Recent work in finding more efficient ways of testing chemicals is almost more important than the potential hazards of the chemicals themselves.
By Catherine Clabby
When UNC-Chapel Hill scientists spilled commonly used fungicides onto mouse brain cells, genes inside those cells changed. Activity levels among some of the genes shifted to distinctive patterns observed in the brains of people with autism.
First hear this: No one has proven that a class of popular crop-field fungicides causes brain damage. For starters, mouse brain cells are not human brain cells (though they are more alike than many people know). And cell studies don’t prove that chemical X produces human brain abnormality Y.
That said, the recently published research is compelling evidence that the potential of human health risks from a popular class of farm fungicides may merit closer study.
“There are so many questions to be answered,” said UNC cell biologist Mark Zylka, who led the chemical-screening project. “Do these chemicals get in the blood? Do they get into the brain? If so, in what concentrations?
Despite uncertainties about a real-world risk, Zylka is confident there is promise in the technique used to make this headline-grabbing discovery. The approach is a step forward in the scientific quest to more rapidly screen chemicals for potential impacts on human health, this time in the brain.
Trial and error
The breadth and accuracy of toxicology studies are vital in today’s chemical-laden world. Their results strongly influence government rulings on safe and unsafe levels of the tens of thousands of manufactured chemicals registered for use in the United States alone.
For nearly 100 years, many toxicology studies exposed mice or other lab animals to chemicals to see if they do harm. Damage to skin, disruption of reproduction and the growth of tumors all flag concerns.
But it isn’t always clear how to extrapolate data from one strain of mice to diverse populations of people. And animal studies can’t clarify precisely how chemicals affect functioning in cells, information that both help prevent and treat illness.
Several federal research agencies, particularly those located in North Carolina, are promoting newer toxicology screens. These use automated laboratory tools, computer analysis and plates of cells or tissues as research subjects to help predict ways that compounds might affect people and the environment.
Automated cell-based screenings broaden the number of chemical targets that can be screened and pick up the pace of studies because they can be cheaper and quicker than animal-based tests, said Scott Auerbach, a molecular toxicologist at the National Toxicology Program in Research Triangle Park.
“The idea is to get us closer more quickly to the things that we should be looking at. Potentially no stone will get left unturned,” said Auerbach, whose program is part of the Toxicology in the 21st Century initiative. So is the National Institute of Environmental Health Sciences, where NTP is based; the Food and Drug Administration; and other federal agencies.
Environmental Protection Agency staff in RTP, also Tox21 participants, compiled nearly 300 chemicals that Zylka’s team screened. Those included pesticides, fungicides, herbicides and compounds used to make plastics, among others.
The UNC researchers employed genomic sequencing tools to capture changes in gene activity in its mouse neurons after they were dosed.
The rates at which genes direct the production of proteins in different types of cells is essential to health. Overactive or underactive genes are less likely to produce the amount of proteins that cells require to grow or function normally.
Increasingly, genetic studies are linking patterns of abnormal gene activity to specific illnesses and disorders. Differences detected in brain cells of people with autism, for instance, include reduced activity of genes involved in transmission of signals between neurons and increased activity among genes involved in immune responses.
Zylka’s team found that exposure to fungicides called quinone outside inhibitors produced those very types of changes in the mouse neurons, in addition to other effects. Among the fungicides screened were pyraclostrobin, trifloxystrobin, famoxadone and fenamidone, agricultural chemicals modeled after an antifungual chemical forest mushrooms emit to protect themselves.
Agricultural chemical companies in North Carolina – BASF, Bayer and Syngenta – are among the producers of products using this class of fungicides. With names such as Cabrio, Reason and Abound, they suppress damaging fruit rot, mold, mildew and other blights on everything from berries to lettuce and nuts and improve farmers’ yields.
In food-crop fields and other agricultural settings, these types of fungicides work by impairing structures inside fungi cells called mitochondria. The structures are important in cells of many organisms because they produce the chemical energy cells can use to power many activities.
In their paper, Zylka’s team cites FDA and U.S. Department of Agriculture data showing that residue from quinone outside inhibitors has been detected on food, including spinach and apples, but in extremely tiny amounts measured in parts per million. Federal studies have documented residue from this class of fungicides on food, particularly leafy green vegetables.
Darren Wallis, a Bayer spokesman, said his company is committed to producing safe products that must undergo rigorous testing before release and are monitored by the company and regulatory agencies around the world once in circulation. Bayer scientists, he said, will certainly review the UNC findings and assess if any next steps are needed.
“The people of Bayer take our jobs of creating products to help improve the lives of people, plants and animals very seriously,” Wallis said.
Effects from exposure to chemicals released into the environment is an active area in autism research. Evidence exists, for instance, that pregnant women living close to farmlands and exposed to a limited number of pesticides are at increased risk of giving birth to children who will be diagnosed with autism, one clue that exposures may increase risk of the disorder.
Given indications that prenatal exposures play a role in autism, good research that sheds light on how chemicals affect neurons is needed, said Geraldine Dawson, director of the Center for Autism and Brain Development at Duke University, who was not involved in the UNC study.
“Zylka and his team have developed a really efficient way to test how a wide range of toxins influence the expressions of genes involved in neuronal development and function,” Dawson said. ”This high-throughput approach will be critical for identifying which combinations of toxins are associated with increased risk.”
Because of what they saw in the brain cells exposed to fungicides, the expanded use of the compounds and their detection on food, the UNC scientists in their paper called for “greater scrutiny” on the fungicides’ effects outside cells too.
There is now reason to observe effects on brains, they said, and behavior.