ADA2b acts to positively regulate blue light-mediated photomorphogenesis in Arabidopsis.

Cryptochromes (CRYs) act as blue light photoreceptors to regulate various plant physiological processes including photomorphogenesis and repair of DNA double strand breaks (DSBs). ADA2b is a conserved transcription co-activator that is involved in multiple plant developmental processes. It is known that ADA2b interacts with CRYs to mediate blue light-promoted DSBs repair. Whether ADA2b may participate in CRYs-mediated photomorphogenesis is unknown. Here we show that ADA2b acts to inhibit hypocotyl elongation and hypocotyl cell elongation in blue light. We found that the SWIRM domain-containing C-terminus mediates the blue light-dependent interaction of ADA2b with CRYs in blue light. Moreover, ADA2b and CRYs act to co-regulate the expression of hypocotyl elongation-related genes in blue light. Based on previous studies and these results, we propose that ADA2b plays dual functions in blue light-mediated DNA damage repair and photomorphogenesis.

A perspective on tumor radiation resistance following high-LET radiation treatment.

High-linear energy transfer (LET) radiation is a promising alternative to conventional low-LET radiation for therapeutic gain against cancer owing to its ability to induce complex and clustered DNA lesions. However, the development of radiation resistance poses a significant barrier. The potential molecular mechanisms that could confer resistance development are translesion synthesis (TLS), replication gap suppression (RGS) mechanisms, autophagy, epithelial-mesenchymal transition (EMT) activation, release of exosomes, and epigenetic changes. This article will discuss various types of complex clustered DNA damage, their repair mechanisms, mutagenic potential, and the development of radiation resistance strategies. Furthermore, it highlights the importance of careful consideration and patient selection when employing high-LET radiotherapy in clinical settings.

The impact of dose rate on responses of human lens epithelial cells to ionizing irradiation.

The knowledge on responses of human lens epithelial cells (HLECs) to ionizing radiation exposure is important to understand mechanisms of radiation cataracts that are of concern in the field of radiation protection and radiation therapy. However, biological effects in HLECs following protracted exposure have not yet fully been explored. Here, we investigated the temporal kinetics of γ-H2AX foci as a marker for DNA double-strand breaks (DSBs) and cell survival in HLECs after exposure to photon beams at various dose rates (i.e., 150 kVp X-rays at 1.82, 0.1, and 0.033 Gy/min, and 137Cs γ-rays at 0.00461 Gy/min (27.7 cGy/h) and 0.00081 Gy/min (4.9 cGy/h)), compared to those in human lung fibroblasts (WI-38). In parallel, we quantified the recovery for DSBs and cell survival using a biophysical model. The study revealed that HLECs have a lower DSB repair rate than WI-38 cells. There is no significant impact of dose rate on cell survival in both cell lines in the dose-rate range of 0.033-1.82 Gy/min. In contrast, the experimental residual γ-H2AX foci showed inverse dose rate effects (IDREs) compared to the model prediction, highlighting the importance of the IDREs in evaluating radiation effects on the ocular lens.

Robustness of carbon-ion radiotherapy against DNA damage repair associated radiosensitivity variation based on a biophysical model.

BACKGROUND: Interpatient variation of tumor radiosensitivity is rarely considered during the treatment planning process despite its known significance for the therapeutic outcome. PURPOSE: To apply our mechanistic biophysical model to investigate the biological robustness of carbon ion radiotherapy (CIRT) against DNA damage repair interference (DDRi) associated patient-to-patient variability in radiosensitivity and its potential clinical advantages against conventional radiotherapy approaches. METHODS AND MATERIALS: The "UNIfied and VERSatile bio response Engine" (UNIVERSE) was extended by carbon ions and its predictions were compared to a panel of in vitro and in vivo data including various endpoints and DDRi settings within clinically relevant dose and linear energy transfer (LET) ranges. The implications of UNIVERSE predictions were then assessed in a clinical patient scenario considering DDRi variance. RESULTS: UNIVERSE tests well against the applied benchmarks. While in vitro survival curves were predicted with an R2 > 0.92, deviations from in vivo RBE data were less than 5.6% The conducted paradigmatic patient plan study implies a markedly reduced significance of DDRi based radiosensitivity variability in CIRT (13% change of D 50 ${{D}_{50}}$ in target) compared to conventional radiotherapy (62%) and that boosting the LET within the target further amplifies this robustness of CIRT (8%). In the case of heightened tumor radiosensitivity, a dose de-escalation strategy for photons allows a reduction of the maximum effective dose within the normal tissue (NT) from a D 2 ${{D}_2}$ of 2.65 to 1.64 Gy, which lies below the level found for CIRT ( D 2 ${{D}_2}$  = 2.41 Gy) for the analyzed plan and parameters. However, even after de-escalation, the integral effective dose in the NT is found to be substantially higher for conventional radiotherapy in comparison to CIRT ( D m e a n ${{D}_{mean}}$ of 0.75, 0.46, and 0.24 Gy for the conventional plan, its de-escalation and CIRT, respectively). CONCLUSIONS: The framework offers adequate predictions of in vitro and in vivo radiation effects of CIRT while allowing the consideration of DRRi based solely on parameters derived from photon data. The results of the patient planning study underline the potential of CIRT to minimize important sources of interpatient divergence in therapy outcome, especially when combined with techniques that allow to maximize the LET within the tumor. Despite the potential of de-escalation strategies for conventional radiotherapy to reduce the maximum effective dose in the NT, CIRT appears to remain a more favorable option due to its ability to reduce the integral effective dose within the NT.

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.

Downregulation of the Rho GTPase pathway abrogates resistance to ionizing radiation in wild-type p53 glioblastoma by suppressing DNA repair mechanisms.

Glioblastoma (GBM), the most common aggressive brain tumor, is characterized by rapid cellular infiltration and is routinely treated with ionizing radiation (IR), but therapeutic resistance inevitably recurs. The actin cytoskeleton of glioblastoma cells provides their high invasiveness, but it remains unclear whether Rho GTPases modulate DNA damage repair and therapeutic sensitivity. Here, we irradiated glioblastoma cells with different p53 status and explored the effects of Rho pathway inhibition to elucidate how actin cytoskeleton disruption affects the DNA damage response and repair pathways. p53-wild-type and p53-mutant cells were subjected to Rho GTPase pathway modulation by treatment with C3 toxin; knockdown of mDia-1, PFN1 and MYPT1; or treatment with F-actin polymerization inhibitors. Rho inhibition increased the sensitivity of glioma cells to IR by increasing the number of DNA double-strand breaks and delaying DNA repair by nonhomologous end-joining in p53-wild-type cells. p53 knockdown reversed this phenotype by reducing p21 expression and Rho signaling activity, whereas reactivation of p53 in p53-mutant cells by treatment with PRIMA-1 reversed these effects. The interdependence between p53 and Rho is based on nuclear p53 translocation facilitated by G-actin and enhanced by IR. Isolated IR-resistant p53-wild-type cells showed an altered morphology and increased stress fiber formation: inhibition of Rho or actin polymerization decreased cell viability in a p53-dependent manner and reversed the resistance phenotype. p53 silencing reversed the Rho inhibition-induced sensitization of IR-resistant cells. Rho inhibition also impaired the repair of IR-damaged DNA in 3D spheroid models. Rho GTPase activity and actin cytoskeleton dynamics are sensitive targets for the reversal of acquired resistance in GBM tumors with wild-type p53.

Radiosensitization of Breast Cancer Cells with a 2-Methoxyestradiol Analogue Affects DNA Damage and Repair Signaling In Vitro.

Radiation resistance and radiation-related side effects warrant research into alternative strategies in the application of this modality to cancer treatment. Designed in silico to improve the pharmacokinetics and anti-cancer properties of 2-methoxyestradiol, 2-ethyl-3-O-sulfamoyl-estra-1,3,5(10)16-tetraene (ESE-16) disrupts microtubule dynamics and induces apoptosis. Here, we investigated whether pre-exposure of breast cancer cells to low-dose ESE-16 would affect radiation-induced deoxyribonucleic acid (DNA) damage and the consequent repair pathways. MCF-7, MDA-MB-231, and BT-20 cells were exposed to sub-lethal doses of ESE-16 for 24 h before 8 Gy radiation. Flow cytometric quantification of Annexin V, clonogenic studies, micronuclei quantification, assessment of histone H2AX phosphorylation and Ku70 expression were performed to assess cell viability, DNA damage, and repair pathways, in both directly irradiated cells and cells treated with conditioned medium. A small increase in apoptosis was observed as an early consequence, with significant repercussions on long-term cell survival. Overall, a greater degree of DNA damage was detected. Moreover, initiation of the DNA-damage repair response was delayed, with a subsequent sustained elevation. Radiation-induced bystander effects induced similar pathways and were initiated via intercellular signaling. These results justify further investigation of ESE-16 as a radiation-sensitizing agent since pre-exposure appears to augment the response of tumor cells to radiation.

The exciting encounter between lncRNAs and radiosensitivity in IR-induced DNA damage events.

Radiation therapy is a commonly used tool in cancer management due to its ability to destroy malignant tumors. Mechanically, the efficacy of radiotherapy mainly depends on the inherent radiosensitivity of cancer cells and surrounding normal tissues, which mostly accounts for molecular dynamics associated with radiation-induced DNA damage. However, the relationship between radiosensitivity and DNA damage mechanism deserves to be further probed. As the well-established RNA regulators or effectors, long noncoding RNAs (lncRNAs) dominate vital roles in modulating ionizing radiation response by targeting crucial molecular pathways, including DNA damage repair. Recently, emerging evidence has constantly confirmed that overexpression or inhibition of lncRNAs can greatly influence the sensitivity of radiotherapy for many kinds of cancers, by driving a diverse array of DNA damage-associated signaling cascades. In conclusion, this review critically summarizes the recent progress in the molecular mechanism of IR-responsive lncRNAs in the context of radiation-induced DNA damage. The different response of lncRNAs when IR exposure. IR exposure can trigger the changes in expression pattern and subcellular localization of lncRNAs that influences the different radiology processes.

RNF126-Mediated MRE11 Ubiquitination Activates the DNA Damage Response and Confers Resistance of Triple-Negative Breast Cancer to Radiotherapy.

Triple-negative breast cancer (TNBC) has higher molecular heterogeneity and metastatic potential and the poorest prognosis. Because of limited therapeutics against TNBC, irradiation (IR) therapy is still a common treatment option for patients with lymph nodes or brain metastasis. Thus, it is urgent to develop strategies to enhance the sensitivity of TNBC tumors to low-dose IR. Here, the authors report that E3 ubiquitin ligase Ring finger protein 126 (RNF126) is important for IR-induced ATR-CHK1 pathway activation to enhance DNA damage repair (DDR). Mechanistically, RNF126 physically associates with the MRE11-RAD50-NBS1 (MRN) complex and ubiquitinates MRE11 at K339 and K480 to increase its DNA exonuclease activity, subsequent RPA binding, and ATR phosphorylation, promoting sustained DDR in a homologous recombination repair-prone manner. Accordingly, depletion of RNF126 leads to increased genomic instability and radiation sensitivity in both TNBC cells and mice. Furthermore, it is found that RNF126 expression is induced by IR activating the HER2-AKT-NF-κB pathway and targeting RNF126 expression with dihydroartemisinin significantly improves the sensitivity of TNBC tumors in the brain to IR treatment in vivo. Together, these results reveal that RNF126-mediated MRE11 ubiquitination is a critical regulator of the DDR, which provides a promising target for improving the sensitivity of TNBC to radiotherapy.

Targeting of p21-Activated Kinase 4 Radiosensitizes Glioblastoma Cells via Impaired DNA Repair.

Glioblastoma is a devastating malignant disease with poor patient overall survival. Strong invasiveness and resistance to radiochemotherapy have challenged the identification of molecular targets that can finally improve treatment outcomes. This study evaluates the influence of all six known p21-activated kinase (PAK) protein family members on the invasion capacity and radio-response of glioblastoma cells by employing a siRNA-based screen. In a panel of human glioblastoma cell models, we identified PAK4 as the main PAK isoform regulating invasion and clonogenic survival upon irradiation and demonstrated the radiosensitizing potential of PAK4 inhibition. Mechanistically, we show that PAK4 depletion and pharmacological inhibition enhanced the number of irradiation-induced DNA double-strand breaks and reduced the expression levels of various DNA repair proteins. In conclusion, our data suggest PAK4 as a putative target for radiosensitization and impairing DNA repair in glioblastoma, deserving further scrutiny in extended combinatorial treatment testing.

The feedback loop of ANKHD1/lncRNA MALAT1/YAP1 strengthens the radioresistance of CRC by activating YAP1/AKT signaling.

Innate radioresistance substantially limits the effectiveness of radiotherapy for colorectal cancer (CRC); thus, a strategy to enhance the radiosensitivity of CRC is urgently needed. Herein, we reported that ankyrin repeat and KH domain containing 1 (ANKHD1) serves as a key regulator of radioresistance in CRC. ANKHD1 was highly expressed in CRC tissues and was highly correlated with Yes-associated protein 1 (YAP1) in CRC. Our results first revealed that ANKHD1 knockdown could increase the radiosensitivity of CRC by regulating DNA-damage repair, both in vitro and in vivo. Furthermore, the interactive regulation between ANKHD1 or YAP1 and lncRNA MALAT1 was revealed by RIP and RNA pull-down assays. Moreover, our results also demonstrated that MALAT1 silencing can radiosensitize CRC cells to IR through YAP1/AKT axis, similar to ANKHD1 silencing. Taken together, we report a feedback loop of ANKHD1/MALAT1/YAP1 that synergistically promotes the transcriptional coactivation of YAP1 and in turn enhances the radioresistance of CRC by regulating DNA-damage repair, probably via the YAP1/AKT axis. Our results suggested that targeting the YAP1/AKT axis downstream of ANKHD1/MALAT1/YAP1 may enhance the radiosensitivity of CRC.

Low-dose but not high-dose γ-irradiation elicits the dominant-negative effect of mutant p53 in vivo.

Contrary to high doses irradiation (HDR), the biological consequences of dose irradiation (LDR) in breast cancer remain unclear due to the complexity of human epidemiological studies. LDR induces DNA damage that activates p53-mediated tumor-suppressing pathways promoting DNA repair, cell death, and growth arrest. Monoallelic p53 mutations are one of the earliest and the most frequent genetic events in many subtypes of cancer including ErbB2 breast cancer. Using MMTV/ErbB2 mutant p53 (R172H) heterozygous mouse model we found differential p53 genotype-specific effect of LDR vs. HDR on mammary tumorigenesis. Following LDR, mutant p53 heterozygous tumor cells exhibit aberrant ATM/DNA-PK signaling with defects in sensing of double-strand DNA brakes and deficient DNA repair. In contrast, HDR-induced genotoxic stress is sufficient to reach the threshold of DNA damage that is necessary for wtp53 induced DNA repair and cell cycle arrest. As a result, mutant p53 endows dominant-negative effect promoting mammary tumorigenesis after low-impact DNA damage leading to the selection of a genetically unstable proliferative population, with negligible mutagenic effect on tumors carrying wtp53 allele.

Dosimetry of heavy ion exposure to human cells using nanoscopic imaging of double strand break repair protein clusters.

The human body is constantly exposed to ionizing radiation of different qualities. Especially the exposure to high-LET (linear energy transfer) particles increases due to new tumor therapy methods using e.g. carbon ions. Furthermore, upon radiation accidents, a mixture of radiation of different quality is adding up to human radiation exposure. Finally, long-term space missions such as the mission to mars pose great challenges to the dose assessment an astronaut was exposed to. Currently, DSB counting using γH2AX foci is used as an exact dosimetric measure for individuals. Due to the size of the γH2AX IRIF of ~ 0.6 µm, it is only possible to count DSB when they are separated by this distance. For high-LET particle exposure, the distance of the DSB is too small to be separated and the dose will be underestimated. In this study, we developed a method where it is possible to count DSB which are separated by a distance of ~ 140 nm. We counted the number of ionizing radiation-induced pDNA-PKcs (DNA-PKcs phosphorylated at T2609) foci (size = 140 nm ± 20 nm) in human HeLa cells using STED super-resolution microscopy that has an intrinsic resolution of 100 nm. Irradiation was performed at the ion microprobe SNAKE using high-LET 20 MeV lithium (LET = 116 keV/µm) and 27 MeV carbon ions (LET = 500 keV/µm). pDNA-PKcs foci label all DSB as proven by counterstaining with 53BP1 after low-LET γ-irradiation where separation of individual DSB is in most cases larger than the 53BP1 gross size of about 0.6 µm. Lithium ions produce (1.5 ± 0.1) IRIF/µm track length, for carbon ions (2.2 ± 0.2) IRIF/µm are counted. These values are enhanced by a factor of 2-3 compared to conventional foci counting of high-LET tracks. Comparison of the measurements to PARTRAC simulation data proof the consistency of results. We used these data to develop a measure for dosimetry of high-LET or mixed particle radiation exposure directly in the biological sample. We show that proper dosimetry for radiation up to a LET of 240 keV/µm is possible.

Individual Response to Radiation of Individuals with Neurofibromatosis Type I: Role of the ATM Protein and Influence of Statins and Bisphosphonates.

Neurofibromatosis type 1 (NF1) is a disease characterized by high occurrence of benign and malignant brain tumours and caused by mutations of the neurofibromin protein. While there is an increasing evidence that NF1 is associated with radiosensitivity and radiosusceptibility, few studies have dealt with the molecular and cellular radiation response of cells from individuals with NF1. Here, we examined the ATM-dependent signalling and repair pathways of the DNA double-strand breaks (DSB), the key-damage induced by ionizing radiation, in skin fibroblast cell lines from 43 individuals with NF1. Ten minutes after X-rays irradiation, quiescent NF1 fibroblasts showed abnormally low rate of recognized DSB reflected by a low yield of nuclear foci formed by phosphorylated H2AX histones. Irradiated NF1 fibroblasts also presented a delayed radiation-induced nucleoshuttling of the ATM kinase (RIANS), potentially due to a specific binding of ATM to the mutated neurofibromin in cytoplasm. Lastly, NF1 fibroblasts showed abnormally high MRE11 nuclease activity suggesting a high genomic instability after irradiation. A combination of bisphosphonates and statins complemented these impairments by accelerating the RIANS, increasing the yield of recognized DSB and reducing genomic instability. Data from NF1 fibroblasts exposed to radiation in radiotherapy and CT scan conditions confirmed that NF1 belongs to the group of syndromes associated with radiosensitivity and radiosusceptibility.

SERPINE2/PN-1 regulates the DNA damage response and radioresistance by activating ATM in lung cancer.

Although the DNA damage response (DDR) is associated with the radioresistance characteristics of lung cancer cells, the specific regulators and underlying mechanisms of the DDR are unclear. Here, we identified the serine proteinase inhibitor clade E member 2 (SERPINE2) as a modulator of radiosensitivity and the DDR in lung cancer. Cells exhibiting radioresistance after ionizing radiation show upregulation of SERPINE2, and SERPINE2 knockdown improves tumor radiosensitivity in vitro and in vivo. Functionally, SERPINE2 deficiency causes a reduction in homologous recombination repair, rapid recovery of cell cycle checkpoints, and suppression of migration and invasion. Mechanistically, SERPINE2 knockdown inhibits the accumulation of p-ATM and the downstream repair protein RAD51 during DNA repair, and RAD51 can restore DNA damage and radioresistance phenotypes in lung cancer cells. Furthermore, SERPINE2 can directly interact with MRE11 and ATM to facilitate its phosphorylation in HR-mediated DSB repair. In addition, high SERPINE2 expression correlates with dismal prognosis in lung adenocarcinoma patients, and a high serum SERPINE2 concentration predicts a poor response to radiotherapy in non-small cell lung cancer patients. In summary, these findings indicate a novel regulatory mechanism by which SERPINE2 modulates the DDR and radioresistance in lung cancer.

Increased Radiosensitivity of Solid Tumors Harboring ATM and BRCA1/2 Mutations.

PURPOSE: Preclinical data indicate that response to radiotherapy (RT) depends on DNA damage repair. In this study, we investigated the role of mutations in genes related to DNA damage repair in treatment outcome after RT. MATERIALS AND METHODS: Patients with solid tumor who participated in next generation sequencing panel screening using biopsied tumor tissue between October 2013 and February 2019 were reviewed and 97 patients that received RT were included in this study. Best response to RT and the cumulative local recurrence rate (LRR) were compared according to absence or presence of missense, nonsense, and frameshift mutations in ATM and/or BRCA1/2. RESULTS: Of the 97 patients, five patients harbored mutation only in ATM, 22 in only BRCA1/2, and six in both ATM and BRCA1/2 (ATMmtBRCAmt). Propensity score matching was performed to select the control group without mutations (ATMwtBRCAwt, n=33). In total, 90 RT-treated target lesions were evaluated in 66 patients. Highest objective response rate of 80% was observed in ATMmtBRCAmt lesions (p=0.007), which was mostly durable. Furthermore, the cumulative 1-year LRR was the lowest in ATMmtBRCAmt lesions and the highest in ATMwtBRCAwt lesions (0% vs. 47.9%, p=0.008). RT-associated toxicities were observed in 10 treatments with no significant difference among the subgroups (p=0.680). CONCLUSION: Tumors with ATM and BRCA1/2 mutations exhibited superior tumor response and local control after RT compared to tumors without these mutations. The results are hypothesis generating and suggest the need for integrating the tumor mutation profile of DNA repair genes during treatment planning.

Global DNA methylation profile at LINE-1 repeats and promoter methylation of genes involved in DNA damage response and repair pathways in human peripheral blood mononuclear cells in response to γ-radiation.

DNA methylation is an epigenetic mechanism, which plays an important role in gene regulation. The present study evaluated DNA methylation profile of LINE1 repeats and promoter methylation of DNA damage response (DDR) and DNA repair (DR) genes (PARP1, ATM, BRCA1, MLH1, XPC, RAD23B, APC, TNFα, DNMT3A, MRE11A, MGMT, CDKN2A, MTHFR) in human peripheral blood mononuclear cells (PBMCs) of healthy donors in response to γ-radiation. Methylation level was correlated with gene expression profile of selected DDR and DR genes (APC, MLH1, PARP1, MRE11A, TNFα, MGMT) to understand their role in gene regulation. Blood samples were collected from 15 random healthy donors, PBMCs were isolated, exposed to 0.1 Gy (low) and 2.0 Gy (high) doses of γ-radiation and proliferated for 48 h and 72 h. Genomic DNA and total RNA were isolated from irradiated PBMCs along with un-irradiated control. Methylation profile was determined from bisulphite converted DNA and amplified by methylation sensitive high resolution melting (MS-HRM) method. Total RNA was converted to cDNA and relative expression was analysed using real time quantitative-PCR. Our results revealed that at 0.1 Gy, MRE11A and TNFα showed significant (P < 0.05) increase in methylation at 72 h. At 2.0 Gy, significant increase (P < 0.05) in methylation profile was observed at LINE1, MRE11A, PARP1, BRCA1, DNMT3A and RAD23B at 48 h and 72 h. PARP1 showed significant positive correlation of methylation status with gene expression. In conclusion, low and high doses of γ-radiation have significant influence on DNA methylation status of LINE1, DDR and DR genes suggesting their potential role as epigenetic signatures in human PBMCs, which can be further explored in human populations.

Activation of efficient DNA repair mechanisms after photon and proton irradiation of human chondrosarcoma cells.

Although particle therapy with protons has proven to be beneficial in the treatment of chondrosarcoma compared to photon-based (X-ray) radiation therapy, the cellular and molecular mechanisms have not yet been sufficiently investigated. Cell viability and colony forming ability were analyzed after X-ray and proton irradiation (IR). Cell cycle was analyzed using flow cytometry and corresponding regulator genes and key players of the DNA repair mechanisms were measured using next generation sequencing, protein expression and immunofluorescence staining. Changes in metabolic phenotypes were determined with nuclear magnetic resonance spectroscopy. Both X-ray and proton IR resulted in reduced cell survival and a G2/M phase arrest of the cell cycle. Especially 1 h after IR, a significant dose-dependent increase of phosphorylated γH2AX foci was observed. This was accompanied with a reprogramming in cellular metabolism. Interestingly, within 24 h the majority of clearly visible DNA damages were repaired and the metabolic phenotype restored. Involved DNA repair mechanisms are, besides the homology directed repair (HDR) and the non-homologous end-joining (NHEJ), especially the mismatch mediated repair (MMR) pathway with the key players EXO1, MSH3, and PCNA. Chondrosarcoma cells regenerates the majority of DNA damages within 24 h. These molecular mechanisms represent an important basis for an improved therapy.

DNA repair inhibitors sensitize cells differently to high and low LET radiation.

The aim of this study was to investigate effects of high LET α-radiation in combination with inhibitors of DDR (DNA-PK and ATM) and to compare the effect with the radiosensitizing effect of low LET X-ray radiation. The various cell lines were irradiated with α-radiation and with X-ray. Clonogenic survival, the formation of micronuclei and cell cycle distribution were studied after combining of radiation with DDR inhibitors. The inhibitors sensitized different cancer cell lines to radiation. DNA-PKi affected survival rates in combination with α-radiation in selected cell lines. The sensitization enhancement ratios were in the range of 1.6-1.85 in cancer cells. ATMi sensitized H460 cells and significantly increased the micronucleus frequency for both radiation qualities. ATMi in combination with α-radiation reduced survival of HEK293. A significantly elicited cell cycle arrest in G2/M phase after co-treatment of ATMi with α-radiation and X-ray. The most prominent treatment effect was observed in the HEK293 by combining α-radiation and inhibitions. ATMi preferentially sensitized cancer cells and normal HEK293 cells to α-radiation. DNA-PKi and ATMi can sensitize cancer cells to X-ray, but the effectiveness was dependent on cancer cells itself. α-radiation reduced proliferation in primary fibroblast without G2/M arrest.