Stem Cell Reports Ar ticle cFOS-SOX9 Axis Reprograms Bone Marrow-Derived Mesenchymal Stem Cells into Chondroblastic Osteosarcoma Yunlong He, 1,7,8 Wentao Zhu, 1,2,7 Min Hwa Shin, 1 Joy Gary, 3 Chengyu Liu, 4 Wendy Dubois, 5 Shelley B. Hoover, 6 Shunlin Jiang, 1 Eryney Marrogi, 1 Beverly Mock, 3 R. Mark Simpson, 6 and Jing Huang 1, * 1 Cancer and Stem Cell Epigenetics Section, Laboratory of Cancer Biology and Genetics, Center for Cancer Research, National Cancer Institute, Building 37, Room 3140A, Bethesda, MD 20892, USA 2 Department of Orthopaedics, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei 430070, China 3 Cancer Genetics Section, Laboratory of Cancer Biology and Genetics, Center for Cancer Research, National Cancer Institute 4 Transgenic Core, National Heart, Lung, and Blood Institute, National Institutes of Health 5 Animal Models Core Facility, Laboratory of Cancer Biology and Genetics, Center for Cancer Research, National Cancer Institute 6 Molecular Pathology Unit, Laboratory of Cancer Biology and Genetics, Center for Cancer Research, National Cancer Institute Bethesda, MD 20892, USA 7 Co-first author 8 Present address: Institute for Cell Engineering and Sidney Kimmel Comprehensive Cancer Center, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA *Correspondence: huangj3@mail.nih.gov http://dx.doi.org/10.1016/j.stemcr.2017.04.029 SUMMARY Bone marrow-derived mesenchymal stem cells (BMSCs) are proposed as the cells of origin of several subtypes of osteosarcoma (OS). How- ever, signals that direct BMSCs to form different subtypes of OS are unclear. Here we show that the default tumor type from spontaneously transformed p53 knockout (p53_KO) BMSCs is osteoblastic OS. The development of this default tumor type caused by p53 loss can be overridden by various oncogenic signals: RAS reprograms p53_KO BMSCs into undifferentiated sarcoma, AKT enhances osteoblastic OS, while cFOS promotes chondroblastic OS formation. We focus on studying the mechanism of cFOS-induced chondroblastic OS for- mation. Integrated genome-wide studies reveal a regulatory mechanism whereby cFOS binds to the promoter of a key chondroblastic transcription factor, Sox9, and induces its transcription in BMSCs. Importantly, SOX9 mediates cFOS-induced cartilage formation in chondroblastic OS. In summary, oncogenes determine tumor types derived from BMSCs, and the cFOS-SOX9 axis is critical for chondro- blastic OS formation. INTRODUCTION Bone marrow-derived mesenchymal stem cells (BMSCs) are multipotent fibroblast-like cells that have the capacity to differentiate into adipocytes, osteocytes, chondrocytes, and fibroblasts. They also maintain bone marrow homeo- stasis by generating osteocytes and providing a niche for hematopoietic stem cells (Bianco et al., 2008; Kfoury and Scadden, 2015). The true in vivo identity of BMSCs has not been firmly established (Bianco et al., 2008; Sacchetti et al., 2016; Worthley et al., 2015; Zhou et al., 2014). BMSCs, in vitro, are loosely defined as a group of cells that have the ability of differentiating into adipocytes, osteoblasts, and chondrocytes. Because of their multipo- tency, BMSCs attract a lot of attention in the tissue regener- ation field. Several key transcription factors govern the development of BMSCs into different cell types. RUNX2 is critical for osteogenic differentiation, SOX9 for chondro- genic differentiation, and peroxisome proliferator acti- vated receptor g (PPARg) for adipogenic differentiation (Bi et al., 1999). In tumor biology, BMSCs are closely rele- vant to sarcomagenesis since they are proposed as the cells of origin of several types of sarcomas, such as osteosarcoma (OS), chondrosarcoma, liposarcoma, and Ewing’s sarcoma (Tirode et al., 2007). For example, both mouse and human BMSCs can give rise to OS through spontaneous transfor- mation or genetic modifications (Calo et al., 2010; Lin et al., 2009; Xiao et al., 2013). OS is the most common cancer arising from bone and mainly affects children and adolescents. The knowledge of the genetic and epigenetic causes of OS remains incom- plete, although TP53 and RB1 losses are commonly involved (Velletri et al., 2016). Around 25% of mice with heterozygous deletion of trp53 (also called p53) develop OS after a long latency, suggesting that a second genetic or epigenetic event is required for the full development of OS (Donehower et al., 1992; Lang et al., 2004; Olive et al., 2004). A recent genome-wide sequencing study of human OS showed that almost all the human OS have point muta- tions, deletions, amplifications, and/or translocations in the TP53 gene or the genes encoding other components (e.g., HDM2) in the p53 signaling pathway, demonstrating the importance of p53 loss in osteosarcomagenesis (Chen et al., 2014). In human OS and patient-derived OS cell lines, TP53 deletions and point mutations are common (Chen et al., 2014; He et al., 2015). Recent whole-genome sequencing studies revealed that the PI3K (phosphoinosi- tide 3-kinase)-AKT (v-akt murine thymoma viral onco- gene)-mTOR (mammalian target of rapamycin) pathway is also involved in osteosarcomagenesis (Chen et al., 1630 Stem Cell Reports j Vol. 8 j 1630–1644 j June 6, 2017 j This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).