Long terminal repeats of endogenous mouse mammary tumour virus contain a long open reading frame which extends into adjacent sequences

Mouse mammary tumor virus (MMTV)-like DNA sequences in the breast tumors of father, mother, and daughter

Background

The diagnosis of late onset breast cancer in a father, mother, and daughter living in the same house for decades suggested the possibility of an environmental agent as a common etiological factor. Both molecular and epidemiological data have indicated a possible role for the mouse mammary tumor virus (MMTV), the etiological agent of breast cancer in mice, in a certain percentage of human breast tumors. The aim of this study was to determine if MMTV might be involved in the breast cancer of this cluster of three family members.

Results

MMTV-like envelope (env) and long terminal repeat (LTR) sequences containing the MMTV superantigen gene (sag) were detected in the malignant tissues of all three family members. The amplified env gene sequences were 98.0%–99.6% homologous to the MMTV env sequences found in the GR, C3H, and BR6 mouse strains. The amplified LTR sequences containing sag sequences segregated to specific branches of the MMTV phylogenetic tree and did not form a distinct branch of their own.

Conclusion

The presence of MMTV-like DNA sequences in the malignant tissues of all three family members suggests the possibility of MMTV as an etiological agent. Phylogenetic data suggest that the MMTV-like DNA sequences are mouse and not human derived and that the ultimate reservoir of MMTV is most likely the mouse. Although the route by which these family members came to be infected with MMTV is unknown, the possibility exists that such infection may have resulted from a shared exposure to mice.

Clonal isolation of different strains of mouse mammary tumor virus-like DNA sequences from both the breast tumors and non-Hodgkin's lymphomas of individual patients diagnosed with both malignancies

PURPOSE

In a previous study, we had detected the presence of mouse mammary tumor virus (MMTV)-like envelope (ENV) gene sequences in both the breast tumors and non-Hodgkin's lymphoma tissue of two of our breast tumor patients who had been diagnosed simultaneously with both malignancies. The aim of this study was to determine if MMTV-like DNA sequences are present in the breast tumors and non-Hodgkin's lymphomas of additional patients suffering from both malignancies and if so to characterize these sequences in detail.

EXPERIMENTAL DESIGN

DNA was extracted from formalin-fixed, paraffin-embedded tissue sample blocks of breast tumors and non-Hodgkin's lymphomas from patients suffering from both malignancies. A 250-bp region of the MMTV ENV gene and a 630-bp region of the MMTV long terminal repeat (LTR) open reading frame (ORF) that encodes the MMTV superantigen (sag) gene were amplified by PCR from the isolated DNA. Amplified products were analyzed by Southern blotting, cloned, and sequenced.

RESULTS

MMTV-like ENV and LTR sequences were detected in both the breast tumors and non-Hodgkin's lymphomas of 6 of 12 patients suffering from both malignancies. A novel mutant of the MMTV ENV gene was identified in these patients. Characterization of the MMTV-like LTR highly variable sag sequences revealed total or nearly total identity to three distinct MMTV proviruses from two different branches of the MMTV phylogenetic tree.

CONCLUSIONS

The presence of MMTV-like ENV and LTR sequences in both the breast tumors and non-Hodgkin's lymphomas of 6 additional patients suggests a possible involvement of these sequences in these two malignancies. MMTV-like LTR sequence homology to different MMTV proviruses revealed the presence of more than one strain of MMTV-like sequences in each individual suggesting the possibility of multiple infections in these patients.

Strain-specific expression of spliced MMTV RNAs containing the superantigen gene

The transmission of milk-borne or exogenous mouse mammary tumor virus (MMTV) requires infection of B cells in the gut-associated lymphoid tissue and expression of the superantigen (Sag) protein at the B-cell surface. Presentation of Sag at the B-cell surface is required for the transmission of MMTV to T cells and subsequent infection of the target mammary gland tissue. Because several different promoters have been reported for MMTV sag mRNA expression, we investigated whether the detection of spliced sag RNAs was dependent upon the cell type infected or the particular MMTV strain examined. In this study, we detected expression of spliced sag RNA from the standard promoter and from an internal U3 promoter in B-cell lines expressing endogenous Mtv-6 by RT-PCR, although expression from the standard promoter appeared to be at least 10-fold higher than that observed from the internal U3 promoter. Sag RNA originating from exogenous C3H MMTV was not observed from either of the U3 promoters in any cell type examined. However, spliced mRNAs containing the exogenous C3H MMTV, endogenous Mtv-8, or endogenous Mtv-17 sag genes could be detected from a previously described promoter in the envelope coding region regardless of the cell type infected. Because sag-specific RNAs can be initiated independently of the LTR promoters, there may be selection for independent control of MMTV sag and structural gene expression.

Identification of three human sequences with viral superantigen-specific primers

The open reading frame (ORF) in the long terminal repeat (LTR) of mouse mammary tumor virus (MMTV) has recently been shown to encode multiple products including a negative acting factor (Naf) and a superantigen (Sag). Expression of superantigens from endogenous MMTV loci in the mouse results in the deletion of whole classes of T cells. In a PCR approach, with primers to the MMTV ORF and hybridization to MMTV specific probes, we have identified three human sequences. Direct sequencing of PCR products revealed that one of these products is related to a human autoantigen that is conserved among many species and is expressed in testes and sperm. The second sequence that we have identified is novel, and no evidence for expression of this sequence could be obtained. Finally, the third ORF-like sequence is a new member of a previously described family of human endogenous retroviruses (RTVL-I). This sequence is transcribed in several human cell lines, including B lymphoblastoid cells, and is thus the first demonstration that an RTVL-I-related sequence can be expressed. Taken together, these findings raise the intriguing possibility that the human genome contains superantigen-like sequences, some of which are also related to endogenous retroviruses, that may influence the T cell repertoire.

Negative-acting factor and superantigen are separable activities of the mouse mammary tumor virus long terminal repeat

The open reading frame contained within the long terminal repeat (LTR) of mouse mammary tumor virus encodes Naf, a negative regulator of transcription, as well as a superantigen activity, Sag, which causes the deletion of specific classes of T cells. In the present study, the effect of Naf expression on different promoters and the coding requirements for Naf and Sag have been investigated. Sag activity was found to require only sequences in the LTR, whereas sequences located within the gag gene were additionally required for functional Naf activity. Surprisingly, both the classic promoter and a recently described promoter located in the LTR can give rise to both functional Naf and Sag. Further analysis of Naf revealed that the downregulatory effect was mediated by sequences located in the LTR and that heterologous promoters were also affected by Naf.

Cloning in a plasmid of an MMTV from a wild Chinese mouse: sequencing of the viral LTR

Plasmid subcloning by conventional techniques of full length exogenous mouse mammary viruses (MMTV) has not been realized because of the involvement of host-mediated structural changes in the viral gag gene. To circumvent this problem, an alternative subcloning method, excision of phagemid (pBluescript SK) from lambda ZAP II, was successfully used to subclone a novel exogenous MMTV (JYG-MMTV) provirus fragment containing an intact gag gene. Sequence analysis revealed that the LTR of this virus is significantly different from the LTR of C3H-MMTV in the U3 region.

Superantigen-induced immune stimulation amplifies mouse mammary tumor virus infection and allows virus transmission

Endogenous and infectious mouse mammary tumor viruses (MMTVs) encode in their 3' long terminal repeat a protein that exerts superantigen activity; that is, it is able to interact with T cells via the variable domain of the T cell receptor (TCR) beta chain. We show here that transmission of an infectious MMTV is prevented when superantigen-reactive cells are absent through either clonal deletion due to the expression of an endogenous MTV with identical superantigen specificity or exclusion due to expression of a transgenic TCR beta chain that does not interact with the viral superantigen. A strict requirement for superantigen-reactive T cells is also seen for a local immune response following MMTV infection. This immune response locally amplifies the number of MMTV-infected B cells, most likely owing to their clonal expansion. Collectively, our data indicate that a superantigen-induced immune response is critical for the MMTV life cycle.

Endogenous superantigen expression controlled by a novel promoter in the MMTV long terminal repeat

Endogenous superantigens are encoded by the open reading frame contained within the mouse mammary tumour virus long terminal repeat (MMTV LTR). Superantigen expression results in T-cell proliferation and, during early ontogeny, T-cell deletion. Here we identify a novel promoter located upstream of the previously described MMTV promoter. Transcripts from this promoter initiate within the U3 region of the MMTV LTR and splice to the acceptor for endogenous superantigen coding region. The novel U3 promoter is active in B lymphocytes, which are cognate antigen-presenting cells for endogenous superantigen, and is able to direct expression of superantigen in the absence of the previously described MMTV promoter.

Polyclonal and monoclonal antibodies monospecific to MMTV LTR orf protein produced in E. coli

Monoclonal and polyclonal antibodies specific to an open reading frame of the mouse mammary tumor virus long terminal repeat were generated using an open reading frame-beta-galactosidase fusion protein produced in E. coli. Both antibodies reacted with the open reading frame-beta-galactosidase fusion protein but not with beta-galactosidase alone using an immunoblotting technique. It is concluded that these antibodies were specific for the protein encoded by the open reading frame of the mouse mammary tumor virus long terminal repeat.

Factors controlling the expression of mouse mammary tumour virus

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V beta 17 T-cell deletion by endogenous mammary tumor virus in wild-type-derived mouse strain

The wild-type-derived mouse strain PWK possesses a beta-chain variable region V beta 17a2 allele, which is expressed on mature T cells as part of the T-cell receptor of most mice expressing I-E, whereas V beta 17 T cells are deleted in all I-E+ laboratory mice bearing a V beta 17a1 allele. However, (PWK x CBA/J)F1 progeny and the wild-type-derived mouse strain MAI, which possesses the V beta 17a2 allele, display deletion of V beta 17 T cells. Analysis of (PWK x CBA/J) x PWK and of (PWK x MAI) x PWK backcrosses demonstrates that endogenous mouse mammary tumor virus MTV-6 from CBA/J and a MTV from strain MAI control the clonal deletion of V beta 17a2 as well as V beta 3 T cells. Furthermore, among I-E- progeny of a (MAI x C57BL/6) x C57BL/6 backcross, we observed that mice inheriting MTV of MAI have a reduced level of V beta 17 T cells, suggesting that the clonal deletion of V beta 17a2 T cells can be mediated in the absence of the I-E molecule. The 3' long terminal repeat of MTV MAI was cloned and translation of the open reading frame was compared to those of MTV known to encode superantigens. Comparisons indicate that MTV MAI has significantly diverged from the other MTVs. However, MTV MAI and MTV-6 share a stretch of 11 identical amino acids at the C terminus, which is divergent in MTV reacting with other V beta s. This suggests that this region is involved in determining the specificity toward V beta s and has been selectively conserved through evolution of the Mus species.

Expression of the mouse mammary tumor virus ORF gene in cultured cells

We have recently shown that expression vectors harboring the open reading frame of the long terminal repeat region of mouse mammary tumor virus direct the synthesis of a product which acts as a superantigen in transgenic mice. The detection of the ORF protein has been hampered by the extremely low levels of expression observed in these mice, as estimated from the low levels of specific mRNA. To study the properties of the ORF protein, we attempted its expression in different cell types in culture. The experiments performed in yeast show that the ORF gene product is a glycoprotein of approximately 45 kDA. As expected from the derived primary sequence, the unglycosylated product made in the presence of tunicamycin has a molecular weight of 36 kDA. No secretion of the glycosylated protein was observed. Curiously, the full-length molecule was made in lower amounts than a truncated version which contains only the C-terminal half of the protein. Transfection experiments in different mammalian cells suggest that high expression of the ORF protein might have an adverse effect on survival of cells in culture.

Detection and characterization of a glycoprotein encoded by the mouse mammary tumor virus long terminal repeat gene

Mouse mammary tumor virus (MMTV) is a retrovirus that causes mammary tumors in susceptible mice. MMTV contains a unique open reading frame (ORF) in the unique 3' region of the proviral long terminal repeat (LTR) with the potential to encode a 36-kDa protein. However, the ORF gene product has not been detected in murine mammary tissues or cell lines. We utilized the baculovirus expression vector system to generate large amounts of the ORF protein. Putative ORF gene products of 36 and 45 kDa were detected as unique proteins in extracts of insect cells infected with recombinant baculovirus (LTR-ORF BV), and the identities of these proteins as viral gene products were confirmed immunologically. Antipeptide antisera were generated in rabbits against peptides chosen from computer-predicted hydrophilic regions of the ORF coding sequence. These antisera reacted specifically by immunoprecipitation and by immunoblot with the proteins expressed in LTR-ORF BV-infected insect cells, as well as with MMTV LTR ORF in vitro translation products. Polyclonal antisera were raised against two putative ORF protein species partially purified from insect cells. These sera specifically immunoprecipitated viral protein products translated in vitro. In vitro translation of MMTV LTR ORF transcripts in the presence of canine pancreatic microsomal membranes generated a higher-molecular-weight ORF gene product, indicating that the ORF protein is modified by N-linked glycosylation. This glycosylated ORF product comigrated with the larger ORF protein species produced in infected insect cells. The gp45 product was metabolically labeled with [3H] mannose, [3H] galactose, and [3H] N-acetyl-D-glucosamine in insect cells, whereas this incorporation was inhibited in the presence of tunicamycin. Digestion of gp45 with endoglycosidase H yielded the lower-molecular-weight ORF protein p36. These observations suggest that the ORF glycoprotein contains hybrid N-linked oligosaccharides. Demonstration of the modified nature of the ORF gene product will facilitate characterization of ORF protein expression in murine tissues.

Sequence analysis of the simian foamy virus type 1 genome

We have cloned the simian foamy virus type 1 genome (SFV1) and determined its nucleotide sequence. Analysis of this genome reveals, in addition to the usual genes encoding retroviral capsid, reverse transcriptase, and envelope protein (respectively, gag, pol, and env), two open reading frames (ORFs) between env and the long terminal repeat with partial homology to the human foamy virus (HFV) bel1 and bel2 genes. The first ORF could code for a polypeptide of 312 amino acids (aa) showing 40% homology with the HFV bel1 putative gene product. A more detailed analysis showed that the protein encoded by this ORF would have features characteristic of known trans-activating proteins. The second ORF could code for a polypeptide of 403 aa showing 38% homology with the putative HFV bel2 gene product. Moreover, the 5' extremity of the RNA genome can be folded into a secondary structure identical to the Tat-response element of human immunodeficiency viruses. A phylogenetic tree of retroviruses, including SFV1 and HFV, was constructed. It showed at the molecular level that Spumavirinae, previously classified on the basis of their morphology and their biological properties, constitute a separate group. The homology between SFV1 and HFV reaches 89% in the reverse transcriptase domain of the pol gene. but is much smaller in other parts of the genome.

naf, a trans-regulating negative-acting factor encoded within the mouse mammary tumor virus open reading frame region

The mouse mammary tumor virus (MMTV) long terminal repeat (LTR) open reading frame (ORF) encodes a negative acting factor (naf). In our test system, naf mediates its effect in trans on another MMTV provirus in which the 5' LTR has been replaced by that of Rous sarcoma virus. naf effects are evidenced at the level of transcriptional initiation rather than as reduced mRNA stability. The introduction of a premature termination codon into the MMTV LTR-encoded ORF abolishes the transcriptional down regulation localizing naf within the ORF. In addition, sequences in the gag/pol genes between +320 and +646 and between +3626 and +4590 relative to the site of transcription initiation are also involved in the MMTV-mediated transcriptional down regulation.

Gene expression in cells infected with gammaherpesvirus saimiri: properties of transcripts from two immediate-early genes

During productive infections of cells with the gammaherpesvirus, herpesvirus saimiri (HVS), a polyadenylated RNA of 2.2-2.4 kb accumulates to form a large fraction of virus-specified RNA. This transcript is from the virus thymidylate synthase (TS) gene and its synthesis, like that of late mRNAs encoding the virus structural proteins, is sensitive to an inhibitor of virus DNA synthesis (phosphonoacetic acid, PAA). Transcription which is insensitive to PAA occurs from many parts of the HVS genome, including the EcoRI-D, EcoRI-E, EcoRI-I, and HindIII-G fragments. A 1.6-kb RNA from EcoRI-I/E and a 1.3-kb RNA from HindIII-G accumulate in HVS-infected cells incubated in the continuous presence of cycloheximide, and thus represent immediate-early (IE) class transcripts. The 1.3-kb message from HindIII-G is the predominant stable RNA under these conditions; accumulation of the 1.6-kb transcript from EcoRI-I/E (which encodes the previously characterized 52-kDa IE phosphoprotein) is markedly more dependent on the multiplicity of infection. The sequence of a 2.5-kbp region of the HindIII-G fragment has been determined and a single major open reading frame is present within the boundaries of the 1.3-kb IE RNA. Comparison of the amino acid sequence of the encoded protein (IE-G) with current databases of protein sequences failed to demonstrate significant similarities with herpesvirus proteins, but did detect a significant similarity with a region of the protein specified by an open reading frame in the LTR of mouse mammary tumor virus. The function of the IE gene in HindIII-G and the basis for the distinctive multiplicity dependence of IE transcription from the 52-kDa gene remain to be established.

Molecular cloning, sequencing, and expression of functional bovine herpesvirus 1 glycoprotein gIV in transfected bovine cells

The gene encoding bovine herpesvirus 1 (BHV-1) glycoprotein gIV was mapped, cloned, and sequenced. The gene is situated between map units 0.892 and 0.902 and encodes a predicted protein of 417 amino acids with a signal sequence cleavage site between amino acids 18 and 19. Comparison of the BHV-1 amino acid sequence with the homologous glycoproteins of other alphaherpesviruses, including herpes simplex virus type 1 glycoprotein gD, revealed significant homology in the amino-terminal half of the molecules, including six invariant cysteine residues. The identity of the open reading frame was verified by expression of the authentic recombinant BHV-1 gIV in bovine cells by using eucaryotic expression vectors pRSDneo (strong, constitutive promoter) and pMSG (weak, dexamethasone-inducible promoter). Constitutive expression of gIV proved toxic to cells, since stable cell lines could only be established when the gIV gene was placed under the control of an inducible promoter. Expression of gIV was cell associated and localized predominantly in the perinuclear region, although nuclear and plasma membrane staining was also observed. Radioimmunoprecipitation revealed that the recombinant glycoprotein was efficiently processed and had a molecular weight similar to that of the native form of gIV expressed in BHV-1-infected bovine cells. Recombinant gIV produced in the transfected bovine cells induced cell fusion, polykaryon formation, and nuclear fusion. In addition, expression of gIV interfered with BHV-1 replication in the transfected bovine cells.

Identification of a second retrotransposon-related element in the genome of Physarum polycephalum

The repetitive fraction of the genome of the eukaryotic slime mould Physarum polycephalum is dominated by Tp1, a family of retrotransposon-like sequences. Tp1 elements are arranged in scrambled clusters probably arising from integration of the element into copies of its own sequence. The present report describes a second sequence family, Tp2, which has been identified within cloned DNA segments of scrambled Tp1 sequences. Like Tp1, the Tp2 element is structurally related to retrotransposons, having long terminal direct repeats and being flanked by an apparent target site duplication, but its relatively short length (1.68 kb) indicates that it is probably incapable of encoding all the functions necessary for its own mobilisation. Analysis of the coding potential of the Tp2 element supports this view, although a striking homology to a nucleic acid binding domain common to many retrotransposons was identified. As with Tp1, putative regulatory signals can be identified in the LTRs of Tp2. Identical arrangements of Tp2 with respect to Tp1 in more than one independently derived clone indicate that non-functional copies of Tp2 may be mobilised as part of a Tp1 transcriptional unit.

Construction and characterization of a packaging cell line for MMTV-based conditional retroviral vectors

A chimeric provirus in which the 5'LTR of a complete biologically active Mouse Mammary Tumour Virus (MMTV) proviral DNA has been replaced with the Rous Sarcoma Virus LTR has been constructed. Upon transfection into permissive cells, this provirus directs the synthesis of the MMTV gag and env structural proteins, but is impaired in packaging of the RNAs that encode these proteins. Supertransfection of these cells with MMTV based vector constructs results in the production of infectious recombinant virus at a higher efficiency than with previously described helper cell lines. Such a retroviral vector system based on MMTV will allow the study of the effects of conditional expression of inserted genes upon infected cells.

Molecular cloning and sequencing of the MTV-1 LTR: evidence for a LTR sequence alteration

The vertically transmitted Mtv-1 provirus is the primary causative factor of mammary neoplasia in certain C3Hf strains that lack the horizontally transmitted mouse mammary tumor virus (MMTV). The studies here report the molecular cloning of the germ line 4.5 kb Mtv-1 3' EcoRI fragment and sequencing of the 3' Mtv-1 LTR. The Mtv-1 LTR sequence is closely related to the 5' Mtv-11 LTR sequence also reported here, as well as to known Mtv-8 and MMTV LTR sequences in the portion of MMTV and Mtv-8 LTRs previously demonstrated to contain transcriptional regulatory sequences. A 91 bp unique sequence region, Mtv-1 bp 862 to 952, exists in the Mtv-1 LTR, which is upstream of the sequence homology with the MMTV transcriptional regulatory domain. The Mtv-1 unique sequence region is distinct from a 117 bp sequence, bp 862 to 978, in the Mtv-11 LTR sequence as well as reported Mtv-8 and MMTV LTR sequences, and is present in the germ line Mtv-1 5' and 3' LTR-containing restriction fragments. S1 nuclease mapping experiments of C3Hf/Se mammary tumor poly(A) RNA with the cloned Mtv-1 and Mtv-11 LTRs exhibited a specific set of S1 protected fragments demonstrating that Mtv transcripts which accumulate in C3Hf spontaneous mammary tumors are encoded by the Mtv-1 provirus.

Mouse mammary tumor virus proviruses in T-cell lymphomas lack a negative regulatory element in the long terminal repeat

The nucleotide sequences of long terminal repeats (LTRs) from several mouse mammary tumor virus (MMTV) proviruses acquired in mouse T-cell lymphomas were determined. All MMTV proviruses cloned from a C57BL/6 lymphoma contained an identical LTR deletion of 491 base pairs (approximately -655 to -165), whereas an MMTV provirus from a BALB/c T-cell lymphoma had a 430-base-pair deletion in the same U3 region. MMTV proviruses with LTR deletions were acquired in these tumors 10 times more frequently than proviruses with intact LTRs. Because the deletions removed a portion of the glucocorticoid response element or "regulated" enhancer, the transcriptional activity of the deleted MMTV LTRs was assessed in both transient expression and stable transfection experiments. Plasmids were constructed in which the deleted or full-length MMTV LTRs were placed upstream of the chloramphenicol acetyltransferase gene. Results from transfection experiments with these constructs showed that the basal expression of the deleted MMTV LTR in the absence of glucocorticoids was higher than that of the full-length Mtv-17 or C3H MMTV LTRs under the same conditions. Moreover, the C3H LTR with a similar deletion (-637 to -255) also promoted high basal levels of chloramphenicol acetyltransferase activity. These results, coupled with the observation in lymphomas of high basal levels of transcription from MMTV proviruses with deleted LTRs, suggested that these proviruses lack negative regulatory elements in their LTRs. Loss of the negative regulatory element may contribute to the selective propagation of proviruses with deleted LTRs.

A transcriptionally defective long terminal repeat within an endogenous copy of mouse mammary tumor virus proviral DNA

Mouse mammary tumor virus proviral DNA is endogenous to most inbred strains of mice but in many strains is not transcriptionally active. This inactivity may be due to defects in the proviruses themselves or to position effects mediated by DNA sequences flanking the proviral units. The transcriptional competence of long terminal repeats (LTRs) derived from endogenous proviral DNA at genetic loci Mtv-8, Mtv-9, and Mtv-17 of the C57BL/6 mouse strain was examined with a transient transfection assay in which gene expression was monitored by expression of chloramphenicol acetyltransferase. LTRs from Mtv-8 and Mtv-9 were able to direct glucocorticoid-induced chloramphenicol acetyltransferase expression in this assay, while the LTR from Mtv-17 was only about 5% as effective. Analysis of chimeric LTRs indicated that the glucocorticoid-inducible transcriptional enhancer element within the Mtv-17 LTR is active when linked to a functional promoter from Mtv-8, whereas the promoter from Mtv-17 is defective in directing hormone-induced gene expression, even when linked to the Mtv-8 glucocorticoid-responsive enhancer. The DNA sequence of transcriptional control regions of the LTRs of all three endogenous proviral units was determined; this analysis revealed that the source of the defect in Mtv-17 is a single G-to-A transition at position-75 with respect to the site of transcription initiation that resides within the previously defined binding site for the transcription factor nuclear factor 1. Competition experiments with a gel electrophoresis mobility shift assay indicated that the affinity of nuclear factor 1 for DNA derived from Mtv-17 is significantly less than for comparable sequences derived from Mtv-8.

Phorbol diester-inducible, cyclosporine-suppressible transcription from a novel promoter within the mouse mammary tumor virus env gene

The mouse T-cell lymphoma cell line EL4.E1 constitutively synthesizes mouse mammary tumor virus (MMTV) transcripts encoding either the entire proviral genome or segments of it. In addition to these conventional mRNAs, however, an mRNA of about 1 kilobase accumulates after induction of these cells with phorbol myristate acetate (PMA). The accumulation of this transcript is strongly inhibited by the immunosuppressive agent cyclosporin A. Its pattern of induction by PMA and suppression by cyclosporin A is thus the same as seen for several lymphokine mRNAs in these cells, including interleukin-2 and granulocyte-macrophage colony-stimulating factor. The short MMTV transcript is the most abundant PMA-induced transcript in EL4.E1 cells, but was not found in a series of other leukocyte tumor cell lines. It is initiated from a novel promoter within the env gene, and a segment of 1,161 nucleotides is then spliced out. The major part of the transcript is a copy of the long terminal repeat (LTR) of MMTV. The MMTV proviral genomes in these cells, and the short transcript, contain a 491-nucleotide deletion in the LTR compared with the normal MMTV provirus. The resulting open reading frame could encode a protein of molecular weight 22,800, which is a likely candidate for an LTR-related protein with a similar molecular weight recently described in this system (J. Racevskis, J. Virol. 58:441-449, 1986).

Multiple regulatory domains in the mouse mammary tumor virus long terminal repeat revealed by analysis of fusion genes in transgenic mice

Transcription initiated within the mouse mammary tumor virus (MTV) long terminal repeat (LTR) is regulated by glucocorticoids, androgens, and estrogen. However, expression of the virus in vivo and transcription of MTV LTR fusion genes in transgenic mice are not readily interpretable solely in terms of the influence of these hormones. To investigate whether there is a regulatory role for sequences within the LTR but outside the region known to be responsible for glucocorticoid induction, we have produced transgenic mice carrying genes in which various regions of the LTR have been linked to the human growth hormone gene. Analysis of expression of the fusion genes in these transgenic mice has demonstrated that the 5' end of the LTR can profoundly influence transcription initiated within the MTV LTR.

Mouse mammary tumor proviruses from a T-cell lymphoma are associated with the retroposon L1Md

Four Charon 4A clones containing mouse mammary tumor virus (MMTV) proviruses and their cellular flanking sequences were obtained from partial EcoRI libraries of a C57BL/6 T-cell lymphoma with both endogenous and newly acquired MMTV proviruses. The cellular flanking sequences of three of four MMTV proviruses contained DNA homologous to the 3' end of the long interspersed retroposon L1Md. Two of the three proviruses were newly acquired in the lymphoma DNA, and these MMTV proviruses appeared to be 5 kilobases downstream and in the same transcriptional orientation as the L1 sequence. The third provirus was endogenous Mtv-9 and was located less than 500 base pairs from the 3' end of L1. Seven additional clones containing MMTV proviruses were isolated from partial MboI libraries of a B6 T-cell lymphoma. Five of the seven clones contained L1 elements in the cellular DNA flanking MMTV DNA. At least two clones (including one with the Mtv-8 provirus) had multiple L1 copies flanking the MMTV provirus, and one clone contained a single MMTV long terminal repeat directly integrated into a truncated L1 sequence. Although the frequencies of B1 and L1 in random library clones were similar, only one MMTV-containing clone hybridized to the abundant repetitive element B1. These data suggest a nonrandom association between MMTV and L1Md.

Multiple regulatory domains in the mouse mammary tumor virus long terminal repeat revealed by analysis of fusion genes in transgenic mice

Transcription initiated within the mouse mammary tumor virus (MTV) long terminal repeat (LTR) is regulated by glucocorticoids, androgens, and estrogen. However, expression of the virus in vivo and transcription of MTV LTR fusion genes in transgenic mice are not readily interpretable solely in terms of the influence of these hormones. To investigate whether there is a regulatory role for sequences within the LTR but outside the region known to be responsible for glucocorticoid induction, we have produced transgenic mice carrying genes in which various regions of the LTR have been linked to the human growth hormone gene. Analysis of expression of the fusion genes in these transgenic mice has demonstrated that the 5' end of the LTR can profoundly influence transcription initiated within the MTV LTR.

Isolation of three kinds of human endogenous retrovirus-like sequences using tRNA(Pro) as a probe

Three kinds of human endogenous retrovirus-like sequences (HuERS-P1, 2 and 3) were isolated from a HeLa cell genomic library using the 3'-half fragment of proline tRNA as a hybridization probe. These elements contained putative primer binding sites complementary to the 3'-terminus of proline tRNA and long terminal repeats (LTRs) characteristic of retrovirus provirus. The LTR sequence of HuERS-P1 consisted of about 690 nucleotides and contained a CAT box, a TATA box and a polyadenylation signal. A complete unit of an Alu family sequence was inserted into the 5'-LTR of one of the clones. HuERS-P2 also contained a TATA box and a polyadenylation signal in its LTR (about 840 nucleotides long), but the LTR sequence of this element was quite different from that of HuERS-P1. Although clone HuERS-P3 contained only the 5'-LTR region, this LTR sequence contained a CAT box, a TATA box and a poly-adenylation signal and was quite similar to the LTR sequence of the recently isolated human retrovirus-related sequence HuRRS-P (Kröger, B. and Horak, I. (1987) J. Virol., 61, 2071-2075). Human and simian DNAs contain 10 to 40 copies of these elements, but mouse DNA does not contain these elements.

Nonrandom distribution of MMTV proviral sequences in the mouse genome

Integrated sequences of mouse mammary tumor virus (MMTV) have been localized in the genomes of five inbred mouse strains (Balb/c, C3H, DBA/2, A.TH, 129-SV) and one mammary tumor cell line (GR). Two major classes of MMTV sequences have been detected in mouse DNA fractions as obtained by Cs2SO4/BAMD (3,6-bis-(acetatomercurimethyl)dioxane) density gradient centrifugation. The first one corresponds to previously described endogenous sequences (Mtv loci), whereas the second one corresponds to endogenous sequences not previously known, and/or recently acquired; in the case of GR cells exogenous sequences may also be present in this class. The genome distribution is somewhat different for the two classes of sequences, the first one being practically only present in the lightest DNA segments of the mouse genome (GC congruent to 38%); the second one being also represented in heavier segments (GC congruent to 43%). This integration pattern suggests that "ancient" endogenous sequences are practically only localized in genome segments of roughly matching composition, whereas exogenous and recently acquired endogenous MMTV sequences may also be present in heavier fractions.

Complete nucleotide sequence of a milk-transmitted mouse mammary tumor virus: two frameshift suppression events are required for translation of gag and pol

We sequenced two recombinant DNA clones constituting a single provirus of the milk-transmitted mouse mammary tumor virus characteristic of BR6 mice. The complete provirus is 9,901 base pairs long, flanked by 6 base-pair duplications of cellular DNA at the site of integration. Five extensive blocks of open reading frame corresponding to the gag gene, the presumed protease, the pol and env genes, and the open reading frame orf within the long terminal repeat of the provirus were readily discernible. Translation of gag, protease, and pol involved three different translational reading frames to produce the three overlapping polyprotein precursors Pr77, Pr110, and Pr160 found in virus-infected cells. Synthesis of the reverse transcriptase and endonuclease therefore required two separate frameshifts to suppress the termination codons at the ends of the Pr77 and Pr110 domains. Direct evidence is presented for translational readthrough of both stop codons in an in vitro protein synthesis system.

A rare common integration site of proviruses of the mouse mammary tumor virus in P-type mammary tumors of mouse strain GR

The mouse mammary tumor virus (MMTV) can induce mammary tumors in mice by proviral activation of the cellular oncogenes int-1 or int-2. Activation of these genes, however, is observed in only a few hormone- and pregnancy-dependent mammary tumors of the mouse strain GR. To study the possible involvement of other oncogenes we cloned three MMTV proviral-host fragments (MT 40, 42, and 53) from different mammary tumors of GR with a single acquired MMTV provirus. From a genomic library of normal mouse DNA we isolated phages with insert DNAs that covered 20-30 kb of the uninterrupted regions. Suitable probes devoid of repetitive DNA sequences were isolated in order to screen other mammary tumors for MMTV proviral integrations in these regions. Only two mammary tumors, MT 40 and 42, showed integration of extra MMTV proviruses within the same region. The integrations occurred only 60 bp apart. The other mammary tumors, however, did not contain MMTV proviral integrations in this region, nor in the MT 53 region. Using mouse-hamster somatic cell hybrid DNA, the MT 40/42 integration region was assigned to mouse chromosome 7, and the second region, MT 53, to chromosome 16. The two regions bear no homology to known cellular oncogenes. We did not observe any mRNA being expressed from these cloned segments either in tumors or in normal mammary glands. These findings indicate that plaque(P)-type mammary tumors in mouse strain GR do not originate from MMTV provirus insertions in a particularly favored integration region, but that there may be a variety of integration sites in these tumors.

Partial transformation of mouse fibroblastic and epithelial cell lines with the v-myc oncogene

To investigate the role of the myc gene in mammalian cell transformation, plasmid constructs containing the v-myc oncogene and a co-selectable G418 resistance marker were introduced into both mouse fibroblasts (NIH-3T3) and bladder epithelial cells (BBN3 and BBN7). After transfection or microinjection of DNA, no transformed foci could be detected on confluent monolayers but, when the cells were cultured under conditions in which individual cells were allowed to grow and form colonies, morphological transformation was observed. Unlike ras-transformed NIH-3T3 cells, v-myc-transformed cells were unable to grow in serum-free medium and therefore still required exogenous growth factors. v-myc-transformed NIH-3T3 cells were poor at forming foci when co-cultivated with untransformed cells; however, the efficiencies could be increased by addition of EGF to the medium. Both v-myc-transformed fibroblasts and epithelial cells acquired the ability to grow in soft agar, though at efficiencies lower than the corresponding ras transformants. Subcutaneous inoculation of v-myc-transformed NIH-3T3 cells into nude mice resulted in no tumours within 6 weeks. After protracted periods (2-3 months) a few tumours were detected, but at a frequency barely above that for spontaneous tumour formation. Epithelial cells transformed by v-myc were either non-tumorigenic or gave a very low incidence of tumours. We conclude that the v-myc oncogene induces morphological changes and anchorage independence in immortal mouse fibroblasts and epithelial cell lines but further events are required for the cells to become tumorigenic.

Spleen specific expression of an MMTV related transcript associated with the Mtv-6 locus in BALB/c mice

We have detected an MMTV related transcript which is expressed in a spleen specific manner in BALB/cHeA mice. Using a recombinant inbred series between BALB/cHeA and STS/A mice (C X S RI series) we have identified RNA associated with the Mtv-3 locus of the STS/A strain. This transcript initiates at the same site in the MMTV LTR as already reported for Mtv-2 and Mtv-8. The novel spleen specific MMTV transcript in the BALB/cHeA strain has a different structure as compared to the transcripts associated with the Mtv-2, Mtv-3, or Mtv-8 loci. We have tentatively identified the Mtv-6 locus as the source of these unique transcripts.

Site-specific rearrangements in the long terminal repeat of extra mouse mammary tumor proviruses in murine T-cell leukemias

Extra MMTV proviruses in T-cell leukemias of GR and C57/BL10 mice contain alterations in their long terminal repeat (LTR) sequence. The four different leukemias studied contain different deletions, but common hallmarks were observed around the recombination sites. At the 5' end of the deletions we observed a common nonamer sequence, AGACAGGTG, in two leukemias and an almost identical sequence, AGAGCAGGTG, in two other leukemias. At the 3' end of the deletions we invariably found a common stretch of five nucleotides, TTAAA. Three of the four leukemias showed nonconserved crossover sites. The deletions in two leukemias were replaced with neighboring sequences, generating direct repeats. The MMTV LTR characteristic open reading frame and glucocorticoid response element were altered in all four rearranged MMTV LTRs. These results demonstrate site specific rearrangements in the LTR of extra MMTV proviruses in T-cell leukemias and suggest that these rearrangements might permit expression of MMTV in a new target cell.

Molecular cloning and long terminal repeat sequences of human endogenous retrovirus genes related to types A and B retrovirus genes

By using a DNA fragment primarily encoding the reverse transcriptase (pol) region of the Syrian hamster intracisternal A particle (IAP; type A retrovirus) gene as a probe, human endogenous retrovirus genes, tentatively termed HERV-K genes, were cloned from a fetal human liver gene library. Typical HERV-K genes were 9.1 or 9.4 kilobases in length, having long terminal repeats (LTRs) of ca. 970 base pairs. Many structural features commonly observed on the retrovirus LTRs, such as the TATAA box, polyadenylation signal, and terminal inverted repeats, were present on each LTR, and a lysine (K) tRNA having a CUU anticodon was identified as a presumed primer tRNA. The HERV-K LTR, however, had little sequence homology to either the IAP LTR or other typical oncovirus LTRs. By filter hybridization, the number of HERV-K genes was estimated to be ca. 50 copies per haploid human genome. The cloned mouse mammary tumor virus (type B) gene was found to hybridize with both the HERV-K and IAP genes to essentially the same extent.

Nucleotide sequence analysis of squirrel monkey retrovirus reveals a novel primer-binding site for tRNALys1,2

Nucleotide sequences of a DNA fragment containing the long terminal repeat (LTR) of squirrel monkey retrovirus (SMRV) were determined. Sequence analysis showed that the SMRV LTR is 456 base pairs (bp) long and is bounded by 2-bp inverted repeats. Within the U3 region, there are two 43-bp repeats and two 42-bp repeats which are homologous to each other. These repeats are likely to provide enhancer activities commonly observed in other enhancer sequences. Following the repeats are transcriptional regulatory sequences including a CAT box, a Goldberg-Hogness box, and a polyadenylation signal, all positioned within the U3 region of SMRV LTR. A 22-nucleotide sequence immediately downstream from the LTR was found to be complementary to tRNALys1,2, suggesting that tRNALys1,2 serves as the primer for the reverse transcription of SMRV viral RNA.

Expression of the protein product of the mouse mammary tumor virus long terminal repeat gene in phorbol ester-treated mouse T-cell-leukemia cells

Exposure of C57BL/6 mouse EL-4 T-cell leukemia cells to phorbol ester (12-O-tetradecanoylphorbol-13-acetate) (TPA) induced the synthesis of protein products encoded by the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) region. Analysis of TPA-treated EL-4 cells with antiserum raised against a synthetic peptide predicted by the MMTV LTR open reading frame sequence detected a polypeptide migrating in gels with an apparent molecular weight of 37,000 Mr, as well as three less prominent proteins with apparent molecular weights of 31,000, 34,000, and 39,000. Tryptic peptide analysis established the identity of the immunoprecipitated cellular proteins with the LTR proteins obtained from in vitro translation of MMTV genomic RNA. All four proteins were glycosylated and were derived from one initial nonglycosylated translation product of 21,000 Mr. The 21,000-Mr apoprotein could be detected after digestion with endoglycosidase F or pretreatment of cells with tunicamycin. Untreated EL-4 cells synthesized three species of MMTV mRNA: 35S, 24S, and 20S. TPA treatment resulted in an increased level of transcription of the three mRNAs and the appearance of a new 1-kilobase mRNA. At least 10 acquired MMTV proviruses are present in the EL-4 genome, and examination of the degree of proviral methylation revealed extensive demethylation. However, no qualitative differences in the state of proviral methylation were apparent between TPA-treated and untreated cells.

Molecular cloning of unintegrated closed circular DNA of porcine retrovirus

Viral DNA of unintegrated closed circular form was isolated from a swine kidney cell line (SKL) which was infected with a porcine retrovirus Tsukuba-1 (PRetV) produced from a swine malignant lymphoma-derived cell line. Shimozuma-1 and cloned using a lambda phage vector, Charon 21A. One of ten independent clones contained the 8.3 kb DNA fragment as an insert, which was thought to be a full length of viral DNA molecule carrying a long terminal repeat (LTR) sequence. We have analyzed this insert by mapping the recognition sites of some restriction endonucleases by Southern blot hybridization with appropriate probes.

A common mouse mammary tumor virus integration site in chemically induced precancerous mammary hyperplasias

Mammary carcinomas can be induced by chemical and hormonal as well as viral carcinogens. Irrespective of the class of inducer, these tumors develop in discrete stages, of which alveolar hyperplasia is one of the earliest identifiable. Since carcinogenesis by the mammary tumor virus is now thought to involve proviral activation of adjacent cell genes at specific loci, we sought to determine if a similar mechanism also played a role in chemical and hormonal carcinogenesis and if its role was stage specific. Three high-tumor-incidence BALB/c hyperplastic alveolar nodule outgrowths of two different etiologies were found to have exogenous mouse mammary tumor virus proviruses integrated at the same site in the genome. This common site of integration is not within the bounds of the int-1 and int-2 loci into which proviruses detected at these loci are clustered in MMTV-induced mammary tumors. All three HANs are commonly impaired in end-point differentiation. We propose that mouse mammary tumor virus integration at this site is responsible for a specific abnormality in differentiation associated with the preneoplastic phenotype.

Binding of the human glucocorticoid receptor to defined regions in the human growth hormone and placental lactogen genes

An in vitro competition assay was used to investigate whether binding sites for the human glucocorticoid receptor occur in the human genes for growth hormone (hGH) and placental lactogen (chorionic somatomammotropin, hCS). These genes display 95% sequence homology. Two receptor-binding regions were found in the hGH gene, one of which is located within 290 bp upstream, and one within 251 bp downstream from the transcription initiation site. Two binding regions homologous to those in the hGH gene were found in the hCS gene. The receptor-binding DNA fragment from the structural part of the genes, but not that from their promoter area, contained a sequence homologous to a 15-bp consensus sequence proposed earlier for the glucocorticoid receptor binding site. It is unlikely that the putative difference in glucocorticoid sensitivity between the hGH and hCS genes is accounted for by major differences in glucocorticoid receptor binding pattern.

Site-specific cleavage of left-handed DNA in pBR322 by lambda-tris(diphenylphenanthroline)cobalt(III)

The chiral complex tris(4,7-diphenyl-1,10-phenanthroline)cobalt(III), lambda-Co(DiP)3(3+), binds to and, with photoactivation, cleaves left-handed DNA helices, thereby providing a unique molecular probe for local DNA conformation. We have mapped the specific left-handed sites where lambda-Co(DiP)3(3+) cleaves in the plasmids pBR322 and pLP32, which is the derivative of pBR322 containing a Z-form d(C-G)16 insert. For pLP32, a primary cleavage is at the insert; for native pBR322, cleavage occurs at four discrete sites: 1.45, 2.3, 3.3, and 4.2 kilobase pairs. These sites correspond to segments of alternating purine-pyrimidines. Moreover, these positions map to the ends of the three distinct coding regions in pBR322: the tetracycline-resistance gene, the origin of replication, and either end of the ampicillin-resistance (beta-lactamase) gene. The locations of these left-handed segments suggest to us that Z-DNA might serve as a conformational punctuation mark to demarcate the ends of genes.

Rearrangements in the long terminal repeat of extra mouse mammary tumor proviruses in T-cell leukemias of mouse strain GR result in a novel enhancer-like structure

Male GR mice develop T-cell leukemia at low frequency late in life. These leukemia cells invariably contain large amounts of mouse mammary tumor virus (MMTV) RNA and MMTV proteins and have extra MMTV proviruses integrated in their DNA. We show here that the extra MMTV proviruses are all derived from the endogenous MMTV provirus associated with the Mtv-2 locus and that the T-cell leukemias are clonal with respect to the acquired MMTV proviruses. The extra MMTV proviruses in six transplantable T-cell leukemia lines studied had rearranged, shortened long terminal repeats (LTRs); each T-cell leukemia, however, had a different LTR rearrangement within its extra MMTV provirus. The alteration within the extra LTRs of T-cell leukemia line 42 involved deletion of 453 nucleotides and generation of a tandem repeat region consisting of regions flanking the deletion. This alteration generated a sequence similar to the adenovirus enhancer core sequence. The viral RNAs in the T-cell leukemias contained corresponding alterations in their U3 regions. These results demonstrate that expression of MMTV in T-cell leukemias of GR mice may be the consequence of the generation of a novel enhancer, which could also stimulate expression of any adjacent cellular oncogene.