Regions of the Moloney murine leukemia virus genome specifically related to induction of promonocytic tumors

Tetherin Inhibits Cell-Free Virus Dissemination and Retards Murine Leukemia Virus Pathogenesis

The relative contributions of cell-free virion circulation and direct cell-to-cell transmission to retroviral dissemination and pathogenesis are unknown. Tetherin/Bst2 is an antiviral protein that blocks enveloped virion release into the extracellular milieu but may not inhibit cell-to-cell virus transmission. We developed live-cell imaging assays which show that tetherin does not affect Moloney murine leukemia virus (MoMLV) spread, and only minimally affects vesicular stomatitis virus (VSV) spread, to adjacent cells in a monolayer. Conversely, cell-free MLV and VSV virion yields and VSV spread to distal cells were dramatically reduced by tetherin. To elucidate the roles of tetherin and cell-free virions during in vivo viral dissemination and pathogenesis, we developed mice carrying an inducible human tetherin (hTetherin) transgene. While ubiquitous hTetherin expression was detrimental to the growth and survival of mice, restriction of hTetherin expression to hematopoietic cells gave apparently healthy mice. The expression of hTetherin in hematopoietic cells had little or no effect on the number of MoMLV-infected splenocytes and thymocytes. However, hTetherin expression significantly reduced cell-free plasma viremia and also delayed MoMLV-induced disease. Overall, these results suggest that MoMLV spread within hematopoietic tissues and cell monolayers involves cell-to-cell transmission that is resistant to tetherin but that virion dissemination via plasma is inhibited by tetherin and is required for full MoMLV pathogenesis.

IMPORTANCE Retroviruses are thought to spread primarily via direct cell-to-cell transmission, yet many have evolved to counteract an antiviral protein called tetherin, which may selectively inhibit cell-free virus release. We generated a mouse model with an inducible tetherin transgene in order to study how tetherin affects retroviral dissemination and on which cell types its expression is required to do so. We first developed a novel in vitro live-cell imaging assay to demonstrate that while tetherin does indeed dramatically reduce cell-free virus spreading, it has little to no effect on direct cell-to-cell transmission of either vesicular stomatitis virus (VSV) or the retrovirus MoMLV. Using our transgenic mouse model, we found that tetherin expression on hematopoietic cells resulted in the specific reduction of MoMLV cell-free plasma viremia but not the number of infected hematopoietic cells. The delay in disease associated with this scenario suggests a role for cell-free virus in retroviral disease progression.

Activation of c-myb by 5' retrovirus promoter insertion in myeloid neoplasms is dependent upon an intact alternative splice donor site (SD') in gag

Alternative splicing in Mo-MuLV recruits a splice donor site, SD', within the gag that is required for optimal replication in vitro. Remarkably, this SD' site was also found to be utilized for production of oncogenic gag-myb fusion RNA in 100% of murine-induced myeloid leukemia (MML) in pristane-treated BALB/c mice. Therefore, we investigated the influence of silent mutations of SD' in this model. Although there was no decrease in the overall incidence of disease, there was a decrease in the incidence of myeloid leukemia with a concomitant increase in lymphoid leukemia. Importantly, there was a complete lack of myeloid tumors associated with 5' insertional mutagenic activation of c-myb, suggesting the specific requirement of the SD' site in this mechanism.

A Moloney murine leukemia virus-based retrovirus with 4070A long terminal repeat sequences induces a high incidence of myeloid as well as lymphoid neoplasms

Retroviruses can be used to accelerate hematopoietic cancers predisposed to neoplastic disease by prior genetic manipulations such as in transgenic or knockout mice. The virus imparts a second neoplastic "hit," providing evidence that the initial hit is transforming. In the present study, a unique retrovirus was developed that can induce a high incidence of myeloid disease and has a broad host range. This agent is a Moloney murine leukemia virus (Mo-MuLV)-based virus that has most of the U3 region of the long terminal repeat (LTR) replaced with that of retrovirus 4070A. Like Mo-MuLV, this virus, called MOL4070LTR, is NB-tropic and not restricted by Fv1 allelles. MOL4070LTR causes myeloid leukemias in ca. 50% of mice, a finding in contrast to Mo-MuLV, which induces almost exclusively lymphoid disease. The data suggest that the LTR of the 4070A virus expands the tissue tropism of the disease to the myeloid lineage. Interesting, MCF recombinant envelope was expressed in the lymphoid but not the myeloid neoplasms of BALB/c mice. This retrovirus has the potential for accelerating myeloid disease in genetically engineered mice.

Casein kinase II phosphorylation site mutations in c-Myb affect DNA binding and transcriptional cooperativity with NF-M

Phosphorylation of c-Myb has been implicated in the regulation of the binding of c-Myb to DNA. We show that murine c-Myb is phosphorylated at Ser-11 and -12 in vivo and that these sites can be phosphorylated in vitro by casein kinase II (CKII), analogous to chicken c-Myb. An efficient method to study DNA binding properties of full-length c-Myb and Myb mutants under nondenaturing conditions was developed. It was found that a Myb mutant in which Ser-11 and -12 were replaced with Ala (Myb Ala-11/12), wild-type c-Myb, and Myb Asp-11/12 bound to the A site of the mim-1 promoter with decreasing affinities. In agreement with this finding, Myb Ala-11/12 transactivated better than wild-type c-Myb and Myb Asp-11/12 on the mim-1 promoter or a synthetic Myb-responsive promoter. Similar observations were made for the myeloid-specific neutrophil elastase promoter. The presence of NF-M or an NF-M-like activity abolished partially the differences seen with the Ser-11/12 mutants, suggesting that the reduced DNA binding due to negative charge at positions 11 and 12 can be compensated for by NF-M. Since no direct interaction of c-Myb and NF-M was observed, we propose that the cooperativity is mediated by a third factor. Our data offer two possibilities for how casein kinase II phosphorylation can influence c-Myb function: first, by reducing c-Myb DNA binding and thereby influencing transactivation, and second, by enhancing the apparent cooperativity between c-Myb and NF-M or an NF-M-like activity.

Modulation of c-Myb-induced transcription activation by a phosphorylation site near the negative regulatory domain

The c-myb protooncogene encodes a highly conserved transcription factor that functions as both an activator and a repressor of transcription. The v-myb oncogenes of E26 leukemia virus and avian myeloblastosis virus encode proteins that are truncated at both the amino and the carboxyl terminus, deleting portions of the c-Myb DNA-binding and negative regulatory domains. This has led to speculation that the deleted regions contain important regulatory sequences. We previously reported that the 42-kDa mitogen-activated protein kinase (p42mapk) phosphorylates chicken and murine c-Myb at multiple sites in the negative regulatory domain in vitro, suggesting that phosphorylation might provide a mechanism to regulate c-Myb function. We now report that three tryptic phosphopeptides derived from in vitro phosphorylated c-Myb comigrate with three tryptic phosphopeptides derived from metabolically labeled c-Myb immunoprecipitated from murine erythroleukemia cells. At least two of these peptides are phosphorylated on serine-528. Replacement of serine-528 with alanine results in a 2- to 7-fold increase in the ability of c-Myb to transactivate a Myb-responsive promoter/reporter gene construct. These findings suggest that phosphorylation serves to regulate c-Myb activity and that loss of this phosphorylation site from the v-Myb proteins may contribute to their transforming potential.

Novel integration sites at the distal 3' end of the c-myb locus in retrovirus-induced promonocytic leukemias

In BALB/c nu/nu and sublethally irradiated DBA/2 mice, promonocytic leukemia was induced by intravenous inoculation of Friend murine leukemia virus (F-MuLV) strain C57 in conjunction with intraperitoneal injection of pristane. These tumors appear to be identical morphologically to previously reported ones induced by other MuLVs, such as Moloney, amphotropic 4070A, and F-MuLV FB29, which most commonly have provirus integrations in the 5' end of the c-myb locus. Interestingly, 2 of the 16 F-MuLV-induced tumors had viruses integrated in the distal 3' end of c-myb. To determine the precise locations of these integrations, it was necessary to clone sequences encoding the 3' c-myb exons and to prepare a physical map of this region. Exons 10 to 15 were positioned on the map, and it was found that the proviruses in the aforementioned tumors were located within narrow region in the beginning of the large (greater than 11 kb) intron 14. The predicted protein product encoded by the affected alleles is truncated by 38 amino acids. This represents a novel virus integration site which is most likely associated with oncogenic activation of the c-myb gene during leukemogenesis.

Different abilities of Friend murine leukemia virus (MuLV) and Moloney MuLV to induce promonocytic leukemia are due to determinants in both psi-gag-PR and env regions

Moloney murine leukemia virus (M-MuLV) is capable of inducing promonocytic leukemia in 50% of adult BALB/c mice that have received peritoneal injections of pristane, but Friend MuLV strain 57 (F-MuLV) is nonleukemogenic under similar conditions. It was shown earlier that these differences could not be mapped to the U3 region of the virus long terminal repeat, indicating the probable influence of structural genes and/or R-U5 sequences. In this study, reciprocal chimeras containing exchanged structural genes and R-U5 sequences from these two closely related viruses were analyzed for differences in ability to induce disease. Results showed that two regions of F-MuLV, psi-gag-PR and env, when substituted for those of M-MuLV were dramatically disease attenuating. The 5'-most region, which is widely distributed, overlaps with the 5' end of the env intron and includes the RNA packaging region, psi, the entire gag coding region, and the viral protease coding region (PR) of pol. It was also found that reciprocal constructs having substitutions of both of these regions of M-MuLV in an F-MuLV background allowed full reestablishment of promonocytic leukemia. These leukemias were positive for c-myb rearrangements which are characteristic of M-MuLV-induced promonocytic leukemias. Neither region alone, however, was sufficient to produce disease with a greater incidence than 13%. Further studies demonstrated that the inability of viruses with psi, gag, PR, or env sequences from F-MuLV to induce leukemia in this model system was not due to their inability to replicate in hematopoietic tissue, to integrate into the c-myb locus early on after infection in vivo, or to express gag-myb mRNA characteristic of M-MuLV-induced preleukemic cells and acute leukemia.

Activation of c-myb is an early bone-marrow event in a murine model for acute promonocytic leukemia

Insertional mutagenesis of c-myb by Moloney murine leukemia virus occurs in 100% of promonocytic leukemias (MMLS) induced by the virus. These leukemias, which resemble acute monocytic leukemia-M5 in humans are induced only in mice undergoing a peritoneal chronic inflammatory response. We have found that two leukemia-specific gag-myb mRNAs in MML provide molecular markers for detection of preleukemic cells in hematopoietic tissue in vivo. The two aberrant RNAs result from splicing of gag to either exon 3 or 4 of c-myb, depending on the site of proviral integration. After reverse transcription-PCR with nested primers and hybridization with specific gag-myb junction probes, one cell, having aberrant c-myb message, could be detected in a minimum of 10(5) liver cells or 10(6) spleen or bone-marrow cells. This approach was used to examine hematopoietic tissues of mice after pristane injection to induce inflammation and virus inoculation. Cells with gag-myb mRNAs could be detected as early as 2 weeks after virus inoculation. In mice receiving both pristane and virus, there was evidence of preleukemic cells in 83% of the mice by 3 weeks after virus infection. Furthermore, 100% of the mice were positive for preleukemic cells by 8 weeks, even though only 50% of mice have been shown to succumb to MML (peak time for disease latency is 12-16 weeks). Cells with these aberrant c-myb messages were initially detected in the bone marrow, but during intermediate stages of disease development these cells disseminated to the spleen, liver, and granuloma. At preleukemic times, from 3 to 8 weeks after virus infection, a lower percentage of mice were positive in the group that did not receive pristane compared with mice in the group receiving pristane. However, at 18 weeks, 100% of the mice in the group receiving virus only had evidence of cells expressing gag-myb RNA in their spleens and/or bone marrow; it is of interest that mice inoculated with virus alone never develop MML. This approach for detecting preleukemic cells will now allow the study of mechanisms by which these preleukemic cells progress to a more transformed state and, perhaps, to a more differentiated state.

Analysis of proviruses integrated in Fli-1 and Evi-1 regions in Cas-Br-E MuLV-induced non-T-, non-B-cell leukemias

The DNAs of the Cas-Br-E MuLV-induced leukemias always contain somatically acquired mink cell focus-forming (MCF) recombinant proviruses. MCF recombinants could be involved during leukemogenesis at both preleukemic times and in late-stage tumors. Among the Cas-Br-E-induced non-T-, non-B-cell leukemias, viral integrations were found in the Fli-1 and Evi-1 region in 71% (36 out of 51) and 22% (16 out of 72) of the tumors analyzed, respectively. As an approach to evaluate the contribution of Cas-Br-E MCF recombinant formation in cis-activation of proto-oncogenes, we analyzed the structure of the Fli-1- and Evi-1-associated proviruses by Southern blot hybridization. In Fli-1, we found that the proviruses, ecotropic as well as MCF, are all integrated within a very short DNA region immediately upstream of the initiator ATG, toward the 3' end of a 5' exon (Ben-David, Giddens, Letwin, and Bernstein, 1991, Genes Dev. 5, 908-918). All proviruses are oriented the same way, in the 5' to 3' transcriptional sense. Both provirus types are able to direct the Fli-1 expression to the same extent presumably via a promoter insertion mechanism. Most of the proviruses had no detectable deletion and contained both 5' and 3' LTR sequences with similar U3 sequences. MCF recombinants did not show any selective advantage over ecotropic proviruses for the Fli-1 locus since the frequency of ecotropic to MCF-recombinant virus at the Fli-1 locus was identical to that observed at any other locus. This suggests that the formation of these MCF recombinants is not essential for activation of Fli-1 and that ecotropic Cas-Br-E already possesses the required sequences for full cis-activation of Fli-1. On the other hand, in Evi-1, there is a strict selection for ecotropic proviruses. Presumably, viral genetic elements outside of the U3 region could be critical for the Evi-1 cis-activation.

New sites of proviral integration associated with murine promonocytic leukemias and evidence for alternate modes of c-myb activation

Murine promonocytic leukemias involving insertional mutagenesis of the c-myb locus can be induced by replication-competent retroviruses. In previously studied promonocytic leukemic cells induced by Moloney murine leukemia virus (called MML), the provirus has been invariably integrated upstream of exons 3 or 4 and the leukemic cells expressed aberrant RNAs with fused virus-myb sequences. Furthermore, Myb expressed by these cells has been shown to be truncated by 47 or 71 amino acids. The present report examines the mechanisms of myb activation in leukemias induced by two other retroviruses, amphotropic virus 4070A and Friend strain FB29 (the leukemias are called AMPH-ML and FB-ML, respectively). This study revealed two additional c-myb proviral insertion sites in these promonocytic leukemias. One FB-ML had a proviral integration in exon 9, and expressed a C-terminally truncated Myb protein of 47 kDa similar to that previously demonstrated to be expressed in the myelomonocytic cell lines NFS60 and VFL-2. However, a sequence of reverse-transcribed and amplified RNA from this leukemia demonstrated that the truncation involved a loss of 248 amino acids compared with a loss of 240 amino acids in the myelomonocytic cell lines. Another leukemia had a provirus integrated in the 5' end of c-myb upstream of exon 2 (in the first intron) and produced a Myb protein that was indistinguishable on sodium dodecyl sulfate-polyacrylamide gel electrophoresis from normal Myb. This latter leukemia (FB-ML R1-4-10) expressed Myb with the smallest N-terminal truncation observed so far in promonocytic leukemias; translation begins at an ATG within c-myb exon 2, leading to loss of only 20 amino acids from the N terminus. Unlike the proteins produced in Moloney murine leukemia virus-induced promonocytic leukemias (MML) that have larger truncations, this protein has an intact DNA binding region and does not contain N-terminal amino acids encoded by gag. However, this protein is similar to all N-terminally truncated Mybs so far studied, in that the truncation resulted in deletion of a casein kinase II phosphorylation site which has been proposed to be involved in regulation of DNA binding.

Acute myeloid leukemia induction by amphotropic murine retrovirus (4070A): clonal integrations involve c-myb in some but not all leukemias

Amphotropic murine retrovirus 4070A was demonstrated to be highly leukemogenic when inoculated intravenously into adult DBA/2 mice that were undergoing an intense chronic inflammatory response, but was nonleukemogenic in the absence of inflammation. The virus-induced promoonocytic leukemias, designated AMPH-ML, are similar morphologically and in cell surface marker expression to monocytic leukemias, called MML and MF-ML, previously shown to be induced by Moloney murine leukemia virus and MF-3 virus (a recombinant between Friend murine leukemia virus and Moloney murine leukemia virus) and resemble certain mature acute monocytic leukemias in humans (AML subtype M5). Approximately two-thirds of the AMPH-MLs (subgroup I) were demonstrated to have alterations in the 5' end of the c-myb locus, an event which occurs in 100% of MML and MF-ML. Data indicate that proviral insertions in AMPH-ML subgroup I resulted in aberrant c-myb mRNA expression and truncation of its translation product at the amino terminus. Approximately one-third of the AMPH-MLs (subgroup II) had not undergone any DNA rearrangements at the c-myb locus. In addition, their transcripts and protein products were of normal size. These latter leukemias also had not undergone DNA rearrangements in c-myc, although retroviruses expressing myc have previously been shown to induce monocyte-macrophage tumors in mice undergoing a chronic inflammation. That subgroup II leukemias had at least one clonal viral insertion suggests that there may be other sites in the cellular genome that can be activated by insertional mutagenesis in these murine acute monocytic leukemias.

Identification of a common viral integration region in Cas-Br-E murine leukemia virus-induced non-T-, non-B-cell lymphomas

The Cas-Br-E murine leukemia virus is a nondefective retrovirus that induces non-T-, non-B-cell lymphomas in susceptible NIH/Swiss mice. By using a DNA probe derived from Cas-Br-E provirus-flanking sequences, we identified a DNA region, originally called Sic-1, rearranged in 16 of 24 tumors analyzed (67%). All proviruses were integrated in a DNA segment smaller than 100 bp and were in the same 5'-to-3' orientation. Ecotropic as well as mink cell focus-forming virus types were found integrated in that specific DNA region. On the basis of Southern blot analysis of somatic cell hybrids and progeny of an interspecies backcross, the Sic-1 region was localized on mouse chromosome 9 near the previously described proto-oncogenes or common viral integration sites: Ets-1, Cbl-2, Tpl-1, and Fli-1. Restriction map analysis shows that this region is identical to the Fli-1 locus identified in Friend murine leukemia virus-induced erythroleukemia cell lines and thus may contain sequences also responsible for the development of mouse non-T-, non-B-cell lymphomas.