DNA double-strand breaks: signaling, repair and the cancer connection

An investigation on the polymorphisms of two DNA repair genes and susceptibility to ESCC and GCA of high-incidence region in northern China

AIM

To investigate the possible association of three SNPs, XRCC2 C41657T, XRCC2 G4234C and XRCC3 A17893G with susceptibility to esophageal squamous cell carcinoma (ESCC) and gastric cardia adenocarcinoma (GCA) in a population of northern China.

METHODS

XRCC2 C41657T, XRCC2 G4234C and XRCC3 A17893G SNP were genotyped by polymerase-chain reaction (PCR)-restriction fragment length polymorphism (RFLP) analysis in 583 cancer patients (329 ESCC and 254 GCA) and 614 healthy controls.

RESULTS

The genotype distribution of the XRCC2 C41657T in ESCC and GCA patients were significantly different from that in healthy controls (P values = 0.04 and 0.04 respectively). And a significant difference was found in the allele distribution of GCA patients from that in controls (P = 0.01). The XRCC2 C41657T polymorphism was associated with a modest enhancement in ESCC risk and GCA risk: OR for C/T genotype was 1.38 (1.01-1.89) in GCA risk and for T/T genotype was 2.24 (1.10-4.57) in ESCC risk. When stratified for age, smoking status and family history of UGIC, the C/T genotype showed a modest significant trend on the risk of GCA patients in the groups of age < or =50 years and non-smokers, the adjusted OR were 2.84 (1.21-6.66) and 1.62 (1.06-2.49). The T/T genotype significantly increased the susceptibility of GCA patients in negative family history of UGIC (3.04, 1.02-8.32) and to ESCC patients in the group of age >50 years (3.03, 1.31-6.98), Negative family of UGIC (3.03, 1.12-7.07) and smokers (2.64, 1.02-6.83). The genotype and allele distribution of XRCC2 G4234C and XRCC3 A17893G in ESCC and GCA patients were not significantly different from that in healthy controls (all P values were above 0.05).

CONCLUSION

In this study, we found that the C41657T polymorphism of XRCC2 genes might modify the risk of ESCC and GCA development.

Accumulation of Ku80 proteins at DNA double-strand breaks in living cells

Ku plays a key role in multiple nuclear processes, e.g., DNA double-strand break (DSB) repair. The regulation mechanism of the localizations of Ku70 and Ku80 plays a key role in regulating the multiple functions of Ku. Although numerous biochemical studies in vitro have elucidated the DNA binding mechanism of Ku, no accumulation mechanisms of Ku70 and Ku80 at DSBs have been clarified in detail in vivo. In this study, we examined the accumulation mechanism of Ku80 at DSBs in living cells. EGFP-Ku80 accumulation at DSBs began immediately after irradiation. On the other hand, our data show that Ku70 alone, which has DNA binding activity independent of Ku80, cannot accumulate at the DSBs, whereas Ku70 bound to Ku80 can. The deletion of the C-terminal DNA-PKcs-binding domain and the mutation at the SUMOylation site of Ku80 had no effect on Ku80 accumulation. Unexpectedly, N-terminal deletion mutants of Ku80 fully lost their accumulation activity, although the mutants retained their Ku70 binding activity. Altogether, these data demonstrate that Ku80 is essential for Ku70 accumulation at DSBs. Furthermore, three domains of Ku80, i.e., the N-terminal alpha/beta, the DNA-binding, and Ku70-binding domains, seem to necessary for the accumulation at or recognition of DSBs in the early stage after irradiation.

Reactive oxygen species, DNA damage, and error-prone repair: a model for genomic instability with progression in myeloid leukemia?

Myelodysplastic syndromes (MDS) comprise a heterogeneous group of disorders characterized by ineffective hematopoiesis, with an increased propensity to develop acute myelogenous leukemia (AML). The molecular basis for MDS progression is unknown, but a key element in MDS disease progression is loss of chromosomal material (genomic instability). Using our two-step mouse model for myeloid leukemic disease progression involving overexpression of human mutant NRAS and BCL2 genes, we show that there is a stepwise increase in the frequency of DNA damage leading to an increased frequency of error-prone repair of double-strand breaks (DSB) by nonhomologous end-joining. There is a concomitant increase in reactive oxygen species (ROS) in these transgenic mice with disease progression. Importantly, RAC1, an essential component of the ROS-producing NADPH oxidase, is downstream of RAS, and we show that ROS production in NRAS/BCL2 mice is in part dependent on RAC1 activity. DNA damage and error-prone repair can be decreased or reversed in vivo by N-acetyl cysteine antioxidant treatment. Our data link gene abnormalities to constitutive DNA damage and increased DSB repair errors in vivo and provide a mechanism for an increase in the error rate of DNA repair with MDS disease progression. These data suggest treatment strategies that target RAS/RAC pathways and ROS production in human MDS/AML.

DNA repair polymorphisms and outcome of chemotherapy for acute myelogenous leukemia: a report from the Children's Oncology Group

Polymorphisms of DNA repair genes RAD51 and XRCC3 increase susceptibility to acute myeloid leukemia (AML) in adults, an effect enhanced by deletion of the glutathione-S-transferase M1 (GSTM1) gene. In this study, we genotyped 452 children with de novo AML treated on CCG protocols 2941 and 2961 and compared genotype frequencies with those of normal blood donors, and analyzed the impact of genotype on outcome of therapy. XRCC3 Thr241Met, RAD51 G135C and GSTM1 genotypes did not increase susceptibility to AML when assessed singly. In contrast, when XRCC3 and RAD51 genotypes were examined together a significant increase in susceptibility to AML was seen in children with variant alleles. Analysis of outcome of therapy showed that patients heterozygous for the XRCC3 Thr241Met allele had improved post-induction disease-free survival compared to children homozygous for the major or minor allele, each of whom had similar outcomes. Improved survival was due to reduced relapse in the heterozygous children, and this effect was most marked in children randomized to therapy likely to generate DNA double-strand breaks (etoposide, daunomycin), compared with anti-metabolite (fludarabine, cytarabine) based therapy. In contrast, RAD51 G135C and the GSTM1 deletion polymorphism did not influence outcome of AML therapy in our study population.

A YY1-INO80 complex regulates genomic stability through homologous recombination-based repair

DNA damage repair is crucial for the maintenance of genome integrity and cancer suppression. We found that loss of the mouse transcription factor YY1 resulted in polyploidy and chromatid aberrations, which are signatures of defects in homologous recombination. Further biochemical analyses identified a YY1 complex comprising components of the evolutionarily conserved INO80 chromatin-remodeling complex. Notably, RNA interference-mediated knockdown of YY1 and INO80 increased cellular sensitivity toward DNA-damaging agents. Functional assays revealed that both YY1 and INO80 are essential in homologous recombination-based DNA repair (HRR), which was further supported by the finding that YY1 preferentially bound a recombination-intermediate structure in vitro. Collectively, these observations reveal a link between YY1 and INO80 and roles for both in HRR, providing new insight into mechanisms that control the cellular response to genotoxic stress.

Telomere length inheritance and aging

Telomere shortening accompanies human aging, and premature aging syndromes are often associated with short telomeres. These two observations are central to the hypothesis that telomere length directly influences longevity. If true, genetically determined mechanisms of telomere length homeostasis should significantly contribute to variations of longevity in the human population. On the other hand, telomere shortening is also observed in the course of many aging-associated disorders but determining whether it is a cause or a consequence is not an easy task. Here, we review the most relevant experimental and descriptive data relating telomere length, as a quantitative trait, to aging and longevity.

Suppression of gross chromosomal rearrangements by a new alternative replication factor C complex

Defects in DNA replication fidelity lead to genomic instability. Gross chromosomal rearrangement (GCR), a type of genomic instability, is highly enhanced by various initial mutations affecting DNA replication. Frequent observations of GCRs in many cancers strongly argue the importance of maintaining high fidelity of DNA replication to suppress carcinogenesis. Recent genome wide screens in Saccharomyces cerevisiae identified a new GCR suppressor gene, ELG1, enhanced level of genome instability gene 1. Its physical interaction with proliferating cell nuclear antigen (PCNA) and complex formation with Rfc2-5p proteins suggest that Elg1 functions to load/unload PCNA onto DNA during a certain DNA metabolism. High level of DNA damage accumulation and enhanced phenotypes with mutations in genes involved in cell cycle checkpoints, homologous recombination (HR), or chromatin assembly in the elg1 strain suggest that Elg1p-Rfc2-5p functions in a fundamental DNA metabolism to suppress genomic instability.

Dysfunctional telomeres activate an ATM-ATR-dependent DNA damage response to suppress tumorigenesis

The POT1 (protection of telomeres) protein binds the single-stranded G-rich overhang and is essential for both telomere end protection and telomere length regulation. Telomeric binding of POT1 is enhanced by its interaction with TPP1. In this study, we demonstrate that mouse Tpp1 confers telomere end protection by recruiting Pot1a and Pot1b to telomeres. Knockdown of Tpp1 elicits a p53-dependent growth arrest and an ATM-dependent DNA damage response at telomeres. In contrast to depletion of Trf2, which activates ATM, removal of Pot1a and Pot1b from telomeres initiates an ATR-dependent DNA damage response (DDR). Finally, we show that telomere dysfunction as a result of Tpp1 depletion promotes chromosomal instability and tumorigenesis in the absence of an ATM-dependent DDR. Our results uncover a novel ATR-dependent DDR at telomeres that is normally shielded by POT1 binding to the single-stranded G-overhang. In addition, our results suggest that loss of ATM can cooperate with dysfunctional telomeres to promote cellular transformation and tumor formation in vivo.

ATM kinase activity modulates Fas sensitivity through the regulation of FLIP in lymphoid cells

Ataxia telangiectasia (A-T) is a rare cancer-predisposing genetic disease, caused by the lack of functional ATM kinase, a major actor of the double strand brakes (DSB) DNA-damage response. A-T patients show a broad and diverse phenotype, which includes an increased rate of lymphoma and leukemia development. Fas-induced apoptosis plays a fundamental role in the homeostasis of the immune system and its defects have been associated with autoimmunity and lymphoma development. We therefore investigated the role of ATM kinase in Fas-induced apoptosis. Using A-T lymphoid cells, we could show that ATM deficiency causes resistance to Fas-induced apoptosis. A-T cells up-regulate FLIP protein levels, a well-known inhibitor of Fas-induced apoptosis. Reconstitution of ATM kinase activity was sufficient to decrease FLIP levels and to restore Fas sensitivity. Conversely, genetic and pharmacologic ATM kinase inactivation resulted in FLIP protein up-regulation and Fas resistance. Both ATM and FLIP are aberrantly regulated in Hodgkin lymphoma. Importantly, we found that reconstitution of ATM kinase activity decreases FLIP protein levels and restores Fas sensitivity in Hodgkin lymphoma-derived cells. Overall, these data identify a novel molecular mechanism through which ATM kinase may regulate the immune system homeostasis and impair lymphoma development.

Deficient repair of particulate hexavalent chromium-induced DNA double strand breaks leads to neoplastic transformation

Hexavalent chromium (Cr(VI)) is a potent respiratory toxicant and carcinogen. The most carcinogenic forms of Cr(VI) are the particulate salts such as lead chromate, which deposit and persist in the respiratory tract after inhalation. We demonstrate here that particulate chromate induces DNA double strand breaks in human lung cells with 0.1, 0.5, and 1 microg/cm(2) lead chromate inducing 1.5, 2, and 5 relative increases in the percent of DNA in the comet tail, respectively. These lesions are repaired within 24 h and require Mre11 expression for their repair. Particulate chromate also caused Mre11 to co-localize with gamma-H2A.X and ATM. Failure to repair these breaks with Mre11-induced neoplastic transformation including loss of cell contact inhibition and anchorage-independent growth. A 5-day exposure to lead chromate induced loss of cell contact inhibition in a concentration-dependent manner with 0, 0.1, 0.5, and 1 microg/cm(2) lead chromate inducing 1, 78, and 103 foci in 20 dishes, respectively. These data indicate that Mre11 is critical to repairing particulate Cr(VI)-induced double strand breaks and preventing Cr(VI)-induced neoplastic transformation.

Breast cancer risk is associated with the genes encoding the DNA double-strand break repair Mre11/Rad50/Nbs1 complex

The evolutionarily conserved Mre11-Rad50-Nbs1 (MRN) complex, consisting of proteins encoded by the genes Mre11, Rad50, and Nbs1, was recently shown to play a crucial role in DNA double-strand break (DSB) repair by recruiting the nuclear protein kinase ataxia telangiectasia mutated to DSB sites, leading to activation of this DNA repair network. Given the fact that carriers of defective mutation and polymorphic variants of ataxia telangiectasia mutated are at higher risk of developing breast cancer, we hypothesized a role of the MRN genes in determining breast cancer susceptibility. This hypothesis was examined both in a case control study of 559 breast cancer patients and 1,125 healthy women of single-nucleotide polymorphisms in Mre11, Rad50, and Nbs1 and by the in vivo detection of binding between Mre11 and BRCA1, encoded by the breast cancer susceptibility gene BRCA1. We were also interested in defining whether any association between MRN genes and breast cancer was modified by reproductive risk factors reflecting the level of estrogen exposure or susceptibility to estrogen exposure, as estrogen is known to initiate breast cancer development due to its metabolites causing DSB formation. Support for the hypothesis came from the observations that (a) one single-nucleotide polymorphism in Nbs1 was significantly associated with breast cancer risk, and a trend toward an increased risk of developing breast cancer was found in women harboring a greater number of putative high-risk genotypes of MRN genes (an adjusted odds ratio of 1.25 for each additional putative high-risk genotype; 95% confidence interval, 1.10-1.44); (b) this association between risk and the number of putative high-risk genotypes was stronger and more significant in women thought to be more susceptible to estrogen, i.e., those with no history of full-term pregnancy, those older (>or=26 years of age) at first full-term pregnancy, or those having had fewer (<2) full-term pregnancies; the risk effect conferred by harboring a higher number of high-risk genotypes of MRN genes was more significant in women without a history of breast feeding; and (c) Mre11 and BRCA1 were shown to form a complex in vivo, and this interaction was increased by irradiation. This study supports the role of the MRN pathway in breast cancer development, further strengthening the suggestion that mechanisms regulating DSB repair may play a mutator role driving breast cancer pathogenesis.

Radioprotection of 4-hydroxy-3,5-dimethoxybenzaldehyde (VND3207) in culture cells is associated with minimizing DNA damage and activating Akt

Vanillin is a naturally occurring compound and food-flavoring agent with antioxidant and antimutagenic activities. In present study, we explored the radioprotective effect of a novel vanillin derivative VND3207 (4-hydroxy-3,5-dimethoxybenzaldehyde). VND3207 has a much higher potential in scavenging hydroxyl radical and superoxide radical than vanillin as indicated in the ESR spin-trapping measurement, and it can effectively protect plasmid DNA against 10-50 Gy gamma-ray induced breaks in vitro at the concentrations as low as 10-20 microM. Using human lymphoblastoid AHH-1 cells and human fibroblastoid HFS cells, we demonstrated that VND3207 at 5-40 microM concentrations significantly attenuated the inhibition of proliferation and occurrence of apoptosis produced by 1-8 Gy gamma-irradiation. In the cultured cells, VND3207 significantly decreased the initial production and residual level of DNA double-strand breaks (DSBs) induced by 2 or 8 Gy irradiation. Treatment of VND3207 enhanced the level of DNA-PKcs protein, a critical component of DNA DSB repair pathway in the cells with or without gamma-irradiation. Consistently, the phosphorylation of Akt protein, a mediator of survival signal, as well as its substrate GSK3beta was concurrently increased by VND3207. Our results suggest that VND3207 has radioprotection effect through its capabilities as a powerful antioxidant, in minimizing DNA damage, and activating survival signal Akt pathway, and it may be of value in the development of radioprotective compounds.

Low dose X-radiation adaptive response in spleen and prostate of Atm knockout heterozygous mice

PURPOSE

To investigate the effect of being heterozygous for a knockout mutation in the ataxia telangiectasia (Atm) gene on radiation adaptive response.

MATERIALS AND METHODS

DNA recombination, as measured by pKZ1 inversion frequency, was quantified by histochemistry in Atm knockout heterozygous prostate and spleen 3 days after treatment with a priming dose of 0.01 or 10 mGy X-radiation 4 h prior to a challenge dose of 1,000 mGy.

RESULTS

In spleen and prostate, a single dose of 0.01 mGy caused an induction in inversion frequency but a dose of 10 mGy prevented the induction of a proportion of endogenous inversions. Both doses induced an adaptive response, of similar magnitude, to a subsequent high challenge dose for chromosomal inversions in both spleen and prostate. The adaptive response completely prevented the induction of inversions from a 1,000 mGy challenge dose and also a proportion of endogenous inversions. The adaptive responses and distribution of inversions across gland cross-sections observed here in Atm knockout heterozygote prostate were similar to those induced in Atm wild-type prostate in a previous study.

CONCLUSIONS

Being heterozygous for a knockout mutation in the Atm gene does not affect the endogenous pKZ1 inversion frequency, the inversion response to single low radiation doses used here, or the induction of a radiation adaptive response for inversions in pKZ1 mouse spleen or prostate.

Attenuated DNA damage repair by trichostatin A through BRCA1 suppression

Recent studies have demonstrated that some histone deacetylase (HDAC) inhibitors enhance cellular radiation sensitivity. However, the underlying mechanism for such a radiosensitizing effect remains unexplored. Here we show evidence that treatment with the HDAC inhibitor trichostatin A (TSA) impairs radiation-induced repair of DNA damage. The effect of TSA on the kinetics of DNA damage repair was measured by performing the comet assay and gamma-H2AX focus analysis in radioresistant human squamous carcinoma cells (SQ-20B). TSA exposure increased the amount of radiation-induced DNA damage and slowed the repair kinetics. Gene expression profiling also revealed that a majority of the genes that control cell cycle, DNA replication and damage repair processes were down-regulated after TSA exposure, including BRCA1. The involvement of BRCA1 was further demonstrated by expressing ectopic wild-type BRCA1 in a BRCA1 null cell line (HCC-1937). TSA treatment enhanced radiation sensitivity of HCC-1937/wtBRCA1 clonal cells, which restored cellular radiosensitivity (D(0) = 1.63 Gy), to the control level (D(0) = 1.03 Gy). However, TSA had no effect on the level of radiosensitivity of BRCA1 null cells. Our data demonstrate for the first time that TSA treatment modulates the radiation-induced DNA damage repair process, in part by suppressing BRCA1 gene expression, suggesting that BRCA1 is one of molecular targets of TSA.

Aberrant V(D)J recombination is not required for rapid development of H2ax/p53-deficient thymic lymphomas with clonal translocations

Histone H2AX is required to maintain genomic stability in cells and to suppress malignant transformation of lymphocytes in mice. H2ax(-/-)p53(-/-) mice succumb predominantly to immature alphabeta T-cell lymphomas with translocations, deletions, and genomic amplifications that do not involve T-cell receptor (TCR). In addition, H2ax(-/-)p53(-/-) mice also develop at lower frequencies B and T lymphomas with antigen receptor locus translocations. V(D)J recombination is initiated through the programmed induction of DNA double-strand breaks (DSBs) by the RAG1/RAG2 endonuclease. Because promiscuous RAG1/RAG2 cutting outside of antigen receptor loci can promote genomic instability, H2ax(-/-)p53(-/-) T-lineage lymphomas might arise, at least in part, through erroneous V(D)J recombination. Here, we show that H2ax(-/-)p53(-/-)Rag2(-/-) mice exhibit a similar genetic predisposition as do H2ax(-/-)p53(-/-) mice to thymic lymphoma with translocations, deletions, and amplifications. We also found that H2ax(-/-)p53(-/-)Rag2(-/-) mice often develop thymic lymphomas with loss or deletion of the p53(+) locus. Our data show that aberrant V(D)J recombination is not required for rapid onset of H2ax/p53-deficient thymic lymphomas with genomic instability and that H2ax deficiency predisposes p53(-/-)Rag2(-/-) thymocytes to transformation associated with p53 inactivation. Thus, H2AX is essential for suppressing the transformation of developing thymocytes arising from the aberrant repair of spontaneous DSBs.

Genetic variants in the H2AFX promoter region are associated with risk of sporadic breast cancer in non-Hispanic white women aged <or=55 years

The histone protein family member X (H2AFX) is important in maintaining chromatin structure and genetic stability. Genetic variants in H2AFX may alter protein functions and thus cancer risk. In this case-control study, we genotyped four common single nucleotide polymorphisms (i.e., -1654A > G [rs643788], -1420G > A [rs8551], and -1187T > C [rs7759] in the H2AFX promoter region and 1057C > T [rs7350] in the 3' untranslated region (UTR)) in 467 patients with sporadic breast cancer and 488 cancer-free controls. All female subjects were non-Hispanic whites aged <or=55 years. We found that significantly increased risk of breast cancer was associated with variant genotypes in the H2AFX promoter: adjusted odds ratio [OR] = 1.80, 95% confidence interval [CI] = 1.38-2.34 for -1654AG/GG; OR = 1.40, 95% CI = 1.07-1.83 for -1420GA/AA; and OR = 1.65, 95% CI = 1.26-2.16 for -1187TC/CC. Furthermore, the number of variant alleles in the promoter haplotypes was associated with increased risks of breast cancer in a dose-response manner (OR = 6.08, 95% CI = 3.25-11.38; OR = 6.83, 95% CI = 3.83-12.18; and OR = 23.61, 95% CI = 3.95-140.99 for one, two, and three variant alleles, respectively) (P (trend) \ < 0.0001). Age at onset of breast cancer significantly decreased as the number of variant alleles increased (P (trend) = 0.024). However, these effects were not observed in the 3'UTR 1057C > T polymorphism. Therefore, we believe that H2AFX promoter polymorphisms may contribute to the etiology of sporadic breast cancer in young non-Hispanic white women. Larger association studies and related functional studies are warranted to confirm these findings.

Endings in the middle: current knowledge of interstitial telomeric sequences

Interstitial telomeric sequences (ITSs) consist of tandem repeats of the canonical telomeric repeat and are common in mammals. They are localized at intrachromosomal sites, including those repeats located close to the centromeres and those found at interstitial sites, i.e., between the centromeres and the telomeres. ITSs might originate from ancestral intrachromosomal rearrangements (inversions and fusions), from differential crossing-over or from the repair of double-strand break during evolution. Three classes of ITSs have been described in the human genome, namely, short ITSs, long subtelomeric ITSs and fusion ITSs. The fourth class of ITSs, pericentromeric ITSs, has been found in other species. The function of ITSs can be inferred from the association of heritable diseases with ITS polymorphic variants, both in copy number and sequence. This is one of the most attractive aspects of ITS studies because it leads to new and useful markers for genetic linkage studies, forensic applications, and detection of genetic instability in tumors. Some ITSs also might be hotspots of chromosome breakage, rearrangement and amplification sites, based on the type of clastogens and the nature of ITSs. This study will contribute new knowledge with respect to ITSs' biology and mechanism, prevalence of diseases, risk evaluation and prevention of related diseases, thus facilitates the design of early detection markers for diseases caused by genomic instability.

The Mre11 complex mediates the S-phase checkpoint through an interaction with replication protein A

The Mre11/Rad50/Nbs1 complex (MRN) plays an essential role in the S-phase checkpoint. Cells derived from patients with Nijmegen breakage syndrome and ataxia telangiectasia-like disorder undergo radioresistant DNA synthesis (RDS), failing to suppress DNA replication in response to ionizing radiation (IR). How MRN affects DNA replication to control the S-phase checkpoint, however, remains unclear. We demonstrate that MRN directly interacts with replication protein A (RPA) in unperturbed cells and that the interaction is regulated by cyclin-dependent kinases. We also show that this interaction is needed for MRN to correctly localize to replication centers. Abolishing the interaction of Mre11 with RPA leads to pronounced RDS without affecting phosphorylation of Nbs1 or SMC1 following IR. Moreover, MRN is recruited to sites at or adjacent to replication origins by RPA and acts there to inhibit new origin firing upon IR. These studies suggest a direct role of MRN at origin-proximal sites to control DNA replication initiation in response to DNA damage, thereby providing an important mechanism underlying the intra-S-phase checkpoint in mammalian cells.

The early and initiation processes of radiation-induced bystander effects involved in the induction of DNA double strand breaks in non-irradiated cultures

The initiation and the early process of bystander response induced by low dose alpha-particle irradiation are very important for understanding the mechanisms underlying the bystander response. Using a 1 cGy alpha-particle to irradiate 50% of the area of a rectangular mylar dish, time-dependent DNA double strand breaks (DSBs) were induced shortly after irradiation in AG1522 cells, located either in the irradiated area or in the non-irradiated bystander area, reaching a maximum 30 min post irradiation. Medium transfer experiments showed that the conditioned medium harvested from the irradiated culture induced excessive DNA DSBs in the medium recipient cells, and the DSB-inducing ability of the medium showed was time-dependent. The medium transfer results indicated that the soluble bystander signalling molecule(s) had been generated very soon (probably less than 2.5 min) after irradiation and exist continuously to 30 min although the production of signalling molecule(s) decreased after 10 min post irradiation. Pre-treatment with dimethyl sulphoxide (DMSO) eliminated the DNA DSB-inducing ability of the conditioned medium, as well as the formation of excessive DNA DSBs in both irradiated and non-irradiated bystander areas, indicating that reactive oxygen/nitrogen species etc. might be involved in these processes.

Zebrafish as a model system to screen radiation modifiers

Zebrafish (Danio rerio) is a bona fide vertebrate model system for understanding human diseases. It allows the transparent visualization of the effects of ionizing radiation and the convenient testing of potential radioprotectors with morpholino-modified oligonucleotides (MO) knockdown. Furthermore, various reverse and forward genetic methods are feasible to decipher novel genetic modifiers of radioprotection. Examined in the review are the radioprotective effects of the proposed radiomodifiers Nanoparticle DF-1 (C-Sixty, Inc., Houston, TX) and Amifostine (WR-2721, Ethyol), the DNA repair proteins Ku80 and ATM, as well as the transplanted hematopoietic stem cells in irradiated zebrafish. The presence of any of these sufficiently rescued the radiation-induced damages in zebrafish, while its absence resulted in mutagenic phenotypes as well as an elevation of time- and dose-dependent radiation-induced apoptosis. Radiosensitizers Flavopiridol and AG1478, both of which block progression into the radioresistant S phase of the cell cycle, have also been examined in zebrafish. Zebrafish has indeed become a favorite model system to test for radiation modifiers that can potentially be used for radiotherapeutic purposes in humans.

Distinct roles for SWR1 and INO80 chromatin remodeling complexes at chromosomal double-strand breaks

INO80 and SWR1 are two closely related ATP-dependent chromatin remodeling complexes that share several subunits. Ino80 was reported to be recruited to the HO endonuclease-induced double-strand break (DSB) at the budding yeast mating-type locus, MAT. We find Swr1 similarly recruited in a manner dependent on the phosphorylation of H2A (gammaH2AX). This is not unique to cleavage at MAT; both Swr1 and Ino80 bind near an induced DSB on chromosome XV. Whereas Swr1 incorporates the histone variant H2A.Z into chromatin at promoters, H2A.Z levels do not increase at DSBs. Instead, H2A.Z, gammaH2AX and core histones are coordinately removed near the break in an INO80-dependent, but SWR1-independent, manner. Mutations in INO80-specific subunits Arp8 or Nhp10 impair the binding of Mre11 nuclease, yKu80 and ATR-related Mec1 kinase at the DSB, resulting in defective end-processing and checkpoint activation. In contrast, Mre11 binding, end-resection and checkpoint activation were normal in the swr1 strain, but yKu80 loading and error-free end-joining were impaired. Thus, these two related chromatin remodelers have distinct roles in DSB repair and checkpoint activation.

Break-induced loss of heterozygosity in fission yeast: dual roles for homologous recombination in promoting translocations and preventing de novo telomere addition

Loss of heterozygosity (LOH), a causal event in tumorigenesis, frequently encompasses multiple genetic loci and whole chromosome arms. However, the mechanisms leading to such extensive LOH are poorly understood. We investigated the mechanisms of DNA double-strand break (DSB)-induced extensive LOH by screening for auxotrophic marker loss approximately 25 kb distal to an HO endonuclease break site within a nonessential minichromosome in Schizosaccharomyces pombe. Extensive break-induced LOH was infrequent, resulting from large translocations through both allelic crossovers and break-induced replication. These events required the homologous recombination (HR) genes rad32(+), rad50(+), nbs1(+), rhp51(+), rad22(+), rhp55(+), rhp54(+), and mus81(+). Surprisingly, LOH was still observed in HR mutants, which resulted predominantly from de novo telomere addition at the break site. De novo telomere addition was most frequently observed in rad22Delta and rhp55Delta backgrounds, which disrupt HR following end resection. Further, levels of de novo telomere addition, while increased in ku70Delta rhp55Delta strains, were reduced in exo1Delta rhp55Delta and an rhp55Delta strain overexpressing rhp51. These findings support a model in which HR prevents de novo telomere addition at DSBs by competing for resected ends. Together, these results suggest that the mechanisms of break-induced LOH may be predicted from the functional status of the HR machinery.

A novel variable number of tandem repeats (VNTR) polymorphism containing Sp1 binding elements in the promoter of XRCC5 is a risk factor for human bladder cancer

X-ray repair cross-complementing 5 (XRCC5) is a gene involved in repair of DNA double-strand breaks. Abnormal expression of the XRCC5 protein is associated with genomic instability and an increased incidence of cancers. In our study, a polymorphism with a variable number of tandem repeats (21-bp repeat elements at position -201 to -160 relative to the initiation of transcription) in the promoter of XRCC5 was identified. As determined with gel-shift and super-shift assays, the binding affinity of the transcription factor Sp1 to the allele with two 21-bp repeats was greater than that for the allele with one 21-bp repeat. As established with a reporter assay, plasmids containing zero or one repeat element had higher transcriptional activities than plasmids containing two repeat elements. Furthermore, fewer tandem repeats in the promoter of XRCC5 was associated with enhanced levels of the XRCC5 protein in bladder cancer patients. Although, in a case-control study, the different genotypes were not associated with the risk of bladder cancer, individuals not carrying the two tandem repeats allele had an increased risk of bladder cancer compared with those carrying the allele with two repeats. These results indicated that, at least in a population in southeastern China, this polymorphism in the promoter of XRCC5 could regulate the expression of XRCC5 and thereby contribute to susceptibility to bladder cancer.

Nonhomologous end joining is essential for cellular resistance to the novel antitumor agent, beta-lapachone

Commonly used antitumor agents, such as DNA topoisomerase I/II poisons, kill cancer cells by creating nonrepairable DNA double-strand breaks (DSBs). To repair DSBs, error-free homologous recombination (HR), and/or error-prone nonhomologous end joining (NHEJ) are activated. These processes involve the phosphatidylinositol 3'-kinase-related kinase family of serine/threonine enzymes: ataxia telangiectasia mutated (ATM), ATM- and Rad3-related for HR, and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) for NHEJ. Alterations in these repair processes can cause drug/radiation resistance and increased genomic instability. beta-Lapachone (beta-lap; also known as ARQ 501), currently in phase II clinical trials for the treatment of pancreatic cancer, causes a novel caspase- and p53-independent cell death in cancer cells overexpressing NAD(P)H:quinone oxidoreductase-1 (NQO1). NQO1 catalyzes a futile oxidoreduction of beta-lap leading to reactive oxygen species generation, DNA breaks, gamma-H2AX foci formation, and hyperactivation of poly(ADP-ribose) polymerase-1, which is required for cell death. Here, we report that beta-lap exposure results in NQO1-dependent activation of the MRE11-Rad50-Nbs-1 complex. In addition, ATM serine 1981, DNA-PKcs threonine 2609, and Chk1 serine 345 phosphorylation were noted; indicative of simultaneous HR and NHEJ activation. However, inhibition of NHEJ, but not HR, by genetic or chemical means potentiated beta-lap lethality. These studies give insight into the mechanism by which beta-lap radiosensitizes cancer cells and suggest that NHEJ is a potent target for enhancing the therapeutic efficacy of beta-lap alone or in combination with other agents in cancer cells that express elevated NQO1 levels.

The BRCA1/BRCA2/Rad51 complex is a prognostic and predictive factor in early breast cancer

BACKGROUND AND PURPOSE

The breast cancer susceptibility genes BRCA1 and BRCA2 interact with Rad51, one of the central components in the homologous recombination repair pathway. This study evaluates the prognostic and predictive role of BRCA1, BRCA2 and Rad51, individually and as a complex, in breast cancer.

MATERIALS AND METHODS

Expression of BRCA1, BRCA2 and Rad51 was investigated using immunohistochemistry in tumours from 224 women with early breast cancer, who were randomised to receive postoperative radiotherapy or adjuvant chemotherapy (CMF).

RESULTS

Fifty-three percent (112/212) of the tumours had reduced expression of the BRCA1/BRCA2/Rad51 complex. Low expression correlated to high histologic grade (p=0.05). Patients with low expression of the complex developed significantly more local recurrences as compared to patients with high expression (RR=3.20, 95% CI 1.48-6.88, p=0.003). Expression of the BRCA1/BRCA2/Rad51 complex was an independent prognostic factor in multivariate analysis (p=0.03). Patients with low expression of the complex responded well to radiotherapy (RR=0.31, 95% CI 0.14-0.70, p=0.005), whereas patients with high expression had few local recurrences and no additional benefit from radiotherapy (RR=1.08, 95% CI 0.40-2.90, p=0.88).

CONCLUSIONS

Low expression of the BRCA1/BRCA2/Rad51 complex is a marker of poor prognosis, but predicts good response to radiotherapy in patients with early breast cancer.

Polymorphisms in XPC, XPD, XRCC1, and XRCC3 DNA repair genes and lung cancer risk in a population of northern Spain

BACKGROUND

Polymorphisms in DNA repair genes have been associated to repair DNA lesions, and might contribute to the individual susceptibility to develop different types of cancer. Nucleotide excision repair (NER), base excision repair (BER), and double-strand break repair (DSBR) are the main DNA repair pathways. We investigated the relationship between polymorphisms in two NER genes, XPC (poly (AT) insertion/deletion: PAT-/+) and XPD (Asp312Asn and Lys751Gln), the BER gene XRCC1 (Arg399Gln), and the DSBR gene XRCC3 (Thr241Met) and the risk of developing lung cancer.

METHODS

A hospital-based case-control study was designed with 516 lung cancer patients and 533 control subjects, matched on ethnicity, age, and gender. Genotypes were determined by PCR-RFLP and the results were analysed using multivariate unconditional logistic regression, adjusting for age, gender and pack-years.

RESULTS

Borderline association was found for XPC and XPD NER genes polymorphisms, while no association was observed for polymorphisms in BER and DSBR genes. XPC PAT+/+ genotype was associated with no statistically significant increased risk among ever smokers (OR = 1.40; 95%CI = 0.94-2.08), squamous cell carcinoma (OR = 1.44; 95%CI = 0.85-2.44), and adenocarcinoma (OR = 1.72; 95%CI = 0.97-3.04). XPD variant genotypes (312Asn/Asn and 751Gln/Gln) presented a not statistically significant risk of developing lung cancer (OR = 1.52; 95%CI = 0.91-2.51; OR = 1.38; 95%CI = 0.85-2.25, respectively), especially among ever smokers (OR = 1.58; 95%CI = 0.96-2.60), heavy smokers (OR = 2.07; 95%CI = 0.74-5.75), and adenocarcinoma (OR = 1.88; 95%CI = 0.97-3.63). On the other hand, individuals homozygous for the XRCC1 399Gln allele presented no risk of developing lung cancer (OR = 0.87; 95%CI = 0.57-1.31) except for individuals carriers of 399Gln/Gln genotype and without family history of cancer (OR = 0.57; 95%CI = 0.33-0.98) and no association was found between XRCC3 Thr241Met polymorphism and lung cancer risk (OR = 0.92; 95%CI = 0.56-1.50), except for the 241Met/Met genotype and squamous cell carcinoma risk (OR = 0.47; 95%CI = 0.23-1.00).

CONCLUSION

In conclusion, we analysed the association between XPC, XPD, XRCC1, and XRCC3 polymorphisms and the individual susceptibility to develop lung cancer in the Spanish population, specifically with a highly tobacco exposed population. We attempt to contribute to the discovery of which biomarkers of DNA repair capacity are useful for screening this high-risk population for primary preventing and early detection of lung cancer.

The Mre11-Rad50-Nbs1 complex acts both upstream and downstream of ataxia telangiectasia mutated and Rad3-related protein (ATR) to regulate the S-phase checkpoint following UV treatment

The Mre11-Rad50-Nbs1 (MRN) complex is required for mediating the S-phase checkpoint following UV treatment, but the underlying mechanism is not clear. Here we demonstrate that at least two mechanisms are involved in regulating the S-phase checkpoint in an MRN-dependent manner following UV treatment. First, when replication forks are stalled, MRN is required upstream of ataxia telangiectasia mutated and Rad3-related protein (ATR) to facilitate ATR activation in a substrate and dosage-dependent manner. In particular, MRN is required for ATR-directed phosphorylation of RPA2, a critical event in mediating the S-phase checkpoint following UV treatment. Second, MRN is a downstream substrate of ATR. Nbs1 is phosphorylated by ATR at Ser-343 when replication forks are stalled, and this phosphorylation event is also important for down-regulating DNA replication following UV treatment. Moreover, we demonstrate that MRN and ATR/ATR-interacting protein (TRIP) interact with each other, and the forkhead-associated/breast cancer C-terminal domains (FHA/BRCT) of Nbs1 play a significant role in mediating this interaction. Mutations in the FHA/BRCT domains do not prevent ATR activation but specifically impair ATR-mediated Nbs1 phosphorylation at Ser-343, which results in a defect in the S-phase checkpoint. These data suggest that MRN plays critical roles both upstream and downstream of ATR to regulate the S-phase checkpoint when replication forks are stalled.

Targeting DNA repair as a promising approach in cancer therapy

An increased DNA-repair activity in tumour cells has been associated with resistance to treatment to DNA-directed drugs, while defects in DNA repair pathways result in hypersensitivity to these agents. In the past years the unravelling of the molecular basis of these DNA pathways, with a better understanding of the DNA damage caused by different anticancer agents, has provided the rationale for the use of some DNA repair inhibitors to optimise the therapeutic use of DNA-damaging agents currently used in the treatment of tumours. In addition, the possibility to specifically target the differences in DNA repair capacity between normal and tumour cells has recently emerged as an exciting possibility. The present review will mainly cover those approaches that are currently under clinical investigation.

Mre11-Rad50-Nbs1 is a keystone complex connecting DNA repair machinery, double-strand break signaling, and the chromatin template

The Mre11-Rad50-Nbs1 (MRN) complex is providing paradigm-shifting results of exceptional biomedical interest. MRN is among the earliest respondents to DNA double-strand breaks (DSBs), and MRN mutations cause the human cancer predisposition diseases Nijmegen breakage syndrome and ataxia telangiectasia-like disorder (ATLD). MRN's 3-protein multidomain composition promotes its central architectural, structural, enzymatic, sensing, and signaling functions in DSB responses. To organize the MRN complex, the Mre11 exonuclease directly binds Nbs1, DNA, and Rad50. Rad50, a structural maintenance of chromosome (SMC) related protein, employs its ATP-binding cassette (ABC) ATPase, Zn hook, and coiled coils to bridge DSBs and facilitate DNA end processing by Mre11. Contributing to MRN regulatory roles, Nbs1 harbors N-terminal phosphopeptide interacting FHA and BRCT domains, as well as C-terminal ataxia telangiectasia mutated (ATM) kinase and Mre11 interaction domains. Current emerging structural and biological evidence suggests that MRN has 3 coupled critical roles in DSB sensing, stabilization, signaling, and effector scaffolding: (1) expeditious establishment of protein--nucleic acid tethering scaffolds for the recognition and stabilization of DSBs; (2) initiation of DSB sensing, cell-cycle checkpoint signaling cascades, and establishment of epigenetic marks via the ATM kinase; and (3) functional regulation of chromatin remodeling in the vicinity of a DSB.

Proteasomes and proteasome activator 200 kDa (PA200) accumulate on chromatin in response to ionizing radiation

Proteasome activator 200 kDa (PA200) forms nuclear foci after exposure of cells to ionizing radiation and enhances proteasome activity in vitro. Within cells, it is unclear whether PA200 responds to radiation alone or in association with proteasomes. In the present study, we identified three forms of cellular PA200 (termed PA200i, ii and iii) at the mRNA and protein levels. Neither PA200ii nor PA200iii appears to associate with proteasomes. All detectable PA200i is associated with proteasomes, which indicates that PA200i and proteasomes function together within the cell. Consistent with this idea, we find that exposure of cells to radiation leads to an equivalent accumulation of both PA200i and core proteasomes on chromatin. This increase in PA200 and proteasomes on chromatin is not specific to the stage of cell cycle arrest since it occurs in cells that arrest in G(2)/M and cells that arrest in G(1)/S after exposure to radiation. These data provide evidence that PA200 and proteasomes function together within cells and respond to a specific radiation-induced damage independent of the stage of cell cycle arrest.

Lessons learned from DNA repair defective syndromes

Genomic instability is the driving force behind cancer development. Human syndromes with DNA repair deficiencies comprise unique opportunities to study the clinical consequences of faulty genome maintenance leading to premature aging and premature cancer development. These syndromes include chromosomal breakage syndromes with defects in DNA damage signal transduction and double-strand break repair, mismatch repair defective syndromes as well as nucleotide excision repair defective syndromes. The same genes that are severely affected in these model diseases may harbour more subtle variations in the 'healthy' normal population leading to genomic instability, cancer development, and accelerated aging at later stages of life. Thus, studying those syndromes and the molecular mechanisms behind can significantly contribute to our understanding of (skin) cancerogenesis as well as to the development of novel individualized preventive and therapeutic anticancer strategies. The establishment of centers of excellence for studying rare genetic model diseases may be helpful in this direction.

DNA repair is limiting for haematopoietic stem cells during ageing

Accumulation of DNA damage leading to adult stem cell exhaustion has been proposed to be a principal mechanism of ageing. Here we address this question by taking advantage of the highly specific role of DNA ligase IV in the repair of DNA double-strand breaks by non-homologous end-joining, and by the discovery of a unique mouse strain with a hypomorphic Lig4(Y288C) mutation. The Lig4(Y288C) mouse, identified by means of a mutagenesis screening programme, is a mouse model for human LIG4 syndrome, showing immunodeficiency and growth retardation. Diminished DNA double-strand break repair in the Lig4(Y288C) strain causes a progressive loss of haematopoietic stem cells and bone marrow cellularity during ageing, and severely impairs stem cell function in tissue culture and transplantation. The sensitivity of haematopoietic stem cells to non-homologous end-joining deficiency is therefore a key determinant of their ability to maintain themselves against physiological stress over time and to withstand culture and transplantation.

Effects of DNA-targeted ionizing radiation produced by 5-[125I]iodo-2'-deoxyuridine on global gene expression in primary human cells

Background

This study assesses the whole-genome gene expression changes in a panel of primary human cell lines in response to DNA damage mediated by decay of DNA-incorporated radioiodinated thymidine analog 5-[¹²⁵I]iodo-2'-deoxyuridine (¹²⁵I-IUdR). Three normal human cell lines of different origin, namely, gingival fibroblasts AG09319, fetal skin fibroblasts GM05388 and neonatal foreskin epidermal keratinocytes (NHFK) were used in this study. DNA molecules were radiolabeled by incubation of cells in culture in a medium supplemented with either 3.7 kBq/ml or 18.5 kBq/ml of ¹²⁵I-IUdR for 24 h followed by incubation in IUdR-free medium for additional 24 hours. Each experiment was carried out in quadruplicate. ¹²⁵I-IUdR uptake was monitored by measuring DNA-associated radioactivity. The whole-genome gene expression changes were evaluated using Agilent Human Whole Genome oligo microarrays containing 44,290 elements representing all known and predicted human genes. DNA microarray dataset was independently partially validated with quantitative real-time PCR (RT-PCR).

Results

AG09319 gingival cells in culture responded to ¹²⁵I-IUdR treatment by changing the expression level of 335 genes in total, whereas under the same conditions GM05388 and NHFK cells differentially expressed 49 genes and 27 genes, respectively. However, for GM05388 cells the number of differentially expressed genes increases with the rise of ¹²⁵I-IUdR concentrations in cell culture media. The key up-regulated biological processes in a chosen panel of cell lines concern the regulation of protein kinase activities and/or cell death. Genes repressed in response to ¹²⁵I-IUdR treatment are involved in cytokinesis, M phase of the cell cycle, chromosome architecture and organization, DNA metabolism, DNA packaging, DNA repair and response to DNA damage. Despite the disparate nature of the gene patterns elicited by ¹²⁵I-induced DNA damage among the different cell lines, the differentially expressed transcripts reveal strikingly non-random chromosomal distribution in all the cell lines we used.

Conclusion

Our data suggest that DNA-targeted ionizing radiation produced by ¹²⁵I-IUdR results in changes in expression of only a limited subset of genes in primary human cells. The responsive genes are distributed non-randomly among the chromosomes; and a significant fraction of them is p53-dependent in the transcriptional regulation.

Saccharomyces cerevisiae Sae2- and Tel1-dependent single-strand DNA formation at DNA break promotes microhomology-mediated end joining

Microhomology-mediated end joining (MMEJ) joins DNA ends via short stretches [5-20 nucleotides (nt)] of direct repeat sequences, yielding deletions of intervening sequences. Non-homologous end joining (NHEJ) and single-strand annealing (SSA) are other error prone processes that anneal single-stranded DNA (ssDNA) via a few bases (<5 nt) or extensive direct repeat homologies (>20 nt). Although the genetic components involved in MMEJ are largely unknown, those in NHEJ and SSA are characterized in some detail. Here, we surveyed the role of NHEJ or SSA factors in joining of double-strand breaks (DSBs) with no complementary DNA ends that rely primarily on MMEJ repair. We found that MMEJ requires the nuclease activity of Mre11/Rad50/Xrs2, 3' flap removal by Rad1/Rad10, Nej1, and DNA synthesis by multiple polymerases including Pol4, Rad30, Rev3, and Pol32. The mismatch repair proteins, Rad52 group genes, and Rad27 are dispensable for MMEJ. Sae2 and Tel1 promote MMEJ but inhibit NHEJ, likely by regulating Mre11-dependent ssDNA accumulation at DNA break. Our data support the role of Sae2 and Tel1 in MMEJ and genome integrity.

Evaluation of genetic variation in the double-strand break repair pathway and bladder cancer risk

The double-strand break DNA repair (DSBR) pathway is implicated in maintaining genomic stability and therefore could affect bladder cancer risk. Here we present data evaluating 39 single-nucleotide polymorphisms (SNPs) in seven candidate genes whose products are involved in DNA break sensing (NBS1, BRCA1 interacting genes BRIP1 and ZNF350), non-homologous end-joining (NHEJ) DNA repair (XRCC4) and homologous recombination (HR) repair (RAD51, XRCC2 and XRCC3). SNPs for RAD51 and XRCC2 covered most of the common variation. Associations with bladder cancer risk were evaluated in 1,150 newly diagnosed cases of urinary bladder transitional cell carcinomas and 1,149 controls conducted in Spain during 1997-2001. We found that the genetic variants evaluated significantly contributed to bladder cancer risk (global likelihood ratio test P = 0.01). Subjects with the ZNF350 R501S (rs2,278,415) variant allele showed significantly reduced risk compared with common homozygote variants, odds ratio (OR) [95% confidence interval (95% CI)]: 0.76 (0.62-0.93) per variant allele. Carriers of a putative functional SNP in intron 7 of XRCC4 (rs1,805,377) had significantly increased bladder cancer risk compared with common homozygotes: 1.33 (1.08-1.64) per variant allele. Lastly, XRCC2 homozygote variants for three promoter SNPs (rs10,234,749, rs6,464,268, rs3,218,373) and one non-synonymous SNP (rs3,218,536, R188H) were associated with reduced bladder cancer risk (ORs ranging from 0.36 to 0.50 compared with common homozygotes). Meta-analysis for XRCC3 T241M (rs861,539) had a significant small increase in risk among homozygote variants: OR (95% CI) = 1.17 (1.00-1.36). Results from this study provide evidence for associations between variants in genes in the DSBR pathway and bladder cancers risk that warrant replication in other study populations.

Positional stability of single double-strand breaks in mammalian cells

Formation of cancerous translocations requires the illegitimate joining of chromosomes containing double-strand breaks (DSBs). It is unknown how broken chromosome ends find their translocation partners within the cell nucleus. Here, we have visualized and quantitatively analysed the dynamics of single DSBs in living mammalian cells. We demonstrate that broken ends are positionally stable and unable to roam the cell nucleus. Immobilization of broken chromosome ends requires the DNA-end binding protein Ku80, but is independent of DNA repair factors, H2AX, the MRN complex and the cohesion complex. DSBs preferentially undergo translocations with neighbouring chromosomes and loss of local positional constraint correlates with elevated genomic instability. These results support a contact-first model in which chromosome translocations predominantly form among spatially proximal DSBs.

Chronic myeloid leukaemia as a model of disease evolution in human cancer

Chronic myeloid leukaemia (CML) can be considered as a paradigm for neoplasias that evolve through a multi-step process. CML is also one of the best examples of a disease that can be targeted by molecular therapy; however, the success of new 'designer drugs' is largely restricted to the chronic phase of the disease. If not cured at this stage, CML invariably progresses and transforms into an acute-type leukaemia undergoing a 'blast crisis'. The causes of this transformation are still poorly understood. What mechanisms underlie this progression, and are they shared by other common cancers?

Arsenite-induced alterations in Ku70-deficient cells: a model to study genotoxic effects

As one of three subunits of DNA-dependent protein kinase (DNA-PK), Ku70 protein plays an important role in repair of DNA double-strand breaks (DNA DSB). To further understand the functions of Ku70 protein and the mechanisms underlying arsenite-induced genotoxic effects, the effects of Ku70 deficiency were examined. The Ku70-deficient cell line HLFK and null vector cell line HLFC were established after recombinant plasmid of Ku70 gene antisense RNA and null pEGFP-C1 vector were transferred into human embryo lung fibroblasts (HLF) cells. Experiments were undertaken to detect DNA DSB damage by neutral single-cell gel electrophoresis assay (SCGE), chromosomal alterations by micronucleus test, and cell cycle progression by flow cytometry in HLFC and HLFK cells treated with control, 1, 2.5, 5, or 10 microM sodium arsenite for 2, 4, or 24 h, respectively. Western blot analysis results showed that Ku70 protein content in HLFK cells decreased to 38% of those in HLFC cells. The median lethal concentrations (LC50) of sodium arsenite to HLFC and HLFK cells for 24 h were 27.38 microM and 21.80 microM, respectively. Results of neutral SCGE assay showed that there were concentration-dependent increases in tail length of DNA DSB, in percent of cells with DNA DSB tails, and in severity of DNA DSB damage in HLFK and HLFC cells. The increases in these indices in HLFK cells were significantly higher than those found in HLFC cells exposed to similar amounts of metal. The ability of DNA DSB to repair in HLFK cells was less than that seen in HLFC cells. Sodium arsenite produced concentration-dependent elevation in micronuclei and abnormal nuclei formation. The Ku70-deficiency enhanced the susceptibility to chromosomal alterations induced by sodium arsenite. Low concentrations of sodium arsenite induced cell arrest at G1; however, at high concentrations of metal this G1 arrest effect disappeared. These results suggested that Ku70 protein plays an important role in repair of DNA DSB damage and for maintainance of genome stability.

Silencing of endo-exonuclease expression sensitizes mouse B16F10 melanoma cells to DNA damaging agents

We previously identified an endo-exonuclease that is highly expressed in cancer cells and plays an important role in DSB repair mechanisms. A small molecular compound pentamidine, which specifically inhibited nuclease activity of the isolated endo-exonuclease from yeast as well as from mammalian cells, was capable of sensitizing tumor cells to DNA damaging agents. In this study, we investigated the effect of precisely silencing the endo-exonuclease expression by small interfering RNA (siRNA) upon treatment with a variety of DNA damaging agents in mouse B16F10 melanoma cells. A maximum of 3.6 to approximately 4-fold reduction in endo-exonuclease mRNA expression was achieved, over a period of 48-72 h of post transfection with a concomitant reduction in protein expression (approximately 4-5 fold), resulting in a substantial reduction (approximately 45-50%) of the corresponding nuclease activity. Suppressed endo-exonuclease expression conferred significant decrease in cell survival, ranging from approximately 30 to approximately 50% cell killing, in presence of DNA damaging drugs methyl methane sulfonate (MMS), cisplatin, 5-fluoro uracil (5-FU) and gamma-irradiation but not at varying dosages of ultra violet (UV) radiation. The data strongly support a role for the endo-exonuclease in repairing DNA damages, induced by MMS, cisplatin, 5-FU and gamma irradiation but not by UV radiation. The results presented in this study suggest that the endo-exonuclease siRNA could be useful as a therapeutic tool in targeting the endo-exonuclease in cancer therapy.

Spreading of mammalian DNA-damage response factors studied by ChIP-chip at damaged telomeres

Phosphorylated histone H2AX (gammaH2AX) is generated in nucleosomes flanking sites of DNA double-strand breaks, triggering the recruitment of DNA-damage response proteins such as MDC1 and 53BP1. Here, we study shortened telomeres in senescent human cells. We show that most telomeres trigger gammaH2AX formation, which spreads up to 570 kb into the subtelomeric regions. Furthermore, we reveal that the spreading patterns of 53BP1 and MDC1 are very similar to that of gammaH2AX, consistent with a structural link between these factors. Moreover, different subsets of telomeres signal in different cell lines, with those that signal tending to equate to the shortest telomeres of the corresponding cell line, thus linking telomere attrition with DNA-damage signalling. Notably, we find that, in some cases, gammaH2AX spreading is modulated in a manner suggesting that H2AX distribution or its ability to be phosphorylated is not uniform along the chromosome. Finally, we observe weak gammaH2AX signals at telomeres of proliferating cells, but not in hTERT immortalised cells, suggesting that low telomerase activity leads to telomere uncapping and senescence in proliferating primary cells.

DNA double-strand break repair capacity and risk of breast cancer

A tumorigenic role of the non-homologous end-joining (NHEJ) pathway for the repair of DNA double-strand breaks (DSBs) has been suggested by our finding of a significant association between increased breast cancer risk and a cooperative effect of single-nucleotide polymorphisms in NHEJ genes. To confirm this finding, this case-control study detected both in vivo and in vitro DNA end-joining (EJ) capacities in Epstein-Barr virus-immortalized peripheral blood mononuclear cells (PBMCs) of 112 breast cancer patients and 108 healthy controls to identify individual differences in EJ capacity to repair DSB as a risk factor predisposing women to breast cancer. PBMCs from breast cancer patients consistently showed lower values of in vivo and in vitro EJ capacities than those from healthy women (P < 0.05). Logistic regression, simultaneously considering the effect of known risk factors of breast cancer, shows that the in vitro EJ capacity above the median of control subjects was associated with nearly 3-fold increased risks for breast cancer (adjusted odds ratio, 2.98; 95% confidence interval, 1.64-5.43). Furthermore, a dose-response relationship was evident between risk for breast cancer and EJ capacity, which was analyzed as a continuous variable (every unit decrease of EJ capacity being associated with an 1.09-fold increase of breast cancer risk) and was divided into tertiles based on the EJ capacity values of the controls (P for trend < 0.01). The findings support the conclusion that NHEJ may play a role in susceptibility to breast cancer.

Synergism between etoposide and 17-AAG in leukemia cells: critical roles for Hsp90, FLT3, topoisomerase II, Chk1, and Rad51

PURPOSE

DNA-damaging agents, such as etoposide, while clinically useful in leukemia therapy, are limited by DNA repair pathways that are not well understood. 17-(Allylamino)-17-demethoxygeldanamycin (17-AAG), an inhibitor of the molecular chaperone heat shock protein 90 (Hsp90), inhibits growth and induces apoptosis in FLT3(+) leukemia cells. In this study, we evaluated the effects of etoposide and 17-AAG in leukemia cells and the roles of Hsp90, FMS-like tyrosine kinase 3 (FLT3), checkpoint kinase 1 (Chk1), Rad51, and topoisomerase II in this inhibition.

EXPERIMENTAL DESIGN

The single and combined effects of 17-AAG and etoposide and the mechanism of these effects were evaluated. FLT3 and the DNA repair-related proteins, Chk1 and Rad51, were studied in small interfering RNA (siRNA)-induced cell growth inhibition experiments in human leukemia cells with wild-type or mutated FLT3.

RESULTS

We found that etoposide and the Hsp90/FLT3 inhibitor 17-AAG, had synergistic inhibitory effects on FLT3(+) MLL-fusion gene leukemia cells. Cells with an internal tandem duplication (ITD) FLT3 (Molm13 and MV4;11) were more sensitive to etoposide/17-AAG than leukemias with wild-type FLT3 (HPB-Null and RS4;11). A critical role for FLT3 was shown in experiments with FLT3 ligand and siRNA targeted to FLT3. An important role for topoisomerase II and the DNA repair-related proteins, Chk1 and Rad51, in the synergistic effects was suggested from the results.

CONCLUSIONS

The repair of potentially lethal DNA damage by etoposide in leukemia cells is dependent on intact and functioning FLT3 especially leukemias with ITD-FLT3. These data suggest a rational therapeutic strategy for FLT3(+) leukemias that combines etoposide or other DNA-damaging agents with Hsp90/FLT3 inhibitors such as 17-AAG.

Epigenetic inactivation of the chromosomal stability control genes BRCA1, BRCA2, and XRCC5 in non-small cell lung cancer

PURPOSE

Lung cancer cells frequently exhibit marked chromosome instability. We postulated that alterations of the double-strand break repair genes (BRCA1, BRCA2, and XRCC5) might be involved in lung cancer.

PATIENTS AND METHODS

We examined the loss of protein and mRNA expression and the 5'CpG hypermethylation and allelic imbalance of the BRCA1, BRCA2, and XRCC5 genes in 98 non-small cell lung cancer (NSCLC) samples. Anchorage-dependent growth after reexpression of these genes was examined in a lung cancer cell line that originally lacked BRCA1 and BRCA2 expression.

RESULTS

The data indicated that low protein expression of BRCA1 and BRCA2 was frequent in lung adenocarcinomas (42-44%), whereas low XRCC5 protein expression was more prevalent among squamous cell carcinoma (32%). In addition, low BRCA1 expression was significantly associated with low RB expression, especially in lung adenocarcinoma. Concurrent alterations in XRCC5 and p53 were the most frequent profiles in smoking patients. Importantly, low mRNA and protein expressions of BRCA1, BRCA2, and XRCC5 were significantly associated with their promoter hypermethylation. 5-Aza-2'-deoxycytidine treatment of NSCLC cells showed demethylation and reexpression of the BRCA1 and BRCA2 genes and reduced anchorage-independent growth.

CONCLUSIONS

Our retrospective study provides compelling evidence that low mRNA and protein expression in the BRCA1/BRCA2 and XRCC5 genes occur in lung adenocarcinoma and squamous cell carcinoma, respectively, and that promoter hypermethylation is the predominant mechanism in deregulation of these genes. Alteration of the double-strand break repair pathway, perhaps by interacting with p53 and RB deregulation, is important in the pathogenesis of a subset of NSCLC.

Polymorphisms in the DNA repair gene XRCC1 associated with basal cell carcinoma and squamous cell carcinoma of the skin in a Korean population

DNA in most cells is regularly damaged by endogenous and exogenous mutagens. Unrepaired damage can result in apoptosis or may lead to unregulated cell growth and cancer. Inheritance of genetic variants at one or more loci results in reduced DNA repair capacity. This hospital-based case-control study examined whether polymorphisms in the DNA repair gene X-ray repair cross-complementing groups 1 (XRCC1) (Arg194Trp[C > T], Arg280His[G > A] and Arg399Gln[G > A]) play a role in susceptibility to skin cancer. We genotyped these polymorphisms for 212 histopathologically confirmed skin cancer cases (n = 114 basal cell carcinoma, n = 98 squamous cell carcinoma) and 207 age- and sex-matched healthy control cases in Korea. We found that individuals with the Arg/Gln and Arg/Gln + Gln/Gln genotypes at XRCC1 Arg399Gln(G > A) had an approximately 2-fold increased risk of basal cell carcinoma compared to individuals with the Arg/Arg genotype (adjusted odds ratio [AOR] = 2.812, 95% confidence interval [CI] 1.32-5.98, and AOR = 2.324, 95% CI 1.11-4.86). However, we observed that the 194Trp allele of the Arg194Trp(C > T) polymorphism was inversely associated with squamous cell carcinoma risk (Trp/Trp, AOR = 0.06, 95% CI 0.006-0.63). Our data suggest that the Arg194Trp and Arg399Gln polymorphisms may be differentially associated with skin cancer risk.

Intact Mre11/Rad50/Nbs1 complex predicts good response to radiotherapy in early breast cancer

PURPOSE

To investigate the expression and predictive role of the Mre11/Rad50/Nbs1 (MRN) complex and the ataxia-telangiectasia mutated protein (ATM) for the outcome of radiotherapy in breast cancer patients.

METHODS AND MATERIALS

The protein expression of ATM and the DNA repair proteins in the MRN complex were investigated using immunohistochemistry in tumors from 224 women with early breast cancer, who were randomized to receive postoperative radiotherapy or adjuvant chemotherapy.

RESULTS

Compared with normal breast tissue, the staining intensity of Mre11, Rad50, Nbs1, and ATM was reduced in a majority of the tumors. Weak expression of the MRN complex was correlated with high histologic grade and estrogen receptor negativity (p = 0.01 and p = 0.0001, respectively). Radiotherapy significantly reduced the risk of local recurrence as compared with chemotherapy (p = 0.04). The greatest benefit of radiotherapy was seen in patients with moderate/strong expression of the MRN complex (relative risk = 0.27, 95% confidence interval = 0.098-0.72, p = 0.009), whereas patients with negative/weak MRN expression had no benefit of radiotherapy compared with adjuvant chemotherapy. These results suggest that an intact MRN complex is important for the tumor cell eradicating effect of radiotherapy.

CONCLUSIONS

Reduced expression of the MRN complex predicts a poor effect of radiotherapy in patients with early breast cancer.

Mus81-Eme1-dependent and -independent crossovers form in mitotic cells during double-strand break repair in Schizosaccharomyces pombe

During meiosis, double-strand breaks (DSBs) lead to crossovers, thought to arise from the resolution of double Holliday junctions (HJs) by an HJ resolvase. In Schizosaccharomyces pombe, meiotic crossovers are produced primarily through a mechanism requiring the Mus81-Eme1 endonuclease complex. Less is known about the processes that produces crossovers during the repair of DSBs in mitotic cells. We employed an inducible DSB system to determine the role of Rqh1-Top3 and Mus81-Eme1 in mitotic DSB repair and crossover formation in S. pombe. In agreement with the meiotic data, crossovers are suppressed in cells lacking Mus81-Eme1. And relative to the wild type, rqh1Delta cells show a fourfold increase in crossover frequency. This suppression of crossover formation by Rqh1 is dependent on its helicase activity. We found that the synthetic lethality of cells lacking both Rqh1 and Eme1 is suppressed by loss of swi5(+), which allowed us to show that the excess crossovers formed in an rqh1Delta background are independent of Mus81-Eme1. This result suggests that a second process for crossover formation exists in S. pombe and is consistent with our finding that deletion of swi5(+) restored meiotic crossovers in eme1Delta cells. Evidence suggesting that Rqh1 also acts downstream of Swi5 in crossover formation was uncovered in these studies. Our results suggest that during Rhp51-dependent repair of DSBs, Rqh1-Top3 suppresses crossovers in the Rhp57-dependent pathway while Mus81-Eme1 and possibly Rqh1 promote crossovers in the Swi5-dependent pathway.

Combination of radiotherapy with EGFR antagonists for head and neck carcinoma

The introduction of biologically sound radiation fractionation regimens and combinations of radiotherapy with chemotherapy have gradually improved both the survival of patients with locally advanced head and neck squamous cell carcinoma (HNSCC) and the prospect of organ preservation. Long-term follow-up, however, has shown that some of the radiation-chemotherapy combinations are associated with increased late toxicity. This observation, in conjunction with advances in tumor biology, has led to the launch of investigations into molecular markers and targets for therapeutic interventions. Research on the epidermal growth factor receptor (EGFR)-mediated signaling pathway has enriched our understanding of the biology of HNSCC, in terms of carcinogenesis and cellular processes governing tumor response to therapy. The finding that the addition of an antibody-based inhibitor of the EGFR pathway to radiotherapy significantly improves locoregional control and overall survival rates in patients with locally advanced HNSCC, without increasing radiation-induced toxicity, has resulted in the growing acceptance of such combined regimens as a frontline therapy option for locally advanced HNSCC. Because such therapy has benefited only an additional 10%-15% of patients, studies are being undertaken to identify markers and mechanisms of resistance to EGFR antagonists that are essential for the further refinement of therapy. Overall, preclinical and clinical studies on EGFR have validated the concept that selective tumor radiation sensitization can be achieved by modulating a specific perturbed signaling pathway, and these studies have increased the enthusiasm for developing and investigating other novel agents targeting other cellular processes.