Abstract B14: Deciphering individual roles of p53 and NF1 in suppressing gliomagenesis

Cancer is difficult to treat because it is a disease caused by multiple genetic alterations. Understanding how individual mutations contribute to pathogenesis and how they cooperate to aid malignancy offers deep insights of tumorigenic mechanisms that will facilitate the design of effective treatment. However, conventional methods, including most mouse cancer models, do not offer the temporal and spatial resolution required to study tumorigenesis in great details, especially because single gene mutations rarely lead to detectable malignancy. To resolve this issue, our lab uses a novel mouse model termed MADM (Mosaic Analysis with Double Markers) to analyze the contribution of individual mutations to aberrant behaviors of tumor cell of origin during pre-transforming stages. Starting with a mouse heterozygous for a TSG, MADM can generate sparse mutant cells that are null for the TSG via Cre-loxP mediated inter-chromosomal mitotic recombination. Concurrently, MADM unambiguously labels mutant cells with GFP and their wild-type sibling cells with RFP, allowing us to precisely analyze behavioral changes of green mutant cells, using their sibling red cells as the internal control. Using MADM to model glioblastoma, the most common and aggressive malignant primary brain tumor, our lab previously demonstrated that oligodendrocyte precursor cell (OPC) is the cell of origin. Using MADM to generate p53 and NF1 double-null OPCs, we found that mutant OPCs dramatically over-expanded at the pre-transforming stage compared to WT sibling cells. Further analysis indicated that mutant OPCs manifest augmented proliferative activities and halted differentiation processes. However, it is still unknown what is the individual contribution of the loss of p53 and NF1 and how their cooperativity leads to malignant transformation. Utilizing MADM, we generated genetic mosaic mice with individual loss of either p53 or NF1 in OPCs and determined the cellular property changes of mutant cells, including proliferative rate, differentiation ability, overall expansion, and malignant transformation. We found that, while the mosaic loss of NF1 alone is sufficient to lead to increased proliferation and impaired differentiation of mutant cells, it is insufficient to lead to malignancy even though mutant cells overwhelm the mouse brain, suggesting that p53 plays critical gatekeeping roles in gliomagenesis. On the other hand, when we examined brains containing p53-null OPCs, we were surprised to find that the loss of p53 alone had no detectable effect on cell number, proliferation and differentiation of mutant cells. We reasoned that the tumor suppressing functions of p53 may only be manifested in an NF1 mutant background, and thus generated a new mosaic model in which all OPCs are NF1-null while green cells are p53 null and red ones are p53 WT. Surprisingly, we still found no differences in cell number, proliferation, or differentiation between green and red cells, suggesting that p53 suppress gliomagenesis through some unknown mechanisms. In summary, our data suggested that NF1 loss leads to glioma initiation by promoting proliferation and inhibiting differentiation of OPCs while p53 loss only contributes to later stage of glioma progressions, and that an effective treatment paradigm would need to address all these mechanisms to halt malignancy. Citation Format: Phillippe Gonzalez, Chong Liu, Hui Zong. Deciphering individual roles of p53 and NF1 in suppressing gliomagenesis. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr B14.