# The Emerging Roles of RNA Modifications in Glioblastoma
## 3 RNA Modifications in GBM
### RNA m\({}^{6}\)A Modification in GBM
#### 3.1.1 RNA m\({}^{6}\)A Writers in GBM  
The expression of METTL3, the most important m\({}^{6}\)A writer, is significantly elevated in GSCs, while it is attenuated during GSCs differentiation [56]. Elevated expression of METTL3 are associated with the clinical aggressiveness of malignant gliomas [57]. An integrated analysis of m\({}^{6}\)A-RNA immunoprecipitation (RIP) and total RNA-Sequencing (RNA-seq) of METTL3-silenced GSCs revealed that METTL3 is the most important writer responsible for m\({}^{6}\)A modification in GSCs [45]. Moreover, METTL3 silencing also reverses RasV12-mediated malignant transformation of mouse immortalized astrocytes and suppresses GBM tumor growth in vitro and in vivo [56; 57]. Furthermore, METTL3 silencing in GSCs enhances their sensitivity to \(\gamma\)-irradiation and reduces DNA repair [56]. Mechanically, METTL3 seems to be responsible for the m\({}^{6}\)A modification in some GSCs-specifically-expressed genes [45]. Particularly, METTL3 mediates m\({}^{6}\)A modification of mRNA of SRY-box transcription factor 2 (SOX2), a pluripotent gene, at three METTL3/m\({}^{6}\)A sites present in its 3\({}^{\prime}\) untranslated region (UTR), leading to the recruitment of human antigen R (HuR) to SOX2 mRNA to enhance its stability, thereby resulting in GSC maintenance and dedifferentiation [56]. In addition to directly affecting the pluripotent genes, an m\({}^{6}\)A-specific methylated RNA immunoprecipitation with NGS (MeRIP-Seq or m\({}^{6}\)A-Seq) analysis also shows that m\({}^{6}\)A modification peaks seem to be enriched at transcripts responsible for metabolic pathways in GSCs, compared with neural progenitor cells [57]. METTL3 also causes alterations such as decrease of adenosine-to-inosine (A-to-I) and cytidine to uridine (C-to-U) RNA editing events in GSCs by downregulating adenosine deaminase RNA specific 1 (ADAR1) and apolipoprotein B mRNA editing enzyme catalytic subunit 3A (APOBEC3A), which are RNA editing enzymes [45]. Furthermore, gene ontology analysis reveals that the direct targets of METTL3 seems to be enriched in some major oncogenic pathways including NOTCH signaling, vascular endothelial growth factor (VEGF) signaling, angiogenesis, glycolysis and Hedgehog signaling pathways, whereas the indirect targets are enriched in the RAS, mitogen-activated protein kinase (MAPK), G protein-coupled receptors (GPCRs), cadherin signaling pathways and cell cycle [45]. Additionally, in GSCs, METTL3-dependent m\({}^{6}\)A modification also affects the levels of serine- and arginine-rich splicing factors (SRSFs) through upregulation BCL-X (Bcl-2-like protein 1, BCL2L1) or nuclear receptor corepressor 2 (NCOR2) isoforms, which are important to prevent YTHDC1-dependent nonsense-mediated mRNA decay (NMD) [57]. lncRNAs with METTL3-dependent m\({}^{6}\)A marks is also highly expressed in GSCs, compared with that of protein-coding genes [45]. m\({}^{6}\)A marks within 3'UTRs seem to hinder the miRNA binding in GSCs [45]. Nevertheless, an opposite study shows that METTL3 and METTL14 may be a tumor suppressor in GBM [59]. Their results show that deficiency of METTL3 or METTL14 significantly enhances the growth, self-renewal, and tumorigenesis of GBM stem cells. Subsequently, m\({}^{6}\)A sequencing and molecular experiments implies that METTL3 or METTL14 knockdown induced alterations in mRNA m\({}^{6}\)A enrichment and the mRNA expression of critical factors, such as a disintegration and metallopeptidase domain 19 (ADAM19), that participate in modulation of GBM stem cells. In contrast, overexpression of METTL3 renders GBM stem cells growth and self-renewal. Another study in established GBM cell line U251 cells also shows the similar results: METTL3 overexpression reduces migration and proliferation ability and induces cellular apoptosis possible by downregulating heat shock protein 70 (HSP70) expression [60]. The controversial conclusion may be due to the samples they used. These results mean that function of m\({}^{6}\)A modifications regulated by METTL3 in GBM should indeed be further confirmed.