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
Fundulus Transcriptome and Genome
Previously, Mark Hahn, Ph.D., Sibel Karchner, Ph.D., and their team performed deep sequencing of the transcriptome of killifish embryos after exposure to 3,3',4,4',5-pentachlorobiphenyl (PCB-126). Using these data, they demonstrated broad de-sensitization of the AHR signaling pathway in NBH fish, and investigators identified two new AHR genes that may be involved in the mechanism of resistance. The transcriptome sequences were subsequently used to help annotate the killifish genome, part of a large collaborative effort made possible in part by SRP-funded research. In September 2012, investigators participated in a Killifish Genome Annotation Workshop, at which the first assembly of the killifish genome was unveiled and its annotation was begun. Once fully annotated, the killifish genome will be a valuable resource that will facilitate the elucidation of mechanisms underlying the evolution of PCB resistance and its impacts.
Targeted mutation of AHR genes in killifish
To further develop the killifish model and to begin to test hypotheses about the relative roles of the four AHR genes, investigators initiated the generation of AHR2a-null killifish using the new zinc-finger nuclease (ZFN) approach for targeted gene inactivation. Fish from ZFN-injected embryos are being raised to adulthood, in collaboration with scientists at the U.S. Environmental Protection Agency, and will be screened for founders (fish that transmit an AHR mutation to their offspring). The AHR-mutant fish will allow us to better understand the roles of these proteins in the response to chemicals and the adaptation to long-term chemical exposure. This will be the first time that targeted mutagenesis using ZFNs has been applied to a fish model other than zebrafish, and in particular an environmental model such as killifish.
Sensitivity of PCB-resistant fish to oxidative stress
To begin to assess the impacts of the PCB-resistance adaptation, investigators performed experiments to determine whether PCB-resistant killifish exhibit altered sensitivity to oxidative stress. Embryos from NBH were more sensitive to toxic effects of tert-butylhydroquinone (tBHQ), a pro-oxidant chemical. NBH embryos also had lower basal expression of antioxidant genes. The results suggest that adaptation to tolerate PCBs has altered the sensitivity of NBH fish to oxidative stress during embryonic development, demonstrating a cost of the PCB resistance adaptation.
Research Translation and Training Activities
Since the last progress report, this project has contributed to the training of four postdoctoral researchers, all of whom have gone on to obtain tenure-track academic positions.
This research explores how natural populations of animals respond to prolonged, high-level exposure to contaminants. Researchers use fish as models to investigate the mechanisms underlying differential sensitivity to the developmental toxicity of dioxin-like chemicals. The existence of PCB-sensitive and PCB-resistant populations of killifish provides a unique opportunity to understand the molecular mechanisms of differential PCB sensitivity and the impact of evolved resistance on the sensitivity of fish to other environmental stressors. The research addresses a key question concerning the extent to which changes in the sensitivity to one class of chemicals have far-reaching effects on the ability of animals to respond to other types of chemicals or environmental stressors. The results will have relevance for ecological risk assessment at Superfund sites and other contaminated sites, and will contribute to the fundamental understanding of cross talk among signaling pathways and its role in the response to chemical exposure.