The loss of p53 functions is considered to compromise the growth-suppression machinery of the cell and facilitate neoplastic change. In humans, genetic alteration in the p53 gene is one of the most frequently observed molecular changes in
tumors, including urinary bladder
carcinomas. We have investigated the susceptibility of heterozygote p53 knockout mice to
N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in terms of urinary bladder
tumor induction. Both p53(+/-) knockout mice and C57BL/6 original parent strain were administered 0, 0.002, 0.004, 0.0075 and 0.025% BBN in the
drinking water for 20 weeks. As compared with the C57BL/6 strain, greater lesion yields were observed in knockout mice after 20 weeks of treatment.
Transitional cell carcinomas were found in 9 (75%) and 12 (100%) of each 12 mice of the 0.0075 and 0.025% BBN treatment groups, respectively, whereas only 1 (11%) and 6 (67%) of each 9 of the C57BL/6 mice demonstrated
tumors. Preneoplastic lesions (dysplasia) were also observed more frequently in the lower dose groups in the knockout mice than C57BL/6 mice. PCR single-strand conformation polymorphism analysis followed by
DNA direct sequencing of the p53 gene (exons 5-8) extracted from
bladder tumors demonstrated mutations in 3 of 11 (27.3%; exon 7) and 8 of 29 (27.6%; exons 5-8)
tumors in C57BL/6 and knockout mice, respectively. There was no significant difference in the mutation rates at the residual p53 gene between the two cases. All mutations observed in knockout mice were restricted to the normal allele, and none were present in the gene-targeted null allele. In a separate experiment,
5-bromo-2'-deoxyuridine labeling indices
after treatment with BBN for 2 or 4 weeks were significantly higher in knockout mice than wild-type mice. Measurement of the urinary concentration of N-butyl-N-(3-carboxypropyl)nitrosamine, a proximate carcinogenic metabolite, revealed no significant differences between knockout and original parent strain after administration of 0.0075% BBN in the
drinking water for 4 weeks. In conclusion, knockout mice are distinctly more sensitive to urinary bladder
carcinogenesis induced by BBN than their original parent strain, as evidenced by elevated
DNA synthesis during
carcinogen administration and an increased
tumor yield. The high susceptibility of p53 knockout mice appeared to be related to the high level of cell proliferation rather than that of N-butyl-N-(3-carboxypropyl)nitrosamine in the urine or that of mutations at the p53 gene.