The high leukemogenic potential of Gross passage A murine leukemia virus maps in the region of the genome corresponding to the long terminal repeat and to the 3' end of env

TRIM5 alpha cytoplasmic bodies are highly dynamic structures

Tripartite motif (TRIM)5 alpha has recently been identified as a host restriction factor that has the ability to block infection by certain retroviruses in a species-dependent manner. One interesting feature of this protein is that it is localized in distinct cytoplasmic clusters designated as cytoplasmic bodies. The potential role of these cytoplasmic bodies in TRIM5 alpha function remains to be defined. By using fluorescent fusion proteins and live cell microscopy, we studied the localization and dynamics of TRIM5 alpha cytoplasmic bodies. This analysis reveals that cytoplasmic bodies are highly mobile, exhibiting both short saltatory movements and unidirectional long-distance movements along the microtubule network. The morphology of the cytoplasmic bodies is also dynamic. Finally, photobleaching and photoactivation analysis reveals that the TRIM5 alpha protein present in the cytoplasmic bodies is very dynamic, rapidly exchanging between cytoplasmic bodies and a more diffuse cytoplasmic population. Therefore, TRIM5 alpha cytoplasmic bodies are dynamic structures more consistent with a role in function or regulation rather than protein aggregates or inclusion bodies that represent dead-end static structures.

Proteasome inhibitors uncouple rhesus TRIM5alpha restriction of HIV-1 reverse transcription and infection

The primate TRIM5alpha proteins have recently been defined as cellular restriction factors, preventing primate infection by retroviruses from different species. For instance, rhesus TRIM5alpha (rhTRIM5alpha) restricts infection by HIV-1. Virtually all TRIM5alpha proteins block the early replication of retroviruses by preventing the accumulation of reverse transcription products, but the underlying mechanism remains unclear. In this article, we find that disrupting proteasome function alters rhTRIM5alpha localization and allows the normal generation of HIV-1 late reverse transcription products, even though HIV-1 infection and the generation of nuclear 1-LTR and 2-LTR viral cDNA forms remain impaired. This finding suggests rhTRIM5alpha restricts HIV infection in two distinct phases: (i) altering the normal passage of the reverse-transcribing viral genome to the nucleus and (ii) targeting the reverse transcription complex to be disrupted by the proteasome. Because proteasome inhibitor blocks the second phase, accumulation of a nonfunctional viral DNA genome can be readily observed. Defining each phase may reveal HIV-1 targets for future antiviral therapy in which dual blockade may be equally as effective as naturally occurring rhTRIM5alpha protein in preventing HIV-1 infection in vivo.

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.

Analysis of the disease potential of a recombinant retrovirus containing Friend murine leukemia virus sequences and a unique long terminal repeat from feline leukemia virus

We have molecularly cloned a feline leukemia virus (FeLV) (clone 33) from a domestic cat with acute myeloid leukemia (AML). The long terminal repeat (LTR) of this virus, like the LTRs present in FeLV proviruses from other cats with AML, contains an unusual structure in its U3 region upstream of the enhancer (URE) consisting of three tandem direct repeats of 47 bp. To test the disease potential and specificity of this unique FeLV LTR, we replaced the U3 region of the LTR of the erythroleukemia-inducing Friend murine leukemia virus (F-MuLV) with that of FeLV clone 33. When the resulting virus, F33V, was injected into newborn mice, almost all of the mice eventually developed hematopoietic malignancies, with a significant percentage being in the myeloid lineage. This is in contrast to mice injected with an F-MuLV recombinant containing the U3 region of another FeLV that lacks repetitive URE sequences, none of which developed myeloid malignancies. Examination of tumor proviruses from F33V-infected mice failed to detect any changes in FeLV U3 sequences other than that in the URE. Like F-MuLV-infected mice, those infected with the F-MuLV/FeLV recombinants were able to generate and replicate mink cell focus-inducing viruses. Our studies are consistent with the idea that the presence of repetitive sequences upstream of the enhancer in the LTR of FeLV may favor the activation of this promoter in myeloid cells and contribute to the development of malignancies in this hematopoietic lineage.

Diversity of the HIV-1 long terminal repeat following mother-to-child transmission

A study of the human immunodeficiency virus Type 1 (HIV-1) 5' long terminal repeat (LTR) was performed to determine the extent of variation found within the LTR from 19 mother-infant pairs in Tanzania and to assess whether the LTR is useful in distinguishing maternal sequences that were transmitted to infants. HIV-1 subtypes A, C, and D as well as intersubtype recombinant LTR sequences were detected in mothers and infants. The LTR subtype was 100% concordant between mothers and their infants. Diversity calculations showed a significant reduction in LTR variation in infants compared to their mothers. However, the overall magnitude of LTR variation was less than that found in the env gene from the same individuals. These data suggest a selective constraint active upon the 5' long terminal repeat that is distinct from immune selective pressure(s) directed against HIV-1 structural genes. Detection of maternal LTR variants that were transmitted to infants may yield important information concerning nonstructural determinants of HIV-1 transmission from mother to infant.

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.

The exogenous form of Jaagsiekte retrovirus is specifically associated with a contagious lung cancer of sheep

Sheep pulmonary adenomatosis ([SPA] ovine pulmonary carcinoma) is a transmissible lung cancer of sheep that has been associated etiologically with a type D- and B-related retrovirus (jaagsiekte retrovirus (JSRV]). To date it has been impossible to cultivate JSRV in vitro and therefore to demonstrate the etiology of SPA by a classical approach. In addition, the presence of 15 to 20 copies of endogenous JSRV-related sequences (enJSRV) has hampered studies at the molecular level. The aim of this study was to investigate whether the expression of exogenous JSRV was specifically associated with neoplasia in SPA-affected animals. Initially, we found that enJSRVs were transcribed in a wide variety of normal sheep tissues. Then, by sequencing part of the gag gene of enJSRV we established a ScaI restriction site in gag as a molecular marker for the exogenous form of JSRV. Restriction enzyme digestion of PCR products obtained from the amplification of cDNA from a total of 65 tissues collected from SPA-affected and unaffected control sheep revealed that the exogenous form of JSRV was exclusively and consistently present in tumor tissues and lung secretions of the affected animals. In addition, exogenous JSRV provirus was detected only in DNA from SPA tumors and not from nontumor tissues of the same animals. This study has demonstrated clearly that the exogenous form of JSRV is specifically associated with SPA tumors.

Basic helix-loop-helix proteins in murine type C retrovirus transcriptional regulation

E boxes, recognition sequences for basic helix-loop-helix (bHLH) transcription factors, are detected in the enhancer and promoter regions of several murine type C retroviruses. Here we show that ALF1, a member of bHLH protein family of transcription factors, in vitro binds with differing affinities to distinct E-box sequences found in the U3 regulatory regions of Friend, Moloney, SL3-3, and Akv murine leukemia viruses (MLVs) as well as Friend spleen focus-forming virus (SFFV). In NIH 3T3 fibroblasts, ALF1 overexpression elevated transcription from the U3 region of Moloney MLV and the complete long terminal repeat regions of Friend SFFV, Akv MLV, and SL3-3 MLV but neither from the U3 region nor from the complete long terminal repeat of Friend MLV. Introduction of mutations in the Akv MLV E boxes showed the E-box cis elements to be required for the function of ALF1 as a transcription factor. ALF1 and the glucocorticoid receptor, with overlapping DNA binding sequences, did not act synergistically with respect to transcriptional trans activation of expression from the Akv MLV promoter-enhancer region. We conclude that ALF1 in vivo may be an important transcription regulator for Akv, SL3-3, and Moloney MLVs as well as for Friend SFFV.

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.

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.

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.

Determinants of the ability of malignant fibroma virus to induce immune dysfunction and tumor dissemination in vivo

The relationship of virus-induced immunological dysfunction and tumor dissemination was studied using two related tumor-causing leporipoxviruses: malignant fibroma virus (MV) and Shope fibroma virus (SFV). Recombinant viruses, produced by transferring MV's 10.7 kb BamHI C fragment to SFV, replicate in lymphocytes and suppress lymphocyte function in vitro. Those recombinants that replicate in lymphocytes and suppress lymphocyte function in vitro share about 3.5 kb from MV's C fragment. Some recombinants mimic MV in producing immune suppression and disseminated virus infection in vivo. Other recombinants, even some that are highly immunosuppressive in vitro (e.g. R71), only variably induce immune suppression in vivo, and do not cause disseminated disease. A segment of DNA from MV that transfers to Shope fibroma virus almost all of MV's virulence in vivo was identified.

Enhancer functions in U3 of Akv virus: a role for cooperativity of a tandem repeat unit and its flanking DNA sequences

cis-Acting transcriptional control elements in the U3 region of the murine retrovirus Akv were analyzed in mouse NIH 3T3 fibroblast cells by using a transient expression vector system based upon a complete long terminal repeat with linked flanking sequences. Deletion analysis pointed to the essential role of sequences within the 99-base-pair direct repeats, and a fragment encompassing the two repeats was found to possess orientation-independent enhancer activity when positioned either upstream or downstream of the transcriptional unit. Removal of one copy of the 99-base-pair repeat led to a reduction in activity of about 2.5-fold when located in an intact U3 environment but to reductions of up to 2 orders of magnitude when placed in other sequence contexts. Our studies of enhancer functions in the presence of one versus two copies of the tandem repeat point to duplicate functions of repeat sequences and sequences flanking the repeat region and emphasize the complex overall organization of this U3 region.

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).

Multiple sequence elements in the U3 region of the leukemogenic murine retrovirus SL3-2 contribute to cell-dependent gene expression

Determination of the U3 sequence of the leukemogenic murine retrovirus SL3-2 revealed close relationships to SL3-3, Akv, and Gross passage A viruses. The SL3-2 and Akv regions showed wide differences in their relative transcriptional activity in four cell lines as determined by U3-driven transient expression assays. The U3 regions of SL3-2 and SL3-3 gave rise to similar but not identical levels of expression. Deletion mapping of the SL3-2 U3 region points to several determinants of expression of different relative importance in the cell lines tested.

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.

Influence of enhancer sequences on thymotropism and leukemogenicity of mink cell focus-forming viruses

Oncogenic mink cell focus-forming (MCF) viruses, such as MCF 247, show a positive correlation between the ability to replicate efficiently in the thymus and a leukemogenic phenotype. Other MCF viruses, such as MCF 30-2, replicate to high titers in thymocytes and do not accelerate the onset of leukemia. We used these two MCF viruses with different biological phenotypes to distinguish the effect of specific viral genes and genetic determinants on thymotropism and leukemogenicity. Our goal was to identify the viral sequences that distinguish thymotropic, nonleukemogenic viruses such as MCF 30-2 from thymotropic, leukemogenic viruses such as MCF 247. We cloned MCF 30-2, compared the genetic hallmarks of MCF 30-2 with those of MCF 247, constructed a series of recombinants, and tested the ability of recombinant viruses to replicate in the thymus and to induce leukemia. The results established that (i) MCF 30-2 and MCF 247 differ in the numbers of copies of the enhancer sequences in the long terminal repeats. (ii) The thymotropic phenotype of both viruses is independent of the number of copies of the enhancer sequences. (iii) The oncogenic phenotype of MCF 247 is correlated with the presence in the virus of duplicated enhancer sequences or with the presence of an enhancer with a specific sequence. These results show that the pathogenic phenotypes of MCF viruses are dissociable from the thymotropic phenotype and depend, at least in part, upon the enhancer sequences. On the basis of these results, we suggest that the molecular mechanisms by which the enhancer sequences determine thymotropism are different from those that determine oncogenicity.

Identification of a common helper provirus integration site in Abelson murine leukemia virus-induced lymphoma DNA

Abelson murine leukemia virus induces oligoclonal pre-B lymphoma in mice. The expression of the v-abl oncogene in target cells does not appear to be sufficient for tumor induction in several mouse strains, and additional genetic events are thought to be required. We postulated that the helper Moloney murine leukemia virus might induce these events, and its potential role as an insertional mutagen was assessed by the search of a common helper provirus integration site in Abelson murine leukemia virus lymphomas. Molecular cloning of cellular sequences adjacent to Moloney proviruses enabled us to identify a cellular region, designated Ahi-1, which was found occupied by the helper proviruses in 16% of Abelson pre-B-cell lymphomas. All proviruses for which the precise integration site within Ahi-1 could be mapped were found to be in the same orientation. Ahi-1 has been mapped to mouse chromosome 10 and represents a new common proviral integration site. These data suggest that the helper virus contributes to the induction of secondary genetic events which may be important for the development of Abelson murine leukemia virus-induced pre-B-cell lymphoma.

Alterations in the U3 region of the long terminal repeat of an infectious thymotropic type B retrovirus

We isolated and characterized a type B thymotropic retrovirus (DMBA-LV) which is highly related to mouse mammary tumor virus (MMTV) isolates and which induces T-cell thymomas with a high incidence and a very short latent period. Regions of nonhomology between the DMBA-LV genome and the MMTV genome were identified by heteroduplex mapping and nucleotide sequence studies. In the electron microscope heteroduplex mapping studies the EcoRI-generated 5' and 3' fragments of the DMBA-LV genome were compared with the corresponding fragments of the MMTV (C3H and GR) genome isolated from mammary tumors. The results indicated that DMBA-LV contained a region of nonhomologous nucleotide sequences in the 3' half of the U3 region of the long terminal repeat (LTR). Nucleotide sequence studies confirmed these results and showed that in this region 440 nucleotides of the MMTV (C3H) sequences were deleted and substituted with a segment of 122 nucleotides. This substituted segment in the form of a tandem repeat structure contained nucleotide sequences derived exclusively from sequences which flanked the substitution loop. The distal glucocorticoid regulatory element was unaltered, and two additional copies of the distal glucocorticoid regulatory element-binding site were present in the substituted region. The restriction endonuclease map of the reconstructed molecular clone of DMBA-LV was identical to that corresponding to unintegrated linear DMBA-LV DNA present in DMBA-LV-induced tumor cell lines. Since the nucleotide sequences of the LTRs present in four different DMBA-LV proviral copies isolated from a single thymoma were identical, we concluded that they were derived from the same parental virus and that this type B retrovirus containing an alteration in the U3 region of its LTR could induce thymic lymphomas. Thus, DMBA-LV represents the first example of a productively replicating type B retrovirus that contains an LTR modified in the U3 region and that has target cell and disease specificity for T cells.

Endogenous murine leukemia viruses: frequency of radiation-activation and novel pathogenic effects of viral isolates

Female C57BL/6 and BALB/c mice were injected i.p. with 0.06 microCi/kg or 0.5 microCi/kg of the short-lived alpha-emitting radionuclide 224radium at 3-day intervals. Infectious N-ecotropic XC+, and xenotropic C-type retroviruses were activated in several tissues in both strains. In C57BL/6 mice the activation of ecotropic and xenotropic virus was dose-dependent as observed 4 weeks after the start of irradiation. In BALB/c mice a few animals showed activation of ecotropic virus after four weeks of irradiation. The expression of xenotropic virus was similar in irradiated mice and controls. Viral antigen, indicative for viraemia, was not detected in irradiated or control animals. Antiviral antibodies were found in both control and irradiated mice but higher titers were found in the irradiated mice. Bone tissue-derived N-tropic XC+ virus isolates were found to be non-oncogenic in newborn mice of the parental strain. In contrast, the same virus isolates induced a novel pattern of disease, such as osteopetrosis and osteomas together with malignant lymphomas in NMRI mice. The data indicate that the pattern of endogenous murine leukemia virus activation by internal alpha-irradiation is dependent on the dose rate, and on the genetics of the mouse strain.

Important role of the long terminal repeat of the helper Moloney murine leukemia virus in Abelson virus-induced lymphoma

The helper virus has been shown to play a critical role in the development of lymphoma induced by the defective Abelson murine leukemia virus (A-MuLV). Indeed, A-MuLV pseudotyped with some viruses, such as the Moloney MuLV, has been shown to be highly lymphogenic, whereas A-MuLV pseudotyped with other viruses, such as the BALB/c endogenous N-tropic MuLV, has been shown to be devoid of lymphogenic potential (N. Rosenberg and D. Baltimore, J. Exp. Med. 147:1126-1141, 1978; C. D. Scher, J. Exp. Med. 147: 1044-1053, 1978). To map the viral DNA sequences encoding the determinant of the lymphogenic potential of Moloney MuLV when complexed with A-MuLV, we constructed chimeric helper viral DNA genomes in vitro between parental cloned infectious viral DNA genomes from Moloney MuLV and from BALB/c endogenous N-tropic MuLV. Chimeric helper MuLVs, recovered after transfection of NIH 3T3 cells were used to rescue A-MuLV, and the pseudotypes were inoculated into newborn NIH Swiss, CD-1, and SWR/J mice to test their lymphogenic potential. We found that a 0.44-kilobase-pair PstI-KpnI long terminal repeat-containing fragment from the Moloney MuLV was sufficient to confer some, but not complete, lymphogenic potential to a chimeric virus (p7M2) in NIH Swiss and SWR/J mice, but not in CD-1 mice. The addition of the 3'-end env sequences (comprising the carboxy terminus of gp70 and all p15E) to the U3 long terminal repeat sequences restored the full lymphogenic potential of the Moloney MuLV. Our data indicate that the 3'-end sequences of the helper Moloney MuLV are somehow involved in the development of lymphoma induced by A-MuLV. The same sequences have previously been found to harbor the determinant of leukemogenicity and of disease specificity of Moloney MuLV when inoculated alone.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation of a Saccharomyces cerevisiae centromere DNA-binding protein, its human homolog, and its possible role as a transcription factor

A protein that binds specifically to Saccharomyces cerevisiae centromere DNA element I was purified on the basis of a nitrocellulose filter-binding assay. This protein, termed centromere-binding protein 1 (CP1), was heat stable and renaturable from sodium dodecyl sulfate (SDS), and assays of eluates from SDS gels indicated a molecular weight of 57,000 to 64,000. An activity with similar specificity and stability was detected in human lymphocyte extracts, and analysis in SDS gels revealed a molecular weight of 39,000 to 49,000. CP1-binding sites occurred not only at centromeres but also near many transcription units, for example, adjacent to binding sites for the GAL4-positive regulatory protein upstream of the GAL2 gene in S. cerevisiae and adjacent to the TATA element of the adenovirus major late promoter. A factor (termed USF) that binds to the latter site and stimulates transcription has been isolated from HeLa cells by others.

Identification of a new common provirus integration site in gross passage A murine leukemia virus-induced mouse thymoma DNA

The Gross passage A murine leukemia virus (MuLV) induced T-cell leukemia of clonal (or oligoclonal) origin in inoculated mice. To study the role of the integrated proviruses in these tumor cells, we cloned several newly integrated proviruses (with their flanking cellular sequences) from a single tumor in procaryotic vectors. With each of the five clones obtained, a probe was prepared from the cellular sequences flanking the provirus. With one such probe (SS8), we screened several Gross passage A MuLV-induced SIM.S mouse tumor DNAs and found that, in 11 of 40 tumors, a provirus was integrated into a common region designated Gin-1. A 26-kilobase-pair sequence of Gin-1 was cloned from two lambda libraries, and a restriction map was derived. All proviruses were integrated as a cluster in the same orientation within a 5-kilobase-pair region of Gin-1, and most of them had a recombinant structure of the mink cell focus-forming virus type. The frequency of Gin-1 occupancy by provirus was much lower in thymoma induced by other strains of MuLV in other mouse strains. Using somatic-cell hybrid DNAs, we mapped Gin-1 on mouse chromosome 19. Gin-1 was not homologous to 16 known oncogenes and was distinct from the other common regions for provirus integration previously described. Therefore, Gin-1 appears to represent a new common provirus integration region. The integration of a provirus within Gin-1 might be an important event leading to T-cell transformation, and the Gin-1 region might harbor sequences which are involved in tumor development.

Correlation of leukemogenic potential of murine retroviruses with transcriptional tissue preference of the viral long terminal repeats

Recombination studies have established that retroviral long terminal repeats (LTRs) are important genetic determinants of the viral capacity to induce hematopoietic tumors and to specify the type of cell making up the tumor. Plasmids containing LTRs of several murine leukemia viruses linked to the chloramphenicol acetyltransferase gene were tested in transient assays to measure relative rates of transcriptional activity in different types of hematopoietic cells. LTRs of the thymomagenic viruses SL3-3, Moloney leukemia virus, and a Moloney mink cell focus-forming virus all expressed to higher levels than other LTRs in T-lymphocyte cell lines. Conversely, the LTRs of Friend leukemia virus and a polycythemic spleen focus-forming virus expressed to higher levels than other LTRs in erythroleukemia cells. The LTR of nonleukemogenic Akv virus induced a relatively low level of activity compared with the others in all cells tested. Thus the relative level of LTR-driven expression in various types of cells corresponds to the type of tumor caused by the intact virus in vivo. These results provide direct evidence that the tissue specificity of the transcriptional activity of LTRs plays a critical role in determining the target cell for retroviral oncogenesis.

Disease specificity of nondefective Friend and Moloney murine leukemia viruses is controlled by a small number of nucleotides

Moloney murine leukemia virus induces T cell lymphomas after injection into NFS mice, whereas the nondefective Friend virus induces erythroleukemias. Previous studies showed that sequences encompassing the viral transcriptional signals in U3 are the primary determinant of this phenotype in recombinants between these two viruses. To more precisely identify the sequences responsible, we constructed additional recombinants, within U3, between Friend and Moloney viruses and assayed these recombinants for for their disease specificity. We found that a fragment 191 bases long that included the direct repeat (enhancer) region plus 22 nucleotides to its 3' side from Friend virus was sufficient to convert Moloney virus to a virus that induced only erythroleukemias. A 171-base-long fragment of Moloney virus, including just the direct repeat, converted Friend virus to a virus that induced primarily lymphomas (about 85% of mice injected). We also constructed Moloney and Friend virus variants with one rather than two copies of the enhancer element. These viruses retained their disease specificity, although they exhibited a marked increase in the latent period of disease induction. Together the results suggest that 25 or fewer nucleotide differences, lying within and also just 3' of the direct repeat, are the primary determinant of the distinct disease specificities of nondefective Friend and Moloney viruses.

Identification of a new common provirus integration site in gross passage A murine leukemia virus-induced mouse thymoma DNA

The Gross passage A murine leukemia virus (MuLV) induced T-cell leukemia of clonal (or oligoclonal) origin in inoculated mice. To study the role of the integrated proviruses in these tumor cells, we cloned several newly integrated proviruses (with their flanking cellular sequences) from a single tumor in procaryotic vectors. With each of the five clones obtained, a probe was prepared from the cellular sequences flanking the provirus. With one such probe (SS8), we screened several Gross passage A MuLV-induced SIM.S mouse tumor DNAs and found that, in 11 of 40 tumors, a provirus was integrated into a common region designated Gin-1. A 26-kilobase-pair sequence of Gin-1 was cloned from two lambda libraries, and a restriction map was derived. All proviruses were integrated as a cluster in the same orientation within a 5-kilobase-pair region of Gin-1, and most of them had a recombinant structure of the mink cell focus-forming virus type. The frequency of Gin-1 occupancy by provirus was much lower in thymoma induced by other strains of MuLV in other mouse strains. Using somatic-cell hybrid DNAs, we mapped Gin-1 on mouse chromosome 19. Gin-1 was not homologous to 16 known oncogenes and was distinct from the other common regions for provirus integration previously described. Therefore, Gin-1 appears to represent a new common provirus integration region. The integration of a provirus within Gin-1 might be an important event leading to T-cell transformation, and the Gin-1 region might harbor sequences which are involved in tumor development.

Isolation of a Saccharomyces cerevisiae centromere DNA-binding protein, its human homolog, and its possible role as a transcription factor

A protein that binds specifically to Saccharomyces cerevisiae centromere DNA element I was purified on the basis of a nitrocellulose filter-binding assay. This protein, termed centromere-binding protein 1 (CP1), was heat stable and renaturable from sodium dodecyl sulfate (SDS), and assays of eluates from SDS gels indicated a molecular weight of 57,000 to 64,000. An activity with similar specificity and stability was detected in human lymphocyte extracts, and analysis in SDS gels revealed a molecular weight of 39,000 to 49,000. CP1-binding sites occurred not only at centromeres but also near many transcription units, for example, adjacent to binding sites for the GAL4-positive regulatory protein upstream of the GAL2 gene in S. cerevisiae and adjacent to the TATA element of the adenovirus major late promoter. A factor (termed USF) that binds to the latter site and stimulates transcription has been isolated from HeLa cells by others.

Oncogenic retrovirus from spontaneous murine osteomas. II. Molecular cloning and genomic characterization

An N-ecotropic murine leukemia virus (OA MuLV), originally isolated from spontaneous osteomas of strain 101 mice, was molecularly cloned. The virus induces osteomas, osteopetrosis, and malignant lymphomas in NMRI mice. The cloned virus was analyzed by heteroduplex analysis, restriction enzyme mapping, and oligonucleotide mapping. The data show a very close relationship to the endogenous Akv prototype virus with some differences in the gag and the env region. The nucleotide sequence of the U3 region of OA MuLV LTR revealed a structure within the presumable enhancer region very similar to the U3 sequences of the FBJ murine sarcoma virus and its associated helper virus. The significance of these specific structures for the oncogenicity of the virus and the development of the typical disease pattern is discussed.

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

To analyze the emergence of radiation leukemia virus (RadLV) variants in primary X-ray-induced C57BL/Ka thymoma and to identify the virus responsible for the very high leukemogenic potential of passaged Kaplan strain BL/VL3 preparation, we cloned several primary and passaged ecotropic RadLV infectious genomes. By restriction analysis, we found that BL/VL3 cells harbor three related but different ecotropic RadLVs. Their restriction map differs significantly from those of primary RadLVs. Hybridization analysis also indicated that BL/VL3 and primary RadLVs differ in their p15E and long terminal repeat (LTR) regions. As compared with the LTR sequence of the putative parental endogenous ecotropic provirus, the LTR sequence of primary weakly leukemogenic RadLV has only one change, a C-rich sequence, generating a 6-base-pair direct repeat just in front of the promotor. The LTR of the primary nonleukemogenic RadLV only showed few base changes, mainly clustered in R and U5. The LTR from a moderately leukemogenic passaged BL/VL3 RadLV had conserved the C-rich sequence and acquired a 43-base-pair direct repeat in U3 and several other point mutations, small insertions, and deletions scattered in U3, R, and U5. All cloned primary RadLVs were fibrotropic, and some were weakly leukemogenic. All cloned BL/VL3 RadLVs were thymotropic and nonfibrotropic. The block of their replication was found to be after the synthesis of unintegrated linear and supercoiled viral DNA. Most of the BL/VL3 RadLVs were moderately leukemogenic, and one (V-13) was highly leukemogenic, being as virulent as the Moloney strain. We propose a model for the emergence of the RadLV variants and show that the virus responsible for the high leukemogenic potential of BL/VL3 preparation is a nondefective, ecotropic, lymphotropic, nonfibrotropic, unique retrovirus which most likely arose from a parental primary RadLV similar to those studied here.

Ecotropic and mink cell focus-forming murine leukemia viruses integrate in mouse T, B, and non-T/non-B cell lymphoma DNA

Structures of somatically acquired murine leukemia virus (MuLV) genomes present in the DNA of a large panel of MuLV-induced C57BL and BALB/c B and non-T/non-B cell lymphomas were compared with those present in MuLV-induced T-cell lymphomas induced in the same low-"spontaneous"-lymphoma-incidence mice. Analyses were performed with probes specific for the gp70, p15E, and U3-long terminal repeat (LTR) regions of ecotropic AKV MuLV and a mink cell focus-forming virus (MCF)-LTR probe annealing with U3-LTR sequences of a unique endogenous xenotropic MuLV, which also hybridizes with U3-LTR sequences of a substantial portion of somatically acquired MCF genomes in spontaneous AKR thymomas. The DNAs of both T- and B-cell tumors induced by neonatal inoculation with the highly oncogenic C57BL-derived MCF 1233 virus predominantly contain integrated MCF proviruses. In contrast, the DNAs of more slowly developing B and non-T/non-B cell lymphomas induced by poorly oncogenic ecotropic or MCF C57BL MuLV isolates mostly contain somatically acquired ecotropic MuLV genomes. Approximately 50% of the spontaneous C57BL lymphoma DNAs contain somatically acquired MuLV genomes. None of the integrated MuLV proviruses annealed with the MCF-LTR probe, which indicates a clear difference in LTR structure with a substantial portion of the somatically acquired MuLV genomes present in the DNA of spontaneous AKR thymomas. This study stresses a dominant role of MuLV with ecotropic gp70 and LTR sequences in the development of slowly arising MuLV-induced B and non-T/non-B cell lymphomas.

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.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Neurotropic Cas-BR-E murine leukemia virus harbors several determinants of leukemogenicity mapping in different regions of the genome

The infectious virus derived from the molecularly cloned genome of the neurotropic ecotropic murine Cas-BR-E retrovirus was previously shown to have retained the ability to induce hind-limb paralysis and leukemia when inoculated into susceptible mice (P. Jolicoeur, N. Nicolaiew, L. DesGroseillers, and E. Rassart, J. Virol. 45:1159-1163, 1983). To map the viral sequences encoding the leukemogenic determinant(s) of this virus, we used chimeric viral genomes constructed in vitro between cloned viral DNAs from the leukemogenic Cas-BR-E murine leukemia virus (MuLV) and from the related nonleukemogenic amphotropic 4070-A MuLV. Infectious chimeric MuLVs, recovered from NIH 3T3 cells microinjected with these DNAs, were inoculated into newborn NIH Swiss, SIM.S, and SWR/J mice to test their leukemogenic potential. We found that each chimeric MuLV, harboring either the long terminal repeat, the gag-pol, or the pol-env region of the Cas-BR-E MuLV genome, was leukemogenic, indicating that this virus harbors several determinants of leukemogenicity mapping in different regions of its genome. This result suggests that the amphotropic 4070-A MuLV has multiple regions along its genome which prevent the expression of its leukemogenic phenotype, and it also shows that substitution of only one of these regions for Cas-BR-E MuLV sequences is sufficient to make it leukemogenic.

Tissue selectivity of murine leukemia virus infection is determined by long terminal repeat sequences

Here we show that the tissue specificity of murine retrovirus infections is determined by the long terminal repeat (LTR) of an otherwise isogenic set of viruses. The isogenic viruses used for this study contain the coding gag, pol, and env genes of the avirulent Akv virus. Recombinant viruses that contain the LTR of a virus that induces T-cell leukemia lymphoma preferentially infect T lymphocytes. Viruses that carry the LTR of a virus that induces erythroleukemia preferentially infect non-T lymphoblastoid cell lines in the marrow and spleen. The Akv virus itself displays no tissue preference for hematopoietic cells. These experiments suggest that retroviruses that carry appropriate enhancer-promoters can be used to infect selectively specific target cells in animals.

Contribution of the gag and pol sequences to the leukemogenicity of Friend murine leukemia virus

Friend murine leukemia virus (F-MuLV) is a highly leukemogenic replication-competent murine retrovirus. Both the F-MuLV envelope gene and the long terminal repeat (LTR) contribute to its pathogenic phenotype (A. Oliff, K. Signorelli, and L. Collins, J. Virol. 51:788-794, 1984). To determine whether the F-MuLV gag and pol genes also possess sequences that affect leukemogenicity, we generated recombinant viruses between the F-MuLV gag and pol genes and two other murine retroviruses, amphotrophic clone 4070 (Ampho) and Friend mink cell focus-inducing virus (Fr-MCF). The F-MuLV gag and pol genes were molecularly cloned on a 5.8-kilobase-pair DNA fragment. This 5.8-kilobase-pair F-MuLV DNA was joined to the Ampho envelope gene and LTR creating a hybrid viral DNA, F/A E+L. A second hybrid viral DNA, F/Fr ENV, was made by joining the 5.8-kilobase-pair F-MuLV DNA to the Fr-MCF envelope gene plus the F-MuLV LTR. F/A E+L and F/Fr ENV DNAs generated recombinant viruses upon transfection into NIH 3T3 cells. F/A E+L virus (F-MuLV gag and pol, Ampho env and LTR) induced leukemia in 20% of NIH Swiss mice after 6 months. Ampho-infected mice did not develop leukemia. F/Fr ENV virus (F-MuLV gag and pol, Fr-MCV env, F-MuLV LTR) induced leukemia in 46% of mice after 3 months. Recombinant viruses containing the Ampho gag and pol, Fr-MCF env, and F-MuLV LTR caused leukemia in 38% of mice after 6 months. We conclude that the F-MuLV gag and pol genes contain sequences that contribute to the pathogenicity of murine retroviruses. These sequences can convert a nonpathogenic virus into a leukemia-causing virus or increase the pathogenicity of viruses that are already leukemogenic.

The involvement of a type-B retrovirus in the induction of thymic lymphomas

A highly leukemogenic virus (DMBA-LV) (in vivo leukemogenic titer 1-5 X 10(6) IU/ml, and 35-40 days to thymic lymphoma detection) is produced by a chemical carcinogen-induced transplanted thymic lymphoma. The virus preparation is a mixture of a type-B retrovirus highly related to exogenous type-B retroviral isolates and a biologically defective type-C retrovirus. The DNA of DMBA-LV-induced-tumors contains new type-B proviruses but no additional type-C proviruses could be detected. The leukemogenicity of DMBA-LV was completely neutralized by a monoclonal antibody against MMTV envelope glycoprotein, but was not affected by a broadly reacting Friend MuLV anti-gp70 serum which effectively neutralizes type-C ecotropic, xenotropic, and recombinant retroviruses and which completely abolishes the leukemogenic activity of Moloney leukemia virus. Three type-B mammary tumor-inducing retroviral isolates, while containing type-C retroviral sequences, were not leukemogenic. A further characterization of the type-C retroviral sequences present in DMBA-LV indicated that sequences characteristic of endogenous, nonxenotropic proviruses are present. In addition, using a variety of type-C-specific retroviral DNA probes, no evidence was obtained for the presence of a type-B-C-recombinant genome in DMBA-LV. Leukemogenesis was absolutely dependent upon the presence of a functional type-B retroviral envelope gp 52 and DMBA-LV does not appear to contain a leukemogenic retroviral type-C genome.

At least four viral genes contribute to the leukemogenicity of murine retrovirus MCF 247 in AKR mice

Nucleotide sequences encoding gp70, Prp15E, and the U3 region of the long terminal repeat (LTR) distinguish mink cell focus-forming (MCF) retroviruses that can induce leukemia in AKR mice from closely related MCF and ecotropic murine retroviruses that are nonleukemogenic in all inbred mouse strains tested (Lung et al., Cold Spring Harbor Symp. Quant. Biol. 44:1269-1274, 1979; Lung et al., J. Virol. 45:275-290, 1983). We used a set of recombinants constructed in vitro from molecular clones of leukemogenic MCF 247 and nonleukemogenic ecotropic Akv to separate and thereby directly test the role of these genetic elements in disease induction. Leukemogenicity tests of recombinants in AKR mice show that introduction of fragments containing either an MCF LTR or MCF gp70 coding sequences can confer only a very low incidence of disease induction on Akv virus, whereas an MCF type Prp15E alone is completely ineffective. Recombinants with an MCF 247 LTR in combination with MCF Prp15E are moderately oncogenic, whereas those with an MCF 247 LTR plus MCF gp70 coding segment are quite highly leukemogenic. Mice infected with the latter virus show a substantial increase in latent period of disease induction relative to MCF 247; this delay can be reduced when Prp15E, and hence the entire 3' half of the genome, is from MCF 247. Surprisingly, sequences in the 5' half of the genome can also contribute to disease induction. We found a good correlation between oncogenicity and recovery of MCF viruses from thymocytes of injected mice, with early recovery and high titers of MCF in the thymus being correlated with high oncogenicity. This correlation held for recombinants with either an MCF or ecotropic type gp70. Together, these results (i) demonstrate that at least four genes contribute to the oncogenicity of MCF viruses in AKR mice and (ii) suggest that recombinants with only some of the necessary MCF type genes induce leukemia because they recombine to generate complete MCF genomes. Although neither Akv nor MCF 247 is leukemogenic in NFS mice, recombinant viruses whose gp70 gene was derived from Akv but whose LTRs were derived from MCF 247 induced a low incidence of leukemia in this mouse strain.

The tandem direct repeats within the long terminal repeat of murine leukemia viruses are the primary determinant of their leukemogenic potential

To map the viral sequences encoding the leukemogenic determinant(s) of nondefective murine leukemia viruses (MuLVs), we constructed chimeric viral genomes in vitro between cloned viral DNAs from the highly leukemogenic Gross passage A (Gross A) MuLV and from the related nonleukemogenic BALB/c N-tropic MuLV. Infectious chimeric MuLVs, recovered from murine cells microinjected with these DNAs, were inoculated into newborn mice to test the leukemogenic potential of these viruses. We found that the U3 long terminal repeat region from Gross A genomes was sufficient to confer an intermediate leukemogenic potential to chimeric MuLVs. Sequencing data indicated that the U3 tandem direct repeat was responsible for this effect. Adding most of the Gross A p15E-coding sequences to the Gross A U3 long terminal repeat enhanced the leukemogenic potential of chimeric viruses significantly. Adding a larger 3'-end env region (all p15E-coding sequences and 345 base pairs of the carboxy terminus of gp70) to the Gross A U3 long terminal repeat restored the full leukemogenic potential of Gross A MuLV. Chimeric viruses harboring only the Gross A 3'-end env region were, however, nonleukemogenic. Similar chimeric MuLVs, constructed with genomes from the parental weakly leukemogenic BALB/c B-tropic MuLVs and nonleukemogenic BALB/c N-tropic MuLVs, were also studied. Our data indicate that the U3 tandem direct repeat sequences appear to be necessary and sufficient to confer some leukemogenic potential to MuLV. However, env 3'-end sequences, mostly the p15E-encoding sequences, are required for the expression of fully leukemic phenotypes.

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

The Moloney murine leukemia virus (MuLV) is a highly leukemogenic virus. To map the leukemogenic potential of Moloney MuLV, we constructed chimeric viral DNA genomes in vitro between parental cloned infectious viral DNA from Moloney and amphotropic 4070-A MuLVs. Infectious chimeric MuLVs were recovered by microinjection of recombinant DNA into NIH/3T3 cells and tested for their leukemogenic potential by inoculation into NIH/Swiss newborn mice. Parental Moloney MuLV and amphotropic 4070-A MuLV induced thymic and nonthymic leukemia, respectively, when inoculated intrathymically. With chimeric MuLVs, we found that the primary determinant of leukemogenicity of Moloney and amphotropic MuLVs lies within the 1.5-kilobase-pair ClaI-PvuI long terminal repeat (LTR)-containing fragment. The presence of additional Moloney env-pol sequences with the Moloney LTR enhanced the leukemogenic potential of a chimeric MuLV significantly, indicating that these sequences were also involved in tumor development. Since parental viruses induced different forms of leukemia, we could also map the viral sequences conferring this disease specificity. We found that the 1.5-kilobase-pair ClaI-PvuI LTR-containing fragment of Moloney MuLV was necessary and sufficient for a chimeric MuLV to induce thymic leukemia. Similarly, the same LTR-containing fragment of amphotropic MuLV was necessary and sufficient for a chimeric MuLV to induce nonthymic leukemia. Therefore, our results suggest that specific sequences within this short LTR-containing fragment determine two important viral functions: the ability to transform cells in vivo (leukemic transformation) and the selection of a specific population of cells to be transformed (disease specificity).