Exposure assessment for endocrine disruptors: some considerations in the design of studies - Endocrine disruptors: mini-monograph

Environmental Health Perspectives, Oct, 2003 by Carol Rice, Linda S. Birnbaum, James Cogliano, Kathryn Mahaffey, Larry Needham, Walter J. Rogan, Frederick S. vom Saal

In studies designed to evaluate exposure--response relationships in children's development from conception through puberty, multiple factors that affect the generation of meaningful exposure metrics must be considered. These factors include multiple routes of exposure; the timing, frequency, and duration of exposure; need for qualitative and quantitative data; sample collection and storage protocols; and the selection and documentation of analytic methods. The methods for exposure data collection and analysis must be sufficiently robust to accommodate the a priori hypotheses to be tested, as well as hypotheses generated from the data. A number of issues that must be considered in study design are summarized here. Key words: developing child, endocrine disruptors, environmental epidemiology, exposure assessment. Environ Health Perspect 111:1683-1690 (2003). doi: 10.1289/ehp.5798 available via http://dx.doi.org/ [Online 18 March 2003]

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The assessment of human exposure to environmental chemicals requires an understanding of the source of the chemical, its transport and environmental fate, and subsequent routes of entry into the body. Once an exposure occurs, it is necessary to have data on its absorption, distribution, metabolism, and elimination. This is a complex undertaking for any environmental chemical and for any potentially exposed population. It is especially challenging in the evaluation of exposures to children at various stages of their development. The activity of the endocrine system is vital throughout all stages of human life, but it is particularly critical during the stages of greatest human development--in utero, during infancy, early childhood, and puberty.

Exogenous chemicals such as endocrine disruptors are of special interest because they mimic, block, or in some way alter the activity of endogenous chemicals that are synthesized by the endocrine system (National Research Council 1999). The endocrine system as used here refers to all compounds involved in communicating information that are secreted by cells. These compounds can have autocrine effects (on the same cell), paracrine effects (on cells nearby), and classic endocrine effects (secretion is into the blood stream, potentially exposing all cells to the compound) (National Research Council 1999). The class of endocrine disruptors about which we know the most are those that mimic or block endogenous estrogens. Examples of synthetic chemicals with reported estrogenic activity are listed in Table 1.

Dietary exposures to some potential endocrine disruptors have been quantified for adults. For example, phytoestrogen consumption (expressed as total bioflavoids) is estimated at 1 g/day, whereas 100 g of wheat germ with 2 ppm zearalenone provides 200 [micro]g/day (National Research Council 1999). The estimated exposure to dichlorodiphenyltrichloroethane (DDT) from all dietary sources in the general U.S. population is 0.01 [micro]g/day, and estimated exposure to polychlorinated biphenyls (PCBs) is 0.002 [micro]g/day (National Research Council 1999). However, exposures to environmental contaminants may vary among countries; for example, in areas where DDT is still used extensively, breast milk may be a significant source of DDT exposure for an infant (Kashyap et al. 1991).

Akland et al. (2000) have modeled the various factors influencing dietary exposures of young children. However, at different human developmental stages, not only do the diets vary but also the primary route of human exposure may vary; for example, Hubal et al. (2000) described the challenge of assessing children's residential exposure to pesticides.

In addition to dietary sources, other aspects of exposure assessment that should be considered in developing a research initiative include timing or age at exposure; the rate of exposure (frequency and duration); qualitative versus quantitative assessment; sample collection and storage; sample analysis; and exposures assessed to test hypotheses and those to generate additional hypotheses. Each of these aspects is treated in more detail below.

Routes of Exposure and Mechanisms of Response

The earliest relevant exposures may occur before conception. For example, the number of female births relative to total births to parents who resided in the most highly dioxin-contaminated area around Seveso, Italy, at the time of the dioxin release in 1976 was statistically higher than expected for the 8 years after the accident, presumably because of high dioxin exposures to the father (Mocarelli et al. 2000).

Epigenetic imprinting of genes is the basis for the finding that chromosomes provided to an embryo by the egg and sperm are fundamentally different (Jiang et al. 1998). Epigenetic imprinting refers to the mechanism(s) by which hormones and chemicals that mimic hormones permanently alter cellular functions when exposure occurs during organogenesis in fetal life; this also occurs inappropriately during carcinogenesis, causing a breakdown in cell cycle control systems. One mechanism that is generating considerable interest is the epigenetic modification of DNA by addition of methyl groups to specific bases located in the promoter region of genes. DNA methylation consists of the covalent addition of methyl groups to the 5-position of cytosines that are 5' to guanine nucleotides in the DNA sequence. CpG dinucleotide sequences can occur as clusters in regions known as CpG islands, which are normally protected from DNA methylation (Gardiner-Garden and Frommer 1987). When methylation occurs at these normally protected sites, changes occur in chromosome structure and in the capacity for the gene controlled by the promoter to be activated. Inappropriate methylation of genes can act in a manner analogous to that of a classic genetic mutation and can cause the lack of a functional protein product. The result can be a subsequent breakdown in homeostasis, producing functional changes that could easily be missed in short-term tests for acute toxicity in adults or in vitro tests for classic gene mutations involving changes in base sequences (lost and Saluz 1993). Thus, current toxicologic testing methods might not reveal this type of functional damage.