BACKGROUND: In their previous analysis of papillary thyroid carcinomas (PTCs) from an Ukrainian‐American cohort that was exposed to iodine‐131 (¹³¹I) from the Chernobyl accident, the authors identified RET/PTC rearrangements and other driver mutations in 60% of tumors.METHODS: In this study, the remaining mutation‐negative tumors from that cohort were analyzed using RNA sequencing (RNA‐Seq) and reverse transcriptase‐polymerase chain reaction to identify novel chromosomal rearrangements and to characterize their relation with radiation dose.RESULTS: The ETS variant gene 6 (ETV6)‐neurotrophin receptor 3 (NTRK3) rearrangement (ETV6‐NTRK3) was identified by RNA‐Seq in a tumor from a patient who received a high ¹³¹I dose. Overall, the rearrangement was detected in 9 of 62 (14.5%) post‐Chernobyl PTCs and in 3 of 151 (2%) sporadic PTCs (P = .019). The most common fusion type was between exon 4 of ETV6 and exon 14 of NTRK3. The prevalence of ETV6‐NTRK3 rearrangement in post‐Chernobyl PTCs was associated with increasing ¹³¹I dose, albeit at borderline significance (P = .126). The group of rearrangement‐positive PTCs (ETV6‐NTRK3, RET/PTC, PAX8‐PPARγ) was associated with significantly higher dose response compared with the group of PTCs with point mutations (BRAF, RAS; P < .001). In vitro exposure of human thyroid cells to 1 gray of ¹³¹I and γ‐radiation resulted in the formation of ETV6‐NTRK3 rearrangement at a rate of 7.9 × 10⁻⁶ cells and 3.0 × 10⁻⁶ cells, respectively.CONCLUSIONS: The authors report the occurrence of ETV6‐NTRK3 rearrangements in thyroid cancer and demonstrate that this rearrangement is significantly more common in tumors associated with exposure to ¹³¹I and has a borderline significant dose response. Moreover, ETV6‐NTRK3 rearrangement can be directly induced in thyroid cells by ionizing radiation in vitro and, thus, may represent a novel mechanism of radiation‐induced carcinogenesis. Cancer 2014;120:799–807. © 2013 American Cancer Society.

Exposure to ionizing radiation during childhood markedly increases the risk of developing papillary thyroid cancer. We examined tissues from 26 Ukrainian patients with thyroid cancer who were younger than 10 years of age and living in contaminated areas during the time of the Chernobyl nuclear reactor accident. We identified nonoverlapping somatic driver mutations in all 26 cases through candidate gene assays and next-generation RNA sequencing. We found that 22 tumors harbored fusion oncogenes that arose primarily through intrachromosomal rearrangements. Altogether, 23 of the oncogenic drivers identified in this cohort aberrantly activate MAPK signaling, including the 2 somatic rearrangements resulting in fusion of transcription factor ETS variant 6 (ETV6) with neurotrophic tyrosine kinase receptor, type 3 (NTRK3) and fusion of acylglycerol kinase (AGK) with BRAF. Two other tumors harbored distinct fusions leading to overexpression of the nuclear receptor PPARγ. Fusion oncogenes were less prevalent in tumors from a cohort of children with pediatric thyroid cancers that had not been exposed to radiation but were from the same geographical regions. Radiation-induced thyroid cancers provide a paradigm of tumorigenesis driven by fusion oncogenes that activate MAPK signaling or, less frequently, a PPARγ-driven transcriptional program.

Background: The Chornobyl accident in 1986 exposed thousands of people to radioactive iodine isotopes, particularly ¹³¹ I; this exposure was followed by a large increase in thyroid cancer among those exposed as children and adolescents, particularly in Belarus, the Russian Federation, and Ukraine. Here we report the results of the first cohort study of thyroid cancer among those exposed as children and adolescents following the Chornobyl accident. Methods: A cohort of 32 385 individuals younger than 18 years of age and resident in the most heavily contaminated areas in Ukraine at the time of the accident was invited to be screened for any thyroid pathology by ultrasound and palpation between 1998 and 2000; 13 127 individuals (44%) were actually screened. Individual estimates of radiation dose to the thyroid were available for all screenees based on radioactivity measurements made shortly after the accident and on interview data. The excess relative risk per gray (Gy) was estimated using individual doses and a linear excess relative risk model. Results: Forty-five pathologically confirmed cases of thyroid cancer were found during the 1998–2000 screening. Thyroid cancer showed a strong, monotonic, and approximately linear relationship with individual thyroid dose estimate ( P <.001), yielding an estimated excess relative risk of 5.25 per Gy (95% confidence interval [CI] = 1.70 to 27.5). Greater age at exposure was associated with decreased risk of radiation-related thyroid cancer, although this interaction effect was not statistically significant. Conclusion: Exposure to radioactive iodine was strongly associated with increased risk of thyroid cancer among those exposed as children and adolescents. In the absence of Chornobyl radiation, 11.2 thyroid cancer cases would have been expected compared with the 45 observed, i.e., a reduction of 75% (95% CI = 50% to 93%). The study also provides quantitative risk estimates minimally confounded by any screening effects. Caution should be exercised in generalizing these results to any future similar accidents because of the potential differences in the nature of the radioactive iodines involved, the duration and temporal patterns of exposures, and the susceptibility of the exposed population.

26 April 2006 marks the 20th anniversary of the Chernobyl accident. On this occasion, the World Health Organization (WHO), within the UN Chernobyl Forum initiative, convened an Expert Group to evaluate the health impacts of Chernobyl. This paper summarises the findings relating to cancer. A dramatic increase in the incidence of thyroid cancer has been observed among those exposed to radioactive iodines in childhood and adolescence in the most contaminated territories. Iodine deficiency may have increased the risk of developing thyroid cancer following exposure to radioactive iodines, while prolonged stable iodine supplementation in the years after exposure may reduce this risk. Although increases in rates of other cancers have been reported, much of these increases appear to be due to other factors, including improvements in registration, reporting and diagnosis. Studies are few, however, and have methodological limitations. Further, because most radiation-related solid cancers continue to occur decades after exposure and because only 20 years have passed since the accident, it is too early to evaluate the full radiological impact of the accident. Apart from the large increase in thyroid cancer incidence in young people, there are at present no clearly demonstrated radiation-related increases in cancer risk. This should not, however, be interpreted to mean that no increase has in fact occurred: based on the experience of other populations exposed to ionising radiation, a small increase in the relative risk of cancer is expected, even at the low to moderate doses received. Although it is expected that epidemiological studies will have difficulty identifying such a risk, it may nevertheless translate into a substantial number of radiation-related cancer cases in the future, given the very large number of individuals exposed.

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Genomics of radiation-induced damage: The potential adverse effects of exposures to radioactivity from nuclear accidents can include acute consequences such as radiation sickness, as well as long-term sequelae such as increased risk of cancer. There have been a few studies examining transgenerational risks of radiation exposure but the results have been inconclusive. Morton et al. analyzed papillary thyroid tumors, normal thyroid tissue, and blood from hundreds of survivors of the Chernobyl nuclear accident and compared them against those of unexposed patients. The findings offer insight into the process of radiation-induced carcinogenesis and characteristic patterns of DNA damage associated with environmental radiation exposure. In a separate study, Yeager et al. analyzed the genomes of 130 children and parents from families in which one or both parents had experienced gonadal radiation exposure related to the Chernobyl accident and the children were conceived between 1987 and 2002. Reassuringly, the authors did not find an increase in new germline mutations in this population. Science , this issue p. eabg2538 , p. 725