Medulloblastoma (MB) accounts for approximately 25% of childhood brain tumors with 70% of the cases occurring in children under 10. Prognosis for children less than three years old is considerably worse and, due to its destructive effects on the developing nervous system, irradiation is largely avoided in this age group. The p53 pathway is considered a key determinant of anti-tumor responses in many tumors; however, its role in the regulation of cell survival, chemo-sensitivity and chemo-resistance in MB is much less well defined. It has been shown that p53 pathway defects, mutations and nuclear levels increase significantly from MB diagnosis to relapse and correlate with an adverse prognosis. We recently reported on the highly novel and unexpected finding that both the genetic (sh-RNA) and chemical silencing of p53 led to a significant increase in cell death by the drug VMY in MB cell lines with elevated basal p53 (e.g. in D556 cells which express wild type p53 and in DAOY cells that contain mutant p53) as measured by colony forming assays, DNA degradation assays and annexin-V staining, suggesting a surprising commonality in the p53 signaling in both cell lines despite the differences in their p53 status. Equally surprising was the observation that the silencing of p53 in D556 cells enhanced cell death by the clinically used drugs doxorubicin and vincristine. Conversely, we now find that suppressing p53 with sh-RNA in D283 cells, which express much lower levels of wild type p53 compared to D556, resulted in the more classical chemoresistance profile, suggesting a form of p53 ‘addiction’ in D556 and DAOY cells. Mechanistically, we observed that suppressing p53 with sh-RNA in D556 cells treated with VMY or doxorubicin significantly increased the levels of phosphorylated Chk1, S6K, gH2AX, and MDM2. Importantly, phospho-mTOR levels were also significantly increased in the D556/p53 sh-RNA cells and the suppression of mTOR enhanced chemoresistance. These somewhat paradoxical findings suggest that activated Akt/mTOR may induce MB cell death and that suppression of mTOR by p53 enhances chemoresistance. Our data provide new mechanistic insights into the role of p53 in primitive neuroectodermal tumors and may provide new approaches for enhancing the clinical outcome of patients with MB.

Citation Format: Aisha Naeem, Muhammad U. Choudhry, Maria L. Avantaggiati, Olga C. Rodriguez, Chris Albanese. The p53 tumor suppressor protein paradoxically drives chemo-resistance in human medulloblastoma cells through suppressing the mTOR AKT pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3178. doi:10.1158/1538-7445.AM2017-3178