Loss of PRC2 enforces a mesenchymal neural crest stem cell phenotype in NF1-deficient cancer through activation of core transcription factors

Genetic alterations that result in the dysregulation of polycomb repressive complex 2 (PRC2) are common features of multiple cancer types. Loss-of-function mutations in the PRC2 genes, SUZ12 or EED, occur at high frequency in malignant peripheral nerve sheath tumors (MPNST), a clinically aggressive sarcoma that arises from NF1-deficient Schwann cells. To define the oncogenic mechanisms mediated by PRC2 loss, we used engineered MPNST model systems that dynamically reassemble a competent PRC2 coupled with single cell sequencing from clinical samples to evaluate the transcriptomic and epigenetic consequences of PRC2 loss. We discovered a two-pronged oncogenic process; first, PRC2 loss leads to remodeling of bivalent chromatin and the enhancer landscape of the tumor cell. These epigenetic changes result in the upregulation of developmentally regulated transcription factors, including FOXC1, which enforce a transcriptional circuit that is a core vulnerability of the cell. Second, PRC2 loss reduces type I interferon signaling and antigen presentation as a downstream consequence of hyperactivated Ras signaling and its crosstalk with STAT and IRF transcription factors. Mapping of the transcriptional program of these PRC2-deficient tumor cells onto a constructed developmental trajectory of the developing neural crest stem cell revealed that changes induced by PRC2 loss enforced a cellular profile characteristic of a primitive mesenchymal neural crest stem cell.