Genotoxicity of uranium contamination in embryonic zebrafish cells

Uranium is a metal used in the nuclear industry and for military applications. Studies on mammals have shown that uranium is genotoxic. However the molecular and cellular mechanisms responsible for the genotoxicity of uranium are poorly known for other types of vertebrates such as fish. Since unrepaired DNA double-strand breaks (DSBs) are considered to be key lesions in cell lethality, the activity of one of the major DSB-repair pathways, i.e. non-homologous end-joining (NHEJ), has been evaluated in embryonic zebrafish cells (ZF4) exposed to uranium. Genotoxicity of uranium in ZF4 cells was further assessed by comet and micronucleus assays. Exposure to uranium results in the production of DSBs a few hours after incubation. These breaks trigger the phosphorylation of H2AX proteins. We showed that the DNA-PK kinase activity, essential for NHEJ, is altered by the presence of uranium. The presence of uranium in cells disturbs but does not inhibit the repair rate of DSBs. Such a result suggests an impact of uranium upon the reparability of DSBs and the potential activation of alternative DSBs repair pathway leading to the propagation of possible misrepaired DSBs. In parallel, we performed a transmission electron microscopy analysis of cells exposed to uranium and were able to localize internalized uranium using an Energy Dispersive X-ray microanalyser. We observed the formation of precipitates in lysosome-like vesicles for 250 μM of uranium in the medium. The appearance of these precipitates is concomitant with the decrease of the number of DSBs per cell. This process might be a part of a defence system whose role in counteracting cytotoxicity calls for further dedicated research.