Endocrine disrupting chemicals modulate expression of O6-methylguanine DNA methyltransferase (O6-MGMT) gene in the hermaphroditic fish, Kryptolebias marmoratus

O6-methylguanine-DNA methyltransferase (O6-MGMT; EC 2.1.1.63) is a key repair enzyme that helps to protect the cell against alkylation on DNA by removing a methyl group from the O6-position of guanine. Here, we cloned and sequenced the full-length O6-MGMT cDNA from the hermaphroditic fish, Kryptolebias marmoratus. Complete Km-O6-MGMT cDNA was 1324 bp in length, and the open reading frame of 567 bp encoded a polypeptide of 188 amino acid residues. Phylogenetic analysis revealed that Km-O6-MGMT was clustered with those of other fish species. Embryo, juveniles, and aged secondary fish had low levels of Km-O6-MGMT mRNA than adults, indicating more susceptibility to DNA damage by alkylating agent exposure during these developmental stages. Km-O6-MGMT mRNA levels differed according to tissue type and was highest in the liver. Exposure to an alkylating agent, N-methyl-N-nitrosourea (MNU) exposure increased the mRNA expression of tumor suppressor gene such as p53 and oncogenes such as R-ras1, R-ras3, N-ras, c-fos as well as Km-O6-MGMT mRNA in a time-dependent manner. On the contrary, several (anti)estrogenic compounds (17β-estradiol 100 ng/L, tamoxifen 10 μg/L, bisphenol A 600 μg/L, and 4-tert-octylphenol 300 μg/L) suppressed mRNA expression of Km-O6-MGMT in most tissues, especially the liver. In juvenile fish, 17β-estradiol, bisphenol A, and 4-tert-octylphenol also decreased the expression of Km-O6-MGMT mRNA in a time-dependent manner. Overall, our finding shows that Km-O6-MGMT mRNA levels can be modulated by environmental estrogenic compounds as well as alkylating agents. This finding will be helpful to improve our knowledge of the effects of estrogenic compounds that contain the genotoxic ability to inhibit the DNA repair process in aquatic animals.