Using fish models to understand the role of aryl hydrocarbon receptor (AHR)-interacting protein (AIP) in controlling sensitivity and resistance to dioxin-like compounds <i>in vivo</i>

Humans are exposed to chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) that cause toxicity through activation of the aryl hydrocarbon receptor (AHR). There is inter-individual variation in sensitivity to the effects of AHR ligands, but it is not fully explained by variation in the AHR. A clue to the genetic mechanisms underlying differential sensitivity to AHR agonists has emerged from studies of Atlantic killifish (Fundulus heteroclitus) populations with evolved tolerance to PCBs, TCDD, and PAHs. Genomic studies of these populations identified AHR-interacting protein (AIP/Ara9/XAP2) as the strongest candidate resistance gene. However, the precise role of AIP in the mechanism of resistance is unknown. To understand the role of AIP in the toxicity of dioxin-like compounds in vivo, we used CRISPR-Cas9 to generate AIP loss-of-function alleles in killifish and zebrafish (Danio rerio). Homozygous mutant killifish and zebrafish die during larval development-by 30 and 12 days post-fertilization, respectively-while heterozygous mutants develop, survive, and reproduce normally. During embryonic and early larval stages, homozygous mutant zebrafish exhibit reduced sensitivity to embryotoxic effects of exposure to 3,3',4,4',5-pentachlorobiphenyl (PCB126) and TCDD. Gene expression profiling of aip-deficient larvae revealed hundreds of differentially expressed genes. PCB126 induced similar sets of well-known AHR-regulated genes in mutant and wild-type larvae, although with reduced magnitude overall in AIP mutants. This study highlights the important role of AIP in fish larval development and demonstrates that AIP status can influence the response of vertebrate embryos to dioxin-like compounds in vivo.