DNA methylation changes in cells regrowing after fractioned ionizing radiation

Background and purpose: Repeated exposure to ionizing radiation (IR) can result in adaptive reactions. While DNA methylation changes in adaption to repeated stress exposure are established for a variety of drugs, their role in fractioned ionizing radiation is largely unknown. Material and methods: MCF7 breast cancer cells were treated 5 times a week with IR in fractions of 2 Gy, resulting in total doses of 10 and 20 Gy. Cells were harvested 48 and 72 h after the last irradiation, as well as after a recovery period of at least 14 d. To identify genes differentially methylated in irradiated versus non-irradiated cells, we used methyl-CpG immunoprecipitation (MCIp) followed by global methylation profiling on CpG island microarrays. Results: MCIp profiling revealed methylation changes in several CpG islands 48 h after FIR with 10 and 20 Gy. Cells receiving a total dose of 10 Gy started regrowing after 14 d and exhibited similar radioresistance as mock-treated cells. Differential methylation of the CpG units associated with FOXC1 (p < 0.001) and TRAPPC9 (p < 0.001) could be confirmed by time-of-flight mass spectrometry (Sequenom). Conclusions: In summary, these data indicate that regrowth of MCF7 cells after 10 Gy FIR is associated with locus-specific alterations in DNA methylation. (C) 2011 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 101 (2011) 116-121