Fallout at Fukushima -- Part 2

Precursor B-cell acute lymphoblastic leukemia
Image: Wikimedia Commons, VashiDonsk

Because it packs enough energy to expulse electrons from atoms and molecules, ionizing radiation can directly damage biomolecules such as DNA, breaking chemical bonds and inducing single and double-strand breaks. When this damage is not efficiently fixed by the DNA-repair machinery, mutations arise: Mutations in somatic cells can lead to cancer, and in germ line cells, genetic defects can be transmitted to offspring. Indeed, radiation from Chernobyl and the atomic bombs dropped on Japan in the middle of last century has been linked to a wide variety of cancers. The question is, are the workers at Japan's Fukushima site being exposed to levels of radiation that pose a serious genetic risk?Cancer risksDespite several long-term studies of the atomic bomb survivors in Japan, radiotherapy patients, and people affected by the Chernobyl disaster -- which have linked exposure to ionizing radiation with leukemia, thyroid, breast, lung, skin, and stomach cancers, among others -- quantifying the specific radiation dose that increases cancer risk is still a point of contention among experts. An increased risk of cancer is epidemiologically detectable starting at exposures of 150 to 200 millisieverts (mSv) of radiation, said linkurl:John Boice,;http://www.vicc.org/dd/display.php?person=boicej_compuserve.com cancer epidemiologist at The Vanderbilt-Ingram Cancer Center and former chief of the Radiation Epidemiology Branch at the National Cancer Institute. This is a concern for the Fukushima plant workers, who may have been exposed to levels ranging from 200-400 mSv per hour, according to linkurl:reports.;http://seattletimes.nwsource.com/html/nationworld/2014516967_quakeradiation16.htmlAlthough ionizing radiation is perhaps the most extensively studied carcinogen, it is actually a weak one. Rather than inducing point mutations, high doses of ionizing radiation usually cause chromosome rearrangements, breaks, and deletions, often killing the cells well before they have a chance to become cancerous.A long-term linkurl:study;http://www.ncbi.nlm.nih.gov/pubmed/12968934 of 86,000 atomic bomb survivors in Hiroshima and Nagasaki, who had been exposed to an average of 170 mSv, with some exposures ranging upward of 3,000 mSv, found that although "there had been 10,000 deaths related to cancer, only 500 were related to the radiation that was received," Boice said.And in the most recent linkurl:assessment;http://www.unscear.org/unscear/en/chernobyl.html of the Chernobyl accident, the United Nations Scientific Committee on the Effects of Atomic Radiation reported that, aside from an increase in thyroid cancers -- largely caused by the ingestion of food-borne radioactive iodine isotopes scattered by the reactor explosion -- "there is no scientific evidence of increases in overall cancer incidence or mortality rates" as a result of the radiation leak.But more evidence of a link between cancer and the Chernobyl may accumulate over time. With the exception of leukemia, which can develop as quickly as two years after significant exposure to ionizing radiation due to the extreme sensitivity of hematopoetic stem cells in the bone marrow, it usually takes decades before radiation damage manifests as cancer.This risk also increases with every cell division because mutations have a chance to accumulate, explained linkurl:Peter Dedon,;http://dedon.mit.edu/index.html professor of toxicology and biological engineering and member of the Radiation Protection Committee at MIT. As a result, undifferentiated and actively dividing cells, such as stem cells, blood precursor cells, hair follicles, and the gut epithelium, are particularly sensitive to ionizing radiation. For the same reason, age at the time of exposure is often a critical determinant of cancer risk: Because the cells of younger people are proliferating more actively than adults, children are much more vulnerable to the cancer risks imposed by radiation exposure. -- Cristina LuiggiHereditary effectsFollowing the detonation of atomic bombs of Hiroshima and Nagasaki, there was fear that survivors had DNA mutations in their germ cells. Such mutations could theoretically be transmitted from generation to generation and lead to serious birth defects, noted linkurl:Kenneth Weiss,;http://www.anthro.psu.edu/faculty_staff/weiss.shtml a professor of anthropology and genetics at Penn State University. If germ line mutations did result from the dropping of the atomic bombs, it would put a significant genetic burden on Japanese society.But that did not happen. In a 1948-1953 linkurl:study;http://www.rerf.jp/radefx/genetics_e/birthdef.html of over 75,000 children of atomic bomb survivors, there was no statistically significant increase in birth defects, and no documented relation between parental radiation dose and birth weight, prematurity, sex ratio, or neonatal death. In another study, Weiss and colleagues found no elevated frequency of protein-coding mutations in the offspring of bomb survivors, as compared to unexposed Japanese and individuals in the Amazon basin, half a globe away. "There was basically no difference in the number of detected mutations," said Weiss. One possible explanation for the lack of trans-generation genetic effects could be the tiny target -- testes and ovaries are quite small in comparison to the rest of the body, so the amount of radiation hitting those cells will be relatively less than other major organs, said Weiss. In addition, major mutations caused by radiation could simply kill a germ cell, or cause a very early miscarriage, of which a woman might not even be aware.For current individuals at risk of radiation exposure in Japan, the type of radiation exposure is dramatically different than that experienced after the atomic bombs, noted Weiss. In the Fukushima area, radioactive isotopes are most likely to be inhaled or ingested. Such exposure does not pose a risk to germ line cells, said Weiss. "Most of the radiation coming off these particles probably wouldn't even penetrate the skin." While particles could lodge in mucus membranes, such as the lungs, and cause mutations there, it is unlikely that there will be damage to the gametes of exposed individuals, he said.-- Megan Scudellari
**__Related stories:__*** linkurl:Fallout at Fukushima;http://www.the-scientist.com/news/display/58085/
[22nd March 2011]*linkurl:Normal today, cancer tomorrow;http://www.the-scientist.com/news/display/57907/
[6th January 2011]

**__Related F1000 Evaluations:__***linkurl:Exposure to low-dose ionizing radiation from medical imaging procedures;http://f1000.com/1166932?key=h68z8cxn689tvpn
R. Fazel et al., N Engl J Med, 361:849-57, 2009. Evaluated by Luna Gargani and Eugenio Picano, Institute of Clinical Physiology of CNR, Italy.*linkurl:Chromosomal instability in unirradiated hematopoietic cells induced by macrophages exposed in vivo to ionizing radiation;http://f1000.com/1124431?key=dd63lgc8qxmhf88
S.A. Lorimore, Cancer Res, 68:8122-6, 2008. Evaluated by Len Neckers National Cancer Institute, NIH.

Fallout at Fukushima -- Part 2

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