Biochemical Characterization and Genetic Modeling of Glioma-Associated Mutations in Isocitrate Dehydrogenases.

Abstract

Gliomas are the most common tumors of the central nervous system. Our lab recently identified mutations in IDH1 and IDH2 as occurring frequently in progressive gliomas. We applied a series of biochemical and genetic approaches to explore the roles of the mutations in tumors and generate models for study. IDH1/2 mutations have the potential to impact a number of metabolic pathways. IDH1/2 convert isocitrate to α-ketoglutarate while simultaneously converting NADP+ to NADPH. To assess changes in metabolism, we completed metabolic profiling and complementary studies in cell lines with and without mutant IDH1 or mutant IDH2. We identified a decrease in hypoxia signaling and a decrease in global 5-hydroxymethylcytosine in cell lines with mutant IDH1/2 .Having observed mutations in IDH1/2 in a large fraction of progressive gliomas, we asked if the mutations were either 1) advantageous for growth in brain parenchyma, or 2) advantageous in a particular cell-of-origin. Sequencing of a series of metastases to the brain from non-central nervous system tumors identified no mutations in IDH1/2, lending less credence to the first hypothesis. To elucidate whether mutations in IDH1/2 can initiate glioma progression and explore the potential cell-of-origin for progressive gliomas, we generated mice in which we induced expression of mutant IDH2 in different populations of cells in the brain, either alone or in combination with TP53 deletion, another frequently altered gene in progressive gliomas. Mice with broad expression of mutant IDH2 developed hydrocephalus and encephalomalacia early in life, but did not develop tumors. Therefore, we restricted expression, and two brain tumors were identified in mice with both IDH2 mutation and TP53 deletion. While this suggests that both mutations might be required for the development of tumors, this is too small a number to draw significant conclusions. Further research with an expanded cohort of mice, utilization of additional drivers of expression, and further characterization of identified tumors will help in elucidating the role of mutant IDH2 and the cell-of-origin for progressive gliomas.