Characterization of the chromosomal binding sites and dimerization partners of the viral oncoprotein Meq in Marek's disease virus-transformed T cells

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.

Marek's disease virus-encoded microRNA-M6-5p facilitates viral latent infection by targeting histone demethylase KDM2B

Marek's disease virus (MDV), a highly contagious and oncogenic avian alphaherpesvirus, establishes a latent infection primarily in CD4+ T cells. Latent infections are necessary for both persistent lifelong MDV infection and viral tumorigenesis. MicroRNAs (miRNAs) play critical roles as post-transcriptional regulators of viral infections. However, the role of miRNAs in regulating MDV latency remains unclear. In this study, we found that an MDV-encoded miRNA, miR-M6-5p, inhibited viral lytic replication in vitro by functional screening and that infection with an MDV mutant lacking miR-M6-5p resulted in impaired MDV latency, proliferation, and tumor formation in vivo. Importantly, we identified lysine-specific demethylase 2b (KDM2B), an important epigenetic factor, as a target of miR-M6-5p. Furthermore, KDM2B knockdown increased the level of the transcriptionally repressive histone mark H3K27me3 on the key lytic gene pp38 promoter, accompanied by suppression of pp38 expression and reduced latent-to-lytic switch in MDV-latently infected cells, while treatment of cells with H3K27me3 inhibitors (GSK126 and Tazemetostat) markedly promoted the expression of pp38 and MDV reactivation from latency. Thus, miR-M6-5p facilitates MDV latency by epigenetically suppressing pp38 expression by targeting KDM2B. These findings unravel the mechanism by which a virus-encoded miRNA plays a critical role in the regulation of latent MDV infection.IMPORTANCESimilar to other herpesviruses, MDV can establish a lifelong latent infection in the host. During the latency, MDV integrates its genome into the host genome to maintain the viral genome, which is considered a prerequisite for tumor formation. Reactivation of the latent viral genome in response to intracellular and extracellular stimuli re-enters lytic replication, resulting in pathological recurrence and/or viral shedding. However, the regulatory mechanisms underlying MDV latency remain poorly understood. In the present study, we investigated the role of virus-encoded miRNAs in MDV latency. We found that miR-M6-5p facilitated MDV latency, proliferation, and tumor formation in vivo. Mechanistically, miR-M6-5p epigenetically suppressed the expression of the viral lytic gene pp38 by directly targeting the histone demethylase KDM2B. These findings will advance our understanding of the role of virus-encoded miRNA in the regulation of viral latency and will help guide the development of novel strategies for the effective control of MDV.

Marek's Disease Virus (MDV) Meq Oncoprotein Plays Distinct Roles in Tumor Incidence, Distribution, and Size

Marek's disease (MD), characterized by the rapid onset of T-cell lymphomas in chickens, is caused by Mardivirus gallidalpha2, an oncogenic alphaherpesvirus commonly known as Marek's disease virus (MDV). MDV encodes a bZIP protein, Meq, which contains a bZIP domain (basic DNA-binding and leucine zipper dimerization domain) at the amino terminus and a transcriptional regulatory domain at the carboxyl end. Meq can transform murine and chicken fibroblasts in vitro and is essential for tumor formation in chickens. Meq homodimerization and heterodimerization through its bZIP domain are involved in Meq-mediated transformation. However, the role of Meq DNA-binding and transcriptional regulatory domains in transformation has not been investigated. In this study, we constructed recombinant Md5 (very virulent MDV) viruses expressing chimeric Meq proteins generated by swapping the DNA-binding and transcriptional regulatory domains of Meq of Md5 and vaccine (CVI988/Rispens) strains. Our results show that these recombinant viruses, rMd5-Md5/CVI-Meq (Md5 DNA-binding domain and CVI transcriptional regulatory domain) and rMd5-CVI/Md5-Meq (CVI DNA-binding domain and Md5 transcriptional regulatory domain), replicated at levels similar to parental rMd5 in cell culture and chickens and could transmit efficiently among chickens. Interestingly, parental rMd5 and chimeric viruses exhibited distinct pathogenic phenotypes in chickens: rMd5 caused 100% mortality, a moderate level of tumor incidence in visceral organs and small visceral tumors; rMd5-Md5/CVI-Meq caused 100% mortality, a high level of tumor incidence in visceral organs, and very large visceral tumors; while rMd5-CVI/Md5-Meq caused an average of 37% mortality, rarely induced tumors in visceral organs, and the visceral tumors were small. In conclusion, our study suggests that the DNA-binding domain of Meq plays an essential role in transformation (tumor incidence), while the transcriptional regulatory domain of Meq influences the distribution and size of MDV-induced tumors.

The Meq Genes of Nigerian Marek's Disease Virus (MDV) Field Isolates Contain Mutations Common to Both European and US High Virulence Strains

BACKGROUND

Marek's disease (MD) is a pathology affecting chickens caused by Marek's disease virus (MDV), an acute transforming alphaherpesvirus of the genus Mardivirus. MD is characterized by paralysis, immune suppression, and the rapid formation of T-cell (primarily CD4+) lymphomas. Over the last 50 years, losses due to MDV infection have been controlled worldwide through vaccination; however, these live-attenuated vaccines are non-sterilizing and potentially contributed to the virulence evolution of MDV field strains. Mutations common to field strains that can overcome vaccine protection were identified in the C-terminal proline-rich repeats of the oncoprotein Meq (Marek's EcoRI-Q-encoded protein). These mutations in meq have been found to be distinct to their region of origin, with high virulence strains obtained in Europe differing from those having evolved in the US. The present work reports on meq mutations identified in MDV field strains in Nigeria, arising at farms employing different vaccination practices.

MATERIALS AND METHODS

DNA was isolated from FTA cards obtained at 12 farms affected by increased MD in the Plateau State, Nigeria. These sequences included partial whole genomes as well as targeted sequences of the meq oncogenes from these strains. Several of the meq genes were cloned for expression and their localization ability to interact with the chicken NF-IL3 protein, a putative Meq dimerization partner, were assessed.

RESULTS

Sequence analysis of the meq genes from these Nigerian field strains revealed an RB1B-like lineage co-circulating with a European Polen5-like lineage, as well as recombinants harboring a combination of these mutations. In a number of these isolates, Meq mutations accumulated in both N-terminal and C-terminal domains.

DISCUSSION

Our data, suggest a direct effect of the vaccine strategy on the selection of Meq mutations. Moreover, we posit the evolution of the next higher level of virulence MDVs, a very virulent plus plus pathotype (vv++).

Oncogenic Animal Herpesviruses

Oncogenic viruses play a pivotal role in oncology due to their unique role in unraveling the complexities of cancer development. Understanding the role viruses play in specific cancers is important to provide basic insights into the transformation process, which will help identify potential cellular targets for treatment. This review discusses the diverse role of animal herpesviruses in initiating and promoting various forms of cancer. We will summarize the mechanisms that underlie the development of animal herpesvirus-induced cancer that may provide a basis for developing potential therapeutic interventions or preventative strategies in the future.

Immune escape of avian oncogenic Marek's disease herpesvirus and antagonistic host immune responses

Marek's disease virus (MDV) is a highly pathogenic and oncogenic alpha herpesvirus that causes Marek's disease (MD), which is one of the most important immunosuppressive and rapid-onset neoplastic diseases in poultry. The onset of MD lymphomas and other clinical diseases can be efficiently prevented by vaccination; these vaccines are heralded as the first demonstration of a successful vaccination strategy against a cancer. However, the persistent evolution of epidemic MDV strains towards greater virulence has recently resulted in frequent outbreaks of MD in vaccinated chicken flocks worldwide. Herein, we provide an overall review focusing on the discovery and identification of the strategies by which MDV evades host immunity and attacks the immune system. We have also highlighted the decrease in the immune efficacy of current MD vaccines. The prospects, strategies and new techniques for the development of efficient MD vaccines, together with the possibilities of antiviral therapy in MD, are also discussed.

Characterization of a Very Short Meq Protein Isoform in a Marek's Disease Virus Strain in Japan

Marek's disease virus (MDV) causes malignant lymphoma (Marek's disease; MD) in chickens. The Meq protein is essential for tumorigenesis since it regulates the expression of host and viral genes. Previously, we reported that the deletion of the short isoform of Meq (S-Meq) decreases the pathogenicity of MDV. Recently, we identified a further short isoform of Meq (very short isoform of Meq, VS-Meq) in chickens with MD in Japan. A 64-amino-acid deletion was confirmed at the C-terminus of VS-Meq. We measured the transcriptional regulation by VS-Meq in three gene promoters to investigate the effect of VS-Meq on protein function. Wild-type VS-Meq decreased the transrepression of the pp38 promoter but did not alter the transactivation activity of the Meq and Bcl-2 promoters. The deletion in VS-Meq did not affect the activity of the pp38 promoter but enhanced the transactivation activities of the Meq and Bcl-2 promoters. Collectively, the deletion of VS-Meq potentially enhanced the activity of the Meq promoter, while other amino acid sequences in wild-type VS-Meq seemed to affect the weak transrepression of the pp38 promoter. Further investigation is required to clarify the effects of these changes on pathogenicity.

Infection phase-dependent dynamics of the viral and host N6-methyladenosine epitranscriptome in the lifecycle of an oncogenic virus in vivo

Dynamic alteration of the epitranscriptome exerts regulatory effects on the lifecycle of oncogenic viruses in vitro. However, little is known about these effects in vivo because of the general lack of suitable animal infection models of these viruses. Using a model of rapid-onset Marek's disease lymphoma in chickens, we investigated changes in viral and host messenger RNA (mRNA) N6-methyladenosine (m⁶ A) modification during Marek's disease virus (MDV) infection in vivo. We found that the expression of major epitranscriptomic proteins varies among viral infection phases, reprogramming both the viral and the host epitranscriptomes. Specifically, the methyltransferase-like 3 (METTL3)/14 complex was suppressed during the lytic and reactivation phases of the MDV lifecycle, whereas its expression was increased during the latent phase and in MDV-induced tumors. METTL3/14 overexpression inhibits, whereas METTL3/14 knockdown enhances, MDV gene expression and replication. These findings reveal the dynamic features of the mRNA m⁶ A modification program during viral replication in vivo, especially in relation to key pathways involved in tumorigenesis.

Identification and Validation of Ikaros (IKZF1) as a Cancer Driver Gene for Marek’s Disease Virus-Induced Lymphomas

Marek’s disease virus (MDV) is the causative agent for Marek’s disease (MD), which is characterized by T-cell lymphomas in chickens. While the viral Meq oncogene is necessary for transformation, it is insufficient, as not every bird infected with virulent MDV goes on to develop a gross tumor. Thus, we postulated that the chicken genome contains cancer driver genes; i.e., ones with somatic mutations that promote tumors, as is the case for most human cancers. To test this hypothesis, MD tumors and matching control tissues were sequenced. Using a custom bioinformatics pipeline, 9 of the 22 tumors analyzed contained one or more somatic mutation in Ikaros (IKFZ1), a transcription factor that acts as the master regulator of lymphocyte development. The mutations found were in key Zn-finger DNA-binding domains that also commonly occur in human cancers such as B-cell acute lymphoblastic leukemia (B-ALL). To validate that IKFZ1 was a cancer driver gene, recombinant MDVs that expressed either wild-type or a mutated Ikaros allele were used to infect chickens. As predicted, birds infected with MDV expressing the mutant Ikaros allele had high tumor incidences (~90%), while there were only a few minute tumors (~12%) produced in birds infected with the virus expressing wild-type Ikaros. Thus, in addition to Meq, key somatic mutations in Ikaros or other potential cancer driver genes in the chicken genome are necessary for MDV to induce lymphomas.

Deletion of the Viral Thymidine Kinase in a Meq-Deleted Recombinant Marek's Disease Virus Reduces Lymphoid Atrophy but Is Less Protective

Marek's disease (MD) is a ubiquitous disease of domesticated chickens and its etiologic agent is the Gallid alphaherpesvirus 2 (GaHV-2), also known as Marek's disease virus (MDV). MD is currently controlled by vaccination using live attenuated strains of MDV (e.g., CVI988/Rispens), non-pathogenic serotypes of MDV (GaHV-3), or non-pathogenic strains of the related Melagrid alphaherpesvirus 1 (MeHV-1). One attractive strategy for the production of new vaccine strains is a recombinant MDV attenuated by the deletion of the major viral oncogene meq. However, meq-deleted variants of MDV cause atrophy of the bursa and thymus in maternal antibody-negative chickens, and the resulting immunosuppression makes them unsuitable. Herein we detail our attempt to mitigate the lymphoid atrophy caused by meq-deleted MDV by further attenuation of the virus through ablation of the viral thymidine kinase (tk) gene. We demonstrate that ablation of the viral tk from the meq-deleted virus rMd5B40/Δmeq resulted in a virus attenuated for replication in vitro and which spared chickens from atrophy of the lymphoid organs in vivo. When the rMd5B40/Δmeq/Δtk/GFP was used as a vaccine it was protective against challenge with the vv+MDV strain 686, but the protection was less than that provided by the CVI988/Rispens vaccine.

Fos Facilitates Gallid Alpha-Herpesvirus 1 Infection by Transcriptional Control of Host Metabolic Genes and Viral Immediate Early Gene

Gallid alpha-herpesvirus 1, also known as avian infectious laryngotracheitis virus (ILTV), continues to cause huge economic losses to the poultry industry worldwide. Similar to that of other herpesvirus-encoded proteins, the expression of viral genes encoded by ILTV is regulated by a cascade, and the underlying regulatory mechanism remains largely unclear. The viral immediate-early (IE) gene ICP4 plays a prominent role in the initiation of the transcription of early and late genes during ILTV replication. In this study, we identified AP-1 as the key regulator of the transcription of ILTV genes by bioinformatics analysis of genome-wide transcriptome data. Subsequent functional studies of the key members of the AP-1 family revealed that Fos, but not Jun, regulates ILTV infection through AP-1 since knockdown of Fos, but not Jun, by gene silencing significantly reduced ICP4 transcription and subsequent viral genome replication and virion production. Using several approaches, we identified ICP4 as a bona fide target gene of Fos that regulated Fos and has Fos response elements within its promoter. Neither the physical binding of Jun to the promoter of ICP4 nor the transcriptional activity of Jun was observed. In addition, knockdown of Fos reduced the transcription of MDH1 and ATP5A1, genes encoding two host rate-limiting enzymes essential for the production of the TCA intermediates OAA and ATP. The biological significance of the transcriptional regulation of MDH1 and ATP5A1 by Fos in ILTV infection was supported by the fact that anaplerosis of OAA and ATP rescued both ICP4 transcription and virion production in infected cells under when Fos was silenced. Our study identified the transcription factor Fos as a key regulator of ILTV infection through its transcription factor function on both the virus and host sides, improving the current understanding of both avian herpesvirus–host interactions and the roles of AP-1 in viral infection.

Genetic evolution of Marek's disease virus in vaccinated poultry farms

Background and Aim:

The Marek’s disease virus (MDV) is a neoplastic disease causing serious economic losses in poultry production. This study aimed to investigate MDV occurrence in poultry flocks in the Lower Egypt during the 2020 breakout and genetically characterized Meq, gL, and ICP4 genes in field strains of MDV.

Materials and Methods:

Forty samples were collected from different breeds from eight Egyptian governorates in 2020. All flocks had received a bivalent vaccine (herpesvirus of turkey FC-126 + Rispens CVI988). However, weight loss, emaciation, reduced egg production, paralysis, and rough/raised feather follicles occurred. Samples were collected from feather follicles, liver, spleen, and nerve tissue for diagnosis by polymerase chain reaction. MDV genetic characterization was then performed by sequencing the Meq, gL, and ICP4 genes of five positive samples representing different governorates and breeds.

Results:

A total of 28 samples were positive for MDV field strains, while two were related to MDV vaccinal strains. All samples tested negative for ALV (A, B, C, D, and J) and REV. Phylogenetic analysis of the Meq gene of sequenced samples revealed that all MDVs were related to the highly virulent European viruses (Gallid herpesvirus 2 ATE and PC12/30) with high amino acid (A.A.) identity 99.2-100%. Alternatively, there was low A.A. identity with the vaccine strains CVI988 and 3004 (up to 82.5%). These results indicate that further investigation of the efficacy of current Egyptian vaccines is required. The Egyptian strains also harbor a specific mutation, allowing clustering into two subgroups (A and B). By mutation analysis of the Meq gene, the Egyptian viruses in our study had R101K, P217A, and E263D mutations present in all Egyptian viruses. Furthermore, R176A and T180A mutations specific to our strains contributed to the high virulence of highly virulent strains. There were no mutations of the gL or ICP4 genes.

Conclusion:

Further studies should evaluate the protection contributed by current vaccines used in Egypt.

Marek's disease virus US3 protein kinase phosphorylates chicken HDAC 1 and 2 and regulates viral replication and pathogenesis

Marek’s disease virus (MDV) is a potent oncogenic alphaherpesvirus that elicits a rapid onset of malignant T-cell lymphomas in chickens. Three MDV types, including GaHV-2 (MDV-1), GaHV-3 (MDV-2) and MeHV-1 (HVT), have been identified and all encode a U_S3 protein kinase. MDV-1 U_S3 is important for efficient virus growth in vitro. To study the role of U_S3 in MDV replication and pathogenicity, we generated an MDV-1 U_S3-null virus and chimeric viruses by replacing MDV-1 U_S3 with MDV-2 or HVT U_S3. Using MD as a natural virus-host model, we showed that both MDV-2 and HVT U_S3 partially rescued the growth deficiency of MDV-1 U_S3-null virus. In addition, deletion of MDV-1 U_S3 attenuated the virus resulting in higher survival rate and lower MDV specific tumor incidence, which could be partially compensated by MDV-2 and HVT U_S3. We also identified chicken histone deacetylase 1 (chHDAC1) as a common U_S3 substrate for all three MDV types while only U_S3 of MDV-1 and MDV-2 phosphorylate chHDAC2. We further determined that U_S3 of MDV-1 and HVT phosphorylate chHDAC1 at serine 406 (S406), while MDV-2 U_S3 phosphorylates S406, S410, and S415. In addition, MDV-1 U_S3 phosphorylates chHDAC2 at S407, while MDV-2 U_S3 targets S407 and S411. Furthermore, biochemical studies show that MDV U_S3 mediated phosphorylation of chHDAC1 and 2 affect their stability, transcriptional regulation activity, and interaction network. Using a class I HDAC specific inhibitor, we showed that MDV U_S3 mediated phosphorylation of chHDAC1 and 2 is involved in regulation of virus replication. Overall, we identified novel substrates for MDV U_S3 and characterized the role of MDV U_S3 in MDV pathogenesis.

Marek’s disease virus (MDV) is a highly contagious and oncogenic avian alphaherpesvirus that causes T-cell lymphomas in chickens. Alphaherpesviruses encoded U_S3 is a multifunctional protein kinase involved in viral replication, apoptosis resistance, and cell-to-cell spread. In this study, we evaluated the importance of MDV U_S3 in regulating MDV replication and pathogenesis in chickens. Our results provide first evidence that MDV U_S3 protein kinase is involved in the replication and pathogenicity of MDV in its natural host. We also identified chicken histone deacetylase 1 and 2 (chHDAC1 and 2) as novel substrates of U_S3 for MDV and characterized the potential impacts of MDV U_S3 induced phosphorylation in their protein stability, transcriptional regulation and protein interactions; to our knowledge, this is the first comparative study of the functions of U_S3 from all three MDV types. This is an important finding towards a better understanding of the functions of alphaherpesviruses encoded U_S3 protein kinase.

Marek's disease virus Meq oncoprotein interacts with chicken HDAC 1 and 2 and mediates their degradation via proteasome dependent pathway

Marek's disease virus (MDV) encodes a basic-leucine zipper (BZIP) protein, Meq, which is considered the major MDV oncoprotein. It has been reported that the oncogenicity of Meq is associated with its interaction with C-terminal binding protein 1 (CtBP), which is also an interaction partner of Epstein-Barr virus encoded EBNA3A and EBNA3C oncoproteins. Since both EBNA3C and CtBP interact with histone deacetylase 1 (HDAC1) and HDAC2, we examined whether Meq shares this interaction with chicken HDAC1 (chHDAC1) and chHDAC2. Using confocal microscopy analysis, we show that Meq co-localizes with chHDAC1 and chHDAC2 in the nuclei of MDV lymphoblastoid tumor cells. In addition, immunoprecipitation assays demonstrate that Meq interacts with chHDAC1 and chHDAC2 in transfected cells and MDV lymphoblastoid tumor cells. Using deletion mutants, interaction domains were mapped to the N-terminal dimerization domain of chHDAC1 and chHDAC2, and the BZIP domain of Meq. Our results further demonstrate that this interaction mediates the degradation of chHDAC1 and chHDAC2 via the proteasome dependent pathway. In addition, our results show that Meq also induces the reduction of global ubiquitinated proteins through a proteasome dependent pathway. In conclusion, our results provide evidence that Meq interacts with chHDAC1 and chHDAC2, and induces their proteasome dependent degradation.

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.

Role of Marek's Disease Virus (MDV)-Encoded US3 Serine/Threonine Protein Kinase in Regulating MDV Meq and Cellular CREB Phosphorylation

Marek's disease (MD) is a neoplastic disease of chickens caused by Marek's disease virus (MDV), a member of the subfamily Alphaherpesvirinae Like other alphaherpesviruses, MDV encodes a serine/threonine protein kinase, U_S3. The functions of U_S3 have been extensively studied in other alphaherpesviruses; however, the biological functions of MDV U_S3 and its substrates have not been studied in detail. In this study, we investigated potential cellular pathways that are regulated by MDV U_S3 and identified chicken CREB (chCREB) as a substrate of MDV U_S3. We show that wild-type MDV U_S3, but not kinase-dead U_S3 (U_S3-K220A), increases CREB phosphorylation, leading to recruitment of phospho-CREB (pCREB) to the promoter of the CREB-responsive gene and activation of CREB target gene expression. Using U_S3 deletion and U_S3 kinase-dead recombinant MDV, we identified U_S3-responsive MDV genes during infection and found that the majority of U_S3-responsive genes were located in the MDV repeat regions. Chromatin immunoprecipitation sequencing (ChIP-seq) studies determined that some U_S3-regulated genes colocalized with Meq (an MDV-encoded oncoprotein) recruitment sites. Chromatin immunoprecipitation-PCR (ChIP-PCR) further confirmed Meq binding to the ICP4/LAT region, which is also regulated by U_S3. Furthermore, biochemical studies demonstrated that MDV U_S3 interacts with Meq in transfected cells and MDV-infected chicken embryonic fibroblasts in a phosphorylation-dependent manner. Finally, in vitro kinase studies revealed that Meq is a U_S3 substrate. MDV U_S3 thus acts as an upstream kinase of the CREB signaling pathway to regulate the transcription function of the CREB/Meq heterodimer, which targets cellular and viral gene expression.IMPORTANCE MDV is a potent oncogenic herpesvirus that induces T-cell lymphoma in infected chickens. Marek's disease continues to have a significant economic impact on the poultry industry worldwide. U_S3 encoded by alphaherpesviruses is a multifunctional kinase involved in the regulation of various cellular pathways. Using an MDV genome quantitative reverse transcriptase PCR (qRT-PCR) array and chromatin immunoprecipitation, we elucidated the role of MDV U_S3 in viral and cellular gene regulation. Our results provide insights into how viral kinase regulates host cell signaling pathways to activate both viral and host gene expression. This is an important step toward understanding host-pathogen interaction through activation of signaling cascades.

The bZIP Proteins of Oncogenic Viruses

Basic leucine zipper (bZIP) transcription factors (TFs) govern diverse cellular processes and cell fate decisions. The hallmark of the leucine zipper domain is the heptad repeat, with leucine residues at every seventh position in the domain. These leucine residues enable homo- and heterodimerization between ZIP domain α-helices, generating coiled-coil structures that stabilize interactions between adjacent DNA-binding domains and target DNA substrates. Several cancer-causing viruses encode viral bZIP TFs, including human T-cell leukemia virus (HTLV), hepatitis C virus (HCV) and the herpesviruses Marek’s disease virus (MDV), Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV). Here, we provide a comprehensive review of these viral bZIP TFs and their impact on viral replication, host cell responses and cell fate.

Research Progress on MicroRNAs Involved in the Regulation of Chicken Diseases

MicroRNAs (miRNAs) are small, non-coding RNA molecules that inhibit protein translation from target mRNAs. Accumulating evidence suggests that miRNAs can regulate a broad range of biological pathways, including cell differentiation, apoptosis, and carcinogenesis. With the development of miRNAs, the investigation of miRNA functions has emerged as a hot research field. Due to the intensive farming in recent decades, chickens are easily influenced by various pathogen transmissions, and this has resulted in large economic losses. Recent reports have shown that miRNAs can play critical roles in the regulation of chicken diseases. Therefore, the aim of this review is to briefly discuss the current knowledge regarding the effects of miRNAs on chickens suffering from common viral diseases, mycoplasmosis, necrotic enteritis, and ovarian tumors. Additionally, the detailed targets of miRNAs and their possible functions are also summarized. This review intends to highlight the key role of miRNAs in regard to chickens and presents the possibility of improving chicken disease resistance through the regulation of miRNAs.

Avian oncogenic herpesvirus antagonizes the cGAS-STING DNA-sensing pathway to mediate immune evasion

The cellular DNA sensor cGMP-AMP synthase (cGAS) detects cytosolic viral DNA via the stimulator of interferon genes (STING) to initiate innate antiviral response. Herpesviruses are known to target key immune signaling pathways to persist in an immune-competent host. Marek’s disease virus (MDV), a highly pathogenic and oncogenic herpesvirus of chickens, can antagonize host innate immune responses to achieve persistent infection. With a functional screen, we identified five MDV proteins that blocked beta interferon (IFN-β) induction downstream of the cGAS-STING pathway. Specifically, the MDV major oncoprotein Meq impeded the recruitment of TANK-binding kinase 1 and IFN regulatory factor 7 (IRF7) to the STING complex, thereby inhibiting IRF7 activation and IFN-β induction. Meq overexpression markedly reduced antiviral responses stimulated by cytosolic DNA, whereas knockdown of Meq heightened MDV-triggered induction of IFN-β and downstream antiviral genes. Moreover, Meq-deficient MDV induced more IFN-β production than wild-type MDV. Meq-deficient MDV also triggered a more robust CD8+ T cell response than wild-type MDV. As such, the Meq-deficient MDV was highly attenuated in replication and lymphoma induction compared to wild-type MDV. Taken together, these results revealed that MDV evades the cGAS-STING DNA sensing pathway, which underpins the efficient replication and oncogenesis. These findings improve our understanding of the virus-host interaction in MDV-induced lymphoma and may contribute to the development of novel vaccines against MDV infection.

Marek’s disease virus (MDV) is an avian oncogenic herpesvirus that causes a fatal disease in poultry worldwide. Chickens infected with MDV become more susceptible to secondary viral or bacterial infections. However, the mechanisms of MDV-induced immunosuppression and tumorigenesis remain largely unknown. The cGAS-STING pathway is crucial for innate immune responses against both microbial pathogens and intrinsic tumors. Here we identified the MDV oncoprotein, Meq, as an inhibitor of the cGAS-STING DNA-sensing pathway. Mechanistically, Meq interacted with STING and IRF7, and impaired the recruitment of TBK1 and IRF7 to the STING complex, thus inhibiting IRF7 activation and IFN-β induction. Loss of Meq potently enhanced innate immune response, while impaired the replication and oncogenesis of MDV in chickens. Our findings reveal an important mechanism of immune evasion of MDV, instructing us on the virus-host interaction in MDV-induced lymphoma and potential new means to develop MDV vaccine.

Marek's disease virus oncoprotein Meq physically interacts with the chicken infectious anemia virus-encoded apoptotic protein apoptin

Marek's disease (MD) is a neoplastic disease of poultry caused by Marek's disease virus (MDV), a highly contagious alphaherpesvirus. Meq, the major MDV oncoprotein, induces neoplastic transformation of T-cells through several mechanisms, including inhibition of apoptosis. In contrast, the chicken anemia virus (CAV)-encoded protein apoptin (VP3) is a powerful inducer of apoptosis of tumor cells, a property that is exploited for anticancer therapeutics. Although the molecular mechanisms of selective induction of tumor cell apoptosis by apoptin are not fully understood, its tumor cell–restricted nuclear translocation is thought to be important. Co-infection with MDV and CAV is common in many countries, CAV antigens are readily detectable in MD lymphomas, and the MDV-transformed T-lymphoblastoid cell lines such as MSB-1 is widely used for propagating CAV for vaccine production. As MDV-transformed cell lines express high levels of Meq, we examined here whether CAV-encoded apoptin interacts with Meq in these cells. Using immunofluorescence microscopy, we found that apoptin and Meq co-localize to the nucleus, and biochemical analysis indicated that the two proteins do physically interact. Using a combination of Meq mutagenesis and co-immunoprecipitation, we demonstrate that apoptin interacts with Meq within a region between amino acids 130 and 140. Results from the IncuCyte assay suggested that Meq inhibits apoptin-induced apoptosis activity. In summary, our findings indicate that Meq interacts with and inhibits apoptin. Insights into this novel interaction between Meq and apoptin will relevance for pathogenesis of coinfections of the two viruses and in CAV vaccine production using MDV-transformed cell lines.

Specific transcriptional and post-transcriptional regulation of the major immediate early ICP4 gene of GaHV-2 during the lytic, latent and reactivation phases

Transcriptional and post-transcriptional mechanisms are involved in the switch between the lytic, latent and reactivation phases of the viral cycle in herpesviruses. During the productive phases, herpesvirus gene expression is characterized by a temporally regulated cascade of immediate early (IE), early (E) and late (L) genes. In alphaherpesviruses, the major product of the IE ICP4 gene is a transcriptional regulator that initiates the cascade of gene expression that is essential for viral replication. In this study, we redefine the infected cell protein 4 (ICP4) gene of the oncogenic Marek's disease virus (MDV or gallid herpesvirus 2) as a 9438 nt gene ended with four alternative poly(A) signals and controlled by two alternative promoters containing essentially ubiquitous functional response elements (GC, TATA and CCAAT boxes). The distal promoter is associated with ICP4 gene expression during the lytic and the latent phases, whereas the proximal promoter is associated with the expression of this gene during the reactivation phase. Both promoters are regulated by DNA methylation during the viral cycle and are hypermethylated during latency. Transcript analyses showed ICP4 to consist of three exons and two introns, the alternative splicing of which is associated with five predicted nested ICP4ORFs. We show that the ICP4 gene is highly and specifically regulated by transcriptional and post-transcriptional mechanisms during the three phases of the GaHV-2 viral cycle, with a clear difference in expression between the lytic phase and reactivation from latency in our model.

Molecular characterization and phylogenetic analysis of a virulent Marek's disease virus field strain in broiler chickens in Japan

Marek's disease is a lymphoproliferative disease causing a serious threat in poultry production. Field strains of Marek's disease virus (MDVs) are continuously re-emerging, causing great economical losses to the poultry industry worldwide in spite of the intensive vaccination and restrictive management policy used. Histopathological and molecular characterizations of MDVs are essential for monitoring the changes of viruses and evaluating the effectiveness of existing vaccines. During 2016, 190 visceral tumour tissues representing 30 vaccinated chicken flocks from the Gifu prefecture, Japan, were analysed. A pathological examination revealed the presence of lymphoproliferative lesions in the visceral organs. Polymerase chain reaction screening of tissue specimens using specific primers for avian leucosis virus, reticuloendotheliosis virus, and MDV was positive only for MDV. The polymerase chain reaction products of meq, pp38, virus-induced IL-8 homology, and glycoprotein MDV genes were sequenced and used for homology, phylogenetic, and similarity level analysis with the published reference of MDVs in the database. The results revealed high similarity between the field isolates, vv and vv+ strains of MDV from the USA and China. Several point mutations in the nucleotide sequence of the field isolates and their deduced amino acid sequences were detected in those genes. The present molecular analyses indicated that nucleotide and amino acid changes could be valuable criteria for differentiation and determination of the pathogenicity and oncogenicity of MDVs according to the Avian Disease and Oncology Laboratory pathotyping in vivo studies. Furthermore, the results suggest that development of a new vaccine must be considered to overcome this devastating avian oncogenic viral disease.

Gga-miR-219b targeting BCL11B suppresses proliferation, migration and invasion of Marek's disease tumor cell MSB1

Marek’s disease (MD), caused by Marek’s disease virus (MDV), is a lymphotropic neoplastic disease. Previous miRNAome analysis showed gga-miR-219b was significantly downregulated in MDV-induced lymphoma, and one of its potential target genes, B-cell chronic lymphocytic /lymphoma 11B (BCL11B) was predicted. In this study, we further investigated the function of gga-miR-219b, and the gain/loss of function assay showed gga-miR-219b inhibited cell migration and reduced cell proliferation by promoting apoptosis not by cell cycle arrest. Gga-miR-219b also suppressed expression of two cell invasion-related genes MMP2 and MMP9. The results indicated suppressive effect of gga-miR-219b on MD tumorigenesis. The gene BCL11B was verified as a direct target gene of gga-miR-219b. RNA interference was performed to block BCL11B. As expected, the effects triggered by BCL11B downregulation were in accordance with that triggered by gga-miR-219b overexpression, suggesting that BCL11B was a stimulative regulator of MD transformation. Moreover, both gga-miR-219b and BCL11B influenced the expression of Meq gene, the most important oncogene in MDV. Additionally, gene expression level of anti-apoptotic genes BCL2 and BCL2L1 was downregulated and pro-apoptotic gene TNFSF10 was upregulated in MSB1 cells with gga-miR-219b overexpression or BCL11B knockdown, which suggested gga-miR-219b promoted cell apoptosis via regulating gene expression in the apoptosis pathways.

Co-Infection with Marek's Disease Virus and Reticuloendotheliosis Virus Increases Illness Severity and Reduces Marek's Disease Vaccine Efficacy

Marek's disease virus (MDV) and reticuloendotheliosis virus (REV) cause Marek's disease (MD) and reticuloendotheliosis (RE), respectively. Co-infection with MDV and REV is common in chickens, causing serious losses to the poultry industry. However, experimental studies of such co-infection are lacking. In this study, Chinese field strains of MDV (ZW/15) and REV (JLR1501) were used as challenge viruses to evaluate the pathogenicity of co-infection and the influence of MD vaccination in chickens. Compared to the MDV-challenged group, the mortality and tumor rates increased significantly by 20.0% (76.7 to 96.7%) and 26.7% (53.3 to 80.0%), in the co-challenged group, respectively. The protective index of the MD vaccines CVI988 and 814 decreased by 33.3 (80.0 to 47.7) and 13.3 (90.0 to 76.7), respectively. These results indicated that MDV and REV co-infection significantly increased disease severity and reduced the vaccine efficacy. The MDV genome load showed no difference in the feather pulps and spleen, and pathogenicity-related MDV gene expression (meq, pp38, vIL-8, and ICP4) in the spleen significantly increased at some time points in the co-challenged group. Clearly, synergistic pathogenicity occurred between MDV and REV, and the protective efficacy of existing MD vaccines was attenuated by co-infection with Chinese field MDV and REV strains.

Complete nucleotide sequence analysis of the oncogene "Meq" from serotype 1 Marek's disease virus isolates from India

1. A study was undertaken to characterise the oncogene Meq at the molecular level for three serotype 1 Marek's disease virus (MDV) field isolates from vaccinated poultry flocks which had encountered a Marek's disease outbreak in the southern part of India. The isolates were named Ind/TN/11/01, Ind/KA/12/02 and Ind/TN/12/03. The oncogene Meq was amplified by PCR and sequenced. 2. The isolates were shown to have a homology for the Meq gene of 99.1-99.8% with various isolates from China and 98.5-99.2% with isolates from Europe and the USA. Alignment analysis of the nucleotide sequences showed that nucleotide mutations at 5 different positions in the Meq gene displayed perfect regularity in MDVs circulating in the southern part of India, which could be considered as features of field MDVs recently prevalent in this area. 3. In addition, the mutation in the Meq gene at positions 251, 260 and 437 was unique and coincides with very virulent strains from China GX0101, GXY2 and a Hungarian strain ATE. The mutation at positions 283 and 300 was unique and coincides with the very virulent strain ATE of Hungary. There were also single nucleotide mutations at positions 155 (A-T), 369 (A-C), 462 (C-A) and 548 (C-T) observed in the isolate Ind/TN/12/03. 4. Phylogenetic analysis of Meq sequences revealed that field MDVs in this area evolved independently but have similarities with very virulent strains from China, and that Meq has more similarities with the very virulent Hungarian strain.

Oncogenic Marek's disease herpesvirus encodes an isoform of the conserved regulatory immediate early protein ICP27 generated by alternative promoter usage

Herpesvirus gene expression is temporally regulated, with immediate early (IE), early (E) and late (L) genes. ICP27, which is involved in post-transcriptional regulation, is the only IE gene product conserved in all herpesviruses. We show here that the ICP27 transcript of the oncogenic Marek's disease virus shares the same polyadenylation signal as the bicistronic glycoprotein K-ICP27 transcript and is regulated by alternative promoter usage, with transcription from its own promoter (pICP27) or that of gK (pgK). The pgK can generate a spliced ICP27 transcript yielding an N-terminal-deleted ICP27 isoform (ICP27ΔN) that, like ICP27, co-localizes with the SR protein in infected cells, but with a diffuse nuclear distribution. The pICP27 includes functional responsive elements (REs) for SP1, AP1 and CREB, is essentially active during the lytic phase and leads to exclusive expression of the native form of ICP27. The alternative promoter, pgK, including active REs for GATA, P53 and CREB, preferentially generates the gK transcript during the lytic phase and the spliced ICP27 transcript (ICP27ΔN) during the latent phase. An analysis of the DNA methylation marks of each promoter showed that pgK was systematically demethylated, whereas pICP27 was methylated during latency and demethylated during the lytic stage. Thus, MDV ICP27 gene expression is dependent on alternative promoters, the usage of which is regulated by DNA methylation, which differs between viral stages.

Genetic evolution of Gallid herpesvirus 2 isolated in China

Gallid herpesvirus 2 (GaHV-2), which causes Marek's disease in chickens and has caused extensive economic losses, has recently evolved increased virulence in China. To better understand the genetic basis of the pathogenic characteristics changed and increased virulence, we sequenced the genomes of six new GaHV-2 strains (LCC, LTS, WC/1203, JL/1404, CC/1409, and HS/1412) isolated from chickens with failed immunisation as well as one previously isolated Chinese GaHV-2 strain, J-1. Based on a multiple sequence alignment, several characteristic point mutations were detected in the open reading frames of the Chinese isolates. In addition, two deletions and an insertion were identified at the unique short region and terminal repeat short region junctions in Chinese isolates, and the insertion was a characteristic of the new Chinese isolates. According to a phylogenetic analysis, the GaHV-2 genome diverged substantially over the last two decades in China. Based on the internal repeat long region, the new isolates were closely related to very virulent or very virulent plus strains. Additionally, the new Chinese isolates diverged from the previously isolated strains J-1 and 814. In conclusion, our results provide evidence that Chinese GaHV-2 strains contain characteristic sequences, especially the new isolates. The observed genetic divergence in the new Chinese GaHV-2 strains over the last two decades may be related to observed changes in pathogenic characteristics and virulence.

Marek's disease: Genetic regulation of gallid herpesvirus 2 infection and latency

Gallid herpesvirus-2 (GaHV-2) is an oncogenic α-herpesvirus that causes Marek's disease (MD), a T cell lymphosarcoma (lymphoma) of domestic fowl (chickens). The GaHV-2 genome integrates by homologous recombination into the host genome and, by modulating expression of viral and cellular genes, induces transformation of latently infected cells. MD is a unique model of viral oncogenesis. Mechanisms implicated in the regulation of viral and cellular genes during GaHV-2 infection operate at transcriptional, post-transcriptional and post-translational levels, with involvement of viral and cellular transcription factors, along with epigenetic modifications, alternative splicing, microRNAs and post-translational modifications of viral proteins. Meq, the major oncogenic protein of GaHV-2, is a viral transcription factor that modulates expression of viral genes, for example by binding to the viral bidirectional promoter of the pp38-pp24/1.8 kb mRNA, and also modulates expression of cellular genes, such as Bcl-2 and matrix metalloproteinase 3. GaHV-2 expresses viral telomerase RNA subunit (vTR), which forms a complex with the cellular telomerase reverse transcriptase (TERT), thus contributing to tumorigenesis, while vTR independent of telomerase activity is implicated in metastasis. Expression of a viral interleukin 8 homologue may contribute to lymphomagenesis. Inhibition of expression of the pro-apoptotic factors JARID2 and SMAD2 by viral microRNAs may promote the survival and proliferation of GaHV-2 latently infected cells, thus enhancing tumorigenesis, while inhibition of interleukin 18 by viral microRNAs may be involved in evasion of immune surveillance. Viral envelope glycoproteins derived from glycoprotein B (gp60 and gp49), as well as glycoprotein C, may also play a role in immune evasion.

Molecular evolution of Marek's disease virus (MDV) field strains in a 40-year time period

Marek's disease (MD) presents a serious threat in poultry production. The disease has been limited for over 40 yr by protective vaccination. The widely applied vaccination against MD is also one of the factors causing evolutionary pressure onto field Marek's disease virus (MDV) virulent strains. Molecular evolution of MDV genes involved in oncogenesis may increase the pathogenicity of MDV virulent strains. The goal of the presented study was to sum up the molecular evolution of MDV field strains isolated in the last 40 yr in Poland. In total, 85 field MDV strains collected between 1974 and 2012 were propagated in chicken embryo fibroblasts. After DNA extraction, three sets of primers were designed for PCR complementary to the MDV076 (RLORF7) region encoding the meq oncogene as well to the MDV077 (23 kDa protein binding alpha-enolase) and MDV077.5 (RLORF6) genes. The obtained 85 MDV076, 60 MDV077, and 58 MDV077.5 cloned fragments were sequenced and aligned with the sequences of reference MDV strains showing different pathogenicity levels. The retrieved nucleotide (nt) and deduced amino acid sequences of RLORF7, 23 kDa protein, and LORF6 of Polish field strains showed several mutations and substitutions homologous to those observed in reference strains with a determined pathogenicity. The observed changes indicated the continuous evolution of field MDV strains. The RLORF7 nt sequence of analyzed MDV isolates showed similarity to virulent and very virulent MDV reference strains. The obtained 23 kDa and LORF6 nt sequences provided more important data and were more similar to mildly pathogenic strains than to virulent and very virulent MDV. The specific nt motifs in all three genes may indicate an increase of MDV virulence and were found in strains starting from 2006. According to the obtained results, the strains isolated in 2012 are similar to the very virulent plus MDV group. The study showed that RLORF7, 23 kDa protein, and RLORF6 fragments harbor sequence motifs that may have some association with MDV pathogenicity level. However, the exact role of the investigated regions in pathogenicity should be further examined by knock-out MDV strains. Also, the true MDV pathotype may only be determined by traditional in vivo experiments.

Temporal expression and DNA hypomethylation profile of CD30 in Marek's disease virus-infected chicken spleens

Marek's disease (MD) is a viral neoplastic disease of chickens caused by Marek's disease virus (MDV), which is serious threat to worldwide poultry industry. Our previous studies showed that the CD30 gene was hypomethylated in MD lymphoma. In this study, we further analyzed differential expression patterns and methylation levels of the CD30 gene between MDV-infected and noninfected spleens at 4, 7, 14, 21, and 28 d postinfection (dpi). The results showed that the expression of CD30 in MDV-infected spleens was significantly lower than that in noninfected spleens at 4 dpi. The expression of CD30 did not present significant difference between MDV-infected and noninfected spleens at 7 and 14 dpi. However, an increased expression of CD30 was presented in MDV-infected spleens at both 21 and 28 dpi. Simultaneously, CD30 showed a lower DNA methylation level in MDV-infected spleens at 14, 21, and 28 dpi. The results indicated that CD30 gene was involved in the whole process of MD tumorigenesis and upregulated expression of CD30 in MDV-infected spleens might be attributed to the hypomethylation of promoter of CD30 gene.

Temporal kinetics of Marek's disease herpesvirus: integration occurs early after infection in both B and T cells

Marek's disease virus (MDV) is an oncogenic α-herpesvirus that induces Marek's disease characterized by fatal lymphomas in chickens. Here, we explored the timing during pathogenesis when the virus integrates into the host genome, the cell type involved, the role of viral integration on cellular transformation, and tumor clonality. Three immune organs of chicken (thymus, bursa, and spleen) were extracted following infection with either an oncogenic or a non-oncogenic strain of MDV. Using molecular cytogenetics, cells were investigated for viral integration at key time points throughout pathogenesis. Integration profiling of tumors (early to late stage) was conducted. Virus integration was widespread in B and T lymphocytes based on their abundance in bursa and thymus, respectively. Viral replication was detected early after infection as was viral integration into the host genome. Integration is a natural part of the MDV herpesvirus life cycle. In addition, our data using a non-oncogenic virus establish that although integration is a hallmark of tumor cell populations, integration alone is not sufficient for cellular transformation. Our results provide evidence for progression of lineage clonality within tumors. Understanding the features of integration provides insight into the mechanisms of herpesvirus pathology which could lead to disease mitigation strategies.

Role of the short telomeric repeat region in Marek's disease virus replication, genomic integration, and lymphomagenesis

Marek's disease virus (MDV) is a cell-associated alphaherpesvirus that causes generalized polyneuritis and T-cell lymphomas in chickens. MDV is able to integrate its genome into host telomeres, but the mechanism of integration is poorly understood. The MDV genome harbors two arrays of telomeric repeats (TMR) at the ends of its linear genome: multiple telomeric repeats (mTMR), with a variable number of up to 100 repeats, and short telomeric repeats (sTMR), with a fixed number of 6 repeats. The mTMR have recently been shown to play an important role in MDV integration and tumor formation; however, the functions of the sTMR have remained unknown. In this study, we demonstrate that deletion of the sTMR in the MDV genome abrogates virus replication, while extensive mutation of the sTMR does not, indicating that the presence of the sTMR but not the sTMR sequence itself is important. Furthermore, we generated a panel of truncation mutants to determine the minimal length of the sTMR and observed a direct correlation between sTMR length and MDV replication. To address the role of sTMR in MDV replication, integration, and tumorigenesis, sTMR sequences were replaced by a scrambled repeated sequence (vsTMR_mut). vsTMR_mut replicated comparably to parental and revertant viruses in vitro. In vivo, however, a significant reduction in disease and tumor incidence was observed in chickens infected with vsTMR_mut that also correlated with a reduced number of viral integration sites in tumor cells. Taken together, our data demonstrate that the sTMR play a central role in MDV genome replication, pathogenesis, and MDV-induced tumor formation.

IMPORTANCE

Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that infects chickens and causes high economic losses in the poultry industry. MDV integrates its genetic material into host telomeres, a process that is crucial for efficient tumor formation. The MDV genome harbors two arrays of telomeric repeats (TMR) at the ends of its linear genome that are identical to host telomeres and that are termed mTMR and sTMR. mTMR have been recently shown to be involved in MDV integration, while the functions of sTMR remain unknown. Here, we demonstrate that the presence and length of sTMR sequence, but not the exact nucleotide sequence, are crucial for MDV replication. Furthermore, the sTMR contribute to the high integration frequency of MDV and are important for MDV pathogenesis and tumor formation. As a number of herpesviruses harbor arrays of telomeric repeats (TMR), MDV serves as a model to determine the role of the herpesvirus TMR in replication, integration, and pathogenesis.

Integrated analyses of genome-wide DNA occupancy and expression profiling identify key genes and pathways involved in cellular transformation by a Marek's disease virus oncoprotein, Meq

Marek's disease (MD) is an economically significant disease in chickens that is caused by the highly oncogenic Marek's disease virus (MDV). A major unanswered question is the mechanism of MDV-induced tumor formation. Meq, a bZIP transcription factor discovered in the 1990s, is critically involved in viral oncogenicity, but only a few of its host target genes have been described, impeding our understanding of MDV-induced tumorigenesis. Using chromatin immunoprecipitation-sequencing (ChIP-seq) and microarray analysis, a high-confidence list of Meq binding sites in the chicken genome and a global transcriptome of Meq-responsive genes were generated. Meq binding sites were found to be enriched in the promoter regions of upregulated genes but not in those of downregulated genes. ChIP-seq was also performed for c-Jun, a known heterodimeric partner of Meq. The close location of binding sites of Meq and c-Jun was noted, suggesting cooperativity between these two factors in modulating transcription. Pathway analysis indicated that Meq transcriptionally regulates many genes that are part of several signaling pathways including the extracellular signal-regulated kinase /mitogen-activated protein kinase (ERK/MAPK), Jak-STAT, and ErbB pathways, which are critical for oncogenesis and/or include signaling mediators involved in apoptosis. Meq activates oncogenic signaling cascades by transcriptionally activating major kinases in the ERK/MAPK pathway and simultaneously repressing phosphatases, as verified using inhibitors of MEK and ERK1/2 in a cell proliferation assay. This study provides significant insights into the mechanistic basis of Meq-dependent cell transformation.

Characterization of Meq proteins from field isolates of Marek's disease virus in Japan

Serotype 1 strains of Marek's disease virus (MDV-1) cause malignant lymphomas in chickens (Marek's disease; MD). Although MD has been controlled by vaccination, field isolates of MDV-1 have tended to increase in virulence and cause MD even in vaccinated chickens. Meq, a putative MDV-1 oncoprotein, resembles the Jun/Fos family of basic leucine zipper (bZIP) transcription factors and can regulate the expression of viral and cellular genes as a homodimer or as a heterodimer with a variety of bZIP family proteins. Sequencing analysis of some of the viral genes of various MDV-1 strains revealed a distinct diversity of and point mutations in Meq, which may contribute to changes in the transcriptional activities of Meq and, consequently, to increases in MDV-1 oncogenicity. However, few reports have characterized MDV-1 strains isolated in Japan. In this study, we established the amino acid sequences of MDV-1 field isolates from Japan in order to determine whether they display a distinct diversity of and point mutations in Meq. In addition, we analyzed the transactivation activities of the Meq proteins in order to evaluate whether the observed mutations affect their functions. Japanese MDV-1 isolates displayed the distinct mutations in basic region 2 (BR2) and proline-rich repeats (PRRs) of the Meq proteins as well as some unique mutations. Reporter assays revealed that the amino acid substitutions in BR2 and the PRRs affected the Meq transactivation activity. These results suggest that the distinct mutations are also present in the Meq proteins of MDV-1 isolates from Japan and affect their transactivation activities.

The oncogenic microRNA OncomiR-21 overexpressed during Marek's disease lymphomagenesis is transactivated by the viral oncoprotein Meq

Gallid herpesvirus 2 (GaHV-2) is an oncogenic herpesvirus that causes T lymphoma in chicken. GaHV-2 encodes a basic leucine zipper (bZIP) protein of the AP-1 family, Meq. Upon formation of homo- or heterodimers with c-Jun, Meq may modulate the expression of viral and cellular genes involved in lymphomagenesis. GaHV-2 also encodes viral microRNAs (miRNAs) involved in latency and apoptosis escape. However, little is known about cellular miRNA deregulation during the development of GaHV-2-associated lymphoma. We determined the cellular miRNA expression profiles of chickens infected with a very virulent strain (RB-1B) or a vaccine strain (CVI988) or noninfected. Among the most deregulated cellular miRNAs, we focused our efforts on gga-miR-21, which is upregulated during GaHV-2 infection. We mapped the gga-miR-21 promoter to the 10th intron of the TMEM49 gene and found it to be driven by AP-1- and Ets-responsive elements. We show here that the viral oncoprotein Meq binds to this promoter, thereby transactivating gga-miR-21 expression. We confirmed that this miRNA targets chicken programmed death cell 4 (PDCD4) and promotes tumor cell growth and apoptosis escape. Finally, gga-miR-21 was overexpressed only during infection with a very virulent strain (RB-1B) and not during infection with a nononcogenic strain (CVI988), providing further evidence for its role in GaHV-2 lymphomagenesis. Our data therefore suggest an additional role for Meq in GaHV-2-mediated lymphomagenesis through the induction of miR-21 expression.

Dynamic equilibrium of Marek's disease genomes during in vitro serial passage

Attenuation of Gallid herpesvirus-2 (GaHV-2), the causative agent of Marek's disease, can occur through serial passage of a virulent field isolate in avian embryo fibroblasts. In order to gain a better understanding of the genes involved in attenuation and associate observed changes in phenotype with specific genetic variations, the genomic DNA sequence of a single GaHV-2 virulent strain (648A) was determined at defined passage intervals. Biological characterization of these "interval-isolates" in chickens previously indicated that the ability to induce transient paralysis was lost by passages 40 and the ability to induce persistent neurological disease was lost after passage 80, coincident with the loss of neoplastic lesion formation. Deep sequencing of the interval-isolates allowed for a detailed cataloguing of the mutations that exist within a single passage population and the frequency with which a given mutation occurs across passages. Gross genetic alterations were identified in both novel and well-characterized genes and cis-acting regions involved in replication and cleavage/packaging. Deletions in genes encoding the virulence factors vLipase, vIL8, and RLORF4, as well as a deletion in the promoter of ICP4, appeared between passages 61 and 101. Three mutations in the virus-encoded telomerase which predominated in late passages were also identified. Overall, the frequency of mutations fluctuated greatly during serial passage and few genetic changes were absolute. This indicates that serial passage of GaHV-2 results in the generation of a collection of genomes with limited sequence heterogeneity.

Selection of a recombinant Marek's disease virus in vivo through expression of the Marek's EcoRI-Q (Meq)-encoded oncoprotein: characterization of an rMd5-based mutant expressing the Meq of strain RB-1B

Marek's disease (MD) is a highly contagious viral disease of chickens (Gallus gallus domesticus) caused by MD virus (MDV), characterized by paralysis, neurologic signs, and the rapid onset of T-cell lymphomas. MDV-induced T-cell transformation requires a basic leucine zipper protein called Marek's EcoRI-Q-encoded protein (Meq). We have identified mutations in the coding sequence of Meq that correlated with virus pathotype (virulent, very virulent, and very virulent plus). The aim of this study was to determine whether recombinant viruses could be isolated based on Meq expression through in vivo selection. Chicken embryo fibroblasts (CEFs) were cotransfected with an rMd5 strain-based Meq deletion virus (rMd5deltaMeq) and meq loci from strains representing different pathotypes of MDV. Transfected CEFs were inoculated into chickens in two independent studies. We were able to isolate a single recombinant virus, rMDV-1137, in a contact-exposed chicken. rMDV-1137 had recombined two copies of the meq gene of RB-1B and was found to have pathogenicity similar to both RB-1B and rMd5 parental strains. We found the RB-1B- and rMd5-induced lymphomas showed differences in composition and that rMDV-1137-induced lymphomas were intermediate in their composition. We were able to establish cell lines from both RB-1B- (MDCC-UD35, -UD37) and rMDV-1137 (MDCC-UD36, -UD38)-induced, but not rMd5-induced, lymphomas. To date, no rMd5- or parent Md5-transformed T-cell lines have been reported. Our results suggest that 1) a recombinant MDV can be selected on the basis of oncogenicity; 2) changes in Meq sequence seem to affect tumor composition and the ability to establish cell lines; and 3) in addition to meq, other genomic loci affect MDV pathogenicity and oncogenicity.

Hypomethylation upregulates the expression of CD30 in lymphoma induced by Marek's disease virus

Epigenetic modification is widely known to be involved in embryo development, aging, tumorigenesis, and many complex diseases. Both hypermethylation of CpG islands at the gene promoters and global hypomethylation are involved in the initiation and progression of carcinogenesis. However, only a small portion of hypomethylation occurs at gene promoters and leads to the overexpression of certain oncogenes. To determine whether DNA methylation plays a role in tumorigenesis of Marek's disease, we selected one putative oncogene and 8 tumor suppressor genes from the gene expression profile for the analysis of DNA methylation variation. Four normal spleen tissues and 4 Marek's disease virus-infected tumor spleen tissues were collected, and the methylation level of the promoter region of each gene was analyzed using MassARRAY. As a result, the promoter region of CD30 was hypomethylated and displayed a significantly higher expression in Marek's disease virus-infected tumor spleen tissues compared with normal ones (P < 0.05). In neoplastic cells, CD30 was known to promote the survival and proliferation of T-cell lymphomas. This result suggests that activation of CD30 is possibly associated with the tumorigenesis of Marek's disease.

Kinetic expression analysis of the cluster mdv1-mir-M9-M4, genes meq and vIL-8 differs between the lytic and latent phases of Marek's disease virus infection

Marek's disease virus (GaHV-2) is an alphaherpesvirus that induces T-cell lymphoma in chickens. The infection includes both lytic and latent stages. GaHV-2 encodes three clusters of microRNAs (miRNAs) located in the internal (I)/terminal (T) repeat (R) regions. We characterized transcripts encompassing the mdv1-mir-M9-M4 and mir-M11-M1 clusters located in the IR(L)/TR(L) region, upstream and downstream from the meq oncogene, respectively. By 5'- and 3'-RACE-PCR and targeted RT-PCR, we showed that mdv1-mir-M9-M4 could be transcribed from an unspliced transcript or from at least 15 alternatively spliced transcripts covering the IR(L)/TR(L) region, encompassing the meq and vIL-8 genes and localizing the mdv1-mir-M9-M4 cluster to the first intron at the 5'-end. However, all these transcripts, whether spliced or unspliced, seemed to start at the same transcriptional start site, their transcription being driven by a single promoter, prmiRM9M4. We demonstrated alternative promoter usage for the meq and vIL-8 genes, depending on the phase of GaHV-2 infection. During the latent phase, the prmiRM9M4 promoter drove transcription of the meq and vIL-8 genes and the mdv1-mir-M9-M4 cluster in the first intron of the corresponding transcripts. By contrast, during the lytic phase, this promoter drove the transcription only of the mdv1-mir-M9-M4 cluster to generate unspliced mRNA, the meq and vIL-8 genes being transcribed principally from their own promoters. Despite the expression of meq and the mdv1-mir-M9-M4 cluster under two different transcriptional processes during the latent and lytic phases, our data provide an explanation for meq expression and mdv1-mir-M4-5P overexpression in miRNA libraries from GaHV-2-infected cells, regardless of the phase of infection.

Chicks and single-nucleotide polymorphisms: an entrée into identifying genes conferring disease resistance in chicken

Marek’s disease (MD) is one of the most serious chronic infectious disease threats to the poultry industry worldwide. Selecting for increased genetic resistance to MD is a control strategy that can augment current vaccinal control measures. Although our previous efforts integrating various genomic screens successfully identified three resistance genes, the main limitation was mapping precision, which hindered our ability to identify and further evaluate high-confidence candidate genes. Towards identifying the remaining genes of this complex trait, we incorporated three additional approaches made substantially more powerful through next-generation sequencing and that exploit the growing importance of expression variation. First, we screened for allele-specific expression (ASE) in response to Marek’s disease virus (MDV) infection, which, when allelic imbalance was identified, is sufficient to indicate a cis-acting element for a specific gene. Second, sequencing of genomic regions enriched by chromatin immunoprecipitation (ChIP) combined with transcript profiling identified motifs bound and genes directly regulated by MDV Meq, a bZIP transcription factor and the viral oncogene. Finally, analysis of genomic sequences from two experimental lines divergently selected for MD genetic resistance allowed inference about regions under selection as well as potential causative polymorphisms. These new combined approaches have resulted in a large number of high-confidence genes conferring MD resistance reflecting the multigenic basis of this trait, which expands our biological knowledge and provides corresponding single-nucleotide polymorhpisms (SNPs) that can be directly evaluated for their genetic contribution towards disease resistance.

MiRNA expression signatures induced by Marek's disease virus infection in chickens

MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression at the post-transcriptional level. Emerging evidence suggests that differential miRNA expression is associated with viral infection and cancer. Marek's disease virus infection induces lymphoma in chickens. However, the host defense response against Marek's disease (MD) progression remains poorly understood. Here, we utilized microarrays to screen miRNAs that were sensitive to Marek's disease virus (MDV) infection. QRT-PCR analysis confirmed the microarray data and revealed expression patterns of some miRNAs in tumor samples. Chicken miRNA gga-miR-15b, which was reduced in infected susceptible chickens and splenic tumors, controlled the expression of ATF2 (activating transcription factor 2). ATF2 was significantly increased in the same group. Our results indicated that differential expression of miRNA in resistant and susceptible chickens was caused by MDV infection, which effectively influenced protein expression of ATF2. This latter result might be related to Marek's disease resistance/susceptibility.

Epigenetic regulation of the latency-associated region of Marek's disease virus in tumor-derived T-cell lines and primary lymphoma

Meq is the major Marek's disease virus (MDV)-encoded oncoprotein and is essential for T-cell lymphomagenesis. Meq and several noncoding RNAs, including three microRNA (MiR) clusters, are expressed from the repeats of the MDV genome during latent infection of T cells. To investigate the state of the chromatin in this and flanking regions, we carried out chromatin immunoprecipitation (ChIP) analysis of covalent histone modifications and associated bound proteins. T-cell lines and a lymphoma were compared. The chromatin around the promoters for Meq and the noncoding RNAs in both cell lines and the lymphoma were associated with H3K9 acetylation and H3K4 trimethylation, which are marks of transcriptionally active chromatin. These correlated with bound Meq-c-Jun heterodimers. The only binding site for Meq homodimers is located at the lytic origin of replication (OriLyt), next to the lytic gene pp38. This region lacked active marks and was associated with repressive histone modifications (H3K27 and H3K9 trimethylation). DNA CpG methylation was investigated using methylated DNA precipitation (MeDP). In cell lines, DNA methylation was abundant across the repeats but noticeably reduced or absent around the active promoters. In primary tumors, CpG methylation occurred less than 2 months after infection, focused within the ICP4 gene. These data suggest that nonrandom de novo DNA methylation occurs early in lymphomagenesis. In addition, the histone data indicate a role for Meq in the epigenetic regulation of the MDV genome repeats in transformed T cells and suggest that the OriLyt region and the Meq/MiR region might be separated by chromatin boundary elements, and preliminary data on CTCF binding are consistent with this.

Analysis of transcriptional activities of the Meq proteins present in highly virulent Marek's disease virus strains, RB1B and Md5

Marek's disease virus (MDV) is an oncogenic herpesvirus that causes malignant lymphomas in chickens. Recent field isolates of MDV have tended to exhibit increasing virulence, and MDV strains are currently classified into four categories based on their relative virulence. Meq, a putative MDV oncoprotein, resembles the Jun/Fos family of basic leucine zipper (bZIP) transcription factors and can regulate the expression of viral and cellular genes as a homodimer or as a heterodimer with a variety of bZIP family proteins. MDV isolates display distinct diversity and point mutations in Meq, which may contribute to changes in the transcriptional activities of Meq and subsequently, to observed increases in MDV oncogenicity. In this study, we introduced mutations into the meq gene and used dual luciferase reporter assays to analyze the transcriptional activities of the resulting Meq proteins to determine whether distinct mutations in Meq could be responsible for differences in transcriptional activity among MDV strains. A proline-to-alanine substitution at position 217, the second position of one of the proline direct repeats in the transactivation domain, enhanced the transactivation activity of Meq. In addition, we found that two substitutions at positions 283 and 320 affected transactivation activity. These results suggest that the distinct diversity of and point mutations in the Meq proteins are responsible for differences in transactivation activity among MDV strains.

Comparative analysis of oncogenic genes revealed unique evolutionary features of field Marek's disease virus prevalent in recent years in China

BACKGROUND

Marek's disease (MD) is an economically important viral disease of chickens caused by Marek's disease virus (MDV), an oncogenic herpesvirus. This disease was well controlled since the widespread use of commercial vaccines, but field MDVs have shown continuous increasing in virulence and acquired the ability to overcome the immune response induced by vaccines. Nowadays, MD continues to be a serious threat to poultry industry, isolation and characterization of MDVs are essential for monitoring changes of viruses and evaluating the effectiveness of existing vaccines.

RESULTS

Between 2008 and 2010, 18 field MDV strains were isolated from vaccinated chicken flocks in Sichuan province, China. Three oncogenic genes including Meq, pp38 and vIL-8 genes of the 18 isolates were amplified and sequenced. Homology analysis showed that the deduced amino acid sequences of these three genes exhibit 95.0-98.8%, 99.3-100% and 97.0-98.5% homology respectively with these of other reference strains published in GenBank. Alignment analysis of the nucleotide and deduced amino acid sequences showed that four amino acid mutations in Meq gene and two amino acid mutations in vIL-8 gene displayed perfect regularity in MDVs circulating in China, which could be considered as features of field MDVs prevalent in recent years in China. In addition, one amino acid mutation in pp38 gene can be considered as a feature of virulent MDVs from USA, and three amino acid mutations in Meq gene were identified and unique in very virulent plus (vv+) MDVs. Phylogenetic analysis based on Meq and vIL-8 protein sequences revealed that field MDVs in China evolved independently. Virulence studies showed that CVI988 could provide efficient protection against the field MDVs epidemic recently in China.

CONCLUSIONS

This study and other published data in the GenBank have demonstrated the features of Meq, pp38 and vIL-8 genes of MDVs circulating in recent years in Sichuan, China. Mutations, deletions or insertions were observed in these three genes, and some mutations could be considered as the unique marks of the MDVs circulating presently in China. The paper supplies some valuable information concerning the evolution of MDV which is useful for the vaccine development and control of MD in China.

MicroRNA-26a-mediated regulation of interleukin-2 expression in transformed avian lymphocyte lines

Background

Micro(mi)RNAs are a class of small non-coding RNAs that play critical roles in the induction of various cancers, including lymphomas induced by oncogenic viruses. While some of the miRNAs are oncogenic, miRNAs such as miR-26a are consistently downregulated in a number of cancers, demonstrating their potential tumor suppressor functions. Global miRNA expression profiles of a number of virus-transformed avian lymphoma cell lines have shown downregulation of gga-miR-26a expression, irrespective of molecular mechanisms of transformation or the viral aetiology. The neoplastic transformation of lymphocytes by many viruses accompanies high levels of proliferative responses, mostly mediated through cytokines such as IL-2. Chicken IL-2 can modulate T-cell proliferation and cytotoxicity in vitro and in vivo and dysregulation of IL-2 expression is observed in diseases such as leukaemia.

Results

The expression levels of gga-miR-26a in chicken lymphoma cells transformed by 3 distinct avian oncogenic viruses, viz Marek's disease virus (MDV), avian leukosis virus (ALV) and Reticuloendotheliosis virus (REV) were consistently downregulated compared to the levels in the normal lymphocytes. This downregulation of miR-26a regardless of the viral etiology and molecular mechanisms of transformation was consistent with the tumor suppressor role of this miRNA. Notwithstanding this well-established role in cancer, we demonstrate the additional role of this miRNA in directly targeting chicken IL-2 through reporter and biochemical assays. The downregulation of miR-26a can relieve the suppressive effect of this miRNA on IL-2 expression.

Conclusions

We show that miR-26a is globally downregulated in a number of avian lymphoma cells irrespective of the mechanisms of transformation, reiterating the highly conserved tumor suppressor function of this miRNA. However, with the potential for directly targeting chicken IL-2, the downregulation of miR-26a in these tumor cells could relieve the inhibitory effect on IL-2 expression assisting in the proliferative features of the transformed lymphocyte lines.

Viral control of vTR expression is critical for efficient formation and dissemination of lymphoma induced by Marek's disease virus (MDV)

Marek's disease virus (MDV) is an alphaherpesvirus that causes lethal T-cell lymphomas in chickens. MDV is unique in that it harbors two copies of a viral telomerase RNA subunit (vTR) in its genome exhibiting 88% sequence identity to the chicken orthologue, chTR. The minimal telomerase ribonucleoprotein complex consists of a protein subunit with reverse transcriptase activity (TERT) and TR. Physiologically, the complex compensates for the progressive telomere shortening that occurs during mitosis and is involved in the process of cellular immortalization. Previous studies showed that MDV vTR performes an auxiliary function during oncogenesis. Comparative in vitro analysis of the viral and chicken TR promoters revealed that the vTR promoter (PvTR) was up to 3-fold more efficient than the chTR promoter (PchTR) in avian cells and that the stronger transcriptional activity of PvTR resulted largely from an E-box located two nucleotides downstream of the transcriptional start site of the vTR gene. To test the hypothesis that PvTR is required for vTR expression and, hence, efficient tumor formation, we generated a recombinant virus, vPchTR+/+, in which the vTR promoter was replaced by that of chTR. In vivo, growth of vPchTR+/+ was indistinguishable from that of parental virus; however, tumor induction was reduced by >50% and lymphomas were smaller and less disseminated when compared to those induced by parental virus. We concluded that PvTR is not required for lytic replication in vivo but is essential for efficient transcription of vTR and thereby critical for efficient MDV lymphoma formation.

Identification of bZIP interaction partners of viral proteins HBZ, MEQ, BZLF1, and K-bZIP using coiled-coil arrays

Basic-region leucine-zipper transcription factors (bZIPs) contain a segment rich in basic amino acids that can bind DNA, followed by a leucine zipper that can interact with other leucine zippers to form coiled-coil homo- or heterodimers. Several viruses encode proteins containing bZIP domains, including four that encode bZIPs lacking significant homology to any human protein. We investigated the interaction specificity of these four viral bZIPs by using coiled-coil arrays to assess self-associations as well as heterointeractions with 33 representative human bZIPs. The arrays recapitulated reported viral-human interactions and also uncovered new associations. MEQ and HBZ interacted with multiple human partners and had unique interaction profiles compared to any human bZIPs, whereas K-bZIP and BZLF1 displayed homospecificity. New interactions detected included HBZ with MAFB, MAFG, ATF2, CEBPG, and CREBZF and MEQ with NFIL3. These were confirmed in solution using circular dichroism. HBZ can heteroassociate with MAFB and MAFG in the presence of MARE-site DNA, and this interaction is dependent on the basic region of HBZ. NFIL3 and MEQ have different yet overlapping DNA-binding specificities and can form a heterocomplex with DNA. Computational design considering both affinity for MEQ and specificity with respect to other undesired bZIP-type interactions was used to generate a MEQ dimerization inhibitor. This peptide, anti-MEQ, bound MEQ both stably and specifically, as assayed using coiled-coil arrays and circular dichroism in solution. Anti-MEQ also inhibited MEQ binding to DNA. These studies can guide further investigation of the function of viral and human bZIP complexes.

Both homo and heterodimers of Marek's disease virus encoded Meq protein contribute to transformation of lymphocytes in chickens

Marek' disease virus serotype-1, also know as Gallid herpesvirus 2 (GaHV-2), elicits T-cell lymphomas in chickens. The GaHV-2 genome encodes an oncoprotein, Meq, with similarity to the Jun/Fos family of proteins. We have previously shown that Meq homodimers are not sufficient to induce lymphomas in chickens. In this study, we investigated the role of Meq heterodimers in the pathogenicity of GaHV-2 by generating a chimeric meq gene, which contains the leucine zipper region of Fos (meqFos). A recombinant virus containing the meqFos gene in place of parental meq, rMd5-MeqFos, was not capable of transforming chicken lymphocytes, indicating that heterodimerization of Meq alone is not sufficient for transformation. In addition, the recovery of the oncogenic phenotype by a recombinant virus encoding one copy each of MeqGCN (homodimer) and MeqFos (heterodimer) conclusively demonstrates that both homo and heterodimerization of Meq are required for oncogenesis.