Integration of spleen focus-forming virus proviruses in Friend tumor cells

Hematopoietic stem cells and retroviral infection

Retroviral induced malignancies serve as ideal models to help us better understand the molecular mechanisms associated with the initiation and progression of leukemogenesis. Numerous retroviruses including AEV, FLV, M-MuLV and HTLV-1 have the ability to infect hematopoietic stem and progenitor cells, resulting in the deregulation of normal hematopoiesis and the development of leukemia/lymphoma. Research over the last few decades has elucidated similarities between retroviral-induced leukemogenesis, initiated by deregulation of innate hematopoietic stem cell traits, and the cancer stem cell hypothesis. Ongoing research in some of these models may provide a better understanding of the processes of normal hematopoiesis and cancer stem cells. Research on retroviral induced leukemias and lymphomas may identify the molecular events which trigger the initial cellular transformation and subsequent maintenance of hematologic malignancies, including the generation of cancer stem cells. This review focuses on the role of retroviral infection in hematopoietic stem cells and the initiation, maintenance and progression of hematological malignancies.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

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.

Retroviral endogenous transcripts related to the envelope gene of Friend spleen focus-forming virus in normal mouse tissues

Retroviral endogenous sequences related to the envelope (env) gene of Friend spleen focus forming virus (SFFV) and of mink cell focus forming viruses (MCF) are present in the genome of various mouse strains. We have examined the transcription of these SFFV/MCF-related sequences in normal tissues of two mouse strains, ICFW and DBA/2. Cytoplasmic Poly A+ RNAs of normal mouse tissues were analyzed by dot-blot and Northern blot hybridizations with a subcloned env SFFV DNA fragment (0.4 kbp BamH I-Sma I). In both mice, the level of SFFV/MCF env related transcripts was very low in bone marrows and spleens whereas it was high in kidneys. Intermediate levels of transcripts were observed in other tissues (thymus, liver and brain). In both mouse strains, the size of SFFV/MCF env related transcripts varied from one tissue to another. Some transcripts in DBA/2 mice were reminiscent of full-size viral message indicating an occasional expression of xenotropic/MCF endogenous virus in this low-leukemic strain. Sizes of the other SFFV/MCF related env transcripts were unusual, but were similar in both strains for each tissue studied. This last result suggests a tissue-specific transcription of endogenous sequences related to the SFFV/MCF env gene. A 1.8 kb SFFV/MCF env RNA was the major transcript in the tissues which expressed a high level of these env transcripts. Treatment of mice with phenylhydrazine which greatly stimulates erythroid differentiation in spleens increased the level of SFFV/MCF related env RNAs only in the spleens, suggesting a possible correlation between the SFFV/MCF env transcription and the stimulation of the erythroid spleen cells.

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

The Friend spleen focus-forming virus (SFFV) is an envelope gene recombinant between the ecotropic Friend murine leukemia virus and the endogenous xenotropic mink cell focus-forming retroviral sequences. We synthesized an octadecanucleotide complementary to the 3' end of the SFFV env gene designed for discriminating the SFFV proviruses from the xenotropic mink cell focus-forming virus and ecotropic exogenous or endogenous viral sequences. Under appropriate hybridization conditions this probe allowed the identification, in addition to few endogenous DNA fragments, of multiple SFFV proviruses integrated in the genome of Friend malignant cells. Therefore this probe should be of interest in further characterizing the SFFV integration sites and possibly the SFFV ancestor gene.

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.