The Clinical and Cellular Landscape of DKK1 in Osteosarcoma

Background: Dickkopf-1 (DKK1) is a secreted protein that acts as a Wnt antagonist and is implicated in osteosarcoma progression. Elevated circulating DKK1 levels have been described at OS diagnosis, but its circulating dynamics, prognostic relevance, and cellular distribution remain incompletely characterized. Methods: We analysed longitudinal plasma DKK1 in 34 patients using mixed-effects models, evaluated DKK1 expression and survival in two independent transcriptomic cohorts, and examined cellular and tissue distribution using single-cell transcriptomics and immunohistochemistry. Cultured mesenchymal stem cells were assessed as potential nonmalignant sources. Results: Circulating DKK1 declined by 12.4% per 30-day period during treatment, without significant changes detected around primary tumour resection or relapse. Metastatic presentation and poor histologic response were associated with higher estimated DKK1 levels. Higher tumour DKK1 expression was associated with worse overall and metastasis-free survival (pooled HRs per SD increase: 1.35 [95% CI 1.02 - 1.78] and 1.48 [1.06 - 2.07], respectively). DKK1 transcripts were predominantly detected in malignant cells across multiple subpopulations and protein staining was enriched in osteoid and matrix-rich regions of primary tumours. Conclusions: Tumour DKK1 expression is associated with adverse outcomes, while circulating concentration decline over treatment time. These findings support further prognostic evaluation and investigation of the associations among DKK1 expression and production, tissue localisation, and circulating levels.