Accumulation Of DNA Double-strand Breaks Research Articles

Oxidative Stress and Replication-Independent DNA Breakage Induced by Arsenic in Saccharomyces cerevisiae

Arsenic is a well-established human carcinogen of poorly understood mechanism of genotoxicity. It is generally accepted that arsenic acts indirectly by generating oxidative DNA damage that can be converted to replication-dependent DNA double-strand breaks (DSBs), as well as by interfering with DNA repair pathways and DNA methylation. Here we show that in budding yeast arsenic also causes replication and transcription-independent DSBs in all phases of the cell cycle, suggesting a direct genotoxic mode of arsenic action. This is accompanied by DNA damage checkpoint activation resulting in cell cycle delays in S and G2/M phases in wild type cells. In G1 phase, arsenic activates DNA damage response only in the absence of the Yku70–Yku80 complex which normally binds to DNA ends and inhibits resection of DSBs. This strongly indicates that DSBs are produced by arsenic in G1 but DNA ends are protected by Yku70–Yku80 and thus invisible for the checkpoint response. Arsenic-induced DSBs are processed by homologous recombination (HR), as shown by Rfa1 and Rad52 nuclear foci formation and requirement of HR proteins for cell survival during arsenic exposure. We show further that arsenic greatly sensitizes yeast to phleomycin as simultaneous treatment results in profound accumulation of DSBs. Importantly, we observed a similar response in fission yeast Schizosaccharomyces pombe, suggesting that the mechanisms of As(III) genotoxicity may be conserved in other organisms.

Pro-crossover factors regulate damage-dependent apoptosis in the Caenorhabditis elegans germ line

During meiosis, DNA double-strand breaks (DSBs) are physiologically induced to start the recombination process and promote the formation of interhomologue crossovers (COs), which are required to ensure faithful chromosome segregation into the gametes. The timely repair of DSBs is an essential part of the meiotic programme, as accumulation of unprocessed DSBs during the pachytene stage of meiotic prophase triggers a DNA damage checkpoint response that induces apoptosis of damaged cells. We show that CO-promoting factors MSH-4, MSH-5, and ZHP-3, but not COSA-1, are required for the apoptotic response of the meiotic DNA damage checkpoint. Lack of MSH-4 or MSH-5 suppresses the apoptotic response observed in some DNA repair-defective mutants such as fcd-2 and brc-1 (orthologues of FANCD2 and BRCA1), irrespectively of the amount of DSBs present in pachytene nuclei. Although ionizing radiation fails to induce apoptosis in msh-4/5-mutant backgrounds, it induces transcriptional activation of the apoptosis-activator egl-1, which is controlled by the Caenorhabditis elegans p53 orthologue CEP-1. This finding suggests that MSH-4/5 involvement in the apoptotic response occurs downstream or independently of damage sensing and checkpoint activation. This study establishes a role for pro-CO factors MSH-4/5 and ZHP-3 in the execution of apoptosis at late meiotic prophase following the accumulation of exogenous or endogenous DNA damage.

A human iPSC model of Ligase IV deficiency reveals an important role for NHEJ-mediated-DSB repair in the survival and genomic stability of induced pluripotent stem cells and emerging haematopoietic progenitors.

DNA double strand breaks (DSBs) are the most common form of DNA damage and are repaired by non-homologous-end-joining (NHEJ) or homologous recombination (HR). Several protein components function in NHEJ, and of these, DNA Ligase IV is essential for performing the final 'end-joining' step. Mutations in DNA Ligase IV result in LIG4 syndrome, which is characterised by growth defects, microcephaly, reduced number of blood cells, increased predisposition to leukaemia and variable degrees of immunodeficiency. In this manuscript, we report the creation of a human induced pluripotent stem cell (iPSC) model of LIG4 deficiency, which accurately replicates the DSB repair phenotype of LIG4 patients. Our findings demonstrate that impairment of NHEJ-mediated-DSB repair in human iPSC results in accumulation of DSBs and enhanced apoptosis, thus providing new insights into likely mechanisms used by pluripotent stem cells to maintain their genomic integrity. Defects in NHEJ-mediated-DSB repair also led to a significant decrease in reprogramming efficiency of human cells and accumulation of chromosomal abnormalities, suggesting a key role for NHEJ in somatic cell reprogramming and providing insights for future cell based therapies for applications of LIG4-iPSCs. Although haematopoietic specification of LIG4-iPSC is not affected per se, the emerging haematopoietic progenitors show a high accumulation of DSBs and enhanced apoptosis, resulting in reduced numbers of mature haematopoietic cells. Together our findings provide new insights into the role of NHEJ-mediated-DSB repair in the survival and differentiation of progenitor cells, which likely underlies the developmental abnormalities observed in many DNA damage disorders. In addition, our findings are important for understanding how genomic instability arises in pluripotent stem cells and for defining appropriate culture conditions that restrict DNA damage and result in ex vivo expansion of stem cells with intact genomes.

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.

Accumulation of DNA Damage-Induced Chromatin Alterations in Tissue-Specific Stem Cells: The Driving Force of Aging?

Accumulation of DNA damage leading to stem cell exhaustion has been proposed to be a principal mechanism of aging. Using 53BP1-foci as a marker for DNA double-strand breaks (DSBs), hair follicle stem cells (HFSCs) in mouse epidermis were analyzed for age-related DNA damage response (DDR). We observed increasing amounts of 53BP1-foci during the natural aging process independent of telomere shortening and after protracted low-dose radiation, suggesting substantial accumulation of DSBs in HFSCs. Electron microscopy combined with immunogold-labeling showed multiple small 53BP1 clusters diffusely distributed throughout the highly compacted heterochromatin of aged HFSCs, but single large 53BP1 clusters in irradiated HFSCs. These remaining 53BP1 clusters did not colocalize with core components of non-homologous end-joining, but with heterochromatic histone modifications. Based on these results we hypothesize that these lesions were not persistently unrepaired DSBs, but may reflect chromatin rearrangements caused by the repair or misrepair of DSBs. Flow cytometry showed increased activation of repair proteins and damage-induced chromatin modifications, triggering apoptosis and cellular senescence in irradiated, but not in aged HFSCs. These results suggest that accumulation of DNA damage-induced chromatin alterations, whose structural dimensions reflect the complexity of the initial genotoxic insult, may lead to different DDR events, ultimately determining the biological outcome of HFSCs. Collectively, our findings support the hypothesis that aging might be largely the remit of structural changes to chromatin potentially leading to epigenetically induced transcriptional deregulation.

In vivo γ-irradiation low dose threshold for suppression of DNA double strand breaks below the spontaneous level in mouse blood and spleen cells

There is a considerable controversy as to whether DNA damage induced by low doses and low dose rates of ionizing radiation is treated by cellular defence mechanisms in ways similar to that induced at high doses and high dose rates, and what downstream delayed effects may be caused by low doses compared to moderate and high doses. This constitutes the major challenge for the linear no-threshold model currently used for radiological risk estimates. Among the various DNA lesions induced by ionizing radiation, DNA double strand breaks (DSBs) are considered the most important due to their potential to cause cell death, mutagenesis and carcinogenesis. This study examined the accumulation of DNA DSBs in mouse blood leucocytes and splenocytes after long-term, chronic low dose γ-irradiation in vivo, and how this exposure may alter cell sensitivity to acute high dose irradiation. Animals were irradiated for 40, 80 or 120 days at a dose rate of 0.15mGy/h, with total accumulated doses of 144, 288 and 432mGy. DNA DSBs were measured in blood leucocytes and splenocytes using the neutral comet assay. We found that after an initial slight increase in the level of DNA DSBs at 40 days of exposure compared to controls, there was a subsequent drop after either 80 (P<0.01) or 120 days of exposure (P=0.066 for blood leucocytes; P=0.024 for splenocytes). Interestingly, the DNA breaks level after both 80 and 120 days of exposure was lower than in control. Similarly, the cells exposed to the chronic radiation for 80 and 120 days were less sensitive to the induction of DNA DSBs by acute 4Gy irradiation, whereas 40 days of exposure did not significantly modify the radiosensitivity. Our results indirectly indicate that low level ionizing radiation in vivo may trigger inducible repair of both endogenous and exogenous DNA DSBs, and that there is a dose threshold for this inducible defence mechanism, below which it does not occur. These data provide new evidence, now at the molecular level in vivo, that the dose–response for DNA DSBs at very low doses and dose rates is not linear.

Abstract LB-213: The antitumor effect of trabectedin (TR) is potentiated by olaparib (OL) in preclinical models of bone and soft tissue sarcomas (STS).

Abstract Background. TR is a DNA-binding alkaloid approved in Europe for the treatment STS as second/third line therapy. TR interferes with gene transcription and nucleotide excision repair, inducing DNA double strand breaks (DSBs). There is a strong clinical interest to increase TR activity by combination with other anti-cancer drugs. PARP-1 inhibitors (PARPi) which disable DNA base-excision repair mechanism causing the accumulation of DSBs, look like a worth to explore TR therapeutic partners. Our preclinical studies were focused on the effects of the combination of TR with the PARPi (OL and veliparib). Methods: We explored DNA damage by comet assay and P-H2AX determination, PARP-1 activity by polyADPribosylation assay in a panel of 18 sarcoma cell lines, evaluating cell viability after 72h treatment with escalating doses of TR, PARPi, and their constant combination. Colony growth, cell cycle, apoptosis and DNA damage were evaluated. Antitumoral activity was checked after 4 weeks of treatment with 3 endovenous injection of TR once a week and OL intraperitoneally 5 days/week, and compared to control and single agents in bone (SJSA-1) and soft tissue sarcoma (DMR) xenografted NOD/SCID mice. TUNEL assay, P-H2AX, PCNA and CD31 immunohistochemistry was done on explanted xenografts. Results: We showed TR-induced DNA damage and PARP-1 activation in sarcoma sensitive cell line. The addition of PARPi completely blocked basal and TR-induced PARP-1 activation. DNA damage response and checkpoints (ATM, ATR, BRCA1, CHK1, CHK2, p53) were activated after single agents and combination treatment inducing cell cycle arrest in S/G2 phase. This block was released after 48h in single agent-treated cells but not in combination-treated cells leading to apoptotic cell death. Colony growth assay and viability tests revealed a strong synergism of the two drugs (Combination Index &amp;lt; 1, based on Chou-Talalay method). Different sarcoma histotypes displayed differences in sensitivity to both drugs and combination (Ewing's sarcoma the most sensitive&amp;gt; myxoid liposarcoma &amp;gt; leiomyosarcoma &amp;gt; osteosarcoma ≥ undifferentiated pleomorphic &amp;gt; fibrosarcoma). The antitumor effect of combined TR and OL was shown after in vivo treatment of SJSA-1 and DMR xenografted mice. A significant tumor volume reduction was observed if compared to both control and single agent and was attributed to increased DNA damage, apoptosis and reduced proliferation, but not to anti-angiogenic potentiation. In fact, a direct effect on tumor vasculature was shown in TR and combination treated group vs control. Conclusions: Our results validate the biological rationale to combine TR and PARPi in sarcoma and suggest exploring this pharmacological approach in the clinical setting. Citation Format: Ymera Pignochino, Federica Capozzi, Carmine Dell'aglio, Marco Basiricò, Lorenzo D'ambrosio, Danilo Galizia, Erica Palesandro, Maria Serena Benassi, Massimo Aglietta, Giovanni Grignani. The antitumor effect of trabectedin (TR) is potentiated by olaparib (OL) in preclinical models of bone and soft tissue sarcomas (STS). [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-213. doi:10.1158/1538-7445.AM2013-LB-213

Abstract 3112: The homeoprotein DLX4 induces topoisomerase IIα expression but reduces sensitivity of tumor cells to topoisomerase II poisons.

Abstract Chemotherapeutic drugs such as anthracyclines and etoposide that target topoisomerase II (TOP2) induce tumor cell death by causing accumulation of DNA double-strand breaks (DSBs). Experimental studies have shown that overexpression of topoisomerase IIα (TOP2A) increases sensitivity to TOP2 poisons. However, the clinical value of TOP2A in predicting responsiveness to TOP2 poisons has been controversial and it is not known why some TOP2A-overexpressing tumors respond poorly to these drugs. We identified that expression of TOP2A, the gene encoding TOP2, is induced by DLX4, a homeoprotein that is overexpressed in breast and ovarian cancers. Analysis of breast cancer datasets revealed that TOP2A-High cases that expressed DLX4 at low levels responded more favorably to anthracycline-based chemotherapy than TOP2A-High cases that also highly expressed DLX4. Overexpression of TOP2A alone in tumor cells increased sensitivity to TOP2 poisons. However, DLX4 decreased sensitivity to these drugs irrespective of its induction of TOP2A. We identified that DLX4 stimulates repair of DSBs and interacts with Ku proteins that are components of the non-homologous end-joining machinery. DLX4 did not reduce levels of TOP2 poison-induced DSBs in Ku-deficient cells, but reduced the level of DSBs when Ku was reconstituted in these cells. Taken together, our findings indicate that DLX4 induces TOP2A expression, but reduces the sensitivity of tumor cells to TOP2 poisons by stimulating Ku-dependent repair of DSBs. These dual and opposing effects of DLX4 could explain why some tumors with elevated TOP2A levels are not highly sensitive to TOP2 poisons. Citation Format: Bon Q. Trinh, Song Yi Ko, Nicolas Barengo, Shiaw-Yih Lin, Honami Naora. The homeoprotein DLX4 induces topoisomerase IIα expression but reduces sensitivity of tumor cells to topoisomerase II poisons. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3112. doi:10.1158/1538-7445.AM2013-3112

Design, Synthesis, and Biological Evaluation of a Series of Benzo[de][1,7]naphthyridin-7(8H)-ones Bearing a Functionalized Longer Chain Appendage as Novel PARP1 Inhibitors

A series of benzo[de][1,7]naphthyridin-7(8H)-ones possessing a functionalized long-chain appendage have been designed and evaluated as novel PARP1 inhibitors. The initial effort led to the first-generation PARP1 inhibitor 26 bearing a terminal phthalazin-1(2H)-one framework and showing remarkably high PARP1 inhibitory activity (0.31 nM) but only moderate potency in the cell. Further effort generated the second-generation lead 41, showing high potency against both the PARP1 enzyme and BRCA-deficient cells, especially for the BRCA1-deficient MDA-MB-436 cells (CC50 < 0.26 nM). Mechanistic studies revealed that the new PARP1 inhibitors significantly inhibited H2O2-triggered PARylation in SKOV3 cells, induced cellular accumulation of DNA double-strand breaks, and impaired cell-cycle progression in BRCA2-deficient cells. Significant potentiation on the cytotoxicity of Temozolomide was also observed. The unique structural character and exceptionally high potency of 41 made it stand out as a promising drug candidate worthy for further evaluation.

Targeting XRCC1 Deficiency in Breast Cancer for Personalized Therapy

XRCC1 is a key component of DNA base excision repair, single strand break repair, and backup nonhomologous end-joining pathway. XRCC1 (X-ray repair cross-complementing gene 1) deficiency promotes genomic instability, increases cancer risk, and may have clinical application in breast cancer. We investigated XRCC1 expression in early breast cancers (n = 1,297) and validated in an independent cohort of estrogen receptor (ER)-α-negative breast cancers (n = 281). Preclinically, we evaluated XRCC1-deficient and -proficient Chinese hamster and human cancer cells for synthetic lethality application using double-strand break (DSB) repair inhibitors [KU55933 (ataxia telangectasia-mutated; ATM inhibitor) and NU7441 (DNA-PKcs inhibitor)]. In breast cancer, loss of XRCC1 (16%) was associated with high grade (P < 0.0001), loss of hormone receptors (P < 0.0001), triple-negative (P < 0.0001), and basal-like phenotypes (P = 0.001). Loss of XRCC1 was associated with a two-fold increase in risk of death (P < 0.0001) and independently with poor outcome (P < 0.0001). Preclinically, KU55933 [2-(4-Morpholinyl)-6-(1-thianthrenyl)-4H-pyran-4-one] and NU7441 [8-(4-Dibenzothienyl)-2-(4-morpholinyl)-4H-1-benzopyran-4-one] were synthetically lethal in XRCC1-deficient compared with proficient cells as evidenced by hypersensitivity to DSB repair inhibitors, accumulation of DNA DSBs, G2-M cell-cycle arrest, and induction of apoptosis. This is the first study to show that XRCC1 deficiency in breast cancer results in an aggressive phenotype and that XRCC1 deficiency could also be exploited for a novel synthetic lethality application using DSB repair inhibitors. Cancer Res; 73(5); 1621-34. ©2012 AACR.

Homologous Recombination DNA Repair Genes Play a Critical Role in Reprogramming to a Pluripotent State

Induced pluripotent stem cells (iPSCs) hold great promise for personalized regenerative medicine. However, recent studies show that iPSC lines carry genetic abnormalities, suggesting that reprogramming may be mutagenic. Here, we show that the ectopic expression of reprogramming factors increases the level of phosphorylated histone H2AX, one of the earliest cellular responses to DNA double-strand breaks (DSBs). Additional mechanistic studies uncover a direct role of the homologous recombination (HR) pathway, a pathway essential for error-free repair of DNA DSBs, in reprogramming. This role is independent of the use of integrative or nonintegrative methods in introducing reprogramming factors, despite the latter being considered a safer approach that circumvents genetic modifications. Finally, deletion of the tumor suppressor p53 rescues the reprogramming phenotype in HR-deficient cells primarily through the restoration of reprogramming-dependent defects in cell proliferation and apoptosis. These mechanistic insights have important implications for the design of safer approaches to creating iPSCs.

Accumulation of DNA double-strand breaks underlies the ageing of oocytes in mice and humans

Accumulation of DNA double-strand breaks underlies the ageing of oocytes in mice and humans

Histone H3K56 Acetylation, Rad52, and Non-DNA Repair Factors Control Double-Strand Break Repair Choice with the Sister Chromatid

DNA double-strand breaks (DSBs) are harmful lesions that arise mainly during replication. The choice of the sister chromatid as the preferential repair template is critical for genome integrity, but the mechanisms that guarantee this choice are unknown. Here we identify new genes with a specific role in assuring the sister chromatid as the preferred repair template. Physical analyses of sister chromatid recombination (SCR) in 28 selected mutants that increase Rad52 foci and inter-homolog recombination uncovered 8 new genes required for SCR. These include the SUMO/Ub-SUMO protease Wss1, the stress-response proteins Bud27 and Pdr10, the ADA histone acetyl-transferase complex proteins Ahc1 and Ada2, as well as the Hst3 and Hst4 histone deacetylase and the Rtt109 histone acetyl-transferase genes, whose target is histone H3 Lysine 56 (H3K56). Importantly, we use mutations in H3K56 residue to A, R, and Q to reveal that H3K56 acetylation/deacetylation is critical to promote SCR as the major repair mechanism for replication-born DSBs. The same phenotype is observed for a particular class of rad52 alleles, represented by rad52-C180A, with a DSB repair defect but a spontaneous hyper-recombination phenotype. We propose that specific Rad52 residues, as well as the histone H3 acetylation/deacetylation state of chromatin and other specific factors, play an important role in identifying the sister as the choice template for the repair of replication-born DSBs. Our work demonstrates the existence of specific functions to guarantee SCR as the main repair event for replication-born DSBs that can occur by two pathways, one Rad51-dependent and the other Pol32-dependent. A dysfunction can lead to genome instability as manifested by high levels of homolog recombination and DSB accumulation.

Dual Functions of the Homeoprotein DLX4 in Modulating Responsiveness of Tumor Cells to Topoisomerase II-Targeting Drugs

Topoisomerase II (TOP2)-targeting poisons such as anthracyclines and etoposide are commonly used for cancer chemotherapy and kill tumor cells by causing accumulation of DNA double-strand breaks (DSB). Several lines of evidence indicate that overexpression of TOP2A, the gene encoding topoisomerase IIα, increases sensitivity of tumor cells to TOP2 poisons, but it is not clear why some TOP2A-overexpressing (TOP2A-High) tumors respond poorly to these drugs. In this study, we identified that TOP2A expression is induced by DLX4, a homeoprotein that is overexpressed in breast and ovarian cancers. Analysis of breast cancer datasets revealed that TOP2A-high cases that also highly expressed DLX4 responded more poorly to anthracycline-based chemotherapy than TOP2A-high cases that expressed DLX4 at low levels. Overexpression of TOP2A alone in tumor cells increased the level of DSBs induced by TOP2 poisons. In contrast, DLX4 reduced the level of TOP2 poison-induced DSBs irrespective of its induction of TOP2A. DLX4 did not stimulate homologous recombination-mediated repair of DSBs. However, DLX4 interacted with Ku proteins, stimulated DNA-dependent protein kinase activity, and increased erroneous end-joining repair of DSBs. Whereas DLX4 did not reduce levels of TOP2 poison-induced DSBs in Ku-deficient cells, DLX4 stimulated DSB repair and reduced the level of TOP2 poison-induced DSBs when Ku was reconstituted in these cells. Our findings indicate that DLX4 induces TOP2A expression but reduces sensitivity of tumor cells to TOP2 poisons by stimulating Ku-dependent repair of DSBs. These opposing activities of DLX4 could explain why some TOP2A-overexpressing tumors are not highly sensitive to TOP2 poisons.

RAD52-Dependent Synthetic Lethality Eradicates Leukemia Stem Cells

Abstract 3▪▪This icon denotes a clinically relevant abstractWe showed that imatinib-naive and imatinib-treated BCR-ABL1 kinase-positive chronic myeloid leukemia in chronic phase (CML-CP) Lin−CD34+CD38− stem cells (LSCs) and Lin−CD34+CD38+ leukemia progenitor cells (LPCs) accumulate high numbers of the reactive oxygen species (ROS)-induced DNA double-strand breaks (DSBs) which, if not repaired, are lethal (Nieborowska-Skorska et al., Blood, 2012). Genome-wide microarray screen indicated that LSCs overexpress numerous genes responsible for homologous recombination repair (HRR) of DSBs. Direct targeting of RAD51, a key element in HRR, by small molecule inhibitor exerted anti-CML effect, but normal cells were also affected, indicating that RAD51 is not a preferable target. In normal cells HRR depends primarily on BRCA1/BRCA2-RAD51 pathway, while RAD52-RAD51 pathway may serve as a back-up. However BRCA1 protein is downregulated in CML, and RAD51(F259V) mutant (RAD52 binding-deficient) induced apoptosis and reduced cell growth in BCR-ABL1 –dependent manner, thus highlighting the potential role of alternative RAD52-driven HRR in CML. The absence of RAD52 (Rad52−/−) inhibited BCR-ABL1 –mediated cell cycle progression and clonogenic activity, induced apoptosis, reduced Lin−Kit+Sca1+CD34−Flt3− long-term LSCs and Lin−Kit+Sca1+CD34+Flt3− short-term LSCs, and abrogated leukemogenesis in murine model of CML. At the same time RAD52 was expendable in normal cells. In addition, RAD52 was essential to prevent accumulation of ROS-induced lethal DSBs in BCR-ABL1 –positive murine LSCs. Thus it appears that RAD52 is necessary to repair numerous ROS-induced DSBs in LSCs to promote leukemogenesis. BCR-ABL1 kinase interacts with and phosphorylates RAD52 on Y104, but phosphorylation-deficient RAD52(Y104F) mutant did not exert any negative effect on BCR-ABL1 kinase-driven leukemogenesis. This observation suggests that RAD52 activity is preserved in imatinib-treated cells. On the other hand, DNA binding-deficient RAD52(F79A) and RAD52(K102A) mutants (disrupt DNA binding domain I and II, respectively) inhibited clonogenic potential of Lin−CD34+ CML-CP cells and BCR-ABL1 –positive Lin−Kit+Sca1+ murine LSCs, but not normal counterparts. Therefore, RAD52-mediated DNA binding activity appears to be essential for BCR-ABL1 kinase-driven leukemogenesis, but not normal hematopoiesis. RAD52 DNA I binding groove containing F79 forms a pocket/niche. Peptide aptamer containing amino acid residues surrounding RAD52 F79, but not that with F79A substitution, inhibited RAD52 foci, RAD52-dependent RAD51 foci and HRR activity, resulting in elevation of the number of lethal DSBs and abrogation of clonogenic activity of BCR-ABL1-positive murine leukemia cells. Aptamer F79 reduced the growth of LSCs and LPCs from CML-CP and more aggressive CML-accelerated phase (CML-AP), depleted quiescent LSCs by attrition and eradicated BCR-ABL1 leukemia from SCID mice. Moreover, aptamer F79 enhanced the anti-CML effects of imatinib. The effect of aptamer F79 in BCR-ABL1 leukemia cells depended on downregulation of BRCA1 implicating “synthetic lethality”. At the same time the aptamer did not exert any measurable negative effects on normal cells and tissues. In conclusion, we postulate that CML-CP/AP cells are “addicted” to RAD52 and that targeting of DNA binding domain of RAD52 may induce “synthetic lethality” to eliminate proliferating LSCs/LPCs and to deplete quiescent LSCs. Moreover, anti-RAD52 F79 aptamer exerted anti-tumor activity in acute myeloid leukemia primary cells, and in cell lines derived from carcinomas of breast, ovary and pancreas displaying BRCA1 and/or BRCA2 deficiency. Thus, BRCA1/BRCA2ness-driven “addiction” to RAD52 may be selectively targeted to achieve “synthetic lethality” in wide-range of tumors while being harmless to normal cells. Disclosures:Copland:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Pfizer: Honoraria.

350 Poly (ADP-ribose) Polymerase-1 (PARP-1) Inhibitors Potentiate Trabectedin Activity in Preclinical Models of Bone and Soft Tissue Sarcomas

Background: Trabectedin (TR) is a DNA-binding alkaloid approved in Europe for the treatment of soft tissue sarcoma as second/third line therapy. TR binds to the minor groove of DNA and interferes with gene transcription and nucleotide excision repair, inducing DNA double strand breaks (DSBs), and S/G2 cell cycle arrest. There is a strong clinical interest to increase TR activity by combination with other anti-cancer drugs. PARP-1 inhibitors (PARP1-i) which disable DNA base-excision repair mechanism causing the accumulation of DSBs, look like a worth to explore TR therapeutic partners. We focused our in vitro studies on the effects of the combination of TR with the PARP1-i olaparib (OL) and veliparib (VEL). Methods:We explored DNA damage by comet assay and phospho-histone H2AX determination and PARP-1 activity by polyADPribosylation assay in a panel of sarcoma cell lines treated with TR, OL,VEL and combinations. We evaluated cell viability after 72h treatment with escalating doses of TR (0−2nM), PARP1-i (VEL:0−80mcM; OL:0−20mcM), and their constant combinations. Following colony formation, cell cycle, apoptosis and DNA damage response were checked. Results: We demonstrated that TR-induced DNA damage and PARP1 activation in sarcoma sensitive cell line. The addition of PARP1i completely blocked basal and TR-induced PARP-1 activation. DNA damage response and checkpoints (ATM, ATR, BRCA1, CHK1, CHK2, p53) are activated after treatments with single agents and combinations inducing cell cycle arrest in S/G2 phase. This block was release after 48h in single agents-treated cells but not in combinations-treated cells leading to apoptotic cell death. Colony growth assay and viability tests revealed a strong synergism of the two drugs (Combination Index leyomiosarcoma > osteosarcoma undifferentiated pleomorphic > fibrosarcoma the most resistant). Determination of molecular markers of sensitivity and in vivo antitumoral activity studies are ongoing. Conclusions: Our results validate the biological rationale to combine TR and PARP1-i in sarcomas and suggest exploring this pharmacological approach in the clinical setting.

351 Temozolomide-sensitizing Effects of ABT-888 Are Limited to Inherently TMZ-sensitive MGMT Hypermethylated GBM Xenografts

Background: Trabectedin (TR) is a DNA-binding alkaloid approved in Europe for the treatment of soft tissue sarcoma as second/third line therapy. TR binds to the minor groove of DNA and interferes with gene transcription and nucleotide excision repair, inducing DNA double strand breaks (DSBs), and S/G2 cell cycle arrest. There is a strong clinical interest to increase TR activity by combination with other anti-cancer drugs. PARP-1 inhibitors (PARP1-i) which disable DNA base-excision repair mechanism causing the accumulation of DSBs, look like a worth to explore TR therapeutic partners. We focused our in vitro studies on the effects of the combination of TR with the PARP1-i olaparib (OL) and veliparib (VEL). Methods:We explored DNA damage by comet assay and phospho-histone H2AX determination and PARP-1 activity by polyADPribosylation assay in a panel of sarcoma cell lines treated with TR, OL,VEL and combinations. We evaluated cell viability after 72h treatment with escalating doses of TR (0−2nM), PARP1-i (VEL:0−80mcM; OL:0−20mcM), and their constant combinations. Following colony formation, cell cycle, apoptosis and DNA damage response were checked. Results: We demonstrated that TR-induced DNA damage and PARP1 activation in sarcoma sensitive cell line. The addition of PARP1i completely blocked basal and TR-induced PARP-1 activation. DNA damage response and checkpoints (ATM, ATR, BRCA1, CHK1, CHK2, p53) are activated after treatments with single agents and combinations inducing cell cycle arrest in S/G2 phase. This block was release after 48h in single agents-treated cells but not in combinations-treated cells leading to apoptotic cell death. Colony growth assay and viability tests revealed a strong synergism of the two drugs (Combination Index leyomiosarcoma > osteosarcoma undifferentiated pleomorphic > fibrosarcoma the most resistant). Determination of molecular markers of sensitivity and in vivo antitumoral activity studies are ongoing. Conclusions: Our results validate the biological rationale to combine TR and PARP1-i in sarcomas and suggest exploring this pharmacological approach in the clinical setting.

Treatment with the PARP-inhibitor PJ34 causes enhanced doxorubicin-mediated cell death in HeLa cells

Adjuvant therapies can incorporate a number of different drugs to minimize the cardiotoxicity of cancer chemotherapy, decrease the development of drug resistance and increase the overall efficacy of the treatment regime. Topoisomerase IIα is a major target of many commonly used anticancer drugs, where cell death is brought about by an accumulation of double-strand DNA breaks. Poly (ADP-ribose) polymerase (PARP)-1 has been extensively studied for its role in the repair of double-strand DNA breaks, but its ability to add highly negative biopolymers (ribosylation) to target proteins provides a vast number of pathways where it can also be important in mediating cell death. In this study, we combine the classical topoisomerase IIα poison doxorubicin with the PARP inhibitor PJ34 to investigate the potentiation of chemotherapeutic efficiency in HeLa cells. We demonstrate that PJ34 treatment has the capacity to increase endogenous topoisomerase IIα protein by about 20%, and by combining doxorubicin treatment with PJ34, we observed a 50% improvement in doxorubicin-mediated cell death in HeLa cells. These results were correlated with the ribosylation of transcription factor specificity factor 1 after doxorubicin treatment, thereby altering its affinity for binding to known regulatory elements within the human topoisomerase IIα promoter. Taken together, these results highlight the synergistic potential of combining PARP inhibitors with classical topoisomerase IIα-targeting drugs.

Preclinical study of trabectedin (TR) and poly(ADP-ribose)polymerase 1 (PARP-1) inhibitor combination in soft tissue sarcoma (STS).

10066 Background: TR is an alkylating agent approved in Europe for the treatment of advanced STS as second/third line therapy. TR binds to the minor groove of DNA and interferes with gene transcription and nucleotide excision repair mechanism, inducing DNA double strand breaks (DSBs), and S/G2 cell cycle arrest. There is a strong clinical interest to increase TR activity combining it with other anti-cancer drugs. PARP-1 inhibitors disable DNA base-excision repair mechanism causing the accumulation of DSBs and look like a reasonable TR partner to be explored. We focused our in vitro studies on the effects of the combination of TR with the PARP-1 inhibitor Olaparib (OL). Methods: We explored the activity of TR-OL combination against a panel of different histotypes of STS cell lines, evaluating cell viability after 72h treatment with escalating doses of TR (0-2 nM), OL (0-20 µM), and their constant combination. Following colony formation, cell cycle, apoptosis (annexin V+/PI+) and DNA damage (phospho-histone H2AX - Ser139) were checked. Results: The TR-OL combination strongly affects STS cell viability, showing synergism (Combination Index &lt; 1, based on Chou–Talalay method) on 8 out of 13 cell lines tested. We observed a strong synergism, as a massive reduction of colony growth (402.91, MES-SA, and DMR-SN-8.4.98 lines) induced by the combination if compared with each single agent (78% vs. ~27% : p&lt;0.05). OL potentiates the S/G2 cell-cycle arrest caused by TR at 48h (Control= 29%, TR= 33.4%, OL= 31.5%, TR-OL= 80.9%), and induces a strong increase of the apoptotic cells at 72h (Control= 17.4%, TR= 28.3%, OL= 33.5%, TR-OL= 56.8%). Furthermore, the TR-OL synergism on DNA damage is confirmed by a significant increase of the DSBs marker (Control= 8.7%, TR= 59.5%, OL= 23.8%, TR-OL= 76.5%). Conclusions: These results validate the biological rationale to combine TR and PARP-1 inhibitors in STS and suggest assessing this drug combination in the clinical setting.

Abstract LB-263: Synthetic lethal targeting of PTEN-associated homologous recombination (HR) deficient melanoma cells by human apurinic/apyrimidinic endonuclease (APE1) inhibitors

Abstract Background Mutation of the tumour suppressor PTEN is well-characterised in many advanced malignancies, including 30-50% of metastatic melanomas. Emerging evidence suggests a new role for PTEN in DNA repair, through transcriptional downregulation of the critical HR protein RAD51. Tumour cells deficient in HR (such as BRCA-deficient breast and ovarian cancers) have been therapeutically targeted in a synthetic lethality (SL) strategy using PARP inhibitors that block single strand break repair (SSBR), a pathway related to base excision repair (BER). We have previously identified novel inhibitors of the critical BER enzyme APE1, and demonstrated these can induce SL in BRCA-deficient cell lines. In the current study, we have evaluated SL relationships in PTEN-proficient and -deficient melanoma systems for a potential novel treatment strategy. Method We screened a panel of melanoma cell lines (MeWo, MM418, SkMel28, SkMel30, HT144, UACC62) for PTEN, APE1 and RAD51 protein expression. We tested for SL using clonogenic survival assays in PTEN- and RAD51-deficient cell lines using our recently developed APE1 DNA repair domain inhibitors, methoxyamine (indirect APE1 inhibitor) and NU1025 (PARP inhibitor), either alone or in combination. Aldehyde reactive probe assay, neutral comet assay, γH2AX immunofluorescence and FACS analyses were performed to confirm biological activity in cells. Results Of the screened cell lines, SkMel28, HT144 and UACC62 were deficient in PTEN and RAD51 protein expression. APE1 inhibitors were more toxic to PTEN-deficient cells compared to PTEN-proficient cells (p&amp;lt;0.01). Significant sensitivity was also seen in PTEN-deficient cells treated with methoxyamine (p&amp;lt;0.05) and PARP inhibitors (p&amp;lt;0.001), confirming that BER modulation results in SL in PTEN deficiency. This increased cytotoxicity in PTEN deficiency is associated with accumulation of DNA double strand breaks (DSB), as assessed by comet assay (p&amp;lt;0.001) and γH2AX accumulation (p&amp;lt;0.05). DSB accumulation leads to G2/M arrest and apoptosis, as demonstrated by FACS analysis. Conclusion Our data provides the first pre-clinical evidence that targeting APE1 in PTEN-RAD51 deficient melanoma cells is a promising new SL strategy. We are complementing the current small molecule studies by developing PTEN knockout systems using shRNA technology. 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 LB-263. doi:1538-7445.AM2012-LB-263