Toxicogenomic assessment of 137Cs in zebrafish embryos across developmental stages using an AOP network approach

With the advancement and widespread adoption of nuclear energy, and the Fukushima nuclear wastewater discharge incident, concerns regarding the environmental risks posed by radioactive elements in aquatic environments are increasing. Cesium-137 (¹³⁷Cs), a prominent anthropogenic radionuclide, exhibits developmental toxicity in various organisms; however, the precise extent of this environmental risk and the stage-specific molecular mechanisms remain inadequately defined. Here, zebrafish embryos were used to investigate the effects of ¹³⁷Cs across various developmental stages, including the proenteral, segmental, hatching, and full cycle stages, and the related mechanisms. Morphological observations at 48 h post-fertilization (hpf) revealed significant (p < 0.05) embryonic malformations in zebrafish exposed during the hatching stage (S3) and full stage (S4) at 194 and 970 Bq/L, respectively. Concentration-dependent transcriptomic analyses indicated that the biological pathways perturbed by ¹³⁷Cs were linked mainly to gene damage repair. These pathways included mRNA 5'-UTR binding, double-strand break repair via break-induced replication, and single-stranded DNA helicase activity, with a point of departure (POD) for effect initiation ≤ 100 Bq/L. Finally, the ¹³⁷Cs-enriched biological pathways were integrated into an adverse outcome pathway (AOP) network to quantitatively compare and mechanistically elucidate stage-specific toxicity, revealing four typical AOPs: a reduction in the DHB4/HSD17B4 ratio leading to liver steatosis, induction of adenoma/carcinoma in the proximal tubular epithelium (renal tubules), inhibition of NADH-ubiquinone oxidoreductase leading to neurodegeneration, and tubulin binding resulting in aneuploid offspring. This study introduces a novel methodology for assessing the environmental risks associated with radioactive substances and establishing stage-specific AOP-informed mechanisms critical for refining safety benchmarks.