In vivo expression patterns of microRNAs of Gallid herpesvirus 2 (GaHV-2) during the virus life cycle and development of Marek's disease lymphomas

OncomiR mdv1-miR-M7-5p promotes avian lymphomatosis by modulating the BCL2/Bax mitochondrial apoptosis signaling pathway

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that infects poultry and causes fatal lymphomas in infected chickens. Notably, the mdv1-miR-M7-5p, a pivotal oncomiR encoded by MDV, is closely associated with viral replication and latency. Here, mdv1-miR-M7-5p was transfected into the chicken lymphoma cell line MSB1, which resulted in the inhibition of lymphoma cell apoptosis and an increase in lymphoma cell proliferation and migration. Additionally, the expression of the tumor suppressor genes p53 and ARRDC3 were significantly downregulated, while the MDV latency-associated genes such as ICP4 and ICP27 were significantly upregulated. The BCL2/Bax ratio was increased while the expression of genes involved in the apoptotic signaling pathway were decreased. Furthermore, our mitochondrial function experiments in MSB1 cells demonstrated that mdv1-miR-M7-5p enhanced mitochondrial ATP release and altered the mitochondrial membrane potential, thereby affecting mitochondrial function and inhibiting lymphoma cell apoptosis. Dual-luciferase assays revealed that mdv1-miR-M7-5p binds to caspase-6. For the in vivo study, a cholesterol-modified inhibitor of mdv1-miR-M7-5p was administered to chickens. Inhibition of mdv1-miR-M7-5p resulted in a lower mortality rate than that in the control groups. Furthermore, the expression levels of the cytokines interferon-gamma (IFN-γ), interleukin (IL)-4, and IL-17 in the plasma of MDV-infected chickens were significantly increased. A marked increase was observed in apoptosis in the spleen tissues, and the expression of apoptosis-related genes including caspase-3 and tumor suppressor gene PTEN in immune organs, including the spleen, bursa of Fabricius, and thymus, were markedly upregulated. In summary, the oncogenic miRNA mdv1-miR-M7-5p promotes MDV latency and may facilitate lymphoma formation by mediating the BCL2/CytC signaling pathway. This mediation enhances mitochondrial function and inhibits lymphoma cell apoptosis, thereby contributing to lymphoma development.

Viral miRNA delivered by exosomes from Marek's disease virus-transformed lymphoma cell line exerts regulatory function in internalized primary chicken embryo fibroblast cells

In the past decade, research has demonstrated that viral miRNAs encoded by a number of viral genomes, particularly by most of the herpesvirus including Marek's disease virus (MDV), play important regulatory roles in viral infection, replication, and regulation of tumorigenesis. As macrovesicles in cells, exosomes can deliver viral miRNAs and exert gene regulatory functions. Whether the exosomes play a role in the replication, pathogenesis/tumorigenesis of avian herpesviruses such as oncogenic Marek's disease virus (MDV) remains unclear. Herein we extracted and identified the exosomes from MDV-transformed T cell line MSB-1 and demonstrated high abundance of MDV-1 miRNA expression. Using dual luciferase-based reporter assay, we also demonstrated that the exosomes derived from MSB-1 can deliver functional miRNA successfully into primary chicken embryo fibroblasts. These findings provide new insights into the role of exosomes and the mechanisms of how virus-encoded miRNA function in MDV latency/activation switching, viral replication, pathogenesis and/or tumorigenesis.

Critical roles of non-coding RNAs in lifecycle and biology of Marek's disease herpesvirus

Over the past two decades, numerous non-coding RNAs (ncRNAs) have been identified in different biological systems including virology, especially in large DNA viruses such as herpesviruses. As a representative oncogenic alphaherpesvirus, Marek's disease virus (MDV) causes an important immunosuppressive and rapid-onset neoplastic disease of poultry, namely Marek's disease (MD). Vaccinations can efficiently prevent the onset of MD lymphomas and other clinical disease, often heralded as the first successful example of vaccination-based control of cancer. MDV infection is also an excellent model for research into virally-induced tumorigenesis. Recently, great progress has been made in understanding the functions of ncRNAs in MD biology. Herein, we give a review of the discovery and identification of MDV-encoded viral miRNAs, focusing on the genomics, expression profiles, and emerging critical roles of MDV-1 miRNAs as oncogenic miRNAs (oncomiRs) or tumor suppressor genes involved in the induction of MD lymphomas. We also described the involvements of host cellular miRNAs, lincRNAs, and circRNAs participating in MDV life cycle, pathogenesis, and/or tumorigenesis. The prospects, strategies, and new techniques such as the CRISPR/Cas9-based gene editing applicable for further investigation into the ncRNA-mediated regulatory mechanisms in MDV pathogenesis/oncogenesis were also discussed, together with the possibilities of future studies on antiviral therapy and the development of new efficient MD vaccines.

Genetic resilience in chickens against bacterial, viral and protozoal pathogens

The genome contributes to the uniqueness of an individual breed, and enables distinctive characteristics to be passed from one generation to the next. The allelic heterogeneity of a certain breed results in a different response to a pathogen with different genomic expression. Disease resistance in chicken is a polygenic trait that involves different genes that confer resistance against pathogens. Such resistance also involves major histocompatibility (MHC) molecules, immunoglobulins, cytokines, interleukins, T and B cells, and CD4+ and CD8+ T lymphocytes, which are involved in host protection. The MHC is associated with antigen presentation, antibody production, and cytokine stimulation, which highlight its role in disease resistance. The natural resistance-associated macrophage protein 1 (Nramp-1), interferon (IFN), myxovirus-resistance gene, myeloid differentiation primary response 88 (MyD88), receptor-interacting serine/threonine kinase 2 (RIP2), and heterophile cells are involved in disease resistance and susceptibility of chicken. Studies related to disease resistance genetics, epigenetics, and quantitative trait loci would enable the identification of resistance markers and the development of disease resistance breeds. Microbial infections are responsible for significant outbreaks and have blighted the poultry industry. Breeding disease-resistant chicken strains may be helpful in tackling pathogens and increasing the current understanding on host genetics in the fight against communicable diseases. Advanced technologies, such as the CRISPR/Cas9 system, whole genome sequencing, RNA sequencing, and high-density single nucleotide polymorphism (SNP) genotyping, aid the development of resistant breeds, which would significantly decrease the use of antibiotics and vaccination in poultry. In this review, we aimed to reveal the recent genetic basis of infection and genomic modification that increase resistance against different pathogens in chickens.

Role of microRNA and long non-coding RNA in Marek's disease tumorigenesis in chicken

Marek's disease virus (MDV), the causative agent of Marek's disease (MD), results in highly infectious phymatosis, lymphatic tissue hyperplasia, and neoplasia. MD is associated with high morbidity and mortality rate. Non-coding RNAs (ncRNAs) entails long non-coding RNA (lncRNA) and microRNA (miRNA). Numerous studies have reported that specific miRNAs and lncRNAs participate in multiple cellular processes, such as proliferation, migration, and tumor cell invasion. Specialized miRNAs and lncRNAs militate a similar role in MD tumor oncogenesis. Despite its growing popularity, only a few reviews are available on ncRNA in MDV tumor oncogenes. Herein, we summarized the role of the miRNAs and lncRNAs in MD tumorigenesis. Altogether, we brought forth the research issues, such as MD prevention, screening, regulatory network formation, novel miRNAs, and lncRNAs analysis in MD that needs to be explored further. This review provides a theoretical platform for the further analysis of miRNAs and lncRNAs functions and the prevention and control of MD and malignancies in domestic animals.

Virus-encoded miR-155 ortholog in Marek's disease virus promotes cell proliferation via suppressing apoptosis by targeting tumor suppressor WWOX

Marek's disease virus serotype 1 (MDV-1) is an important oncogenic α-herpesvirus that induces immunosuppressive and rapid-onset T-cell lymphomatous disease in poultry commonly referred to as Marek's disease (MD). As an excellent biomodel for the study of virally-induced cancers in natural hosts, MDV-1 encoded microRNAs (miRNAs) have been previously demonstrated with the potential roles to act as critical regulators in virus replication, latency, pathogenesis and especially in oncogenesis. Similar to the oncogenic γ-herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV), miR-M4-5p, the cellular microRNA-155 (miR-155) ortholog encoded by MDV-1, is also involved in MD oncogenesis. In lymphoblastoid cell lines derived from MDV-induced T-cell lymphomas, miR-M4-5p has been shown to be highly expressed and participate in inducing MD lymphomagenesis by regulating multiple signal pathways. Herein we report the new identification of the host WW domain-containing oxidoreductase (WWOX) as a biological target for miR-M4-5p. Further experiments revealed that as a critical oncomiRNA, miR-M4-5p promotes the proliferations of both chicken embryo fibroblast (CEF) and MSB-1 cells via suppressing cell apoptosis by targeting WWOX, a well-known tumor suppressor. Our data presents a novel insight in elucidating the regulatory mechanisms mediated by the viral analog of miR-155 that potentially contribute to MD tumorigenesis.

Marek's Disease Virus (Gallid alphaherpesvirus 2)-Encoded miR-M2-5p Simultaneously Promotes Cell Proliferation and Suppresses Apoptosis Through RBM24 and MYOD1-Mediated Signaling Pathways

MicroRNAs (miRNAs) have been demonstrated for their involvement in virus biology and pathogenesis, including functioning as key determinants of virally-induced cancers. As an important oncogenic α-herpesvirus affecting poultry health, Marek’s disease virus serotype 1 [Gallid alphaherpesvirus 2 (GaHV-2)] induces rapid-onset T-cell lymphomatous disease commonly referred to as Marek’s disease (MD), an excellent biological model for the study of virally-induced cancer in the natural hosts. Previously, we have demonstrated that GaHV-2-encoded miRNAs (especially those within the Meq-cluster) have the potential to act as critical regulators of multiple processes such as virus replication, latency, pathogenesis, and/or oncogenesis. In addition to miR-M4-5p (miR-155 homolog) and miR-M3-5p, we have recently found that miR-M2-5p possibly participate in inducing MD lymphomagenesis. Here, we report the identification of two tumor suppressors, the RNA-binding protein 24 (RBM24) and myogenic differentiation 1 (MYOD1), being two biological targets for miR-M2-5p. Our experiments revealed that as a critical miRNA, miR-M2-5p promotes cell proliferation via regulating the RBM24-mediated p63 overexpression and MYOD1-mediated IGF2 signaling and suppresses apoptosis by targeting the MYOD1-mediated Caspase-3 signaling pathway. Our data present a new strategy of a single viral miRNA exerting dual role to potentially participate in the virally-induced T-cell lymphomagenesis by simultaneously promoting the cell proliferation and suppressing apoptosis.

Efficient Mutagenesis of Marek's Disease Virus-Encoded microRNAs Using a CRISPR/Cas9-Based Gene Editing System

The virus-encoded microRNAs (miRNAs) have been demonstrated to have important regulatory roles in herpesvirus biology, including virus replication, latency, pathogenesis and/or tumorigenesis. As an emerging efficient tool for gene editing, the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 system has been successfully applied in manipulating the genomes of large DNA viruses. Herein, utilizing the CRISPR/Cas9 system with a double-guide RNAs transfection/virus infection strategy, we have established a new platform for mutagenesis of viral miRNAs encoded by the Marek's disease virus serotype 1 (MDV-1), an oncogenic alphaherpesvirus that can induce rapid-onset T-cell lymphomas in chickens. A series of miRNA-knocked out (miR-KO) mutants with deletions of the Meq- or the mid-clustered miRNAs, namely RB-1B∆Meq-miRs, RB-1B∆M9-M2, RB-1B∆M4, RB-1B∆M9 and RB-1B∆M11, were generated from vvMDV strain RB-1B virus. Interestingly, mutagenesis of the targeted miRNAs showed changes in the in vitro virus growth kinetics, which is consistent with that of the in vivo proliferation curves of our previously reported GX0101 mutants produced by the bacterial artificial chromosome (BAC) clone and Rec E/T homologous recombination techniques. Our data demonstrate that the CRISPR/Cas9-based gene editing is a simple, efficient and relatively nondisruptive approach for manipulating the small non-coding genes from the genome of herpesvirus and will undoubtedly contribute significantly to the future progress in herpesvirus biology.

RNA Sequencing revealed differentially expressed genes functionally associated with immunity and tumor suppression during latent phase infection of a vv + MDV in chickens

Very virulent plus Marek's disease (MD) virus (vv + MDV) induces tumors in relatively resistant lines of chickens and early mortality in highly susceptible lines of chickens. The vv + MDV also triggers a series of cellular responses in both types of chickens. We challenged birds sampled from a highly inbred chicken line (line 6₃) that is relatively resistant to MD and from another inbred line (line 7₂) that is highly susceptible to MD with a vv + MDV. RNA-sequencing analysis was performed with samples extracted from spleen tissues taken at 10-day and 21-day post infection (dpi). A total of 64 and 106 differentially expressed genes was identified in response to the vv + MDV challenge at latent phase in the resistant and susceptible lines of chickens, respectively. Direct comparisons between samples of the two lines identified 90 and 126 differentially expressed genes for control and MDV challenged groups, respectively. The differentially expressed gene profiles illustrated that intensive defense responses were significantly induced by vv + MDV at 10 dpi and 21 dpi but with slight changes in the resistant line. In contrast, vv + MDV induced a measurable suppression of gene expression associated with host defense at 10 dpi but followed by an apparent activation of the defense response at 21 dpi in the susceptible line of chickens. The observed difference in gene expression between the two genetic lines of chickens in response to MDV challenge during the latent phase provided a piece of indirect evidence that time points for MDV reactivation differ between the genetic lines of chickens with different levels of genetic resistance to MD. Early MDV reactivation might be necessary and potent to host defense system readiness for damage control of tumorigenesis and disease progression, which consequently results in measurable differences in phenotypic characteristics including early mortality (8 to 20 dpi) and tumor incidence between the resistant and susceptible lines of chickens. Combining differential gene expression patterns with reported GO function terms and quantitative trait loci, a total of 27 top genes was selected as highly promising candidate genes for genetic resistance to MD. These genes are functionally involved with virus process (F13A1 and HSP90AB1), immunity (ABCB1LB, RGS5, C10ORF58, OSF-2, MMP7, CXCL12, GAL1, GAL2, GAL7, HVCN1, PDE4D, IL4I1, PARP9, EOMES, MPEG1, PDK4, CCLI10, K60 and FST), and tumor suppression (ADAMTS2, LXN, ARRDC3, WNT7A, CLDN1 and HPGD). It is anticipated that these findings will facilitate advancement in the fundamental understanding on mechanisms of genetic resistance to MD. In addition, such advancement may also provide insights on tumor virus-induced tumorigenesis in general and help the research community recognize MD study may serve as a good model for oncology study involving tumor viruses.

A Tiny RNA that Packs a Big Punch: The Critical Role of a Viral miR-155 Ortholog in Lymphomagenesis in Marek's Disease

MicroRNAs (miRNAs) are small non-coding RNAs that have been identified in animals, plants, and viruses. These small RNAs play important roles in post-transcriptional regulation of various cellular processes, including development, differentiation, and all aspects of cancer biology. Rapid-onset T-cell lymphoma of chickens, namely Marek’s disease (MD), induced by Gallid alphaherpesvirus 2 (GaHV2), could provide an ideal natural animal model for herpesvirus-related cancer research. GaHV2 encodes 26 mature miRNAs derived from 14 precursors assembled in three distinct gene clusters in the viral genome. One of the most highly expressed GaHV2 miRNAs, miR-M4-5p, shows high sequence similarity to the cellular miR-155 and the miR-K12-11 encoded by Kaposi’s sarcoma-associated herpesvirus, particularly in the miRNA “seed region.” As with miR-K12-11, miR-M4-5p shares a common set of host and viral target genes with miR-155, suggesting that they may target the same regulatory cellular networks; however, differences in regulatory function between miR-155 and miR-M4-5p may distinguish non-viral and viral mediated tumorigenesis. In this review, we focus on the functions of miR-M4-5p as the viral ortholog of miR-155 to explore how the virus mimics a host pathway to benefit the viral life cycle and trigger virus-induced tumorigenesis.

Putative roles as oncogene or tumour suppressor of the Mid-clustered microRNAs in Gallid alphaherpesvirus 2 (GaHV2) induced Marek's disease lymphomagenesis

In the last decade, numerous microRNAs (miRNAs) have been identified in diverse virus families, particularly in herpesviruses. Gallid alphaherpesvirus 2 (GaHV2) is a representative oncogenic alphaherpesvirus that induces rapid-onset T-cell lymphomas in its natural hosts, namely Marek’s disease (MD). In the GaHV2 genome there are 26 mature miRNAs derived from 14 precursors assembled into three clusters, namely the Meq-cluster, Mid-cluster and LAT-cluster. Several GaHV2 miRNAs, especially those in the Meq-cluster (e.g. miR-M4-5p), have been demonstrated to be critical in MD pathogenesis and/or tumorigenesis. Interestingly the downstream Mid-cluster is regulated and transcribed by the same promoter as the Meq-cluster in the latent phase of the infection, but the role of these Mid-clustered miRNAs in GaHV2 biology remains unclear. We have generated the deletion mutants of the Mid-cluster and of its associated individual miRNAs in GX0101 virus, a very virulent GaHV2 strain, and demonstrated that the Mid-clustered miRNAs are not essential for virus replication. Using GaHV2-infected chickens as an animal model, we found that, compared with parental GX0101 virus, the individual deletion of miR-M31 decreased the mortality and gross tumour incidence of infected chickens while the deletion individually of miR-M1 or miR-M11 unexpectedly increased viral pathogenicity or oncogenicity, similarly to the deletion of the entire Mid-cluster region. More importantly, our data further confirm that miR-M11-5p, the miR-M11-derived mature miRNA, targets the viral oncogene meq and suppresses its expression in GaHV2 infection. We report here that members of the Mid-clustered miRNAs, miR-M31-3p and miR-M11-5p, potentially act either as oncogene or tumour suppressor in MD lymphomagenesis.

MicroRNAs in large herpesvirus DNA genomes: recent advances

MicroRNAs (miRNAs) are small non-coding RNAs (ncRNAs) that regulate gene expression. They alter mRNA translation through base-pair complementarity, leading to regulation of genes during both physiological and pathological processes. Viruses have evolved mechanisms to take advantage of the host cells to multiply and/or persist over the lifetime of the host. Herpesviridae are a large family of double-stranded DNA viruses that are associated with a number of important diseases, including lymphoproliferative diseases. Herpesviruses establish lifelong latent infections through modulation of the interface between the virus and its host. A number of reports have identified miRNAs in a very large number of human and animal herpesviruses suggesting that these short non-coding transcripts could play essential roles in herpesvirus biology. This review will specifically focus on the recent advances on the functions of herpesvirus miRNAs in infection and pathogenesis.

Hyperediting by ADAR1 of a new herpesvirus lncRNA during the lytic phase of the oncogenic Marek's disease virus

Marek's disease virus, or Gallid herpesvirus 2 (GaHV-2), is an avian alphaherpesvirus that induces T-cell lymphoma in chickens. During transcriptomic studies of the RL region of the genome, we characterized the 7.5 kbp gene of the ERL lncRNA (edited repeat-long, long non-coding RNA), which may act as a natural antisense transcript (NAT) of the major GaHV-2 oncogene meq and of two of the three miRNA clusters. During infections in vivo and in vitro, we detected hyperediting of the ERL lncRNA that appeared to be directly correlated with ADAR1 expression levels. The ERL lncRNA was expressed equally during the lytic and latent phases of infection and during viral reactivation, but its hyperediting increased only during the lytic infection of chicken embryo fibroblasts. We also showed that chicken ADAR1 expression was controlled by the JAK/STAT IFN-response pathway, through an inducible promoter containing IFN-stimulated response elements that were functional during stimulation with IFN-α or poly(I:C). Like the human and murine miR-155-5p, the chicken gga-miR-155-5p and the GaHV-2 analogue mdv1-miR-M4-5p deregulated this pathway by targeting and repressing expression of suppressor of cytokine signalling 1, leading to the upregulation of ADAR1. Finally, we hypothesized that the natural antisense transcript role of the ERL lncRNA could be disrupted by its hyperediting, particularly during viral lytic replication, and that the observed deregulation of the innate immune system by mdv1-miR-M4-5p might contribute to the viral cycle.

Chicken gga-miR-181a targets MYBL1 and shows an inhibitory effect on proliferation of Marek's disease virus-transformed lymphoid cell line

Marek's disease (MD), caused by Marek's disease virus (MDV), is a lymphoproliferative neoplastic disease of chickens and is characterized by MD lymphoma in multiple visceral organs of chicken. It causes great damage to poultry health. Recently, miRNA has been reported to be involved in Marek's disease lymphomagenesis. Our previous study showed that gga-miR-181a was downregulated in MDV-induced lymphoma, and its target gene, v-myb myeloblastosis viral oncogene homolog-like 1 (MYBL1), was predicted. In this study, the interaction between gga-miR-181a and MYBL1 was further verified by detecting protein expression levels of MYBL1 after transfecting miR-181a mimic into MD lymphoma cell line, MSB1. The result showed that protein level of MYBL1 was lower in gga-miR-181a mimic transfecting group than that in the negative control group at 96 h post transfection, which indicated that MYBL1 was a target gene of gga-miR-181a. Additionally, we found that the expression of MYBL1 was higher in MDV-infected samples than that in non-infected controls, which agreed with the proposition that miRNA showed a negatively correlated expression pattern with its target gene. We observed the inhibitory effect of gga-miR-181a on MSB1 cell proliferation. Collectively, the aberrant expression of gga-miR-181a and MYBL1 in MD lymphoma suggested that they might be involved in MD tumor transformation and played important roles.