The PDGF/PI3K/AKT Axis in Mesenchymal Chondrosarcoma: Functional Characterization and Implications for Anticancer Therapy

Mesenchymal chondrosarcoma is a high-grade, malignant, primitive mesenchymal tumor. It accounts for 2%~4% of all chondrosarcomas and mainly affects adolescents and young adults. Mesenchymal chondrosarcoma has a strong trend toward late local and metastatic recurrence, and the outcome for these patients is poor. We previously described the HEY1-NCOA2 as a recurrent gene fusion in mesenchymal chondrosarcoma, an important breakthrough for characterizing this disease; however, little study had been done to functionally characterize the fusion protein, in large part due to a lack of suitable models for evaluating the impact of HEY1-NCOA2 expression in the appropriate cellular context. We recently developed an in vitro model for studying mesenchymal chondrosarcoma tumorigenesis, using stably transduced iPSC-derived mesenchymal stem cells (iPSC-MSCs) with inducible-expression of HEY1-NCOA2. With the in vitro model, we performed genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) and RNA-seq to identify HEY1-NCOA2-dependent transcriptional regulation. We demonstrated that HEY1-NCOA2 fusion protein preferentially binds to promoter regions of canonical HEY1 targets, resulting in transactivation of HEY1 targets, and significantly enhances cell proliferation. Intriguingly, we identified that both PDGFB and PDGFRA are directly targeted and upregulated by HEY1-NCOA2; and the fusion protein, but not wild-type HEY1 or NCOA2, dramatically increased phospho-AKT (Ser473) expression. Of note, a previous study on mesenchymal chondrosarcoma patients’ tumor samples revealed increased expression of PKC-a, PDGFR-a and mTOR by immunohistochemistry staining. Hence, we hypothesize that the PDGF/PI3K/AKT axis is the critical pathway upregulated by HEY1-NCOA2 fusion in mesenchymal chondrosarcoma and can be rationally targeted as an innovative, mechanism-based therapeutic strategy for mesenchymal chondrosarcoma patients. The specific aims in this proposal are therefore designed to 1) Determine the impact on chondrogenic lineage differentiation and dysregulated pathways resulting from HEY1-NCOA2 fusion expression and further elaborate PDGF signaling pathway in mesenchymal chondrosarcoma tumorigenesis and/or tumor growth; and 2) Assess the preclinical rationale for PDGF/PI3K/AKT inhibition in the treatment of mesenchymal chondrosarcoma. Successful completion of our proposed study will help understanding the mechanisms by which transcription factor fusion HEY1-NCOA2 impair human mesenchymal stem cell-chondroprogenitor-chondrocyte ontogeny and opening a new therapeutic venue for mesenchymal chondrosarcoma. Specifically, our work will determine whether anti-PDGF/PI3K/AKT axis is an effective treatment for mesenchymal chondrosarcoma. Notably, multiple PDGF and mTOR signaling inhibitors are already in clinical trials for the treatment of a variety of human tumors and could thus be rapidly translated to trials for mesenchymal chondrosarcoma patients.