Amplification of elephant p53 (EP53) was recently described as a potential mechanism for cancer resistance in elephants. Osteosarcoma is the most common pediatric bone tumor and also occurs frequently in pet dogs. Both human and canine osteosarcoma contain a very high rate of TP53 alterations leading to genomic instability. The purpose of our study was to determine if EP53 could enhance and/or restore p53 function in osteosarcoma and trigger p53-mediated cell death. We expressed various EP53 proteins in canine and human osteosarcoma cell lines (OSCA-40, U-2 OS, and Saos-2) by transfection or viral transduction. Expression of EP53 was confirmed by Western blot. Apoptosis of cells transfected/transduced with EP53 was compared to cells transfected/transduced with negative control vectors. Apoptosis was measured by fluorescence microscopy and caspase activity. We observed a significant increase in caspase activity (normalized to cell viability) of U-2 OS (TP53-wild type) cells expressing EP53 compared to negative control cells (p<0.0001). Saos-2 (TP53-null) and OSCA-40 cells underwent apoptosis as visualized by time-lapse fluorescence microscopy. Experiments to confirm these results with caspase activity assays are currently under way. Increased expression of p21, a direct p53 target, was observed in Saos-2 cells by Western blot. Taken together, our results indicate that EP53 enhances and restores p53-mediated apoptosis in osteosarcoma. These data suggest for the first time that EP53 can function in both human and canine osteosarcoma cells to promote cell death. Efforts to define the mechanism of action of EP53 in osteosarcoma cells are ongoing. Our results support further exploration of EP53-based osteosarcoma therapeutics for both humans and dogs.

Citation Format: Lisa M. Abegglen, Cristhian Toruno, Lauren N. Donovan, Rosann Robinson, Mor Goldfeder, Genevieve Couldwell, Wendy K. Kiso, Dennis L. Schmitt, Aleah F. Caulin, Carlo C. Maley, Avi Schroeder, Joshua D. Schiffman. Elephant p53 (EP53) enhances and restores p53-mediated apoptosis in human and canine osteosarcoma [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr A25.