B-lymphoma induction by reticuloendotheliosis virus: characterization of a mutated chicken syncytial virus provirus involved in c-myc activation

An outbreak of lymphomas in a layer chicken flock previously infected with fowlpox virus containing integrated reticuloendotheliosis virus

Visceral lymphomas occurred in a 236-day-old layer flock previously diagnosed with reticuloendotheliosis virus (REV)-integrated fowlpox virus (FPV) infection at the age of 77 days. Common pathologic lesions were multiple neoplastic nodules of homogeneous lymphocytes in the livers and spleens of all submitted chickens. All neoplastic tissues were positive for the REV envelope (env) gene by PCR. In a retrospective molecular study of FPV-infected 77-day-old chickens from the same flock, we identified nearly full-length REV provirus integrated into the genome of FPV as well as the REV env gene in trachea samples, whereas only the REV LTR region was present in the FPV strain used to vaccinate this flock. The 622-bp REV env gene nucleotide sequence derived from the trachea and neoplastic tissues was identical. Commercial ELISA of serum samples revealed that all chickens aged between 17 and 263 days in this flock were positive for REV but not for avian leukosis virus. Taken together, the evidence suggests that the visceral lymphomas were caused by a REV-integrated FPV field strain. FPV infections of commercial chickens should be followed up by careful monitoring for manifestations of REV infection, including lymphomas and immune depression, considering the ease with which the REV provirus appears to be able to integrate into the FPV genome.

Pathogenicity and Transmission of Reticuloendotheliosis Virus Isolated from Endangered Prairie Chickens

The pathogenicity and transmission of a field isolate of reticuloendotheliosis virus (REV) was studied using an experimental model in Japanese quail. Oncogenicity was also evaluated after inoculations in chickens and turkeys. The original REV (designated APC-566) was isolated from Attwater's prairie chickens (Tympanuchus cupido attwateri), an endangered wild avian species of the southern United States. The transmissibility of the REV isolate was studied in young naive Japanese quail in contact with experimentally infected quail. Vertical transmission was not detected by virus isolation and indirect immunofluorescence. Seroconversion was detected in few contact quails, suggesting horizontal transmission. The APC-566 isolate induced tumors beginning at 6 wk of age in quails infected as embryos. Most of the tumors detected in Japanese quail were lymphosarcomas, and 81% of these neoplasias contained CD3+ cells by immunoperoxidase. REV APC-566 was also oncogenic in chickens and turkeys infected at 1 day of age, with tumors appearing as early as 58 days after infection in chickens and at 13 wk of age in turkeys. This study was conducted in part as an attempt to understand the potential for pathogenicity and transmission of REV isolated from endangered avian species.

Full genome sequence and some biological properties of reticuloendotheliosis virus strain APC-566 isolated from endangered Attwater's prairie chickens

Reticuloendotheliosis virus (REV) causes runting, high mortality, immunosuppression, and chronic neoplasia associated with T and/or B cell lymphomas in a variety of domestic and wild birds, including Attwater's prairie chickens (APC) (Tympanuchus cupido attwateri). The complete proviral sequence of a recent REV isolate from APC (REV APC-566) was determined. This virus was isolated from an APC maintained in captivity in a reproduction program intended to avoid its extinction. REV APC-566 was determined to be oncogenic in Japanese quail (Coturnix coturnix japonica), chickens (Gallus gallus) and turkeys (Meleagris gallopavo). Immune responses against bacteria and viruses were significantly reduced in turkeys infected with REV APC-566. The proviral genome is 8286 nucleotides in length and exhibits a genetic organization characteristic of replication-competent gammaretroviruses. The REV APC-566 provirus contains two identical long terminal repeats (LTR) and a complete set of genes including gag, gag-pol and env. As previously reported, alignments with other REV sequences showed high similarity with sequences found in the gag and pol genes from other REVs. The REV APC-566 env gene showed high nucleotide sequence homology with REV sequences inserted in fowl poxvirus (99.8%), and with spleen necrosis virus (SNV) (95.1%). Sequences coding for a previously reported immunosuppressive peptide contained in the transmembrane region of the env gene are well conserved among all REV sequences analyzed. The LTR was the most divergent region, exhibiting various deletions and insertions. REV APC-566 has a unique insertion of 23 bp in U3 and shares deletions of 19 and 5 bp with chicken syncytial virus and REV inserts in fowlpox virus.

Molecular characterization of reticuloendotheliosis virus insertions in the genome of field and vaccine strains of fowl poxvirus

Evidence of the widespread occurrence of reticuloendotheliosis virus (REV) sequence insertions in fowl poxvirus (FPV) genome of field isolates and vaccine strains has increased in recent years. However, only those strains carrying a near intact REV provirus are more likely to cause problems in the field. Detection of the intact provirus or REV protein expression from FPV stocks has proven to be technically difficult. The objective of the present study was to evaluate current and newly developed REV and FPV polymerase chain reaction (PCR) assays to detect the presence of REV provirus in FPV samples. The second objective was to characterize REV insertions among recent "variant" FPV field isolates and vaccine strains. With REV, FPV, and heterologous REV-FPV primers, five FPV field isolates and four commercial vaccines were analyzed by PCR and nucleotide sequence analysis. Intact and truncated REV 5' long terminal repeat (LTR) sequences were detected in all FPV field isolates and vaccine strains, indicating heterogeneous REV genome populations. However only truncated 3' LTR and envelope sequences were detected among field isolates and in one vaccine strain. Amplifications of the REV envelope and 3' LTR provided strong evidence to indicate that these isolates carry a near intact REV genome. Three of the four FPV vaccine strains analyzed carried a solo complete or truncated 5' LTR sequence, indicating that intact REV provirus was not present. Comparison of PCR assays indicated that assays amplifying REV envelope and REV 3' LTR sequences provided a more accurate assessment of REV provirus than PCR assays that amplify the REV 5' LTR region. Therefore, to differentiate FPV strains that carry intact REV provirus from those that carry solo 5' LTR sequences, positive PCR results with primers that amplify the 5' LTR should be confirmed with more specific PCR assays, such as the envelope, or the REV 3' LTR PCR.

Field and vaccine strains of fowlpox virus carry integrated sequences from the avian retrovirus, reticuloendotheliosis virus

For baculoviruses and herpesviruses, integration of transposons or retroviruses into the virus genome has been documented. We report here that field and vaccine strains of fowlpox virus (FPV) carry integrated sequences from the avian retrovirus, reticuloendotheliosis virus (REV). Using PCR and hybridization analysis we observed that vaccine and field strains of FPV carry REV sequences integrated into a previously uncharacterized region of the right 1/3 of the FPV genome. Long-range PCR, hybridization, and nucleotide sequence determination demonstrated that one vaccine strain (FPV S) and recently isolated field strains carry a near-full-length REV provirus. For another vaccine strain (FPV M) a rearranged remnant of the LTR was found at the same insertion site. By Western blotting and reverse transcriptase assays we were unable to demonstrate free REV in supernatants of FPV S cultures. The near-full-length REV provirus integrated into the FPV genome is infectious since FPV S DNA gave rise to REV upon transfection into chicken embryo fibroblasts. Upon infection of chickens with FPV S, all chickens developed high-titered antibodies to REV, and REV was isolated from the blood of half of the inoculated chickens. Our observations add to the list of targets for retrovirus integration into DNA virus genomes. The integration of a near-full-length, and apparently infectious, REV provirus into FPV provides additional transmission routes for the retrovirus by way of the infectious cycle of FPV, including the possibility of mechanical transmission by biting insects since FPV is believed to be transmitted by this route. For large DNA viruses, including the poxviruses, retrovirus integration with attendant possibilities of gene transduction may be an important mechanism for virus evolution, including the acquisition of cellular genes with the potential to modify virus virulence and pathogenicity.

Retroviral insertional activation in a herpesvirus: transcriptional activation of US genes by an integrated long terminal repeat in a Marek's disease virus clone

Insertional activation of host proto-oncogenes has been recognized as a basic mechanism by which nonacute retroviruses induce cancer. Our previous work has demonstrated that retroviruses can efficiently integrate into DNA virus genomes. Specifically, coinfection of cultured fibroblasts with a chicken herpesvirus, Marek's disease virus (MDV), and a chicken retrovirus results in frequent stable retroviral insertions into the herpesvirus genome. Such insertions could alter the expression of herpesvirus genes, possibly resulting in novel phenotypic properties. In this article, we report the characterization of a replication-competent clone of MDV with integrated retroviral sequences. This virus was isolated from a chicken following injection of fibroblasts coinfected with MDV and the retrovirus, reticuloendotheliosis virus. Transcripts originating from the reticuloendotheliosis virus long terminal repeat promoters were found to encode the adjoining MDV genes, SORF2, US1, and US10. This virus replicates well in culture but has an unusual phenotype in chickens, characterized by an attenuated virulence which produces no nerve lesions but, rather, severe thymic atrophy. While the causal relationship between the insertion and the observed phenotypes remains to be established, our data provide the first evidence of retroviral insertional activation of herpesvirus genes.

Retroviral insertions into a herpesvirus are clustered at the junctions of the short repeat and short unique sequences

We previously described the integration of a nonacute retrovirus, reticuloendotheliosis virus (REV), into the genome of a herpesvirus, Marek disease virus (MDV), following both long-term and short-term coinfection in cultured fibroblasts. The long-term coinfection occurred in the course of attenuating the oncogenicity of the JM strain of MDV and was sustained for > 100 passages. The short-term coinfection, which spanned only 16 passages, was designed to recreate the insertion phenomenon under controlled conditions. We found that REV integrations into MDV were common and could occur within the first passage following coinfection. Now we have mapped the integration sites. After 5-16 passages in vitro, 17 out of 19 REV junction sites are clustered in two 1-kilobase regions at the junctions of the short unique and short repeat region of MDV. In the long-term cocultivation experiment, 6 out of 10 insertions also mapped in this region. In both cases, integrated proviruses are unstable and undergo subsequent recombinative deletion, often leaving a solitary long terminal repeat. The long terminal repeat sequences are, however, stably maintained for many rounds of passaging in vitro. This clustering of insertions presumably is influenced by selection for viable and fast-growing viruses, and occurs in a region of the MDV genome which shows significant size heterogeneity in several strains.

Reticuloendotheliosis virus long terminal repeat elements are efficient promoters in cells of various species and tissue origin, including human lymphoid cells

Promiscuous transcriptional activity of the reticuloendotheliosis virus (REV) long terminal repeat (LTR) was detected in transient expression assays using LTR-chloramphenicol acetyltransferase-encoding gene chimeras, and cells of diverse species and tissue type; levels of expression from two different REV LTRs correlate with reports of pathogenicity of the respective viruses in vivo. REVs do not encode a transactivator targeted to the viral LTR, and cells infected with Marek's disease virus, a herpesvirus with an overlapping host range, do not express factors that preferentially enhance expression from REV or avian sarcoma/leukemia virus LTRs. REV LTRs work efficiently in human lymphoid cells, and are viable alternatives to promoters commonly used for expression of cloned genes. They may also prove useful in the identification of new, ubiquitous cellular transcription factors.

Transcriptional interaction between retroviral long terminal repeats (LTRs): mechanism of 5' LTR suppression and 3' LTR promoter activation of c-myc in avian B-cell lymphomas

Chicken syncytial viruses induce bursal lymphomas by integrating into the c-myc locus and activating myc expression by 3' long terminal repeat (LTR) promoter insertion. In contrast to wild-type proviruses, in which transcription initiates predominantly in the 5'LTR, these myc-associated proviruses exhibit a predominance of transcription from the 3' LTR and little transcription from the 5' LTR. Most of these proviruses contain deletions within the 5' end of their genome that spare the 5' LTR. We report the identification of a 0.3-kb viral leader sequence that modulates 5' and 3' LTR transcriptional activities. In the presence of this sequence, transcription from the 5' LTR predominates, but in its absence, the 3' LTR promoter becomes activated, resulting in a high level of myc expression. This viral sequence does not behave like a classical enhancer; it activates transcription only when located downstream from the promoter and in the sense orientation. In this regard, it resembles the recently described human immunodeficiency virus RNA enhancer. This study suggests that retroviruses contain internal sequences which directionally activate the 5' LTR promoter to facilitate transcription of the viral genome and that deletion of these sequences is one step in the activation of the 3' LTR of myc-associated proviruses in avian bursal lymphomas.

Retrovirus insertion into herpesvirus in vitro and in vivo

Retroviruses and herpesviruses are naturally occurring pathogens of humans and animals. Coinfection of the same host with both these viruses is common. We report here that a retrovirus can integrate directly into a herpesvirus genome. Specifically, we demonstrate insertion of a nonacute retrovirus, reticuloendotheliosis virus (REV), into a herpesvirus, Marek disease virus (MDV). Both viruses are capable of inducing T lymphomas in chickens and often coexist in the same animal. REV DNA integration into MDV occurred in a recently attenuated strain of MDV and in a short-term coinfection experiment in vitro. We also provide suggestive evidence that REV has inserted into pathogenic strains of MDV in the past. Sequences homologous to the REV long terminal repeat are found in oncogenic MDV but not in nononcogenic strains. These results raise the possibility that retroviral information may be transmitted by herpesvirus and that herpesvirus expression can be modulated by retroviral elements. In addition, retrovirus may provide a useful tool to characterize herpesviral function by insertional mutagenesis.

Transcriptionally active genome regions are preferred targets for retrovirus integration

We have analyzed the transcriptional activity of cellular target sequences for Moloney murine leukemia virus integration in mouse fibroblasts. At least five of the nine random, unselected integration target sequences studied showed direct evidence for transcriptional activity by hybridization to nuclear run-on transcripts prepared from uninfected cells. At least four of the sequences contained multiple recognition sites for several restriction enzymes that cut preferentially in CpG-rich islands, indicating integration into 5' or 3' ends or flanking regions of genes. Assuming that only a minor fraction (less than 20%) of the genome is transcribed in mammalian cells, we calculated the probability that this association of retroviral integration sites with transcribed sequences is due to chance to be very low (1.6 x 10(-2]. Thus, our results strongly suggest that transcriptionally active genome regions are preferred targets for retrovirus integration.

Avian proto-myc genes promoted by defective or nondefective retroviruses are single-hit transforming genes in primary cells

Lymphomas of certain strains of chickens infected by retroviruses frequently contain recombinant transforming genes in which the promoter of the cellular proto-myc gene is replaced by that of a defective rather than an intact retrovirus. Here we ask whether the resulting hybrid genes are sufficient for tumorigenic transformation like viral myc genes. Further, we ask whether retroviruses must be defective in order to mutate proto-myc to a transforming gene or whether the defectiveness plays a transformation-independent function in tumorigenesis. For this purpose the defective provirus of proviral-proto-myc recombinants from lymphomas were repaired, or intact proviruses were recombined with proto-myc genes in vitro, and then compared to recombinant proto-myc genes with defective proviruses for transforming function in quail embryo fibroblasts. It was found that a single copy of a provirus-proto-myc recombinant gene with an intact provirus is sufficient to transform a quail embryo cell in vitro. Moreover, our analyses showed that multiple internal retroviral deletions [corrected] eliminate or inhibit provirus expression. The effect of these deletions [corrected] was detectable only because the inactive proviruses were linked to the selectable, transforming proto-myc gene marker. It is consistent with our results that proviral defectiveness of recombinant proto-myc genes is necessary in vivo for the clonal growth of a transformed cell into a tumor to escape antiviral immunity. The large discrepancy between the probabilities of provirus insertion and tumorigenesis is suggested to reflect the low probabilities of spontaneous deletion of the provirus and of rare, strain-specific defects of tumor-resistance genes of the host.

Multiple proto-oncogene activations in avian leukosis virus-induced lymphomas: evidence for stage-specific events

We have examined avian leukosis virus-induced B-cell lymphomas for multiple, stage-specific oncogene activations. Three targets for viral integration were identified: c-myb, c-myc, and a newly identified locus termed c-bic. The c-myb and c-myc genes were associated with different lymphoma phenotypes. The c-bic locus was a target for integration in one class of lymphomas, usually in conjunction with c-myc activation. The data indicate that c-myc and c-bic may act synergistically during lymphomagenesis and that c-bic is involved in late stages of tumor progression.

Replication-defective vectors of reticuloendotheliosis virus transduce exogenous genes into somatic stem cells of the unincubated chicken embryo

Replication-defective vectors derived from reticuloendotheliosis virus were used to transduce exogenous genes into early somatic stem cells of the chicken embryo. One of these vectors transduced and expressed the chicken growth hormone coding sequence. The helper cell line, C3, was used to generate stocks of vector containing about 10(4) transducing units per ml. Injection of 5- to 20-microliters volumes of vector directly beneath the blastoderm of unincubated chicken embryos led to infection of somatic stem cells. Infected embryos and adults contained unrearranged integrated proviral DNAs. Embryos expressed the transduced chicken growth hormone gene and contained high levels of serum growth hormone. Blood, brain, muscle, testis, and semen contained from individuals injected as embryos contained vector DNA. Replication-defective vectors of the reticuloendotheliosis virus transduced exogenous genes into chicken embryonic stem cells in vivo.

Multiple proto-oncogene activations in avian leukosis virus-induced lymphomas: evidence for stage-specific events

We have examined avian leukosis virus-induced B-cell lymphomas for multiple, stage-specific oncogene activations. Three targets for viral integration were identified: c-myb, c-myc, and a newly identified locus termed c-bic. The c-myb and c-myc genes were associated with different lymphoma phenotypes. The c-bic locus was a target for integration in one class of lymphomas, usually in conjunction with c-myc activation. The data indicate that c-myc and c-bic may act synergistically during lymphomagenesis and that c-bic is involved in late stages of tumor progression.

Functional interaction between transcriptional elements in the long terminal repeat of reticuloendotheliosis virus: cooperative DNA binding of promoter- and enhancer-specific factors

Transcription from reticuloenodotheliosis virus strain T (REV-T), an avian retrovirus unrelated to avian leukosis and sarcoma viruses, is modulated by sequences in at least five functional domains. A promoter containing a TATA and multiple CCAAT motifs in U3 of the long terminal repeat was absolutely required for transcription. Transcriptional efficiency was greatly augmented by an enhancer immediately upstream, which contained a 22-base-pair repeated sequence. Transcription was further influenced by a negative-acting domain in the 5' region of U3 and two downstream domains in the transcribed non-protein-coding region. One of these latter domains contained a consensus enhancer core sequence and positively affected transcription in both mammalian and avian cells; the other acted negatively in a dog cell line. Transcription from REV-T in vivo required cellular factors which could be competed for specifically by the promoter or enhancer domain. The downstream domains competed with reporter genes containing these domains, but not directly with the U3 sequences. The promoter, enhancer, and the positive-acting downstream domains formed multiple complexes with distinct classes of cellular factors in both avian and mammalian cell extracts. Binding of factors to the promoter and enhancer domains was cooperative when these domains were joined in cis.

Transient expression analysis of the reticuloendotheliosis virus long terminal repeat element

A region of the Reticuloendotheliosis virus (REV) long terminal repeat (LTR) harbouring single or duplicated copies of 46-bp and 26-bp sequence elements is implicated in enhancer activity. Sequences residing upstream from the proviral 3' LTR did not contribute to activity of the intact LTR. Gene expression regulated by a combination of REV enhancer and SV40 early region promoter was 50-fold less than from the analogous construct containing the chicken syncytial virus promoter. Deletion of LTR sequences immediately downstream of the CAP site, which include a region capable of forming a stable hairpin in the mRNA, decreased expression by 70%. Expression assays and S1 nuclease mapping showed that a second transcriptional start site, identified by transcription in vitro using HeLa cell lysates and purified DNA templates, was not used in vivo in the cell lines examined.

Functional interaction between transcriptional elements in the long terminal repeat of reticuloendotheliosis virus: cooperative DNA binding of promoter- and enhancer-specific factors

Transcription from reticuloenodotheliosis virus strain T (REV-T), an avian retrovirus unrelated to avian leukosis and sarcoma viruses, is modulated by sequences in at least five functional domains. A promoter containing a TATA and multiple CCAAT motifs in U3 of the long terminal repeat was absolutely required for transcription. Transcriptional efficiency was greatly augmented by an enhancer immediately upstream, which contained a 22-base-pair repeated sequence. Transcription was further influenced by a negative-acting domain in the 5' region of U3 and two downstream domains in the transcribed non-protein-coding region. One of these latter domains contained a consensus enhancer core sequence and positively affected transcription in both mammalian and avian cells; the other acted negatively in a dog cell line. Transcription from REV-T in vivo required cellular factors which could be competed for specifically by the promoter or enhancer domain. The downstream domains competed with reporter genes containing these domains, but not directly with the U3 sequences. The promoter, enhancer, and the positive-acting downstream domains formed multiple complexes with distinct classes of cellular factors in both avian and mammalian cell extracts. Binding of factors to the promoter and enhancer domains was cooperative when these domains were joined in cis.

Localization and footprinting of an enhancer within the avian sarcoma virus gag gene

A cis-acting regulatory element within the gag gene of avian retroviruses has been localized by deletion analysis, and sites of protein interaction have been studied by DNase I footprinting. Unidirectional deletions were made from both the 5' and 3' ends of a 656-base-pair fragment of the gag gene of Fujinami sarcoma virus. These deletion mutants were tested for enhancer activity in a chloramphenicol acetyltransferase transient expression assay. A sharp 5' boundary for enhancer activity was observed between 776 and 786 nucleotides downstream from the transcription initiation site. In contrast, deletion from the 3' side resulted in a gradual loss of enhancer activity, reaching a near basal level of activity by nucleotide 868. Internal deletion of 76 nucleotides just downstream of the 5' boundary abolished enhancement. Mutagenesis of a consensus enhancer core sequence (GTGGTTTG) showed that this sequence was not necessary for enhancer activity in our transient assays. DNase I footprinting with both a highly purified enhancer-binding protein from rat liver (EBP20) and a partially purified chicken liver nuclear extract showed specific protection of nucleotides 813 to 872 within the localized enhancer region. Footprinting of unidirectional deletion mutants that had lost activity indicated that this binding was not sufficient to confer enhancement.

Provirus tagging as an instrument to identify oncogenes and to establish synergism between oncogenes

Insertional mutagenesis is one of the mechanisms by which retroviruses can transform cells. Once a provirus was found in the vicinity of c-myc, with the concomitant activation of this gene, other proto-oncogenes were shown to be activated by proviral insertion in retrovirally-induced tumors. Subsequently, cloning of common proviral insertion sites led to the discovery of a series of new (putative) oncogenes. Some of these genes have been shown to fulfill key roles in growth and development. In this review I shall describe how proviruses can be used to identify proto-oncogenes, and list the loci, identified by this method. Furthermore, I shall illuminate the potential of provirus tagging by showing that it not only can mark new oncogenes, but can also be instrumental in defining sets of (onco)genes that guide a normal cell in a step-by-step fashion to its fully transformed, metatasizing, counterpart.