Abstract A023: MYC overexpression functionally drives aggressive osteosarcoma progression

Abstract

Recurrent and metastatic osteosarcoma (OS) remain major therapeutic challenges in pediatric oncology. MYC amplification and upregulation have been associated with high-risk disease, but MYC copy-number status is an imperfect surrogate for MYC activity, and the functional consequences of MYC overexpression during OS progression remain incompletely defined. Here, we investigate the role of MYC overexpression in OS tumorigenesis and progression and its relationship to genomic amplification. We engineered MYC overexpression in bone marrow-derived mesenchymal stem cells, osteoprogenitor cells, and tumor cells derived from an Osx-cre; Trp53-null; Rb1-null genetically engineered mouse model (GEMM), as well as in human OS cell lines with low endogenous MYC expression. Phenotypic effects were assessed using in vitro proliferation and migration assays, and in vivo tumorigenicity and metastasis assays. To determine the relationship between MYC amplification and MYC expression or clinical behavior, we also analyzed TARGET OS cases with matched MYC copy-number, mRNA, and survival data. Functionally, MYC overexpression produced context-dependent effects across osteoblastic lineage cells. In GEMM-derived mesenchymal stem cells and mutant osteoprogenitor cells, MYC increased proliferation and enhanced migration capacity. In GEMM-derived tumor cells, MYC also enhanced migration but did not promote proliferation. In human OS models, MYC expression varied independently of amplification status, and engineered MYC overexpression in models with low endogenous MYC expression enhanced migration, but baseline proliferation was not increased. In vivo, MYC overexpression conferred tumorigenicity and metastatic propensity to otherwise non-tumorigenic cell lines HOS and MG63. In an orthotopic SaOS-2 xenograft model, MYC overexpression increased tumor engraftment frequency compared to controls. Among tumors that engrafted successfully, growth kinetics were similar between groups, suggesting that MYC primarily enhances tumor-initiating and metastatic capacity rather than growth. In TARGET OS samples, MYC mRNA levels were not significantly different across diploid, gain, and amplified tumors, although a modest upward trend was observed. In this cohort, MYC amplification status did not significantly stratify overall survival, whereas high MYC mRNA expression was associated with inferior overall survival compared with low MYC mRNA expression. These data demonstrate that MYC overexpression is a functional driver of aggressive OS behavior and suggest that MYC expression is more informative than MYC amplification alone. MYC enhances proliferation in early osteogenic contexts but promotes tumorigenesis and metastasis in established osteosarcoma cells without increasing proliferation rate. Together, these results support MYC overexpression, not amplification, as a driver of high-risk OS and establish models for studying MYC-dependent tumorigenicity and metastasis. Generative artificial intelligence was used to assist with language editing of this abstract.