The Role of Non-coding RNAs in Viral Myocarditis

Viral myocarditis (VMC) is a disease characterized as myocardial parenchyma or interstitium inflammation caused by virus infection, especially Coxsackievirus B3 (CVB3) infection, which has no accurate non-invasive examination for diagnosis and specific drugs for treatment. The mechanism of CVB3-induced VMC may be related to direct myocardial damage of virus infection and extensive damage of abnormal immune response after infection. Non-coding RNA (ncRNA) refers to RNA that is not translated into protein and plays a vital role in many biological processes. There is expanding evidence to reveal that ncRNAs regulate the occurrence and development of VMC, which may provide new treatment or diagnosis targets. In this review, we mainly demonstrate an overview of the potential role of ncRNAs in the pathogenesis, diagnosis and treatment of CVB3-induced VMC.


INTRODUCTION
Viral myocarditis (VMC) is a non-ischemic inflammatory disease caused by viral infection and related autoimmune disorders, occupying the vast majority of myocarditis. Epidemiological studies have shown that the incidence of VMC is estimated at 1.0-2.2/million (Vos et al., 2015;Olejniczak et al., 2020). VMC usually presents with chest pain, palpitation and dyspnea, and some patients may progress to heart failure, which has bring huge economic burden on patients and societies (Grun et al., 2012). The gold diagnosis standard of VMC is endomyocardial biopsy (EMB), which has a limited clinical application, so the incidence rate of VMC is likely underestimated. So far, there is no specific blood test has been established to diagnose VMC reliably (Olejniczak et al., 2020). Besides, immunosuppressant, antiviral, and circulatory support are the major treatment methods of VMC, but there still are many patients who have progressed to heart failure after these treatments and thus need heart transplantation (Schultz et al., 2009;Pollack et al., 2015;Tschope et al., 2019). Therefore, the exploration of potentially less traumatic diagnosis methods and new therapeutic targets of VMC is important.
As we all know, lots of viruses can cause VMC, including enterovirus, adenovirus, cytomegalovirus, influenza virus, hepatitis C virus, parvovirus, and other viruses (Fairweather et al., 2012;Lobo et al., 2014;Verdonschot et al., 2016;Minhas et al., 2017;Ntusi, 2017;Spartalis et al., 2017). Coxsackievirus B3 (CVB3) is a single positive strand enterovirus, which is the most common pathogen in VMC etiology (Fairweather et al., 2012). Since the first time using CVB3 to induce myocarditis in mice in 1974 by Woodruff, most of the subsequent experimental models of VMC were induced by CVB3 (Gauntt and Huber, 2003). As the same pattern with many other viruses, CVB3 causes VMC through direct damage to host and indirect injury induced by the abnormal immune response of host immune system (Kindermann et al., 2012;Fung et al., 2016). Therefore, the molecular mechanism research of CVB3 induced VMC model can provide a theoretical basis for the clinical diagnosis and treatment.
Non-coding RNAs (ncRNAs) refer to RNAs that are not translated into protein, including transfer RNA (tRNA), ribosomal RNA (rRNA), small nuclear RNA (snRNA), piwi interacting RNA (piRNA), micro RNA (miRNA), long noncoding RNA (lncRNA), and circular RNA (circRNA) (Panni et al., 2019;Smolle et al., 2019;Zhang W. et al., 2019). ncRNAs were once regarded as transcriptional "garbage." But recent studies suggested that ncRNAs play an important role in viral infection and host antiviral immunity (Chen et al., 2018). It is generally known that a series of pathophysiological changes after the viral infection is determined by the mutual battle between the virus and the host (Barbu et al., 2020). When the virus is in a dominant position, the host is damaged. On the contrary, the host clears the virus through immune response. Therefore, after viral infection, some ncRNAs may change to help the virus escape from the host's immune system, while the other ncRNAs may change to help the host clear the virus (Nicolas, 2017). Future therapies may focus on helping the host clear viruses or decreasing the viral replication by changing the expression of these important ncRNAs. In addition, considering that some ncRNAs of blood will change significantly in the state of disease and have good stability in peripheral circulation, so they have great potential in the field of VMC diagnosis (Regouc et al., 2020;Santos et al., 2020). In this review, we will mainly describe the potential roles of miRNAs, lncRNAs, and circRNAs in the pathogenesis, diagnosis, and treatment of CVB3-induced viral myocarditis.

VMC MODEL INDUCED BY CVB3
Although there is no detailed epidemiological incidence data of CVB3-induced VMC, CVB3 is a leading causative pathogen of VMC (Gauntt and Huber, 2003). Therefore, many teams have studied the formation structure, life cycle, cell infection preference of CVB3 and the CVB3-induced VMC model (Fairweather et al., 2012;Garmaroudi et al., 2015).

Structure of CVB3
CVB3 virus particle is an icosahedral particle without coating, and its diameter is about 30 nm (Gauntt and Huber, 2003). Capsid protein comprises VP1, VP2, VP3, and VP4. Among these, the first three form the external capsid, while the VP4 located at the inner layer of capsid. The viral genome is a linear single-stranded RNA molecule of about 7.4 kb in size. The genome comprises an open reading frame (ORF) in the middle and an untranslated region on both sides. The ORF encodes 11 proteins (four capsid proteins and 2A, 2B, 2C, 3A, 3B, 3C, 3D) (Garmaroudi et al., 2015). Among them, 2A and 3C are viral proteases, which can induce cell death by cutting the host translation process related proteins such as elF4G and DAP5. Besides, 2B and 2C are involved in RNA synthesis, in which 2B can lead to cardiomyocyte death through largely increasing intracellular calcium level caused by inserting itself into endoplasmic reticulum and Golgi membrane. Moreover, 3A and 3B are vital for viral replication (Peischard et al., 2019). 3D acts as an RNA dependent RNA polymerase, which plays an important role in the synthesis of new viral RNA genome (Garmaroudi et al., 2015).

Life Cycle of CVB3
CVB3 life cycle mainly includes three critical steps: virus entry, viral translation and replication, virus assembly and release (Freimuth et al., 2008;Knowlton, 2008;Pinkert et al., 2009). DAF and CAR are membrane proteins required for CVB3 to enter the cell, and the entry mode is internalization (Bergelson et al., 1995;Shafren et al., 1995;Bewley et al., 1999;Coyne and Bergelson, 2006). After virus entry, viral genome transcribes, and translates various functional and structural proteins. The viral RNAs are copied by RNA polymerase 3D protein, and then the copied genome is transferred to the structural protein to assemble complete viral particles. Finally, the virus promotes host cell death to facilitate the release of virus particles.

Tissue Specificity and Gender Preference of CVB3 Infection
CVB3 susceptible cells are mainly distributed in the heart and pancreas, while the lung and kidney are relatively tolerant (Cheung et al., 2005;Harvala et al., 2005). In addition, CVB3 infection has a difference between males and females. Clinically, two-thirds of cases of myocarditis occur in men. It was also found that male mice were more likely to develop CVB3-induced myocarditis than females (Gauntt and Huber, 2003). The sex difference may be related to the different immune responses induced by sex hormones.

Mechanism of CVB3-Induced Myocarditis
As mentioned before, the pathogenesis of CVB3-induced VMC model includes direct myocardial injury by virus and indirect myocardial injury mediated by the immune system. In the process of direct myocardial injury, the functional proteins 2A and 3C of CVB3 block the translation process of host proteins and hinder the metabolism of host cells by cutting the important proteins elF4G and DAP5 (Chau et al., 2007;Lewis et al., 2008). Besides, the breakdown of the endoplasmic reticulum and Golgi apparatus leads to cell death during the process of virus release (van Kuppeveld et al., 1997). In the process of indirect myocardial injury, the virus promotes the release of inflammatory factors by activating innate immune cells (macrophages, NK cells) and acquired immune cells (Th cells). Moreover, the continuous synthesis of virus products in infected host cells can induce the production of cytokines, inflammatory factors and chemokines, thus stimulating the cardiac infiltration of inflammatory cells. In this process, IL-1α/−5 /−6 /−7 can be massively synthesized by infected cardiomyocytes, while IL-1β/−4 /−6 /−8, TNF α, and TNF β can be massively synthesized and released by inflammatory infiltrated leukocytes (Gauntt and Huber, 2003). Subsequently, a large number of inflammatory factors will further promote myocardial injury.

THE ROLE OF miRNAs IN CVB3-INDUCED VMC
MiRNAs are a type of endogenous single-stranded RNAs constructed by about 20-25 nucleotides, which can regulate gene expression by binding to the target mRNAs Ha and Kim, 2014;Jonas and Izaurralde, 2015;Bracken et al., 2016;Gebert and MacRae, 2019;Treiber et al., 2019). With the deeper and deeper research in recent years, the synthesis mechanism and the function of miRNAs are clarified. The synthesis process of miRNAs contains several steps: transcription, cleavage, export, further cleavage, strand selection, and interaction with mRNA. First, the gene encoding miRNA is transcribed into primary miRNA transcripts which named pri-miRNA with cap structure and polyadenylated tail by RNA polymerase II or RNA polymerase III . Then, Pri-miRNA is cleaved into about 70-80 nucleotide precursor miRNAs (pre-miRNAs) with hairpin structure by a protein complex consisted of Drosha and its cofactor Pasha (Lee et al., 2003;Denli et al., 2004;Gregory et al., 2004;Han et al., 2004;Landthaler et al., 2004). Subsequently, pre-miRNAs are transported from the nucleus to the cytoplasm by the transporter exportin-5 (Yi et al., 2003;Bohnsack et al., 2004;Kim, 2004;Lund et al., 2004). The pre-miRNAs are cut by Dicer enzyme to form 21 to 25 nt doublestranded miRNA (miRNA duplex) (Grishok et al., 2001;Ketting et al., 2001). After the unwinding of the double-strand, one strand of miRNA duplex was degraded and the other one became of the mature miRNA which then directed into RNA induced silencing complex (RISC) (Gregory et al., 2005;Kobayashi and Tomari, 2016). At last, mature miRNAs regulate target mRNA expression by binding to the target mRNA. When the miRNAs and target mRNAs are completely complementary, miRNAs can lead to degradation of target mRNAs, and the binding site is usually in the coding region of mRNAs (Lai, 2002). By contrast, when they are not completely complementary, miRNAs can inhibit the translation process by binding to the 3 ′ untranslated region (3 ′ UTR) of target mRNAs, and finally affect the expression of protein (Bartel, 2004). Recently, it has been found that some miRNAs can also promote gene expression in some cases (Xiao et al., 2017). In the course of VMC, miRNAs have been proved to be differentially expressed, which may regulate the disease development by affecting the life cycle of virus and host immune (Figure 1).

The Influence of miRNAs on the Life Cycle of CVB3
Viral infection initiates a battle between virus and host. As a type of micro-organism that completely depends on cells to survive, the integrity of life cycle is of great significance for viral infection and infection expansion. Viruses complete their life cycle depends on controlling the survival of host cells. Before the viral load reaches a certain threshold in the cell, the virus inhibits the apoptosis of the host cell, to facilitate viral replication, translation, and assembly. Once viral load reaches the threshold, the virus promotes cell apoptosis to release the amplified virus particles. At the same time, the body is also making efforts to fight against the virus. In the acute phase of virus infection, the body can reduce the infection by inhibiting virus replication. Recent studies have shown that CVB3 can regulate viral replication and host cell apoptosis by affecting miRNAs expression (Chen et al., 2018).

MiRNAs on the Replication of CVB3
It has been proven that the replication process is indispensable in the viral life cycle, so inhibiting this process may help to block VMC in the early stages. Undoubtedly, many miRNAs are involved in this process ( Table 1; Sodroski et al., 2019;Su et al., 2020). MiRNAs have two patterns to regulate viral replication: one is the direct pattern, and the other is the indirect pattern.
The direct pattern means that miRNAs directly target the viral genome sequence to inhibit or promote viral replication. Wang et al. have found that miR-342-5p may target the 2Ccoding region of the viral genome by bioinformatic analysis. Further experiments confirmed that miR-342-5p can directly inhibit viral replication by targeting the 2C-coding region of CVB3 (Wang et al., 2012). Unlike with most negatively regulated traditional miRNAs, some miRNAs can promote mRNA expression. Wang's group found that miR-10a * directly promotes the viral biosynthesis by targeting 3D-coding region (nt6818-nt6941) of CVB3. Additionally, miR-10a * is abundant in heart of Balb/c mice which indicated that miR-10a * may affect VMC via inducing CVB3 replication (Tong et al., 2013). These findings implied that miRNAs might be a useful treatment by directly limiting viral replication.
The indirect pattern means that miRNAs indirectly control viral replication by targeting the host genome. Many genes, such as zinc finger protein (ZFP)-148, sprout 1 (SPRY1), sproutyrelated EVH1 domain-containing protein 1 (SPRED1), and Etwenty six 1/2 (ETS1/2), regulate replication in viral infection. Studies have shown that the upregulation miRNAs, such as miR-203 and miR-20b, targeted ZFP-148 to promote viral replication and cell survival in CVB3 induced VMC Xu et al., 2017). The miRNAs secreted by cells may also act as complex regulators on viral infection. Germano et al. showed that the levels of miR-590-5p increased in vesicles shed from infected cells, and this miRNA prolong viral replication and enhance cell survival via targeting SPRY1 (Germano et al., 2019). Besides, extracellular signal-regulated kinase 1/2 (ERK1/2) and Wnt/β-catenin pathways were involved in the VMC. CVB3 infection promotes ERK1/2 phosphorylation, which activated ETS transcriptional protein activity, causing miR-126 expression upregulation. Then miR-126 blocked the target gene SPRED1, lipoprotein receptor-related protein 6 (LRP6) and Wnt responsive Cdc42 homolog 1 (WRCH1) to promote the crosstalk with ERK1/2 and Wnt/β-catenin pathways. Finally, the ERK1/2 pathway accelerated the viral replication, and the Wnt/β-catenin pathway promoted virus-mediated cell death and viral release . Therefore, CVB3 infection can induce the upregulation of miR-126 and promote virus replication and release by regulating ERK1/2 and Wnt/β-catenin pathways.
All in all, these studies confirmed that the expression of miR-342-5p, miR-10a * , miR-203, miR-20b, miR-590-5p, and miR-126 could promote viral replication, and the body could also reactively promote the expression of miR-221/-222 to reduce viral replication. In the future, we can reduce CVB3 replication by targeting these miRNAs in VMC treatment.

The Effect of miRNAs on Host Cell Apoptosis During CVB3 Infection
The virus completes its life cycle by controlling apoptosis (Kvansakul, 2017). In the early stage of viral infection, the virus promotes cell survival by inhibiting cell apoptosis to ensure sufficient amplification. In the later stage of infection, the virus promotes cell apoptosis to facilitate release and spread. MiR-34a, miR-222, miR-98, and miR-21 play an important role in cell apoptosis Tong et al., 2019;Zheng et al., 2019;Shen et al., 2020). Therefore, many researchers focus on the role of miRNAs in virus-induced cell apoptosis. Jiang et al. found that inhibition of miR-34a expression largely decreased the level change of apoptosis-related protein, including Bax and Bcl-2 . Sirtuin 1 (SIRT1) was a validated target gene of miR-34a (Castro et al., 2013;Yang et al., 2015;Carloni et al., 2016). Then the researchers confirmed miR-34a act as a pro-apoptotic molecule in VMC via targeting the SIRT1-p53 signaling pathway . In contrast, miR-222, miR-98, and miR-21 may act as anti-apoptotic factors in VMC (He et al., 2013;Zhang B. Y. et al., 2016;. Zhang et al. found that the expression of adenosine deaminase, RNA-specific (ADAR1) and miR-222 was increased in CVB3 induced VMC model, and the study in vitro showed that ADAR1 combined Dicer increased cell viability by inducing miR-222 synthesis which decreased the confirmed target phosphatase and tensin homolog (PTEN) expression . Previous studies have proven that PTEN is an apoptosis protein (Dupont et al., 2002;Lin et al., 2007; Cheng et al., 2009). Hence, ADAR1-Dicer Complex may inhibit cell apoptosis by miR-222 in CVB3 induced VMC. Additionally, MiR-98 decreased cell apoptosis by targeting the FAS/FASL gene (Zhang B. Y. et al., 2016), and miR-21 reduced cell apoptosis by targeting programmed cell death 4 (PDCD4) (He et al., 2013), a well-known apoptosis gene (Gaur et al., 2011;Stagakis et al., 2011;Junker et al., 2015). Furthermore, another study confirmed that miR-21 reduced cell apoptosis by targeting mitogen-activated protein kinase kinase 3 (MAP2K3) . Interestingly, although both studies showed that miR-21 has an anti-apoptosis effect, the expression trend of miR-21 was not consistent after CVB3 infection in different studies ( Table 2). The conflicting results may be related to the different viral strains used and different sample acquisition time in two studies. All in all, these studies show that the virus can complete its life cycle by influencing the level of miRNAs to regulate host cell apoptosis.
In the future, we can target these miRNAs to destroy the life cycle of the virus to reduce body injury.

MiRNAs on Host Immune and Inflammatory Response
The CVB3-induced immune response is very complex, including the activation of innate immunity and acquired immunity. In the early stage of CVB3 infection, innate immune-related cells (macrophages and NK cells) infiltrate the heart, producing chemokines and Interferon (IFNs). CVB3 infection can promote the polarization of M2 macrophages toward M1 macrophages, which increases the inflammation Gou et al., 2018). Besides, the 2A protein of CVB3 inhibits the production of IFNs by targeting TLR3 and MDA5/RIF-I pathways, which are vital pathways in host antiviral immunity (Lind et al., 2016). In the subsequent stage, acquired immunerelated cells (Th1, Th2, and Th17 cells) are activated to produce cytokines and inflammatory factors (Gauntt and Huber, 2003). Th1 cells mainly produce IFN-γ, which can promote inflammation. Moreover, Th2 cells mainly produce IL-10, which has an anti-inflammatory effect. Furthermore, Th17 cells mainly produce IL-17, which plays a key role in promoting CVB3 induced infection (Garmaroudi et al., 2015). In recent years, researchers also focus on the field that miRNAs participate in the inflammation process by activating innate and acquired immune responses during CVB3 infection ( Table 3).

MiRNAs on T Cells and Macrophages During CVB3 Infection
Monocytes-macrophages and T cells are representative cells in innate and adaptive immune in VMC, respectively (Gauntt and Huber, 2003). Monocytes differentiate into macrophages after they reach the tissues, among which M1 macrophages have the effect of promoting inflammation, while M2 macrophages have the effect of anti-inflammation (Huang et al., 2018a). Th cells (mainly including Th1, Th2, and Th17 cells) also play a role in CVB3 infection (Garmaroudi et al., 2015). Previous studies have shown that IFN-γ knockout mice are free from VMC after CVB3 treatment, while IFN-γ overexpression is susceptible (Huber and Sartini, 2005). Therefore, Th1 cells can secrete IFNγ to promote the development of VMC. Besides, IL-10 is a vital anti-inflammatory factor, and Th2 cells mainly secrete IL-10 to act as an anti-inflammatory role. As we all know, Th17 cells can secrete IL-17. Previous studies have found that inhibition of IL-17 by neutralizing antibodies can reduce viral replication and myocardial damage in myocarditis, suggesting that Th17 cells are important pro-inflammatory cells (Fan et al., 2011). Proinflammatory factors, such as miR-21 and miR-146b, have been identified that were increased in VMC. Inhibition of miR-21 or miR-146b decreased Th17 cells and relieved myocardial injury by targeting retinoid-related orphan receptor gamma-t (ROR-γt) . Moreover, miR-155 also acts as a pro-inflammatory factor in some cases and it was upregulated in CVB3 induced VMC model and human myocarditis specimen (Corsten M.F. et al., 2012;. A research conducted by Corsten et al. illustrated that inhibition of miR-155 relieved cardiac injury by inhibiting macrophage infiltration and T cell activation, and the direct target of miR-155, purinerich box 1 (PU.1), may function in this process (Corsten M.F. et al., 2012). A further study directed by Zhang et al. showed that inhibition of miR-155 increased M2 macrophages and decreased M1 macrophages to attenuate cardiac inflammatory . However, Bao et al. found that miR-155 acts as an anti-inflammatory factor to reduce cardiac injury by targeting v-rel avian reticuloendotheliosis viral oncogene homolog A (RelA) (Bao and Lin, 2014), a vital component of nuclear factor kappa-B (NF-κB) inflammatory pathways (Gasparini et al., 2014;Mukherjee et al., 2015;Kabacaoglu et al., 2019). These contradictory results may be due to the use of different models. Moreover, miR-223, which also acts as an anti-inflammatory factor, has been verified that was decreased in VMC. Forced expression of miR-223 activated M1 macrophages toward M2 anti-inflammatory phenotype through targeting PBX/knotted 1 homeobox 1 (PKNOX1), and thus relieving myocardial inflammation (Gou et al., 2018). Therefore, CVB3 infection can promote immune cells-mediated inflammation by regulating the expression of miRNAs. In the future, we can reduce the degree of myocardial inflammation by targeting these miRNAs.
Potentiality of miRNAs in the Treatment of CVB3-Induced VMC As mentioned above, after CVB3 infection, miRNAs function by influencing the viral life cycle and immune response. Therefore, miRNAs may be potential therapeutic targets for VMC. For example, miRNAs such as miR-342-5p, miR-10a * , miR-203, miR-20b, miR-590-5p, and miR-126 can directly or indirectly affect virus replication. We can design inhibitors or analogs of these miRNAs to inhibit CVB3 replication, thereby reducing the viral load in infected host cells. In addition, miRNAs such as miR-21, miR-146b, miR-233, miR-155, miR-27b, miR-30a, miR-181d, miR-214, miR-10a, miR-381, and miR-133b can regulate the CVB3-induced inflammatory response by affecting the differentiation of immune cells or the release of cytokines, so we can design inhibitors or analogs of miRNAs to reduce myocardial inflammation and damage. Clinical trials of miRNAs based therapeutics have been used in other diseases, so it may become a reliable treatment for VMC in the future (Beg et al., 2017). Although miRNAs-based therapeutics might be a promising treatment in VMC, there are still some problems. First, finding the method to help the miRNAs targeted transport to the heart is critical. MiRNAs can reach all organs of the body through blood circulation, which may cause other organs injury under miRNAs untargeted transporting. Second, one miRNA can target multiple genes at the same time, and one gene may be controlled by multiple miRNAs. MiRNAs may hence have other unpredictable side effects. Thirdly, a large number of miRNAs are only studied in animals. As we all know, mice and the human body have great differences. MiRNAs with functions in mice may not work in human body. Finally, miRNAs are also expressed in physiological state, so the determination of its therapeutic dose is also a difficulty. Therefore, further research may be needed to solve these problems.

Potentiality of miRNAs in VMC Diagnosis
In VMC, miRNAs are differentially expressed not only in cells but also in blood. The stability of miRNAs in the blood can be maintained by binding RNA binding proteins and enveloping them within extracellular vesicles (Santos et al., 2020). There has been other disease using miRNAs as candidate diagnostic tool , but VMC has not yet been. Therefore, miRNAs in the blood may be used as a disease marker for VMC. Expression change of miRNAs in the circulation may help us assess the severity of VMC. Wang et al. evaluated the predictive value of miR-1 and miR-146b in VMC. MiR-1 decreased significantly in VMC, which was negatively correlated with the left ventricular shortening fraction and left ventricular ejection fraction. MiR-146b was significantly up-regulated in VMC, which was positively correlated with those two indexes (Wang D. et al., 2016). Therefore, miR-1 and miR-146b may be potential disease markers of VMC. Moreover, Corsten et al. Found that miR-208b and miR-499 were significantly increased in VMC, and correlated with blood troponin levels (Corsten et al., 2010). Furthermore, in a study on the correlation between miRNAs and child VMC pathology, the level of miRNA-208a increased significantly in the acute phase of the disease. Compared with the chronic phase, the level of miRNA-21 in the acute phase was also significantly higher (Goldberg et al., 2018). These studies indicated that miR-208a, miR-21, miR-1, miR-146b, miR-208b, and miR-499 will be promising biomarkers for VMC diagnosis. Compared with EMB, circulating miRNAs have the characteristics of less trauma, low cost, good sensitivity, and high specificity, so they may be used as potential disease markers for VMC differential diagnosis in the future. However, as a disease marker, miRNAs also meet some challenges. First of all, the quantitative methods, units of measurement and thresholds of miRNAs are different in different laboratories, which may limit their further use.
Secondly, different methods of extracting miRNAs in different laboratories may produce contradictory results. Thirdly, the level of miRNAs has been fluctuating in different stage of VMC, so the point in time of obtaining miRNAs still needs further study.

THE ROLE OF lncRNAs IN VMC
LncRNAs belongs to non-coding RNAs with a length longer than 200 nucleotides (Li J. et al., 2018;Barangi et al., 2019;Chi et al., 2019). In many ways, lncRNAs are similar to mRNAs. However, compared with mRNAs, the transcription number and species conservation of lncRNA is low. The secondary structure of lncRNA, such as hairpin structure and stem ring structure formed by post-transcriptional modification, interacts with chromatin and protein to function. Based on the relative position to the protein-coding gene, lncRNAs can be divided into the following five categories: bidirectional lncRNA, the intron lncRNA, intergenic lncRNA, sense lncRNA, and antisense lncRNA. LncRNAs function mainly through chromatin remodeling, transcription and post-transcriptional regulation to influence gene expression (Atianand and Fitzgerald, 2014;Quinn and Chang, 2016;Dykes and Emanueli, 2017;Huang et al., 2018b). Previous studies have found that lncRNAs are involved in many pathophysiological processes, such as growth and development, immune, inflammation and tumor Alessio et al., 2019;Chi et al., 2019;Wang et al., 2019;Zhang K. et al., 2019).
Liu et al. confirmed that lncRNAs were differentially expressed in child acute fulminant myocarditis. Among them, 1645 lncRNAs were up-regulated and 1456 lncRNAs were downregulated. Further bioinformatic analysis showed that T cell activation, T cell receptor complex, negative regulation of complement activation, immune response and T-helper 17 cell differentiation process may participate in the progress of VMC . In addition, Zhang et al. recently found that lncRNA Ak085865 can promote the expression of anti-inflammatory M2 macrophages, reduce the number of proinflammatory M1 macrophages. LcRNA Ak085865 knockout mice were more susceptible to CVB3 induced VMC, suggesting that lncRNA Ak085865 may play a vital role in the pathogenesis of VMC by affecting macrophage polarization . Furthermore, in isoproterenol-induced myocardial fibrosis model of VMC, lncRNA ROR promotes myocardial fibrosis by regulating the expression of C-Myc, suggesting that lncRNA act a vital factor in the chronicity of VMC (Zhang and Sun, 2019). From these existing studies, we can conclude that lncRNAs may be a potential therapeutic target by influencing macrophage polarization and myocardial fibrosis. However, there are few studies on lncRNAs in VMC, and more studies are needed to elucidate the role of lncRNAs.

THE ROLE OF CIRCRNAS IN VMC
CircRNAs are non-coding RNAs widely expressed in eukaryotic cells, and different from the traditional linear RNAs because of they are circular molecules (Kristensen et al., 2019;Liu C. et al., 2019;Vo et al., 2019;Wilusz, 2019;Zlotorynski, 2019). The characteristics of circRNA are high abundance, structure stable and highly tissue-specific expression. They are mainly produced by the splicing of exon or intron sequences. Besides, reverse complementary sequences or RNA binding proteins (RBPs) are necessary for circRNAs production (Kristensen et al., 2019). Recent studies have shown that circRNAs act as a miRNA sponge to adsorb miRNA, protein sponge, protein scaffold and coding small peptide to complete their biological function (Xie et al., 2018;Zhang M. et al., 2018;Dong et al., 2019;Jiang and Ning, 2019;Liu J. et al., 2019;Lu et al., 2019). Many researchers have found that circRNAs play an important role in cardiovascular diseases such as myocardial infarction, heart failure, and atherosclerosis (Wang K. et al., 2016;Wang et al., 2017;Zhang F. et al., 2018). Zhang et al. confirmed that circRNAs were differentially expressed in fulminant myocarditis, and further bioinformatic analysis showed that many inflammatory or immune pathways, including TNF signaling pathway, Th1 and Th2 cell differentiation and T cell receptor signaling pathway, were involved in VMC . Therefore, circRNAs may be a new therapeutic target for VMC by regulating the immune-inflammatory pathway. In addition, considering the stability of circRNAs, circulating circRNAs are also potential biomarkers for VMC. However, there is no detailed study on the mechanism of circRNAs affecting the development of VMC.

CONCLUSIONS AND PERSPECTIVE
VMC is characterized as a localized or diffuse disease of myocardial parenchyma or interstitium caused by virus infection. The mechanism of VMC may be related to direct myocardial damage of virus infection and extensive damage of abnormal immune response after infection. Although large progress on VMC has been obtained in recent decades, the diagnosis, and treatment of viral myocarditis are still facing great challenges. With the discovery of more and more non-coding RNAs, non-coding RNAs have become the vital molecules of gene regulation in our cognitive range now. In the progress of VMC, non-coding RNAs regulate virus life cycle, immune and inflammatory response by targeting virus or host genes. By reviewing the mechanism of ncRNAs in CVB3-induced myocarditis, we summarized the non-coding RNA related potential disease markers and therapeutic targets of VMC. MiRNAs of VMC have been largely studied by researchers. However, the lncRNA and circRNA field of VMC is still virgin land to be developed. In the future, our researchers should pay more attention to the lncRNA and circRNA area.

AUTHOR CONTRIBUTIONS
ZY, CZ, YX, ZP, and LZ conceptualized the study. CZ and YX prepared the draft. CZ, YX, and ZY reviewed and edited the manuscript. CZ, YX, and ZL contributed to the visualization.
ZY and HZ supervised the study. All authors contributed to the article and approved the submitted version.

FUNDING
This study was supported by the grants from the National Natural Science Foundation of China (81670220), the project of Guangdong Provincial Science and Technology Plan (2015A020212013), the Science and Technology Project of Guangzhou City (201803010008), and the International Scientific and Technological Cooperation Project of Guangzhou Economic and Technological Development Zone (2017GH13).