Analysis of carcinogen-exposed Japanese medaka ( Oryzias latipes) for mutations in K -ras and p53 genes

One of the best developed laboratory models for comparative carcinogenesis studies is the Japanese medaka ( Oryzias latipes). The medaka has been used for carcinogenicity testing for well over a decade. Our laboratory has been addressing the molecular basis of tumorigenesis in this fish model. Cellular oncogenes and tumor suppressor genes regulate normal cell growth and differentiation. Mutations which affect their expression and/or protein structure may contribute to multistage carcinogenesis. We have exposed medaka to the known carcinogens: diethylnitrosamine (DEN), methylazoxymethanol acetate (MAMAc), N-methyl- N′-nitro- N-nitrosoguanidine and methylene chloride. We are examining tumors induced by these chemicals for mutations in the K -ras cellular oncogene and p53 tumor suppressor gene. We have cloned and sequenced both the medaka K- ras and p53 genes. We have observed mutations at the 12th codon in K- ras in DEN- and MAMAc-exposed medaka. We are using a rapid mutation screening method based on the nonisotopic RNase cleavage assay to screen for other mutations in both K- ras and p53.