Transformation of Human Mesenchymal Stem Cells into High-Grade Sarcomas by YAP1 and K-RAS Reflects the Undifferentiated Pleomorphic Sarcoma-Myxofibrosarcoma Disease Spectrum

High-grade complex karyotype sarcomas are a heterogeneous group of more than seventy tumors that vary in histology, clinical course, and patient demographics. Despite these differences, sarcomas are treated similarly with varying efficacy. This is in part due to the rarity of the disease and the lack of preclinical models for the study of individual sarcoma subtypes. Here, we describe the development of a model of high-grade sarcoma formation from human mesenchymal stem cells by introducing genetic changes that are seen in complex karyotype sarcomas. RB1 and P53 are tumor suppressors that are often mutated or functionally inactivated in sarcomas and these were targeted using CRISPR-Cas9 technology which, alone did not result in sarcoma formation. To identify drivers, a lentiviral library screen was performed and the results were validated in a secondary screen. From this, YAP1 and K-RAS were found to robustly drive the formation of undifferentiated pleomorphic sarcoma and myxofibrosarcoma which exist on a disease spectrum. Other drivers were identified that resulted in high grade sarcomas. The majority of these tumors were undifferentiated pleomorphic sarcoma although pleomorphic leiomyosarcoma and osteosarcoma were present in a subset of tumors driven by CDK4 and PIK3CA. This tool allows for the study of human sarcoma subtype development from mesenchymal stem cells and provides a platform for further mechanistic studies, investigations of metastasis, and drug development for this aggressive and heterogeneous disease. ### Competing Interest Statement The authors have declared no competing interest.