Characterization of endogenous and exogenous mouse mammary tumor virus proviral DNA with site-specific molecular clones

Mouse mammary tumor virus infects human cells

Mouse mammary tumor virus (MMTV) has long been speculated to be involved in human breast cancer and more recently in human primary biliary cirrhosis. Despite complete proviral sequences markedly homologous to MMTV being identified in human breast cancer tissue, no convincing evidence has been presented to date that MMTV can infect human cells. Using both wild-type and a genetically marked virus (MMTV-EGFP), we show here the successful infection of a number of different human cells by MMTV. Furthermore, infection of human cells is shown to be almost as efficient as the infection of murine mammary epithelial cells. Sequencing of PCR products from integrated proviruses reveals that reverse transcription and integration of the viral genome has occurred as expected. Furthermore, sequencing of two independent MMTV proviral integration sites reveal them to be present only in the human and not in the mouse genome. Infection requires an intact MMTV envelope protein and is blocked either by heat inactivation of the virus or by specific neutralizing anti-MMTV serum, ruling out a nonspecific mechanism of viral transfer. Thus, MMTV can infect human cells and this finding provides a possible explanation for the detection by others of MMTV sequences in human breast cancer patients.

Localization of self-interacting domains within betaretrovirus Gag polyproteins

The Betaretrovirus genus is characterized by the ability to preassemble immature capsids within the cytoplasm. For Mason-Pfizer monkey virus (M-PMV) this ability depends in part upon the unique Internal Scaffold Domain (ISD) within the p12 region of Gag. In this study, we have further characterized the ability of M-PMV p12 to promote Gag-Gag interaction and have examined the Gag polyprotein of the related mouse mammary tumor virus (MMTV) to potentially identify a region with equivalent function. Using the yeast two-hybrid system, we confirmed that both Gag polyproteins strongly interact, primarily through the CA-NC regions, but also through additional domains N-terminal to CA. For M-PMV, this auxiliary interaction domain was p12. For MMTV, no single strongly self-interacting protein was identified. Instead, MMTV Gag appears to utilize the weak contributions of several protein domains to support the main interaction of its CA-NC. Our findings suggest that, in addition to the canonical NC "I-domain" interaction, MMTV Gag self-association results from the concerted action of multiple regions of the polyprotein while M-PMV Gag relies mainly on its p12 domain.

Cloning of an infectious milk-borne mouse mammary tumor virus (MMTV) DNA from a mammary tumor that developed in an endogenous MMTV-free wild mouse

Molecular characterization of infectious mouse mammary tumor viruses (MMTVs) has been hampered due to the problem of cloning a full-length exogenous virus into a plasmid. The present report describes our strategy for obtaining a full-length clone of an exogenous MMTV from a mouse mammary tumor that arose spontaneously in a wild Chinese mouse free of endogenous MMTV and shows that the cloned virus (JYG-MMTV) is expressed in rat RBA cells. Four-week-old C58/J x CBA/CaJ female mice, free of both endogenous and exogenous MMTVs, were injected with virus-secreting RBA cells. The progeny of these mice were bred, and their offspring were tested for the presence of MMTV. These third-generation mice were found to actively produce MMTV that was shed in their milk and transmitted to their offspring. The virus was detected not only in the mammary glands of these young mice, but also in their spleens and bone marrow. These results suggest that our plasmid-cloned exogenous JYG-MMTV is infectious. This virus can now be used effectively in manipulating the various genes of JYG-MMTV and other MMTV strains to understand their structure/function relationships.

Mouse mammary tumor virus and mammary tumorigenesis in wild mice

The current knowledge of the distribution of the mouse mammary tumor virus (MMTV) proviral genomes and the mechanism of mammary tumorigenesis by MMTV in mice, with the main emphasis on Asian feral mice, is reviewed. The relevant earlier discoveries on the mode of MMTV transmission are summarized to provide an outline of the biology of MMTV. Finally, the viral etiology of human breast cancer will be discussed.

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.

Mice deficient for the 55 kd tumor necrosis factor receptor are resistant to endotoxic shock, yet succumb to L. monocytogenes infection

The multiple biological activities of tumor necrosis factor (TNF) are mediated by two distinct cell surface receptors of 55 kd (TNFRp55) and 75 kd (TNFRp75). Using gene targeting, we generated a TNFRp55-deficient mouse strain. Cells from TNFRp55-/-mutant mice lack expression of TNFRp55 but display normal numbers of high affinity TNFRp75 molecules. Thymocyte development and lymphocyte populations are unaltered, and clonal deletion of potentially self-reactive T cells is not impaired. However, TNF signaling is largely abolished, as judged by the failure of TNF to induce NF-kappa B in T lymphocytes from TNFRp55-deficient mice. The loss of TNFRp55 function renders mice resistant to lethal dosages of either lipopolysaccharides or S. aureus enterotoxin B. In contrast, TNFRp55-deficient mice are severely impaired to clear L. monocytogenes and readily succumb to infection. Thus, the 55 kd TNFR plays a decisive role in the host's defense against microorganisms and their pathogenic factors.

CD4 expression is differentially required for deletion of MLS-1a-reactive T cells

Clonal deletion of thymocytes expressing potentially self-reactive T cell receptors (TCRs) occurs during thymocyte ontogeny. Mice deficient for CD4 expression provide a unique model system to study the contribution of the CD4 molecule in negative selection of T cells reactive against the major histocompatibility complex class II-associated retroviral self-superantigen, Mls-1a. In the presence of Mls-1a determinants, mature CD8+ T cells expressing V beta 6, 8.1, and 9 were deleted in CD4-deficient mice, thus demonstrating that TCR affinity for Mls-1a is sufficient for deletion and that a signal through CD4 was not required. However, in instances where the TCR affinity for Mls-1a is low, as in the case of V beta 7+ T cells, CD4 expression was required for clonal deletion. These results demonstrate that for Mls-1a-mediated clonal deletion of T cells, the requirement for the accessory or coreceptor function of CD4 depends on the affinity of the TCR.

Isolation of a pathogenic clone of mouse mammary tumor virus

Exogenous mouse mammary tumor virus (MMTV) was cloned from a GR mammary tumor. Clone lambda GRT39 contained a full-length integrated MMTV(GR) provirus and both 5' and 3' host flanking DNA. The lambda GRT39 provirus had no apparent structural changes associated with cloning and retained the exogenous MMTV gag gene poison sequence. When introduced into rat mammary adenocarcinoma LA7 cells, the lambda GRT39 provirus was fully expressed. lambda GRT39-transfected LA7 cells made MMTV RNA, had gp52 SU protein on the cell surface, and produced B-type retrovirus particles characteristic of MMTV. Mammary tumors developed in hormone-stimulated BALB/c females injected with MMTV from lambda GRT39-transfected LA7 cells [MMTV (lambda GRT39)]. The tumors had new, clonally integrated copies of the MMTV(lambda GRT39) provirus and were expressing MMTV antigen. These data indicate that the lambda GRT39 provirus is biologically active and pathogenic.

Construction of a clonable, infectious, and tumorigenic mouse mammary tumor virus provirus and a derivative genetic vector

Molecular genetic studies of mouse mammary tumor virus (MMTV) have been hampered by the difficulty of cloning proviruses of milk-borne strains because of inhibitory sequences located in the gag gene. To surmount this barrier we have constructed a hybrid MMTV provirus composed of clonable 5' sequences (encompassing gag) from an endogenous MMTV provirus of C3H mice (Mtv-1) and 3' sequences from the milk-borne strain of MMTV in C3H mice, MMTV(C3H). Virions produced from XC cells transfected with this hybrid provirus are infectious in cell culture and tumorigenic in BALB/cJ mice. A vector derived from this provirus, containing the neomycin phosphotransferase gene (neo) and origins of replication from simian virus 40 and pBR322, is capable of transferring G418 resistance by virus infection in cell culture when supplied with viral proteins from either MMTV(C3H) or the hybrid MMTV. Expression of both hybrid and vector proviruses is inducible by dexamethasone in infected cells.

Expression of human sequences related to those of mouse mammary tumor virus

Sequences related to those of the mouse mammary tumor virus (MuMTV) genome have been cloned from human DNA by screening a library prepared from the DNA of a human breast cancer cell line with MuMTV gag-pol DNA. Nine distinct groups of (MuMTV-related) sequences were identified among 100 lambda recombinants by cross-hybridization experiments with subcloned fragments containing gag-pol-related DNA. The largest group, of 64 recombinants, contains the MuMTV-related sequences cloned by others. The other eight groups contain MuMTV-related sequences that have not been described previously. The gag-pol regions of one recombinant from each of the nine groups were hybridized to RNA prepared from five human breast cancer cell lines, from placenta, and from two cell lines derived from other malignancies. RNAs were detected by probes for seven of the groups. The RNAs ranged in size from 1.2 to 12 kilobases. Probes for six of the groups detected large RNAs that could represent transcripts of full-length proviral DNA. Two of the probes detected RNA in one breast cancer cell line only. Most of the RNAs were detected in more than one cell line.

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.

Insertion elements and transitions in cloned mouse mammary tumour virus DNA: further delineation of the poison sequences

The provirus of mouse mammary tumour virus (MMTV) is reputed to contain sequences within the viral gag gene that prevent or inhibit its propagation as a recombinant DNA clone in Escherichia coli. Here we report the successful isolation of several lambda and plasmid clones comprising the 5' virus-host DNA junction fragments from integrated MMTV proviruses in BR6 mice. Although the lambda clones appeared intact, almost all of the plasmids were found to contain the bacterial insertion sequences IS1 or IS2 within a small region of the gag gene. One nondisrupted clone was recovered which had undergone multiple G to A transitions, some of which created stop codons in gag. These results have provided more precise information as to the location of the poison sequences and are discussed in relation to possible explanations for the phenomenon.

Structure of a human retroviral sequence related to mouse mammary tumor virus

Human sequences that are related to the mouse mammary tumor virus (MuMTV) genome were cloned from breast tumor cell DNA. Of 100 recombinants, only 1 hybridized with two different probes from separate regions of the MuMTV genome (gag-pol and long terminal repeat [LTR]). This sequence, NMWV 4, was shown to have a proviruslike structure. Hybridization to digests of normal and tumor cell DNA indicated that NMWV 4 and a few closely related sequences are endogenous to the human genome. The regions that contain homology to the MuMTV LTR were sequenced. Long repeated sequences with the hallmarks of retroviral LTRs were identified. The NMWV 4 LTR contains transcription initiation and termination signals and is flanked by a polypurine tract (5' LTR) and a primer-binding site (3' LTR). The primer-binding site is complementary to tRNA lysine, the primer used by MuMTV and HTLV-III. The polypurine tract is also similar to those of these two retroviruses.

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.

Characterization, chromosome assignment, and segregation analysis of endogenous proviral units of mouse mammary tumor virus

In the course of analyzing sites of proviral integration in tumors induced by mouse mammary tumor virus (MMTV), we have isolated recombinant DNA clones corresponding to the 5' and 3' ends of four endogenous MMTV proviruses present in BALB/c and BR6 mice. This has permitted the structural characterization of each locus by detailed restriction mapping and the preparation of DNA probes specific for the cellular sequences flanking each provirus. These probes have been used to trace the segregation patterns of the proviruses, designated Mtv-8, Mtv-9, Mtv-17, and Mtv-21, in a panel of inbred strains of laboratory mice and to map Mtv-17 and Mtv-21 to mouse chromosomes 4 and 8, respectively. The unambiguous resolution of these four proviruses on Southern blots has greatly facilitated the analysis of other endogenous MMTV proviruses in these inbred mice.

Restriction endonuclease map of endogenous mouse mammary tumor virus loci in GR, DBA, and NFS mice

Mouse mammary tumor virus (MMTV) is integrated in the genome of most mice as an endogenous provirus. Two of these MMTV proviral loci (Mtv-1 and Mtv-2) are associated with virus expression and tumorigenicity. We prepared restriction endonuclease maps of the endogenous MMTV proviruses in two strains, DBA and GR, which contain the Mtv-1 and Mtv-2 loci, plus a third strain, NFS, which has a low mammary tumor incidence. We find that all these mouse strains have certain MMTV loci in common even though their origins are widely divergent. We also find that some integrated MMTV proviruses appear to have undergone alterations or deletions when compared with MMTV exogenous proviral DNA. We have thus made a comprehensive characterization of MMTV loci in these mouse strains which could serve as a basis for the study of their differences in expression.

Molecular cloning and characterization of mouse mammary tumor proviruses from a T-cell lymphoma

Five mouse mammary tumor virus proviruses and their flanking cellular DNA sequences have been cloned from a transplanted C57BL/6 (B6) T-cell lymphoma containing additional copies of mouse mammary tumor virus DNA. Characterization of these proviruses and their flanking DNA indicates that B6 lymphomas contain many newly integrated mouse mammary tumor virus copies synthesized by a mechanism(s) which generates polymorphism or deletions or both.

Production of mouse mammary tumor virus upon transfection of a recombinant proviral DNA into cultured cells

We have investigated the intracellular proteins synthesized in rat XC and feline kidney cells transfected with endogenous mouse mammary tumor virus (MMTV) proviral DNA. The endogenous provirus GR40, associated with the Mtv-8 locus, directs the synthesis of gag proteins indistinguishable from those found in MMTV-infected cells. The env precursor Pr73env and the mature gp52 proteins could not be detected in these cells. Instead an env-related protein of 68K is synthesized. In contrast to this endogenous provirus, a cloned exogenous proviral variant directs the synthesis of apparently normal env proteins upon transfection into the same cell lines. These results suggest that the env gene of the endogenous MMTV provirus GR40 is defective. The exogenous proviral variant is not expected to synthesize virus particles since it carries a rearrangement in the gag gene. In order to obtain an MMTV provirus capable of correctly expressing both gag and env functions, we have constructed a hybrid endogenous-exogenous provirus containing the 5' long terminal repeat (LTR)-gag of GR40 and the pol-env-3' LTR of the exogenous provirus. Upon transfection into feline kidney cells, this hybrid provirus directed the synthesis of apparently authentic gag and env proteins. Further, virus particles can be detected in the culture medium of the transfected cells by electron microscopy. Viral proteins obtained from viral particles banded in a sucrose gradient were detected by immunoprecipitation.

Proviral unit II of endogenous mouse mammary tumour virus is selectively amplified and expressed in C57B1/10 mammary tumours induced by non-viral carcinogens

Restriction enzyme analysis and molecular hybridization assay of DNA isolated from C57B1/10 mammary adenocarcinomas induced by a combination of dimethylbenzanthracene, oestrogen, and prolactin, revealed the presence of four extra copies of endogenous mouse mammary tumour virus (MMTV). PstI restriction pattern of the amplified proviral sequences indicated their identity with the proviral Unit II of endogenous MMTV. The amplified proviruses are hypomethylated and expressed in a hormone-dependent fashion. Their internal structure is slightly modified, since an additional EcoRI recognition site is present within the proviral genomic DNA. Selective amplification of Unit II MMTV provirus in the course of mammary tumourigenesis initiated by chemical carcinogens and hormones is compatible with the accepted multifactorial nature of this process, and is interpreted in terms of the insertional mutagenesis model for MMTV-induced oncogenesis. However, sequences of cellular DNA, adjacent to the amplified Unit II proviruses, show no homology to the integration domains int-1 and int-2 common to exogenous MMTV.

Sequence organization and molecular cloning of mouse mammary tumor virus DNA endogenous to C57BL/6 mice

The sequence organization of mouse mammary tumor virus DNA endogenous to the C57BL/6 inbred mouse strain was characterized by Southern blot analysis, utilizing probes specific for particular regions of the mouse mammary tumor virus provirus and by molecular cloning of endogenous mouse mammary tumor virus DNA. The genome of C57BL/6 mice contains three apparently intact, endogenous proviral units; two of these units comprise the Mtv-8 (unit II) and Mtv-9 (unit III) genetic loci that are also present in the DNA of BALB/c mice. The third unit is defined by EcoRI restriction fragments of 10.0 and 8.4 kilobases that contain the 5' and 3' portions of the provirus, respectively. This unit, termed unit XI and encoded by the genetic locus Mtv-17, has not been previously recognized in C57BL/6 DNA, but it can be clearly distinguished from the proviral units at Mtv-8 and Mtv-9 by Southern blot analysis under appropriate conditions. The proviral unit at Mtv-17 is not present in BALB/c DNA. DNAs comprising the entire Mtv-8 locus and the 3' portions of Mtv-9 and Mtv-17 were cloned. Analysis of the cloned DNA revealed no obvious deletions or rearrangements that would render proviral DNA defective; however, these endogenous genes are normally not transcriptionally active.

Interaction of the heterozygous nude gene with the asplenia trait in mammary tumorigenesis

The BALB/c mouse strain has been shown to contain endogenous mouse mammary tumor virus (MMTV) proviral sequences. However, no exogenous MMTV particles have been detected in their tissues. Female BALB/c mice from our colonies exhibit a very low incidence of spontaneous mammary tumors (SMT); less than 1% at up to 20 mo of age. Immunodeficient BALB/c mice heterozygous for the nude gene (nu/+, +/+), for the dominant hemimelia gene associated with asplenia (+/+, Dh/+), or for both traits (nu/+, Dh/+) have been examined for SMT incidence and the presence of MMTV proviruses. Based on restriction digestion with Eco RI, Bam HI, and Pst I, the immunodeficient mice have an MMTV provirus copy number and organization identical to the BALB/cCrgl strain. This MMTV DNA pattern is distinct from the MMTV proviruses in C3H/He, C57BL/6J and CBA/CaJ mice, which were parental strains of the immunodeficient mutants. Normal female BALB/c or BALB/c heterozygous for the asplenic trait do not develop significant numbers of SMT at up to 19 mo of age. In contrast, an incidence of 23.8% and 57.7% SMT was observed in BALB/c nu/+ heterozygotes, and in BALB/c nu/+, Dh/+ heterozygotes, respectively. These results indicate that agenesis of the spleen, concomitant with the presence of the heterozygous nude gene, contribute to a high incidence of SMT in the low-SMT BALB/c mouse strain.

Organization and expression of mouse mammary tumor virus sequences in normal and neoplastic C3Hf/HeSed mouse tissues

The organization and expression of germinally transmitted mouse mammary tumor virus (MMTV) proviruses in C3Hf/HeSed mouse tissues were examined. Digestion with the restriction enzymes EcoRI, BamHI, and HindIII and hybridization with cloned probes specific for the long terminal repeat and the 5' and 3' regions of the MMTV genome revealed three full-length (units Ib, II, and V) and two subgenomic (units I and IX) MMTV proviruses in C3Hf/HeSed mouse germ line DNA. The EcoRI fragments (15.0 and 5.7 kilobase pairs [kbp]) that contained unit Ib were previously described as separate, subgenomic MMTV proviruses. The methylated state of each full-length MMTV provirus was examined in DNA from C3Hf/HeSed mouse livers, spleens, mammary glands, and mammary tumors by digestion with EcoRI or BamHI in combination with the methyl-sensitive restriction enzymes HhaI or HpaII. Unit Ib contained HhaI- and HpaII-sensitive sites in spleen, mammary gland, and mammary tumor DNA but was completely methylated in liver DNA. Units II and V contained HhaI- and HpaII-sensitive sites in mammary gland and mammary tumor DNA, but the sites were extensively methylated in spleen and liver DNA. The HhaI-sensitive sites were mapped to the 5' end of the 5' and 3' long terminal repeats of each full-length MMTV provirus. C3Hf/HeSed mouse tissue RNA was examined for MMTV transcripts. Mammary glands contained MMTV RNA species of 9.0, 3.8, and 1.7 kb. Mammary tumors contained high levels of the 9.0- and 3.8-kb transcripts but lacked the 1.7-kb species. A very low level of the 3.8-kb MMTV transcript was present in spleens. Livers lacked detectable MMTV RNA. These results implicate mammary tissue as the site of unit V activation in the formation of MMTV virions.

The methylation pattern of endogenous mouse mammary tumor virus proviral genes is tissue specific and stably inherited

The methylation pattern of mouse mammary tumor virus (MMTV) proviral genes endogenous to the mouse strains C3H, 020, FM/JmsA, C57BL6, and BALB/c were investigated in various organs and mammary tumor tissue. Digestion of DNA with EcoRI or with EcoRI and HpaII followed by Southern blotting analysis and hybridization to a nick-translated MMTV DNA, allowed the distinction between the fully methylated and hypomethylated gene copies. MMTV proviral gene methylation was found to be organ specific, and the methylation pattern is stably inherited. The same proviral units present in different strains of mice exhibit the same organ-specific methylation patterns. Although proviral genes are normally heavily methylated in all tissues, hypomethylation of endogenous proviral genes was found in organs not known to express MMTV.

Expression of an 86-kilodalton glycoprotein in an idiopathic mammary adenocarcinoma of a BALB/c mouse

An epithelial cell line (BALB/c-ST) was established from a mammary adenocarcinoma that developed spontaneously in a 20-month-old BALB/c mouse. Like uninfected normal tissues, the cell line was found to contain three endogeneous murine mammary tumor virus (MuMTV) proviruses, but MuMTV particles and antigens were not detected. The cultured cells, after being inoculated subcutaneously, produced tumors in syngeneic mice, and sera from a high percentage of the tumor-bearing mice specifically immunoprecipitated an 86-kilodalton antigen from the extracts of BALB/c-ST cells. This antigen was found to be glycosylated, but whether or not it was exposed to the cell surface could not be demonstrated by cell-surface iodination and immunoprecipitation studies. The 86-kilodalton antigen, a glycoprotein designated gp86, was not detected in immunoprecipitates from extracts of normal mammary cells or from the extracts of GR, C3H, and BALB/cfC3H mammary tumor cells. After being infected in vitro with RIII-derived MuMTV but not with C3H-derived MuMTV, the BALB/c-ST cells appeared to undergo a phenotypic change in that they did not produce tumors in syngeneic mice, and the expression of gp86 was inhibited. Our results indicate that the expression of gp86 in the mammary cells of BALB/c mice is a consequence of neoplastic transformation and that MuMTV infection modulates its expression.

The human genome contains multiple sequences of varying homology to mouse mammary tumour virus DNA

Sequences related to the mouse mammary tumour virus (MuMTV) DNA were isolated from a genomic library of human DNA by screening under conditions of relaxed stringency. It is estimated that there are in the order of 50 MuMTV-like sequences per haploid genome and that the homology between the different human sequences and MuMTV varies by 15%.

Characterization and chromosomal location of endogenous mouse mammary tumor virus loci in GR, NFS, and DBA mice

Endogenous mouse mammary tumor virus (MMTV) proviral copies were characterized in three genetically dissimilar mouse strains: GR, a high-tumor-incidence strain bred in Europe that carries an MMTV proviral copy associated with early mammary tumors; DBA, a high-tumor-incidence laboratory strain bred in the USA with an endogenous copy that is associated with MMTV antigen expression in the milk; and NFS, a recently inbred line of the low-tumor-incidence NIH Swiss mouse. MMTV proviral loci were studied using restriction endonuclease analysis and the Southern transfer procedure in genetic crosses and in somatic cell hybrids. By studying the segregation of MMTV-specific EcoRI, BamHI, and PstI fragments, the organization of these fragments into MMTV proviral loci was determined and it was shown that (1) many homologous proviral loci are present in these three mouse strains, (2) these MMTV proviruses differ in their pattern of internal restriction sites, and (3) the MMTV loci are distributed on multiple chromosomes including 1 and 7.

Sequences related to mouse mammary tumor virus genome in tumor cells and lymphocytes from patients with breast cancer

Cloned murine mammary tumor virus (MuMTV) sequences allowed us to search for murine mammary tumor virus related sequences in the DNA of surgically removed human breast tumors. Out of 28 tumors so far examined two were found to contain an Eco RI DNA fragment homologous to the long terminal repeat-group antigen (LTR-Gag) and the Envelope (Env) sequences of MuMTV. We have taken the lymphocytes of these patients and cultured them. Rapid growth of lymphocytes, mostly of T origin, occurred in the presence of T cell growth factor (TCGF). Whereas DNA extracted from fresh lymphocytes is negative, that extracted from the 3-day cultured lymphocytes showed MuMTV related sequences. Long term cultures of T cells and a similar culture derived from a healthy person donor were negative at all stages. DNA extracted from the Ebstein Barr Virus-transformed B cells of the patient does not contain the MuMTV related sequences.

Amplification and novel locations of endogenous mouse mammary tumor virus genomes in mouse T-cell lymphomas

Endogenous mouse mammary tumor virus genomes are amplified and located in novel cell DNA sequences in many mouse T-cell lymphomas. Transplanted tumors recovered from the same mouse strain and shown to be of independent origin by chromosomal analysis, by the presence of JH immunoglobulin gene rearrangements, or by the integration patterns of exogenous Moloney MuLV genomes frequently showed similar patterns of novel mouse mammary tumor virus-containing cell DNA fragments. This process of amplification and relocation can occur within a limited number of cell generations and in C57BL/6 mice does not lead to the synthesis of mature virus-encoded proteins. In some instances, amplified mouse mammary tumor virus genomes contained novel restriction cleavage sites in the gag-pol region. The restricted time course of occurrence, lack of synthesis of mature virion proteins, and apparent site specificity indicate that this process of retrovirus amplification differs significantly from virus replication after exogenous infection.

Correlation of glucocorticoid receptor binding sites on MMTV proviral DNA with hormone inducible transcription

Steroid hormones, when complexed to their receptors, recognize and bind specific DNA sequences and subsequently induce increased levels of transcription. The mechanisms of steroid hormone action were analyzed by constructing chimeric DNA molecules from portions of mouse mammary tumor virus envelope and long terminal repeat (LTR) regions ligated to the thymidine kinase (tk) gene of herpes simplex virus. This construction allowed the tk gene to be expressed in a hormone-responsive fashion upon transfection into Ltk- cells. Comparison of transcription data with in vitro binding data showed that hormone-responsive transcription can be directly correlated to the presence of steroid hormone receptor binding sites on the DNA. There are at least two such receptor binding sites in the LTR region, one between -202 and -137 and another between -137 and -50 base pairs from the RNA cap site, as well as a site near the 5' end of the envelope region. These results strengthen the hypothesis that steroid-receptor complexes regulate genes primarily by binding to DNA sites near the promoter region and thereby modulate transcription.

Evidence for a prokaryotic promoter in the murine mammary tumor virus long terminal repeat

The long terminal repeat (LTR) of C3H murine mammary tumor virus (MuMTV) is approx. 1.3 kb long. HaeIII digestion of a cloned PstI fragment containing the left-end LTR generated four fragments of sizes 0.56, 0.41, 0.34 and 0.14 kb, one of which (0.41 kb) had a promoter activity in Escherichia coli. This was demonstrated by replacing the bacterial promoter for the neomycin-resistance (NmR) gene in the plasmid pKC56 with the HaeIII fragments. Only the 0.41-kb fragment that contains sequences from the U3 region of the LTR was found to contain a promoter, as shown by the expression of the drug-resistance phenotype in the recombinant plasmid. The strength of this promoter was comparable to or greater than that found with the parental NmR gene promoter. S1 nuclease mapping of the NmR gene transcript indicated that the initiation of this transcript occurred within the 0.41-kb LTR fragment from a site approx. 10 bp upstream from the 3' end. A comparison of the known DNA sequences in the MuMTV LTR with those found in bacterial promoters revealed that a 'Pribnow box', the initiation signal for the prokaryotic promoters, is present in the 0.41-kb LTR fragment upstream from the initiation site. Furthermore, in a recombinant plasmid that contained the complete LTR the same promoter sequences appeared to be involved in the initiation of RNA transcription. The 0.34-kb LTR fragment, which contains sequences derived from the U3 and U5 regions of the LTR, did not possess promoter activity in E. coli. However, it was found to induce deletions of adjacent plasmid DNA sequences. The deletions were specifically initiated from the downstream end of the LTR-fragment insert. The presence of a prokaryotic promoter in the MuMTV LTR, together with the observation that certain LTR sequences can induce deletions, analogous to those caused by transposable elements, in recombinant plasmids suggest that the MuMTV LTR may have evolved from such elements.

Glucocorticoids and chromosomal position modulate murine mammary tumor virus transcription by affecting efficiency of promoter utilization

The rate of transcription of murine mammary tumor virus (MTV) sequences in MTV-infected rat hepatoma tissue culture cells is strongly affected by both glucocorticoid hormones and the chromosomal position of provirus integration. We have characterized MTV RNAs produced in J2.17 and M1.54, independent isolates containing, respectively, 1 and 10 proviruses integrated at distinct chromosomal loci. M1.54, but not J2.17, synthesized MTV RNA in the absence of glucocorticoids; the rate of hormone-stimulated viral gene transcription in M1.54 was 50- to 100-fold higher than in J2.17. In each case in which MTV genes were expressed (J2.17 induced, M1.54 basal and induced), the viral RNAs produced were indistinguishable. RNA blotting revealed accumulation of two transcripts, 7.8 and 3.8 kilobases; the latter was likely produced from the former by RNA splicing. Sites used for transcription initiation, polyadenylation, and splicing have been identified from the sizes of end-labeled hybridization probes protected from digestion with mung bean nuclease; the unique initiation and polyadenylation sites were both encoded within the MTV long-terminal-repeat sequence. The efficiencies of splicing and of utilization of the polyadenylation signal did not appear to vary as functions of chromosomal position or hormonal stimulation. Differences in rates of viral gene transcription were reflected in the differential accumulation of the 5'-terminal 136 nucleotides of MTV RNA. Thus, glucocorticoids and chromosomal position appeared to affect solely the efficiency of utilization of the MTV promoter, leaving unchanged the sites of initiation, splicing, and polyadenylation, as well as the efficiencies of the latter two processes.

Mouse mammary tumour virus related sequences are present in human DNA

MuMTV-related sequences have been identified in the DNA of human breast cancer cells using the Southern transfer technique and hybridisation with cloned MuMTV DNA under conditions in which partially mismatched sequences form stable hybrids. Hybridisation with cloned fragments of the MuMTV genome showed that the gag-pol region shares the most homology (estimated to be greater than 80%) with the human MuMTV-related sequences, however, DNA fragments partially homologous to the MuMTV LTR, gag ad env regions were also detected. Analysis of several human DNA samples suggests that the majority of the human MuMTV-related sequences are genetically transmitted but additional Eco R1 fragments were detected in the DNA of one out of three breast cancer cell lines, MCF7. These sequences are potential probes for the human MuMTV-related retroviral sequences and will allow their possible role in human breast cancer to be evaluated.

Subfragments of the large terminal repeat cause glucocorticoid-responsive expression of mouse mammary tumor virus and of an adjacent gene

After transfection of mouse mammary tumor virus (MMTV) proviral DNA into cultured cells, the DNA is transcribed in a glucocorticoid-sensitive fashion. The large terminal repeat (LTR) region of MMTV is 1,328 nucleotides long and contains the regulatory information necessary for the hormonal response. We have constructed a MMTV LTR-thymidine kinase (tk) chimeric gene and have tested the biological activity of molecules containing various deletions in the LTR after transformation of LTK- APRT- mouse cells. In the TK+ transformants, both a LTR- tk chimeric RNA and an authentic tk RNA are correctly initiated and transcribed. The synthesis of the chimeric RNA as well as that of the tk RNA is hormonally regulated. A plasmid containing 202 nucleotides of LTR DNA 5' to the RNA initiation site is fully sensitive to glucocorticoids; 50 nucleotides still cause a residual inducibility.

Tumorigenesis by mouse mammary tumor virus: evidence for a common region for provirus integration in mammary tumors

We have prepared specific probes for unique-sequence cellular DNA adjacent to each of the newly integrated proviruses in tumors induced by mouse mammary tumor virus (MMTV). The use of such probes to screen a large number of independent mammary tumors in the BR6 strain of mouse has indicated that in at least 17 out of the 40 tumors examined so far, an MMTV provirus has integrated into a common chromosomal domain. A 10 kb Eco RI fragment of single copy DNA from this region has been isolated and partially characterized by restriction enzyme mapping. Of the proviruses located within this fragment in different tumors, all but one are complete, in the same orientation, and clustered within about 3 kb of cellular DNA. These findings are consistent with an insertional mutagenesis model for tumorigenesis by MMTV, in which the integration of a provirus in a particular region of cellular DNA may activate a neighboring oncogene. The region we describe here appears to be different from that reported for mammary tumors in the C3H strain of mouse.

Glucocorticoids and chromosomal position modulate murine mammary tumor virus transcription by affecting efficiency of promoter utilization

The rate of transcription of murine mammary tumor virus (MTV) sequences in MTV-infected rat hepatoma tissue culture cells is strongly affected by both glucocorticoid hormones and the chromosomal position of provirus integration. We have characterized MTV RNAs produced in J2.17 and M1.54, independent isolates containing, respectively, 1 and 10 proviruses integrated at distinct chromosomal loci. M1.54, but not J2.17, synthesized MTV RNA in the absence of glucocorticoids; the rate of hormone-stimulated viral gene transcription in M1.54 was 50- to 100-fold higher than in J2.17. In each case in which MTV genes were expressed (J2.17 induced, M1.54 basal and induced), the viral RNAs produced were indistinguishable. RNA blotting revealed accumulation of two transcripts, 7.8 and 3.8 kilobases; the latter was likely produced from the former by RNA splicing. Sites used for transcription initiation, polyadenylation, and splicing have been identified from the sizes of end-labeled hybridization probes protected from digestion with mung bean nuclease; the unique initiation and polyadenylation sites were both encoded within the MTV long-terminal-repeat sequence. The efficiencies of splicing and of utilization of the polyadenylation signal did not appear to vary as functions of chromosomal position or hormonal stimulation. Differences in rates of viral gene transcription were reflected in the differential accumulation of the 5'-terminal 136 nucleotides of MTV RNA. Thus, glucocorticoids and chromosomal position appeared to affect solely the efficiency of utilization of the MTV promoter, leaving unchanged the sites of initiation, splicing, and polyadenylation, as well as the efficiencies of the latter two processes.

Acquisition of proviral DNA of mouse mammary tumor virus in thymic leukemia cells from GR mice

Male mice of strain GR develop T-cell leukemia at a low frequency late in life. These leukemia cells contain large amounts of mouse mammary tumor virus (MMTV) RNA and MMTV proteins in a precursor form (Nusse et al., J. Virol. 32:251-258, 1979). We used restriction enzyme analysis and molecular hybridization to identify MMTV proviruses in the DNA of these leukemia cells. GR leukemia cells contained additional integrated MMTV proviruses at various sites in the genome. This amplification of MMTV proviruses in GR leukemia cells is not restricted to one particular endogenous MMTV provirus of strain GR. The number and location of the extra MMTV proviruses present in transplants of GR leukemia cells did not change upon serial transplantation of the leukemia cells. Acquisition of MMTV proviruses was also found in a similar leukemia, L1210 of the DBA/2 mouse strain, but not in three other leukemias, SL2 of DBA/2, BW5147 of AKR, and a spontaneous thymoma of BALB/c. The two main classes of MMTV RNA, 35S and 24S, were present in the cytoplasmic RNA of GR leukemia cells, indicating that the aberrant processing of MMTV precursor proteins is not due to anomolously sized RNAs. We could not detect extra RNAs in GR leukemia cells which would represent read-through transcripts of cellular genes adjacent to the extra MMTV proviruses, initiated by a promoter signal in the right MMTV long terminal repeat sequence. These data suggest that acquisition of MMTV proviruses may coincide with the onset of leukemogenesis in GR male mice.

Mouse mammary tumor virus proviral sequences congenital to C3H/Sm mice are differentially hypomethylated in chemically induced, virus-induced, and spontaneous mammary tumors

C3H/Sm mice have lost the exogenous milk-borne mouse mammary tumor virus (MMTV) characteristic of the C3H strain and have a very low (1.5%) incidence of spontaneous mammary tumors, yet they are highly susceptible to mammary carcinogenesis by either chemical carcinogens or infection with the milk-borne virus. We have analyzed the MMTV proviral DNA content of normal tissues and of spontaneous, virus-induced, and chemically induced mammary tumors by restriction endonuclease digestion and Southern blot analysis. Although the results clearly showed additional MMTV sequences in the virus-induced tumor which are not present in normal liver DNA, none of the spontaneous or chemically induced tumors could be shown to contain either newly acquired exogenous or amplified endogenous MMTV sequences. Interestingly, mammary tumors arising in C3H/Sm mice treated simultaneously with infectious MMTV (C3H) and dimethylbenz[a]anthracene (DMBA) possessed new exogenous MMTV DNA even though no quantitative change in tumor production was observed when these mice were compared with C3H/Sm mice treated with DMBA alone (Smith et al., Int. J. Cancer 26:373-379, 1980). Our data indicate that the endogenous MMTV proviral units are extensively methylated in normal tissues, such as livers and normal nonlactating mammary glands. In the absence of MMTV (C3H), we found that in the rare, spontaneously occurring C3H/Sm mammary tumors, certain endogenous MMTV sequences were specifically hypomethylated. Hypomethylation of endogenous MMTV sequences was also noted in the chemically induced mammary tumors, even though radioimmune competition assays for MMTV gp52 and p28 are negative (Smith et al., Int. J. Cancer 27:81-86, 1981). Our results support the conclusion that amplification of endogenous MMTV sequences is not intrinsic to C3H/Sm mouse mammary tumors arising spontaneously or after induction by chemicals. On the other hand, integration of exogenous MMTV DNA into the genome was a constant feature of mammary tumors developing in MMTV (C3H)-infected C3H/Sm mice, even when DMBA was used as the carcinogen. Hypomethylation of some endogenous MMTV sequences is characteristic of C3H/Sm mammary tumors, whether spontaneous or induced by chemicals, which suggests that these sequences are located in actively transcribing regions of the tumor cell genome.

Restriction endonuclease mapping of the proviral DNA of the exogenous RIII murine mammary tumor virus

Cellular DNA containing integrated murine mammary tumor virus (MuMTV) was isolated from FeI/C6 feline kidney cells and CCL64 mink lung cells infected with milkborne RIII MuMTV. By using restriction enzyme HpaI, intact RIII MuMTV provirus (length, 8.7 kilobases [kb]) was excised from the cellular DNA. Subsequent restriction endonuclease analysis of this HpaI fragment with KpnI, HindIII, EcoRI, BamHI, BglII, PstI, SstI, SalI, and XhoI enabled us to construct a map of the RIII virus genome. A comparison of this map with the maps of the GR and C3H MuMTV's revealed that there are greater sequence differences between the RIII virus and the GR and C3H MuMTV proviruses than there are between the GR and C3H proviruses. The following are features of the restriction map unique to the RIII provirus: the presence of three BamHI and two EcoRI cleavage sites, a HpaI cleavage site in the terminal 3'-5' repeat unit of the provirus, and the absence of an XhoI cleavage site. Another distinguishing feature of the RIII provirus is that the sizes of some of the restriction fragments produced by cleavage of the RIII provirus with PstI are different from the sizes of the fragments obtained by PstI cleavage of the GR and C3H proviruses. Like the GR proviral DNA, the RIII proviral DNA has three SstI (SacI) cleavage sites, whereas the C3H provirus has only two SstI sites. HpaI digestion of MuMTV-infected mink lung cell DNA revealed only one class of provirus (an 8.7-kb fragment); however, we observed several minor classes of RIII proviral DNA in addition to the major class of provirus DNA in infected cat kidney cells. PstI digestion of the HpaI 8.7-kb fragments from both feline and mink cells generated a 3.7-kb DNA fragment identical in size to a PstI-generated fragment that has been found in GR and C3H milkborne virus-infected cells. Although a fragment similar in size to the milkborne 3.7-kb PstI fragment has been found as an endogenous component in many C3H and GR mouse tissues, we did not observe such an endogenous fragment in the RIII mouse strain. Therefore, the 3.7-kb fragment may be useful as a marker for the milkborne RIII MuMTV provirus in RIII mice.

Delimitation of a DNA sequence which confers inducibility by glucocorticoid hormones

A chimeric long terminal repeat-thymidine kinase (LTR-tk) gene has been used to define the sequence requirements for glucocorticoid induction of gene expression. The original LTR-tk gene contains an entire mouse mammary tumor virus (MMTV) LTR preceding the tk gene. This gene can be expressed in a hormone-responsive fashion upon transfection into L tk--cells to produce a chimeric LTR-tk mRNA. Stepwise deletion of nucleotide sequences 5' of the viral RNA initiation site revealed that 202 nucleotides upstream of the viral cap site are sufficient for the hormonal regulation. Deletion of 5' sequences up to 59 nucleotides upstream of the viral cap site abolished RNA initiation in the LTR and hormonal induction.

Mammary tumor virus DNA contains sequences required for its hormone-regulated transcription

Glucocorticoids regulate the rate of transcription of integrated murine mammary tumor virus (MTV) genes in most clones of MTV-infected rat hepatoma tissue culture (HTC) cells. To determine whether hormonal regulation is mediated from flanking cellular sequences or rather from within the viral DNA, we analyzed the relative rates of transcription of MTV and adjacent HTC sequences in two lines of infected HTC cells, J2.15 and J2.17, each of which contains a single insertion of MTV DNA. In addition, we measured in uninfected HTC cells the transcriptional rates of the corresponding "preinsertion fragments," which may be viewed as the equivalent sequences bearing deletions of MTV DNA; the two genomic segments into which integration occurred in the two lines are unrelated. Previous work showed that glucocorticoids induce the accumulation of MTV RNA in J2.17, whereas viral transcripts are not detected in J2.15, RNA pulse-labeling experiments indicate that glucocorticoids stimulate that rate of MTV gene transcription in J2.17 but not in J2.15; in contrast, no labeled RNA hybridizing to flanking sequences was detected either in the uninfected or in the infected cells. We conclude that the host site of integration cells. We conclude that the host site of integration of MTV need not be transcriptionally active or hormonally responsive to permit viral gene expression. Furthermore, these rate measurements indicate that glucocorticoid-stimulated MTV RNA synthesis in J2.17 does not reflect readthrough transcription from a regulated cellular promoter. This notion is independently supported by transcript mapping experiments showing that the 5' terminus of nuclear MTV RNA is at a site on MTV DNA approximately 1.2 kb downstream from the host-viral junction. Thus our data are consistent with the presence of a hormone-responsive element within the provirus.

Structure, stability, methylation, expression and glucocorticoid induction of endogenous and transfected proviral genes of mouse mammary tumor virus in mouse fibroblasts

A sequence of mouse genomic DNA containing an endogenous mouse mammary tumor virus (MMTV) provirus, which was isolated by molecular cloning in lambda vector phage, has been reintroduced into cultured mouse L cells, using the thymidine-kinase cotransfection technique. Individual cell clones that acquired the transfected MMTV proviral DNA have been isolated. The transfected DNA remains stable in these cells and does not become methylated. It is transcribed into MMTV RNA, And the transcription is stimulated by glucocorticoid hormones.

The endogenous proviral mouse mammary tumor virus genes of the GR mouse are not identical and only one corresponds to the exogenous virus

The endogenous proviral copies of mouse mammary tumor virus (MMTV) were selected from a gene library of GR mouse DNA. We obtained five different lambda. MMTV recombinant clones. Four of them correspond to the 3' Eco RI fragments of the endogenous proviruses an one comprises an intact MMTV provirus with 2 to 3 kb of flanking mouse genomic DNA. Heteroduplex formation followed by S1 digestion under stringent conditions shows that there is nucleotide sequence heterology among the cloned endogenous proviral copies. Only one endogenous proviral copy, associated with the mtv-2 locus, was found to be totally homologous to the exogenous proviral DNA.