Modulation of benzo[a]pyrene-induced covalent DNA modifications in adult and fetal mouse tissues by gestation stage

In these studies, we investigated the influence of gestation age on the induction of covalent DNA modifications by benzo[a]pyrene (B[a]P). Timed-pregnant ICR mice were given a single treatment of B[a]P (80 mg/kg, p.o.) on different days of gestation, killed 24 h later and analyzed for the presence of B[a]P-induced DNA adducts using the P1 nuclease version of the 32P-postlabeling method. Our results showed that B[a]P bound to embryonic, placental, fetal and maternal DNA throughout gestation with gestation-stage dependency. Overall, B[a]P bound less to maternal DNA during organogenesis and placentation compared to other stages of gestation and to the non-pregnant stage. The ontogenesis of B[a]P-induced DNA adducts in fetal tissues exhibited organ specificity that had two different types of profiles. With advancing gestation age, one type (lung, carcass and placenta) exhibited a steady linear increase, and the other type [gastrointestinal tract (GIT) and skin] a biphasic increase. In the fetal and maternal organs, adduct levels peaked 2 days before parturition. Over the course of gestation, fetal adduct levels were 70-100% of adult levels in the skin, 7-12% in the GIT, 25-40% in the liver and 15-80% in the lung. The adduct levels in many fetal organs exhibited little relationship to placental adduct levels throughout gestation. Collectively, our results indicate that: (i) transplacental DNA damage induced by B[a]P is determined mainly by fetal competence in metabolic activation and/or detoxification of B[a]P; and (ii) events occurring during placentation and organogenesis inhibit B[a]P binding to maternal tissues.