ANALYSIS OF THE K-RAS AND P53 PATHWAYS IN X-RAY-INDUCED LUNG-TUMORS IN THE RAT

The risk from exposure to low-dose radiation in conjunction with cigar ette smoking has not been estimated due in part to limited knowledge s urrounding the molecular mechanisms underlying radiation-induced cance rs. The purpose of this investigation was to determine the frequency f or alterations in genes within the K-ras and p53 signal and cell cycle regulatory pathways, respectively, in X-ray-induced lung tumors in th e F344/N rat. These tumors were examined for genetic alterations in th e K-ras, c-raf-1, p53, mdm2 and cip1 genes. No K-ras mutations were de tected by sequencing in 18 squamous cell carcinomas (SCCs) or 17 adeno carcinomas. However, using a K-ras codon 12 mutation selection assay, a codon 12 GGT --> GAT mutation was detected in one SCC, suggesting th at activation of the K-ras proto-oncogene is both a rare and late even t. Single-strand conformation polymorphism (SSCP) analysis of the kina se-binding domain of the c-raf-1 gene did not detect any polymorphisms . Three of 18 SCCs but none of the adenocarcinomas showed p53 nuclear immunoreactivity. Single-strand conformation polymorphism analysis of exons 4-9 of the p53 gene detected only an exon 9 mutation in one SCC. Mutations were not detected in the three SCCs with immunoreactive p53 protein. No amplification of the mdm2 gene was detected; however, nuc lear mdm2 immunoreactivity was present in one of the three SCCs tat st ained positive for the p53 protein. Thus the increased level of p53 pr otein in one SCC may stem from stabilization by the mdm2 gene product. The complete cDNA of the rat cip1 gene comprising 810 bases was clone d and sequenced. Overall homology between the rat and human cip1 genes was 74%. Homology between the rat and mouse genes was 90%. The freque ncy of somatic mutations in exon 2 of the cip1 gene was determined by SSCP analysis. No alterations in electrophoretic mobility were detecte d. The results of this investigation indicate that alterations in the K-ras and p53 pathways do not play a major role in the genesis of X-ra y-induced lung tumors in the rat. (C) 1996 by Radiation Research Socie ty