Wells, PG; Bhuller, Y; Mccallum, GP; Preston, TJ; Wiley, MJ; Wong, AW
Endogenous or xenobiotic-enhanced embryonic formation of reactive oxygen species (ROS) may cause embryopathies by oxidatively damaging cellular macromolecules and/or altering signal transduction. We have used drugs (phenytoin, thalidomide, methamphetamine), environmental chemicals (benzo[a]pyrene) and gamma irradiation to examine this hypothesis in mouse and rabbit models. Embryonic prostaglandin H synthases (PHSs) and lipoxygenases can bioactivate xenobiotics to free radical intermediates that initiate ROS formation, resulting in oxidation of proteins, lipids and DNA. Oxidative DNA damage and embryopathies are reduced in PHS knockout mice, and in mice treated with PHS inhibitors, antioxidative enzymes, antioxidants and free radical trapping agents. Thalidomide causes embryonic DNA oxidation in susceptible (rabbit) but not resistant (mice) species. Embryopathies are increased in mutant mice deficient in the antioxidative enzyme glucose-6-phosphate dehydrogenase (G6PD), or by reductions in glutathione (GSH) or GSH-dependent enzymes. Inducible nitric oxide synthase (iNOS) knockout mice are partially protected, but the absence of embryonic iNOS protein expression in wild-type controls suggests extra-embryonic diffusible factors. Inhibition of Ras or NF-kappaB pathways with chemical or antisense inhibitors reduces embryopathies, implicating ROS-mediated signal transduction. Transgenic mice expressing the bacterial DNA repair protein fpg exhibit lower constitutive DNA oxidation, and stably transfected cells expressing either fpg or its human homolog Ogg1 are protected from DNA oxidation and cytotoxicity caused by ROS initiators. Conversely, Atm, p53, ogg1 and CSB knockout mice deficient in DNA damage response/repair exhibit enhanced oxidative DNA damage, embryopathies and/or postnatal neurodevelopmental deficits, suggesting a teratological role for DNA damage. Even endogenous embryonic oxidative stress carries a risk, since untreated G6PD- or ATM-deficient mice have increased embryopathies. Thus, oxidative DNA damage may constitute an embryopathic molecular lesion, and embryonic processes regulating the balance of ROS signaling, oxidative DNA damage and repair may be important determinants of teratological risk.