FAP expression as a marker of malignant transformation enabling in vivo characterization in peripheral nerve sheath tumors: a multimodal and translational study.

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive sarcomas and a major cause of mortality in neurofibromatosis type 1 (NF-1). Distinguishing MPNSTs from benign neurofibromas remains challenging. We investigated fibroblast activation protein alpha (FAP) as a malignancy biomarker and theranostic target in peripheral nerve sheath tumors. Therefore, we integrated publicly available bulk transcriptomics, spatial transcriptomics, and single-cell RNA sequencing with immunohistochemistry (IHC) on independent archival samples. We further directly assessed clinical translatability using FAP-targeted PET/CT in an NF-1 patient undergoing work-up for suspected malignant transformation. Across independent bulk datasets, FAP was consistently up-regulated in MPNSTs compared with neurofibromas. In the TCGA sarcoma dataset, FAP varied by histotype but was clearly expressed in MPNSTs. Spatial transcriptomics revealed enrichment of FAP-high regions in MPNSTs and co-localization with tumor cell markers. Single-cell analysis showed FAP expression in MPNST tumor cells and cancer-associated fibroblasts, with the highest levels in neural crest-like tumor subpopulations previously linked to adverse prognosis; pseudotime analysis indicated decreasing FAP expression along trajectories toward Schwann cell precursor-like states linking FAP expression to a more primitive, dedifferentiated tumor cell state. IHC confirmed strong, predominantly tumor cell-intrinsic FAP expression in MPNSTs, with minimal staining in neurofibromas and normal tissues. Plexiform neurofibromas exhibited intermediate FAP expression. In clinical imaging, FAP-PET demonstrated higher tracer uptake in histologically proven MPNSTs than in benign lesions within the same patient, including a neurofibroma that was FDG-avid but FAP-negative, supporting added diagnostic specificity over FDG-PET/CT. In summary, FAP is robustly overexpressed in MPNSTs at transcript and protein levels, potentially concentrates in high-risk tumor cell states, and is detectable by targeted PET imaging. These findings identify FAP as a clinically relevant biomarker for malignancy in NF-1-associated tumors and support implementation of FAP-directed diagnostics and therapeutics in peripheral nerve sheath tumor work-up.