Tenosynovial Giant Cell Tumor (TGCT) Cell Models for Research

Tenosynovial giant cell tumor (TGCT) is a rare, locally aggressive mesenchymal neoplasm arising from the synovium of joints, bursae, and tendon sheaths. The World Health Organization (WHO) Classification of Soft Tissue Tumors (5th edition, 2020) categorizes TGCT into localized (L-TGCT) and diffuse (D-TGCT) types. The incidence is approximately 1.8 per million person-years for localized and 0.4 per million for diffuse forms, with a slight female predominance (WHO, 2020). TGCT is not malignant but can cause significant morbidity due to joint destruction, pain, and recurrence. The 5-year survival is near 100% for localized disease, but diffuse TGCT has a recurrence rate of 14-50% after surgery (NCI, PDQ). No specific mortality data are available due to its benign nature, but quality of life is severely impacted. Risk factors include age (typically 30-50 years) and a history of joint trauma. Research is critical to understand the molecular drivers and develop targeted therapies to reduce recurrence and improve surgical outcomes.

TGCT is an ideal model for studying tumor microenvironment interactions and CSF1R signaling. The disease is driven by overexpression of colony-stimulating factor 1 (CSF1) due to chromosomal translocations, leading to massive infiltration of CSF1R-positive macrophages. This unique mechanism makes TGCT a paradigm for paracrine oncogenesis. Public datasets, such as those from the The Cancer Genome Atlas (TCGA) and the Catalogue of Somatic Mutations in Cancer (COSMIC), provide genomic and transcriptomic data. However, TGCT is not included in TCGA; instead, data are available from small case series and the NCBI Gene Expression Omnibus (GEO). Open questions include the role of secondary mutations, the heterogeneity of CSF1 expression, and the development of resistance to CSF1R inhibitors like pexidartinib. Gene-edited cell models are essential to dissect these mechanisms.

The primary oncogenic pathway in TGCT involves CSF1 overexpression. The steps are:

1. Chromosomal translocation (e.g., t(1;2) or t(1;5)) places the CSF1 gene under the control of a strong promoter, such as COL6A3 or other genes.

2. Overexpression of CSF1 leads to secretion of the ligand into the tumor microenvironment.

3. CSF1 binds to CSF1R on macrophages and monocytes, promoting their recruitment, proliferation, and survival.

4. The accumulated macrophages produce growth factors and cytokines that support tumor growth and tissue invasion.

Secondary pathways include activation of the PI3K/AKT/mTOR axis in the neoplastic cells, which may contribute to cell survival and proliferation. Additionally, dysregulation of the NF-κB pathway has been implicated in macrophage-mediated inflammation.

Data from COSMIC (v100) and NCBI Gene. Note: TGCT is not in TCGA; frequencies are derived from small cohorts and case reports.

Key signaling networks in TGCT include:

• • CSF1/CSF1R axis: Central driver; activates MAPK and PI3K/AKT pathways in macrophages.
• • PI3K/AKT/mTOR: Promotes cell survival and proliferation in neoplastic cells.
• • NF-κB: Mediates inflammatory responses and macrophage activation.
• • Wnt/β-catenin: May contribute to synovial cell proliferation.
• • Key nodes:
• • CSF1R: Receptor tyrosine kinase; target of pexidartinib.
• • SRC: Downstream kinase involved in cytoskeletal remodeling.
• • STAT3: Transcription factor activated by CSF1R signaling.

Organoids: Patient-derived organoids (PDOs) from TGCT are emerging but not yet widely established. Advantages include preserving the tumor microenvironment and CSF1 expression. However, they are technically challenging due to the rarity of the disease.

• • Patient-derived xenografts (PDX): Implantation of TGCT tissue into immunodeficient mice; useful for drug testing.
• • Genetically engineered mouse models (GEMM): Overexpression of CSF1 under a synovial-specific promoter; recapitulates macrophage infiltration.
• • Induced models: Injection of CSF1-overexpressing cells into mice to mimic tumor formation.

Limitations: PDX models may lose CSF1 expression over passages; GEMMs are time-consuming to generate.

CRISPR-based isogenic cell lines are powerful tools for studying TGCT. For example, knocking out CSF1 in a CSF1-overexpressing cell line (e.g., derived from a TGCT patient) can validate its role in macrophage recruitment. Similarly, introducing CSF1R mutations into monocytic cell lines (e.g., THP-1) can model resistance to CSF1R inhibitors. Commercially available, sequence-verified gene-edited models (e.g., CSF1 knockout in HEK293T or U937) accelerate research by providing consistent, reproducible systems. These models are essential for functional validation and drug screening.

Knockout and knock-in lines are used to validate the role of CSF1 and other genes in TGCT. For example, CRISPR-mediated knockout of CSF1 in a TGCT-derived cell line reduces its ability to recruit macrophages in co-culture assays. Knock-in of CSF1R mutations (e.g., D802V) into monocytic cells can confer resistance to pexidartinib, confirming the oncogenic driver.

Isogenic pairs (e.g., CSF1 knockout vs. wild-type) are used in high-throughput screens to identify compounds that specifically target CSF1-dependent growth. Resistance models are generated by exposing cells to increasing concentrations of CSF1R inhibitors and selecting for surviving clones, then identifying mutations via sequencing.

CRISPR synthetic lethality screens can identify genes that are essential in CSF1-overexpressing cells but not in normal cells. For example, knocking out CSF1R in a CSF1-dependent cell line may reveal compensatory pathways. These screens help identify novel biomarkers and therapeutic targets.