Abstract A78: Using primary osteosarcoma samples to study tumor heterogeneity
Authors: Sayles Leanne C. , Vincent-Tompkins Justin , Chan Charles , Weissman Irving , Sweet-Cordero E. Alejandro 
Published: May 2015

Abstract
Osteosarcoma (OS) is an aggressive pediatric bone malignancy that affects 400 children per year in the United States. It has one of the lowest 5 yr. survival rates of any pediatric cancer. In order to gain a better understanding of the pathogenesis of OS, we have established a panel of patient derived xenografts (PDX). Through a three institution collaboration, we have obtained 52 OS patient samples, 38 pre-chemotherapy biopsies and 16 post induction chemotherapy resections. These samples were implanted into immunocompromised mice either subrenal or intradermal and allowed to expand. We were able to generate 20 PDX samples (38% take rate) and have characterized 7 of them. This includes one pre and post-chemo pair from the same patient. The PDX samples had similar histology to the original patient biopsy including formation of osteoid, which is a hallmark of OS.
Tumor propagating cells (TPCs) are a rare subset of cells with in the tumor that are thought to contain the properties necessary to propagate and give rise to all cells found in the original tumor. TPCs are also thought to be resistant to chemotherapy and thus, it is important to identify and study these cells to gain a better understand the biology that underlies the TPCs ability to propagate the tumor. To identify the TPCs in OS, we selected a panel of cell surface markers that have been associated with TPCs in other cancer types and assessed for their expression across multiple OS PDX samples. CD49f, CD146 and CD90 were expressed heterogeneously in most PDX samples. There was also a high degree of overlap in the expression of these three markers in a subset of OS cells. To determine if the heterogenenous staining observed has a biological significance, we sorted CD49f positive and negative cells and implanted them in serial dilutions into immunocompromized mice. Although no difference in tumor instance was observed, CD49f + tumor cells formed significantly larger tumors than those arising from CD49f- tumor cells. Thus, CD49f expression may mark a more proliferative subset of cells within the bulk tumor. Further studies are currently ongoing to define the functional relevance of CD146 and CD90 expression and their role in tumor propagation and tumor morphology using OS PDX samples.
To further characterize these OS PDX samples, we have employed RNA sequencing technology. To date we have obtained RNAseq on 4 pre-chemotherapy and 3 post-chemotherapy patient samples. Analysis of this dataset is ongoing and may provide insight into mechanisms of chemoresistance.
Citation Format: Leanne C. Sayles, Justin Vincent-Tompkins, Charles Chan, Irving Weissman, E. Alejandro Sweet-Cordero. Using primary osteosarcoma samples to study tumor heterogeneity. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr A78.