Identification and characterization of neoplastic cells in keratin-positive giant cell-rich tumors.

Keratin-positive giant cell-rich tumor (KPGCT) is a newly described bone and soft tissue tumor. The tumor is characterized by scattered keratin-positive cells and the presence of HMGA2::NCOR2 fusions. It is not known if the HMGA2::NCOR2 fusion is located in the keratin-positive cells, and little is known about how KPGCT develops. KPGCT shares some histologic features with tenosynovial giant cell tumor (TGCT), a soft tissue tumor with CSF1 rearrangements. Single-nuclei RNA sequencing and Xenium spatial transcriptomics were used to elucidate the most likely mechanisms driving KPGCT and compare KPGCT to TGCT. We identified a population of cells coexpressing keratin, HMGA2, and CSF1 as the likely neoplastic cells; these cells constitute the minority of the cells in the tumor. These cells express no synovial markers, confirming KPGCT as a distinct entity, separate from TGCT. The bulk of the tumor consists of CSF1R-expressing macrophages and multinucleated giant cells (MNGCs), suggesting an important role for CSF1-CSF1R signaling. In addition, we find that the likely neoplastic cell population shows activation of the hippo signaling pathway, a possible central pathway in KPGCT tumorigenesis. The CSF1-CSF1R axis likely stimulates the migration and proliferation of macrophages, which form the majority of cells in the tumor, as well as their differentiation into MNGCs. These results provide a rationale for the use of CSF1R inhibitors, which have already shown efficacy in TGCT, as a therapy for KPGCT.