# Position-specific methyl substitution on benzo[a]pyrene drives AHR-dependent fin duplication in zebrafish

## TL;DR

This study shows that specific methyl substitutions on a type of pollution chemical called benzo[a]pyrene cause unique effects in zebrafish, helping understand how such chemicals might impact health.

## Contribution

The study reveals that position-specific methyl substitution on benzo[a]pyrene leads to AHR-dependent fin duplication in zebrafish, highlighting structural toxicity mechanisms.

## Key findings

- 8-methylbenzo[a]pyrene causes fin duplication in zebrafish at low concentrations.
- Toxicity of 8-methylbenzo[a]pyrene is Ahr2 dependent with protective role of Cyp1a.
- Transcriptomic responses differ based on methyl position and concentration.

## Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a contaminant class characterized by fused aromatic rings, formed through the incomplete combustion of organic materials and petrogenic sources. Despite the abundance and toxicity of alkyl-substituted PAHs, most research and regulation focus on unsubstituted parent PAHs. Alkyl substitution of Benzo[a]pyrene (BaP), one of the most well-studied parent PAHs, drastically alters its bioactivity in zebrafish. In larval zebrafish exposed from 6 h post-fertilization (hpf), BaP caused behavioral effects but no morphological effects up to 50 µM at 120 hpf. In contrast, 8-methylbenzo[a]pyrene caused a distinct fin duplication phenotype by 0.26 µM and additional morphological effects by 1 µM. Alkyl substitution in different positions (7-, 6-, 9-, and 10-MBaP) did not elicit morphological effects at similar concentrations. This study characterized the morphological effects of 8-MBaP in zebrafish and investigated its mechanism(s) of action. Using knock-out lines, we demonstrated that 8-MBaP toxicity is Ahr2 dependent and that Cyp1a served a protective role. To identify underlying transcriptomic changes, embryos were exposed to 3 concentrations of BaP, 6-MBaP, and 8-MBaP. Whole embryos/larvae were collected at 48 and 72 hpf, which was before and during phenotype onset, respectively. Collecting RNA and morphological effects across concentration, time, and chemicals facilitated the identification of concentration-dependent transcriptional responses linked to the downstream morphological phenotypes unique to BaP methylation at the eighth position. This study improves environmental and human health hazard assessment by identifying critical structural features and mechanisms of action contributing to the toxicity of PAH mixtures in the environment.