Loss of Mdm2 or Mdm4 leads to embryo lethal phenotypes that arep53-dependent. To determine whether Mdm2 and Mdm4 inhibitp53 function redundantly in a more restricted cell type, conditionalalleles were crossed to a neuronal specific Cre transgene to deleteMdm2 and Mdm4 in the CNS. Mice lacking Mdm2 in the CNSdeveloped hydranencephaly at embryonic day 12.5 due to apoptosis,whereas Mdm4 deletion showed a proencephaly phenotypeat embryonic day 17.5 because of cell cycle arrest andapoptosis. The deletion of both genes, strikingly, contributed to aneven earlier and more severe CNS phenotype. Additionally, Mdm2and Mdm4 had a gene dosage effect, because loss of three of thefour Mdm alleles also showed a more accelerated CNS phenotypethan deletion of either gene alone. All phenotypes were rescued bydeletion of p53. Thus, these in vivo data demonstrate the importanceof Mdm4 independent of Mdm2 in inhibition of p53

The p53 protein integrates multiple upstream signals andfunctions as a tumor suppressor by activating distinctdownstream genes1–3. At the cellular level, p53 inducesapoptosis, cell cycle arrest and senescence. A rare mutant formof p53 with the amino acid substitution R175P, found in humantumors, is completely defective in initiating apoptosis butstill induces cell cycle arrest4,5. To decipher the functionalimportance of these pathways in spontaneous tumorigenesis,we used homologous recombination to generate mice withmutant p53-R172P (the mouse equivalent of R175P in humans).Mice inheriting two copies of this mutation (Trp53515C/515C)escape the early onset of thymic lymphomas that characterizeTrp53-null mice. At 7 months of age, 90% of Trp53-null micehad died, but 85% of Trp53515C/515C mice were alive andtumor-free, indicating that p53-dependent apoptosis was notrequired for suppression of early onset of spontaneous tumors.The lymphomas and sarcomas that eventually developed inTrp53515C/515C mice retained a diploid chromosome number, insharp contrast to aneuploidy observed in tumors and cells fromTrp53-null mice. The ability of mutant p53-R172P to induce apartial cell cycle arrest and retain chromosome stability arecrucial for suppression of early onset tumorigenesis.

To understand the relevance of p53 missense mutations in vivo, we generated a mouse containing an arg-to-his substitution at p53 amino acid 172, which corresponds to the R175H hot-spot mutation in human tumors by homologous recombination. Inadvertently, this mouse contains the additional deletion of a G nucleotide at a splice junction that attenuates levels of mutant p53 to near wild-type levels. Mice heterozygous for the mutant allele differed from p53___ mice in tumor spectrum, with a significant increase in the number of carcinomas and a slight decrease in the number of lymphomas. More importantly, the osteosarcomas and carcinomas that developed in these mutant mice frequently metastasized (69% and 40%, respectively). In contrast, metastasis is rare in osteosarcomas of p53___ mice. Loss of heterozygosity studies of tumors indicated loss of heterozygosity in only 1 of 11 tumors. These data indicate clear differences between a p53 missense mutation and a null allele in tumorigenesis in vivo and suggest that the p53R172H_g mutant represents a gain-of-function allele.

Mice lacking one or two copies of the p53 gene haveprovided invaluable insight into the process of tumorigenesis.The importance of apoptosis in suppression of tumorigenesisin vivo became evident from analysis of these mice.Moreover, the timing and kinds of tumors that develop inthese mice are altered by the presence of additional inheritedmutations, by strain differences, and by food intake. Developmentalabnormalities are also visible in mice with loss ofp53 and with overexpression of p53 suggesting that p53levels are critical for normal cellular processes. While mice donot necessarily recapitulate all the tumor types found ininherited cancers, they offer the unique opportunity to decipherthe critical pathways in tumorigenesis. These findingscan then be applied to humans.

Progression through the cell cycle is regulated by numerous proteins, one of which is the cyclin-dependent kinase inhibitor, p21. A new study identifies a novel protein complex that stabilizes p21. The stability of this complex is critical in effecting the p53-mediated cell cycle checkpoint.