DNA methylation as a potential biomarker for radiation exposure

Abstract

Epigenetic changes, including DNA methylation, microRNA expression and histone modification, are heritable and stable alterations in regulation of gene transcription. Although the changes are required for normal health and development, they can also contribute to diseases including cancers. The epigenetic changes could be a missing link among radiation exposure, radiation-induced genomic instability, and radiation-induced carcinogenesis. While recent studies have shown that ionizing radiation (IR) can affect epigenetic changes in animal and cancer cell lines, there is a lack of information on epigenetic alterations following irradiation and involved mechanisms in human. This study aimed to evaluate DNA methylation changes, one of the epigenetic alterations, in normal human blood irradiated with gamma rays in vitro and investigate the effects of low-dose radiation exposure on DNA methylation in peripheral blood DNA from nuclear power plant workers. I also evaluated whether radiation-induced DNA methylation states were associated with genomic instability. I evaluated global DNA methylation levels using DNA methylation quantification kits and examined the methylation levels of specific locus and repeat element such as long interspersed element-1 (LINE-1) and Satellite 2 (Sat2) by MethyLight assay in human blood. I also evaluated the association between DNA methylation and the frequency of micronuclei or chromosomal aberrations, indicators of genomic instability in radiation exposed workers. The expression of DNA methyltransferases (DNMTs) was measured by using quantitative real-time PCR to investigate the mechanisms of radiation-induced DNA methylation alterations. In vitro study revealed that g-irradiation led to global DNA hypomethylation in normal human leukocytes and the expression of DNMTs was decreased with g-irradiation. These results indicated that global hypomethylation detected in this study may be the consequence of decreased DNMTs expression by g-irradiation. LINE-1 hypermethylation after 0.5, 1, and 2 Gy of g irradiation was observed in g-ray exposed blood. It was also observed to be significantly methylated in Mut L homologue-1 (MLH1) promoter region in cells irradiated with 2Gy g radiation, and the expression of MLH1 mRNA was also decreased. This study found that global DNA methylation levels were lower in radiation-exposed workers than in controls. The methylation levels were negatively associated with recent 1.5 year radiation dose, while the levels were increased with total cumulative dose in radiation-exposed workers. Global DNA methylation levels were increased with DNMT1 expression, and the expression levels were correlated with recent 1.5 year radiation dose. These findings suggest that recent 1.5 year radiation exposure caused a decrease of DNMT1 expression, which resulted in global DNA hypomethylation. LINE-1 methylation levels were higher in radiation-exposed workers than in controls, while Sat2 methylation levels had no significant differences between workers and controls. Total cumulative radiation dose was significantly associated with LINE-1 methylation levels after adjusting for age, smoking status and alcohol intake in radiation-exposed workers. Global DNA methylation levels were also correlated with chromosomal aberrations in workers, while the association was not found in controls. In contrary to this result, the frequency of micronuclei was not correlated with global DNA methylation and repeat element methylation levels. This finding suggests that global DNA hypomethylation is associated with radiation-induced double strand breaks. These results support the hypothesis that exposure to ionizing radiation may influence DNA methylation in human blood. These findings of this study suggest the possible use of DNA methylation levels as a potential biomarker to monitor IR exposure for radiation-related workers. In addition, the findings of this study support a link between global DNA methylation and genomic instability in radiation-exposed individuals.