Mouse Mesenchymal Stem Cells Expressing PAX-FKHR Form Alveolar Rhabdomyosarcomas by Cooperating with Secondary Mutations

Abstract

Alveolar rhabdomyosarcomas (ARMS) are highly malignant soft-tissue sarcomas that arise in children, adolescents, and young adults. Although formation and expression of the PAX-FKHR fusion genes is thought to be the initiating event in this cancer, the role of PAX-FKHR in the neoplastic process remains largely unknown in a progenitor cell that is undefined. We hypothesize that PAX-FKHR determine the ARMS progenitor to the skeletal muscle lineage, which when coupled to the inactivation and/or activation of critical cell signaling pathways leads to the formation of ARMS. Because a number of studies have proposed that mesenchymal stem cells (MSC) are the progenitor for several of the sarcomas, we tested this hypothesis in MSCs. We show that PAX-FKHR induce skeletal myogenesis in MSCs by transactivating MyoD and myogenin. Despite exhibiting enhanced growth in vitro, the PAX-FKHRexpressing populations do not form colonies in soft agar or tumors in mice. Expression of dominant-negative p53, or the SV40 early region, elicits tumor formation in some of the PAX-FKHR-expressing populations. Additional activation of the Ras signaling pathway leads to highly malignant tumor formation for all of the populations. The PAX-FKHR-expressing tumors were shown to have histologic, immunohistochemical, and gene expression profiles similar to human ARMS. Our results show the critical role played by PAX-FKHR in determining the molecular, myogenic, and histologic phenotype of ARMS. More importantly, we identify MSCs as a progenitor that can give rise to ARMS. [Cancer Res 2008;68(16):6587-97]

Modeling sarcomagenesis using multipotent mesenchymal stem cells

“…Additional expression of the constitutively active H-RAS G12V leads to tumor formation in all of the PAX-FKHR-expressing populations. These PAX-FKHRexpressing tumors display histological features and gene expression profiles similar to human ARMS [74].…”

“…However, PAX3-FKHR and PAX7-FKHR fusions induced skeletal myogenesis but not transformation when introduced in BM-mMSCs [74]. Nevertheless, the expression of a dominant-negative form of p53 or SV40 large T antigen (which inactivates both p53 and Rb) elicits tumor formation in a proportion of the PAX-FKHR-expressing cells.…”

“…In this regard, the inactivation of p53 is sufficient to transform mMSCs [31] and generate sarcomas in mesenchymal tissues of mouse models [36]. Similarly, p53 mutation has also been successfully used as a cooperating transforming hit in the development of several models of fusion gene-associated sarcomas from mMSCs [40,54,74] or in mouse models [58,76] (Table 3). According to the available literature, the frequency of p53 mutations in human sarcomas ranges from ~6% in well-differentiated/dedifferentiated liposacomas to 23% in osteosarcomas [114].…”

FUS-CHOP Fusion Protein Expression Coupled to p53 Deficiency Induces Liposarcoma in Mouse but Not in Human Adipose-Derived Mesenchymal Stem/Stromal Cells

“…It has been demonstrated that BM-mMSCs provided a permissive environment for tumoral transformation and sarcoma development mediated by several sarcoma-associated fusion genes [2][3][4][5]. Nevertheless, a human sarcoma model remains to be developed.…”

“…Several types of sarcomas have been reproduced in vivo on overexpression of specific fusion oncoproteins in bone marrow (BM)-derived mouse MSCs (mMSCs). These models include the recapitulation of Ewing's sarcoma by expression in BM-mMSCs of EWS-FLI-1 [2,3], mixoid liposarcoma (MLS) by expression of FUS (FUsed in Sarcoma; also termed TLS, Translocated in LipoSarcoma)-CHOP (C/EBP HOmologous Protein; also termed DDIT3, DNA Damage-Inducible Transcript 3) [4], and alveolar rhabdomyosarcoma by expression of PAX-FKHR [5]. Furthermore, cancer-initiating cells displaying MSC properties have been recently identified in Ewing's Sarcoma [6].…”