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.

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

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.

negaIndividualswith Li-Fraumeni syndrome carry inherited tive p53 mutants can inhibit the function of the normalmutations in the p53 tumor suppressor gene and are p53 protein, usually through protein-protein interactionspredisposed to tumor development. To examine the (Milner et al., 1991). The dominant-negative hypothesismechanistic nature of these p53 missense mutations, is strongly supported by the observations that manywe generated mice harboring a G-to-A substitution at mutant p53 proteins have an increased half-life (Finlaynucleotide 515 of p53 (p53_/515A) corresponding to the et al., 1988; Hinds et al., 1990; Slingerland et al., 1993)p53R175H hot spot mutation in human cancers. Al- and that they oligomerize with wild-type p53, inhibitingthough p53_/515Amice display a similar tumor spectrum its function (Farmer et al., 1992; Jeffrey et al., 1995;and survival curve as p53_/_mice, tumors fromp53_/515A Milner et al., 1991; Sturzbecher et al., 1992). The formamicemetastasized with high frequency. Correspond- tion of mixed wild-type and mutant p53 molecules coningly,the embryonic fibroblasts from the p53515A/515A verts wild-type p53 into themutant conformation in vitromutant mice displayed enhanced cell proliferation, (Milner et al., 1991). Gain-of-function mutations, on theDNA synthesis, and transformation potential. The dis- other hand, are those missense mutations in which muruptionof p63 and p73 in p53_/_ cells increased trans- tant p53 has additional functions not seen in wild-typeformation capacity and reinitiated DNA synthesis to p53. For example, the p53R175H mutant, when overexlevelsobserved in p53515A/515A cells. Additionally, p63 pressed in a nontransformed cell line lacking p53, yieldsand p73 were functionally inactivated in p53515A cells. tumors in nude mice, while the parental cell line doesThese results provide in vivo validation for the gain-of- not (Dittmer et al., 1993). Transgenic mice overexpressfunctionproperties of certain p53missensemutations ing the human p53R175H mutation in epithelial cellsand suggest a mechanistic basis for these pheno- exhibit an increased susceptibility to chemical carcinotypes.