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.

To study the importance of phosphorylation for p53transactivation function, we generated mutations ateach of its known phosphorylated serine amino acids.Mutations of murine p53 serine residues individually toeither alanine or glutamic acid at positions 7, 9, 12, 18,37, 312, and 389 resulted in equivalent levels of transcriptionalactivation in standard transient transfectionexperiments. However, when p53 transcriptional activitywas measured in cells that attain G1 arrest uponcontact inhibition, wild-type p53 was inactive, and onlyalteration at serine 389 to glutamic acid resulted in afunctional p53 protein. This Ser 3 Glu mutant also hasan increased ability to bind DNA. Elimination of thephosphorylation site by substitution of an alanineamino acid resulted in loss of transcriptional activity.We also demonstrated that specific phosphorylation ofp53 at serine 389 is induced by cyclin E overexpressionin high-density cells. Our data establish for the first timethat phosphorylation of p53 at serine 389 is important inactivating its function in vivo.

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.