-1 tended to occur in cells with higher levels of RAD54L and RAD54B, which are Swi2/Snf2-related translocases known to dissociate RAD51 filaments from dsDNA. In PC3 prostate cancer cells, RS-1-induced lethality was accompanied by the formation of microscopically visible RAD51 nuclear protein foci occurring in the absence of any DNA-damaging treatment. Treatment with RS-1 promoted significant antitumor responses in a mouse model, providing proof-of-principle for this novel therapeutic strategy.

DNA repair and cytotoxic drugs: the potential role of RAD51 in clinical outcome of non-small-cell lung cancer patients

Many of the cytotoxic drugs used in the treatment of non-small-cell lung carcinoma patients can interfere with DNA activity and the definition of an individual DNA repair profile could be a key strategy to achieve better response to chemotherapeutic treatment. Although DNA repair mechanisms are important factors in the prevention of carcinogenesis, these molecular pathways are also involved in therapy response. RAD51 is a crucial element in DNA repair by homologous recombination and has been shown to interfere with the prognosis of patients treated with chemoradiotherapy. There is increasing evidence that genetic polymorphisms in repair enzymes can influence DNA repair capacity and, consequently, affect chemotherapy efficacy. We conducted this review to show the possible influence of the RAD51 genetic variants in damage repair capacity and treatment response in non-small-cell lung carcinoma patients.

Contributions of Rad9 to tumorigenesis

Rad9 plays a crucial role in maintaining genomic stability by regulating cell cycle checkpoints, DNA repair, telomere stability, and apoptosis. Rad9 controls these processes mainly as part of the heterotrimeric 9-1-1 (Rad9-Hus1-Rad1) complex. However, in recent years it has been demonstrated that Rad9 can also act independently of the 9-1-1 complex as a transcriptional factor, participate in immunoglobulin class switch recombination, and show 3'-5' exonuclease activity. Aberrant Rad9 expression has been associated with prostate, breast, lung, skin, thyroid, and gastric cancers. High expression of Rad9 is causally related to, at least, human prostate cancer growth. On the other hand, deletion of Mrad9, the mouse homolog, is responsible for increased skin cancer incidence. These results reveal that Rad9 can act as an oncogene or tumor suppressor. Which of the many functions of Rad9 are causally related to initiation and progression of tumorigenesis and the mechanistic details by which Rad9 induces or suppresses tumorigenesis are presently not known, but are crucial for the development of targeted therapeutic interventions.

Genetic variability of Xrcc3 and Rad51 modulates the risk of head and neck cancer

A case-control study was conducted to analyze the possible associations between the head and neck cancer (HNC) risk and fourteen single nucleotide polymorphisms (SNPs) and haplotypes in Xrcc3 and Rad51 genes. This study involved 81 HNC cases and 111 healthy control subjects. A significant risk-increasing effect of rs3212057 (p.Arg94His) SNP in Xrcc3 (OR=6.6; p<0.01) was observed. On the other hand, risk-decreasing effect was found for rs5030789 (g.3997A>G) and rs1801321 (c.-60G>T) in 5' near gene and 5'UTR regions of Rad51, respectively (OR=0.3 and OR=0.2, p<0.05, respectively). Moreover, these effects were shown to be modulated by tobacco-smoking status and gene-gene interactions. Concluding, the genetic variability of Xrcc3 and/or Rad51 genes might be of relevance with respect to HNC risk.

RAD51 can inhibit PDGF-B-induced gliomagenesis and genomic instability

Faithful replication and DNA repair are vital for maintenance of genome integrity. RAD51 is a central protein in homologous recombination repair and during replication, when it protects and restarts stalled replication forks. Aberrant RAD51 expression occurs in glioma, and high expression has been shown to correlate with prolonged survival. Furthermore, genes involved in DNA damage response (DDR) are mutated or deleted in human glioblastomas, corroborating the importance of proper DNA repair to suppress gliomagenesis. We have analyzed DDR and genomic instability in PDGF-B-induced gliomas and investigated the role of RAD51 in glioma development. We show that PDGF-B-induced gliomas display genomic instability and that co-expression of RAD51 can suppress PDGF-B-induced tumorigenesis and prolong survival. Expression of RAD51 inhibited proliferation and genomic instability of tumor cells independent of Arf status. Our results demonstrate that the RAD51 pathway can prevent glioma initiation and maintain genome integrity of induced tumors, suggesting reactivation of the RAD51 pathway as a potential therapeutic avenue.

ATM modulates the loading of recombination proteins onto a chromosomal translocation breakpoint hotspot

Chromosome translocations induced by DNA damaging agents, such as ionizing radiation and certain chemotherapies, alter genetic information resulting in malignant transformation. Abrogation or loss of the ataxia-telangiectasia mutated (ATM) protein, a DNA damage signaling regulator, increases the incidence of chromosome translocations. However, how ATM protects cells from chromosome translocations is still unclear. Chromosome translocations involving the MLL gene on 11q23 are the most frequent chromosome abnormalities in secondary leukemias associated with chemotherapy employing etoposide, a topoisomerase II poison. Here we show that ATM deficiency results in the excessive binding of the DNA recombination protein RAD51 at the translocation breakpoint hotspot of 11q23 chromosome translocation after etoposide exposure. Binding of Replication protein A (RPA) and the chromatin remodeler INO80, which facilitate RAD51 loading on damaged DNA, to the hotspot were also increased by ATM deficiency. Thus, in addition to activating DNA damage signaling, ATM may avert chromosome translocations by preventing excessive loading of recombinational repair proteins onto translocation breakpoint hotspots.

Stereoselective synthesis of chiral IBR2 analogues

Two stereoselective routes were developed to synthesize optically pure IBR2 analogues 1-16. The first features addition of N-Boc-3-bromoindole 26 to the sulfinamide 25, providing a 1:1 ratio of the separable diasteroisomers 27 and 28 in good yield. In a straightforward fashion, the sulfinamides 27 and 28 were conveniently converted into the key amines 39 and 47 over 8 steps, respectively, from which a series of 3,4-dihydroisoquinolinyl IBR2 analogues 1-14 containing fluorinated and trifluoromethylated benzyl groups were prepared. Another route highlights the highly enantioselective addition of indole to the sulfonyl amide 50 with bifunctional aminothioureas 57 and 58 as catalysts. After the reaction conditions were optimized, the desired sulfonyl amides (R)-55 and (S)-55 were obtained in 99% ee and 98% ee, respectively. Acylation of (R)-55 and (S)-55 separately and subsequent allylation gave compounds 60 and 63, respectively, which were further subjected to RCM to furnish compounds 61 and 64 and, after removal of the Boc groups, the desired IBR2 analogues 15 and 16.

Overexpression of RAD51 suppresses recombination defects: a possible mechanism to reverse genomic instability

RAD51, a key protein in the homologous recombinational DNA repair (HRR) pathway, is the major strand-transferase required for mitotic recombination. An important early step in HRR is the formation of single-stranded DNA (ss-DNA) coated by RPA (a ss-DNA-binding protein). Displacement of RPA by RAD51 is highly regulated and facilitated by a number of different proteins known as the 'recombination mediators'. To assist these recombination mediators, a second group of proteins also is required and we are defining these proteins here as 'recombination co-mediators'. Defects in either recombination mediators or co-mediators, including BRCA1 and BRCA2, lead to impaired HRR that can genetically be complemented for (i.e. suppressed) by overexpression of RAD51. Defects in HRR have long been known to contribute to genomic instability leading to tumor development. Since genomic instability also slows cell growth, precancerous cells presumably require genomic re-stabilization to gain a growth advantage. RAD51 is overexpressed in many tumors, and therefore, we hypothesize that the complementing ability of elevated levels of RAD51 in tumors with initial HRR defects limits genomic instability during carcinogenic progression. Of particular interest, this model may also help explain the high frequency of TP53 mutations in human cancers, since wild-type p53 represses RAD51 expression.

The consequences of Rad51 overexpression for normal and tumor cells

The Rad51 recombinase is an essential factor for homologous recombination and the repair of DNA double strand breaks, binding transiently to both single stranded and double stranded DNA during the recombination reaction. The use of a homologous recombination mechanism to repair DNA damage is controlled at several levels, including the binding of Rad51 to single stranded DNA to form the Rad51 nucleofilament, which is controlled through the action of DNA helicases that can counteract nucleofilament formation. Overexpression of Rad51 in different organisms and cell types has a wide assortment of consequences, ranging from increased homologous recombination and increased resistance to DNA damaging agents to disruption of the cell cycle and apoptotic cell death. Rad51 expression is increased in p53-negative cells, and since p53 is often mutated in tumor cells, there is a tendency for Rad51 to be overexpressed in tumor cells, leading to increased resistance to DNA damage and drugs used in chemotherapies. As cells with increased Rad51 levels are more resistant to DNA damage, there is a selection for tumor cells to have higher Rad51 levels. While increased Rad51 can provide drug resistance, it also leads to increased genomic instability and may contribute to carcinogenesis.

Estrogen-dependent cell signaling and apoptosis in BRCA1-blocked BG1 ovarian cancer cells in response to plumbagin and other chemotherapeutic agents

BACKGROUND

Cellular response to chemotherapeutic drugs in the absence of BRCA1 either completely or partially had drawn less attention. The present study evaluated whether there is a differential inhibition of cell growth by selected compounds with respect to BRCA1 status in estrogen receptor (ER)-positive ovarian cancer cells.

MATERIALS AND METHODS

The BG1 ovarian cancer cells used in the experiments were antisensely blocked with BRCA1 gene. Growth inhibition and apoptotic induction were analyzed to evaluate the cytotoxic effects. Small interfering RNA (SiRNA) transfection, western blot analysis, RT-PCR analysis and molecular modeling were carried out to analyze the estrogen-dependent action of plumbagin.

RESULTS

Although we found that all the compounds studied induce apoptosis, the induction was in the order of plumbagin > doxorubicin > tamoxifen > cisplatin. Plumbagin can bind to the active site of ER-alpha. Plumbagin, however, induced ER-alpha 46 kDa truncated isoform, which was found abundantly preempted in the cytoplasm compared with a 66-kDa full-length isoform. The truncated isoform is known to inhibit classical ER-alpha signaling pathways. SiRNA-transfected cells for ER-alpha exhibited lower cytotoxicity upon plumbagin treatment than the control-transfected cells.

CONCLUSION

Taken together, this study indicates that plumbagin has chemotherapeutic potential in BRCA1-mutated/defective ER-positive cancers.

Differential regulation of DNA repair protein Rad51 in human tumour cell lines exposed to doxorubicin

Radiotherapy and chemotherapy often induce DNA double-strand breaks in both normal and malignant cells. The proteins involved in the repair of such lesions are central to cancer prognosis and treatment, as they can be overexpressed in many cancers, accelerating malignant transformation and increasing repair capacity, potentially leading to cellular resistance. If malignant cells can be selectively targeted repair proteins could also be candidates for targeted therapy. In this study, two keyplayers in eukaryotic DNA double-strand break repair, Rad51 and DNA-dependent protein kinase catalytic subunit, were analysed in noncancerous human breast cells (MCF12A) and the breast cancer cell lines (MDA MB 231 and MCF7) in response to treatment with doxorubicin. A cell cycle-independent increase in Rad51 protein levels (a recombinase involved in homologous recombination repair) was observed 24 and 48 h after treatment in MDA MB 231 and MCF12A when exposed to low levels of doxorubicin, whereas MCF7 cells displayed a continuous decrease in Rad51 protein with increasing drug concentration. DNA-dependent protein kinase catalytic subunit, which is involved in nonhomologous end joining of DNA lesions, remained unaltered under all conditions tested. Topoisomerase II-alpha protein, the primary target of doxorubicin, was upregulated at low concentrations of doxorubicin in all cell lines tested. Here we show that Rad51 protein levels can be differentially regulated in normal and malignant breast cell lines in response to doxorubicin, independent of cell cycle state. These observations have direct relevance to chemosensitivity and add an additional prognostic factor that could be taken into account when designing targeted therapeutic regimes.

Genome-wide genetic analysis of polyploidy in yeast

Polyploidy, increased sets of chromosomes, occurs during development, cellular stress, disease and evolution. Despite its prevalence, little is known about the physiological alterations that accompany polyploidy. We previously described 'ploidy-specific lethality', where a gene deletion that is not lethal in haploid or diploid budding yeast causes lethality in triploids or tetraploids. Here we report a genome-wide screen to identify ploidy-specific lethal functions. Only 39 out of 3,740 mutations screened exhibited ploidy-specific lethality. Almost all of these mutations affect genomic stability by impairing homologous recombination, sister chromatid cohesion, or mitotic spindle function. We uncovered defects in wild-type tetraploids predicted by the screen, and identified mechanisms by which tetraploidization affects genomic stability. We show that tetraploids have a high incidence of syntelic/monopolar kinetochore attachments to the spindle pole. We suggest that this defect can be explained by mismatches in the ability to scale the size of the spindle pole body, spindle and kinetochores. Thus, geometric constraints may have profound effects on genome stability; the phenomenon described here may be relevant in a variety of biological contexts, including disease states such as cancer.

A specific RAD51 haplotype increases breast cancer risk in Jewish non-Ashkenazi high-risk women

While the precise genes involved in determining familial breast cancer risk in addition to BRCA1/2 are mostly unknown, one strong candidate is RAD51. Jewish non-Ashkenazi women at high-risk for breast/ovarian cancer and ethnically matched controls were genotyped using four single nucleotide polymorphisms spanning the RAD51 genomic region, and the resulting haplotypes were constructed using the GERBIL algorithm. A total of 314 individuals were genotyped: 184 non-Ashkenazi high-risk women (119 with breast cancer), and 130 unaffected, average-risk ethnically matched controls. Using GEBRIL, three frequent haplotypes were constructed. One of the haplotypes (TGTA - coined haplotype 3) was present in 7.3% (19/260 haplotypes) of controls (n=130) and in 16.8% (40/238 haplotypes) of high-risk breast cancer patients (n=119, P=0.001). A specific RAD51 haplotype is more prevalent among non-Ashkenazi Jewish high-risk women than in average-risk population.

Rad51-related changes in global gene expression

High expression of Rad51, the catalytic component in homologous recombination, has been reported to contribute to genomic instability. To elucidate biological processes related to Rad51, we performed global gene expression analysis on human fibrosarcoma cells induced to express variable Rad51 levels. The results indicate that Rad51 overexpression mediates late rather than early transcriptional responses. Using Gene Ontology analysis, we extracted functional annotations for Rad51-related changes in gene expression that were independent of general cell culture effects. High Rad51 levels conferred increased expression of genes involved in actin remodelling. These changes were accompanied by alterations in cell morphology. Moreover, core components of the mismatch repair (MMR) machinery were down-regulated in response to increased Rad51 expression. Given the role of MMR in the correction of DNA mismatches during replication and recombination, a concurrent increase in Rad51 levels and decrease in the expression of MMR genes could conceivably act synergistically towards genomic instability.

Nuclear Translocation of Phospholipase C-δ1 Is Linked to the Cell Cycle and Nuclear Phosphatidylinositol 4,5-Bisphosphate

Nuclear phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate, fluctuate throughout the cell cycle and are linked to proliferation and differentiation. Here we report that phospholipase C-delta(1) accumulates in the nucleus at the G(1)/S boundary and in G(0) phases of the cell cycle. Furthermore, as wild-type protein accumulated in the nucleus, nuclear phosphatidylinositol 4,5-bisphosphate levels were elevated 3-5-fold, whereas total levels were decreased compared with asynchronous cultures. To test whether phosphatidylinositol 4,5-bisphosphate binding is important during this process, we introduced a R40D point mutation within the pleckstrin homology domain of phospholipase C-delta(1), which disables high affinity phosphatidylinositol 4,5-bisphosphate binding, and found that nuclear translocation was significantly reduced at G(1)/S and in G(0). These results demonstrate a cell cycle-dependent compartmentalization of phospholipase C-delta(1) and support the idea that relative levels of phosphoinositides modulate the portioning of phosphoinositide-binding proteins between the nucleus and other compartments.

Impaired homologous recombination DNA repair and enhanced sensitivity to DNA damage in prostate cancer cells exposed to anchorage-independence

During metastases, cancer cells are temporarily exposed to the condition in which interactions with extracellular environment can be restricted (anchorage-independence). We demonstrate that the sensitivity of prostate cancer cell lines, DU145 and PC-3, to genotoxic treatment (cisplatin and gamma-irradiation) increased several folds when cells were forced to grow in anchorage-independence. This enhanced drug sensitivity was associated with a severe impairment of homologous recombination-directed DNA repair (HRR). The mechanism involves Rad51, which is the major enzymatic component of HRR. The protein level of Rad51 and its recruitment to DNA double-strand breaks (DSBs) were both attenuated. Rad51 deficiency in anchorage-independence was not associated with Rad51 promoter activity, and was not compensated by a constitutive overexpression of Rad51 cDNA. Instead, Rad51 protein level and its ability to colocalize with DSBs were restored in the presence of proteosome inhibitors, or when cells from the suspension cultures were allowed reattachment. Presented results indicate that anchorage-independence sensitizes prostate cancer cells to genotoxic agents; however, it also attenuates faithful component of DNA repair by targeting stability of Rad51. This temporal attenuation of HRR may contribute to the accumulation mutations after DNA damage, and possibly the selection of new adaptations in cells, which survived genotoxic treatment.

Therapy-related acute myeloid leukemia–like MLL rearrangements are induced by etoposide in primary human CD34+ cells and remain stable after clonal expansion

Rearrangements involving the MLL gene on chromosome band 11q23 are a hallmark of therapy-related acute myeloid leukemias following treatment with topoisomerase II poisons including etoposide. Therapy-related and de novo genomic translocation breakpoints cluster within a well-characterized 8.3-kb fragment of MLL. Repair of etoposide-stabilized DNA topoisomerase II covalent complexes may initiate MLL rearrangements observed in patients. We used a culture system of primary human hematopoietic CD34+ cells and inverse polymerase chain reaction to characterize the spectrum of stable genomic rearrangements promoted by etoposide exposure originating within an MLL translocation hotspot in therapy-related leukemia. Alterations to the region were observed at a readily detectable frequency in etoposide-treated cells. Illegitimate repair events after minimal repair included MLL tandem duplications and translocations, with minor populations of deletions or insertions. In stably repaired cells that proliferated for 10 to 14 days, the significant majority of illegitimate events were MLL tandem duplications, and several deletions, inversions, insertions, and translocations. Thus, etoposide promotes specific rearrangements of MLL consistent with the full spectrum of oncogenic events identified in leukemic samples. Although etoposide-initiated rearrangements are frequent, only a small subset of translocations occurs in cells that proliferate significantly.

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.

Ligand dependent and independent internalization and nuclear translocation of fibroblast growth factor (FGF) receptor 1

Basic fibroblast growth factor (FGF-2) is one of the prototype members of a rapidly expanding family of polypeptides. FGF-2 acts on cells via a dual-receptor system consisting of high-affinity tyrosine kinase receptors (FGFR) and low-affinity receptors comprised of heparan sulfate proteoglycans. Following ligand binding and subsequent internalization, both FGF-2 and FGFR1 are translocated to the nucleus where they have activities distinct from those expressed at the cell surface. Despite the growing number of growth factors and receptors shown to translocate to the nucleus, little is known about the mechanisms of internalization and translocation and how these processes are regulated. In the studies reported in this paper, we examined the roles of clathrin-dependent and -independent endocytosis in the uptake of FGFR1 and one of its ligands, FGF-2. While the uptake of FGF-2 occurred at least partly by a caveolar-dependent mechanism, that of FGFR1 was independent of both caveolae and coated pits. Surprisingly, neither the uptake of FGF-2 nor FGFR1 required the activity of the receptor tyrosine kinase. In addition, we identified a cell cycle-dependent pathway of FGFR1 nuclear translocation that appears to be independent of ligand binding.

Normal expression of DNA repair proteins, hMre11, Rad50 and Rad51 but protracted formation of Rad50 containing foci in X-irradiated skin fibroblasts from radiosensitive cancer patients

About 5% of oncology patients treated by radiation therapy develop acute or late radiotoxic effects whose molecular mechanisms remain poorly understood. In this study, we evaluated the potential role of DNA repair proteins in the hypersensitivity of cancer patients to radiation therapy. The expression levels and focal nuclear distribution of DNA repair proteins, hMre11, Rad50 and Rad51 were investigated in skin fibroblasts strains derived from cancer patients with adverse early skin reaction to radiotherapy using Western blot and foci immunofluorescence techniques, respectively. Cells from cancer patients with normal reaction to radiotherapy as well as cells from apparently healthy subjects served as controls. Cellular radiosensitivity after in vitro irradiation was assessed by the clonogenic survival assay. The clonogenic survival assay and Western blot analysis of the DNA repair proteins did not reveal any abnormalities in cellular radiosensitivity in vitro and in protein expression levels or their migration patterns in the fibroblasts derived from cancer patients with hypersensitive reaction to radiotherapy. In contrast, in vitro irradiated cells from radiosensitive patients exhibited a significantly higher number of nuclei with focally concentrated Rad50 protein than in both control groups. The observed alteration of the distribution of radiation-induced Rad50 foci in cells derived from cancer patients with acute side reactions to radiotherapy might contribute to their radiation therapy outcome. These data suggest the usefulness of the Rad50 foci analysis for predicting clinical response of cancer patients to radiotherapy.

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.

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.

Recombinational DNA repair and human disease

We review the genes and proteins related to the homologous recombinational repair (HRR) pathway that are implicated in cancer through either genetic disorders that predispose to cancer through chromosome instability or the occurrence of somatic mutations that contribute to carcinogenesis. Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and an ataxia-like disorder (ATLD), are chromosome instability disorders that are defective in the ataxia telangiectasia mutated (ATM), NBS, and Mre11 genes, respectively. These genes are critical in maintaining cellular resistance to ionizing radiation (IR), which kills largely by the production of double-strand breaks (DSBs). Bloom syndrome involves a defect in the BLM helicase, which seems to play a role in restarting DNA replication forks that are blocked at lesions, thereby promoting chromosome stability. The Werner syndrome gene (WRN) helicase, another member of the RecQ family like BLM, has very recently been found to help mediate homologous recombination. Fanconi anemia (FA) is a genetically complex chromosomal instability disorder involving seven or more genes, one of which is BRCA2. FA may be at least partially caused by the aberrant production of reactive oxidative species. The breast cancer-associated BRCA1 and BRCA2 proteins are strongly implicated in HRR; BRCA2 associates with Rad51 and appears to regulate its activity. We discuss in detail the phenotypes of the various mutant cell lines and the signaling pathways mediated by the ATM kinase. ATM's phosphorylation targets can be grouped into oxidative stress-mediated transcriptional changes, cell cycle checkpoints, and recombinational repair. We present the DNA damage response pathways by using the DSB as the prototype lesion, whose incorrect repair can initiate and augment karyotypic abnormalities.

Analysis of mouse Rad54 expression and its implications for homologous recombination

Homologous recombination is one of the major pathways for repair of DNA double-strand breaks (DSBs). Important proteins in this pathway are Rad51 and Rad54. Rad51 forms a nucleoprotein filament on single-stranded DNA (ssDNA) that mediates pairing with and strand invasion of homologous duplex DNA with the assist of Rad54. We estimated that the nucleus of a mouse embryonic stem (ES) cells contains on average 4.7x10(5) Rad51 and 2.4x10(5) Rad54 molecules. Furthermore, we showed that the amount of Rad54 was subject to cell cycle regulation. We discuss our results with respect to two models that describe how Rad54 stimulates Rad51-mediated DNA strand invasion. The models differ in whether Rad54 functions locally or globally. In the first model, Rad54 acts in cis relative to the site of strand invasion. Rad54 coats the Rad51 nucleoprotein filament in stoichiometric amounts and binds to the target duplex DNA at the site that is homologous to the ssDNA in the Rad51 nucleoprotein filament. Subsequently, it promotes duplex DNA unwinding. In the second model, Rad54 acts in trans relative to the site of strand invasion. Rad54 binds duplex DNA distant from the site that will be unwound. Translocation of Rad54 along the duplex DNA increases superhelical stress thereby promoting duplex DNA unwinding.

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

After exposure of mammalian cells to DNA damage, the endogenous Rad51 recombination protein is concentrated in multiple discrete foci, which are thought to represent nuclear domains for recombinational DNA repair. Overexpressed Rad51 protein forms foci and higher-order nuclear structures, even in the absence of DNA damage, in cells that do not undergo DNA replication synthesis. This correlates with increased expression of the cyclin-dependent kinase (Cdk) inhibitor p21. Following DNA damage, constitutively Rad51-overexpressing cells show reduced numbers of DNA breaks and chromatid-type chromosome aberrations and a greater resistance to apoptosis. In contrast, Rad51 antisense inhibition reduces p21 protein levels and sensitizes cells to etoposide treatment. Downregulation of p21 inhibits Rad51 foci formation in both normal and Rad51-overexpressing cells. Collectively, our results show that Rad51 expression, Rad51 foci formation and p21 expression are interrelated, suggesting a functional link between mammalian Rad51 protein and p21-mediated cell cycle regulation. This mechanism may contribute to a highly effective recombinational DNA repair in cell cycle-arrested cells and protection against DNA damage-induced apoptosis.