Delayed mitogenic stimulation decreases DNA damage assessed by micronucleus assay in human peripheral blood lymphocytes after (60)co irradiation

The relative biological effectiveness of high-energy clinical 3 and 6 MV X-rays for micronucleus induction in human lymphocytes

PURPOSE

In the modern era of radiotherapy, use of conventional radiation modalities (based on γ-rays) is being replaced by high-energy linear accelerator-based X-rays. As a result of mishandling of equipment or mechanical errors, health workers can be exposed to these high-energy X-rays. Especially in the absence of personnel monitoring devices, biodosimetry with a lower energy X-ray calibration curve may not provide an acceptable dose estimate. Moreover, the relative biological effectiveness (RBE) value assigned for X-rays is the same (ONE) regardless of beam energy (V), employed in diagnosis, interventional medicine, and radiotherapy. Therefore, the purpose of the study is to examine the induced biological effects, measured through micronucleus (MN) formation, of X-rays of different energies (3 and 6 MV X-rays), and to investigate the RBE relative to 225 kVp X-rays.

MATERIALS AND METHODS

Peripheral blood lymphocytes (PBLs) from healthy donors (n = 6), were irradiated with 225 kVp, 3 MV, and 6 MV energy X-rays and induced biological damage was quantified as MN formation using the cytokinesis blocked MN (CBMN) assay.

RESULTS

The MN per cell in the X-irradiated samples for the three different X-ray energies showed a significant (p<.0001) dose-dependent increase, when compared to unexposed samples. Aberration frequencies obtained at the same dose for the three different energies showed significant (p<.05) difference for the MN per cell among the energy levels; however, the in vitro dose-response curve parameters (slope, intercept, and coefficient) did not show any significant differences. The estimated dose in the blinded sample was within the 95% confidence intervals of each of the calibration curves. However, overall, the 6 MV dose-response curve coefficients yielded the closest dose estimate to that of the true dose. The calculated RBE values at 5% induced MN for 3 and 6 MV LINAC X-rays were 2.0 ± 0.04 and 0.70 ± 0.01, respectively, and the average RBE for the complete dose-response curves were 1.13 ± 0.04 and 0.80 ± 0.02 relative to 225 kVp X-rays as standard radiation.

CONCLUSION

The established dose-response curves obtained for PBL exposed to different energy levels of X-rays of 225 kVp, 3 MV, and 6 MV are ready to use for biological dosimetry purposes. The calculated RBE values for the higher energies of X-rays relative to 225 kVp X-rays in this study suggest that RBE of X-rays may not be equal to one, with the true value dependent on the beam energy, the dose and dose rate, and the endpoint investigated.

Exploring the differentially expressed genes in human lymphocytes upon response to ionizing radiation: a network biology approach

Purpose

The integration of large-scale gene data and their functional analysis needs the effective application of various computational tools. Here we attempted to unravel the biological processes and cellular pathways in response to ionizing radiation using a systems biology approach.

Materials and Methods

Analysis of gene ontology shows that 80, 42, 25, and 35 genes have roles in the biological process, molecular function, the cellular process, and immune system pathways, respectively. Therefore, our study emphasizes gene/protein network analysis on various differentially expressed genes (DEGs) to reveal the interactions between those proteins and their functional contribution upon radiation exposure.

Results

A gene/protein interaction network was constructed, which comprises 79 interactors with 718 interactions and TP53, MAPK8, MAPK1, CASP3, MAPK14, ATM, NOTCH1, VEGFA, SIRT1, and PRKDC are the top 10 proteins in the network with high betweenness centrality values. Further, molecular complex detection was used to cluster these associated partners in the network, which produced three effective clusters based on the Molecular Complex Detection (MCODE) score. Interestingly, we found a high functional similarity from the associated genes/proteins in the network with known radiation response genes.

Conclusion

This network-based approach on DEGs of human lymphocytes upon response to ionizing radiation provides clues for an opportunity to improve therapeutic efficacy.

Extension of lymphocyte viability for radiation biodosimetry: Potential implications for radiological/nuclear mass casualty incidents

Dicentric chromosome assay (DCA) is routinely used for estimating the absorbed radiation dose in exposed humans. Optimal lymphocyte viability is crucial for reliable dose estimation and most cytogenetic laboratories prefer the receipt of blood samples within 24 to 36 hours after collection. Delays in the shipment/receipt of samples can occur sometimes under certain unforeseen circumstances: (1) Adverse weather conditions, (2) distant location of blood collection sites, and (3) shipping and handling of a large number of samples after radiological/nuclear mass casualty incident(s). To circumvent some of these limitations, we evaluated the suitability of ex vivo irradiated blood samples stored in the presence of phytohemagglutinin (PHA) for 7 days at ambient temperature (22-24°C) for radiation biodosimetry. Blood samples stored in the presence of PHA for up to 7 days showed a higher mitotic index than blood samples stored without PHA. To verify the use of stored blood samples for DCA, frequencies of X-rays induced dicentric chromosomes were analyzed in the blood samples that were cultured either 24 hours after exposure or 7 days later after storage. Our results indicate that storage of ex vivo irradiated blood samples in the presence of PHA at ambient temperature was found optimal for DCA and that the radiation doses estimated by dicentric chromosome frequencies were grossly similar between the fresh and stored blood samples. Our study suggests that reliable and accurate biodosimetry results can be obtained for triage using blood samples stored for up to a week at ambient temperature in the presence of PHA.

Assessment of Correlation between Chromosomal Radiosensitivity of Peripheral Blood Lymphocytes after In vitro Irradiation and Normal Tissue Side Effects for Cancer Patients Undergoing Radiotherapy

Patients receiving identical radiation treatments experience different effects, from undetectable to severe, on normal tissues. A crucial factor of radiotherapy related side effects is individual radiosensitivity. It is difficult to spare surrounding normal tissues delivering radiation to cancer cells during radiotherapy. Therefore, it may be useful to develop a simple routine cytogenetic assay which would allow the screening of a large number of individuals for radiosensitivity optimizing tumor control rates and minimizing severe radiotherapy effects with possibility to predict risk level for developing more severe early normal tissue adverse events after irradiation. This study was conducted to assess the correlation between in vitro radiosensitivity of peripheral blood lymphocytes from cancer patients who are undergoing radiotherapy using the cytokinesis-block micronucleus (CBMN), G2 chromosomal radiosensitivity assays, and normal tissue acute side effects. The CBMN and G2 chromosomal radiosensitivity assays were performed on blood samples taken from cancer patients before radiotherapy, after first fractionation, and after radiotherapy. Acute normal tissue reactions were graded according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer. This study suggests that there is a correlation between higher frequency of micronuclei after in vitro irradiation of blood samples and higher degree of normal tissue reactions. In addition, higher number of chromatid breaks was observed in patients with more severe normal tissue reactions. This pilot study included only 5 cancer patients, and therefore, further studies with a bigger cohort are required to identify radiosensitive patients.

Strong Correlation among Three Biodosimetry Techniques Following Exposures to Ionizing Radiation

Three in vitro dose calibration curves for biodosimetry such as dicentric chromosome assay, fluorescence in situ hybridization (FISH) assay for translocation, and micronuclei (MNs) in binucleated cell assay were established after exposure to ionizing radiation. Peripheral blood lymphocyte samples obtained from healthy donors were irradiated with ⁶⁰Co source at a dose rate of 0.5 Gy/min to doses of 0.1–6 Gy. The results from three in vitro dose calibration curves for biodosimetry were analyzed to understand the relationship among biodosimetry assay techniques. Our comparison demonstrates that there is a very strong positive correlation among the dicentric assay, FISH, and MNs analysis, and these three biodosimetry assays strongly support the in vitro dose reconstruction and the emergency preparedness of public or occupational radiation overexposure.

Frequency of gamma H2AX foci in healthy volunteers and health workers occupationally exposed to X-irradiation and its relevance in biological dosimetry

Gamma-H2AX (γ-H2AX) assay is a marker to measure double-strand breaks in the deoxyribonucleic acid. Variables such as age, oxidative stress, temperature, genetic factors and inter-individual variation have been reported to influence the baseline γ-H2AX focus levels. Therefore, knowledge on baseline frequency of γ-H2AX foci in a targeted population would facilitate reliable radiation triage and dose estimation. The objective of the present study was to establish the baseline data using blood samples from healthy volunteers (n = 130) differing in age, occupation and lifestyle as well as from occupationally exposed health workers (n = 20). The γ-H2AX focus assay was performed using epifluorescence microscopy. In vitro dose-response curve for γ-H2AX foci was constructed in blood samples (n = 3) exposed to X-rays (30 min post-exposure). The mean γ-H2AX focus frequency obtained in healthy volunteers was 0.042 ± 0.001 and showed an age-related increase (p < 0.001). Significantly higher (p < 0.005) focus frequencies were observed in health workers (0.066 ± 0.005) than in healthy volunteers. A sub-group analysis did not show a significant (p > 0.1) difference in γ-H2AX focus frequency among sexes. Blood exposed in vitro to X-rays showed dose-dependent increase in γ-H2AX foci frequency (Y = 0.1902 ± 0.1363 + 2.9020 ± 0.3240 * D). Baseline frequency of γ-H2AX foci obtained from different age groups showed a significant (p < 0.01) influence on the dose-response coefficients. The overall results demonstrated that the γ-H2AX assay can be used as a reliable biomarker for radiation triage and estimating the radiation absorbed dose by considering variables such as age, occupation and lifestyle factors.