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

Mammalian TIMELESS is required for ATM-dependent CHK2 activation and G2/M checkpoint control

Timeless (Tim), a core circadian clock gene in Drosophila, is retained in mammals but has no apparent mammalian circadian clock function. Mammalian TIM is essential for ATR-dependent Chk1 activation and S-phase arrest. We report that TIM is likewise essential for ATM-dependent Chk2-mediated signaling of doxorubicin-induced DNA double strand breaks. TIM depletion attenuates doxorubicin-induced G(2)/M cell cycle arrest and sensitizes cancer cells to doxorubicin-induced cytotoxicity. TIM is, thereby, a potential novel anticancer drug target whose inhibition may enhance the therapeutic cytotoxicity of agents that activate DNA damage pathways as part of their mechanism.

Influence of DNA repair RAD51 gene variants in overall survival of non-small cell lung cancer patients treated with first line chemotherapy

PURPOSE

Lung cancer continues to be the most frequent cancer with approximately one million people worldwide dying of this disease each year. Non-small-cell lung cancer (NSCLC) accounts for approximately 80% of all lung cancers. The RAD51 protein is the key protein for homologous recombination, an evolutionarily conserved mechanism for DNA damage repair and the generation of genetic diversity. We conducted this study in order to investigate the effect of the RAD51 G135C polymorphism in treatment response to combined platinum taxanes/gemcitabine first line chemotherapy in NSCLC patients.

METHODS

We analysed RAD51 G135C polymorphism in 243 NSCLC patients using PCR-RFLP methodology.

RESULTS

There were no statistically significant differences between the groups of NSCLC patients with the different genotypes regarding tumour stage (p = 0.232). Our results indicate that the mean survival rates were statistically different according to the patient's genotypes. The group of patients carrying the C allele presented a higher mean survival rate than the other patients (56.0 months vs. 41.7 months; p = 0.024). Moreover, regarding smoking history, our results demonstrate that overall survival time differed significantly according to the patient's genotypes in smoker and ex-smoker individuals (p = 0.034). No statistically significant differences were found in the genotype frequencies and overall survival rate among non-smoker NSCLC patients (p = 0.413).

CONCLUSIONS

This is the first study evaluating the effect of the RAD51 G135C polymorphism in NSCLC patient survival. Our results suggest that RAD51 genotypes could be useful molecular markers for predicting the clinical outcome of NSCLC patients.

Logical network of genotoxic stress-induced NF-κB signal transduction predicts putative target structures for therapeutic intervention strategies

Genotoxic stress is induced by a broad range of DNA-damaging agents and could lead to a variety of human diseases including cancer. DNA damage is also therapeutically induced for cancer treatment with the aim to eliminate tumor cells. However, the effectiveness of radio- and chemotherapy is strongly hampered by tumor cell resistance. A major reason for radio- and chemotherapeutic resistances is the simultaneous activation of cell survival pathways resulting in the activation of the transcription factor nuclear factor-kappa B (NF-κB). Here, we present a Boolean network model of the NF-κB signal transduction induced by genotoxic stress in epithelial cells. For the representation and analysis of the model, we used the formalism of logical interaction hypergraphs. Model reconstruction was based on a careful meta-analysis of published data. By calculating minimal intervention sets, we identified p53-induced protein with a death domain (PIDD), receptor-interacting protein 1 (RIP1), and protein inhibitor of activated STAT y (PIASy) as putative therapeutic targets to abrogate NF-κB activation resulting in apoptosis. Targeting these structures therapeutically may potentiate the effectiveness of radio-and chemotherapy. Thus, the presented model allows a better understanding of the signal transduction in tumor cells and provides candidates as new therapeutic target structures.

Association of TP53 and MDM2 polymorphisms with survival in bladder cancer patients treated with chemoradiotherapy

Platinum-based chemoradiotherapy (CRT) as bladder conservation therapy has shown promising results for muscle-invasive bladder cancer. However, CRT might diminish survival as a result of the delay in cystectomy for some patients with non-responding bladder tumors. Because the p53 tumor suppression pathway, including its MDM2 counterpart, is important in chemotherapy- and radiotherapy-associated effects, functional polymorphisms in the TP53 and MDM2 genes could influence the response to treatment and the prognosis following CRT. We investigated associations between two such polymorphisms, and p53 overexpression, and response or survival in bladder cancer patients treated with CRT. The study group comprised 96 patients who underwent CRT for transitional cell carcinoma of the bladder. Single nucleotide polymorphisms (SNPs) in TP53 (codon 72, arginine > proline) and MDM2 (SNP309, T > G) were genotyped using PCR-RFLP, and nuclear expression levels of p53 were examined using immunohistochemistry. None of the genotypes or p53 overexpression was significantly associated with response to CRT. However, patients with MDM2 T / G + G / G genotypes had improved cancer-specific survival rates after CRT (P = 0.009). In multivariate analysis, the MDM2 T / G + G / G genotypes, and more than two of total variant alleles in TP53 and MDM2, were independently associated with improved cancer-specific survival (P = 0.031 and P = 0.015, respectively). In addition, MDM2 genotypes were significantly associated with cystectomy-free survival (P = 0.030). These results suggest that the TP53 and MDM2 genotypes might be useful prognostic factors following CRT in bladder cancer, helping patient selection for bladder conservation therapy.

N terminus of CtIP is critical for homologous recombination-mediated double-strand break repair

DNA double-strand breaks (DSBs) represent one of the most lethal types of DNA damage cells encounter. CtIP (also known as RBBP8) acts together with the MRN (MRE11-RAD50-NBS1) complex to promote DNA end resection and the generation of single-stranded DNA, which is critically important for homologous recombination repair. However, it is not yet clear exactly how CtIP participates in this process. Here, we demonstrate that besides the known conserved C terminus, the N terminus of CtIP protein is also required in DSB end resection and DNA damage-induced G(2)/M checkpoint control. We further show that both termini of CtIP can interact with the MRN complex and that the N terminus of CtIP, especially residues 22-45, binds to MRN and plays a critical role in targeting CtIP to sites of DNA breaks. Collectively, our results highlight the importance of the N terminus of CtIP in directing its localization and function in DSB repair.

The Mre11/Rad50/Nbs1 complex functions in resection-based DNA end joining in Xenopus laevis

The repair of DNA double-strand breaks (DSBs) is essential to maintain genomic integrity. In higher eukaryotes, DNA DSBs are predominantly repaired by non-homologous end joining (NHEJ), but DNA ends can also be joined by an alternative error-prone mechanism termed microhomology-mediated end joining (MMEJ). In MMEJ, the repair of DNA breaks is mediated by annealing at regions of microhomology and is always associated with deletions at the break site. In budding yeast, the Mre11/Rad5/Xrs2 complex has been demonstrated to play a role in both classical NHEJ and MMEJ, but the involvement of the analogous MRE11/RAD50/NBS1 (MRN) complex in end joining in higher eukaryotes is less certain. Here we demonstrate that in Xenopus laevis egg extracts, the MRN complex is not required for classical DNA-PK-dependent NHEJ. However, the XMRN complex is necessary for resection-based end joining of mismatched DNA ends. This XMRN-dependent end joining process is independent of the core NHEJ components Ku70 and DNA-PK, occurs with delayed kinetics relative to classical NHEJ and brings about repair at sites of microhomology. These data indicate a role for the X. laevis MRN complex in MMEJ.

Radiation carcinogenesis in context: how do irradiated tissues become tumors?

It is clear from experimental studies that genotype is an important determinant of cancer susceptibility in general, and for radiation carcinogenesis specifically. It has become increasingly clear that genotype influences not only the ability to cope with DNA damage but also influences the cooperation of other tissues, like the vasculature and immune system, necessary for the establishment of cancer. Our experimental data and that of others suggest that the carcinogenic action of ionizing radiation (IR) can also be considered a two-compartment problem: while IR can alter genomic sequence as a result of DNA damage, it can also induce signals that alter multicellular interactions and phenotypes that underpin carcinogenesis. Rather than being accessory or secondary to genetic damage, we propose that such non-targeted radiation effects create the critical context that promotes cancer development. This review focuses on experimental studies that clearly define molecular mechanisms by which cell interactions contribute to cancer in different organs, and addresses how non-targeted radiation effects may similarly act though the microenvironment. The definition of non-targeted radiation effects and their dose dependence could modify the current paradigms for radiation risk assessment since radiation non-targeted effects, unlike DNA damage, are amenable to intervention. The implications of this perspective in terms of reducing cancer risk after exposure are discussed.

Functional redundancy of exon 12 of BRCA2 revealed by a comprehensive analysis of the c.6853A>G (p.I2285V) variant

Variants of unknown significance (VUS) in BRCA1 and BRCA2 are common, and present significant challenges for genetic counseling. We observed that BRCA2: c.6853A>G (p.I2285V) (Breast Cancer Information Core [BIC] name: 7081A>G; http://research.nhgri.nih.gov/bic/) co-occurs in trans with the founder mutation c.5946delT (p.S1982RfsX22) (BIC name: 6174delT), supporting the published classification of p.I2285V as a neutral variant. However, we also noted that when compared with wild-type BRCA2, p.I2285V resulted in increased exclusion of exon 12. Functional assay using allelic complementation in Brca2-null mouse embryonic stem cells revealed that p.I2285V, an allele with exon 12 deleted and wild-type BRCA2 were all phenotypically indistinguishable, as measured by sensitivity to DNA-damaging agents, effect on irradiation-induced Rad51 foci formation, homologous recombination, and overall genomic integrity. An allele frequency study showed the p.I2285V variant was identified in 15 out of 722 (2.1%) Ashkenazi Jewish cases and 10 out of 475 (2.1%) ethnically-matched controls (odds ratio, 0.99; 95% confidence interval: 0.44-2.21; P=0.97). Thus the p.I2285V variant is not associated with an increased risk for breast cancer. Taken together, our clinical and functional studies strongly suggest that exon 12 is functionally redundant and therefore missense variants in this exon are likely to be neutral. Such comprehensive functional studies will be important adjuncts to genetic studies of variants.

The DNA-damage response in human biology and disease

The prime objective for every life form is to deliver its genetic material, intact and unchanged, to the next generation. This must be achieved despite constant assaults by endogenous and environmental agents on the DNA. To counter this threat, life has evolved several systems to detect DNA damage, signal its presence and mediate its repair. Such responses, which have an impact on a wide range of cellular events, are biologically significant because they prevent diverse human diseases. Our improving understanding of DNA-damage responses is providing new avenues for disease management.

Role for Kruppel-like factor 4 in determining the outcome of p53 response to DNA damage

Cells are incessantly exposed to many sources of genotoxic stress. A critical unresolved issue is how the resulting activation of the p53 tumor suppressor can lead to either cell cycle arrest or apoptosis depending on the extent of DNA damage. The present study shows that the level of Krüppel-like factor 4 (KLF4) expression is inversely correlated with the extent of DNA damage. KLF4 is activated by p53 following cytostatic, mild DNA damage, whereas it is strongly repressed via enhanced turnover of mRNA on severe DNA damage that irreversibly drives cells to apoptosis. Blocking the repression of KLF4 on severe DNA damage suppresses p53-mediated apoptosis, whereas ablation of the KLF4 induction on mild DNA damage shifts the p53 response from cell cycle arrest to cell death. Our results suggest that coordinate regulation of KLF4 expression depending on the extent of DNA damage may be an important mechanism that dictates the life and death decisions of p53.

Aberrantly resolved RAG-mediated DNA breaks in Atm-deficient lymphocytes target chromosomal breakpoints in cis

Canonical chromosomal translocations juxtaposing antigen receptor genes and oncogenes are a hallmark of many lymphoid malignancies. These translocations frequently form through the joining of DNA ends from double-strand breaks (DSBs) generated by the recombinase activating gene (RAG)-1 and -2 proteins at lymphocyte antigen receptor loci and breakpoint targets near oncogenes. Our understanding of chromosomal breakpoint target selection comes primarily from the analyses of these lesions, which are selected based on their transforming properties. RAG DSBs are rarely resolved aberrantly in wild-type developing lymphocytes. However, in ataxia telangiectasia mutated (ATM)-deficient lymphocytes, RAG breaks are frequently joined aberrantly, forming chromosomal lesions such as translocations that predispose (ATM)-deficient mice and humans to the development of lymphoid malignancies. Here, an approach that minimizes selection biases is used to isolate a large cohort of breakpoint targets of aberrantly resolved RAG DSBs in Atm-deficient lymphocytes. Analyses of this cohort revealed that frequently, the breakpoint targets for aberrantly resolved RAG breaks are other DSBs. Moreover, these nonselected lesions exhibit a bias for using breakpoints in cis, forming small chromosomal deletions, rather than breakpoints in trans, forming chromosomal translocations.

Do DNA repair genes OGG1, XRCC3 and XRCC7 have an impact on susceptibility to bladder cancer in the North Indian population?

OBJECTIVE

Polymorphisms in DNA repair genes may be associated with altered DNA repair capacity, thereby influencing an individual's susceptibility to smoking-related cancers such as bladder cancer. Therefore, we sought to examine the correlation between single nucleotide polymorphisms in DNA repair genes and bladder cancer.

METHODOLOGY

We undertook a case-control study of 212 urothelial bladder cancer (UBC) cases and 250 controls to investigate the association between OGG1 (C1245G rs1052133), XRCC3 (C18067T, rs861539) and XRCC7 (G6721T, rs7003908) polymorphisms and bladder cancer susceptibility by PCR-RFLP and the ARMS method. We also investigated gene-environment interactions.

RESULTS

The OGG1 GG genotype was associated with an elevated risk of urothelial bladder cancer (UBC) (OR, 2.10; p, 0.028). XRCC7 + 6721 GG was also associated with increased susceptibility to UBC (OR, 4.45; p, 0.001). In a recessive model, the OGG1 GG genotype showed an increased risk of TaG(2,3) + T1G(1-3) tumors. Additionally, the OGG1 GG genotype in non-smokers represented a 2.46-fold greater risk (OR, 2.46; p, 0.035) in bladder cancer patients. Subsequent analysis demonstrated more pronounced association of XRCC7 with smokers (OR, 4.39; p, 0.001). XRCC7 also showed increased association with TaG(2,3) + T1G(1-3) tumors and muscle invasive tumors (OR, 3.16; p, 0.001 and OR, 4.24; p, 0.001, respectively). Multiple Cox regression analysis in non-muscle invasive bladder tumor (NMIBT) patients demonstrated an association of the OGG1 GG polymorphism with a high risk of recurrence in patients on cystoscopic surveillance (HR, 4.04; p, 0.013). Subsequently, shorter recurrence-free survival (log rank p, 0.024; CC/GG, 42/24) was observed.

CONCLUSION

Our data suggest association of a variant (GG) genotype of OGG1 with increased UBC susceptibility and a high risk of tumor recurrence in NMIBT patients on cystoscopic surveillance. XRCC7 G allele carriers (TG+GG) are also at an elevated risk for susceptibility to UBC as evidenced by a high odds ratio throughout the analysis.

Glyceraldehyde 3-phosphate dehydrogenase depletion induces cell cycle arrest and resistance to antimetabolites in human carcinoma cell lines

Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is a multifunctional protein that acts at the intersection of energy metabolism and stress response in tumor cells. To elucidate the role of GAPDH in chemotherapy-induced stress, we analyzed its activity, protein level, intracellular distribution, and intranuclear mobility in human carcinoma cells A549 and UO31 after treatment with cytarabine, doxorubicin, and mercaptopurine. After treatment with cytosine arabinoside (araC), enzymatically inactive GAPDH accumulated in the nucleus. Experiments on fluorescence recovery after photobleaching with green fluorescent protein-GAPDH fusion protein in the live cells treated with araC demonstrated reduced mobility of green fluorescent protein-GAPDH inside the nucleus, indicative of interactions with nuclear macromolecular components after genotoxic stress. Depletion of GAPDH with RNA interference stopped cell proliferation, and induced cell cycle arrest in G(1) phase via p53 stabilization, and accumulation of p53-inducible CDK inhibitor p21. Neither p21 accumulation nor cell cycle arrest was detected in GAPDH-depleted p53-null NCI-H358 cells. GAPDH-depleted A549 cells were 50-fold more resistant to treatment with cytarabine (1.68 +/- 0.182 microM versus 0.03 +/- 0.015 microM in control). Depletion of GAPDH did not significantly alter cellular sensitivity to doxorubicin (0.05 +/- 0.023 microM versus 0.035 +/- 0.0154 microM in control). Induction of cell cycle arrest in p53-proficient carcinoma cells via GAPDH abrogation suggests that GAPDH-depleting agents may have a cytostatic effect in cancer cells. Our results define GAPDH as an important determinant of cellular sensitivity to antimetabolite chemotherapy because of its regulatory functions.

DNA damage responses by human ELG1 in S phase are important to maintain genomic integrity

Genomic integrity depends on DNA replication, recombination and repair, particularly in S phase. We demonstrate that a human homologue of yeast Elg1 plays an important role in S phase to preserve genomic stability. The level of ELG1 is induced during recovery from a variety of DNA damage. In response to DNA damage, ELG1 forms distinct foci at stalled DNA replication forks that are different from DNA double strand break foci. Targeted gene knockdown of ELG1 resulted in spontaneous foci formation of gamma-H2AX, 53BP1 and phosphorylated-ATM that mark chromosomal breaks. Abnormal chromosomes including fusions, inversions and hypersensitivity to DNA damaging agents were also observed in cells expressing low level of ELG1 by targeted gene knockdown. Knockdown of ELG1 by siRNA reduced homologous recombination frequency in the I-SceI induced double strand break-dependent assay. In contrast, spontaneous homologous recombination frequency and sister chromatin exchange rate were upregulated when ELG1 was silenced by shRNA. Taken together, we propose that ELG1 would be a new member of proteins involved in maintenance of genomic integrity.

The DNA-damage response: new molecular insights and new approaches to cancer therapy

The DNA of all cells is continually under assault from a wide range of DNA-damaging agents. To counter this threat to their genetic integrity, cells possess systems, collectively known as the DDR (DNA-damage response), to detect DNA damage, signal its presence and mediate its repair. In the present article, I provide an overview of the DDR and then describe how work in my laboratory and elsewhere has identified some of the key protein players that mediate cellular responses to the most cytotoxic form of DNA damage: the DNA DSB (double-strand break). I also discuss some of my laboratory's recent work, which has revealed that the way cells respond to DSBs is modulated in a cell-cycle-dependent manner to ensure that the cell uses the DSB repair system that is most suited to its cell-cycle stage. Finally, I explain how our increasing knowledge of the DDR is suggesting new avenues for treating cancer and provide an example of a DDR-inhibitory drug that is showing promise in clinical trials.

DNA double-strand breaks by asbestos, silica, and titanium dioxide: possible biomarker of carcinogenic potential?

DNA double-strand breaks (DSBs) can result in cell death or genetic alterations when cells are subjected to radiation, exposure to toxins, or other environmental stresses. A complex DNA-damage-response pathway is activated to repair the damage, and the inability to repair these breaks can lead to carcinogenesis. One of the earliest responses to DNA DSBs is the phosphorylation of a histone, H2AX, at serine 139 (gamma-H2AX), which can be detected by a fluorescent antibody. A study was undertaken to compare the induction of DNA DSBs in normal (small airway epithelial) cells and cancer cells (A549) after exposure to asbestos (crocidolite), a proven carcinogen, silica, a suspected carcinogen, and titanium dioxide (TiO(2)), an inert particle recently reported to be carcinogenic in animals. The results indicate that crocidolite induced greater DNA DSBs than silica and TiO(2), regardless of cell type. DNA DSBs caused by crocidolite were higher in normal cells than in cancer cells. Silica and TiO(2) induced higher DNA DSBs in cancer cells than in normal cells. The production of reactive oxygen species was found to be highest in cells exposed to crocidolite, followed, in potency, by silica and TiO(2). The generation of reactive oxygen species was higher in normal cells than in cancer cells. Cell viability assay indicated that crocidolite caused the greatest cytotoxicity in both cell types. Apoptosis, measured by caspase 3/7 and poly (ADP-Ribose) polymerase activation, was highest in crocidolite-exposed cells, followed by TiO(2) and silica. The results of this study indicate that crocidolite has a greater carcinogenic potential than silica and TiO(2), judged by its ability to cause sustained genomic instability in normal lung cells.

Double-strand break formation during nucleotide excision repair of a DNA interstrand cross-link

The DNA interstrand cross-link (ICL) resulting from the C4'-oxidized abasic site (C4-AP) is a unique clustered lesion comprised of a cross-link adjacent to a nick. The ICL is a substrate for the UvrABC nucleotide excision repair system. The strand containing the nick is preferentially incised, but the nick influences the cleavage sites. Moreover, in approximately 15% of the molecules, the strand opposite the nick is incised, resulting in a more toxic double-strand break. This is the first example in which an interstrand cross-link is converted by nucleotide excision misrepair into a more deleterious double-strand break.

Chromatin loops, illegitimate recombination, and genome evolution

Chromosomal rearrangements frequently occur at specific places ("hot spots") in the genome. These recombination hot spots are usually separated by 50-100 kb regions of DNA that are rarely involved in rearrangements. It is quite likely that there is a correlation between the above-mentioned distances and the average size of DNA loops fixed at the nuclear matrix. Recent studies have demonstrated that DNA loop anchorage regions can be fairly long and can harbor DNA recombination hot spots. We previously proposed that chromosomal DNA loops may constitute the basic units of genome organization in higher eukaryotes. In this review, we consider recombination between DNA loop anchorage regions as a possible source of genome evolution.

WRN helicase promotes repair of DNA double-strand breaks caused by aberrant mismatch repair of chromium-DNA adducts

Recent studies in yeast have found that processing of DNA double-strand breaks (DSB) for recombination repair involves Sgs1 helicase. Human cells have five Sgs1 homologues with unknown selectivity and significance for repair of different DSB types. Here we examined the importance of WRN helicase in repair of G(2)-specific DSB caused by abnormal mismatch repair (MMR) of ternary Cr-DNA adducts. We found that Cr(VI) induced a rapid dispersal of WRN from the nucleolus resulting in its prolonged retention in the nucleoplasm. The loss of MSH2 or MLH1 MMR proteins abolished the long-term but not the initial WRN relocalization. WRN-deficient fibroblasts were hypersensitive to Cr(VI)-induced clonogenic death and contained high levels of persistent DSB detected by gamma-H2AX/53BP1 foci and pulsed-field gel electrophoresis. WRN was involved in recombination repair of Cr-induced DNA damage, as evidenced by WRN-RAD51 colocalization and defective formation of RAD51 foci in the absence of WRN. The accumulation of unrepaired DSB in WRN-depleted cells was rescued by the inactivation of MMR, indicating that MMR-generated DSB were a key substrate for WRN action in Cr(VI)-treated cells. Competition for the limited amounts of WRN in primary cells between G(2) processes of telomere rebuilding and recombinational repair is expected to increase persistence of Cr-induced DSB and may cause telomeric abnormalities in tissues of chronically chromate-exposed workers. Our work provides the first demonstration of the major importance of WRN in repair of a specific class of DSB in human cells.

In vitro senescence of rat mesenchymal stem cells is accompanied by downregulation of stemness-related and DNA damage repair genes

Mesenchymal stem cells (MSCs) are of particular interest because they are being tested using cell and gene therapies for a number of human diseases. MSCs represent a rare population in tissues. Therefore, it is essential to grow MSCs in vitro before putting them into therapeutic use. This is compromised by senescence, limiting the proliferative capacity of MSCs. We analyzed the in vitro senescence of rat MSCs, because this animal is a widespread model for preclinical cell therapy studies. After initial expansion, MSCs showed an increased growth doubling time, lost telomerase activity, and expressed senescence-associated beta-galactosidase. Senescence was accompanied by downregulation of several genes involved in stem cell self-renewal. Of interest, several genes involved in DNA repair also showed a significant downregulation. Entry into senescence occurred with characteristic changes in Retinoblastoma (RB) expression patterns. Rb1 and p107 genes expression decreased during in vitro cultivation. In contrast, pRb2/p130 became the prominent RB protein. This suggests that RB2/P130 could be a marker of senescence or that it even plays a role in triggering the process in MSCs.

Increased sister chromatid cohesion and DNA damage response factor localization at an enzyme-induced DNA double-strand break in vertebrate cells

The response to DNA damage in vertebrate cells involves successive recruitment of DNA signalling and repair factors. We used light microscopy to monitor the genetic dependencies of such localization to a single, induced DNA double strand break (DSB) in vertebrate cells. We used an inducible version of the rare-cutting I-SceI endonuclease to cut a chromosomally integrated I-SceI site beside a Tet operator array that was visualized by binding a Tet repressor-GFP fusion. Formation of γ-H2AX foci at a single DSB was independent of ATM or Ku70. ATM-deficient cells showed normal kinetics of 53Bp1 recruitment to DSBs, but Rad51 localization was retarded. 53Bp1 and Rad51 foci formation at a single DSB was greatly reduced in H2AX-null DT40 cells. We also observed decreased inter-sister chromatid distances after DSB induction, suggesting that cohesin loading at DSBs causes elevated sister chromatid cohesion. Loss of ATM reduced DSB-induced cohesion, consistent with cohesin being an ATM target in the DSB response. These data show that the same genetic pathways control how cells respond to single DSBs and to multiple lesions induced by whole-cell DNA damage.

Chromosomal translocations caused by either pol32-dependent or pol32-independent triparental break-induced replication

Double-strand breaks (DSBs) are harmful DNA lesions that can generate chromosomal rearrangements or chromosome losses if not properly repaired. Despite their association with a number of genetic diseases and cancer, the mechanisms by which DSBs cause rearrangements remain unknown. Using a newly developed experimental assay for the analysis of translocations occurring between two chromosomes in Saccharomyces cerevisiae, we found that a single DSB located on one chromosome uses a short homologous sequence found in a third chromosome as a bridge to complete DSB repair, leading to chromosomal translocations. Such translocations are dramatically reduced when the short homologous sequence on the third chromosome is deleted. Translocations rely on homologous recombination (HR) proteins, such as Rad51, Rad52, and Rad59, as well as on the break-induced replication-specific protein Pol32 and on Srs2, but not on Ku70. Our results indicate that a single chromosomal DSB efficiently searches for short homologous sequences throughout the genome for its repair, leading to triparental translocations between heterologous chromosomes. Given the abundance of repetitive DNA in eukaryotic genomes, the results of this study open the possibility that HR rather than nonhomologous end joining may be a major source of chromosomal translocations.

The LKB1-AMPK pathway: metabolism and growth control in tumour suppression

In the past decade, studies of the human tumour suppressor LKB1 have uncovered a novel signalling pathway that links cell metabolism to growth control and cell polarity. LKB1 encodes a serine-threonine kinase that directly phosphorylates and activates AMPK, a central metabolic sensor. AMPK regulates lipid, cholesterol and glucose metabolism in specialized metabolic tissues, such as liver, muscle and adipose tissue. This function has made AMPK a key therapeutic target in patients with diabetes. The connection of AMPK with several tumour suppressors suggests that therapeutic manipulation of this pathway using established diabetes drugs warrants further investigation in patients with cancer.

Targeting DNA repair pathways: a novel approach to reduce cancer therapeutic resistance

Increased chemo-resistance and radio-resistance of cancer cells is a major obstacle in the treatment and management of malignant cancers. An important mechanism that underlies the development of such therapeutic resistance is that cancer cells recognize DNA lesions induced by DNA-damaging agents and by ionizing radiation, and repair these lesions by activating various DNA repair pathways. Therefore, Use of pharmacological agents that can inhibit certain DNA repair pathways in cancer cells has the potential for enhancing the targeted cytotoxicity of anticancer treatments and reversing the associated therapeutic resistance associated with DNA repair; such agents, offering a promising opportunity to achieve better therapeutic efficacy. Here we review the major DNA repair pathways and discuss recent advances in the development of novel inhibitors of DNA repair pathways; many of these agents are under preclinical/clinical investigation.

The BRCA1 3'-UTR: 5711+421T/T_5711+1286T/T genotype is a possible breast and ovarian cancer risk factor

BACKGROUND

A significant proportion of familial and early-onset breast and ovarian cancers occur in individuals without coding mutations of BRCA1 and BRCA2.

AIMS

We identified genetic variation at 3'-untranslated region (UTR) of BRCA1 in familial and early-onset breast and ovarian cancer patients both with and without BRCA1/2 mutation in the coding regions (BRCA1/2 pos and BRCA1/2 neg), and verified the possible cancer risk factor of the specific 3'-UTR variation using functional analysis.

METHODS

BRCA1 SNP analysis was screened in 46 patients and 103 unaffected Thais by heteroduplex analysis and DNA sequencing. After chi-square test for the potential cancer association of the specific 3'-UTR genotypes, the functional tests were conducted using several strategies of the luciferase gene expression model.

RESULTS

We document the existence of two 3'-UTR polymorphic sites, the 5711+421(G or T) and the 5711+1286(C or T). Frequency of homozygous genotype 5711+421T/T_5711+1286T/T (or T/T-T/T) in the group of BRCA1/2 neg cancer patients was triple of that seen in unaffected persons and showed a significant cancer association (p = 0.007). Functional analysis of these polymorphic sites using luciferase experiments showed an obvious significant reduction in activity associated with the T allele at both sites.

CONCLUSION

These results suggest that the inheritance of specific 3'-UTR polymorphisms may predispose individuals to early-onset or familial breast or ovarian cancer.

Lung cancer susceptibility and prognosis associated with polymorphisms in the nonhomologous end-joining pathway genes: a multiple genotype-phenotype study

BACKGROUND

Nonsmall cell lung cancer (NSCLC) frequently exhibits genomic instability, such as high fractional allelic loss (FAL). Genomic instability may result from unrepaired or misrepaired double-strand breaks (DSBs). The authors of this report postulated that polymorphisms in genes of the nonhomologous end-joining (NHEJ) pathway, which is the major DSB repair pathway in mammalian cells, may modulate lung cancer susceptibility and prognosis.

METHODS

Patients with NSCLC (n = 152) and a group of appropriate age-matched and sex-matched controls (n = 162) were subjected to genotype analysis of the NHEJ pathway genes x-ray repair complementing defective repair in Chinese hamster cells 6 (Ku70) (reference single nucleotide polymorphism number [rs] 2267437), x-ray repair complementing defective repair in Chinese hamster cells 5 (Ku80) (rs3835), x-ray repair complementing defective repair in Chinese hamster cells 4 (XRCC4) (rs1805377), and DNA ligase IV (LIG4) (rs1805388). The gene-gene interaction (joint effect), genotype-environmental (ie, smoking) correlation, and genotype-phenotype (ie, FAL) correlation were examined. The Kaplan-Meier method and log-rank tests were used to assess the prognostic effect.

RESULTS

There was a significant association between the XRCC4 and LIG4 genotypes with NSCLC risk in an analysis of individual polymorphism associations, and the risk of NSCLC increased further in a combined analysis of multiple polymorphisms (adjusted odds ratio [OR], 8.74). The patients who had a homozygous variant guanine/guanine genotype of the XRCC4 gene had a poorer prognosis compared with other patients (P = .015). There was a significant difference between the patient smokers and controls for XRCC4 (adjusted OR, 2.67) and LIG4 (adjusted OR, 2.04). In addition, polymorphisms in XRCC4 and LIG4 were linked significantly with patients who had high FAL (adjusted OR, 2.03-3.84).

CONCLUSIONS

To the authors' knowledge, this is the first nested case-control study to demonstrate a significant association between the polymorphisms of genes in the NHEJ pathway and lung cancer susceptibility and prognosis. The results may be useful for risk assessment and disease monitoring of patients with NSCLC.

Receptor signaling as a regulatory mechanism of DNA repair

Radiotherapy plays a crucial role in the treatment of many malignancies; however, locoregional disease progression remains a critical problem. This has stimulated laboratory research into understanding the basis for tumor cell resistance to radiation and the development of strategies for overcoming such resistance. We know that some cell signaling pathways that respond to normal growth factors are abnormally activated in human cancer and that these pathways also invoke cell survival mechanisms that lead to resistance to radiation. For example, abnormal activation of the epidermal growth factor receptor (EGFR) promotes unregulated growth and is believed to contribute to clinical radiation resistance. Molecular blockade of EGFR signaling is an attractive strategy for enhancing the cytotoxic effects of radiotherapy and, as shown in numerous reports, the radiosensitizing effects of EGFR antagonists correlate with a suppression of the ability of the cells to repair radiation-induced DNA double strand breaks (DSBs). The molecular connection between the EGFR and its governance of DNA repair capacity appears to be mediated by one or more signaling pathways downstream of this receptor. The purpose of this review is to highlight what is currently known regarding EGFR signaling and the processes responsible for repairing radiation-induced DNA lesions that would explain the radiosensitizing effects of EGFR antagonists.

Use of Phage Display to Isolate Specific Human Monoclonal Antibody Fragments Against a Potential Target for Multiple Myeloma

Introduction: Multiple myeloma (MM), a malignancy of plasma cells, accounts for 10% of all haematological malignancies and is currently incurable. Although it can be treated, the disease tends to relapse after several years and becomes increasingly resistant to conventional therapy. Investigations into using humoral therapy for MM are now underway with a view that novel therapeutic agents may provide a more targeted therapy for MM. Materials and Methods: Here, phage display, a faster and more efficient method compared to classical hybridoma fusion technology, was used as a proof-of-concept to isolate several single-chain Fragment variables (scFv) against Ku86. Results: Anti-Ku86 polyclonal scFvs biopanning was successful where third round scFvs (A450~1.1) showed a 1/3 increase in binding as compared to the first round scFvs (A450~0.4) with 100ug/mL of antigen (purified human Ku86). Subsequent selection and verification of monoclonal antibodies using third round biopanning revealed 4 good affinity binding clones ranging from A450~0.1 to A450~0.15 on 12.5ug/mL of antigen as compared to low binders (A450~0.07) and these antibodies bind to Ku86 in a specific and dose-dependent manner. Comparative studies were also performed with commercially available murine antibodies and results suggest that 2 of the clones may bind close to the following epitopes aa506-541 and aa1-374. Conclusions: These studies using phage display provide an alternative and viable method to screen for antibodies quickly and results show that good affinity antibodies against Ku86 have been successfully isolated and they can be used for further studies on MM and form the basis for further development as anti-cancer therapeutic agents. Key words: Antibody isolation, Ku86, Phage display, ScFv

HSSB1 and hSSB2 form similar multiprotein complexes that participate in DNA damage response

hSSB1 (human single strand DNA-binding protein 1) has been shown to participate in homologous recombination (HR)-dependent repair of DNA double strand breaks (DSBs) and ataxia telangiectasia-mutated (ATM)-mediated checkpoint pathways. Here we present evidence that hSSB2, a homolog of hSSB1, plays a role similar to hSSB1 in DNA damage-response pathways. This was evidenced by findings that hSSB2-depleted cells resemble hSSB1-depleted cells in hypersensitivity to DNA-damaging reagents, reduced efficiency in HR-dependent repair of DSBs, and defective ATM-dependent phosphorylation. Notably, hSSB1 and hSSB2 form separate complexes with two identical proteins, INTS3 and hSSBIP1 (C9ORF80). Cells depleted of INTS3 and hSSBIP1 also exhibited hypersensitivity to DNA damage reagents, chromosomal instability, and reduced ATM-dependent phosphorylation. hSSBIP1 was rapidly recruited to laser-induced DSBs, a feature also similar to that reported for hSSB1. Depletion of INTS3 decreased the stability of hSSB1 and hSSBIP1, suggesting that INTS3 may provide a scaffold to allow proper assembly of the hSSB complexes. Thus, our data demonstrate that hSSB1 and hSSB2 form two separate complexes with similar structures, and both are required for efficient HR-dependent repair of DSBs and ATM-dependent signaling pathways.

Dramatic response to platinum in a patient with cancer with a germline BRCA2 mutation

We present a case of dramatic response of poor prognosis cancer in a lady with a germline mutation in the BRCA2 gene who was exposed to platinum containing chemotherapy. She is cancer-free 10 years' later. Such cases provide clinical scenarios for the basis of trials of platinum-like agents in individuals with BRCA mutations who develop cancer.

Suppression of nonhomologous end joining repair by overexpression of HMGA2

Understanding the molecular details associated with aberrant high mobility group A2 (HMGA2) gene expression is key to establishing the mechanism(s) underlying its oncogenic potential and effect on the development of therapeutic strategies. Here, we report the involvement of HMGA2 in impairing DNA-dependent protein kinase (DNA-PK) during the nonhomologous end joining (NHEJ) process. We showed that HMGA2-expressing cells displayed deficiency in overall and precise DNA end-joining repair and accumulated more endogenous DNA damage. Proper and timely activation of DNA-PK, consisting of Ku70, Ku80, and DNA-PKcs subunits, is essential for the repair of DNA double strand breaks (DSB) generated endogenously or by exposure to genotoxins. In cells overexpressing HMGA2, accumulation of histone 2A variant X phosphorylation at Ser-139 (gamma-H2AX) was associated with hyperphosphorylation of DNA-PKcs at Thr-2609 and Ser-2056 before and after the induction of DSBs. Also, the steady-state complex of Ku and DNA ends was altered by HMGA2. Microirradiation and real-time imaging in living cells revealed that HMGA2 delayed the release of DNA-PKcs from DSB sites, similar to observations found in DNA-PKcs mutants. Moreover, HMGA2 alone was sufficient to induce chromosomal aberrations, a hallmark of deficiency in NHEJ-mediated DNA repair. In summary, a novel role for HMGA2 to interfere with NHEJ processes was uncovered, implicating HMGA2 in the promotion of genome instability and tumorigenesis.

Genetic polymorphisms in XRCC1, OGG1, APE1 and XRCC3 DNA repair genes, ionizing radiation exposure and chromosomal DNA damage in interventional cardiologists

Interventional cardiologists working in high-volume cardiac catheterization laboratory are exposed to significant occupational radiation risks. Common single-nucleotide polymorphisms (SNPs) in DNA repair genes are thought to modify the effects of low-dose radiation exposure on DNA damage, the main initiating event in the development of cancer and hereditary disease. The aim of this study was to determine the relationship between XRCC1 (Arg194Trp and Arg399Gln), OGG1 (Ser326Cys), APE1 (Asp148Glu) and XRCC3 (Thr241Met) SNPs and chromosomal DNA damage. We enrolled 77 subjects: 40 interventional cardiologists (27 male, 41.3+/-9.4 years and 13 female, 37.8+/-8.4 years) and 37 clinical cardiologists (26 male, 39.4+/-9.5 years and 11 female, 35.0+/-9.8 years) without radiation exposure as the control group. Micronucleus (MN) assay was performed as biomarker of chromosomal DNA damage and an early predictor of cancer. MN frequency was significantly higher in interventional cardiologists than in clinical physicians (19.7+/-7.8 per thousand vs. 13.5+/-6.3 per thousand, p=0.0003). Within the exposed group, individuals carrying a XRCC3 Met241 allele had higher frequency than homozygous XRCC3 Thr241 (21.2+/-7.8 per thousand vs. 16.6+/-7.1 per thousand, p=0.03). Individuals with two or more risk alleles showed a higher MN frequency when compared to subjects with one or no risk allele (18.4+/-6.6 per thousand vs. 14.4+/-6.1 per thousand, p=0.02). An interactive effect was found between smoking, exposure >10 years and the presence of the two or more risk alleles on the MN frequency (F=6.3, p=0.02). XRCC3 241Met alleles, particularly in combination with multiple risk alleles of DNA repair genes, contribute to chromosomal DNA damage levels in interventional cardiologists.

p38 Mitogen-activated protein kinase- and HuR-dependent stabilization of p21(Cip1) mRNA mediates the G(1)/S checkpoint

Activation of p38 mitogen-activated protein kinase (MAPK) plays an important role in the G(2)/M cell cycle arrest induced by DNA damage, but little is known about the role of this signaling pathway in the G(1)/S transition. Upregulation of the cyclin-dependent kinase inhibitor p21(Cip1) is thought to make a major contribution to the G(1)/S cell cycle arrest induced by gamma radiation. We show here that inhibition of p38 MAPK impairs p21(Cip1) accumulation and, as a result, the ability of cells to arrest in G(1) in response to gamma radiation. We found that p38 MAPK induces p21(Cip1) mRNA stabilization, without affecting its transcription or the stability of the protein. In particular, p38 MAPK phosphorylates the mRNA binding protein HuR on Thr118, which results in cytoplasmic accumulation of HuR and its enhanced binding to the p21(Cip1) mRNA. Our findings help to understand the emerging role of p38 MAPK in the cellular responses to DNA damage and reveal the existence of p53-independent networks that cooperate in modulating p21(Cip1) levels at the G(1)/S checkpoint.

Lithium-mediated protection of hippocampal cells involves enhancement of DNA-PK-dependent repair in mice

Long-term neurological deficiencies resulting from hippocampal cytotoxicity induced by cranial irradiation (IR) present a challenge in the treatment of primary and metastatic brain cancers, especially in children. Previously, we showed that lithium protected hippocampal neurons from IR-induced apoptosis and improved neurocognitive function in treated mice. Here, we demonstrate accelerated repair of IR-induced chromosomal double-strand breaks (DSBs) in lithium-treated neurons. Lithium treatment not only increased IR-induced DNA-dependent protein kinase (DNA-PK) threonine 2609 foci, a surrogate marker for activated nonhomologous end-joining (NHEJ) repair, but also enhanced double-strand DNA end-rejoining activity in hippocampal neurons. The increased NHEJ repair coincided with reduced numbers of IR-induced gamma-H2AX foci, well-characterized in situ markers of DSBs. These findings were confirmed in vivo in irradiated mice. Consistent with a role of NHEJ repair in lithium-mediated neuroprotection, attenuation of IR-induced apoptosis of hippocampal neurons by lithium was dramatically abrogated when DNA-PK function was abolished genetically in SCID mice or inhibited biochemically by the DNA-PK inhibitor IC86621. Importantly, none of these findings were evident in glioma cancer cells. These results support our hypothesis that lithium protects hippocampal neurons by promoting the NHEJ repair-mediated DNA repair pathway and warrant future investigation of lithium-mediated neuroprotection during cranial IR, especially in the pediatric population.

Twilight effects of low doses of ionizing radiation on cellular systems: a bird's eye view on current concepts and research

The debate about the health risks from low doses of radiation is vigorous and often acrimonious since many years and does not appear to weaken. Being far from completeness, this review presents only a bird's eye view on current concepts and research in the field. It is organized and divided in two parts. The first is dedicated to molecular responses determined by radiation-induced DNA ruptures. It focuses its attention on molecular pathways that are activated by ATM and tries to describe the variegated functions and specific roles of Chk2 and p53 and other proteins in sensing, promoting and executing DNA repair. The second part is more concerned with the risk associated with exposure to low dose radiation and possible effects that the radiation-affected cell may undergo. These effects include induction of apoptosis and mitotic catastrophe, bystander effect and genomic instability, senescence and hormetic response. Current hypotheses and research on these issues are briefly discussed.

Rapid rates of sperm DNA damage after activation in tench (Tinca tinca: Teleostei, Cyprinidae) measured using a sperm chromatin dispersion test

Spermatozoal haplotypic DNA is prone to damage, leading to male fertility problems. So far, the assessment of sperm DNA breakage has been challenging because protamines render the nuclear chromatin highly compacted. Here, we report the application of a new test to quantify DNA fragmentation in spermatozoa of an externally fertilizing teleost fish. The sperm chromatin dispersion (SCD) test uses a species-specific lysing solution to generate controlled protein depletion that, followed by DNA-specific fluorescent labelling, allows an easy morphological discrimination between nuclei affected by DNA damage. Using tench (Tinca tinca) as our model, we first trialled the test against established, but more technically demanding, assays employing in situ nick translation (ISNT) and the comet assay. The SCD test showed high concordance with ISNT, comet assay measures and a chromatin-swelling test, confirming the application of this straightforward SCD technique to various aspects of reproductive biology. Second, we examined between-male variation in DNA damage, and measured changes through time following spermatozoal activation. Between-male variation in the basal levels of average DNA damage ranged from 0 to 20% of sperm showing damage, and all showed increases in DNA fragmentation through time (0-60 min). The rates of DNA damage increase are the fastest so far recorded in sperm for a living organism, and may relate to the external fertilization mode. Our findings have relevance for broodstock selection and optimizing IVF protocols routinely used in modern aquaculture.

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.

Impaired lymphocyte development and antibody class switching and increased malignancy in a murine model of DNA ligase IV syndrome

Hypomorphic mutations in DNA ligase IV (LIG4) cause a human syndrome of immunodeficiency, radiosensitivity, and growth retardation due to defective DNA repair by the nonhomologous end-joining (NHEJ) pathway. Lig4-null mice are embryonic lethal, and better mouse models are needed to study human LigIV syndrome. We recently identified a viable mouse strain with a Y288C hypomorphic mutation in the Lig4 gene. Lig4Y288C mice exhibit a greater than 10-fold reduction of LigIV activity in vivo and recapitulate the immunodeficiency and growth retardation seen in human patients. Here, we have demonstrated that the Lig4Y288C mutation leads to multiple defects in lymphocyte development and function, including impaired V(D)J recombination, peripheral lymphocyte survival and proliferation, and B cell class switch recombination. We also highlight a high incidence of thymic tumors in the Lig4Y288C mice, suggesting that wild-type LigIV protects against malignant transformation. These findings provide explanations for the complex lymphoid phenotype of human LigIV syndrome.

Recent advance in our understanding of the molecular nature of chromosomal abnormalities

The completion of the human genome project has enabled researchers to characterize the breakpoints for various chromosomal structural abnormalities including deletions, duplications or translocations. This in turn has shed new light on the molecular mechanisms underlying the onset of gross chromosomal rearrangements. On the other hand, advances in genetic manipulation technologies for various model organisms has increased our knowledge of meiotic chromosome segregation, errors which, contribute to chromosomal aneuploidy. This review focuses on the current understanding of germ line chromosomal abnormalities and provides an overview of the mechanisms involved. We refer to our own recent data and those of others to illustrate some of the new paradigms that have arisen in this field. We also discuss some perspectives on the sexual dimorphism of some of the pathways that leads to these chromosomal abnormalities.

Mutational inactivation of the nijmegen breakage syndrome gene (NBS1) in glioblastomas is associated with multiple TP53 mutations

Nijmegen breakage syndrome caused by NBS1 germline mutations is a rare autosomal recessive disease with clinical features that include microcephaly, increased radiosensitivity, and predisposition to cancer. NBS1 plays a key role in DNA double-strand break repair and the maintenance of genomic stability. We screened 87 glioblastomas for NBS1 mutations (all 16 exons). Single-strand conformation polymorphism followed by direct DNA sequencing revealed 12 NBS1 mutations (8 missense and 4 intronic mutations) in 9 (32%) of 28 primary (de novo) glioblastomas carrying 2 or more TP53 mutations. None of the NBS1 mutations has been previously reported as a germline mutation in Nijmegen breakage syndrome patients. NBS1 mutations were not detected in 19 primary glioblastomas with 1 TP53 mutation or in 21 primary glioblastomas without TP53 mutations. Secondary glioblastomas that developed through progression from low-grade or anaplastic astrocytoma had TP53 mutations in 16 (84%) of 19 cases, but none contained mutations of the NBS1 gene. These results suggest that multiple TP53 mutations in glioblastomas are due to deficient repair of DNA double-strand breaks caused by mutational inactivation of the NBS1 gene.

Involvement of Exo1b in DNA damage-induced apoptosis

Apoptosis is essential for the maintenance of inherited genomic integrity. During DNA damage-induced apoptosis, mechanisms of cell survival, such as DNA repair are inactivated to allow cell death to proceed. Here, we describe a role for the mammalian DNA repair enzyme Exonuclease 1 (Exo1) in DNA damage-induced apoptosis. Depletion of Exo1 in human fibroblasts, or mouse embryonic fibroblasts led to a delay in DNA damage-induced apoptosis. Furthermore, we show that Exo1 acts upstream of caspase-3, DNA fragmentation and cytochrome c release. In addition, induction of apoptosis with DNA-damaging agents led to cleavage of both isoforms of Exo1. The cleavage of Exo1 was mapped to Asp514, and shown to be mediated by caspase-3. Expression of a caspase-3 cleavage site mutant form of Exo1, Asp514Ala, prevented formation of the previously observed fragment without any affect on the onset of apoptosis. We conclude that Exo1 has a role in the timely induction of apoptosis and that it is subsequently cleaved and degraded during apoptosis, potentially inhibiting DNA damage repair.

RAD50 and NBS1 form a stable complex functional in DNA binding and tethering

The RAD50/MRE11/NBS1 protein complex (RMN) plays an essential role during the early steps of DNA double-strand break (DSB) repair by homologous recombination. Previous data suggest that one important role for RMN in DSB repair is to provide a link between DNA ends. The striking architecture of the complex, a globular domain from which two extended coiled coils protrude, is essential for this function. Due to its DNA-binding activity, ability to form dimers and interact with both RAD50 and NBS1, MRE11 is considered to be crucial for formation and function of RMN. Here, we show the successful expression and purification of a stable complex containing only RAD50 and NBS1 (RN). The characteristic architecture of the complex was not affected by absence of MRE11. Although MRE11 is a DNA-binding protein it was not required for DNA binding per se or DNA-tethering activity of the complex. The stoichiometry of NBS1 in RMN and RN complexes was estimated by SFM-based volume analysis. These data show that in vitro, R, M and N form a variety of stable complexes with variable subunit composition and stoichiometry, which may be physiologically relevant in different aspects of RMN function.

DNA breakage and induction of DNA damage response proteins precede the appearance of visible mutant huntingtin aggregates

Huntington's disease (HD) is a neurodegenerative disorder that follows an autosomal-dominant inheritance pattern. The pathogenesis of the disease depends on the degree of expansion of triplet (CAG) repeats located in the first exon on the gene. An expanded polyglutamine tract within the protein huntingtin (Htt) enables a gain-of-function phenotype that is often exhibited by a dysfunctional oligomerization process and the formation of protein aggregates. How this process leads to neurodegeneration remains undefined. We report that expression of a Htt-fragment containing an expanded glutamine tract induces DNA damage and activates the DNA damage response pathway. Both single-strand and double-strand breaks are observed as the mutant protein accumulates in the cell; these breaks precede the appearance of detectable protein aggregates containing mutant Htt. We also observe activation of H2AX, ATM, and p53 in cells expressing mutant Htt, a predictable response in cells containing chromosomal breakage. Expression of wild-type Htt does not affect the integrity of DNA, nor does it activate the same pathway. Furthermore, DNA damage and activated H2AX are present in HD transgenic mice before the formation of mutant Htt aggregates and HD pathogenesis. Taken together, our data suggest that the expression of mutant Htt causes an accumulation of DNA breaks that activates the DNA damage response pathway, a process that can disable cell function. Because these events can lead to apoptosis, it is possible that the DNA damage response pathway activated by single- and double-strand breaks that we found contributes to neurodegeneration.

Visualizing the disassembly of S. cerevisiae Rad51 nucleoprotein filaments

Rad51 is the core component of the eukaryotic homologous recombination machinery and assembles into elongated nucleoprotein filaments on DNA. We have used total internal reflection fluorescence microscopy and a DNA curtain assay to investigate the dynamics of individual Saccharomyces cerevisiae Rad51 nucleoprotein filaments. For these experiments the DNA molecules were end-labeled with single fluorescent semiconducting nanocrystals. The assembly and disassembly of the Rad51 nucleoprotein filaments were visualized by tracking the location of the labeled DNA end in real time. Using this approach, we have analyzed yeast Rad51 under a variety of different reaction conditions to assess parameters that impact the stability of the nucleoprotein filament. We show that Rad51 readily dissociates from DNA in the presence of ADP or in the absence of nucleotide cofactor, but that free ATP in solution confers a fivefold increase in the stability of the nucleoprotein filaments. We also probe how protein dissociation is coupled to ATP binding and hydrolysis by examining the effects of ATP concentration, and by the use of the nonhydrolyzable ATP analogue adenosine 5'-(beta, gamma-imido) triphosphate and ATPase active-site mutants. Finally, we demonstrate that the Rad51 gain-of-function mutant I345T dissociates from DNA with kinetics nearly identical to that of wild-type Rad51, but assembles 30% more rapidly. Together, these results provide a framework for studying the biochemical behaviors of S. cerevisiae Rad51 nucleoprotein filaments at the single-molecule level.