GSE123892
Stranded RNA-seq were performed on total RNA following ribosomal RNAs depletion (Ribo-zero removal kit, illumina) for 3 brain , 8 IDHwt and 5 IDHmut glioma samples.
Aberrant DNA methylation is a well-defined feature of cancer cells that is commonly associated to transcriptional alteration. Nonetheless, a primary role of this defect in genome-wide cancer-associated gene misregulation is still being questioned.  Here, we used adult glioma, a widespread type of brain tumor, to evaluate the relative contribution of DNA methylation-dependent and -independent mechanisms on transcriptional alteration at CpG-island/promoter-associated genes in cancer cells.   By extensive molecular analyses conducted in both IDH wild-type (IDHwt) and IDH mutated (IDHmut) samples, we show that DNA hypermethylation affects only a minority of genes that show a loss or a gain of expression. In IDHwt samples, for instance, more than 75% of aberrantly repressed genes do not display DNA methylation defect at their CpG-island promoter. Rather, an alteration in the H3K27me3 signature is the predominant molecular defect at misregulated genes. We also observed that a bivalent chromatin signature in stem cell might not only predispose genes to hypermethylation, as widely documented, but more generally to all type of transcriptional alterations. In addition, we evidenced that transcriptional strength in healthy brain influences the choice between DNA methylation- or H3K27me3- associated silencing pathway at repressed genes in glioma. Combined our study supports a model whereby the altered developmental control of H3K27me3 dynamics, and more specifically defects in the interplay between polycomb protein complexes and brain-specific transcriptional machinery, is the main cause of transcriptional alteration in glioma cells.  Together our study provides a comprehensive and revisited picture of epigenetic gene deregulations in cancer. Our observations are also of importance for the design of drugs that aims to target cancer epigenetic defects. Overall design: Total RNA following ribosomal RNAs depletion (Ribo-zero removal kit, illumina) for 3 brain , 8 IDHwt and 5 IDHmut glioma samples.