Differential Effect of Gestation Stage on Benzo(a)pyrene-induced Micronucleus Formation and/or Covalent DNA Modifications in Mice

Benzo(a)pyrene (BP), an environmental carcinogen, binds ubiquitously to the DNA of maternal and fetal tissues (Lu et al. Cancer Res., 46: 3046–3054, 1986). These studies further investigated the effect of gestation age on the induction of genetic damage by BP. Timed-pregnant ICR mice were treated with one dose of BP on various days of gestation and sacrificed 24–120 h after treatment. At the molecular level, BP covalently bound to the DNA of adult bone marrow and fetal liver of mice at all gestation ages. Compared to the nonpregnant mice (adduct level = 15 adducts/108 bases), the adduct levels in the pregnant adult bone marrow were decreased up to 50% during early gestation (days 3–9) and then increased steadily to a 4–fold excess over nonpregnant values during late gestation (days 15–18). In the fetal liver, adduct levels exhibited little variation (3–4 adducts/108 bases) between days 11 and 15 of gestation and then increased sharply to 14 adducts/108 bases after day 16. At the cellular level, a higher percentage of polychromatic RBCs from adult and fetal mice after BP treatment contained micronuclei (MN) than controls (solvent or untreated). Bone marrow from pregnant mice exhibited greater increases in the formation of MN during early gestation (days 3–9) relative to late gestation (days 15–18), compared to the nonpregnant mice. In the fetuses, the amounts of MN formed were higher than those found in the adult nonpregnant or maternal mice, but these amounts decreased with gestation progression. Thus, MN induction with gestation progression differed from DNA adduction in adults and fetuses. In addition, the dose and time responses of MN formation also differed from those of covalent DNA modifications, when analyzed in the bone marrow of pregnant mice treated on gestation day 5. Collectively, our results showed that pregnancy and development modulate different types of genetic damage in different ways. Fetal tissues may be more sensitive than maternal tissues to genetic damage. Factors in addition to DNA adduct formation may be responsible for MN induction.