The role of genetics in determining developmental disorders has been realized for many decades. But it is only relatively recently that a number of environmental chemicals have been recognized to affect developing tissues. One particular class of these, the dioxinlike compounds, are persistent environmental contaminants. Humans are exposed to these chemicals through a number of sources, and thus have a ''background'' body burden. Work in experimental animals indicates that these compounds affect cell proliferation and differentiation, are tumor promoters, and are potent immune, developmental, and reproductive toxicants. These studies also indicate that developing systems are sensitive targets for these chemicals. More recent studies examining children exposed to levels of these compounds that are within a range of background exposures are suggestive that subtle, yet significant, neuroendocrine and immune effects occur. All evidence to date indicates that these compounds produce their biological and toxicological effects by binding to a gene regulatory protein, the Ah receptor, the normal function and endogenous ligand of which have not been identified. Data on the biochemistry of this protein indicates well-regulated and conserved pathways for it to control the synthesis and activity of a number of hormones and growth factors. These and other data suggest that the Ah receptor may be necessary for the normal development of many tissues, including those in the neuroendocrine and immune systems. Further research is necessary to determine whether low-level exposure to these chemicals is related to neuroendocrine-related developmental disabilities. In particular, an exposure level that does or does not lead to functional developmental effects needs to be determined. The ability of these chemicals to act as co-factors in genetically related disabilities also needs to be assessed.
In the absence of this information, avoidance of excess exposure to these chemicals is recommended. This should be guided by knowledge of sources and concentrations in foods and coupled with sound and nutritionally based public health advisories at both the state and local levels.
1997 Wiley-Liss, Inc.