Mercury (II) impairs nucleotide excision repair (NER) in zebrafish (Danio rerio) embryos by targeting primarily at the stage of DNA incision

Mercuric ion (Hg2+) is the most prevalent form of inorganic Hg found in polluted aquatic environment. As inhibition of DNA damage repair has been proposed as one of the mechanisms of Hg2+-induced genotoxicity in aquatic animals and mammalian cells, this study explored the susceptibility of different stages of nucleotide excision repair (NER) in zebrafish (Danio rerio) embryos to Hg2+ using UV-damaged DNA as the repair substrate. Exposure of embryos at 1 h post fertilization (hpf) to HgCl2 at 0.1-2.5 μM for 9 h caused a concentration-dependent inhibition of NER capacity monitored by a transcription-based DNA repair assay. The extracts of embryos exposed to 2.5 μM Hg2+ almost failed to up-regulate UV-suppressed marker cDNA transcription. No inhibition of ATP production was observed in all Hg2+-exposed embryos. Hg2+ exposure imposed either weak inhibitory or stimulating effects on the gene expression of NER factors, while band shift assay showed the inhibition of photolesion binding activities to about 40% of control in embryos treated with 1-2.5 μM HgCl2. The damage incision stage of NER in zebrafish embryos was found to be more sensitive to Hg2+ than photolesion binding capacity due to the complete loss of damage incision activity in the extracts of embryos exposed to 1-2.5 μM Hg2+. NER-related DNA incision was induced in UV-irradiated embryos based on the production of short DNA fragments matching the sizes of excision products generated by eukaryotic NER. Pre-exposure of embryos to Hg2+ at 0.1-2.5 μM all suppressed DNA incision/excision in UV-irradiated embryos, reflecting a high sensitivity of DNA damage incision/excision to Hg2+. Our results showed the potential of Hg2+ at environmental relevant levels to disturb NER in zebrafish embryos by targeting primarily at the stage of DNA incision/excision.