Evidence for Ras-dependent signal transduction in phenytoin teratogenicity.

Reactive oxygen species (ROS) can transduce intracellular signals or damage macromolecules, including mutational activation of the K-ras oncogene, particularly in A/J mice, which are reported to be highly sensitive to the ROS-initiating teratogen phenytoin. Here, we determined in embryo culture whether the Ras pathway mediated phenytoin embryopathy at the protein or gene level. Embryos from pregnant inbred A/J dams and outbred CD-1 dams were cultured with a therapeutic concentration of phenytoin, with or without alpha-hydroxyfarnesylphosphonic acid, an inhibitor of the enzyme farnesyl-protein transferase, which is required for posttranslational Ras activation. A/J and CD-1 embryos were similarly susceptible to phenytoin embryopathies, with reductions in anterior neuropore closure, turning, yolk sac diameter, and somite development (p < 0.05). The farnesyl-protein transferase inhibitor blocked phenytoin embryotoxicity in A/J embryos for all parameters except yolk sac diameter (p < 0.05) and completely blocked embryotoxicity in CD-1 embryos (p < 0.05). Embryonic DNA did not show phenytoin-initiated mutations in codon 12 of the K-ras gene in either A/J or CD-1 embryos, but phenytoin substantially increased the levels of GTP-bound Ras in both CD-1 and A/J embryos. These results provide the first direct evidence that Ras proteins may be involved in the teratogenicity of phenytoin, likely via a mechanism other than mutational activation.