2,3,7,8-Tetrachlorodibenzo-p-dioxin: Segregation of Toxicity with the Ah Locus

2,3,7,8-Tetrachlorodibenzo- p -dioxin (TCDD) and related halogenated aromatic hydrocarbons (a) produce a similar pattern of toxic responses; (b) induce a battery of enzymes, including aryl hydrocarbon hydroxylase (AHH) activity; and (c) bind reversibly and with a high affinity to a cytosol protein that is the receptor for this enzyme induction response. To test the hypothesis that the toxic effects produced by these compounds are mediated by their binding to the induction receptor, we examined the genetic segregation of two toxic responses, with the Ah locus, which determines this receptor. The dose-response relationship for thymic atrophy produced by TCDD was examined in C57BL/6J mice, which have a high affinity receptor and are sensitive to enzyme induction, DBA/2J mice, which have a lower affinity receptor and are less sensitive to induction of AHH activity, and hybrid B6D2F 1 /J mice. C57BL/6J mice were approximately 10-fold more sensitive to thymic involution than DBA/2J mice and the hybrid B6D2F 1 /J were intermediate between the two parental strains. The progeny of a B6D2F 1 /J x DBA/2J mating were phenotyped (as Aa or aa), and the mice heterozygous for the high affinity receptor were more sensitive to thymic atrophy. The capacity of other halogenated aromatic hydrocarbons to produce thymic atrophy corresponded to their capacity to bind to the induction receptor, and nonhalogenated compounds which are agonists for the receptor (3-methylcholanthrene, β-naphthoflavone) also produced this toxic response. TCDD produced a dose-related incidence of cleft palate formation in the fetuses of C57BL/6J mice. A single dose of 30 µg/kg of TCDD on day 10 of pregnancy produced an incidence of cleft palates of 54% in C57BL/6J fetuses, 13% in B6D2F 1 /J fetuses and only 2% in DBA/2J fetuses. Five inbred strains of mice with a low affinity receptor developed only a 0-3% incidence of cleft palates from TCDD (30 µg/kg), while four of five inbred strains with a high affinity receptor developed a 50% or greater incidence. For TCDD and congeners, (a) the correspondence of the structure-activity relationship for receptor binding and for toxic potency and (b) the segregation of thymic involution and probably teratogenesis with the Ah locus indicate that binding of these compounds to the receptor is an essential step in their mechanism of toxicity.