The restriction endonuclease map of Marek's disease virus (MDV) serotype 2 and collinear relationship among three serotypes of MDV

Marek's disease virus research in the post-sequencing era: new tools for the study of gene functions and virus-host interactions

Despite the fact that the causative agent of Marek's disease was described more than 30 years ago, and that subsequently many classical biological studies have been carried out on the Marek's disease virus (MDV), detailed analysis of its gene functions has been hampered by lack of suitable research tools. Information on the primary structure of MDV-1 and its serologically related viruses, MDV-2 and herpesvirus of turkeys, is now available. This review focuses on the introduction of the modern and highly efficient technology of bacterial artificial chromosome (BAC) cloning and mutagenesis for rapid manipulation of the MDV genome, with the aim of studying the functions of its genes and non-coding regions. Constructed MDV BACs carry the complete genome of MDV that can be multiplied in Escherichia coli and manipulated using the tools provided by bacterial genetics. The novel approach of MDV DNA mutagenesis using BAC technology will be explained by examples, and we will discuss gene functions in comparison with their counterparts in other herpesviruses. In addition, we have shown that MDV BAC DNA can be used as a polynucleotide vaccine to protect against Marek's disease, thus opening a new chapter in strategies for control of this disease.

Marek's disease virus (MDV) encodes an interleukin-8 homolog (vIL-8): characterization of the vIL-8 protein and a vIL-8 deletion mutant MDV

Chemokines induce chemotaxis, cell migration, and inflammatory responses. We report the identification of an interleukin-8 (IL-8) homolog, termed vIL-8, encoded within the genome of Marek's disease virus (MDV). The 134-amino-acid vIL-8 shares closest homology to mammalian and avian IL-8, molecules representing the prototype CXC chemokine. The gene for vIL-8 consists of three exons which map to the BamHI-L fragment within the repeats flanking the unique long region of the MDV genome. A 0.7-kb transcript encoding vIL-8 was detected in an n-butyrate-treated, MDV-transformed T-lymphoblastoid cell line, MSB-1. This induction is essentially abolished by cycloheximide and herpesvirus DNA polymerase inhibitor phosphonoacetate, indicating that vIL-8 is expressed with true late (gamma2) kinetics. Baculovirus-expressed vIL-8 was found to be secreted into the medium and shown to be functional as a chemoattractant for chicken peripheral blood mononuclear cells but not for heterophils. To characterize the function of vIL-8 with respect to MDV infection in vivo, a recombinant MDV was constructed with a deletion of all three exons and a soluble-modified green fluorescent protein (smGFP) expression cassette inserted at the site of deletion. In two in vivo experiments, the vIL-8 deletion mutant (RB1BvIL-8DeltasmGFP) showed a decreased level of lytic infection in comparison to its parent virus, an equal-passage-level parent virus, and to another recombinant MDV containing the insertion of a GFP expression cassette at the nonessential US2 gene. RB1BvIL-8DeltasmGFP retained oncogenicity, albeit at a greatly reduced level. Nonetheless, we have been able to establish a lymphoblastoid cell line from an RB1BvIL-8DeltasmGFP-induced ovarian lymphoma (MDCC-UA20) and verify the presence of a latent MDV genome lacking vIL-8. Taken together, these data describe the identification and characterization of a chemokine homolog encoded within the MDV genome that is dispensable for transformation but may affect the level of MDV in vivo lytic infection.

Identification of the Marek's disease virus serotype 2 genes homologous to the glycoprotein B (UL27), ICP18.5 (UL28) and major DNA-binding protein (UL29) genes of herpes simplex virus type 1

We determined the nucleotide sequence of non-pathogenic Marek's disease virus serotype 2 (MDV2) strain HPRS24 glycoprotein B (gB) (UL27), ICP18.5 (UL28) and major DNA-binding protein (MDBP) (UL29) genes homologous to herpes simplex virus type 1 (HSV-1). The sequence data revealed that important motives in the proteins are conserved in MDV2 ICP18.5 and MDBP, however the sequence of viral DNA replication origin which exists in the regions between the UL29 and UL30 genes of other alphaherpesviruses was not found in the regions of the MDV2 genome. By northern blot analyses, we also demonstrated that 8.9, 5.0 and 2.6 kb transcripts were actually transcribed from the sequenced region in MDV2-infected cells. The MDV2 UL28 and UL29 genes have not been reported in other serotypes of MDV.

Identification and transcriptional analysis of the homologues of the herpes simplex virus type 1 UL30 to UL40 genes in the genome of nononcogenic Marek's disease virus serotype 2

Studies on Marek's disease virus serotype 2 (MDV2) are important for understanding the natural nononcogenic phenotype of MDV. This study reports a 27,535 bp nucleotide sequence of part of the MDV2 genome located in the central unique long (U(L)) region. The analysis revealed 11 complete ORFs with high amino acid sequence identities to the products of other alphaherpesviruses. The MDV2 ORFs were arranged collinearly with the prototype sequence of herpes simplex virus type 1, ranging from the UL30 to UL40 genes. Sequences that were particularly well conserved among alphaherpesviruses were the putative functional domain of the DNA polymerase (UL30) and the ribonucleotide reductase large and small subunits (UL39 and UL40). On the other hand, in contrast to oncogenic MDV1, MDV2 did not contain the conserved proline-repeat region in the UL36 homologue. All the genes identified were confirmed to be transcribed as 3'-coterminal mRNAs and/or unique transcripts in virus-infected cells.

Identification and sequence analysis of the Marek's disease virus serotype 2 homologous genes of the herpes simplex virus type 1 UL25, UL26 and UL26.5 genes

We identified and determined the nucleotide sequence of Marek's disease virus serotype 2 (MDV2) UL25, UL26 and UL26.5 homologous genes of herpes simplex virus type 1 (HSV-1). The UL25, UL26 and UL26.5 genes of HSV-1 encode virion proteins (UL25 and UL26.5) and serine protease (UL26). The deduced amino acid sequences of the three proteins show a high degree of homology to counterparts of HSV-1. By northern blot analyses we found that four transcripts whose sizes are 4.9, 3.9, 2.0 and 1.3 kb are transcribed from the domains of MDV2 genome containing the three genes. This is the first report dealing with UL25, UL26 and UL26.5 homologues of HSV-1 in MDV serotypes.

Identification and sequence analysis of the Marek's disease virus serotype 2 gene homologous to the herpes simplex virus type 1 UL52 protein

The gene of Marek's disease virus serotype 2 (MDV2) homologous to the UL52 gene of herpes simplex virus type 1 (HSV-1) was identified and characterized. The MDV2 UL52 homologous gene encodes 1,071 amino acids with a molecular weight of 118.7 kDa, which includes putative metal-binding site and overlapping region with the UL53 homologous gene. Although a putative polyadenylation signal sequence was found in the downstream of the MDV2 UL52 gene, a MDV2 UL52 DNA probe reacted only with the polycistronic 6.3 kb transcript, representing the UL52 and the downstream genes of UL53 and UL54. Transcriptional pattern of this region of MDV2 was somewhat different from corresponding regions of HSV-1 and infectious laryngotracheitis virus.

Identification and DNA sequence analysis of the Marek's disease virus serotype 2 genes homologous to the herpes simplex virus type 1 UL20 and UL21

We determined 3,135 bp of the nucleotide sequence located in an 8.5 kb EcoRI-E fragment in the unique long (UL) genome region of Marek's disease virus serotype 2 (MDV2), and identified UL20 and UL21 homologous genes of herpes simplex virus type 1 (HSV-1). The UL20 and UL21 homologous genes of MDV2 are arranged colinearly with the prototype sequence of HSV-1. In addition, an open reading frame (MDV2 ORF 273), which has been identified within the UL21 homologous gene of MDV2, has no apparent relation to any other known herpesvirus genes. Northern blot analysis and reverse transcriptase polymerase chain reaction confirmed the existance of RNA transcripts related to the UL20 and ORF 273 genes in MDV2-infected cells, except no transcript related to the UL21 gene being detected. The putative protein product of the MDV2 UL20 gene had a relatively low homology but that of the MDV2 UL21 gene had a moderate homology among herpesviruses. Further, the possible functions and features of the predicted proteins encoded within the sequenced region are discussed.

Identification and structure of the Marek's disease virus serotype 2 glycoprotein M gene: comparison with glycoprotein M genes of Herpesviridae family

We determined the nucleotide sequence of a portion of BamHI-C fragment of Marek's disease virus serotype 2 (MDV2) strain HPRS24 which was suspected to contain the homologue of the herpes simplex virus type 1 (HSV-1) gene UL10, encoding glycoprotein M (gM). An open reading frame whose translation product exhibited significant similarities to HSV-1 gM protein and respective proteins of other herpesviruses of 37.5% and 45.5% to 31.8%, respectively, was identified. A number of distinct transcriptional consensus sequences were found upstream of the first putative start codon of MDV2 UL10 protein. In transcriptional analysis, the gene was transcribed into an 1.5 kb RNA. The primary translation product comprises 424 amino acids with a predicted molecular weight of 46.9 kDa. The predicted MDV2 UL10 protein contains eight hydrophobic domains with sufficient length and hydrophobicity to span the lipid bilayer conserved in the genomes of all herpesviruses which have been sequenced so far. In the region located between the first and second hydrophobic domains, two potential N-linked glycosylation sites were presented. Interestingly, highly charged residues were abundantly possessed in the carboxy-terminal part of the MDV2 UL10 protein. By comparison of the amino acid sequence of the MDV2 UL10 gene with the homologues from other herpesviruses, the data might contribute for further evidence of the evolution of herpesviruses from a common progenitor and an ancient example of MDV2 belonging to the Alphaherpesvirinae subfamily. In addition, the existence of corresponding genes in human, mammalian, and avian herpesvirus genomes, suggests indirectly an important role for gM in the natural life cycle of the virus.

Gene arrangement and RNA transcription of the BamHI fragments K and M2 within the non-oncogenic Marek's disease virus serotype 2 unique long genome region

We determined the nucleotide sequence of a 6593 bp fragment of the Marek's disease virus serotype 2 (MDV2) unique long region located in the right part of genomic BamHI-M2 and the adjacent part of BamHI-K fragments. Within this region five complete open reading frames (ORFs) were identified whose deduced amino acid sequences exhibited homology to the UL53 (glycoprotein K), UL54 (immediate early regulatory protein ICP27), and UL55 gene products of herpes simplex virus type 1 (HSV-1). Homologue to the HSV-1 UL56 was not detected. However, we identified a gene between the MDV2 UL54 and UL55 genes with homology to the first ORF (ORF-1) of equine herpesvirus type 1 and corresponding gene identified in pseudorabies virus. Two adjacent ORFs contained in the BamHI-K fragment, ORF 873s and ORF 873, were found by computer analysis to have the properties of an intron encoding a glycoprotein: ORF 873s encodes a 84 amino acid polypeptide with a stretch of a hydrophobic signal sequence in the C-terminus, and ORF 873 encodes a 873 amino acid polypeptide with a transmembrane domain and putative three N-linked glycosylation sites. All the identified genes were confirmed to be transcribed with 3'-coterminal transcripts and/or a unique transcript in the virus-infected cells. Especially, 3.5 kb mRNA of ORF 873s and ORF 873 are transcribed from a potential promoter region of ORF 873s, and splice donor and acceptor sites are used to splice the mRNA after cleavage of a 113 bp-nucleotide sequence.

The genetic organization and transcriptional analysis of the short unique region in the genome of nononcogenic Marek's disease virus serotype 2

Studies on the Marek's disease virus (MDV) serotype 2 (MDV2) genome may be important for understanding the naturally nononcogenic nature of the virus. To determine the complete DNA sequence of MDV2 unique short (Us) region, genomic BamHI fragments F, M1 and R were sequenced. The MDV2 Us region is 12109 bp long and contains 12 potential open reading frames (ORFs) likely to encode for proteins. Seven of them exhibit homologies to herpes simplex virus type 1 (HSV-1) US1 (ICP22), US2, US3 (protein kinase), US6 (gD), US7 (gI), US8 (gE) and US10 genes. These ORFs are conserved in a similar arrangement with those of HSV-1, except for US10 which is transposed in the Us regions of all three MDV serotypes. The predicted amino acid sequence of MDV2 ORF6 is homologous to SORF3 of the other serotypes of MDV serotype 1 (MDV1) and herpesvirus of turkeys (HVT) and to infectious laryngotracheitis virus SR1. In addition, four ORFs, which have been identified around the Us and inverted repeat junction regions, have no apparent relation to any other known herpesvirus genes. The identified ORFs in the MDV2 Us region were more colinear with their previously reported locations of MDV1 than with those of HVT and other alphaherpesviruses. Ten of the 12 ORFs in the MDV2 Us region were expressed and transcribed with 3'-coterminal transcripts and/or a unique transcript in the virus-infected cells. Compared to other MDV serotypes, the MDV2 Us-encoded proteins showed 46-70% and 33-59% identities with equivalent of MDV1 and HVT at the amino acid level, respectively. Our present data will be useful to understand the different pathogenicity among serotypes of MDV and to allow precise manipulation of the genes for a possible use in genetically engineered vaccines.

Identification and DNA sequence analysis of the Marek's disease virus serotype 2 gene homologous to the herpes simplex virus type 1 glycoprotein H

Marek's disease virus (MDV) serotype 2 (MDV2) gene homologous to the glycoprotein H (gH) gene of herpes simplex virus type 1 was identified and sequenced. The predicted region encoding for the MDV2 gH gene was 2436 nucleotide and the primary translation product was 812 amino acids with a molecular weight of 89.4 kDa. The protein encoded by MDV2 gH gene has a number of features characteristic of a membrane-associated glycoprotein. First, there are 9 potential N-linked glycosylation sites and 11 cysteine residues, and 6 of the sites and 8 of the residues were conserved among all of the three MDV serotypes. Second, this protein had N-terminal and C-terminal hydrophobic regions, which were a signal sequence and a transmembrane-anchor domain, respectively. From the northern blot analysis, it was suggested that a transcript encoding MDV2 gH and a poly-cistronic transcript encoding MDV2 thymidine kinase, gH, and possibly other genes of downstream on this strand existed. Alignment of the amino acid sequences of the gH homologues among the three MDV serotypes showed 57.5% (MDV1 and MDV2), 56.2% (MDV1 and HVT), and 50.1% (MDV2 and HVT) identities.

Characterization and expression of the Marek's disease virus serotype 2 glycoprotein E in recombinant baculovirus-infected cells: initial analysis of its DNA sequence and antigenic properties

In Marek's disease virus (MDV) serotype 2 (MDV2) genome, a gene equivalent to the glycoprotein E (gE) of other alphaherpesviruses was identified and sequenced. The primary translation product comprises 488 amino acids with a M(r) of 54.3 kDa. The predicted amino acid sequence possesses several characteristics typical of membrane glycoproteins, including a N-terminal hydrophobic signal sequence, C-terminal transmembrane and cytoplasmic domains, and extra-cellular region containing four potential N-linked glycosylation sites. Compared with other MDV serotypes, MDV2 gE showed 47.3% identity with MDV1 gE, and 38.9% identity with HVT gE at the amino acid level. In transcriptional analyses, a 2.0 kb mRNA which starts between 65 and 86 bps upstream of the potential translational initiation codon of gE was identified as the gE-specific transcript. By a recombinant baculovirus, this potential gE coding region was expressed as several specific products from 66 to 72 kDa. These products were susceptible to tunicamycin treatment, indicating that they were glycoprotein in nature. Further, the expressed gE reacted with all chicken-antisera raised to each of the three serotypes of MDV (strains GA, SB-1, and FC126), suggesting that gE is expressed by all three serotypes of MDV in infected cells and conserves common antigenic epitope(s) beyond those that are serotype specific.

Identification and DNA sequence analysis of the Marek's disease virus serotype 2 genes homologous to the thymidine kinase and UL24 genes of herpes simplex virus type 1

The thymidine kinase (TK) gene has been used as a safe and convenient locus for expression of heterologous proteins in some alphaherpesviruses including herpesvirus of turkeys (HVT) antigenically related to Marek's disease virus (MDV) serotypes 1 (MDV1) and 2 (MDV2). In MDV2 strain HPRS 24 genome, genes equivalent to the TK and UL24 homologues of herpes simplex virus type 1 were identified and sequenced. The MDV2 UL24 gene overlaps the 5' end of the TK gene in a head-to-head orientation. The predicted region encoding for the MDV2 TK gene is 1,056 nucleotides, corresponding to a polypeptide of 352 amino acids in length. Putative nucleotide- and thymidine-binding sites were identified within the predicted amino acid sequence. The predicted region encoding for the UL24 gene is 948 nucleotides, corresponding to a polypeptide of 316 amino acids in length. By northern blot analyses using MDV2 TK- and UL24-specific DNA probes, four transcripts of approximately 7.8, 5.0, 3.5, and 1.1 kb for the TK gene, and a transcript of 3.8 kb for the UL24 gene were detected in MDV2-infected cells. Alignment of the amino acid sequence of MDV2 TK homologue with those published for TK homologues of other MDV serotypes showed 73.9% (MDV1 vs. MDV2), 58.2% (MDV1 vs. HVT), and 56.8% (MDV2 vs. HVT) identities. Comparison to other alphaherpesvirus TK homologues revealed amino acid sequence homologies varying from 34.5% to 27.8%. The putative MDV2 UL24 homologous protein had identity with the well conserved five motifs among alphaherpesviruses.

Identification of a potential Marek's disease virus serotype 2 glycoprotein D gene with homology to herpes simplex virus glycoprotein D

The gene of Marek's disease virus (MDV) serotype 2 (MDV2) homologous to glycoprotein D (gD) of herpes simplex virus (HSV) was identified and characterized by its nucleotide and predicted amino acid sequences. The MDV2 gD homologous gene contains an open reading frame capable of specifying a polypeptide of 385 amino acids, which include N- and C-terminal hydrophobic domains consistent with signal and anchor regions, respectively, and two potential N-linked glycosylation sites, one of which was located in a highly conserved region when compared to MDV serotype 1 (MDV1) and herpesvirus of turkeys (HVT). By northern blot analysis using a MDV2 gD-specific DNA probe, two highly abundant polycistronic 6.0 and 4.2 kb transcripts were detected in MDV2-infected cells. The genes encoding MDV2 protein kinase (PK), gD, and glycoprotein I (gI) homologues are transcribed to form 3' coterminal mRNAs of 6.0 kb (encoding PK, gD and gI) and 4.2 kb (encoding gD and gI), respectively. By using rapid amplification cDNA end (RACE) method, several RNA start sites, to be thought those of the 4.2 kb mRNA, were detected in the upstream of MDV2 gD homologue.

Preparation of monoclonal antibodies against Marek's disease virus serotype 1 glycoprotein D expressed by a recombinant baculovirus

A recombinant baculovirus, the genome of which contains DNA encoding Marek's disease virus serotype 1 (MDV1) homolog of glycoprotein D (gD) of herpes simplex virus under the polyhedrin promoter was constructed and designated rAcMDV1gD. Five monoclonal antibodies (MAbs) which recognize the MDV1 homolog of gD (MDV1 gD) in Spodoptera frugiperda cells infected with rAcMDV1gD were prepared. The MAbs reacted with proteins ranging from 52 to 49 kDa in rAcMDV1gD-infected cell lysates by immunoblot analysis. These molecular weights were coincident with molecular weights predicted from the open reading frame of MDV1 gD. By ELISA additivity test, the 5 MAbs were divided into 3 groups which seemed to recognize 3 different epitopes. In addition, all of the 5 MAbs were reactive with chick embryo fibroblasts (CEFs) expressing MDV1 gD. The MAbs are considered to be useful to study the role of MDV1 gD in MDV1 infection.

Transactivation activity of Meq, a Marek's disease herpesvirus bZIP protein persistently expressed in latently infected transformed T cells

Marek's disease virus (MDV) is an avian herpesvirus that induces a variety of diseases, including T-cell lymphomas, in chickens. In latently infected, transformed lymphoid cells, very few viral transcripts or proteins are detected. We previously described a gene, meq (MDV EcoQ), which is persistently expressed in MDV-transformed tumor samples and cell lines. meq codes for a 339-amino-acid protein with a basic-leucine zipper domain near its N terminus and a proline-rich domain near its C terminus. The basic-leucine zipper domain shows homology with Jun/Fos family proteins, whereas the proline-rich domain resembles that of the WT-1 tumor suppressor protein. These structural features raise the possibility that Meq functions as a transcription factor in regulating viral latency or oncogenesis. In this report, we show that the proline-rich domain is a potent transcription activator when fused to the yeast (Saccharomyces cerevisiae) Gal4(1-147) DNA-binding domain. The transactivation activity maps to the C-terminal 130 amino acids, with the last 33 amino acids essential. In the absence of these 33 amino acids, a two-and-one-half proline-rich repeat structure was found to exhibit repression activity. We further show that Meq is able to dimerize not only with itself but also with c-Jun. Meq/c-Jun heterodimers bind to an AP1-like sequence in the meq promoter region with an affinity much greater than that of Meq/Meq or c-Jun/c-Jun homodimers. Cotransfection chloramphenicol acetyltransferase assays suggest that the Meq/c-Jun heterodimers can up-regulate Meq expression in both chicken embryo fibroblasts and F9 cells. Our data provide the first biochemical evidence that Meq is a transcriptional factor and identify c-Jun as one of Meq's interacting partners.

Expression of Marek's disease virus (MDV) serotype 2 gene which has partial homology with MDV serotype 1 pp38 gene

We constructed the recombinant baculovirus expressing the gene of non-pathogenic Marek's disease virus (MDV) serotype 2 (MDV2) which encodes a polypeptide with partial homology to MDV serotype 1 (MDV1) pp38, an antigen associated with transformed cells. The recombinant MDV2 protein was detected as a band of 32 kDa in immunoblot analysis with MDV2-infected chicken serum. Mouse serum against insect Spodoptera frugiperda cells infected with the recombinant baculovirus immunoprecipitated a 38 kDa molecule from the lysate of MDV2-infected chicken embryo fibroblasts (CEFs) but did not immunoprecipitate the MDV1 pp38 from the lysate of MDV1-infected CEFs. This result indicates that the recombinant MDV2 protein has no epitopes shared with the MDV1 pp38.

Expression of a novel Marek's disease virus type 1 (MDV1)-specific protein p40 in insect cells by a baculovirus vector and in chick embryo fibroblasts infected with MDV1

We found that the open reading frame of 1053 nucleotides within the short unique sequence of Marek's disease virus (MDV) type 1 (MDV1) expresses a 40 kDa protein, using a baculovirus vector in insect cells. Mouse antisera against the 40 kDa protein reacted with a 40 kDa protein in MDV1-infected cells. A cross-reactive polypeptide of 37 kDa was also detected in cells infected with herpesvirus of turkey.