Mdm2 inhibits the function of the p53 tumor suppressor. Mdm2 is overexpressed in many tumors with wild-type p53 suggesting an alternate mechanism of loss of p53 activity in tumors. An Mdm2-binding protein (MTBP) was identified using a yeast two-hybrid screen. In tissue culture, MTBP inhibits Mdm2 self-ubiquitination, leading to stabilization of Mdm2 and increased degradation of p53. To address the role of MTBP in the regulation of the p53 pathway in vivo, we deleted the Mtbp gene in mice. Homozygous disruption of Mtbp resulted in early embryonic lethality, which was not rescued by loss of p53. Mtbp þ /À mice were not tumor prone. When mice were sensitized for tumor development by p53 heterozygosity, we found that the Mtbp þ /À p53 Introduction p53 is a transcription factor that activates numerous downstream genes with roles in cell cycle arrest, apoptosis, DNA repair and senescence (Levine et al., 2004). As such, disruption of the p53 pathway is a critical event in human cancer. Mutations or deletions in the p53 gene occur in approximately 50% of cancers (Bartek et al., 1991;Levine, 1993;Vogelstein et al., 2000). The p53 pathway is also inactivated by overexpression of the p53 inhibitor, Mdm2. Overexpression of Mdm2 by several mechanisms occurs in about 30% of sarcomas and many other types of tumors (Iwakuma and Lozano, 2003). ARF (alternative reading frame of the Ink 4a locus), another important regulator of this pathway, binds and inhibits Mdm2 by relocating Mdm2 from nucleus to nucleolus (Sharpless and DePinho, 1999;Weber et al., 1999). Therefore, ARF loss in tumors releases Mdm2, leading to increased binding to and inactivation of p53. Thus, the ARF-Mdm2-p53 pathway is a key pathway that must be bypassed in tumor development.Experiments using genetically engineered mice have provided direct evidence for the importance of these factors in tumorigenesis. Mice with homozygous or heterozygous deletion of p53 develop tumors (Donehower et al., 1992;Tsukada et al., 1993;Jacks et al., 1994;Purdie et al., 1994). Mice with missense mutations identical to those found in humans with Li-Fraumeni syndrome develop a variety of tumors with frequent metastasis (Lang et al., 2004;Olive et al., 2004 MTBP was isolated as a novel Mdm2-binding protein (Boyd et al., 2000). The MTBP-Mdm2 interaction was observed using purified proteins in vitro and by overexpression in cells. Overexpression of MTBP protects Mdm2 from self-ubiquitination, leading to Mdm2 stabilization and p53 degradation (Brady et al., 2005). We therefore hypothesized that MTBP affects tumor development through modulation of p53 activity. To test this hypothesis and examine the in vivo physiological function of MTBP, we generated mice with disruption of the Mtbp gene. Our results show that Mtbp-null mice were embryonic lethal, and this phenotype was p53 independent. Mtbp þ /À mice were normal, but when crossed with p53 þ /À mice, Mtbpþ /À mice developed significantly more metastatic tumors compared to p53 þ /À mice. In vitro invasion assays clearly supported the in ...

BALB/c mice are predisposed to developing spontaneous mammary tumors, which are further increased in a p53 heterozygous state. C57BL/6J mice are resistant to induced mammary tumors and develop less than 1% mammary tumors in both wild-type and p53+/- states. To map modifiers of mammary tumorigenesis, we have established F1 and F2 crosses and backcrosses to BALB/cJ (N2-BALB/cJ) and C57BL/6J (N2-C57BL/6J) strains. All cohorts developed mammary carcinomas in p53+/- females, suggesting that multiple loci dominantly and recessively contributed to mammary tumorigenesis. We mapped two modifiers of mammary tumorigenesis in the BALB/cJ strain. Mtsm1 (mammary tumor susceptibility modifier), a dominant-acting modifier, is located on chromosome 7. Mtsm1 is suggestive for linkage to mammary tumorigenesis (p = 0.001). We have analyzed the Mtsm1 region to locate candidate genes by comparing it to a rat modifier region, Mcs3, which shares syntenic conservation with Mtsm1. Expression data and SNPs were also taken into account. Five potential candidate genes within Mtsm1 are Aldh1a3, Chd2, Nipa2, Pcsk6, and Tubgcp5. The second modifier mapped is Mtsm2, a recessive-acting modifier. Mtsm2 is located on chromosome X and is significantly linked to mammary tumorigenesis (p = 1.03 x 10(-7)).