LAUSR

The majority of cells in Tenosynovial Giant Cell Tumor (TGCT) are macrophages responding to CSF1 that is overproduced by a small number of neoplastic cells with a chromosomal translocation involving the CSF1 gene. Treatment with inhibitors of the CSF1 pathway has been clinically effective. An autocrine loop was postulated where the neoplastic cells are stimulated through the CSF1 receptor (CSF1R) expressed on their surface. Here we show that the neoplastic cells themselves do not express CSF1R and therefore may be unaffected by current therapies. We identified a new marker for synoviocytes, GFPT2, that highlights the tumor cells in TCGT and is associated with activation of the YAP1/TAZ pathway. The neoplastic cells in TGCT are highly similar non-neoplastic synoviocytes. Finally, we provide molecular support for the osteoclast-like features of the giant cells in TGCT that correlate with the destructive effects of TGCT on bone.