Abstract SY16-03: Studying lineage potential in tumorigenesis using MADM, a genetic mosaic system.
Authors: Zong Hui 
Published: August 2014

Abstract
Human cancers frequently arise from the loss of both copies of a tumor suppressor gene (TSG) in sporadic cells. We have established a mouse genetic system termed MADM (Mosaic Analysis with Double Markers) that limits TSG inactivation in very few cells for physiologically relevant cancer modeling. MADM unequivocally labels sporadic mutant and wildtype sibling cells with GFP and RFP respectively, to confer the in vivo single cell resolution and lineage tracing capability. The research theme of our lab is focused on brain tumor modeling with two questions that are particularly interesting to us: 1) What is the developmental origin of brain tumors? 2) How do tumor cells evolve along their lineage potentials during malignant transformation?
The cell-of-origin for glioma has long been thought to be neural stem cells (NSCs) based on two observations. First, purified tumor cells manifest stem cell features. Second, the introduction of p53 and NF1 mutations into NSCs in mouse models led to glioma formation. However, endpoint features may not reliably reflect the nature of tumor initiating cells, thus the analysis should focus on early, pre-transforming stages. Furthermore, conceptually there is a critical difference between cell-of-origin and cell-of-mutation. The former is the cell type that transforms into malignancy, while the latter is the one in which initial mutations occur but may not directly transform. We used MADM to probe into early phases of gliomagenesis, and surprisingly found the lack of overpopulation of mutant NSCs. Among NSC-derived cell types, we only detected dramatic over-expansion of mutant oligodendrocyte precursor cells (OPCs) at pre-transforming stages. Consistently, terminal-stage tumor cells displayed salient OPC features by both histological criteria and transcriptome profiling. Most importantly, introducing the same mutations directly into OPCs was sufficient for malignant transformation. Our findings strongly implicate OPC as a cell-of-origin for glioma, and highlight the importance of analyzing early phases of tumorigenesis to pinpoint its origin.
Our lab also generated a medulloblastoma model by introducing TSG mutations into uni-potent granule neuron precursors (GNPs). Preliminary findings indicated that tumor GNPs divert from their uni-potency to give rise to glial cells. We are currently investigating two critical questions on these tumor-derived glial cells: 1) how do uni-potent GNPs reprogram themselves during malignant transformation to generate cell types beyond the original lineage potential? 2) how do glial cells contribute to the progression of medulloblastoma?
In summary, our work has started unraveling the importance of lineage potentials during the tumorigenic process. While the same mutations could occur in many cell types, often times only one cell lineage would be able to transform into malignancy. Teasing apart the differences in signaling context between transforming and non-responsive cell types could provide critical insights for devising highly effective treatment strategies. It's also important to note that tumor cells have the capacity to deviate from normal developmental program and alter their original lineage potentials. The mechanisms and significance of “tumor-reprogramming” still await further investigations.
Citation Format: Hui Zong. Studying lineage potential in tumorigenesis using MADM, a genetic mosaic system. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr SY16-03. doi:10.1158/1538-7445.AM2013-SY16-03