Low freqeuncy production of recombinant subgroup E avian leukosis viruses by uninfected V-15B chicken cells

Role of methylation in the induced and spontaneous expression of the avian endogenous virus ev-1: DNA structure and gene products

The endogenous avian provirus ev-1 is widespread in white leghorn chickens. Although it has no major structural defects, ev-1 has not been associated with any phenotype and is ordinarily expressed at a very low level. In this report, we describe a chicken embryo (Number 1836) cell culture containing both ev-1 and ev-6 which spontaneously expressed the ev-1 provirus. This culture released a high level of noninfectious virions containing a full complement of virion structural (gag) proteins but devoid of reverse transcriptase activity or antigen. These virions contained 70S RNA closely related to the genome of Rous-associated virus type 0, but identifiable as the ev-1 genome by oligonucleotide mapping. A fraction of the RNA molecules in the 70S complex were unusual in that they were polyadenylated 100 to 200 nucleotides downstream of the usual polyadenylation site. Eight sibling embryo cultures did not share this unusual phenotype with 1836, indicating that it was not inherited. However, an identical phenotype was inducible in the sibling cultures by treatment with 5-azacytidine, an inhibitor of DNA methylation, and the induced expression was stable for more than 10 generations. Analysis of chromatin structure and DNA methylation of the ev-1 provirus in 1836 cells revealed the presence (in a fraction of the proviruses) of both DNase I hypersensitive sites in the long terminal repeats and in gag and a pattern of cleavage sites for methyl-sensitive restriction endonuclease not found in a nonexpressing sibling. These results lend strong support to the role of DNA methylation in the control of gene expression. Additionally, they explain the lack of phenotype associated with ev-1 as due to a combination of its low expression and defectiveness in pol and env.

Role of methylation in the induced and spontaneous expression of the avian endogenous virus ev-1: DNA structure and gene products

The endogenous avian provirus ev-1 is widespread in white leghorn chickens. Although it has no major structural defects, ev-1 has not been associated with any phenotype and is ordinarily expressed at a very low level. In this report, we describe a chicken embryo (Number 1836) cell culture containing both ev-1 and ev-6 which spontaneously expressed the ev-1 provirus. This culture released a high level of noninfectious virions containing a full complement of virion structural (gag) proteins but devoid of reverse transcriptase activity or antigen. These virions contained 70S RNA closely related to the genome of Rous-associated virus type 0, but identifiable as the ev-1 genome by oligonucleotide mapping. A fraction of the RNA molecules in the 70S complex were unusual in that they were polyadenylated 100 to 200 nucleotides downstream of the usual polyadenylation site. Eight sibling embryo cultures did not share this unusual phenotype with 1836, indicating that it was not inherited. However, an identical phenotype was inducible in the sibling cultures by treatment with 5-azacytidine, an inhibitor of DNA methylation, and the induced expression was stable for more than 10 generations. Analysis of chromatin structure and DNA methylation of the ev-1 provirus in 1836 cells revealed the presence (in a fraction of the proviruses) of both DNase I hypersensitive sites in the long terminal repeats and in gag and a pattern of cleavage sites for methyl-sensitive restriction endonuclease not found in a nonexpressing sibling. These results lend strong support to the role of DNA methylation in the control of gene expression. Additionally, they explain the lack of phenotype associated with ev-1 as due to a combination of its low expression and defectiveness in pol and env.

Selective shedding and congenital transmission of endogenous avian leukosis viruses

Shedding and congenital transmission of endogenous avian leukosis viruses were studied in viremic White Leghorn hens exogenously infected with viruses with endogenous long terminal repeats (LTRs) and in four semicongenic lines of hens that naturally express infectious endogenous viruses (EVs). Relatively high titers of infectious virus EV7 (encoded at locus ev7), Rous-associated virus-0 (RAV-0), and recombinant 882/-16 RAV-0 were detected in blood cells and sera from exogenously infected hens, but marked differences were noted in the incidence of congenitally infected progeny. In enzyme immunoassays that detect viral group-specific antigen, little or no p27 was detected in albumens from dams infected with RAV-0. However, hatchmates infected with either EV7 or recombinant 882/-16 RAV-0, which was constructed with an RAV-0 LTR, shed high titers of p27. Similarly, semicongenic hens that expressed RAV-0 (EV2) (encoded at locus ev2) shed little or no p27 into albumens, but hens that harbored ev10, ev11, and ev12 shed high titers of p27. A slower electrophoretic mobility of p27, considered to be characteristic of EVs that are restricted in congenital transmission, was not associated with low levels of shedding or congenital transmission; p27 from other EVs and p27 from an avian leukosis virus field strain, all of which are shed at high levels, had mobilities identical to that of p27 from RAV-0. Although shedding and congenital transmission appear to be controlled by the viral genome, there was no correlation between low efficiency of shedding or congenital transmission and endogenous LTR or p27 sequences.

Characterization of endogenous viral loci in five lines of white Leghorn chickens

Five lines of chickens have been examined for the presence of DNA sequences related to the endogenous avian retrovirus. Five new loci have been identified, based upon analysis with the restriction endonucleases SacI and BamHI. One locus has been associated with the production of infectious endogenous virus. Restriction endonuclease mapping suggested a limited similarity between the flanking cellular sequences of two of these loci, ev-17 and ev-18, and several endogenous loci, including ev-1, already characterized. The data suggested that these two loci might have been generated by chromosomal duplication. Hybridization analysis with a probe containing the cellular sequences that flank ev-1, however, revealed that these flanking sequences shared no detectable homology with the cellular sequences that surround ev-17, ev-18, or nine other endogenous loci that were examined. These results are consistent with the hypothesis that several of the endogenous viral loci resulted either from independent infections of the germ line or from virus transpositions.

Rous sarcoma virus p19 and gp35 can be chemically crosslinked to high molecular weight complexes. An insight into virus assembly

We have used the method of chemical crosslinking in order to determine the spatial interactions between components of Rous sarcoma virus. A high molecular weight complex formed by crosslinking has been isolated by ultracentrifugation on sucrose density gradients containing 0.1% (w/v) sodium dodecyl sulphate. This complex is composed of the two viral glycoproteins gp85 and gp35, the gag protein p19, and the viral RNA. Two types of bonding are important for the formation and stability of the complex: first, native disulphide bonds between gp85 and gp35 and between individual p19 molecules; and second, hetero-crosslinking between gp35 and p19 as well as homo-crosslinking between p19. Although viral RNA is quantitatively present in the complex, experiments with RNase treatment show that it is not essential for its formation or stability. A small amount of lipid is present in the complex and appears to be crosslinked to p19. In vitro-labelling of purified virus with the lipophilic photoactivatable reagent [125I] iodonaphthylazide resulted in the labelling of gp35 and p19/23. In vivo-labelling of virus with [3H]palmitate resulted in only gp35 becoming labelled. These results substantiate the membrane association of these proteins. The significance of the interactions in the high molecular weight complex for the stability of the virus and, by implication, the role which they may play in viral assembly are discussed.

Characterization of a solitary long terminal repeat of avian endogenous virus origin

A recombinant lambda phage library constructed with a partial EcoR1 digest of DNA from a normal RPRL line 15B chicken was screened using 32P-labeled plasmid containing Rous-associated virus (pRAV-2). Nucleotide sequence analyses of a fragment of one subclone revealed the presence of a solitary long terminal repeat (LTR) that is similar to the LTRs of avian endogenous retroviruses ev1 and ev2. This LTR is flanked by unique 6 bp direct repeats characteristic of the target site for duplication of avian leukosis viruses.

Identification of the viral sequence required for the generation of recovered avian sarcoma viruses and characterization of a series of replication-defective recovered avian sarcoma viruses

The ability of transformation-defective deletion mutants of Schmidt-Ruppin Rous sarcoma virus to induce tumors and generate recovered sarcoma viruses (rASVs) was correlated with the partial src sequences retained in the transformation-defective viral genomes. Since all the transformation-defective viruses that were capable of generating rASVs retained a portion of the 3' src sequence, regardless of the extent of the 5' src deletion, and those lacking the 3' src were unable to generate rASVs, it appears that the 3', but most likely not the 5', src sequence retained in the transformation-defective viral genome is essential for rASV formation. However, rASVs derived from a particular mutant, td109, which retained a portion of the 3' src sequence, but lacked most (if not all) of the 5' src sequence, were all found to be defective in replication. Analyses of the genomic sequences of 13 isolates of td109-derived rASVs revealed that they contained various deletions in viral envelope (env), polymerase (pol), and structural protein (gag) genes. Ten isolates of rASVs contained env deletions. One isolate (rASV3812) contained a deletion of env and the 3' half of pol, and one isolate (rASV398) contained a deletion of env and pol. The one with the most extensive deletion (rASV374) had a deletion from the p12-coding sequence through pol and env. In addition, the 5' src region of td109-derived rASVs were heterogeneous. Among the 7 isolates analyzed in detail, one isolate of rASV had a small deletion of the 5' src sequence, whereas three other isolates contained extra new sequences upstream from src. Both env- and env- pol- rASVs were capable of directing the synthesis of precursor and mature gag proteins in the infected nonproducer cells. We attribute the deletions in the replication-defective rASVs to the possibility that the 5' recombination site between the td109 and c-src sequence, involved regions of only partial homology due to lack of sufficient 5' src sequence in the td109 genome for homologous recombination. A model of recombination between the viral genome and the c-src sequence is proposed to account for the requirement of the 3' src sequence and the basis for the generation of deletions in td109-derived rASVs.

High spontaneous mutation rate of Rous sarcoma virus demonstrated by direct sequencing of the RNA genome

Direct and extensive sequencing of RSV RNA genome is reported. More than 10,000 nt of the T1 RNase resistant RSV RNA fragments (1) have been sequenced and shown to cover 3900 nt of RSV genome. The frequent sequence variations found indicate that RSV supports a very high incidence of spontaneous mutations in the course of replication, one very probable cause of the genetic diversity among the avian retroviruses. Sequences of the structured RSV RNAs allowed us also to precisely characterize the structured domains of the retroviral genome and show that the src gene is not structured.

The influence of the ev 3 locus on the inducibility of serum antibody reactivity for envelope glycoprotein group-specific determinants

Chickens segregating for the ev 3 locus were bred by backcross matings of line 6(3) to line 15B. Analysis of RAV-1-infected segregants indicated that inducibility of antibody reactivity for envelope glycoprotein group-specific determinants correlated with the absence of ev 3, whereas noninducibility correlated with the presence of ev 3. Since the ev 3(+) and ev 3(-) segregants possessed similar genetic backgrounds, these results provide direct evidence that the ev 3 locus determines the phenotype of noninducibility.

Genomes of endogenous and exogenous avian retroviruses

The endogenous viruses of chickens are closely related to the exogenous avian leukosis viruses (ALV) yet as a group differ from these viruses in their host range, growth rate, and oncogenicity. The present study was undertaken to determine the patterns of relationship among the genomes of endogenous and exogenous ALVs. Complete or partial T1 oligonucleotide maps were prepared from the genomes of endogenous viruses that reside at eight distinct loci in chickens. Selected endogenous viruses and recombinants of endogenous or endogenous and exogenous viruses were characterized for host range and growth rate. From these data we could infer the following: (1) Endogenous viruses form a distinct lineage of ALVs with the most distinctive differences occurring in the portion of env that encodes host range and the U3 portion of the long terminal repeat; (2) The U3 sequences of endogenous ALVs determine the low growth rates of these viruses; and (3) Endogenous ALVs have distinctive oligonucleotide markers that allow them to be subclassified into distinct lineages. Our results suggest that endogenous viruses are derived from one another and not from exogenous field strains of ALV. This phenomenon may be related to the unique env encoded host range of endogenous ALVs, their unique U3 encoded growth rates, or perhaps their unique access, as residents of germ line DNA, to germ line cells.

Differential reactivity of serum antibody from tumor-bearing 15I5 X 7(2) chickens for cross-reactive species of endogenous retroviral envelope glycoprotein

Experiments were undertaken to determine if the sera of avian sarcoma virus-infected 15I5 X 7(2) chickens exhibit antibody reactivity for species of endogenous retroviral envelope glycoprotein expressed in 15I5 X 7(2) fibroblasts. Two viruses were used for infection of the 15I5 X 7(2) chickens, Pr-A and cl. 85; the envelope glycoprotein of Pr-A, but not of cl. 85, is antigenically cross-reactive with 15I5 X 7(2) endogenous retroviral envelope glycoprotein. Both the Pr-A and cl. 85-infected 15I5 X 7(2) chickens exhibited serum antibody reactivity for the envelope glycoprotein of the endogenous retrovirus RAV-O. In contrast, neither group of infected chickens exhibited detectable serum antibody reactivity for a distinct species of endogenous retroviral envelope glycoprotein, one which though antigenically cross-reactive with the envelope glycoprotein of RAV-O is expressed to much higher levels in 15I5 X 7(2) fibroblasts. Possible mechanisms to account for the observed pattern of antibody reactivity are discussed.

Host Susceptibility to endogenous viruses: defective, glycoprotein-expressing proviruses interfere with infections

Three defective endogenous avian leukosis viruses, ev3, ev6 and ev9, interfered with subgroup E virus infections, ev3, ev6, and ev9 expressed high levels of subgroup E envelope glycoproteins. These glycoproteins reduced the activity of cellular receptors for subgroup E viruses. ev3 and ev6 protected chickens and cultured cells from subgroup E virus infections.

Clonal analysis of the integration and expression of endogenous avian retroviral DNA acquired by exogenous viral infection

Rous-associated virus-0 is one of several endogenous avian retroviruses that are transmitted vertically and that can be isolated from different inbred lines of chickens. These viruses, referred to here as induced-leukosis viruses bearing a subgroup E glycoprotein (ILV-E), are all closely related. Clonal populations of fibroblasts from line 15B and line 100 inbred chickens have been examined for the presence and expression of exogenously acquired ILV-E sequences. Restriction enzyme analysis of uniform populations of line 15B fibroblasts, prepared by cloning cells either before or after infection with ILV-E, indicates that viral sequences were inserted at multiple sites within the cell genome. Analysis of 49 clonal populations of line 100 fibroblasts containing between one and five copies of exogenous ILV-E sequences demonstrated that each clone was characterized by a unique set of viral DNA insertions within the cell genome. The expression of the exogenous ILV-E sequences within these fibroblast clones was examined by using reverse transcriptase activity as a measure of virus production. Some clones produced an amount of virus equivalent to that produced by an equal number of the uncloned ILV-E-infected parental fibroblasts. Other clones produced 5- to 10-fold less virus. Still other clones produced no detectable virus at all. Among nine clones derived from cells containing a single copy of the ILV-E provirus, the level of virus production differed more than 100-fold. DNA from these clones was analyzed with several different restriction endonucleases to characterize the location and arrangement of the ILV-E sequences. All nine clones consisted of cells that appeared to contain a complete provirus inserted (i) in a different site within the cellular DNA and (ii) in an orientation that was colinear with the viral genomic RNA. It was observed that several cleavage sites potentially affected by methylation were equally available for cleavage in all clones regardless of the level of viral production.