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The oncogene Mdmx is overexpressed in many human malignancies, and together with Mdm2, negatively regulates the p53 tumor suppressor. However, a p53-independent function of Mdmx that impacts genome stability has been described, but this function is not well understood. In the present study, we determined that of the thirteen different cancer types evaluated, 6–90% of those that had elevated levels of Mdmx had concurrent inactivation (mutated or deleted) of p53. We show elevated levels of Mdmx inhibited double-strand DNA break repair and induced chromosome and chromatid breaks independent of p53, leading to genome instability. Mdmx impaired early DNA damage response signaling, such as phosphorylation of the serine/threonine-glutamine motif, mediated by the ATM kinase. Moreover, we identified Mdmx associated with Nbs1 of the Mre11-Rad50-Nbs1 (MRN) DNA repair complex, and this association increased upon DNA damage and was detected at chromatin. Elevated Mdmx levels also increased cellular transformation in a p53-independent manner. Unexpectedly, all Mdmx-mediated phenotypes also occurred in cells lacking Mdm2 and were independent of the Mdm2-binding domain (RING) of Mdmx. Therefore, Mdmx-mediated inhibition of the DNA damage response resulted in delayed DNA repair and increased genome instability and transformation independent of p53 and Mdm2. Our results reveal a novel p53- and Mdm2-independent oncogenic function of Mdmx that provides new insight into the many cancers that overexpress Mdmx.

Aging is thought to negatively affect multiple cellular processes, including the ability to maintain chromosome stability. Chromosome instability (CIN) is a common property of cancer cells and may be a contributing factor to cellular transformation. The types of DNA aberrations that arise during aging prior to tumor development and that contribute to tumorigenesis are currently unclear. Mdm2, a key regulator of the p53 tumor suppressor and modulator of DNA break repair, is frequently overexpressed in malignancies and contributes to CIN. To determine the relationship between aging and CIN and the role of Mdm2, pre-cancerous wild-type C57Bl/6 and littermate-matched Mdm2 transgenic mice at various ages were evaluated. Metaphase analyses of wild-type cells showed a direct correlation between age and increased chromosome and chromatid breaks, chromosome fusions, and aneuploidy, but the frequency of polyploidy remained stable over time. Elevated levels of Mdm2 in pre-cancerous mice increased both the numerical and the structural chromosomal abnormalities observed. Chromosome and chromatid breaks, chromosome fusions, aneuploidy, and polyploidy were increased in older Mdm2 transgenic mice compared to wild-type littermates. Unexpectedly, chromosome fusions, aneuploidy, and polyploidy rates in Mdm2 transgenic mice, but not chromosome and chromatid breaks, showed cooperation between Mdm2 overexpression and age. Notably, Mdm2 overexpression promoted gains in one or more chromosomes with age, while it did not affect the rate of chromosome loss. Therefore, aging increased specific forms of genomic instability, and elevated Mdm2 expression cooperated with aging to increase the likelihood of gaining certain chromosomal abnormalities of the kind thought to lead to cancer development.