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

B. Bardot (1, 2) , R. Bouarich-Bourimi (2, 1) , J. Leemput (2, 1) , V. Lejour (3) , Ann-Leig Hamon (4) , L. Plancke (2, 1) , A.G. Jochemsen (3) , I. Simeonova (2, 1) , M. Fang (2, 1) , F. Toledo (2, 1)

1 DIG CANCER - Dynamique de l'information génétique : bases fondamentales et cancer
2 UPMC - Université Pierre et Marie Curie - Paris 6
3 LUMC - Leiden University Medical Center
4 MSSMat - Laboratoire de mécanique des sols, structures et matériaux

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.

Domaines: Sciences de l'ingénieur [physics]

B. Bardot, R. Bouarich-Bourimi, J. Leemput, V. Lejour, Ann-Leig Hamon, et al.. Mice engineered for an obligatory Mdm4 exon skipping express higher levels of the Mdm4-S isoform but exhibit increased p53 activity. Oncogene, 2015, 34, pp.2943-2948. ⟨10.1038/onc.2014.230⟩. ⟨hal-02457193⟩