Profound DNA methylomic differences between single- and multi-fraction alpha irradiations of lung fibroblasts
Alpha-radiation is a ubiquitous environmental agent with epigenotoxic effects. Human exposure to alpha-radiation at potentially harmful levels can occur repetitively over the long term via inhalation of naturally occurring radon gas that accumulates in enclosed spaces, or as a result of a single exposure from a nuclear accident. Alterations in epigenetic DNA methylation (DNAm) have been implicated in normal aging and cancer pathogenesis. Nevertheless, the effects of aberrations in the methylome of human lung cells following exposure to single or multiple alpha-radiation events on these processes remain unexplored. Here, we performed genome-wide DNAm profiling of human embryonic lung fibroblasts from control and irradiated cells using americium-241 alpha sources. Cells were alpha-irradiated in quadruplicates to seven doses using two exposure regimens, a single-fraction (SF) where the total dose was given at once, and a multi-fraction (MF) method, where the total dose was equally distributed over 14 consecutive days. Our results revealed that SF irradiations were prone to a decrease in methylation levels, while MF irradiations mostly increased methylation. The analysis also showed that the gene body (i.e. exons and introns) was the region mostly altered by both the SF hypomethylation and the MF hypermethylation. Additionally, the MF irradiations induced the highest number of differentially methylated regions in genes associated with DNAm biomarkers of aging, carcinogenesis, and cardiovascular disease. The DNAm profile of the oncogenes and tumour suppressor genes suggested that the fibroblasts elicited a defensive response to the MF alpha-irradiation. Key DNAm events of ionizing radiation exposure including changes in methyl levels in mitochondria dysfunction-related genes were mainly identified in the MF groups. However, these alterations were under-represented, indicating that the mitochondria undergo adaptive mechanisms, aside from methylation, in response to radiation-induced oxidative stress. In this study, we identified a contrasting methylomic profile in the lung fibroblasts alpha-irradiated to SF compared with MF exposures. These findings demonstrate that the methylome response of the lung cells to alpha-radiation is highly dependent on both the total dose and the exposure regimen. They also provide novel insights into potential biomarkers of alpha-radiation which may lead to the development of innovative approaches to detect, prevent and treat alpha-particle related diseases.