||Detecting Cancer-Relevant DNA Damage Due to Heavy Ion Exposure
Benjamin, Blyth ,
Tsuruoka, Chizuru ,
Yamada, Yutaka ,
Kakinuma, ShizukoShimada, Yoshiya
Introduction: High LET radiation produces a different spectrum of DNA damage to that of similar dose of low LET radiation. However, the relative biological effectiveness depends on the parameters of the exposure and the endpoint, since increasing DNA damage levels can also result in reduced cell fitness and survival. For assessing high LET radio-therapy-induced second cancer risk, the key ques-tions are the relative ability of heavy ions to induce cancer in normal tissues, and the mechanisms by which heavy ions produce cancer-initiating DNA lesions.J-SHARE: The National Institute of Radiological Sciences has established J-SHARE (Japan Store-house of Animal Radiobiology Experiments) a pub-lic database containing the experimental and tumour pathology data for large series of modern lifespan/cancer studies in mice and rats for which tissue samples are available. The studies investigate dose, dose-rate, fractionation protocols, and age-at-exposure for whole body exposures to X-rays, gam-ma-rays, neutrons, and carbon ions, as used at our institute’s heavy ion treatment hospital. The variety of standard laboratory strains, hybrids, cancer sus-ceptible strains and transgenic models used allows us to investigate tumours arising in a range of tissues. Materials and Methods: In this study (1), we compared a series of 100 T cell lymphomas induced in 1 week old B6C3F1 mice by carbon ion irradiation (HIMAC: 290 MeV/n, 13 kev/μm) and compared them with similar tumours arising after 137Cs gamma ray irradiation (these tumours do not arise in unirra-diated mice). This whole-body carbon ion exposure reproduces the component of the carbon beam trav-ersing the normal tissue ahead of the tumour vol-ume, where the LET is substantially lower than that in the target volume. For high LET radiotherapy, it is this lower LET pre-Bragg peak exposure of the nor-mal tissue which is most relevant for second cancer risk assessment. Tumours were induced by 4 or 4.8 Gy of gamma-rays or carbon ions, either as a single acute dose or split over four doses each given 1 week apart (modelling normal tissue exposure during a standard clincal protocol). T cell lymphomas in B6C3F1 mice are well characterised, induced by mutations at a small number of known genomic loci including Notch1, Bcl11b, Ikzf1 and Pten. This al-lowed us to examine whether the mechanism for activating/inactivating these driver genes differed between low LET photon and pre-Bragg peak heavy ion exposures, using sequencing and DNA copy number analysis.Results: A single 4 Gy of carbon ion irradiation resulted in higher lymphoma incidence and de-creased survival time compared to the same dose of gamma rays. The effect of fractionation showed interactions with dose, with competition between early occurring lymphomas and late occurring solid tumours. Direct comparison of the various muta-tional mechanisms revealed that most features were common between the two irradiation sources (e.g. Trisomy 15, and Notch1, Ikzf1 and Trp53 muta-tions); however, loss of heterozygosity at the Bcl11b locus was significantly increased, and Pten muta-tion/null expression was significantly decreased in carbon ion- versus gamma ray-induced tumours. DNA copy number analysis revealed that loss of Bcl11b by deletion or recombination extending to the nearby telomere was similar between the gamma and carbon groups, but that the increased rate of Bcl11b loss in carbon ion tumours was due to inter-stial deletion or recombination events not observed following gamma ray exposure.Across the whole genome, the frequency of large interstial deletions was almost three times higher in carbon ion-induced tumours, with many of these not involving known cancer-initiating genes. This sug-gests that large interstial deletions may be more read-ily induced by heavy ion irradiation, even ahead of the high LET region, and may be a signature of the exposure even if not the cause of tumour initiation. Conclusion: Both the risk and phenotype of tu-mours induced by heavy ions may be influenced by DNA lesions that occur more readily at even moder-ately increased LET. Yet, most carcinogenic events in radiation-induced tumours are acquired later dur-ing tumour progession, as would be expected to be common to low and high LET. J-SHARE provides a unique resource to examine the nature of DNA le-sions following heavy ion exposures, and to investi-gate dose, dose-rate and age-at-exposure effects. Acknowledgments: This project was funded by NIRS and a grant from the Radiation Effects Asso-ciation (Japan). References: 1. Genetic analysis of T Cell Lymphomas in Carbon Ion-Irradiated Mice Reveals Frequency Interstitial Chromosomal Deletions: Implications for Second Cancer Induction in Normal Tissues during Carbon Ion Radiotherapy (B. Blyth et al.), PLOS ONE, (2015).
14th International Workshop on Radiation Damage to DNA