Summary Alveolar soft part sarcoma (ASPS), a deadly soft tissue malignancy with a predilection for adolescents and young adults, associates consistently with t(X;17) translocations that generate the fusion gene ASPSCR1-TFE3. We proved the oncogenic capacity of this fusion gene by driving sarcomagenesis in mice from conditional ASPSCR1-TFE3 expression. The completely penetrant tumors were indistinguishable from human ASPS by histology and gene expression. They formed preferentially in the anatomic environment highest in lactate--the cranial vault--, expressed high levels of lactate importers, harbored abundant mitochondria, metabolized lactate as a metabolic substrate and responded to the administration of exogenous lactate with tumor cell proliferation and angiogenesis. These data demonstrate lactate’s role as a driver of alveolar soft part sarcomagenesis.

Osteosarcomas remain an enigmatic group of malignancies that share in common the presence of transformed cells producing osteoid matrix, even if these cells comprise a minority of the tumor volume. The differentiation state of osteosarcomas has therefore become a topic of interest and challenge to those who study this disease. In order to test how the cell of origin contributes to the final state of differentiation in the transformed cells, we compared the relative tumorigenicity of Cre-LoxP conditional disruption of the cell cycle checkpoint tumor suppressor genes Trp53 and Rb1 using Prx1-Cre, Collagen-1α1-Cre, and Osteocalcin-Cre to transform undifferentiated mesenchyme, pre-osteoblasts, and mature osteoblasts, respectively. The Prx1 and Col1α1 lineages developed tumors with nearly complete penetrance, as anticipated. Osteosarcomas also developed in 44 percent of Oc-Cre;Rb1fl/fl;Trp53fl/fl mice. We confirmed using EdU click chemistry that the Oc-Cre lineage includes very few actively cycling cells. By assessing radiographic mineralization and histologic osteoid production, the differentiation state of tumors did not correlate with the differentiation state of the lineage of origin. Some of the osteocalcin-lineage-derived osteosarcomas were among the least osteoblastic. Osteocalcin immunohistochemistry in tumors correlated well with expression of DNA methyl transferases, suggesting that silencing of these epigenetic regulators may influence the final differentiation state of an osteosarcoma. Transformation of differentiated, minimally proliferative osteoblasts is possible, but may require such an epigenetic reprogramming that the tumors no longer resemble their differentiated origins.

β-catenin is a master regulator in the cellular biology of development and neoplasia. Its dysregulation is implicated as a driver of colorectal carcinogenesis and the epithelial-mesenchymal transition in other cancers. Nuclear β-catenin staining is a poor prognostic sign in synovial sarcoma, the most common soft-tissue sarcoma in adolescents and young adults. We show through genetic experiments in a mouse model that expression of a stabilized form of β-catenin greatly enhances synovial sarcomagenesis. Stabilization of β-catenin enables a stem-cell phenotype in synovial sarcoma cells, specifically blocking epithelial differentiation and driving invasion. β-catenin achieves its reprogramming in part by upregulating transcription of TCF/LEF target genes. Even though synovial sarcoma is primarily a mesenchymal neoplasm, its progression towards a more aggressive and invasive phenotype parallels the epithelial-mesenchymal transition observed in epithelial cancers, where β-catenin's transcriptional contribution includes blocking epithelial differentiation.