Kinetics Of Double-strand Breaks Research Articles

Kinetics of double strand breaks of DNA in tritiated water evaluated using single molecule observation method

Double strand breaks (DSBs) of DNA molecules in tritiated water was examined under sterilized and non-sterilized conditions using a single molecule observation method. The genome DNA of bacteriophage T4 GT7 was immersed in sterilized tritiated water (5.2 MBq/cm3) and non-sterilized tritiated water (4.2 MBq/cm3) for 1, 7 and 14 day(s). Then the length of DNA molecules was measured using a fluorescence microscope after intercalation of fluorescent dye. The dose rate was 1.4–1.7 × 10−2 Gy/h and the dose level was 0.41–5.8 Gy. The rate of DSBs induced by β-rays from tritium was successfully evaluated under the sterilized conditions and the value comparable with the DSB rate under γ-ray irradiation (Noda et al., Scientific Reports 7 (2017) 8557) was obtained. The length of DNA molecules in non-sterilized tritiated water was clearly shorter than that in the sterilized tritiated water. This observation suggested that the effects of tritium was far weaker than that of microorganisms (e.g. bacteria) and impurities in water even at the tritium concentration as high as 5.2 MBq/cm3.

CSA and CSB play a role in the response to DNA breaks.

CS proteins have been involved in the repair of a wide variety of DNA lesions. Here, we analyse the role of CS proteins in DNA break repair by studying histone H2AX phosphorylation in different cell cycle phases and DNA break repair by comet assay in CS-A and CS-B primary and transformed cells. Following methyl methane sulphate treatment a significant accumulation of unrepaired single strand breaks was detected in CS cells as compared to normal cells, leading to accumulation of double strand breaks in S and G2 phases. A delay in DSBs repair and accumulation in S and G2 phases were also observed following IR exposure. These data confirm the role of CSB in the suppression of NHEJ in S and G2 phase cells and extend this function to CSA. However, the repair kinetics of double strand breaks showed unique features for CS-A and CS-B cells suggesting that these proteins may act at different times along DNA break repair.The involvement of CS proteins in the repair of DNA breaks may play an important role in the clinical features of CS patients.

Intrinsic LINE-1 Hypomethylation and Decreased Brca1 Expression are Associated with DNA Repair Delay in Irradiated Thyroid Cells.

Exposure to ionizing radiation greatly increases the risk of developing papillary thyroid carcinoma (PTC), especially during childhood, mainly due to gradual inactivation of DNA repair genes and DNA damages. Recent molecular characterization of PTC revealed DNA methylation deregulation of several promoters of DNA repair genes. Thus, epigenetic silencing might be a plausible mechanism for the activity loss of tumor suppressor genes in radiation-induced thyroid tumors. Herein, we investigated the impact of ionizing radiation on global methylation and CpG islands within promoter regions of homologous recombination (HR) and non-homologous end joining (NHEJ) genes, as well as its effects on gene expression, using two well-established normal differentiated thyroid cell lines (FRTL5 and PCCL3). Our data reveal that X-ray exposure promoted G2/M arrest in normal thyroid cell lines. The FRTL5 cells displayed a slower kinetics of double-strand breaks (DSB) repair and a lower long interspersed nuclear element-1 (LINE-1) methylation than the PCCL3 cells. Nevertheless, acute X-ray exposure does not alter the expression of genes involved in HR and NHEJ pathways, apart from the downregulation of Brca1 in thyroid cells. On the other hand, HR and NHEJ gene expressions were upregulated in radiation-induced senescent thyroid cells. Taken together, these data suggest that FRTL5 cells intrinsically have less efficient DNA DSB repair machinery than PCCL3 cells, as well as genomic instability, which could predispose the FRTL5 cells to unrepaired DSB lesions and, therefore, gene mutations.

DSB repair model for mammalian cells in early S and G1 phases of the cell cycle: Application to damage induced by ionizing radiation of different quality

The purpose of this work is to test the hypothesis that kinetics of double strand breaks (DSB) repair is governed by complexity of DSB. To test the hypothesis we used our recent published mechanistic mathematical model of DSB repair for DSB induced by selected protons, deuterons, and helium ions of different energies representing radiations of different qualities. In light of recent advances in experimental and computational techniques, the most appropriate method to study cellular responses in radiation therapy, and exposures to low doses of ionizing radiations is using mechanistic approaches. To this end, we proposed a ‘bottom–up’ approach to study cellular response that starts with the DNA damage. Monte Carlo track structure method was employed to simulate initial damage induced in the genomic DNA by direct and indirect effects. Among the different types of DNA damage, DSB are known to be induced in simple and complex forms. The DSB repair model in G1 and early S phases of the cell cycle was employed to calculate the repair kinetics. The model considers the repair of simple and complex DSB, and the DSB produced in the heterochromatin. The inverse sampling method was used to calculate the repair kinetics for each individual DSB. The overall repair kinetics for 500 DSB induced by single tracks of the radiation under test were compared with experimental results. The results show that the model is capable of predicting the repair kinetics for the DSB induced by radiations of different qualities within an accepted range of uncertainty.

A mathematical model of P53 gene regulatory networks under radiotherapy

P53, a vital anticancer gene, controls the transcription and translation of a series of genes, and implement the cell cycle arrest and cell apoptosis by regulating their complicated signal pathways. Under radiotherapy, cell can trigger internal self-defense mechanisms in fighting against genome stresses induced by acute ion radiation (IR). To simulate the investigating of cellular responding acute IR at single cell level further, we propose a model of P53 gene regulatory networks under radiotherapy. Our model can successfully implement the kinetics of double strand breaks (DSBs) generating and their repair, ataxia telangiectasia mutated (ATM) activation, as well as P53-MDM2 feedback regulating. By comparing simulations under different IR dose, we can try to find the optimal strategy in controlling of IR dose and therapy time, and provide some theoretical analysis to obtain much better outcome of radiotherapy further.

Survival Curves with Shoulders: Damage Interaction, Unsaturated but Dose-Dependent Rejoining Kinetics or Inducible Repair of DNA Double-Strand Breaks?

Double-strand breaks (DSBs) are considered as critical lesions for radiation-induced cell killing. Two processes, identified by the rejoining kinetics of DNA DSBs induced in yeast at radiation doses which are also applied in survival studies, can lead to survival curves with shoulders: firstly, "DSB interaction" when repair time is unrestricted, and secondly, a biphasic and unsaturated DSB rejoining, where the fractions of the two rejoining components are dose-dependent and repair time is restricted. Thus the unsaturated rejoining kinetics of DSBs is not in contradiction to the observation of survival curves with shoulders. While these two mechanisms are derived from studies on cells kept under nongrowth conditions during postirradiation incubation, experiments are reviewed which were designed to study inducibility of DSB rejoining. Some of these studies suggest that rejoining of DSBs under growth conditions may include an inducible process.