Human low grade gliomas have long been viewed as an exceptionally heterogenous group of tumors of the central nervous system. This view has been challenged by the recent discovery that the majority of these tumors harbor an identical missense mutation in the catalytic pocket of the metabolic enzyme isocitrate dehydrogenase (IDH). The mutant protein loses its normal enzymatic activity and gains the new ability to produce the oncometabolite 2-hydroxyglutarate (2HG). 2HG is structurally similar to alpha-ketoglutarate (AKG), accumulates at high levels (> 100-fold) in tumor tissue, and competitively inhibits the activity of AKG- dependent enzymes. This family of enzymes includes histone lysine demethylases, 5-methylcytosine hydroxylases likely involved in DNA demethylation, the HIF prolyl hydroxylase, and other enzymes with diverse functions. In high grade gliomas that have evolved from lower grade gliomas, IDH1 mutation is associated with a CpG island methylator phenotype (CIMP) and increased RNA levels of genes expressed during normal neuronal differentiation (the proneural gene expression subgroup). While these observations point toward a unique molecular pathogenesis of IDH1-mutant glioma, no experimental evidence thus far has incriminated mutant IDH1 as the cause of CIMP or documented its role in glioma initiation or maintenance. Using experimental glioma models derived by my group, we recently uncovered that expression of mutant IDH1 is sufficient to trigger genome-wide DNA hypermethylation (Turcan et al., Nature, in press), induces the expression of the neuronal differentiation marker Tuj-1 (Lu et al., Nature, in press), and is required for the in- vivo growthof IDH1-mutant human glioma xenografts. These new findings provide the first direct evidence for a role of mutant IDH1 in gliomagenesis and provide an experimental angle for further mechanistic studies.
Aim 1 will dissect the precise temporal sequence of DNA and histone lysine methylation during the development of G-CIMP, as well as their reversibility in response to pharmacologic inhibition of the mutant enzyme.
Aim 2 will determine the effects of mutant IDH1 on differentiation potential and self-renewal in distinct cellular compartments within the neurogenic-gliogenic axis.
Aim 3 will identify the downstream target of 2HG-inhibition that promotes tumor maintenance. Results of our studies will shed light on mechanisms through which an oncometabolite can regulate the molecular pathogenesis of cancer and refine our view of mutant IDH1 as a potential therapeutic target in human glioma.
Mutations in the metabolic enzyme isocitrate dehydrogenase (IDH) were recently discovered in the majority of of human low grade gliomas. IDH1 mutations occur early during the growth of these tumors and may represent the molecular 'driver' of this disease. This project aims to understand how the mutant IDH1 enzyme contributes to the biology of human glioma and leverages insights and experimental models developed by my group.
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