Molecular dosimetry of DNA adducts in the medaka small fish model

Small fish models are being used with increasing frequency for carcinogenicity testing and comparative cancer research in the US, Canada and Europe. However, there is a need to further define the early biochemical events of carcinogenesis in these species. Identification and quantitation of DNA adducts can integrate all of the various factors involved in chemical exposure, uptake, distribution and biotransformation of a putative carcinogen. In the present study, Japanese medaka (Oryzias latipes) were exposed to the alkylating agent, diethylnitrosamine (DEN), in the ambient water. Liver DNA was analyzed for O6-ethylguanine (O6EG), O4-ethylthymidine (O4ET) and O2-ethylthymidine (O2ET) by the immuno-slot-blot technique, using monoclonal antibodies against each adduct of interest. While fish exposed to 10 p.p.m. DEN had liver DNA adduct concentrations at or only slightly higher than background levels, those exposed to 100 p.p.m. DEN averaged 34 and 53 pmol O6EG/micromol guanine, 15 and 41 pmol O2ET/micromol thymidine and 2 and 6 pmol O4ET/micromol thymidine at 0 and 24 h post-exposure, respectively. The results of this study show that, under these short-term exposure conditions, ethyl-DNA adducts appear to accumulate in medaka liver tissue in a sublinear (i.e. non-linear) fashion after aqueous exposure to DEN. Thus, critical DNA repair enzymes such as O6-alkylguanine DNA alkyltransferase, which are relatively efficient at lower carcinogen levels, are probably saturated at the 100 p.p.m. concentration level of DEN.