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Mice engineered for an obligatory Mdm4 exon skipping express higher levels of the Mdm4-S isoform but exhibit increased p53 activity

Oncogene volume 34, pages 2943–2948 (2015)Cite this article

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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 (Mdm4ΔE6) 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 p53ΔP/ΔP context, the Mdm4ΔE6 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.

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Acknowledgements

We thank members of the Curie technological platforms for their contribution to this study: I Grandjean, C Daviaud and M Verlhac from the Animal facility, C Alberti, E Belloir and N Mebirouk from the transgenesis platform, M Richardson, A Nicolas, R Leclere and M Huerre from the pathology service. Our lab received funding as an Equipe Labellisée by the Ligue Nationale contre le Cancer. The project was initiated by grants from the Fondation de France (Comité Tumeurs), the Ligue Nationale contre le Cancer (Comité Ile de France) and the Fondation ARC. JL was supported by a post-doctoral fellowship from the Cancéropôle Ile de France.

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Author notes

  1. R Bouarich-Bourimi and J Leemput: These authors contributed equally to this work.

Authors and Affiliations

  1. Institut Curie, Centre de recherche, Genetics of Tumor Suppression (Equipe Labellisée Ligue 2014), Paris, France

    B Bardot, R Bouarich-Bourimi, J Leemput, V Lejour, A Hamon, L Plancke, I Simeonova, M Fang & F Toledo

  2. UPMC Univ Paris 06, Paris, France

    B Bardot, R Bouarich-Bourimi, J Leemput, V Lejour, A Hamon, L Plancke, I Simeonova, M Fang & F Toledo

  3. CNRS UMR 3244, Paris, France

    B Bardot, R Bouarich-Bourimi, J Leemput, V Lejour, A Hamon, L Plancke, I Simeonova, M Fang & F Toledo

  4. Leiden University Medical Center, Leiden, The Netherlands

    A G Jochemsen

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  1. B Bardot
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Correspondence to F Toledo.

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Bardot, B., Bouarich-Bourimi, R., Leemput, J. et al. Mice engineered for an obligatory Mdm4 exon skipping express higher levels of the Mdm4-S isoform but exhibit increased p53 activity. Oncogene 34, 2943–2948 (2015). https://doi.org/10.1038/onc.2014.230

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