Epigenetic reprograming in myalgic encephalomyelitis/chronic fatigue syndrome: A narrative of latent viruses

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic disease presenting with severe fatigue, post‐exertional malaise, and cognitive disturbances—among a spectrum of symptoms—that collectively render the patient housebound or bedbound. Epigenetic studies in ME/CFS collectively confirm alterations and/or malfunctions in cellular and organismal physiology associated with immune responses, cellular metabolism, cell death and proliferation, and neuronal and endothelial cell function. The sudden onset of ME/CFS follows a major stress factor that, in approximately 70% of cases, involves viral infection, and ME/CFS symptoms overlap with those of long COVID. Viruses primarily linked to ME/CFS pathology are the symbiotic herpesviruses, which follow a bivalent latent–lytic lifecycle. The complex interaction between viruses and hosts involves strategies from both sides: immune evasion and persistence by the viruses, and immune activation and viral clearance by the host. This dynamic interaction is imperative for herpesviruses that facilitate their persistence through epigenetic regulation of their own and the host genome. In the current article, we provide an overview of the epigenetic signatures demonstrated in ME/CFS and focus on the potential strategies that latent viruses—particularly Epstein–Barr virus—may employ in long‐term epigenetic reprograming in ME/CFS. Epigenetic studies could aid in elucidating relevant biological pathways impacted in ME/CFS and reflect the physiological variations among the patients that stem from environmental triggers, including exogenous viruses and/or altered viral activity.

Multiple viral infections have long been implicated in the onset and chronicity of ME/CFS pathology, but no single pathogen has been identified.Viruses predominantly linked to ME/CFS belong to the family of herpesviruses that include large double-stranded DNA viruses, notably Epstein-Barr virus (EBV) [51][52][53][54][55][56][57], human herpesvirus-6 (HHV-6) [58][59][60], HHV-7 [58,59], herpes simplex virus-1 (HSV-1), and HSV-2 [3,7,61].Herpesviruses are common infectious agents within the general population that establish life-long latency in humans.Sporadic reactivation of herpesviruses facilitates transmission to uninfected individuals, hence ensuring their survival over time [62].It may occur spontaneously, albeit most commonly following a stressful event, such as an infection, septic shock, trauma, or emotional or mental stress, and it is related to immune suppression and loss of T-cell surveillance [5].EBV infection is a risk factor in a subgroup of ME/CFS patients [56], and a higher rate of active EBV infection has been demonstrated in a subgroup of patients as well [51].Multiple studies support the hypothesis of altered EBV activity in ME/CFS [54,[63][64][65][66][67][68][69].However, the association between EBV infection and ME/CFS is not established yet.
The persistence of ME/CFS symptoms after the triggering infection has ceased implies that effector mechanisms are continuously present, whereas the initial trigger in ME/CFS, and similarly in long COVID, is a transient event.

Epigenetic reprograming in ME/CFS
First clues for a genetic component in ME/CFS emerged from the observation that mothers and children diagnosed with ME/CFS share similar symptoms, in contrast to fathers and their children, suggesting that ME/CFS results from a combined effect of sex-biased genetic predisposition and environmental factors [70].A single-gene variation is less probable to account for the complex pathology of ME/CFS, and most studies suggest a varied genetic contribution based on the association between a small number of polymorphisms.The most significant gene alterations in ME/CFS have been summarized by Deumer et al. [71] and indicate pathways related to immunity, inflammation, neurotransmission, oxidative stress, the catecholamine pathway, the serotoninergic system, and circadian rhythm.A systematic review by Tziastoudi et al. [72] on gene alterations-common in ME/CFS and long COVID-highlighted the involve-ment of genes related to inflammation (mediated by chemokine-and cytokine-signaling pathways), T-cell activation, and Toll-like receptor-signaling pathways.
Epigenetic mechanisms play a crucial role in bridging external environmental signals with internal cellular responses, and their reversible nature makes them particularly relevant when considering the onset of complex acquired diseases such as ME/CFS.ME/CFS has been associated with epigenetic alterations, including changes in DNA methylation (DNAm) profiles (Table 1), modifications in chromatin acetylation, and the differential expression of non-coding RNAs (ncRNAs) (Table 2).The multitude of systemic alterations and molecular pathways previously implicated in ME/CFS pathology align with the discoveries from epigenetic studies, as outlined in Fig. 1.The suggested downstream effects of epigenetic alterations are briefly summarized in Fig. 2.

DNA methylation studies in ME/CFS
Fourteen studies have examined alterations in DNAm profile in patients with ME/CFS and are summarized in Table 1.Among them, eight studies opted for a genome-wide perspective on DNAm, whereas six studies focused on the methylation status of individual genes.
Most studies on DNAm patterns suggest alterations in immune responses [73][74][75][76][77][78][79][80].The primary sample investigated was peripheral blood mononuclear cells (PBMCs), with a few studies focusing on isolated subpopulations such as CD4 + (cluster of differentiation 4 positive) [80] and CD3 + (cluster of differentiation 3 positive T cells) [79].In the latter, a significant association of DNAm patterns with single-nucleotide polymorphisms in neighboring genes was demonstrated [79].Correlations of DNAm patterns with symptom severity (and particularly PEM) have been documented [76], with changes in the patients' health status observed longitudinally, including a relapse and a recovery cycle [77].A comparison of two geographically distinct cohorts yielded similar outcomes [75].Depending on sample type, the comparison of DNAm patterns among studies showed both overlapping and distinct signatures, hence validating that methylation changes were specific to the distinct physiological functions of different cell types [73].
Central nervous system and ANS dysfunction in ME/CFS are likewise reflected in DNAm patterns [73,77].The hypothalamic-pituitary-adrenal (HPA) axis is responsible for regulating the stress response.Differences in glucocorticoid sensitivity in vitro-correlated with epigenetic patterns related to HPA axis signaling-further support the hypothesis of a dysfunctional HPA axis [78].
Six studies have explored the DNAm status of single genes in ME/CFS patients.Brain-derived neurotropic factor (BDNF) hypomethylation in exon 9 accounted for the altered BDNF levels in the serum [81].CpG methylation pattern of seven sites in the perforin (PRF1) gene was correlated with PRF1 levels [82].The glucocorticoid receptor (GR) NR3C1 gene was explored in two studies and found to be hypomethylated compared to controls [83,84].Two studies investigated the association of the methylation status with genetic variability for serotonin receptor 2A (HTR2A) and catechol-O-methyltransferase (COMT) genes.The promoter of HTR2A presented with the polymorphism- 1438G/A (rs6311) exhibited altered methylation profiles, affecting the binding of GR and other transcription factors [85].DNAm in the S-COMT promoter was associated with COMT haplotypes and enzymatic activity in patients with ME/CFS and fibromyalgia (FM), whereas increased DNAm in the MB-COMT promoter was reported only among the patient groups.COMT regulates nociceptive processing and inflammation [86].

Histone modifications in ME/CFS
Increased histone deacetylase (HDAC) activity was reported in PBMC samples, which correlated with low cortisol levels [87], and levels of HDAC-2 and HDAC-3 were found to be upregulated in ME/CFS [87].Significant changes in proteins involved in histone methylation at H3K4 (activating gene expression) and H3K27 (repressing gene expression) were reported in ME/CFS [88].Following an exercise test, responsive genes demonstrated elevated levels of HDAC-1 and HDAC-2 binding sites and changes in 21 proteins with regulatory functions in acetylation and deacetylation [89].Patterns of serum metabolites in ME/CFS link a state of hypermetabolism and hypoacetylation, which was suggested to correlate with the development of PEM [90].

Non-coding RNAs in ME/CFS
Eleven studies have investigated the expression levels of ncRNAs in ME/CFS (Table 2).Most of them focused on the investigation of circulating micro-RNAs (miRNAs) because the latter could potentially be used as noninvasive biomarkers for disease diagnosis and prognosis.
Similarly to the DNAm studies, alterations in the expression patterns of circulating miRNAs in plasma, in PBMCs, or in isolated cytotoxic cell subpopulations were related to immune functions [91][92][93][94][95]. Differences were observed both at baseline and following post-exertional stress challenge [92,93] and could discriminate ME/CFS from FM patients, a frequent comorbidity with overlap in symptomatology [94].An attempt to associate changes in plasma miRNAs with a herpesvirus infection (HHV-6A/B) showed no associations with HHV-6A/B positivity [95].Conversely, miRNA expression patterns were associated with sex and the nutritional status of the patients, highlighting the importance of subjects' metabolic state [92].
Signatures of miRNA expression related to neuronal and endocrine pathways come from a study in which the miRNomes were analyzed simultaneously in PBMCs and extracellular vesicles [96].The emerging pathways in PBMCs included posttranscriptional silencing by small RNAs, circadian clock, chromatin organization and modifying enzymes, and several others related to the X-chromosome encoded methyl-CpG-binding protein 2 (MECP2) gene.Pathways related to MECP2 were proposed to involve environmental sex-biased effectors for ME/CFS, whereas those related to neurotrophic receptor tyrosine kinase signaling were associated with cognitive disability and sensorial dysfunctions.
The induction of a set of miRNAs-which were previously reported to regulate endothelial function in ME/CFS plasma samples-was found independent of disease severity and was confirmed in PBMC microarray data from public miRNA data repositories [97].Furthermore, HDAC-1 represented the most relevant node within the network.
Alterations in the pattern of ncRNAs in local biological fluids such as cerebrospinal fluid (CSF) naturally point toward pathways related to the function of the proximal organ/system.Alterations in miRNA expression in CSF following exercise were related to cleavage of amyloids, apoptosis, and trans-endothelial migration [98].Interestingly, comparison with Gulf War Illness (GFI)-which presents with substantial symptom overlap to ME/CFS-demonstrated distinct miRNA patterns that were suggested to account for different mechanisms for PEM in GFI and ME/CFS patients.
Long ncRNAs (lncRNAs) constitute nearly 68% of the transcriptome and were suggested to have a significant regulatory role in antiviral interferon (IFN) response, viral transcription, and latency [99].Analysis of a selected panel of very long ncR-NAs (>5 kb) in PBMCs could differentiate ME/CFS from healthy donors and relate to altered responses to oxidative stress, chronic viral infection, and hypoxemia [100].The expression profile of circR-NAs in the peripheral blood of ME/CFS patients following PEM induction is related to histone modification, cellular catabolic processes, protein modification processes, and cellular component organization [101].

EBV in ME/CFS pathology
EBV infects nearly 95% of adults worldwide.It is primarily transmitted through the saliva, infecting B cells and epithelial cells in the oral cavity, and though in most cases the infection is asymptomatic or mild, in young adults and adolescents it may lead to infectious mononucleosis (IM) [102].EBV infection is a key feature in a subgroup of ME/CFS patients.IM may be a risk factor for ME/CFS in adolescents since, after 6, 12, and 24 months of IM, 13%, 7%, and 4% of subjects, respectively, met the disease diagnostic criteria [103].In adults, persisting illness and fatigue have been correlated with evidence of EBV infection [104].The mechanisms of immune escape that develop in latently infected cells after primary EBV infection [57] resemble features of immune dysregulation described in ME/CFS (e.g., compromised cytotoxic and T-cell responses, augmented Th2 and T-regulatory responses) [5].Incompetent immunosurveillance could potentially result in increased proliferation of EBV-latent cells.A hypothetical scenario could be that the trigger of EBV-associated ME/CFS could be the latently infected cells rather than the viral load [57].In our study [54], SARS-CoV-2 infection resulted in elevated antibodies against Epstein-Barr nuclear antigen 1 (EBNA1) exclusively in ME/CFS patients, contrary to anti-viral capsid antigen (VCA) antibodies.EBNA1 is the only EBV-protein produced in all EBV-latency stages, whereas VCA is produced during recent infection/reactivation [105].

Epigenetic mechanisms in latent-lytic lifecycle and reactivation of EBV
The initial site of herpesvirus infection usually supports lytic replication, resulting in the fulminant production of infectious progeny.Establishment of persistent infection at secondary anatomical sites or cell types (e.g., memory B cells) facilitates viral latency.The EBV genome can adopt four latency types (types 0 and I-III) with associated transcriptional repertoires [106].EBV DNA in virions is "epigenetically naïve," but following infection, EBV maintains its genome in the host cell as an extrachromosomal circular episome.Following the chromatinization of the viral DNA by host histones, it is subsequently subjected to epigenetic modification.Latency is hence achieved when viral genes required for lytic replication are epigenetically repressed, whereas genes required for latent state are maintained in an active chromatin state [107].
Epigenetic silencing of viral DNA may have originally evolved as an innate antiviral defense mechanism that EBV successfully subverted to avoid detection by the host adaptive immune system and persist life-long in the host [108].Epigenetic modifications occurring for the establishment of herpesvirus latency and reactivation are briefly described below and reviewed in detail in Refs.[109][110][111][112].During EBV infection, the expression of DNMT1, DNMT3A, and DNMT3B is dysregulated and linked to latent membrane protein 1 (LMP1).LMP1 has been shown to induce DNMT1.Modification of the host genes evident by promoter hypermethylation included ELL3 (Elongation Factor RNA Polymerase II-Like 3), RBM5 (involved in RNA splicing), the distal promoter of IRF5, SMAD4 (mothers against decapentaplegic homolog 4, involved in transforming growth factor beta signaling), and retinoic acid receptor-β2 [108].

DNA, histone, and chromatin modifications
The methylation status of the EBV genome controls the transition from latent to lytic state [113] and requires the expression of two early immediate genes, BZLF1 and BRLF1, which encode the transcription factors Zta (also known as ZEBRA, EB1, and Z) and Rta (also known as R) [114].Z is the only known transcription factor to preferentially activate the methylated form of a target promoter [115].The activity of Z (Zp) and R (Rp) promoters is primarily regulated by host transcription factors, and although DNAm of the Rp inhibits its activa-tion, it enhances the ability of Z to activate the Rp [116].Rp activation has been suggested as the first step required for Z disruption of viral latency.EBV induces DNAm in most lytic genes in B cells, but the Zp region remains unmethylated to facilitate activation by specific host transcription factors [117].
EBV has evolved multiple ways to overcome epigenetic silencing by the host.One mechanism involves packaging factors into the tegument of viral particles that inhibit epigenetic repression by the host cells.For example, the tegument protein BNRF1 localizes to promyelocytic leukemia nuclear bodies (PML-NBs) and disassembles the histone chaperone complex Daxx-ATRX by binding Daxx and displacing ATRX.The consequence is the restriction of the PML-NBs complex to deposit repressive forms of histones on the viral promoter Wp [118].EBV regulates patterns of repressive histone modifications, leading to aberrant DNAm and reprograming of host cells and their progeny.EBV can induce the expression of DNMTs and/or polycomb group (PcG) proteins such as EZH2.The regulation of BZLF1 expression is a critical determinant of whether EBV remains in a latent or reactivated state.The Zp region-which controls BZLF1 expression-is primarily regulated by histone modifications, including suppressive markers, such as H3K27me3, H3K9me2/3, and H4K20me3, during latency.These repressive histone markers help keep the promoter silent, whereas histone acetylation, phosphorylation, and H3K4me3 are associated with viral reactivation.Among these modifications, H3K27me3 appears to be crucial for maintaining latency.These epigenetic markers strike a balance, allowing for repression during latency while permitting activation upon stimulation [118,119].

EBV-derived ncRNAs
EBV infection triggers a cascade of events that collectively induce the expression of both viral and host ncRNAs [120].EBV ncRNAs are grouped into EBV-encoded small RNAs (EBERs) and rightward transcripts from the Bam HIA region (BARTs) [121].The patterns of expression of EBV-derived ncRNAs have been associated with different EBV latency types in various cancers, and they have significantly contributed to understanding EBVinduced carcinogenesis [109,117].The EBERs are expressed during all the latency states and are markers for latently infected cells [117].The BART transcripts give rise to 44 miRNAs, whereas 3 miR-NAs originate from the vicinity of the BHRF1 ORF.EBV-derived miRNAs expressed during latency may both inhibit the expression of viral lytic genes and regulate host gene expression [122].EBV miR-NAs are secreted in exosomes as well, thus extending the reach of their regulatory effects [123].
The primary targets of EBV-miRNAs are host genes; they regulate multiple biological processes (apoptosis, tumor suppression, and immune regulation) and cell-signaling pathways (NFκB, Wnt, and MAP3K2), many of which are also naturally controlled by host miRNAs.EBV-miRNAs facilitate viral persistence by inhibiting T-cell responses and facilitate recognition escape by the host immune system.Examples include direct targeting of IL-6R (miR-BART6-3p), dampening of EBV-triggered RIG-I-like receptor signaling and type I IFN response, direct targeting of IFN-γ (miR-BART20-5p), and escape of NK-cell-mediated killing [120,124].
EBV-induced lncRNAs can viral gene transcription and host gene expression by modifying histone marks at gene promoters.They may affect cellular processes such as the IFN responses, the unfolded protein response, and mitochondrial antiviral signaling [99].EBV can generate circR-NAs, including spliced transcripts in the BART region [125] and have been found in clinical samples from EBV-related cancers [126].The function of viral circRNAs is unknown, though their similar structure to the host circRNAs implies that they have equivalent roles in restricting viral overproliferation as a means of immune escape [127].

Viral-induced perturbations in cellular metabolism and epigenetics
Viral infections-including herpesviruses-have profound effects on cellular metabolism and mitochondrial function.They may induce alterations in mitochondrial morphology, a reduction in respiration or mitochondrial membrane potential [128,129].Viral proteins and nucleic acids can disrupt metabolic pathways, leading to altered energy production, decreased ATP synthesis, increased reactive oxygen species production, and a metabolic shift toward glycolysis [130,131].EBV targets the mitochondrial one-carbon metabolism that drives nucleotide, mitochondrial NADPH, and glutathione production [132].Disturbances in cellular metabolism and mitochondrial dysfunction in ME/CFS patients could potentially result from a persistent/recurrent viral stimulus [133], which could arise either from the reactivation of latent viruses and/or insufficient management and elimination by the immune system.
Alterations in cellular metabolism are dynamically integrated into the regulation of epigenetic mechanisms and ensuing gene transcription.Epigenetic modifications are mediated by enzymes (e.g., DNA methyltransferases or hydroxylases, histone acetyltransferases or deacetylases), the activities of which are regulated in part by the concentrations of their required tricarboxylic acid cycle-derived metabolic substrates or cofactors [134,135].
A typical infection triggers immune activation that is characterized by a metabolic shift toward glycolysis.This shift is essential for immune cells to meet the energy demands for their proliferation and the availability of intermediate metabolites that are necessary for effector protein function.Those metabolic shifts are transient; however, in severe or persistent/recurrent infections, their prolongation may contribute to epigenetic changes that eventually might cause immunosuppression.Immunosuppression may result either from epigenetic silencing of cytokine genes or by de-repression of immune inhibitory factors [136].In this context, the observed transition toward glycolysis in individuals with ME/CFS has been suggested to be associated with a combination of factors, including histone deacetylation, a sustained decrease in acetate production through glycolysis, and disturbances in the acetylation of cytoplasmic and mitochondrial enzymes [90].
Histone lactylation is a recently described epigenetic mechanism that involves metabolic intermediates produced during glycolysis and oxidative phosphorylation and is sensitive to lactate levels.Lactate produced from glycolysis can yield lactyl-CoA, which can affect histone lactylation [137,138] and may promote histone H3K27 acetylation [139].Aberrant lactate production and/or regulation in patients with ME/CFS presents as elevated lactate levels that have been confirmed both systemically (and linked to the severity of PEM) [140], and locally in various regions of the brain [24].The cause of lactate deregulation in ME/CFS is not fully understood, though in vitro experiments have demonstrated that serum factors in severe ME/CFS can induce excessive lactate secretion and elevated mitochondrial respiration [141].
Collectively, studies on viruses and ME/CFS suggest that both the viral activity itself and the immune response may contribute to the symptomatology of ME/CFS.Herpesviruses can acquire epigenetic control of their own genomes, but they also alter the epigenetic landscape of the host immune cells [107].Chronic/Recurrent viral infection may stand as a long-term driving force for epigenetic reprograming, which ultimately may lead to immune cell exhaustion, tolerance, and anergy, rendering the host more vulnerable to subsequent infectious triggers [142].

Human endogenous retroviruses (HERVs) and ME/CFS
Human endogenous retroviruses (HERVs) are retroelements that have been acquired during human evolution and comprise a significant portion (8%) of the human genome.Although HERVs are replication deficient, epigenetic, or transcriptional activity of endogenous retroelements is evident both in vitro and in human samples [143].
HERVs have emerged as a significant source of cell type-specific regulatory elements, including promoters, enhancers, chromatin boundary elements, and regulatory RNAs.They play a notable role in regulating immune responses, acting as enhancers and promoters, particularly in interferon-stimulated genes.They constitute a substantial proportion (15%-30%) of enhancers in immune cells, and their regulatory elements are enriched in chromatin landscapes associated with immune cells and immune responses [153].Moreover, HERVs can function as non-coding regulatory RNAs, such as lncRNAs, derived from HERV insertions and implicated in the evolution of non-functional alternative isoforms of genes they promote, potentially regulating protein function through decoy isoforms [153].The dysregulation of HERVs suggests broader regulatory consequences, though not limited to their pathological implications.
Transcriptional activity of HERVs (HERV-K genes) was documented in PBMCs of individuals with ME/CFS [154], whereas epigenetic studies have demonstrated extensive alterations in methylation patterns of non-coding genetic elements [155].
Serology data from our study also support the transcriptional activation of HERV-K following SARS-CoV-2 infection [54].HERVs are kept silenced through various epigenetic mechanisms-mainly DNAm and chromatin modifications-and transcriptional activation has been linked to various immune, neurodegenerative diseases, and cancer [156].However, it is not clear whether HERV activation precedes and contributes to pathology or if it is a bystander effect of the disease state.The virallike nature of HERV-derived products (DNA, RNA, and proteins) allows them to be recognized by different pattern recognition receptors and induces innate immune responses [157].Moreover, specific HERVs (e.g., HERV-K18 and HERV-W) can act as superantigens, causing non-antigen-specific immune hyperactivation that, in the long term, may contribute to immune exhaustion and T-cell unresponsiveness (anergy) [146,158].Immunological abnormalities reported in ME/CFS are consistent with this contradictory co-existence of immune hyperdrive and incompetence.At the same time, HERV expression could be detrimental for the brain due to the toxic effect of HERV envelope proteins on mature neurons, potentially leading to neurocognitive pathologies [159,160].A plausible scenario could be that HERV activationdriven by the requirement of reinforced immune responses-has a collateral effect on the cognitive and neurologic abnormalities in ME/CFS [156].

Epigenetic reprograming in long COVID
The clinical overlap between ME/CFS and long COVID [7] [169], and in most of the studies, it remains only correlative [170][171][172].Still, the effects of active EBV infection could be a potential mechanism contributing to perturbation of immune responses in long COVID [173], potentially involving epigenetic mechanisms.

Limitations of epigenetic studies in ME/CFS
A significant limitation in ME/CFS epigenetic studies stems from the clinical case definitions used for patient diagnosis and study inclusions.Although some studies employ stricter criteria such as the Canadian Consensus Criteria [1], most have utilized the Fukuda criteria [174], which lack key symptoms such as PEM and neurocognitive dysfunction, potentially leading to the inclusion of patients with psychiatric disorders [175].ME/CFS's inherent heterogeneity-with varying disease severity, duration, trigger, and comorbidities (such as FM, postural orthostatic tachycardia syndrome, inflammatory bowel syndrome, Hashimoto thyroiditis, hypermobility, and Ehlers-Danlos syndrome) [3,176]-further complicates epigenetic signatures.Although there is currently no consensus on ME/CFS patient stratification, it is important to take into consideration these factors in the interpretation of epigenetic patterns.Additionally, the condition's sex bias toward females [177] has led to predominantly female study cohorts, potentially introducing bias.The diversity of sample types used adds complexity to the identification of collective findings from epigenetic analysis.Small sample sizes, regional variations, and differences in analytical methods and statistical thresholds also contribute to the variability in collected data, highlighting the need for larger, more standardized, and targeted studies.

Conclusion
The epigenetic landscape in ME/CFS reflects the heterogeneity and varying severity of the symptomatology observed among patients.ME/CFS, like most chronic diseases, arises from an underlying genetic predisposition combined with environmental triggers that disrupt the patient's health baseline.Following the initial trigger, the disease progresses to a chronic state with intermittent relapses and partial recovery, rendering patients more susceptible to subsequent environmental triggers.
The prolonged nature of epigenetic changes implies that the primary trigger of transcriptional regulation does not need to be constantly present.
Virally induced epigenetic changes may persist in the absence of continued viral triggers, as in the case of viral latency (i.e., herpesviruses in ME/CFS) and/or after clearance of the viral genome (following SARS-CoV-2 infection in long COVID cases).Such epigenetic changes could have lasting effects on the deterioration of cellular processes such as immune responses and cellular metabolism.
Although immune suppression temporarily limits the casualties of an overly exuberant immune response, in the long run, it makes individuals more susceptible to secondary infectious triggers.In this scenario, virally induced epigenetic changes may provide a rational explanation for a "hit-andrun" model of infectious-triggered ME/CFS and long COVID.
Epigenetic mechanisms are imperative in determining the outcome of host-virus interactions.Latent viruses such as EBV, which follow a bivalent lifecycle of latency and reactivation, succeed in persisting life-long in the host by modulating the host epigenome and by acquiring their own epigenetic features.In the context of ME/CFS, in which the viral activity of EBV and other herpesviruses is prominent, the investigation of the epigenetic profiles of both host and viral genes is thus crucial.The outlining of temporal alterations that reflect physiological changes, such as symptom diversity and severity, could provide insights into the underlying pathology of ME/CFS.
Currently, there is no standard treatment for ME/CFS patients.However, a few treatment approaches, either in clinical trials or off-label therapies, have shown encouraging results and significant improvement, albeit in certain subsets of patients [2].Epigenetic studies could be a key parameter in patient subtyping, facilitating the determination of the appropriate interventions to target those ME/CFS subgroups that could benefit the most.

Table 1 .
Studies on DNA methylation in patients with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS).

Table 2 .
Studies on RNA-mediated epigenetic elements in patients with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS).