Low-dose study finds no effects

Wednesday, 16 May 2012

Mice exposed to low-dose radiation for an extended period showed no signs of DNA damage, said a study from Massachusetts Institute of Technology (MIT) that challenges existing radiation guidelines.

Living cells are constantly bombarded by ionizing radiation in various forms and from various sources. All this has the potential to damage DNA and unless this damage is corrected by self-repair mechanisms it can result in cell malfunction or the malignancy known as cancer. For most people the additional exposure from nuclear weapons detonation, nuclear power operations or nuclear accidents is a tiny fraction of the total, but this is not the case for everyone. The MIT team said their study contributed to asking the question, "How much additional radiation is too much?"

"Instead of being conservative, it makes more sense to look at a best estimate of how hazardous radiation really is."

Jacqueline Yanch, MIT

The effects of radiation on DNA have been clearly shown to be significant for high radiation doses, such as those received by Japanese survivors of atomic bombs. What is less well understood and far harder to study are the effects of lower doses over longer times. The prevailing method to deal with this area of uncertainty is to extrapolate the observable effects of high doses and assume the same relationship applies to low doses with no observable effect i.e. assume that all levels of exposure come with a commensurate health risk, no matter how small. This approach is used in practice as a basis for the management of occupational and public exposure worldwide.

One of the authors of the report was Jacquelyn Yanch. She noted that radioactive contamination can force people from their homes, as seen at Fukushima: "And there you really want to call into question how conservative in your analysis of the radiation effect you want to be. Instead of being conservative, it makes more sense to look at a best estimate of how hazardous radiation really is."

Addressing this, the MIT study exposed one group of mice to low-level radiation for five weeks and compared the effects of this with another group exposed to the same amount of radiation in one burst as well as a third control group exposed only to normal background levels. The researchers saw the low-dose rate group showing "no significant change" in the levels of various kinds of DNA damage compared to the control.

The title of the paper is Integrated molecular analysis indicates undetectable DNA damage in mice after continuous irradiation at ~400-fold natural background radiation. It was published in the journal Environmental Health Perspectives.

MIT said that the DNA strand in each living cell is subject to at least about 10,000 changes per day, but that self-repair mechanisms are able to correct these. Exposure to radiation at 400 times background levels resulted in only around 12 extra changes. "These studies suggest that exposure to continuous radiation at a dose rate that is orders of magnitude higher than background does not significantly impact several key DNA damage and DNA damage responses," said the paper.

The study included 112 mice in total which all received 10.5 cGy (roughly equivalent in biological effect to 105 millisieverts in humans), an amount previously shown to affect DNA when delivered acutely. One group received its dose over a five-week period spent near an iodine-125 source, the other in the course of a 1.4 minute exposure to X-rays. Samples were taken from the animals' blood, spleen, bone marrow and pancreas and analysed for DNA damage and the expression of genes that indicate a response to DNA damage. The exposure from iodine-125 is "reasonable surrogate" for that from the long-term contaminant from nuclear power accidents, caesium-137, said the researchers.

The paper concluded: "Taken together, studies of animals that live under conditions of prolonged continuous exposure to radiation at ~400x background do not show any evidence of increased levels of base damage... nor double strand breaks... nor induction of a DNA damage response... Importantly, when delivered acutely, the same total dose induced micronuclei and induced key genes involved in the DNA damage response."

MIT said the absense of the genes in the low-dose group indicated a threshold for cell morphology and DNA damage responses, something that is absent from current extrapolation-based guidelines.

One of the authors, Bevin Engelward, said that the work provides a framework for additional research and careful evaluation of our current guidelines. "It is interesting that, despite the evacuation of roughly 100,000 residents, the Japanese government was criticised for not imposing evacuations for even more people. From our studies, we would predict that the population that was left behind would not show excess DNA damage - that is something we can test using technologies recently developed in our laboratory," said Engelward.
 
Researched and written
by World Nuclear News