(P 59) COMPREHENSIVE SINGLE CELL TRANSCRIPTOMICS ANALYSIS OF MURINE OSTEOSARCOMA UNCOVERS SKP2 FUNCTION IN METASTASIS, GENOMIC INSTABILITY AND IMMUNE ACTIVATION AND REALS ADDITIONAL TARGET PATHWAYS

Objective: Osteosarcoma (OS) is the most common primary pediatric bone malignancy. One promising new therapeutic target is SKP2, encoding a substrate recognition factor of the SCF E3 ubiquitin ligase responsible for ubiquitination and proteasome degradation of substrate p27, thus driving cellular proliferation. We have shown previously that knockout of Skp2 in an immunocompetent transgenic mouse model of OS improved survival, drove apoptosis, and induced tumor inflammation. However, tumor formation is not totally blocked after Skp2 disruption. We wish to study the underlying mechanism of this survival benefit and Skp2 function loss escape.

Methods: We applied single-cell RNA-sequencing (scRNA-seq) to study primary OS tumors derived from Osx-Cre driven conditional knockout of Rb1 and Trp53 (DKO). We further compared this model with OS models with functional disruption of Skp2: one with Skp2 knockout(TKO) and the other with the Skp2-p27 interaction disrupted (DKOAA, resulting in p27 overexpression).

Results: We showed that murine OS models recapitulate the tumor heterogeneity and microenvironment complexity observed in patient tumors. We found reduction of T cell exhaustion and upregulation of interferon activation, along with evidence of replicative and endoplasmic reticulum-related stress in the Skp2 disruption models, and showed that interferon induction was correlated with improved survival in OS patients. Additionally, our scRNA-seq analysis uncovered decreased activities of metastasis-related gene signatures in the Skp2-disrupted OS, which we validated by observation of a strong reduction in lung metastasis in the Skp2 knockout mice. Finally, we report several potential mechanisms of escape from targeting Skp2 in OS, including upregulation of Myc targets, DNA copy number amplification and overexpression of alternative E3 ligase genes, and potential alternative lineage activation.

Conclusion: These mechanistic insights into OS tumor biology and Skp2 function suggest novel targets for new, synergistic therapies, while the data and our comprehensive analysis may serve as a public resource for further big data-driven OS research.