Enhanced spontaneous and benzo(a)pyrene-induced mutations in the lung of Nrf2-deficient gpt delta mice

The lung is an organ that is sensitive to mutations induced by chemicals in ambient air, and transgenic mice harboring guanine phosphoribosyltransferase (gpt) gene as a target gene are a well-established model system for assessing genotoxicity in vivo. Transcription factor Nrf2 mediates inducible and constitutive expression of cytoprotective enzymes against xenobiotics and mutagens. To address whether Nrf2 is also involved in DNA protection, we generated nrf2^+/-::gpt and nrf2^-/-::gpt mice. The spontaneous mutation frequency of the gpt gene in the lung was approximately three times higher in nrf2-null (nrf2^-/-) mice than nrf2 heterozygous (nrf2^+/-) and wild-type (nrf2^+/+) mice, whereas in the liver, the mutation frequency was higher in nrf2^-/- and nrf2 ^+/- mice than in nrf2^+/+ wild-type mice. By contrast, no difference in mutation frequency was observed in testis among the three genotypes. A single intratracheal instillation of benzo(a)pyrene (BaP) increased the lung mutation frequency 3.1- and 6.1-fold in nrf^2+/- and nrf2^-/- mice, respectively, compared with BaP-untreated nrf2 ^+/- mice, showing that nrf2^-/- mice are more susceptible to genotoxic carcinogens. Surprisingly, mutation profiles of the gpt gene in BaP-treated nrf^2+/- mice was substantially different from that in BaP-untreated nrf2^-/- mice. In nrf2^-/- mice, spontaneous and BaP-induced mutation hotspots were observed at nucleotides 64 and 140 of gpt, respectively. These results thus show that Nrf2 aids in the prevention of mutations in vivo and suggest that Nrf2 protects genomic DNA against certain types of mutations.