Replication stress induced site-specific phosphorylation targets WRN to the ubiquitin-proteasome pathway

Discovery of WRN inhibitor HRO761 with synthetic lethality in MSI cancers

The Werner syndrome RecQ helicase WRN was identified as a synthetic lethal target in cancer cells with microsatellite instability (MSI) by several genetic screens1-6. Despite advances in treatment with immune checkpoint inhibitors7-10, there is an unmet need in the treatment of MSI cancers11-14. Here we report the structural, biochemical, cellular and pharmacological characterization of the clinical-stage WRN helicase inhibitor HRO761, which was identified through an innovative hit-finding and lead-optimization strategy. HRO761 is a potent, selective, allosteric WRN inhibitor that binds at the interface of the D1 and D2 helicase domains, locking WRN in an inactive conformation. Pharmacological inhibition by HRO761 recapitulated the phenotype observed by WRN genetic suppression, leading to DNA damage and inhibition of tumour cell growth selectively in MSI cells in a p53-independent manner. Moreover, HRO761 led to WRN degradation in MSI cells but not in microsatellite-stable cells. Oral treatment with HRO761 resulted in dose-dependent in vivo DNA damage induction and tumour growth inhibition in MSI cell- and patient-derived xenograft models. These findings represent preclinical pharmacological validation of WRN as a therapeutic target in MSI cancers. A clinical trial with HRO761 (NCT05838768) is ongoing to assess the safety, tolerability and preliminary anti-tumour activity in patients with MSI colorectal cancer and other MSI solid tumours.

The importance of staged surgery for giant atypical central neurocytoma

The aim of this article is to discuss the importance of staged surgeries when approaching atypical central neurocytoma in children. Also, we show the preoperative embolization of the lesion as a maneuver to reduce the intraoperative bleeding. Central neurocytomas represent less than 0.5% of all intracranial tumors, and atypical central neurocytomas usually have unfavorable outcome, with high recurrence rate. The intraventricular location is frequent, with a predilection for the lateral ventricles. When completely resected, these lesions have a good prognosis. We report a case of a 12-year old male patient that presented with a history of headache for about 6 months, which worsened for 1 week prior to admission. Magnetic resonance imaging (MRI) brain showed a massive lesion occupying both lateral ventricles. He underwent a microsurgical treatment of a highly vascularized lesion, but the perioperative bleeding required interruption of the surgery. Thus, a preoperative embolization was able to occlude most arterial feeders and allowed subtotal resection in a second surgery. The patient had complete neurological recovery despite immediate post-operative deficits, and the histopathology was suggestive of atypical neurocytoma. Two-stage surgery with preoperative adjuvant embolization is a feasible strategy for treatment of large central neurocytomas in children.

Factors Affecting the Re-Endothelialization of Endothelial Progenitor Cell

The vascular endothelium, which plays an essential role in maintaining the normal shape and function of blood vessels, is a natural barrier between the circulating blood and the vascular wall tissue. The endothelial damage can cause vascular lesions, such as atherosclerosis and restenosis. After the vascular intima injury, the body starts the endothelial repair (re-endothelialization) to inhibit the neointimal hyperplasia. Endothelial progenitor cell is the precursor of endothelial cells and plays an important role in the vascular re-endothelialization. However, re-endothelialization is inevitably affected in vivo and in vitro by factors, which can be divided into two types, namely, promotion and inhibition, and act on different links of the vascular re-endothelialization. This article reviews these factors and related mechanisms.

Pharmacological targeting of differential DNA repair, radio-sensitizes WRN-deficient cancer cells in vitro and in vivo

Werner (WRN) expression is epigenetically downregulated in various tumors. It is imperative to understand differential repair process in WRN-proficient and WRN-deficient cancers to find pharmacological targets for radio-sensitization of WRN-deficient cancer. In the current investigation, we showed that pharmacological inhibition of CHK1 mediated homologous recombination repair (HRR), but not non-homologous end joining (NHEJ) repair, can causes hyper-radiosensitization of WRN-deficient cancers. This was confirmed in cancer cell lines of different tissue origin (osteosarcoma, colon adenocarcinoma and melanoma) with WRN silencing and overexpression. We established that WRN-depleted cells are dependent on a critical but compromised CHK1-mediated HRR-pathway for repairing ionizing radiation (IR) induced DSBs for their survival. Mechanistically, we unraveled a new finding that the MRE11, CTIP and WRN proteins are largely responsible for resections of late and persistent DSBs. In response to IR-treatment, MRE11 and CTIP-positively and WRN-negatively regulate p38-MAPK reactivation in a CHK1-dependent manner. A degradation resistant WRN protein, mutated at serine 1141, abrogates p38-MAPK activation. We also showed that CHK1-p38-MAPK axis plays important role in RAD51 mediated HRR in WRN-silenced cells. Like CHK1 inhibition, pharmacological-inhibition of p38-MAPK also hyper-radiosensitizes WRN-depleted cells by targeting HR-pathway. Combination treatment of CHK1-inhibitor (currently under various clinical trials) and IR exhibited a strong synergy against WRN-deficient melanoma tumor in vivo. Taken together, our findings suggest that pharmacological targeting of CHK1-p38-MAPK mediated HRR is an attractive strategy for enhancing therapeutic response of radiation treatment of cancer.

Using a Human Papillomavirus Model to Study DNA Replication and Repair of Wild Type and Damaged DNA Templates in Mammalian Cells

Human papillomaviruses have 8kbp DNA episomal genomes that replicate autonomously from host DNA. During initial infection, the virus increases its copy number to 20-50 copies per cell, causing torsional stress on the replicating DNA. This activates the DNA damage response (DDR) and HPV replicates its genome, at least in part, using homologous recombination. An active DDR is on throughout the HPV life cycle. Two viral proteins are required for replication of the viral genome; E2 binds to 12bp palindromic sequences around the A/T rich origin of replication and recruits the viral helicase E1 via a protein-protein interaction. E1 forms a di-hexameric complex that replicates the viral genome in association with host factors. Transient replication assays following transfection with E1-E2 expression plasmids, along with an origin containing plasmid, allow monitoring of E1-E2 replication activity. Incorporating a bacterial lacZ gene into the origin plasmid allows for the determination of replication fidelity. Here we describe how we exploited this system to investigate replication and repair in mammalian cells, including using damaged DNA templates. We propose that this system has the potential to enhance the understanding of cellular components involved in DNA replication and repair.

Werner Syndrome Protein (WRN) Regulates Cell Proliferation and the Human Papillomavirus 16 Life Cycle during Epithelial Differentiation

Human papillomaviruses recruit a host of DNA damage response factors to their viral genome to facilitate homologous recombination replication in association with the viral replication factors E1 and E2. We previously demonstrated that SIRT1 deacetylation of WRN promotes recruitment of WRN to E1-E2 replicating DNA and that WRN regulates both the levels and fidelity of E1-E2 replication. The deacetylation of WRN by SIRT1 results in an active protein able to complex with replicating DNA, but a protein that is less stable. Here, we demonstrate an inverse correlation between SIRT1 and WRN in CIN cervical lesions compared to normal control tissue, supporting our model of SIRT1 deacetylation destabilizing WRN protein. We CRISPR/Cas9 edited N/Tert-1 and N/Tert-1+HPV16 cells to knock out WRN protein expression and subjected the cells to organotypic raft cultures. In N/Tert-1 cells without WRN expression, there was enhanced basal cell proliferation, DNA damage, and thickening of the differentiated epithelium. In N/Tert-1+HPV16 cells, there was enhanced basal cell proliferation, increased DNA damage throughout the epithelium, and increased viral DNA replication. Overall, the results demonstrate that the expression of WRN is required to control the proliferation of N/Tert-1 cells and controls the HPV16 life cycle in these cells. This complements our previous data demonstrating that WRN controls the levels and fidelity of HPV16 E1-E2 DNA replication. The results describe a new role for WRN, a tumor suppressor, in controlling keratinocyte differentiation and the HPV16 life cycle.IMPORTANCE HPV16 is the major human viral carcinogen, responsible for around 3 to 4% of all cancers worldwide. Our understanding of how the viral replication machinery interacts with host factors to control/activate the DNA damage response to promote the viral life cycle remains incomplete. Recently, we demonstrated a SIRT1-WRN axis that controls HPV16 replication, and here we demonstrate that this axis persists in clinical cervical lesions induced by HPV16. Here, we describe the effects of WRN depletion on cellular differentiation with or without HPV16; WRN depletion results in enhanced proliferation and DNA damage irrespective of HPV16 status. Also, WRN is a restriction factor for the viral life cycle since replication is disrupted in the absence of WRN. Future studies will focus on enhancing our understanding of how WRN regulates viral replication. Our goal is to ultimately identify cellular factors essential for HPV16 replication that can be targeted for therapeutic gain.

MIB1-mediated degradation of WRN promotes cellular senescence in response to camptothecin treatment

Werner syndrome protein (WRN) plays critical roles in DNA replication, recombination, and repair, as well as transcription and cellular senescence. Ubiquitination and degradation of WRN have been reported, however, the E3 ubiquitin ligase of WRN is little known. Here, we identify mindbomb E3 ubiquitin protein ligase 1 (MIB1) as a novel E3 ubiquitin ligase for WRN protein. MIB1 physically interacts with WRN in vitro and in vivo and induces ubiquitination and degradation of WRN in the ubiquitin-proteasome pathway. Camptothecin (CPT) enhances the interaction between MIB1 and WRN, and promotes WRN degradation in a MIB1-dependent manner. In addition, CPT-induced cellular senescence is facilitated by the expression of MIB1 and attenuated by WRN expression. Our results show that MIB1-mediated degradation of WRN promotes cellular senescence and reveal a novel model executed by MIB1 and WRN to regulate cellular senescence.

MiRNA-144-3p inhibits high glucose induced cell proliferation through suppressing FGF16

As a major cause of blindness, diabetic retinopathy (DR) is often found in the developed countries. Our previous study identified a down-regulated miRNA: miR-144-3p in response to hyperglycemia. The present study aims to investigate the role of miR-144-3p in proliferation of microvascular epithelial cells. Endothelial cells were treated with different concentrations of glucose, after which miR-144-3p were detected with real-time PCR assay. MiR-144-3p mimics or inhibitors were used to increase or knockdown the level of this miRNA. Western blotting assay and ELISA assay were used to measure the expression and concentration of VEGF protein. 5-Bromo-2-deoxyUridine (BrdU) labeled cell cycle assay was used to detect cells in S phase. MiRNA targets were predicted by using a TargetScan tool, and were further verified by luciferase reporter assay. In the present study, we focussed on a significantly down-regulated miRNA, miR-144-3p, and investigated its role in high glucose (HG) induced cell proliferation. Our data showed that miR-144-3p mimics significantly inhibited HG induced cell proliferation and reduced the percentage of cells in S phase. HG induced up-regulation of VEGF was also prohibited by miR-144-3p mimics. Through wound-healing assay, we found that miR-144-3p suppressed cell migration after HG treatments. Moreover, we predicted and proved that fibroblast growth factor (FGF)16 is a direct target of miR-144-3p. Finally, miR-144-3p attenuated HG induced MAPK activation. In conclusion, we demonstrated that miR-144-3p inhibited high glucose-induced cell proliferation through suppressing FGF16 and MAPK signaling pathway, suggesting a possible role of miR-144-FGF16 in the development of DR.

Human RIF1-Protein Phosphatase 1 Prevents Degradation and Breakage of Nascent DNA on Replication Stalling

RIF1 is a multifunctional protein implicated in controlling DNA replication and repair. Here, we show that human RIF1 protects nascent DNA from over-degradation at stalled replication forks. The major nuclease resecting nascent DNA in the absence of RIF1 is DNA2, operating with WRN as an accessory helicase. We show that RIF1 acts with protein phosphatase 1 to prevent over-degradation and that RIF1 limits phosphorylation of WRN at sites implicated in resection control. Protection by RIF1 against inappropriate degradation prevents accumulation of DNA breakage. Our observations uncover a crucial function of human RIF1 in preventing genome instability by protecting forks from unscheduled DNA2-WRN-mediated degradation.

Werner Helicase Control of Human Papillomavirus 16 E1-E2 DNA Replication Is Regulated by SIRT1 Deacetylation

Human papillomaviruses (HPV) are double-stranded DNA viruses causative in a host of human diseases, including several cancers. Following infection, two viral proteins, E1 and E2, activate viral replication in association with cellular factors and stimulate the DNA damage response (DDR) during the replication process. E1-E2 uses homologous recombination (HR) to facilitate DNA replication, but an understanding of host factors involved in this process remains incomplete. Previously, we demonstrated that the class III deacetylase SIRT1, which can regulate HR, is recruited to E1-E2-replicating DNA and regulates the level of replication. Here, we demonstrate that SIRT1 promotes the fidelity of E1-E2 replication and that the absence of SIRT1 results in reduced recruitment of the DNA repair protein Werner helicase (WRN) to E1-E2-replicating DNA. CRISPR/Cas9 editing demonstrates that WRN, like SIRT1, regulates the quantity and fidelity of E1-E2 replication. This is the first report of WRN regulation of E1-E2 DNA replication, or a role for WRN in the HPV life cycle. In the absence of SIRT1 there is an increased acetylation and stability of WRN, but a reduced ability to interact with E1-E2-replicating DNA. We present a model in which E1-E2 replication turns on the DDR, stimulating SIRT1 deacetylation of WRN. This deacetylation promotes WRN interaction with E1-E2-replicating DNA to control the quantity and fidelity of replication. As well as offering a crucial insight into HPV replication control, this system offers a unique model for investigating the link between SIRT1 and WRN in controlling replication in mammalian cells.IMPORTANCE HPV16 is the major viral human carcinogen responsible for between 3 and 4% of all cancers worldwide. Following infection, this virus activates the DNA damage response (DDR) to promote its life cycle and recruits DDR proteins to its replicating DNA in order to facilitate homologous recombination during replication. This promotes the production of viable viral progeny. Our understanding of how HPV16 replication interacts with the DDR remains incomplete. Here, we demonstrate that the cellular deacetylase SIRT1, which is a part of the E1-E2 replication complex, regulates recruitment of the DNA repair protein WRN to the replicating DNA. We demonstrate that WRN regulates the level and fidelity of E1-E2 replication. Overall, the results suggest a mechanism by which SIRT1 deacetylation of WRN promotes its interaction with E1-E2-replicating DNA to control the levels and fidelity of that replication.

MDM2-mediated degradation of WRN promotes cellular senescence in a p53-independent manner

MDM2 (Murine double minute 2) acts as a key repressor for p53-mediated tumor-suppressor functions, which includes cellular senescence. We found that MDM2 can promote cellular senescence by modulating WRN stability. Werner syndrome (WS), caused by mutations of the WRN gene, is an autosomal recessive disease, which is characterized by premature aging. Loss of WRN function induces cellular senescence in human cancer cells. Here, we found that MDM2 acts as an E3 ligase for WRN protein. MDM2 interacts with WRN both in vivo and in vitro. MDM2 induces ubiquitination of WRN and dramatically downregulates the levels of WRN protein in human cells. During DNA damage response, WRN is translocated to the nucleoplasm to facilitate its DNA repair functions; however, it is degraded by the MDM2-mediated ubiquitination pathway. Moreover, the senescent phenotype induced by DNA damage reagents, such as Etoposide, is at least in part mediated by MDM2-dependent WRN degradation as it can be significantly attenuated by ectopic expression of WRN. These results show that MDM2 is critically involved in regulating WRN function via ubiquitin-dependent degradation and reveal an unexpected role of MDM2 in promoting cellular senescence through a p53-independent manner.

Werner Syndrome Protein and DNA Replication

Werner Syndrome (WS) is an autosomal recessive disorder characterized by the premature development of aging features. Individuals with WS also have a greater predisposition to rare cancers that are mesenchymal in origin. Werner Syndrome Protein (WRN), the protein mutated in WS, is unique among RecQ family proteins in that it possesses exonuclease and 3' to 5' helicase activities. WRN forms dynamic sub-complexes with different factors involved in DNA replication, recombination and repair. WRN binding partners either facilitate its DNA metabolic activities or utilize it to execute their specific functions. Furthermore, WRN is phosphorylated by multiple kinases, including Ataxia telangiectasia mutated, Ataxia telangiectasia and Rad3 related, c-Abl, Cyclin-dependent kinase 1 and DNA-dependent protein kinase catalytic subunit, in response to genotoxic stress. These post-translational modifications are critical for WRN to function properly in DNA repair, replication and recombination. Accumulating evidence suggests that WRN plays a crucial role in one or more genome stability maintenance pathways, through which it suppresses cancer and premature aging. Among its many functions, WRN helps in replication fork progression, facilitates the repair of stalled replication forks and DNA double-strand breaks associated with replication forks, and blocks nuclease-mediated excessive processing of replication forks. In this review, we specifically focus on human WRN's contribution to replication fork processing for maintaining genome stability and suppressing premature aging. Understanding WRN's molecular role in timely and faithful DNA replication will further advance our understanding of the pathophysiology of WS.

Three-dimensional Culture of Human Airway Epithelium in Matrigel for Evaluation of Human Rhinovirus C and Bocavirus Infections

OBJECTIVE

Newly identified human rhinovirus C (HRV-C) and human bocavirus (HBoV) cannot propagate in vitro in traditional cell culture models; thus obtaining knowledge about these viruses and developing related vaccines are difficult. Therefore, it is necessary to develop a novel platform for the propagation of these types of viruses.

METHODS

A platform for culturing human airway epithelia in a three-dimensional (3D) pattern using Matrigel as scaffold was developed. The features of 3D culture were identified by immunochemical staining and transmission electron microscopy. Nucleic acid levels of HRV-C and HBoV in 3D cells at designated time points were quantitated by real-time polymerase chain reaction (PCR). Levels of cytokines, whose secretion was induced by the viruses, were measured by ELISA.

RESULTS

Properties of bronchial-like tissues, such as the expression of biomarkers CK5, ZO-1, and PCK, and the development of cilium-like protuberances indicative of the human respiration tract, were observed in 3D-cultured human airway epithelial (HAE) cultures, but not in monolayer-cultured cells. Nucleic acid levels of HRV-C and HBoV and levels of virus-induced cytokines were also measured using the 3D culture system.

CONCLUSION

Our data provide a preliminary indication that the 3D culture model of primary epithelia using a Matrigel scaffold in vitro can be used to propagate HRV-C and HBoV.

A game of substrates: replication fork remodeling and its roles in genome stability and chemo-resistance

During the hours that human cells spend in the DNA synthesis (S) phase of the cell cycle, they may encounter adversities such as DNA damage or shortage of nucleotides. Under these stresses, replication forks in DNA may experience slowing, stalling, and breakage. Fork remodeling mechanisms, which stabilize slow or stalled replication forks and ensure their ability to continue or resume replication, protect cells from genomic instability and carcinogenesis. Fork remodeling includes DNA strand exchanges that result in annealing of newly synthesized strands (fork reversal), controlled DNA resection, and cleavage of DNA strands. Defects in major tumor suppressor genes BRCA1 and BRCA2, and a subset of the Fanconi Anemia genes have been shown to result in deregulation in fork remodeling, and most prominently, loss of kilobases of nascent DNA from stalled replication forks. This phenomenon has recently gained spotlight as a potential marker and mediator of chemo-sensitivity in cancer cells and, conversely, its suppression - as a hallmark of acquired chemo-resistance. Moreover, nascent strand degradation at forks is now known to also trigger innate immune response to self-DNA. An increasingly sophisticated molecular description of these events now points at a combination of unbalanced fork reversal and end-resection as a root cause, yet also reveals the multi-layered complexity and heterogeneity of the underlying processes in normal and cancer cells.

Hypoxia tolerance, longevity and cancer-resistance in the mole rat Spalax - a liver transcriptomics approach

The blind subterranean mole rat Spalax shows a remarkable tolerance to hypoxia, cancer-resistance and longevity. Unravelling the genomic basis of these adaptations will be important for biomedical applications. RNA-Seq gene expression data were obtained from normoxic and hypoxic Spalax and rat liver tissue. Hypoxic Spalax broadly downregulates genes from major liver function pathways. This energy-saving response is likely a crucial adaptation to low oxygen levels. In contrast, the hypoxia-sensitive rat shows massive upregulation of energy metabolism genes. Candidate genes with plausible connections to the mole rat’s phenotype, such as important key genes related to hypoxia-tolerance, DNA damage repair, tumourigenesis and ageing, are substantially higher expressed in Spalax than in rat. Comparative liver transcriptomics highlights the importance of molecular adaptations at the gene regulatory level in Spalax and pinpoints a variety of starting points for subsequent functional studies.

Recent Advances in Understanding Werner Syndrome

Aging, the universal phenomenon, affects human health and is the primary risk factor for major disease pathologies. Progeroid diseases, which mimic aging at an accelerated rate, have provided cues in understanding the hallmarks of aging. Mutations in DNA repair genes as well as in telomerase subunits are known to cause progeroid syndromes. Werner syndrome (WS), which is characterized by accelerated aging, is an autosomal-recessive genetic disorder. Hallmarks that define the aging process include genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulation of nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. WS recapitulates these hallmarks of aging and shows increased incidence and early onset of specific cancers. Genome integrity and stability ensure the normal functioning of the cell and are mainly guarded by the DNA repair machinery and telomeres. WRN, being a RecQ helicase, protects genome stability by regulating DNA repair pathways and telomeres. Recent advances in WS research have elucidated WRN’s role in DNA repair pathway choice regulation, telomere maintenance, resolution of complex DNA structures, epigenetic regulation, and stem cell maintenance.

Repurposing DNA repair factors to eradicate tumor cells upon radiotherapy

Cancer is the leading cause of death worldwide. Almost 50% of all cancer patients undergo radiation therapy (RT) during treatment, with varying success. The main goal of RT is to kill tumor cells by damaging their DNA irreversibly while sparing the surrounding normal tissue. The outcome of RT is often determined by how tumors recognize and repair their damaged DNA. A growing body of evidence suggests that tumors often show abnormal expression of DNA double-strand break (DSB) repair genes that are absent from normal cells. Defects in a specific DNA repair pathway make tumor cells overly dependent on alternative or backup pathways to repair their damaged DNA. These tumor cell-specific abnormalities in the DNA damage response (DDR) machinery can potentially be used as biomarkers for treatment outcomes or as targets for sensitization to ionizing radiation (IR). An improved understanding of genetic or epigenetic alterations in the DNA repair pathways specific to cancer cells has paved the way for new treatments that combine pharmacological exploitation of tumor-specific molecular vulnerabilities with IR. Inhibiting DNA repair pathways has the potential to greatly enhance the therapeutic ratio of RT. In this review, we will discuss DNA repair pathways in active cells and how these pathways are deregulated in tumors. We will also describe the impact of targeting cancer-specific aberrations in the DDR as a treatment strategy to improve the efficacy of RT. Finally, we will address the current roadblocks and future prospects of these approaches.

RAD51 interconnects between DNA replication, DNA repair and immunity

RAD51, a multifunctional protein, plays a central role in DNA replication and homologous recombination repair, and is known to be involved in cancer development. We identified a novel role for RAD51 in innate immune response signaling. Defects in RAD51 lead to the accumulation of self-DNA in the cytoplasm, triggering a STING-mediated innate immune response after replication stress and DNA damage. In the absence of RAD51, the unprotected newly replicated genome is degraded by the exonuclease activity of MRE11, and the fragmented nascent DNA accumulates in the cytosol, initiating an innate immune response. Our data suggest that in addition to playing roles in homologous recombination-mediated DNA double-strand break repair and replication fork processing, RAD51 is also implicated in the suppression of innate immunity. Thus, our study reveals a previously uncharacterized role of RAD51 in initiating immune signaling, placing it at the hub of new interconnections between DNA replication, DNA repair, and immunity.

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.