Abstract AP15: IDENTIFICATION OF P53 PRIONS AS AN INDEPENDENT PROGNOSTIC MARKER FOR SURVIVAL IN HIGH-GRADE SEROUS OVARIAN CANCER

PURPOSE: Although the discovery of prions was rewarded with a Nobel Prize, their existence was only attributed to a limited number of diseases. Recent evidence suggests that their role has been underestimated and several other proteins carry prion-like properties, like ß-amyloid, and most recently p53. High-grade serous ovarian cancers (HGSOC) harbor TP53 mutations in about 96% of cases. These mutations promote p53 aggregation, which might be responsible for complete abrogation of tumor suppressor functions, resulting in dominant-negative activity and oncogenic gain-of-function. Here, we describe the use of an ELISA-based technology for p53 prion detection in fresh-frozen tumor tissue and their clinical relevance in ovarian cancer. EXPERIMENTAL PROCEDURES: Fresh-frozen tumor tissue specimens of 81 HGSOC patients, who previously had been included in the EU-funded OVCAD study, were analyzed. For each of these patients at least 5-year follow-up data were available. For the detection of p53 prions the Seprion-ELISA, previously designed for the detection of BSE and scrapie was adapted and optimized. To investigate the impact of p53 aggregation on clinical outcomes (overall survival and progression-free survival), we performed a complete-case analysis. In a subset of patients the Ki67 proliferation index and a homologous recombination deficiency (HRD) score, based on the Myriad score, were available. Correlations were analyzed using ANOVA and t-test. RESULTS: In 39 of 46 (84.8%) patients with missense mutated cancers a p53 prion specific signal was observed. The aggregation propensity varied considerably within samples carrying the same mutations. Multivariable Cox regression models, with respect to other prognostic factors significantly associated with overall survival in patients with late-stage serous epithelial ovarian cancer (age, FIGO stage and presence of a residual tumor), show superiority of the group with extensive p53 aggregation in overall survival and in progression-free survival. No and moderate p53 aggregation are associated with a worse overall survival in contrast to high p53 aggregation (P values 0.025 and 0.011). Similar results in these groups are assessed for progression-free survival (P values 0.030 and 0.008). Interestingly, the group with extensive p53 aggregation was associated with a non-significant trend toward higher HR deficiency. Furthermore, this group had a significantly higher Ki67 index compared to patients with moderate p53 aggregation (P value 0.033). CONCLUSIONS: This study reports the first specific and quantitative screening for p53 prions in patient material. We were able to demonstrate that the p53-Seprion-ELISA is a robust and highly sensitive detection tool for p53 prions. Our data show that the aggregation propensity is not only depending on the TP53 mutation and that other cofactors may be involved. Moreover, we show that p53 aggregation is an independent prognostic marker for survival. The higher Ki67 proliferation index and the trend towards higher genomic instability in patients with extensive p53 aggregation suggest that these tumors have an increased likelihood of response to platinum-containing therapy. To conclude, we demonstrated the high potential of p53 aggregation as a biomarker for patients' survival, suggesting that classification of patients based on the amount of aggregated p53 could allow therapy decisions. Citation Format: Nicole Heinzl, Elisabeth Maritschnegg, Katarzyna Koziel, Stuart Wilson, Georg Heinze, Christine Wallisch, Reinhard Horvat, Jalid Sehouli, Ioana Braicu, Ignace Vergote, Els van Nieuwenhuysen, Sven Mahner, Eva Obermayr, Eva Schuster, Barbara Holzer, Nicole Concin, Robert Zeillinger. IDENTIFICATION OF P53 PRIONS AS AN INDEPENDENT PROGNOSTIC MARKER FOR SURVIVAL IN HIGH-GRADE SEROUS OVARIAN CANCER [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr AP15.