p53 wild-type and p53 nullizygous big blue transgenic mice have similar frequencies and patterns of observed mutation in liver, spleen and brain

Transgenic mouse mutation detection systems offer a powerful tool for analysis of spontaneous and induced mutations in vivo. Mice doubly transgenic for a null mutation of the p53 tumor suppressor gene and a lambda shuttle vector harboring the lacI gene were utilized to examine the rate and pattern of spontaneous somatic mutation of the lad transgene in vivo. Three somatic tissues were examined: liver, spleen and brain. At 6 weeks of age, three p53 wild type (+/+) and three p53 nullizygous (-/-), lacI (+/-) male mice were analysed. The mutation frequencies in the two genotypes were similar. The mutant frequencies for wild type (+/+) and nullizygous (-/-) p53 genotypes were, respectively, 4.2×10^-5 and 3.6×10^-5 in the liver, 4.3 × 10^-5 and 3.4 × 10^-5 in the spleen and 2.8 × 10^-5 and 3.0 × 10^-5 in the brain. When the data from the three tissues were combined, the mutant frequency was 3.7 × 10^-5 for the (+/+) genotype and 3.3×10^-5 for the (-/-) genotype. By sequencing both strands in the DNA-binding region of the lacI gene, 91 mutations were found. When recurrent mutations in the same mouse were excluded, a total of 67 definitely independent mutations were found. No statistically significant differences were found in the mutational spectra between the two genotypes when the three tissues were analysed individually or combined (P = 0.58). These findings suggest a need to reconsider the general form of hypothesis that the p53 gene serves as the 'guardian of the genome'.