Abstract 4875: Identification of the cellular origin and "stemness" phenotype of Malignant Rhabdoid Tumors (MRT) may represent a new therapeutic approach in paediatric oncology
Authors: Ramli Ras A. , Finetti Martina A. , Selby Matthew P. , Grabovska Yura , Crosier Stephen , Smith Amanda J. , Clifford Steven C. , Williamson Daniel 
Published: July 2017

Abstract
Malignant Rhabdoid Tumors (MRT) are highly aggressive childhood malignancies characterized by a single mutation; biallelic inactivation of SMARCB1, a component of the SWI/SNF chromatin remodeling complex. These tumors may occur anywhere, most frequently in the brain (Atypical Teratoid/Rhabdoid Tumor, ATRT) and in the kidneys and soft tissues (Extra Cranial Rhabdoid tumor, ECRT). MRT presents immune markers from multiple lineages exhibiting a teratoid phenotype, characterized by cells with heterogeneous morphology within the same tumor leading to speculation that the MRT cell of origin is a type of multipotent stem cell. Despite recent advances in treating other solid tumors, treatment for MRT still remains ineffective and we hypothesized that MRT “stemness” characteristics are key to its aggressive clinical course and resistance to treatment.
Here we analyze the expression profiles of primary MRT (n= 134) and catalog the expression of a program of “stemness” genes capable of driving aberrant self-renewal. Further, we show by re-expression of SMARCB1 in MRT cells that several of these key “stemness” genes are aberrantly activated by SMARCB1 mutation. One such gene, BMI1 was further demonstrated to be critical to MRT tumorigenesis by shRNA knockout, and a novel anti-BMI1 drug, PTC209 was tested to show efficacy in MRT. Knockdown of BMI1 in MRT cells reduced the self-renewal capability of cells as seen from the number of tumorspheres formed in limiting dilution assay (LDA). Interestingly, BMI1 loss triggers upregulation of p16(INK4a) expression and this mimics the expression profile when SMARCB1 was re-expressed into MRT cells.
To identify a putative MRT cell of origin we performed a meta-analysis cross-referencing expression profiles from primary MRT (n= 20), and functional models in which SMARCB1 was re-expressed (n=5 lines) with expression profiles from multiple stem cell types including epithelial, embryonic, neural mesenchymal and neural crest (n= 446). Analysis by t-SNE and NMF consensus clustering suggested that MRT bore the closest resemblance to either early neural progenitors (ATRT) or early neural crest cells (ECRT). In MRT cells in which SMARCB1 was forcibly re-expressed, cells became gradually more differentiated and their expression profile altered from that of an early neural crest to a more differentiated mesenchymal pattern.
For the first time, we show evidence that ECRTs and ATRTs appear to have different cells of origin, despite having the same mutation and tumor appearance. We further demonstrate that SMARCB1 expression is necessary for MRT cells to maintain a de-differentiated “stem-like” state and finally that the SMARCB1-dependent stemness gene BMI1 shows potential as a novel therapeutic target in MRT.
Citation Format: Ras A. Ramli, Martina A. Finetti, Matthew P. Selby, Yura Grabovska, Stephen Crosier, Amanda J. Smith, Steven C. Clifford, Daniel Williamson. Identification of the cellular origin and "stemness" phenotype of Malignant Rhabdoid Tumors (MRT) may represent a new therapeutic approach in paediatric oncology [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4875. doi:10.1158/1538-7445.AM2017-4875