Age-related mutation accumulation at a lacZ reporter locus in normal and tumor tissues of Trp53-deficient mice

Increased genomic instability has been found associated with cancer and aging. The p53 tumor suppressor protein is a major determinant of genomic instability as a regulator of cell cycle control and apoptosis in response to DNA damage. To investigate the rate of age-related mutation accumulation in the absence of p53, we crossed Trp53 null mice with transgenic mice harboring a lacZ mutational target gene. In the hybrid animals, lacZ mutation frequencies at early age (i.e. at about 2 months) were found to be the same as in the control lacZ animals. However, up until about 6 months, when the Trp53-knockout mice usually die from cancer, mutations were found to accumulate with age in the spleen, and to a lesser extent in the liver, at a more rapid rate than in the control Trp53^+/+ or Trp53^+/-, lacZ hybrid mice. Treatment of 2-3-month-old Trp53^-/-, lacZ hybrid mice with the powerful mutagen ethyl nitrosourea (ENU) resulted in a higher number of mutations induced in the liver but not in the spleen, as compared to the Trp53^+/+, lacZ mice. These results suggest that p53 is not an important determinant of gene mutation induction, either spontaneously during development or after treatment with a mutagen. The accelerated age-related accumulation of mutations in normal spleen and liver could be explained by the defect in apoptosis, which would prevent severely damaged cells from being eliminated.