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 IDH1(R132H). 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 IDH1(R132H) 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.