Age-Stratified Transcriptomic Profiling Reveals Biologically Distinct Molecular Phenotypes Across Pediatric, Adolescent, and Adult Osteosarcoma

Background/Objectives: Osteosarcoma exhibits bimodal age distribution with distinct clinical behaviors between pediatric and adult patients. Despite genomic evidence supporting age-related molecular heterogeneity, systematic transcriptomic characterization remains lacking. This study aimed to delineate age-associated transcriptional differences and develop a pediatric-specific prognostic signature. Methods: Bulk RNA sequencing was performed on tumor specimens from 70 osteosarcoma patients stratified into pediatric (≤14 years, n = 37), adolescent (15–18 years, n = 22), and adult (≥19 years, n = 11) groups. Differential expression, functional enrichment, and immune infiltration analyses were conducted. A pediatric-specific signature was validated in the TARGET-OS cohort (n = 87). Results: Pediatric osteosarcoma exhibited a hyperproliferative phenotype, enriched in E2F targets, G2M checkpoint, and DNA replication pathways. Adolescent tumors showed heightened immune–inflammatory signatures, while adult tumors activated osteogenic differentiation programs. Regarding the immune microenvironment, only adolescent tumors demonstrated active immune infiltration; pediatric and adult groups exhibited immunologically “cold” features. We identified a 10-gene pediatric-specific transcriptomic signature that declined with increasing age. High signature scores were significantly associated with inferior overall survival (hazard ratio [HR] = 5.6, 95% confidence interval [CI]: 1.2–26.2, p = 0.01) and progression-free survival (HR = 2.1, 95% CI: 1.1–4.2, p = 0.03). These findings showed concordant trends in the independent TARGET-OS cohort. Conclusions: Pediatric, adolescent, and adult osteosarcoma harbor distinct transcriptional profiles representing biologically different disease entities. The pediatric-specific 10-gene signature may serve as a clinically actionable biomarker for risk stratification and guide age-adapted therapeutic strategies.