FREQUENT AND SPECIFIC MUTATIONS OF THE RAT P53 GENE IN HEPATOCARCINOMAS INDUCED BY TAMOXIFEN

Tamoxifen (TAM) is a triphenylethylene antiestrogen used for the treat ment, and in clinical trials for the prevention, of breast cancer in w omen. In rats, TAM is a strong liver carcinogen which induces the form ation of liver DNA adducts. The DNA of 24 hepatocarcinomas (HCCs) coll ected at necropsy from individual female Sprague-Dawley rats that were given 22.6 mg/kg TAM daily for 12 months was studied for the presence of mutations in exons 5-9 of the p53 gene by single-strand conformati on polymorphism and DNA sequencing analysis. The sequences of introns 5-8 of the rat p53 gene were determined in order to design primers hom ologous to regions located in these introns. p53 mutations were found in 50% (12 of 24) of the HCCs. These mutations were all specifically c lustered in two sites, codons 231 (exon 6-7) and 294 (exon 8). Nine HC Cs contained a transition from adenine to guanine in the second base o f codon 231 (CAC to CGC), which resulted in a histidine to arginine am ino acid substitution; 4 HCCs contained a nonmiscoding transition from cytosine to thymidine in the third base of codon 294 (TGC to TGT; cys teine to cysteine). One HCC contained both mutations. The present repo rt supports previous observations on the genotoxicity of TAM in rodent s and raises concerns about its use as a chemopreventive agent against breast cancer in women.