Comparison of Individual Radiosensitivity to γ-Rays and Carbon Ions

Analysis of nuclear maturation, DNA damage and repair gene expression of bovine oocyte and cumulus cells submitted to ionizing radiation

Radiotherapy causes destruction of tumor cells, but also threatens the integrity and survival of surrounding normal cells. Then, woman submitted to irradiation for cancer treatment may present permanent ovary damage, resulting in impaired fertility. The objective of this study was to investigate the effects of therapeutic doses of ionizing radiation (IR), used for ovarian cancer treatment in humans, on bovine cumulus-oocyte complexes (COCs) as experimental model. Bovine ovaries were exposed to 0.9 Gy, 1.8 Gy, 3.6 Gy or 18.6 Gy IR, and then COCs were collected and used to evaluate: (a) oocyte nuclear maturation; (b) presence of phosphorylated H2A.X (γH2AX), as an indicator of DNA double-strand breaks (DSBs); and (c) expression of genes involved in DNA repair (TP53BP1, RAD52, ATM, XRCC6 and XRCC5) and apoptosis (BAX). The radiation doses tested in this study had no detrimental effects on nuclear maturation and did not increase γH2AX in the oocytes. However, IR treatment altered the mRNA abundance of RAD52 (RAD52 homolog, DNA repair protein) and BAX (BCL2-associated X protein). We conclude that although IR doses had no apparent effect on oocyte nuclear maturation and DNA damage, molecular pathways involved in DNA repair and apoptosis were affected by IR exposure in cumulus cells.

DNA double-strand breaks in cancer cells as a function of proton linear energy transfer and its variation in time

PURPOSE

The complex relationship between linear energy transfer (LET) and cellular response to radiation is not yet fully elucidated. To better characterize DNA damage after irradiations with therapeutic protons, we monitored formation and disappearance of DNA double-strand breaks (DNA DSB) as a function of LET and time. Comparisons with conventional γ-rays and high LET carbon ions were also performed.

MATERIALS AND METHODS

In the present work, we performed immunofluorescence-based assay to determine the amount of DNA DSB induced by different LET values along the 62 MeV therapeutic proton Spread out Bragg peak (SOBP) in three cancer cell lines, i.e. HTB140 melanoma, MCF-7 breast adenocarcinoma and HTB177 non-small lung cancer cells. Time dependence of foci formation was followed as well. To determine irradiation positions, corresponding to the desired LET values, numerical simulations were carried out using Geant4 toolkit. We compared γ-H2AX foci persistence after irradiations with protons to that of γ-rays and carbon ions.

RESULTS

With the rise of LET values along the therapeutic proton SOBP, the increase of γ-H2AX foci number is detected in the three cell lines up to the distal end of the SOBP, while there is a decrease on its distal fall-off part. With the prolonged incubation time, the number of foci gradually drops tending to attain the residual level. For the maximum number of DNA DSB, irradiation with protons attain higher level than that of γ-rays. Carbon ions produce more DNA DSB than protons but not substantially. The number of residual foci produced by γ-rays is significantly lower than that of protons and particularly carbon ions. Carbon ions do not produce considerably higher number of foci than protons, as it could be expected due to their physical properties.

CONCLUSIONS

In situ visualization of γ-H2AX foci reveal creation of more lesions in the three cell lines by clinically relevant proton SOBP than γ-rays. The lack of significant differences in the number of γ-H2AX foci between the proton and carbon ion-irradiated samples suggests an increased complexity of DNA lesions and slower repair kinetics after carbon ions compared to protons. For all three irradiation types, there is no major difference between the three cell lines shortly after irradiations, while later on, the formation of residual foci starts to express the inherent nature of tested cells, therefore increasing discrepancy between them.

Induction of Genomic Instability in a Primary Human Fibroblast Cell Line Following Low-Dose Alpha-Particle Exposure and the Potential Role of Exosomes

PURPOSE

To study the induction of genomic instability (GI) in the progeny of cell populations irradiated with low doses of alpha-particles and the potential role of exosome-encapsulated bystander signalling.

METHODS

The induction of GI in HF19 normal fibroblast cells was assessed by determining the formation of micronuclei (MN) in binucleate cells along with using the alkaline comet assay to assess DNA damage.

RESULTS

Low dose alpha-particle exposure (0.0001-1 Gy) was observed to produce a significant induction of micronuclei and DNA damage shortly after irradiation (assays performed at 5 and 1 h post exposure, respectively). This damage was not only still evident and statistically significant in all irradiated groups after 10 population doublings, but similar trends were observed after 20 population doublings. Exosomes from irradiated cells were also observed to enhance the level of DNA damage in non-irradiated bystander cells at early times.

CONCLUSION

very low doses of alpha-particles are capable of inducing GI in the progeny of irradiated cells even at doses where <1% of the cells are traversed, where the level of response was similar to that observed at doses where 100% of the cells were traversed. This may have important implications with respect to the evaluation of cancer risk associated with very low-dose alpha-particle exposure and deviation from a linear dose response.

Radiomic Analysis of CT Predicts Tumor Response in Human Lung Cancer with Radiotherapy

PURPOSE

Radiomics features can be positioned to monitor changes throughout treatment. In this study, we evaluated machine learning for predicting tumor response by analyzing CT images of lung cancer patients treated with radiotherapy.

EXPERIMENTAL DESIGN

For this retrospective study, screening or standard diagnostic CT images were collected for 100 patients (mean age, 67 years; range, 55-82 years; 64 men [mean age, 68 years; range, 55-82 years] and 36 women [mean age, 65 years; range, 60-72 years]) from two institutions between 2013 and 2017. Radiomics analysis was available for each patient. Features were pruned to train machine learning classifiers with 50 patients, then trained in the test dataset.

RESULT

A support vector machine classifier with 2 radiomic features (flatness and coefficient of variation) achieved an area under the receiver operating characteristic curve (AUC) of 0.91 on the test set.

CONCLUSION

The 2 radiomic features, flatness, and coefficient of variation, from the volume of interest of lung tumor, can be the biomarkers for predicting tumor response at CT.

Biological effects and inter-individual variability in peripheral blood lymphocytes of healthy donors exposed to 60 MeV proton radiotherapeutic beam

Purpose: The aim of our study was to investigate the amount of initial DNA damage and cellular repair capacity of human peripheral blood lymphocytes exposed to the therapeutic proton beam and compare it to X-rays. Materials and methods: Lymphocytes from 10 healthy donors were irradiated in the Spread Out Bragg Peak of the 60 MeV proton beam or, as a reference, exposed to 250 kV X-rays. DNA damage level was assessed using the alkaline version of the comet assay method. For both sources of radiation, dose-DNA damage response (0-4 Gy) and DNA repair kinetics (0-120 min) were estimated. The observed DNA damage was then used to calculate the relative biological effectiveness (RBE) of the proton beam in comparison to that of X-rays. Results: Dose-response relationships for the DNA damage level showed linear dependence for both proton beam and X-rays (R² = 0.995 for protons and R² = 0.993 for X-rays). Within the dose range of 1-4 Gy, protons were significantly more effective in inducing DNA damage than were X-rays (p < .05). The average RBE, calculated from the proton and X-ray doses required for the iso-effective, internally standardized tail DNA parameter (sT-DNA) was 1.28 ± 0.57. Similar half-life time of residual damage and repair efficiency of induced DNA damage for both radiation types were observed. In the X-irradiated group, significant inter-individual differences were observed. Conclusions: Proton therapy was more effective at high radiation doses. However, DNA damage repair mechanism after proton irradiation seems to differ from that following X-rays.

Dose-response curves for analyzing of dicentric chromosomes and chromosome translocations following doses of 1000 mGy or less, based on irradiated peripheral blood samples from five healthy individuals

In terms of biological dosimetry at the time of radiation exposure, the dicentric chromosome (Dic) assay (DCA) is the gold standard for assessing for the acute phase and chromosome translocation (Tr) analysis is the gold standard for assessing the chronic phase. It is desirable to have individual dose-response curves (DRCs) for each laboratory because the analysis criteria differ between laboratories. We constructed the DRCs for radiation dose estimation (with three methods) using peripheral blood (PB) samples from five healthy individuals. Aliquots were irradiated with one of eight gamma-ray doses (0, 10, 20, 50, 100, 200, 500 or 1000 mGy), then cultured for 48 h. The number of chromosome aberrations (CAs) was analyzed by DCA, using Giemsa staining and centromere-fluorescence in situ hybridization (centromere-FISH) and by chromosome painting (chromosome pairs 1, 2 and 4) for Tr analysis. In DCA, there was large variation between individuals in the frequency of Dics formed, and the slopes of the DRCs were different. In Tr analysis, although variation was observed in the frequency of Tr, the slopes of the DRCs were similar after adjusting the background for age. Good correlation between the irradiation dose and the frequency of CAs formed was observed with these three DRCs. However, performing three different biological dosimetry assays simultaneously on PB from five donors nonetheless results in variation in the frequency of CAs formed, especially at doses of 50 mGy or less, highlighting the difficulty of biological dosimetry using these methods. We conclude that it might be difficult to construct universal DRCs.

Evaluation of ATM heterozygous mutations underlying individual differences in radiosensitivity using genome editing in human cultured cells

Ionizing radiation (IR) induces DNA double-strand breaks (DSBs), which are an initial step towards chromosomal aberrations and cell death. It has been suggested that there are individual differences in radiosensitivity within human populations, and that the variations in DNA repair genes might determine this heterogeneity. However, it is difficult to quantify the effect of genetic variants on the individual differences in radiosensitivity, since confounding factors such as smoking and the diverse genetic backgrounds within human populations affect radiosensitivity. To precisely quantify the effect of a genetic variation on radiosensitivity, we here used the CRISPR-ObLiGaRe (Obligate Ligation-Gated Recombination) method combined with the CRISPR/Cas9 system and a nonhomologous end joining (NHEJ)-mediated knock-in technique in human cultured cells with a uniform genetic background. We generated ATM heterozygous knock-out (ATM ^+/-) cell clones as a carrier model of a radiation-hypersensitive autosomal-recessive disorder, ataxia-telangiectasia (A-T). Cytokinesis-blocked micronucleus assay and chromosome aberration assay showed that the radiosensitivity of ATM ^+/- cell clones was significantly higher than that of ATM ^+/+ cells, suggesting that ATM gene variants are indeed involved in determining individual radiosensitivity. Importantly, the differences in radiosensitivity among the same genotype clones were small, unlike the individual differences in fibroblasts derived from A-T-affected family members.

Exosome-Mediated Telomere Instability in Human Breast Epithelial Cancer Cells after X Irradiation

In directly irradiating cells, telomere metabolism is altered and similar effects have been observed in nontargeted cells. Exosomes and their cargo play dominant roles in communicating radiation-induced bystander effects with end points related to DNA damage. Here we report novel evidence that exosomes are also responsible for inducing telomere-related bystander effects. Breast epithelial cancer cells were exposed to either 2 Gy X rays, or exposed to irradiated cell conditioned media (ICCM), or exosomes purified from ICCM. Compared to control cells, telomerase activity decreased in the 2 Gy irradiated cells and both bystander samples after one population doubling. At the first population doubling, telomere length was shorter in the 2 Gy irradiated sample but not in the bystander samples. By 24 population doublings telomerase activity recovered to control levels in all samples; however, the 2 Gy irradiated sample continued to demonstrate short telomeres and both bystander samples acquired shorter telomeres. RNase treatment of exosomes prevented the bystander effects on telomerase and telomere length that were observed at 1 population doubling and 24 population doublings, respectively. Thermal denaturation by boiling eliminated the reduction of telomere length in bystander samples, suggesting that the protein fraction of exosomes also contributes to the telomeric effect. RNase treatment plus boiling abrogated all telomere-related effects in directly irradiated and bystander cell populations. These findings suggest that both proteins and RNAs of exosomes can induce alterations in telomeric metabolism, which can instigate genomic instability in epithelial cancer cells after X-ray irradiation.