Mechanisms of glioma initiation by IDH oncogenes - PROJECT SUMMARY Lower grade gliomas, as well as the high grade gliomas that arise from them, are now diagnostically defined by the presence of mutations in genes encoding isocitrate dehydrogenase (IDH) enzymes. Mutant IDH enzymes synthesize the oncometabolite (R)-2HG. (R)-2HG accumulates to millimolar levels in IDH-mutant glioma cells and inhibits 2-oxoglutarate-dependent enzymes, including the TET and KDM dioxygenases that catalyze DNA and histone demethylation, respectively. Thus, (R)-2HG cells causes chromatin hypermethylation and epigenetic reprogramming. IDH mutations occur first in the series of genetic alterations that cause lower grade gliomas, suggesting that they play an important role in tumor initiation. However, our understanding of the precise targets of (R)-2HG-induced epigenetic reprogramming that drive neural cell transformation is limited. This limitation is tied to the difficulty in using primary samples from established tumors to retrospectively study tumor initiation. To address this issue, we created a genetic mouse model of astrocytoma and performed time- resolved single-cell multi-omics analyses of the alterations caused by mutant IDH during premalignancy and tumor initiation. We found that mutant IDH repressed neuroblast and interneuron cell lineages and expanded oligodendrocyte precursor cells prior to tumor formation. Moreover, we found that oligodendrocyte precursor cells, but not neuroblasts and interneurons, were permissive to transformation. These data suggest that altered lineage specification of neural progenitor cells may be a key mechanism of glioma promotion by IDH oncogenes. To identify molecular changes that cause this effect, we performed a cross-species transcriptomic analysis of IDH-mutant and IDH-wildtype tissues. We identified mutant IDH-induced silencing of a lineage- specific transcription factor that is required for interneuron differentiation. This silencing event was associated with DNA hypermethylation of an associated CpG island. Based on these findings, we hypothesize that epigenetic reprogramming by (R)-2HG drives glioma initiation by inhibiting neuronal specification of neural progenitor cells, thereby expanding the pool of oligodendrocyte precursor cells that are susceptible to transformation by glioma-associated mutations. We will test this hypothesis through three specific aims. In Specific Aim #1, we will examine the relationship between DNA hypermethylation and silencing of the identified interneuron-specifying transcription factor in human and mouse samples. In Specific Aim #2, we will test whether ectopic expression of this transcription factor reverses mutant IDH-induced lineage reprogramming of neural progenitor cells in vivo. In Specific Aim #3, we will ask if altered neural progenitor cell lineage priming drives gliomagenesis downstream of IDH mutations and whether a gene involved in this process may serve as a biomarker of response to mutant IDH inhibitors, which have shown anti-glioma activity in the clinic. If successful, our work may reveal the molecular and cellular dynamics that drive IDH-mutant glioma initiation and nominate new approaches to monitor and predict response to IDH inhibitor therapies.
