Abstract 4290: Mad1 promotes tumor progression through destabilization of p53

Mitotic arrest deficient 1 (Mad1) plays a well-characterized role in the mitotic checkpoint. However, interphase roles of Mad1 that do not impact mitotic checkpoint function are still largely uncharacterized. We have found that upregulation of Mad1, which is common in human breast cancer, results in decreased protein stability of the tumor suppressor p53 in multiple cell types. To gain mechanistic insight into this process, we first determined whether increasing expression of Mad1 alters its localization. Upregulated Mad1 localizes to puncta within interphase nuclei in both breast cancer tissue and cultured cells. We found that upregulated Mad1 localizes to ProMyelocytic Leukemia Nuclear Bodies (PML NBs), which have been implicated in the stabilization of p53. Immunoprecipitation results indicate that Mad1 and PML interact and that the C-terminal domain (CTD) of Mad1 and the N-terminal domain of PML are required for this interaction. Within the CTD of Mad1, the SUMO-Interacting Motif (SIM) is critical for the localization of Mad1 to PML NBs. MDM2 is an E3 ubiquitin ligase that targets p53 for degradation. In response to DNA damage, PML sequesters Mdm2 to the nucleolus, which stabilizes p53. In cells with elevated levels of Mad1, Mad1 interrupts the interaction between PML and MDM2. Mad1, rather than MDM2, is sequestered to the nucleolus after DNA damage, and Mad1 displaces MDM2 from PML. The displaced MDM2 is no longer sequestered in the nucleolus, freeing it to ubiquitinate p53, resulting in p53 destabilization. Upregulation of Mad1 promotes the growth of orthotopic mammary tumors. Mammary tumors with upregulated Mad1 show decreased levels of p53 and its downstream effector p21. Our results show that, in addition to causing a low rate of chromosome missegregation, Mad1 upregulation has an unexpected interphase role in promoting tumor formation and progression by destabilizing p53. Thus, Mad1 upregulation promotes tumors in through at least two distinct pathways. Citation Format: Jun Wan, Beth A. Weaver. Mad1 promotes tumor progression through destabilization of p53 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4290.