Direct radiation exposure caused DNA breaks that led to thyroid cancer, but didn't impact future children. Radiation exposure from the 1986 Chernobyl disaster — the world's deadliest nuclear accident — raised the risk of certain mutations linked to thyroid cancer, but it didn't cause new mutations in DNA that parents who cleaned up after the nuclear accident passed along to their children, two new studies find. The new research is a step forward in understanding the mechanisms that drive human thyroid cancer, said Stephen Chanock, the director of the division of cancer epidemiology and genetics at the U.S. National Cancer Institute (NCI) and the senior author on both research papers. It's also reassuring for those exposed to radiation in events such as the 2011 Fukushima nuclear power plant disaster and who plan to start families, Chanock told Live Science. "People who had very high-dose radiation didn't have more mutations in the next generation," he said. "That's telling us that if there's any effect it's very, very subtle and very rare." Chernobyl fallout The nuclear accident at Chernobyl that occured on April 26, 1986, exposed residents of Ukraine, Belarus and the nearby Russian Federation to a cloud of radioactive contamination. Epidemiological research has shown that those exposed had a higher risk than the unexposed for a particular kind of thyroid cancer called papillary thyroid carcinoma. (Fortunately, this type of cancer is treatable and has a high survival rate, according to the American Thyroid Association.) The younger the person is at time of radiation exposure, the higher the risk of developing papillary thyroid carcinoma in the future. In the new study, Chanock and his colleagues analyzed tissue from the thyroid carcinoma tumors held in the Chernobyl Tissue Bank, comparing the genetics of tumors from 359 people who were exposed to Chernobyl radiation before adulthood with that of tumors from people from the same region who were born more than nine months after the Chernobyl accident and thus not directly exposed. Radiation exposure in these individuals was well-studied, so researchers could determine not only if a person had radiation exposure, but how much. The researchers found that with more radiation exposure, tumor tissue showed higher levels of double-stranded DNA breaks, in which the two strands that make up DNA snap at the same point. Cells have repair mechanisms to fix such breaks, but the findings showed that the tumors had errors in these repair mechanisms, too, particularly one called non-homologous end-joining (NEHJ). "They have just one major error that drives the cancer," Chanock said, adding that this was the first time that researchers have been able to identify such a driver in a human cancer. These errors aren't unique to radiation-caused cancers, Chanock said. The same mutations occurred in non-exposed people with tumors, just at a lower rate. They also occur in other types of cancer along with additional mutations, Chanock said. For that reason, he is hopeful that the results could lead to new drug studies that target these genes and the cellular processes they direct. The next generation In a second study, researchers looked for possible multigenerational effects of radiation exposure. Previous studies on atomic bomb survivors from Hiroshima and Nagasaki have not found evidence of major congenital defects, stillbirth or newborn deaths in babies conceived after the exposure, though a recent re-analysis of the data suggests the possibility of increased risk. The current study focused on living children of a group known as the liquidators —— people who worked at the plant to clean up the radioactive mess in the months after the disaster. Researchers sequenced the entire genomes of 130 children born between 1987 and 2002 to these individuals, who were exposed to very high radiation levels. The research team was looking for de novo mutations, or totally new genetic mutations found in the child's DNA that were not in either parents' genome. Finding an increase in genetic mutations found in the child but not the parents would suggest that radiation was damaging the sperm or the egg. Finding no increase in de novo mutations would suggest that children largely escape damage to their DNA from their parents' exposure. Between 50 and 100 of these mutations occur naturally in each generation, and the results showed that the mutations occurred at a similar rate in children of Chernobyl liquidators. There was no effect of radiation. "This is extraordinary work," said Daniel Stram, a professor of preventive medicine at the Keck School of Medicine at the University of Southern California who was not involved in the research. "It's really bringing together the genetic side and the radiation epidemiology side." There are long-standing concerns that radiation exposure from work or from cancer therapies might affect future children, but the new research is reassuring, Stram told Live Science. "People have talked about doing this kind of work for decades," he said. "It's only now that we have the technology to be able to actually address the questions." Source
