Radiation Repair Research Articles

Transcriptomic Evidence of Base and Nucleotide Excision Repair Mechanisms in Response to UV-B Radiation in an Arctic Fungus Pseudogymnoascus australis Strain HNDR4

Polar micro-fungi are known to have developed protective mechanisms and responses against the harmful effects of UV radiation. In this study, physiological, cellular, and transcriptomic responses of an Arctic fungal strain of Pseudogymnoascus australis towards UV-B radiation were examined. Fungal isolate was cultured on 0.45μm nylon membranes overlayed on Czapex-Dox agar (CDA) at 15ºC for 10 d in dark condition to reach the mid-log phase. To assess DNA damage and recovery, the isolates were exposed to UV-B radiation (UV dosage of 6.1 kj m-2 d-1) for 130 min, followed by 0, 2 and 6 hr incubation under light condition. Differential gene expression in DNA repair after UV-B treatment at these time points was analysed based on RNA-Seq data output of 40 million reads per sample from an Illumina NovaSeq 6000 system platform. The results revealed that radiation-repair (RAD) genes were upregulated and the photoreactivation (PHR1) gene was downregulated after UV-B exposure in light condition. Downregulation of oxidoreductase activity was observed from the GO enrichment analysis. The KEGG pathway enrichment analysis exhibited upregulation of base excision repair (BER) genes and nucleotide excision repair (NER) genes upon UV-B exposure. This is the fi rst RNASeq transcriptomic study on the DNA repair mechanisms employed by a polar fungus to mitigate UV-induced DNA damage.

Clinicopathological Study of Xeroderma Pigmentosa: A Series of Eight Cases

Abstract Background: Xeroderma pigmentosa (XP) is a rare inherited (autosomal recessive) disease, resulting from impairment in DNA repair that involves recognition and repair of ultraviolet radiation (UVR)-induced DNA damage in the nucleotide excision repair pathway. This results in increased photosensitivity, UVR-induced damage to skin and eye, increased susceptibility to cutaneous and ocular cancers, and progressive neurodegeneration in some patients. Aims and Objective: The objective of the study was to describe the clinicopathological spectrum of eight cases of XP. Materials and Methods: An ambispective case series was conducted in a pediatric tertiary care hospital in eastern India during a 10-year period from 2013 to 2022. Results: All the children of our cohort were born of consanguineous marriage. The mean age of presentation was 1.2 years (range: 7 months–3 years), whereas three children presented during their infancy. The male-to-female ratio was 5:3. The most common findings were cutaneous (100%), followed by ophthalmic (75%) and/or neurological symptoms (25%). Patients had normal skin at birth but soon developed extreme photosensitivity followed by abnormal skin pigmentation and subsequently progressive xerosis, atrophy, wrinkling, and poikiloderma over time. Six patients had varied degrees of ocular involvement, whereas three of them had severe manifestations including madarosis, tylosis, ectropion, lagophthalmos, phthisis bulbi, clouding and scarring of the cornea with complete or partial loss of vision, and ophthalmic malignancies. Fifty percent (n = 4) of cases had cutaneous and ocular premalignant (actinic keratosis) and malignant lesions including melanoma and nonmelanoma skin cancer such as squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) in their early childhood. One patient had simultaneous occurrence of multiple malignancies together (SCC, BCC, and melanoma). Neurological abnormalities (subnormal intelligence) were found in two cases. There was no evidence of sensory neural hearing loss, microcephaly, neuroregression, or neurodeficits. Conclusion: Although XP is associated with increased mortality and morbidity, early diagnosis followed by persistent vigorous photoprotection and regular screening for early detection of malignancies along with psychological support can drastically improve patients’ quality of life and life expectancy. Besides genetic counseling, further research is required on formulating optimal management of XP, specifically the role and possibilities of gene therapy in XP.

Long non-coding RNA PANDAR promoted radiation and cisplatin-induced DNA damage repair through ATR/CHK1 in NSCLC.

DNA-damaging agents, including radiation and platinum-based chemotherapy, are indispensable treatments for non-small cell lung cancer (NSCLC) patients. However, cancer cells tend to be resistant to both radiation and chemotherapy, thus resulting in treatment failure or recurrence. The purpose of this study was to explore the effect and mechanism of long non-coding RNA (lncRNA) PANDAR (promoter of CDKN1A antisense DNA damage-activated RNA) on NSCLC sensitivity to radiation and chemotherapy. Cell counting kit (CCK-8), colony formation and flow cytometry were respectively performed to determine the cell cycle and apoptosis of NSCLC cells treated with γ-ray radiation and cisplatin. The extent of DNA damage was evaluated using a comet assay and immunofluorescence staining against γH2AX. In addition, we explored the role of PANDAR in DNA damage response pathways through western blot analysis. Finally, a nude mouse subcutaneous xenograft model was established to assess the sensitivity to radiation and chemotherapy in vivo. In cell experiments, PANDAR knockdown can increase the sensitivity of NSCLC cells to radiation and cisplatin. The CCK-8 results showed that cell viability was significantly increased in the overexpression group after radiation and cisplatin treatments. The overexpression group also showed more colonies, less apoptosis and DNA damage, and G2/M phase arrest was aggravated to provide the time necessary for DNA repair. Contrary to PANDAR overexpression, the trends were reversed in the PANDAR knockdown group. Furthermore, PANDAR knockdown inhibited radiation and cisplatin-activated phosphorylation levels of ATR and CHK1 in NSCLC cells. Finally, our in vivo model showed that targeting PANDAR significantly sensitized NSCLC to radiation and cisplatin. Our study showed that PANDAR knockdown promoted sensitivity to radiation and cisplatin in NSCLC by regulating the ATR/CHK1 pathway, thus providing a novel understanding as well as a therapeutic target for NSCLC treatment. In NSCLC cells, lncRNA PANDAR negatively regulates sensitivity to radiation and cisplatin. PANDAR can promote the repair of radiation and cisplatin-induced DNA damage and activation of the G2/M checkpoint through the ATR/CHK1 pathway. PANDAR knockdown results in defects in DNA damage repair accompanied by more cell apoptosis.

Comprehensive Temporal Protein Dynamics during Postirradiation Recovery in Deinococcus radiodurans.

Deinococcus radiodurans (D. radiodurans) is an extremophile that can tolerate ionizing radiation, ultraviolet radiation, and oxidation. How D. radiodurans responds to and survives high levels of ionizing radiation is still not clear. In this study, we performed label-free proteomics to explore the proteome dynamics during postirradiation recovery (PIR). Surprisingly, proteins involved in translation were repressed during the initial hours of PIR. D. radiodurans also showed enhanced DNA repair and antioxidative response after 6 kGy of gamma irradiation. Moreover, proteins involved in sulfur metabolism and phenylalanine metabolism were enriched at 1 h and 12 h, respectively, indicating different energy and material needs during PIR. Furthermore, based on these findings, we proposed a novel model to elucidate the possible molecular mechanisms of robust radioresistance in D. radiodurans, which may serve as a reference for future radiation repair.

Comparison of the Medical Uses and Cellular Effects of High and Low Linear Energy Transfer Radiation.

Exposure to ionizing radiation can occur during medical treatments, from naturally occurring sources in the environment, or as the result of a nuclear accident or thermonuclear war. The severity of cellular damage from ionizing radiation exposure is dependent upon a number of factors including the absorbed radiation dose of the exposure (energy absorbed per unit mass of the exposure), dose rate, area and volume of tissue exposed, type of radiation (e.g., X-rays, high-energy gamma rays, protons, or neutrons) and linear energy transfer. While the dose, the dose rate, and dose distribution in tissue are aspects of a radiation exposure that can be varied experimentally or in medical treatments, the LET and eV are inherent characteristics of the type of radiation. High-LET radiation deposits a higher concentration of energy in a shorter distance when traversing tissue compared with low-LET radiation. The different biological effects of high and low LET with similar energies have been documented in vivo in animal models and in cultured cells. High-LET results in intense macromolecular damage and more cell death. Findings indicate that while both low- and high-LET radiation activate non-homologous end-joining DNA repair activity, efficient repair of high-LET radiation requires the homologous recombination repair pathway. Low- and high-LET radiation activate p53 transcription factor activity in most cells, but high LET activates NF-kB transcription factor at lower radiation doses than low-LET radiation. Here we review the development, uses, and current understanding of the cellular effects of low- and high-LET radiation exposure.

DNA double-strand break repair and adaptive responses of low-dose radiation in normal and tumor lung cell lines

The adaptive response (AR), which can be induced by low-dose ionizing radiation (LD), may influence the therapeutic ratio of cancer treatment. We investigated the AR and the DNA double-strand break (DSB) repair pathway in human lung tumor cells and normal cells. We measured viability and proliferation of normal lung cells (MRC-5) and lung cancer cells (QU-DB) using the MTT and colony formation assays. Flow cytometric analysis of γ-H2AX was used to measure DNA-DSBs induction, repair, and residual damages. AR was seen in the normal cells but not in the cancer cells. Our findings suggest that LD stimulates DSB repair and that this may contribute to distinctive AR in normal vs. cancer cells.

The gut microbiome and melanoma: A review.

Disturbances in the microbial ecosystem have been implemented in chronic inflammation, immune evasion and carcinogenesis, with certain microbes associated with the development of specific cancers. In recent times, the gut microbiome has been recognised as a potential novel player in the pathogenesis and treatment of malignant melanoma. It has been shown that the composition of gut microbiota in early-stage melanoma changes from in situ to invasive and then to metastatic disease. The gut bacterial and fungal profile has also been found to be significantly different in melanoma patients compared to controls. Multiple studies of immune checkpoint inhibitor (ICI) therapies have shown that the commensal microbiota may have an impact on anti-tumor immunity and therefore ICI response in cancer patients. When it comes to chemotherapy and radiotherapy treatments, studies demonstrate that gut microbiota are invaluable in the repair of radiation and chemotherapy-induced damage and therapeutic manipulation of gut microbiota can be an effective strategy to deal with side effects. Studies demonstrate the oncogenic and tumor-suppressive properties of the gut microbiome, which may play a role in the pathogenesis of melanoma. Despite this, investigations into specific interactions are still in its infancy, but starting to gain momentum as more significant and clinically relevant effects are emerging.

Histone chaperone FACT complex inhibitor CBL0137 interferes with DNA damage repair and enhances sensitivity of medulloblastoma to chemotherapy and radiation.

Medulloblastoma (MB) is a malignant pediatric brain tumor with a poor prognosis. Post-surgical radiation and cisplatin-based chemotherapy have been a mainstay of treatment, which often leads to substantial neurocognitive impairments and morbidity, highlighting the need for a novel therapeutic target to enhance the sensitivity of MB tumors to cytotoxic therapies. We performed a comprehensive study using a cohort of 71MB patients' samples and pediatric MB cell lines and found that MB tumors have elevated levels of nucleosome remodeling FACT (FAcilitates Chromatin Transcription) complex and DNA repair enzyme AP-endonuclease1 (APE1). FACT interacts with APE1 and facilitates recruitment and acetylation of APE1 to promote repair of radiation and cisplatin-induced DNA damage. Further, levels of FACT and acetylated APE1 both are correlate strongly with MB patients' survival. Targeting FACT complex with CBL0137 inhibits DNA repair and alters expression of a subset of genes, and significantly improves the potency of cisplatin and radiation in vitro and in MB xenograft. Notably, combination of CBL0137 and cisplatin significantly suppressed MB tumor growth in an intracranial orthotopic xenograft model. We conclude that FACT complex promotes chemo-radiation resistance in MB, and FACT inhibitor CBL0137 can be used as a chemo-radiation sensitizer to augment treatment efficacy and reduce therapy-related toxicity in high-risk pediatric patients.

Folate Depletion as a Radiosensitizing Strategy for Solid Tumors

<h3>Purpose/Objective(s)</h3> Folate is an essential substrate for DNA synthesis, therefore important for propagation of cancer cells through cell cycle following repair of radiation induced DNA damage. We hypothesized folate depletion could improve radiosensitivity by impairing cell cycle progression, therefore potentially the efficacy of radiation therapy. <h3>Materials/Methods</h3> We investigated folate depletion as a novel radiosensitization strategy <b>in-vitro</b> using a range of cancer cell lines; melanoma (A375), pancreas (PANC1), cervix (Ca Ski) and lung (A549), to recapitulate varying degree of radiosensitivity observed in clinic. Cell lines were irradiated to a number of target doses (0, 2, 4, 8, 16 and 24 Gy) using an intensity-modulated radiation therapy-based radiobiology platform and subjected to varying levels of folate depletion, achieved with carboxypeptidase G (0, 0.00006, 0.0002, 0.0006, 0.002 and 0.006 units [U]). Cell viability was assessed with MTS assay and Loewe combination index values were calculated to assess interaction between radiation and folate depletion (< 1 synergy; = 1 additivity; > 1 antagonism). <h3>Results</h3> Reproducible, dose dependent and statistically significant reduction in cell viability seen for radiation as well as to folate depletion for A375, PANC1 and Ca Ski cell lines (<i>P</i> < 0.0001, two-way ANOVA), and similar results observed for A549 cell line although the magnitude of cell kill was less marked for radiation (<i>P</i> = 0.0013). A375 cell line demonstrated antagonism to lower carboxypeptidase G concentrations that was more pronounced for larger radiation doses, but increasing drug concentrations resulted in synergy. PANC1 cell line showed low level antagonism for 0.00006 U carboxypeptidase G, whereas at 0.0002 U synergy was seen which was more apparent for lower radiation doses, and 0.0006 U produced even better synergy (Table 1). Ca Ski cell line showed additivity with the lowest concentration of carboxypeptidase G but increasing concentrations demonstrated higher degree of synergy. A549 cell line showed antagonism with lower concentrations of drug, however, in contrast with other cell lines, high antagonism was observed with low radiation doses, and when synergy was seen, it was for larger radiation doses. <h3>Conclusion</h3> We demonstrate folate depletion as a novel radiosensitizing strategy <b>in-vitro. In-vivo</b> validation would advance this approach for further exploration to enhance efficacy of radiation therapy in solid tumors.

Regulation of Pkc1 Hyper-Phosphorylation by Genotoxic Stress.

The cell wall integrity (CWI) signaling pathway is best known for its roles in cell wall biogenesis. However, it is also thought to participate in the response to genotoxic stress. The stress-activated protein kinase Mpk1 (Slt2, is activated by DNA damaging agents through an intracellular mechanism that does not involve the activation of upstream components of the CWI pathway. Additional observations suggest that protein kinase C (Pkc1), the top kinase in the CWI signaling cascade, also has a role in the response to genotoxic stress that is independent of its recognized function in the activation of Mpk1. Pkc1 undergoes hyper-phosphorylation specifically in response to genotoxic stress; we have found that this requires the DNA damage checkpoint kinases Mec1 (Mitosis Entry Checkpoint) and Tel1 (TELomere maintenance), but not their effector kinases. We demonstrate that the casein kinase 1 (CK1) ortholog, Hrr25 (HO and Radiation Repair), previously implicated in the DNA damage transcriptional response, associates with Pkc1 under conditions of genotoxic stress. We also found that the induced association of Hrr25 with Pkc1 requires Mec1 and Tel1, and that Hrr25 catalytic activity is required for Pkc1-hyperphosphorylation, thereby delineating a pathway from the checkpoint kinases to Pkc1. We used SILAC mass spectrometry to identify three residues within Pkc1 the phosphorylation of which was stimulated by genotoxic stress. We mutated these residues as well as a collection of 13 phosphorylation sites within the regulatory domain of Pkc1 that fit the consensus for CK1 sites. Mutation of the 13 Pkc1 phosphorylation sites blocked hyper-phosphorylation and diminished RNR3 (RiboNucleotide Reductase) basal expression and induction by genotoxic stress, suggesting that Pkc1 plays a role in the DNA damage transcriptional response.

Sox9-expressing cells promote regeneration after radiation-induced lung injury via the PI3K/AKT pathway

BackgroundRadiation-induced lung injury (RILI) is considered one of the most common complications of thoracic radiation. Recent studies have focused on stem cell properties to obtain ideal therapeutic effects, and Sox9 has been reported to be involved in stem cell induction and differentiation. However, whether Sox9-expressing cells play a role in radiation repair and regeneration remains unknown.MethodsWe successfully obtained Sox9CreER, RosatdTomato and RosaDTA mice and identified Sox9-expressing cells through lineage tracing. Then, we evaluated the effects of the ablation of Sox9-expressing cells in vivo. Furthermore, we investigated the underlying mechanism of Sox9-expressing cells during lung regeneration via an online single-cell RNA-seq dataset.ResultsIn our study, we demonstrated that Sox9-expressing cells promote the regeneration of lung tissues and that ablation of Sox9-expressing cells leads to severe phenotypes after radiation damage. In addition, analysis of an online scRNA-Seq dataset revealed that the PI3K/AKT pathway is enriched in Sox9-expressing cells during lung epithelium regeneration. Finally, the AKT inhibitor perifosine suppressed the regenerative effects of Sox9-expressing cells and the AKT pathway agonist promotes proliferation and differentiation.ConclusionsTaken together, the findings of our study suggest that Sox9-expressing cells may serve as a therapeutic target in lung tissue after RILI.

Cycling Stem Cells Are Radioresistant and Regenerate the Intestine

SUMMARYA certain number of epithelial cells in intestinal crypts are DNA damage resistant and contribute to regeneration. However, the cellular mechanism underlying intestinal regeneration remains unclear. Using lineage tracing, we show that cells marked by an Msi1 reporter (Msi1+) are right above Lgr5high cells in intestinal crypts and exhibit DNA damage resistance. Single-cell RNA sequencing reveals that the Msi1+ cells are heterogeneous with the majority being intestinal stem cells (ISCs). The DNA damage-resistant subpopulation of Msi1+ cells is characterized by low-to-negative Lgr5 expression and is more rapidly cycling than Lgr5high radiosensitive crypt base columnar stem cells (CBCs). This enables an efficient repopulation of the intestinal epithelium at early stage when Lgr5high cells are not emerging. Furthermore, relative to CBCs, Msi1+ cells preferentially produce Paneth cells during homeostasis and upon radiation repair. Together, we demonstrate that the DNA damage-resistant Msi1+ cells are cycling ISCs that maintain and regenerate the intestinal epithelium.

Совершенствование технологии ремонта изоляционных конструкций электроподвижного состава Восточного полигона тепловым излучением

Совершенствование технологии ремонта изоляционных конструкций электроподвижного состава Восточного полигона тепловым излучением

Radiation repair models for clinical application

A number of newlyemerging clinical techniques involve non-conventional patterns of radiation delivery which require an appreciation of the role played by radiation repair phenomena. This review outlines the main models of radiation repair, focussing on those which are of greatest clinical usefulness and which may be incorporated into biologically effective dose assessments. The need to account for the apparent "slowing-down" of repair rates observed in some normal tissues is also examined, along with a comparison of the relative merits of the formulations which can be used to account for such phenomena. Jack Fowler brought valuable insight to the understanding of radiation repair processes and this article includes reference to his important contributions in this area.

The endonuclease EEPD1 mediates synthetic lethality in RAD52-depleted BRCA1 mutant breast cancer cells

BackgroundProper repair and restart of stressed replication forks requires intact homologous recombination (HR). HR at stressed replication forks can be initiated by the 5′ endonuclease EEPD1, which cleaves the stalled replication fork. Inherited or acquired defects in HR, such as mutations in breast cancer susceptibility protein-1 (BRCA1) or BRCA2, predispose to cancer, including breast and ovarian cancers. In order for these HR-deficient tumor cells to proliferate, they become addicted to a bypass replication fork repair pathway mediated by radiation repair protein 52 (RAD52). Depleting RAD52 can cause synthetic lethality in BRCA1/2 mutant cancers by an unknown molecular mechanism.MethodsWe hypothesized that cleavage of stressed replication forks by EEPD1 generates a fork repair intermediate that is toxic when HR-deficient cells cannot complete repair with the RAD52 bypass pathway. To test this hypothesis, we applied cell survival assays, immunofluorescence staining, DNA fiber and western blot analyses to look at the correlation between cell survival and genome integrity in control, EEPD1, RAD52 and EEPD1/RAD52 co-depletion BRCA1-deficient breast cancer cells.ResultsOur data show that depletion of EEPD1 suppresses synthetic lethality, genome instability, mitotic catastrophe, and hypersensitivity to stress of replication of RAD52-depleted, BRCA1 mutant breast cancer cells. Without HR and the RAD52-dependent backup pathway, the BRCA1 mutant cancer cells depleted of EEPD1 skew to the alternative non-homologous end-joining DNA repair pathway for survival.ConclusionThis study indicates that the mechanism of synthetic lethality in RAD52-depleted BRCA1 mutant cancer cells depends on the endonuclease EEPD1. The data imply that EEPD1 cleavage of stressed replication forks may result in a toxic intermediate when replication fork repair cannot be completed.

091 Vemurafenib impairs the repair of UV radiation induced DNA damage

091 Vemurafenib impairs the repair of UV radiation induced DNA damage

Radiological changes with magnetic resonance imaging and computed tomography after irradiating minipig mandibles: The role of T2-SPIR mixed signal intensities in the detection of osteoradionecrosis

PurposeRadiotherapy in the head and neck can induce several radiologically detectable changes in bone, osteoradionecrosis (ORN) among them. The purpose is to investigate radiological changes in mandibular bone after irradiation with various doses with and without surgery and to determine imaging characteristics of radiotherapy and ORN in an animal model. Materials and methodsSixteen Göttingen minipigs were divided into groups and were irradiated with two fractions with equivalent doses of 0, 25, 50 and 70 Gray. Thirteen weeks after irradiation, left mandibular teeth were removed and dental implants were placed. CT-scans and MR-imaging were made before irradiation and twenty-six weeks after. Alterations in the bony structures were recorded on CT-scan and MR-imaging and scored by two head-neck radiologists. ResultsIncreased signal changes on MR-imaging were associated with higher radiation doses. Two animals developed ORN clinically. Radiologically mixed signal intensities on T2-SPIR were seen. On CT-scans cortical destruction was found in three animals. Based on imaging, three animals were diagnosed with ORN. ConclusionIrradiation of minipig mandibles with various doses induced damages of the mandibular bone. Imaging with CT-scan and MR-imaging showed signal and structural changes that can be interpreted as prolonged and insufficient repair of radiation induced bone damages.

Tissue breathing and topology of rats thymocytes surface under acute total γ-irradiation

Assessment of the effect of single total γ irradiation to the parameters of mitochondrial oxidation and the topology of the thymocyte surface. The study was performed in sexually mature white outbreeding male rats divided into three groups: two experimental and one control. The states of energy metabolism were determined by the rate of oxygen consumption by the thymus tissues on endogenous substrates at the presence of 2,4 dinitrophenol, uncoupler of a tissue breathing (TB) and oxidative phosphorylation (OP) after a single total γ irradiation at a dose of 1.0 Gy at 3, 10, 40 and 60 days. The topology of thymus cells was assessed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). On the 3rd and 10th days after total gamma irradiation at a dose of 1.0 Gy, a significant decrease in respira tory activity was determined in thymus tissues on endogenous substrates. Simultaneously, on the 3rd day, pro nounced changes in the morphological parameters of thymocytes (height, volume, area of contact with the sub strate) and the topology of their surface were also observed. On the 10th day after irradiation, most of the morpho logical parameters of thymocytes, except for their volume, were characterized by restoration to normal. In the long term (on the 30th and 60th days after exposure), a gradual but not complete recovery of the respiratory activity of thymocytes was observed, accompanied by an increase in the degree of dissociation of TD and OP. The obtained data reflect and refine mechanisms of post radiation repair of lymphopoiesis, showing the presence of conjugated changes in the parameters of aerobic energy metabolism of thymocytes, morphology and topology of their surface. The synchronism of changes in the parameters under study is a reflection of the state of the cytoskeleton, the functional activity of which largely depends on the level and efficiency of mitochondrial oxidation.

O36. Mitochondrial dynamics as a potential target for cancer therapy

Introduction Mitochondria are important organelles that control metabolism and cell death. Cellular mitochondrial status, including mitochondrial dynamics, may determine tumour radioresponsiveness. The aim of this study was to characterise mitochondrial status in cancer cells, modulate mitochondrial dynamics and assess changes in cell survival and response to radiation therapy. Materials and methods Mitochondrial dynamics was assessed using the MitoTracker red staining technique and microscopic evaluation. Respiratory status and mitochondrial membrane potential were analysed and correlated with radiation survival in several cancer cell lines. Modulation of mitochondrial dynamics was achieved using the fission inhibitor Mdivi-1. Cell survival assays were employed to determine the effect of Mdivi-1 on cellular radiation response. Mechanistic aspects were explored by assessment of apoptosis, cell cycle dynamics and cytogenetic effects of Mdivi-1. Results Inhibition of fission by Mdivi-1 induced a measureable increase in fusion and caused significant cytotoxicity. Mdivi-1 was shown to significantly increase cellular radiosensitivity. Differences in cell survival were associated with cell cycle arrest and increased apoptosis. Mdivi-1 treatment induced micronuclei and enhanced radiation-induced cytogenetic damage. No influence on radiation repair was detected. Conclusion Results of this study suggest that regulation of mitochondrial dynamics is an important aspect of cellular function that may influence the response of cancer cells to therapy. While no firm correlation could be established between mitochondrial status and radiosensitivity, a novel scoring system was developed to assess mitochondrial fragmentation. Notably, modulation of mitochondrial dynamics was found to facilitate both cytotoxicity and enhanced cellular response to radiotherapy. Therefore, modulation of mitochondrial dynamics may represent a novel approach to targeted cancer therapy.

O43. Estimating the effectiveness of high LET radiations to induce damage to DNA in human lymphocytes and modelling the repair thereof

Introduction Neutron and ion beam irradiations have an increased efficiency in cell killing, resulting from their high linear-energy-transfer (LET) which in turn induces clustered and complex DNA breaks. Similarly, Auger electron irradiations have been shown to a have an increased relative biological effectiveness (RBE) when compared to low-LET radiations. We report on our investigations into the RBE of the DNA incorporated Auger electron emitting 123 I and a model which may estimate the induction and repair of γ -H2AX foci (markers of DNA double strand breaks (DSBs)) in human lymphocytes after exposure to fast neutrons. Materials and methods The RBE of 123 I was determined by comparison of micronuclei inductions in T-lymphocytes using 5-[ 123 I]-iodo-2-deoxyuridine and similar inductions by graded doses of 60 Co γ -rays. Additionally, a theoretical model which describes the time-based repair pathways of DNA DSBs using a set of dependent partial differential equations, was investigated. The validity of the model was assessed by comparing the γ -H2AX foci kinetics predicted by the model to experimental curves observed for lymphocytes exposed to neutrons (20 keV/ μ m) and 60 Co γ -rays. Results The dose-limiting RBE of the DNA incorporated 123 I was found to range from 3 to 11. These observed RBE values relate well to those obtained in studies with 125 I incorporated into rodent cell lines. The higher micronuclei frequencies noted for Auger electrons in the study is indicative of the high-LET nature of these particles. Similar to experimental observations, the theoretical repair model predicted that a larger number of DSBs remain unrepaired after neutron irradiation compared to low-LET γ -irradiation. The model was shown to accurately describe and estimate the kinetics of γ -H2AX foci in human lymphocytes after exposure to fast neutrons and 60 Co γ -rays. Conclusion With respect to their role in radiation therapy and protection, there is a need for investigating and developing models which may predict the induction and repair of high-LET radiation induced DNA damage. Acknowledgement This work was supported by a University Development Cooperation VLIR Own Initiative Programme between Belgium and South Africa.