Derivation of sarcomas from mesenchymal stem cells via inactivation of the Wnt pathway

Malignant fibrous histiocytoma (MFH), now termed high-grade undifferentiated pleomorpbic sarcoma, is a commonly diagnosed mesenchymal tumor, yet both the underlying molecular mechanisms of tumorigenesis and cell of origin remain unidentified. We present evidence demonstrating that human mesenchymal stem cells (hMSCs) are the progenitors of MFH. DKK1, a Writ inhibitor and mediator of hMSC proliferation, is overexpressed in MFH. Using recombinant proteins, antibody depletion, and siRNA knockdown strategies of specific Writ elements, we show that DKK1 inhibits hMSC commitment to differentiation via Wnt2/beta-catenin canonical signaling and that Wnt5a/JNK noncanonical signaling regulates a viability checkpoint independent of Dkk1. Finally, we illustrate that hMSCs can be transformed via inhibition of Writ signaling to form MFH-like tumors in nude mice, and conversely, MFH cells in which Writ signaling is appropriately reestablished can differentiate along mature connective tissue lineages. Our results provide mechanistic insights regarding the cell of origin of MFH, establish what we believe is a novel tumor suppressor role for Writ signaling, and identify a potential therapeutic differentiation strategy for sarcomas.