Tumor-treating fields elicit a conditional vulnerability to ionizing radiation via the downregulation of BRCA1 signaling and reduced DNA double-strand break repair capacity in non-small cell lung cancer cell lines

The use of tumor-treating fields (TTFields) has revolutionized the treatment of recurrent and newly diagnosed glioblastoma (GBM). TTFields are low-intensity, intermediate frequency, alternating electric fields that are applied to tumor regions and cells using non-invasive arrays. The predominant mechanism by which TTFields are thought to kill tumor cells is the disruption of mitosis. Using five non-small cell lung cancer (NSCLC) cell lines we found that there is a variable response in cell proliferation and cell killing between these NSCLC cell lines that was independent of p53 status. TTFields treatment increased the G2/M population, with a concomitant reduction in S-phase cells followed by the appearance of a sub-G1 population indicative of apoptosis. Temporal changes in gene expression during TTFields exposure was evaluated to identify molecular signaling changes underlying the differential TTFields response. The most differentially expressed genes were associated with the cell cycle and cell proliferation pathways. However, the expression of genes found within the BRCA1 DNA-damage response were significantly downregulated (P<0.05) during TTFields treatment. DNA double-strand break (DSB) repair foci increased when cells were exposed to TTFields as did the appearance of chromatid-type aberrations, suggesting an interphase mechanism responsible for cell death involving DNA repair. Exposing cells to TTFields immediately following ionizing radiation resulted in increased chromatid aberrations and a reduced capacity to repair DNA DSBs, which were likely responsible for at least a portion of the enhanced cell killing seen with the combination. These findings suggest that TTFields induce a state of ‘BRCAness' leading to a conditional susceptibility resulting in enhanced sensitivity to ionizing radiation and provides a strong rationale for the use of TTFields as a combined modality therapy with radiation or other DNA-damaging agents.

The Mechanisms of Action of Tumor Treating Fields

Tumor treating fields (TTFields), a new modality of cancer treatment, are electric fields transmitted transdermally to tumors. The FDA has approved TTFields for the treatment of glioblastoma multiforme and mesothelioma, and they are currently under study in many other cancer types. While antimitotic effects were the first recognized biological anticancer activity of TTFields, data have shown that tumor treating fields achieve their anticancer effects through multiple mechanisms of action. TTFields therefore have the ability to be useful for many cancer types in combination with many different treatment modalities. Here, we review the current understanding of TTFields and their mechanisms of action.

miR-125a-5p Functions as Tumor Suppressor microRNA And Is a Marker of Locoregional Recurrence And Poor prognosis in Head And Neck Cancer

MicroRNAs (miRNAs) are short single-stranded RNAs, measuring 21 to 23 nucleotides in length and regulate gene expression at the post-transcriptional level through mRNA destabilization or repressing protein synthesis. Dysregulation of miRNAs can lead to tumorigenesis through changes in regulation of key cellular processes such as cell proliferation, cell survival, and apoptosis. miR-125a-5p has been implicated as a tumor suppressor miRNA in malignancies such as non-small cell lung cancer and colon cancer. However, the role of miR-125a-5p has not been fully investigated in head and neck squamous cell carcinoma (HNSCC). We performed microRNA microarray profiling of HNSCC tumor samples obtained from a prospective clinical trial evaluating the role of postoperative radiotherapy in head and neck cancer. We also mined through The Cancer Genome Atlas to evaluate expression and survival data. Biological experiments, including cell proliferation, flow cytometry, cell migration and invasion, clonogenic survival, and fluorescent microscopy, were conducted using HN5 and UM-SCC-22B cell lines. miR-125a-5p downregulation was associated with recurrent disease in a panel of high-risk HNSCC and then confirmed poor survival associated with low expression in HNSCC via the Cancer Genome Atlas, suggesting that miR-125a-5p acts as a tumor suppressor miRNA. We then demonstrated that miR-125a-5p regulates cell proliferation through cell cycle regulation at the G₁/S transition. We also show that miR-125a-5p can alter cell migration and modulate sensitivity to ionizing radiation. Finally, we identified putative mRNA targets of miR-125a-5p, including ERBB2, EIF4EBP1, and TXNRD1, which support the tumor suppressive mechanism of miR-125a-5p. Functional validation of ERBB2 suggests that miR-125a-5p affects cell proliferation and sensitivity to ionizing radiation, in part, through ERBB2. Our data suggests that miR-125a-5p acts as a tumor suppressor miRNA, has potential as a diagnostic tool and may be a potential therapeutic target for the management and treatment of squamous cell carcinoma of the head and neck.

Metastasis-associated protein 1/histone deacetylase 4-nucleosome remodeling and deacetylase complex regulates phosphatase and tensin homolog gene expression and function

Metastasis-associated protein 1 (MTA1) is widely overexpressed in human cancers and is associated with malignant phenotypic changes contributing to morbidity in the associated diseases. Here we discovered for the first time that MTA1, a master chromatin modifier, transcriptionally represses the expression of phosphatase and tensin homolog (PTEN), a tumor suppressor gene, by recruiting class II histone deacetylase 4 (HDAC4) along with the transcription factor Yin-Yang 1 (YY1) onto the PTEN promoter. We also found evidence of an inverse correlation between the expression levels of MTA1 and PTEN in physiologically relevant breast cancer microarray datasets. We found that MTA1 up-regulation leads to a decreased expression of PTEN protein and stimulation of PI3K as well as phosphorylation of its signaling targets. Accordingly, selective down-regulation of MTA1 in breast cancer cells increases PTEN expression and inhibits stimulation of the PI3K/AKT signaling. Collectively, these findings provide a mechanistic role for MTA1 in transcriptional repression of PTEN, leading to modulation of the resulting signaling pathways.

An overview of potential novel mechanisms of action underlying Tumor Treating Fields-induced cancer cell death and their clinical implications

Traditional cancer therapy choices for clinicians are surgery, chemotherapy, radiation and immune therapy which are used either standalone therapies or in various combinations. Other physical modalities beyond ionizing radiation include photodynamic therapy and heating and the more recent approach referred to as Tumor Treating Fields (TTFields). TTFields are intermediate frequency, low-intensity, alternating electric fields that are applied to tumor regions and cells using noninvasive arrays. TTFields have revolutionized the treatment of newly diagnosed and recurrent glioblastoma (GBM) and unresectable and locally advanced malignant pleural mesothelioma (MPM). TTFields are thought to kill tumor cells predominantly by disrupting mitosis; however it has been shown that TTFields increase efficacy of different classes of drugs, which directly target mitosis, replication stress and DNA damage pathways. Hence, a detailed understanding of TTFields' mechanisms of action is needed to use this therapy effectively in the clinic. Recent findings implicate TTFields' role in different important pathways such as DNA damage response and replication stress, ER stress, membrane permeability, autophagy, and immune response. This review focuses on potentially novel mechanisms of TTFields anti-tumor action and their implications in completed and ongoing clinical trials and pre-clinical studies. Moreover, the review discusses advantages and strategies using chemotherapy agents and radiation therapy in combination with TTFields for future clinical use.

Kidney protein profiling of Wilms' tumor patients by analysis of formalin-fixed paraffin-embedded tissue samples

Wilms' tumor (nephroblastoma, WT) is the most frequent renal cancer in children. However, molecular details leading to WT have not been characterized sufficiently yet. Proteomic studies might provide new insights but are hampered by limited availability of fresh frozen tissue specimen. Therefore, we tested formalin-fixed paraffin-embedded (FFPE) tissue sections routinely collected for pathological inspection for their use in in-depth-proteomic analyses of WT samples in comparison to fresh frozen specimen. The overlap of the proteins identified was over 65%. Thus we used FFPE material from 7 patients for tandem mass spectrometry based comparison of the proteomes of WT and healthy renal tissues. We detected 262 proteins, which were differentially expressed in tumor compared to healthy renal tissue. The majority of these proteins displayed lower levels in the tumor tissue and only 30% higher levels. For selected candidates data were confirmed by immunohistochemical staining. Correlation analysis of blastemal proportions in WT and protein intensities revealed candidates for tumor stratification.

CONCLUSION

This proof of principle proteomic study of FFPE tissue sections from WT patients demonstrates that these archived tissues constitute a valuable resource for larger in-depth proteomic studies to identify markers to follow chemotherapy efficiency or for stratification of tumor subtypes.

Proteome analysis reveals new mechanisms of Bcl11b-loss driven apoptosis

The Bcl11b protein was shown to be important for a variety of functions such as T cell differentiation, normal development of central nervous system, and DNA damage response. Malignant T cells undergo apoptotic cell death upon BCL11B down-regulation, however, the detailed mechanism of cell death is not fully understood yet. Here we employed two-dimensional difference in-gel electrophoresis (2D-DIGE), mass spectrometry and cell biological experiments to investigate the role of Bcl11b in malignant T cell lines such as Jurkat and huT78. We provide evidence for the involvement of the mitochondrial apoptotic pathway and observed cleavage and fragments of known caspase targets such as myosin, spectrin, and vimentin. Our findings suggest an involvement of ERM proteins, which were up-regulated and phosphorylated upon Bcl11b down-regulation. Moreover, the levels of several proteins implicated in cell cycle entry, including DUT-N, CDK6, MCM4, MCM6, and MAT1 were elevated. Thus, the proteome data presented here confirm previous findings concerning the consequences of BCL11B knock-down and provide new insight into the mechanisms of cell death and cell cycle disturbances induced by Bcl11b depletion.

Abstract 3998: miRNA associated with distal metastasis and local recurrence after post-operative radiotherapy in high-risk head and neck cancer

Distal metastasis and local-regional recurrence after radiation therapy is a major cause of treatment failure for patients with head and neck squamous cell carcinoma. miRNA have been proposed as biomarkers in cancer to predict therapeutic outcomes and has been associated with the cellular response to radiation. The advantages of miRNA as a biomarker lies in its stability in tissues as well as body fluids, hence the potential for non-invasive diagnosis and prognosis. In this study, we performed miRNA expression profiling using tumor samples from 114 head and neck cancer patients treated by post-operative radiotherapy (PORT) at M.D. Anderson Cancer Center from 1992 to 1999. All patients were considered to be at high-risk for recurrence having histologically proven advanced squamous cell carcinoma. Amongst these samples, 41 had distal metastasis (DM), 24 recurred locally or local/regional (LR) and 49 responded to PORT (no evidence of disease (NED)). Total RNA was extracted using a modified protocol of the Qiagen RNeasy Plus kit which retains small RNA fractions. miRNA profiling was performed using miRCURY 7th gen LNA™ microRNA Array from Exiqon. Microarray data were background-subtracted using mis-match control probes and normalized with median scaling. An ANOVA model based on clinical outcome was used to compare of miRNA expression between DM vs. NED and LR vs NED. There are 94 miRNAs that differentially expressed in DM and 60 miRNAs differentially expressed in LR with statistical significance (p &lt; 0.05, Fold change &gt; 1.4). Among them, miR-551a and miR-551b-3p are significantly associated with the DM group and have been validated using qPCR. By transfection of either mimic or an inhibitor of these miRNAs using the HN5 head and neck cancer cell line, we have confirmed that both miR-551a and miR-551b-3p promote cell proliferation, enhance cell migration and invasion. We have further discovered GLIPR2, an autophagy inhibitor, as a direct binding target of both miRNAs using luciferase assay. By over-expressing miR-551a/miR-551b-3p or suppressing GLIPR2 expression, we detected higher level autophagy with increased LC3 foci and overexpression of lipidated LC3B. Base on the above data, we propose a model that miR-551a and miR-551b-3p enhance radio-resistance and metastasis by promoting autophagy via suppressing GLIPR2 expression. A panel of 49 head and neck cancer cell lines were used for aCGH study and miRNA profiling. Further analysis is underway to correlate the patterns of DNA copy number variation and miRNA expression with radio-sensitivity. The findings in these cell lines will also be validated in tumor tissue samples. In summary, our data demonstrated the potential of miRNAs as biomarkers that predict therapeutic outcomes and mechanistic studies of miRNA markers will lead to novel pathway discovery and further improvement of cancer treatment.

Citation Format: Lianghao Ding, Narasimha Kumar Karanam, John S. Yordy, Uma Giri, Michael D. Story. miRNA associated with distal metastasis and local recurrence after post-operative radiotherapy in high-risk head and neck cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3998. doi:10.1158/1538-7445.AM2015-3998

Abstract 1371: Tumor treating fields exposure causes an imbalance of reactive oxygen homeostasis likely through the cytosolic function of the fanconi′s anemia genes

The use of Tumor Treating Fields (TTFields) has been shown to significantly benefit patients with non-resectable locally advanced or metastatic glioblastoma and malignant pleural mesothelioma. Although the disruption of mitosis that was initially attributed as the mechanism of tumor cell killing, in subsequent studies, it has been shown that TTFields inhibits DNA damage repair by downregulating DNA repair genes and proteins, including those associated with Fanconi's Anemia (FA). As a result, radiation-induced DNA damage repair is reduced and DNA replication fork protection is impaired, leading to replication stress and replication fork collapse. As the FA genes play a critical role in multiple repair pathways, TTFields is likely to increase susceptibility to other chemo agents besides radiation that cause DNA damage which is repaired by FA-dependent DNA repair and replication fork maintenance pathways in nuclei. In addition to their nuclear role, the FA proteins also regulate mitophagy in the cytosol. Using a panel of non-small cell lung cancer cell lines, it was found that TTFields exposure disrupted the clearance of damaged mitochondria despite the presence of a well-defined membrane markers for autophagosomal destruction of damaged mitochondria, that being PINK1 and parkin. Analysis of electron micrographs revealed disrupted mitochondrial cristae, while radical oxygen metabolism studies identified an imbalance leading to the production of reactive oxygen species (ROS) and ultimately, cell death. The downregulation of key mitochondrial genes including DAPIT, OSCP1, ATP5S, ATP5B and COXIV that are related to electron transport and mitochondrial integrity was confirmed by gene expression analysis. The small molecule dichloroacetate (DCA), an inhibitor of pyruvate dehydrogenase kinase, changes the metabolic state of cancer cells from cytoplasmic glycolysis to mitochondrial glucose oxidation, resulting in a decrease in mitochondrial membrane potential and an increase in ROS production in cancer cells. A combination of DCA with TTFields is therefore being investigated as a means of exploiting mitochondrial dysfunction caused by TTFields. The inhibition of mitophagy suggests a new mechanism of action for TTFields exposure tied to the downregulation of the FA pathway genes, the result being both nuclear and cytosolic perturbations in cell homeostasis. This study offers an evidence-based rationale for combining TTFields with various chemotherapy agents that may cause DNA damage, induce replication stress, and expose cells to radical oxygen byproducts of disrupting electron transport chains and the disruption of mitochondrial integrity.

Citation Format: Narasimha Kumar Karanam, Michael Story. Tumor treating fields exposure causes an imbalance of reactive oxygen homeostasis likely through the cytosolic function of the fanconi′s anemia genes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1371.

Abstract 3316: Tumor Treating Fields in combination with radiation cause significant delay in tumor growth in in-vivo mice modelsignificant delay in tumor growth in in-vivo mice model

New physical cancer treatment modality called Tumor Treating Fields (TTFields) deliver low-intensity, intermediate frequency, alternating electric fields non-invasively to the tumor. TTFields is believed to inhibit mitosis as its primary mechanism of action. We previously showed that TTFields exposure decreased the expression of FANC/BRCA1 pathway genes, which play an essential role in DNA double strand break (DSB) repair. As a result, DNA DSB repair under IR exposure is downregulated, and most importantly, the TTFields alone increased gH2AX foci and chromatid aberrations as a function of TTFields time exposure. The length of newly replicated DNA decreased and R-loop formation increased after TTFields treatment, suggesting replication stress is induced by TTFields. Our research revealed that TTFields are involved in DNA damage and replication stress pathways besides mitosis, thus, we predicted that, by applying TTFields first, a conditional vulnerability environment would be created, rendering cells more susceptible to agents like radiation and chemotherapy. In accordance with our hypothesis, radiation susceptibility increased when cells were exposed to TTFields prior to IR treatment compared to IR treatment followed by TTFields. The combination of TTFields together with Radiation has been tested in-vivo with the newly designed Inovivo system. To measure tumor growth delay caused by TTFields treatment, three distinct syngenic tumor mouse models were utilized, namely LLC (Murine Lewis Lung Carcinoma), KPC63 (Pancreatic Cancer) and MC38 (Colon Cancer). Although TTField exposure decreased tumor volume in the three mouse models tested, there was no statistically significant difference in tumor growth between the heat and TTField groups. In IR combination experiments, the MC38 mouse model was chosen since TTFields induced a greater TGD effect in this model. In the first Inovivo experiment, we combined TTFields and radiation therapy, which delayed tumor growth significantly. Following that, we tested the combined efficacy of two rounds of TTFields and radiation treatment. The tumor growth delay was more pronounced after two rounds of TTFields treatment compared to one, which confirms the hypothesis that TTFields treatment induces conditional vulnerability. The results are consistent with observations made by the researchers in the clinic that high compliance patients (who have received more exposure to TTFields) have better prognosis compared to low compliance patients. Our inovivo experiment results not only confirm our earlier in vitro results, but also support the use of TTFields in IR combination therapies.

Citation Format: Narasimha Kumar Karanam, Zengfu Shang, Michael D. Story, Debabrata Saha. Tumor Treating Fields in combination with radiation cause significant delay in tumor growth in in-vivo mice modelsignificant delay in tumor growth in in-vivo mice model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3316.

Abstract 559: Proteomic analysis of lung cancer cells exposed to tumor treating fields identified the dysregulation of the E2F-Rb-CDK4/6 axis rendering tumor cells susceptible to novel combination therapies that target CDK4/6 and/or E2F

TTFields is a novel non-invasive physical modality of cancer therapy now approved for recurrent and newly diagnosed glioblastoma multiforme (GBM) in combination with temozolomide, and unresectable locally advanced or metastatic malignant pleural mesothelioma (MPM) in combination with platinum based chemotherapy. Clinical trials are ongoing for other cancers, including lung, pancreatic, and ovarian cancers. TTFields are low-intensity, intermediate frequency, alternating electric fields that are applied to tumor regions and cells using non-invasive arrays. One mechanism described for TTFields induced cell death has been via the disruption of mitosis while a more recent examination suggests that TTFields causes replication stress, and down-regulates DNA repair and cell cycle checkpoint genes. However, the exact cause of the downregulation of DNA repair and cell cycle checkpoint genes has been elusive. To that end, we employed relative quantitative proteomic analysis using tandem mass tags (TMT). All samples underwent trypsin digestion and labelling with different TMT reagents. They were then combined and the mixture was analyzed on an Orbitrap Fusion mass spectrometry device. Peptide quantitation was accomplished by comparing the intensities of the TMT reporter ions. STRING DB analysis of differentially expressed proteins revealed interaction networks that included cell cycle, DNA damage repair and replication, and transcriptional and translational regulation. Upstream analysis of key genes associated with cell cycle checkpoint and DNA repair identified reduced expression of the transcriptional activators E2F1 and E2F2 and increased expression of the transcriptional repressor E2F6, suggesting that TTFields affects the CDK-RB-E2F axis. For example, the downregulation of key DNA repair genes including RAD51, BRCA1 and BRCA2 could be explained through the upregulation of the transcriptional repressors E2F4 and E2F6 (a known repressor of BRCA1). These proteins are involved in homologous recombination repair and nucleotide excision repair, but also with replication fork maintenance, replication fork collapse and overall replication stress, the latter of which likely leads to cell death. Therefore, TTFields was combined with the E2F inhibitor HLM006474 with or without the CDK4/6 inhibitor abemaciclib. TTFields in combination with either inhibitor enhanced cell killing synergistically, as compared to TTFields alone, while the triple combination was found to be highly lethal (&gt;90% by 72 h) as measured by clonogenic assay followed by the Highest Single Agent approach to determine synergy. Taken together our results identify the CDK-RB-E2F axis as a novel druggable target that can be used in combination with TTFields for cancer therapy.

Citation Format: Narasimha Kumar Karanam, Sivaramakrishna Yadavalli, Michael Story. Proteomic analysis of lung cancer cells exposed to tumor treating fields identified the dysregulation of the E2F-Rb-CDK4/6 axis rendering tumor cells susceptible to novel combination therapies that target CDK4/6 and/or E2F [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 559.

Abstract 1051: Targeting replication stress pathway provides an avenue for novel combination therapy options including TTfields plus chemo agents which increase replication stress

Tumor Treating Fields (TTFields) is a new physical therapeutic modality, which has been FDA approved for the treatment of recurrent glioblastoma (GBM) as monotherapy, for newly diagnosed GBM in combination with temozolomide, and for unresectable locally advanced or metastatic malignant pleural mesothelioma (MPM) in combination with pemetrexed and a platinum-based chemotherapy. Clinical trials are ongoing for other cancers, including lung, pancreatic, and ovarian cancers. TTFields are low-intensity, intermediate frequency, alternating electric fields which are loco-regionally applied to tumor sites using non-invasive arrays. The initial mechanism by which TTFields was thought to kill tumor cells was the disruption of mitosis. Using gene expression analysis and functional characterization studies, we discovered a novel role of TTFields in DNA damage repair and replication stress pathways. TTFields treatment decreases Fanconi Anemia (FA) pathway signaling proteins thereby impairs ionizing radiation (IR)-induced DNA damage repair processes. The length of newly replicated DNA slowed as a function of TTFields exposure time and that TTFields increased R-loop formation, which indicates that TTFields induced replication stress. Hence, we hypothesized TTFields increase sensitivity of chemotherapy agents that target and increase replication stress in novel combination therapy options. PARP1 protects DNA breaks, by recruiting DNA repair and checkpoint proteins to the sites of damage and recruiting MRE11 for DNA end processing (required for replication restart and enhanced Chk1 activation). Indeed, TTFields treatment concomitant with the PARP1 inhibitor olaparib followed by IR was found to be synergistic compared to IR or olaparib alone or in combination. However, the degree of sensitization and synergy varied across non-small cell lung cancer cell lines. TTFields increased cell killing efficacy of etoposide synergistically, which forms a ternary complex with topoisomerase II and prevents re-ligation of DNA strands to elicit DNA strand breaks and induce replication stress. The advantage of combining TTFields with AZD6738 (an ataxia telangiectasia and Rad3-related protein (ATR) inhibitor) was tested. ATR is an essential kinase regulator of the replication checkpoint that plays a critical role in safeguarding genome integrity from replication stress. Another novel combination of TTFields together with irinotecan, which traps topoisomerase I- DNA in ternary cleavage complex and inhibits initial cleavage reaction and re-ligation steps to increase replication stress, is also tested. Taken together, our results suggest that different chemo agents that target replication stress pathways can be used in combination with TTFields for cancer therapy, which need to be tested in in-vivo studies and clinical trials.

Citation Format: Narasimha Kumar Karanam, Michael Dean Story. Targeting replication stress pathway provides an avenue for novel combination therapy options including TTfields plus chemo agents which increase replication stress [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 1051.

EXTH-05. THERAPEUTIC IMPLICATIONS OF TTFIELDS INDUCED DNA DAMAGE AND REPLICATION STRESS IN NOVEL COMBINATIONS FOR CANCER TREATMENT

TTFields are low-intensity, intermediate frequency, alternating electric fields which are applied to tumor regions using non-invasive arrays. TTFields is approved for the treatment of glioblastoma and mesothelioma with clinical trials ongoing in other cancer types. The mechanism of action for TTFields includes interference with mitosis, reduced DNA double strand break (DSB) repair capacity and the frank induction of DNA DSBs. The mechanism by which TTFields induces DNA DSBs appears to be through the enhancement of DNA replication stress with continued TTFields exposure. The induction of DNA DSBs appears to be as a result of significantly reduced expression of the DNA replication complex genes MCM6 and MCM10 as well as the Fanconi’s Anemia (FA) pathway genes. TTFields treatment increases the number of RPA foci, decreases nascent DNA length and increases R-loop formation which are markers of DNA replication stress. These results suggest that TTFields-induced replication stress is the underlying mechanism and cellular endogenous source of DNA DSB generation via replication fork collapse. The current study suggests that TTFields exposure causes a conditional vulnerability environment that renders cells more susceptible to chemotherapeutic agents that induce DNA damage and/or cause replication stress. Supporting this is the synergistic cell killing seen with TTFields exposure concomitant with cisplatin, TTFields plus concomitant PARP inhibition with or without subsequent radiation, or radiation given at the completion of a TTFields exposure. Finally, TTFields-induced mitotic aberrations and DNA damage/replication stress events, although intimately linked to one another as one can expose the other, are likely initiated independently of one another as suggested by the gene expression analysis of 47 key mitosis regulator genes. These results establish that enhanced replication stress and reduced DNA repair capacity are also major mechanisms of TTFields effects, effects for which there are therapeutic implications.

Abstract 3217: Newly identified role of tumor treating fields in DNA damage repair and replication stress pathways

Tumor treating fields (TTFields) are low-intensity, intermediate frequency, alternating electric fields non-invasively applied to the region of the tumor. TTFields have revolutionized the treatment of recurrent and newly diagnosed glioblastoma and Phase III clinical trials are ongoing for brain metastasis. Phase II trials are ongoing in non-small cell lung, ovarian and pancreatic cancers. The primary mechanism of TTFields function is thought to be the disruption of mitosis; however, other mechanisms are under investigation. Using a panel of 5 non-small cell lung cancer cell lines (NSCLC), we found that cell proliferation and killing as a result of TTFields exposure was variable. To understand the molecular mechanisms underlying the biological effects of TTFields exposure we examined temporal gene expression changes in these NSCLC cell lines after TTFields treatment. Interestingly we found that the expression of the BRCA1 DNA damage repair pathway genes were significantly downregulated (P &lt; 0.05) upon TTFields treatment which was confirmed at the protein level by western blot. Furthermore, TTFields treatment slowed the repair of ionizing radiation-induced DNA damage compared to radiation alone which was evident by an increased number of DNA double strand break repair foci at any given time. Moreover, we found that TTFields treatment alone increased the number of γH2AX foci and the incidence of chromatid aberrations. Given the role of BRCA1/FANC gene downregulation, we examined the status of replication fork integrity as a result of TTFields exposure and determined that the length of newly replicated DNA, slowed as a function of TTFields exposure time. Furthermore, we showed that TTFields increased R-loop formation (DNA:RNA hybrid structures) which was quantified using a DNA-RNA hybrid specific antibody. These results clearly indicate that TTFields induce DNA damage and increase replication stress. Based on newly identified mechanisms of TTFields action we hypothesized that by applying TTFields first, a conditional lethality environment would develop, rendering cells more susceptible to agents such as radiation or in the case of BRCA1 downregulation PARP inhibition. Indeed, by applying TTFields before, rather than after radiation, all cell lines were more susceptible to death. Studies of PARP inhibition in combination with TTFields and TTFields plus radiation are ongoing. Finally, we propose that future use of TTFields in a clinical setting may be more durable if provided post-surgery but prior to or concomitantly with chemotherapy or radiation therapy.

Citation Format: Narasimha Kumar Karanam, Lianghao Ding, Brock Sishc, Debabrata Saha, Michael D. Story. Newly identified role of tumor treating fields in DNA damage repair and replication stress pathways [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3217.

EXTH-76. DEVELOPING THE NOVEL COMBINATION THERAPY OPTIONS FOR CANCER THERAPY USING TUMOR TREATING FIELDS TOGETHER WITH THE CHEMO AGENTS TARGETING THE REPLICATION STRESS PATHWAY

Tumor Treating Fields (TTFields) as a component of cancer therapy has been shown to provide significant clinical benefit. The disruption of mitosis was identified as the first mechanism of action, however, a novel role for TTFields outside of mitosis where TTFields downregulates the Fanconi’s Anemia genes and proteins results in replication stress and reduced DNA repair capacity. Given these results we hypothesized that TTFields would increase the sensitivity of tumor cells to chemotherapy agents that increase replication stress. An analysis of agents that target replication stress in different ways was initiated. PARP1: Targeting PARP1 protects DNA breaks by recruiting DNA repair and checkpoint proteins to the sites of damage. Using the PARP1 inhibitor olaparib followed by radiation resulted in synergistic cell killing compared to radiation or olaparib alone or in combination. Etoposide: Etoposide forms a ternary complex with topoisomerase II and prevents re-ligation of DNA strands to elicit DNA strand breaks and replication stress. When combined with TTFields cell killing increased synergistically vs. etoposide alone. AZD6738: ATR is an essential kinase regulator of the replication checkpoint that plays a critical role in safeguarding genome integrity from replication stress. The advantage of combining TTFields with the ATR inhibitor-AZD6738 was tested for cell killing and the combination vs. AZD6738 was found to be synergistic. Irinotecan: Irinotecan traps topoisomerase I- DNA in a ternary cleavage complex and inhibits the initial cleavage reaction and re-ligation steps resulting in increased replication stress. Irinotecan and TTFields was tested and found to be highly effective at tumor cell killing. Collectively, our results suggest that it is likely of considerable clinical benefit to combine TTFields with chemotherapy agents that cause replication stress. This notion may explain the results of a number of clinical trials and suggests that there may be novel TTFields/replication stress-inducing chemotherapy agent combinations worth exploring.

Abstract 2138: Tumor treatment fields downregulate the BRCA1/FA pathway genes leading to reduced DNA repair capacity, the inhibition of mitophagy and enhanced cell death

The application of new physical cancer treatment modality utilizing alternating electric fields termed tumor treatment fields (TTFields) has revolutionized the treatment of recurrent and newly diagnosed glioblastoma. This non-invasive exposure to low-intensity, intermediate frequency, alternating electric fields to the region of the tumor has resulted in a significant increase in overall survival when compared to standard therapy with very minimal side effects. Clinical trials are recruiting or ongoing at additional tumor sites including lung, pancreatic, and ovarian cancer. The primary mechanism of TTField cell killing is thought to be the disruption of mitosis; however, other potential mechanisms are under investigation. Using a panel of five NSCLC cell lines we found that TTFields treatment alone inhibits cell proliferation, and decreases survival, though the degree of inhibition varies between cell lines. To understand the molecular mechanisms underlying the biological effects of TTField exposure we studied temporal gene expression changes in the NSCLC cell lines after TTField treatment. We observed that most differentially expressed genes are part of cell cycle and proliferation pathways which is in agreement with earlier findings. Interestingly we found that the expression of BRCA1 DNA damage repair pathway genes were significantly downregulated (P &lt; 0.05) upon TTField treatment. We confirmed the downregulation of BRCA1/FA pathway proteins by western blot. When examining the nuclear role of the BRCA1/FA pathway genes we found that TTField treatment slowed the repair of ionizing radiation-induced DNA damage compared to radiation alone which is evident by an increased number of DNA double strand break repair foci at any given time. Moreover, we found that TTField treatment increased the incidence of chromatid aberrations. We also examined the newly identified BRCA1/FA pathway genes cytosolic role in mitophagy where we observed alterations in mitophagy related gene (PINK1, OSCP1, ATP5 and DAPIT) expression and confirmed the same at the protein level by western blot. We hypothesized that TTFields disrupt the clearance of damaged mitochondria due to the downregulation of BRCA1/FA pathway players, causing an imbalance in oxygen metabolism leading to the production of high levels of radical oxygen species (ROS) and as a result, cell death. Using CellROX dye we found that TTField treatment did result in increased ROS production suggesting a new mechanism of action for TTField exposure. Novel chemotherapy agents, particularly PARP inhibitors, in combination with DNA damaging agents like radiation and TTFields may be advantageous through the conditional vulnerability of down-regulated BRCA1.

Citation Format: Narasimha Kumar Karanam, Lianghao Ding, Brock Sishc, Debabrata Saha, Michael D. Story. Tumor treatment fields downregulate the BRCA1/FA pathway genes leading to reduced DNA repair capacity, the inhibition of mitophagy and enhanced cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2138. doi:10.1158/1538-7445.AM2017-2138

Abstract 3939: Exploiting tumor treating fields induced downregulation of BRCA1 pathway for novel combination therapies

A new physical cancer treatment modality called Tumor Treating Fields (TTFields) has demonstrated effectiveness in the treatment of solid tumors in vitro and in vivo. TTFields therapy is a non-invasive cancer treatment modality that delivers low intensity (1-3 V/cm), intermediate frequency (100-300 kHz) alternating electric fields to the tumor. The TTFields delivery device called Optune (NovoCure), has been approved for recurrent and newly diagnosed glioblastoma and clinical trials are ongoing for other cancers. The primary mechanism of TTFields action is thought to be interference with mitosis. We monitored temporal gene expression changes in 5 non-small cell lung cancer (NSCLC) cell lines whose response to TTFields is variable and found that the expression of the BRCA1 DNA damage repair pathway, as well as other DNA repair/checkpoint pathways were significantly downregulated (P&lt;0.05) upon TTFields exposure. We monitored the dynamics of DNA double strand break repair to study functional relevance of BRCA1 pathway downregulation and found that TTFields treatment slowed the repair of ionizing radiation (IR)-induced DNA damage and interestingly, TTFields alone increased the number of γH2AX foci and the incidence of chromatid aberrations. Furthermore, as a function of TTFields exposure time, decreased replication fork speed and increased R-loop formation was identified, suggesting that TTFields induce replication stress. We carried out DNA/RNA immunoprecipitation sequencing (DRIP-Seq) analysis and are currently mapping R-loop formation within the genome. Based upon these newly identified mechanisms of TTFields action, i.e., enhancing DNA damage, reducing DNA repair capacity and increasing replication stress, led us to hypothesize that by applying TTFields first, a conditional vulnerability environment would develop rendering cells more susceptible to novel combination therapies. For example, cell killing by ionizing radiation exposure is enhanced when NSCLCs are exposed to TTFields after radiation. However, TTFields exposure prior to IR treatment is more effective compared to IR treatment prior to TTFields treatment. Furthermore, the combination of cisplatin together with TTFields also suggests synergistic effects in NSCLC cells. TTFields exposure concomitant with the PARP inhibitor Olaparib followed by radiation was synergistic compared to radiation or Olaparib alone or in combination, although the degree of sensitization and synergy varied across the different NSCLC cell lines. Taken together our results suggest that TTFields may enhance the efficacy of radiation if used as a neoadjuvant therapy or as concomitant therapy with different chemotherapy agents to improve patient outcome in clinic.

Citation Format: Narasimha Kumar Karanam, Liang-Hao Ding, Brock Sishc, Debabrata Saha, Michael D. Story. Exploiting tumor treating fields induced downregulation of BRCA1 pathway for novel combination therapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3939.