Insulator dysfunction and oncogene activation in IDH mutant gliomas

Abstract

Gain-of-function IDH mutations are initiating events that define major clinical and prognostic classes of gliomas ^{1, 2} . Mutant IDH protein produces a novel onco-metabolite, 2-hydroxyglutarate (2-HG), that interferes with iron-dependent hydroxylases, including the TET family of 5′-methylcytosine hydroxylases ^{3– 7} . TET enzymes catalyze a key step in the removal of DNA methylation ^{8, 9} . IDH mutant gliomas thus manifest a CpG island methylator phenotype (G-CIMP) ^{10, 11} , though the functional significance of this altered epigenetic state remains unclear. Here we show that IDH mutant gliomas exhibit hyper-methylation at CTCF binding sites, compromising binding of this methylation-sensitive insulator protein. Reduced CTCF binding is associated with loss of insulation between topological domains and aberrant gene activation. We specifically demonstrate that loss of CTCF at a domain boundary permits a constitutive enhancer to aberrantly interact with the receptor tyrosine kinase gene PDGFRA, a prominent glioma oncogene. Treatment of IDH mutant gliomaspheres with demethylating agent partially restores insulator function and down-regulates PDGFRA. Conversely, CRISPR-mediated disruption of the CTCF motif in IDH wildtype gliomaspheres up-regulates PDGFRA and increases proliferation. Our study suggests that IDH mutations promote gliomagenesis by disrupting chromosomal topology and allowing aberrant regulatory interactions that induce oncogene expression.