Double strand break induction and kinetics indicate preserved hypersensitivity in keratinocytes to subtherapeutic doses for 7weeks of radiotherapy

The Effect of High-Dose-Rate Pulsed Radiation on the Survival of Clinically Relevant Radioresistant Cells

We demonstrated that low dose pulsed radiation (0.25 Gy) at a high-dose-rate, even for very short intervals (10 s), decreases cell survival to a greater extent than single exposure to a similar total dose and dose rate. The objective of this study was to clarify whether high-dose-rate pulsed radiation is effective against SAS-R, a clinically relevant radioresistant cell line. Cell survival following high-dose-rate pulsed radiation was evaluated via a colony assay. Flow cytometry was utilized to evaluate γH2AX, a molecular marker of DNA double-strand breaks and delayed reactive oxygen species (ROS) associated with radiation-induced apoptosis. Increased cytotoxicity was observed in SAS-R and parent SAS cells in response to high dose rate pulsed radiation compared to single dose, as determined by colony assays. Residual γH2AX in both cells subjected to high-dose-rate pulsed radiation showed a tendency to increase, with a significant increase observed in SAS cells at 72 h. In addition, high-dose-rate pulsed radiation increased delayed ROS more than the single exposure did. These results indicate that high-dose-rate pulsed radiation was associated with residual γH2AX and delayed ROS, and high-dose-rate pulsed radiation may be used as an effective radiotherapy procedure against radioresistant cells.

Radiation Biomarkers in Large Scale Human Health Effects Studies

Following recent developments, the RENEB network (Running the European Network of biological dosimetry and physical retrospective dosimetry) is in an excellent position to carry out large scale molecular epidemiological studies of ionizing radiation effects, with validated expertise in the dicentric, fluorescent in situ hybridization (FISH)-translocation, micronucleus, premature chromosome condensation, gamma-H2AX foci and gene expression assays. Large scale human health effects studies present complex challenges such as the practical aspects of sample logistics, assay costs, effort, effect modifiers and quality control/assurance measures. At Public Health England, the dicentric, automated micronucleus and gamma-H2AX radiation-induced foci assays have been tested for use in a large health effects study. The results of the study and the experience gained in carrying out such a large scale investigation provide valuable information that could help minimise random and systematic errors in biomarker data sets for health surveillance analyses going forward.

Exposure of Human Skin Organoids to Low Genotoxic Stress Can Promote Epithelial-to-Mesenchymal Transition in Regenerating Keratinocyte Precursor Cells

For the general population, medical diagnosis is a major cause of exposure to low genotoxic stress, as various imaging techniques deliver low doses of ionizing radiation. Our study investigated the consequences of low genotoxic stress on a keratinocyte precursor fraction that includes stem and progenitor cells, which are at risk for carcinoma development. Human skin organoids were bioengineered according to a clinically-relevant model, exposed to a single 50 mGy dose of γ rays, and then xeno-transplanted in nude mice to follow full epidermis generation in an in vivo context. Twenty days post-xenografting, mature skin grafts were sampled and analyzed by semi-quantitative immuno-histochemical methods. Pre-transplantation exposure to 50 mGy of immature human skin organoids did not compromise engraftment, but half of xenografts generated from irradiated precursors exhibited areas displaying focal dysplasia, originating from the basal layer of the epidermis. Characteristics of epithelial-to-mesenchymal transition (EMT) were documented in these dysplastic areas, including loss of basal cell polarity and cohesiveness, epithelial marker decreases, ectopic expression of the mesenchymal marker α-SMA and expression of the EMT promoter ZEB1. Taken together, these data show that a very low level of radiative stress in regenerating keratinocyte stem and precursor cells can induce a micro-environment that may constitute a favorable context for long-term carcinogenesis.

Epidermal Keratinocyte Depletion during Five Weeks of Radiotherapy is Associated with DNA Double-Strand Break Foci, Cell Growth Arrest and Apoptosis: Evidence of Increasing Radioresponsiveness and Lack of Repopulation; the Number of Melanocytes Remains Unchanged

During fractionated radiotherapy, epithelial cell populations are thought to decrease initially, followed by accelerated repopulation to compensate cell loss. However, previous findings in skin with daily 1.1 Gy dose fractions indicate continued and increasing cell depletion. Here we investigated epidermal keratinocyte response with daily 2 Gy fractions as well as accelerated and hypofractionation. Epidermal interfollicular melanocytes were also assessed. Skin-punch biopsies were collected from breast cancer patients before, during and after mastectomy radiotherapy to the thoracic wall with daily 2 Gy fractions for 5 weeks. In addition, 2.4 Gy radiotherapy four times per week and 4 Gy fractions twice per week for 5 weeks, and two times 2 Gy daily for 2.5 weeks, were used. Basal keratinocyte density of the interfollicular epidermis was determined and immunostainings of keratinocytes for DNA double-strand break (DSB) foci, growth arrest, apoptosis and mitosis were quantified. In addition, interfollicular melanocytes were counted. Initially minimal keratinocyte loss was observed followed by pronounced depletion during the second half of treatment and full recovery at 2 weeks post treatment. DSB foci per cell peaked towards the end of treatment. p21-stained cell counts increased during radiotherapy, especially the second half. Apoptotic frequency was low throughout radiotherapy but increased at treatment end. Mitotic cell count was significantly suppressed throughout radiotherapy and did not recover during weekend treatment gaps, but increased more than threefold compared to unexposed skin 2 weeks post-radiotherapy. The number of melanocytes remained constant over the study period. Germinal keratinocyte loss rate increased gradually during daily 2 Gy fractions for 5 weeks, and similarly for hypofractionation. DSB foci number after 2 Gy irradiation revealed an initial radioresistance followed by increasing radiosensitivity. Growth arrest mediated by p21 strongly suggests that cells within or recruited into the cell cycle during treatment are at high risk of loss and do not contribute significantly to repopulation. It is possible that quiescent (G₀) cells at treatment completion accounted for the accelerated post-treatment repopulation. Recent knowledge of epidermal tissue regeneration and cell cycle progression during genotoxic and mitogen stress allows for a credible explanation of the current finding. Melanocytes were radioresistant regarding cell depletion.

Radiation Biology and Its Role in the Canadian Radiation Protection Framework

The linear no-threshold (linear-non-threshold) model is a dose-response model that has long served as the foundation of the international radiation protection framework, which includes the Canadian regulatory framework. Its purpose is to inform the choice of appropriate dose limits and subsequent as low as reasonably achievable requirements, social and economic factors taken into account. The linear no-threshold model assumes that the risk of developing cancer increases proportionately with increasing radiation dose. The linear no-threshold model has historically been applied by extrapolating the risk of cancer at high doses (>1,000 mSv) down to low doses in a linear manner. As the health effects of radiation exposure at low doses remain ambiguous, reducing uncertainties found in cancer risk dose-response models can be achieved through in vitro and animal-based studies. The purpose of this critical review is to analyze whether the linear no-threshold model is still applicable for use by modern nuclear regulators for radiation protection purposes, or if there is sufficient scientific evidence supporting an alternate model from which to derive regulatory dose limits.

UV-Radiation Response Proteins Reveal Undifferentiated Cutaneous Interfollicular Melanocytes with Hyperradiosensitivity to Differentiation at 0.05 Gy Radiotherapy Dose Fractions

To date, the response activated in melanocytes by repeated genotoxic insults from radiotherapy has not been explored. We hypothesized that the molecular pathways involved in the response of melanocytes to ionizing radiation and ultraviolet radiation (UVR) are similar. Skin punch biopsies, not sun-exposed, were collected from prostate cancer patients before, as well as at 1 and 6.5 weeks after daily doses of 0.05-1.1 Gy. Interfollicular melanocytes were identified by ΔNp63- and eosin-periodic acid Schiff staining. Immunohistochemistry and immunofluorescence were performed to detect molecular markers of the melanocyte lineage. Melanocytes were negative for ΔNp63, and the number remained unchanged over the treatment period. At radiation doses as low as 0.05 Gy, melanocytes express higher protein levels of microphthalmia-associated transcription factor (MITF) and Bcl-2. Subsets of MITF- and Bcl-2-negative melanocytes were identified among interfollicular melanocytes in unexposed skin; the cell number in both subsets was reduced after irradiation in a way that indicates low-dose hyperradiosensitivity. A corresponding increase in MITF- and Bcl-2-positive cells was observed. PAX3 and SOX10 co-localized to some extent with MITF in unexposed skin, more so with radiation exposure. Low doses of ionizing radiation also intensified c-KIT and DCT staining. Nuclear p53 and p21 were undetectable in melanocytes. Apoptosis and proliferation could not be observed. In conclusion, undifferentiated interfollicular melanocytes were identified, and responded with differentiation in a hypersensitive manner at 0.05 Gy doses. Radioresistance regarding cell death was maintained up to fractionated doses of 1.1 Gy, applied for 7 weeks. The results suggest that the initial steps of melanin synthesis are common to ionizing radiation and UVR, and underline the importance of keratinocyte-melanocyte interaction behind hyperpigmentation and depigmentation to radiotherapy.