title: Mice engineered for an obligatory Mdm4 exon skipping express higher levels of the Mdm4-S isoform but exhibit increased p53 activity.
abstract: Mdm4, a protein related to the ubiquitin-ligase Mdm2, is an essential inhibitor of tumor suppressor protein p53. In both human and mouse cells, the Mdm4 gene encodes two major transcripts: one encodes the full-length oncoprotein (designated below as Mdm4-FL), whereas the other, resulting from a variant splicing that skips exon 6, encodes the shorter isoform Mdm4-S. Importantly, increased Mdm4-S mRNA levels were observed in several human cancers, and correlated with poor survival. However, the role of Mdm4-S in cancer progression remains controversial, because the Mdm4-S protein appeared to be a potent p53 inhibitor when overexpressed, but the splice variant also leads to a decrease in Mdm4-FL expression. To unambiguously determine the physiological impact of the Mdm4-S splice variant, we generated a mouse model with a targeted deletion of the Mdm4 exon 6, thereby creating an obligatory exon skipping. The mutant allele (Mdm4(ΔE6)) prevented the expression of Mdm4-FL, but also led to increased Mdm4-S mRNA levels. Mice homozygous for this allele died during embryonic development, but were rescued by a concomitant p53 deficiency. Furthermore in a hypomorphic p53(ΔP/ΔP) context, the Mdm4(ΔE6) allele led to p53 activation and delayed the growth of oncogene-induced tumors. We next determined the effect of Mdm4(+/ΔE6) heterozygosity in a hypermorphic p53(+/Δ31) genetic background, recently shown to be extremely sensitive to Mdm4 activity. Mdm4(+/ΔE6) p53(+/Δ31) pups were born, but suffered from aplastic anemia and died before weaning, again indicating an increased p53 activity. Our results demonstrate that the main effect of a skipping of Mdm4 exon 6 is not the synthesis of the Mdm4-S protein, but rather a decrease in Mdm4-FL expression. These and other data suggest that increased Mdm4-S mRNA levels might correlate with more aggressive cancers without encoding significant amounts of a potential oncoprotein. Hypotheses that may account for this apparent paradox are discussed.
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: B
authors.initials: B
authors.lastname: Bardot
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: R
authors.initials: R
authors.lastname: Bouarich-Bourimi
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: J
authors.initials: J
authors.lastname: Leemput
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: V
authors.initials: V
authors.lastname: Lejour
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: A
authors.initials: A
authors.lastname: Hamon
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: L
authors.initials: L
authors.lastname: Plancke
authors.affiliation: Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: A G
authors.initials: AG
authors.lastname: Jochemsen
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: I
authors.initials: I
authors.lastname: Simeonova
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: M
authors.initials: M
authors.lastname: Fang
authors.affiliation: 1] Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France [2] UPMC Univ Paris 06, Paris, France [3] CNRS UMR 3244, Paris, France.
authors.forename: F
authors.initials: F
authors.lastname: Toledo
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004622:Embryo, Mammalian; D005091:Exons / Q000235:genetics; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D008819:Mice, Nude; D008822:Mice, Transgenic; D009361:Neoplasm Invasiveness; D009369:Neoplasms / Q000235:genetics / Q000473:pathology; D011247:Pregnancy; D020033:Protein Isoforms / Q000235:genetics; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D022821:RNA Splice Sites / Q000235:genetics*; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000235:genetics*
chemical_list: C461497:Mdm4 protein, mouse; D020033:Protein Isoforms; D011518:Proto-Oncogene Proteins; D022821:RNA Splice Sites; D016159:Tumor Suppressor Protein p53; D044767:Ubiquitin-Protein Ligases
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