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

Mercuric ion (Hg) 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 Hg-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 Hg using UV-damaged DNA as the repair substrate. Exposure of embryos at 1h post fertilization (hpf) to HgCl at 0.1-2.5μM for 9h caused a concentration-dependent inhibition of NER capacity monitored by a transcription-based DNA repair assay. The extracts of embryos exposed to 2.5μM Hg almost failed to up-regulate UV-suppressed marker cDNA transcription. No inhibition of ATP production was observed in all Hg-exposed embryos. Hg 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 1-2.5μM HgCl. The damage incision stage of NER in zebrafish embryos was found to be more sensitive to Hg than photolesion binding capacity due to the complete loss of damage incision activity in the extracts of embryos exposed to 1-2.5μM Hg. 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 Hg at 0.1-2.5μM all suppressed DNA incision/excision in UV-irradiated embryos, reflecting a high sensitivity of DNA damage incision/excision to Hg. Our results showed the potential of Hg at environmental relevant levels to disturb NER in zebrafish embryos by targeting primarily at the stage of DNA incision/excision.