Role of DNA Damage and Epigenetic DNA Methylation Changes in Radiation-Induced Genomic Instability and Bystander Effects in Germline In Vivo

Abstract

Ionizing radiation (IR) is a curative treatment for many human malignancies, an important diagnostic modality, and a pivotal preparative regimen for bone marrow transplantation. On the other hand, IR is a potent damaging agent that can affect a variety of processes in directly exposed cells, in their descendents, and in neighboring un-irradiated naïve 'bystander' cells. Accumulation of DNA damage caused by IR in conjunction with disrupted cellular regulation processes can lead to genome instability in the germline, and therefore to transgenerational genome instability in offspring of exposed males. The exact mechanisms of IR-induced genome instability in directly exposed and in naïve bystander germ cells remain obscure, yet accumulating evidence points to the role of DNA damage and DNA methylation changes in genome instability development. In the current study, we used a well-established murine model to define the role of DNA methylation, DNA damage, as well as two important germline regulators of DNA methylation, Brother of the Regulator of Imprinted Sites (BORIS) and CCCTC binding factor (CTCF), in IR and bystander responses of the male germline. Here we report that irradiation leads to a significant accumulation of DNA damage in the exposed and bystander testis tissue, and to altered DNA methylation and dysregulated BORIS expression in the exposed testis tissue. The possible molecular mechanisms and biological consequences of the observed changes are discussed.

Ionizing radiation-induced metabolic oxidative stress and prolonged cell injury

“…Further, the processing of closely spaced oxidative lesions (oxidatively-induced clustered clustered DNA lesions or OCDLs) can induce DNA double strand breaks, a serious type of DNA lesion that leads to cell death or long-term stressful effects in surviving cells [69; 70; 71]. Increasing evidence indicates that inflammatory cells in circulating blood of patients that received partial body irradiation may also induce DNA damage at sites that are distant from the irradiated target [4; 72], hence contributing to ‘out-of-field’or abscopal effects [5; 73; 74; 75]. Macrophages also secrete cytokines [76] that may perturb physiological functions in normal surrounding cells.…”

“…Hence, the effects of localized energy deposition events in a cell may not be assessed independently of neighboring cells. Whereas, certain genetic and epigenetic changes in targeted and non-targeted cells may be observed shortly after exposure [5; 6; 54; 77], others require several generations to be expressed [10; 78; 79; 80]. Certain effects (e.g.…”

“…However, oxidative changes may continue to arise for days and months after the initial exposure presumably because of continuous generation of reactive oxygen (ROS) and nitrogen (RNS) species [3]. Remarkably, these processes occur not only in the irradiated cells but also in their progeny [2; 4; 5]. Furthermore, radiation-induced oxidative stress may spread from targeted cells to non-targeted bystander cells through intercellular communication mechanisms [6; 7; 8; 9].…”

Are Epigenetic Mechanisms Involved in Radiation-Induced Bystander Effects?

“…Initial theories to explain genomic instability within the existing framework centered around ideas that a mutator phenotype had been activated (Loeb, 2011) but the very high yields of non-clonal genetic damage inducible even by culture medium from irradiated cells made this unlikely as did the persistent nature of the effect which was neither selected out nor ultimately dominant (Seymour and Mothersill, 1988, 1997; Mendonca et al, 1989). It has been suggested that an external epigenetic driver is involved such as oxy-radicals (Hamada et al, 2011), methylation changes (Kaup et al, 2006; Tamminga and Kovalchuk, 2011), or miRNA mediated signaling (Ilnytskyy et al, 2009; Kovalchuk et al, 2010) but because a single exposure to radiation can turn on the process indefinitely both in vitro and in vivo (O’Reilly et al, 1994; Mothersill et al, 2000, 2010) this hypothesis requires the driver to be permanently activated both in time and in space following a single exposure to ultra-low doses of radiation. An important conceptual point here is that signal generation and response to the signal are separate processes and may not both occur in a single system (Vines et al, 2008).…”

Health Risks of Space Exploration: Targeted and Nontargeted Oxidative Injury by High-Charge and High-Energy Particles

“…Importantly, space flight-induced effects on the immune system (204) and the ensuing inflammation and perturbations in oxidative metabolism (91,156,237) increase the risk of cancer (237,295). When macrophages and neutrophils are recruited to sites of inflammation, they generate ROS and RNS, which can cause a large spectrum of oxidative damage in neighboring cells (48,75), hence contributing to out-of-field or abscopal effects (142,192,212,269). Interestingly, low doses/low fluences of protons and HZE particles have been shown to trigger numerical and phenotypic changes in immune/inflammatory cells (112)(113)(114)259) and changes in cytokine levels (236,246) in mice.…”