Cellular effects of ionizing radiation : Relevant for understanding cancer risk after medical and environmental radiation exposures

Abstract: Radiation-induced cancers are stochastic and delayed effects of exposure to ionizing radiation. The dose-response relationship for radiation-induced cancers at both low dose/low dose rates and high doses (doses encountered during radiotherapy) remains unclear. Uncertainties observed in epidemiological studies at low doses and dose rates hamper cancer risk estimation at this dose level. Assessing dose-response relationships for radiotherapy-induced cancers is also complicated due to the inherent difficulty in assessing the doses absorbed by tissues at the site of tumours. In addition, the modulatory effect of chemotherapy on the incidence of radiotherapy-induced cancer risk has been debated. Although included as a modifying factor of the incidence of radiotherapy-induced cancers, results from epidemiological studies do not provide sufficient evidence to support this claim. This thesis summarizes studies conducted to improve the understanding of the association of cancer incidence and radiation dose at clinically relevant (high) doses, low doses and low dose rates, as well as the modulatory role of platinum-based chemotherapy on radiation-induced carcinogenesis. In Paper I, we investigated the competitive relationship between cell killing and the accumulation of DNA damage and genomic instability using two normal cell types (VH10 fibroblasts and AHH-1 lymphoblasts). Dose fractionation schemes were designed based on the cell growth characteristics of each cell type. Cells were irradiated at 0.25, 0.5, 1.0, or 2 Gy per fraction, representing the various dose levels within a radiation field, to simulate the heterogeneous dose distribution across normal tissue during radiotherapy. Following fractionated radiation exposure, the effects on cell growth, cell survival, radiosensitivity, and the accumulation of residual DNA damage and genomic instability were analyzed as a function of dose per fraction and the total absorbed dose. The accumulation of DNA damage and markers of genomic instability associated with DNA damage depended on cell type-specific factors.In Paper II, we investigated the modulatory effects of combining cisplatin and radiation on the accumulation of micronuclei (a biomarker of DNA damage and carcinogenesis) in peripheral blood lymphocytes of patients receiving treatment for gynaecological cancers. We also determined the modulatory effects of the combination of both agents on cell death and cell proliferation, by scoring the frequency of apoptotic and binucleated cells. We compared the frequency of these markers between patients receiving treatment with radiotherapy alone and a combination of cisplatin and radiotherapy. There was a decline in the frequency of micronuclei in patients receiving a combination of cisplatin and radiotherapy.We conducted in vitro experiments in Paper III using AHH-1 and VH10 cells. We investigated the effects of the concurrent combination of cisplatin treatment and multifractionated radiation exposure at 1 Gy per fraction on cell growth, cell survival, cell death, changes in radiosensitivity, accumulation of DNA damage, and other markers of genomic instability as well as the expression of cancer stem cell markers. We also investigated the interaction between cisplatin and radiation exposure in our schedule. The concurrent combination of cisplatin and radiation did not increase the accumulation of markers of genomic instability.In Paper IV, we investigated the short and long-term effects of radiation exposure at low doses and low dose rates on global gene expression, cell growth and cell survival of VH10 fibroblasts to identify unique dose rate signatures that could be useful biomarkers in determining if the application of DDREF is accurate. Except for the differential expression of DMXL2, the long-term effects of LDLDR exposure on global gene expression, cell growth and cell survival of VH10 fibroblasts were negligible. These results suggest that the accumulation of DNA damage and other markers of genomic instability is regulated by cell type-specific factors at these dose levels.