Involvement of the oncoprotein c-Myc in viral telomerase RNA gene regulation during Marek's disease virus-induced lymphomagenesis

The role of telomerase and viruses interaction in cancer development, and telomerase-dependent therapeutic approaches

Human telomerase reverse transcriptase (hTERT) is an enzyme that is critically involved in elongating and maintaining telomeres length to control cell life span and replicative potential. Telomerase activity is continuously expressed in human germ-line cells and most cancer cells, whereas it is suppressed in most somatic cells. In normal cells, by reducing telomerase activity and progressively shortening the telomeres, the cells progress to the senescence or apoptosis process. However, in cancer cells, telomere lengths remain constant due to telomerase's reactivation, and cells continue to proliferate and inhibit apoptosis, and ultimately lead to cancer development and human death due to metastasis. Studies demonstrated that several DNA and RNA oncoviruses could interact with telomerase by integrating their genome sequence within the host cell telomeres specifically. Through the activation of the hTERT promoter and lengthening the telomere, these cells contributes to cancer development. Since oncoviruses can activate telomerase and increase hTERT expression, there are several therapeutic strategies based on targeting the telomerase of cancer cells like telomerase-targeted peptide vaccines, hTERT-targeting dendritic cells (DCs), hTERT-targeting gene therapy, and hTERT-targeting CRISPR/Cas9 system that can overcome tumor-mediated toleration mechanisms and specifically apoptosis in cancer cells. This study reviews available data on the molecular structure of telomerase and the role of oncoviruses and telomerase interaction in cancer development and telomerase-dependent therapeutic approaches to conquest the cancer cells.

Role of DNA Methylation and CpG Sites in the Viral Telomerase RNA Promoter during Gallid Herpesvirus 2 Pathogenesis

Previous studies demonstrated that telomerase RNAs possess functions that promote tumor development independent of the telomerase complex. vTR is a herpesvirus-encoded telomerase RNA subunit that plays a crucial role in virus-induced tumorigenesis and is expressed by a robust viral promoter that is highly regulated by the c-Myc oncoprotein binding to the E-boxes. Here, we demonstrated that the DNA methylation patterns in the functional c-Myc response elements of the vTR promoter change upon reactivation from latency, and that demethylation strongly increases telomerase activity in virus-infected cells. Moreover, the introduction of mutation in the CpG dinucleotides of the c-Myc binding sites resulted in decreased vTR expression and complete abrogation of tumor formation. Our study provides further confirmation of the involvement of specific DNA methylation patterns in the regulation of vTR expression and vTR importance for virus-induced tumorigenesis.

Gallid herpesvirus type 2 (GaHV-2) is an oncogenic alphaherpesvirus that induces malignant T-cell lymphoma in chicken. GaHV-2 encodes a viral telomerase RNA subunit (vTR) that plays a crucial role in virus-induced tumorigenesis, enhances telomerase activity, and possesses functions independent of the telomerase complex. vTR is driven by a robust viral promoter, highly expressed in virus-infected cells, and regulated by two c-Myc response elements (c-Myc REs). The regulatory mechanisms involved in controlling vTR and other genes during viral replication and latency remain poorly understood but are crucial to understanding this oncogenic herpesvirus. Therefore, we investigated DNA methylation patterns of CpG dinucleotides found in the vTR promoter and measured the impact of methylation on telomerase activity. We demonstrated that telomerase activity was considerably increased following viral reactivation. Furthermore, CpG sites within c-Myc REs showed specific changes in methylation after in vitro reactivation and in infected animals over time. Promoter reporter assays indicated that one of the c-Myc REs is involved in regulating vTR transcription, and that methylation strongly influenced vTR promoter activity. To study the importance of the CpG sites found in c-Myc REs in virus-induced tumorigenesis, we generated recombinant virus containing mutations in CpG sites of c-Myc REs together with the revertant virus by two-step Red-mediated mutagenesis. Introduced mutations in the vTR promoter did not affect the replication properties of the recombinant viruses in vitro. In contrast, replication of the mutant virus in infected chickens was severely impaired, and tumor formation completely abrogated. Our data provides an in-depth characterization of c-Myc oncoprotein REs and the involvement of DNA methylation in transcriptional regulation of vTR.

IMPORTANCE Previous studies demonstrated that telomerase RNAs possess functions that promote tumor development independent of the telomerase complex. vTR is a herpesvirus-encoded telomerase RNA subunit that plays a crucial role in virus-induced tumorigenesis and is expressed by a robust viral promoter that is highly regulated by the c-Myc oncoprotein binding to the E-boxes. Here, we demonstrated that the DNA methylation patterns in the functional c-Myc response elements of the vTR promoter change upon reactivation from latency, and that demethylation strongly increases telomerase activity in virus-infected cells. Moreover, the introduction of mutation in the CpG dinucleotides of the c-Myc binding sites resulted in decreased vTR expression and complete abrogation of tumor formation. Our study provides further confirmation of the involvement of specific DNA methylation patterns in the regulation of vTR expression and vTR importance for virus-induced tumorigenesis.

A Common Live-Attenuated Avian Herpesvirus Vaccine Expresses a Very Potent Oncogene

Marek’s disease virus (MDV) is one of several oncogenic herpesviruses and causes fatal lymphomas in chickens. The current “gold standard” vaccine is the live-attenuated MDV strain CVI988/Rispens (CVI), which is widely used and efficiently prevents tumor formation. Intriguingly, CVI expresses two predominant isoforms of the major MDV oncogene meq: one variant with a regular size of meq (Smeq) and one long isoform (Lmeq) harboring an insertion of 180 bp in the transactivation domain. In our study, we could break the long-standing assumption that the Lmeq isoform is an indicator for virus attenuation. Using recombinant viruses that express the different CVI-meq isoforms, we could demonstrate that both isoforms drastically differ in their abilities to promote pathogenesis and tumor formation in infected chickens.

Vaccines play a crucial role in the protection of animals and humans from deadly pathogens. The first vaccine that also protected against cancer was developed against the highly oncogenic herpesvirus Marek’s disease virus (MDV). MDV infects chickens and causes severe immunosuppression, neurological signs, and fatal lymphomas, a process that requires the viral oncogene, meq. The most frequently used Marek’s disease vaccine is the live-attenuated CVI988/Rispens (CVI) strain, which efficiently protects chickens and prevents tumorigenesis. Intriguingly, CVI expresses at least two isoforms of meq; however, it remains unknown to what extent these isoforms contribute to virus attenuation. In this study, we individually examined the contribution of the two CVI-meq isoforms to the attenuation of the vaccine. We inserted the respective isoforms into a very virulent MDV (strain RB-1B), thereby replacing its original meq gene. Surprisingly, we could demonstrate that the longer isoform of meq strongly enhanced virus-induced pathogenesis and tumorigenesis, indicating that other mutations in the CVI genome contribute to virus attenuation. On the contrary, the shorter isoform completely abrogated pathogenesis, demonstrating that changes in the meq gene can indeed play a key role in virus attenuation. Taken together, our study provides important evidence on attenuation of one of the most frequently used veterinary vaccines worldwide.

IMPORTANCE Marek’s disease virus (MDV) is one of several oncogenic herpesviruses and causes fatal lymphomas in chickens. The current “gold standard” vaccine is the live-attenuated MDV strain CVI988/Rispens (CVI), which is widely used and efficiently prevents tumor formation. Intriguingly, CVI expresses two predominant isoforms of the major MDV oncogene meq: one variant with a regular size of meq (Smeq) and one long isoform (Lmeq) harboring an insertion of 180 bp in the transactivation domain. In our study, we could break the long-standing assumption that the Lmeq isoform is an indicator for virus attenuation. Using recombinant viruses that express the different CVI-meq isoforms, we could demonstrate that both isoforms drastically differ in their abilities to promote pathogenesis and tumor formation in infected chickens.

Overexpression of cellular telomerase RNA enhances virus-induced cancer formation

The telomerase RNA subunit (TR) is overexpressed in many tumors; however, the contribution of TR in cancer formation remains elusive. The most frequent clinically diagnosed cancer in the animal kingdom is caused by the highly oncogenic herpesvirus Marek's disease virus (MDV). MDV encodes a TR (vTR) that plays an important role in virus-induced tumorigenesis and shares 88% sequence identity with its cellular homologue. To determine if the cellular TR possesses pro-oncogenic activity, we replaced vTR with the cellular homologue in the virus genome. Insertion of cellular TR resulted in a strong overexpression in virus infected cells, while virus replication was not affected. Strikingly, cellular TR promoted tumor formation as efficient as vTR, while tumorigenesis was severely impaired in the absence of vTR. Our data provide the first evidence that overexpression of cellular TR can contribute to tumor formation in vivo using this natural virus-host model for herpesvirus-induced oncogenesis.

MYC drives overexpression of telomerase RNA (hTR/TERC) in prostate cancer

Telomerase consists of at least two essential elements, an RNA component hTR or TERC that contains the template for telomere DNA addition and a catalytic reverse transcriptase (TERT). While expression of TERT has been considered the key rate-limiting component for telomerase activity, increasing evidence suggests an important role for the regulation of TERC in telomere maintenance and perhaps other functions in human cancer. By using three orthogonal methods including RNAseq, RT-qPCR, and an analytically validated chromogenic RNA in situ hybridization assay, we report consistent overexpression of TERC in prostate cancer. This overexpression occurs at the precursor stage (e.g. high-grade prostatic intraepithelial neoplasia or PIN) and persists throughout all stages of disease progression. Levels of TERC correlate with levels of MYC (a known driver of prostate cancer) in clinical samples and we also show the following: forced reductions of MYC result in decreased TERC levels in eight cancer cell lines (prostate, lung, breast, and colorectal); forced overexpression of MYC in PCa cell lines, and in the mouse prostate, results in increased TERC levels; human TERC promoter activity is decreased after MYC silencing; and MYC occupies the TERC locus as assessed by chromatin immunoprecipitation (ChIP). Finally, we show that knockdown of TERC by siRNA results in reduced proliferation of prostate cancer cell lines. These studies indicate that TERC is consistently overexpressed in all stages of prostatic adenocarcinoma and that its expression is regulated by MYC. These findings nominate TERC as a novel prostate cancer biomarker and therapeutic target. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Telomeres and Telomerase: Role in Marek's Disease Virus Pathogenesis, Integration and Tumorigenesis

Telomeres protect the ends of vertebrate chromosomes from deterioration and consist of tandem nucleotide repeats (TTAGGG)_n that are associated with a number of proteins. Shortening of the telomeres occurs during genome replication, thereby limiting the replication potential of somatic cells. To counteract this shortening, vertebrates encode the telomerase complex that maintains telomere length in certain cell types via de novo addition of telomeric repeats. Several herpesviruses, including the highly oncogenic alphaherpesvirus Marek's disease virus (MDV), harbor telomeric repeats (TMR) identical to the host telomere sequences at the ends of their linear genomes. These TMR facilitate the integration of the MDV genome into host telomeres during latency, allowing the virus to persist in the host for life. Integration into host telomeres is critical for disease and tumor induction by MDV, but also enables efficient reactivation of the integrated virus genome. In addition to the TMR, MDV also encodes a telomerase RNA subunit (vTR) that shares 88% sequence identity with the telomerase RNA in chicken (chTR). vTR is highly expressed during all stages of the virus lifecycle, enhances telomerase activity and plays an important role in MDV-induced tumor formation. This review will focus on the recent advances in understanding the role of viral TMR and vTR in MDV pathogenesis, integration and tumorigenesis.

Immortalization of chicken preadipocytes by retroviral transduction of chicken TERT and TR

The chicken is an important agricultural animal and model for developmental biology, immunology and virology. Excess fat accumulation continues to be a serious problem for the chicken industry. However, chicken adipogenesis and obesity have not been well investigated, because no chicken preadipocyte cell lines have been generated thus far. Here, we successfully generated two immortalized chicken preadipocyte cell lines through transduction of either chicken telomerase reverse transcriptase (chTERT) alone or in combination with chicken telomerase RNA (chTR). Both of these cell lines have survived >100 population doublings in vitro, display high telomerase activity and have no sign of replicative senescence. Similar to primary chicken preadipocytes, these two cell lines display a fibroblast-like morphology, retain the capacity to differentiate into adipocytes, and do not display any signs of malignant transformation. Isoenzyme analysis and PCR-based analysis confirmed that these two cell lines are of chicken origin and are free from inter-species contamination. To our knowledge, this is the first report demonstrating the generation of immortal chicken cells by introduction of chTERT and chTR. Our established chicken preadipocyte cell lines show great promise as an in vitro model for the investigation of chicken adipogenesis, lipid metabolism, and obesity and its related diseases, and our results also provide clues for immortalizing other avian cell types.

Alternative splicing of a viral mirtron differentially affects the expression of other microRNAs from its cluster and of the host transcript

Interplay between alternative splicing and the Microprocessor may have differential effects on the expression of intronic miRNAs organized into clusters. We used a viral model - the LAT long non-coding RNA (LAT lncRNA) of Marek's disease oncogenic herpesvirus (MDV-1), which has the mdv1-miR-M8-M6-M7-M10 cluster embedded in its first intron - to assess the impact of splicing modifications on the biogenesis of each of the miRNAs from the cluster. Drosha silencing and alternative splicing of an extended exon 2 of the LAT lncRNA from a newly identified 3' splice site (SS) at the end of the second miRNA of the cluster showed that mdv1-miR-M6 was a 5'-tailed mirtron. We have thus identified the first 5'-tailed mirtron within a cluster of miRNAs for which alternative splicing is directly associated with differential expression of the other miRNAs of the cluster, with an increase in intronic mdv1-miR-M8 expression and a decrease in expression of the exonic mdv1-miR-M7, and indirectly associated with regulation of the host transcript. According to the alternative 3SS used for the host intron splicing, the mdv1-miR-M6 is processed as a mirtron by the spliceosome, dispatching the other miRNAs of the cluster into intron and exon, or as a canonical miRNA by the Microprocessor complex. The viral mdv1-miR-M6 mirtron is the first mirtron described that can also follow the canonical pathway.

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.

Latency and tumorigenesis in Marek's disease

Despite the remarkable progress in our understanding of Marek's disease (MD) and the causative Marek's disease virus (MDV) biology, a number of major features of this complex viral disease remain unknown. Significant information on critical aspects of virus latency in lymphoid cells, and the virus-host interaction in MDV-induced lymphoma, remains to be identified. Moreover, the nature of the unique milieu of the feather follicle epithelial cell that allows cytolytic infection to continue, despite maintaining the latent infection in the lymphoid cells, is not fully understood. Although there has been significant progress in our understanding of the functions of a number of viral genes in the pathogenesis of the disease, the characteristics of the latent infection, how it differs from tumor phase, and whether latency is a prerequisite for the tumor phase are all important questions still to be answered. Reticuloendotheliosis virus-transformed cell lines have been shown to support MDV latency in a manner almost identical to that seen in MDV-transformed cell lines. There are increasing data on the role of epigenetic regulation, including DNA methylation and histone modifications, in maintaining viral latency. Onset of MD tumor is relatively rapid, and recent studies based on chromosomal integration and T-cell repertoire analysis demonstrated the clonal nature of MD lymphomas. Among the viral determinants of oncogenicity, the basic leucine zipper protein Meq is considered to be the most important and the most extensively studied. Deleting the Meq proteins or abolishing some of the important interactions does affect the oncogenicity of the virus. In addition, the noncoding sequences in the viral genome, such as the viral telomerase RNA and the virus-encoded microRNAs, also have significant influence on MDV-encoded oncogenesis.

The human telomerase catalytic subunit and viral telomerase RNA reconstitute a functional telomerase complex in a cell-free system, but not in human cells

The minimal vertebrate telomerase enzyme is composed of a protein component (telomerase reverse transcriptase, TERT) and an RNA component (telomerase RNA, TR). Expression of these two subunits is sufficient to reconstitute telomerase activity in vitro, while the formation of a holoenzyme comprising telomerase-associated proteins is necessary for proper telomere length maintenance. Previous reports demonstrated the high processivity of the human telomerase complex and the interspecies compatibility of human TERT (hTERT). In this study, we tested the function of the only known viral telomerase RNA subunit (vTR) in association with human telomerase, both in a cell-free system and in human cells. When vTR is assembled with hTERT in a cell-free environment, it is able to interact with hTERT and to reconstitute telomerase activity. However, in human cells, vTR does not reconstitute telomerase activity and could not be detected in the human telomerase complex, suggesting that vTR is not able to interact properly with the proteins constituting the human telomerase holoenzyme.

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.

ICP27 protein of Marek's disease virus interacts with SR proteins and inhibits the splicing of cellular telomerase chTERT and viral vIL8 transcripts

All herpesviruses have a post-transcriptional regulatory protein that prevents precursor mRNA splicing and leads to the shutting off of host protein synthesis. The ICP27 protein of herpes simplex virus 1 (HSV-1) is the prototype of these proteins. Marek's disease virus (MDV-1), an alphaherpesvirus that induces lymphoma in birds, also has an ICP27 protein that is produced in lytic MDV-1-infected cells. We characterized this protein. We demonstrated ICP27 production in latently infected MSB-1 cells, but only on MDV-1 reactivation. ICP27 was found predominantly in specific structures within the nucleus. The ICP27 of MDV-1 colocalized and interacted with SR proteins. We demonstrated inhibitory effects of MDV-1 ICP27 on the splicing of both the viral vIL8 and cellular chTERT (telomerase reverse transcriptase) genes. Thus, the ICP27 of MDV-1 plays a similar role to the ICP27 of HSV-1 and may be involved in MDV-1 replication and the development of Marek's disease.

A p53-dependent promoter associated with polymorphic tandem repeats controls the expression of a viral transcript encoding clustered microRNAs

The tumor suppressor protein p53 plays a role in cellular responses to cancer-initiating events by regulating progress through the cell cycle. Several recent studies have shown that p53 transactivates expression of the members of the proapoptotic microRNA-34 family, which are underexpressed in several cancers. We demonstrate here that the latency-associated cluster of microRNAs (miRNA) encoded by an oncogenic herpesvirus, gallid herpesvirus 2 (GaHV-2), is a direct target of p53. Robust transcriptional activity was induced in three avian cell lines by a sequence mapping 600 base pairs (bp) upstream of the cluster of miRNAs. We found transcription start sites for the pri-miRNA transcript at the 3' end of this transcription-inducing sequence. The promoter has no consensus core promoter element, but is organized into a variable number of tandem repeats of 60-bp harboring p53-responsive elements (RE). The minimal functional construct consists of two tandem repeats. Mutagenesis to change the sequence of the p53 RE abolished transcriptional activity, whereas p53 induction enhanced mature miRNA expression. The identification of a viral miRNA promoter regulated by p53 is biologically significant, because all avirulent GaHV-2 strains described to date lack the corresponding regulatory sequence, whereas all virulent, very virulent, and hypervirulent strains possess at least two tandem repeats harboring the p53 RE.

Alternative splicing and nonsense-mediated decay regulate telomerase reverse transcriptase (TERT) expression during virus-induced lymphomagenesis in vivo

BACKGROUND

Telomerase activation, a critical step in cell immortalization and oncogenesis, is partly regulated by alternative splicing. In this study, we aimed to use the Marek's disease virus (MDV) T-cell lymphoma model to evaluate TERT regulation by splicing during lymphomagenesis in vivo, from the start point to tumor establishment.

RESULTS

We first screened cDNA libraries from the chicken MDV lymphoma-derived MSB-1 T- cell line, which we compared with B (DT40) and hepatocyte (LMH) cell lines. The chTERT splicing pattern was cell line-specific, despite similar high levels of telomerase activity. We identified 27 alternative transcripts of chicken TERT (chTERT). Five were in-frame alternative transcripts without in vitro telomerase activity in the presence of viral or chicken telomerase RNA (vTR or chTR), unlike the full-length transcript. Nineteen of the 22 transcripts with a premature termination codon (PTC) harbored a PTC more than 50 nucleotides upstream from the 3' splice junction, and were therefore predicted targets for nonsense-mediated decay (NMD). The major PTC-containing alternatively spliced form identified in MSB1 (ie10) was targeted to the NMD pathway, as demonstrated by UPF1 silencing. We then studied three splicing events separately, and the balance between in-frame alternative splice variants (d5f and d10f) plus the NMD target i10ec and constitutively spliced chTERT transcripts during lymphomagenesis induced by MDV indicated that basal telomerase activity in normal T cells was associated with a high proportion of in-frame non functional isoforms and a low proportion of constitutively spliced chTERT. Telomerase upregulation depended on an increase in active constitutively spliced chTERT levels and coincided with a switch in alternative splicing from an in-frame variant to NMD-targeted variants.

CONCLUSIONS

TERT regulation by splicing plays a key role in telomerase upregulation during lymphomagenesis, through the sophisticated control of constitutive and alternative splicing. Using the MDV T-cell lymphoma model, we identified a chTERT splice variant as a new NMD target.

Herpesvirus telomerase RNA(vTR)-dependent lymphoma formation does not require interaction of vTR with telomerase reverse transcriptase (TERT)

Telomerase is a ribonucleoprotein complex involved in the maintenance of telomeres, a protective structure at the distal ends of chromosomes. The enzyme complex contains two main components, telomerase reverse transcriptase (TERT), the catalytic subunit, and telomerase RNA (TR), which serves as a template for the addition of telomeric repeats (TTAGGG)(n). Marek's disease virus (MDV), an oncogenic herpesvirus inducing fatal lymphoma in chickens, encodes a TR homologue, viral TR (vTR), which significantly contributes to MDV-induced lymphomagenesis. As recent studies have suggested that TRs possess functions independently of telomerase activity, we investigated if the tumor-promoting properties of MDV vTR are dependent on formation of a functional telomerase complex. The P6.1 stem-loop of TR is known to mediate TR-TERT complex formation and we show here that interaction of vTR with TERT and, consequently, telomerase activity was efficiently abrogated by the disruption of the vTR P6.1 stem-loop (P6.1mut). Recombinant MDV carrying the P6.1mut stem-loop mutation were generated and tested for their behavior in the natural host in vivo. In contrast to viruses lacking vTR, all animals infected with the P6.1mut viruses developed MDV-induced lymphomas, but onset of tumor formation was significantly delayed. P6.1mut viruses induced enhanced metastasis, indicating functionality of non-complexed vTR in tumor dissemination. We discovered that RPL22, a cellular factor involved in T-cell development and virus-induced transformation, directly interacts with wild-type and mutant vTR and is, consequently, relocalized to the nucleoplasm. Our study provides the first evidence that expression of TR, in this case encoded by a herpesvirus, is pro-oncogenic in the absence of telomerase activity.

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.

Vaccination against Marek's disease reduces telomerase activity and viral gene transcription in peripheral blood leukocytes from challenged chickens

We investigated whether telomerase activity and viral gene transcription were associated with protection against the RB-1B strain of Marek's disease virus (MDV) in chickens vaccinated with Rispens CVI988 or the herpes virus of turkey (HVT). Telomerase activity in peripheral blood leukocytes (PBLs) seemed to be an appropriate marker of lymphoma and levels of viral transcription were correlated with the virulence of MDV strains. Vaccinated protected birds had lower levels of telomerase activity and RB-1B viral gene transcription than unvaccinated chickens infected with RB-1B. The decrease in RB-1B viral transcription was more marked in chickens vaccinated with CVI988 than in those vaccinated with HVT. Indeed, RB-1B viral transcription was not detectable after 14 days post-challenge. In conclusion, telomerase activity and gene transcription in challenge MDV strains are potential new reliable criteria of protection in vaccinated chickens.