Mutations in TP53, the gene that encodes the tumour suppressor p53, are found in 50% of human cancers, and increased levels of its negative regulators MDM2 and MDM4 (also known as MDMX) downregulate p53 function in many of the rest. Understanding p53 regulation remains a crucial goal to design broadly applicable anticancer strategies based on this pathway. This Review of in vitro studies, human tumour data and recent mouse models shows that p53 post-translational modifications have modulatory roles, and MDM2 and MDM4 have more profound roles for regulating p53. Importantly, MDM4 emerges as an independent target for drug development, as its inactivation is crucial for full p53 activation.

Drug-selected intrachromosomal gene amplification by breakage-fusion-bridge (BFB) cycles is well documented in mammalian cells, but factors governing this mechanism are not clear. Here, we show that only some clastogenic drugs induce drug resistance through intrachromosomal amplification. We strictly correlate triggering of BFB cycles to induction of fragile site expression. We demonstrate a dual role for fragile sites in intrachromosomal amplification: a site telomeric to the selected gene is involved in initiation, while a centromeric site defines the size and organization of early amplified units. The positions of fragile sites relative to boundaries of amplicons found in human cancers support the hypothesis that fragile sites play a key role in the amplification of at least some oncogenes during tumor progression.

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The mechanisms by which Mdm2 and Mdm4 (MdmX) regulate p53 remain controversial. We generated a mouse encoding p53 lacking the proline-rich domain (p53DeltaP). p53DeltaP exhibited increased sensitivity to Mdm2-dependent degradation and decreased transactivation capacity, correlating with deficient cell cycle arrest and reduced apoptotic responses. p53DeltaP induced lethality in Mdm2-/- embryos, but not in Mdm4-/- embryos. Mdm4 loss did not alter Mdm2 stability but significantly increased p53DeltaP transactivation to partially restore cycle control. In contrast, decreasing Mdm2 levels increased p53DeltaP levels without altering p53DeltaP transactivation. Thus, Mdm4 regulates p53 activity, while Mdm2 mainly controls p53 stability. Furthermore, Mdm4 loss dramatically improved p53DeltaP-mediated suppression of oncogene-induced tumors, emphasizing the importance of targeting Mdm4 in chemotherapies designed to activate p53.

P53 is an unstable transcription factor that is mutated in a majority of human cancers. With a significant role in initiating cell elimination programs, a network has evolved to fine-tune P53 transcriptional output and prevent errant activation. Modifications of the C terminus have long been viewed as critical binary determinants of P53 stability or activation. However, these conclusions are based on in vitro transfection or biochemical analyses where the stoichiometries between P53 and its regulators are perturbed. Therefore, we tested the importance of the C-terminal regulatory region for P53 control in mice where the seven C-terminal lysines were changed to arginine (Trp-53 7KR ). Surprisingly, the homozygous mutant mice are viable and phenotypically normal. We have functionally characterized the mutant protein in both MEFs and thymocytes, revealing the unexpected result that Trp-53 7KR exhibits a normal half-life and functions like WT P53 in cell cycle arrest and apoptosis, and in an E1A-ras xenograft tumor suppression assay. However, a significant difference is that P53 7KR is activated more easily by DNA damage in thymus than WT P53. Importantly, although MEFs encoding WT P53 spontaneously emerge from crisis to become immortal in a 3T3 growth protocol, we do not observe any such escape with the P53 7KR cells. We propose that the C-terminal modifications believed to be critical for proper P53 regulation are not essential, but contribute to a fine-tuning mechanism of homeostatic control in vivo.Mdm2 ͉ p53 ͉ tumor suppressor ͉ Trp-53 ͉ ubiquitination P 53 can respond to a variety of stimuli to either initiate cell cycle arrest or initiate apoptosis. Therefore, it is critical to control the output of this short-lived transcription factor (1). It is generally accepted that P53 regulation involves competition between the E3 ubiquitin ligase MDM2 and histone acetyl transferases (HATs). MDM2-mediated ubiquitination of the conserved C-terminal lysines of P53 initiates proteasome-dependent degradation, while the HATs acetylate the same lysines, resulting in P53 stabilization and activation (2, 3). Other studies report that C-terminal lysine methylation is required for P53 stabilization (4), neddylation inhibits transactivation (3), and sumoylation may have positive or negative roles (5). Regardless of the modification, the C terminus is thought to be critical for P53 stability or activation. However, these conclusions are often based on nonnative conditions, where the ratios between P53 and its regulators are altered. Therefore, we tested the importance of the C-terminal regulatory region for P53 control in mice where the seven C-terminal lysines were changed to arginine (Trp-53 7KR ). MethodsTargeting Construct. Using ET recombination (GeneBridges, Dresden Germany), we recovered the WT Trp-53 locus from a 129 strain mouse BAC (a generous gift from Manuel Serrano, Spanish National Cancer Center, Madrid) with the following primers 5Ј-GAT AAG CAA GAC ATT GTA CCA GCA ATT AAC CCC CCA CCC CCA ACT CCA TAG CGG CCG CTG AAG ...

The gene TP53, encoding transcription factor p53, is mutated or deleted in half of human cancers, demonstrating the crucial role of p53 in tumor suppression. Importantly, p53 inactivation in cancers can also result from the amplification / overexpression of its specific inhibitors MDM2 and MDM4 (also known as MDMX). The presence of wild-type p53 in those tumors with MDM2 or MDM4 overexpression stimulates the search for new therapeutic agents to selectively reactivate it. This short survey highlights recent insights into MDM2 and MDM4 regulatory functions and their implications for the design of future p53-based anticancer strategies. We now know that MDM2 and MDM4 inhibit p53 in distinct and complementary ways: MDM4 regulates p53 activity, while MDM2 mainly regulates p53 stability. Upon DNA damage, MDM2-dependent degradation of itself and MDM4 contribute significantly to p53 stabilization and activation. These and other data imply that the combined use of MDM2 and MDM4 antagonists in cancer cells expressing wild type p53 should activate p53 more significantly than agents that only antagonize MDM2, resulting in more effective anti-tumor activity.

Two‐colour in situ hybridization with probes for two co‐amplified markers located several megabases apart on chromosome 1 has been used to analyse early stages of adenylate deaminase 2 (AMPD2) gene amplification in Chinese hamster cells. In the amplified chromosomal structures, the distribution of hybridization spots identifies megabase‐long inverted repeats. Their organization is remarkably well accounted for if breakage‐fusion‐bridge cycles involving sister chromatids drive the amplification process at these early stages. During interphase the markers often segregate into distinct nuclear domains. Many nuclei have bulges or release micronuclei, carrying several copies of one or both markers. These observations indicate that the amplified units destabilize the nuclear organization and eventually lead to DNA breakage during interphase. We propose a model in which interphase breakage has a role in the progression of gene amplification.

Mutations in p53, although frequent in human cancers, have not been implicated in telomere-related syndromes. Here, we show that homozygous mutant mice expressing p53Δ31, a p53 lacking the C-terminal domain, exhibit increased p53 activity and suffer from aplastic anemia and pulmonary fibrosis, hallmarks of syndromes caused by short telomeres. Indeed, p53Δ31/Δ31 mice had short telomeres and other phenotypic traits associated with the telomere disease dyskeratosis congenita and its severe variant the Hoyeraal-Hreidarsson syndrome. Heterozygous p53+/Δ31 mice were only mildly affected, but decreased levels of Mdm4, a negative regulator of p53, led to a dramatic aggravation of their symptoms. Importantly, several genes involved in telomere metabolism were downregulated in p53Δ31/Δ31 cells, including Dyskerin, Rtel1, and Tinf2, which are mutated in dyskeratosis congenita, and Terf1, which is implicated in aplastic anemia. Together, these data reveal that a truncating mutation can activate p53 and that p53 plays a major role in the regulation of telomere metabolism.