p53 and Ha-ras mutations in chemically induced hamster buccal pouch carcinomas.

Well-differentiated squamous cell carcinomas were induced in hamster buccal pouch epithelium by twice weekly topical applications of N-methyl-N-benzylnitrosamine (MBN) or 7,12-dimethylbenz[a]anthracene (DMBA) over a period of 15 weeks. Each of the 22 tumors induced (14 MBN and eight DMBA) were evaluated by single-strand conformation polymorphism and DNA sequencing to identify mutations in conserved exons (E5-E8) of the p53 tumor suppressor gene and codons 12/13 and 61 of Ha-ras. In addition, Northern blot analysis of 10 MBN tumors and five DMBA tumors was performed to determine whether the mdm-2 gene was overexpressed, p53 mutations were detected in five of 14 (35%) MBN-induced carcinomas and in two of eight (25%) DMBA-induced carcinomas. Ha-ras mutations were detected in three of 14 (21%) MBN-induced carcinomas and in three of eight (37%) DMBA-induced carcinomas. One MBN-induced carcinoma exhibited a mutation in both the p53 and Ha-ras genes. The majority (five of seven of p53/Ha-ras mutations induced by MBN were G->A transitions and two of these occurred at hamster p53 codon 248, which corresponds to human p53 codon 245, a known mutational tumor 'hot spot'. A->T transversion at Ha-ras codon 61 accounted for three of five (60%) DMBA-induced mutations. There was no evidence of mdm-2 overexpression in any of the tumors evaluated. Overall, the results provide additional support for the validity of the hamster buccal pouch model of oral carcinogenesis, as applied to sequential cellular and molecular analysis and cancer chemoprevention studies.