Experimental and computational analysis of the DNA damage induced in zebrafish, Danio rerio, early life stages after exposure to tritiated thymidine.

Tritium is an ubiquitous radioactive hydrogen isotope. It is found in all environmental compartments, in three different forms: tritiated water (HTO), gaseous tritium (HT) and organically bound tritium (OBT). Once internalized in the organism, it can either be found free in the tissues (TFWT) or bound to organic matter (OBT). This study aims to assess if tritiated thymidine, an organic form of tritium, induces DNA breaks once internalized in a model organism and its DNA. To do so, both experimental procedures and nanodosimetry simulations have been used. Zebrafish embryos (3.5 hpf, hours post fertilization) were exposed to three tritiated thymidine activity concentrations (7.5, 40, 110 kBq/mL, leading to internal dose rates of 22, 170 and 270 μGy/h) for four days. Individuals were sampled after 1 and 4 days of exposure and DNA break levels were assessed by the comet assay. Results showed that, even at the lowest activity concentration, tritiated thymidine induced DNA breaks in both embryos (1 dpf) and larvae (4 dpf). It was also highlighted that there was no increase nor decrease in DNA break level between 1 and 4 dpf, except in the case of the exposure to 170 μGy/h, where a slight decrease was observed. Geant4-DNA Monte Carlo simulations, performed on two spherical zebrafish nuclei of two different radii (2.5 and 5 μm), highlighted that organic tritium mainly induced single strand breaks (SSB). The results also showed that most of the damage was indirectly induced. Those results, combined with various experimentations, expose tritiated thymidine genotoxic pathways that could lead to both short- and long-term health effects.