Abstract A22: Social behaviors in medulloblastoma: Functional analysis of tumor-derived, supportive glial cells

Understanding the interactions between tumor and supportive cells carries great promise of effective and novel treatment methods. However, conventional methods, including mouse cancer models, lack the spatial resolution to tease apart tumor versus supportive cell types for mechanistic studies. To circumvent this problem, our lab makes use of a novel mouse genetic system termed MADM (mosaic analysis with double markers) for cancer modeling. Starting with a mouse heterozygous for a tumor suppressor gene (TSG), MADM can generate sporadic mutant cells that are null for the TSG via Cre-loxP mediated interchromosomal mitotic recombination events. Most importantly, MADM unequivocally labels mutant cells with GFP, enabling us to trace the lineage of mutant cells and easily distinguish them from neighboring normal cells. We used MADM to model medulloblastoma, the most common type of pediatric brain tumor. It is known that such tumors, especially the desmoplastic subtype, are originated from granule neuron precursors (GNPs). Normal GNPs divide exponentially after birth followed by a prompt cease of expansion by postnatal day 21 (P21) in mice and up to 1 year after birth in humans to give rise to mature granule neurons. Medulloblastoma forms when GNPs stop responding to differentiation signals and continue to proliferate beyond the normal developmental time window. To establish our mouse model for medulloblastoma, we used Math1-Cre that expresses specifically in GNPs to generate green mutant cells. The tumor model is successful in two aspects. First, Math1-Cre faithfully induced recombination in GNPs, which gave rise to granule neurons but not other cell types in wild-type cerebellum. Second, medulloblastoma formed in all mice at the age of 2–3 months, and tumors consisted of GFP-labeled mutant cells generated by MADM. Surprisingly, a closer look at the tumor mass led to the observation of a population of green cells with large cell bodies, reminiscent of glial cells rather than GNPs. The fact that the glial cells are GFP-positive suggests that they are derived from mutant GNPs, although normal GNPs are known to be unipotent and should not give rise to any glial cells. We next stained tumor sections with glia-specific markers GFAP and BLBP to further confirm the identity of these cells in the tumor mass, and found 100% co-localization of the glial markers with these cells. Now that MADM-based model revealed an aberrant fate switch in medulloblastoma, the more important question is whether these glial cells have any function in the tumor. To answer this question, we incorporated a TK transgene driven by the GFAP promoter into our mouse model, which can specifically ablate glial cells upon the injection of a drug called GCV. Preliminary data showed that 1 week after the injection of GCV, tumors regressed completely in our mouse model. In summary, these findings suggest that there is a self-organizing social behavior in medulloblastoma: mutant GNPs divert their developmental fate restriction to give rise to glial cells, which in turn provide critical support for the tumor growth. Currently we are focusing on determining the molecular mechanisms by which tumor glial cells are generated from transformed GNPs, and by which they sustain the tumor growth. Dissecting the mode of support provided by tumor-generated glia should shed light on novel targets for more specific therapeutic strategies, circumventing the devastating side effects of current surgical, radio-, and chemotherapies for medulloblastoma patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr A22.