Citation:

Fay, K., C. LaLone, D. Feifarek, J. Doering, J. Cavallin, Dan Villeneuve, AND G. Ankley. Addressing species diversity in biotransformation: Variability in expressed transcripts of Phase I and II hepatic enzymes among fishes. SETAC North America, Minneapolis, MN, November 12 - 16, 2017.

Impact/Purpose:

Adverse Outcome Pathways (AOPs) offer a mechanistic means to connect biological effects measured in vitro to outcomes relevant for chemical risk assessment. To employ AOPs for risk assessment, the chemical exposure at the target site must be known; however, an organism’s ability to biotransform xenobiotics is one of the greatest uncertainties in making a prediction of target site concentration. The present study characterizes the diversity of hepatic phase I and II xenobiotic metabolizing enzymes among fishes, using mined available fish liver transcriptome data and conducting full-transcript, isoform sequencing on liver samples from two dozen phylogenetically diverse fish species. This novel RNAseq approach eliminated the need for transcriptome reconstruction resulting in reference genomes of the highest precision, allowing for detection of enzyme isoform orthologs among the species, as well as the nuclear receptors that control expression of the enzymes. Species were selected for broad phylogenetic coverage, as well as economic, research, and conservation importance. In addition to comparing information across fish species, the resolved isoforms were compared to human xenobiotic metabolizing enzymes. This comparison aids in evaluating the utility of human-based biotransformation tools such as ToxCast chemical screening assays or metabolism prediction software (i.e. QSARs) for potential relevance in fish. The present work addresses AOPDD product needs regarding taxonomic applicability (Task 1.1C) as well as the marriage of AOPs with Aggregate Exposure Pathway (AEP; Task 1.4B).

Description:

The ability of an organism to metabolize a pollutant is critical to understanding the risk the chemical poses to the organism. In the environment, fish are uniquely exposed to pollutants found in agricultural runoff and discharges from industry and wastewater treatment plants. Most research on chemical toxicity in fish rely on a few model fish species, like the zebrafish. For other fish, scientists make predictions of pollutant effects based on the model species. However, fish are extremely diverse. There is already good evidence that fish possess very different metabolizing enzymes which act on specific types of pollutants, making predictions of chemical toxicity challenging. This study examines the variability in metabolizing enzymes found among diverse fish species, including: model species, endangered species (e.g., American eel), species which have changed very little over time (e.g., sturgeon), and the most highly-evolved species (e.g., pufferfish). There is increasing evidence that diverse xenobiotic metabolizing enzymes exist among fishes, potentially resulting in different chemical sensitivities and accumulation, but this has never been systematically evaluated. One concern is that model test species such as rainbow trout, zebrafish and fathead minnows may not adequately represent the xenobiotic metabolizing capacity of other fish species. Our current study mined available fish liver transcriptome data and performed full-transcript, isoform sequencing on liver samples from two dozen phylogenetically diverse fish species. This novel RNAseq approach eliminated the need for transcriptome reconstruction resulting in reference genomes of the highest precision, allowing for detection of enzyme isoform orthologs among the species, as well as the nuclear receptors that control expression of the enzymes. Species were selected for broad phylogenetic coverage, as well as economic, research, and conservation importance, and included: sea lamprey (Petromyzon marinus), lake sturgeon (Acipenser fluvenscens), American eel (Anguilla rostrate), alligator gar (Atractosteus spatula), paddlefish (Polyodon spathula), rainbow trout (Oncorhynchus mykiss), rainbow smelt (Osmerus mordax), fathead minnow (Pimephales promelas), Antarctic icefish (Trematomus loennbergii), common carp (Cyprinus carpio), and channel catfish (Ictalurus punctatus). In addition to comparing information across fish species, the resolved isoforms were compared to human xenobiotic metabolizing enzymes. This comparison aids in evaluating the utility of human-based biotransformation tools such as ToxCast chemical screening assays or metabolism prediction software for potential relevance in fish. The content of this presentation neither constitute nor necessarily reflect US EPA policy..

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