Maiden Voyage of the Collaborative Cross Mouse: Exploring Variability in Animals’ Response to Perchloroethylene

Researchers have spent more than a decade perfecting an alternative laboratory mouse model—the Collaborative Cross (CC)—that is designed to mimic the genetic diversity of the human population. Environmental Health Perspectives contains the first published toxicology study to be performed with CC mice: an exploration of how the toxic effects of perchloroethylene vary from animal to animal. Most inbred mouse strains are genetically identical, which means that individuals from the same strain respond similarly to the same chemical exposure. This limits researchers’ ability to study how differences in metabolism and other factors influence the effects of exposure and to understand how effects might vary in genetically diverse human populations. To address that shortcoming, researchers began to develop the CC model in 2002. First, they identified eight “founder” strains from the three major laboratory and wild subspecies of Mus musculus, otherwise known as the house mouse. They crossbred these strains and then crossbred their offspring. This unique breeding process generated inbred CC strains that each have a random sampling of the genetic diversity contained in the founder mice. Although individuals from a given CC strain are genetically identical to one another, the strains themselves are genetically distinct. Therefore, using a mouse population comprising multiple CC strains produces a facsimile of a genetically diverse human population. Several dozen genetically unique CC strains have been commercialised since the breeding effort began. For the present study, males from 45 CC strains were exposed to perchloroethylene, or perc, an industrial solvent that is also a common environmental contaminant. Perc is metabolised primarily to trichloroacetate (TCA), which has been shown to cause liver cancer in certain strains of rodents. The goal of the study was to look for strain-specific differences in perc’s toxicokinetics (i.e., how much perc and how much TCA accumulate in various organs) and its toxicodynamics (i.e., the effects caused by perc and TCA at target sites within the body). Investigators from Texas A&M University gave the animals a single high oral dose of 1,000 mg/kg body weight. After 1, 2, 4, 12, or 24 hours, they euthanised one animal from each strain and collected serum and tissue samples for analysis. According to their results, the toxicokinetics of perc and TCA varied significantly from one strain to the next. The amounts in liver, for instance, spanned a roughly 8-fold difference from the strain with the highest level to the strain with the lowest level. The toxicodynamics results, meanwhile, were less straightforward. The study’s first author, Joseph Cichocki, a postdoctoral research fellow at Texas A&M’s Department of Veterinary Integrative Biosciences, says that liver effects from perc were previously attributed to the formation of TCA, which in turn triggers peroxisome proliferator–activated receptors (PPARs), including PPAR?, in exposed cells. PPARs are known to be involved in cell proliferation and in the onset of liver cancer in mice. Cichocki and his colleagues homed in on two PPAR?-induced genes—Acox1 and Cyp4a10—and found that their induction was highly variable in the different strains. According to Cichocki, it had previously been assumed that Acox1 and Cyp4a10 expression levels would rise proportionately with increasing doses of perc. However, their results showed that this was not the case: Some mice had low Acox1 and Cyp4a10 expression levels despite being exposed to high doses of the chemical; in other mice, the opposite was true. The implication, Cichocki says, is that perc doses may not reliably predict the magnitude of their toxic effects. He asserts that additional factors, some that are related to toxicokinetics and others that are not, are responsible for interindividual variability in susceptibility to perc-induced toxicity. “Our study was not without limitations,” Cichoki says. “But the takeaway message is that by using CC mice, we could characterise and quantify interindividual variability across the population.” Steven Munger, an assistant professor at Jackson Laboratory, where development of the CC mouse first began, says that traditional single-strain toxicology studies provide only a limited view of the potential responses to a given chemical. “With Cichocki’s study, we’ve seen a distribution that looks a lot more like what you’d expect to see in exposed humans,” he says. “Some strains are outliers for particular phenotypes, just as a minority of people might exhibit a response that many others do not. And we still need to account for these people in our risk assessments.” Cichocki explains that risk assessors ordinarily use default “uncertainty factors” to estimate interindividual variation in chemical effects. By providing a glimpse into the biological nature of that variation, experts anticipate that experiments with CC mice could enable risk assessors to replace those default values with real data. “This is what we’re concluding with our paper,” Cichocki says. “We’re trying to generate the experimental data for population-level variability that regulators need.”

Environmental Health Perspectives, 24 July 2017 ;