Chromosome stability, in the absence of apoptosis, is critical for suppression of tumorigenesis in Trp53 mutant mice

Abstract

The p53 protein integrates multiple upstream signals and functions as a tumor suppressor by activating distinct downstream genes 1-3 . At the cellular level, p53 induces apoptosis, cell cycle arrest and senescence. A rare mutant form of p53 with the amino acid substitution R175P, found in human tumors, is completely defective in initiating apoptosis but still induces cell cycle arrest 4,5 . To decipher the functional importance of these pathways in spontaneous tumorigenesis, we used homologous recombination to generate mice with mutant p53-R172P (the mouse equivalent of R175P in humans). Mice inheriting two copies of this mutation (Trp53 515C/515C ) escape the early onset of thymic lymphomas that characterize Trp53-null mice. At 7 months of age, 90% of Trp53-null mice had died, but 85% of Trp53 515C/515C mice were alive and tumor-free, indicating that p53-dependent apoptosis was not required for suppression of early onset of spontaneous tumors. The lymphomas and sarcomas that eventually developed in Trp53 515C/515C mice retained a diploid chromosome number, in sharp contrast to aneuploidy observed in tumors and cells from Trp53-null mice. The ability of mutant p53-R172P to induce a partial cell cycle arrest and retain chromosome stability are crucial for suppression of early onset tumorigenesis.We introduced the 515G→C mutation into the Trp53 locus by homologous recombination into embryonic stem (ES) cells and cre-loxP-mediated excision in mice (Fig. 1). Sequencing of the entire coding region of the Trp53 transcript from a homozygous mutant mouse found no other changes. The allele Trp53 515C expressed a fulllength mutant p53 protein with the amino acid substitution R172P, which was more abundant than wild-type p53 (Fig. 1d).To assay the cell cycle arrest function, we treated subconfluent cultures of wild-type, Trp53-null, and Trp53 515C/515C mouse embryonic fibroblasts (MEFs) with γ-radiation and labeled them with 5-bromo-deoxyuridine (BrdU) to measure the number of cells in S phase 6 . The ratio of cells in S phase among cells treated with γ-radiation versus untreated cells was significantly lower in Trp53 515C/515C MEFs than in Trp53-null MEFs (Fig. 2a), but the

Krüppel-like factor 4 exhibits antiapoptotic activity following γ-radiation-induced DNA damage

“…Apoptosis is an evolutionarily conserved process that enables an organism to remove unwanted or damaged cells (Yu and Zhang, 2005). Several studies have indicated that the induction of apoptosis is an essential function of the tumor suppressor, p53 (Symonds et al, 1994;Schmitt et al, 2002;Liu et al, 2004), and that this function has been shown to exist in both normal (Clarke et al, 1993;Lowe et al, 1993a, b) and cancerous cells (Bunz et al, 1999). Other studies have indicated that this function is conserved in lower eukaryotes, at least in response to DNA damage (Jin et al, 2000;Schumacher et al, 2001;Brodsky et al, 2004).…”

P53 in blind subterranean mole rats – loss-of-function versus gain-of-function activities on newly cloned Spalax target genes

“…Furthermore, several mutations from arginine to leucine at codon 175 (Klein et al, 2001), isoleucine or serine (Ryan and Vousden, 1998) ablate apoptosis activity, but function as wild types in regard to cell cycle arrest, similar to the amino acid adaptation observed in Spalax p53. In a mouse model (Liu et al, 2004) in which p53 R175P mutation was introduced, the initiation of apoptosis was abrogated, whereas cell cycle arrest remained intact. Finally, although apaf1 and mdm2 genes display a high degree of conservation between Spalax and other species, and the effect of Spalax p53 on both human and Spalax target genes is similar, it cannot be ruled out that the altered activity…”

Protéger et sévir : p53, métabolisme et suppression tumorale

“…La suppression tumorale a été testée vis-à-vis des deux types de tumorigenèse (spontanée et induite) dans les souris des quatre premières colonnes, et d'un seul (tumorigenèse spontanée ou induite) dans les souris des deux dernières colonnes. *Les souris p53 R172P/R172P sont protégées de la tumorigenèse induite par un dysfonctionnement des télomères, probablement grâce à l'induction de la sénescence, mais pas contre la tumorigenèse doublement induite par un dysfonctionnement télomérique combiné à un traitement par un carcinogène chimique [43,52]. **La suppression de la tumorigenèse spontanée chez les souris p53 25,26/25,26 est controversée [29].…”

“…L'inhibition de l'autophagie en constitue possiblement une autre facette : liée également à une expression basale de p53 (dans le cytoplasme), elle prive les cellules d'une souplesse métabolique nécessaire pour leur survie dans des conditions fréquemment rencontrées par les tumeurs naissantes [47]. Le maintien du nombre de centrosomes et de la ploïdie contribue aussi probablement à cette suppression tumorale constitutive [43]. La normalisation des cellules peut éventuellement impliquer un arrêt réversible du cycle cellulaire (flèche brisée) si l'activité de p53 est un peu plus élevée.…”