Addition Of m6A Research Articles

Nuclear retention coupled with sequential polyadenylation dictates post-transcriptional m6A modification in the nucleus

N6-methyladenosine (m6A) modification is deemed to be co-transcriptionally installed on pre-mRNAs, thereby influencing various downstream RNA metabolism events. However, the causal relationship between m6A modification and RNA processing is often unclear, resulting in premature or even misleading generalizations on the function of m6A modification. Here, we develop 4sU-coupled m6A-level and isoform-characterization sequencing (4sU-m6A-LAIC-seq) and 4sU-GLORI to quantify the m6A levels for both newly synthesized and steady-state RNAs at transcript and single-base-resolution levels, respectively, which enable dissecting the relationship between m6A modification and alternative RNA polyadenylation. Unexpectedly, our results show that many m6A addition events occur post-transcriptionally, especially on transcripts with high m6A levels. Importantly, we find higher m6A levels on shorter 3' UTR isoforms, which likely result from sequential polyadenylation of longer 3' UTR isoforms with prolonged nuclear dwelling time. Therefore, m6A modification can also take place post-transcriptionally to intimately couple with other key RNA metabolism processes to establish and dynamically regulate epi-transcriptomics in mammalian cells.

Increased PTCHD4 expression via m6A modification of PTCHD4 mRNA promotes senescent cell survival.

RNA modifications, including N6-methyladenosine (m6A), critically modulate protein expression programs in a range of cellular processes. Although the transcriptomes of cells undergoing senescence are strongly regulated, the landscape and impact of m6A modifications during senescence are poorly understood. Here, we report a robust m6A modification of PTCHD4 mRNA, encoding Patched Domain-Containing Protein 4, in senescent cells. The METTL3/METTL14 complex was found to incorporate the m6A modification on PTCHD4 mRNA; addition of m6A rendered PTCHD4 mRNA more stable and increased PTCHD4 production. MeRIP RT-qPCR and eCLIP analyses were used to map this m6A modification to the last exon of PTCHD4 mRNA. Further investigation identified IGF2BP1, but not other m6A readers, as responsible for the stabilization and increased abundance of m6A-modified PTCHD4 mRNA. Silencing PTCHD4, a transmembrane protein, enhanced growth arrest and DNA damage in pre-senescent cells and sensitized them to senolysis and apoptosis. Our results indicate that m6A modification of PTCHD4 mRNA increases the production of PTCHD4, a protein associated with senescent cell survival, supporting the notion that regulating m6A modification on specific mRNAs could be exploited to eliminate senescent cells for therapeutic benefit.

WTAP Targets the METTL3 m6A-Methyltransferase Complex to Cytoplasmic Hepatitis C Virus RNA to Regulate Infection.

Modification of the hepatitis C virus (HCV) positive-strand RNA genome by N6-methyladenosine (m6A) regulates the viral life cycle. This life cycle takes place solely in the cytoplasm, while m6A addition on cellular mRNA takes place in the nucleus. Thus, the mechanisms by which m6A is deposited on the viral RNA have been unclear. In this work, we find that m6A modification of HCV RNA by the m6A-methyltransferase proteins methyltransferase-like 3 and 14 (METTL3 and METTL14) is regulated by Wilms' tumor 1-associating protein (WTAP). WTAP, a predominantly nuclear protein, is an essential member of the cellular mRNA m6A-methyltransferase complex and known to target METTL3 to mRNA. We found that HCV infection induces localization of WTAP to the cytoplasm. Importantly, we found that WTAP is required for both METTL3 interaction with HCV RNA and m6A modification across the viral RNA genome. Further, we found that WTAP, like METTL3 and METTL14, negatively regulates the production of infectious HCV virions, a process that we have previously shown is regulated by m6A. Excitingly, WTAP regulation of both HCV RNA m6A modification and virion production was independent of its ability to localize to the nucleus. Together, these results reveal that WTAP is critical for HCV RNA m6A modification by METTL3 and METTL14 in the cytoplasm. IMPORTANCE Positive-strand RNA viruses such as HCV represent a significant global health burden. Previous work has described that HCV RNA contains the RNA modification m6A and how this modification regulates viral infection. Yet, how this modification is targeted to HCV RNA has remained unclear due to the incompatibility of the nuclear cellular processes that drive m6A modification with the cytoplasmic HCV life cycle. In this study, we present evidence for how m6A modification is targeted to HCV RNA in the cytoplasm by a mechanism in which WTAP recruits the m6A-methyltransferase METTL3 to HCV RNA. This targeting strategy for m6A modification of cytoplasmic RNA viruses is likely relevant for other m6A-modified positive-strand RNA viruses with cytoplasmic life cycles such as enterovirus 71 and SARS-CoV-2 and provides an exciting new target for potential antiviral therapies.

Epitranscriptomic addition of m6A regulates HIV-1 RNA stability and alternative splicing.

Previous work has demonstrated that the epitranscriptomic addition of m6A to viral transcripts can promote the replication and pathogenicity of a wide range of DNA and RNA viruses, including HIV-1, yet the underlying mechanisms responsible for this effect have remained unclear. It is known that m6A function is largely mediated by cellular m6A binding proteins or readers, yet how these regulate viral gene expression in general, and HIV-1 gene expression in particular, has been controversial. Here, we confirm that m6A addition indeed regulates HIV-1 RNA expression and demonstrate that this effect is largely mediated by the nuclear m6A reader YTHDC1 and the cytoplasmic m6A reader YTHDF2. Both YTHDC1 and YTHDF2 bind to multiple distinct and overlapping sites on the HIV-1 RNA genome, with YTHDC1 recruitment serving to regulate the alternative splicing of HIV-1 RNAs. Unexpectedly, while YTHDF2 binding to m6A residues present on cellular mRNAs resulted in their destabilization as previously reported, YTHDF2 binding to m6A sites on HIV-1 transcripts resulted in a marked increase in the stability of these viral RNAs. Thus, YTHDF2 binding can exert diametrically opposite effects on RNA stability, depending on RNA sequence context.

N6-methyladenosine modification of HIV-1 RNA suppresses type-I interferon induction in differentiated monocytic cells and primary macrophages.

N6-methyladenosine (m6A) is a prevalent RNA modification that plays a key role in regulating eukaryotic cellular mRNA functions. RNA m6A modification is regulated by two groups of cellular proteins, writers and erasers that add or remove m6A, respectively. HIV-1 RNA contains m6A modifications that modulate viral infection and gene expression in CD4+ T cells. However, it remains unclear whether m6A modifications of HIV-1 RNA modulate innate immune responses in myeloid cells that are important for antiviral immunity. Here we show that m6A modification of HIV-1 RNA suppresses the expression of antiviral cytokine type-I interferon (IFN-I) in differentiated human monocytic cells and primary monocyte-derived macrophages. Transfection of differentiated monocytic U937 cells with HIV-1 RNA fragments containing a single m6A-modification significantly reduced IFN-I mRNA expression relative to their unmodified RNA counterparts. We generated HIV-1 with altered m6A levels of RNA by manipulating the expression of the m6A erasers (FTO and ALKBH5) or pharmacological inhibition of m6A addition in virus-producing cells, or by treating HIV-1 RNA with recombinant FTO in vitro. HIV-1 RNA transfection or viral infection of differentiated U937 cells and primary macrophages demonstrated that HIV-1 RNA with decreased m6A levels enhanced IFN-I expression, whereas HIV-1 RNA with increased m6A modifications had opposite effects. Our mechanistic studies indicated that m6A of HIV-1 RNA escaped retinoic acid-induced gene I (RIG-I)-mediated RNA sensing and activation of the transcription factors IRF3 and IRF7 that drive IFN-I gene expression. Together, these findings suggest that m6A modifications of HIV-1 RNA evade innate immune sensing in myeloid cells.

Targeted RNA m6A Editing Using Engineered CRISPR-Cas9 Conjugates.

N6-methyladenosine (m6A) is a major epitranscriptomic mark exerting crucial diverse roles in RNA metabolisms, including RNA stability, mRNA translation, and RNA structural rearrangement. m6A modifications at different RNA regions may have distinct molecular effects. Here, we describe a CRISPR-Cas9-based approach that enables targeted m6A addition or removal on endogenous RNA molecules without altering the nucleotide sequence. By fusing a catalytically inactive Cas9 with engineered m6A modification enzymes, the programmable m6A editors are capable of achieving RNA methylation and demethylation at desired sites, facilitating dissection of regional effects of m6A and diversifying the toolkits for RNA manipulation.

The m6A-Related mRNA Signature Predicts the Prognosis of Pancreatic Cancer Patients

N6-methyladenosine (m6A) has an important epitranscriptomic modification that controls cancer self-renewal and cell fate. The addition of m6A to mRNA is a reversible modification. The deposition of m6A is encoded by a methyltransferase complex involving three homologous factors, jargonized as “writers,” “erasers,” and “readers.” However, their roles in pancreatic adenocarcinoma (PAAD) are underexploited. With the use of The Cancer Genome Atlas (TCGA) and International Cancer Genome Consortium (ICGC) databases, we provided an mRNA signature that may improve the prognostic prediction of PAAD patients based on the genetic status of m6A regulators. PAAD patients with genetic alteration of m6A regulators had worse disease-free and overall survival. After comparing PAAD groups with/without genetic alteration of m6A regulators, we identified 196 differentially expressed genes (DEGs). Then, we generated a 16-mRNA signature score system through least absolute shrinkage and selection operator (LASSO) Cox regression analysis. Multivariate cox regression analysis demonstrated that a high-risk score significantly correlates with poor prognosis. Moreover, time-dependent receiver operating characteristic (ROC) curves revealed it was effective in predicting the overall survival in both training and validation sets. PAH, ZPLD1, PPFIA3, and TNNT1 from our signature also exhibited an independent prognostic value. Collectively, these findings can improve the understanding of m6A modifications in PAAD and potentially guide therapies in PAAD patients.

N6-methyladenosine modification enables viral RNA to escape recognition by RNA sensor RIG-I.

Internal N6-methyladenosine (m6A) modification is one of the most common and abundant modifications of RNA. However, the biological role(s) of viral RNA m6A remains elusive. Using human metapneumovirus (hMPV) as a model, we demonstrate that m6A serves as a molecular marker for innate immune discrimination of self from nonself RNAs. We show that hMPV RNAs are m6A methylated and that viral m6A methylation promotes hMPV replication and gene expression. Inactivating m6A addition sites with synonymous mutations or demethylase resulted in m6A deficient recombinant hMPVs and virion RNAs that induced significantly higher expression of type I interferon (IFN) which was dependent on the cytoplasmic RNA sensor RIG-I, not MDA5. Mechanistically, m6A-deficient virion RNA induces higher expression of RIG-I, binds more efficiently to RIG-I, and facilitates the conformational change of RIG-I, leading to enhanced IFN expression. Furthermore, m6A-deficient rhMPVs triggered higher IFN in vivo and were significantly attenuated in cotton rats yet retained high immunogenicity. Collectively, our results highlight that (i) virus acquires m6A in their RNAs as a means of mimicking cellular RNA to avoid detection by innate immunity; and (ii) viral RNA m6A can serve as a target to attenuate hMPV for vaccine purposes.

Altered m6A Modification of Specific Cellular Transcripts Affects Flaviviridae Infection.

The RNA modification N6-methyladenosine (m6A) modulates mRNA fate and thus affects many biological processes. We analyzed m6A across the transcriptome following infection by dengue virus (DENV), Zika virus (ZIKV), West Nile virus (WNV), and hepatitis C virus (HCV). We found that infection by these viruses in the Flaviviridae family alters m6A modification of specific cellular transcripts, including RIOK3 and CIRBP. During viral infection, the addition of m6A to RIOK3 promotes its translation, while loss of m6A in CIRBP promotes alternative splicing. Importantly, viral activation of innate immune sensing or the endoplasmic reticulum (ER) stress response contributes to the changes in m6A in RIOK3 or CIRBP, respectively. Further, several transcripts with infection-altered m6A profiles, including RIOK3 and CIRBP, encode proteins that influence DENV, ZIKV, and HCV infection. Overall, this work reveals that cellular signaling pathways activated during viral infection lead to alterations in m6A modification of host mRNAs to regulate infection.

The m6A Dynamics of Profilin in Neurogenesis.

Our understanding of the biological role of N 6-methyladenosine (m6A), a ubiquitous non-editing RNA modification, has increased greatly since 2011. More recently, work from several labs revealed that m6A methylation regulates several aspects of mRNA metabolism. The “writer” protein METTL3, known as MT-A70 in humans, DmIme4 in flies, and MTA in plants, has the catalytic site of the METTL3/14/16 subunit of the methyltransferase complex that includes many other proteins. METTL3 is evolutionarily conserved and essential for development in multicellular organisms. However, until recently, no study has been able to provide a mechanism that explains the essentiality of METTL3. The addition of m6A to gene transcripts has been compared with the epigenetic code of histone modifications because of its effects on gene expression and its reversibility, giving birth to the field of epitranscriptomics, the study of the biological role of this and similar RNA modifications. Here, we focus on METTL3 and its likely conserved role in profilin regulation in neurogenesis. However, this and many other subunits of the methyltransferase complex are starting to be identified in several developmental processes and diseases. A recent plethora of studies about the biological role of METTL3 and other components of the methyltransferase complex that erase (FTO) or recognize (YTH proteins) this modification on transcripts revealed that this RNA modification plays a variety of roles in many biological processes like neurogenesis. Our work in Drosophila shows that the ancient and evolutionarily conserved gene profilin (chic in Drosophila) is a target of the m6A writer. Here, we discuss the implications of our study in Drosophila and how it unveils a conserved mechanism in support of the essential function of METTL3 in metazoan development. Profilin (chic) is an essential gene of ancient evolutionary origins, present in sponges (Porifera), the oldest still extant metazoan phylum of the common metazoan ancestor Urmetazoa. We propose that the relationship between profilin and METTL3 is conserved in metazoans and it provides insights into possible regulatory roles of m6A modification of profilin transcripts in processes such as neurogenesis.

Abstract SY43-02: RNA modifications and cell identity: The importance of forgetting the past to embrace the future

Abstract Impaired gene regulation lies at the heart of many disorders, including cancer. Gene expression is controlled through multiple mechanisms that are coordinated to ensure the proper and timely expression of each gene. From transcription to translation, every step of mRNA processing is controlled by interaction between the mRNA transcript and regulatory factors, such as RNA binding proteins or small noncoding RNAs. We now understand that these regulatory interactions can be modulated by RNA modifications. Presence of a modified nucleotide can create, or abrogate, sites for regulatory interactions by at least two distinct mechanisms: by inducing changes in RNA structure or through direct recognition by the RNA binding protein of the modified nucleotide. The RNA modification N6-methyladenosine (m6A) is present in several classes of RNAs, including mRNAs and lncRNAs, where it is the most abundant internal RNA modification. In mRNAs and lncRNAs, the m6A modification is present in a well-defined RNA motif, RRACH (R = A or G, H = A, C or U), and is enriched at specific transcript landmarks such as the last transcribed exon, near the STOP codon. The METTL3/METTL14 heterodimer is at the core of the large protein complex that catalyzes addition of m6A to mRNAs. To date, two enzymes capable of removing m6A have been identified: fat mass and obesity-associated protein (FTO) and alkB homolog 5 (ALKBH5). Our studies on the role of m6A in ESCs revealed that thousands of messenger and long noncoding RNAs are m6A-modified, including transcripts encoding core pluripotency transcription factors such as Nanog, Sox2, and Myc. Our observations suggest that m6A marks unstable transcripts, including transcripts that need to be turned over upon differentiation. Loss of Mettl3 did not affect self-renewal, but resulted in impaired ESC exit from self-renewal towards differentiation to several lineages. Mettl3 KO cells did not differentiate into cardiomyocites or neurons in vitro, and subcutaneous injection of Mettl3 KO cells formed tumors consistent in morphology with teratomas that tended to be larger than tumors derived from wild-type cells. Furthermore, teratomas derived from Mettl3 KO cells were predominantly composed of poorly differentiated cells with very high mitotic indices and numerous apoptotic bodies, whereas wild-type cells differentiated predominantly into neuroectoderm. The role of m6A in facilitating transition between cell states is not limited to embryonic stem cells. The m6A interacting protein YTHDC2 is required for a clean transition from mitosis to meiosis in male and female germ cells. Ythdc2-/- male germ cells attempt to enter meiotic prophase but appear to have a mixed identity, maintaining expression of mitotic Cyclin A2 and failing to properly express many meiotic markers. Instead of executing meiotic prophase, the cells attempt an abnormal mitotic-like division, then quickly die. YTHDC2 is required during the early stages of meiotic entry in mice to both downregulate the previous mitotic program and facilitate proper expression of meiotic and differentiation genes. We propose that m6A makes the transition between cell states possible by facilitating a reset mechanism between stages, as occurs in ESCs and during gametogenesis. In contrast to epigenetic mechanisms that provide cellular memory of gene expression states, m6A enforces the transience of genetic information, helping cells to forget the past and thereby embrace the future. Citation Format: Pedro J. Batista. RNA modifications and cell identity: The importance of forgetting the past to embrace the future [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr SY43-02.

Addition of m6A to SV40 late mRNAs enhances viral structural gene expression and replication.

Polyomaviruses are a family of small DNA tumor viruses that includes several pathogenic human members, including Merkel cell polyomavirus, BK virus and JC virus. As is characteristic of DNA tumor viruses, gene expression in polyomaviruses is temporally regulated into an early phase, consisting of the viral regulatory proteins, and a late phase, consisting of the viral structural proteins. Previously, the late transcripts expressed by the prototypic polyomavirus simian virus 40 (SV40) were reported to contain several adenosines bearing methyl groups at the N6 position (m6A), although the precise location of these m6A residues, and their phenotypic effects, have not been investigated. Here, we first demonstrate that overexpression of the key m6A reader protein YTHDF2 induces more rapid viral replication, and larger viral plaques, in SV40 infected BSC40 cells, while mutational inactivation of the endogenous YTHDF2 gene, or the m6A methyltransferase METTL3, has the opposite effect, thus suggesting a positive role for m6A in the regulation of SV40 gene expression. To directly test this hypothesis, we mapped sites of m6A addition on SV40 transcripts and identified two m6A sites on the viral early transcripts and eleven m6A sites on the late mRNAs. Using synonymous mutations, we inactivated the majority of the m6A sites on the SV40 late mRNAs and observed that the resultant viral mutant replicated more slowly than wild type SV40. Alternative splicing of SV40 late mRNAs was unaffected by the reduction in m6A residues and our data instead suggest that m6A enhances the translation of viral late transcripts. Together, these data argue that the addition of m6A residues to the late transcripts encoded by SV40 plays an important role in enhancing viral gene expression and, hence, replication.

Epitranscriptomic Enhancement of Influenza A Virus Gene Expression and Replication

Many viral RNAs are modified by methylation of the N6 position of adenosine (m6A). m6A is thought to regulate RNA splicing, stability, translation, and secondary structure. Influenza A virus (IAV) expresses m6A-modified RNAs, but the effects of m6A on this segmented RNA virus remain unclear. We demonstrate that global inhibition of m6A addition inhibits IAV gene expression and replication. In contrast, overexpression of the cellular m6A "reader" protein YTHDF2 increases IAV gene expression and replication. To address whether m6A residues modulate IAV RNA function in cis, we mapped m6A residues on the IAV plus (mRNA) and minus (vRNA) strands and used synonymous mutations to ablate m6A on both strands of the hemagglutinin (HA) segment. These mutations inhibited HA mRNA and protein expression while leaving other IAV mRNAs and proteins unaffected, and they also resulted in reduced IAV pathogenicity in mice. Thus, m6A residues in IAV transcripts enhance viral gene expression.

Settling the m6A debate: methylation of mature mRNA is not dynamic but accelerates turnover.

Post-transcriptional modification of RNA nucleosides has been implicated as a pivotal regulator of mRNA biology. In this issue of Genes & Development, Ke and colleagues (pp. 990-1006) provide insights into the temporal and spatial distribution of N6-methyladenosine (m6A) in RNA transcripts by analyzing different subcellular fractions. Using a recently developed biochemical approach for detecting m6A, the researchers show that m6A methylations are enriched in exons and are added to transcripts prior to splicing. Although m6A addition is widely thought to be readily reversible, they demonstrate in HeLa cells that once RNA is released from chromatin, the modifications are surprisingly static. This study integrates data from previous publications to clarify conflicting conclusions regarding the role of m6A in mRNA biogenesis and function. Ke and colleagues found that m6A methylation levels negatively correlate with transcript half-life but are not required for most pre-mRNA splicing events.

Viral Epitranscriptomics.

Although it has been known for over 40 years that eukaryotic mRNAs bear internal base modifications, it is only in the last 5 years that the importance of these modifications has begun to come into focus. The most common mRNA modification, the addition of a methyl group to the N6 position of adenosine (m6A), has been shown to affect splicing, translation, and stability, and m6A is also essential for embryonic development in organisms ranging from plants to mice. While all viral transcripts examined so far have been found to be extensively m6A modified, the role, if any, of m6A in regulating viral gene expression and replication was previously unknown. However, recent data generated using HIV-1 as a model system strongly suggest that sites of m6A addition not only are evolutionarily conserved but also enhance virus replication. It is therefore likely that the field of viral epitranscriptomics, which can be defined as the study of functionally relevant posttranscriptional modifications of viral RNA transcripts that do not change the nucleotide sequence of that RNA, is poised for a major expansion in scientific interest and may well fundamentally change our understanding of how viral replication is regulated.

Detection of N 6-methyladenosine in urine samples by electrochemiluminescence using a heated ITO electrode

A sensitive and rapid electrochemiluminescence (ECL) method for the detection of N 6-Methyladenosine (m6A) in urine samples on a heated indium-tin-oxide (ITO) electrode is presented. The ECL intensity of Tris(2,2′-bipyridyl) dichlororuthenium(II)hexahydrate (Ru(bpy) 3 2+) can be enhanced by the presence of m6A. Experimental results showed that the change of ECL intensities (Δ I) of the Ru(bpy) 3 2+ between before and after addition of m6A was affected by the working electrode surface temperature ( T e ); the highest Δ I occurred at 31 °C. Under optimum conditions, the Δ I had a linear relationship with the m6A concentration in the range of 1.9 × 10 −9–3.9 × 10 −6 mol/L and a detection limit of 7.7 × 10 −10 mol/L ( S/ N = 3) at T e = 31 °C. The recovery of m6A standards added to urine samples verified the accuracy of the proposed method.