Number and location of mouse mammary tumor virus proviral DNA in mouse DNA of normal tissue and of mammary tumors

Regulatory T cell adoptive transfer alters uterine immune populations, increasing a novel MHC-IIlow macrophage associated with healthy pregnancy

Intrauterine fetal demise (IUFD) - fetal loss after 20 weeks - affects 6 pregnancies per 1,000 live births in the United States, and the majority are of unknown etiology. Maternal systemic regulatory T cell (Treg) deficits have been implicated in fetal loss, but whether mucosal immune cells at the maternal-fetal interface contribute to fetal loss is under-explored. We hypothesized that the immune cell composition and function of the uterine mucosa would contribute to the pathogenesis of IUFD. To investigate local immune mechanisms of IUFD, we used the CBA mouse strain, which naturally has mid-late gestation fetal loss. We performed a Treg adoptive transfer and interrogated both pregnancy outcomes and the impact of systemic maternal Tregs on mucosal immune populations at the maternal-fetal interface. Treg transfer prevented fetal loss and increased an MHC-IIlow population of uterine macrophages. Single-cell RNA-sequencing was utilized to precisely evaluate the impact of systemic Tregs on uterine myeloid populations. A population of C1q+, Trem2+, MHC-IIlow uterine macrophages were increased in Treg-recipient mice. The transcriptional signature of this novel uterine macrophage subtype is enriched in multiple studies of human healthy decidual macrophages, suggesting a conserved role for these macrophages in preventing fetal loss.

A V beta 8.2-specific superantigen from exogenous mouse mammary tumor virus carried by FM mice

A number of endogenous mouse mammary tumor virus (MMTV) proviruses encode superantigen that have the ability to stimulate T cells with a certain T cell receptor (TCR) beta-chain variable region (V beta) and to mediate the V beta-specific clonal deletion. The tumorigenic milk-borne MMTV carried by C3H and GR mice also have superantigenic properties in vivo. In the present study we identified and characterized a novel V beta 8.2-specific superantigen of exogenous MMTV carried by FM mice. The open reading frame (ORF) in the 3' long terminal repeat of the MMTV was cloned by polymerase chain reaction with primers corresponding to conserved regions spanning the ORF coding region. Sequence analysis of the ORF revealed that there is no sequence identical to those in other known MMTV in the carboxy terminus implicated in TCR V beta recognition. Subcutaneous injection of the virus into adult BALB/c mice induced an approximately three- to fourfold enlargement of draining lymph nodes and a substantial increase of V beta 8.2+ CD4+ T cells in the lymph nodes within 6 days. The exposure of newborn BALB/c mice to the virus by foster nursing resulted in a marked deletion of V beta 8.2+ cells both in CD4+ and CD8+ T cells. Thus, a novel milk-borne MMTV in FM mice expresses strong superantigenic properties capable of stimulating V beta 8.2+ T cells. V beta 8.2+ T cells have been demonstrated to be frequently involved in recognition of conventional antigens and responsible for autoimmune diseases such as experimental allergic encephalomyelitis. Therefore, the MMTV (FM) may provide a new mouse model system for inducing immunodeficiency or autoimmune disease by retroviral infection.

Stably integrated mouse mammary tumor virus long terminal repeat DNA requires the octamer motifs for basal promoter activity

In the mouse mammary tumor virus promoter, a tandem of octamer motifs, recognized by ubiquitous and tissue-restricted Oct transcription factors, is located upstream of the TATA box and next to a binding site for the transcription factor nuclear factor I (NF-I). Their function was investigated with mutant long terminal repeats under different transfection conditions in mouse Ltk- cells and quantitative S1 nuclease mapping of the transcripts. In stable transfectants, which are most representative of the state of proviral DNA with respect to both number of integrated DNA templates and chromatin organization, a long terminal repeat mutant of both octamer sites showed an average 50-fold reduction of the basal transcription level, while the dexamethasone-stimulated level was unaffected. DNase I in vitro footprinting assays with L-cell nuclear protein extracts showed that the mutant DNA was unable to bind octamer factors but had a normal footprint in the NF-I site. I conclude that mouse mammary tumor virus employs the tandem octamer motifs of the viral promoter, recognized by the ubiquitous transcription factor Oct-1, for its basal transcriptional activity and the NF-I binding site, as previously shown, for glucocorticoid-stimulated transcription. A deletion mutant with only one octamer site showed a marked base-level reduction at high copy number but little reduction at low copies of integrated plasmids. The observed transcription levels may depend both on the relative ratio of transcription factors to DNA templates and on the relative affinity of binding sites, as determined by oligonucleotide competition footprinting.

Stably integrated mouse mammary tumor virus long terminal repeat DNA requires the octamer motifs for basal promoter activity

In the mouse mammary tumor virus promoter, a tandem of octamer motifs, recognized by ubiquitous and tissue-restricted Oct transcription factors, is located upstream of the TATA box and next to a binding site for the transcription factor nuclear factor I (NF-I). Their function was investigated with mutant long terminal repeats under different transfection conditions in mouse Ltk- cells and quantitative S1 nuclease mapping of the transcripts. In stable transfectants, which are most representative of the state of proviral DNA with respect to both number of integrated DNA templates and chromatin organization, a long terminal repeat mutant of both octamer sites showed an average 50-fold reduction of the basal transcription level, while the dexamethasone-stimulated level was unaffected. DNase I in vitro footprinting assays with L-cell nuclear protein extracts showed that the mutant DNA was unable to bind octamer factors but had a normal footprint in the NF-I site. I conclude that mouse mammary tumor virus employs the tandem octamer motifs of the viral promoter, recognized by the ubiquitous transcription factor Oct-1, for its basal transcriptional activity and the NF-I binding site, as previously shown, for glucocorticoid-stimulated transcription. A deletion mutant with only one octamer site showed a marked base-level reduction at high copy number but little reduction at low copies of integrated plasmids. The observed transcription levels may depend both on the relative ratio of transcription factors to DNA templates and on the relative affinity of binding sites, as determined by oligonucleotide competition footprinting.

Factors controlling the expression of mouse mammary tumour virus

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Rapid identification of DNA fragments containing promoters for RNA polymerase II

We describe a direct procedure for screening genomic recombinant DNA libraries or restriction fragments of cloned DNA regions for RNA polymerase II promoters. Cellular polyadenylated mRNA is chemically de-capped by beta-elimination reaction and enzymatically re-capped with [alpha-32P]GTP by vaccinia guanylyl transferase. Since this enzyme only accepts di- or triphosphorylated 5' termini as a substrate, the mRNAs are labeled exclusively at the first nucleotide, irrespective of whether the mRNA was intact or fragmented before in vitro capping. By using in vitro-capped mRNA as a hybridization probe, recombinant DNA molecules or restriction fragments that carry a cap site (and thus likely an RNA polymerase II promoter) can directly be identified. Here, we demonstrate the applicability of this procedure by the isolation and characterization of several genomic DNA clones containing RNA polymerase II promoter sequences, that are highly active in liver.

An amplified endogenous retroviral sequence on the murine Y chromosome related to murine leukemia viruses and viruslike 30S sequences

A highly repeated sequence on the murine Y chromosome was cloned and characterized. The DNA sequence of the viral long terminal repeats (LTRs) showed that the 5' and 3' LTRs were approximately 90% homologous. The LTRs are generally unrelated to any previously reported viral LTR but are somewhat similar to the viruslike 30S sequences.

Different int-1 region DNA rearrangements within different zones of a single mouse mammary tumor

Fragments were taken from separate parts of hormone-dependent (HD) primary GR mouse mammary tumors and serially transplanted in estrone plus progesterone treated or hormonally untreated castrated mice. The transplants were examined with respect to int-1 DNA rearrangement, proviral integrations of the murine mammary tumor virus (MMTV), and estrogen and progesterone receptor content. One of the fragments (b) taken from the primary tumor of line TSI 96 produced transplants that showed int-1 rearrangement in one allele and also MMTV proviral integrations not at the int-1 gene, whereas transplants from another fragment (a) only had the normal germ-line int-1 arrangement and no extra MMTV provirus. These respective genotypes were retained when the tumors became hormonally independent during further transplantations. The results indicate that int-1 rearrangement was not present in the originally transformed cell but occurred in a HD cell during growth of the tumor. Furthermore they indicate that loss of hormonal dependence in GR mammary tumors is due to a mutational event, unrelated to int-1 rearrangement.

Intracisternal A-particle genes in Mus musculus: a conserved family of retrovirus-like elements

The structural organization of intracisternal A-particle genes has been studied, using isolates from a mouse gene library in lambda phage Charon 4A. The predominant gene form among the isolates was 7.3 kilobases (kb) in length. R-loops between the 7-kb (35S) A-particle genomic ribonucleic acid and several of these genes were colinear, with no visible evidence of intervening deoxyribonucleic acid sequences. One recombinant was found with an A-particle gene that contained a 1.7-kb deletion. Using the deletion as a reference, the deoxyribonucleic acid and ribonucleic acid homology regions were localized with respect to one another and to the restriction map: the 5' terminus of the ribonucleic acid was several hundred base pairs within the 5' end of the deoxyribonucleic acid homology region. Restriction endonuclease fragments encompassing the 5' and 3' regions of one 7.3-kb gene were separately subcloned into pBR322. Heteroduplexes between the two subclones revealed an approximately 300-base pair segment of terminally redundant sequences. The cloned 3' fragment hybridized with restriction fragments from the 5' end of several other A-particle genes, demonstrating the presence of common (though not necessarily identical) terminally repeated sequences. A-particle genes varied in the occurrence of specific restriction sites at characteristic internal loci. However, heteroduplexes between several variant 7.3-kb genes showed continuous homology regions even when spread under stringent hybridization conditions. The relative abundance of restriction site variants was highly conserved in 12 laboratory strains of Mus musculus, in embryonic and adult tissues of a single inbred strain, and in the SC-1 cell line of feral mouse origin, but appeared to differ in a feral Japanese substrain, Mus musculus molossinus. Some evidence suggests that subsets of A-particle genes may have similar flanking sequences. The results are discussed in terms of the evolution of this multigene family.

Activation of int-1 and int-2 loci in GRf mammary tumors

The Mtv-2 locus is known to be associated with a high mammary tumor incidence (97%) and early development of mammary tumors (3-13 months) in GR mice. However, it was not previously known whether the provirus which resides at the Mtv-2 locus is tumorigenic in and of itself or whether reintegration of proviruses generated from Mtv-2 is required for tumorigenesis. Foster-nursing GR mice on C57/BL mice eliminates the milk-borne source of GR virus, and allows the study of Mtv-2 derived proviruses alone. Using this approach, we have tested predictions which follow from the "positional" versus "reintegrational" models of tumorigenesis. Specifically, we have examined tumors from primary foster-nursed (GRf) mice to determine if MMTV proviruses derived from Mtv-2 were scattered randomly throughout the genome or were clustered in the vicinity of the int-1 and int-2 loci, which are thought to be associated with mammary tumorigenesis. It was found that the majority of spontaneous GRf mammary tumors that were tested have MMTV proviral integrations in either or both of the int-1 and int-2 loci and have transcription of either or both of the int loci. Tumors induced by Mtv-2, therefore, appear to have arisen via a mechanism similar to the activation of the int loci by exogenous (milk-borne) MMTV proviruses.

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.

Exogenous mouse mammary tumor virus proviral DNA isolated from a kidney adenocarcinoma cell line contains alterations in the U3 region of the long terminal repeat

Mouse mammary tumor virus (MMTV) is a B-type retrovirus which induces predominantly mammary carcinomas after a relatively long latency period. To date, very little is known about the reasons for the strict tissue specificity of MMTV. The BALB/cf/Cd strain of mice, which was infected with milk-borne MMTV (C3H), shows a high incidence of kidney adenocarcinomas, and our data suggest that MMTV might be involved in the formation of these tumors. Newly integrated exogenous MMTV proviruses were found in the genome of transplanted tumor cells as well as in the DNA of a cell line derived from one tumor, but not in normal cells of BALB/cf/Cd mice. The MMTV DNA in these tumor cells was transcribed and viral RNA synthesis was strongly stimulated by glucocorticoid hormones. Viral structural polypeptides, comparable in size and antigenicity to MMTV polypeptides of infected mammary tumor cells were synthesized and processed normally in the cell line and were organized correctly into intracytoplasmic particles. Heteroduplex analysis of the molecularly cloned MMTV proviral DNAs of kidney and mammary tumor origin revealed a high degree of homology in the gag, pol, and env genes. A striking difference, however, was observed in the U3 region of the two LTRs that might relate to the different tissue specificity of the two viruses.

Differential transformation of C3H10T1/2 cells by v-mos: sequential expression of transformation parameters

Extremely small quantities of the product of the transforming gene v-mos of Moloney murine sarcoma virus are able to efficiently transform cells. Recent data indicate the existence of a threshold level for v-mos transformation of NIH3T3 cells. Using mouse mammary tumor virus long terminal repeat sequences or hybrid promoters consisting of mouse mammary tumor virus and Moloney murine sarcoma virus long terminal repeat elements to express v-mos in C3H10T1/2 cells, we established cell lines representing different stages of morphological transformation in vitro. The threshold level for v-mos transformation was considerably lower than that for NIH3T3 cells, because no treatment with dexamethasone or primary selection other than transformation was necessary during standard transfection procedures. Using the cell lines mentioned we established an association of the level of v-mos expression with the transformation parameters examined, but not with p53 levels. Furthermore, the characterization of the different promoters showed (i) that the distal binding site confers hormone responsiveness to Moloney murine sarcoma virus promoter elements and (ii) that artifactual transcription initiation sites can be detected in mouse mammary tumor virus-Moloney murine sarcoma virus hybrid promoters which are, however, not regulated by the hormone.

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.

Transfer, by selective breeding, of the pathogenic Mtv-2 endogenous provirus from the GR strain to a wild mouse line free of endogenous and exogenous mouse mammary tumor virus

The GR laboratory mouse strain has five endogenous proviral copies of the mouse mammary tumor virus (MMTV). One of these, Mtv-2, is unique because it causes mammary carcinomas in virtually 100% of breeding GR females prior to 1 year of age. To facilitate studies of this locus in particular, and mammary tumorigenesis in general, we genetically tailored a new mouse line, WXG-2, which bears Mtv-2 as its only endogenous MMTV provirus. The WXG-2 line was constructed by making hybrids between the GR strain and a wild mouse line free of both endogenous and exogenous MMTV, backcrossing to the MMTV-free line, and fixing the Mtv-2 locus in a population with the desired genotype. Mammary tumors were observed in 5 of the 20 hybrid females carrying the endogenous Mtv-2 provirus. The WXG-2 line represents a new model system for studying MMTV-induced mammary tumorigenesis in the absence of multiple endogenous proviruses.

Differential transformation of C3H10T1/2 cells by v-mos: sequential expression of transformation parameters

Extremely small quantities of the product of the transforming gene v-mos of Moloney murine sarcoma virus are able to efficiently transform cells. Recent data indicate the existence of a threshold level for v-mos transformation of NIH3T3 cells. Using mouse mammary tumor virus long terminal repeat sequences or hybrid promoters consisting of mouse mammary tumor virus and Moloney murine sarcoma virus long terminal repeat elements to express v-mos in C3H10T1/2 cells, we established cell lines representing different stages of morphological transformation in vitro. The threshold level for v-mos transformation was considerably lower than that for NIH3T3 cells, because no treatment with dexamethasone or primary selection other than transformation was necessary during standard transfection procedures. Using the cell lines mentioned we established an association of the level of v-mos expression with the transformation parameters examined, but not with p53 levels. Furthermore, the characterization of the different promoters showed (i) that the distal binding site confers hormone responsiveness to Moloney murine sarcoma virus promoter elements and (ii) that artifactual transcription initiation sites can be detected in mouse mammary tumor virus-Moloney murine sarcoma virus hybrid promoters which are, however, not regulated by the hormone.

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.

New acceptor cell for transfected genomic DNA: oncogene transfer into a mouse mammary epithelial cell line

A line of mouse mammary epithelial cells (NMuMG) has been characterized for its ability to be stably transfected with exogenous DNA. A transfection frequency of at least 1 cell per 1,000 was obtained with the pSV2neo plasmid. Several thousand G418-resistant NMuMG cell clones can easily be generated in cotransfection of genomic DNA and pSV2neo. The NMuMG cells were isolated from normal mammary glands and do not form malignant lesions when injected into nude mice. We have cotransfected NMuMG cells with pSV2neo and genomic DNA from the human EJ bladder carcinoma line, a cell line which contains an activated c-rasH oncogene. When a pool of 4,700 G418-resistant colonies was injected into nude mice, tumors were obtained. These tumors contain a transfected human rasH gene. Genomic DNA transfection into a line of mouse epithelial cells, in combination with the selection of stable transfectants and tumor induction in nude mice, can be used to screen human tumor DNA for the presence of activated oncogenes.

Molecular cloning, characterization, and genetic mapping of an endogenous murine mammary tumor virus proviral unit I of C3H/He mice

We have cloned and characterized a novel endogenous murine mammary tumor virus proviral unit of the C3H/He strain of mice. The cloned proviral unit is 16 kilobase pairs (kbp) in size and is composed of a 5.6-kbp 5' EcoRI segment of an endogenous provirus with 10.4-kbp flanking cellular sequences. A comparison of the restriction map of the cloned proviral DNA with that of an endogenous provirus of the GR strain of mice has revealed minor differences in restriction sites on the two proviruses. The restriction enzyme SstI, which does not cleave the 5' EcoRI fragment of GR DNA, cleaves the C3H/He proviral sequences once; MspI has an additional site in the C3H/He proviral sequences. By using a subcloned fragment containing unique cellular sequences as a hybridization probe, we (i) mapped the C3H/He proviral unit to chromosome 14 by using mouse-hamster somatic cell hybrids, and (ii) demonstrated that this proviral unit is also present in the genome of DBA/2 mice. From these results we conclude that the C3H/He strain of mice acquired this proviral unit from DBA stock by genetic transmission. Our data also indicate that the murine mammary tumor virus sequences present in the gag-specific proviral unit of C3H/He mice extend at least 2.45 kbp downstream of the EcoRI site in the genomic DNA. Since the structural organization and chromosomal location of this proviral unit are distinct from those of previously reported proviral units represented by similar-sized (16.7-kbp) EcoRI fragments, we tentatively propose to designate this proviral unit Mtv-7a.

Molecular basis of altered mouse mammary tumor virus expression in the D-2 hyperplastic alveolar nodule line of BALB/c mice

The preneoplastic D-2 hyperplastic outgrowth line, which was derived from a hormone-induced hyperplastic alveolar nodule (HAN) of a BALB/c mouse, was used for a detailed analysis of mouse mammary tumor virus (MMTV) expression. The D-2 HAN line has previously been shown to express viral RNA representative of the entire genome, although viral particles have been noted only rarely. The MMTV-specific mRNA, protein, and DNA content of the D-2 tissues was defined in an effort to better understand the molecular basis of the aberrant virus expression. Northern blotting techniques demonstrated the presence of properly processed 8.9 kb (genomic) and 3.6 kb (envelope) mRNA. Protein electroblotting procedures established the presence of properly processed viral core protein p28. In contrast, the envelope precursor polyprotein was not processed into detectable levels of gp52. Analysis of MMTV proviral content by Southern blot methodology revealed the presence of a newly acquired provirus which serves as a marker for the clonal nature of the D-2 line. The origin of the new provirus is unknown. Methylation studies established that the new proviral insert is hypomethylated and, therefore, is likely serving as the template for the MMTV expression observed in the D-2 HAN line. These characteristics of the D-2 line make it an excellent system in which to study the role, if any, of MMTV in the progression of D-2 preneoplastic tissues to the tumor phenotype.

Characterization and chromosomal distribution of endogenous mouse mammary tumor viruses of European mouse strains STS/A and GR/A

The endogenous mouse mammary tumor virus (MMTV) proviral copies in two genetically dissimilar mouse strains, STS/A, a European mouse strain, and BALB/c, were characterized. STS/A carries the same four MMTV proviral copies as GR.Mtv-2-; these strains share also most of the isoenzyme markers and are therefore highly related. Cellular DNA of GR.Mtv-2- contains a partial MMTV provirus that is not present in STS/A. GR.Mtv-2- is derived from GR; they differ in the locus Mtv-2 that contains one MMTV provirus. Expression of this Mtv-2 endogenous MMTV provirus is directly linked to mammary tumorigenesis in GR. MMTV proviral loci were studied using restriction enzyme analysis and the Southern transfer procedure using liver DNAs from recombinant inbred strains between BALB/c and STS/A. All segregating MMTV-specific EcoRI fragments were identified to MMTV proviral loci and most of these were localized by studying the cosegregation of the Mtv units and known chromosomal markers. Since STS/A, GR.Mtv-2-, and GR are highly related, the five complete endogenous MMTV proviruses of GR were located on the following chromosomes: Mtv-2 on chromosome 18, Mtv-3 on 11, Mtv-19 on 1, Mtv-20 on 4, whereas Mtv-8 has tentatively been located on chromosome 18 by Callahan et al. (R. Callahan, D. Gallahan, and Ch. Kozak (1984), J. Virol. 49, 1005-1008). GR and GR.Mtv-2 furthermore contain two incomplete MMTV proviral elements, one of which is also present in STS/A.

Hygromycin B phosphotransferase as a selectable marker for DNA transfer experiments with higher eucaryotic cells

The DNA coding sequence for the hygromycin B phosphotransferase gene was placed under the control of the regulatory sequences of a cloned long terminal repeat of Moloney sarcoma virus. This construction allowed direct selection for hygromycin B resistance after transfection of eucaryotic cell lines not naturally resistant to this antibiotic, thus providing another dominant marker for DNA transfer in eucaryotic cells.

Members of the Amy-2 alpha-amylase gene family of mouse strain CE/J contain duplicated 5' termini

Several members of the Amy-2 alpha-amylase multigene family from the CE/J strain of mouse have been cloned in cosmid vectors. Structural analysis of these recombinants reveals that the cloned Amy-2 copies contain tandem 5' termini. The duplicated 5'-terminal elements, which lie upstream from the Amy-2 cap site, are separated from their Amy-2 homologues by about 8000 bases. The orientation of these 5' orphons is the same as that of Amy-2. Gene titration and cloning experiments suggest that at least four of the approximately 15 Amy-2 copies present in the CE/J genome contain 5' orphon elements. The extent of sequence homology between 5' orphons and their gene homologues has been determined by DNA sequence analysis. All the orphons are identical and contain the entire 185 base-pairs of the first exon, 49 base-pairs of the first intron and more than 400 base-pairs of the Amy-2 5' flanking region. Intron and flanking-region sequences of the orphons differ by about 20% from their Amy-2 counterparts, and the exon by about 8%. The TATA box and the cap site are conserved, while the ATG translation initiation signal is mutated to ATA in the orphon. No transcription initiation has been detected at the orphon cap site using run-off transcription in isolated pancreatic nuclei in vitro.

New acceptor cell for transfected genomic DNA: oncogene transfer into a mouse mammary epithelial cell line

A line of mouse mammary epithelial cells (NMuMG) has been characterized for its ability to be stably transfected with exogenous DNA. A transfection frequency of at least 1 cell per 1,000 was obtained with the pSV2neo plasmid. Several thousand G418-resistant NMuMG cell clones can easily be generated in cotransfection of genomic DNA and pSV2neo. The NMuMG cells were isolated from normal mammary glands and do not form malignant lesions when injected into nude mice. We have cotransfected NMuMG cells with pSV2neo and genomic DNA from the human EJ bladder carcinoma line, a cell line which contains an activated c-rasH oncogene. When a pool of 4,700 G418-resistant colonies was injected into nude mice, tumors were obtained. These tumors contain a transfected human rasH gene. Genomic DNA transfection into a line of mouse epithelial cells, in combination with the selection of stable transfectants and tumor induction in nude mice, can be used to screen human tumor DNA for the presence of activated oncogenes.

Integration of type B retroviral DNA in virus-induced primary murine thymic lymphomas

In previous studies we described the isolation and characterization of a highly leukemogenic virus, DMBA-LV, isolated from a transplanted, chemical carcinogen-induced thymic lymphoma. The virus is composed of a mixture of two unrelated retroviral genomes, one highly related to type B milk-borne mouse mammary tumor virus isolates and the other partially related to type C viral genomes. In the present study, primary thymic lymphomas induced by DMBA-LV in CFW/D, NIH Swiss, C3H/Bi/Ka, and C57BL/Ka mice were assessed for the presence of newly integrated type B retroviral DNA. All 46 primary thymic lymphomas examined contained one to four newly acquired murine mammary tumor virus proviruses. Based on the sizes of provirus-cell DNA junction fragments, the integration of newly acquired murine mammary tumor virus proviruses did not appear to be random.

Hygromycin B phosphotransferase as a selectable marker for DNA transfer experiments with higher eucaryotic cells

The DNA coding sequence for the hygromycin B phosphotransferase gene was placed under the control of the regulatory sequences of a cloned long terminal repeat of Moloney sarcoma virus. This construction allowed direct selection for hygromycin B resistance after transfection of eucaryotic cell lines not naturally resistant to this antibiotic, thus providing another dominant marker for DNA transfer in eucaryotic cells.

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.

Mouse kidney and submaxillary gland renin genes differ in their 5' putative regulatory sequences

The genomic DNAs that specify the renin mRNAs found in the submaxillary gland and in the kidney of Swiss mice have been isolated by molecular cloning in phage lambda. The Ren1 and Ren2 genes, encoding the kidney and submaxillary renins, respectively, span about 9.5 kilobases each. Restriction maps of both renin genes and their flanking sequences are presented. To correlate the differential expression of the renin genes with their structures, a sequence analysis of the putative 5' regulatory sequences of both genes was conducted. The results show that the leader sequences and promoter regions of both genes are highly homologous to a point located 179 nucleotides upstream from the transcription start point; the sequences diverge beyond this position.

Rapid size determination of mRNAs complementary to cloned DNA sequences: plaque and colony hybrid-selection of cDNAs

We describe a novel screening method for the rapid size determination of mRNAs. This method has been used successfully to identify the cDNA and genomic DNA clones that had been constructed using the plasmid vector pBR322 or phage vectors M13mp7 and lambda EMBL3. Poly(A) RNA is reverse-transcribed into 32P-labeled cDNA under conditions that yield a high proportion of full-length cDNA copies. This radioactive cDNA is then hybridized in situ to bacterial colonies or phage plaques harboring recombinant DNA molecules. Colonies or plaques containing sequences complementary to abundant or moderately abundant mRNAs are detected by autoradiography and excised from nitrocellulose filters. The hybridized cDNA is eluted from the filter by alkaline denaturation and sized directly by alkaline agarose gel electrophoresis. The mRNAs characterized thus far by this technique measure between 450 and 2100 nucleotides and account for 0.03% to 15% of the mass of cytoplasmic poly(A) RNA.

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.

Protoneoplasia: the molecular biology of murine mammary hyperplasia

The mouse mammary tumor virus has provided a window into the inner workings of the mammary epithelial cell at the earliest stages of neoplasia. Techniques of molecular biology permitted us to look through that window revealing a new biology which deserves consideration as a model for mammary tumorigenesis in all species. According to this model the neoplastic process originates in a single mammary epithelial cell as a result of a critical genetic alteration, such as integration of MuMTV (MuMHV) into a key site in the mouse genome (Fig. 4). The genetic alteration immortalizes the cell and provides it with selective growth advantages which result in a clonal proliferation. This original proliferation emerges as the protoneoplastic mammary hyperplasia. The protoneoplastic cells have limited growth potential and are not obligated to undergo malignant transformation but they represent a genetically evolving population highly susceptible to full blown malignancy after exposure to carcinogens. Protoneoplastic cells which undergo further genetic alterations that provide additional selective growth advantages proliferate and emerge as malignant tumors. The genetic alterations are sometimes reflected by changes in viral DNA but this is not essential and most mouse mammary tumors probably do not occur as the result of new host-virus interactions. No doubt the current work on the mouse "int" loci will help define the genes responsible for the induction and maintenance of the protoneoplastic state. Since such host genes have proven so ubiquitous, one must also predict that analogous genes will be found in human mammary protoneoplasias. Detection of such sequences may help distinguish protoneoplastic processes from nonneoplastic, low risk hyperplasias in the human breast. Finally, the gene or genes involved in the more lethal malignant transformation await elucidation. Based on past and current progress one can be sure that the mouse mammary tumor system will help point the way.

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.

Regulation of endogenous murine mammary tumor virus expression in C57BL mouse lactating mammary glands: transcription of functional mRNA with a block at the translational level

Expression of endogenous murine mammary tumor viruses (MuMTVs) in various mouse strains is regulated in different ways, and in the absence of exogenous MuMTV, this regulation influences the incidence of spontaneous mammary tumors. Two mouse strains with low mammary tumor incidence, BALB/c and C57BL, control endogenous MuMTV expression at different stages. Neither of the strains had any detectable MuMTV polypeptides in its lactating mammary glands (LMG). However, in C57BL LMG, substantial amounts of MuMTV RNA were present, whereas very little viral RNA was detected in BALB/c LMG. By determining MuMTV RNA levels in LMG of hybrids and backcrosses of BALB/c and C57BL mice, we found that there are three unlinked, independently segregating genetic loci in C57BL mice that are responsible for the presence of moderately high amounts of MuMTV RNA in LMG. The viral RNA in C57BL LMG was processed and transported to the cytoplasm where it was found to cosediment with EDTA-sensitive polysomes. No viral proteins were detected in run-off reactions that permit completion of nascent polypeptide synthesis with polysomes from C57BL LMG, and sensitive radioimmunoassays failed to detect any MuMTV proteins in these tissues. In contrast, MuMTV mRNA purified from C57BL LMG did direct the synthesis of both gag and env MuMTV polypeptides when added to a heterologous rabbit reticulocyte lysate cell-free translation system. We propose that MuMTV mRNA in C57BL LMG, for unknown reasons, is blocked at the translational level.

Two promoters of different strengths control the transcription of the mouse alpha-amylase gene Amy-1a in the parotid gland and the liver

We show that two promoters of different strengths are involved in the tissue-specific expression of the alpha-amylase gene Amy-1a in the parotid gland and the liver of mouse. The weaker of the two promoters directs the synthesis of mRNA with a liver-type leader sequence. This promoter is active in both tissues. A promoter that is about 30-fold stronger is exclusively active in the parotid, where it directs the synthesis of an mRNA with a parotid-specific leader sequence. Neither the parotid nor the liver promoter is used in tissues that do not contain cytoplasmic alpha-amylase mRNAs, such as brain, kidney, and spleen. Nuclear transcripts that are initiated several kilobases upstream of the parotid cap site are detected in several tissues. They are most abundant in brain, and are apparently not processed into alpha-amylase mRNA.

Structural analysis of a 1.7-kilobase mouse mammary tumor virus-specific RNA

We have detected a mouse mammary tumor virus (MMTV)-specific 1.7-kilobase (kb) polyadenylated RNA in mammary glands of several mouse strains. In BALB/c mice, it is the only MMTV-specific RNA species present. C3H and GR mammary glands and tumors contain, in addition, 3.8- and 7.8-kb MMTV RNAs. Nuclease S1 analysis was performed to map 1.7-kb polyadenylated RNA. It contains predominantly long terminal repeat (LTR) sequences. The 5' end maps approximately 134 nucleotides upstream from the 3' end of the LTR. Colinearity with complete proviral DNA continues to a site about 153 nucleotides downstream from the left (5') LTR. No sequences from the middle part of proviral DNA were found. Colinearity with proviral DNA is resumed 72 nucleotides upstream from the right (3') LTR. The nucleotide sequence in this area is TTCCAGT, which is a splice acceptor consensus sequence. The anatomy of 1.7-kb RNA indicates that it may serve as a messenger for the 36,700-dalton protein encoded by the LTRs of MMTV.

Further evidence for the protein coding potential of the mouse mammary tumor virus long terminal repeat: nucleotide sequence of an endogenous proviral long terminal repeat

The 3' half of an endogenous mouse mammary tumor virus from a C3H mouse was cloned in the Charon 4A vector phage. A comparison of the proviral clone with previously published endogenous mouse mammary tumor virus restriction maps identified it as endogenous unit II (J. Cohen and H. Varmus, Nature [London] 278:418-423, 1979), which is present in all inbred mouse strains derived from the original Bagg albino x DBA cross. The nucleotide sequence of the unit II long terminal redundancy (LTR) was determined and compared with the sequence previously determined for the exogenous C3H virus LTR (Donehower et al., J. Virol. 37:226-238, 1981). Virtually all sequence differences between the two LTRs were base substitutions. The total amount of sequence divergence was 6.6%. The large open reading frame reported previously in the exogenous LTR was preserved in the endogenous LTR. In addition, the pattern of sequence divergence was highly nonrandom with respect to the putative amino acid codons of the two open reading frames. Most of the base substitutions in this region resulted in silent or conservative amino acid codon changes. The nonrandom divergence pattern indicates that selective forces are operating on this segment of DNA and argues that the putative protein is functional in the life cycle of mouse mammary tumor virus. Possible roles for the protein and its mode of expression are discussed.

Targeted integration of baboon endogenous virus in the BEVI locus on human chromosome 6

The infection of cultured human cells with baboon endogenous virus (BEV) frequently leads to an association of viral DNA with a specific genetic locus (termed BEVI, for baboon endogenous virus infection) on chromosome 6. Restriction endonuclease digestion of DNA from BEV-infected human cells and their derived somatic cell clones frequently revealed a common cellular DNA sequence in the proximity of one of the junctions between cellular DNA and the integrated virus. We propose that a short cellular DNA sequence, repeated on chromosome 6 and separated by unique DNA sequences, presents a high-affinity target for the integration of BEV in human cells.

Integration of new endogenous mouse mammary tumor virus proviral DNA at common sites in the DNA of mammary tumors of C3Hf mice and hypomethylation of the endogenous mouse mammary tumor virus proviral DNA in C3Hf mammary tumors and spleens

To understand the molecular mechanisms by which the endogenous murine mammary tumor virus (MuMTV) proviruses are expressed and produce late-occurring mammary tumors in C3Hf mice, we analyzed, by the use of restriction enzymes and the Southern transfer procedure, genomic DNA from normal organs of mammary tumor-bearing and tumor-free mice and from 12 late-occurring C3Hf mammary tumors. We found, by using the restriction enzymes EcoRI and HindIII, that in addition to the preexisting endogenous MuMTV proviruses, new MuMTV-specific proviral DNA was integrated into new sites in the host genome in all 12 of the tumors that we examined. PstI digests of C3Hf tumor DNA revealed that the new proviral DNA found in C3Hf tumors was of endogenous origin. Moreover, the respective sizes of at least one of the new DNA fragments generated by EcoRI or HindIII digestion were the same in at least 50% of the C3Hf tumors analyzed, suggesting that the integration site of this new proviral DNA could be at the same location in the host genome of these tumors. Our results may imply that mammary tumorigenesis in C3Hf mice results from activation of cellular oncogenes by an MuMTV proviral DNA promoter. Specific hypomethylation of MuMTV proviral DNA was detected in the mammary tumors and spleens of C3Hf tumor-bearing mice. Our results indicated that most, if not all, of the hypomethylated MuMTV proviral DNA sequences were derived from the endogenous MuMTV provirus located at the MTV-1 locus, a locus responsible for the production of MuMTV antigens and increased incidence of mammary carcinoma in C3Hf mice. In spleens of non-tumor-bearing mice of ages 3, 6, 9, and 12 months, there was progressive hypomethylation of proviral DNA with increasing age, suggesting a possible correlation between demethylation of MuMTV proviral DNA in the spleens of C3Hf mice and the expression of endogenous MuMTV.

Cloned mouse mammary tumor virus DNA exhibits glucocorticoid-dependent expression in simian virus 40-transformed mink cells

Mink lung epithelial cells were transfected with two cloned mouse mammary tumor virus (MMTV) DNAs, a 9-kilobase clone derived from an unintegrated exogenous viral genome and a 14-kilobase clone containing an integrated endogenous provirus along with cellular flanking sequences. Mink lung cells were chosen because they do not contain endogenous MMTV sequences. On the basis of our observation that simian virus 40 DNA efficiently transforms these cells, we isolated cell clones containing MMTV DNA by using transformation with simian virus 40 DNA as a selective marker in cotransfection experiments. Levels of the 9-kilobase MMTV mRNA representing the entire viral genome and of the spliced 4.4-kilobase mRNA which codes for the viral envelope proteins were glucocorticoid dependent in transformed cells. Expression of low levels of Pr77gag, the precursor of the group-specific viral core proteins, and of gPr73env, the precursor of the viral envelope proteins, was also hormone dependent. We conclude that these cloned MMTV DNAs contain all the information necessary for the synthesis of normal viral RNAs and proteins. These findings also provide further evidence that the DNA sequences involved in the hormone responsiveness of MMTV expression are contained within the viral genome.

Isolation and analysis of genomic DNA clones encoding the third component of mouse complement

A gene library was constructed with DNA from strain A mice by using the phage lambda vector lambda 1059. By screening with cloned cDNA for the third component of mouse complement, C3, four different C3 genomic clones were isolated from this library. Two of the recombinant phages carry insertions of 14 and 18 kilobase pairs, respectively, which together cover one complete copy of the C3 gene and several hundred nucleotides of its 5' and 3' flanking sequences. The distance from the 5' end of the gene, which includes the hexanucleotide T-A-T-A-A-A and a translation initiation codon, to its 3' end as defined by the poly(A) attachment site is 24 kilobase pairs. From the genomic DNA sequence, a signal peptide of 24 amino acid residues is predicted at the NH2 terminus of the initial translation product. The signal peptide and the next two amino acids are encoded by the first exon of this gene.

Molecular cloning of retrovirus-like genes present in multiple copies in the Syrian hamster genome

Endogenous retrovirus-like sequences homologous to intracisternal type-A particle (IAP) genes, which are present in the inbred mouse (Mus musculus) genome, were cloned from a Syrian hamster gene library. A typical hamster IAP gene was 7 kb long and segments homologous to long terminal repeat (IAP) sequences present in Mus musculus IAP genes were located at both ends of the gene. Contrary to the pattern found in the Mus musculus IAP genes, the organization of the cloned hamster IAP genes was not markedly polymorphic and deletion was not observed among these cloned genes. A sequence about 0.8 kb long and located close to the 3' end of the hamster IAP gene was well conserved in both IAP gene families, although they showed less overall homology with one another. The reiteration frequency of the hamster IAP genes was calculated to be 950 copies per haploid genome. Since such IAP genes with the above properties were not found in the genome of the Chinese hamster, whose progenitors diverged from those of the Syrian hamster about 7.5 Myr ago, the integration of a huge number of Syrian hamster IAP genes must have occurred subsequent to such divergence.

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.