The Zn-finger domain of MdmX suppresses cancer progression by promoting genome stability in p53-mutant cells

The MDMX (MDM4) oncogene is amplified or overexpressed in a significant percentage of human tumors. MDMX is thought to function as an oncoprotein by binding p53 tumor suppressor protein to inhibit p53-mediated transcription, and by complexing with MDM2 oncoprotein to promote MDM2-mediated degradation of p53. However, down-regulation or loss of functional MDMX has also been observed in a variety of human tumors that are mutated for p53, often correlating with more aggressive cancers and a worse patient prognosis. We have previously reported that endogenous levels of MdmX can suppress proliferation and promote pseudo-bipolar mitosis in primary and tumor cells derived from p53-deficient mice, and that MdmX-p53 double deficient mice succumb to spontaneously formed tumors more rapidly than p53-deficient mice. These results suggest that the MdmX oncoprotein may act as a tumor-suppressor in cancers with compromised p53 function. By using orthotopic transplantation and lung colonization assays in mice we now establish a p53-independent anti-oncogenic role for MdmX in tumor progression. We also demonstrate that the roles of MdmX in genome stability and in proliferation are two distinct functions encoded by the separate MdmX protein domains. The central Zn-finger domain suppresses multipolar mitosis and chromosome loss, whereas the carboxy-terminal RING domain suppresses proliferation of p53-deficient cells. Furthermore, we determine that it is the maintenance of genome stability that underlies MdmX role in suppression of tumorigenesis in hyperploid p53 mutant tumors. Our results offer a rationale for the increased metastatic potential of p53 mutant human cancers with aberrant MdmX function and provide a caveat for the application of anti-MdmX treatment of tumors with compromised p53 activity

Repeated cleavage failure does not establish centrosome amplification in untransformed human cells

Transient cleavage failure in dividing cells is not sufficient to establish stable populations of cells with extra centrosomes

Approches génétique et moléculaire de la duplication des corps basaux chez paramecium tetraurelia

La paramécie est un organisme recouvert de cils, chacun ancré sur un corps basal, structure polarisée se dupliquant à chaque cycle cellulaire et analogue structural des centrioles du centrosome. Chez le protozoaire Paramécie, plusieurs mutations affectant la duplication ont été décrites. Le mutant kin241-1, présente une hyperduplication des corps basaux, des anomalies de leur polarité et des zones morphogénétiques et de la différenciation nucléaire durant les processus sexuels. Le gène KIN241 a été isolé et cloné par complémentation fonctionnelle. Il code une protéine contenant 4 domaines: une isomérase-cyclophiline, un domaine d'interaction avec l'ARN, des répétition d'un dipeptide E-K et un C-terminal riche en sérines. Cette protéine, nommée CRIP (Cyclophiline RNA Interacting Protein), fusionnée avec la GFP, est localisée dans le noyau conformément aux signaux NLS (Nuclear Localisation Signal) de la partie Ct. L'introduction d'une délétion provoquant la disparition de la moitié des signaux NLS, diminue l'efficacité de l'adressage: la localisation est nucléaire et cytoplasmique. La mutation originale, kin241-1, (insertion de 2 nucléotides) introduit prématurément un signal de fin de traduction et donne une isoforme dépourvue de sa partie Ct. Cette isoforme n'est retrouvée ni dans le noyau ni dans le cytoplasme. Le phénotype de la mutation kin241-1 est donc dû à l'absence de la protéine dans la cellule. Ceci a été confirmé par des expériences d'extinction génique consistant à inactiver l'ARNm endogène et donc à prévenir la synthèse de la protéine CRIP. Nous concluons que CRIP est un facteur de processing d'une classe d'ARNm spécifiques des processus morphogénétiques. CRIP existe chez de nombreux organismes: levure, plantes, insectes et mammifères. Aucune de ces protéines n'a encore été étudiées. Cette thèse est un premier pas dans l'étude d'une nouvelle famille de protéines pouvant jouer un rôle dans le processing d'ARNm impliqués dans la morphogénèse.Paramecium is a unicellular organism covered by numerous cilia. Each of them is anchored on a basal body, a polarized structure, which duplicates during the cell cycle and is analogous to centrioles of the centrosome. In Paramecium, several mutants defective in basal body duplication have already been described. The kin241-1 mutation displays a highly pleiotropic effect on cortical organization, inducing hyperduplication of basal bodies and affecting nuclear reorganization during sexual processes. I have cloned the KIN241 gene by functional complementation. It encodes a new protein with 4 domains: a cyclophilin type isomerase, an RNA recognition motif, a domain rich in the dipeptide E-K and a C-terminal string of serines. This protein, named CRIP (Cyclophilin-RNA Interacting Protein) is localized in the nucleus via nuclear localization signals (NLS). A deletion which eliminates half of the NLS, decreases nuclear transport, so that the protein is localized in both the nucleus and the cytoplasm. The original mutation kin241-1 (insertion of 2 nucleotides) introduces a premature signal to end translation and leads to a protein truncated of its C-terminal domain. This isoform is unstable in vivo. This allowed me to conclude that the phenotype of the kin241-1 mutation is caused by the jack of CRIP protein. This hypothesis was confirmed by gene silencing experiments. In these experiments, the endogenous mRNA is degraded precluding the wild type protein synthesis. We have suggested that CRIP could be a factor involved in processing of a subclass of mRNA specific for morphological processes. The protein CRIP is present in different organisms including: yeast, plants, insects and mammals. These proteins have not been yet studied. This thesis thus raports the identification and first steps in investigation of the role this new family of proteins.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

MdmX regulates transformation and chromosomal stability in p53-deficient cells

The cellular homologues Mdm2 and MdmX play critical roles in regulating the activity of the p53 tumor suppressor in damaged and non-damaged cells and during development in mice. Recently, we have utilized genetically defined primary cells and mice to reveal that endogenous levels of MdmX can also suppress multipolar mitosis and transformation in hyperploid p53-deficient cells and tumorigenesis in p53-deficient mice. These MdmX functions are not shared by Mdm2, and are distinct from the well-established ability of MdmX to complex with and inhibit p53 activity. Here we discuss some of the ramifications of MdmX loss in p53-deficient cells and mice, and we explore further the fate of MdmX/p53-double null embryonic fibroblasts undergoing multi-polar cell division using time-lapse video microscopy. We also discuss the relationship between chromosomal loss, cell proliferation, and the tumorigenic potential of p53-deficient cells lacking MdmX