Radiation-induced DNA Double-strand Breaks Research Articles

Structural Differences between the Genomes of Deinococcus radiodurans Strains from Different Laboratories.

The bacterium Deinococcus radiodurans is known to efficiently and accurately reassemble its genome after hundreds of DNA double-strand breaks (DSBs). Only at very large amounts of radiation-induced DSBs is this accuracy affected in the wild-type D. radiodurans, causing rearrangements in its genome structure. However, changes in its genome structure may also be possible during the propagation and storage of cell cultures. We investigate this possibility by listing structural differences between three completely sequenced genomes of D. radiodurans strains with a recent common ancestor-the type strain stored and sequenced in two different laboratories (of the ATCC 13939 lineage) and the first sequenced strain historically used as the reference (ATCC BAA-816). We detected a number of structural differences and found the most likely mechanisms behind them: (i) transposition/copy number change in mobile interspersed repeats-insertion sequences and small non-coding repeats, (ii) variable number of monomers within tandem repeats, (iii) deletions between long direct DNA repeats, and (iv) deletions between short (4-10 bp) direct DNA repeats. The most surprising finding was the deletions between short repeats because it indicates the utilization of a less accurate DSB repair mechanism in conditions in which a more accurate one should be both available and preferred. The detected structural differences, as well as SNPs and short indels, while being important footprints of deinococcal DNA metabolism and repair, are also a valuable resource for researchers using these D. radiodurans strains.

BUB1 regulates non-homologous end joining pathway to mediate radioresistance in triple-negative breast cancer

BackgroundTriple-negative breast cancer (TNBC) is a highly aggressive form of breast cancer subtype often treated with radiotherapy (RT). Due to its intrinsic heterogeneity and lack of effective targets, it is crucial to identify novel molecular targets that would increase RT efficacy. Here we demonstrate the role of BUB1 (cell cycle Ser/Thr kinase) in TNBC radioresistance and offer a novel strategy to improve TNBC treatment.MethodsGene expression analysis was performed to look at genes upregulated in TNBC patient samples compared to other subtypes. Cell proliferation and clonogenic survivals assays determined the IC50 of BUB1 inhibitor (BAY1816032) and radiation enhancement ratio (rER) with pharmacologic and genomic BUB1 inhibition. Mammary fat pad xenografts experiments were performed in CB17/SCID. The mechanism through which BUB1 inhibitor sensitizes TNBC cells to radiotherapy was delineated by γ-H2AX foci assays, BLRR, Immunoblotting, qPCR, CHX chase, and cell fractionation assays.ResultsBUB1 is overexpressed in BC and its expression is considerably elevated in TNBC with poor survival outcomes. Pharmacological or genomic ablation of BUB1 sensitized multiple TNBC cell lines to cell killing by radiation, although breast epithelial cells showed no radiosensitization with BUB1 inhibition. Kinase function of BUB1 is mainly accountable for this radiosensitization phenotype. BUB1 ablation also led to radiosensitization in TNBC tumor xenografts with significantly increased tumor growth delay and overall survival. Mechanistically, BUB1 ablation inhibited the repair of radiation-induced DNA double strand breaks (DSBs). BUB1 ablation stabilized phospho-DNAPKcs (S2056) following RT such that half-lives could not be estimated. In contrast, RT alone caused BUB1 stabilization, but pre-treatment with BUB1 inhibitor prevented stabilization (t1/2, ~8 h). Nuclear and chromatin-enriched fractionations illustrated an increase in recruitment of phospho- and total-DNAPK, and KAP1 to chromatin indicating that BUB1 is indispensable in the activation and recruitment of non-homologous end joining (NHEJ) proteins to DSBs. Additionally, BUB1 staining of TNBC tissue microarrays demonstrated significant correlation of BUB1 protein expression with tumor grade.ConclusionsBUB1 ablation sensitizes TNBC cell lines and xenografts to RT and BUB1 mediated radiosensitization may occur through NHEJ. Together, these results highlight BUB1 as a novel molecular target for radiosensitization in women with TNBC.

Manipulating Radiation-Sensitive Z-DNA Conformation for Enhanced Radiotherapy.

DNA double-strand breaks (DSBs) yield highly determines radiotherapy efficacy. However, improving the inherent radiosensitivity of tumor DNA to promote radiation-induced DSBs remains a challenge. Using theoretical and experimental models, the underexplored impact of Z-DNA conformations on radiosensitivity, yielding higher DSBs than other DNA conformations, is discovered. Thereout, a radiosensitization strategy focused on inducing Z-DNA conformation, utilizing CBL@HfO2 nanocapsules loaded with a Z-DNA inducer CBL0137, is proposed. A hollow mesoporous HfO2 (HM-HfO2) acts as a delivery and an energy depositor to promote Z-DNA breakage. The nanocapsule permits the smart DSBs accelerator that triggers its radiosensitization with irradiation stimulation. Impressively, the CBL@HfO2 facilitates the B-Z DNA conformational transition, augmenting DSBs about threefold stronger than irradiation alone, generating significant tumor suppression with a 30% cure rate. The approach enables DSBs augmentation by improving the inherent radiosensitivity of DNA. As such, it opens up an era of Z-DNA conformation manipulation in radiotherapy.

Radiation-induced DNA double-strand breaks in cortisol exposed fibroblasts as quantified with the novel foci-integrated damage complexity score (FIDCS)

Without the protective shielding of Earth’s atmosphere, astronauts face higher doses of ionizing radiation in space, causing serious health concerns. Highly charged and high energy (HZE) particles are particularly effective in causing complex and difficult-to-repair DNA double-strand breaks compared to low linear energy transfer. Additionally, chronic cortisol exposure during spaceflight raises further concerns, although its specific impact on DNA damage and repair remains unknown. This study explorers the effect of different radiation qualities (photons, protons, carbon, and iron ions) on the DNA damage and repair of cortisol-conditioned primary human dermal fibroblasts. Besides, we introduce a new measure, the Foci-Integrated Damage Complexity Score (FIDCS), to assess DNA damage complexity by analyzing focus area and fluorescent intensity. Our results show that the FIDCS captured the DNA damage induced by different radiation qualities better than counting the number of foci, as traditionally done. Besides, using this measure, we were able to identify differences in DNA damage between cortisol-exposed cells and controls. This suggests that, besides measuring the total number of foci, considering the complexity of the DNA damage by means of the FIDCS can provide additional and, in our case, improved information when comparing different radiation qualities.

Changes in repair pathways of radiation-induced DNA double-strand breaks at the midblastula transition in Xenopus embryo.

Ionizing radiation (IR) causes DNA damage, particularly DNA double-strand breaks (DSBs), which have significant implications for genome stability. The major pathways of repairing DSBs are homologous recombination (HR) and nonhomologous end joining (NHEJ). However, the repair mechanism of IR-induced DSBs in embryos is not well understood, despite extensive research in somatic cells. The externally developing aquatic organism, Xenopus tropicalis, serves as a valuable model for studying embryo development. A significant increase in zygotic transcription occurs at the midblastula transition (MBT), resulting in a longer cell cycle and asynchronous cell divisions. This study examines the impact of X-ray irradiation on Xenopus embryos before and after the MBT. The findings reveal a heightened X-ray sensitivity in embryos prior to the MBT, indicating a distinct shift in the DNA repair pathway during embryo development. Importantly, we show a transition in the dominant DSB repair pathway from NHEJ to HR before and after the MBT. These results suggest that the MBT plays a crucial role in altering DSB repair mechanisms, thereby influencing the IR sensitivity of developing embryos.

Abstract 705: miR-21-5p induces radioresistance through reinforced DNA damage response in oral squamous cell carcinoma

Abstract Cancers arising from oral cavity and oropharynx are commonly treated with radiotherapy, whereas tumor radioresistance results in poor outcome. The prerequisite for developing more effective interventions for these patients is that the molecular mechanisms leading to a radioresistant phenotype in these tumors are clear to researchers. Unfortunately, the radioresistant mechanisms remain elusive to date. We recently identified that a highly conserved and cancer-related microRNA, i.e. miR-21-5p, plays an important role in radioresistance of oral squamous cell carcinoma (OSCC). We analyzed the RNA-sequencing data of 290 tissue samples from TCGA database, and found the expression of miR-21-5p was significantly upregulated in many OSCC samples (N=271) when comparing to adjacent normal tissues (N=19). The difference was further confirmed on our freshly collected OSCC tissues from patients in operation room. We performed RNA-sequencing analysis in 11 OSCC samples and their adjacent normal tissues. Among the 2106 differentially expressed genes between OSCC and normal control groups, we found that several important DNA repair genes (RAD50 double strand break repair protein gene, RAD50; DNA Polymerase Theta, POLQ; Ataxia Telangiectasia and Rad3-related, ATR) were significantly overexpressed in OSCC samples (P<0.05), while positive correlations between miR-21-5p expression and DNA repair genes RAD51 and XRCC4 were identified in the OSCC cohort. Notably, ATR is an important DNA damage response gene, while XRCC4, RAD51, and POLQ are involved in DNA double strand break (DSB) repair through non-homologous end joining, homologous recombination, and microhomology-mediated end joining, respectively. OSCC cell lines were further recruited to study the relations of miR-21-5p with radiation-induced DSB repair. By analyzing kinetics of 53BP1 foci, a biomarker for detecting DSBs, we found that anti-miR-21-5p transfected cells showed more residual foci, indicating of unfinished repair of DSBs, at 5 hours and 18 hours post irradiation. The deficient DSB repair resulted from anti-miR-21-5p transfection were further confirmed by single cell gel electrophoresis with comet assay. Consistently, the same treatment resulted in fewer clonogenic survivors in colony formation assay, demonstrating increased radiosensitivity of OSCC cells when miR-21-5p was antagonized. This work provided novel insights about the mechanism of miR-21-5p-induced radioresistance in OSCC, which may give rise to novel therapeutic strategy for improved treatment of OSCC patients. Citation Format: Qi Liu, Weitao Hu, Xinghan Li, Yongqiang Deng, Lin Ma. miR-21-5p induces radioresistance through reinforced DNA damage response in oral squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 705.

Senescence-Associated Heterochromatin Foci Suppress γ-H2AX Focus Formation Induced by Radiation Exposure.

DNA damage is induced by both endogenous and exogenous factors. Repair of DNA double-strand break (DSB), a serious damage that threatens genome stability, decreases with senescence. However, the molecular mechanisms underlying the decline in DNA repair capacity during senescence remain unclear. We performed immunofluorescence staining for phosphorylated histone H2AX (γ-H2AX) in normal human fetal lung fibroblasts and human skin fibroblasts of different ages after chronic irradiation (total dose, 1 Gy; dose rate, 1 Gy/day) to investigate the effect of cellular senescence and organismal aging on DSB repair. Accumulation of DSBs was observed with cellular senescence and organismal aging, probably caused by delayed DSB repair. Importantly, the formation of γ-H2AX foci, an early event in DSB repair, is delayed with cellular senescence and organismal aging. These results suggest that the delay in γ-H2AX focus formation might delay the overall DSB repair. Interestingly, immediate γ-H2AX foci formation was suppressed in cells with senescence-associated heterochromatin foci (SAHF). To investigate the relationship between the γ-H2AX focus formation and SAHF, we used LiCl to relax the SAHFs, followed by irradiation. We demonstrated that LiCl rescued the delayed γ-H2AX foci formation associated with cellular senescence. This indicates that SAHF interferes with γ-H2AX focus formation and inhibits DSB repair in radiation-induced DSB. Our results suggest that therapeutic targeting of SAHFs have potential to resolve DSB repair dysfunction associated with cellular senescence.

Abstract A001: Discovery of imidazo [4,5-c]pyridin-2-ones as selective inhibitors of DNA-dependent protein kinase and effective radiosensitizers

Abstract Inhibition of the repair of radiation-induced DNA double strand breaks (DSB) offers the potential to sensitize tumors to radiation therapy. The dominant role of non-homologous end-joining in the repair of radiation-induced DSBs indicates that DNA-dependent protein kinase (DNA-PK) is an excellent target for the development of radiosensitizers. We report the discovery of a new chemical class of kinase inhibitor based on the imidazo[4,5-c]pyridin-2-one scaffold. The class was developed by scaffold-hopping from the pan phosphoinositide 3-kinsase (PI3K) and PI3K-like kinase (PIKK) inhibitor dactolisib. Iterative development culminated in the identification of SN39536 as a low nM DNA-PK inhibitor with excellent selectivity. Further modification led to the discovery of the more potent and more selective SN40905. Both SN39536 and SN40905 were effective inhibitors of DNA-PK kinase activity in a biochemical assay with substantial selectivity for DNA-PK across the PIKK and PI3K families. Both compounds selectively inhibited growth of HAP1 PRKDC wild-type cells when combined with radiation, but not the corresponding PRKDC-/- cells. SN39536 and SN40905 were effective radiosensitizers of colorectal carcinoma (HCT116), non-small cell lung cancer (H460, H1299, and A549), pancreatic (BxPC-3, PANC-1, and MiaPaCa-2) and head and neck squamous cell carcinoma (FaDu and UT-SCC-74B) cells determined using a clonogenic survival endpoint. Both SN39536 and SN40905 displayed high oral bioavailability. When administered PO at a range of non-toxic doses, both SN39536 and SN40905 provided significant additional tumor cell killing of HCT116, UT-SCC-74B, and BxPC-3 tumor xenografts in combination with a single (13 Gy) radiation treatment determined by ex vivo clonogenic survival assays. SN39536 also provided substantial tumor growth inhibition of HCT116 tumor xenografts in combination with RAD (10 Gy). SN39536 and SN40905 represent a new, potent, and selective class of DNA-PK inhibitors with significant potential as radiosensitizers for the treatment of human cancers. Citation Format: Michael P. Hay, Cho R. Hong, Lydia P. Liew, Benjamin D. Dickson, Way W. Wong, Sophia F. O'Brien-Gortner, Rebecca Airey, Jagdish Jaiswal, William R. Wilson, Stephen M. Jamieson. Discovery of imidazo [4,5-c]pyridin-2-ones as selective inhibitors of DNA-dependent protein kinase and effective radiosensitizers [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr A001.

Abstract A151: Establishing assays to investigate combinations of fractioned radiotherapy with DNA damage response agents in vitro and in vivo to enable investigation of radiosensitization and improved anti-tumour responses

Abstract Introduction: Radiotherapy is a main treatment modality for multiple cancer types, with approximately half of all cancer patients receiving radiotherapy as part of their treatment regimen. In addition to direct cytotoxic effects on cancer cells, it is now appreciated that radiosensitization using specific drugs inhibiting DNA damage response (DDR) pathways can enhance the sensitivity of tissues or cells to radiation therapy. Establishing assays to screen for potential radiosensitizing compounds is therefore valuable to inform drug discovery research. Methods: The MDA-MB-436 human breast cancer cell line was selected for assay optimization. DDR inhibitors of DNA-dependent protein kinase (DNA-PK) (AZD7648) and Poly-(ADP-Ribose)-Polymerase (PARP) (niraparib) were used as tool radiosensitizing compounds, as these proteins play key roles in the repair of radiation-induced DNA double strand breaks. Effects on cell viability, cell cycle progression and DDR biomarkers were determined under a variety of irradiation (IR) and compound conditions. In parallel, in vivo validation experiments were performed. Results and Conclusions: Both AZD7648 and niraparib radiosensitize MDA-MB-436 cells to IR treatment. A dose of 2 Gray (Gy) reduced the observed anti-proliferative compound IC50 compared to IR treatment or compound treatment alone. Furthermore, combined treatment of 2 Gy IR with either DDR inhibitor induces S-phase cell cycle arrest and increases cell death. Analysis of DDR biomarkers confirms their induction by radiotherapy in combination with DDR inhibitors. Similar changes to DDR biomarker expression were also observed in the in vivo xenograft mouse model. In summary, in vitro and in vivo assays have been established to investigate radiosensitization through the combination of IR and DDR inhibitors. Furthermore, these assays can identify novel compounds acting via a synergistic mechanism. Citation Format: Charlotte Bell, Stewart Brown, Amy Cantrell, Paul Farrington, Miguel Garcia, Ben Hodgson, Yin Xin Ho, Allan Jordan, Theoni Katopodi, Jane Kendrew, Wen Hao Neo, Stuart Thomson, Graeme E Walker, Emily Wright. Establishing assays to investigate combinations of fractioned radiotherapy with DNA damage response agents in vitro and in vivo to enable investigation of radiosensitization and improved anti-tumour responses [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A151.

Most DNA repair defects do not modify the relationship between relative biological effectiveness and linear energy transfer in CRISPR-edited cells.

Cancer is a highly heterogeneous disease, driven by frequent genetic alterations which have significant effects on radiosensitivity. However, radiotherapy for a given cancer type is typically given with a standard dose determined from population-level trials. As a result, a proportion of patients are under- or over-dosed, reducing the clinical benefit of radiotherapy. Biological optimization would not only allow individual dose prescription but also a more efficient allocation of limited resources, such as proton and carbon ion therapy. Proton and ion radiotherapy offer an advantage over photons due to their elevated Relative Biological Effectiveness (RBE) resulting from their elevated Linear Energy Transfer (LET). Despite significant interest in optimizing LET by tailoring radiotherapy plans, RBE's genetic dependence remains unclear. The aim of this study is to better define the RBE/LET relationship in a panel of cell lines with different defects in DSB repair pathways, but otherwise identical biological features and genetic background to isolate these effects. Normal human cells (RPE1), genetically modified to introduce defects in DNA double-strand break (DSB) repair genes, ATM, BRCA1, DCLRE1C, LIG4, PRKDC and TP53, were used to map the RBE-LET relationship. Cell survival was measured with clonogenic assays after exposure to photons, protons (LET 1 and 12keV/µm) and alpha particles (129keV/µm). Gene knockout sensitizer enhancement ratio (SER) values were calculated as the ratio of the mean inactivation dose (MID) of wild-type cells to repair-deficient cells, and RBE values were calculated as the ratio of the MID of X-ray and particle irradiated cells. 53BP1 foci were used to quantify radiation-induced DSBs and their repair following irradiation. Deletion of NHEJ genes had the greatest impact on photon sensitivity (ATM-/- SER=2.0 and Lig4-/- SER=1.8), with genes associated with HR having smaller effects (BRCA1-/- SER=1.2). Wild-type cells showed RBEs of 1.1, 1.3, 5.0 for low- and high-LET protons and alpha particles respectively. SERs for different genes were independent of LET, apart from NHEJ knockouts which proved to be markedly hypersensitive across all tested LETs. Due to this hypersensitivity, the impact of high LET was reduced in cell models lacking the NHEJ repair pathway. HR-defective cells had moderately increased sensitivity across all tested LETs, but, notably, the contribution of HR pathway to survival appeared independent of LET. Analysis of 53BP1 foci shows that NHEJ-defective cells had the least DSB repair capacity after low LET exposure, and no visible repair after high LET exposure. HR-defective cells also had slower repair kinetics, but the impact of HR defects is not as severe as NHEJ defects. DSB repair defects, particularly in NHEJ, conferred significant radiosensitivity across all LETs. This sensitization appeared independent of LET, suggesting that the contribution of different DNA repair pathways to survival does not depend on radiation quality.

Fisetin overcomes non-targetability of mutated KRAS induced YB-1 signaling in colorectal cancer cells and improves radiosensitivity by blocking repair of radiation-induced DNA double-strand breaks

Background and purposeKRAS is frequently mutated, and the Y-box binding protein 1 (YB-1) is overexpressed in colorectal cancer (CRC). Mutant KRAS (KRASmut) stimulates YB-1 through MAPK/RSK and PI3K/AKT, independent of epidermal growth factor receptor (EGFR). The p21-activated kinase (PAK) family is a switch-site upstream of AKT and RSK. The flavonoid compound fisetin inhibits RSK-mediated YB-1 signaling. We sought the most effective molecular targeting approach that interferes with DNA double strand break (DSB) repair and induces radiosensitivity of CRC cells, independent of KRAS mutation status. Materials and MethodsKRAS activity and KRAS mutation were analyzed by Ras-GTP assay and NGS. Effect of dual targeting of RSK and AKT (DT), the effect of fisetin as well as targeting PAK by FRAX486 and EGFR by erlotinib on YB-1 activity was tested by Western blotting after irradiation in vitro and ex vivo. Additionally, the effect of DT and FRAX486 on DSB repair pathways was tested in cells expressing reporter constructs for the DSB repair pathways by flow cytometry analysis. Residual DSBs and clonogenicity were examined by γH2AX- and clonogenic assays, respectively. ResultsErlotinib neither blocked DSB repair nor inhibited YB-1 phosphorylation under KRAS mutation condition in vitro and ex vivo. DT and FRAX486 effectively inhibited YB-1 phosphorylation independent of KRAS mutation status and diminished homologous recombination (HR) and alternative non-homologous end joining (NHEJ) repair. DT and FRAX486 inhibited DSB repair in CaCo2 but not in isogenic KRASG12V cells. Fisetin inhibited YB-1 phosphorylation, blocked DSB repair and increased radiosensitivity, independent of KRAS mutation status. ConclusionCombination of fisetin with radiotherapy may improve CRC radiation response, regardless of KRASmut status.

Kinetics of radiation-induced DNA double-strand breaks through coarse-grained simulations

Double-strand breaks (DSBs), i.e., the covalent cut of the DNA backbone over both strands, are a detrimental outcome of cell irradiation, bearing chromosomal aberrations and leading to cell apoptosis. In the early stages of the evolution of a DSB, the disruption of the residual interactions between the DNA moieties drives the fracture of the helical layout; in spite of its biological significance, the details of this process are still largely uncertain. Here, we address the mechanical rupture of DNA by DSBs via coarse-grained molecular dynamics simulations: the setup involves a 3855-bp DNA filament and diverse DSB motifs, i.e., within a range of distances between strand breaks (or DSB distance). By employing a coarse-grained model of DNA, we access the molecular details and characteristic timescales of the rupturing process. A sequence-nonspecific, linear correlation is observed between the DSB distance and the internal energy contribution to the disruption of the residual (Watson-Crick and stacking) contacts between DNA moieties, which is seemingly driven by an abrupt, cooperative process. Moreover, we infer an exponential dependence of the characteristic rupture times on the DSB distances, which we associate to an Arrhenius-like law of thermally-activated processes. This work lays the foundations of a detailed, mechanistic assessment of DSBs in silico as a benchmark to both numerical simulations and data from single-molecule experiments.

DNA-PKcs regulates myogenesis in an Akt-dependent manner independent of induced DNA damage.

Skeletal muscle regeneration relies on muscle stem (satellite) cells. We previously demonstrated that satellite cells efficiently and accurately repair radiation-induced DNA double-strand breaks (DSBs) via the DNA-dependent kinase DNA-PKcs. We show here that DNA-PKcs affects myogenesis independently of its role in DSB repair. Consequently, this process does not require the accumulation of DSBs and it is also independent of caspase-induced DNA damage. We report that in myogenic cells DNA-PKcs is essential for the expression of the differentiation factor Myogenin in an Akt2-dependent manner. DNA-PKcs interacts with the p300-containing complex that activates Myogenin transcription. We show also that SCID mice that are deficient in DNA-PKcs, and are used for transplantation and muscle regeneration studies, display altered myofiber composition and delayed myogenesis upon injury. These defects are exacerbated after repeated injury/regeneration events resulting in reduced muscle size. We thus identify a novel, caspase-independent, regulation of myogenic differentiation, and define a differentiation phase that does not involve the DNA damage/repair process.

DIPG-24. BRD4 INHIBITION AS A RADIOSENSITIZER THROUGH BLOCKING DNA REPAIR FOR THE TREATMENT OF DIFFUSE MIDLINE GLIOMA

Abstract Diffuse intrinsic pontine glioma (DIPG) is one of the devastating childhood cancers. Radiation therapy (RT) remains the only effective treatment yet provides a 5-year survival rate of only 1%. Several clinical trials have attempted to enhance RT efficacy by combining it with radiosensitizing agents, though none have been successful in doing so. Given this, there is a critical need to identify effective therapeutics to enhance the RT anti-tumor activity in DIPG. Here, we identified BRD4 inhibitors as candidate radiosensitizers from a high throughput drug screening. DIPG cells show increased H3 K27 acetylation (H3K27ac) levels, which bind to BET bromodomain protein 4 (BRD4) and are strongly associated with active transcription. We tested two BRD4 inhibitors (BRD4i: AZD5153 and JQ-1) as well as genetic BRD4 depletion, and observed enhancement of radiation-induced DNA damage, confirming BRD4i as potential radiosensitizers in the treatment of DIPG. We evaluated the effects of BRD4i on gene expression using RNAseq, and observed significant inhibition of DNA-repair proteins such as BRCA1 and RAD51. CUT-RUN qPCR showed decreased H3K27ac at BRCA1 and RAD51 promoters. Combination of BRD4i and RT inhibited cell viability significantly using clonogenic survival assay, apoptosis assay, and also showed cell cycle arrest. Radiation-induced DNA double-strand break (DSB) repair was prolonged with BRD4i with high levels of gH2AX and 53BP1 likely due to inhibition of DNA-repair. In vivo studies revealed increased survival of animals treated with combination therapy of RT and BRD4i in comparison to either monotherapy. Together, these results highlight BRD4i as a potential radiosensitizer and provide a rationale for developing combination therapy with radiation in the treatment of DIPG.

Evaluation of Low-dose Radiation-induced DNA Damage and Repair in 3D Printed Human Cellular Constructs.

DNA double-strand breaks (DSBs) induced by ionizing radiation (IR) are considered to be the most critical lesion that when unrepaired or misrepaired leads to genomic instability or cell death depending on the radiation exposure dose. The potential health risks associated with exposures of low-dose radiation are of concern since they are being increasingly used in diverse medical and non-medical applications. Here, we have used a novel human tissue-like 3-dimensional bioprint to evaluate low-dose radiation-induced DNA damage response. For the generation of 3-dimensional tissue-like constructs, human hTERT immortalized foreskin fibroblast BJ1 cells were extrusion printed and further enzymatically gelled in a gellan microgel-based support bath. Low-dose radiation-induced DSBs and repair were analyzed in the tissue-like bioprints by indirect immunofluorescence using a well-known DSB surrogate marker, 53BP1, at different post-irradiation times (0.5 h, 6 h, and 24 h) after treatment with various doses of γ rays (50 mGy, 100 mGy, and 200 mGy). The 53BP1 foci showed a dose dependent induction in the tissue bioprints after 30 min of radiation exposure and subsequently declined at 6 h and 24 h in a dose-dependent manner. The residual 53BP1 foci number observed at 24 h post-irradiation time for the γ-ray doses of 50 mGy, 100 mGy, and 200 mGy was not statistically different from mock treated bioprints illustrative of an efficient DNA repair response at these low-dose exposures. Similar results were obtained for yet another DSB surrogate marker, γ-H2AX (phosphorylated form of histone H2A variant) in the human tissue-like constructs. Although we have primarily used foreskin fibroblasts, our bioprinting approach-mimicking a human tissue-like microenvironment-can be extended to different organ-specific cell types for evaluating the radio-response at low-dose and dose-rates of IR.

UbcH5c-dependent activation of DNA-dependent protein kinase in response to replication-mediated DNA double-strand breaks

Camptothecin (CPT) exhibits strong cytotoxicity by inducing DNA double-strand breaks (DSBs) through DNA replication. Unlike radiation-induced DSBs, which have two DNA ends, CPT-induced DSBs are considered to have only one DNA end. However, the differences in cellular responses to one-ended and two-ended DSBs are not well understood. Our previous study showed that proteasome inhibitor treatment suppressed CPT-induced activation of DNA-PK, a factor required for non-homologous end-joining in DSB repair, suggesting that the ubiquitin-proteasome pathway is involved in DNA-PK activation in response to one-ended DSBs. To identify the ubiquitination factors required for DNA-PK activation, we screened an siRNA library against E2 ubiquitin-conjugating enzymes and identified UbcH5c. Knockdown of UbcH5c suppressed DNA-PK activation caused by CPT, but not by the radio-mimetic drug neocarzinostatin. UbcH5c-dependent DNA-PK activation occurred independent of DNA end resection. Furthermore, loss of UbcH5c reduced DNA-PK-dependent chromosomal aberrations and suppressed the activation of cell cycle checkpoint in response to CPT. These results suggest that UbcH5c regulates DNA-PK activation in response to one-ended DSBs caused by replication fork collapse. To our knowledge, this is the first report of a DSB repair-related factor that is differentially involved in the response to one- and two-ended DSBs.

Unraveling astrocyte behavior in the space brain: Radiation response of primary astrocytes.

Exposure to space conditions during crewed long-term exploration missions can cause several health risks for astronauts. Space radiation, isolation and microgravity are major limiting factors. The role of astrocytes in cognitive disturbances by space radiation is unknown. Astrocytes' response toward low linear energy transfer (LET) X-rays and high-LET carbon (12C) and iron (56Fe) ions was compared to reveal possible effects of space-relevant high-LET radiation. Since astronauts are exposed to ionizing radiation and microgravity during space missions, the effect of simulated microgravity on DNA damage induction and repair was investigated. Primary murine cortical astrocytes were irradiated with different doses of X-rays, 12C and 56Fe ions at the heavy ion accelerator GSI. DNA damage and repair (γH2AX, 53BP1), cell proliferation (Ki-67), astrocytes' reactivity (GFAP) and NF-κB pathway activation (p65) were analyzed by immunofluorescence microscopy. Cell cycle progression was investigated by flow cytometry of DNA content. Gene expression changes after exposure to X- rays were investigated by mRNA-sequencing. RT-qPCR for several genes of interest was performed with RNA from X-rays- and heavy-ion-irradiated astrocytes: Cdkn1a, Cdkn2a, Gfap, Tnf, Il1β, Il6, and Tgfβ1. Levels of the pro inflammatory cytokine IL-6 were determined using ELISA. DNA damage response was investigated after exposure to X-rays followed by incubation on a 2D clinostat to simulate the conditions of microgravity. Astrocytes showed distinct responses toward the three different radiation qualities. Induction of radiation-induced DNA double strand breaks (DSBs) and the respective repair was dose-, LET- and time-dependent. Simulated microgravity had no significant influence on DNA DSB repair. Proliferation and cell cycle progression was not affected by radiation qualities examined in this study. Astrocytes expressed IL-6 and GFAP with constitutive NF-κB activity independent of radiation exposure. mRNA sequencing of X-irradiated astrocytes revealed downregulation of 66 genes involved in DNA damage response and repair, mitosis, proliferation and cell cycle regulation. In conclusion, primary murine astrocytes are DNA repair proficient irrespective of radiation quality. Only minor gene expression changes were observed after X-ray exposure and reactivity was not induced. Co-culture of astrocytes with microglial cells, brain organoids or organotypic brain slice culture experiments might reveal whether astrocytes show a more pronounced radiation response in more complex network architectures in the presence of other neuronal cell types.

Abstract 2406: Targeting prmt5 inhibits DNA repair and enhances the radiosensitivity of GBM cells

Abstract Glioblastoma (GBM) is considered highly radioresistant. To improve the efficacy of GBM radiotherapy, one approach is the addition of a targeted radiosensitizer, which requires an understanding of the molecules that determine GBM radioresponse. Along these lines, PRMT5 is a type II arginine methyltransferase that symmetrically dimethylates histone and non-histone substrates to regulate chromatin structure and mRNA splicing, two parameters that can influence radiosensitivity. As an initial test of PRMT5 as a target for GBM radiosensitization, PRMT5 knockdown using siRNA was shown to increase the in vitro radiosensitivity of U251 GBM cells. PRMT5 was then targeted using the small molecule inhibitor LLY-283, which penetrates the blood brain barrier. Exposure of U251 cells to LLY-283 (24h) reduced the levels of symmetric dimethylated arginine as determined by western blot, indicating an inhibition of PRMT5 activity. Based on clonogenic analysis, LLY-283 exposure beginning 1h prior to irradiation significantly enhanced the radiosensitivity of U251 cells. Whereas treatment of the normal fibroblast cell line MRC9 with LLY-283 reduced PRMT5 activity, no effect was detected on radiosensitivity, suggesting tumor cell selective radiosensitization. As an initial investigation of the radiosensitizing mechanism, U251 were treated with LLY-283 1h prior irradiation (2Gy) and the number of γH2AX foci, marker for DNA double strand breaks (DSBs), was determined. Whereas LLY-283 had no effect on the initial level of radiation-induced γH2AX foci, the dispersal of foci was significantly delayed in LLY-283 treated cells. These results suggest that LLY-283 inhibits the repair of radiation-induced DSBs. The neutral comet assay, an alternative measure of radiation-induced DSBs showed that LLY-283 delivered 1h prior irradiation (10Gy) significantly increased the comet tail moment detected at 6 and 24h as compared to 10Gy only, consistent with an inhibition of DSB repair. Based on these two approaches LLY-283-induced radiosensitization appears to be mediated by an inhibition of DSB repair. Extending these studies to GBM stem-like cell lines (GSCs) showed that LLY-283 inhibited PRMT5 activity and when delivered 1h prior to irradiation enhanced radiosensitivity. Analysis of γH2AX foci showed a delay in foci dispersal in LLY-283 treated cells consistent with an inhibition of DSB repair. Because a 1h LLY-283 pretreatment was sufficient for the inhibition of DSB repair and enhanced radiosensitivity, it appears that the radiosensitization was primarily the result of altered chromatin function rather than changes in gene expression due to alternative mRNA splicing. Altogether, these results suggest targeting PRMT5 as a potential strategy for enhancing the radiosensitivity of GBMs. Citation Format: Charlotte M. Degorre, Steven Lohard, Philip J. Tofilon. Targeting prmt5 inhibits DNA repair and enhances the radiosensitivity of GBM cells [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 2406.

UBA1 inhibition contributes radiosensitization of glioblastoma cells via blocking DNA damage repair.

Glioblastoma multiforme (GBM) is a brain tumor with high mortality and recurrence rate. Radiotherapy and chemotherapy after surgery are the main treatment options available for GBM. However, patients with glioblastoma have a grave prognosis. The major reason is that most GBM patients are resistant to radiotherapy. UBA1 is considered an attractive potential anti-tumor therapeutic target and a key regulator of DNA double-strand break repair and genome replication in human cells. Therefore, we hypothesized that TAK-243, the first-in-class UBA1 inhibitor, might increase GBM sensitivity to radiation. The combined effect of TAK-243 and ionizing radiation on GBM cell proliferation, and colony formation ability was detected using CCK-8, colony formation, and EdU assays. The efficacy of TAK-243 combined with ionizing radiation for GBM was further evaluated in vivo, and the mechanism of TAK-243 sensitizing radiotherapy was preliminarily discussed. The results showed that TAK-243, in combination with ionizing radiation, significantly inhibited GBM cell proliferation, colony formation, cell cycle arrest in the G2/M phase, and increased the proportion of apoptosis. In addition, UBA1 inhibition by TAK-243 substantially increased the radiation-induced γ-H2AX expression and impaired the recruitment of the downstream effector molecule 53BP1. Therefore, TAK-243 inhibited the radiation-induced DNA double-strand break repair and thus inhibited the growth of GBM cells. Our results provided a new therapeutic strategy for improving the radiation sensitivity of GBM and laid a theoretical foundation and experimental basis for further clinical trials.

ВЛИЯНИЕ ПРЕПАРАТА ГУМИНО-ФУЛЬВОВЫХ КИСЛОТ НА КОЛИЧЕСТВЕННЫЙ ВЫХОД ОСТАТОЧНЫХ ФОКУСОВ γH2AX И ПРОЛИФЕРАТИВНУЮ АКТИВНОСТЬ В ОБЛУЧЕННЫХ МЕЗЕНХИМАЛЬНЫХ СТРОМАЛЬНЫХ КЛЕТКАХ ЧЕЛОВЕКА

Purpose: To evaluate the influence of a humic-fulvic acid substance on the quantitative yield of residual foci of the DNA double-strand break (DSB) repair protein-marker - phosphorylated histone H2AX (γH2AX) and proliferation activity in a culture of human mesenchymal stromal cells (MSCs) 24, 48, and 72 h after exposure to X-ray radiation at doses of 2, 4 and 10 Gy. Material and methods: Through 24 hours after incubation of MSCs with a substance of humic-fulvic acids (Humic Complex, OOO Sistema-BioTechnologies, Russia) at a dilution of 1/1000. Cells were irradiated on an X-ray biological device RUB RUST-M1 at a voltage of 200 kV, beam current 2×5 mA, aluminum filter 1.5 mm, absorbed dose rate 0.85 Gy/min. Immunocytochemical staining was used to quantify the residual γH2AX foci and the percentage of proliferating cells using antibodies to γH2AX and Ki-67 (a marker protein for cell proliferation), respectively. Statistical analysis of the obtained data was carried out using the statistical software package Statistica 8.0 (StatSoft). To assess the significance of differences between samples, Student’s t-test was used. Results and conclusion: The conducted studies showed that on the cell model used and under the above experimental conditions, the humic-fulvic acid substance does not affect the efficiency of repair of radiation-induced DNA DSBs, however, it significantly reduces the proliferation activity of both irradiated and non-irradiated MSCs. It is advisable to conduct detailed studies of the molecular and cellular mechanisms of the antiproliferative effect of humic and fulvic acids.