Superfund Research Program
Mechanisms and Impacts of PCB Resistant Fish
Project Leader: Mark E. Hahn (Woods Hole Oceanographic Institution)
Co-Investigators: Sibel I. Karchner (Woods Hole Oceanographic Institution), Neelakanteswar Aluru (Woods Hole Oceanographic Institution)
Grant Number: P42ES007381
Funding Period: 1995-2020
The objective of this project is to understand the impact of long-term, multi-generational exposure to high levels of dioxin-like chemicals, including certain polychlorinated biphenyls (PCBs). To address this problem, Dr. Hahn and his research team are studying the Atlantic killifish (Fundulus heteroclitus) inhabiting New Bedford Harbor, MA, a Superfund site highly contaminated with PCBs and other chemicals. Previously, the project investigators showed that New Bedford Harbor killifish are about 14-fold less sensitive to dioxin-like compounds than killifish from a reference site, Scorton Creek, in Sandwich, MA. The investigators also found that this “dioxin resistance” is heritable. They identified two killifish aryl hydrocarbon receptors (AHR1 and AHR2) and AHR repressor (AHRR), a protein that inhibits effects occurring through the AHRs. The specific objective of their research in this project is to determine the role of AHR1, AHR2, and AHRR in producing the dioxin/PCB resistance that has evolved in New Bedford Harbor fish.
This year project investigators expanded their studies of AHR gene polymorphisms (multiple forms of the gene that lead to variant AHR proteins) and their possible role in the mechanism of resistance. The investigators collected fish from New Bedford Harbor and Scorton Creek in the summer of 2003 and characterized their sensitivity to 3,3’,4,4’,5-pentachlorobiphenyl as well as their genotype at the AHR1 and AHR2 loci. PCB treatment caused induction of cytochrome P450-1A1 (CYP1A1, a gene regulated by AHRs) and AHRR in Scorton Creek fish but not New Bedford Harbor fish. Thus, the PCB-resistant phenotype has persisted in New Bedford Harbor despite the partial remediation of the most contaminated part of the harbor in 1995. Importantly, the investigators observed substantial inter-individual variability among Scorton Creek fish in the responsiveness to PCB treatment, suggesting genetic variability in their responsiveness to PCBs.
To examine AHR genotypes in these fish, the researchers measured the frequency of single nucleotide polymorphisms (SNPs) in AHR1 and AHR2 genes of individual fish (26 fish per site). They identified more than two dozen SNPs in AHR1 and a similar number in AHR2. For both genes, there were statistically significant differences in allele frequencies between the Scorton Creek and New Bedford Harbor populations.
The results of this research are providing new information about how natural populations of animals respond to prolonged, high-level exposure to contaminants. In addition, this research uses fish as models to investigate the mechanisms underlying differential sensitivity to the developmental toxicity of dioxin-like chemicals. The results obtained this year contribute to the understanding of dioxin/PCB resistance in fish and its molecular basis, particularly the possible role of AHR polymorphisms. The researchers found that killifish AHR1 and AHR2 genes both are highly polymorphic, and obtained evidence that certain AHR alleles are under-represented in a population of PCB-resistant fish. These results suggest that such altered allele frequencies could be involved in the mechanism of resistance. Understanding how PCB/dioxin resistance occurs will improve our ability to predict the sensitivity of humans and wildlife to these compounds.