IDH1R132H is intrinsically tumor-suppressive but functionally attenuated by the glutamate-rich cerebral environment.

Recurrent heterozygous mutation of isocitrate dehydrogenase 1 gene (IDH1), predominantly resulting in histidine substitution at arginine 132, was first identified in glioma. The biological significance of IDH1R132H, however, has been controversial, and its prevalent association with glioma remains enigmatic. Although recent studies indicate that IDH1R132H is nonessential to tumor growth or even anti-tumor growth, whether IDH1R132H initiates gliomagenesis remains obscure. In this study, we report that IDH1R132H is intrinsically tumor-suppressive but the activity can be attenuated by glutamate-the cerebral neurotransmitter. We observed that IDH1R132H was highly suppressive of subcutaneous tumor growth driven by platelet-derived growth factor B (PDGFB), but IDH1R132H tumor growth and glioma penetrance were virtually indistinguishable from those of IDH1-wildtype tumors in orthotopic models. In vitro, addition of glutamate compromised IDH1R132H inhibition of neurosphere genesis, indicating glutamate promotion of oncogenic dominance. Furthermore, we observed that IDH1R132H expression was markedly decreased in tumors but became more permissible upon the deletion of tumor-suppressor gene Cdkn2a. To provide direct evidence for the opposing effect of IDH1R132H on PDGFB-driven glioma development, we explored tandem expression of the two molecules from a single transcript to preclude selection against IDH1R132H expression. Our results demonstrate that when juxtaposed with oncogenic PDGFB, IDH1R132H overrides the oncogenic activity and obliterates neurosphere genesis and gliomagenesis even in the glutamate-rich microenvironment. We propose therefore that IDH1R132H is intrinsically suppressive of glioma initiation and growth but such tumor-suppressive activity is compromised by the glutamate-rich cerebral cortex, thereby offering a unifying hypothesis for the perplexing role of IDH1R132H in glioma initiation and growth.

Functional requirement of a wild-type allele for mutant IDH1 to suppress anchorage-independent growth through redox homeostasis.

Mutations of isocitrate dehydrogenase 1 (IDH1) gene are most common in glioma, arguably preceding all known genetic alterations during tumor development. IDH1 mutations nearly invariably target the enzymatic active site Arg132, giving rise to the predominant IDH1R132H. Cells harboring IDH1 R132H -heterozygous mutation produce 2-hydroxyglutarate (2-HG), which results in histone and DNA hypermethylation. Although exogenous IDH1 R132H transduction has been shown to promote anchorage-independent growth, the biological role of IDH1R132H in glioma remains debatable. In this study, we demonstrate that heterozygous IDH1 R132H suppresses but hemizygous IDH1 R132H promotes anchorage-independent growth. Whereas genetic deletion of the wild-type allele in IDH1 R132H -heterozygous cells resulted in a pronounced increase in neurosphere genesis, restoration of IDH1 expression in IDH1 R132H -hemizygous cells led to the contrary. Conversely, anchorage-independent growth was antagonistic to the mutant IDH1 function by inhibiting gene expression and 2-HG production. Furthermore, we identified that in contrast to IDH1 R132H -hemizygous neurosphere, IDH1 R132H -heterozygous cells maintained a low level of reducing power to suppress neurosphere genesis, which could be bypassed, however, by the addition of reducing agent. Taken together, these results underscore the functional importance of IDH1 mutation heterozygosity in glioma biology and indicate functional loss of mutant IDH1 as an escape mechanism underlying glioma progression and the pathway of redox homeostasis as potential therapeutic targets.