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A little more than a decade ago, Deborah Cory-Slechta was studying the effects of prenatal low-level lead exposure on children’s IQ.
Most of the children she studied lived in poorer sections of Rochester, she recalls. “We’d send taxis to get the moms, and some would say things like, ‘I don’t have money to pay the rent, for insurance, for food, and you want me to worry about lead?’
“It was an epiphany to me,” says Cory-Slechta, who is now professor of environmental medicine and pediatrics at University of Rochester. “It put toxic exposure in a different context. Clearly, in these communities, other factors affected child development—a lot of them acting on the same biological systems as lead, setting up an infrastructure for interactions: the perfect formula for enhanced toxicity.
“There is this focus in science on studying one factor at a time, but diseases and disorders—particularly the most intractable ones—don’t arise this way.”
The development of the brain, in particular, is a highly complex process that can be derailed by diverse forces, including environmental toxicants like lead. Toxicology has traditionally studied such substances in isolation, but through the work of researchers like Cory-Slechta a more dynamic, nuanced, interactive picture is emerging.
Stressful circumstances, such as drew Cory-Slechta’s attention, genetic differences, and toxic exposures are strands in a multifactorial weave that researchers are only beginning to unravel. The thread that ties them all together, some suspect, is epigenetics, the process through which experience and environment shape how genes behave.
“A lot of this is early work,” says Joel Nigg, professor of psychiatry, pediatrics, and behavioral neuroscience at Oregon Health Sciences University. “It may be premature to declare a new era, but I believe we are entering one. This is where the action will be, going forward.”
DNA makes a difference
Some of the interactions we understand best so far are between environmental toxicants and the genes that regulate how the body handles them. Prenatal exposure to organophosphate pesticides, for example, can adversely affect cognitive and motor development, and increase risk of autistic-type disorders. Several groups of researchers have identified a gene, PON1, that influences susceptibility to these effects.
“PON1 determines how fast the liver metabolizes these toxicants,” says Joel Nigg, commenting on this research. “In the offspring of mothers with the fast-metabolizing version of the gene, there was less association between exposure and subsequent cognitive development; with the slow-metabolizing gene, a strong association.”
Nigg is trying to show a similar gene interaction with lead, which is known to increase ADHD risk. “Lead exposure is ubiquitous,” he says. “Every child has some lead in their blood, and there is strong evidence that no level is safe.”
His research focuses on the Hfe gene, which regulates absorption and metabolism of iron and lead. In a study (as yet unpublished), Nigg and his colleagues show that in children with one common mutation, the association between blood lead levels and ADHD risk was much stronger than in the general population, while in those with another, it was much weaker.
“We don’t expect to show lead to be a major cause of ADHD; our purpose is to nail down an exemplar case of how a gene-environment interaction can cause the disorder,” he says.
Indeed, in his view, toxicants represent a particularly clear-cut aspect of the environment, but the range of gene-environment interactions is far broader, including lifestyle, diet and stress, and their effects extend to the full spectrum of neurological and psychiatric disorders.
Jason Richardson, deputy director of the joint graduate program in toxicology at Rutgers University, hopes to delve more deeply into such interactions. A recent study he led combined animal and human data to explore the role of pyrethroids, compounds widely used in pesticides and thought be reasonably safe, in ADHD. In the animal model, mice exposed to modest levels of pyrethroids displayed ADHD-like behavior—increased activity and impulsivity, and reduced working memory and attention. As in human ADHD, these phenomena were more pronounced in males.
Post-mortem brain examination found increased numbers of dopaminergic transporters and D1 dopamine receptors in the pyrethroid-exposed animals. Human studies suggest that similar dopaminergic dysregulation plays a role in ADHD.
To confirm the pyrethroid-ADHD link, the investigators analyzed data from the National Health and Nutrition Examination Survey (NHANES). Among more than 2000 children, those with detectable urinary levels of pyrethroid metabolites were more than twice as likely to be diagnosed with ADHD. Another analysis of NHANES data showed 50% more hyperactivity with each 10-fold increase in pyrethroid metabolites.
But there’s more to the story, Richardson is convinced. “Differential exposure contributes [to ADHD risk], but as 90% of people in the US have measurable levels of pyrethroids in their urine, we need to pay attention to [other factors],” he says. “We’re planning to explore interactions from a mechanistic standpoint.”
As one of the first groups funded under the new Virtual Consortium for Translational/Transdisciplinary Environmental Research (ViCTER) program of the National Institute of Environmental Health Sciences, Richardson and a team including a geneticist and epidemiologist are dissecting the mechanisms linking genes, environment, pyrethroid exposure, and ADHD-like phenomena.
They aim to identify genes that increase ADHD risk and biomarkers that reflect events within the affected brain. Using stem cell technology, Richardson says, they hope to grow dopaminergic neurons from blood samples of children whose genes raise ADHD risk and explore how pyrethroids affect them.
A matter of stress
Other researchers are exploring the interaction between environmental toxicants and psychosocial factors.
Pondering how the difficult lives of the mothers in her Rochester study might affect their children’s development, Deborah Cory-Slechta noted that prenatal stress is affected by the hypothalamus-pituitary-adrenal (HPA) axis, which is also affected by metals like lead. “They had the same targets, and the same outcomes. So I got into the business of looking at the combined effects of lead and prenatal stress.”
Her research, primarily with animal models, documented the interaction she hypothesized: The effects of lead on learning and memory were accentuated in the offspring of mice subjected to prenatal stress.
“We expanded to measures of impulsivity and of executive function, and we started looking at other neurotoxicants: did they also interact with prenatal stress? The answer was yes,” Cory-Slechta says.
In a 2014 study, neither prenatal stress nor low levels of methylmercury (a common contaminant of fish) alone impaired cognitive function—but combined, they did. “The interaction unmasked an effect never seen before,” she said.
Her recent research looks beyond prenatal influences. “We know from the behavioral science literature that early experience is a huge determinant of what happens later in life,” Cory-Slechta says. In one study, she and colleagues took litters of rats that had been exposed to lead and prenatal stress, putting some in enriched environments, and subjecting others to negative experiences.
“If you follow the effects of lead, they look like completely different brains,” she says. “Clearly, in the interaction between prenatal stress and lead exposure, the nature of early behavioral experience can change the final outcome.”
A recent study suggested comparable interactions in the real-life setting of New York City. The Columbia Center for Children’s Environmental Health has been following one group of children, now 16 years old, from before birth. “During this time, we’ve collected a great deal of data and biospecimens, blood and urine, from mothers and children,” says principal investigator Frederica Perera. The researchers also gathered information on air pollution, and on social and psychological factors that might influence early development.
“Now we’re increasingly interested in putting them all together, to look at possible interactions,” says Perera, professor of environmental health sciences at Columbia.
Her 2015 paper in the journal Neurotoxicology and Teratology analyzed the effects of an air pollution component, polycyclic aromatic hydrocarbons (PAH), and stress. Specifically, the researchers measured levels of PAH bonded to DNA in maternal and umbilical cord blood, which Perera describes as “a complex measure of both exposure and individual response [including genetic] factors.”
The measure of stress was “material hardship”—unmet needs for food, housing, and clothing, as reported by the mother during pregnancy or in the child’s early years.
The researchers found that the higher mothers’ PAH-DNA levels were, the worse their children scored on tests of IQ, perceptual reasoning, and working memory—but only in offspring of those who reported prenatal or recurring material hardship.
Chronic prenatal or early life stress may interfere with the normal ability to detoxify compounds like PAH, Perera conjectures. In addition, “stress is thought to involve inflammatory mediators, as are these pollutants; we think to some extent they potentiate each other through a shared pathway, and neuroinflammation is particularly important in the developing brain.”
These findings, Perera says, underline the need to ameliorate both air pollution and poverty, and confirm the importance of “the prenatal window for prevention.”
But research also raises the possibility of redressing prenatal damage, Cory-Slechta says. The molecular mechanisms behind toxicant-stress interactions remain unclear, and she and other researchers, including Joel Nigg, think the answer may lie in epigenetics, the way experience and environment change whether and when genes are expressed. How the brain develops in the face of physical and psychosocial challenges could be materially affected by such processes.
Since epigenetic modifications are plastic, they could explain how the right kind of early life experience might cancel out the effects of toxicants or prenatal stress, and the wrong kind make them worse. “We just don’t know enough about how these different experiences interact,” she says.
“The question for us is: What kind of epigenetic changes do you see in the brain in response to lead alone, and in response to prenatal stress, and how does later experience modify these changes?”
Her lab and others are working on it.