Controlled laboratory experiments were conducted in which adult tomcod were collected from the HR or Miramichi River (MR) and treated with graded doses of TCDD, coplanar PCBs, and PAHs. High levels of hepatic CYP1A1 mRNA expression were observed with all chemicals in tomcod from the MR - maximal levels of gene induction ranged between 50 and 460-fold (Courtenay et al., 1999; Yuan, 2003). Surprisingly, tomcod from the HR responded very differently than those from the MR to treatments with TCDD and four coplanar PCBs, but not to B[a]P and a second PAH. Much higher treatment doses of PCBs or TCDD were needed to induce significant CYP1A1 expression in tomcod from the HR, although maximum levels of gene induction for each compound were similar for the two populations (Fig. 30.2).
Genetic polymorphisms were reported in the coding regions of CYP1A1 (Roy et al., 1995) and AHR (Roy and Wirgin, 1997) among tomcod populations. For both genes, tomcod from the HR exhibited genotypes that were absent in tomcod from elsewhere. It is tempting to speculate that these polymorphisms contribute to the reduced inducibility of CYP1A1, however empirical evaluations of the functional importance of the variant alleles have yet to be conducted.
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