Low Dose Radiation-Induced Epigenetic Alterations Found in Agouti Mouse Model
In a paper published in The FASEB Journal, Low Dose researchers from Duke University, University of Wisconsin-Madison, and the University of Michigan showed that exposures to low-doses of ionizing radiation (LDIR) in utero can alter the epigenetic response of the agouti viable yellow (Avy) mouse. This alteration resulted in increased DNA methylation, which plays an important role for epigenetic gene regulation in development and disease. It also increased the frequency of offspring showing the agouti-brown coat color.
This study is the first to demonstrate that LDIR exposures of embryonic stem (ES) cells during early development cause both dose- and sex-dependent epigenetically induced adaptive changes at the Avy locus of this mouse. The epigenetic effects were more pronounced in the male offspring. The researchers also determined that that this response is in part dependent on a cellular oxidative stress response.
At the lowest exposure dose of 0.4 cGy, offspring coat color distribution was not significantly altered from that of sham-irradiated control offspring. In contrast, graded exposures from 0.7 to 7.6 cGy significantly shifted the coat color distribution of the offspring toward heavily mottled and brown.
Twice as many offspring irradiated with low doses had brown coats than observed in the control offspring for each dose. At 7.6 cGy, there were significantly more heavily mottled animals than in the control offspring, whereas the incidence of brown offspring was more comparable to that observed in the control offspring. This indicated that at the highest dose used in this study, the coat color distribution was returning to that of the controls.
The researchers also showed that antioxidants given as supplements to the pregnant mice blocked the shift toward brown coat color from low doses of radiation and the potential for a positive adaptive response to low doses of radiation. The manuscript also discusses previous literature showing that hypomethylation of DNA at high doses of radiation and hypermethylation at low doses indicates that the mechanisms of action are different as a function of radiation dose. This would suggest that the shape of the dose response for this response is nonlinear and would be sublinear in the low dose region.
Why it matters: Humans are exposed to low-dose ionizing radiation (LDIR) from a number of environmental and medical sources. In addition to inducing genetic mutations, there is concern that LDIR may also alter the epigenome—the chemical compounds that modify, or mark, the genome. These epigenetic changes could result in detrimental biological outcomes, and this has been demonstrated for a number of chemical compounds. Such heritable effects early in life can result in phenotypes that are either detrimental or potentially protective against subsequent environmental exposures.
Several experimental diets were able to shift the frequency of these two different phenotypes in the offspring with more yellow mice being produced. Thus, according to lead researcher Dr. Randy Jirtle, University of Wisconsin-Madison, "This unique mouse strain is an exquisitely sensitive biosensor for environmental agents that alter the fetal epigenome."
In these studies the radiation exposure in the low dose region produced an increase in the frequency of brown offspring. These mice have a lower frequency of obesity, diabetes, and cancer than those that are yellow, suggesting that low doses of radiation alter the epigenome in a way that is beneficial to the offspring in this mouse strain.
Says Jirtle, "Because the epigenome varies markedly between species, the effect of LDIR on the epigenome in multiple generations needs to be defined in humans." This may be possible using some of the stored tissues from human exposures.
Methods: The scientists exposed pregnant mice to LDIR of 0.4, 0.7, 1.4, 3.0, or 7.6 cGy. The exposure was scheduled at the time when the animals were most sensitive to chemically induced epigenetic changes. Some of the mice were also fed a diet high in antioxidants starting a week before they were mated to agouti mice of varying coat colors. This made it possible to evaluate the influence of reactive oxidative stress on the radiation-induced epigenetic changes in both male and female offspring. The offspring were weaned 22 days after birth, and categorized by five coat color classes: yellow, slightly mottled, mottled, heavily mottled, and brown. A control group was treated the same but was sham-irradiated.
The epigenetic changes induced in the coat color influenced the risk for development of obesity, diabetes, and cancer. The level of methylation was measured in the DNA to determine if the DNA was hyper- or hypo-methylated, which can also influence cancer risk.
Reference: Bernal AJ, DC Dolinoy, D Huang, DA Skaar, C Weinhouse, and RL Jirtle. 2013. "Adaptive radiation-induced epigenetic alterations mitigated by antioxidants." The FASEB Journal 27:665-671. DOI: 10.1096/fj.12-220350.