TENT5A Maintains MYC mRNA Stability to Enhance Osteosarcoma Stemness

MYC genomic amplification and pathway activation are associated with aggressive behavior and poor prognosis in osteosarcoma. However, a discordance exists between copy-number gains and transcriptional output from MYC in osteosarcoma, and defining these mechanisms is critical to understanding and intercepting persistent MYC signaling. In this study, we showed that cytoplasmic mRNA poly(A) sustains MYC activation in osteosarcoma. Multiomics profiling and single-cell transcriptomics identified TENT5A, a noncanonical RNA-binding poly(A) polymerase, as selectively upregulated in MYC-activated tumors and enriched in proliferative, stem-like populations. Biochemical and genetic evidence demonstrated that TENT5A directly bound MYC mRNA via its PAP/OAS1 domain, extended its poly(A) tail, and stabilized the transcript, thereby reinforcing MYC-driven stemness and chemoresistance. Gain- and loss-of-function assays, orthotopic xenografts, and patient-derived organoids confirmed that elevated TENT5A enhanced tumor-initiating capacity and reduced chemotherapy sensitivity. Pharmacologic inhibition of TENT5A disrupted MYC mRNA stabilization, shortened poly(A) tails, and reversed chemoresistance in preclinical models. These findings delineate a posttranscriptional RNA stabilization pathway that reconciles the disconnect between MYC genetic alterations and transcriptional activity and nominate the RNA-binding protein TENT5A as a therapeutically tractable target in osteosarcoma. SIGNIFICANCE: Enhanced MYC mRNA stability mediated by TENT5A-induced poly(A) elongation supports cancer stem cell maintenance in osteosarcoma, highlighting TENT5A inhibition as a potential strategy to block MYC activity in cancer.