Targeting MDM4 in Rare Refractory Ovarian Cancer Sub-Types

Activation of wild-type (wt) p53 tumor suppressor is sufficient to kill tumor cells, even if other gene defects are present. Cisplatin is a p53-dependent drug that is heavily used in treating cancer. However, in several cancers harboring predominantly wt-p53, its benefit, if any, is short-lived. For instance, response rate

to cisplatin in the Clear Cell ovarian cancer (OvCa) sub-type is a low 25%, resulting in an overall survival of only 10%. Thus, cisplatin resistance is a significant impediment, and this has created an unmet need to identify novel strategies that would lead to targeted therapeutic options for rational development against the refractory

OvCa disease. A major factor in resistance is the loss of p53 function, and not necessarily through mutation. This proposal is focused on wt-p53 since there is now acute awareness from our work and those of others that this genotype is ubiquitous in many advanced clinically resistant cancers, such as mesothelioma and osteo-

sarcoma. In this proposal, our focus is on refractory OvCa harboring wt-p53, which is the genotype present predominantly (~90%) in the rare OvCa sub-types, constituting 30% of all ovarian cancers. Importantly, these sub-types occur at an earlier age and, therefore, death is observed in younger women. But, current therapy of

these sub-types remains empirical, due largely to profound knowledge gaps that exist in this disease. We have established that tumor cells become resistant to cisplatin when this drug fails to activate wt- p53 due to silencing of the specific kinase that is vital for p53 phosphorylation and, thereby, p53-dependent

transactivation function. Normally, wt-p53 is maintained in an inactive state by its endogenous natural inhibitors. The activity of partially released p53 through pharmacological targeting of such inhibitors, however, is insufficient to exceed the critical threshold for apoptotic function. Since p53 phosphorylation is vital for fully

functionalizing p53 for its apoptotic function, we propose a novel hypothesis that targeted reduction in MDM4 will fully release p53 and allow access of the critical p53-site to endogenous active kinase(s) and induce phosphorylation. We will test this hypothesis through two specific aims: 1) Establish basal phosphorylation of

p53 by targeting MDM4 in refractory OvCa sub-type tumor models in vitro and in vivo; 2) Identify the active kinase that phosphorylates p53 and assess its potential role for therapy. We will utilize pharmacologic tools to define tumor sensitivity following MDM4 modulation, biochemical and molecular tools to identify and modulate

the kinase and delineate the basis for reversal of cisplatin resistance, and graphically model antitumor effects of rational combinations to establish synergy. This high-risk project has the potential to establish a conceptual foundation for therapeutic activation of wt-p53 in rare OvCa that will enable follow-up studies and likely change

the trajectory of treatment outcomes for large numbers of patients diagnosed with cisplatin-resistant OvCa sub- types. Importantly, there is a high potential that the outcome from the proposed studies will likely be tumor agnostic and, thereby, substantially impact a wider spectrum of wt-p53 cancers resistant to other drugs.