Genetic transmission of mammary tumour virus by GR mice

Prevalent de novo somatic mutations in superantigen genes of mouse mammary tumor viruses in the genome of C57BL/6J mice and its potential implication in the immune system

Background

Superantigens (SAgs) of mouse mammary tumor viruses (MMTVs) play a crucial role in T cell selection in the thymus in a T cell receptor (TCR) Vβ-specific manner and SAgs presented by B cells activate T cells in the periphery. The peripheral T cell repertoire is dynamically shaped by the steady induction of T cell tolerance against self antigens throughout the lifespan. We hypothesize that de novo somatic mutation of endogenous MMTV SAgs contributes to the modulation of the peripheral T cell repertoire.

Results

SAg coding sequences were cloned from the genomic DNAs and/or cDNAs of various tissues of female C57BL/6J mice. A total of 68 unique SAg sequences (54 translated sequences) were identified from the genomic DNAs of liver, lungs, and bone marrow, which are presumed to harbor only three endogenous MMTV loci (Mtv-8, Mtv-9, and Mtv-17). Similarly, 69 unique SAg sequences (58 translated sequences) were cloned from the cDNAs of 18 different tissues. Examination of putative TCR Vβ specificity suggested that some of the SAg isoforms identified in this study have Vβ specificities different from the reference SAgs of Mtv-8, Mtv-9, or Mtv-17.

Conclusion

The pool of diverse SAg isoforms, generated by de novo somatic mutation, may play a role in the shaping of the peripheral T cell repertoire including the autoimmune T cell population.

Reducing mammary cancer risk through premature stem cell senescence

The reproductive capacity of the mammary epithelial stem cell is reduced coincident with the number of symmetric divisions it must perform. In a study of FVB/N mice with the transgene, WAP-TGFbeta1, we discovered that mammary epithelial stem cells were prematurely aged due to ectopic expression of TGF-beta1. To test whether premature aging of mammary epithelial stem cells would have an impact on susceptibility or resistance to mammary cancer, female littermates from FVB/N x WAP-TGF-beta1 mating were injected with mouse mammary tumor virus (MMTV) at 8-10 weeks of age. A total of 44 females were inoculated, maintained as breeders and observed for tumor development for up to 18 months. Only one mammary tumor appeared in 17 TGF-beta1 females while 15 were collected from 29 wild type sisters. Premalignant mammary epithelial cells in infected glands were identified by transplantation of single cell (1 x 10(5)) suspensions into nulliparous hosts and testing for hyperplastic outgrowth. Although the number of positive takes was significantly reduced with TGF-beta1 cells, both MMTV-infected TGF-beta1 and wild type cells produced hyperplastic outgrowths suggesting that premalignant transformation was achieved in each group. The results suggest a positive correlation between the procreative life-span of mammary epithelial stem cells and mammary cancer risk.

Molecular cloning and functional analysis of three type D endogenous retroviruses of sheep reveal a different cell tropism from that of the highly related exogenous jaagsiekte sheep retrovirus

Integrated into the sheep genome are 15 to 20 copies of type D endogenous loci that are highly related to two exogenous oncogenic viruses, jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV). The exogenous viruses cause infectious neoplasms of the respiratory tract in small ruminants. In this study, we molecularly cloned three intact type D endogenous retroviruses of sheep (enJS56A1, enJS5F16, and enJS59A1; collectively called enJRSVs) and analyzed their genomic structures, their phylogenies with respect to their exogenous counterparts, their capacity to form viral particles, and the expression specificities of their long terminal repeats (LTRs). In addition, the pattern of expression of enJSRVs in vivo was studied by in situ hybridization. All of the three enJSRV proviruses had open reading frames for at least one of the structural genes. In particular, enJS56A1 had open reading frames for all structural genes, but it could not assemble viral particles when highly expressed in human 293T cells. We localized the defect for viral assembly in the first two-thirds of the gag gene by making a series of chimeras between enJS56A1 and the exogenous infectious molecular clone JSRV(21). Phylogenetic analysis distinguished five ovine type D retroviruses: enJSRV groups A and B, ENTV, and two exogenous JSRV groups (African versus United Kingdom/North America isolates). Transient transfection assays indicated that the LTRs of the three enJSRVs were not preferentially active in differentiated lung epithelial cells. This suggests that the pulmonary tropic JSRV developed from a type D retrovirus that did not have lung specificity. Consistent with this, in situ hybridization of a panel of normal ovine tissues revealed high expression of enJSRV mRNA in the luminal epithelium and glandular epithelium of the uterus; lower expression was localized in the lamina propria of the gut and in the bronchiolar epithelium of the lungs.

MMTV-induced mammary tumorigenesis: gene discovery, progression to malignancy and cellular pathways

The study of the mouse mammary tumor virus (MMTV) has provided important insights into the mechanisms of gene transcription regulation by steroid hormones, the mode of action of heritable super antigens and the progressive nature of neoplastic transformation in the mammary gland. Here we describe the current situation with respect to the latter aspect of MMTV biology and the prospects for further advance in our understanding of breast cancer in humans that may be expected from a continued study of MMTV-induced mammary neoplasia. MMTV is a heritable somatic mutagen whose target range is limited. Commonly, the tumorigenic capacity of MMTV is restricted to mammary gland, whereas infection is found in a variety of cell types. In order to replicate, proviral DNA must be inserted into the cell DNA and cell division is required to fix the mutation. Yet only in the mammary epithelium does this lead to neoplastic transformation. This suggests a unique relationship between MMTV and mammary epithelium. In evaluating this relationship, we and others have discovered genes and potential gene pathways that are pertinent in mammary differentiation and neoplasia. In addition, the clonal nature of these progressive events from normal to malignant phenotype has become increasingly clear. The weight of these observations compel us to conclude that mammary neoplasms arise from multipotent mammary epithelial cells through a process of acquired mutations that are reflected in the increasingly malignant nature of the population of progeny produced by these damaged stem cells.

In vivo effects of a recombinant vaccinia virus expressing a mouse mammary tumor virus superantigen

Early after infection, the mouse mammary tumor virus (MMTV) expresses a superantigen (SAg) at the surface of B lymphocytes. Interaction with the T-cell receptor Vbeta domain induces a polyclonal proliferative response of the SAg-reactive T cells. Stimulated T cells become anergic and are deleted from the T-cell repertoire. We have used a recombinant vaccinia virus encoding the MMTV(GR) SAg to dissect the effects of the retroviral SAg during an unrelated viral infection. Subcutaneous infection with this recombinant vaccinia virus induces a very rapid increase of Vbeta14 T cells in the draining lymph node. This stimulation does not require a large Plumber of infectious particles and is not strictly dependent on the expression of the major histocompatibility complex class II I-E molecule, as it is required after MMTV(GR) infection. In contrast to MMTV infection during which B cells are infected, we do not observe any clonal deletion of the reactive T cells following the initial stimulation phase. Our data show that contrary to the case with MMTV, macrophages but not B cells are the targets of infection by vaccinia virus in the lymph node, indicating the ability of these cells to present a retroviral SAg. The altered SAg expression in a different target cell observed during recombinant vaccinia virus infection therefore results in significant changes in the SAg response.

A novel V beta 2-specific endogenous mouse mammary tumor virus which is capable of producing a milk-borne exogenous virus

We have previously reported new Mtv loci, Mtv-48 and -51, in the Japanese laboratory mouse strains CS and NC. Here we show by backcross analysis that both Mtv-48 and -51 cosegregate with very slow deletion of T cells bearing V beta 2. The nucleotide sequences of the open reading frames in the 3' long terminal repeats of Mtv-48 and -51 were very similar to those of Mtv-DDO, mouse mammary tumor virus C4 [MMTV(C4)], and MMTV(BALB/cV), which encode V beta 2-specific superantigens. Furthermore, backcross female mice carrying Mtv-48 but not Mtv-51 were found to be able to produce milk-borne MMTV(CS), which can vigorously stimulate V beta 2-expressing T cells after local injection in vivo in an I-E-dependent manner. On the other hand, mice carrying Mtv-51 but not Mtv-48 could not produce such an MMTV in milk. The nucleotide sequences of MMTV(CS) open reading frame were completely matched with those of Mtv-48. These results indicate that the provirus Mtv-48 but not Mtv-51 is capable of producing a milk-borne virus of which the superantigen stimulates V beta 2-expressing T cells.

Dietary effects on gene expression in mammary tumorigenesis

Studies were undertaken to determine the effect a high fat diet has on the hormonally controlled transcription of the Mtv-1 locus in C3Hf mice. The expression of this locus in the initiating event in mammary tumor development in the C3Hf mouse. Mice were weaned at 21 days to either a high fat diet containing 23.5 percent corn oil or to a low fat diet containing 5 percent corn oil. Mice were sacrificed at first, second, and third parity, or when they had developed mammary tumors, and their mammary glands and mammary tumors were isolated. RNA was isolated from all mammary glands and breast tumors and analyzed. The high fat diet accelerated the transcription of the Mtv-1 locus. The transcripts of this locus, which are never seen in C3Hf mouse mammary glands until second parity, were present in first parity mammary glands of 6 out of 10 high fat diet C3Hf mice which were studied. The mammary glands of 15 first parity C3Hf mice which were on the low fat diet were analyzed and none contained the Mtv-1 specific transcripts. In addition, mammary tumor development was detected earlier (11 vs 17.8 months) and after fewer litters (2.1 vs. 4.2) on the average in high fat diet C3Hf mice. One C3Hf mouse on the high fat diet developed a breast tumor at six months without going through pregnancy. These results indicate that a high fat diet of 23.5 percent corn oil can accelerate hormonally controlled gene expression specifically linked to mammary tumorigenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary fat: gene expression and mammary tumorigenesis

Although discovered as an exogenous agent of mammary carcinoma, the mouse mammary tumor virus (MMTV) is also transmitted vertically as endogenous proviral DNA present in the germ line of all inbred mice. In the C3Hf mouse, which receives no exogenous virus, the initial event of mammary tumorigenesis is the transcription of the endogenous MMTV proviral DNA present at the Mtv-1 locus. Transcription occurs as a result of the hormonal effects of pregnancy, and Mtv-1 specific transcripts are seen in second-parity lactating mammary glands of these mice. As a means of studying the effects of diet on mammary carcinoma at the molecular/genetic level, we have studied the transcription of the Mtv-1 locus in C3Hf mice on a high-fat diet containing 46% fat in calories or a low-fat diet containing 10% fat in calories. We have detected an accelerated transcription of the Mtv-1 locus (first- vs. second-parity lactating mammary glands) in > 50% of the C3Hf mice on the high-fat diet. In addition, mice on the high-fat diet developed mammary tumors earlier (11 vs. 17.8 mos) and after fewer litters (2.1 vs. 4.2). Our results indicate that fat in the diet can affect gene expression related to mammary carcinoma.

Mouse mammary tumor virus-induced tumorigenesis in sag transgenic mice: a laboratory model of natural selection

Transgenic mice that expressed the superantigen protein encoded in the C3H exogenous mouse mammary tumor virus long terminal repeat deleted their V beta 14+ T cells during the shaping of their immune repertoire and showed no evidence of virus production in their mammary glands after infection by milk-borne C3H exogenous virus. However, they developed mammary gland tumors that had newly integrated copies of C3H exogenous virus, although the latency of tumor formation was much longer than in their nontransgenic littermates that retained their V beta 14+ T cells. After four generations, infectious C3H virus was completely eliminated from the transgenic mouse pedigree. These data support the hypothesis that endogenous mouse mammary tumor proviruses are retained in the genome as protection against exogenous virus infection and subsequent tumorigenesis and show that there may be natural selection against the virus in vivo.

Nonspecific augmentation of lymph node T cells and I-E-independent selective deletion of V beta 14+ T cells by Mtv-2 in the DDD mouse

DDD/1 (DDD) mice were characterized by marked paucity of T cells in lymph nodes (LN). In DDD-Mtv-2/Mtv-2 (DDD-Mtv-2) congenics, T cells were 4- to 18-fold increased depending on ages but B cells doubled at the most. Thymus weight also increased. In DDDfDDD-Mtv-2, DDD neonatally infected with Mtv-2-derived exogenous MMTV (MMTV-2), neither LN cells nor thymus weight increased. The V beta 5+ and V beta 8+ T cell contents in LN were practically the same among three strains. The Mtv-2-induced expansion of LN T cells was polyclonal and appeared indigenous to DDD mice. Both Mtv-2 and MMTV-2 induced progressive age-dependent deletion of V beta 14+CD4+ LN cells. Mtv-2 but not MMTV-2 caused deletion of V beta 14+CD8+ LN cells and mature V beta 14+CD4+ thymocytes. Thus, Mtv-2- and MMTV-2-induced V beta 14+ T cell deletion may reflect intrathymic and peripheral elimination, respectively. The absence of I-E gene expression in DDD indicates that V beta 14+ T cell deletion advances independently of I-E molecules in this experimental system.

Host genetic background effect on the frequency of mouse mammary tumor virus-induced rearrangements of the int-1 and int-2 loci in mouse mammary tumors

The frequency with which int-1 and int-2 are rearranged in mouse mammary tumors by mouse mammary tumor virus (MMTV)-induced insertional mutagenesis is a consequence of the host genetic background. In 75% of C3H mammary tumors, int-1 is rearranged by MMTV insertion, whereas only 30% of BALB/cfC3H tumors contain a virus-induced rearrangement of int-1. This difference is significant (P less than 0.005) and could not be accounted for by the potentially additive effect of the genetically transmitted Mtv-1-encoded virus in C3H mice. Similarly, MMTV-induced rearrangement of the int-2 gene in mammary tumors of the R111 mouse strain (59%) occurred at a significantly (P less than 0.025) higher frequency than in BALB/cfR111 (25%) mammary tumors. Moreover, in BALB/cfR111 mammary tumors, there is evidence that rearrangement of int-1 and int-2 does not occur independently (P less than 0.025). These results suggest that the long history of inbreeding for high tumor incidence of C3H and R111 mouse strains has selected for the fixation of host mutations which either complement the action of the particular int gene or affect the sensitivity of specific subpopulations of mammary epithelium to infection by particular strains of MMTV.

Establishment of DDD/1-Mtv-2/Mtv-2, nu/nu and DDD/1-Mtv-2/Mtv-2, nu/+ mice: preliminary characterization in relation to MTV antigen expression and mammary tumorigenesis

To investigate the effects of the T-cell deprivation on viral mammary tumorigenesis, two double congenic mouse strains of the DDD genetic background, DDD/1-Mtv-2/Mtv-2, nu/nu and DDD/1-Mtv-2/Mtv-2, nu/+, were produced by the cross between DDD/1-Mtv-2/Mtv-2 (DDD-Mtv-2) and DDD/1-nu/nu mice, followed by repeated intercross breedings. Expression of the mouse mammary tumor virus (MTV)-gp52 antigen was demonstrated in the mammary glands of mice from 14 days on, in both -nu/nu and -nu/+ females. Mammary gland development was comparable in both strains, but, the incidence of mammary cancer was lower in the T-cell-deprived mice.

Mouse mammary tumor virus (MMTV) infection in SWISS and RIII mice. Correlation between resistance to exogenous infection and anti-MMTV serum response

Host-virus relationships were examined in mice from the two mouse mammary tumor virus (MMTV)-infected strains SWISS MB+ and RIII, which harbour the same MMTV variant, and from the derived sublines Swiss MB- and RIIIf, which were freed of milk-borne MMTV by foster-nursing. These two strains are not phylogenetically related, the SWISS strain bearing the endogenous Mtv-3 locus in its DNA. In RIII and SWISS MB+ mice, the incidence of early mammary tumors, which was of 96% and 8%, respectively, was correlated to the level of MMTV expression in milk. In the SWISS MB-line, a non-coordinate expression of the provirus associated with the Mtv-3 locus was observed in the mammary glands, the salivary glands and the spleen. This expression was not tumorigenic and was characterized by the presence of the p28 gag antigen and the absence of the gp52 env antigen, except, however, in mammary glands of elder mice where traces of gp52 were found. In the mammary glands of SWISS MB+ mice, the expression of the Mtv-3 locus was masked by large amounts of antigens resulting from exogenous virus expression. RIIIf mice were MMTV-negative. Viral antigens coexisted with anti-MMTV antibodies in the serum of infected and tumor-bearing mice, but not in the form of immune complexes as verified by a method that allowed to detect specific antigen-containing-soluble immune complexes. An anti-MMTV serum reactivity was also detected in SWISS MB- and RIIIf mice. However, the serum response was higher in the two SWISS lines than in the two RIII lines. Except in tumor-bearing mice, the anti-MMTV response was not significantly modified by the presence of exogenous virus and thus resulted essentially from exposure to endogenous MMTV expression. In experimental infection studies, RIII mice were more susceptible to MMTV infection than SWISS mice. The correlation between resistance to MMTV infection and serum response to endogenous MMTV expression, suggests that the non-tumorigenic expression of an endogenous provirus can protect at least partially, against exogenous MMTV infection.

DNA rearrangements of the int region in spontaneous mouse mammary tumors of SHN/S and SLN/S mice

SHN and SLN mice originating from the same Swiss albino stock are genetically very close to each other. The incidence and latent period of mammary tumor development in SHN mice were higher and shorter than those in SLN. To elucidate these differences in the behavior of mammary tumorigenesis, the frequency of insertion of mammary tumor viral genes within the int-1 and int-2 regions in spontaneous mammary tumors from their two substrains, SHN/S and SLN/S, were compared. The frequency of provirus integration into either int-1 or int-2 in DNAs from mammary tumors was 52% (11/21) in SHN/S and 45% (5/11) in SLN/S. The frequency of insertion within int-1 or int-2 could not account for the different susceptibilities of SHN/S and SLN/S.

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

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

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.

Genetic resistance to mammary tumorigenesis in a mouse strain with high murine mammary tumor virus expression

Although II-TES mice release large amounts of murine mammary tumor virus (MMTV) in milk, they are resistant to mammary tumorigenesis. High mammary tumor incidence was observed in (BALB/ca X II-TES)F1 and (C57BL/6N X II-TES)F1, whereas no mammary tumors developed in BALB/ca X OZ-F)F1. Mammary tumors developed in 68% of (OZ-F X (OZ-F X II-TES)F1 and 45% of (II-TES X (OZ-F X II-TES)F1). These results suggest that the II-TES mouse carries a recessive gene for mammary tumor resistance which does not inhibit MMTV release, and two independent dominant mammary tumor promoting genes which are inhibited by the resistant gene.

A standardized nomenclature for endogenous mouse mammary tumor viruses

We propose a revised standardized nomenclature for endogenous mouse mammary tumor viruses based on characterization by molecular cloning techniques and genetic segregation data.

Activation of int-1 and int-2 mammary oncogenes in hormone-dependent and -independent mammary tumors of GR mice

Mammary tumors in the GR mouse strain are caused by the expression of an endogenous provirus of the mouse mammary tumor virus (MMTV). The tumors progress from a hormone-dependent growth phase to autonomous, hormone-independent growth. We studied proviral insertion of MMTV at the int-1 and int-2 mammary oncogenes and the transcription of these genes during progression of a series of transplanted mammary tumors. During the hormone-dependent phase, 6 of 15 transplanted tumors were positive for proviral insertion at int-1 or int-2 or both. These tumors were oligoclonal with respect to the fraction of tumor cells with novel int-1 and int-2 restriction fragments and, apparently, consisted of different tumor cells with proviruses integrated at different oncogenes, including genes that are not yet known. In 10 tumors we detected expression of the int genes, indicating that most tumors contain minor populations of cells with int-1 or int-2 activations. On transplantation the tumors remained oligoclonal during the hormone-dependent phase. The hormone-independent variants of the tumors emerged as clonal outgrowths of cells with MMTV proviruses that could be traced back in the hormone-dependent tumors, but not always those of cells that were positive for insertions near int-1 or int-2. The maintenance of oligoclonality during the hormone-dependent phase suggests a growth-controlling effect of different populations of cells on each other. The clonal, hormone-independent tumors that arise later seem to be the result of mutations that are unrelated to int activation.

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

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

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.

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.

An endogenous mouse mammary tumor virus genome common in inbred mouse strains is located on chromosome 6

We have examined EcoRI-restricted cellular DNA from mouse-hamster somatic cell hybrids. Results of this analysis show that the unit II mouse mammary tumor virus proviral genome is located on mouse chromosome 6. Restriction analysis of cellular DNA from (C3H/OuJ X Czech II) X Czech II backcross mice showed a strong linkage between unit II and Igk. The gene order of these markers on chromosome 6 relative to the Raf and Kirsten murine sarcoma virus ras-2 proto-oncogenes was established.

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.

Immunohistochemical expression of mammary tumor virus antigens in mammary glands of virgin mice, in relation to Mtv genes

MTV antigens in the resting mammary glands of GRS/A, SHN, C3H, and Balb/c virgin mice were detected by immunoperoxidase techniques using antiserum against MTV whole virion, gp 52 or p 25 to differentiate the expression between endogenous and exogenous MTV. Balb/c mice were crossed to infect MTV into each reciprocal hybrid or fosternursed inbred. Immunohistochemical stainings of gp 52 in the formol-fixed sections were almost the same as those of the whole MTV virion, and the results on various cases were as follows: In the mice with endogenous GR-MTV, positivity was first observed at the age of 14 days, while the first expression of exogenous GR-MTV was delayed to the age of 140 days. The mice with endogenous and/or exogenous SHN-MTV showed the first antigen appearance at the age of 65 days, and those with exogenous C3H-MTV did at the age of 80 days. The virgins with only endogenous C3H-MTV came to express the antigen after the age of 200 days. Staining of p 25 in Carnoy-fixed sections of MTV-positive mammary glands was found in the supranuclear cytoplasm and apical surface of the glandular cells and the lumen, all of which are the site of A and B particles. By means of preembedding method for gp 52, the reaction products were ultrastructurally detected not only on the MTV-budding apical surface, together with the intraluminal B particles, but also on the MTV-free apical cell membrane of the glandular cells in the mammary gland of the GR virgins.

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.

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.

Two genetically transmitted BALB/c mouse mammary tumor virus genomes located on chromosomes 12 and 16

We have examined EcoRI-restricted cellular DNA from BALB/c mouse-hamster somatic cell hybrids by blot hybridization for the presence of mouse mammary tumor virus-related sequences. Results of this analysis show that mouse mammary tumor virus-related proviral copies are located on chromosomes 16 (16-kilobase-pair fragment) and 12 (10.5- and 7.7-kilobase-pair fragments).

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.

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.

Effect of hormones on the expression of proviral genes Mtv-2 and Mtv-3 in mouse mammary gland

We have investigated the expression of the Mtv-2 and Mtv-3 proviral genes in mouse mammary glands by examining the effect of hormones on levels of mammary tumor virus (MTV) proteins p27 (gag) and gp52 (env) in mouse mammary explants. We also investigated the effect of the hormones on DNA synthesis in the explants. The mammary glands were derived from inbred GR and 020 mice, and from the respective congenic mouse strains GR/Mtv-2- and 020/Mtv-2+. The addition of insulin to the culture medium caused increases in p27 and gp52 levels in GR and 020/Mtv-2+ glands; a further increase in the viral proteins was obtained by also adding dexamethasone. Prolactin in combination with progesterone enhanced p27 and gp52 levels, but to a lesser extent than did dexamethasone. Dexamethasone caused a slight but significant increase in p27 protein in mammary explants from GR/Mtv-2- mice. Our data indicate that the Mtv-2 locus and the Mtv-3 locus in mouse mammary gland are under separate glucocorticoid control, and that Mtv-2 expression is also stimulated by the prolactin and progesterone combination. Whereas dexamethasone enhances MTV protein levels in mouse mammary explants, it inhibits DNA synthesis in the explants.

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.

Simple sandwich enzyme immunoassay for quantification of mouse mammary tumor virus in mouse milk

We have developed a sandwich enzyme immunoassay in order to measure quantitatively mouse mammary tumor virus (MMTV) in mouse milk. In this assay, the antibody-beta-D-galactosidase complex and antibody-bound silicon rubber pieces pieces as solid phase are used. The assay is able to detect 10 ng/ml of MMTV in the milk sample.

Genetic mapping of endogenous mouse mammary tumor viruses: locus characterization, segregation, and chromosomal distribution

The restriction endonuclease EcoRI has been used to study the inheritance of strain difference in endogenous mouse mammary tumor virus DNA sequences. This enzyme, which cleaves at only one site within the nondefective viral genome, generates DNA fragments containing mouse mammary tumor virus sequences which vary in size according to the locations of EcoRI restriction sites in the flanking mouse sequences, thereby defining unique integration sites of the viral genome. Recombinant inbred strains of mice have been used to study the inheritance of these DNA fragments which hybridize to mouse mammary tumor virus cDNA sequences. The results define 11 segregating units consisting of 1 or 2 fragments. These units were shown to segregate among the recombinant inbred strains, and in some instances linkage was established. Two units were shown to be linked on chromosome 1. Another unit was mapped to chromosome 7, which is presumably identical to the previously defined genetic locus Mtv- 1. One other mouse mammary tumor virus locus was tentatively assigned to chromosome 6. The results are consistent with the view that integration of mouse mammary tumor virus can take place at numerous sites within the genome, and once inserted, these proviruses appear to be relatively stable genetic entities.

Mammary tumor virus proviral DNA in normal murine tissue and non-virally induced mammary tumors

The Southern DNA filter transfer technique was used to study the involvement of the endogenous mouse mammary tumor virus (MMTV) in the development of mammary tumors of nonviral etiology. The presence of extra MMTV proviruses in the genomes of these non-virally induced mammary tumors would indicate an integration of the provirus of an activated endogenous MMTV. Acquisition of MMTV proviruses did not seem to be an absolute requirement for the development of hormone or carcinogenically induced mammary tumors in strain BALB/c nor for hormone-induced mammary tumors in mouse strains 020, C57BL, and C3Hf. In some hormone-induced mammary tumors we did observe extra MMTV proviruses in submolar quantities, indicating that reintegration may occasionally occur and that only a part of the tumor cells acquired new MMTV DNA information. Hormone-dependent and -independent primary mammary tumors of the mouse strain GR, which are controlled by the Mtv-2 mammary tumor induction gene, all acquired extra MMTV proviruses. Most of these extra MMTV proviral-DNA-containing fragments appeared present in submolar quantities, suggesting that only part of the tumor cells acquired extra MMTV proviral information. These findings indicate that the initially transformed mammary gland cells of non-virally induced mammary tumors do not necessarily acquire extra MMTV proviral DNA information, in contrast to the MMTV-induced mammary tumors, in which all tumor cells contain extra MMTV DNA information.

Mammary tumor induction loci in GR and DBAf mice contain one provirus of the mouse mammary tumor virus

The mammary tumor induction genes Mtv-1 in mouse strain DBAf and Mtv-2 in strain GR control the complete expression of the endogenous mouse mammary tumor virus (MMTV). We have used a combination of genetic, biochemical and molecular biological methods to identify and correlate specific copies of the endogenous MMTV proviral genes with the biological properties of the tumor induction genes Mtv-1 and Mtv-2. These Mtv induction genes contain specific MMTV proviral information, as was concluded from restriction enzyme analysis and molecular hybridization of DNAs of congenic mouse strains and of progenitors of backcross populations. The congenic strains differed from the parental strains GR and 020 only in the Mtv-2 gene, one lacking the Mtv-2 gene (GR/Mtv-2-) and one having obtained this gene (020/Mtv-2+). The gain or loss coincided with two Eco RI cellular DNA fragments containing MMTV DNA information. Since Eco RI cuts the exogenous proviral variant of MMTV DNA once, we assume that these two cellular DNA fragments contain one MMTV provirus. The same cellular DNA fragments containing MMTV DNA information segregated together with MMTV expression in the offspring population of the backcross. In a similar backcross analysis of the induction gene Mtv-1 it was also demonstrated that the Mtv-1 gene comprises one MMTV provirus. These data indicate that Mtv induction genes contain specific but different MMTV proviral genes and that nly a limited number of the MMTV proviruses present in the cellular DNA is associated with the control of proviral expression.

Localization of a gene for expression of mouse mammary tumor virus antigens in the GR/Mtv-2- mouse strain

The GR/Mtv-2- mouse strain is congenic to the GR strain but lacks the Mtv-2 gene for high amounts of mouse mammary tumor virus (MMTV) virion particles in the milk and early mammary tumors. With a sensitive competition radioimmunoassay for individual viral proteins of MMTV, substantial amounts of the gag proteins p27 and p10 could still be detected in extracts of the mammary glands of GR/Mtv-2- mice, but essentially no viral envelope antigens. The genetic transmission of the MMTV gag expression in the GR/Mtv-2- strain was investigated. In a cross with the virus-negative BALB/c strain, the MMTV p27 expression behaved as a dominant feature. Double backcross analysis proved that the p27 expression was governed by a single gene located on chromosome 11, cloe to the Es-3 locus. The gene was thereby not allelic to any of the previously described MMTV induction genes, Mtv-1 and Mtv-2, and is therefore called Mtv-3. It is concluded that the total MMTV expression in the GR strain is under control of two separate loci, Mtv-2 on chromosome 18, inducing high levels of complete virus particles and also early mammary tumors; and Mtv-3 on chromosome 11, coding for partial MMTV expression.

Proviruses of mouse mammary tumor virus in normal and neoplastic tissues from GR and C3Hf mouse strains

We analyzed two experimental situations to assess the role of endogenous mouse mammary tumor virus (MMTV) DNA in the genesis of mammary carcinomas. (i) GR mice carry in their germ line one or more proviruses indistinguishable by limited restriction mapping from the proviruses introduced into cells by experimental infection with the highly tumorigenic virus isolated from GR mouse milk, MMTV(GR). Most tumors arising in GR mice contain one or more proviruses at various sites in tumor DNA in addition to those present endogenously. Detection of these new proviruses is possible as a consequence of the clonal or quasiclonal character of the tumors. (ii) C3H/He mice carry three units of endogenous viral DNA, none of which resembles the DNA of the commonly encountered strains of milk-borne MMTV. Nevertheless, MMTV-associated tumors arise late in life when these animals are removed from the influence of milk-borne virus; the responsible agent, MMTV(C3Hf), can also produce tumors in BALB/c mice. We found that tumors arising in both C3Hf/He mice and BALB/c mice infected with MMTV(C3Hf) were clonal or quasiclonal and contained one or more new copies of proviral DNA at various sites in the host genome. These new proviruses were readily distinguished from the proviruses of the common milk-borne virus strains and closely resembled unit II of endogenous MMTV DNA (Cohen et al., J. Virol., 32:483-496). Thus, in both experimental systems, we found evidence for new proviruses in mammary tumors, despite the preexistence of similar or identical proviruses in the germ line. The results suggest that the repositioning of MMTV proviruses may be required for the full expression of the oncogenic potential of endogenous MMTV DNA.