Overexpression of Mdm2 in mice reveals a p53-independent role for Mdm2 in tumorigenesis

Abstract

The Mdm2 proto-oncogene is amplified to high copy numbers in human sarcomas and is overexpressed in a wide variety of other human cancers. Because Mdm2 protein forms a complex with the p53 tumor suppressor protein and down-regulates p53 function, the oncogenic potential of Mdm2 is presumed to be p53-dependent. To model these conditions in mice, we have used the entire Mdm2 gene, under transcriptional control of its native promoter region, as a transgene to create mice that overexpress Mdm2. The transgenic mice are predisposed to spontaneous tumor formation, and the incidence of sarcomas observed in the Mdm2-transgenic mice in the presence or absence of functional p53 demonstrates that, in addition to Mdm2-mediated inactivation of p53, there exists a p53-independent role for Mdm2 in tumorigenesis.

Figure 1

Southern analysis of the offspring of Mdm2-transgenic, Mdm2 heterozygous mice crossed with Mdm2 heterozygous mice. Genomic DNA was isolated from tail biopsies, digested with EcoRI, and run on a 1% agarose gel. Southern analysis by using a Mdm2 genomic probe confirmed the presence of the transgene and the genotype of the mice. Arrows indicate those mice which are Mdm2-deficient and which are rescued by the presence of the transgene. Controls in first two lanes are DNA isolated from an Mdm2-heterozygous (+/−) mouse and from a wild-type (+/+) mouse

Figure 2

Northern analysis of RNA isolated from wild-type and Mdm2-transgenic mouse tissues. Northern analysis of total RNA was performed by using an Mdm2 full-length cDNA probe. Human glyceraldehyde-3-phosphate dehydrogenase was probed to provide a phosphorimaging control for RNA load and integrity.

Figure 3

Tumorigenesis in Mdm2-transgenic mice. Twenty Mdm2-transgenic mice (diamond), 11 homozygous (2×) transgenic mice (circle), and 20 Mdm2-transgenic/p53-null mice (triangle) were assayed for tumor formation. The rates of tumorigenesis in these populations was compared with the rates of tumorigenesis in 42 p53-null mice (hatched square) and 30 wild-type mice (open square).