m6A RNA Methylation in Cardiovascular Diseases
Authors: Siyi Wu, Shuchen Zhang, Xiaoguang Wu, Xiang Zhou
Journal: Molecular Therapy (2020)

Cardiovascular diseases (CVDs) remain the leading cause of death and disability worldwide, despite marked improvements in prevention, diagnosis, and early intervention. There is an urgent need to discover more effective therapeutic strategies, which would be facilitated by a more in-depth understanding of CVDs and their underlying molecular mechanisms. Recent advances in knowledge about epigenetic mechanisms, especially RNA methylation, have revealed a close relationship between epigenetic modifications and CVDs and have brought to potential novel targets for diagnosis and treatment. Here, we provide a review of recent studies exploring RNA N6-methyladenosine (m6A) modification, with particular emphasis on its role in CVDs, such as coronary heart disease, hypertension, cardiac hypertrophy, and heart failure. We also introduce the "life cycle" of m6A and its dominant function in several biological processes. Finally, we highlight the prospects of treatment based on interfering with m6A, which could have a transformative effect on clinical medicine. Cardiovascular diseases (CVDs) remain the leading cause of death and disability worldwide, despite marked improvements in prevention, diagnosis, and early intervention. There is an urgent need to discover more effective therapeutic strategies, which would be facilitated by a more in-depth understanding of CVDs and their underlying molecular mechanisms. Recent advances in knowledge about epigenetic mechanisms, especially RNA methylation, have revealed a close relationship between epigenetic modifications and CVDs and have brought to potential novel targets for diagnosis and treatment. Here, we provide a review of recent studies exploring RNA N6-methyladenosine (m6A) modification, with particular emphasis on its role in CVDs, such as coronary heart disease, hypertension, cardiac hypertrophy, and heart failure. We also introduce the "life cycle" of m6A and its dominant function in several biological processes. Finally, we highlight the prospects of treatment based on interfering with m6A, which could have a transformative effect on clinical medicine. Cardiovascular diseases (CVDs), which encompass a wide range of disorders, including coronary heart disease (CHD), hypertension, cardiac hypertrophy, and heart failure (HF), are the main cause of death and disability worldwide. Attempts to identify the molecular mechanisms underlying CVDs, especially those involving RNA methylation, which is increasingly recognized to play an important role in pathological cardiovascular events, have recently accelerated, and there is hope that new therapeutic options may be on the horizon. Research into epigenetic mechanisms has achieved several notable breakthroughs, including dramatic improvements in the understanding of DNA methylation, histone modifications, and the role of non-coding RNA. The past decade has witnessed significant progress in RNA-based therapeutics for CVDs,1Lu D. Thum T. RNA-based diagnostic and therapeutic strategies for cardiovascular disease.Nat. Rev. Cardiol. 2019; 16: 661-674Crossref PubMed Scopus (80) Google Scholar, 2Zhi Y. Xu C. Sui D. Du J. Xu F.J. Li Y. Effective Delivery of Hypertrophic miRNA Inhibitor by Cholesterol-Containing Nanocarriers for Preventing Pressure Overload Induced Cardiac Hypertrophy.Adv. Sci. (Weinh.). 2019; 6: 1900023PubMed Google Scholar, 3Feng Y. Xu W. Zhang W. Wang W. Liu T. Zhou X. LncRNA DCRF regulates cardiomyocyte autophagy by targeting miR-551b-5p in diabetic cardiomyopathy.Theranostics. 2019; 9: 4558-4566Crossref PubMed Scopus (25) Google Scholar which pave the way toward precision medicine. Despite these important advances, factors such as instability, inadequate binding affinity, ease of delivery, immunogenicity, and off-target effects remain barriers to the widespread use of RNA-based therapeutics. Nevertheless, RNA-based therapeutics, such as small interfering RNAs, microRNAs (miRNAs), antisense oligonucleotides, and RNA editing, have already shown great potential to target a majority of the existing undruggable genes and to create new therapeutic paradigms. It seems likely that RNA methylation could be a promising new addition to this field. RNA methylation, which is the most prevalent epigenetic modification of RNA nucleotides, with over 170 different modifications reported so far, occurs primarily as 7-methylguanosine (m7G), 5-methylcytosine (m5C), 5-hydroxymethylcytosine (5-hmC), N1-methyladenosine (m1A), N6-methyladenosine (m6A), N6,2′-O-dimethyladenosine (m6Am), and 2′-O′ methylation (2′-OMe).4Boccaletto P. Machnicka M.A. Purta E. Piatkowski P. Baginski B. Wirecki T.K. de Crécy-Lagard V. Ross R. Limbach P.A. Kotter A. et al.MODOMICS: a database of RNA modification pathways. 2017 update.Nucleic Acids Res. 2018; 46: D303-D307Crossref PubMed Scopus (684) Google Scholar m6A is the most prevalent internal RNA modification (accounting for approximately 97.4%) and is highly conserved and hard coded in eukaryotic species.5Huang H. Weng H. Chen J. The Biogenesis and Precise Control of RNA m6A Methylation.Trends Genet. 2020; 36: 44-52Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar So far, m6A has been identified in messenger RNA (mRNA), ribosomal RNA, transfer RNA (tRNA), small nucleolar RNA, long non-coding RNA, circular RNA, and miRNA.6Xuan J.J. Sun W.J. Lin P.H. Zhou K.R. Liu S. Zheng L.L. Qu L.H. Yang J.H. RMBase v2.0: deciphering the map of RNA modifications from epitranscriptome sequencing data.Nucleic Acids Res. 2018; 46: D327-D334Crossref PubMed Scopus (123) Google Scholar m6A was first identified in the 1970s and has recently become a hot topic in research because of its critical roles in many fundamental biological processes, including control of gene expression7Meyer K.D. Jaffrey S.R. The dynamic epitranscriptome: N6-methyladenosine and gene expression control.Nat. Rev. Mol. Cell Biol. 2014; 15: 313-326Crossref PubMed Scopus (461) Google Scholar and regulation of mRNA stability and homeostasis,8Wang X. Lu Z. Gomez A. Hon G.C. Yue Y. Han D. Fu Y. Parisien M. Dai Q. Jia G. et al.N6-methyladenosine-dependent regulation of messenger RNA stability.Nature. 2014; 505: 117-120Crossref PubMed Scopus (1517) Google Scholar,9Edupuganti R.R. Geiger S. Lindeboom R.G.H. Shi H. Hsu P.J. Lu Z. Wang S.Y. Baltissen M.P.A. Jansen P.W.T.C. Rossa M. et al.N6-methyladenosine (m6A) recruits and repels proteins to regulate mRNA homeostasis.Nat. Struct. Mol. Biol. 2017; 24: 870-878Crossref PubMed Scopus (212) Google Scholar as well as its association with a variety of diseases. Meyer and Jaffrey7Meyer K.D. Jaffrey S.R. The dynamic epitranscriptome: N6-methyladenosine and gene expression control.Nat. Rev. Mol. Cell Biol. 2014; 15: 313-326Crossref PubMed Scopus (461) Google Scholar indicated that m6A was located in a large section of the transcriptome in specific regions of mRNA and might undergo post-transcriptional methylation to regulate its fate and function. However, Slobodin et al.10Slobodin B. Han R. Calderone V. Vrielink J.A.F.O. Loayza-Puch F. Elkon R. Agami R. Transcription Impacts the Efficiency of mRNA Translation via Co-transcriptional N6-adenosine Methylation.Cell. 2017; 169: 326-337.e12Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar demonstrated that m6A modification of mRNAs was co-transcriptional. The two conclusions may be contradict or coexist. There are three main components in the "life cycle" of m6A: adenosine methyltransferases (writers), m6A-binding proteins (readers), and m6A demethylating enzymes (erasers). In addition to m6A, several other forms of methylation affect mRNA metabolism and other biological processes. m1A, with an estimated average transcript level of 20% in humans, associates with translation initiation sites and the first splice site and dynamically responds to changing physiological and stress conditions. A recent study showed that m1A around the start codon correlates with higher protein levels, indicating a functional role in promoting translation of mRNA.11Dominissini D. Nachtergaele S. Moshitch-Moshkovitz S. Peer E. Kol N. Ben-Haim M.S. Dai Q. Di Segni A. Salmon-Divon M. Clark W.C. et al.The dynamic N(1)-methyladenosine methylome in eukaryotic messenger RNA.Nature. 2016; 530: 441-446Crossref PubMed Scopus (428) Google Scholar The functional role of m5C, which is widely distributed throughout coding and non-coding RNA sequences, is only just beginning to emerge, but it appears to regulate cell proliferation, differentiation, and development. m5C also stabilizes tRNA molecules and modifies mRNAs by known methyltransferases NSUN, TRDMT1, and NOP2.12Gkatza N.A. Castro C. Harvey R.F. Heiß M. Popis M.C. Blanco S. Bornelöv S. Sajini A.A. Gleeson J.G. Griffin J.L. et al.Cytosine-5 RNA methylation links protein synthesis to cell metabolism.PLoS Biol. 2019; 17: e3000297Crossref PubMed Scopus (35) Google Scholar,13Squires J.E. Patel H.R. Nousch M. Sibbritt T. Humphreys D.T. Parker B.J. Suter C.M. Preiss T. Widespread occurrence of 5-methylcytosine in human coding and non-coding RNA.Nucleic Acids Res. 2012; 40: 5023-5033Crossref PubMed Scopus (497) Google Scholar A recent study in zebrafish revealed an unexpected mechanism of RNA m5C-regulated maternal mRNA stabilization, involving preferential recognition of m5C-modified mRNAs by Y-box binding protein 1 during maternal-to-zygotic transition.14Yang Y. Wang L. Han X. Yang W.L. Zhang M. Ma H.L. Sun B.F. Li A. Xia J. Chen J. et al.RNA 5-Methylcytosine Facilitates the Maternal-to-Zygotic Transition by Preventing Maternal mRNA Decay.Mol. Cell. 2019; 75: 1188-1202.e11Abstract Full Text Full Text PDF PubMed Scopus (72) Google Scholar Like m5C, 5-hmC plays an important role in switching genes on and off but shows a greater difference in tissue distribution than m5C.15Li W. Liu M. Distribution of 5-hydroxymethylcytosine in different human tissues.J. Nucleic Acids. 2011; 2011: 870726Crossref PubMed Scopus (200) Google Scholar It has recently been suggested that 5-hmC regulates the expression of genes involved in cellular differentiation and pluripotency and might mediate fetal heart and lung growth, although the relationship is not fully understood. m7G is widely acknowledged to act as a cap structure in mRNA, which affects RNA stability by blocking the 5′ termini and acts as a novel epitranscriptomic marker by promoting mRNA translation.16Furuichi Y. LaFiandra A. Shatkin A.J. 5′-Terminal structure and mRNA stability.Nature. 1977; 266: 235-239Crossref PubMed Scopus (339) Google Scholar,17Malbec L. Zhang T. Chen Y.S. Zhang Y. Sun B.F. Shi B.Y. Zhao Y.L. Yang Y. Yang Y.G. Dynamic methylome of internal mRNA N7-methylguanosine and its regulatory role in translation.Cell Res. 2019; 29: 927-941Crossref PubMed Scopus (29) Google Scholar m6Am has been shown to be the preferred cellular substrate for fat mass and obesity-associated protein (FTO) and also to stabilize mRNA, predominantly by reducing decapping and impairing miRNA-mediated mRNA degradation.18Mauer J. Luo X. Blanjoie A. Jiao X. Grozhik A.V. Patil D.P. Linder B. Pickering B.F. Vasseur J.J. Chen Q. et al.Reversible methylation of m6Am in the 5′ cap controls mRNA stability.Nature. 2017; 541: 371-375Crossref PubMed Scopus (436) Google Scholar Although there are undoubtedly many other known methyl modifications, we will focus on the impact of m6A on RNA regulation and will discuss in detail the m6A "life cycle" (writers, readers, and erasers). We will also emphasize the role of m6A in CVDs, since this modification has been shown to be pivotal in cardiovascular homeostasis. The deposition of m6A in mammals is carried out by a multicomponent methyltransferase complex (Figure 1). A key protein of this complex, methyltransferase-like 3 (METTL3), was first identified as an S-adenosyl methionine-binding protein with methyltransferase capacity.19Bokar J.A. Shambaugh M.E. Polayes D. Matera A.G. Rottman F.M. Purification and cDNA cloning of the AdoMet-binding subunit of the human mRNA (N6-adenosine)-methyltransferase.RNA. 1997; 3: 1233-1247PubMed Google Scholar Recent studies have unveiled other components of the complex, including METTL14, METTL16, Wilms tumor 1-associated protein (WTAP), RNA-binding motif protein 15 (RBM15), and KIAA1429. Silencing of METTL3 was recently reported to inhibit apoptosis of cardiomyocytes subjected to hypoxia/reoxygenation.20Song H. Feng X. Zhang H. Luo Y. Huang J. Lin M. Jin J. Ding X. Wu S. Huang H. et al.METTL3 and ALKBH5 oppositely regulate m6A modification of TFEB mRNA, which dictates the fate of hypoxia/reoxygenation-treated cardiomyocytes.Autophagy. 2019; 15: 1419-1437Crossref PubMed Scopus (112) Google Scholar This may be associated with the methylation of TFEB, a key regulator of lysosomal biogenesis and autophagy genes, by METTL3 and subsequent reduction in expression levels. In the methyltransferase complex, METTL3 acts as the catalytic subunit, whereas METTL14 plays only a structural role and activates METTL3 through allostery and recognition of RNA substrates.21Wang P. Doxtader K.A. Nam Y. Structural Basis for Cooperative Function of Mettl3 and Mettl14 Methyltransferases.Mol. Cell. 2016; 63: 306-317Abstract Full Text Full Text PDF PubMed Scopus (355) Google Scholar Biochemical and structural studies have demonstrated that m6A is only generated by this METTL3/14 enzyme complex.22Schöller E. Weichmann F. Treiber T. Ringle S. Treiber N. Flatley A. Feederle R. Bruckmann A. Meister G. Interactions, localization, and phosphorylation of the m6A generating METTL3-METTL14-WTAP complex.RNA. 2018; 24: 499-512Crossref PubMed Scopus (121) Google Scholar Using crosslinking and analysis of cDNA, METTL16 has recently been shown to regulate splicing by targeting pre-mRNAs and various non-coding RNAs.23Warda A.S. Kretschmer J. Hackert P. Lenz C. Urlaub H. Höbartner C. Sloan K.E. Bohnsack M.T. Human METTL16 is a N6-methyladenosine (m6A) methyltransferase that targets pre-mRNAs and various non-coding RNAs.EMBO Rep. 2017; 18: 2004-2014Crossref PubMed Scopus (209) Google Scholar WTAP, a regulatory subunit localizing METTL3 and METTL14 to nuclear speckles and interacting with the METTL3-METTL14 complex, assists the catalytic activity of m6A methyltransferases and regulates gene expression and alternative splicing.24Ping X.L. Sun B.F. Wang L. Xiao W. Yang X. Wang W.J. Adhikari S. Shi Y. Lv Y. Chen Y.S. et al.Mammalian WTAP is a regulatory subunit of the RNA N6-methyladenosine methyltransferase.Cell Res. 2014; 24: 177-189Crossref PubMed Scopus (837) Google Scholar,25Liu J. Yue Y. Han D. Wang X. Fu Y. Zhang L. Jia G. Yu M. Lu Z. Deng X. et al.A METTL3-METTL14 complex mediates mammalian nuclear RNA N6-adenosine methylation.Nat. Chem. Biol. 2014; 10: 93-95Crossref PubMed Scopus (1121) Google Scholar Recent research has shown that m6A formation is mediated by RBM15 and RBM15B, which bind the m6A-methylation complex and recruit it to specific sites in RNA.26Patil D.P. Chen C.K. Pickering B.F. Chow A. Jackson C. Guttman M. Jaffrey S.R. m(6)A RNA methylation promotes XIST-mediated transcriptional repression.Nature. 2016; 537: 369-373Crossref PubMed Scopus (590) Google Scholar In addition, KIAA1429 has similarly been found to interact with METTL3, but the relevance of this interaction needs further investigation.27Schwartz S. Mumbach M.R. Jovanovic M. Wang T. Maciag K. Bushkin G.G. Mertins P. Ter-Ovanesyan D. Habib N. Cacchiarelli D. et al.Perturbation of m6A writers reveals two distinct classes of mRNA methylation at internal and 5′ sites.Cell Rep. 2014; 8: 284-296Abstract Full Text Full Text PDF PubMed Scopus (580) Google Scholar The m6A readers involved in RNA metabolism are generally classified into three major classes, the DF family (YTHDF1, -2, and -3), YTHDC1, and YTHDC2, all of which can specifically recognize m6A modification to control mRNA maturation, translation, and decay (Figure 1). Human YTHDF1 selectively recognizes m6A-modified mRNAs and promotes translational output via interactions with initiation factors and ribosomes, suggesting a dual role of directly facilitating initiation of translation and delivering cellular mRNAs to the translation machinery.28Wang X. Zhao B.S. Roundtree I.A. Lu Z. Han D. Ma H. Weng X. Chen K. Shi H. He C. N(6)-methyladenosine Modulates Messenger RNA Translation Efficiency.Cell. 2015; 161: 1388-1399Abstract Full Text Full Text PDF PubMed Scopus (1144) Google Scholar It has previously been reported that YTHDF2, which is localized in membrane-less cytoplasmic P granules, reduces the stability of m6A-modified mRNA.8Wang X. Lu Z. Gomez A. Hon G.C. Yue Y. Han D. Fu Y. Parisien M. Dai Q. Jia G. et al.N6-methyladenosine-dependent regulation of messenger RNA stability.Nature. 2014; 505: 117-120Crossref PubMed Scopus (1517) Google Scholar Although both YTHDF1 and YTHDF2 are located in the cytoplasm, YTHDF2-mediated decay controls the lifetime of target transcripts, whereas YTHDF1-mediated promotion augments translation efficiency. YTHDF3 plays an important role in the initial stages of translation, but the details remain unclear.29Li A. Chen Y.S. Ping X.L. Yang X. Xiao W. Yang Y. Sun H.Y. Zhu Q. Baidya P. Wang X. et al.Cytoplasmic m6A reader YTHDF3 promotes mRNA translation.Cell Res. 2017; 27: 444-447Crossref PubMed Scopus (306) Google Scholar YTHDC1, a nuclear RNA-binding protein, mediates m6A-regulated mRNA splicing.30Roundtree I.A. He C. Nuclear m(6)A Reader YTHDC1 Regulates mRNA Splicing.Trends Genet. 2016; 32: 320-321Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar Unlike the other YTH proteins, YTHDC2 is abundantly expressed in testes, although its function has not been fully elucidated and varies in different studies.31Wojtas M.N. Pandey R.R. Mendel M. Homolka D. Sachidanandam R. Pillai R.S. Regulation of m6A Transcripts by the 3′→5′ RNA Helicase YTHDC2 Is Essential for a Successful Meiotic Program in the Mammalian Germline.Mol. Cell. 2017; 68: 374-387.e12Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar Recently, FTO, which has efficient reversible demethylation activity targeting the m6A residues in RNA, has been categorized as an eraser (Figure 1) and has aroused much interest among scientific researchers. FTO belongs to the non-heme Fe(II)- and α-ketoglutarate-dependent dioxygenase AlkB family of proteins, which also contains the human AlkB homologs ABH1–8 and the ten-eleven translocator family (TET1–3).32Jia G. Fu Y. Zhao X. Dai Q. Zheng G. Yang Y. Yi C. Lindahl T. Pan T. Yang Y.G. He C. N6-methyladenosine in nuclear RNA is a major substrate of the obesity-associated FTO.Nat. Chem. Biol. 2011; 7: 885-887Crossref PubMed Scopus (1563) Google Scholar FTO was initially shown to correlate with human obesity by a genome-wide association study (GWAS)33Church C. Moir L. McMurray F. Girard C. Banks G.T. Teboul L. Wells S. Brüning J.C. Nolan P.M. Ashcroft F.M. Cox R.D. Overexpression of Fto leads to increased food intake and results in obesity.Nat. Genet. 2010; 42: 1086-1092Crossref PubMed Scopus (454) Google Scholar but has now emerged with a new identity as an m6A demethylase. Obesity-associated single nucleotide polymorphisms (SNPs) within FTO have been reported to be functionally connected with regulation of expression of the neighboring gene, IRX3. Moreover, body-mass-index-associated SNPs in human brain are associated with the expression of IRX3 but not FTO.34Smemo S. Tena J.J. Kim K.H. Gamazon E.R. Sakabe N.J. Gómez-Marín C. Aneas I. Credidio F.L. Sobreira D.R. Wasserman N.F. et al.Obesity-associated variants within FTO form long-range functional connections with IRX3.Nature. 2014; 507: 371-375Crossref PubMed Scopus (723) Google Scholar FTO can bind multiple RNA species, including mRNA, small nuclear RNA (snRNA), and tRNA, and can thus demethylate cap m6Am in mRNA, internal and cap m6Am in snRNAs, and m1A in tRNA. FTO-mediated demethylation in mRNA, which influences the transcript level of target mRNA, has a preference for m6A over cap m6Am.35Wei J. Liu F. Lu Z. Fei Q. Ai Y. He P.C. Shi H. Cui X. Su R. Klungland A. et al.Differential m6A, m6Am, and m1A Demethylation Mediated by FTO in the Cell Nucleus and Cytoplasm.Mol. Cell. 2018; 71: 973-985.e5Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar Additionally, FTO is a potent regulator of nuclear mRNA processing events, including alternative splicing and 3′ end mRNA processing.36Bartosovic M. Molares H.C. Gregorova P. Hrossova D. Kudla G. Vanacova S. N6-methyladenosine demethylase FTO targets pre-mRNAs and regulates alternative splicing and 3′-end processing.Nucleic Acids Res. 2017; 45: 11356-11370Crossref PubMed Scopus (158) Google Scholar As studies have progressed, FTO has been found to be involved in many fundamental physiological processes. Some data indicate that FTO affects mitochondrial content and fat metabolism by modulating m6A levels in hepatocytes.37Kang H. Zhang Z. Yu L. Li Y. Liang M. Zhou L. FTO reduces mitochondria and promotes hepatic fat accumulation through RNA demethylation.J. Cell. Biochem. 2018; 119: 5676-5685Crossref PubMed Scopus (31) Google Scholar A recent case-controlled study also found that decreased protein expression levels of FTO are responsible for the increase in m6A and may further increase the risk of complications of premature ovarian insufficiency.38Ding C. Zou Q. Ding J. Ling M. Wang W. Li H. Huang B. Increased N6-methyladenosine causes infertility is associated with FTO expression.J. Cell. Physiol. 2018; 233: 7055-7066Crossref PubMed Scopus (43) Google Scholar FTO also plays a critical role in the development and prognosis of many types of cancer. It influences cancer stem cell function and promotes the growth, proliferation, and metastasis of cancer cells, thereby providing a promising novel target for diagnosis and treatment.39Chen J. Du B. Novel positioning from obesity to cancer: FTO, an m6A RNA demethylase, regulates tumour progression.J. Cancer Res. Clin. Oncol. 2019; 145: 19-29Crossref PubMed Scopus (40) Google Scholar In addition to FTO, alkB homolog 5 RNA demethylase (ALKBH5) also catalyzes the demethylation of m6A-containing RNA with an activity comparable to that of FTO. ALKBH5, which is localized in the nucleus, plays important roles in mRNA export and in the association of nuclear speckle proteins and RNA metabolism. In mice, ALKBH5 deficiency leads to impaired fertility because of aberrant spermatogenesis and apoptosis in the testes.40Zheng G. Dahl J.A. Niu Y. Fedorcsak P. Huang C.M. Li C.J. Vågbø C.B. Shi Y. Wang W.L. Song S.H. et al.ALKBH5 is a mammalian RNA demethylase that impacts RNA metabolism and mouse fertility.Mol. Cell. 2013; 49: 18-29Abstract Full Text Full Text PDF PubMed Scopus (1295) Google Scholar A total of 1,551 differentially expressed genes were associated with this defect. These genes cover broad functional categories, including spermatogenesis-related mRNAs involved in the p53 functional interaction network, suggesting that ALKBH5-mediated m6A demethylation has fundamental and broad functions in mammalian cells. Since m6A is the most predominant RNA modification, we will pay particular attention to m6A methylation and reveal the dictative role of RNA modification upon mRNA. In-depth research into the role of m6A has been carried out in a broad range of eukaryotic species, including mammals, silkworms, plants, and yeast. m6A can modulate almost every step in mRNA metabolism and dictates distinct fates of mRNAs via differential processing, translation, and procedural decay.41Zhao B.S. Roundtree I.A. He C. Post-transcriptional gene regulation by mRNA modifications.Nat. Rev. Mol. Cell Biol. 2017; 18: 31-42Crossref PubMed Scopus (648) Google Scholar m6A occurs in highly conserved mRNA regions, with a consensus sequence discerned as [G/A/U] [G > A] m6AC [U > A > C].42Dominissini D. Moshitch-Moshkovitz S. Schwartz S. Salmon-Divon M. Ungar L. Osenberg S. Cesarkas K. Jacob-Hirsch J. Amariglio N. Kupiec M. et al.Topology of the human and mouse m6A RNA methylomes revealed by m6A-seq.Nature. 2012; 485: 201-206Crossref PubMed Scopus (1891) Google Scholar m6A is enriched in 3′ untranslated regions (UTRs) and within internal long exons but is relatively sparse in intronic regions, suggesting that methylation could occur either before or at the same time as RNA splicing.43Fu Y. Dominissini D. Rechavi G. He C. Gene expression regulation mediated through reversible m6A RNA methylation.Nat. Rev. Genet. 2014; 15: 293-306Crossref PubMed Scopus (690) Google Scholar The "life cycle" of m6A methylation starts during transcription. The writing and erasing of m6A takes place primarily during the nuclear phase, while reading occurs during both the nuclear and cytoplasmic phases, when m6A binds specific readers. According to a study from the Roundtree group,44Roundtree I.A. Luo G.Z. Zhang Z. Wang X. Zhou T. Cui Y. Sha J. Huang X. Guerrero L. Xie P. et al.YTHDC1 mediates nuclear export of N6-methyladenosine methylated mRNAs.eLife. 2017; 6: e31311Crossref PubMed Scopus (320) Google Scholar m6A is able to enhance nuclear mRNA export though the interaction between YTHDC1 and nuclear export adaptor protein SRSF3. In detail, YTHDC1, the m6A "reader," could facilitate binding of RNA to both SRSF3 and the mRNA export receptor NXF1 and thus play a critical role in targeting m6A-modified mRNAs to the export pathway. Various studies have connected m6A to enhancement of translation, which can occur by three distinct mechanisms. The first involves the reader YTHDF1, which is proposed to bind the translation initiation factor eIF3 and recruit it to the stop codon and 3′ UTRs.28Wang X. Zhao B.S. Roundtree I.A. Lu Z. Han D. Ma H. Weng X. Chen K. Shi H. He C. N(6)-methyladenosine Modulates Messenger RNA Translation Efficiency.Cell. 2015; 161: 1388-1399Abstract Full Text Full Text PDF PubMed Scopus (1144) Google Scholar Another mechanism of m6A-mediated upregulation of translation is reported to involve direct binding of the 5′ UTR m6A to eIF3 in the absence of the cap-binding factor eIF4E.45Meyer K.D. Patil D.P. Zhou J. Zinoviev A. Skabkin M.A. Elemento O. Pestova T.V. Qian S.B. Jaffrey S.R. 5′ UTR m(6)A Promotes Cap-Independent Translation.Cell. 2015; 163: 999-1010Abstract Full Text Full Text PDF PubMed Scopus (749) Google Scholar The third mechanism involves immediate activation of translation by the writer METTL3.46Lin S. Choe J. Du P. Triboulet R. Gregory R.I. The m(6)A Methyltransferase METTL3 Promotes Translation in Human Cancer Cells.Mol. Cell. 2016; 62: 335-345Abstract Full Text Full Text PDF PubMed Scopus (594) Google Scholar In the case of heterogeneous nuclear ribonucleoprotein G, m6A is suggested to be able to modify RNA structures, thus altering RNA-protein interactions and influencing the expression and alternative splicing patterns of target mRNAs.47Liu N. Zhou K.I. Parisien M. Dai Q. Diatchenko L. Pan T. N6-methyladenosine alters RNA structure to regulate binding of a low-complexity protein.Nucleic Acids Res. 2017; 45: 6051-6063Crossref PubMed Scopus (251) Google Scholar A mechanism termed "the m6A-switch" by Liu et al.48Liu N. Dai Q. Zheng G. He C. Parisien M. Pan T. N(6)-methyladenosine-dependent RNA structural switches regulate RNA-protein interactions.Nature. 2015; 518: 560-564Crossref PubMed Scopus (798) Google Scholar suggests that m6A controls the accessibility of single-stranded RBMs and thus affects biological regulation due to RNA-protein interactions. Liu et al.48Liu N. Dai Q. Zheng G. He C. Parisien M. Pan T. N(6)-methyladenosine-dependent RNA structural switches regulate RNA-protein interactions.Nature. 2015; 518: 560-564Crossref PubMed Scopus (798) Google Scholar also showed that m6A-switches are able to regulate both the abundance and alternative splicing of target mRNAs, which means that m6A can logically be considered as a remodeler of RNA structure. There are around 2,798 high-confidence m6A-switches identified by photoactivatable-ribonucleoside-enhanced crosslinking and immunoprecipitation (PAR-CLIP) and anti-m6A immunoprecipitation (MeRIP). In the future, the intricate system surrounding methylation deposition, recognition, and removal may be exploited to target specific mRNAs to regulate biological processes and physiological responses. m6A is present at a frequency of ∼3 modifications per average mRNA molecule, and m6A peaks are found to be enriched near the stop codon in human transcripts. More than 5% m6A on mRNA inhibits translation, and 100% m6A blocks translation. m6A is also more prevalent in alternatively spliced exons and introns.42Dominissini D. Moshitch-Moshkovitz S. Schwartz S. Salmon-Divon M. Ungar L. Osenberg S. Cesarkas K. Jacob-Hirsch J. Amariglio N. Kupiec M. et al.Topology of the human and mouse m6A RNA methylomes revealed by m6A-seq.Nature. 2012; 485: 201-206Crossref PubMed Scopus (1891) Google Scholar,49Meyer K.D. Saletore Y. Zumbo P. Elemento O. Mason C.E. Jaffrey S.R. Comprehensive analysis of mRNA methylation reveals enrichment in 3′ UTRs and near stop codons.Cell. 2012; 149: 1635-1646Abstract Full Text Full Text PDF PubMed Scopus (1678) Google Scholar Comparative analysis revealed that m6A has gene- and cell-type-specific features. Increased m6A was found to promote the reprogramming of mouse embryonic fibroblasts into pluripotent stem cells, indicating that it can determine stem cell fate by regulating pluripotency transition toward differentiation.50Chen T. Hao Y.J. Zhang Y. Li M.M. Wang M. Han W. Wu Y. Lv Y. Hao J. Wang L. et al.m(6)A RNA methylation is regulated by microRNAs and promotes reprogramming to pluripotency.Cell Stem Cell. 2015; 16: 289-301Abstract Full Text Full Text PDF PubMed Scopus (308) Google Scholar Another study confirmed that m6A erasure on target genes impaired exit of embryonic stem cells from self-renewal toward differentiation.51Batista P.J. Molinie B. Wang J. Qu K. Zhang J. Li L. Bouley D.M. Lujan E. Haddad B. Daneshvar K