XRCC2 and XRCC3, new human Rad51-family members, promote chromosome stability and protect against DNA cross-links and other damages

Expression pattern analysis of DNA repair-related and DNA damage response genes revealed by 55K oligomicroarray upon UV-B irradiation in the intertidal copepod, Tigriopus japonicus

Ultraviolet-B (UV-B) radiation affects the genome stability of aquatic organisms by absorption of certain wavelength at the molecular level. Recently, extensive gene information has been identified from the intertidal copepod, Tigriopus japonicus. Here, we developed a 55K (54,254 genes) oligomicroarray and tested its usefulness to identify the effect of single dose of UV-B irradiation (12 kJ/m(2)) on transcriptomes of the copepod T. japonicus. A total of 35,361 spots were identified to be significantly modulated on the 55K oligomicroarray by hierarchical clustering after exposure to UV-B irradiation over 48 h (6, 12, 24, and 48 h). Of them, 1300 and 588 genes were observed to be up-regulated and down-regulated at all time points, respectively. Particularly, it was observed that several genes involved in DNA repair mechanism were significantly modulated in the UV-B-exposed T. japonicus by microarray and quantitative real-time RT-PCR analysis. In detail, UV-B irradiation specifically up-regulated some genes in non-homologous end-joining (NHEJ), homologous recombination (HR), base excision repair (BER), and mismatch repair (MMR) pathways. On the other hand, a majority of down-regulated genes were representatives for the nucleotide excision repair (NER) mechanism. These results demonstrated that DNA damage would be induced by UV-B irradiation in this species, resulting in reliable induction or repression of various DNA repair mechanism on UV-B-induced DNA damage. In this report, we suggest that a high density microarray-based approach for risk assessment of UV-B irradiation would be useful to elucidate the mechanistic analysis in a non-model organism. This study could also provide a better understanding of molecular mechanisms of cellular protection against UV-B-induced stress.

Role of genetic susceptibility in development of treatment-related adverse outcomes in cancer survivors

Clear and unambiguous associations have been established between therapeutic exposures and specific complications. However, considerable interindividual variability is observed in the risk of developing an outcome for a given therapeutic exposure. Genetic predisposition and especially its interaction with therapeutic exposures can potentially exacerbate the toxic effect of treatment on normal tissues and organ systems, and can possibly explain the interindividual variability. This article provides a brief overview of the current knowledge about the role of genomic variation in the development of therapy-related complications. Relatively common outcomes with strong associations with therapeutic exposures, including cardiomyopathy, obesity, osteonecrosis, ototoxicity, and subsequent malignancies are discussed here. To develop a deeper understanding of the molecular underpinnings of therapy-related complications, comprehensive and near-complete collection of clinically annotated samples is critical. Methodologic issues such as study design, definition of the endpoints or phenotypes, identification of appropriate and adequately sized study population together with a reliable plan for collecting and maintaining high-quality DNA, and selection of an appropriate approach or platform for genotyping are also discussed. Understanding the etiopathogenetic pathways that lead to the morbidity is critical to developing targeted prevention and intervention strategies, optimizing risk-based health care of cancer survivors, thus minimizing chronic morbidities and improving quality of life.

RNAi-mediated targeting of noncoding and coding sequences in DNA repair gene messages efficiently radiosensitizes human tumor cells

Human tumor cell death during radiotherapy is caused mainly by ionizing radiation (IR)-induced DNA double-strand breaks (DSB), which are repaired by either homologous recombination repair (HRR) or nonhomologous end-joining (NHEJ). Although siRNA-mediated knockdown of DNA DSB repair genes can sensitize tumor cells to IR, this approach is limited by inefficiencies of gene silencing. In this study, we show that combining an artificial miRNA (amiR) engineered to target 3'-untranslated regions of XRCC2 (an HRR factor) or XRCC4 (an NHEJ factor) along with an siRNA to target the gene coding region can improve silencing efficiencies to achieve more robust radiosensitization than a single approach alone. Mechanistically, the combinatorial knockdown decreased targeted gene expression through both a reduction in mRNA stability and a blockade to mRNA translation. Together, our findings establish a general method of gene silencing that is more efficient and particularly suited for suppressing genes that are difficult to downregulate by amiR- or siRNA-based methods alone.

Molecular and cellular pharmacology of the hypoxia-activated prodrug TH-302

TH-302 is a 2-nitroimidazole triggered hypoxia-activated prodrug (HAP) of bromo-isophosphoramide mustard currently undergoing clinical evaluation. Here, we describe broad-spectrum activity, hypoxia-selective activation, and mechanism of action of TH-302. The concentration and time dependence of TH-302 activation was examined as a function of oxygen concentration, with reference to the prototypic HAP tirapazamine, and showed superior oxygen inhibition of cytotoxicity and much improved dose potency relative to tirapazamine. Enhanced TH-302 cytotoxicity under hypoxia was observed across 32 human cancer cell lines. One-electron reductive enzyme dependence was confirmed using cells overexpressing human NADPH:cytochrome P450 oxidoreductase and radiolytic reduction established the single-electron stoichiometry of TH-302 fragmentation (activation). Examining downstream effects of TH-302 activity, we observed hypoxia-dependent induction of γH2AX phosphorylation, DNA cross-linking, and cell-cycle arrest. We used Chinese hamster ovary cell-based DNA repair mutant cell lines and established that lines deficient in homology-dependent repair, but not lines deficient in base excision, nucleotide excision, or nonhomologous end-joining repair, exhibited marked sensitivity to TH-302 under hypoxia. Consistent with this finding, enhanced sensitivity to TH-302 was also observed in lines deficient in BRCA1, BRCA2, and FANCA. Finally, we characterized TH-302 activity in the three-dimensional tumor spheroid and multicellular layer models. TH-302 showed much enhanced potency in H460 spheroids compared with H460 monolayer cells under normoxia. Multicellular layers composed of mixtures of parental HCT116 cells and HCT116 cells engineered to express an oxygen-insensitive bacterial nitroreductase showed that TH-302 exhibits a significant bystander effect.

RAD51 paralogs: roles in DNA damage signalling, recombinational repair and tumorigenesis

Chromosomal double-strand breaks (DSBs) have the potential to permanently arrest cell cycle progression and endanger cell survival. They must therefore be efficiently repaired to preserve genome integrity and functionality. Homologous recombination (HR) provides an important error-free mechanism for DSB repair in mammalian cells. In addition to RAD51, the central recombinase activity in mammalian cells, a family of proteins known as the RAD51 paralogs and consisting of five proteins (RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3), play an essential role in the DNA repair reactions through HR. The RAD51 paralogs act to transduce the DNA damage signal to effector kinases and to promote break repair. However, their precise cellular functions are not fully elucidated. Here we discuss recent advances in our understanding of how these factors mediate checkpoint responses and act in the HR repair process. In addition, we highlight potential functional similarities with the BRCA2 tumour suppressor, through the recently reported links between RAD51 paralog deficiencies and tumorigenesis triggered by genome instability.

Fanconi anemia: at the crossroads of DNA repair

Fanconi anemia (FA) is an autosomal disorder that causes genome instability. FA patients suffer developmental abnormalities, early-onset bone marrow failure, and a predisposition to cancer. The disease is manifested by defects in DNA repair, hypersensitivity to DNA crosslinking agents, and a high degree of chromosomal aberrations. The FA pathway comprises 13 disease-causing genes involved in maintaining genomic stability. The fast pace of study of the novel DNA damage network has led to the constant discovery of new FA-like genes involved in the pathway that when mutated lead to similar disorders. A majority of the FA proteins act as signal transducers and scaffolding proteins to employ other pathways to repair DNA. This review discusses what is known about the FA proteins and other recently linked FA-like proteins. The goal is to clarify how the proteins work together to carry out interstrand crosslink repair and homologous recombination-mediated repair of damaged DNA.

Diabetes causes multiple genetic alterations and downregulates expression of DNA repair genes in the prostate

The molecular impact of diabetes mellitus on prostate gland has not been elucidated. In this study, we performed a whole-genome cDNA microarray analysis using a streptozotocin-induced diabetic rat model to identify the effects of diabetes on the gene expression profiles in prostate. Our study shows that diabetes causes changes in the expression of multiple genes, particularly those related to cell proliferation and differentiation, oxidative stress, DNA damage repair, cell cycle checkpoints, angiogenesis and apoptosis. These findings were confirmed by real-time polymerase chain reaction and immunohistochemical staining using rat and human prostate tissue. We also used a cell culture model (human normal prostatic RWPE-1 cell line) to study the direct effect of high glucose. We found that high glucose caused increased intracellular oxidative stress and DNA damage, as well as downregulation of anti-oxidative enzymes and DNA damage repair genes MRE11 and XRCC3. Our findings provide important insights into understanding the pathogenesis of the diabetes-induced changes in prostate as well as identifying potential therapeutic targets for future studies.

Mechanism of cell death resulting from DNA interstrand cross-linking in mammalian cells

DNA interstrand cross-links (ICLs) are critical cytotoxic lesions produced by cancer chemotherapeutic agents such as the nitrogen mustards and platinum drugs; however, the exact mechanism of ICL-induced cell death is unclear. Here, we show a novel mechanism of p53-independent apoptotic cell death involving prolonged cell-cycle (G₂) arrest, ICL repair involving HR, transient mitosis, incomplete cytokinesis, and gross chromosomal abnormalities resulting from ICLs in mammalian cells. This characteristic ‘giant' cell death, observed by using time-lapse video microscopy, was reduced in ICL repair ERCC1- and XRCC3-deficient cells. Collectively, the results illustrate the coordination of ICL-induced cellular responses, including cell-cycle arrest, DNA damage repair, and cell death.

Regulation of the DNA Damage Response to DSBs by Post-Translational Modifications

Damage to the genetic material can affect cellular function in many ways. Therefore, maintenance of the genetic integrity is of primary importance for all cells. Upon DNA damage, cells respond immediately with proliferation arrest and repair of the lesion or apoptosis. All these consequences require recognition of the lesion and transduction of the information to effector systems. The accomplishment of DNA repair, but also of cell cycle arrest and apoptosis furthermore requires protein-protein interactions and the formation of larger protein complexes. More recent research shows that the formation of many of these aggregates depends on post-translational modifications. In this article, we have summarized the different cellular events in response to a DNA double strand break, the most severe lesion of the DNA.

A role for XRCC2 gene polymorphisms in breast cancer risk and survival

BACKGROUND

The XRCC2 gene is a key mediator in the homologous recombination repair of DNA double strand breaks. It is hypothesised that inherited variants in the XRCC2 gene might also affect susceptibility to, and survival from, breast cancer.

METHODS

The study genotyped 12 XRCC2 tagging single nucleotide polymorphisms (SNPs) in 1131 breast cancer cases and 1148 controls from the Sheffield Breast Cancer Study (SBCS), and examined their associations with breast cancer risk and survival by estimating ORs and HRs, and their corresponding 95% CIs. Positive findings were further investigated in 860 cases and 869 controls from the Utah Breast Cancer Study (UBCS) and jointly analysed together with available published data for breast cancer risk. The survival findings were further confirmed in studies (8074 cases) from the Breast Cancer Association Consortium (BCAC).

RESULTS

The most significant association with breast cancer risk in the SBCS dataset was the XRCC2 rs3218408 SNP (recessive model p=2.3×10(-4), minor allele frequency (MAF)=0.23). This SNP yielded an OR(rec) of 1.64 (95% CI 1.25 to 2.16) in a two-site analysis of SBCS and UBCS, and a meta-OR(rec) of 1.33 (95% CI 1.12 to 1.57) when all published data were included. This SNP may mark a rare risk haplotype carried by two in 1000 of the control population. Furthermore, the XRCC2 coding R188H SNP (rs3218536, MAF=0.08) was significantly associated with poor survival, with an increased per-allele HR of 1.58 (95% CI 1.01 to 2.49) in a multivariate analysis. This effect was still evident in a pooled meta-analysis of 8781 breast cancer patients from the BCAC (HR 1.19, 95% CI 1.05 to 1.36; p=0.01).

CONCLUSIONS

These findings suggest that XRCC2 SNPs may influence breast cancer risk and survival.

Genetic biomonitoring of inhabitants exposed to uranium in the north region of Brazil

Aiming to assess the susceptibility of populations in the Brazilian Amazon region to ionizing radiation emitted from uranium, mutations frequencies in the DNA repair genes XRCC1 and XRCC3 and in the metabolic gene GSTM1 were evaluated. The XRCC1 allele frequencies for the 194Trp polymorphism in the municipalities of Monte Alegre, Prainha and Alenquer were, respectively, 0.12, 0.13 and 0.07, and for 399Gln polymorphism they were, respectively, 0.28, 0.30 and 0.32. Frequencies for GSTM1 gene deletion homozygotes were, respectively, 0.36, 0.31 and 0.40 for all municipalities. These frequencies are comparable to those described for Brazilian individuals from other regions of the country. Also, allele frequencies of XRCC3 241Met polymorphism of the Monte Alegre and Alenquer populations were 0.28 and 0.33, respectively. In conclusion, frequencies of important polymorphic features of cellular DNA repair and metabolic apparatus in the populations studied do not differ from those of populations in other regions of Brazil.

Interactions among Trypanosoma brucei RAD51 paralogues in DNA repair and antigenic variation

Homologous recombination in Trypanosoma brucei is used for moving variant surface glycoprotein (VSG) genes into expression sites during immune evasion by antigenic variation. A major route for such VSG switching is gene conversion reactions in which RAD51, a universally conserved recombinase, catalyses homology-directed strand exchange. In any eukaryote, RAD51-directed strand exchange in vivo is mediated by further factors, including RAD51-related proteins termed Rad51 paralogues. These appear to be ubiquitously conserved, although their detailed roles in recombination remain unclear. In T. brucei, four putative RAD51 paralogue genes have been identified by sequence homology. Here we show that all four RAD51 paralogues act in DNA repair, recombination and RAD51 subnuclear dynamics, though not equivalently, while mutation of only one RAD51 paralogue gene significantly impedes VSG switching. We also show that the T. brucei RAD51 paralogues interact, and that the complexes they form may explain the distinct phenotypes of the mutants as well as observed expression interdependency. Finally, we document the Rad51 paralogues that are encoded by a wide range of protists, demonstrating that the Rad51 paralogue repertoire in T. brucei is unusually large among microbial eukaryotes and that one member of the protein family corresponds with a key, conserved eukaryotic Rad51 paralogue.

Choosing the right path: does DNA-PK help make the decision?

DNA double-strand breaks are extremely harmful lesions that can lead to genomic instability and cell death if not properly repaired. There are at least three pathways that are responsible for repairing DNA double-strand breaks in mammalian cells: non-homologous end joining, homologous recombination and alternative non-homologous end joining. Here we review each of these three pathways with an emphasis on the role of the DNA-dependent protein kinase, a critical component of the non-homologous end joining pathway, in influencing which pathway is ultimately utilized for repair.

The Ku-dependent non-homologous end-joining pathway contributes to low-dose radiation-stimulated cell survival

Low-dose (≤0.1 Gy) radiation-induced adaptive responses could protect cells from high-challenge dose radiation-induced killing. The protective role is believed to promote the repair of DNA double-strand breaks (DSBs) that are a severe threat to cell survival. However, it remains unclear which repair pathway, homologous recombination repair (HRR) or non-homologous end-joining (NHEJ), is promoted by low-dose radiation. To address this question, we examined the effects of low-dose (0.1 Gy) on high-challenge dose (2-4 Gy) induced killing in NHEJ- or HRR-deficient cell lines. We showed that 0.1 Gy reduced the high-dose radiation-induced killing for wild-type or HRR-deficient cells, but enhanced the killing for NHEJ-deficient cells. Interestingly, low-dose radiation also enhanced the killing for wild-type cells exposed to high-challenge dose radiation with high-linear energy transfer (LET). Because it is known that high-LET radiation induces an inefficient NHEJ, these results support that the low-dose radiation-stimulated protective role in reducing high-challenge dose (low-LET)-induced cell killing might depend on NHEJ. In addition, we showed that low-dose radiation activated the DNA-PK catalytic subunit (DNA-PKcs) and the inhibitor of DNA-PKcs destroyed the low-dose radiation-induced protective role. These results suggest that low-dose radiation might promote NHEJ through the stimulation of DNA-PKcs activity and; therefore, increase the resistance of cells to high-challenge dose radiation-induced killing.

The effect of acute dose charge particle radiation on expression of DNA repair genes in mice

The space radiation environment consists of trapped particle radiation, solar particle radiation, and galactic cosmic radiation (GCR), in which protons are the most abundant particle type. During missions to the moon or to Mars, the constant exposure to GCR and occasional exposure to particles emitted from solar particle events (SPE) are major health concerns for astronauts. Therefore, in order to determine health risks during space missions, an understanding of cellular responses to proton exposure is of primary importance. The expression of DNA repair genes in response to ionizing radiation (X-rays and gamma rays) has been studied, but data on DNA repair in response to protons is lacking. Using qPCR analysis, we investigated changes in gene expression induced by positively charged particles (protons) in four categories (0, 0.1, 1.0, and 2.0 Gy) in nine different DNA repair genes isolated from the testes of irradiated mice. DNA repair genes were selected on the basis of their known functions. These genes include ERCC1 (5' incision subunit, DNA strand break repair), ERCC2/NER (opening DNA around the damage, Nucleotide Excision Repair), XRCC1 (5' incision subunit, DNA strand break repair), XRCC3 (DNA break and cross-link repair), XPA (binds damaged DNA in preincision complex), XPC (damage recognition), ATA or ATM (activates checkpoint signaling upon double strand breaks), MLH1 (post-replicative DNA mismatch repair), and PARP1 (base excision repair). Our results demonstrate that ERCC1, PARP1, and XPA genes showed no change at 0.1 Gy radiation, up-regulation at 1.0 Gy radiation (1.09 fold, 7.32 fold, 0.75 fold, respectively), and a remarkable increase in gene expression at 2.0 Gy radiation (4.83 fold, 57.58 fold and 87.58 fold, respectively). Expression of other genes, including ATM and XRCC3, was unchanged at 0.1 and 1.0 Gy radiation but showed up-regulation at 2.0 Gy radiation (2.64 fold and 2.86 fold, respectively). We were unable to detect gene expression for the remaining four genes (XPC, ERCC2, XRCC1, and MLH1) in either the experimental or control animals.

Expression of double strand DNA breaks repair genes in pterygium

PURPOSE

The alteration in the expression of some genes and proteins responsible for chosen DNA repair pathways in pterygium pathogenesis were studied. This study was focused on the examination of the expression of genes and RAD50 protein taking part in homologous recombination.

METHODS

Peripheral blood lymphocytes, samples of pterygium tissue, samples of conjunctiva of patients suffering from pterygium as well as peripheral blood lymphocytes and conjunctiva of patients from the control group were examined. In order to identify genes products from RNA, Ribonuclease Protection Assai method was applied. LIM15, RAD50, RAD54, RAD52, MRE11, XRCC2, XRCC3, RAD51, RAD51B, RAD51C, RAD51D genes transcripts were detected. Expression of RAD50 protein was analyzed immunohistochemically.

RESULTS

Peripheral blood lymphocytes analyses revealed lower level of RAD50 gene expression in the pterygium patients compared to the control group and the increased expression of XRCC2, XRCC3 and RAD51 genes in patients with pterygium, who declared the recurrence of the lesion in comparison to the patients with primary pterygium. Lower expression of the RAD54 gene in pterygium tissue comparing to conjunctiva from the eyes with pterygium was found. An expression of RAD50 gene in the conjunctiva originating from eyes with pterygium in comparison to the conjunctiva of control group was shown to be considerably higher. Expression of RAD50 protein in pterygium squamous epithelial cells was significantly higher than in conjunctiva from control group.

CONCLUSION

There may exist a relationship between pterygium pathogenesis and damages of double strand DNA, however, the elucidation of its exact nature needs further study.

Targeting abnormal DNA double strand break repair in cancer

A major challenge in cancer treatment is the development of therapies that target cancer cells with little or no toxicity to normal tissues and cells. Alterations in DNA double strand break (DSB) repair in cancer cells include both elevated and reduced levels of key repair proteins and changes in the relative contributions of the various DSB repair pathways. These differences can result in increased sensitivity to DSB-inducing agents and increased genomic instability. The development of agents that selectively inhibit the DSB repair pathways that cancer cells are more dependent upon will facilitate the design of therapeutic strategies that exploit the differences in DSB repair between normal and cancer cells. Here, we discuss the pathways of DSB repair, alterations in DSB repair in cancer, inhibitors of DSB repair and future directions for cancer therapies that target DSB repair.

Using synthetic DNA interstrand crosslinks to elucidate repair pathways and identify new therapeutic targets for cancer chemotherapy

Many cancer chemotherapeutic agents form DNA interstrand crosslinks (ICLs), extremely cytotoxic lesions that form covalent bonds between two opposing DNA strands, blocking DNA replication and transcription. However, cellular responses triggered by ICLs can cause resistance in tumor cells, limiting the efficacy of such treatment. Here we discuss recent advances in our understanding of the mechanisms of ICL repair that cause this resistance. The recent development of strategies for the synthesis of site-specific ICLs greatly contributed to these insights. Key features of repair are similar for all ICLs, but there is increasing evidence that the specifics of lesion recognition and synthesis past ICLs by DNA polymerases are dependent upon the structure of ICLs. These new insights provide a basis for the improvement of antitumor therapy by targeting DNA repair pathways that lead to resistance to treatment with crosslinking agents.

RAD51C: a novel cancer susceptibility gene is linked to Fanconi anemia and breast cancer

Germline mutations in many of the genes that are involved in homologous recombination (HR)-mediated DNA double-strand break repair (DSBR) are associated with various human genetic disorders and cancer. RAD51 and RAD51 paralogs are important for HR and in the maintenance of genome stability. Despite the identification of five RAD51 paralogs over a decade ago, the molecular mechanism(s) by which RAD51 paralogs regulate HR and genome maintenance remains obscure. In addition to the known roles of RAD51C in early and late stages of HR, it also contributes to activation of the checkpoint kinase CHK2. One recent study identifies biallelic mutation in RAD51C leading to Fanconi anemia-like disorder. Whereas a second study reports monoallelic mutation in RAD51C associated with increased risk of breast and ovarian cancer. These reports show RAD51C is a cancer susceptibility gene. In this review, we focus on describing the functions of RAD51C in HR, DNA damage signaling and as a tumor suppressor with an emphasis on the new roles of RAD51C unveiled by these reports.

Role for the mammalian Swi5-Sfr1 complex in DNA strand break repair through homologous recombination

In fission yeast, the Swi5-Sfr1 complex plays an important role in homologous recombination (HR), a pathway crucial for the maintenance of genomic integrity. Here we identify and characterize mammalian Swi5 and Sfr1 homologues. Mouse Swi5 and Sfr1 are nuclear proteins that form a complex in vivo and in vitro. Swi5 interacts in vitro with Rad51, the DNA strand-exchange protein which functions during HR. By generating Swi5^−/− and Sfr1^−/− embryonic stem cell lines, we found that both proteins are mutually interdependent for their stability. Importantly, the Swi5-Sfr1 complex plays a role in HR when Rad51 function is perturbed in vivo by expression of a BRC peptide from BRCA2. Swi5^−/− and Sfr1^−/− cells are selectively sensitive to agents that cause DNA strand breaks, in particular ionizing radiation, camptothecin, and the Parp inhibitor olaparib. Consistent with a role in HR, sister chromatid exchange induced by Parp inhibition is attenuated in Swi5^−/− and Sfr1^−/− cells, and chromosome aberrations are increased. Thus, Swi5-Sfr1 is a newly identified complex required for genomic integrity in mammalian cells with a specific role in the repair of DNA strand breaks.

Our genome constantly undergoes DNA damage as a result of agents in the environment, as well as from metabolic processes. One method of repairing DNA damage is homologous recombination (HR), in which genetic information from a duplicate sequence (the sister chromatid) is copied into the damaged site in DNA. In model organisms (the yeasts), a protein complex termed Swi5-Sfr1 functions in DNA damage repair by HR. In this study, we characterize mouse homologues of this complex. We find that mouse cells lacking this complex are sensitive to DNA damaging agents, in particular, those that cause breaks in DNA strands and that serve as cancer chemotherapeutics. These cells also have increased numbers of chromosome aberrations when exposed to DNA damaging agents. Moreover, HR is decreased in Swi5 and Sfr1 mutant cells under conditions where the cell is challenged. Together, these results demonstrate a requirement for the Swi5-Sfr1 protein complex in maintaining genomic integrity in mammalian cells.

SG2285, a novel C2-aryl-substituted pyrrolobenzodiazepine dimer prodrug that cross-links DNA and exerts highly potent antitumor activity

The pyrrolobenzodiazepines (PBD) are naturally occurring antitumor antibiotics, and a PBD dimer (SJG-136, SG2000) is in phase II trials. Many potent PBDs contain a C2-endo-exo unsaturated motif associated with the pyrrolo C-ring. The novel compound SG2202 is a PBD dimer containing this motif. SG2285 is a water-soluble prodrug of SG2202 in which two bisulfite groups inactivate the PBD N10-C11 imines. Once the bisulfites are eliminated, the imine moieties can bind covalently in the DNA minor groove, forming an interstrand cross-link. The mean in vitro cytotoxic potency of SG2285 against human tumor cell lines is GI(50) 20 pmol/L. SG2285 is highly efficient at producing DNA interstrand cross-links in cells, but they form more slowly than those produced by SG2202. Cellular sensitivity to SG2285 was primarily dependent on ERCC1 and homologous recombination repair. In primary B-cell chronic lymphocytic leukemia samples, the mean LD(50) was significantly lower than in normal age-matched B and T lymphocytes. Antitumor activity was shown in several human tumor xenograft models, including ovarian, non-small cell lung, prostate, pancreatic, and melanoma, with cures obtained in the latter model with a single dose. Further, in an advanced-stage colon model, SG2285 administered either as a single dose, or in two repeat dose schedules, was superior to irinotecan. Our findings define SG2285 as a highly active cytotoxic compound with antitumor properties desirable for further development.

Sequence conversion by single strand oligonucleotide donors via non-homologous end joining in mammalian cells

Double strand breaks (DSBs) can be repaired by homology independent nonhomologous end joining (NHEJ) pathways involving proteins such as Ku70/80, DNAPKcs, Xrcc4/Ligase 4, and the Mre11/Rad50/Nbs1 (MRN) complex. DSBs can also be repaired by homology-dependent pathways (HDR), in which the MRN and CtIP nucleases produce single strand ends that engage homologous sequences either by strand invasion or strand annealing. The entry of ends into HDR pathways underlies protocols for genomic manipulation that combine site-specific DSBs with appropriate informational donors. Most strategies utilize long duplex donors that participate by strand invasion. Work in yeast indicates that single strand oligonucleotide (SSO) donors are also active, over considerable distance, via a single strand annealing pathway. We examined the activity of SSO donors in mammalian cells at DSBs induced either by a restriction nuclease or by a targeted interstrand cross-link. SSO donors were effective immediately adjacent to the break, but activity declined sharply beyond approximately 100 nucleotides. Overexpression of the resection nuclease CtIP increased the frequency of SSO-mediated sequence modulation distal to the break site, but had no effect on the activity of an SSO donor adjacent to the break. Genetic and in vivo competition experiments showed that sequence conversion by SSOs in the immediate vicinity of the break was not by strand invasion or strand annealing pathways. Instead these donors competed for ends that would have otherwise entered NHEJ pathways.

Rad51C is essential for embryonic development and haploinsufficiency causes increased DNA damage sensitivity and genomic instability

Homologous recombination is essential for repair of DNA interstrand cross-links and double-strand breaks. The Rad51C protein is one of the five Rad51 paralogs in vertebrates implicated in homologous recombination. A previously described hamster cell mutant defective in Rad51C (CL-V4B) showed increased sensitivity to DNA damaging agents and displayed genomic instability. Here, we identified a splice donor mutation at position +5 of intron 5 of the Rad51C gene in this mutant, and generated mice harboring an analogous base pair alteration. Rad51C(splice) heterozygous animals are viable and do not display any phenotypic abnormalities, however homozygous Rad51C(splice) embryos die during early development (E8.5). Detailed analysis of two CL-V4B revertants, V4B-MR1 and V4B-MR2, that have reduced levels of full-length Rad51C transcript when compared to wild type hamster cells, showed increased sensitivity to mitomycin C (MMC) in clonogenic survival, suggesting haploinsufficiency of Rad51C. Similarly, mouse Rad51C(splice/neo) heterozygous ES cells also displayed increased MMC sensitivity. Moreover, in both hamster revertants, Rad51C haploinsufficiency gives rise to increased frequencies of spontaneous and MMC-induced chromosomal aberrations, impaired sister chromatid cohesion and reduced cloning efficiency. These results imply that adequate expression of Rad51C in mammalian cells is essential for maintaining genomic stability and sister chromatid cohesion to prevent malignant transformation.

The importance of XRCC2 in RAD51-related DNA damage repair

The repair of DNA damage by homologous recombination (HR) is a key pathway for the maintenance of genetic stability in mammalian cells, especially during and following DNA replication. The central HR protein is RAD51, which ensures high fidelity DNA repair by facilitating strand exchange between damaged and undamaged homologous DNA segments. Several RAD51-like proteins, including XRCC2, appear to help with this process, but their roles are not well understood. Here we show that XRCC2 is highly conserved and that most substantial truncations of the protein destroy its ability to function. XRCC2 and its partner protein RAD51L3 are found to interact with RAD51 in the 2-hybrid system, and XRCC2 is shown to be important but not essential for the accumulation of RAD51 at the sites of DNA damage. We visualize the localization of XRCC2 protein at the same sites of DNA damage for the first time using specialized irradiation conditions. Our data indicate that an important function of XRCC2 is to enhance the activity of RAD51, so that the loss of XRCC2 results in a severe delay in the early response of RAD51 to DNA damage.

BRCA1 and XRCC1 polymorphisms associated with survival in advanced gastric cancer treated with taxane and cisplatin

This study evaluated the influence of genetic polymorphism influencing drug metabolism on survival in taxane- and cisplatin-treated advanced gastric cancer (AGC). Peripheral blood samples from 207 AGC patients treated with first-line chemotherapy of taxane and cisplatin were used. We investigated polymorphisms that influenced the metabolism of taxane (ATP-binding cassette transporter B1 (ABCB1)), cisplatin (glutathione S-transferase M1 (GSTM1), glutathione S-transferase P1 (GSTP1), glutathione S-transferase T1 (GSTT1), excision repair cross complementing 1 (ERCC1), X-ray Cross Complementing group 3 (XRCC3), X-ray Cross Complementing group 4 (XRCC4), X-ray Cross Complementing group 1 (XRCC1), breast cancer (BRCA1)), and 5-fluorouracil (methylene tetrahydrofolate reductase (MTHFR), thymidylate synthase (TYMS)). A total of 207 patients were enrolled between May 2004 and Dec 2008, and 200 patients were analyzed. The overall response rate was 38.5%. Time to progression and overall survival time were 4.3 +/- 0.19 months and 11.9 +/- 1.05 months, respectively. There was no significant association between genetic polymorphism and response rate. However, the BRCA1 mutant TT homozygote was associated with significant prolongation of overall survival (hazard ratio [HR] = 0.43; 95% confidence interval [CI], 0.20-0.92; P = 0.03) and progression-free survival (HR = 0.51; 95% CI, 0.26-1.00; P = 0.05). Also, the XRCC1 194 CT genotype was associated with inferior overall survival, relative to the XRCC1 194 CC homozygotes (HR = 1.49; 95% CI, 0.11-2.07; P = 0.018).These findings suggest that BRCA1 TT and XRCC1 194 CT genotypes could be modest prognostic markers of AGC response in taxane- and cisplatin-treated patients.

Emergence of rationally designed therapeutic strategies for breast cancer targeting DNA repair mechanisms

Accumulating evidence suggests that many cancers, including BRCA1- and BRCA2-associated breast cancers, are deficient in DNA repair processes. Both hereditary and sporadic breast cancers have been found to have significant downregulation of repair factors. This has provided opportunities to exploit DNA repair deficiencies, whether acquired or inherited. Here, we review efforts to exploit DNA repair deficiencies in tumors, with a focus on breast cancer. A variety of agents, including PARP (poly [ADP-ribose] polymerase) inhibitors, are currently under investigation in clinical trials and available results will be reviewed.

53BP1 inhibits homologous recombination in Brca1-deficient cells by blocking resection of DNA breaks

Defective DNA repair by homologous recombination (HR) is thought to be a major contributor to tumorigenesis in individuals carrying Brca1 mutations. Here, we show that DNA breaks in Brca1-deficient cells are aberrantly joined into complex chromosome rearrangements by a process dependent on the nonhomologous end-joining (NHEJ) factors 53BP1 and DNA ligase 4. Loss of 53BP1 alleviates hypersensitivity of Brca1 mutant cells to PARP inhibition and restores error-free repair by HR. Mechanistically, 53BP1 deletion promotes ATM-dependent processing of broken DNA ends to produce recombinogenic single-stranded DNA competent for HR. In contrast, Lig4 deficiency does not rescue the HR defect in Brca1 mutant cells but prevents the joining of chromatid breaks into chromosome rearrangements. Our results illustrate that HR and NHEJ compete to process DNA breaks that arise during DNA replication and that shifting the balance between these pathways can be exploited to selectively protect or kill cells harboring Brca1 mutations.

Human cell toxicogenomic analysis of bromoacetic acid: a regulated drinking water disinfection by-product

The disinfection of drinking water is a major achievement in protecting the public health. However, current disinfection methods also generate disinfection by-products (DBPs). Many DBPs are cytotoxic, genotoxic, teratogenic, and carcinogenic and represent an important class of environmentally hazardous chemicals that may carry long-term human health implications. The objective of this research was to integrate in vitro toxicology with focused toxicogenomic analysis of the regulated DBP, bromoacetic acid (BAA) and to evaluate modulation of gene expression involved in DNA damage/repair and toxic responses, with nontransformed human cells. We generated transcriptome profiles for 168 genes with 30 min and 4 hr exposure times that did not induce acute cytotoxicity. Using qRT-PCR gene arrays, the levels of 25 transcripts were modulated to a statistically significant degree in response to a 30 min treatment with BAA (16 transcripts upregulated and nine downregulated). The largest changes were observed for RAD9A and BRCA1. The majority of the altered transcript profiles are genes involved in DNA repair, especially the repair of double strand DNA breaks, and in cell cycle regulation. With 4 hr of treatment the expression of 28 genes was modulated (12 upregulated and 16 downregulated); the largest fold changes were in HMOX1 and FMO1. This work represents the first nontransformed human cell toxicogenomic study with a regulated drinking water disinfection by-product. These data implicate double strand DNA breaks as a feature of BAA exposure. Future toxicogenomic studies of DBPs will further strengthen our limited knowledge in this growing area of drinking water research.

Detection of novel recombinases in bacteriophage genomes unveils Rad52, Rad51 and Gp2.5 remote homologs

Homologous recombination is a key in contributing to bacteriophages genome repair, circularization and replication. No less than six kinds of recombinase genes have been reported so far in bacteriophage genomes, two (UvsX and Gp2.5) from virulent, and four (Sak, Redβ, Erf and Sak4) from temperate phages. Using profile–profile comparisons, structure-based modelling and gene-context analyses, we provide new views on the global landscape of recombinases in 465 bacteriophages. We show that Sak, Redβ and Erf belong to a common large superfamily adopting a shortcut Rad52-like fold. Remote homologs of Sak4 are predicted to adopt a shortcut Rad51/RecA fold and are discovered widespread among phage genomes. Unexpectedly, within temperate phages, gene-context analyses also pinpointed the presence of distant Gp2.5 homologs, believed to be restricted to virulent phages. All in all, three major superfamilies of phage recombinases emerged either related to Rad52-like, Rad51-like or Gp2.5-like proteins. For two newly detected recombinases belonging to the Sak4 and Gp2.5 families, we provide experimental evidence of their recombination activity in vivo. Temperate versus virulent lifestyle together with the importance of genome mosaicism is discussed in the light of these novel recombinases. Screening for these recombinases in genomes can be performed at http://biodev.extra.cea.fr/virfam.

DNA interstrand crosslink repair in mammalian cells: step by step

Interstrand DNA crosslinks (ICLs) are formed by natural products of metabolism and by chemotherapeutic reagents. Work in E. coli identified a two cycle repair scheme involving incisions on one strand on either side of the ICL (unhooking) producing a gapped intermediate with the incised oligonucleotide attached to the intact strand. The gap is filled by recombinational repair or lesion bypass synthesis. The remaining monoadduct is then removed by nucleotide excision repair (NER). Despite considerable effort, our understanding of each step in mammalian cells is still quite limited. In part this reflects the variety of crosslinking compounds, each with distinct structural features, used by different investigators. Also, multiple repair pathways are involved, variably operative during the cell cycle. G(1) phase repair requires functions from NER, although the mechanism of recognition has not been determined. Repair can be initiated by encounters with the transcriptional apparatus, or a replication fork. In the case of the latter, the reconstruction of a replication fork, stalled or broken by collision with an ICL, adds to the complexity of the repair process. The enzymology of unhooking, the identity of the lesion bypass polymerases required to fill the first repair gap, and the functions involved in the second repair cycle are all subjects of active inquiry. Here we will review current understanding of each step in ICL repair in mammalian cells.

Homologous recombination contributes to the repair of DNA double-strand breaks induced by high-energy iron ions

To test the contribution of homologous recombinational repair (HRR) in repairing DNA damage sites induced by high-energy iron ions, we used (1) HRR-deficient rodent cells carrying a deletion in the RAD51D gene and (2) syngeneic human cells impaired for HRR by RAD51D or RAD51 knockdown using RNA interference. We found that in response to exposure to iron ions, HRR contributed to cell survival in rodent cells and that HRR deficiency abrogated RAD51 focus formation. Complementation of the HRR defect by human RAD51D rescues both enhanced cytotoxicity and RAD51 focus formation. For human cells irradiated with iron ions, cell survival was decreased, and in p53 mutant cells, the levels of mutagenesis were increased when HRR was impaired. Human cells synchronized in S phase exhibited a more pronounced resistance to iron ions compared with cells in G(1) phase, and this increase in radioresistance was diminished by RAD51 knockdown. These results indicate a role for RAD51-mediated DNA repair (i.e. HRR) in removing a fraction of clustered lesions induced by charged-particle radiation. Our results are the first to directly show the requirement for an intact HRR pathway in human cells in ensuring DNA repair and cell survival after exposure to high-energy high-LET radiation.

Cellular radiosensitivity: how much better do we understand it?

PURPOSE

Ionising radiation exposure gives rise to a variety of lesions in DNA that result in genetic instability and potentially tumourigenesis or cell death. Radiation extends its effects on DNA by direct interaction or by radiolysis of H(2)O that generates free radicals or aqueous electrons capable of interacting with and causing indirect damage to DNA. While the various lesions arising in DNA after radiation exposure can contribute to the mutagenising effects of this agent, the potentially most damaging lesion is the DNA double strand break (DSB) that contributes to genome instability and/or cell death. Thus in many cases failure to recognise and/or repair this lesion determines the radiosensitivity status of the cell. DNA repair mechanisms including homologous recombination (HR) and non-homologous end-joining (NHEJ) have evolved to protect cells against DNA DSB. Mutations in proteins that constitute these repair pathways are characterised by radiosensitivity and genome instability. Defects in a number of these proteins also give rise to genetic disorders that feature not only genetic instability but also immunodeficiency, cancer predisposition, neurodegeneration and other pathologies.

CONCLUSIONS

In the past 50 years our understanding of the cellular response to radiation damage has advanced enormously with insight being gained from a wide range of approaches extending from more basic early studies to the sophisticated approaches used today. In this review we discuss our current understanding of the impact of radiation on the cell and the organism gained from the array of past and present studies and attempt to provide an explanation for what it is that determines the response to radiation.

Plant DNA recombinases: a long way to go

DNA homologous recombination is fundamental process by which two homologous DNA molecules exchange the genetic information for the generation of genetic diversity and maintain the genomic integrity. DNA recombinases, a special group of proteins bind to single stranded DNA (ssDNA) nonspecifically and search the double stranded DNA (dsDNA) molecule for a stretch of DNA that is homologous with the bound ssDNA. Recombinase A (RecA) has been well characterized at genetic, biochemical, as well as structural level from prokaryotes. Two homologues of RecA called Rad51 and Dmc1 have been detected in yeast and higher eukaryotes and are known to mediate the homologous recombination in eukaryotes. The biochemistry and mechanism of action of recombinase is important in understanding the process of homologous recombination. Even though considerable progress has been made in yeast and human recombinases, understanding of the plant recombination and recombinases is at nascent stage. Since crop plants are subjected to different breeding techniques, it is important to know the homologous recombination process. This paper focuses on the properties of eukaryotes recombinases and recent developments in the field of plant recombinases Dmc1 and Rad51.

XPF-ERCC1 participates in the Fanconi anemia pathway of cross-link repair

Interstrand cross-links (ICLs) prevent DNA strand separation and, therefore, transcription and replication, making them extremely cytotoxic. The precise mechanism by which ICLs are removed from mammalian genomes largely remains elusive. Genetic evidence implicates ATR, the Fanconi anemia proteins, proteins required for homologous recombination, translesion synthesis, and at least two endonucleases, MUS81-EME1 and XPF-ERCC1. ICLs cause replication-dependent DNA double-strand breaks (DSBs), and MUS81-EME1 facilitates DSB formation. The subsequent repair of these DSBs occurs via homologous recombination after the ICL is unhooked by XPF-ERCC1. Here, we examined the effect of the loss of either nuclease on FANCD2 monoubiquitination to determine if the nucleolytic processing of ICLs is required for the activation of the Fanconi anemia pathway. FANCD2 was monoubiquitinated in Mus81(-/-), Ercc1(-/-), and XPF-deficient human, mouse, and hamster cells exposed to cross-linking agents. However, the monoubiquitinated form of FANCD2 persisted longer in XPF-ERCC1-deficient cells than in wild-type cells. Moreover, the levels of chromatin-bound FANCD2 were dramatically reduced and the number of ICL-induced FANCD2 foci significantly lower in XPF-ERCC1-deficient cells. These data demonstrate that the unhooking of an ICL by XPF-ERCC1 is necessary for the stable localization of FANCD2 to the chromatin and subsequent homologous recombination-mediated DSB repair.

DNA repair gene polymorphisms and prostate cancer risk in South Australia--results of a pilot study

OBJECTIVE

Single nucleotide polymorphisms (SNPs) in DNA repair genes may impact on DNA damage, and cancer risk. To elucidate the role of SNPs in DNA repair genes in prostate cancer (PC) we conducted a case-control study comprising of 118 Caucasian men affected with late onset PC and 132 age-matched healthy controls from South Australia.

METHODS AND MATERIALS

We examined the association between PC risk with nonsynonymous SNPs (nsSNPs) in 5 genes involved in 3 DNA-repair pathways: (1) base excision repair (BER): hOGG1 C1245G (Ser326Cys) and XRCC1 G28152A (Arg399Gln); (2) nucleotide excision repair (NER): XPD G23591A (Asp312Asn); (3) homologous recombination repair: RAD51 G135C (in 5' untranslated region) and XRCC3 C18067T (Thr241Met).

RESULTS

Prostate cancer risk was significantly increased only for carriers of the G allele of the C1245G polymorphism in the hOGG1 gene (OR = 2.28; 95% CI = 1.36-3.83; P = 0.002).

CONCLUSION

Our results suggest that this common nsSNP in a gene involved in repair of oxidative damage to DNA may contribute to PC susceptibility in South Australian men.

Activity of ribonucleotide reductase helps determine how cells repair DNA double strand breaks

Mammalian cells can choose either nonhomologous end joining (NHEJ) or homologous recombination (HR) for repair of chromosome breaks. Of these two pathways, HR alone requires extensive DNA synthesis and thus abundant synthesis precursors (dNTPs). We address here if this differing requirement for dNTPs helps determine how cells choose a repair pathway. Cellular dNTP pools are regulated primarily by changes in ribonucleotide reductase activity. We show that an inhibitor of ribonucleotide reductase (hydroxyurea) hypersensitizes NHEJ-deficient cells, but not wild type or HR-deficient cells, to chromosome breaks introduced by ionizing radiation. Hydroxyurea additionally reduces the frequency of irradiated cells with a marker for an early step in HR, Rad51 foci, consistent with reduced initiation of HR under these conditions. Conversely, promotion of ribonucleotide reductase activity protects NHEJ-deficient cells from ionizing radiation. Importantly, promotion of ribonucleotide reductase activity also increases usage of HR in cells proficient in both NHEJ and HR at a targeted chromosome break. Activity of ribonucleotide reductase is thus an important factor in determining how mammalian cells repair broken chromosomes. This may explain in part why G1/G0 cells, which have reduced ribonucleotide reductase activity, rely more on NHEJ for DSB repair.

Specific targeted gene repair using single-stranded DNA oligonucleotides at an endogenous locus in mammalian cells uses homologous recombination

The feasibility of introducing point mutations in vivo using single-stranded DNA oligonucleotides (ssON) has been demonstrated but the efficiency and mechanism remain elusive and potential side effects have not been fully evaluated. Understanding the mechanism behind this potential therapy may help its development. Here, we demonstrate the specific repair of an endogenous non-functional hprt gene by a ssON in mammalian cells, and show that the frequency of such an event is enhanced when cells are in S-phase of the cell cycle. A potential barrier in using ssONs as gene therapy could be non-targeted mutations or gene rearrangements triggered by the ssON. Both the non-specific mutation frequencies and the frequency of gene rearrangements were largely unaffected by ssONs. Furthermore, we find that the introduction of a mutation causing the loss of a functional endogenous hprt gene by a ssON occurred at a similarly low but statistically significant frequency in wild type cells and in cells deficient in single strand break repair, nucleotide excision repair and mismatch repair. However, this mutation was not induced in XRCC3 mutant cells deficient in homologous recombination. Thus, our data suggest ssON-mediated targeted gene repair is more efficient in S-phase and involves homologous recombination.

Cellular redistribution of Rad51 in response to DNA damage: novel role for Rad51C

Exposure of cells to DNA-damaging agents results in a rapid increase in the formation of subnuclear complexes containing Rad51. To date, it has not been determined to what extent DNA damage-induced cytoplasmic to nuclear transport of Rad51 may contribute to this process. We have analyzed subcellular fractions of HeLa and HCT116 cells and found a significant increase in nuclear Rad51 levels following exposure to a modest dose of ionizing radiation (2 grays). We also observed a DNA damage-induced increase in nuclear Rad51 in the Brca2-defective cell line Capan-1. To address a possible Brca2-independent mechanism for Rad51 nuclear transport, we analyzed subcellular fractions for two other Rad51-interacting proteins, Rad51C and Xrcc3. Rad51C has a functional nuclear localization signal, and although we found that the subcellular distribution of Xrcc3 was not significantly affected by DNA damage, there was a damage-induced increase in nuclear Rad51C. Furthermore, RNA interference-mediated depletion of Rad51C in HeLa and Capan-1 cells resulted in lower steady-state levels of nuclear Rad51 as well as a diminished DNA damage-induced increase. Our results provide important insight into the cellular regulation of Rad51 nuclear entry and a role for Rad51C in this process.

Choreography of recombination proteins during the DNA damage response

Genome integrity is frequently challenged by DNA lesions from both endogenous and exogenous sources. A single DNA double-strand break (DSB) is lethal if unrepaired and may lead to loss of heterozygosity, mutations, deletions, genomic rearrangements and chromosome loss if repaired improperly. Such genetic alterations are the main causes of cancer and other genetic diseases. Consequently, DNA double-strand break repair (DSBR) is an important process in all living organisms. DSBR is also the driving mechanism in most strategies of gene targeting, which has applications in both genetic and clinical research. Here we review the cell biological response to DSBs in mitotically growing cells with an emphasis on homologous recombination pathways in yeast Saccharomyces cerevisiae and in mammalian cells.

Enhanced radiosensitization of human glioma cells by combining inhibition of poly(ADP-ribose) polymerase with inhibition of heat shock protein 90

Glioblastoma multiforme (GBM) are the most common primary brain tumor and are resistant to standard therapies. The nondividing nature of normal brain provides an opportunity to enhance the therapeutic ratio by combining radiation with inhibitors of replication-specific DNA repair pathways. Based on our previous findings that inhibition of poly(ADP-ribose) polymerase (PARP) increases radiosensitivity of human glioma cells in a replication-dependent manner and generates excess DNA breaks that are repaired by homologous recombination (HR), we hypothesized that inhibition of HR would amplify the replication-specific radiosensitizing effects of PARP inhibition. Specific inhibitors of HR are not available, but the heat shock protein 90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) has been reported to inhibit HR function. The radiosensitizing effects of 17-AAG and the PARP inhibitor olaparib were assessed, and the underlying mechanisms explored. 17-AAG down-regulated Rad51 and BRCA2 protein levels, abrogated induction of Rad51 foci by radiation, and inhibited HR measured by the I-Sce1 assay. Individually, 17-AAG and olaparib had modest, replication-dependent radiosensitizing effects on T98G glioma cells. Additive radiosensitization was observed with combination treatment, mirrored by increases in gammaH2AX foci in G(2)-phase cells. Unlike olaparib, 17-AAG did not increase radiation sensitivity of Chinese hamster ovary cells, indicating tumor specificity. However, 17-AAG also enhanced radiosensitivity in HR-deficient cells, indicating that its effects were only partially mediated by HR inhibition. Additional mechanisms are likely to include destabilization of oncoproteins that are up-regulated in GBM. 17-AAG is therefore a tumor-specific, replication-dependent radiosensitizer that enhances the effects of PARP inhibition. This combination has therapeutic potential in the management of GBM.

Cellular and molecular consequences of defective Fanconi anemia proteins in replication-coupled DNA repair: mechanistic insights

The Fanconi anemia (FA) molecular network consists of 15 "FANC" proteins, of which 13 are associated with mutations in patients with this cancer-prone chromosome instability disorder. Whereas historically the common phenotype associated with FA mutations is marked sensitivity to DNA interstrand crosslinking agents, the literature supports a more global role for FANC proteins in coping with diverse stresses encountered by replicative polymerases. We have attempted to reconcile and integrate numerous observations into a model in which FANC proteins coordinate the following physiological events during DNA crosslink repair: (a) activating a FANCM-ATR-dependent S-phase checkpoint, (b) mediating enzymatic replication-fork breakage and crosslink unhooking, (c) filling the resulting gap by translesion synthesis (TLS) by error-prone polymerase(s), and (d) restoring the resulting one-ended double-strand break by homologous recombination repair (HRR). The FANC core subcomplex (FANCA, B, C, E, F, G, L, FAAP100) promotes TLS for both crosslink and non-crosslink damage such as spontaneous oxidative base damage, UV-C photoproducts, and alkylated bases. TLS likely helps prevent stalled replication forks from breaking, thereby maintaining chromosome continuity. Diverse DNA damages and replication inhibitors result in monoubiquitination of the FANCD2-FANCI complex by the FANCL ubiquitin ligase activity of the core subcomplex upon its recruitment to chromatin by the FANCM-FAAP24 heterodimeric translocase. We speculate that this translocase activity acts as the primary damage sensor and helps remodel blocked replication forks to facilitate checkpoint activation and repair. Monoubiquitination of FANCD2-FANCI is needed for promoting HRR, in which the FANCD1/BRCA2 and FANCN/PALB2 proteins act at an early step. We conclude that the core subcomplex is required for both TLS and HRR occurring separately for non-crosslink damages and for both events during crosslink repair. The FANCJ/BRIP1/BACH1 helicase functions in association with BRCA1 and may remove structural barriers to replication, such as guanine quadruplex structures, and/or assist in crosslink unhooking.

RAD51D- and FANCG-dependent base substitution mutagenesis at the ATP1A1 locus in mammalian cells

Elaborate processes act at the DNA replication fork to minimize the generation of chromatid discontinuity when lesions are encountered. To prevent collapse of stalled replication forks, mutagenic translesion synthesis (TLS) polymerases are recruited temporarily to bypass DNA lesions. When a replication-associated (one-ended) double-strand break occurs, homologous recombination repair (HRR) can restore chromatid continuity in what has traditionally been regarded as an "error-free" process. Our previous mutagenesis studies show an important role for HRR in preventing deletions and rearrangements that would otherwise result from error-prone nonhomologous end joining (NHEJ) after fork breakage. An analogous, but distinct, role in minimizing mutations is attributed to the proteins defective in the cancer predisposition disease Fanconi anemia (FA). Cells from FA patients and model systems show an increased proportion of gene-disrupting deletions at the hprt locus as well as decreased mutation rates in the hprt assay, suggesting a role for the FANC proteins in promoting TLS, HRR, and possibly also NHEJ. It remains unclear whether HRR, like the FANC pathway, impacts the rate of base substitution mutagenesis. Therefore, we measured, in isogenic rad51d and fancg CHO mutants, mutation rates at the Na(+)/K(+)-ATPase alpha-subunit (ATP1A1) locus using ouabain resistance, which specifically detects base substitution mutations. Surprisingly, we found that the spontaneous mutation rate was reduced approximately 2.5-fold in rad51d knockout cells, an even greater extent than observed in fancg cells, when compared with parental and isogenic gene-complemented control lines. A approximately 2-fold reduction in induced mutations in rad51d cells was seen after treatment with the DNA alkylating agent ethylnitrosurea while a lesser reduction occurred in fancg cells. Should the model ATP1A1 locus be representative of the genome, we conclude that at least 50% of base substitution mutations in this mammalian system arise through error-prone polymerase(s) acting during HRR-mediated restart of broken replication forks.