Prenatal Programming and Toxicity (PPTOX) Introduction

Abstract

The developmental origin of health and disease hypothesis posits that early-life exposures, including prenatal, can influence disease outcomes throughout the entire lifespan of an organism. Over the past 30 years, scientific researchers have compiled robust epidemiological and mechanistic data showing the effects of early-life nutrition, chemical exposures, and stress on prenatal programing and toxicity. Using novel techniques in genomics and epigenetics, science is now establishing strong links between low-level early-life environmental exposures and the later development of noncommunicable diseases, such as cardiovascular disease, obesity, diabetes, neurodevelopmental and neurodegenerative disease, reproductive effects, immune system function and cancer. Now scientists must engage with communities, industry, policy makers, and clinicians to leverage our newfound understanding of prenatal programing and toxicity into better health outcomes across the lifespan.

The pre- and perinatal time period constitutes the most sensitive developmental window during which human health and wellness may be adversely impacted. Nutrition, exposure to environmental contaminants, and stress can all lead to adverse health effects. Often these are seen as direct effects (eg, low birth weight, premature parturition, birth defects), whereas other adverse health outcomes associated with early-life exposures do not manifest until years or decades later. The developmental origin of health and disease (DOHaD) hypothesis posits that early life exposures can influence disease outcomes throughout the entire lifespan of an organism (1).

Although scientists and clinicians have recognized lasting effects of prenatal malnutrition and chemical exposures for many decades, the DOHaD hypothesis did not gain significant recognition until the mid-1980s when Dr David Barker published a series of studies showing an association between low birth weight and increased incidence of ischemic heart disease in adulthood (2, 3). Later, Barker et al also linked fetal nutrition to increased coronary heart disease (4). These early studies led to a better understanding and acceptance of how maternal nutrition and, by extension fetal nutrient availability, can lead to health effects throughout the entire lifespan (reviewed in Ref. 5).

Similarly, the potential lifelong effects of chemical exposures have recognized for over 40 years. The most prominent example, diethylstilbestrol (DES), was a drug used clinically to prevent miscarriages in pregnant women from 1940 to 1970. Exposure to this xenobiotic estrogen in utero before the 18th week of pregnancy altered normal fetal organogenesis within the developing female reproductive tract, and later in life caused increased incidence of vaginal and cervical clear cell adenocarcinomas in young women (reviewed by Ref. 6), among a host of other effects in DES daughters and sons. The later-life effects of early exposure to DES were seen with pharmacological doses of the exogenous hormone, but in the early 1990s researchers began to report evidence of DOHaD effects in response to low-dose and environmental exposures, many of which did not exhibit overt birth defects (7–9).

Although DOHaD was first identified in epidemiologic observations by Barker and others, the last 25 years have provided significant advances in our understanding of the mechanisms of prenatal programing and toxicity (PPTOX). When exposed to environmental stressors, adult organisms generally respond with activational physiologic responses within a defined range of plasticity. These changes within biological systems allow the organism to adapt to a wide variety of environmental conditions, maintain fitness, and return to steady state after the stressor is no longer present. In contrast, the same environmental exposure during a critical window of early development can create organizational changes within biological systems. Organizational changes permanently alter the biological system and may cause disease, or predispose the organism to dysfunction and disease at a later time. Organizational changes provide the platform on which the DOHaD principle is built.

DOHaD effects are being increasingly seen due to perturbation of the endocrine system, because it is particularly sensitive to low-level exposures to endocrine disrupting chemicals (EDCs) (10). Hormones produce effects at extremely low concentrations and are tightly regulated within the body (11), and even very small alterations in ligand binding or receptor expression can have large and devastating consequences. During gestation, the endocrine system closely regulates the development of a wide range of biological systems, so low-level EDC exposure at these critical developmental windows has the potential to alter critical organizational events (12). This is true both in male and female offspring, but the effects of EDCs may be discordant between the sexes.

Advanced molecular techniques are keys to developing further our understanding of how early-life exposures can lead to disease later in life. The greatest advances in our mechanistic understanding of the underlying factors of DOHaD come from new genomic tools/technologies and from the field of epigenetics. The National Institutes of Health (NIH)-sponsored Human Genome Project (13) revolutionized researchers' ability to quickly and cost-effectively assess DNA coding, structure, and function. Advanced microarray, sequencing, multiparameter flow cytometry, immunoprecipitation, and inmmunohybridization techniques have allowed researchers to identify subtle cell-signaling and gene expression changes associated with DNA transcription, translation, or chromatin structure as a result of low level chemical exposures. Further, advances in epigenetics have provided great insights as to how fetal programming of imprinted genes and those with metastable epialleles may contribute to later-life disease through histone modification and/or direct DNA modification. Several reviews have been published that expound upon the role of epigenetic research in DOHaD (14–18).

In addition to providing a mechanism for direct effects of chemical exposure, epigenetics provides a plausible mechanism for transgenerational effects as well. Animal studies have shown that adverse effects of some chemical exposures are able to be heritably transmitted to future generations that were never exposed to the compound, ie, at or beyond the F3 generation. It has been proposed that this noncoded inheritability is due to incomplete erasure events during early gestation. Although the transmission of imprinted epigenetic information is likely involved in this process, there are likely other processes that are yet to be identified (19, 20).

Although nearly all toxicological research focuses on the effects of single chemicals, humans are exposed to hundreds of chemicals before they are even born. The issue of how to evaluate complex mixtures has long been a primary challenge for toxicologists and epidemiologists. Environment-wide association studies (EWAS) show great promise in the search for environmental factors associated with specific diseases (21), including those developmental exposures that lead to adult-onset disease. Advances in bioinformatics now allow investigators to describe how single exposures lead to secondary exposures, or what coexposures may be common. Understanding how complex exposures are correlated is necessary for defining what combinations of environmental exposures are relevant to human health outcomes. Applying EWAS to the exposome, the totality of exposures throughout a lifetime, and specifically to the exposures that are present during sensitive developmental windows, will allow researchers to better understand PPTOX. Currently, based on a specific exposure identified from an EWAS, it is very difficult to establish whether the exposure is independently associated with a disease or whether the exposure is a correlated secondary exposure or coexposure. That being said, EWAS data show great promise in identifying coexposures that may contribute to a variety of disease states (22).

The scientific evidence supporting PPTOX and the DOHaD hypothesis has grown most significantly in recent years and is now sufficiently robust. Unfortunately, improved public health cannot be achieved using a single-chemical approach to research. The scientific community needs to focus on health outcomes with a systems approach to identify specific windows of susceptibility and the biological pathways that are being impacted. Cardiovascular disease, obesity, diabetes, neurodevelopmental and neurodegenerative disease, reproductive effects, immune system function, and cancer have all been associated with prenatal chemical exposures (23–25). Because they may originate during early development, investigating these noncommunicable conditions is now a priority for children's health research.

Prospective cohort studies provide tremendous opportunities for studying gene-environment interaction (26). For over a decade, the NIH-sponsored National Children's Study was in design as a prospective cohort with goals of working across disciplines to address outstanding questions in children's health, including DOHaD and PPTOX. With the abrupt cancellation of the National Children's Study this past year, it will be important for the scientific community to establish a large prospective birth cohort that provides multiple early measures of maternal exposures, frequent observations of offspring over time, clinical databases for subsequent analyses, and biological repositories of stored samples. Although only in the initial planning phase, the Precision Medicine initiative, announced by President Obama and being established by the NIH (27), may offer great opportunities to advance our understanding of PPTOX and the etiology of disease.

Ultimately, the goal of the entire PPTOX community is to provide the sound scientific information necessary to be protective of human health, both for ourselves and future generations. But scientists cannot provide these protections alone. Scientists must engage with a variety of stakeholders who have the ability to directly impact human health. Scientists from across government, industry and academia need to interface with clinicians, policy makers, and, most importantly, directly with consumers to ensure health and wellness for future generations.