Understanding Chemical Changes in mRNA and New Directions in Research

N6-methyladenosine (m⁶A) is the most extensively studied mRNA modification, with over 7000 publications indexed in PubMed (Figure 1). This modification is dynamically regulated by the METTL3–METTL14 methyltransferase complex, which installs the mark, and by the demethylases FTO and ALKBH5, which remove it. Reader proteins such as YTHDF1–3 and YTHDC1 interpret m⁶A marks to influence mRNA fate. m⁶A is typically enriched in the 3′ untranslated region (UTR) and near stop codons, where it promotes regulated mRNA decay, translational control, alternative splicing, and nuclear export.

Functionally, m⁶A marks have been shown to fine-tune transcript dosage in response to developmental and environmental cues. In embryonic stem cells, m⁶A destabilizes pluripotency factors to drive differentiation. Under stress conditions, m⁶A enhances translation of heat-shock proteins. Circadian rhythm, synaptic plasticity, and antiviral responses also depend on m⁶A-mediated regulation. Dysregulation of m⁶A is implicated in several diseases: METTL3 overexpression in glioblastoma promotes tumorigenicity by stabilizing oncogenic transcripts, while FTO upregulation contributes to obesity and chemoresistance in leukemia. The structural positioning of m⁶A and its surrounding sequence context (DRACH motif) underlies its selective functional roles (reviewed in [25,26]).

Pseudouridine (Ψ) is an isomer of uridine in which the uracil base is attached to the ribose via a carbon–carbon instead of a nitrogen–carbon glycosidic bond, increasing base stacking and RNA stability (Figure 1). While Ψ has long been studied in tRNA and rRNA, transcriptome-wide mapping (using Pseudo-seq and CeU-seq) has revealed widespread presence in mRNA, especially under stress conditions [27]. Ψ can be introduced either cotranscriptionally by PUS enzymes or chemically during therapeutic RNA synthesis [28].

Functionally, Ψ enhances mRNA translational capacity and prevents innate immune recognition, which is critical for mRNA therapeutics [29]. In COVID-19 vaccines, synthetic mRNAs are modified with Ψ to reduce Toll-like receptor activation and increase translational efficiency [29]. Ψ also accumulates in stress granules and may contribute to selective mRNA stabilization during oxidative or nutrient stress [30]. Although its functions are still being elucidated, Ψ appears to act as a broad modulator of RNA stability, translation, and immunogenicity [31].

The modification 5-methylcytosine (m⁵C) (Figure 1) is catalyzed by the NSUN family of methyltransferases, particularly NSUN2 and NSUN6, and is enriched in both the coding region and 3′ UTR of mRNAs [32]. Unlike m⁶A, m⁵C lacks a well-characterized reader protein repertoire, though ALYREF has been proposed to bind m⁵C-modified mRNA and facilitate nuclear export [32].

Recent studies suggest that m⁵C enhances mRNA stability and influences ribosome loading. NSUN2-mediated m⁵C is upregulated in response to stress and during cell differentiation. Loss-of-function mutations in NSUN2 cause neurodevelopmental disorders, underscoring its importance. In cancer, m⁵C marks are elevated in tumor samples, where they correlate with poor prognosis and resistance to chemotherapy. Although less well-studied than m⁶A, m⁵C represents a growing area of interest in epitranscriptomics due to its context-specific roles in mRNA fate and disease [33,34,35].

Recent evidence reveals extensive crossregulation between the m⁶A and m⁵C RNA methylation systems, including reciprocal modifications of their effector transcripts, suggesting post-transcriptional feedback loops [32]. Proteomic analyses uncovered coordinated cellular responses and interactions—such as between ALKBH5 (m⁶A eraser) and NSUN4 (m⁵C writer)—linked to diverse pathways including mitochondrial function, proteasome activity, and post-translational modifications [32]. These networks also converge on neurological processes, with in vitro data showing colocalization of m⁶A-marked RNAs and the m⁵C reader ALYREF following synaptic activation [32].

Inosine is produced through the enzymatic deamination of adenosine by adenosine deaminases acting on RNA (ADARs) (Figure 1), and it represents one of the most widespread and functionally significant RNA modifications in metazoans [36]. During translation, inosine is interpreted as guanosine by the ribosome, effectively reprogramming codons and enabling the generation of protein isoforms not directly encoded by the genome [36]. This editing commonly occurs in coding regions, 3′ untranslated regions (UTRs), and especially within microRNA (miRNA) seed sequences, where it can drastically alter target specificity [37].

Functionally, A-to-I editing expands transcriptomic and proteomic diversity and plays a critical role in regulating innate immunity, particularly by preventing inappropriate activation of cytosolic RNA sensors such as MDA5 [38]. It also modulates mRNA splicing, nuclear retention, and subcellular localization [39]. In the central nervous system, high levels of editing are essential for the proper function of neurotransmitter receptors and ion channels, such as the GluA2 subunit of AMPA receptors and voltage-gated potassium channels [40].

Dysregulation of A-to-I editing has been implicated in a wide range of diseases, including autoimmune disorders (e.g., Aicardi–Goutières syndrome), neurological diseases (e.g., epilepsy, ALS, schizophrenia), and multiple cancers, where editing imbalances may contribute to immune evasion or altered cell signaling [41,42].

N1-methyladenosine (m¹A) is a reversible RNA modification in which a methyl group is added to the N1 position of adenosine (Figure 1), introducing a positive charge at physiological pH [43]. This structural alteration disrupts canonical Watson–Crick base pairing and significantly affects RNA secondary structure, stability, and interaction with ribosomes [43]. The modification m¹A is commonly enriched in 5′ untranslated regions (5′ UTRs), especially near translation start codons, suggesting a role in fine-tuning translation initiation [44]. It is installed primarily by the methyltransferase complex TRMT6/TRMT61A and removed by the demethylase ALKBH3, indicating tight enzymatic regulation [45].

Functionally, m¹A can promote or inhibit translation in a position-dependent manner—enhancing initiation when placed proximally to the start codon, but potentially repressing translation when located elsewhere [46]. Its levels are responsive to various cellular stimuli, including nutrient availability, oxidative stress, and heat shock, making m¹A a dynamic regulator in stress-adaptive gene expression programs [47]. In mitochondria, m¹A also appears in mitochondrial tRNAs and rRNAs, where it contributes to proper translation of mitochondrial-encoded proteins [48].

Pathologically, elevated m¹A levels have been observed in multiple cancer types, where they are thought to support oncogenic growth by enabling translational reprogramming toward stress-resilient and proliferation-promoting mRNAs [49]. Overexpression of m¹A writers such as TRMT6/61A or impaired ALKBH3 activity has been correlated with poor prognosis and chemoresistance [49]. Because of its dynamic nature and disease relevance, m¹A is emerging as a biomarker for tumor progression and a potential therapeutic target [49].

Technological advances such as m¹A-selective chemical labeling, antibody-based enrichment (m¹A-seq), and nanopore-based detection are now enabling more precise, transcriptome-wide mapping of m¹A at single-nucleotide resolution [50]. Future studies will clarify the tissue-specific and context-dependent roles of m¹A in development, differentiation, and disease [51].

N6,2′-O-dimethyladenosine (m⁶Am) is a methylated nucleotide located immediately adjacent to the canonical 7-methylguanosine (m⁷G) cap at the first transcribed nucleotide of certain mRNAs, typically when the initiating base is adenosine. This dual methylation—on both the N6 position of the adenine base and the 2′-O position of the ribose—creates a structurally distinct cap-proximal mark [52,53]. The modification m⁶Am is deposited by the methyltransferase PCIF1 (also known as CAPAM) in a sequence-dependent manner and removed by the m⁶A demethylase FTO, linking it to the broader dynamic regulation of m⁶A-related pathways [52,53].

Functionally, m⁶Am enhances mRNA transcript stability and has been associated with increased resistance to decapping and exonucleolytic decay, particularly through the DCP2-dependent pathway [52]. As such, it prolongs mRNA half-life and can influence gene expression levels at the post-transcriptional level. Importantly, the presence of m⁶Am at the 5′ end may reduce binding affinity of certain decapping enzymes and modulate translation efficiency under basal or stress conditions [52].

The effects of m⁶Am are context-dependent, varying across tissues, developmental stages, and stress responses [54]. For instance, under hypoxic or nutrient-limiting conditions, dynamic changes in PCIF1 expression or FTO demethylation activity can lead to remodeling of the m⁶Am epitranscriptome, thereby adjusting the stability of transcripts encoding key regulatory proteins. This fine-tuning has been implicated in cellular adaptation to environmental cues [54].

From a disease perspective, altered m⁶Am levels have been observed in cancer, where FTO overexpression may drive oncogenic gene expression by removing m⁶Am and destabilizing tumor suppressor mRNAs [52,53]. Conversely, reduced FTO activity has been linked to metabolic disorders and neurodevelopmental abnormalities [52,53]. However, disentangling the specific contributions of m⁶Am from internal m⁶A sites remains a technical challenge [54].

Recent advances in m⁶Am-specific mapping techniques—such as m⁶Am-seq and cap-specific chemical labeling approaches—are beginning to shed light on the m⁶Am epitranscriptome with greater resolution [54]. Future studies will further elucidate how m⁶Am integrates into broader regulatory networks and how its reversible deposition influences RNA fate in health and disease [52,53].

Eukaryotic mRNAs are cotranscriptionally modified at their 5′ ends with a series of cap structures that play essential roles in RNA stability, nuclear export, translation, and immune recognition [55,56]. These modifications occur in a hierarchical manner and are installed by a coordinated set of enzymes: RNGTT (RNA guanylyltransferase and 5′-phosphatase), RNMT (RNA guanine-N7 methyltransferase), and the cap-specific 2′-O methyltransferases CMTR1 and CMTR2 [57].

The Cap0 structure (m⁷GpppN, where N is any nucleotide) is formed when RNGTT first adds a guanosine cap to the nascent RNA and RNMT methylates the guanosine at the N7 position [57]. This modification is essential for transcript stability and the recruitment of the cap-binding complex (CBC), which facilitates nuclear export and translation initiation [57].

Cap1 and Cap2 modifications involve additional 2′-O-methylation of the ribose sugar of the first and second transcribed nucleotides, respectively. CMTR1 methylates the first nucleotide to form Cap1, and CMTR2 modifies the second nucleotide to yield Cap2 [55,56]. These methylations are crucial for distinguishing endogenous mRNAs from viral RNAs, as innate immune sensors such as RIG-I and MDA5 recognize and respond to improperly capped or unmethylated RNA species [55,56]. Thus, Cap1 and Cap2 modifications act as a molecular “self” signature that prevents erroneous activation of interferon-stimulated gene expression [55,56].

Beyond immune evasion, cap methylation status influences translation efficiency. Cap1 and Cap2 modifications enhance the binding affinity of translation initiation factors (e.g., eIF4E) and are increasingly recognized as modulators of transcript-specific translation under stress or developmental cues. For example, selective capping patterns may allow cells to prioritize translation of certain mRNAs during viral infection, inflammation, or differentiation.

Emerging evidence also suggests dynamic regulation of capping enzymes in cancer, immune disorders, and viral pathogenesis. Some viruses, such as flaviviruses and coronaviruses, encode their own cap-modifying enzymes to mimic host cap structures and evade immune detection.

In summary, 5′ cap modifications (Cap0, Cap1, and Cap2) function not only as mechanical gatekeepers of RNA processing but as immunological checkpoints and regulators of translational prioritization, underscoring their centrality in gene expression regulation and host-pathogen interactions [55,56].

Also referred to as ribothymidine, 5-methyluridine (m⁵U) is a methylated form of uridine where a methyl group is added to the fifth carbon of the uracil ring [58]. Historically, this modification has been best characterized in transfer RNAs (tRNAs), where it plays a well-established role in stabilizing the TΨC loop and facilitating accurate codon-anticodon pairing during translation [58]. In tRNA, m⁵U is catalyzed by enzymes such as TRMT2A and TRMT2B (tRNA methyltransferase 2 homologs A and B), which are conserved across eukaryotes [58].

More recently, attention has turned to the presence and potential functions of m⁵U in messenger RNA (mRNA) and other noncoding RNAs, although its prevalence and biological significance in these contexts remain incompletely understood [59]. Emerging transcriptomic and mass spectrometry evidence suggests that m⁵U may exist in low stoichiometry in certain mRNAs, potentially acting as a regulatory modification under specific physiological or stress conditions [59].

Mechanistically, m⁵U in mRNA may influence local secondary structure, RNA–protein interactions, or ribosome dynamics during translation [59]. It could also affect mRNA stability or splicing, depending on its location within the transcript. For example, methylation within untranslated regions (UTRs) or near splice sites may modulate transcript half-life or alternative exon usage [59].

The enzymatic machinery responsible for m⁵U deposition in mRNA is still under investigation. While TRMT2A/B are known to methylate tRNAs, whether they also modify mRNAs directly—or whether other, yet-uncharacterized RNA methyltransferases are involved—remains an open question [59]. Likewise, demethylases or “erasers” for m⁵U in RNA have not yet been definitively identified, raising the possibility that this mark may be either stable or dynamically regulated through nonenzymatic turnover pathways [59].

The biological significance of m⁵U in mRNA is further underscored by emerging links to cancer biology and neurological diseases, where dysregulation of TRMT2 enzymes and associated pathways has been observed [60]. Additionally, some viral RNAs have been found to incorporate m⁵U, potentially as a mechanism to mimic host RNA and evade immune detection [60].

In summary, although best studied in tRNA, m⁵U is an underexplored but promising RNA modification in mRNA, with potential roles in fine-tuning gene expression, especially under stress or developmental regulation. Future studies using direct RNA sequencing and m⁵U-specific antibodies or chemical labeling methods will be crucial to uncover its distribution, function, and regulatory dynamics in the transcriptome.

In the ribonucleotide modification 2′-O-methyladenosine (Am), a methyl group is added to the 2′ hydroxyl (OH) position of the ribose sugar on adenosine (Figure 1). While extensively characterized in transfer RNA (tRNA) and ribosomal RNA (rRNA)—where it contributes to RNA stability, secondary structure, and resistance to nucleolytic cleavage—recent research suggests that Am also appears in messenger RNA (mRNA), particularly near the 5′ end and within untranslated regions (UTRs) [61,62].

In tRNA and rRNA, 2′-O-methylations such as Am help maintain proper folding and enhance resistance to enzymatic degradation. The methylation of adenosine to form Am can prevent base pairing and increase thermal stability, which is particularly important in RNAs exposed to structural fluctuations during processes like translation [63].

The FTSJ1 enzyme (FtsJ RNA 2′-O-methyltransferase 1), a human homolog of the bacterial FtsJ methyltransferase family, is one of the key methyltransferases responsible for installing Am modifications, particularly in tRNA [64]. FTSJ1 has also been implicated in neuronal development, and mutations in this gene are associated with intellectual disability syndromes, suggesting that Am modification may be critical for neurodevelopmental gene regulation [65].

In mRNA, Am has been observed in cap-adjacent positions, where it contributes to the Cap1 and Cap2 structures (see Section 3.1.7), but may also occur internally within transcripts [66]. Cap-associated Am modifications are known to modulate innate immune recognition by reducing the immunogenicity of foreign RNAs and enhancing translational efficiency [66].

Although Am in internal mRNA regions is less well-characterized, emerging evidence points to dynamic regulation under stress conditions and potential roles in alternative splicing, RNA localization, or translational fine-tuning [67]. The presence of Am may influence RNA-binding protein affinity or change the structural dynamics of the mRNA molecule in ways that are still being actively investigated [67].

Am is also thought to contribute to resistance to exonucleolytic degradation, much like other 2′-O-methyl modifications [68]. This may allow Am-containing transcripts to persist longer in the cytoplasm, thereby shaping post-transcriptional gene expression patterns [68].

Overall, 2′-O-methyladenosine represents an evolutionarily conserved but underexplored RNA modification that may play broader roles in the regulation of gene expression, especially under conditions that challenge transcript stability or immune tolerance. Further advances in site-specific detection methods, such as RiboMethSeq [69] or nanopore sequencing [70] approaches adapted for sugar methylation, are expected to shed light on the full extent of Am′s regulatory roles in the transcriptome [61,62].

N4-acetylcytidine (ac4C) is a conserved RNA modification in which an acetyl group is added to the amino group at the N4 position of cytidine (Figure 1) [71]. While ac4C was historically studied in tRNA and rRNA—where it contributes to codon–anticodon pairing accuracy and ribosomal stability—recent studies have identified ac4C in messenger RNA (mRNA), particularly in the coding sequences (CDS) of highly expressed genes [71].

The primary “writer” enzyme responsible for installing ac4C in RNA is NAT10 (N-acetyltransferase 10), a protein with both acetyltransferase and helicase domains [72]. NAT10 targets cytidine residues and acetylates them in a sequence- and structure-dependent manner, enhancing the structural integrity of RNA and facilitating accurate base pairing during translation [72]. This modification leads to improved translational fidelity, reducing the frequency of codon misreading or ribosomal stalling [72].

Beyond translation, ac4C also enhances mRNA stability, likely by promoting structural rigidity and protecting transcripts from exonucleolytic decay [73]. This effect may be particularly important under conditions of cellular stress, where global mRNA degradation is elevated, and the selective stabilization of certain transcripts becomes essential for survival [73]. Indeed, ac4C levels are dynamically regulated in response to stress, including oxidative and heat shock conditions, where the modification helps ensure the continued translation of critical survival genes [73].

Intriguingly, elevated levels of ac4C and increased NAT10 expression have been observed in several human cancers, including colorectal and liver cancers [74]. This suggests that ac4C may play a role in tumorigenesis, possibly by enabling the overexpression of oncogenic proteins with greater stability and translational efficiency [74]. Inhibition of NAT10 has been shown to suppress tumor growth in some models, positioning it as a potential therapeutic target [74].

Emerging evidence also links ac4C with the regulation of immune responses and stress granule formation, hinting at broader roles in post-transcriptional control [75]. Moreover, because NAT10 is involved in DNA damage repair and nuclear architecture, its RNA-modifying functions may interface with other cellular pathways beyond translation [75].

In summary, ac4C is a versatile RNA modification that not only enhances translation accuracy and mRNA stability but contributes to cellular adaptation under stress and may promote pathological states such as cancer. As technologies improve for detecting ac4C sites transcriptome-wide (e.g., acRIP-seq and ac4C-specific antibodies), our understanding of its regulatory network and disease relevance is expected to grow significantly [76].

The modification m⁷G forms part of the 5′ cap structure but has also been detected internally in some mRNAs. It plays roles in nuclear export and translation. Recent studies have suggested that mRNA internal m⁷G and its writer protein METTL1 (methyltransferase 1, tRNA methylguanosine) are closely related to cell metabolism and cancer regulation. The IGF2BP (insulin growth factor 2 binding protein)-family proteins IGF2BP1-3 can preferentially bind internal mRNA m⁷G and regulate mRNA stability [77].

In the post-transcriptional RNA modification 2′-O-methylguanosine (Gm), a methyl group is added to the 2′-hydroxyl (2′-OH) group of the ribose sugar in guanosine [78]. This methylation can occur at both internal sites within the RNA molecule and at cap-adjacent positions, particularly within the first few nucleotides of the mRNA 5′ end, such as in Cap1 (m⁷GpppNm) and Cap2 (m⁷GpppNmNm) structures [78].

The enzymatic writers for Gm modifications include members of the FTSJ (FtsJ methyltransferase) family and CMTR2 (Cap Methyltransferase 2) for cap-adjacent Gm. These enzymes catalyze 2′-O-methylation in a sequence- and structure-specific manner, and their activity is often regulated by stress, developmental stage, or cell type [78].

Functionally, Gm contributes to mRNA stability by protecting transcripts from exonucleolytic cleavage. The 2′-O-methyl group enhances the resistance of the phosphodiester backbone to degradation by RNases, especially those that recognize unmodified ribose moieties [78]. This protection against decay increases the half-life of mRNAs and may promote sustained translation of essential proteins, particularly under stress or during rapid proliferation [78].

Additionally, Gm plays a significant role in regulating translation efficiency. By altering RNA secondary structure and reducing backbone flexibility, Gm can enhance ribosome processivity and reduce translational pausing [79]. When located near the 5′ cap, Gm modifications can improve the binding affinity of the eukaryotic initiation factor eIF4E and facilitate the formation of the cap-binding complex, thereby boosting cap-dependent translation initiation [79].

Importantly, 2′-O-methylation also functions as a self-identifying molecular signature that distinguishes endogenous mRNAs from viral or foreign RNAs [80]. The innate immune sensor RIG-I, for instance, is known to preferentially detect RNAs lacking 2′-O-methyl modifications, leading to the activation of antiviral responses [80]. Thus, Gm and related cap-proximal modifications help evade immune detection, maintaining self-tolerance and preventing unwanted inflammation [80].

Emerging evidence suggests that dysregulation of Gm deposition or recognition may contribute to diseases such as cancer and neurodegeneration, where aberrant translation and immune activation are commonly observed [81]. However, the specific distribution and functional significance of internal Gm residues in mRNA are still being actively explored with the help of high-resolution techniques like RiboMeth-seq and LC-MS/MS [82,83].

In summary, 2′-O-methylguanosine (Gm) is a versatile RNA modification that enhances transcript stability, improves translational efficiency, and contributes to immune self-recognition—underscoring its critical role in both normal physiology and disease [84].

In the ribose modification 2′-O-methylcytidine (Cm), a methyl group is added to the 2′-hydroxyl (2′-OH) group of cytidine (Figure 1) [78]. Like other 2′-O-methylated nucleotides, Cm is found in both internal sites of the RNA body and at cap-adjacent positions as part of higher-order cap structures (Cap1 and Cap2) [78]. It is deposited by methyltransferases such as FTSJ1 and CMTR1/2, often in a sequence- and structure-dependent manner [78].

Functionally, Cm contributes to mRNA stability by enhancing resistance to RNase-mediated degradation. The methylation at the 2′-OH position sterically hinders endonucleolytic cleavage, thereby prolonging transcript half-life and enabling sustained protein production—especially important under conditions of cellular stress or immune challenge [68,85].

Cm also plays a critical role in modulating translation efficiency. By rigidifying the ribose–phosphate backbone and altering local RNA secondary structure, it can promote more efficient ribosomal scanning and elongation [68,85]. When located near the 5′ cap, Cm may also facilitate the recruitment of initiation factors such as eIF4E and eIF4G, enhancing cap-dependent translation initiation [68,85]. This may be especially important in highly proliferative or metabolically active cells where translational throughput is critical.

In the innate immune system, Cm and other 2′-O-methylations serve as self-marks that prevent activation of immune sensors such as RIG-I and MDA5, which preferentially detect hypomethylated or nonself RNA [86]. This capacity for immune evasion has been exploited by some viruses, which encode their own 2′-O-methyltransferases to mimic host mRNAs [86]. Dysregulation of Cm methylation may therefore contribute to autoimmune disorders or altered antiviral defenses [86].

Though less studied than other modifications such as m⁶A, emerging evidence from RiboMeth-seq, LC-MS/MS, and Nanopore direct RNA sequencing indicates that Cm occurs at conserved positions in both coding and noncoding RNAs [87]. Its distribution may vary by cell type and environmental context, and changes in Cm levels have been linked to cancer progression and neurological disorders [88].

In summary, 2′-O-methylcytidine (Cm) serves as a multifunctional RNA modification that enhances mRNA stability, supports efficient translation, and helps maintain immune homeostasis. Its expanding functional repertoire underscores the complexity of the epitranscriptome and the need for further high-resolution mapping and functional studies [84].

A modified cytidine residue, 5-hydroxymethylcytidine (hm⁵C) is characterized by a hydroxymethyl group (-CH₂OH) at the 5-position of the pyrimidine ring [89]. Although well-characterized in the context of DNA as an intermediate in the active demethylation of 5-methylcytosine (5mC) by TET (ten–eleven translocation) enzymes, the presence of hm⁵C in RNA suggests a potential parallel regulatory role in the epitranscriptome [89]. Its discovery in multiple RNA classes—including mRNA, rRNA, and tRNA—has raised important questions about its biogenesis, function, and dynamics in gene expression regulation [90].

In RNA, hm⁵C is likely derived from the oxidation of 5-methylcytidine (m⁵C) via TET family dioxygenases, particularly TET2, although the full set of responsible enzymes and their cofactor requirements (e.g., Fe²⁺, α-ketoglutarate) remain to be fully elucidated [90]. The modification may be transient, serving as a signaling intermediate or as a stable modification depending on context and cell type [90].

Functionally, hm⁵C is hypothesized to influence RNA structure, binding protein recruitment, and post-transcriptional regulation, much like its m⁵C precursor. Its enrichment in coding sequences and untranslated regions suggests possible involvement in mRNA stability, translation regulation, or nuclear export [90], although these functions remain speculative in the absence of comprehensive mechanistic studies.

The epitranscriptomic role of hm⁵C may also mirror its epigenetic behavior in DNA, wherein it acts as both a stable regulatory mark and a reversible intermediate during demethylation. This raises the possibility of dynamic RNA demethylation cycles, potentially responsive to environmental stimuli, cellular stress, or developmental cues [90]. In support of this, emerging studies have found context-dependent patterns of hm⁵C in mammalian tissues, including neurons, and in disease states such as cancer and neurodegeneration [90].

Recent advances in mapping technologies such as chemical labeling, antibody-based enrichment, and direct RNA nanopore sequencing have enabled more confident detection of hm⁵C sites in the transcriptome [91]. However, its stoichiometry and biological significance remain poorly defined, in part because of a lack of selective reader proteins and functional assays [91].

In conclusion, while hm⁵C is a relatively understudied RNA modification, its structural similarity and biosynthetic overlap with DNA methylation pathways suggest it may represent a novel layer of epitranscriptomic regulation, potentially contributing to cell state plasticity, translational control, and disease pathogenesis. Further studies are warranted to clarify its readers, erasers, and downstream functional consequences (reviewed in [33]).

The discovery of comodified RNA loci, particularly those harboring both N6-methyladenosine (m⁶A) and pseudouridine (Ψ) modifications, represents an emerging frontier in the field of epitranscriptomics [92]. These co-occurring marks, located on the same RNA molecule and sometimes within the same nucleotide neighborhood, suggest a layer of combinatorial regulation that mirrors the complexity of histone modification crosstalk in chromatin biology [92]. While m⁶A is known to influence RNA splicing, export, and decay through reader proteins such as YTH domain-containing proteins, Ψ contributes to RNA stability and secondary structure by enhancing base stacking and altering hydrogen bonding potential [92].

The presence of m⁶A and Ψ at adjacent or overlapping sites raises key questions about how these modifications interact structurally and functionally [92]. It is possible that m⁶A-induced conformational changes may prime an RNA region for subsequent pseudouridylation, or vice versa, and that together they cooperatively modulate translation efficiency, RNA–protein interactions, or decay rates. For instance, m⁶A may promote recruitment of RNA-binding proteins while Ψ stabilizes the structure of the binding motif, fine-tuning accessibility and recognition in a context-dependent manner.

Long-read nanopore direct RNA sequencing has emerged as a powerful tool for detecting comodified sites because of its ability to read full-length RNA molecules while preserving native modifications [92]. This allows for precise mapping of modification co-occurrence on individual transcripts and even single molecules—something not easily achievable with traditional short-read or antibody-based approaches. Studies using this technology have already revealed previously unrecognized modification patterns in stress-responsive and developmentally regulated mRNAs [8].

Functionally, comodified sites may play roles in adaptive gene expression, particularly under stress conditions or during development when dynamic remodeling of RNA fate is essential. Preliminary findings suggest that dual m⁶A/Ψ marks are enriched in mRNAs involved in cell fate decisions, metabolic control, and immune signaling, though mechanistic details remain under investigation [92].

In sum, comodified m⁶A/Ψ loci represent a nascent but promising area of study that expands our understanding of multilayered RNA regulation. These sites may act as epitranscriptomic integration hubs, where multiple signals converge to direct transcript-specific outcomes. As tools for multimodification mapping improve, future research will be crucial for uncovering how comodified RNAs are written, read, erased, and functionally interpreted in health and disease [8].