Researchers explore transgenerational effects of ancestral exposures
By Eddy Ball
A new NIEHS-funded study offers more evidence about the transgenerational actions of environmental compounds on reproductive disease. The findings result from a productive collaboration between environmental researchers at Washington State University (WSU) and Department of Defense (DoD) scientists, and the DoD Telemedicine and Advanced Technology Research Center.
The findings from a research team led by NIEHS grantees and WSU professor Michael Skinner, Ph.D., appeared online May 3 in the journal PLoS One. The paper outlines transgenerational effects similar to ones induced genetically, but are caused instead by epigenetic modifications of gene expression that do not alter DNA itself.
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The team reports effects on adult-onset ovarian disease in F3 generation female rats, resulting from a direct exposure, experienced by F0 generation female rats and their F1 progeny, to a panel of five compounds and mixtures selected for their relevance to anticipated exposures by military personnel. An earlier study by the group, published Feb. 28 in PLoS One, found transgenerational disease in rats from exposure to all of the compounds used in the newest study.
While the evidence supports the notion that epigenetic modifications, triggered by environmental exposures, can result in transgenerational inheritance of disease — the so-called you are what your great-grandmother ate hypothesis — the researchers offered a caveat up front. “The current study used pharmacological doses and administration to assess potential transgenerational actions on ovarian disease, and should not be considered a risk assessment analysis.” However, they argue that, based on the significance of their findings, such a risk assessment analysis should be a follow-up to their study.
Exposure during a critical window of development
The researchers studied five treatment groups of pregnant F0 outbred rats during days 8-15 of fetal development, a time of gonadal sex determination, exposed through injection to one of the compounds or mixtures, including a fungicide, a pesticide mixture, a plastic mixture, dioxin, and a hydrocarbon mixture (see text box). Because the researchers observed a drop in litter size for the F1 generation of the plastics group, they added another treatment group exposed to only half of the original dose.
The researchers injected only the original F0 pregnant females with the treatment compounds. F1 generation rats bred normally with other F1 animals of the same treatment group, generating F2 rats that were bred similarly to produce the F3 generation of animals.
The team evaluated ovaries taken from F0-F3 rats at one year of age, and the researchers performed microarray transcriptome analysis and methylated DNA immunoprecipitation to identify gene expression and epigenetic modification. They also performed confirmational experiments on cultured neonatal rat ovaries from four-day-old females.
The team found increased numbers of ovarian cysts, a hallmark of polycystic ovarian disease, in all the treated lineage groups compared to control lineage groups. Mean decreases in the primordial follicle counts of 35 and 60 percent occurred in both the F1 generation, which was exposed across the placenta, and the F3 generation, which was exposed transgenerationally.
Although the researchers determined that treatment with the various toxicants can lead to different transgenerational phenotypes, all of the treatments resulted in the same increase in small ovarian cysts and the same decrease in the primordial follicle pool.
Public health implications
In their call for further research, that they consider amply justified by the findings of their study, the authors conceded, “The current ovarian abnormalities and disease in rats cannot be directly correlated to the human polycystic ovarian syndrome nor human primary ovarian insufficiency and loss of fertility.”
Nevertheless, they add, there are compelling similarities between the morphological changes found in the rats and those found in the ovarian diseases that affect up to 18 percent of women, who often also experience insulin resistance and a higher risk for diabetes, in addition to infertility and other reproductive abnormalities.
“These results suggest a new paradigm be considered for the [genetic and epigenetic] etiology of [adult-onset] ovarian disease,” the researchers concluded. “Further elucidation of the etiology of ovarian disease and epigenetic transgenerational inheritance will provide insights into prevention and therapeutic strategies for female health.”
Manikkam M, Guerrero-Bosagna C, Tracey R, Haque MM, Skinner MK. 2012. Transgenerational actions of environmental compounds on reproductive disease and identification of epigenetic biomarkers of ancestral exposures. PLoS One 7(2):e31901.
Nilsson E, Larsen G, Manikkam M, Guerrero-Bosagna C, Savenkova MI, Skinner MK. 2012. Environmentally induced epigenetic transgenerational inheritance of ovarian disease. PLoS One 7(5):e36129.
A real life scenario of environmental exposure
According to the researchers, with valuable input from DOD, the team selected an exposure panel of widely used chemicals with some evidence of toxic, reproductive, or epigenetic effects in animals. The exposure compounds were the following substances and mixtures:
- Vinclozolin, an agricultural fungicide previously shown to cause transgenerational epigenetic disease
- A mixture of permethrin, the most commonly used human insecticide shown to have minor toxicologic effects in mammals, and DEET, an insect repellent reported to have negligible toxic effects
- A plastic mixture of bisphenol A (BPA), dibutyl phthalate (DBP), and bis(2-ethylhexyl)phthalate (DEHP), all plasticizer chemicals that commonly appear together in plastics with in vitro and in vivo toxic effects
- Dioxin (TCDD), a byproduct of some commercial chemical syntheses that has been shown to induce adult-onset diseases, including premature failure to exhibit estrous cycle activity
- Jet fuel (jet propellant 8, JP8), a hydrocarbon mixture often used for dust control on road surfaces, with known toxicologic effects, but not known to induce reproductive defects
The team used pharmacologic doses of all the compounds and mixtures.