Truncated <scp>FOS</scp> impairs osteogenic differentiation and induces prostaglandin and <scp>NFκB</scp> signalling in an <i>in vitro</i> cell‐of‐origin model for osteoid osteoma and osteoblastoma

Osteoid osteoma and osteoblastoma are non-malignant bone-forming tumours of the skeleton, characterised by the presence of irregular trabeculae of woven bone. Rearrangements in FOS, and less frequently FOSB, have recently been identified in osteoid osteoma and osteoblastoma. Identical rearrangements in FOS were previously detected in epithelioid haemangioma, where these led to truncation of the FOS protein in the C-terminal domain, causing increased protein stability due to impaired degradation. Since FOS plays a role in osteogenic differentiation, the effect of FOS truncation on osteogenic differentiation and proliferation was investigated in an in vitro model for osteoid osteoma and osteoblastoma. In this model, truncated FOS (FOSΔ) was overexpressed in human foetal mesenchymal stem cells through a lentiviral vector. Osteogenic differentiation - assessed by measuring mineralisation, ALPL expression, and ALP activity - and proliferation rate were reduced in cells overexpressing FOSΔ compared to mesenchymal stem cells with an empty lentiviral vector (pLV). Transcriptome-sequencing and differential gene expression analysis revealed decreased gene expression of genes in pathways involving cell cycling and mitosis and osteogenic differentiation, including WNT signalling, extracellular matrix organisation, and matrix metalloproteinases (MMPs), in FOSΔ as compared to empty vector cells, indicating decreased proliferation and osteogenesis. Instead, FOSΔ cells showed upregulation of genes involved in prostaglandin signalling and NF-kB inflammatory pathways. These findings highlight that FOSΔ compromises cellular growth and osteogenesis, which is in line with the morphological features of osteoid osteoma and osteoblastoma with woven bone formation instead of mature lamellar bone, as well as the indolent clinical behaviour. Additionally, FOSΔ promotes inflammatory signalling instead, which correlates with clinically exquisite response to non-steroid anti-inflammatory drugs. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.