Characterization of synthetic lethality between Mdm2 overexpression and genotoxic agents and identification of a novel function of the Mdmx oncogene in DNA repair
Carrillo, Alexia Melo
The oncogenes Mdm2 and Mdmx are often overexpressed in many human cancers. In addition to being negative regulators of the tumor suppressor p53, there is evidence these proteins have important p53-independent functions that are not clearly understood. Recently, a novel function of Mdm2 was identified where overexpression of Mdm2 inhibited double-strand DNA break repair through interacting with an important DNA repair protein, Nbs1, and this function of Mdm2 was independent of p53. Little was known about p53-independent functions of Mdmx and the contribution these functions may have on tumorigenesis. Here, I identify a novel interaction between Mdmx and Nbs1. Furthermore, I characterize a novel function of Mdmx that inhibits DNA damage response signaling and double-strand DNA break repair independent of both p53 and Mdm2. Specifically, Mdmx overexpression results in a decrease in the phosphorylation of the histone variant H2AX in addition to serine/glutamine or theronine/gluatamine motifs on substrates of the DNA-damage induced kinase ATM. Additionally, Mdmx overexpression promotes genome instability and confers transformation potential. Because increased levels of Mdm2 and Mdmx inhibit DNA break repair and promote genome instability, targeting this family of proteins pharmacologically has the potential to increase the efficacy of genotoxic drugs. Here, I demonstrate that the small molecule, Nutlin-3, causes an increase in Mdm2 protein levels, which results in an inhibition of double-strand DNA break repair and DNA damage response signaling independent of p53. In ovarian cancer cells with inactivated p53, Nutlin-3 combined with genotoxic agents, such as cisplatin, has a cooperative effect resulting in increased apoptosis. My results provide insight into generating novel treatment options for cancers with inactivated p53. Overall, my findings illustrate a conserved function of Mdm2 and Mdmx in the inhibition in double-strand DNA break repair. This function can be beneficial for developing novel therapeutic options for cancers with inactivated p53.