Transmission of (LTR, v-src, LTR) without recombination with a helper virus

Remembering Jan Svoboda: A Personal Reflection

The Czech scientist Jan Svoboda was a pioneer of Rous sarcoma virus (RSV). In the 1960s, before the discovery of reverse transcriptase, he demonstrated the long-term persistence of the viral genome in non-productive mammalian cells, and he supported the DNA provirus hypothesis of Howard Temin. He showed how the virus can be rescued in the infectious form and elucidated the replication-competent nature of the Prague strain of RSV later used for the identification of the src oncogene. His studies straddled molecular oncology and virology, and he remained an active contributor to the field until his death last year. Throughout the 50 years that I was privileged to know Svoboda as my mentor and friend, I admired his depth of scientific inquiry and his steadfast integrity in the face of political oppression.

Cross- and Co-Packaging of Retroviral RNAs and Their Consequences

Retroviruses belong to the family Retroviridae and are ribonucleoprotein (RNP) particles that contain a dimeric RNA genome. Retroviral particle assembly is a complex process, and how the virus is able to recognize and specifically capture the genomic RNA (gRNA) among millions of other cellular and spliced retroviral RNAs has been the subject of extensive investigation over the last two decades. The specificity towards RNA packaging requires higher order interactions of the retroviral gRNA with the structural Gag proteins. Moreover, several retroviruses have been shown to have the ability to cross-/co-package gRNA from other retroviruses, despite little sequence homology. This review will compare the determinants of gRNA encapsidation among different retroviruses, followed by an examination of our current understanding of the interaction between diverse viral genomes and heterologous proteins, leading to their cross-/co-packaging. Retroviruses are well-known serious animal and human pathogens, and such a cross-/co-packaging phenomenon could result in the generation of novel viral variants with unknown pathogenic potential. At the same time, however, an enhanced understanding of the molecular mechanisms involved in these specific interactions makes retroviruses an attractive target for anti-viral drugs, vaccines, and vectors for human gene therapy.

On board a raft or boat in the retrovirus sea

This article summarizes the essential steps in understanding the chicken Rous sarcoma virus (RSV) genome association with a nonpermissive rodent host cell genome. This insight was made possible by in-depth study of RSV-transformed rat XC cells, which were called virogenic because they indefinitely carry virus genetic information in the absence of any infectious virus production. However, the virus was rescued by association of XC cells with chicken fibroblasts, allowing cell fusion between both partners. This and additional studies led to the interpretation that the RSV genome gets integrated into the host cell genome as a provirus. Study of additional rodent virogenic cell lines provided evidence that the transcript of oncogene v-src can be transmitted to other retroviruses and produce cell transformation by itself. As discussed in the text, two main questions related to nonpermissiveness to retrovirus infection remain to be solved. The first is changes in the retrovirus envelope gene allowing virus entry into a nonpermissive cell. The second is the nature of the permissive cell functions required by the nonpermissive cell to ensure infectious virus production. Both lines of investigation are being pursued.

Epidemiological study of fowl glioma-inducing virus in chickens in Asia and Germany

Fowl glioma-inducing virus (FGV), which belongs to avian leukosis virus (ALV) subgroup A, induces fowl glioma. This disease is characterized by multiple nodular gliomatous growths of astrocytes and has been previously reported in Europe, South Africa, Australia, the United States and Japan. FGV and FGV variants have spread to ornamental Japanese fowl, including Japanese bantams (Gallus gallus domesticus), in Japan. However, it is unclear how and where FGV emerged and whether FGV is related to the past fowl glioma in European countries. In this study, the prevalence of FGV in European, Asian and Japanese native chickens was examined. FGV could not be isolated from any chickens in Germany and Asian countries other than Japan. Eighty (26%) out of 307 chickens reared in Japan were positive by FGV-screening nested polymerase chain reaction and 11 FGV variants with an FGV-specific sequence in their 3' untranslated region were isolated. In addition, four other ALVs lacking the FGV-specific sequence were isolated from Japanese bantams with fowl glioma and/or cerebellar hypoplasia. These isolates were considered to be distinct recombinant viruses between FGV variants and endogenous/exogenous avian retroviruses. These results suggest that the variants as well as distinct recombinant ALVs are prevalent among Japanese native chickens in Japan and that FGV may have emerged by recombination among avian retroviruses in the chickens of this country.

The transcription factor EGR1 regulates metastatic potential of v-src transformed sarcoma cells

Metastatic spreading of cancer cells is a highly complex process directed primarily by the interplay between tumor microenvironment, cell surface receptors, and actin cytoskeleton dynamics. To advance our understanding of metastatic cancer dissemination, we have developed a model system that is based on two v-src transformed chicken sarcoma cell lines-the highly metastatic parental PR9692 and a non-metastasizing but fully tumorigenic clonal derivative PR9692-E9. Oligonucleotide microarray analysis of both cell lines revealed that the gene encoding the transcription factor EGR1 was downregulated in the non-metastatic PR9692-E9 cells. Further investigation demonstrated that the introduction of exogenous EGR1 into PR9692-E9 cells restored their metastatic potential to a level indistinguishable from parental PR9692 cells. Microarray analysis of EGR1 reconstituted cells revealed the activation of genes that are crucial for actin cytoskeleton contractility (MYL9), filopodia formation (MYO10), the production of specific extracellular matrix components (HAS2, COL6A1-3) and other essential pro-metastatic abilities.

Recombinant avian leukosis viruses of subgroup J isolated from field infected commercial layer chickens with hemangioma and myeloid leukosis possess an insertion in the E element

Background

Five isolates (JS09GY2, JS09GY3, JS09GY4, JS09GY5, and JS09GY6) of avian leukosis virus subgroup J (ALV-J) were isolated from six infected commercial layer flocks displaying both hemangioma and myeloid leukosis (ML), which shared the same parental line, in China in 2009.

Results

All six of the commercial layer chickens examined showed hemangiomas on their body surface or feet. Some developed hemangiomas in their internal organs, causing hepatorrhexis and blood loss. Histopathologically different stages of hemangiomas with ML in the liver, heart, and spleen, were observed. Five viral isolates were obtained from infected DF1 cells incubated with the spleen tissue or serum of the birds from the six flocks. By full genome sequences analysis, a 19-nucleotide repeat sequence was identified in the primer binding site (PBS)-leader region of isolates JS09GY3 and JS09GY6, located between sites 249 and 250 according to the sequence of reference strain HPRS103, and also present in Rous sarcoma virus strain Schmidt–Ruppin B (RSV-SRB), Rous associated virus type 1 (RAV-1), and Rous associated virus type 2 (RAV-2). The predicted Gp85 proteins of isolates JS09GY2, JS09GY3, JS09GY5, and JS09GY6 were highly variable. Interestingly, the E elements of these four examined isolates showed a key deletion at site 30, which produced a new c-Ets-1 binding site. An 11-bp insertion was also found in the E element of isolate JS09GY3 located between bp 66 and 67 according to the sequence of reference strain HPRS103, while almost all previously reported Chinese strains showed an almost identical deletion of 127 bp in the same region.

Conclusions

Five ALV-J isolates were obtained from six field infected commercial layer chickens. Coexistence of hemangioma and ML were observed in these infected cases both macro- and microscopically. Complete proviral genome sequences of two isolates (JS09GY3 and JS09GY6) and the partial sequences of the other two isolates (JS09GY2 and JS09GY5) were determined. The isolates were found to be recombinants of ALV-J with a PBS-leader sequence originating from other retroviruses. The Gp85 protein with an amino acid deletion, a contiguous 11-bp insertion mutation in the E element, and a novel binding site, were noted in the proviral genomes.

Molecular and Biological Characterization of a Naturally Occurring Recombinant Subgroup B Avian Leukosis Virus with a Subgroup J–Like Long Terminal Repeat

Infection of broiler chickens with subgroup J avian leukosis virus (ALV) results in the induction of myeloid tumors. However, although egg-type chickens are susceptible to infection with ALV-J, the tumor incidence is very low, and on rare occasions the tumors observed are of the myeloid lineage. We recently described the isolation of an ALV (AF115-4) from commercial egg-type chickens suffering from myeloid leukosis. AF115-4 was initially identified as an ALV-J isolate based on PCR analysis of the long terminal repeat (LTR). However, further characterization of the viral envelope indicated that the virus is recombinant with subgroups B envelope and J LTR. Here we further characterize this recombinant virus at both the molecular and biological levels. We show that the AF115-4 isolate expresses a recombinant envelope glycoprotein encoded by a subgroup B gp85 region and a subgroup E gp37 region. The host range ofAF115-4 was analyzed using cells resistant to infection by subgroups A/B, J, or E; this shows that no ALV-J was present in the isolates obtained from the affected chickens. Additional antigenic characterization of AF115-4 using chicken sera specific for subgroups B or J indicated that no ALV-J was present in the samples examined. Inoculation of AF 115-4 into ALV-susceptible 1515 X 71 chickens resulted in the induction of lymphoid leukosis but not the expected myeloid leukosis affecting the commercial chickens. These results suggest that differences in the genetic makeup of the chickens from which AF115-4 was isolated and the line 1515 X 71 used in the present experiments may be responsible for the observed differences in pathogenicity. In addition, the results suggest that ALV-J continues to evolve by recombination, generating new viruses with different pathological properties.

Postulation of and evidence for provirus existence in RSV-transformed cells and for an oncogenic activity associated with only part of the RSV genome

This article gives a historical insight into the establishment of suitable models allowing the postulation that chicken Rous sarcoma virus (RSV) becomes integrated in different cells as a provirus. This is documented by the correspondence between two laboratories involved in these investigations. Special attention is paid to RSV-transformed mammalian cells, their virogenic nature, virus rescue by cell fusion, and finally their use for the oncogene v-src characterization. Two sets of experiments are mentioned, which provided an early indication of a transforming gene present in RSV.

The isopycnic, compartmentalized integration of Rous sarcoma virus sequences

Rous sarcoma virus (RSV) can cause tumors in hamsters, which harbor complete or partially deleted RSV sequences, in their genomes. Here we have studied the localization of RSV sequences integrated into the genome of cell lines derived from six independent hamster tumors. We have found that integration occurred in the isochores richest in guanine + cytosine, of the host genome, as it had been previously observed for bovine leukemia and hepatitis B viral sequences. The integration of RSV proviral sequences is, therefore, 'isopycnic' (i.e., it takes place in host genome sequences which compositionally match the viral sequences) and compartmentalized (i.e., it occurs in a small compositional compartment of the host genome). The hamster genome compartment hosting RSV sequences precisely corresponds to a compartment of the human genome which is the most active in both transcription and recombination. The notion of a compartmentalized, isopycnic integration of RSV proviral sequences fits, therefore, with the viral integration into transcriptionally active and recombinogenic regions of the host genome observed by other authors, but is broader, in that it includes, in addition, the requirement for a compositional match between host genome sequences and expressed viral sequences.

A transformation-competent recombinant between v-src and Rous-associated virus RAV-1

The LTR, v-src, LTR provirus, which arose by the reverse transcription and integration of src mRNA in the H-19 hamster tumor, has been successfully rescued by fusion with chicken fibroblasts infected with Rous-associated virus RAV-1. One rescued virus, E6, acquired 1 kilobase of the 5' end of the gag gene structure. Recombination took place in the region of 15-nucleotide homology exactly between v-src exon (position 7054) and gag (position 1417). This recombination resulted in the alteration of src splice acceptor site sequences, but this site is maintained as a functional splice acceptor site. The nucleotide structure of the long terminal repeat of recombinant E6 virus suggests that it arose by the intermolecular jump of reverse transcription from RAV-1 to src mRNA and then the switch of templates between already depicted regions of homology. The second jump of reverse transcription was apparently an intramolecular event. The acquisition of 1 kilobase of the 5' gag by E6 resulted in maintaining the balance of unspliced and spliced E6 RNAs and assured the replication advantage of rescued E6 virus over rescued F6 virus, the genome of which corresponds to that present in ancestral H-19 cells.

The LTR, v-src, LTR provirus generated in the mammalian genome by src mRNA reverse transcription and integration

Different types of altered proviruses of Rous sarcoma virus (RSV) have been detected in mammalian tumor cell lines. We cloned and sequenced one of these altered proviruses with the structure LTR, v-src, LTR. The presence of an intact viral splice junction, as well as duplications of the chromosomal sequence GCGGGG flanking the two 2-base-pair-deleted LTRs, demonstrated reverse transcription and normal retroviral integration of src mRNA in mammalian cells. In addition, a 1-nucleotide deletion 2 bases upstream from the AAUAAA polyadenylation signal is suspected to be responsible for the absence of a poly(A) track in the src mRNA present in virions of rescued viruses.

Lack of competition results in efficient packaging of heterologous murine retroviral RNAs and reticuloendotheliosis virus encapsidation-minus RNAs by the reticuloendotheliosis virus helper cell line

We constructed recombinant reticuloendotheliosis virus (Rev)-derived and murine leukemia virus-derived vectors to characterize the specificity of packaging retroviral RNAs in Rev proteins. Using this approach, we further localized the Rev encapsidation sequence (E) to a 144-nucleotide region and determined that there are sequences in both the 5' and 3' halves of this region which are necessary in cis for viral replication. We found that the Rev E, like the murine leukemia virus E (psi), is position independent (R. Mann and D. Baltimore, J. Virol. 54:401-407, 1986). Also, a 156-nucleotide region of the Rev intron enhanced replication in a cis-acting fashion in the presence, but not in the absence, of helper virus. Finally, we showed that packaging of E- and heterologous retroviral genomes occurred efficiently in the Rev helper cell in the absence of competing E-containing (E+) viral RNAs.