DNA DSB Repair Research Articles

Nimotuzumab Enhances the Radiosensitivity of Cancer Cells In Vitro by Inhibiting Radiation-Induced DNA Damage Repair

BackgroundNimotuzumab is a humanized IgG1 monoclonal antibody specifically targeting EGFR. In this study, we aimed to investigate the molecular mechanisms of nimotuzumab in its effects of enhancing cancer cell radiosensitivity.Principal FindingLung cancer A549 cells and breast cancer MCF-7 cells were pretreated with or without nimotuzumab for 24 h before radiation to perform the clonogenic survival assay and to analyze the cell apoptosis by flow ctyometry. γ-H2AX foci were detected by confocal microscopy to assess the effect of nimotuzumab on radiation induced DNA repair. EGFR activation was examined and the levels of DNA damage repair related proteins in A549 cells at different time point and at varying doses exposure after nimotuzumab and radiation treatment were examined by Western blot. Pretreatment with nimotuzumab reduced clonogenic survival after radiation, inhibited radiation-induced EGFR activation and increased the radiation-induced apoptosis in both A549 cells and MCF-7 cells. The foci of γ-H2AX 24 h after radiation significantly increased in nimotuzumab pretreated cells with different doses. The phosphorylation of AKT and DNA-PKcs were remarkably inhibited in the combination group at each dose point as well as time point.ConclusionsOur results revealed that the possible mechanism of nimotuzumab enhancing the cancer radiosensitivity is that nimotuzumab inhibited the radiation-induced activation of DNA-PKcs through blocking the PI3K/AKT pathway, which ultimately affected the DNA DSBs repair.

Regenerative capacity of old muscle stem cells declines without significant accumulation of DNA damage.

The performance of adult stem cells is crucial for tissue homeostasis but their regenerative capacity declines with age, leading to failure of multiple organs. In skeletal muscle this failure is manifested by the loss of functional tissue, the accumulation of fibrosis, and reduced satellite cell-mediated myogenesis in response to injury. While recent studies have shown that changes in the composition of the satellite cell niche are at least in part responsible for the impaired function observed with aging, little is known about the effects of aging on the intrinsic properties of satellite cells. For instance, their ability to repair DNA damage and the effects of a potential accumulation of DNA double strand breaks (DSBs) on their regenerative performance remain unclear. This work demonstrates that old muscle stem cells display no significant accumulation of DNA DSBs when compared to those of young, as assayed after cell isolation and in tissue sections, either in uninjured muscle or at multiple time points after injury. Additionally, there is no significant difference in the expression of DNA DSB repair proteins or globally assayed DNA damage response genes, suggesting that not only DNA DSBs, but also other types of DNA damage, do not significantly mark aged muscle stem cells. Satellite cells from DNA DSB-repair-deficient SCID mice do have an unsurprisingly higher level of innate DNA DSBs and a weakened recovery from gamma-radiation-induced DNA damage. Interestingly, they are as myogenic in vitro and in vivo as satellite cells from young wild type mice, suggesting that the inefficiency in DNA DSB repair does not directly correlate with the ability to regenerate muscle after injury. Overall, our findings suggest that a DNA DSB-repair deficiency is unlikely to be a key factor in the decline in muscle regeneration observed upon aging.

BRCA2 is epistatic to the RAD51 paralogs in response to DNA damage

Homologous recombination plays an important role in the high-fidelity repair of DNA double-strand breaks. A central player in this process, RAD51, polymerizes onto single-stranded DNA and searches for homology in a duplex donor DNA molecule, usually the sister chromatid. Homologous recombination is a highly regulated event in mammalian cells: some proteins have direct enzymatic functions, others mediate or overcome rate-limiting steps in the process, and still others signal cell cycle arrest to allow repair to occur. While the human BRCA2 protein has a clear role in delivering and loading RAD51 onto single-stranded DNA generated after resection of the DNA break, the mechanistic functions of the RAD51 paralogs remain unclear. In this study, we sought to determine the genetic interactions between BRCA2 and the RAD51 paralogs during DNA DSB repair. We utilized siRNA-mediated knockdown of these proteins in human cells to assess their impact on the DNA damage response. The results indicate that loss of BRCA2 alone imparts a more severe phenotype than the loss of any individual RAD51 paralog and that BRCA2 is epistatic to each of the four paralogs tested.

AZD5438, an Inhibitor of CDK 1, 2, and 9, Enhances the Radiosensitivity of Non-small Cell Lung Carcinoma Cells

Radiation therapy (RT) is one of the primary treatment modalities for the treatment of non-small cell lung cancer (NSCLC). However, due to the intrinsic radiation resistance of these tumors, many patients experience RT failure which leads to considerable tumor progression including regional lymph node and distant metastasis. The purpose of this pre-clinical study is to evaluate the efficacy of a new class of CDK inhibitor, AZD5438, as a radiosensitizer in several NSCLC models that are specifically resistant to conventional fractionated RT. The combined effect of ionizing radiation and AZD5438, a highly specific inhibitor of CDK 1, 2 and 9, was determined in vitro by surviving fraction, cell cycle distribution, apoptosis, DNA double strand break (DSBs) repair and homologous recombination (HR) assays in 3 NSCLC cell lines (A549, H1299 and H460). For the in vivo studies human xenograft animal models in athymic nude mice were used. Treatment of NSCLC cells with AZD5438 significantly augments cellular radiosensitivity (DER 1.4 to 1.75). The degree of radiosensitization by AZD5438 is larger in radioresistant cell lines (A549 and H1299). This enhanced radiosensitivity is due to a prolonged G2-M arrest, inhibition of HR, delayed DNA-DSB repair and increased apoptosis. Combined treatment of AZD5438 and IR also enhances tumor growth delay with an enhancement factor (EF) ranging from 1.2 to 1.7. This study supports the evaluation of newer generation CDK inhibitors, such as AZD5438, as potent radiosensitizers in NSCLC models, especially in tumors that demonstrate variable intrinsic radiation response.

Abstract 365: DNA double-strand break imaging with anti-γH2AX antibodies does not alter DNA repair kinetics

Abstract Introduction: DNA double-strand breaks (dsb) are caused by exposure to radiotherapy and various anticancer drugs and are very deleterious. Upon dsb formation, histone H2AX is phosphorylated to form γH2AX foci at the break site. We have reported the development of a non-invasive in vivo DNA dsb imaging agent, based on anti-γH2AX IgG, modified by addition of the cell penetrating and nuclear localizing peptide, TAT. It was demonstrated that fluorescent and radiolabelled anti-γH2AX-TAT was internalized into cancer cells, and retained specifically in cells expressing γH2AX foci, through binding to γH2AX. Using in vivo fluorescence and SPECT imaging, we showed tumour uptake correlates with the extent of DNA dsb damage after radiation therapy (Cornelissen et al. Cancer Res 2011 71:4539). Ideally, imaging γH2AX in this way should not alter the physiology/ biology of the imaged organism. Therefore, we investigated the influence of anti-γH2AX-Tat on DNA dsb repair kinetics using comet assays and γH2AX and 53BP1 foci kinetics. These data were applied using a published mathematical model of non-homologous end joining (NHEJ) by Cucinotta et al. (Rad Res 2008 169, 214-222). Methods: A panel of four carcinoma cells (MCF7, MDA-MB-231/H2N, MDA-MB-468, MCF7) was exposed to a range of concentrations of anti-γH2AX-Tat (0 - 0.5 μg/mL) and irradiated 1 h later (IR; 137Cs, 0.95 Gy/min, 0-4 Gy). At selected time points, cells were analysed using γH2AX and 53BP1 foci staining and counting and comet assays. Foci counting was performed automatically using an InCell analyser. Comet assay results were analysed using software developed in house and expressed as Olive Tail Moment. γH2AX foci kinetics data were modelled in silico using a concatenary model or an extended version of the NHEJ model, allowing for anti-γH2AX-Tat binding to γH2AX. The kinetics of dsb formation and resolution were calculated from the model (dsbcal), and validated using comet assay data. Results: The number of γH2AX foci/cell was maximal at 0.5-1 h post IR. The affinity (KD) of anti-γH2AX-Tat for γH2AX was 25 nM. There was a trend towards a dose-dependent increase of the maximum number of foci. The concatenary model revealed no significant differences between treated and non-treated cells. However, using the NHEJ model, a dose-dependent decrease in the rate of phosphorylation of γH2AX was demonstrated (in MCF7 cells exposed to 0-0.5 μg/mL anti-γH2AX-Tat, κPγ = 2507-1462 copy−1 h−1; p<0.01), as well as a lower affinity of DNA-PKcs for H2AX (κM = 0.03 - 0.10 in MCF7 cells; p<0.01). Conversely, there was no significant effect on the kinetics of the calculated number of dsbs (p > 0.05). This was confirmed by comet assays. Conclusion: Anti-γH2AX-TAT binding to γH2AX reduces the rate of formation of new γH2AX foci, but has no significant influence on DNA dsb repair. Therefore, the use of γH2AX-Tat for imaging DNA dsbs does not alter DNA dsb status. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 365. doi:1538-7445.AM2012-365

Abstract 4680: DNA dependent protein kinase as a new molecular target for treatment of non-small cell lung cancer

Abstract Prognosis with non-small cell lung cancer (NSCLC) has improved with the recent introduction of molecular targeted therapy. However, it is limited to patients with epidermal growth factor receptor (EGFR) mutations, who benefit from EGFR tyrosine kinase inhibitors (EGFR-TKI). A new molecular target is needed with other NSCLC patients. In general, chemotherapeutic agents cause DNA double-strand breaks (DNA DSB), leading to apoptosis of cancer cells; DNA repair reverses their effect, resulting in resistance to chemotherapy. DNA DSB repair can be mediated via two major pathways: homologous recombination (HR) and non-homologous end-joining (NHEJ). More than 90% of DSBs in mammalian cells are repaired by NHEJ, which is accomplished by core protein components including Ku70/Ku86, DNA-dependent protein kinase (DNA-PK), XRCC4, DNA ligase IV, Artemis, and Cernunnos-XLF. We recently reported that NK314, a dual inhibitor of topoisomerase IIα (Top2β) and DNA-PK, evidences a potent anti-tumor effect in adult T-cell leukemia-lymphoma (ATL). NK314 induced DNA DSBs and inhibited DNA repair mediated through degradation of the DNA-PK catalytic subunit (DNA-PKcs). In the present study, we examined the effect of NK314 in NSCLC cell lines and assessed DNA-PK as a possible molecular target for NSCLC. Twenty separate NSCLC cell lines were used, with IC50 varying from 46-193 nM. Levels of DNA-PKcs protein varied among the cell lines, whereas those of Ku70 and Ku86 did not. The inhibitory effect of cell growth was closely related to level of DNA-PKcs protein, but not to levels of other DNA-damage-activated protein kinases such as ATM or ATR. Establishment of predictive markers concomitantly with the development of a novel molecular target is indispensable for clinical application. Because detection of DNA-PKcs in peripheral blood is difficult, we investigated an alternative marker for predicting DNA-PKcs inhibitor. We previously found that heterogeneous nuclear ribonucleoprotein B1 (hnRNP B1), an RNA binding protein, interacts with DNA-PK, and we established a system for detecting hnRNP B1 mRNA in peripheral blood. We therefore examined the suitability of hnRNP B1 as an alternative biomarker for DNA-PK inhibitors. Close correlation between levels of DNA-PKcs and hnRNP B1 proteins was established using Western blot analysis with NSCLC cell lines (p=0.995). These results suggest that DNA-PK is a novel molecular target for NSCLC, and detection of hnRNP B1 might serve as an alternative molecular marker for the amount of DNA-PK inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4680. doi:1538-7445.AM2012-4680

Chronic hypoxia compromises repair of DNA double-strand breaks to drive genetic instability

Hypoxic cells have been linked to genetic instability and tumor progression. However, little is known about the exact relationship between DNA repair and genetic instability in hypoxic cells. We therefore tested whether the sensing and repair of DNA double-strand breaks (DNA-dsbs) is altered in irradiated cells kept under continual oxic, hypoxic or anoxic conditions. Synchronized G0-G1 human fibroblasts were irradiated (0-10 Gy) after initial gassing with 0% O(2) (anoxia), 0.2% O(2) (hypoxia) or 21% O(2) (oxia) for 16 hours. The response of phosphorylated histone H2AX (γ-H2AX), phosphorylated ataxia telangiectasia mutated [ATM(Ser1981)], and the p53 binding protein 1 (53BP1) was quantified by intranuclear DNA repair foci and western blotting. At 24 hours following DNA damage, residual γ-H2AX, ATM(Ser1981) and 53BP1 foci were observed in hypoxic cells. This increase in residual DNA-dsbs under hypoxic conditions was confirmed using neutral comet assays. Clonogenic survival was also reduced in chronically hypoxic cells, which is consistent with the observation of elevated G1-associated residual DNA-dsbs. We also observed an increase in the frequency of chromosomal aberrations in chronically hypoxic cells. We conclude that DNA repair under continued hypoxia leads to decreased repair of G1-associated DNA-dsbs, resulting in increased chromosomal instability. Our findings suggest that aberrant DNA-dsb repair under hypoxia is a potential factor in hypoxia-mediated genetic instability.

MRE11 and RAD50, but not NBS1, are essential for gene targeting in the moss Physcomitrella patens

The moss Physcomitrella patens is unique among plant models for the high frequency with which targeted transgene insertion occurs via homologous recombination. Transgene integration is believed to utilize existing machinery for the detection and repair of DNA double-strand breaks (DSBs). We undertook targeted knockout of the Physcomitrella genes encoding components of the principal sensor of DNA DSBs, the MRN complex. Loss of function of PpMRE11 or PpRAD50 strongly and specifically inhibited gene targeting, whilst rates of untargeted transgene integration were relatively unaffected. In contrast, disruption of the PpNBS1 gene retained the wild-type capacity to integrate transforming DNA efficiently at homologous loci. Analysis of the kinetics of DNA-DSB repair in wild-type and mutant plants by single-nucleus agarose gel electrophoresis revealed that bleomycin-induced fragmentation of genomic DNA was repaired at approximately equal rates in each genotype, although both the Ppmre11 and Pprad50 mutants exhibited severely restricted growth and development and enhanced sensitivity to UV-B and bleomycin-induced DNA damage, compared with wild-type and Ppnbs1 plants. This implies that while extensive DNA repair can occur in the absence of a functional MRN complex; this is unsupervised in nature and results in the accumulation of deleterious mutations incompatible with normal growth and development.

The Role of Survivin for Radiation Oncology: Moving Beyond Apoptosis Inhibition

Alterations in the expression of apoptosis-related proteins, like the inhibitor of apoptosis (IAP) protein family, display a pivotal pathway by which cancer cells acquire resistance to therapeutic treatment. Among this family, survivin, the smallest and structural unique member, deserves growing attention due to its universal over-expression in human tumors, and its prominent role in disparate networks of cellular division, intracellular signaling and apoptosis. Several preclinical studies have demonstrated that targeting survivin expression by the use of small interfering RNAs, dominant negative mutants, antisense-oligonucleotides and small molecule repressors sensitized tumor cells towards chemotherapy and irradiation and reduced tumor growth potential. Due to these properties, survivin has been proposed as a molecular target for anticancer therapies. Recent studies further revealed that radio-sensitization achieved by survivin inhibition seems to be multifaceted and involves caspase-dependent and caspase-independent mechanisms. In general, an enhanced rate of apoptosis, and pronounced cell cycle arrest have been observed. More recently, a hampered DNA-damage response has been noted, indicating a distinct role of the protein in radiation-induced double strand break repair. These properties were linked to a nuclear import and physical interrelationship with members of the DNA-DSB repair machinery such as phospho-histone H2AX and DNA dependent Protein Kinase (DNA-PKcs). The applicability of survivin-driven strategies in clinical practice is currently under investigation as the first survivin inhibitors successfully entered phase I/II trials. Although these trials do not include radiation therapy at present, survivin inhibitors may represent a novel type of molecular antagonists to improve the effectiveness of radiation therapy or chemoradio-therapy.

Bortezomib Induced BRCAness Sensitizes Multiple Myeloma Cells to PARP Inhibitors

AbstractAbstract 789 Background:Poly-ADP-ribose-polymerase (PARP) inhibitors are cytotoxic to tumor cells with impaired DNA damage repair machinery (DRR), in particular those with a deficient homology directed repair (HR) of DNA double stranded breaks (DSB). Multiple Myeloma (MM) cells are characterized by a highly unstable genome and while the exact mechanisms for this karyotypic instability is largely unknown, their DDR machinery is thought to be highly stressed. The ubiquitin-proteasome system (UPS) is involved in the regulation of several cellular functions including DDR and in particular HR. In addition proteasome inhibitors are reported to induce an unfolded protein response (UPR) in MM cells resulting in their apoptotic death. We have postulated that inhibition of the 26S proteasome also alters the DNA-DSB repair machinery leading to a BRCAness state in MM cells, sensitizing them to PARP inhibitors. Methods and results:In order to biochemically inhibit PARP in MM cells, we used a novel selective inhibitor of PARP1 and PARP2, 2-(R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide or ABT-888. We first demonstrated inhibition of PARP activity as measured by a reduction in poly-ADP-ribose (PAR) polymer levels (western blotting) in human MM cell lines (MM1S, U266, H929, RPMI8226, KMS-11, OPM2, INA-6) treated with ABT-888 (5 μM). PARP inhibition and the reduction of PAR levels resulted in DNA damage as evidenced by ATM phosphorylation and induced DNA-DSBs with increased γH2AX (phospho-Ser139-H2AX) levels within 6–12 hours of MM cells treatment with ABT-888. Increased γH2AX foci formation was also detected by immunofluorescent staining within 6–12 hours of ABT-888 treatment and nearly fully resolved by 24 hours, consistent with repair of resultant DNA-DSBs. As expected treatment with ABT-888 alone had no effect on the viability of MM cells consistent with their ability to repair DNA-DSBs resulting from PARP inhibition.We then examined the effect of bortezomib on HR-mediated repair of DNA-DSBs, in particular on the BRCA/FA pathway. A significant reduction of MM cells' FANCD2, BRCA1, BRCA2 and RAD51 mRNA levels (qRT-PCR) was observed within 6–12 hours of bortezomib treatment (10 nM). Similar results were observed at the protein level indicating that bortezomib impedes homology-directed DNA-DSBs repair and results in an operational BRCAness state in MM cells. Therefore, we next tested whether this bortezomib-induced BRCAness was sufficient to sensitize MM cells to PARP inhibition with ABT-888. Consistent with our hypothesis, we observed that co-treatment of MM cell lines with bortezomib and ABT-888 lead to persistent and increased γH2AX foci at 24 hours compared to treatment with ABT-888 alone. Co-treatment also significantly potentiated cell death (Annexin V/PI staining) compared to treatment with bortezomib alone. Similar results were observed in CD138+ primary MM cells (n=8) with strong synergistic effect (CI < 1) between bortezomib and ABT-888. Importantly, no impaired viability (Annexin/PI staining) or function (colony forming unit assay) was noted for CD138− cells or CD34+ peripheral blood stem cells after bortezomib and ABT-888 co-treatment. Mechanistic studies have also shown that apoptotic events (caspase 3, caspase 8 and PARP cleavage) are markedly enhanced by this combination.Based on our in vitro data, we evaluated in vivo the activity of ABT-888 in combination with bortezomib in a Scid murine xenograft model of human MM. Significant inhibition of tumour growth (p<0.005) was noted in mice treated with the combination of bortezomib and ABT-888 compared to bortezomib alone or control-treated mice. This tumour growth inhibition also resulted in a significant increase in survival (p<0.05) of the animals. No toxicity (e.g. weight loss, ruffled coats, paralysis, etc.) was observed in mice treated with the combination. Induction of DNA-DSBs was also confirmed in vivo as shown by an increase in 53BP1 and γH2AX foci formation in tumors of mice treated with the combination compared to bortezomib alone. Conclusion:Our studies indicate that bortezomib induces a BRCAness state in MM cells by impairing HR-mediated repair of DNA-DSBs and results in a contextual synthetic lethality when combined with the PARP inhibitor ABT-888. These data provide the scientific basis for the future clinical testing of PARP inhibitors in combination with proteasome inhibitors for the treatment of MM. Disclosures:No relevant conflicts of interest to declare.

The mTOR Inhibitor Rapamycin Suppresses DNA Double-Strand Break Repair

mTOR (mammalian target of rapamycin) signaling plays a key role in the development of many tumor types. Therefore, mTOR is an attractive target for cancer therapeutics. Although mTOR inhibitors are thought to have radiosensitization activity, the molecular bases remain largely unknown. Here we show that treating MCF7 breast cancer cells with rapamycin (an mTOR inhibitor) results in significant suppression of homologous recombination (HR) and nonhomologous end joining (NHEJ), two major mechanisms required for repairing ionizing radiation-induced DNA DSBs. We observed that rapamycin impaired recruitment of BRCA1 and Rad51 to DNA repair foci, both essential for HR. Moreover, consistent with the suppressive role of rapamycin on both HR and NHEJ, persistent radiation-induced DSBs were detected in cells pretreated with rapamycin. Furthermore, the frequency of chromosome and chromatid breaks was increased in cells treated with rapamycin before and after irradiation. Thus our results show that radiosensitization by mTOR inhibitors occurs via disruption of the major two DNA DSB repair pathways.

The radiosensitizing effect of Ku70/80 knockdown in MCF10A cells irradiated with X-rays and p(66)+Be(40) neutrons

BackgroundA better understanding of the underlying mechanisms of DNA repair after low- and high-LET radiations represents a research priority aimed at improving the outcome of clinical radiotherapy. To date however, our knowledge regarding the importance of DNA DSB repair proteins and mechanisms in the response of human cells to high-LET radiation, is far from being complete.MethodsWe investigated the radiosensitizing effect after interfering with the DNA repair capacity in a human mammary epithelial cell line (MCF10A) by lentiviral-mediated RNA interference (RNAi) of the Ku70 protein, a key-element of the nonhomologous end-joining (NHEJ) pathway. Following irradiation of control and Ku-deficient cell lines with either 6 MV X-rays or p(66)+Be(40) neutrons, cellular radiosensitivity testing was performed using a crystal violet cell proliferation assay. Chromosomal radiosensitivity was evaluated using the micronucleus (MN) assay.ResultsRNAi of Ku70 caused downregulation of both the Ku70 and the Ku80 proteins. This downregulation sensitized cells to both X-rays and neutrons. Comparable dose modifying factors (DMFs) for X-rays and neutrons of 1.62 and 1.52 respectively were obtained with the cell proliferation assay, which points to the similar involvement of the Ku heterodimer in the cellular response to both types of radiation beams. After using the MN assay to evaluate chromosomal radiosensitivity, the obtained DMFs for X-ray doses of 2 and 4 Gy were 2.95 and 2.66 respectively. After neutron irradiation, the DMFs for doses of 1 and 2 Gy were 3.36 and 2.82 respectively. The fact that DMFs are in the same range for X-rays and neutrons confirms a similar importance of the NHEJ pathway and the Ku heterodimer for repairing DNA damage induced by both X-rays and p(66)+Be(40) neutrons.ConclusionsInterfering with the NHEJ pathway enhanced the radiosensitivity of human MCF10A cells to low-LET X-rays and high-LET neutrons, pointing to the importance of the Ku heterodimer for repairing damage induced by both types of radiation. Further research using other high-LET radiation sources is however needed to unravel the involvement of DNA double strand break repair pathways and proteins in the cellular response of human cells to high-LET radiation.

Purification and characterization of exonuclease-free Artemis: Implications for DNA-PK-dependent processing of DNA termini in NHEJ-catalyzed DSB repair

Artemis is a member of the β-CASP family of nucleases in the metallo-β-lactamase superfamily of hydrolases. Artemis has been demonstrated to be involved in V(D)J-recombination and in the NHEJ-catalyzed repair of DNA DSBs. In vitro, both DNA-PK independent 5′–3′ exonuclease activities and DNA-PK dependent endonuclease activity have been attributed to Artemis, though mutational analysis of the Artemis active site only disrupts endonuclease activity. This suggests that either the enzyme contains two different active sites, or the exonuclease activity is not intrinsic to the Artemis polypeptide. To distinguish between these possibilities, we sought to determine if it was possible to biochemically separate Artemis endonuclease activity from exonuclease activity. Recombinant [His] 6-Artemis was expressed in a Baculovirus insect-cell expression system and isolated using a three-column purification methodology. Exonuclease and endonuclease activities, the ability to be phosphorylated by DNA-PK, and Artemis antibody reactivity was monitored throughout the purification and to characterize final pools of protein preparation. Results demonstrated the co-elution of exonuclease and endonuclease activities on a Ni–agarose affinity column but separation of the two enzymatic activities upon fractionation on a hydroxyapatite column. An exonuclease-free fraction of Artemis was obtained that retained DNA-PK dependent endonuclease activity, was phosphorylated by DNA-PK and reacted with an Artemis specific antibody. These data demonstrate that the exonuclease activity thought to be intrinsic to Artemis can be biochemically separated from the Artemis endonuclease.

Unrelated hematopoietic stem cell transplantation for Cernunnos‐XLF deficiency

Cernunnos-XLF deficiency is a rare CI characterized by a defective DNA DSB repair mechanism. Its clinical manifestations are growth retardation, dysmorphic features, malformations, and severe B- and T-cell lymphopenia. BM failure may complicate the clinical picture. To date, there have been no described patients with CSy undergoing allogeneic HSCT. We report a case of CSy treated successfully with unrelated allogeneic HSCT after a reduced-intensity conditioning regimen. Two yr after HSCT, the patient maintains full donor engraftment, normal hematopoiesis, and progressively improving immune competence, thus suggesting that HSCT may be the treatment of choice for CSy.

Cell cycle checkpoint defects contribute to genomic instability in PTEN deficient cells independent of DNA DSB repair

Chromosomes in PTEN deficient cells display both numerical as well as structural alterations including regional amplification. We found that PTEN deficient cells displayed a normal DNA damage response (DDR) as evidenced by the ionizing radiation (IR)-induced phosphorylation of Ataxia Telangiectasia Mutated (ATM) as well as its effectors. PTEN deficient cells also had no defect in Rad51 expression or DNA damage repair kinetics post irradiation. In contrast, caffeine treatment specifically increased IR-induced chromosome aberrations and mitotic index only in cells with PTEN, and not in cells deficient for PTEN, suggesting that their checkpoints were defective. Furthermore, PTEN-deficient cells were unable to maintain active spindle checkpoint after taxol treatment. Genomic instability in PTEN deficient cells could not be attributed to lack of PTEN at centromeres, since no interaction was detected between centromeric DNA and PTEN in wild type cells. These results indicate that PTEN deficiency alters multiple cell cycle checkpoints possibly leaving less time for DNA damage repair and/or chromosome segregation as evidenced by the increased structural as well as numerical alterations seen in PTEN deficient cells.

Pretreatment with heat does not affect double-strand breaks DNA rejoining in Chlamydomonas reinhardtii

We aimed to clarify if heat pretreatment could protect Chlamydomonas reinhardtii cells from gamma rays DNA damaging action. It was studied whether: (1) heat pretreatment could accelerate DNA DSB rejoining; (2) chloroplast chaperones (HSP70B, HSP90C) could be involved in protection from radiation-induced DNA DSB. It was obtained that heat pretreatment (37–42 °C) induced minor DNA DSB levels which might be insufficient as signals for DNA DSB repair induction. No correlation between chaperones overproduction and DNA DSB rejoining was shown. These are probably the first data that HSP70B and HSP90C do not protect DNA against radiation-induced damage in a plant model system.

DNA repair gene expression profile and risk of locoregional relapse in locally advanced breast cancer patients

528 Background: The cytotoxicity of radiation therapy appears mainly mediated through the induction of DNA-double strand breaks. We explore whether DNA repair gene expression could be associated with the risk of locoregional recurrence (LRR) in locally advanced breast cancer patients. Methods: mRNA levels of 21 selected DNA repair genes were measured in tumors samples of 97 locally advanced breast cancer patients included in a phase III trial (CRH cohort), using quantitative reverse-transcriptase polymerase chain reaction. Normalized mRNA levels were evaluated for association with LRR-free survival (LRR-FS) and overall survival (OS). Results were validated in an independant cohort (Netherlands Cancer Institute: NKI cohort). Multivariate analysis, including known prognostic factors, was done to assess the association between gene expression profile of DNA repair genes and outcomes. Results: Overexpression of RAD51, PRKDC, and XRCC6 were associated with a higher risk of LRR in the CRH cohort. RAD51 was the only gene associated with LRR in the NKI cohort. With a median follow-up of 126 months (CRH cohort), the 5-year LRR-FS rates were 100% in patients (n = 61) with low RAD51, compared with 70% in patients (n = 36) with high RAD51 (p &lt; 0.0001). The 5-year OS rates were 95% in patients with low RAD51, compared with 69% in patients with high RAD51 (p = 0.00026). RAD51 overexpression was associated with a higher risk of LRR (multiadjusted hazards ratio [HR], 12.83, 95% CI: 3.6 - 45.6) and a higher risk of death (multiadjusted hazards ratio [HR], 4.10, 95% CI: 1.7 - 9.7). RAD51 was also significantly associated with shorter LRR-FS and OS in the NKI cohort. Conclusions: Our results suggest that overexpression of RAD51, a key component of the homologous recombination and the DNA DSBs repair, is associated with a higher risk of LRR and death, and may be a prognostic marker of LRR in locally advanced breast cancer patients. No significant financial relationships to disclose.

Tumor Hypoxia as a Modifier of DNA Strand Break and Cross-Link Repair

Hypoxia is a common characteristic of many solid tumors and is associated with poor prognosis. Cells with low oxygen levels can have altered sensitivity to radiotherapy and chemotherapy secondary to changes in the incidence of DNA single- and double-strand breaks (DNA-ssb, DNA-dsb), DNA base damage, DNA-DNA cross-links and DNA-protein cross-links. Recent evidence also supports that cells exposed to chronic hypoxia have a decreased capacity of DNA-dsb repair. This review will examine the influence of short-term and prolonged hypoxia on the two major pathways of DNA-dsb repair: homologous recombination (HR) and non-homologous end-joining (NHEJ). Novel treatment strategies designed to exploit the hypoxic tumor microenvironment are also discussed. Modification of DNA damage sensing and repair due to fluctuating oxygen levels within a dynamic tumor microenvironment may have profound implications for tumor progression and treatment.

Acute Normal Tissue Reactions in Head-and-Neck Cancer Patients Treated With IMRT: Influence of Dose and Association With Genetic Polymorphisms in DNA DSB Repair Genes

To investigate the association between dose-related parameters and polymorphisms in DNA DSB repair genes XRCC3 (c.-1843A>G, c.562-14A>G, c.722C>T), Rad51 (c.-3429G>C, c.-3392G>T), Lig4 (c.26C>T, c.1704T>C), Ku70 (c.-1310C>G), and Ku80 (c.2110-2408G>A) and the occurrence of acute reactions after radiotherapy. The study population consisted of 88 intensity-modulated radiation therapy (IMRT)-treated head-and-neck cancer patients. Mucositis, dermatitis, and dysphagia were scored using the Common Terminology Criteria (CTC) for Adverse Events v.3.0 scale. The population was divided into a CTC0-2 and CTC3+ group for the analysis of each acute effect. The influence of the dose on critical structures was analyzed using dose-volume histograms. Genotypes were determined by polymerase chain reaction (PCR) combined with restriction fragment length polymorphism or PCR-single base extension assays. The mean dose (D(mean)) to the oral cavity and constrictor pharyngeus (PC) muscles was significantly associated with the development of mucositis and dysphagia, respectively. These parameters were considered confounding factors in the radiogenomics analyses. The XRCC3c.722CT/TT and Ku70c.-1310CG/GG genotypes were significantly associated with the development of severe dysphagia (CTC3+). No association was found between the investigated polymorphisms and the development of mucositis or dermatitis. A risk analysis model for severe dysphagia, which was developed based on the XRCC3c.722CT/TT and Ku70c.-1310CG/GG genotypes and the PC dose, showed a sensitivity of 78.6% and a specificity of 77.6%. The XRCC3c.722C>T and Ku70c.-1310C>G polymorphisms as well as the D(mean) to the PC muscles were highly associated with the development of severe dysphagia after IMRT. The prediction model developed using these parameters showed a high sensitivity and specificity.

1-Methylxanthine enhances the radiosensitivity of tumor cells

Purpose: To determine the efficacy of a caffeine derivative 1-methylxanthine (1-MTX) in increasing radiosensitivity of cancer cells and elucidate the underlying mechanisms in vitro.Materials and methods: RKO human colorectal cancer cells carrying wild type protein 53 kDa (p53) were incubated with 3 mM 1-MTX for 30 min, exposed to 4 Gy ionizing radiation, and further incubated with 1-MTX for three days. The clonogenic cell death was determined, and the cell cycle distribution and apoptosis were studied with flow cytometry at different times after irradiation. The DNA double strand break (DNA DSB) was examined using phosphorylated Histone2A (γ-H2AX) foci formation, and the expression/activity of checkpoint 2 kinase (Chk2), cell division cycle 25 (Cdc25) phosphatase and cyclin B1/Cdc2 kinase were also investigated using western blotting and in vitro kinase assays.Results: The treatment with 3 mM 1-MTX increased the radiation-induced clonogenic and apoptotic cell death. The radiation-induced phosphorylation of Chk2 and Cdc25c and the radiation-induced increase in the cyclin B1/Cdc2 kinas activity were little affected by 1-MTX. The radiation-induced G2/M arrest was only slightly shortened and the expression of radiation-induced γ-H2AX was markedly prolonged by 1-MTX.Conclusions: 1-MTX significantly increased the radiosensitivity of RKO human colorectal cancer cells carrying wild type p53 mainly by inhibiting the repair of radiation-induced DNA DSB without causing significant alteration in radiation-induced G2/M arrest. Such a radiosensitization occurred at 1-MTX concentrations almost non-toxic to the target tumor cells.