Abstract
Heterozygous mutations in the isocitrate dehydrogenase 1 (IDH1) gene are most common in glioma, resulting in predominantly arginine to histidine substitution at codon 132. Because IDH1R132H requires a wild-type allele to produce (D)-2-hydroxyglutarate for epigenetic reprogramming, glioma cells with loss of the remaining wild-type allele (hence IDH1R132H-hemizygous) exhibit an IDH1-wild-type-like glioma phenotype and aggressive tumor growth. Although previous studies reported that transgenic IDH1R132H induced the expression of nestin—a glioma stem-cell marker, the underlying mechanism remains unclear. Furthermore, this finding seems at odds with the better outcome of IDH-mutant glioma owing to a negative association of nestin with overall survival. To gain a comprehensive understanding of glioma stem-cell marker gene regulation in IDH-mutant glioma, we compared gene expression between IDH1R132H-heterozygous and IDH1R132H-hemizygous glioma cells under adherent and spheroid growth conditions and found that glioma stem-cell marker genes, including CD44, NES, and PROM1, are generally downregulated in the spheroid growth of IDH1R132H-heterozygous cells. This result was validated in patient samples of IDH-mutant glioma compared with those of IDH-wildtype glioma, even though modest NES upregulation was observed in the adherent growth of IDH1R132H-hemizygous glioma cells. In contrast, CD24 is specifically upregulated in the spheroid growth of IDH1R132H-heterozygous cells and patient samples of IDH-mutant glioma and is apparently associated with better survival. Mechanistically, CD24 and NES expression responds differentially to alteration of (D)-2-hydroxyglutarate levels. CD24 upregulation is associated with histone and DNA demethylation as opposed to hypermethylation in those downregulated glioma stem-cell marker genes. Therefore, the better outcome of IDH-mutant glioma is orchestrated exquisitely through epigenetic reprogramming that directs bidirectional expression of glioma stem-cell marker genes.
LGR5, a novel functional glioma stem cell marker, promotes EMT by activating the Wnt/β-catenin pathway and predicts poor survival of glioma patients