Driving Brain Tumorigenesis: Generation and Biological Characterization of a Mutant IDH1 Mouse Model
Despite decades worth of research, glioblastoma remains one of the most lethal cancers. The identification of <italic>IDH1</italic> as a major cancer gene in glioblastoma provides an exceptional opportunity for improving our understanding, diagnostics, and treatment of this disease. In addition to mutations in <italic>IDH1</italic>, recent studies from our laboratory have characterized the genetic landscape of gliomas and have shown the cooperation between IDH1 mutations and other oncogenic alterations such at TP53 mutations. Normally, IDH1 functions in the oxidative decarboxylation of isocitrate to α–ketoglutarate, however the mutant form confers neomorphic enzymatic activity by producing 2–hydroxyglutarate, an oncometabolite responsible for aberrant methylation in IDH1–mutated tumors, among other mutant <italic>IDH1</italic>–mediated phenotypes. To determine the role of mutant IDH1 <italic>in vivo</italic>, we generated a conditional knock–in mouse model. This genetically faithful system is both biologically and clinically relevant and will promote the understanding of mutant IDH1–mediated tumorigenesis while offering a route for therapeutic targeting.
We observed that broad expression of mutant IDH1 throughout the brain leads to hydrocephalus in 80% of animals. In assessing the earliest effects of mutant IDH1 on the brain, we determined mutant IDH1 confers a decrease in the proliferative cells of the subventricular zone of the lateral ventricle, the area which houses the neural stem cells in embryonic and adult animals. Additionally, a perturbation to the normal neural stem cell niche was observed in these animals. Combined, this data suggests that mutant IDH1 may be affecting the signaling pathways involved in differentiation in this population of cells. <italic>In vivo</italic> and <italic>in vitro</italic> studies will further elucidate mutant IDH1's effects on the differentiation patterns of neural stem cells expressing mutant IDH1.
To express mutant IDH1 in a more restricted manner and harness spatiotemporal control, we crossed mutant animals to a Nestin–CreER<super>T2</super> strain of mouse that permits expression of floxed alleles upon treatment with tamoxifen. Animals were sacrificed at the onset of symptoms or at 1–year of age. We observed the development of both low– and high–grade gliomas in approximately 15–percent of E18.5 tamoxifen–treated animals. All tumors were found in a TP53–deleted background with mutant IDH1 being detected in only those tumors with the mutant allele. Lastly, to decrease the latency and increase the penetrance of tumor formation, an orthotopic intracranial injection model was generated to allow for visualization of tumor formation and development, as well as investigation of therapeutic modalities. The models generated and the knowledge gained from these studies will offer an understanding of the biological effects of the most common mutations found in the astrocytic subset of gliomas, bringing us strides closer to determining mechanisms and therapeutic targets for <italic>IDH1</italic>–mutated cancers.
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