Using Patient-Derived Gliomaspheres to Molecularly Characterize and Dissect Distinctive Traits of Isocitrate Dehydrogenase I Mutant Gliomas for Therapeutic Benefit
Glioblastoma (GBM) is the most common and malignant primary brain tumor. Despite maximal therapy prognosis remains poor. In the preceding decade, histological methods of categorization have begun to give way to molecular and genetic methods. Most prominently, the IDH1 mutation, when found in glioblastoma correlates with a better prognosis than those glioblastomas in which the mutation is not found. It is not known why this correlation exists. The mutant IDH1 enzyme is able to reduce alpha-ketoglutarate to create extremely high levels of 2-hydroxyglutarate but the tumorigenic significance of this reaction is unknown.
In this dissertation we use an extensive collection of IDH1mutant and IDH1wildtype patient-derived gliomaspheres to dissect genetic and metabolic differences that distinguish the IDH1mutant subgroup. We found that the IDH1mutant gliomas were readily identifiable by their slower growth and distinctive expression profile. However, pharmacologic and genetic inhibition of the IDH1mutant enzyme did not change this signature in fact it did not seem to have any effect on these cells other than to decrease the levels of 2-hydroxyglutarate.
The expression profile of IDH1mutant cells showed a large set of down-regulated genes in comparison to IDH1wildtype cells. One of the few genes to be up-regulated was Olig2 which upon further study we revealed to be essential for growth and a regulator of other important genes such as Tet2. Further, Olig2 expression may be targetable by the histone deacetylase inhibitor (HDACi) class of drugs in particular valproic acid.
When we subjected the expression profiles of IDH1mutant glioma cells to metabolic analysis we found that IDH1mutant cells were enriched for pathways involved in DNA repair following radiation while IDH1wildtype glioma cells were enriched for pathways involved in de novo DNA synthesis. Following these results we confirmed that IDH1mutant cells do show better recovery following radiation and IDH1wildtype cells show a greater utilization and dependence on the de novo pathway for DNA synthesis.
Historically, glioblastoma has been defined based on histologic criteria and while two glioblastoma samples may contain two mutually exclusive lists of mutations they would be considered minor variants within the larger fairly homogenous class. It is now becoming clear constitute a distinct group of brain tumors with different characteristics, prognosis and response to therapy and thus need to studied separately from IDH1wildtype glioblastomas.