Analysis of integrated proviral DNA sequences with an octadecanucleotide probe designed for specific identification of spleen focus-forming virus in the mouse genome

General method to label antisense oligonucleotides with radioactive halogens for pharmacological and imaging studies

Evaluation of oligonucleotides for biomedical applications requires different in vivo and in vitro approaches (pharmacokinetics, biodistribution, macro- and microimaging, metabolism,.), that are performed with different radioisotopes according to the temporal and spatial resolution needed. A method to introduce radioactive isotopes of halogens (fluorine, bromine, and iodine) in a small and stable molecule has been developed. Radiosynthons can then be conjugated with any given oligonucleotide in one step to create the appropriate radiotracer. This general radiolabeling procedure for oligonucleotides is efficient to synthesize (18)F-, (76)Br-, and (125)I-oligonucleotides for biological needs. Applications of the method to biodistribution, metabolism, in vivo and ex vivo imaging of (125)I- and (18)F-labeled oligonucleotides are reported.

Mutations in the env gene of friend spleen focus-forming virus overcome Fv-2r-mediated resistance to Friend virus-induced erythroleukemia

Although Fv-2r homozygous mice are resistant to leukemias induced either by an erythropoietin-encoding virus or by wild-type Friend virus (FV) (M. E. Hoatlin, S. L. Kozak, F. Lilly, A. Chakraborti, C. A. Kozak, and D. Kabat, Proc. Natl. Acad. Sci. USA 87:9985-9989, 1990), they are susceptible to some variants of FV (R. A. Steeves, E. A. Mirand, A. Bulba, and P. J. Trudel, Int. J. Cancer 5:349-356, 1970; R. W. Geib, M. B. Seaward, M. L. Stevens, C.-L. Cho, and M. Majumdar, Virus Res. 14:161-174, 1989). To localize the virus gene involved in influencing the host range, we cloned and sequenced the env gene of the BB6 variant of FV (Steeves et al., Int. J. Cancer 5:349-356, 1970). In comparison with the wild-type env gene, the BB6 variant contains a 159-bp deletion that eliminates the membrane-proximal portion of the extracellular domain and 58 point mutations resulting in 13 amino acid changes. Substitution of the variant env gene for the wild-type env gene resulted in a recombinant virus that produced a Friend virus-like disease in Fv-2r homozygotes. Our results identify the spleen focus-forming virus env gene as the viral gene involved in this virus-host interaction. Additionally, they suggest that the product of the Fv-2r gene modifies the interaction between the spleen focus-forming virus envelope protein and the erythropoietin receptor.

A common site for immortalizing proviral integrations in Friend erythroleukemia: molecular cloning and characterization

By using a tagged derivative of Friend spleen focus-forming virus, we previously obtained evidence that proviral integration(s) in the host genome can cause erythroblast immortality by abrogating the commitment of the cell to differentiate (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 62:4129-4135, 1988). Exploiting the fact that each leukemia was a single clone that contained one tagged provirus, we have now molecularly cloned and characterized one common genomic site for immortalizing proviral integrations.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

A tagged helper-free Friend virus causes clonal erythroblast immortality by specific proviral integration in the cellular genome

A colinear molecular clone of the Lilly-Steeves polycythemia strain of Friend spleen focus-forming virus (SFFV) was modified by inserting a 215-base-pair tag of simian virus 40 DNA into its nonfunctional pol gene region. The DNA was then transfected into psi-2 packaging cells, and helper-free tagged SFFV was recovered in the culture medium. Injection of this helper-free virus into NIH/Swiss mice caused transient mild splenomegaly and formation of spleen foci at 9 to 10 days. Although the vast majority of infected erythroblast clones then differentiated and died out, rare cell clones that were present in only 20 to 30% of the mice grew extensively by 26 to 33 days to form transplantable leukemias. The clonality of these leukemias was established by Southern blot analysis of their DNAs by using several restriction endonucleases and the simian virus 40 tag as a hybridization probe. All transplantable leukemias lacked helper virus contamination and contained a single tagged SFFV provirus that expressed the mitogenic env gene product gp55. The SFFV proviruses in these leukemias also appeared to be integrated into a few tightly clustered sites in the cellular genome. Although the tagged SFFV caused polycythemia during the polyclonal early stage of erythroblastosis, growth of the helper-free clonal erythroleukemias caused severe anemia. These results suggest that a single SFFV can cause mitosis of erythroblasts, and that cell immortalization also occurs when the provirus integrates into a critical site in the host genome. We propose that mice with clonal-stage leukemia become anemic because the immortalizing proviral integrations interfere with the cellular commitment to differentiate.

Loss of pathogenicity of spleen focus-forming virus after pseudotyping with Akv

Friend virus complex (FV), which comprises replication-competent Friend murine leukemia virus (FMuLV) plus replication-defective spleen focus-forming virus (SFFV), induces a multistage erythroleukemia. We have examined the role of replication-competent helper virus in the early and late stages of FV disease by replacing FMuLV, the native helper, with Akv, the endogenous ecotropic MuLV of AKR mice. SFFVP/FRE, an established fibroblast line nonproductively infected with the polycythemic strain of SFFV, was superinfected with FMuLV or with Akv. Although supernatants from these cells showed similar titers in the XC plaque assay, supernatants from Akv-infected SFFVP/FRE cells showed 100- to 5,000-fold less activity than did those from FMuLV-infected cells with respect to spleen focus induction in vivo. Since virions isolated from these two supernatants contained similar ratios of SFFV to helper virus genomic RNA, it did not appear that the difference was due to a relative inability of Akv to package SFFV. Although FMuLV- and Akv-rescued SFFV are equally infectious in a mouse fibroblast cell line (NIH 3T3), FMuLV-rescued SFFV was far more efficient in inducing erythroid bursts in cultured primary bone marrow cells. Adding Akv to preparations of FMuLV-rescued SFFV did not significantly interfere with burst induction. Helper-free SFFV induced 50- to 500-fold more spleen foci when coinjected with FMuLV than it did with Akv. Helper virus also affected mortality rates that reflect the late stage of the disease. When FMuLV- or Akv-rescued SFFV was injected into NIH Swiss mice at dosage levels adjusted to give equal numbers of spleen foci, all mice receiving FMuLV-rescued SFFV developed splenomegaly and died, whereas no mice receiving Akv-rescued SFFV died or developed detectable splenomegaly. When FMuLV was coinjected with Akv-rescued SFFV, the mortality rate rose from 0 to 100%. Injection of helper-free SFFV alone did not induce mortality, but coinjection of helper-free SFFV with FMuLV resulted in 100% mortality. Thus, the helper virus used to rescue SFFV plays at least a quantitatively important role in the early stage of FV disease and a crucial role in the late stage of the disease in vivo.

Spi-1 is a putative oncogene in virally induced murine erythroleukaemias

Retroviral insertional mutagenesis has been proposed as an efficient mechanism to turn on or to increase the expression of oncogenes in several avian or mammal models. Integration site studies of avian leukosis virus, murine leukaemia and murine mammary tumour viruses led to the coleutification of highly conserved genes whose expression is induced or increased during leukaemogenesis, probably through enhancer elements present in the retroviral long terminal repeats. This is reminiscent of the activation of cellular proto-oncogenes or putative oncogenes in numerous human tumours and leukaemias as a result of chromosomal translocations or DNA rearrangements. Here we report the characterization of a new putative oncogene isolated from a murine erythroleukaemia induced by the acute leukaemogenic retrovirus spleen focus forming virus (SFFV). An important and unusual feature of this genomic locus Spi-1 (for SFFV proviral integration) is that rearrangements due to SFFV integration were found in 95% of the erythroid tumours studied. A 4.0-kilobase messenger RNA was detected in rearranged tumours. No Spi-1 rearrangement was detected in other virally induced myeloid, lymphoid or erythroid tumours tested.

Multistage Friend erythroleukemia: independent origin of tumor clones with normal or rearranged p53 cellular oncogenes

The erythroleukemia induced by Friend virus complex in adult mice is a multistage malignancy characterized by the emergence, late in the disease, of tumorigenic cell clones. We have previously shown that a significant proportion of these clones have unique rearrangements in their cellular p53 oncogene. The clonal relationships among Friend tumor cells isolated in the late stages of Friend erythroleukemia were analyzed by examining the unique integration site of Friend murine leukemia virus and the unique rearrangement in their cellular p53 oncogene. The majority of clones isolated from individual mice infected with Friend virus were clonally related as judged by the site of Friend murine leukemia virus integration. However, Southern gel analysis of DNA from individual Friend cell clones indicated that all of the clones with a normal p53 gene from the same mice were clonally related, but were unrelated to the Friend cell lines with a rearranged p53 gene. These results suggest that Friend tumor cells with rearrangements in their p53 gene arise as the result of a unique transformation event, rather than by progression from already existing tumor cells with a normal p53 gene. They also suggest that such rearrangements in the p53 gene confer a strong selective advantage to these cells in vivo.