Mapping the viral sequences conferring leukemogenicity and disease specificity in Moloney and amphotropic murine leukemia viruses

Early events in retrovirus XMRV infection of the wild-derived mouse Mus pahari

A novel gammaretrovirus, xenotropic murine leukemia virus-related virus (XMRV), has been identified in patients with prostate cancer and in patients with chronic fatigue syndromes. Standard Mus musculus laboratory mice lack a functional XPR1 receptor for XMRV and are therefore not a suitable model for the virus. In contrast, Gairdner's shrew-mice (Mus pahari) do express functional XPR1. To determine whether Mus pahari could serve as a model for XMRV, primary Mus pahari fibroblasts and mice were infected with cell-free XMRV. Infection of cells in vitro resulted in XMRV Gag expression and the production of XMRV virions. After intraperitoneal injection of XMRV into Mus pahari mice, XMRV proviral DNA could be detected in spleen, blood, and brain. Intravenous administration of a green fluorescent protein (GFP) vector pseudotyped with XMRV produced GFP(+) CD4(+) T cells and CD19(+) B cells. Mice mounted adaptive immune responses against XMRV, as evidenced by the production of neutralizing and Env- and Gag-specific antibodies. Prominent G-to-A hypermutations were also found in viral genomes isolated from the spleen, suggesting intracellular restriction of XMRV infection by APOBEC3 in vivo. These data demonstrate infection of Mus pahari by XMRV, potential cell tropism of the virus, and immunological and intracellular restriction of virus infection in vivo. These data support the use of Mus pahari as a model for XMRV pathogenesis and as a platform for vaccine and drug development against this potential human pathogen.

Promoter Choice for Retroviral Vectors: Transcriptional Strength Versus Trans-Activation Potential

Gene expression from retroviral vectors can be driven by either the retroviral long terminal repeat (LTR) promoter or by cellular or viral promoters located internally in an LTR-deleted self-inactivating vector design. Adverse events in a gene therapy clinical trial for X-linked severe combined immune deficiency have led to the realization that the enhancer/promoter elements contained within integrated vectors may also act outside the vector genome to trans-activate host genes. Ideally, the gene expression system chosen for a vector should possess a low probability of trans-activation while still being able to support adequate levels of transgene expression. However, the parameters that define these specific characteristics are unknown. To gain insight into the mechanism of trans-activation, we compared a panel of commonly used retroviral LTRs and cellular and viral promoters for their ability to drive gene expression and to trans-activate a nearby minimal promoter in three different cell lines. These studies identified two elements, the cytomegalovirus enhancer/chicken beta-actin (CAG) and elongation factor (EF)-1alpha promoters, as being of potential value for use in vectors targeting lymphoid cells, as these elements exhibited both high levels of reporter gene expression and relatively low levels of trans-activation in T cells.

Genotoxicity of retroviral integration in hematopoietic cells

The experience of the past 3 years, since the first case of leukemia was reported in a child cured of X-linked severe combined immunodeficiency (X-SCID) by gene therapy, indicates that the potential genotoxicity of retroviral integration in hematopoietic cells will remain a consideration in evaluating the relative risks versus benefits of gene therapy for specific blood disorders. Although many unique variables may have contributed to an increased risk in X-SCID patients, clonal dominance or frank neoplasia in animal models, clonal dominance in humans with chronic granulomatous disease, and the ability of retroviral integration to immortalize normal bone marrow cells or convert factor-dependent cells to factor independence suggest that transduction of cells with an integrating retrovirus has the potential for altering their subsequent biologic behavior. The selective pressure imposed during in vitro culture or after engraftment may uncover a growth or survival advantage for cells in which an integration event has affected gene expression. Such cells then carry the risk that subsequent mutations may lead to neoplastic evolution of individual clones. Balancing that risk is that the vast majority of integration events seem to be neutral and that optimizing vector design may diminish the probability of altering gene expression by an integrated vector genome. Several cell culture systems and animal models designed to empirically evaluate the safety of vector systems are being developed and should provide useful data for weighing the relative risks and benefits for specific diseases and patient populations. Gene therapy interventions continue to have enormous potential for the treatment of disorders of the hematopoietic system. The future of such efforts seems bright as we continue to evolve and improve various strategies to make such interventions both effective and as safe as possible.

Retroviral insertional mutagenesis: past, present and future

Retroviral insertion mutagenesis screens in mice are powerful tools for efficient identification of oncogenic mutations in an in vivo setting. Many oncogenes identified in these screens have also been shown to play a causal role in the development of human cancers. Sequencing and annotation of the mouse genome, along with recent improvements in insertion site cloning has greatly facilitated identification of oncogenic events in retrovirus-induced tumours. In this review, we discuss the features of retroviral insertion mutagenesis screens, covering the mechanisms by which retroviral insertions mutate cellular genes, the practical aspects of insertion site cloning, the identification and analysis of common insertion sites, and finally we address the potential for use of somatic insertional mutagens in the study of nonhaematopoietic and nonmammary tumour types.

The use of retroviral vectors for gene therapy-what are the risks? A review of retroviral pathogenesis and its relevance to retroviral vector-mediated gene delivery

Retroviral vector-mediated gene transfer has been central to the development of gene therapy. Retroviruses have several distinct advantages over other vectors, especially when permanent gene transfer is the preferred outcome. The most important advantage that retroviral vectors offer is their ability to transform their single stranded RNA genome into a double stranded DNA molecule that stably integrates into the target cell genome. This means that retroviral vectors can be used to permanently modify the host cell nuclear genome. Recently, retroviral vector-mediated gene transfer, as well as the broader gene therapy field, has been re-invigorated with the development of a new class of retroviral vectors which are derived from lentiviruses. These have the unique ability amongst retroviruses of being able to infect non-cycling cells. Vectors derived from lentiviruses have provided a quantum leap in technology and seemingly offer the means to achieve significant levels of gene transfer in vivo.The ability of retroviruses to integrate into the host cell chromosome also raises the possibility of insertional mutagenesis and oncogene activation. Both these phenomena are well known in the interactions of certain types of wild-type retroviruses with their hosts. However, until recently they had not been observed in replication defective retroviral vector-mediated gene transfer, either in animal models or in clinical trials. This has meant the potential disadvantages of retroviral mediated gene therapy have, until recently, been seen as largely, if not entirely, hypothetical. The recent clinical trial of gammac mediated gene therapy for X-linked severe combined immunodeficiency (X-SCID) has proven the potential of retroviral mediated gene transfer for the treatment of inherited metabolic disease. However, it has also illustrated the potential dangers involved, with 2 out of 10 patients developing T cell leukemia as a consequence of the treatment. A considered review of retroviral induced pathogenesis suggests these events were qualitatively, if not quantitatively, predictable. In addition, it is clear that the probability of such events can be greatly reduced by relatively simple vector modifications, such as the use of self-inactivating vectors and vectors derived from non-oncogenic retroviruses. However, these approaches remain to be fully developed and validated. This review also suggests that, in all likelihood, there are no other major retroviral pathogenetic mechanisms that are of general relevance to replication defective retroviral vectors. These are important conclusions as they suggest that, by careful design and engineering of retroviral vectors, we can continue to use this gene transfer technology with confidence.

Sequence analysis of porcine endogenous retrovirus long terminal repeats and identification of transcriptional regulatory regions

Porcine cells express endogenous retroviruses, some of which are infectious for human cells. To better understand the replication of these porcine endogenous retroviruses (PERVs) in cells of different types and animal species, we have performed studies of the long terminal repeat (LTR) region of known gammaretroviral isolates of PERV. Nucleotide sequence determination of the LTRs of PERV-NIH, PERV-C, PERV-A, and PERV-B revealed that the PERV-A and PERV-B LTRs are identical, whereas the PERV-NIH and PERV-C LTRs have significant sequence differences in the U3 region between each other and with the LTRs of PERV-A and PERV-B. Sequence analysis revealed a similar organization of basal promoter elements compared with other gammaretroviruses, including the presence of enhancer-like repeat elements. The sequences of the PERV-NIH and PERV-C repeat element are similar to that of the PERV-A and PERV-B element with some differences in the organization of these repeats. The sequence of the PERV enhancer-like repeat elements differs significantly from those of other known gammaretroviral enhancers. The transcriptional activities of the PERV-A, PERV-B, and PERV-C LTRs relative to each other were similar in different cell types of different animal species as determined by transient expression assays. On the other hand, the PERV-NIH LTR was considerably weaker in these cell types. The transcriptional activity of all PERV LTRs was considerably lower in porcine ST-IOWA cells than in cell lines from other species. Deletion mutant analysis of the LTR of a PERV-NIH isolate identified regions that transactivate or repress transcription depending on the cell type.

Ikaros, a lymphoid-cell-specific transcription factor, contributes to the leukemogenic phenotype of a mink cell focus-inducing murine leukemia virus

Mink cell focus-inducing (MCF) viruses induce T-cell lymphomas in AKR/J strain mice. MCF 247, the prototype of this group of nonacute murine leukemia viruses, transforms thymocytes, in part, by insertional mutagenesis and enhancer-mediated dysregulation of cellular proto-oncogenes. The unique 3' (U3) regions in the long terminal repeats of other murine leukemia viruses contain transcription factor binding sites known to be important for enhancer function and for the induction of T-cell lymphomas. Although transcription factor binding sites important for the biological properties of MCF 247 have not been identified, pathogenesis studies from our laboratory suggested to us that binding sites for Ikaros, a lymphoid-cell-restricted transcriptional regulator, affect the biological properties of MCF 247. In this report, we demonstrate that Ikaros binds to predicted sites in U3 sequences of MCF 247 and that site-directed mutations in these sites greatly diminish this binding in vitro. Consistent with these findings, ectopic expression of Ikaros in murine cells that do not normally express this protein significantly increases transcription from the viral promoter in transient gene expression assays. Moreover, site-directed mutations in specific Ikaros-binding sites reduce this activity in T-cell lines that express Ikaros endogenously. To determine whether the Ikaros-binding sites are functional in vivo, we inoculated newborn mice with a variant MCF virus containing a mutant Ikaros-binding site. The variant virus replicated in thymocytes less efficiently and induced lymphomas with a delayed onset compared to the wild-type virus. These data are consistent with the hypothesis that the Ikaros-binding sites in the U3 region of MCF 247 are functional and cooperate with other DNA elements for optimal enhancer function in vivo.

Increased expression of MIP-1 alpha and MIP-1 beta mRNAs in the brain correlates spatially and temporally with the spongiform neurodegeneration induced by a murine oncornavirus

The chimeric murine oncornavirus FrCas(E) causes a rapidly progressive paralytic disease associated with spongiform neurodegeneration throughout the neuroaxis. Neurovirulence is determined by the sequence of the viral envelope gene and by the capacity of the virus to infect microglia. The neurocytopathic effect of this virus appears to be indirect, since the cells which degenerate are not infected. In the present study we have examined the possible role of inflammatory responses in this disease and have used as a control the virus F43. F43 is an highly neuroinvasive but avirulent virus which differs from FrCas(E) only in 3' pol and env sequences. Like FrCas(E), F43 infects large numbers of microglial cells, but it does not induce spongiform neurodegeneration. RNAase protection assays were used to detect differential expression of genes encoding a variety of cytokines, chemokines, and inflammatory cell-specific markers. Tumor necrosis factor alpha (TNF-alpha) and TNF-beta mRNAs were upregulated in advanced stages of disease but not early, even in regions with prominent spongiosis. Surprisingly there was no evidence for upregulation of the cytokines interleukin-1 alpha (IL-1 alpha), IL-1 beta, and IL-6 or of the microglial marker F4/80 at any stage of this disease. In contrast, increased levels of the beta-chemokines MIP-1 alpha and -beta were seen early in the disease and were concentrated in regions of the brain rich in spongiosis, and the magnitude of responses was similar to that observed in the brains of mice injected with the glutamatergic neurotoxin ibotenic acid. MIP-1alpha and MIP-1beta mRNAs were also upregulated in F43-inoculated mice, but the responses were three- to fivefold lower and occurred later in the course of infection than was observed in FrCas(E)-inoculated mice. These results suggest that the robust increase in expression of MIP-1 alpha and MIP-1 beta in the brain represents a correlate of neurovirulence in this disease, whereas the TNF responses are likely secondary events.

Tandemization of a subregion of the enhancer sequences from SRS 19-6 murine leukemia virus associated with T-lymphoid but not other leukemias

Most simple retroviruses induce tumors of a single cell type when infected into susceptible hosts. The SRS 19-6 murine leukemia virus (MuLV), which originated in mainland China, induces leukemias of multiple cellular origins. Indeed, infected mice often harbor more than one tumor type. Since the enhancers of many MuLVs are major determinants of tumor specificity, we tested the role of the SRS 19-6 MuLV enhancers in its broad disease specificity. The enhancer elements of the Moloney MuLV (M-MuLV) were replaced by the 170-bp enhancers of SRS 19-6 MuLV, yielding the recombinants DeltaMo+SRS(+) and DeltaMo+SRS(-) M-MuLV. M-MuLV normally induces T-lymphoid tumors in all infected mice. Surprisingly, when neonatal mice were inoculated with DeltaMo+SRS(+) or DeltaMo+SRS(-) M-MuLV, all tumors were of T-lymphoid origin, typical of M-MuLV rather than SRS 19-6 MuLV. Thus, the SRS 19-6 MuLV enhancers did not confer the broad disease specificity of SRS 19-6 MuLV to M-MuLV. However, all tumors contained DeltaMo+SRS M-MuLV proviruses with common enhancer alterations. These alterations consisted of tandem multimerization of a subregion of the SRS 19-6 enhancers, encompassing the conserved LVb and core sites and adjacent sequences. Moreover, when tumors induced by the parental SRS 19-6 MuLV were analyzed, most of the T-lymphoid tumors had similar enhancer alterations in the same region whereas tumors of other lineages retained the parental SRS 19-6 MuLV enhancers. These results emphasize the importance of a subregion of the SRS 19-6 MuLV enhancer in induction of T-cell lymphoma. The relevant sequences were consistent with crucial sequences for T-cell lymphomagenesis identified for other MuLVs such as M-MuLV and SL3-3 MuLV. These results also suggest that other regions of the SRS 19-6 MuLV genome contribute to its broad leukemogenic spectrum.

Cloning of E6 and E7 genes of human papilloma virus type 18 and transformation potential of E7 gene and its mutants

E6 and E7 genes of human papilloma virus type 18 have been subcloned from plasmid pC7, carrying an insert of DNA from squamous cell carcinoma of cervix. Both genes in comparison to prototype variant contain one mutation that changes asparagine to leucine. In the case of E6 gene this mutation is mapped in codon 129, in the case of E7 the same change AAC to AAA mapped in codon 92. In addition both genes contain few point mutations that do not change the aminoacid sequences of the protein. Two mutants of E7 gene have been constructed by site directed mutagenesis based on PCR technology-one in codon 10 (change Asp to Asn) and one in codon 24 (change Asp to Gly). The first type of mutation did not influence the transformation potential of the E7 gene in comparison to the parental one with mutation in codon 92. The mutation in codon 24 (region responsible for the interaction with Rb protein) eliminate the transformation potential of the gene. The cells transformed with E7 mutants in codons 10 and 92 were tumorigenic for syngenic rats.

Molecular and phylogenetic analysis of SRS 19-6 murine leukemia virus

The complete nucleotide sequence of the genome of Solid-type Reticulum cell Sarcoma 19-6 murine leukemia virus (SRS 19-6 MuLV) was determined. This virus was isolated in mainland China from laboratory mice that had been separated from western mice since the 1930s. The genome is 8,256 nucleotides in length and exhibits a genetic organization characteristic of replication competent MuLVs. Phylogenies constructed from reverse transcriptase (RT) domains showed that SRS 19-6 MuLV is closely related to other MuLV-related retroviruses; however, it has clearly diverged from previously isolated MuLVs. Comparative sequence analysis of the env sequences indicated that SRS 19-6 MuLV encodes a surface (SU) glycoprotein that is related to other ecotropic MuLVs in the VR-A and VR-B variable regions. However, SRS 19-6 MuLV env glycoprotein was distinct from all other MuLVs (ecotropic and non-ecotropic) in the proline-rich hypervariable region. No evidence for recombination with endogenous MuLV env sequences in generation of SRS 19-6 MuLV was observed. Comparisons of long terminal repeat (LTR) sequences revealed that the GV 1.4 molecular clone of Graffi MuLV contained 96% sequence identity to SRS 19-6 MuLV's LTR with 99% identity when comparisons were restricted to the U3 regions of the two viruses. The consensus enhancer binding motifs contained in the U3 regions of the two viruses were nearly identical. Nevertheless the two viruses have previously been shown to induce distinct patterns of disease. Comparisons between 196 and Graffi GV1.4 MuLVs may provide insights into the mechanisms of disease specificity induced by MuLVs.

Sequence-specific and/or stereospecific constraints of the U3 enhancer elements of MCF 247-W are important for pathogenicity

The oncogenic potential of many nonacute retroviruses is dependent on the duplication of the enhancer sequences present in the unique 3' (U3) region of the long terminal repeat (LTR). In a molecular clone (MCF 247-W) of the murine leukemia virus MCF 247, a leukemogenic mink cell focus-inducing (MCF) virus, the U3 enhancer sequences are tandemly repeated in the LTR. We mutated the enhancer region of MCF 247-W to test the hypothesis that the duplicated enhancer sequences of this virus have a sequence-specific and/or a stereospecific role in enhancer function required for transformation. In one virus, we inserted 14 nucleotide bp into the novel sequence generated at the junction of the two enhancers to generate an MCF virus with an interrupted enhancer region. In the second virus, only one copy of the enhancer sequences was present. This second virus also lacked the junction sequence present between the two enhancers of MCF 247-W. Both viruses were less leukemogenic and had a longer mean latency period than MCF 247-W. These data indicate that the sequence generated at the junction of the two enhancers and/or the stereospecific arrangement of the two enhancer elements are required for the full oncogenic potential of MCF 247-W. We analyzed proviral LTRs within the c-myc locus in tumor DNAs from mice injected with the MCF virus with the interrupted enhancer region. Some of the proviral LTRs integrated upstream of c-myc contain enhancer regions that are larger than those of the injected virus. These results are consistent with the suggestion that the virus with an interrupted enhancer changes in vivo to perform its role in the transformation of T cells.

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.

In vivo footprinting of the enhancer sequences in the upstream long terminal repeat of Moloney murine leukemia virus: differential binding of nuclear factors in different cell types

The enhancer sequences in the Moloney murine leukemia virus (M-MuLV) long terminal repeat (LTR) are of considerable interest since they are crucial for virus replication and the ability of the virus to induce T lymphomas. While extensive studies have identified numerous nuclear factors that can potentially bind to M-MuLV enhancer DNA in vitro, it has not been made clear which of these factors are bound in vivo. To address this problem, we carried out in vivo footprinting of the M-MuLV enhancer in infected cells by in vivo treatment with dimethyl sulfate (DMS) followed by visualization through ligation-mediated PCR (LMPCR) and gel electrophoresis. In vivo DMS-LMPCR footprinting of the upstream LTR revealed evidence for factor binding at several previously characterized motifs. In particular, protection of guanines in the central LVb/Ets and Core sites within the 75-bp repeats was detected in infected NIH 3T3 fibroblasts, Ti-6 lymphoid cells, and thymic tumor cells. In contrast, factor binding at the NF-1 sites was found in infected fibroblasts but not in T-lymphoid cells. These results are consistent with the results of previous experiments indicating the importance of the LVb/Ets and Core sequences for many retroviruses and the biological importance especially of the NF-1 sites in fibroblasts and T-lymphoid cells. No evidence for factor binding to the glucocorticoid responsive element and LVa sites was found. Additional sites of protein binding included a region in the GC-rich sequences downstream of the 75-bp repeats (only in fibroblasts), a hypersensitive guanine on the minus strand in the LVc site (only in T-lymphoid cells), and a region upstream of the 75-bp repeats. These experiments provide concrete evidence for the differential in vivo binding of nuclear factors to the M-MuLV enhancers in different cell types.

Members of the GATA family of transcription factors bind to the U3 region of Cas-Br-E and graffi retroviruses and transactivate their expression

Cas-Br-E and Graffi are two murine viruses that induce myeloid leukemia in mice: while Cas-Br-E induces mostly non-T, non-B leukemia composed of very immature cells, Graffi causes exclusively a granulocytic leukemia (E. Rassart, J. Houde, C. Denicourt, M. Ru, C. Barat, E. Edouard, L. Poliquin, and D. Bergeron, Curr. Top. Microbiol. Immunol. 211:201-210, 1995). In an attempt to understand the basis of the myeloid specificity of these two retroviruses, we used DNase I footprinting analysis and gel mobility shift assays to identify a number of protein binding sites within the Cas-Br-E and Graffi U3 regions. Two protected regions include potential GATA binding sites. Methylation interference analysis with different hematopoietic nuclear extracts showed the importance of the G residues in these GATA sites, and supershift assays clearly identified the binding factors as GATA-1, GATA-2, and GATA-3. Transient assays with long terminal repeat (LTR)-chloramphenicol acetyltransferase constructs showed that these three GATA family members are indeed able to transactivate Cas-Br-E and Graffi LTRs. Thus, the availability and relative abundance of the various members of the GATA family of transcription factors in a given cell type could influence the transcriptional tissue specificity of murine leukemia viruses and hence their disease specificity.

T lymphocytes with a normal ADA gene accumulate after transplantation of transduced autologous umbilical cord blood CD34+ cells in ADA-deficient SCID neonates

Adenosine deaminase-deficient severe combined immunodeficiency was the first disease investigated for gene therapy because of a postulated production or survival advantage for gene-corrected T lymphocytes, which may overcome inefficient gene transfer. Four years after three newborns with this disease were given infusions of transduced autologous umbilical cord blood CD34+ cells, the frequency of gene-containing T lymphocytes has risen to 1-10%, whereas the frequencies of other hematopoietic and lymphoid cells containing the gene remain at 0.01-0.1%. Cessation of polyethylene glycol-conjugated adenosine deaminase enzyme replacement in one subject led to a decline in immune function, despite the persistence of gene-containing T lymphocytes. Thus, despite the long-term engraftment of transduced stem cells and selective accumulation of gene-containing T lymphocytes, improved gene transfer and expression will be needed to attain a therapeutic effect.

Amphotropic murine leukemia viruses induce spongiform encephalomyelopathy

Recombinants of amphotropic murine leukemia virus (A-MuLV) have found widespread use in retroviral vector systems due to their ability to efficiently and stably infect cells of several different species, including human. Previous work has shown that replication-competent recombinants containing the amphotropic env gene, encoding the major SU envelope glycoprotein that determines host tropism, induce lymphomas in vivo. We show here that these viruses also induce a spongiform encephalomyelopathy in mice inoculated perinatally. This fatal central nervous system disease is characterized by noninflammatory spongiform lesions of nerve and glial cells and their processes, and is associated with moderate astro- and microgliosis. The first clinical symptoms are ataxia, tremor, and spasticity, progressing to complete tetraparesis and incontinence, and finally death of the animal. Sequences within the amphotropic env gene are necessary for disease induction. Coinfection of A-MuLV recombinants with nonneuropathogenic ecotropic or polytropic MuLV drastically increases the incidence, degree, and distribution of the neurodegenerative disorder. The consequence of these results in view of the use of A-MuLV recombinants in the clinic is discussed.

Amplified and tissue-directed expression of retroviral vectors using ping-pong techniques

Ping-pong amplification is an efficient process by which helper-free retrovirions replicate in cocultures of cell lines that package retroviruses into distinct host-range envelopes [11]. Transfection of a retroviral vector DNA into these cocultures results in massive virus production, with potentially endless cross-infection between different types of packaging cells. Because the helper-free virus spreads efficiently throughout the coculture, it is unnecessary to use dominant selectable marker genes, and the retroviral vectors can be simplified and optimized for expressing a single gene of interest. The most efficient ping-pong vector, pSFF, derived from the Friend erythroleukemia virus, has been used for high-level expression of several genes that could not be expressed with commonly employed two-gene retroviral vectors. Contrary to previous claims, problems of vector recombination are not inherent to ping-pong methods. Indeed, the pSFF vector has not formed replication-competent recombinants as shown by stringent assays. Here we review these methods, characterize the ping-pong process using the human erythropoietin gene as a model, and describe a new vector (pSFY) designed for enhanced expression in T lymphocytes. Factors that limit tissue-specific expression are reviewed.

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.

Identification of a region of a murine leukemia virus long terminal repeat with novel transcriptional regulatory activities

The 93-bp region downstream of the enhancer (DEN) in the long terminal repeat (LTR) of the mink cell focus-forming virus (MCF13) has been shown to be important for transcriptional activation and viral lymphomagenicity (J. C. Tupper, H. Chen, E. F. Hays, G. C. Bristol, and F. K. Yoshimura, J. Virol. 66:7080-7088, 1992). In this report, we have further explored the role of the DEN region in transcriptional activation. We observed that it has enhancer-like abilities as well as some unique LTR properties. Transcriptional activation by the DEN region involved interactions with enhancer sequences that were either synergistic or additive, depending on the cell type. The most intriguing property of the DEN region is its ability to induce transcription in activated T cells. This activity is unique for the LTR in that no other LTR region can do this. We also examined the role of the DEN region in retroviral lymphomagenesis. We cloned and sequenced proviral LTRs integrated upstream of the cellular c-myc gene from DNA obtained from thymic tumors induced by DEN region deletion mutant viruses in AKR mice. We determined that for transcriptional activation of the c-myc proto-oncogene, enhancer sequences can substitute for the DEN region. This study identifies the significance of non-enhancer sequences in the LTR for the oncogenesis of the MCF13 retrovirus.

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.

Viral determinants of simian immunodeficiency virus (SIV) virulence in rhesus macaques assessed by using attenuated and pathogenic molecular clones of SIVmac

To identify viral determinants of simian immunodeficiency virus (SIV) virulence, two pairs of reciprocal recombinants constructed from a pathogenic (SIVmac239) and a nonpathogenic (SIVmac1A11) molecular clone of SIV were tested in rhesus macaques. A large 6.2-kb fragment containing gag, pol, env, and the regulatory genes from each of the cloned (parental) viruses was exchanged to produce one pair of recombinant viruses (designated SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11gag-env/239 to indicate the genetic origins of the 5'/internal/3' regions, respectively, of the virus). A smaller 1.4-kb fragment containing the external env domain of each of the parental viruses was exchanged to create the second pair (SIVmac1A11/239env/1A11 and SIVmac239/1A11env/239) of recombinant viruses. Each of the two parental and four recombinant viruses was inoculated intravenously into four rhesus macaques, and all 24 animals were viremic by 4 weeks postinoculation (p.i.). Virus could not be isolated from peripheral blood mononuclear cells (PBMC) of any animals infected with SIVmac1A11 after 6 weeks p.i. but was consistently isolated from all macaques inoculated with SIVmac239 for 92 weeks p.i. Virus isolation was variable from animals infected with recombinant viruses; SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11env/239 were isolated most frequently. Animals inoculated with SIVmac239 had 10 to 100 times more virus-infected PBMC than those infected with recombinant viruses. Three animals infected with SIVmac239 died with simian AIDS (SAIDS) during the 2-year observation period after inoculation, and the fourth SIVmac239-infected animal had clinical signs of SAIDS. Two animals infected with recombinant viruses died with SAIDS; one was infected with SIVmac239/1A11gag-env/239, and the other was infected with SIVmac1A11/239gag-env/1A11. The remaining 18 macaques remained healthy by 2 years p.i., and 13 were aviremic. One year after inoculation, peripheral lymph nodes of some of these healthy, aviremic animals harbored infected cells. All animals seroconverted within the first few weeks of infection, and the magnitude of antibody response to SIV was proportional to the levels and duration of viremia. Virus-suppressive PBMC were detected within 2 to 4 weeks p.i. in all animals but tended to decline as viremia disappeared. There was no association of levels of cell-mediated virus-suppressive activity and either virus load or disease progression. Taken together, these results indicate that differences in more than one region of the viral genome are responsible for the lack of virulence of SIVmac1A11.

Characterization of a protein that binds multiple sequences in mammalian type C retrovirus enhancers

Mammalian type C retrovirus enhancer factor 1 (MCREF-1) is a nuclear protein that binds several directly repeated sequences (CNGGN6CNGG) in the Moloney and Friend murine leukemia virus (MLV) enhancers (N. R. Manley, M. O'Connell, W. Sun, N. A. Speck, and N. Hopkins, J. Virol. 67:1967-1975, 1993). In this paper, we describe the partial purification of MCREF-1 from calf thymus nuclei and further characterize the binding properties of MCREF-1. MCREF-1 binds four sites in the Moloney MLV enhancer and three sites in the Friend MLV enhancer. Ethylation interference analysis suggests that the MCREF-1 binding site spans two adjacent minor grooves of DNA.

Two factors that bind to highly conserved sequences in mammalian type C retroviral enhancers

The transcriptional enhancers of the Moloney and Friend murine leukemia viruses (MLV) are important determinants of viral pathogenicity. We used electrophoretic mobility shift and methylation interference assays to study nuclear factors which bind to a region of these enhancers whose sequence is identical between Moloney and Friend viruses and particularly highly conserved among 35 mammalian type C retroviruses whose enhancer sequences have been aligned (E. Golemis, N. A. Speck, and N. Hopkins, J. Virol. 64:534-542, 1990). Previous studies identified sites for the leukemia virus factor b (LVb) and core proteins in this region (N. A. Speck and D. Baltimore, Mol. Cell. Biol. 7:1101-1110, 1987) as well as a site, overlapping those for LVb and core, for a third factor (N. R. Manley, M. A. O'Connell, P. A. Sharp, and N. Hopkins, J. Virol. 63:4210-4223, 1989). Surprisingly, the latter factor appeared to also bind two sites identified in the Friend MLV enhancer, Friend virus factor a and b1 (FVa and FVb1) sites, although the sequence basis for the ability of the protein to bind these diverse sites was not apparent. Here we describe the further characterization of this binding activity, termed MCREF-1 (for mammalian type C retrovirus enhancer factor 1), and the identification of a consensus sequence for its binding, GGN8GG. We also identify a factor, abundant in mouse T-cell lines and designated LVt, which binds to two sites in the Moloney MLV enhancer, overlapping the previously identified LVb and LVc binding sites. These sites contain the consensus binding site for the Ets family of proteins. We speculate on how distinct arrays of these factors may influence the disease-inducing phenotype.

Contributions to transcriptional activity and to viral leukemogenicity made by sequences within and downstream of the MCF13 murine leukemia virus enhancer

We have identified nucleotide sequences that regulate transcription in both a cell-type-specific and general manner in the long terminal repeat of the MCF13 murine leukemia virus. Besides the enhancer element, we have observed that the region between the enhancer and promoter (DEN) has a profound effect on transcription in different cell types. This effect, however, was dependent on the copy number of enhancer repeats and was detectable in the presence of a single repeat. When two enhancer repeats were present, the effect of DEN on transcription was abrogated except in T cells. DEN also makes a significant contribution to the leukemogenic property of the MCF13 retrovirus. Its deletion from the MCF13 virus dramatically reduced the incidence of thymic lymphoma and increased the latency of disease in comparison with the wild-type virus. This effect was most marked when one rather than two enhancer repeats was present in the mutant viruses. We also observed that the removal of one repeat alone remarkably reduced leukemogenicity by the MCF13 virus. A newly identified protein-binding site (MLPal) located within DEN affects transcription only in T cells, and its deletion attenuates the ability of an MCF13 virus with a single enhancer repeat to induce thymic lymphoma. This observation suggests that the MLPal protein-binding site contributes to the effect of the DEN region on T-cell-specific transcription and viral leukemogenicity. This study identifies the importance of nonenhancer sequences in the long terminal repeat for the oncogenesis of the MCF13 retrovirus.

Phenotypes of murine leukemia virus-induced tumors: influence of 3' viral coding sequences

Murine leukemia viruses (MuLVs) induce leukemias and lymphomas in mice. We have used fluorescence-activated cell sorter analysis to determine the hematopoietic phenotypes of tumor cells induced by a number of MuLVs. Tumor cells induced by ecotropic Moloney, amphotropic 4070A, and 10A1 MuLVs and by two chimeric MuLVs, Mo(4070A) and Mo(10A1), were examined with antibodies to 13 lineage-specific cell surface markers found on myeloid cell, T-cell, and B-cell lineages. The chimeric Mo(4070A) and Mo(10A1) MuLVs, consisting of Moloney MuLV with the carboxy half of the Pol region and nearly all of the Env region of 4070A and 10A1, respectively, were constructed to examine the possible influence of these sequences on Moloney MuLV-induced tumor cell phenotypes. In some instances, these phenotypic analyses were supplemented by Southern blot analysis for lymphoid cell-specific genomic DNA rearrangements at the immunoglobulin heavy-chain, the T-cell receptor gamma, and the T-cell receptor beta loci. The results of our analysis showed that Moloney MuLV, 4070A, Mo(4070A), and Mo(10A1) induced mostly T-cell tumors. Moloney MuLV and Mo(4070A) induced a wide variety of T-cell phenotypes, ranging from immature to mature phenotypes, while 4070A induced mostly prothymocyte and double-negative (CD4- CD8-) T-cell tumors. The tumor phenotypes obtained with 10A1 and Mo(10A1) were each less variable than those obtained with the other MuLVs tested. 10A1 uniformly induced a tumor consisting of lineage marker-negative cells that lack lymphoid cell-specific DNA rearrangements and histologically appear to be early undifferentiated erythroid cell-like precursors. The Mo(10A1) chimera consistently induced an intermediate T-cell tumor. The chimeric constructions demonstrated that while 4070A 3' pol and env sequences apparently did not influence the observed tumor cell phenotypes, the 10A1 half of pol and env had a strong effect on the phenotypes induced by Mo(10A1) that resulted in a phenotypic consistency not seen with other viruses. This result implicates 10A1 env in an active role in the tumorigenic process.

A viral long terminal repeat expressed in CD4+CD8+ precursors is downregulated in mature peripheral CD4-CD8+ or CD4+CD8- T cells

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.

A viral long terminal repeat expressed in CD4+CD8+ precursors is downregulated in mature peripheral CD4-CD8+ or CD4+CD8- T cells

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.

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.

Purification of core-binding factor, a protein that binds the conserved core site in murine leukemia virus enhancers

The Moloney murine leukemia virus causes thymic leukemias when injected into newborn mice. A major genetic determinant of the thymic disease specificity of the Moloney virus genetically maps to two protein binding sites in the Moloney virus enhancer, the leukemia virus factor b site and the adjacent core site. Point mutations introduced into either of these sites significantly shifts the disease specificity of the Moloney virus from thymic leukemia to erythroleukemia (N. A. Speck, B. Renjifo, E. Golemis, T. Frederickson, J. Hartley, and N. Hopkins, Genes Dev. 4:233-242, 1990). We have purified several polypeptides that bind to the core site in the Moloney virus enhancer. These proteins were purified from calf thymus nuclear extracts by selective pH denaturation, followed by chromatography on heparin-Sepharose, nonspecific double-stranded DNA-cellulose, and core oligonucleotide-coupled affinity columns. We have achieved greater than 13,000-fold purification of the core-binding factors (CBFs), with an overall yield of approximately 19%. Analysis of purified protein fractions by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis reveals more than 10 polypeptides. Each of the polypeptides was recovered from an SDS-polyacrylamide gel, and those in the molecular size range of 19 to 35 kDa were demonstrated to have core-binding activity. The purified CBFs were shown by DNase I footprint analyses to bind the core site in the Moloney virus enhancer specifically, and also to core motifs in the enhancers from a simian immunodeficiency virus, the immunoglobulin mu chain, and T-cell receptor gamma-chain genes.

Purification of core-binding factor, a protein that binds the conserved core site in murine leukemia virus enhancers

The Moloney murine leukemia virus causes thymic leukemias when injected into newborn mice. A major genetic determinant of the thymic disease specificity of the Moloney virus genetically maps to two protein binding sites in the Moloney virus enhancer, the leukemia virus factor b site and the adjacent core site. Point mutations introduced into either of these sites significantly shifts the disease specificity of the Moloney virus from thymic leukemia to erythroleukemia (N. A. Speck, B. Renjifo, E. Golemis, T. Frederickson, J. Hartley, and N. Hopkins, Genes Dev. 4:233-242, 1990). We have purified several polypeptides that bind to the core site in the Moloney virus enhancer. These proteins were purified from calf thymus nuclear extracts by selective pH denaturation, followed by chromatography on heparin-Sepharose, nonspecific double-stranded DNA-cellulose, and core oligonucleotide-coupled affinity columns. We have achieved greater than 13,000-fold purification of the core-binding factors (CBFs), with an overall yield of approximately 19%. Analysis of purified protein fractions by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis reveals more than 10 polypeptides. Each of the polypeptides was recovered from an SDS-polyacrylamide gel, and those in the molecular size range of 19 to 35 kDa were demonstrated to have core-binding activity. The purified CBFs were shown by DNase I footprint analyses to bind the core site in the Moloney virus enhancer specifically, and also to core motifs in the enhancers from a simian immunodeficiency virus, the immunoglobulin mu chain, and T-cell receptor gamma-chain genes.

Substitution of leucine for isoleucine in a sequence highly conserved among retroviral envelope surface glycoproteins attenuates the lytic effect of the Friend murine leukemia virus

Friend murine leukemia virus is a replication-competent retrovirus that contains no oncogene and that exerts lytic and leukemogenic properties. Thus, newborn mice inoculated with Friend murine leukemia virus develop severe early hemolytic anemia before appearance of erythroleukemia. To identify the retroviral determinants regulating these effects, we used chimeric infectious constructions and site-directed point mutations between a virulent Friend murine leukemia virus strain and a naturally occurring variant attenuated in lytic and leukemogenic effects. We found that severe hemolytic anemia was always associated with higher numbers of blood reticulocytes with budding retroviral particles. Furthermore, a remarkably conservative leucine to isoleucine change in the extracellular SU component of the retroviral envelope was sufficient to attenuate this lytic effect. Also, this leucine at position 348 of the envelope precursor protein was located within the only stretch of five amino acids that is conserved in the extracellular SU component of all murine, feline, and primate type C and type D retroviral envelopes. This observation suggested an important structural function for this yet undescribed conserved sequence of the envelope. Lastly, we observed that lytic and leukemogenic effects were attenuated by a deletion of a second repeat in the transcriptional enhancer region of the viral long terminal repeats of the variant strain.

Infection by mink cell focus-forming viruses confers interleukin 2 (IL-2) independence to an IL-2-dependent rat T-cell lymphoma line

The development of T-cell lymphomas in rodents infected with type C retroviruses has been linked to the generation of a class of envelope (env) recombinant viruses called mink cell focus-forming viruses (MCF viruses) in the preleukemic thymus. To determine whether infection by MCF viruses altered the growth phenotype of retrovirus-induced T-cell lymphomas, a Moloney murine leukemia virus-induced interleukin-2 (IL-2)-dependent rat T-cell lymphoma line (4437A) was infected with MCF-247, modified MCF-V33 (mMCF-V33), or NZB-xenotropic (NZB-X) virus. The effects of virus infection on the IL-2 dependence of these cells was examined by cultivating them in the absence of IL-2. After IL-2 withdrawal, the uninfected and NZB-X-infected cells went through a crisis period characterized by massive death. All the independently maintained cultures of MCF- and mMCF-V33-infected cells, on the other hand, became IL-2 independent without a crisis. All the polytropic virus-infected IL-2-independent cultures contained a population of cells that was polyclonal with regard to polytropic provirus integration. Over this polyclonal background each culture produced multiple clones of cells that were selected rapidly after IL-2 withdrawal. Furthermore, the resulting MCF- or mMCF-V33-infected IL-2-independent cells retained the expression of IL-2 receptor. These data show that MCF and mMCF-V33 viruses may alter the growth phenotype of a T-cell lymphoma line and suggest that their effect on cell growth may be due to the direct interaction of the MCF envelope glycoprotein with cellular components, perhaps the IL-2 receptor.

Nuclear factor 1 activates the feline leukemia virus long terminal repeat but is posttranscriptionally down-regulated in leukemia cell lines

A recombinant feline leukemia virus (FeLV) proviral clone (T17T-22) with a long terminal repeat (LTR) which differs from prototype FeLV by a point mutation within a conserved nuclear factor 1 (NF1)-binding motif in the LTR enhancer domain was found to be poorly expressed after DNA transfection. The NF1 point mutation reduced in vitro protein binding as assessed by gel shift analysis and reduced promoter activity significantly (2- to 10-fold). However, the degree of promoter impairment due to the NF1 site mutation varied according to cell type and was least severe in a feline leukemia cell line (T3) which had low levels of nuclear NF1 DNA-binding activity. Low NF1 DNA-binding activity was observed in three FeLV-induced leukemia cell lines (T3, T17, and FL74) and in murine F9 embryonal carcinoma cells. While similar levels of NF1 gene mRNA transcripts were detected in all cell lines, Western immunoblot analysis of F9, T17, and FL74 but not T3 nuclear extracts revealed very low levels of nuclear NF1 protein. These results indicate that NF1 activity is down-regulated in FeLV-induced leukemia cells by diverse posttranscriptional mechanisms. We suggest that NF1 down-regulation may be an important characteristic of target cells susceptible to FeLV transformation in vivo and may provide the selective pressure which favors duplication of the LTR core enhancer sequence in T-cell leukemogenic FeLV variants.

A novel HIV-1 isolate containing alterations affecting the NF-kappa B element

Three molecular clones of HIV-1, derived from a single isolate (AL1), exhibited distinct replicative and cytopathic properties during propagation in a human T cell line. The phenotypic differences observed were attributable, in large part, to changes affecting the viral LTR. Nucleotide sequence and PCR analyses demonstrated the presence of novel duplications or deletions involving the NF-kappa B motif. These changes in the enhancer element were identified in the original AL1 virus stock. Subcloning of the variant NF-kappa B segments into LTR-driven CAT expression vectors confirmed a correlation between promoter activity and replicative/cytopathic capacity.

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.

Substitution of the U3 long terminal repeat region of the neurotropic Cas-Br-E retrovirus affects its disease-inducing potential

The Cas-Br-E and ts-Mo BA-1 murine leukemia viruses (MuLV) induce a spongiform neurodegenerative disease with different clinical manifestations, namely, either hind limb paralysis (Cas-Br-E) or tremors, spasticity, and hind limb weakness (ts-Mo Ba-1). We constructed the chimeric NEBA-1 MuLV by replacing the long terminal repeat of Cas-Br-E MuLV with that of ts-Mo BA-1 MuLV. In SWR/J or CFW/D mice, NEBA-1 MuLV induced an ataxic neurological disease characterized by clinical signs different from those induced by both parents. Although NEBA-1 MuLV did not induce lesions in novel brain areas, the spongiform lesions were more severe in deep cerebellar nuclei and in the spinal cord than those found in paralyzed mice inoculated with Cas-Br-E MuLV. By in situ hybridization, we found that the distribution of the spongiform lesions closely correlated with the distribution of the infected central nervous system cells. In the spinal cord, a close correlation was found between the number of infected cells and the severity of the spongiform degeneration. Sequencing of the substituted ts-BA-1 MuLV fragment and comparison with homologous sequences of Cas-Br-E and Moloney MuLV showed differences mainly in the U3 tandem direct repeats. Our results show that a few modifications within the U3 long terminal repeat allow the virus to cause more severe lesions in some central nervous system regions and that the severity of the spongiform degeneration correlates with the level of viral replication.

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).

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.

Alignment of U3 region sequences of mammalian type C viruses: identification of highly conserved motifs and implications for enhancer design

We aligned published sequences for the U3 region of 35 type C mammalian retroviruses. The alignment reveals that certain sequence motifs within the U3 region are strikingly conserved. A number of these motifs correspond to previously identified sites. In particular, we found that the enhancer region of most of the viruses examined contains a binding site for leukemia virus factor b, a viral corelike element, the consensus motif for nuclear factor 1, and the glucocorticoid response element. Most viruses containing more than one copy of enhancer sequences include these binding sites in both copies of the repeat. We consider this set of binding sites to constitute a framework for the enhancers of this set of viruses. Other highly conserved motifs in the U3 region include the retrovirus inverted repeat sequence, a negative regulatory element, and the CCAAT and TATA boxes. In addition, we identified two novel motifs in the promoter region that were exceptionally highly conserved but have not been previously described.

Mutation of the core or adjacent LVb elements of the Moloney murine leukemia virus enhancer alters disease specificity

Transcriptional enhancers of replication-competent mouse C-type retroviruses are potent determinants of the distinct disease-inducing phenotypes of different viral isolates and can also strongly influence the incidence and latent period of disease induction. To study the contribution of individual protein-binding sites to viral pathogenicity, we introduced mutations into each of the known nuclear factor-binding sites in the enhancer region of the Moloney murine leukemia virus and injected viruses with these mutations into newborn NFS mice. All viruses induced disease. Viruses with mutations in both copies of the leukemia virus factor a (LVa) site, leukemia virus factor c (LVc) site, or in just the promoter proximal copy of the glucocorticoid response element (GRE) had a latent period of disease onset and disease specificity indistinguishable from that of the wild-type Moloney virus. Viruses with mutations in two or three of the GREs, in both copies of the leukemia virus factor b (LVb) site, in two of the four nuclear factor 1 (NF1) consensus motifs, or in both copies of the conserved viral core element showed a significant delay in latent period of disease induction. Strikingly, viruses with mutations in the core element induced primarily erythroleukemias, and mutations in the LVb site also resulted in a significant incidence of erythroleukemias. These and other genetic and biochemical studies suggest models for how subtle alterations in the highly conserved structure of mouse C-type retrovirus enhancers can produce a dramatic effect on disease specificity.

Differential DNA binding of nuclear proteins to a long terminal repeat region of the MCF13 and Akv murine leukemia viruses

Long terminal repeat (LTR) sequences of murine leukemia viruses (MLVs) have been demonstrated to be mainly responsible for the pathogenic differences in these retroviruses. A region of the LTR which is downstream of the enhancer elements has been shown to contribute both to enhancer activity as well as to disease specificity of MLVs. We have identified protein-DNA complexes generated by this region of a lymphomagenic MLV (MCF13) and one which is nonpathogenic (Akv). One protein-DNA complex we have observed for this region is unique to MCF13 DNA sequences. Detection of protein involved in this unique MCF13 complex in different cell lines revealed that it was ubiquitous.

Nuclear factors that bind to the enhancer region of nondefective Friend murine leukemia virus

Nondefective Friend murine leukemia virus (MuLV) causes erythroleukemia when injected into newborn NFS mice, while Moloney MuLV causes T-cell lymphoma. Exchange of the Friend virus enhancer region, a sequence of about 180 nucleotides including the direct repeat and a short 3'-adjacent segment, for the corresponding region in Moloney MuLV confers the ability to cause erythroid disease on Moloney MuLV. We have used the electrophoretic mobility shift assay and methylation interference analysis to identify cellular factors which bind to the Friend virus enhancer region and compared these with factors, previously identified, that bind to the Moloney virus direct repeat (N. A. Speck and D. Baltimore, Mol. Cell. Biol. 7:1101-1110, 1987). We identified five binding sites for sequence-specific DNA-binding proteins in the Friend virus enhancer region. While some binding sites are present in both the Moloney and Friend virus enhancers, both viruses contain unique sites not present in the other. Although none of the factors identified in this report which bind to these unique sites are present exclusively in T cells or erythroid cells, they bind to three regions of the enhancer shown by genetic analysis to encode disease specificity and thus are candidates to mediate the tissue-specific expression and distinct disease specificities encoded by these virus enhancer elements.

Mapping of functional regions of murine retrovirus long terminal repeat enhancers: enhancer domains interact and are not independent in their contributions to enhancer activity

We have used deletion and recombinant long terminal repeat (LTR) mutants to examine enhancer activity differences between LTRs of the nonpathogenic Akv and the thymus lymphomagenic MCF13 murine retroviruses. Deletion mutant analysis revealed that major control regions for MCF13 and Akv LTR enhancer activity were similar but not identical. For both LTRs, major control regions were distinctly different in a murine T-cell and a fibroblast cell line. Recombinant enhancer analysis showed that LTRs could be divided into three regions capable of altering the level of enhancer activity through cooperative or antagonistic interaction. The contribution of each region to enhancer activity was dependent on its context with respect to the other regions. LTR enhancer function in different cell types appears to be the result of the interaction of enhancer modular elements.

Retrovirus vectors containing an internal attachment site: evidence that circles are not intermediates to murine retrovirus integration

Murine cells were infected with a retrovirus vector containing a defective native attachment (att) site, an internal att site, and a neo gene. Analysis of the proviruses by virus rescue and Southern blots demonstrated that internal att sites were not utilized for integration and could not complement defects in the native site. These data suggest that murine retroviruses do not integrate in vivo through tandem long terminal repeat circular DNA intermediates.