Integrated state of oncornavirus DNA in normal chicken cells and in cells transformed by avian myeloblastosis virus

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Characterization of avian myeloblastosis-associated virus DNA intermediates

The major species of unintegrated linear viral DNA identified in chicken embryonic fibroblasts infected with either the avian myeloblastosis-associated viruses (MAV-1, MAV-2) or the standard avian myeloblastosis virus complex (AMV-S) has a mass of 5.3 X 10(6) daltons. An additional minor DNA component observed only in AMV-S-infected cells has a mass of 4.9 X 10(6) daltons. The unintegrated linear viral DNAs and integrated proviruses of MAV-1 and MAV-2 have been analyzed by digestion with the restriction endonucleases EcoRI and HindIII. MAV-2 lacks a HindIII site present in MAV-1. These fragments have been compared to those generated by EcoRI and HindIII digestion of linear viral DNAs of AMV-S. Restriction enzyme digestion of AMV-S viral DNA produced unique fragments not found with either MAV-1 or MAV-2 viral DNAs. The major viral component present in AMV-S stocks has the HindIII restriction pattern of MAV-1. Restriction enzyme analysis of the 5.3 X 10(6)-dalton unintegrated MAV viral DNAs and their integrated proviruses suggests that the DNAs have a direct terminal redundancy of approximately 0.3 megadaltons and integrate colinearly with respect to the unintegrated linear DNA.

Integration of Rous-associated virus type O provirus in susceptible chicken cells

The number of viral genome equivalents per haploid cell genome was determined in normal chicken embryos from three selected chicken lines and in cultured fibroblasts (CEF) from these embryos. The cellular concentration of endogenous proviral DNA is similar in embryos from chickens of lines SPAFAS, 7, 15, 7 x 15, and 100. The concentration of proviral DNA is not affected by in vitro cultivation in CEF from lines that do not spontaneously produce virus, nor in CEF from line 7, which lacks receptors for Rous-associated virus type 0 (RAV-0). There is, however, a restricted increase in the number of integrated proviral genome equivalents in CEF from line 7 x 15, which produces RAV-0 and can support replication of this virus, and in CEF from line 15 experimentally infected with RAV-0.

Integration of proviral DNA in chicken cells infected with Schmidt-Ruppin Rous sarcoma virus is not enhanced by DNA repair

The effect DNA repair might have on the integration of exogenous proviral DNA into host cell DNA was investigated by comparing the efficiency of proviral DNA integration in normal chicken embryonic fibroblasts and in chicken embryonic fibroblasts treated with UV or 4-nitroquinoline-1-oxide. The cells were treated with UV or 4-nitroquinoline-1-oxide at various time intervals ranging from 6 h before to 24 h after infection with Schmidt-Ruppin strain A of Rous sarcoma virus. The chicken embryonic fibroblasts were subsequently cultured for 18 to 21 days to ensure maximal integration and elimination of nonintegrated exogenous proviral DNA before DNA was extracted. Integration of proviral DNA into the cellular genome was quantitated by hybridization of denatured cellular DNA on filters with an excess of (3)H-labeled 35S viral RNA. The copy number of the integrated proviruses in normal cells and in infected cells was also determined from the kinetics of liquid RNA-DNA hybridization in DNA excess. Both RNA excess and DNA excess methods of hybridization indicate that two to three copies of the endogenous provirus appear to be present per haploid normal chicken cell genome and that two to three copies of the provirus of Schmidt-Ruppin strain A of Rous sarcoma virus become integrated per haploid cell genome after infection. The copy number of viral genome equivalents integrated per cell treated with UV or 4-nitroquinoline-1-oxide at different time intervals before or after infection did not differ from the copy number in untreated but infected cells. This finding supports our previous report that the integration of oncornavirus proviral DNA is restricted to specific sites in the host cell DNA and suggests a specific mechanism for integration.

Variations in integration site of avian oncornaviruses in different hosts

We examined the integration site of avian oncornaviruses in the genome of different hosts with respect to the repetitive frequency of the cellular DNA sequences adjacent to the integrated proviral DNA. The following systems were studied: avian sarcoma virus (B-77) and avian leukosis virus (Rous-associated virus-61) in cultured duck embryonic cells and B-77 in cultured mouse 3T3 cells. These systems represent different host responses to viral infection, i.e., one in which both cellular transformation and viral replication occur (B-77-infected duck cells), one in which viral replication, but not transformation, occurs (Rous-associated virus-61-infected duck cells), and one in which transformation, but not viral replication, occurs (B-77-infected 3T3 cells). Two sequential hybridizations were used. First, large denatured DNA fragments (2.8 X 10(6) daltons) were reassociated to different C0t (mole-seconds per liter) values. Next, DNA remaining single stranded at different C0t values was isolated by hydroxylapatite column chromatography, immobilized on nitrocellulose filters, and hybridized with an excess of 3H-labeled 35S viral RNA to titrate the concentration of proviral DNA. Results show that B-77 sarcoma virus and Rous-associated virus-61 integrate in the unique region of duck DNA, whereas B-77 proviral DNA is associated with both repeated and unique host DNA sequences in transformed mouse 3T3 cells.

Host restriction of Friend leukemia virus: synthesis and integration of the provirus

Host restriction of exogenous infection by murine leukemia viruses is controlled in vitro predominantly by the murine Fv-1 locus. The mechanism of this host restriction was investigated by comparing the early events in the replication of N-tropic versus B-tropic Friend leukemia virus in NIH 3T3 cells. These cells, which are Fv-1nn in type, are permissive for the N-tropic strain, but nonpermissive for the B-tropic strain, which replicates permissively in Balb/c cells. We have studied the synthesis, intracellular location, and molecular form of virus-specific DNA early in replication by means of molecular hybridization with a virus-specific DNA probe. Our results suggest that in the permissive infection viral DNA rapidly becomes integrated with cellular DNA. However, in the nonpermissive infection, although almost equal amounts of both positive and negative strand viral DNA are synthesized, integration of the provirus does not occur.

Evidence for tandem integration of avian myeloblastosis virus DNA with endogenous provirus in leukemic chicken cells

The integration site of avian myeloblastosis virus (AMV) proviral DNA in DNA from leukemia chicken myeloblasts has been studied by three sequential nucleic acid hybridizations that can localize the proviral DNA according to the repetitiveness of the adjacent cellular DNA regions. First, large denatured cellular DNA fragments (2.1 x 10(6) daltons) were reassociated and fractionated according to sequence reiteration frequenct. Next, DNA remaining single-stranded in each fraction was immobilized on nitrocellulose filters hybridized with an excess of unlabeled 70S RNA from Rous-associated virus-0 to saturate the endogenous proviral DNA sequences.

Synethesis and integration of viral DNA in chicken cells at different time after infection with various multiplicities of avian oncornavirus

To see if integration of the provirus resulting from RNA tumor virus infection is limited to specific sites in the cell DNA, the variation in the number of copies of virus-specific DNA produced and integrated in chicken embryo fibroblasts after RAV-2 infection with different multiplicities has been determined at short times, long times, and several transfers after infection. The number of copies of viral DNA in cells was determined by initial hybridization kinetics of single-stranded viral complementary DNA with a moderate excess of cell DNA. The approach took into account the different sizes of cell DNA and complementary DNA in the hybridization mixture. It was found that uninfected chicken embryo fibroblasts have approximately seven copies, part haploid genome of DNA sequences homologous to part of the Rous-association virus 2 (RAV-2) genome. Infection with RAV-2 adds additional copies, and different sequences, of RAV -2- specific DNA. By 13 h postinfection, there are 3 to 10 additional copies per haploid genome. This number can not be increased by increasing the multiplicity of infection, and stays relatively constant up to 20 h postinfection, when some of the additional viral DNA is integrated. Between 20 and 40 h postinfection, the cells accumulated up to 100 copies per haploid genome of viral DNA. Most of these are unintegrated. This number decreases with cell transfer, until cells are left with one to three copies of additional viral DNA sequences per haploid genome, of which most are integrated. The finding that viral infection causes the permanent addition of one to three copies of integrated viral DNA, despite the cells being confronted with up to 100 copies per haploid genome after infection, is consistent with a hypothesis that chicken cells contain a limited number of specific integration sites for the oncornavirus genome.

Use of alkaline sucrose gradients in a zonal rotor to detect integrated and unintegrated avian sarcoma virus-specific DNA in cells

We have attempted to distinguish integrated and unintegrated forms of avian sarcoma virus-specific DNA in cells by sedimentaton through an alkaline sucrose gradient in a slowly reorienting zonal rotor. Results obtained with this procedure are similar to those obtained by the more convenient analysis of networks of high-molecular-weight cell DNA. Most, if not all, viral DNA appears completely integrated into the host cell genome in an avian sarcoma virus-transformed mammalian cell and in normal chicken cells (in which viral DNA is genetically transmitted). Fully transformed duck cells and duck embryo fibroblasts infected for 20 to 72 h contain both integrated and unintegrated viral DNA; up to one copy per cell is integrated within 20 h after infection, and four to eight copies are integrated in fully transformed cells. The amount of unintegrated DNA varies but may comprise over 75% of the viral DNA in acutely infected cells and from 20 to 70% of the viral DNA in fully transformed cells. The unintegrated DNA in either case consists principally of duplexes with "minus" strands the length of a subunit of the viral genome (2.5 X 10(6) to 3 X 10(6) daltons) and relatively short "plus" strands (0.5 X 10(6) to 1.0 X 10(6) daltons).

Restricted addition of proviral DNA in target tissues of chickens infected with avian myeloblastosis virus

Proviral DNA is synthesized within an hour after infection of chicken cells with an avian oncornavirus and is integrated into nuclear cellular DNA within a short time. The viral DNA appears to be synthesized as double-stranded molecules of approximately 6 X 10(6) daltons some of which are converted into supercoiled cricles perhaps as a requisite for integration. The endogenous v-DNA in normal chicken cells and both the endogenous and amv v-DNA in leukemic chicken myeloblasts are covalently linked with chromosomal DNA. There is no detectable free DNA either circular or linear present in leukemic cells several weeks after infection. The endogenous v-DNA which is transmitted vertically from parents to offspring is uniformly and stably distributed in all chicken organs. There are about 1-2 copies of endogenous provirus per haploid genome of all normal cells. This DNA is very closely related to RAV-O RNA. After infection with AMV it seems that target cells such as leukemic myeloblasts, RBC and nephroblasts acquire complete copies of AMV DNA. Interestingly, only these target cells can be converted to neoplastic cells in the chicken as well as in vitro. The target cells acquire 1-2 copies of AMV specific DNA per haploid genome in addition to the endogenous v-DNA. All the available evidence shows that leukemic and kidney tumor cells have acquired AMV v-DNA. It remains to be elucidated whether the newly added viral DNA is alone responsible for neoplastic changes or does so in conjunction with endogenous viral information.

Host induced alteration of avian sarcoma virus B-77 genome

The genome of an avian oncornavirus was altered after infection of a heterologous host. This was studied with avian sarcoma virus B-77 in duck embryonic fibroblasts (DEF) and chicken embryonic fibroblasts (CEF). To detect alteration of the viral genome, we hybridized 35S B-77 RNA with normal duck DNA by either one of two techniques:when viral RNA was in excess and when DNA was in excess. The RNA of B-77 passaged only in gs minus chf minus CEF does not have homology with duck DNA. However, after several passages of B-77 through DEF the viral genome acquired duck specific RNA sequences. After 4 and 10 passages, B-77 RNA acquired 2.2 and 6.6%, respectively, complementarity to normal duck DNA. The duck specific RNA sequences were found to be covalently linked to the B-77 RNA genome. Also, the host specific sequences acquired by the virus appear to be from a region of the duck DNA which is repeated four to six times per cell. After 5 back passages in CEF some of the duck specific RNA sequences in the viral genome were lost.

Infectious DNA recovered from avian tumor-virus-producing cells

A single treatment of chick embryo fibroblasts with DNA recovered from chick embryo fibroblasts productively infected and transformed with four different strains of RSV, or productively infected with two different strains of RAV, resulted in virus production and cell transformation (in the case of RSV) two or three passages after treatment (8-25 days). The virus recovered from cultures was phenotypically identical to that produced by the donor cells. No virus production nor cell transformation resulted from treatment of control cultures with DNA digested with DNAse. Infectious RSV-DNA was recovered from purified donor cell nuclei and was associated with the precipitable fraction of DNA prepared according to the method of Hirt (1967). It also sedimented with cellular DNA in density gradients, and with high molecular weight DNA (2-4 times 10-7 daltons) in sucrose gradients, which suggests that it is associated and may be integrated with chromosomal DNA. In some experiments, DNA fractions of lower molecular weight (down to 6 times 10-6 daltons) were also infectious. DNA from virus-producing RSV-transformed cells also gave rise to virus and Rous cells in cultures of fibroblasts from gs- embryos. However, the amount of DNA required for successful infection varied widely between experiments, and no reproducible dose-effect relationship was observed. The frequency of DNA-treated cells which produced virus remained low, even when the assay cultures were pretreated with 5-bromodeoxyuridine.

Fate of viral RNA of murine leukemia virus after infection

[3H]Uridine-labeled Rauscher leukemia virus was used to infect mouse embryo fibroblasts. After the infected cells were separated into nuclear and cytoplasmic fractions nucleic acid was extracted by sodium dodecyl sulfate-phenol-chloroform treatment and analyzed by Cs2SO4 and sucrose density gradient centrifugation. Between 45 and 70 min after infection a transient and synchronized shift of the acid-insoluble radioactive peak toward the RNA-DNA hybrid region occurred in both the nuclear and cytoplasmic fractions. The density of the cytoplasmic hybrid shifted to 1.56 g/ml (RNA equals about 50%), while the sedimentation rate decreased from 36 S to 14 S; however, the density of the nuclear hybrid shifted to 1.58-1.48 g/ml (RNA equals 57-17%, respectively), while its sedimentation rate remained about 65 S. The hybrids in both the nuclear and the cytoplasmic fractions still showed hybrid density after heat denaturation. The processes of the early stages of RNA tumor virus infection are discussed with regard to the functions of viral RNA-dependent DNA polymerase (reverse transcriptase) and a possible integration of viral genetic information into the host chromosome.

Differences between the integration of avian myeloblastosis virus DNA in leukemic cells and of endogenous viral DNA in normal chicken cells

The nature of integrated viral DNA in normal and leukemic chicken cells has been studied by sequential nucleic acid hybridization procedures that localize the viral specific DNA in cellular DNA regions differing in reiteration frequency. First, DNA.DNA reassociation was employed to fractionate cellular DNA sequences according to their reiteration frequencies. Next, the DNA in each fraction was denatured, immobilized on nitrocellulose filters, and then hybridized with viral [(3)H]RNA. In normal cells, endogenous viral DNA appears to be associated with cell sequences reiterated 1200 times, and each integration unit appears to have a maximal size approximately equivalent to the 35S RNA subunit of the virion. In infected cells, additional viral sequences are found which reassociate as if they integrated adjacent to unique cellular DNA, or in tandem with endogenous viral DNA.