The Mdm2-a splice variant alters development, lifespan, cellular growth and senescence in a p53-dependent manner

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Many tumor types have been shown to express splice variants of the oncoprotein MDM2. Childhood rhabdomyosarcomas in particular, often contain the Mdm2-a splice variant, the function of which is currently unknown. To assess the role of this alternate splice form, we generated an Mdm2-a transgenic mouse model. Mdm2-a homozygous mice were not viable and died peri-natally; however, this phenotype can be rescued by reduction of the p53 gene dosage. These data support previous studies that splice variants such as MDM2-A interact with full-length MDM2 and consequently activate p53. In vitro, we found that MEFs expressing MDM2-A exhibited a p53-dependent growth inhibition relative to wild-type MEFs. Surprisingly, however, p53 activity was not significantly enhanced in the transgenic MEFs and no elevation in p53 transcriptional activity was measurable. We further characterized Mdm2-a in hemizygous mice and found that while these animals developed normally and were not tumor-prone, they displayed a significant reduction in lifespan relative to wild-type littermates. This phenotype is frequently observed in models of aging due to enhanced p53 activation; however, Mdm2-a mice did not display any of the expected phenotypes of accelerated aging such as reductions in body and organ mass, dermal adipose thickness, bone density and hair re-growth. In vitro, Mdm2-a MEFs displayed enhanced senescence relative to wild-type MEFs, as evidenced by an increase in senescence-associated beta-galactosidase activity. An increase in expression of the senescence marker p21 was also observed in Mdm2-a MEFs and its role, as well as that of p53, in MDM2-A induced senescence is currently under investigation. In conclusion, MDM2-A acts to inhibit growth and enhance senescence both in vitro and in vivo, a phenotype that appears contradictory to its expression in tumors.
 This work was supported by NIH Grants CA92401, CA21765 and the American Lebanese Syrian Associated Charities (ALSAC).