Abstract A009: Synthetic lethality in the context of STAG2-mutant Ewing sarcoma

Abstract

Abstract Introduction: Ewing sarcoma is an aggressive type of pediatric bone cancer, characterized by a fusion transcription factor (EWSR1-FLI1 in 85% of cases1,2). Recurrent secondary mutations are limited to alterations in TP53, CDKN2A and most frequently STAG2 (15-22%3-6). STAG2-loss-of-function (LOF) mutations correlate with poor prognosis, metastasis and relapse3,5,6, and are shown to induce a metastatic phenotype in preclinical studies7,8. Despite intensive multimodal treatment (surgery, radiation and chemotherapy), the estimated 5-year survival is a mere 65-75% for patients with localized disease, and below 30% for those presenting with metastasis or relapse9. Targeted treatment is currently not available. Our current work aims at identifying synthetic lethal (SL) interactions with STAG2-LOF mutations in the context of Ewing sarcoma, which could represent potential avenues for targeted therapy. Methods: In order to identify STAG2-SL interactions, we have performed genome-wide and subsequent targeted CRISPR knockout (KO) screening in Ewing sarcoma cell lines and their isogenic CRISPR-engineered STAG2-KO clones. To capture Ewing sarcoma inter- and intra-tumoral heterogeneity, we engineered additional bulk STAG2-KO populations across a wide range of cell lines and primary samples, using a lentiviral approach. These isogenic (STAG2 +/-) models are used for high throughput drug screening, validation of screening results and investigation of underlying SL mechanisms. Results: The quality of our genetic screening is validated by the recurrent emergence of (untargetable) paralog STAG1 as the top STAG2-SL hit. Additional preliminary results include a targetable candidate that emerged from both CRISPR- and drug-based screens in most of our tested models. Validation experiments across a broader panel of isogenic cell lines and primary models are currently ongoing. Conclusion: In this work, we employ genetic and drug-based screening approaches across a panel of isogenic (STAG2+/-) Ewing sarcoma cell lines, aiming to identify STAG2-SL interactions. These approaches have yielded potential hits that are currently being investigated across a broader panel of Ewing cell lines and primary models. Ultimately, this work aims to identify novel targeted therapeutic strategies for this aggressive subtype of Ewing sarcoma.