Developmental plasticity is the concept that an organism adapts epigenetically during in utero
development to the anticipated external environment via cues available within the maternal-fetal microenvironment. As a consequence, the developing fetus is exquisitely sensitive to not only nutritional factors within the maternal circulation, but also to toxicants such as heavy metals, pesticides and endocrine-disrupting compounds [37
]. Among these endocrine disruptors, TCDD and structurally-related compounds have been known to disturb steroid action related to maintenance of pregnancy following adult exposures [10
]. However, early life exposure to this toxicant may have an even greater potential to alter reproductive success by disrupting reproductive tract development via epigenetic modification of critical genes. Thus, there is growing concern that prenatal and neonatal toxicant exposure may promote reproductive disorders which do not become apparent for many years [2
] and perhaps persist for multiple generations.
Although a better understanding of human health issues related to early life toxicant exposure will be critical for making future policy decisions related to exposure risks, we must first utilize animal models that can reveal potential toxicant-associated epigenetic changes under controlled laboratory conditions. In this manner, using a murine model, we have previously shown that developmental exposure to TCDD leads to reduced uterine sensitivity to progesterone in adult female animals [19
], suggesting that reproductive success may be compromised in exposed animals. In the current study, we have examined whether the TCDD-mediated loss of progesterone responsiveness in similarly exposed mice acts to disrupt either fertility or maintenance of pregnancy. In addition, we examined whether the phenotype previously observed in TCDD exposed F1 mice persists in future generations, which would suggest that early life toxicant exposure results in heritable epigenetic alterations that can negatively affect fertility.
Within our current TCDD exposure studies, we found that singly-exposed F1 mice and dually exposed F1 mice exhibited similar reproductive tract defects with regard to fertility and maintenance of pregnancy. Specifically, both treatment groups exhibited reduced fertility compared to control mice, with approximately half of the mice in each exposure group able to achieve visible pregnancy (). Infertility correlated with diminished uterine PR expression as determined by immunohistochemistry. Significantly, in the first series of studies we also found that the majority of TCDD exposed F1 mice which were able to become pregnant in either treatment group failed to sustain their pregnancy and delivered preterm. Although most pups appeared to be viable at birth, (due to direct visualization or the presence of a milkspot), all premature mice died within 24 hours of birth. Surviving (full-term) female offspring were mated at adulthood and were similarly examined for fertility and pregnancy outcome. With the exception of the F2 mice, which were descended from the fertile but dually TCDD-exposed dams, we found that each successive generation of animals exhibited similar fertility and pregnancy outcomes as the F1 mice. All toxicant exposed mice, or descendants of exposed mice, fared worse with regard to fertility or pregnancy outcomes compared to any generation of control mice.
Not surprisingly, F1 mice in the current study which were exposed to TCDD at multiple times during development (in utero
, 4 weeks and 9 weeks) exhibited complete infertility, likely due in part to their profound loss of PR expression described within in our previous report [19
]. However, it is also probable that multiple exposures to TCDD impact other reproductive organs and systems which can affect fertility. For example, defects in ovarian steroid synthesis [39
] and neural development [40
] have also been noted following exposure to this toxicant. For these reasons, our ongoing murine studies will focus primarily on the impact of a single in utero
TCDD exposure since our data suggests that a single exposure more closely mimics the spectrum of reproductive phenotypes observed in TCDD-exposed human populations [11
Perhaps, the most profound observation of our study came following the realization that our colony had been unexpectedly exposed to mouse parvovirus (MPV). MPV is a DNA virus of the Parvoviridae
family; although common in laboratory animal facilities, this virus does not usually cause overt reproductive failure [33
]. Since the virus is shed in urine and feces, it is highly contagious and spreads quickly within a mouse colony. MPV infection is known to impede development of grafted tumors in mice [43
], but little is currently known about the impact of this virus on reproductive tract function. Our data, presented herein, suggests that although MPV infection alone did not affect fertility or pregnancy outcomes, this virus dramatically affected the length of pregnancy in mice with a history of developmental toxicant exposure.
Similar to MPV infections in mice, chronic viral infection is common among humans; however, the inflammatory stress induced by such infection may have little impact on the length of pregnancy in women with a robust responsiveness to progesterone at the maternal-fetal interface. Our MPV data, albeit unintentional, suggests that the TCDD-mediated decrease in progesterone responsiveness may heighten sensitivity to inflammation, leading to PTB in mice additionally exposed to this virus. Specifically, the incidence of PTB among singly-exposed F1 mice in the presence and absence of MPV was 86% and 36%, respectively. Thus, we surmise that the higher incidence of PTB observed in the first study was due to the combination of increased sensitivity to inflammation in the presence of an underlying, occult inflammatory stressor (MPV). In order to explore this hypothesis, we exposed pregnant, virus-free control (conF1) and virus-free TCDD exposed F1 mice to a low dose of LPS that would not independently affect pregnancy length. As expected, control mice were resistant to this inflammatory stressor while all mice with a history of developmental toxicant exposure delivered within 16 hours of LPS treatment. Although speculative at this juncture, humans commonly are infected with numerous viruses that do not cause reproductive failure (i.e., herpes simplex [cold sores] and herpes zoster [shingles]); thus a dormant viral infection or other minor stressor (smoking, depression, allergies) might be risk factors only in susceptible women (i.e., with a genetic polymorphism or history of toxicant exposure).
The rate of PTB in women in industrialized countries continues to increase despite better health care and patient awareness. Our data supports the possibility of environmental factors which lead to reduced endometrial progesterone responsiveness and an increased sensitivity to inflammation as a risk for reproductive tract failure. More specifically, known risk factors for PTB among women (e.g. stress, dental caries, asthma) may only truly be risk factors for women with a heightened sensitivity to inflammation. Although our study did not assess the potential role of genetic polymorphisms, our data certainly supports a possible role of gene-environment interactions in the loss of reproductive tract function observed in certain women. Understanding the impact of “minor” stressors on inflammatory processes which lead to reproductive dysfunction, such as PTB, should allow development of better management strategies for both clinicians and patients.