Development of metastasis is the major cause of death in osteosarcoma, which is the most common malignant bone tumor in children and young adults. However, detection of metastasis relies solely on imaging techniques and targeted treatments of metastatic patients are still not available to improve the dismal outcome. Using an immunoproteomic approach, we identified that p27 autoantibody was significantly elevated in the plasma of high-risk osteosarcoma patients. Results of immunohistochemistry showed that cytoplasmic mislocalization of p27 occurred in a majority of osteosarcoma cases and in highly metastatic osteosarcoma cell lines. We demonstrated that ectopic expression of cytoplasmic p27 and shRNA-mediated gene silencing promoted and inhibited, respectively, the migration and invasion of osteosarcoma cells. Additionally, a mutation in S10 or T198, but not T157, phosphosite abolished the pro-migration and invasion phenotypes of cytoplasmic p27, possibly through a RhoA-related mechanism. Furthermore, mice injected with cells carrying cytoplasmic p27 increased the development of pulmonary metastasis when compared to an empty vector control. Lastly, using a large cohort of serum samples (n = 233), we showed that a higher level of the p27 autoantibody significantly correlated with poor overall and event-free survival of osteosarcoma (p < 0.05). Together, our results suggest that the circulating p27 autoantibody can be used as a prognostic biomarker at diagnosis and cytoplasmic mislocalization of p27 is a frequent event in osteosarcoma, which can promote tumor cell motility and invasiveness as well as metastasis formation. Targeting specific p27 phosphorylations may provide a novel therapeutic approach for osteosarcoma patients with metastasis.

Citation Format: Manjula Nakka, Yiting Li, Aaron J. Kelly, Ching C. Lau, Mark Krailo, Donald A. Barkauskas, John Hicks, Tsz-Kwong Man. Cytoplasmic mislocalization of p27 promotes metastasis and its autoantibody correlates with prognosis in osteosarcoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2445.