Search

Through an shRNA screen, we identified the protein arginine methyltransferase Prmt1 as a vulnerable intervention point in murine p53/Rb-null osteosarcomas, the human counterpart of which lacks effective therapeutic options. Depletion of Prmt1 in p53-deficient cells impaired tumor initiation and maintenance in vitro and in vivo Mechanistic studies reveal that translation-associated pathways were enriched for Prmt1 downstream targets, implicating Prmt1 in translation control. In particular, loss of Prmt1 led to a decrease in arginine methylation of the translation initiation complex, thereby disrupting its assembly and inhibiting translation. p53/Rb-null cells were sensitive to p53-induced translation stress, and analysis of human cancer cell line data from Project Achilles further revealed that Prmt1 and translation-associated pathways converged on the same functional networks. We propose that targeted therapy against Prmt1 and its associated translation-related pathways offer a mechanistic rationale for treatment of osteosarcomas and other cancers that exhibit dependencies on translation stress response. Cancer Res; 77(17); 4613-25. ©2017 AACR ., (©2017 American Association for Cancer Research.)

Osteosarcoma is the most common primary bone tumor, yet there have been no substantial advances in treatment or survival in three decades. We examined 59 tumor/normal pairs by whole-exome, whole-genome, and RNA-sequencing. Only the TP53 gene was mutated at significant frequency across all samples. The mean nonsilent somatic mutation rate was 1.2 mutations per megabase, and there was a median of 230 somatic rearrangements per tumor. Complex chains of rearrangements and localized hypermutation were detected in almost all cases. Given the intertumor heterogeneity, the extent of genomic instability, and the difficulty in acquiring a large sample size in a rare tumor, we used several methods to identify genomic events contributing to osteosarcoma survival. Pathway analysis, a heuristic analytic algorithm, a comparative oncology approach, and an shRNA screen converged on the phosphatidylinositol 3-kinase/mammalian target of rapamycin (PI3K/mTOR) pathway as a central vulnerability for therapeutic exploitation in osteosarcoma. Osteosarcoma cell lines are responsive to pharmacologic and genetic inhibition of the PI3K/mTOR pathway both in vitro and in vivo.

Tumors are thought to be sustained by a reservoir of self-renewing cells, termed tumor-initiating cells or cancer stem cells. Osteosarcomas are high-grade sarcomas derived from osteoblast progenitor cells and are the most common pediatric bone malignancy. In this report we show that the stem cell transcription factor Sox2 is highly expressed in human and murine osteosarcoma (mOS) cell lines as well as in the tumor samples. Osteosarcoma cells have increased ability to grow in suspension as osteospheres, that are greatly enriched in expression of Sox2 and the stem cell marker, Sca-1. Depletion of Sox2 by short-hairpin RNAs in independent mOS-derived cells drastically reduces their transformed properties in vitro and their ability to form tumors. Sox2-depleted osteosarcoma cells can no longer form osteospheres and differentiate into mature osteoblasts. Concomitantly, they exhibit decreased Sca-1 expression and upregulation of the Wnt signaling pathway. Thus, despite other mutations, these cells maintain a requirement for Sox2 for tumorigenicity. Our data indicate that Sox2 is required for osteosarcoma cell self renewal, and that Sox2 antagonizes the pro-differentiation Wnt pathway that can in turn reduce Sox2 expression. These studies define Sox2 as a survival factor and a novel biomarker of self renewal in osteosarcomas, and support a tumor suppressive role for the Wnt pathway in tumors of mesenchymal origin. Our findings could provide the basis for novel therapeutic strategies based on inhibiting Sox2 or enhancing Wnt signaling for the treatment of osteosarcomas.

Osteosarcoma is the most common primary malignant tumor of bone. Analysis of familial cancer syndromes and sporadic cases has strongly implicated both p53 and pRb in its pathogenesis; however, the relative contribution of these mutations to the initiation of osteosarcoma is unclear. We describe here the generation and characterization of a genetically engineered mouse model in which all animals develop short latency malignant osteosarcoma. The genetically engineered mouse model is based on osteoblast-restricted deletion of p53 and pRb. Osteosarcoma development is dependent on loss of p53 and potentiated by loss of pRb, revealing a dominance of p53 mutation in the development of osteosarcoma. The model reproduces many of the defining features of human osteosarcoma including cytogenetic complexity and comparable gene expression signatures, histology, and metastatic behavior. Using a novel in silico methodology termed cytogenetic region enrichment analysis, we demonstrate high conservation of gene expression changes between murine osteosarcoma and known cytogentically rearranged loci from human osteosarcoma. Due to the strong similarity between murine osteosarcoma and human osteosarcoma in this model, this should provide a valuable platform for addressing the molecular genetics of osteosarcoma and for developing novel therapeutic strategies.