HIGH SUSCEPTIBILITY OF P53(+ -) KNOCKOUT MICE IN N-BUTYL-N-(4-HYDROXYBUTYL)NITROSAMINE URINARY-BLADDER CARCINOGENESIS AND LACK OF FREQUENT MUTATION IN RESIDUAL ALLELE/

The Loss of p53 functions is considered to compromise the growth-suppr ession machinery of the cell and facilitate neoplastic change, In huma ns, genetic alteration in the p53 gene is one of the most frequently o bserved molecular changes in tumors, including urinary bladder carcino mas, We have investigated the susceptibility of heterozygote p53 knock out mice to N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in terms of ur inary 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 C5 7BL/6 strain, greater lesion yields were observed in knockout mice aft er 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 trea tment groups, respectively, whereas only 1 (11%) and 6 (67%) of each 9 of the C57BL/6 mice demonstrated tumors, Preneoplastic lesions (dyspl asia) were also observed more frequently in the lower dose groups in t he knockout mice than C57BL/6 mice. PCR single-strand conformation pol ymorphism analysis followed by DNA direct sequencing of the p53 gene ( exons 5-8) extracted from bladder tumors demonstrated mutations in 3 o f 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 s eparate experiment, 5-bromo-2'-deoxyuridine labeling indices after tre atment 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 meta bolite, revealed no significant differences between knockout and origi nal parent strain after administration of 0.0075% BBN in the drinking water for 4 weeks. In conclusion, knockout mice are distinctly more se nsitive to urinary bladder carcinogenesis induced by BBN than their or iginal parent strain, as evidenced by elevated DNA synthesis during ca rcinogen administration and an increased tumor yield, The high suscept ibility of p53 knockout mice appeared to be related to the high level of cell proliferation rather than that of N-butyl-N-(3-carboxypropyl)n itrosamine in the urine or that of mutations at the p53 gene.