MOLECULAR CHARACTERIZATION OF MURINE MODELS OF ASTROCYTOMA
Creators: name: Vitucci, Mark
Publisher: name: University of North Carolina at Chapel Hill
Year: 2013

description: Astrocytomas are some of the most lethal diffuse gliomas, and glioblastoma (GBM, Grade IV astrocytoma) has a median survival of 12-15 months with therapy. The last decade has seen increased efforts to define the molecular landscape of human GBM, and led to a focus on genetic abnormalities within the receptor tyrosine kinase (RTK), RB cell cycle, and P53 signaling pathways. Genetically-engineered mouse (GEM) models have been designed based upon the data from these studies and have helped determine some of the requirements for gliomagenesis depending on the cellular and developmental context. Despite these efforts gliomagenesis requirements and progression are not completely defined, and more importantly, it is often unclear which molecular subtype is modeled by these GEM. In this work, we employ GEM with conditional, inducible mutations in the RB cell cycle, MAPK, and PI3K pathways to effect astrocytoma initiation followed by stochastic progression in astrocytes throughout the brain in adult mice. We define the requirements for astrocytoma initiation and the effect they have on gene expression and copy number. Stochastic progression to high-grade astrocytoma (HGA) and GBM are characterized by detection via contrast-enhancing MRI, rapid growth, genotype-dependent survival, acquisition of copy number abnormalities (CNA), and gene expression subtypes that resemble human GBM. These subtypes correlate with brain region rather than original genotype. In parallel, we isolated astrocytes from pups containing the same genetic mutations and induced recombination in culture to create G1/S-defective astrocytes with activated Kras and/or Pten deletion. We examined how these individual and combined mutations affected gene expression and phenotypic hallmarks of astrocytoma tumorigenesis including cell growth, migration, and invasion. Combined disruption of MAPK and PI3K signaling led to the most aggressive, invasive astrocytes (TRP) with stem-like and proneural expression profiles. These TRP astrocytes were confirmed to have stem cell properties in vitro and in vivo. After orthotopic injection into syngeneic mice, these TRP astrocytes formed HGA with high incidence, short latency, and reproducible survival, supporting its utility as a preclinical model. We replicated standard of care GBM treatment consisting of radiation with concurrent temozolomide and showed that TRP allografts were susceptible to radiation but not temozolomide. Similar to TRP astrocytes in vitro, the allograft HGA expression profiles were proneural, but after radiation treatment most were most similar to the mesenchymal subtype. Overall, this research defines the requirements for astrocytoma in adult murine astrocytes and raises important questions about whether mutations, cell type, or location determines molecular subtype. We develop several models which will be useful to further elucidate the molecular nuances of astrocytoma and their effects on initiation, progression, and signaling pathways. These models will also serve as the basis for future subtype specific preclinical models in which to develop novel gene signatures, biomarkers, and molecularly targeted therapies.