Organization of mouse mammary tumor virus-specific DNA endogenous to BALB/c mice

Exploring the link between viruses and cancer in companion animals: a comprehensive and comparative analysis

Currently, it is estimated that 15% of human neoplasms globally are caused by infectious agents, with new evidence emerging continuously. Multiple agents have been implicated in various forms of neoplasia, with viruses as the most frequent. In recent years, investigation on viral mechanisms underlying tumoral transformation in cancer development and progression are in the spotlight, both in human and veterinary oncology. Oncogenic viruses in veterinary medicine are of primary importance not only as original pathogens of pets, but also in the view of pets as models of human malignancies. Hence, this work will provide an overview of the main oncogenic viruses of companion animals, with brief notes of comparative medicine.

Mouse Mammary Tumor Virus (MMTV) and MMTV-like Viruses: An In-depth Look at a Controversial Issue

Since its discovery as a milk factor, mouse mammary tumor virus (MMTV) has been shown to cause mammary carcinoma and lymphoma in mice. MMTV infection depends upon a viral superantigen (sag)-induced immune response and exploits the immune system to establish infection in mammary epithelial cells when they actively divide. Simultaneously, it avoids immune responses, causing tumors through insertional mutagenesis and clonal expansion. Early studies identified antigens and sequences belonging to a virus homologous to MMTV in human samples. Several pieces of evidence fulfill a criterion for a possible causal role for the MMTV-like virus in human breast cancer (BC), though the controversy about whether this virus was linked to BC has raged for over 40 years in the literature. In this review, the most important issues related to MMTV, from its discovery to the present days, are retraced to fully explore such a controversial issue. Furthermore, the hypothesis of an MMTV-like virus raised the question of a potential zoonotic mouse–man transmission. Several studies investigate the role of an MMTV-like virus in companion animals, suggesting their possible role as mediators. Finally, the possibility of an MMTV-like virus as a cause of human BC opens a new era for prevention and therapy.

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.

Quantitation of endogenous mouse mammary tumor virus superantigen expression by lymphocyte subsets

Superantigens (SAg) encoded by endogenous mouse mammary tumor viruses (Mtv) interact with the V beta domain of the T cell receptor (TcR-V beta). Presentation of Mtv SAg can lead to stimulation and/or deletion of the reactive T cells, but little is known about the quantitative aspects of SAg presentation. Although monoclonal antibodies have been raised against Mtv SAg, they have not been useful in quantitating SAg protein, which is present in very low amounts in normal cells. Alternative attempts to quantitate Mtv SAg mRNA expression are complicated by the fact that Mtv transcription occurs from multiple loci and in different overlapping reading frames. In this report we describe a novel competitive polymerase chain reaction assay which allows the locus-specific quantitation of SAg expression at the mRNA level in lymphocyte subsets from mouse strains with multiple endogenous Mtv loci. In B cells as well as T cells (CD4+ or CD8+), Mtv-6 SAg is expressed at the highest levels, followed by Mtv-7 SAg and (to a much lesser extent) Mtv-8,9. Consistent with functional Mtv-7 SAg presentation studies, we find that Mtv-7 SAg expression is higher in B cells than in CD8+ T cells and very low in the CD4+ subset. The overall hierarchy in Mtv SAg expression (i.e. Mtv-6 > Mtv-7 > Mtv 8,9) was also observed for mRNA isolated from neonatal thymus. Furthermore, the kinetics of intrathymic deletion of the corresponding TcR-V beta domains during ontogeny correlated with the levels of Mtv SAg expression. Collectively our data suggest that T cell responses to Mtv SAg are largely controlled by SAg expression levels on presenting cells.

Structure and biological activity of the subgenomic Mtv-6 endogenous provirus

The Mtv-6 provirus has an incomplete genome, but retains a functional superantigen gene (sag) which directs the thymic deletion of CD4+ T cells expressing T cell receptors containing the V beta 3 or V beta 5 chains. To better understand the Mtv-6 superantigen, the structure and biological activity of the Mtv-6 provirus was analyzed. First, the complete nucleotide sequence was determined, and the mutation producing the subgenomic provirus was identified. Second, the nucleotide sequence of the 5' end of the sag gene transcript (including the splice junction) was determined by sequence analysis of a cDNA clone. Third, the superantigen activity of Mtv-6 was analyzed in mice carrying the Mtv-6 provirus isolated by selective breeding on a genetic background free of endogenous and exogenous mouse mammary tumor virus (MMTV). These studies demonstrate that (i) the Mtv-6 provirus contains a 6.2-kb deletion between two 12-bp direct repeats encompassing the central portion of the provirus but not affecting sag gene splicing or translation, (ii) the sag gene transcript has the structure predicted from previous S1 nuclease mapping studies, and (iii) the Mtv-6 superantigen can direct thymic deletion of target V beta 3+ and V beta 5+ T cells in the absence of gene products from full-length MMTV proviruses.

Factors controlling the expression of mouse mammary tumour virus

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

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

Influence of mammary cell differentiation on the expression of proteins encoded by endogenous BALB/c mouse mammary tumor virus genes

The interactions between differentiation-associated cellular events in the intact mammary gland or in cultured mammary cells and the post-transcriptional activity of the endogenous mouse mammary tumor virus (MMTV) loci were investigated. The transcriptional activities of the endogenous MMTV proviruses of the BALB/c mouse strain (Mtv-6, Mtv-8 and Mtv-9) appear to be regulated differentially during pregnancy-induced mammary gland development (J.E. Knepper, D. Medina and J.S. Butel, J. Virol. 59, 518-521, 1986). Analysis of MMTV-specific proteins at various stages of mammary gland development (virgin, midpregnant, lactating, regressing) established the presence of steady-state levels of a 67,000-Mr env precursor-type polypeptide at all physiological stages. However, processing to lower-molecular-weight env-specific proteins, including a predominant 50,000-Mr species, was detected only with the transition to the functional mammary gland phenotype. The contributions of cell proliferation, cell-matrix interactions, and modulation of functional activity to the pattern of endogenous MMTV protein expression were investigated using a 3-dimensional collagen type I culture system. Growth and cell-matrix interactions (cell polarization, lumen formation) leading to formation of 3-dimensional duct-like structures were permissive for the synthesis and processing of MMTV-specific proteins; accumulation of high levels of the 50,000-Mr env-specific polypeptide was associated with the onset of the fully functional mammary cell phenotype. Expression of MMTV-specific proteins was not due to amplification of a specific cell subpopulation. The potential of the full-length Mtv-8 and Mtv-9 proviruses to be transcribed, as indicated by their methylation status, was not dramatically different between differentiated and undifferentiated mammary cells in culture. This study indicates that MMTV transcriptional activity is reflected at the protein level in mammary tissue of BALB/c mice and that viral protein synthesis and processing may serve as important markers of different physiological stages of mammary epithelial cells. These observations also suggest a general approach to the examination of potential modulatory effects of cellular interactions (cell-cell, cell-matrix or both) known to be important in various differentiated epithelial cell systems for the expression of viral genes.

Spontaneous progression of hyperplastic outgrowths of the D1 lineage to mammary tumors: expression of mouse mammary tumor virus and cellular proto-oncogenes

Mammary cancer in mice is characterized by progression through defined stages of preneoplasia, with the most common preneoplastic stage being the hyperplastic alveolar nodule (HAN). We determined the relative levels of RNA expression of various cellular proto-oncogenes and endogenous mouse mammary tumor virus genes in outgrowths and tumors of three sublines of the transplantable D1 HAN preneoplastic outgrowth line. The three sublines differed in relative tumor-producing capabilities. Subline D1B produced a high incidence of tumors with short latency periods, whereas sublines D1C and D1D produced low incidences of tumors with long latency periods. No consistent alteration in proto-oncogene expression correlated with relative tumorigenicity, although tumors frequently contained higher levels of one or more proto-oncogene transcripts as compared with preneoplastic tissue. Slightly elevated (2- to 6-fold) levels of different oncogene transcripts were detected in 13 of 17 tumors as compared with outgrowth tissue, including abl (2 tumors), fps (5 tumors), Ha-ras (6 tumors), and Ki-ras (8 tumors). One tumor contained 45 times more Ki-ras-specific RNA than outgrowth tissue because of a comparable amplification of Ki-ras DNA sequences. Elevated levels of Ha-ras occurred more frequently in tumors of a high-incidence subline than in a less-aggressive subline (5/10 vs 1/7), but this difference was not statistically significant. However, consistent changes in MMTV expression accompanied progression from preneoplastic tissues to mammary tumors. All 17 tumors displayed reduced levels of the MMTV-specific long terminal repeat (LTR) transcript (1.6 kb) as compared with HAN tissue; tumors with moderate levels of LTR transcript expressed the 3.8-kb envelope message as well, one not detected in HANs. Expression of the LTR transcript is apparently influenced by factors in addition to the methylation status of endogenous mouse mammary tumor virus genes, which was similar in outgrowths and tumors. As the survey of representative proto-oncogenes failed to identify a uniform change between HAN and tumors, it is likely that other genes are involved in tumor progression in the mammary gland.

Endogenous mouse mammary tumor virus is expressed in several organs in addition to the lactating mammary gland

We studied the transcription of endogenous mouse mammary tumor virus (MMTV) in transgenic and normal strains of mice of different genetic backgrounds. Although the lactating mammary gland in all strains showed the highest level of MMTV expression, detectable levels of viral RNA were also found in the lungs, kidneys, salivary glands, seminal vesicles and/or prostate gland, testes, and lymphoid tissue in mice which contain different endogenous proviruses. Transcription in transgenic mice containing the MMTV long terminal repeat linked to the simian virus 40 early region also occurred in these same organs. These results indicate that the MMTV long terminal repeat functions in several organs in addition to the lactating mammary gland.

Activation of endogenous MMTV proviruses in murine mammary cancer induced by chemical carcinogen

A study was undertaken to determine whether activation of expression of silent endogenous mouse mammary tumor virus (MMTV) proviruses may occur during tumor induction by a chemical carcinogen. A series of transplantable mammary tumors induced in BALB/c mice by treatment with dimethylbenz(alpha)anthracene (DMBA), pituitary isograft, or both was examined. The results obtained suggest that chemical carcinogens may induce mammary tumors through more than one pathway. Two of 9 tumor lines produced virus-specific products at levels above those observed during the course of normal mammary gland development. One tumor contained high levels of MMTV-specific envelope [3.8 kilobase (kb)] and genomic length (8.9 kb) RNAs. This tumor expressed core- and envelope-related proteins detectable by immunoblotting (including p28, gp52, and gp36), displayed an acquired provirus with a restriction map different from those of described exogenous MMTV strains, and contained abundant virus particles. The other tumor that expressed high levels of MMTV gene products contained envelope-specific (3.8 kb) and long-terminal-repeat-specific (1.6 kb) messages but no full-length RNA. It exhibited an aberrant 39 kDa, envelope-related protein, but no virus particles. Methylation data implicated the usually silent endogenous Mtv-8 provirus as the source of the abnormal envelope protein. None of the tumors expressed RNA from the putative mammary oncogenes, int-1 or int-2. We propose that chemical carcinogens may activate different cellular genes by mutation and that, in a subset of DMBA-induced mammary tumors, the target genes include endogenous MMTV proviruses that are normally not expressed. The effect on provirus expression varies from tumor to tumor, but is stable over passage of a given tumor. MMTV may be of etiological importance in the genesis of those DMBA-induced tumors which contain high levels of MMTV-specific products, but its action in the BALB/c system is not mediated through enhanced expression of the int-1 or int-2 preferred integration regions.

The endogenous retrovirus Mtv-8 on mouse chromosome 6 maps near several kappa light chain markers

Endogenous retroviruses are known to affect expression of cellular genes in the vicinity of their integration sites. The endogenous mouse mammary tumor provirus (Mtv-8) previously has been reported to reside on mouse chromosome 6 near the immunoglobulin kappa chain locus. Using pairs of mouse strains on the BALB/c (Mtv-8 positive) and C58 (Mtv-8 negative) backgrounds which are congenic for chromosome 6 genetic markers, we have confirmed the chromosome assignment of this provirus. Moreover, we have analyzed the N1 progeny of a (B6 X C58) X C58 backcross to determine the segregation of the Mtv-8 provirus with respect to polymorphisms in the Igk-VSer and Igk-J loci. The results with congenic and backcross mice together with results of others suggest that Mtv-8 is located approximately 0.52 cM from several closely linked kappa markers on chromosome 6.

Inheritance of a mutant histocompatibility gene and a new mammary tumor virus genome in the B6.KH-84 mouse strain

The potential association between integration or deletion of mouse mammary tumor virus (MMTV) retroviral sequences and the appearance of non-H-2 histocompatibility (H) antigen mutations was investigated. Genomic blots from inbred strains carrying 22 loss, gain-loss, and gain mutations on the BALB/c and C57BL/6 backgrounds were hybridized with probes homologous to the long terminal repeat (LTR) and envelope (env) regions of MMTV. Twenty-one mutants were identical in restriction patterns to the respective background strains with all tested restriction enzymes and both probes. However, genomic blots of one gain mutant, B6.C-KH-84, exhibited restriction fragments which were not exhibited by either of the parental strains, C57BL/6 or BALB/c. An additional 5.5 kb Eco RI fragment was observed with the env probe and additional 9.2 kb and 5.5 kb fragments were observed with the LTR probe. These observations were substantiated by hybridization of these two probes with genomic blots generated with additional restriction enzymes. Assuming that the new provirus contains a single, internal Eco RI site as has been observed for other MMTV proviral sequences, it is presumed that the new provirus includes both 5' and 3' LTRs in addition to the env region. Based on the unique sizes of the observed restriction fragments relative to other identified MMTV proviral sequences, this provirus has been designated Mtv-22. The potential role of Mtv-22 in the genesis of the gained histocompatibility antigen in B6.C-KH-84 is discussed.

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.

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.

Differential expression of endogenous mouse mammary tumor virus genes during development of the BALB/c mammary gland

Expression of endogenous mouse mammary tumor virus sequences varied over the course of development of the mammary gland during primary pregnancy and lactation in virus-free BALB/c mice. Although RNA from all regions of the genome was detected, both the level and temporal regulation of expression were different for long terminal repeat-, env-, and gag-pol-specific RNAs. Analysis of the methylation status of proviral DNA indicated differential accessibility of the three endogenous units during development. The results demonstrated noncoordinate regulation of mouse mammary tumor virus expression with respect to provirus template utilized and specific transcripts accumulated.

Direct detection of exogenous mouse mammary tumor virus sequences in lymphoid cells of BALB/cfC3H female mice

The presence of exogenous mouse mammary tumor virus (MMTV) (C3H) DNA sequences in lymphoid tissue (spleen, bone marrow, and thymus) and nonlymphoid tissue (liver and kidney) of BALB/cfC3H female mice was directly assessed by DNA hybridization methods. Lymphoid tissues were found positive for integrated MMTV(C3H) sequences in females as young as 4 weeks. In most samples, the level of splenic MMTV(C3H) infection was low (2 to 5%). Infection remained throughout the life of the animal. The percentage of spleen samples found positive for exogenous viral infection was significantly higher in females bearing mammary tumors, whether virgin or multiparous. Liver and kidney DNAs were negative for exogenous MMTV sequences, suggesting tissue type selectivity in MMTV infection.

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

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

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

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

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

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

Cellular genes in the mouse regulate in trans the expression of endogenous mouse mammary tumor viruses

The transcriptional activities of the eleven mouse mammary tumor virus (MMTV) proviruses endogenous to two sets of recombinant inbred (RI) mouse strains, BXD and BXH, were characterized. Comparison of the levels of virus-specific RNA quantitated in each strain showed no direct relationship between the presence of a particular endogenous provirus or with increasing numbers of proviruses. Association of specific genetic markers with the level of MMTV-specific RNA was examined by using multiple regression analysis. Several cellular loci as well as proviral loci were identified that were significantly associated with viral expression. Importantly, these cellular loci associated with MMTV expression segregated independently of viral sequences.

Mouse mammary tumor virus gene expression regulated in trans by Lps locus

Expression of mouse mammary tumor virus (MMTV) in the lactating mammary glands of uninfected mice varies between strains of mice in a manner largely independent of the proviral content. Previous linkage analysis in the mouse suggested that the Lps locus was associated with steady-state levels of MMTV RNA. The Lps locus mediates the mouse's response to the injection of lipopolysaccharide (LPS) in the responder mouse while mice with the deficient allele are incapable of responding. Injecting LPS-responder mice, C3HfB/HeN, and nonresponder mice, C3Hf/HeJ, with LPS resulted in a threefold increase in the level of MMTV RNA in responder mice but had no effect on nonresponders. The increased level was due to only one of the possible MMTV transcripts: the 1.7-kb transcript containing the open reading frame (orf) of the long terminal repeat (LTR). The level of MMTV-specific transcripts, then, is regulated by the Lps locus, a cellular gene which is not linked to any viral coding sequences and therefore must act in trans.

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

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

Identification and characterization of a mouse mammary tumor virus protein uniquely expressed on the surface of BALB/cV mammary tumor cells

A unique subline of BALB/c mice, designated BALB/cV, exhibits an intermediate mammary tumor incidence (47%) and harbors a distinct milk-transmitted mouse mammary tumor virus (MMTV). The BALB/cV subline was used to study the molecular basis of potential virus-host interactions involving cell surface-expressed MMTV proteins. Cell surface iodination identified virus-specific proteins expressed on BALB/cV primary mammary tumor cells grown in culture. In contrast to (C3H)MMTV-producing cell lines which expressed MMTV gp52, BALB/cV tumor cells lacked gp52 and expressed instead a 68K, env-related protein. The 68Kenv protein was also detected on the surface of metabolically labeled BALB/cV tumor cells by an external immunoprecipitation technique. The expression of 68Kenv was restricted to mammary tissues of BALB/cV mice that also expressed other MMTV proteins. Biochemical analysis established that 68Kenv was not modified by N-linked glycosylation. 125I-labeled 68Kenv was rapidly released into the media of tumor cell cultures and was recovered both in the form of a soluble protein and in a 100,000 g pellet. The biologic function of this cell surface-expressed viral protein remains unknown.

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

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

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.

Mammary tumorigenesis in C3Hf/Ki mice: examination of germinal mouse mammary tumor viruses and the int-1 and int-2 putative proto-oncogenes

The organization and expression of endogenous mouse mammary tumor virus (MMTV) proviruses in normal and neoplastic C3Hf/Ki tissues were examined. MMTV-containing EcoRI, HindIII, BamHI and PstI restriction fragments of C3Hf/Ki DNA were identical to those of C3H/StWi DNA. The full-length endogenous MMTV Units Ia (Mtv-7), II (Mtv-8), III (Mtv-9) and IV (Mtv-10), in addition to the subgenomic endogenous MMTV Units I (Mtv-6) and IX (Mtv-14), were germinally transmitted in C3Hf/Ki DNA. The previously uncharacterized Mtv-7 was contained in EcoRI fragments of 16.7 and 11.7 kbp. The endogenous MMTV Unit V (Mtv-1), which is responsible for virus production and mammary tumorigenesis in C3Hf/He mice, was absent from C3Hf/Ki DNA. The 9.0 kb gag-pol, the 3.8 kb env and the 1.7 kb LTR MMTV RNA transcripts were present in C3Hf/Ki mammary glands. MMTV proviruses, in addition to the endogenous C3Hf/Ki MMTV complement, were not detected in C3Hf/Ki mammary tumor DNA. The DNA organization and RNA expression of the putative mammary proto-oncogene regions int-1 and int-2 were also examined in C3Hf/Ki mammary tumors. The int-1 and int-2 regions did not appear rearranged, amplified, or expressed in C3Hf/Ki mammary tumors. These studies indicate that MMTV proviral activation of the int proto-oncogenes is not necessary for C3Hf/Ki mammary tumorigenesis.

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

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

Bacterial chromosomal restriction endonuclease analysis of the homology of Bacteroides species

Chromosomal DNAs of selected Bacteroides organisms whose relatedness had been previously determined by "conventional" filter-annealing studies (J. L. Johnson, Int. J. Syst. Bacteriol. 28:245, 1978) were further analyzed by restriction endonuclease analysis coupled with the Southern hybridization procedure (E. M. Southern, J. Mol. Biol. 98:503, 1975). By comparing their EcoRI restriction fragment patterns in agarose gel electrophoresis, each Bacteroides strain could be clearly differentiated. As a simple and direct means for comparison purposes this method was particularly useful for differentiating genetically similar organisms such as Bacteroides strains of the same species which shared greater than 75% homology. In contrast, bacterial chromosomal restriction endonuclease analysis in conjunction with Southern hybridizations was most effectively used to determine the significance of low levels of homology (less than 24%) as this technique provided additional information on the nature and relative distribution of that homology when the areas of homology were displayed as reproducible bands in autoradiograms.

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.

Proteins encoded by the long terminal repeat region of mouse mammary tumor virus: identification by hybrid-selected translation

The long terminal repeat (LTR) region of mouse mammary tumor virus (MMTV) is known to contain an open reading frame of sufficient length to code for a protein of 36,000 Mr. The coding capacity of the 3' sequences of MMTV genomic RNA has been demonstrated by in vitro translation studies, which have reported the synthesis of four related proteins: p36, p24, p21, and p18. These proteins are overlapping translation products of the same open reading frame, with the smaller ones initiating at internal methionine codons. From the predicted amino acid sequence of the LTR protein, we have selected a region likely to be antigenic, obtained a synthetic peptide of that region, and raised antiserum to the peptide. The antipeptide serum specifically immunoprecipitated all four proteins from in vitro translated genomic 3' MMTV RNA, plus an additional one of 32,000 Mr. Published sequence data of MMRV LTRs show an internal AUG codon at a position which could initiate a protein of 32,000 Mr. The three smaller in vitro translation products (p24, p21, and p18) were consistently synthesized in much greater amounts than the p36 or p32 protein. The relative amount of each in vitro synthesized protein from genomic MMTV RNA could be predicted and was in good agreement with the postulated effect of flanking nucleotides on the efficiency of the respective AUG initiation codon. Polyadenylated RNAs, isolated from various mouse tissues, were selected by hybridization to plasmid DNA containing MMTV LTR sequences immobilized on nitrocellulose. In vitro translation of hybrid-selected mRNAs isolated from BALB/c mouse lactating mammary glands and carcinogen-induced mammary tumors, followed by immunoprecipitation with antipeptide serum, revealed that only one polypeptide was synthesized by the MMTV LTR-specific mRNA, the 36,000 Mr species.

Mouse mammary tumor virus DNA methylation: tissue-specific variation

Mouse mammary tumor virus-specific DNA sequences endogenous to the BALB/c mouse are shown to exhibit variable levels of methylation in a tissue-specific manner. In DNA from both lactating mammary gland and spleen, MMTV-specific sequences were hypomethylated at specific HpaII and HhaI sites. These variably methylated sites were found in the terminal repetitive sequences of the endogenous viral genomes. The specific hypomethylation of a HpaII site in Mtv-9 is associated with expression of a 1.6 kb transcript in the lactating mammary gland.

Tumorigenesis by mouse mammary tumor virus: proviral activation of a cellular gene in the common integration region int-2

Approximately 50% of tumors induced by mouse mammary tumor virus (MMTV) contain an acquired provirus within a limited region of chromosomal DNA, termed int-2. We have extended our previous characterization of this locus and have mapped provirus integration sites in 21 independent tumors. Although integration occurs at multiple sites, proviruses within int-2 are distributed into two oppositely oriented groups whose transcription is directed away from a central domain. Provirus insertion in int-2 is accompanied by expression of RNA derived, at least in part, from this central domain. Since the RNA is not detected in normal mammary tissue, we conclude that MMTV integration activates the expression of a cellular gene within int-2 and that this event may contribute to tumorigenesis.

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.

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.

Immunological selection of variant mouse lymphoid cells with altered glucocorticoid responsiveness

We have devised an immunological procedure to separate cells on the basis of expression of mouse mammary tumor virus (MMTV) gene products. Plastic petri dishes coated with specific antibodies against MMTV proteins bind cells with an efficiency that correlates with the level of MMTV gene expression. Glucocorticoid-sensitive mouse thymoma cell line W7 was infected with MMTV. Clones from the infected population retain the relatively slow cytolytic glucocorticoid response and, in addition, exhibit a rapid induction of MMTV-specific RNA and proteins. By combining our immunological selection with the selection for resistance to hormone-mediated cytolysis, we have isolated variant cells which are resistant to the cytotoxic effect of glucocorticoids but which retain the induction of viral gene products and must therefore have a functional glucocorticoid receptor protein.

Immunological selection of variant mouse lymphoid cells with altered glucocorticoid responsiveness

We have devised an immunological procedure to separate cells on the basis of expression of mouse mammary tumor virus (MMTV) gene products. Plastic petri dishes coated with specific antibodies against MMTV proteins bind cells with an efficiency that correlates with the level of MMTV gene expression. Glucocorticoid-sensitive mouse thymoma cell line W7 was infected with MMTV. Clones from the infected population retain the relatively slow cytolytic glucocorticoid response and, in addition, exhibit a rapid induction of MMTV-specific RNA and proteins. By combining our immunological selection with the selection for resistance to hormone-mediated cytolysis, we have isolated variant cells which are resistant to the cytotoxic effect of glucocorticoids but which retain the induction of viral gene products and must therefore have a functional glucocorticoid receptor protein.

Androgen regulation of cell proliferation and expression of viral sequences in mouse mammary tumour cells

The role of steroids in promoting cell proliferation is well established but the molecular mechanisms are not clear. The S115 mouse mammary tumour cell line provides a model system for molecular studies in vitro in that it exhibits in tissue culture both a positive proliferative response to androgens and a change from a transformed phenotype in the presence of androgen to a normal phenotype when androgen is removed. We have considered here the possible involvement of mouse mammary tumour virus (MMTV) in these processes. We have demonstrated the presence in S115 cells of MMTV-related sequences which are transcribed into RNA only in the long-term presence of androgen. Prolonged culture in the absence of androgen, which results in loss of proliferative response to androgen, is accompanied by loss of MMTV-related RNA and increased methylation of MMTV-related sequences.

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.

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.

Transcription of mouse mammary tumor virus: identification of a candidate mRNA for the long terminal repeat gene product

We have examined an assortment of preneoplastic and neoplastic mouse mammary tissues for the presence of an mRNA which could encode the putative long terminal repeat gene product of mouse mammary tumor virus. We report here the detection of a novel mouse mammary tumor virus-specific, polyadenylic acid-containing transcript in certain preneoplastic and neoplastic mammary tissue of BALB/c mice. The molecule is 1.6 kilobases in length and contains sequences from the transcriptional leader and the U3 region of the proviral DNA. The upstream terminus of the 3' information lies 75 to 80 nucleotides from the beginning of the long terminal repeat open reading frame, in close proximity to a consensus splice acceptor in the DNA. The transcript was detected in hormonally or chemically induced neoplastic, preneoplastic, and lactating mammary tissue of BALB/c mice, but not in preneoplastic or tumor tissue induced by exogenous viruses in any strain of mice examined. This implies that the RNA we observed is transcribed from an endogenous provirus template.