Isolation and characterization of a gp 70+ non producer Friend tumor cell clone

Germ-line deletion of p53 reveals a multistage tumor progression in spi-1/PU.1 transgenic proerythroblasts

Activation of the spi-1/PU.1 proto-oncogene and loss of p53 function are genetic alterations associated with the emergence of Friend malignant erythroleukemic cells. To address the role of p53 during erythroleukemogenesis, spi-1 transgenic mice (spi-1-Tg) which develop erythroleukemia were bred with p53-deficient mice. Three classes of spi-1 transgenic mice differing in their p53 functional status (p53(+/+), p53(+/-) and p53(-/-)) were generated. These mice developed a unique pattern of erythroleukemia. In wild-type p53 spi-1-Tg mice, none of the primary erythroleukemic spleen cells displayed autonomous growth in vitro and in vivo. In contrast, in p53(+/-) spi-1-Tg mice, erythroleukemic cells gave rise to growth factor-independent cell lines and generated tumors in vivo. Malignancy was associated with loss of the wild-type p53 allele. The p53(-/-) spi-1-Tg mice developed erythroleukemia with a total incidence and a reduced latency compared to the two other genotypes. Unexpectedly, 50% of p53(-/-) spi-1-Tg erythroleukemic spleens generated cell lines that were strictly dependent upon erythropoietin (Epo) for proliferation, whereas the remainder proliferated independently of cytokines. Moreover, only 70% of these spleen cells were tumorigenic. These findings indicate that p53 germ-line deletion did not confer malignancy to spi-1-transgenic proerythroblasts. Moreover Epo independence and tumorigenicity appear as separable phenotypic characteristics revealing that the spi-1-Tg proerythroblasts progress towards malignancy through multiple oncogenic events.

Murine retrovirus-induced spongiform encephalomyelopathy: host and viral factors which determine the length of the incubation period

A molecular clone of wild mouse ecotropic retrovirus CasBrE (clone 15-1) causes a spongiform neurodegenerative disease with a long incubation period, greater than or equal to 6 months. This virus infects the central nervous system (CNS) at low levels. In contrast, a chimeric virus, FrCasE, containing env and 3' pol sequences of 15-1 in a Friend murine leukemia virus background, infects the CNS at high levels and causes a rapid neurodegenerative disease with an incubation period of only 16 days. With both viruses, the induction of neurologic disease is dependent on inoculation during the perinatal period. Since the length of the incubation period of this disease appears to be a function of the relative level of CNS infection, we have attempted to identify the viral and host factors which determine the relative level of virus infection of the CNS. It was previously shown that the CNS is susceptible to infection only during the perinatal period (M. Czub, S. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 65:2539-2544, 1991). Here we have found that the susceptibility of the CNS wanes progressively or gradually as a function of the age of the host, this age-dependent resistance being complete by 12 to 14 days of age. Utilizing a group of chimeric viruses, we found that the relative level of CNS infection achieved after inoculation of mice at 1 day of age was a function of the kinetics of virus replication and spread in peripheral organs. Viruses which reached peak viremia titers early (5 to 7 days of age) infected the CNS at high levels, and viruses which reached peak titers later infected the CNS at lower levels. Among the group of viruses examined in the current study, the kinetics of peripheral virus replication and spread appeared to be influenced primarily by sequences within the R-U5-5' leader region of the viral genome. These results suggested that the relative level of CNS infection was determined very early in life and appeared to be a function of a dynamic balance between the kinetics of virus replication in the periphery and a progressively developing restriction of virus replication in the CNS.

A putative truncated cytokine receptor gene transduced by the myeloproliferative leukemia virus immortalizes hematopoietic progenitors

The myeloproliferative leukemia virus (MPLV) is an acute leukemogenic murine replication-defective retrovirus. By sequencing the envelope gene of a biologically active MPLV clone, we found that this region comprises a novel oncogene named v-mpl in phase with two parts of the Friend murine leukemia virus envelope gene. The MPLV env region could encode an env-mpl fusion polypeptide that presents the characteristics of a transmembrane protein. We show that in vitro infection of bone marrow cells with helper-free MPLV readily yields immortalized factor-independent hematopoietic cell lines of different lineages. In mice, the c-mpl proto-oncogene is expressed in hematopoietic tissues as a 3 kb mRNA. Since v-mpl shares strong structural analogies with the hematopoietin receptor superfamily, it is likely that MPLV has transduced a truncated form of an as yet unidentified hematopoietic growth factor receptor.

Neurodegenerative disease induced by the wild mouse ecotropic retrovirus is markedly accelerated by long terminal repeat and gag-pol sequences from nondefective Friend murine leukemia virus

The wild mouse ecotropic retrovirus (WM-E) induces a spongiform neurodegenerative disease in mice after a variable incubation period of 2 months to as long as 1 year. We isolated a molecular clone of WM-E (15-1) which was weakly neurovirulent (incidence, 8%) but was highly leukemogenic (incidence, 45%). Both lymphoid and granulocytic leukemias were observed, and these leukemias were often neuroinvasive. A chimeric virus was constructed containing the env and 3' pol sequences of 15-1 and long terminal repeat (LTR), gag, and 5' pol sequences from a clone of Friend murine leukemia virus (FB29). FB29 has been shown previously to replicate to high levels in the central nervous system (CNS) but is not itself neurovirulent. This finding was confirmed at the DNA level in the current study. Surprisingly, intraperitoneal inoculation of neonatal IRW mice with the chimeric virus (FrCasE) caused an accelerated neurodegenerative disease with an incubation period of only 16 days and was uniformly fatal by 23 days postinoculation. Introduction of the LTR of 15-1 into the FrCasE genome yielded a virus (FrCasEL) with a degree of neurovirulence intermediate between those of 15-1 and FrCasE. No differences were found in the levels of viremia or the relative levels of viral DNA in the spleens of mice inoculated with 15-1, FrCasE, or FrCasEL. However, the levels of viral DNA in the CNS correlated with the relative degrees of neurovirulence of the respective viruses (FrCasE greater than FrCasEL greater than 15-1). Thus, the env and 3' pol sequences of WM-E (15-1) were required for neurovirulence, but elements within the LTR and gag-pol regions of FB29 had a profound influence on the level of CNS infection and the rate of development of neurodegeneration.

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.

Frequent involvement of the fim-3 region in Friend murine leukemia virus-induced mouse myeloblastic leukemias

fim-3 is a new proviral integration region involved in 23% (16 of 68) of Friend murine leukemia virus-induced myeloblastic leukemias. This region is distinct from 20 oncogenes and from putative oncogenes tested so far. Proviruses are integrated in a 16-kilobase region, always in the same orientation. No RNA expression of fim-3 was detected.

Genetic analysis of myeloproliferative leukemia virus, a novel acute leukemogenic replication-defective retrovirus

The myeloproliferative leukemia virus (MPLV) is a new acute leukemogenic, nonsarcomatogenic retroviral complex that is generated during the in vivo passage of a molecularly cloned Friend ecotropic helper virus. Examination of viral RNA expression in MPLV-producing cells revealed the presence of two distinct molecular species that hybridized with a long terminal repeat or an ecotropic env-specific probe but not with a xenotropic mink cell focus-forming virus env-specific probe derived from a spleen focus-forming virus: an 8.2-kilobase species corresponding to a full-length Friend murine leukemia virus (F-MuLV) and a deleted species with a genomic size of 7.4 kilobases. This deleted virus was biologically cloned by limiting dilutions and single cell cloning in Mus dunni fibroblasts. Three nonproducer clones with normal morphologies and containing one single integrated copy of the deleted virus were superinfected with F-MuLV, Moloney murine leukemia virus, Gross murine leukemia virus, mink cell focus-forming virus (HIX), or the amphotropic 1504 murine leukemia virus. All pseudotypes caused macroscopic and microscopic abnormalities in mice that were similar to those seen in the parental stock. A comparison of the physical maps of F-MuLV and MPLV, which was deduced from the restriction enzyme digests of unintegrated proviral DNAs, indicated that the MPLV-defective genome (i) is probably derived from F-MuLV, (ii) has conserved the F-MuLV gag and pol regions, and (iii) is deleted and rearranged in the env region in a manner that is clearly distinct from that of Friend or Rauscher spleen focus-forming viruses.

fim-1 and fim-2: two new integration regions of Friend murine leukemia virus in myeloblastic leukemias

The Friend helper murine leukemia virus (F-MuLV) induces in mice a high percentage of myeloblastic leukemias. Myeloblastic transformation is also observed after in vitro infection of long-term bone marrow cultures. To investigate the molecular events leading to the generation of myeloblastic leukemias, we first screened a panel of leukemic cells for rearrangement or amplification of known oncogenes or previously described specific integration sites. No modification of these genes was observed. Therefore, we searched for common integration sites by constructing a genomic library from a myeloblastic cell line harboring only five integrated proviruses. This library was screened with a virus-specific probe, and virus-host cellular junction fragments were subcloned. Two flanking cellular sequences corresponding to two different integrated proviruses were used to analyze additional myeloblastic leukemias. The first probe detected rearrangements in 2 of 42 myeloblastic leukemias, and the second probe detected rearrangements in 6 of 42. We demonstrated that, in each case, the rearrangement was the result of F-MuLV integration, with all proviruses in the same orientation and clustering in a region less than 3 kilobases long. The two regions, named fim-1 and fim-2, were different from 15 oncogenes tested. Rearrangements of these two regions were found in F-MuLV-induced myeloblastic leukemias but not in 20 lymphoid or erythroid leukemias induced by the same virus.

Friend murine leukemia virus and spleen focus-forming virus expression in highly malignant interferon-sensitive and interferon-resistant Friend leukemia cells

Analysis of expression of the Friend murine leukemia virus (F-MuLV) and of the spleen focus forming virus (SFFV) has been undertaken in highly malignant interferon (IFN)-sensitive (745) and IFN-resistant (3Cl-8) Friend leukemia cells (FLC), serially passaged intraperitoneally in DBA/2 mice. In vivo passaged 745 cells, as well as the clones derived thereof, did not release Friend virus (FV). Western blot analysis of the plasma membrane fractions of the virus nonproducer 745 cells revealed the lack of gp69/70 glycoprotein expression. At least 10 intraperitoneal passages of virus producer in vitro passaged of virus producer in vitro passaged 745 cells were necessary to obtain the selection of the virus nonproducer phenotype. In contrast in vivo passaged 3Cl-8 cells continued to produce FV even after 100 in vivo passages and expressed gp69/70 antigens to a similar extent as the original in vitro passaged FLC. The expression of F-MuLV and SFFV RNAs in virus producer and virus nonproducer FLC clones has been investigated by means of Northern blot technique using probes specific for either F-MuLV or SFFV. No F-MuLV specific RNA sequences were detected in virus nonproducer 745 clones. SFFV specific RNA transcripts and gp52/55 glycoprotein production could be revealed in all the FLC tested. Southern blot analysis showed the presence of F-MuLV specific sequences in the cellular DNA of virus nonproducer 745 clones. As both in vivo passaged F-MuLV producer 3Cl-8 and F-MuLV nonproducer 745 cells were equally barely immunogenic and highly malignant when injected into syngeneic DBA/2 mice, these results indicate that F-MuLV expression does not result per se in a high immunogenic potential of tumor cells. For the time being, as a specific property of 3Cl-8 versus 745 cells is the interferon-resistant phenotype, it is tempting to speculate that the selection of virus nonproducer cell variants after in vivo passages of interferon-sensitive 745 cells could depend on the presence of low levels of endogenous interferon in normal young mice.

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.

Monoclonal proliferation of Friend murine leukemia virus-transformed myeloblastic cells occurs early in the leukemogenic process

Integrated Friend murine leukemia virus copies were analyzed by the Southern blotting procedure in myeloblastic cell lines obtained after in vitro infection of long-term mouse bone marrow cultures. Several steps leading to the generation of malignant myeloblastic cells after a long latency period were observed in the evolution of infected cultures. Shortly after infection, a random distribution of integrated provirus copies was observed in the DNA of normally differentiating myeloid cells. In contrast, a distinct pattern of integrated Friend murine leukemia virus copies was evident in the first non-differentiating immature myeloblastic cells appearing in cultures, suggesting a monoclonal origin of these cells. For each cell line, characteristic hybridizing fragments were conserved during the 1-year culture period necessary for the acquisition of tumorigenic properties and were also observed in tumors grafted in vivo. We can conclude that monoclonality is effective very early in the myeloid transformation process, as soon as the precursor cells are blocked in their differentiation.

Organization and state of methylation of endogenous type C retroviral sequences in 129 mouse differentiated and undifferentiated teratocarcinoma cell lines

Attempts to activate type C endogenous viruses in 129 mouse fibroblasts and in teratocarcinoma-derived cell lines have never been successful, although the genome of these cells contains xenotropic virus-related sequences. We have investigated the arrangement of these sequences and their methylation state by DNA restriction endonuclease digestion, electrophoresis of digests in agarose gels, Southern blotting and hybridization with specific probes. Our results show that the majority of the sequences are organized into two complete provirus families integrated at multiple sites in the cell genome and that they are hypermethylated in embryonal carcinoma cells as compared with differentiated cells. Having previously found a higher expression of viral RNA in 129 derived embryonal carcinoma cells, our data indicate an apparent direct correlation between methylation and type C virogenes expression.

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

Using the Southern blot procedure, we studied the presumed spleen focus-forming virus (SFFV) provirus integration sites in the genome of the premalignant and the malignant cells isolated during the course of Friend erythroleukemia. Two restriction endonucleases, PstI and BamHI, discriminated the presumed integrated SFFV proviruses from the endogenous xenotropic-mink cell focus-forming viral sequences. No SFFV integration sites were detectable in the premalignant cells, suggesting a random integration of SFFV proviruses in the genome of these cells. In contrast, SFFV proviruses were detected at a single or very few sites in the genome of all malignant cells we analyzed. These results indicate that the event leading to the malignant transformation in acute Friend leukemia is clonal. In two of the six animals examined, tumors cells isolated from the spleens and the livers of individual mice showed identical SFFV integration patterns. This last result suggests that in some cases different tumors in a same leukemic animal could be derived from a unique clonal event.

Expression of c-ras and c-myc oncogenes in murine erythroleukemias induced by Friend viruses

We have examined the expression of three cellular oncogenes (c-Ki-ras, c-Ha-ras and c-myc) in different stages of murine erythroleukemias induced in ICFW mice by Friend viruses (F-MuLV and SFFV) in comparison to normal mice spleens and bone marrows, including erythropoietically-stimulated spleens from phenylhydrazine-treated mice. There is no evidence of c-Ki-ras RNA expression in any of the tissues tested. c-Ha-ras RNA was found at similar levels in erythroleukemic cells and normal erythroid cells. In contrast, increased levels of normal 2.3 Kb and short 1.8 Kb c-myc RNA transcripts were detected in both early preleukemic and late leukemic phases of the diseases, as compared to normal erythroid cells. Apparently, neither myc gene amplification nor myc gene rearrangement was observed in erythroleukemias. Our results suggest a possible involvement of c-myc gene either in erythroid cell differentiation or in an early proliferation step of the erythroleukemic process.