Once in a great while, a scientific conference takes place that later proves to have been a turning point in a particular field—a seminal event remembered long after the name tags have been discarded and the posters recycled. Although it’s too soon to be certain, participants say the Forum on Endocrine Disrupting Chemicals, held 3 June 2005 in San Diego, may well come to be seen as a landmark in both the growth of the discipline and the progress of the science itself.
With the stated intent of bringing the science of endocrine-disrupting chemicals (EDCs) to the forefront, The Endocrine Society convened the workshop the day before its 87th annual meeting. Although EDCs have been on the society’s agenda before, the forum was its first day-long, formally organized event devoted to the subject.
“We’ve had very scattered presentations of results from the NIEHS and the Environmental Protection Agency at the endocrine meeting from time to time, but it was really hard in the past to produce a great deal of interest,” says Kenneth Korach, program director of the NIEHS Environmental Diseases and Medicine Program, who delivered the keynote address at the forum. “Now The Endocrine Society is taking a much more active role in expanding its interactions and development regarding the endocrine disruptor field, and there seems to be a strong commitment for supporting endocrine disruptor research and establishing a formal society program in EDCs.”
Attendance at the forum—organized by prominent EDC researchers Andrea Gore, R. Thomas Zoeller, and Jerrold Heindel—included more than 200 toxicologists, epidemiologists, clinicians, and other members of the endocrinology community, indicating that the effort to reach across disciplines and encourage translation has been successful. Korach believes that increasing awareness of EDCs among clinicians is particularly important. “Some of the effects of EDCs that will be seen in humans will be picked up by . . . endocrinologists in their diagnosis of disorders as these patients present to them, so educating them will be very worthwhile in terms of translational research.”
Korach and fellow NIEHS scientists Retha Newbold and John McLachlan (now at Tulane University) were among the pioneers in EDC research; today, Korach is encouraged by the field’s progress. “Twenty-five or thirty years ago, it was a very small group, but now we’re drawing more and more people into it from other disciplines,” he said, “and that’s a real success in terms of the education taking place at this forum.”
In Utero and Beyond
Korach, Newbold, and several other speakers familiarized attendees with the latest and most significant concepts in the field, focusing to a large extent on the growing belief that exposure to EDCs in utero can result in gene–environment interactions that will cause susceptibility to disease or reproductive problems later in life. The developing embryo is thought to be particularly sensitive to exposure to even low doses of exogenous EDCs during critical periods in early development, especially during sexual differentiation and organ development. The synthetic estrogen diethylstilbestrol, a well-known human carcinogen, is perhaps the most famous example of an EDC with a potentially devastating impact following fetal exposure. This compound was prescribed to more than 5 million women to prevent miscarriages from the 1940s through the 1970s, and many of the children born to those women have experienced immune system problems, cancers, and reproductive maladies such as deformed reproductive organs and infertility.
The forum also featured presentations reporting a variety of highly significant recent findings regarding the effects of EDCs. Work published 3 June 2005 in Science and presented by Michael Skinner, director of the Center for Reproductive Biology at Washington State University, could prove to have extraordinarily far-reaching implications.
Skinner’s group was researching the effects of two model EDCs—the anti-androgenic fungicide vinclozolin and the estrogenic pesticide methoxychlor—on embryonic testis development in mice. They discovered that exposing gestating mothers to the EDCs caused reduced sperm generation in adult male offspring. More importantly, as they continued to breed the animals, they found that the altered phenotype was retained all the way to the fourth generation—the male germ line had been permanently reprogrammed. “The human analogy is that your grandmother may have been exposed to an environmental toxicant, and two generations later, you might have a disease, even though you’ve never seen the toxicant—and then you could potentially pass it on to your grandchildren,” says Skinner.
The transgenerational effect was found to be epigenetic, caused by a chemical modification of DNA through methylation, as opposed to a normal base DNA mutation. Ninety percent of the offspring in each generation inherited the phenotype, a very high transmission frequency compared to that seen with genetic mutational events, which is typically 1% or less.
“Now we need to consider this trans-generational effect in our future analyses, doing transgenerational studies to see if the effects of a toxicant can actually be transferred to the subsequent generations,” says Skinner. It will also be important to identify the types of compounds, including other EDCs, that may tend to have this effect. Skinner says this is among several lines of investigation his team will pursue.
Beyond the effects on male fertility, the group also found that as both the male and female animals aged, they experienced other disease states such as premature aging, prostate disease, kidney disease, and tumor development. This implies that the epigenetic transgenerational effects of an environmental exposure may actually constitute a previously unrecognized disease mechanism. Identifying the genes with this reprogrammed methylation pattern that are transgenerational could lead to the development of many new diagnostic markers or therapeutic targets. Skinner and his colleagues are now in the early stages of establishing candidate genes and exploring their correlation with certain diseases.
Finally, the discovery raises the possibility that any environmental factor with an epigenetic effect could play a significantly more important role in evolutionary biology than previously suspected. “If you have a subpopulation of animals that are exposed and gain this permanent genetic phenotype, then you actually could potentially change the evolution of that species,” Skinner explains. “This could explain and provide a mechanism for some unknown parameters in evolutionary biology.”
Reproduction Ramifications
In another presentation, Shanna Swan, a professor of obstetrics and gynecology at the University of Rochester Medical Center, shared the results of her group’s investigation—the first of its kind in humans—of the effects of prenatal phthalate exposure on male testicular development. Phthalates are a common class of chemicals used in many household products, plastics, and cosmetics, and population studies have shown that virtually everyone carries some level of body burden of the compounds. In a study published in the August 2005 issue of EHP, the team found, consistent with previous rodent studies on phthalate exposure, an association between elevated intrauterine phthalate concentrations and adverse effects on male infants’ genital characteristics that serve as markers for normal sexual development, particularly the distance between the anus and the base of the penis.
“The idea is that the development of the testes is interrupted in fetal life, and that this has consequences in adult life, as well as at birth,” says Swan. “We’ve certainly seen that [effect] in rodents, and this is the first evidence that it may occur in humans as well.” [For more information on this study, see “Phthalates and Baby Boys,” p. A542 this issue.]
Two other presentations at the forum were particularly noteworthy. In one, Frederick vom Saal, a professor in the Division of Biological Sciences at the University of Missouri–Columbia, described several of his group’s studies into the interactive effects between varying levels of bisphenol A and fetal endogenous estradiol.
Bisphenol A is used extensively as a liner in canned goods, and in polycarbonate plastic products such as baby bottles, water bottles, and laboratory equipment including animal cages and feeding and watering equipment. The chemical has been shown to leach in bioactive amounts from such products, particularly when they are scratched or worn. According to vom Saal, this could represent a previously unrecognized source of disruption to laboratory experiments, with bisphenol A exposure impacting hormonal activity in experimental animals.
Even at very low doses of bisphenol A—well below the “no effect” concentrations recognized in current regulations—vom Saal and colleagues have seen prostate deformities in experimental animals associated with minute changes in background fetal estradiol levels. He told attendees that, given the understanding that the fetus is extremely sensitive to very small changes in estrogen, it seems clear that the levels of bisphenol A that leach out of products constitute a threat to human health.
In the other presentation, graduate student Stefanie Whish of Northern Arizona University in Flagstaff described research to be presented at the main meeting suggesting that uranium in its soluble form is an EDC, and may contribute to reproductive health problems in the Navajo people. Whish and her colleagues treated ovariectomized mice with uranyl nitrate in drinking water. Despite the absence of ovaries, the animals exhibited estrogen-like responses to the uranyl nitrate exposure, suggesting that the compound is estrogenic and possibly an EDC. These results were observed at the EPA’s safe drinking water concentration level for uranyl nitrate, which is exceeded in many drinking water sources in the Four Corners region of the Navajo nation.
With scientific evidence mounting that EDCs may have profound and complex effects upon human health, it is becoming increasingly clear that a comprehensive, multidisciplinary approach will be needed to gain the knowledge necessary to accurately assess risk and develop therapies. Only time will tell, but this forum may be seen as a landmark event in achieving the critical mass of cross-disciplinary interest, enthusiasm, and communication that will result in important new discoveries in the future.
Beginning a lifetime of vulnerability.
A recent meeting highlighted new data showing that in utero exposures to endocrine-disrupting chemicals can initiate changes leading to disease later in life.