Research Highlight: Using Zebrafish As Models for Toxicology Investigations

The genome is an organism's hereditary information, encoded in DNA. Genomics is the study of the structure, function, evolution, and mapping of genomes. A xenobiotic (in Greek, xenos means “stranger” and biotic means “related to living beings”) is any substance that is foreign to the body. Drugs, pesticides, and carcinogens are examples of xenobiotics. Xenobiotic disposition, or drug metabolism, refers to the biochemical changes (biotransformation) made by enzymes in the body that break down and eliminate drugs and other foreign substances from the body.

Our Genomics of Xenobiotic Disposition Area of Research Emphasis (ARE) focuses on understanding why individuals metabolize drugs differently. Why do some people break down and eliminate a drug or toxin relatively quickly compared to others who retain it longer? People who retain a xenobiotic are exposed to it for a longer time and are thus potentially more vulnerable for harmful health effects.

Factors that influence individual differences in xenobiotic disposition include genetics, previous exposure to the xenobiotic (or other xenobiotics), nutritional status, and the individual’s stage of development.

It is known that xenobiotics such as cadmium, copper, and certain pesticides interfere with the sense of smell in fish. Fish depend on their sense of smell to find prey, sense predators, and find their way to natal streams. Salmon are particularly at risk because they rely so much on their sense of smell.

Genomics of Xenobiotic Disposition ARE core leader Evan Gallagher is working to understand what happens at the cellular, molecular, and biochemical levels to the olfactory system of fish when they are exposed to copper.

In a recent study, his team exposed adult zebrafish for 24 hours to 3 concentrations of copper, concentrations within the range of copper concentrations from urban runoff. The olfactory systems of the fish were then harvested and analyzed to discover whether genes involved in pathways in the sense of smell, the olfactory system, behaved differently when the fish were exposed to copper.

Current research suggests that copper exposure causes epigenetic changes in the genes in the olfactory pathway. Epigenetic changes are changes in the activity of genes not caused by changes in the DNA sequence. Epigenetic changes regulate whether the genes are turned on or off.

In this study, the researchers looked at epigenetic changes caused by microRNAs, or miRNAs, small molecules found in plants and animals that regulate genes. The research team identified changes in the concentrations of several miRNAs that regulate various genes in the olfactory system. Some of the dysregulated genes are involved in neurogenesis, the creation of new nerve cells needed to heal after an injury to the nervous system. The change in miRNA levels was greater as the concentration of copper increased.

These results suggest that changes in the concentration of miRNA molecules are a possible mechanism by which copper damages the olfactory system. The next step is to focus on the target genes identified in this study and explore their specific roles in the olfactory system in fish.

This study appears to be the first to look at miRNA regulation as the mechanism by which copper damages the olfactory system in fish. The results provide new information about the potential role of miRNA molecules in gene regulation when fish are exposed to copper.

Gallagher’s work on metal exposure in fish doesn’t discourage him from eating fish. An avid fly fisherman, he says, “It's my opinion that our salmon are overwhelmingly beneficial for us, especially in the Puget Sound region. They have all the things you find in over-the-counter vitamins and supplements – Vitamin E, antioxidants, omega-3 fatty acids – that protect against heart disease and age-related disorders.”

Event Highlight: CEEH Outreach at UW Science and Engineering Fest in Yakima

UW Freshman Jose Lopez helped at the Pesticides & Health table hosted by the Center's Outreach Group 
Along with other UW programs, the Center's Community Outreach and Ethics Core (COEC) hosted a table at the UW Genome Sciences Education Outreach (GSEO) annual Science and Engineering Festival in Yakima on March 25-26. The audience was junior high and high school students and the community.

The theme of our display was Pesticides and Health: Ethical Implications for Genetic Testing in the Workplace. We offered this verbal spiel, in English and Spanish:

  • Pesticides are chemicals that kill insects and weeds. Do you think something intended to kill insects are good for people? (Our visitors thought not).
  • Gene-environment interactions help explain why some people get sick from being exposed to toxic substances like pesticides, and others don't.
  • We all have a gene called PON1, for paraoxonase 1. The gene holds the recipe for an enzyme that's also called PON1. Because of small differences in the PON1 gene, different people make very different amounts of the PON1 enzyme.
  • The job of the PON1 enzyme is to break down foreign molecules in our bodies. PON1 breaks down organophosphate pesticides. 
  • People who have less PON1 enzyme have a higher risk of getting sick from being around pesticides because the pesticide stays in their body longer.
Visitors answered 4 ethical questions by dropping a colored stone in a graduated cylinder labeled Yes or No. The questions were:
  1. If you had a genetic variant that made you more likely to get sick from pesticides, would you want to know?
  2. Should genetic testing be required for employees who work with pesticides?
  3. If a worker has been tested for PON1 status, to measure if they are more likely than average to get sick from pesticides, should their employer be told the test result?
  4. Should family members who live with a farm worker, and those who live or go to school near farm fields, also be tested for PON1 status to find out if they are more likely to get sick from pesticides?
We offered our handout about PON1. We also have a fact sheet, Health Risks of Pesticides in Food.

The majority answered Yes to all questions. Nearly everyone answered Yes to Q1, wanting to know their own risk. More visitors answered No to each subsequent question. Students were more likely than adults to answer Yes, they would share genetic information with employers Adults were more hesitant, and some even debated the pros and cons out loud. Some feared workers would be discriminated against if test results were known. 

For Q4, a larger minority answered No, they didn't think family members should be tested. Some shrugged, perhaps feeling it didn't matter, or there was nothing they could do about it. One mother told of living in a home surrounded by orchards with a young child who had severe asthma. She asked the orchardist if he could let the family know when pesticide spraying would be done so they could take the child away. The orchardist was unable to tell her when the spraying would happen.

Among our visitors were farmworkers, orchardists, and student and adult community members who lived near orchards and fields. Everyone seemed to know that pesticides are dangerous. Several reported that personal protective equipment is highly regulated and used, which was good to hear.

Many visitors wanted to know how to protect themselves. We offered to stay inside during spraying; wash fruit and vegetables; eat organic produce if possible; eat green tea and blueberries--foods like these that are high in antioxidants can increase the activity of PON1.

Pesticides, health, and genetics were pertinent topics to bring to the agricultural Yakima Valley. It's valuable to go to communities with our Center research, both to share what we know and to listen and learn from them.

- Marilyn Hair