Studies on emerging radiation leukemia virus variants in C57BL/Ka mice

Hotspots of MLV integration in the hematopoietic tumor genome

Extensive research has been performed regarding the integration sites of murine leukemia retrovirus (MLV) for the identification of proto-oncogenes. To date, the overlap of mutations within specific oligonucleotides across different tumor genomes has been regarded as a rare event; however, a recent study of MLV integration into the oncogene Zfp521 suggested the existence of a hotspot oligonucleotide for MLV integration. In the current review, we discuss the hotspots of MLV integration into several genes: c-Myc, Stat5a and N-myc, as well as ZFP521, as examined in tumor genomes. From this, MLV integration convergence within specific oligonucleotides is not necessarily a rare event. This short review aims to promote re-consideration of MLV integration within the tumor genome, which involves both well-known and potentially newly identified and novel mechanisms and specifications.

In vitro murine leukemia retroviral integration and structure fluctuation of target DNA

Integration of the retroviral genome into host DNA is a critical step in the life cycle of a retrovirus. Although assays for in vitro integration have been developed, the actual DNA sequences targeted by murine leukemia retrovirus (MLV) during in vitro reproduction are unknown. While previous studies used artificial target sequences, we developed an assay using target DNA sequences from common MLV integration sites in Stat5a and c-myc in the genome of murine lymphomas and successfully integrated MLV into the target DNA in vitro. We calculated the free energy change during folding of the target sequence DNA and found a close correlation between the calculated free energy change and the number of integrations. Indeed, the integrations closely correlated with fluctuation of the structure of the target DNA segment. These data suggest that the fluctuation may generate a DNA structure favorable for in vitro integration into the target DNA. The approach described here can provide data on the biochemical properties of the integration reaction to which the target DNA structure may contribute.

Complete genome sequences of the two viral variants of the Graffi MuLV: Phylogenetic relationship with other murine leukemia retroviruses

A detailed phylogenetic analysis of two variants of the Graffi murine retrovirus, GV-1.2 and GV-1.4, showed that they are closely related to SRS 19-6 and Moloney MuLVs. Two stretches of sequence testify to the divergence between Graffi and SRS 19-6 MuLVs, one corresponding to a recombination event of Graffi MuLV with a xenotropic virus. Moloney MuLV was found more distant, particularly in the GAG region. Our study encompasses every class of MuLVs (ecotropic, amphotropic, xenotropic, polytropic) with some focus on exogenous ecotropic viruses and further adds to previous phylogenetic studies. Graffi, SRS 19-6, Moloney, Friend and Rauscher MuLVs form a cluster that appears to share a common ancestor with the Casitas-amphotropic and -ecotropic MuLVs but are more distant to the Akv-type and xenotropic MuLVs. The analysis also revealed that the ENV region of HEMV, the prototype of the MuLV ancestor, was closely related to the corresponding region of Cas-Br-E.

Radiation leukemia virus common integration at the Kis2 locus: simultaneous overexpression of a novel noncoding RNA and of the proximal Phf6 gene

Retroviral tagging has been used extensively and successfully to identify genes implicated in cancer pathways. In order to find oncogenes implicated in T-cell leukemia, we used the highly leukemogenic radiation leukemia retrovirus VL3 (RadLV/VL3). We applied the inverted PCR technique to isolate and analyze sequences flanking proviral integrations in RadLV/VL3-induced T lymphomas. We found retroviral integrations in c-myc and Pim1 as already reported but we also identified for the first time Notch1 as a RadLV common integration site. More interestingly, we found a new RadLV common integration site that is situated on mouse chromosome X (XA4 region, bp 45091000). This site has also been reported as an SL3-3 and Moloney murine leukemia virus integration site, which strengthens its implication in murine leukemia virus-induced T lymphomas. This locus, named Kis2 (Kaplan Integration Site 2), was found rearranged in 11% of the tumors analyzed. In this article, we report not only the alteration of the Kis2 gene located nearby in response to RadLV integration but also the induction of the expression of Phf6, situated about 250 kbp from the integration site. The Kis2 gene encodes five different alternatively spliced noncoding RNAs and the Phf6 gene codes for a 365-amino-acid protein which contains two plant homology domain fingers, recently implicated in the Börjeson-Forssman-Lehmann syndrome in humans. With the recent release of the mouse genome sequence, high-throughput retroviral tagging emerges as a powerful tool in the quest for oncogenes. It also allows the analysis of large DNA regions surrounding the integration locus.

Sequence and insertion sites of murine melanoma-associated retrovirus

We previously showed that B16 melanoma cells produce ecotropic melanoma-associated retrovirus (MelARV) which encodes a melanoma-associated antigen recognized by MM2-9B6 monoclonal antibody. The biological significance of MelARV in melanoma formation remains unknown. We found that infection of normal melanocytes with MelARV resulted in malignant transformation. It is likely that MelARV emerged from the defective Emv-2 provirus, a single copy of ecotropic provirus existing in the genome of C57BL/6 mice. In the present study, we cloned and sequenced the full-length MelARV genome and its insertion sites and we completed sequencing of the Emv-2 provirus. Our data show that MelARV has a typical full-length retroviral genome with high homology (98.54%) to Emv-2, indicating a close relationship between both viruses. MelARV probably emerged as a result of recombination between Emv-2 and an endogenous nonecotropic provirus. Some observed differences in the gag and pol regions of MelARV might account for the restoration of productivity and infectivity of a novel retrovirus that somatically emerged during melanoma formation. MelARV does not contain any oncogene and therefore might induce transformation by insertional mutagenesis. We sequenced two insertion sites of MelARV. The first insertion site represents the 3' coding region of the c-maf proto-oncogene at 67.0 centimorgans (cM) on chromosome 8. The c-maf proto-oncogene encodes a basic leucine zipper protein homologous to c-fos and c-jun. Insertion of MelARV in BL6 melanoma cells resulted in the up-regulation of c-maf. It is noteworthy that the Emv-2 provirus is also inserted into a noncoding region at 61.0 cM on the same chromosome 8. The second insertion site is the 3' noncoding region of the DNA polymerase gamma (PolG) gene on chromosome 7. The expression of PolG was not affected by the MelARV insertion. Further investigation of the biological significance of MelARV in melanoma formation is being undertaken.

Murine leukemia provirus-mediated activation of the Notch1 gene leads to induction of HES-1 in a mouse T lymphoma cell line, DL-3

Constitutive activation of Notch signaling is known to be associated with tumorigenesis. In a mouse T lymphoma cell line, DL-3, we found that a murine leukemia provirus was inserted in the Notch1 locus, which led to marked expression of a virus-Notch1 fusion mRNA encoding an intracellular portion of the Notch1 protein. Furthermore, expression and nuclear localization of this constitutively active form of Notch1 protein were confirmed. Corresponding to this finding, the transcription of the hairy/enhancer of split (HES-1) gene, a known target of Notch1 signaling, was elevated in this cell line. A potential role for overexpressed HES-1 in the development of the lymphoma was discussed.

Nuclear factors that bind to the U3 region of two murine myeloid leukemia-inducing retroviruses, Cas-Br-E and Graffi

Cas-Br-E and Graffi are two myeloid leukemia-inducing murine viruses. Cas-Br-E induces, in NIH-Swiss mice, mostly non-T, non-B leukemia composed of very immature cells with no specific characteristics (Bergeron et al. (1993). Leukemia 7, 954-962). The Graffi murine leukemia virus causes exclusively myeloid leukemia, but the tumor cells are clearly of granulocytic nature (Ru et al. (1993). J. Virol. 67, 4722). We were interested to understand the role of the long terminal repeat (LTR) U3 region in the myeloid specificity of these two retroviruses. We used DNase I footprinting and gel mobility shift assays to identify a number of protein binding sites within Cas-Br-E and Graffi U3 regions. The pattern of protected regions is highly similar for the two viruses. Some factors identified in other murine leukemia viruses, like the core binding factor, also bind to Cas-Br-E and Graffi LTR; however, other binding sites seem specific for these two viruses. Only one difference between them was noted, at the 5' end of the U3 region. Transcriptional activity of both LTRs was also analyzed in various cell lines and compared with other murine leukemia viruses. The results show a slight myeloid specificity for the two LTRs, and indicate that the Graffi enhancer is quite strong in a broad range of cell types.

Infection of WHV/c-myc transgenic mice with Moloney murine leukaemia virus and proviral insertion near the syndecan-4 gene in an early liver tumour

The capacity of Moloney murine leukaemia virus (MoMLV) to infect neonatal hepatocytes and to accelerate liver carcinogenesis was examined in a transgenic mouse model. WHV/c-myc mice which are highly susceptible to the development of liver tumours were infected with MoMLV shortly after birth, when expression of the murine ecotropic retroviral receptor gene was still detectable in the neonatal liver. All MoMLV-infected transgenic mice and non-transgenic littermates succumbed to T-cell lymphomas within 2-9 months; during this period of time, three infected transgenic animals developed primary hepatocellular carcinomas. Remarkably, one of these liver tumours arose significantly faster than tumours from uninfected WHV/c-myc controls, and it harboured a unique MoMLV provirus. The provirus integration site was located 5.5 kb upstream of the first exon of the syndecan-4 gene, which encodes a heparan sulphate proteoglycan implicated in growth factor activation and protein kinase C distribution in focal adhesions. Our data provide evidence for clonal MoMLV provirus integration in a hepatocellular carcinoma, and indicate that parenchymal liver cells may be susceptible to MoMLV infection following neonatal inoculation.

The murine AIDS virus Gag precursor protein binds to the SH3 domain of c-Abl

The Pr60gag protein of the murine AIDS (MAIDS) defective virus promotes the proliferation of the infected target B cells and is responsible for inducing a severe immunodeficiency disease. Using the yeast two-hybrid system, we identified the SH3 domain of c-Abl as interacting with the proline-rich p12 domain of Pr60gag. The two proteins were shown to associate in vitro and in vivo in MAIDS virus-infected B cells. Overexpression of Pr60(gag) in these cells led to a detectable increase of the levels of c-Abl protein and to its translocation at the membrane. These results suggest that this viral protein serves as a docking site for signaling molecules and that c-Abl may be involved in the proliferation of infected B cells.

Association of repeat sequences with integrated retroviruses in a murine leukaemia cell line

An analysis was made of the retroviral integration sites for retroviruses in a murine lymphoid precursor cell line, C1-V13D, derived following in vitro infection with RadLV, an ecotropic murine retrovirus. A genomic library was constructed and lambda clones were selected for their capacity to hybridize with the specific RadLV gp70 ecotropic env probe. Analysis of these clones by a combination of approaches, including subcloning, partial restriction mapping and sequencing, has confirmed the existence of multiple recombinant and defective viruses in C1-V13D. To check for the presence of coding sequences in flanking genomic DNA, 32P-labelled cDNA from C1-V13D was used to probe HindIII- and Psti-digested virus-positive lambda clones by Southern analysis. Regions hybridizing specifically with 32P-labelled C1-V13D cDNA were subcloned and analysed. A notable feature of these cDNA+ regions was the frequent presence of B1, B2 and simple repeats. These repeat elements were found to be present in high frequency in the genomic regions flanking the proviruses, in numbers higher than expected for the genome as a whole. All full-length viruses isolated appeared to represent integration events into regions rich in repeat elements. Some B1 and B2 repeats have been shown to code for functional proteins and to play regulatory roles. Viral integration in the vicinity of these genetic elements could contribute to oncogenesis if the integration event were to disrupt normal gene function.

Leukaemogenic progression: the importance of differentiation and associated genetic events

A feature common to many blood cell cancers is the uncoupling of normal proliferative and differentiative events, both of which are intimately linked in the cell's developmental programme. In some cancers, further differentiative events have been associated with oncogenic progression and, in other cancers, terminal differentiation of cells has been shown to result in reversal of malignancy and death of the cancer cell. Clearly the development of cancer is not the result of a single oncogenic event, but rather a myriad of events which appear to proceed in concert in a step-wise fashion and which are likely to be influenced by the cellular environment. Here we review some of the major genetic changes which occur in leukaemogenesis and discuss the possible role of differentiative events in the development of leukaemia.

Induction of leukemia in mice using a radiation leukemia virus-induced cell line: a model system for studying oncogenic progression

Leukemogenesis induced by slowly transforming retroviruses is a multistep process which is difficult to dissect because of its long latency and the problem of distinguishing oncogenic from differentiative events. A method for leukemia induction in mice has been developed using a cell line isolated following in vitro infection with the slowly transforming murine radiation leukemia virus (RadLV). The CI-V13D cell line represents a lymphoid precursor cell type at an early stage in cell transformation and can develop subcutaneous tumors in irradiated syngeneic hosts but not in allogeneic mice even after sublethal irradiation. Selective growth in allogeneic (CBA/H) mouse thymus has been demonstrated, but this requires preirradiation of the recipient. Upon reisolation from CBA/H thymus, C1-V13D progeny clones displayed increased tumorigenic potential in comparison to the 'parental' CI-V13D cell line. Tumorigenicity was shown to increase with serial passage through thymus and electron micrographs of clones also revealed increased production of C-type retroviruses. This new model for oncogenic progression should be more amenable to analysis of early genetic changes occurring during replication of leukemia in the thymus.

Long-range mapping of Mis-2, a common provirus integration site identified in murine leukemia virus-induced thymomas and located 160 kilobase pairs downstream of Myb

The nondefective Moloney murine leukemia virus (MuLV) induces clonal or oligoclonal T-cell tumors in mice or rats. The proviruses of these nondefective MuLVs have been shown to act as insertion mutagens most frequently activating an adjacent cellular gene involved in cell growth control. Mutations by provirus insertions, recognized as common provirus integration sites, have been instrumental in identifying novel cellular genes involved in tumor formation. We have searched for new common provirus integration sites in Moloney MuLV-induced thymomas. Using cellular sequences flanking a provirus cloned from one of these tumors, we found one region, designated Mis-2, which was the target of provirus integration in a low (3%) percentage of these tumors. Mis-2 was mapped on mouse chromosome 10, approximately 160 kbp downstream of myb. The Mis-2 region may contain a novel gene involved in tumor development.

Graffi murine leukemia virus: molecular cloning and characterization of the myeloid leukemia-inducing agent

The Graffi murine leukemia virus (MuLV) is a retroviral mixture that induces predominantly myeloid leukemia in several inbred strains of mice. To analyze the viral component responsible for the myeloid leukemogenesis, we cloned several proviruses from a Graffi MuLV-infected cell line. Several infectious molecular clones were obtained that could be classified into two distinct groups of infectious MuLV. Both types of MuLV were nondefective, ecotropic, and NB tropic and induced granulocytic leukemia in BALB/c and NFS mice. Restriction enzyme analysis and molecular hybridization with several MuLV probes on one molecular clone from each group revealed that both groups are closely related to each other but are clearly distinct from all known retroviruses. One component of MuLV, however, induced leukemia with a shorter latency period and harbored a lengthier long terminal repeat. The long terminal repeat of the more leukemogenic component of MuLV had acquired a 60-bp perfect duplication in the U3 region. Analysis of the tumor DNAs with probes for the mouse T-cell receptor and immunoglobulin heavy chain genes revealed frequent rearrangements with one or both probes. This concomitant expression by leukemic cells of markers of different lineages, observed in human leukemias, has been termed "lineage infidelity" and confirms that the latter rearrangements are not restricted to hematopoietic precursors committed to lymphoid differentiation.

Retroviral gene expression as a possible biomarker of aging

We examined effects of aging on endogenous retrovirus gene expression of mouse lymphocytes with a hypothesis that it may be a useful biomarker of aging. Mice have endogenous murine leukemia viruses (MuLVs) in their chromosomes. We detected the gene expression of long terminal repeats (LTRs) of MuLVs. Brains, livers and spleens were taken from young (3 months old) and old (27 months old) male C57BL/6 mice. In addition to these control (C) mice, we also determined gene expression in dietary restricted (DR) mice, in which rates of aging are known to be slowed. RNA was extracted from the tissues and converted into cDNA. The MuLV-LTR portion of cDNA was amplified by polymerase chain reaction (PCR). The PCR products were analyzed by agarose gel electrophoresis. Gene expressions of young mice were found to be tissue-specific. Expressed LTRs from brains, livers and spleens were that of 370 base-pairs (bp), those of 370 and 620 bp, and those of 370, 400 and 620 bp, respectively. Old mice of C group, however, decreased tissue specificity: expressed LTRs became those of 370-400 bp in any tissues. In contrast the tissue specific gene expression was conserved in old DR mice which had to get prolonged life span and decreased lymphoma incidence. Thereby, gene expression of endogenous retroviruses appears to change during aging and to be modifiable by life-prolonging DR. It may be therefore used as a biomarker of aging in mice. Humans are known to have similar gene elements like MuLV. The present findings demonstrate a possibility of application of endogenous gene expressions to the epidemiology of aging.

U3 long terminal repeat-mediated induction of intracellular immunity by a murine retrovirus: a novel model of latency for retroviruses

BL/VL3 radiation leukemia virus (RadLV) is a thymotropic, highly leukemogenic murine leukemia virus (MuLV) which is unable to replicate in vitro in mouse fibroblasts. We have previously reported that the U3 long terminal repeat region of its genome is responsible for this block (E. Rassart, Y. Paquette, and P. Jolicoeur, J. Virol. 62:3840-3848, 1988). By using hybrids of permissive and resistant cells infected with BL/VL3 RadLV or fibrotropic MuLV, we found that the resistant phenotype was dominant. Investigation to determine at which step of the virus cycle the block operates revealed that integration, transcription, and translation of the BL/VL3 viral genome occurred at normal levels in nonpermissive cells. The BL/VL3 RadLV Pr65gag proteins made in nonpermissive cells were also myristylated and located at the membrane, and the levels of their cleaved products were similar to those of fibrotropic MuLV. However, processing of BL/VL3 RadLV Pr85env was impaired in nonpermissive cells. Virions were not released into the culture medium of nonpermissive cells, as measured by reverse transcriptase activity and by content in p30 or gp70 protein and as documented by lower levels of budding particles seen by electron microscopy. These results indicate that BL/VL3 RadLV replication is blocked at a late stage of the virus cycle, i.e., at virion assembly. Interestingly, these BL/VL3 RadLV-infected nonpermissive fibroblasts were resistant to superinfection by fibrotropic Moloney MuLV, and this resistance also occurred at a late step of the Moloney virus cycle. Since this block is dominant, it appears that the U3 long terminal repeat region of the BL/VL3 viral genome has the ability to induce a cellular suppressor factor(s), thus bringing intracellular immunity against itself and against other ecotropic MuLVs.

Determinants of thymotropism in Kaplan radiation leukemia virus and nucleotide sequence of its envelope region

Radiation leukemia viruses (RadLVs) are a group of murine leukemia viruses which are induced by radiation and cause T-cell leukemia. Viral clones isolated from the BL/VL3 lymphoid cell line derived from a thymoma show variable tropism and leukemogenic potential. We have constructed chimeric viruses by in vitro recombination between two viruses, a RadLV that is thymotropic and an endogenous ecotropic virus that is nonthymotropic. We show here that, in contrast to thymotropism determinants identified previously, which lie in the long terminal repeat (LTR), it is the envelope region that is responsible for the thymotropism of BL/VL3 RadLV. The nonthymotropic virus which we have rendered thymotropic by transfer of the env region of RadLV in the present study has been shown previously to become thymotropic when the LTR of another thymotropic virus is inserted in its genome. Thus, the LTR and envelope gene may be involved in complementary action to lead to thymotropism.

The diversity of retroviral diseases of the immune system

Retroviruses have been implicated as causative agents for a range of diseases including neoplasia, autoimmunity and immunosuppression. No two retroviruses carry the same complement of genes and for this reason it is not surprising that they induce a variety of different disease states. One common element in retroviral evolution has been the need to avoid immune recognition in order to persist within the host. A comparative approach, looking at various persistent retroviruses, has been used to pin-point the types of genetic adaptations adopted by retroviruses to remain hidden, often within the T cell compartment. Most of these retroviruses are T-cell-tropic and the diseases which they induce usually reflect the effect of the retrovirus on normal lymphocyte function.

Identification of a novel gene, Vin-1, in murine leukemia virus-induced T-cell leukemias by provirus insertional mutagenesis

The BL/VL3 radiation leukemia virus is a nondefective retrovirus which induces clonal or oligoclonal T-cell leukemia in mice. To study the role of provirus insertional mutagenesis in the development of these neoplasias, we searched for common provirus integration sites in BL/VL3 radiation leukemia virus-induced tumors. Using cellular sequences flanking a provirus cloned from one of these thymomas, we found that the viral genome was integrated into a common region, designated Vin-1, in a low percentage (5%) of these tumors. The proviruses found in this locus were integrated in the same orientation, close to a CpG-rich island, at proximity of a transcriptional unit encoding a 6-kb RNA. Vin-1 RNA was detected in several organs of the adult mouse. Vin-1 RNA levels were high in tumors having a provirus inserted within the Vin-1 region but were also high in some other tumors whose Vin-1 region was not found to be rearranged. Vin-1 was found to be well conserved among mammalian species and was mapped to mouse chromosome 6, between raf and K-ras-2. Vin-1 appears to be a novel gene which may be involved in tumor development.

Preferential expression of a common T cell receptor structure by T cells induced to proliferate in vitro with a murine retrovirus

We have successfully isolated continuous T cell lines from murine spleen which have been induced to proliferate after in vitro exposure to the murine leukemia virus RadLV. Cell lines isolated from several strains of mice have an "immature" phenotype and are immortalized CD4- CD8- CD3+ cell lines. Cell lines of similar phenotype have now been derived from many individual mice, after spleens have been infected with two different RadLV viruses, a leukemogenic and a nonleukemogenic isolate. Among cell lines induced with RadLV/C6VL, an unusually high proportion of cells was found to bind the 124-40 anticlonotypic antibody specific for the alpha beta TCR expressed by C6VL/1 cells which produces RadLV/C6VL. This was not reflected in cell lines induced with the RadLV/V13 isolate nor in various lymphocyte subsets freshly isolated from normal mice, or induced to proliferate in culture. Cells expressing a common TCR structure would appear to be appropriate targets for in vitro proliferation and transformation induced by RadLV.

The specificity of the disease induced by defective murine retroviruses containing abl, fos, or Ha-ras is usually not determined by their LTR

The long terminal repeats (LTR) of the defective murine sarcoma viruses (MSV) containing v-abl, v-Ha-ras, or v-fos were exchanged for LTRs from other retroviruses having different tissue tropism. The new chimeric MSV were found to induce the same diseases as the parental viruses, indicating that sequences outside the LTR, most likely those of the oncogene, are responsible for the disease specificity of these defective MSV.

Characterization of the gag/fusion protein encoded by the defective Duplan retrovirus inducing murine acquired immunodeficiency syndrome

Murine acquired immunodeficiency syndrome is induced by a defective retrovirus. Sequencing of this defective viral genome revealed a long open reading frame which encodes a putative gag/fusion protein, N-MA-p12-CA-NC-COOH, (D. C. Aziz, Z. Hanna, and P. Jolicoeur, Nature (London) 338:505-508, 1989). We raised a specific antibody to the unique p12 domain of this gag fusion precursor, Pr60gag. We found that Pr60gag was indeed encoded by the defective viral genome both in cell-free translation reticulocyte extracts and in infected mouse fibroblasts. Pr60gag was found to be myristylated, phosphorylated, and attached to the cell membrane, like other helper murine leukemia virus (MuLV) gag precursors. Pr60gag was not substantially cleaved within the nonproducer cells and was not released from these cells. However, in the presence of helper MuLV proteins, it formed phenotypically mixed particles. In these particles, Pr60gag was only partially cleaved. In helper MuLV-producing cells harboring the defective virus, a gag-related p40 intermediate was generated both intracellularly and extracellularly. In these cells, Pr60gag appeared to behave as a dominant negative mutant, interfering with proper cleavage of helper Pr65gag. Our data indicate that Pr60gag is a major (and possibly the only) gene product of the defective murine acquired immunodeficiency syndrome virus and is likely to harbor some determinants of pathogenicity of this virus.

Preferential usage of the V beta 8 gene family by CD4-CD8-T cell lines derived from spleen

Receptors encoded by the V beta 8 gene family and identified by the F23.1 antibody are commonly expressed amongst the CD4-CD8-T cell lines isolated from spleen cells infected in vitro with the RadLV retrovirus. All but one out of 12 cell lines showed between 50 and 85% F23.1+ cells in the uncloned cell population which is noticably higher than the approximately 13% level amongst the Ig- normal spleen cell population. There was a high frequency (approximately 50%) of F23.1+ clones from five of these cell lines. The frequency of F23.1 binding cells in the Ig-, CD4/CD8-depleted spleen population is only 0.2%, which gives a precursor frequency in spleen of less than 0.002%. This reflects selective isolation of CD4-CD8- alpha beta+ cells which express V beta 8 gene products by this culture scheme. The requirement for RadLV in induction of these cell lines has been established, suggesting that this retrovirus may selectively stimulate CD4-CD8-F23.1+ T cells. These cells may represent an autoimmune subset present in peripheral lymphoid tissue.

DNA-binding proteins that interact with the long terminal repeat of radiation leukemia virus

We used the electrophoretic mobility shift assay to identify the interactions of nuclear proteins with the long terminal repeat of leukemogenic, thymotropic BL/VL3 radiation leukemia virus (RadLV). In the promoter region, we identified a CCAAT box-binding protein (CBP) that has the same binding characteristics as the CCAAT box-binding protein that binds to the Moloney murine sarcoma virus promoter and most likely represents the CP1 factor. In the upstream enhancer region unique to BL/VL3, we detected several sequence-specific complexes, one with T-lymphocyte extracts but not with fibroblast extracts. This U3 region, UEB, may be important for the T-cell specificity of BL/VL3 RadLV. In the enhancer, which has been uniquely rearranged in this virus, we identified three specific protein-binding sites. Two of them showed characteristics of the LVb and core binding sites previously described for other murine retroviruses. But one binding site, identified as Rad-1, is unique to BL/VL3 RadLV and was found downstream, only 1 nucleotide from the core sequence. Rad-1 has a corelike motif on the minus strand, and the factor that binds to it could be competed by a BL/VL3 core-containing fragment. Moreover, the protein-DNA contacts involve the typical three core Gs separated by one T. These results suggest that Rad-1 binds a factor identical or similar to the core-binding factor. Our data suggest that the LVb, core, and Rad-1 motifs may be sufficient for this enhancer, most likely in association with other U3 long terminal repeat sequences, to promote a high rate of transcription of BL/VL3 RadLV in its specific target cells (thymocytes).

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

Increased H-2Dd expression following infection by a molecularly cloned ecotropic MuLV

The biological consequences of radiation leukemia virus (RadLV) infection include the stimulation of H-2Dd antigen expression in resistant mouse strains and thymoma induction in susceptible strains. In an effort to understand the genetic basis of these phenomena, the integrated ecotropic RadLV genome has been examined in a number of primary RadLV-induced tumors, as well as thymomas adapted to in vitro passage; considerable heterogeneity was observed. Examination of these polymorphic viral sequences should help define the viral gene(s) involved in the biological effects of RadLV infection; toward this end, integrated RadLV genomes were molecularly cloned and examined. The genomes and their flanking sequence were characterized by restriction enzyme analysis. Three unique viral genomes were obtained which represent four integration sites. The three RadLV genomes are shown to carry polymorphisms of the original tumor. Following DNA transfection, one of the three genomes replicated in and reinfected both mouse thymocytes and fibroblasts, but not mink fibroblasts in vitro. Virus encoded by the other two DNA genomes could not be recovered following transfection into any of the three cell types. One of these two apparently defective retroviruses encodes a truncated p15E molecule, while the other has elongated long terminal repeats (LTRs). The non-defective ecotropic isolate was collected from in vitro tissue culture supernatants, concentrated, and used to infect mice. Thymocytes of infected, resistant mice were shown to express elevated levels of H-2Dd antigen as early as 12 days post infection, a hallmark of RadLV infection.

Measurement of binding specificity between T cell lymphomas and murine leukemia viruses

We have previously reported the presence of receptors on radiation leukemia virus (RadLV)-induced thymomas and malignant thymocytes from AKR mice which specifically bind retrovirus produced by these T cell clones. These receptors have been shown to have specificity for virus reminiscent of an immune-specific receptor. Previous studies on T cell lymphoma binding to retroviruses have involved measurement of the interaction of labelled virus with cells using fluorescence-activated cell sorter (FACS) analysis (McGrath et al., J. Virol. (1978) 25, 923; McGrath and Weissman, Cell (1979) 17, 65; Weissman and McGrath, Curr. Top. Microbiol. Immunol. (1982) 98, 103). Here we report development of an assay for measuring lymphoma binding to virus, prepared as an immunoabsorbent adhered to a microtiter plate. Using this assay, we have shown that only T and not B cell lymphomas can bind to T cell-tropic viruses, and some cell lines have greatest specificity for homologous virus. The AKR-derived T cell lymphomas, SL3 and KKT-2, show greater specificity for leukemogenic AKR viruses, than for an AKR xenotropic virus or the recombinant AKR virus, MCF247. The RadLV-induced T cell lines, C6VL/1 and BL/VL3, have been found to bind cross-reactively to several different murine leukemia viruses (MuLVs). RadLV-induced T cell lymphomas do have greater specificity for their cognate retroviruses since free, homologous retrovirus can best block the interaction between cells and virus adhered to the wells of a microtiter plate. Cross-reactive interactions are more easily demonstrated by this assay, probably because low avidity interactions are stabilized as a result of the mode of virus presentation. Binding specificity for retroviral envelope determinants has been demonstrated using a rat anti-retroviral antiserum prepared as an F(ab)1 fragment. This antiserum can inhibit the interaction between the C6VL/1 thymoma and its RadLV virus. Specificity of this antibody for a gp70-like protein was confirmed by NaDodSO4-polyacrylamide gel electrophoresis (PAGE) and by loss of this activity after absorption of antibody on virus. Antibodies specific for RadLV/VL3 gp70 determinants can inhibit the interaction of C6VL/1 with RadLV/VL3 suggesting that cross-reactive binding to heterologous virus is also specific for a gp70 viral env determinant.

Radiation leukemia virus and its effect on H-2 gene expression

In this report we demonstrate that lowered expression of the H-2 antigens on RadLV-induced tumour cells is a result of depressed levels of stable mRNA in these cells. Whether this observation is a result of lowered transcription or of mRNA instability is under investigation. In an effort to determine which viral sequences are essential for mediating both the H-2 regulatory function and the transforming function of RadLV, we have begun to assemble newly integrated proviral genomes from tumours. The restriction enzyme cleavage sites of four isolates are presented; these isolates differ substantially from RadLV genomes previously presented. One of these molecular clones is shown to encode a non-defective B-tropic, ecotropic virus which when reinjected into resistant mouse strains can mediate the up-regulation of H-2Dd antigen expression. Finally, possible mechanisms of H-2 regulation are discussed.

Severe immunodeficiency disease induced by a defective murine leukaemia virus

Different classes of retroviruses have been shown to induce immunodeficiency diseases in various animal species. These animal models may provide an insight into our understanding of AIDS but, with the exception of one strain of feline leukaemia virus, the determinants of pathogenicity have not yet been mapped to these viral genomes. The immunodeficiency-inducing feline leukaemia virus is replication-defective, harbouring the determinant of pathogenicity within its env sequences. We have studied the Duplan strain of murine leukaemia virus which induces, in C57BL/6 mice, a severe immunodeficiency disease with striking similarities to human AIDS. We have identified the aetiological agent of this murine immunodeficiency disease as another defective retrovirus, with a genome of 4.8 kilobases. Molecular cloning and sequencing of this DNA showed that the pol and env genes have been deleted, but that the complete gag region has been conserved and has a novel sequence encoding the p12 protein. As with the feline leukaemia virus, these results provide evidence for the role of defective retroviruses in inducing immunodeficiency and facilitate the study of the mechanisms underlying the pathogenesis of retrovirus-induced immunodeficiency syndromes, including AIDS.

Evidence of recombinant ecotropic provirus integration in thymic lymphomas induced by direct or indirect radiation effects

Several investigators described the occurrence of ecotropic recombinant proviruses in the DNA of in-vivo or in-vitro propagated radio-induced lymphomas, but such proviruses were never detected in primary tumors. To assess their biological significance in the tumorigenic process, we reinvestigated the presence of new proviruses chiefly in primary radio-induced tumors and in models of radioleukemogenesis which could give additional support for their role. Such models included thymic lymphomas originating after (i) graft of non-irradiated thymuses in thymectomized irradiated mice and (ii) the injection of a B-ecotropic retrovirus (T1223/B) in association with a subleukemogenic dose of irradiation. We report for the first time that new ecotropic proviral sequences are encountered in a significant number (30%) of primary lymphomas induced directly by irradiation or indirectly in non-irradiated thymuses grafted in irradiated hosts. The existence of a 3.5-kbp Kpn1 restriction fragment with ecotropic sequences in the digested DNA of these tumor cells indicates that these new sequences belong to an ecotropic provirus recombinant in the gag-pol region. We observed that most of the primary radio-induced tumors in which novel recombinant provirus could be detected, displayed the integration at a single or at a few sites, demonstrating their clonality with respect to viral integration. The same was observed in thymic lymphomas arising after T1223/B virus injection and irradiation and in in-vivo or in-vitro propagated tumors. Altogether, these data bring the first evidence of the integration of ecotropic recombinant proviral genomes in a significant number of primary radiation induced thymic lymphomas and of their possible role in view of their frequent occurrence in grafted thymomas.

Inability of Kaplan radiation leukemia virus to replicate on mouse fibroblasts is conferred by its long terminal repeat

The molecularly cloned infectious Kaplan radiation leukemia virus has previously been shown to be unable to replicate on mouse fibroblasts (E. Rassart, M. Shang, Y. Boie, and P. Jolicoeur, J. Virol. 58:96-106, 1986). To map the viral sequences responsible for this, we constructed chimeric viral DNA genomes in vitro with parental cloned infectious viral DNAs from the nonfibrotropic (F-) BL/VL3 V-13 radiation leukemia virus and the fibrotropic (F+) endogenous BALB/c or Moloney murine leukemia viruses (MuLV). Infectious chimeric MuLVs, recovered after transfection of Ti-6 lymphocytes with these recombinant DNAs, were tested for capacity to replicate on mouse fibroblasts in vitro. We found that chimeric MuLVs harboring the long terminal repeat (LTR) of a fibrotropic MuLV replicated well on mouse fibroblasts. Conversely, chimeric MuLVs harboring the LTR of a nonfibrotropic MuLV were restricted on mouse fibroblasts. These results indicate that the LTR of BL/VL3 radiation leukemia virus harbors the primary determinant responsible for its inability to replicate on mouse fibroblasts in vitro. Our results also show that the primary determinant allowing F+ MuLVs (endogenous BALB/c and Moloney MuLVs) to replicate on mouse fibroblasts in vitro resides within the LTR.

Mechanism of interaction between endogenous ecotropic murine leukemia viruses in (BALB/c X C57BL/6) hybrid cells

The germline ecotropic murine leukemia (MuLV) proviruses of BALB/c and C57BL/6 (B6) mice were analyzed to determine the molecular basis of low virus expression in these mouse strains and to determine the mechanism of interaction of these two proviruses. Previous work had demonstrated that the BALB/c endogenous ecotropic provirus was infectious but unable to induce XC cell syncytia formation, and that induced (BALB/c X B6) hybrid cells expressed 10- to 50-fold more XC syncytia than induced parental cells. Two independently isolated DNA clones of the B6 endogenous ecotropic provirus were noninfectious following transfection into cells, and cell lines that expressed this viral genome produced noninfectious MuLV. Nucleotide sequencing of the mutant region of the B6 provirus indicated that the defective nature of this provirus resulted from an amino acid substitution of proline for alanine in the central portion of reverse transcriptase. From the analysis of the virus produced by induced hybrid cells, and the patterns of steady-state viral RNA in induced cells, we propose that the enhanced XC cell syncytia formation observed in hybrid cells is due to trans-complementation of viral proteins and not viral recombination or trans-activation of viral genome expression.

Nucleotide sequence of a radiation leukemia virus genome

The complete nucleotide sequence of an infectious molecular clone of a radiation murine leukemia proviral DNA RadLV/VL3(T+L+) has been determined. The sequence of the RNA genome is 8318 nucleotides long and contains three large open reading frames encoding the gag, pol, and env gene products. With the exception of a xenotropiclike R peptide and the LTR which bears structural similarities to a xenotropic LTR, displaying typical enhancerlike sequences, the remaining sequences are strikingly similar to the endogenous, ecotropic Akv murine leukemia virus. Therefore, it could be postulated that the leukemogenic properties of RadLV/VL3(T+L+) were generated by a recombination event between a xenotropic virus and an Akv-like ecotropic virus.

Cas-Br-E murine leukemia virus: sequencing of the paralytogenic region of its genome and derivation of specific probes to study its origin and the structure of its recombinant genomes in leukemic tissues

The ecotropic Cas-Br-E murine leukemia virus (MuLV) and its molecularly cloned derivative pBR-NE-8 MuLV are capable of inducing hind-limb paralysis and leukemia after inoculation into susceptible mice. T1 oligonucleotide fingerprinting, molecular hybridization, and restriction enzyme analysis previously showed that the env gene of Cas-Br-E MuLV diverged the most from that of other ecotropic MuLVs. To analyze proviruses in leukemic tissues, we derived DNA probes specific to Cas-Br-E sequences: two from the env region and one from the U3 long terminal repeat. With these probes, we found that this virus induced clonal (or oligoclonal) tumors and we documented the presence of typical mink cell focus-forming-type proviruses in leukemic tissues and the possible presence of other recombinant MuLV proviruses. Since the region harboring the determinant of paralysis was mapped within the pol-env region of the virus (L. DesGroseillers, M. Barrette, and P. Jolicoeur, J. Virol. 52:356-363, 1984), we performed the complete nucleotide sequence of this region covering the 3' end of the genome. We compared the deduced amino acid sequences of the pol carboxy-terminal domain and of the env gene products with those of other nonparalytogenic, ecotropic, and mink cell focus-forming MuLVs. This amino acid comparison revealed that this part of the pol gene product and the p15E diverged very little from homologous proteins of other MuLVs. However, the Cas-Br-E gp70 sequence was found to be quite divergent from that of other MuLVs, and the amino acid changes were distributed all along the protein. Therefore, gp70 remains the best candidate for harboring the determinant of paralysis.

Molecular analysis of emerging radiation leukemia virus variants of C57BL/Ka mice

Molecular cloning of several primary or passaged RadLV variants and their biological characterization has allowed us to propose a model of their emergence following X-ray irradiation of C57BL/6 mouse.