Distinct downstream targets manifest p53-dependent pathologies in mice.

Mdm2, the principal negative regulator of p53, is critical for survival, a fact clearly demonstrated by the p53-dependent death of germline or conditional mice following deletion of Mdm2. On the other hand, Mdm2 hypomorphic (Mdm2Puro/Δ7-12) or heterozygous (Mdm2+/-) mice that express either 30 or 50% of normal Mdm2 levels, respectively, are viable but present distinct phenotypes because of increased p53 activity. Mdm2 levels are also transcriptionally regulated by p53. We evaluated the significance of this reciprocal relationship in a new hypomorphic mouse model inheriting an aberrant Mdm2 allele with insertion of the neomycin cassette and deletion of 184-bp sequence in intron 3. These mice also carry mutations in the Mdm2 P2-promoter and thus express suboptimal levels of Mdm2 entirely encoded from the P1-promoter. Resulting mice exhibit abnormalities in skin pigmentation and reproductive tissue architecture, and are subfertile. Notably, all these phenotypes are rescued on a p53-null background. Furthermore, these phenotypes depend on distinct p53 downstream activities as genetic ablation of the pro-apoptotic gene Puma reverts the reproductive abnormalities but not skin hyperpigmentation, whereas deletion of cell cycle arrest gene p21 does not rescue either phenotype. Moreover, p53-mediated upregulation of Kitl influences skin pigmentation. Altogether, these data emphasize tissue-specific p53 activities that regulate cell fate.