Formation Of Strand Breaks Research Articles

Tumor Treating Fields (TTFields) downregulate the Fanconi Anemia-BRCA pathway and increase the efficacy of chemotherapy in malignant pleural mesothelioma preclinical models

ObjectivesTumor Treating Fields (TTFields) are low intensity, intermediate frequency, alternating electric fields with antimitotic effects on cancerous cells. TTFields concomitant with pemetrexed and a platinum agent are approved in the US and EU as first line therapy for unresectable, locally advanced or metastatic malignant pleural mesothelioma (MPM). The goal of the current study was to characterize the mechanism of action of TTFields in MPM cell lines and animal models. MethodsHuman MPM cell lines MSTO-211H and NCI-H2052 were treated with TTFields to determine the frequency that elicits maximal cytotoxicity. The effect of TTFields on DNA damage and repair, and the cytotoxic effect of TTFields in combination with cisplatin and/or pemetrexed were examined. Efficacy of TTFields concomitant with cisplatin and pemetrexed was evaluated in orthotopic IL-45 and subcutaneous RN5 murine models. ResultsTTFields at a frequency of 150 kHz demonstrated the highest cytotoxicity to MPM cells. Application of 150 kHz TTFields resulted in increased formation of DNA double strand breaks, elevated expression of DNA damage induced cell cycle arrest proteins, and reduced expression of Fanconi Anemia (FA)-BRCA DNA repair pathway proteins. Co-treatment of TTFields with cisplatin or pemetrexed significantly increased treatment efficacy versus each modality alone, with additivity and synergy exhibited by the TTFields-pemetrexed and TTFields-cisplatin combinations, respectively. In animal models, tumor volume was significantly lower for the TTFields-cisplatin-pemetrexed combination compared to control, accompanied by increased DNA damage within the tumor. ConclusionThis research demonstrated that the efficacy of TTFields for the treatment of MPM is associated with reduced expression of FA-BRCA pathway proteins and increased DNA damage. This mechanism of action is consistent with the observed synergism for TTFields-cisplatin vs additivity for TTFields-pemetrexed, as cisplatin-induced DNA damage is repaired via the FA-BRCA pathway.

Abstract 225: Computational analysis of 5-fluorouracil antitumor activity in colon cancer using a mechanistic pharmacokinetic/pharmacodynamic model

Abstract The purpose of this study is to develop a mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model to quantitatively analyze the treatment outcome of 5-fluorouracil (5-FU) in colon cancer. Predicting the outcome of chemotherapeutic agents plays an essential role in improving dose strategies. To that end, we have developed an integrated mechanistic PK/PD model to analyze the kinetics of tumor response to 5-FU treatment. Compared to the existing PK/PD models, our model is novel in the way it incorporates the 5-FU metabolism pathway and downstream antiproliferative events to provide a comprehensive model of treatment outcome. 5-FU has been widely used to treat cancers, including colon cancer. 5-FU metabolites can be incorporated into RNA, DNA and also interfere with thymidylate synthase (TS) catalytic activity, which is the main mechanism of 5-FU cytotoxicity. TS activity has been shown to be the predictor for treatment efficacy. The conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate catalyzed by TS is the sole intracellular de novo source of thymidylate. The formation of a ternary complex between fluorodeoxyuridylate (FdUMP) and TS obstructs the access of dUMP to the TS nucleotide-binding site. Resultant thymidine deprivation and dUMP accumulation give rise to the deoxynucleotide triphosphate (dNTP) pool imbalance, which could trigger S-phase specific DNA fragmentation, cell cycle arrest and apoptosis. In our PK model, compartmental analysis is developed to track the distribution of 5-FU from its entry into body to interstitial fluid outside the tumor cells and to its cellular uptake. In the cell model, the connection of TS inhibition and dNTP pool imbalance is implemented. In addition to the central plasma and peripheral tissue compartments, our model includes compartments for: 5-FU concentration in the extracellular fluids; intracellular 5-FU; 5-FU anabolites; genomic 5-FU in RNA, and in DNA; and, TS - FdUMP complex. Posttreatment free TS level is used to compute the perturbed TS catalytic activity. Using the model outcome on TS inhibition, we modeled the kinetics of dNTP pool perturbation. The dNTP pool imbalance leads to double strand break (DSB) formation that initiates DNA damage repair response, kinetics of which are also modeled. Our model uses the law of mass action and Michaelian kinetics and each molecular reaction is expressed as a nonlinear differential equation. Time course data from the literature are used in parameter estimation. Our simulation results successfully captured the kinetics of treatment outcome from the administration of the 5-FU to downstream effects in the tumor. The next step in our effort is to expand the model to include the tumor growth kinetics. Citation Format: Chenhui Ma, Alexandru Almasan, Evren Gurkan-Cavusoglu. Computational analysis of 5-fluorouracil antitumor activity in colon cancer using a mechanistic pharmacokinetic/pharmacodynamic model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 225.

Natural Merosesquiterpenes Activate the DNA Damage Response via DNA Strand Break Formation and Trigger Apoptotic Cell Death in p53-Wild-type and Mutant Colorectal Cancer.

Simple SummaryBowel cancer is a serious disease, which affects many people worldwide. Unfortunately, the disease is often diagnosed in an advanced stage, which impairs the chance of survival. Furthermore, resistance to therapy occurs frequently. Thus, novel therapeutic approaches are required to improve cancer therapy. Here, we studied whether merosesquiterpenes might be useful for cancer treatment. These compounds occur in marine sponges and were isolated by our group. We were able to identify three compounds with potent cytotoxic activity in different cell lines established from human large bowel cancer. Our experiments provided evidence that the compounds cause DNA damage and trigger cell death, so-called mitochondrial apoptosis, which was attested in cancer cells with expression of wild-type and mutated p53 tumor suppressor. Finally, we show that merosesquiterpenes also kill intestinal tumor organoids, an ex vivo model of large bowel cancer.Colorectal cancer (CRC) is a frequently occurring malignant disease with still low survival rates, highlighting the need for novel therapeutics. Merosesquiterpenes are secondary metabolites from marine sponges, which might be useful as antitumor agents. To address this issue, we made use of a compound library comprising 11 isolated merosesquiterpenes. The most cytotoxic compounds were smenospongine > ilimaquinone ≈ dactylospontriol, as shown in different human CRC cell lines. Alkaline Comet assays and γH2AX immunofluorescence microscopy demonstrated DNA strand break formation in CRC cells. Western blot analysis revealed an activation of the DNA damage response with CHK1 phosphorylation, stabilization of p53 and p21, which occurred both in CRC cells with p53 knockout and in p53-mutated CRC cells. This resulted in cell cycle arrest followed by a strong increase in the subG1 population, indicative of apoptosis, and typical morphological alterations. In consistency, cell death measurements showed apoptosis following exposure to merosesquiterpenes. Gene expression studies and analysis of caspase cleavage revealed mitochondrial apoptosis via BAX, BIM, and caspase-9 as the main cell death pathway. Interestingly, the compounds were equally effective in p53-wild-type and p53-mutant CRC cells. Finally, the cytotoxic activity of the merosesquiterpenes was corroborated in intestinal tumor organoids, emphasizing their potential for CRC chemotherapy.

Novel inexpensive ‘turn-on’ fluorescent biosensor for the sensitive detection of DNA damage induced by epirubicin

For several decades, DNA has been well-known as the cellular target for various cytotoxic anticancer drugs. An active research area between chemistry, medicine and molecular biology is studying mechanism of interaction of drug molecules with DNA. In this study, we introduce a novel inexpensive and simple fluorescent biosensor to directly quantitate the DNA damage induced by epirubicin, an anticancer agent, and investigate the underlying molecular mechanisms of damage. The proposed fluorescent biosensor is established using terbium chloride. In damaged DNA, there are ssDNA regions facing the damaged nucleobases. In the damaged DNA, the terbium biosensor coordinates directly with the unpaired nucleobases accompanied by enhancement of its fluorescence, generating a measureable signal proportionate to the amount of damage. Moreover, dithiothreitol is used in this study to asses any possible DNA damage mechanisms that may occur other than the DNA oxidative damage that is commonly induced by various anthracyclines. In this work, the kinetics of the damage induced in DNA by epirubicin in absence and presence of dithiothreitol were compared using the proposed Tb3+ fluorescence biosensor. The results were confirmed using MALDI-TOF mass spectrometric analysis. It revealed that in absence of dithiothreitol, epirubicin induces oxidative DNA damage due to the interaction with reactive oxygen species causing strand breaks where the mass spectra show a significant reduction of the molecular-ion-peak of DNA, and appearance of peaks of smaller m/z indicating the DNA strand-breaks formation. While, in presence of dithiothreitol, epirubicin covalently binds to DNA to form DNA adducts with m/z values larger than the molecular-ion-peak of DNA target. Furthermore, the potency of epirubicin was examined on calf-thymus natural DNA samples with the proposed fluorescence biosensor. Whilst the interaction mechanism of epirubicin with DNA in presence of dithiothreitol shows slower DNA damage kinetics, but it has more potent damaging effect on DNA than in the case when no dithiothreitol is added. This introduces a novel alternative anticancer therapy that requires further investigation. Moreover, the low cost of the proposed fluorescent biosensor allows it to be an automatic simple process for the investigation of any DNA-drug interactions without the need to couple it with any other analytical method.

Repair of O6-carboxymethylguanine adducts by O6-methylguanine-DNA methyltransferase in human colon epithelial cells.

The protein O6-methylguanine-DNA methyltransferase (MGMT) is able to repair the mutagenic O6-methylguanine (O6-MeG) adduct back to guanine. In this context, it may protect against colorectal cancer formation associated with N-nitroso compounds. Such compounds may be endogenously formed by nitrosylation of amino acids, which can give rise to mutagenic O6-MeG and O6-carboxymethylguanine (O6-CMG) adducts. It is well established that O6-MeG is repaired by MGMT. However, up to now, whether O6-CMG is repaired by this enzyme remains unresolved. Therefore, the aim of the present study was to analyze the fate of both types of O6-guanine adducts in the presence and absence of MGMT activity. To this end, MGMT activity was efficiently blocked by its chemical inhibitor O6-benzylguanine in human colon epithelial cells (HCECs). Exposure of cells to azaserine (AZA) caused significantly higher levels of both O6-MeG and O6-CMG adducts in MGMT-inhibited cells, with O6-CMG as the more abundant DNA lesion. Interestingly, MGMT inhibition did not result in higher levels of AZA-induced DNA strand breaks in spite of elevated DNA adduct levels. In contrast, MGMT inhibition significantly increased DNA strand break formation after exposure to temozolomide (TMZ), a drug that exclusively generates O6-MeG adducts. In line with this finding, the viability of the cells was moderately reduced by TMZ upon MGMT inhibition, whereas no clear effect was observed in cells treated with AZA. In conclusion, our study clearly shows that O6-CMG is repaired by MGMT in HCEC, thereby suggesting that MGMT might play an important role as a tumor suppressor in diet-mediated colorectal cancer.

An evaluation of DNA double strand break formation and excreted guanine species post whole body PET/CT procedure.

Ionizing radiation-induced oxidation and formation of deoxyribonucleic acid (DNA) double strand breaks (DSBs) are considered the exemplar of genetic lesions. Guanine bases are most prone to be oxidized when DNA and Ribonucleic acid (RNA) are damaged. The repair processes that are initiated to correct this damage release multiple oxidized guanine species into the urine. Hence, the excretion of guanine species can be related with the total repair process. Our study quantified the total DSBs formation and the amount of guanine species in urine to understand the DNA break and repair process after whole body (WB) exposure to 18F-FDG positron emission tomography/computed tomography (PET/CT). A total of 37 human participants were included with control and test groups and the average radiation dose was 27.50 ± 2.91 mSv. γ-H2AX foci assay in the collected blood samples was performed to assess the DSBs, and excreted guanine species in urine were analyzed by a competitive ELISA method. We observed a significant increase of DNA damage that correlated well with the increasing dose (p-value 0.009) and body weight (p-value 0.05). In the test group, excreted guanine species in urine sample significantly increased (from 24.29 ± 5.82 to 33.66 ± 7.20 mg/mmol creatinine). A minimum (r2 = 0.0488) correlation was observed between DSBs formation and excreted guanine species. A significant difference of DNA damage and 8-OHdG formation was seen in the test group compared to controls. Larger population studies are needed to confirm these observations, describe the fine-scale timing of changes in the biomarker levels after exposure, and further clarify any potential risks to patients from PET/CT procedures.

Effect of 4‐Nitroquinoline‐1‐Oxide on DNA damage and repair in head and neck squamous cell carcinoma cells

Background Tobacco exposure and human papillomavirus (HPV) infection are among the main factors for development of head and neck squamous cell carcinoma (HNSCC). Interestingly, recent studies show that HPV positive (+) smokers and non-smokers have similar mutational profiles, which suggests that mutations induced in smokers could be associated with the protective effects of HPV. Our preliminary data showed that 4NQO, a mutagen and carcinogen which serves as a surrogate for tobacco exposure induces oxidative DNA damage and DSB formation in HNSCC cells at 6 hrs. HPV+ HNSCC cells also showed a relative increase in activation of homologous recombination (HR) DNA repair proteins compared to HPV negative (-) HNSCC cells at 6 hrs. In this study, we wanted to investigate the effect of 4NQO on DNA damage and repair activation at 24 hrs in HPV+ and HPV- HNSCC cells. Methods NOK E6.E7 +/-, UPCI-SCC40 (HPV-), UM-SCC104 and UM-SCC47 (HPV+) cells were utilized to check effects of 4NQO on DNA repair. Oxidative DNA damage, double strand break formation (DSBs) and protein expression were analyzed by FPG-modified alkaline comet assay, γH2AX staining and Western analysis following treatment with 4NQO for 6 hr, replacement with fresh media, and sample collection at 24 hr. Results 4NQO increased oxidative DNA damage, single strand breaks, and yH2AX foci at 6 hrs; however, these lesions were no longer present at 24 hrs. This suggests DNA repair had probably occurred upon removal of 4NQO. We no longer observed an increase in phosphorylation of BRCA1, Rad51, H2AX, and p21 levels in HNSCC cells at 24 hr although it was present at 6 hrs. Conclusion Our in-vitro data suggests that HPV may have a role in the activation of DNA repair proteins by 4NQO at 6 hrs. This may have allowed for successful repair of DNA damage in both HPV+ and HPV- HNSCC cells by 24 hrs. Taken together, these data suggest that HPV may protect against smoking-induced mutations by preferentially increasing HR DNA repair protein activation.

In vitro Assessment of the DNA Damage Response in Dental Mesenchymal Stromal Cells Following Low Dose X-ray Exposure

Stem cells contained within the dental mesenchymal stromal cell (MSC) population are crucial for tissue homeostasis. Assuring their genomic stability is therefore essential. Exposure of stem cells to ionizing radiation (IR) is potentially detrimental for normal tissue homeostasis. Although it has been established that exposure to high doses of ionizing radiation (IR) has severe adverse effects on MSCs, knowledge about the impact of low doses of IR is lacking. Here we investigated the effect of low doses of X-irradiation with medical imaging beam settings (<0.1 Gray; 900 mGray per hour), in vitro, on pediatric dental mesenchymal stromal cells containing dental pulp stem cells from deciduous teeth, dental follicle progenitor cells and stem cells from the apical papilla. DNA double strand break (DSB) formation and repair kinetics were monitored by immunocytochemistry of γH2AX and 53BP1 as well as cell cycle progression by flow cytometry and cellular senescence by senescence-associated β-galactosidase assay and ELISA. Increased DNA DSB repair foci, after exposure to low doses of X-rays, were measured as early as 30 min post-irradiation. The number of DSBs returned to baseline levels 24 h after irradiation. Cell cycle analysis revealed marginal effects of IR on cell cycle progression, although a slight G2/M phase arrest was seen in dental pulp stromal cells from deciduous teeth 72 h after irradiation. Despite this cell cycle arrest, no radiation-induced senescence was observed. In conclusion, low X-ray IR doses (< 0.1 Gray; 900 mGray per hour), were able to induce significant increases in the number of DNA DSBs repair foci, but cell cycle progression seems to be minimally affected. This highlights the need for more detailed and extensive studies on the effects of exposure to low IR doses on different mesenchymal stromal cells.

PRD1, a homologous recombination initiation factor, is involved in spindle assembly in rice meiosis.

The bipolar spindle structure in meiosis is essential for faithful chromosome segregation. PUTATIVE RECOMBINATION INITIATION DEFECT 1 (PRD1) previously has been shown to participate in the formation of DNA double strand breaks (DSBs). However, the role of PRD1 in meiotic spindle assembly has not been elucidated. Here, we reveal by both genetic analysis and immunostaining technology that PRD1 is involved in spindle assembly in rice (Oryza sativa) meiosis. We show that DSB formation and bipolar spindle assembly are disturbed in prd1 meiocytes. PRD1 signals display a dynamic pattern of localization from covering entire chromosomes at leptotene to congregating at the centromere region after leptotene. Centromeric localization of PRD1 signals depends on the organization of leptotene chromosomes, but not on DSB formation and axis establishment. PRD1 exhibits interaction and co-localization with several kinetochore components. We also find that bi-orientation of sister kinetochores within a univalent induced by mutation of REC8 can restore bipolarity in prd1. Furthermore, PRD1 directly interacts with REC8 and SGO1, suggesting that PRD1 may play a role in regulating the orientation of sister kinetochores. Taken together, we speculate that PRD1 promotes bipolar spindle assembly, presumably by modulating the orientation of sister kinetochores in rice meiosis.

Mitochondrial DNA as a Sensitive Biomarker of UV-Induced Cellular Damage in Human Skin.

Mitochondrial DNA (mtDNA) has been demonstrated to be a reliable biomarker of UV-induced genetic damage in both animal and human skin. Properties of the mitochondrial genome which allow for its use as a biomarker of damage include its presence in multiple copies within a cell, its limited repair mechanisms, and its lack of protective histones. To measure UV-induced mtDNA damage (particularly in the form of strand breaks), real-time quantitative PCR (qPCR) is used, based on the observation that PCR amplification efficiency is decreased in the presence of high levels of damage. Here, we describe the measurement of UV-induced mtDNA damage which includes the extraction of cellular DNA, qPCR to determine the relative amount of mtDNA, qPCR to determine UV-induced damage within a long strand of mtDNA, and the verification of the amplification process using gel electrophoresis.

Dioxane-Linked Amide Derivatives as Novel Bacterial Topoisomerase Inhibitors against Gram-Positive Staphylococcus aureus.

In recent years, novel bacterial topoisomerase inhibitors (NBTIs) have been developed as future antibacterials for treating multidrug-resistant bacterial infections. A series of dioxane-linked NBTIs with an amide moiety has been synthesized and evaluated. Compound 3 inhibits DNA gyrase, induces the formation of single strand breaks to bacterial DNA, and achieves potent antibacterial activity against a variety of Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA). Optimization of this series of analogues led to the discovery of a subseries of compounds (22-25) with more potent anti-MRSA activity, dual inhibition of DNA gyrase and topoisomerase IV, and the ability to induce double strand breaks through inhibition of S. aureus DNA gyrase.

MiR-181a enhances drug sensitivity of mixed lineage leukemia-rearranged acute myeloid leukemia by increasing poly(ADP-ribose) polymerase1 acetylation

Chromosomal translocations and rearrangements involving Mixed Lineage Leukemia (MLL) gene is associated with poor prognosis in AML. Extensive epigenetic changes were found in this group of patients. In clinical study, we found miR-181a expression level was significantly lower in MLL-rearranged AML. As an important epi-miRNA, the role of miR-181a as an epigenetic regulator in leukemia has not been investigated before. In this study, we found miR-181a overexpression enhanced total protein acetylation in THP-1 cells, which harbor MLL-AF9 fusion gene, and protein Mass Spectrum identified poly(ADP-ribose) polymerase 1 (PARP1) was a major downstream target. Increased PARP1 acetylation was mediated by down-regulation of histone deacetylase Sirtuin1 (Sirt1). MiR-181a overexpression resulted in DNA trapping of PARP1, increased DNA double strand break formation and increased chemosensitivity of leukemia cells both in vitro and in vivo. This study indicates miR-181a-Sirt1-PARP1 acetylation pathway could be a promising target for this special group of AML.

Assessing Homologous Recombination and Interstrand Cross-Link Repair in Embryonal Carcinoma Testicular Germ Cell Tumor Cell Lines.

Testicular germ cell tumors (TGCTs) are typically exquisitely sensitive to DNA interstrand cross-link (ICLs) agents. ICLs covalently link both strands of the DNA duplex, impeding fundamental cellular processes like DNA replication to cause cell death. A leading drug used for the treatment of TGCTs is cisplatin, which introduces ICLs and leads to formation of double strand breaks (DSBs), a DNA lesion that can be repaired in the S/G2 phases of the cell cycle by homologous recombination (HR, also termed homology-direct repair). Although most TGCTs respond to cisplatin-induced ICLs, a fraction is resistant to treatment. One proposed mechanism of TGCT resistance to cisplatin is an enhanced ability to repair DSBs by HR. Other than HR, repair of the ICL-lesions requires additional DNA repair mechanisms, whose action might also be implemented in therapy-resistant cells. This chapter describes GFP assays to measure (a) HR proficiency following formation of a DSB by the endonuclease I-SceI, and (b) HR repair induced by site-specific ICL formation involving psoralen. These experimental approaches can be used to determine the proficiency of TGCT cell lines in DSB repair by HR in comparison to HR repair of ICLs, providing tools to better characterize their recombination profile. Protocols of these assays have been adapted for use in Embryonal Carcinoma (EC) TGCT cell lines. Assays only require transient introduction of plasmids within cells, affording the advantage of testing multiple cell lines in a relatively short time.

The RECQL helicase prevents replication fork collapse during replication stress.

Most tumors lack the G1/S phase checkpoint and are insensitive to antigrowth signals. Loss of G1/S control can severely perturb DNA replication as revealed by slow replication fork progression and frequent replication fork stalling. Cancer cells may thus rely on specific pathways that mitigate the deleterious consequences of replication stress. To identify vulnerabilities of cells suffering from replication stress, we performed an shRNA-based genetic screen. We report that the RECQL helicase is specifically essential in replication stress conditions and protects stalled replication forks against MRE11-dependent double strand break (DSB) formation. In line with these findings, knockdown of RECQL in different cancer cells increased the level of DNA DSBs. Thus, RECQL plays a critical role in sustaining DNA synthesis under conditions of replication stress and as such may represent a target for cancer therapy.

Abstract 1368: 4NQO shows formation of yH2AX foci, oxidative base damage and activates DNA repair pathway preferentially in HPV positive HNSCC cells

Abstract Background: There is an epidemic of HPV associated head and neck squamous cell carcinoma (HNSCC) There is controversy as to whether HPV positive SCC patients who have a &amp;gt;10 pack year smoking history have a worse prognosis than HPV associated non-smokers. Tobacco exposure is associated with alteration of DNA damage repair (DDR) pathway which is often activated in several cancers including HNSCC. A mutational profile of HPV associated smokers determined there was no significant difference in the mutational profile between HPV associated non-smokers and smokers. Hence we wanted to test this observation by analyzing the effects of 4NQO, a mutagen and carcinogen which serves as a surrogate for tobacco exposure, in HPV positive and HPV negative HNSCC cells. Methods: Protein and mRNA expression were analyzed by western blotting and real time PCR respectively. yH2AX staining and oxidative DNA damage analysis were performed using immunofluorescence and FPG-modified alkaline comet assay respectively. Results: Our preliminary data shows that 4NQO activates DDR pathway preferably in NOK E6/E7 and HPV positive UMSCC47 and SCC104 HNSCC cells as seen by increased phosphorylation of BRCA1, Rad50 and RAD51, chk1 and increased p21 levels in a time- and concentration-dependent fashion. Real time PCR data showed higher mRNA transcript levels of RAD51 and BRCA1 in NOK E6/E7 and HPV positive UMSCC47 and SCC104 HNSCC cells as compared to NOK and HPV negative SCC40 HNSCC cells upon 4NQO treatment. 4NQO causes yH2AX foci formation suggesting double strand breaks (DSBs) formation and an increase in DNA damage in HNSCC cells. 4NQO increased oxidative base damage in NOK E6/E7, UMSCC47 and SCC104 HPV positive HNSCC cells, and to a lesser degree in NOK and SCC40 HPV negative HNSCC cells. Conclusion: Our preliminary study shows for the first time that HPV E6/E7 oncogenes promote DNA repair in NOK and HPV positive HNSCC cells in response to 4NQO treatment. This could explain why HPV associated HNSCC have a better prognosis irrespective of smoking history. Citation Format: Gauri Shishodia, Rhodee Ric G. Toledo, Xiahuao Rong, Adam Y. Xiao, Lynn Harrison, Cherie Ann Nathan. 4NQO shows formation of yH2AX foci, oxidative base damage and activates DNA repair pathway preferentially in HPV positive HNSCC cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1368.

Single-molecule live-cell imaging reveals RecB-dependent function of DNA polymerase IV indouble strand break repair.

Several functions have been proposed for the Escherichia coli DNA polymerase IV (pol IV). Although much research has focused on a potential role for pol IV in assisting pol III replisomes in the bypass of lesions, pol IV is rarely found at the replication fork in vivo. Pol IV is expressed at increased levels in E. coli cells exposed to exogenous DNA damaging agents, including many commonly used antibiotics. Here we present live-cell single-molecule microscopy measurements indicating that double-strand breaks induced by antibiotics strongly stimulate pol IV activity. Exposure to the antibiotics ciprofloxacin and trimethoprim leads to the formation of double strand breaks in E. coli cells. RecA and pol IV foci increase after treatment and exhibit strong colocalization. The induction of the SOS response, the appearance of RecA foci, the appearance of pol IV foci and RecA-pol IV colocalization are all dependent on RecB function. The positioning of pol IV foci likely reflects a physical interaction with the RecA* nucleoprotein filaments that has been detected previously in vitro. Our observations provide an in vivo substantiation of a direct role for pol IV in double strand break repair in cells treated with double strand break-inducing antibiotics.

Protective effect of leaf extract of Abutilon indicum on DNA damage and peripheral blood lymphocytes in combating the oxidative stress

The current research explores in vitro antioxidant characteristics, radiation-induced DNA damage protection and quenching effects of the oxidative stress by the ethanolic leaf extract of Abutilon indicum (EEAI) on human peripheral blood lymphocytes (PBLs). PBLs were incubated with various concentrations of EEAI accompanied by pre- and post-treatment with hydrogen peroxide. Cell viability was investigated by MTT assay. In addition, quenching of free radicals were measured in vitro using DPPH, superoxide anion, hydrogen peroxide, reducing power and nitric oxide radical scavenging assays. These activities were compared with ascorbic acid as standard antioxidants. Furthermore, inhibition of UV radiation-induced strand break formation in plasmid pBR322 DNA and anti-Fenton reactions in calf thymus DNA was evaluated. Cytotoxic effects of hydrogen peroxide on PBLs were significantly reduced with EEAI pre-treatment compared to post-treatment in a dose-dependent manner comparable with similar cytoprotective effects of ascorbic acid (p > 0.05). EEAI has shown strong antioxidant effects in the scavenging of DPPH, superoxide anion, hydrogen peroxide, and nitric oxide. EEAI also has a strong protective effect of UV-induced plasmid pBR322 DNA cleavage and Fenton-induced DNA damage. Overall, the results revealed that Abutilon indicum has a cytoprotective, potent antioxidant and DNA protective effect that provide pharmacological credence to justify its overall biological activity. Furthermore, future studies to identifying bioactive molecules and its molecular mechanisms responsible for promising therapeutic applications in the rescue of disease-induced cellular oxidative damage.

Involvement of POLA2 in Double Strand Break Repair and Genotoxic Stress.

Cellular survival is dependent on the efficient replication and transmission of genomic information. DNA damage can be introduced into the genome by several different methods, one being the act of DNA replication. Replication is a potent source of DNA damage and genomic instability, especially through the formation of DNA double strand breaks (DSBs). DNA polymerase alpha is responsible for replication initiation. One subunit of the DNA polymerase alpha replication machinery is POLA2. Given the connection between replication and genomic instability, we decided to examine the role of POLA2 in DSB repair, as little is known about this topic. We found that loss of POLA2 leads to an increase in spontaneous DSB formation. Loss of POLA2 also slows DSB repair kinetics after treatment with etoposide and inhibits both of the major double strand break repair pathways: non-homologous end-joining and homologous recombination. In addition, loss of POLA2 leads to increased sensitivity to ionizing radiation and PARP1 inhibition. Lastly, POLA2 expression is elevated in glioblastoma multiforme tumors and correlates with poor overall patient survival. These data demonstrate a role for POLA2 in DSB repair and resistance to genotoxic stress.

Sensitive spectrofluorimetric and mass spectroscopic methods for the determination of nucleic acid damage induced by photosensitized anti-inflammatory drugs: Comparative study

Anti-inflammatory drugs are reported to induce changes in nucleic-acids upon UV-irradiation. Such changes have the potential to cause apoptosis, carcinogenesis, and mutagenesis. In this work, the kinetics of the damage induced in DNA by some anti-inflammatory drugs were compared after UV-irradiation. Five commonly used anti-inflammatory drugs; diclofenac, ketoprofen, leflunomide, piroxicam and tolmetin, were studied. Simple, sensitive and eco-friendly methods for the analysis of DNA-damage were proposed including absorption spectroscopy, MALDI-TOF mass spectrometry and fluorescence using TbCl3. Results show that all drugs induced DNA-damage after UV-irradiation. Absorption spectroscopy results demonstrated hyperchromic shift in the absorption band characteristic to DNA, indicating distortion of the double-strand. Mass spectra showed a significant decrease of the molecular-ion-peak of DNA, together with peaks of smaller m/z that indicated the formation of DNA strand-breaks. TbCl3 fluorescence was observed to increase with incubation time of each drug with DNA, indicating the presence of more single-stranded regions in DNA due to damage. TbCl3 fluorescence was used to obtain the kinetics of the induced damage. Results show that DNA-damage occurred via photoinduced oxidative mechanism. Also, the potency of the studied drugs was examined on calf-thymus real DNA samples using TbCl3 fluorescence with ketoprofen and leflunomide being the most photogenotoxic anti-inflammatory drugs.

The Impact of Chromatin Structure on the Formation of Double Strand Breaks by CRISPR‐CAS9 in the Yeast Genome

DNA is persistently at risk for being damaged by endogenous and exogenous factors. One type of DNA damage is a double‐strand break (DSB), which occurs when both strands of the helix become severed. DSBs are important because upon repair, they could lead to mutations or genomic alteration. The packaging of the DNA into chromatin may affect the susceptibility of an area to DSBs and thus could make the area more prone to mutation. The purpose of this study is to look at how the chromatin environment affects the susceptibility of genes to DSBs in Saccharomyces cerevisiae. A new system, tetracycline‐inducible CRISPR‐Cas9, was used in this experiment to induce DSBs at different sites in the yeast genome Histone modification levels were measured at each site to correlate these chromatin marks with DSB formation frequency. DSB frequency at each genomic locus was measured using quantitative PCR and directly compared to those same sites in unchromatinized DNA to see how chromatin environment and the surrounding histone modifications affects susceptibility to DSBs.To measure the frequency of DSBs at different genomic sites using qPCR, the break site was amplified. DSBs would be indicated through a decrease in product when compared to an uncut control site NFS1 on Chromosome III. While correlation analysis of histone modifications and DSB frequency is still ongoing, preliminary data suggests that repressed genes have higher levels of cutting than active genes after three hours of DSB induction. Of the repressed genes investigated, PPE1 had 82.06% DSB formation and RRT8 had 66.67% after three hours of induction. Active genes RPL22A, TDH2, and TDH3 had on average about 15%, 9.5%, and 56% DSB formation after three hours of DSB induction, respectively. The level of DSB formation in these sites will be directly compared to DSB formation of the same sites on an unchromatinized plasmid, which is currently underway.This research is important because it shows which areas of the genome may be at higher risk for DSBs and whether or that is affected by chromatin structure. Understanding this could help deepen the knowledge of how the genome evolves through spontaneous DSB formation and repair.