G → A mutation of ras genes and infrequent p53 gene mutation in rat transplantable thyroid carcinoma lines from tumors induced in vivo by N-bis(2-hydroxypropyl)nitrosamine

In a comparative study, we have investigated mutational activation of three kinds of ras genes and the p53 gene (exons 5–8) in 19 rat transplantable thyroid carcinoma lines derived from in vivo tumors induced by DHPN. Mutations were identified using single-strand conformation polymorphism and DNA sequencing analysis, and activated ras oncogenes were detected in 6 lines (31%). These all had mutations in Ki-ras codons at 12 or 63, and one of them also possessed a Ha-ras mutation at codon 12 as a double mutation. Three mutations of Ki-ras at codon 12 involved the second nucleotide and two the first position, the other being found at the first nucleotide in codon 63. Base alterations of p53 gene were found in two lines. One had an insertion of 1 base (thymine) between codon 206 and codon 207. Histologically, it was diagnosed as a poorly differentiated papillary carcinoma (scirrhous pattern). In the another case, there was no amino acid change although one base substitution occurred at codon 283 (GAG → GAA) of exon 8. These results indicate that, in the ras family, DHPN induces Ki-ras gene activation preferentially and that p53 mutation may be infrequent in thyroid carcinogenesis in rats, our data thus corresponding well with the previous reports that an inactivated p53 gene only plays a major role in human undifferentiated thyroid carcinomas.