Elevated susceptibility of the p53 knockout mouse esophagus to methyl-N-amylnitrosamine carcinogenesis

Mutations of the p53 tumor suppressor gene constitute one of the most frequent molecular changes in a wide variety of human cancers, including those in the esophagus. Mice deficient in p53 have recently attracted attention for their potential to identify chemical genotoxins. In this study we investigated the susceptibility of p53 nullizygous (-/-), heterozygous (+/-) and wild-type (+/+) mice to methyl-N-amylnitrosamine (MNAN), which specifically induces esophageal tumors in mice and rats. The p53 (+/-) and (+/+) mice were treated with 5 or 15 p.p.m. MNAN in their drinking water for 8 weeks then maintained without further treatment for an additional 7 or 17 weeks, being killed at experimental weeks 15 or 25. An additional group of p53 (-/-) mice were given 5 p.p.m. MNAN for 8 weeks and killed at week 15. At 15 weeks in the 5 p.p.m. groups, squamous cell carcinomas (SCCs) were observed in 10/12 (83.3%) p53 (-/-) and 1/15 (6.7%) p53 (+/-) mice, but in none of the p53 (+/+) mice. In the animals receiving 15 p.p.m., 2/14 (14.3%) p53 (+/-) and 1/11 (9.1%) p53 (+/+) mice developed SCCs. At 25 weeks, the incidence of SCCs was 7/16 (43.8%) and 8/14 (57.1%) in p53 (+/-) mice and 1/13 (7.7%) and 2/10 (20.0%) in p53 (+/+) mice at 5 and 15 p.p.m., respectively. Of the SCCs examined by PCR-single strand conformation polymorphism analysis, 61% (14/23) from p53 (+/-) and 50% (6/12) from p53 (+/+) mice demonstrated mutations in thep53 gene (exons 5-8). These results indicate the order of susceptibility to MNAN-induced esophageal tumorigenesis to be as follows: nullizygotes (-/-) > heterozygotes (+/-) > wild-type (+/+), and provide strong evidence of involvement of p53 mutations in the development of esophageal SCCs.