TENT5A Maintains MYC mRNA Stability to Enhance Osteosarcoma Stemness

MYC genomic amplification and pathway activation is associated with aggressive behavior and poor prognosis in osteosarcoma (OS). However, a discordance exists between copy-number gains and transcriptional output from MYC in OS, and defining these mechanisms is critical to understand and intercept persistent MYC signaling. Here, we showed that cytoplasmic mRNA (poly(A)) sustains MYC activation in OS. Multi-omics profiling and single-cell transcriptomics identified TENT5A, a non-canonical 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 post-transcriptional 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 OS.