Low-grade gliomas (LGGs) are primary brain tumors affecting young adults. Although they initially grow relatively slowly, they eventually transform t more aggressive high-grade gliomas, leading to neurologic deterioration and death. The questions of cell of origin and significance of known mutations in early low-grade gliomagenesis remain unanswered, thus limiting the ability to develop sensitive detection methods and new therapies. In up to 80% of LGGs, gain-of-function mutations are found in the gene encoding the cytosolic isoform of Isocitrate DeHydrogenase (IDH), usually due to an arginine to histidine substitution at position 132 (R132H). While the wild-type enzyme normally functions to convert isocitrate to a-ketoglutarate (aKG), R132H-IDH1 catalyzes the production of R-2-hydroxyglutarate (2HG), which leads to genome-wide epigenetic modifications that may be related to tumorigenesis. The putative role of IDH1 mutations in gliomagenesis has been supported by in vitro observations in human astrocytes and glioma cells, as well as the fact that patients with Ollier disease, which is due to mosaic germline mutations of IDH1, occasionally develop gliomas. LGGs bearing IDH1 mutations are predominantly located in the frontal lobes in close proximity to the frontal horns of the ventricular system. Because the subventricular zone around the lateral ventricles is an area of active neurogenesis, we postulate that the cell of origin in IDH1-mutated LGGs is a component of the neurogenic niche in the subventricular zone. Mouse models of brain-specific mutant IDH1 expression suffer from perinatal lethality and fail to show tumorigenesis. To test the hypothesis that mutant IDH1 represents a driver alteration in LGG initiation and to investigate which brain cells are predisposed by mutant IDH1 to undergo oncogenic transformation, we propose a new approach that overcomes limitations associated with in vitro and in vivo mouse models. Our strategy makes use of human embryonic stem cells to inducibly express R132H-IDH1 in specific cell types within the human neural lineage: neural stem cells, neuroblasts, astrocytes and oligodendrocytes. We propose to test the hypothesis that inducible expression of R132H-IDH1 alters the self-renewal, differentiation potential, proliferation rate, metabolome and epigenetic/transcriptional profile of neural stem cells or other components of the human neural lineage in vitro. Furthermore, to test the hypothesis that mutant IDH1 expression in specific human neural cell types contributes to initiation of LGG formation, we will transplant these target cells expressing R132H-IDH1 into the mouse brain and assess their ability to form invasive tumors. The proposed research will address the important question of whether IDH1 mutations in human embryonic stem cell-derived neural lineages alter cellular physiology and facilitate oncogenic transformation. Successful completion of this project will lead to a disease model, which will allow detailed analysis of the metabolome, epigenome and transcriptome of early human LGGs. Furthermore, such a model can be used for translational applications, such as high-throughput drug screening and biomarker identification.
Mutations in the gene encoding the enzyme isocitrate dehydrogenase are found in the majority of low-grade gliomas, a slow-growing but deadly type of brain tumor. Our research proposal will investigate the hypothesis that the mutant enzyme facilitates oncogenic transformation of human neural cells that we derive from human embryonic stem cells. Our experiments will not only improve our understanding of the early phases of human gliomagenesis, but will also provide an experimental model suitable for basic and translational investigation of low-grade glioma biology.
