Fractionated Low-Dose Radiation Exposure Leads to Accumulation of DNA Damage and Profound Alterations in DNA and Histone Methylation in the Murine Thymus

Abstract

Thymus, an important component of hematopoietic tissue, is a well-documented ''target'' of radiation carcinogenesis. Both acute and fractionated irradiation result in a high risk of leukemia and thymic lymphoma. However, the exact mechanisms underlying radiation-induced predisposition to leukemia and lymphoma are still unknown, and the contributions of genetic and epigenetic mechanisms in particular have yet to be defined. Global DNA hypomethylation is a well-known characteristic of cancer cells. Recent studies have also shown that tumor cells undergo prominent changes in histone methylation, particularly a substantial loss of trimethylation of histone H4-Lys 20 and demethylation of genomic DNA. These losses are considered a universal marker of malignant transformation. In the present study, we investigated the effect of low-dose radiation exposure on the accumulation of DNA lesions and alterations of DNA methylation and histone H4-Lys 20 trimethylation in the thymus tissue using an in vivo murine model. For the first time, we show that fractionated whole-body application of 0.5 Gy X-ray leads to decrease in histone H4-Lys 20 trimethylation in the thymus. The loss of histone H4-Lys 20 trimethylation was accompanied by a significant decrease in global DNA methylation as well as the accumulation of DNA damage as monitored by persistence of histone ;H2AX foci in the thymus tissue of mice exposed to fractionated irradiation. Altered DNA methylation was associated with reduced expression of maintenance (DNMT1) and, to a lesser extent, de novo DNA methyltransferase DNMT3a in exposed animals. Expression of another de novo DNA methyltransferase DNMT3b was decreased only in males. Irradiation also resulted in f20% reduction in the levels of methyl-binding proteins MeCP2 and MBD2. Our results show the involvement of epigenetic alterations in radiation-induced responses in vivo. These changes may play a role in genome destabilization that ultimately leads to cancer. (Mol Cancer Res 2005;3(10):553 -61)

Epigenetic changes and nontargeted radiation effects—Is there a link?

“…The DNA hypomethylation observed after irradiation was related to DNA repair . It also correlated with the radiationinduced alterations in the expression of DNA methyltransferases, especially de novo methyltransferases DNMT3a and DNMT3b Pogribny et al, 2005]. Most importantly, the radiation-induced global genome DNA hypomethylation appeared to be linked to genome instability in the exposed tissue Raiche et al, 2004;Loree et al, 2006].…”

Global DNA Hypomethylation in Liver Cancer Cases and Controls: A Phase I Preclinical Biomarker Development Study

“…18 Endogenous factors acting through three secondary pathways related to decreased DNA methyltransferase expression, 19,20 non-coding RNA silencing 21,22 and defective DNA repair, 23,24 may also have a direct causal role in global DNA hypomethylation. Factors that activate these three endogenous pathways may lead directly to a loss of global DNA methylation, may also cause chromatin modifications leading to hypomethylation, or may only be an intermediate step leading to histones modifications that are linked to global DNA hypomethylation.…”

The study of hypermethylation in blood leukocytes of irradiated parents and their children

“…For example, observed epigenetic changes were mostly in DNA hypermethylation at repetitive elements SAT2 and MLT1A in progeny of irradiated HPV-G cells after 20 passages, but hypomethylation, which is not identified with respect to the sequence context, was also detected [6]. Reduction of various epigenetic indicators (the levels of global genomic hypomethylation, trimethylation of histone H4-Lys20, methyltransferases Dnmt1 and Dnmt3a/b, methyl-binding proteins MeCP2 and MBD2) has been observed in experiments using rodents exposed to acute (2.5 Gy) or chronic (0.5 Gy) irradiation [7,8]. Hypermethylation of the p16 tumorsupressor gene in mouse exposed to chronic low-dose radiation (X-ray, 0.5 Gy, 10 days) was revealed.…”

“…For example, it was revealed that there are numerous epigenetic differences in MZ twins and that such epigenetic variation can serve as a new source of putative molecular substrates for discordance of MZ twins [19]. A number of environmental factors, both chemical and radiation have been found to alter the epigenome, including DNA methylation [5][6][7][8][9][10]. It should be emphasized that most of the investigations devoted to evaluation of the epigenetic effects of radiation (X -rays and γ -rays, range of doses of 0.1 to 2.5 Gy) was performed using cell cloning techniques.…”

“…Сhanges in DNA methylation patterns include global hypomethylation, genespecific hypermethylation/ hypomethylation and loss of imprinting. The reality of these epigenetic modifications, induced by radiation, is shown in experiments using cell cultures and different animal models [5][6][7][8][9]. The bulk of the research is devoted to the assessment of DNA methylation in long interspersed nuclear element-1 (LINE-1) and Alu repetitive elements as a surrogate of genome-wide methylation.…”