Uncertain Inheritance: Transgenerational Effects of Environmental Exposures

Andrea Cupp made a serendipitous discovery when she was a postdoctoral fellow at Washington State University: While investigating how chemicals affect sex determination in embryonic animals, she bred the offspring of pregnant rats that had been dosed with an insecticide called methoxyclor. When the males from that litter grew into adults, they had decreased sperm counts and higher rates of infertility. Cupp had seen these same abnormalities in the animals’ fathers, which had been exposed to methoxyclor in the womb. But this latest generation hadn’t been exposed that way, which suggested that methoxyclor’s toxic effects had carried over generations. “At first I couldn’t believe it,” says Cupp’s advisor, Michael Skinner, a biochemist and Washington State professor. “But then we repeated the breeding experiments and found that the results held up. ” Skinner and Cupp, who is now a professor at the University of Nebraska–Lincoln, published their findings in 2005.1 Since that paper—which showed that reproductive effects not just from methoxyclor but also from the fungicide vinclozolin persisted for at least four generations—the number of published articles reporting similar transgenerational findings has increased steadily. “In the last year and half there’s been an explosion in studies showing transgenerational effects from exposure to a wide array of environmental stressors,” says Lisa Chadwick, a program administrator at the National Institute of Environmental Health Sciences (NIEHS). “This is a field that’s really starting to take off. ” According to Chadwick, the new findings compel a reevaluation of how scientists perceive environmental health threats. “We have to think more long-term about the effects of chemicals that we’re exposed to every day,” she says. “This new research suggests they could have consequences not just for our own health and for that of our children, but also for the health of generations to come. ” Figure 1 Glossary The NIEHS recently issued requests for applications totaling $3 million for research on transgenerational effects in mammals.2 Chadwick says funded studies will address two fundamental data needs, one pertaining to potential transgenerational mechanisms and another to the number of chemicals thought to exert these effects. These studies will extend to what’s known as the F3 generation—the great-grandchildren of the originally exposed animal. That’s because chemicals given to pregnant females (the F0 generation) interact not only with the fetal offspring (the F1 generation) but also the germ cells developing within those offspring, which mature into the sperm and eggs that give rise to the F2 generation. Thus, the F3 animals are the first generation to be totally unexposed to the original agent. Effects that extend to the F2 generation are known as “multigenerational,” whereas those that extend to the F3 generation are known as “transgenerational. ”3 Transgenerational effects have now been reported for chemicals including permethrin, DEET, bisphenol A, certain phthalates, dioxin, jet fuel mixtures, nicotine, and tributyltin, among others. Most of these findings come from rodent studies.4,5,6,7 But preliminary evidence that chemical effects can carry over generations in humans is also emerging, although no F3 data have been published yet. Given the challenges of tracking effects over multiple human lifespans, the evidence is more difficult to interpret, particularly with respect to potential mechanisms, says Tessa Roseboom, a professor of early development and health at the Academic Medical Center in Amsterdam, the Netherlands. Still, some reports have linked nutritional deficiencies from famine and exposure to diethylstilbestrol (DES)—a nonsteroidal estrogen used to protect against miscarriage from the 1940s to the 1970s—to effects that persist among the grandchildren of exposed women.8,9,10,11,12,13