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

RIII/Sa mice with a high incidence of mammary tumors express two exogenous strains and one potential endogenous strain of mouse mammary tumor virus

The inbred mouse strain RIII has long been known for shedding large amounts of mouse mammary tumor virus (MMTV) particles in milk and for the development of hormone-dependent early mammary tumors at a very high incidence (>90%). We have established one RIII subline (RIII/Sa) that shows a pattern of virus expression and tumor incidence similar to that in RIII mice. In the present study, we analyzed the milk and mammary tumors of RIII/Sa mice for virus characterization by reverse transcriptase PCR (RT-PCR) cloning and sequencing of the open reading frame (ORF) of the MMTV long terminal repeats (LTRs). Our results show that these mice express a mixture of at least three different MMTV strains, two of which, designated here as RIII/Sa MMTV-1 and RIII/Sa MMTV-2, are exogenous. The third virus, RIII/Sa MMTV-3, appears to carry the signature of an endogenous provirus, Mtv-17. Similar studies done with the milk and mammary glands of another subline, RIIIS/J, revealed that they do not express MMTV in their milk. The RIII/Sa and RIIIS/J mice also exhibited differences in their endogenous proviral contents. Twelve spontaneously developed mammary tumors of RIII/Sa mice were examined for possible Wnt-1 and/or int-2/Fgf3 mutations that are usually found to occur in most mouse mammary tumors as a consequence of MMTV proviral integration. This work led to the isolation of one MMTV-Wnt-1 junction fragment and one MMTV-int-2/Fgf3 junction fragment from 2 of the 12 tumors. Further analyses showed that both junction fragments contained the RIII/Sa MMTV-2-specific LTR ORF, indicating that this virus was involved in the development of both tumors. Whether RIII/Sa MMTV-1 and/or RIII/Sa MMTV-3 plays any role in mammary tumor development in RIII/Sa mice remains to be established. Overall, the present study demonstrates, to our surprise, that (i) RIII/Sa mice express, unlike other native mouse strains, three strains of MMTVs; and (ii) the virions are completely different from the virus expressed by another subline of RIII mice, the BR6 mice.

Sequence and genetic analyses of the 3' terminus and integration sites of the RIII/Sa mouse mammary tumor (MMTV) exogenous provirus

The RIII/Sa mouse mammary tumor virus (MMTV) is one of several exogenous MMTV viruses transmitted to suckling mice through the milk. We report herein the nucleotide sequence of the exogenous RIII/Sa provirus from the central Eco RI site through to the end of the U5 region encoded by the 3' LTR. We also provide a detailed sequence analysis often different 3' terminal exogenous MMTV proviral integration sites within mammary tumor DNA obtained by the technique of genome walking. Using a combination of Southern blotting with 3' end probes and PCR utilizing a unique RIII/Sa specific 3' primer, we confirm that the RIII/Sa provirus integrates multiple times in mouse mammary tumors but that little or no integration occurs in various normal tissues. Southern blotting analyses with 3' end probes also indicate that RIII/Sa mice contain two additional endogenous MMTV loci, mtv-6 and mtv- 17, not previously reported. The combined genetic analyses reported herein distinguish between exogenous proviral integrants and endogenous germline MMTV.

A Philadelphia negative chronic myelogenous leukemia with the chimeric BCR/ABL gene on chromosome 9 and a b3-a2 splice junction

Approximately 5% of patients with chronic myelogenous leukemia (CML) do not reveal the Philadelphia (Ph) chromosome cytogenetically and are termed Ph-negative CML cases. We report one such case, which appeared normal by routine banding techniques. The BCR/ABL rearrangement was detected by reverse transcriptase-polymerase chain reaction and Southern blotting analysis, which suggested a b3-a2 splice junction. Dual color fluorescence in situ hybridization (FISH) with BCR and ABL DNA probes showed that the chimeric fusion gene was localized on chromosome 9q34, rather than at the typical location on chromosome 22q11. The BCR/ABL rearrangement was detected in 75% of the patient's bone-marrow population, whereas the remaining 25% of the cells appeared normal. The use of dual color FISH in the diagnosis of CML is extremely valuable not only in identifying cases of Ph-negative CML, but also in quantifying the proportion of transformed cell populations. This information ultimately results in an enhancement of our ability to monitor therapy, follow disease progression, and determine transplant eligibility.

Mouse mammary tumor virus and mammary tumorigenesis in wild mice

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

Complete Nucleotide Sequence of Mouse Mammary Tumor Virus from JYG Chinese Wild Mice: Absence of Bacterial Insertion Sequences in the Cloned Viral gag Gene

Mammary tumors of a newly isolated strain of Chinese wild mouse (JYG mouse) harbor exogenous mouse mammary tumor virus (MMTV). The complete nucleotide sequence of exogenous JYG-MMTV was determined on the proviral 5' long terminal repeat (LTR)(partial)-gag-pol-env-3' LTR (partial) fragment cloned into a plasmid vector and the cDNA sequence from JYG-MMTV producing cells. Similarly to the other MMTV species the LTR of JYG-MMTV contains an open reading frame (ORF). The amino acid sequence of the JYG-MMTV ORF resembles that of SW-MMTV (92% identity) and endogenous Mtv-7 (93% identity) especially at the C-terminal region. Thus, a functional similarity in T-cell receptor V beta recognition as a superantigen is implicated among these MMTV species. Analysis of the viral gag nucleotide sequence revealed that this gene is not disrupted by the bacterial insertion sequence IS1 or IS2, which have been reported to be present in the majority of the plasmids containing the gag region. Comparison of amino acid sequences of JYG-MMTV with those of BR6-MMTV showed that over 96% of the amino acids of gag, pol, protease and env products are identical. These results suggest the intact nature of the nucleotide sequence of the near full-length MMTV genome cloned in the plasmid.

Mouse Mammary Tumor Virus Proviral Integration in the DD/Tbr Mice

The patterns of mouse mammary tumor virus (MMTV) integration in the DNA of spontaneous-mammary tumors, salivary glands and livers of DD/Tbr mice were examined using MMTV env, int -1c and int-2c probes. The MMTV env probe revealed 1 to 7 new proviral insertions in all mammary tumors. MMTV integration into int-1 was observed in 10 of 18 mammary tumors, whereas that into int-2 was seen in only 2 of 18 tumors. Of the 13 salivary glands examined, only 3 showed new MMTV proviral integrations, but rearrangement in int-1 or int-2 loci by MMTV was not observed. Immuno-collidal gold electron microscopy revealed the presence of MMTV particles both in mammary tumors and in salivary glands, but no tumors were found to be developed in salivary glands. Taken together these results suggest that salivary glands support MMTV replication, but the virions thus produced may not lead to salivary gland tumorigenesis. It is suggested that the salivary gland is the source of horizontally transmitted MMTV in DD/Tbr mice.

Direct visualization of the transposed ABL gene in a duplicated masked Ph chromosome

In a small percentage of cases of chronic myelogenous leukemia (CML), where the Ph chromosome is masked because of highly complex translocations and sub-microscopic rearrangements, precise identification of chromosomal aberrations by routine banding techniques has been difficult. We report on a new case of CML in which a single copy of a masked Ph chromosome was duplicated during blast crisis, i.e., the karyotype was 47,XY,dir ins(22;9)(q11;q34.1q34.2),t(1;22) (q21;q11), + der (22)t(1;22)(q21;q11). The chromosome in situ suppression hybridization (CISS) technique with whole chromosome 1 and 22 specific painting probes demonstrated that 22q11-qter had been translocated to 1q21, whereas 22q11 was the recipient of 1q21-qter. Furthermore, a cosmid probe identified the location of the ABL gene on only one chromosome 9 (band q34). The other ABL gene could be detected on both derivative chromosomes 22 at band q11 which was flanked by the translocated part of the long arm of chromosome 1, thus providing direct visualization of the ABL insertion in a double masked Ph chromosome. A breakpoint within the 5.8 kb major breakpoint cluster [M-BCR] region was shown by Southern blotting.

Prothymosin alpha gene in humans: organization of its promoter region and localization to chromosome 2

A genomic clone encoding prothymosin alpha (gene symbol: PTMA), a nuclear-targeted protein associated with cell proliferation, was isolated and the 5'-regulatory region subcloned and sequenced. Because of previously reported discrepancies between several cDNA clones and a genomic clone for prothymosin alpha, we determined the sequence of the first exon and of a 1.7-kb region 5' to the first exon. The sequence of the genomic clone reported here corresponds to the published cDNA sequences, suggesting that the previously noted discrepancies may be due to genetic polymorphism in this region. In addition, our sequence data extend the known 5'-upstream sequence by an additional 1.5 kb allowing the identification of numerous, potential cis-acting regulatory sites. This 5'-flanking cloned probe permitted us to localize the prothymosin gene to chromosome 2 in humans.

Localization of the gene coding for ventricular myosin regulatory light chain (MYL2) to human chromosome 12q23-q24.3

Human myosin light chain-2 (MYL2) is an important protein involved in the regulation of myosin ATPase activity in smooth muscle. In cardiac muscle, the precise role of MYL2 is not well understood; however, an increase in ventricular MYL2 is observed during myocardial hypertrophy in cardiac patients with valve stenosis. The chromosomal location of the gene coding for MYL2 was identified using a cloned cDNA for human MYL2. Southern blot analysis of DNA from a human/rodent somatic cell hybrid mapping panel showed that the BamHI fragment that hybridized with this cDNA probe was concordant with chromosome 12. The 768-bp cDNA was hybridized to human metaphase chromosomes. The results revealed a significant clustering of silver grains over chromosome 12 bands q23-q24.3, indicating that the gene coding for MYL2 is located in this region.

Alterations in the U3 region of the long terminal repeat of an infectious thymotropic type B retrovirus

We isolated and characterized a type B thymotropic retrovirus (DMBA-LV) which is highly related to mouse mammary tumor virus (MMTV) isolates and which induces T-cell thymomas with a high incidence and a very short latent period. Regions of nonhomology between the DMBA-LV genome and the MMTV genome were identified by heteroduplex mapping and nucleotide sequence studies. In the electron microscope heteroduplex mapping studies the EcoRI-generated 5' and 3' fragments of the DMBA-LV genome were compared with the corresponding fragments of the MMTV (C3H and GR) genome isolated from mammary tumors. The results indicated that DMBA-LV contained a region of nonhomologous nucleotide sequences in the 3' half of the U3 region of the long terminal repeat (LTR). Nucleotide sequence studies confirmed these results and showed that in this region 440 nucleotides of the MMTV (C3H) sequences were deleted and substituted with a segment of 122 nucleotides. This substituted segment in the form of a tandem repeat structure contained nucleotide sequences derived exclusively from sequences which flanked the substitution loop. The distal glucocorticoid regulatory element was unaltered, and two additional copies of the distal glucocorticoid regulatory element-binding site were present in the substituted region. The restriction endonuclease map of the reconstructed molecular clone of DMBA-LV was identical to that corresponding to unintegrated linear DMBA-LV DNA present in DMBA-LV-induced tumor cell lines. Since the nucleotide sequences of the LTRs present in four different DMBA-LV proviral copies isolated from a single thymoma were identical, we concluded that they were derived from the same parental virus and that this type B retrovirus containing an alteration in the U3 region of its LTR could induce thymic lymphomas. Thus, DMBA-LV represents the first example of a productively replicating type B retrovirus that contains an LTR modified in the U3 region and that has target cell and disease specificity for T cells.

Structural alterations in the long terminal repeat of an acquired mouse mammary tumor virus provirus in a T-cell leukemia of DBA/2 mice

ML, a transplantable T-cell leukemia of DBA/2 mice, expresses the gag and env gene products of the murine mammary tumor virus (MuMTV). Analysis of the genomic DNA of ML cells using the restriction enzyme HindIII and hybridization with MuMTV-specific probes revealed that the ML cells contained two or more newly integrated MuMTV proviruses (ML-MuMTV). Further analysis of these proviruses with a combination of Mspl and Pstl enzymes showed that the long terminal repeat (LTR) (ML-MuMTV LTR) of the ML-MuMTV provirus(es) was structurally different from the LTRs of both exogenous and endogenous MuMTV proviruses of DBA/2 mice. In order to characterize the nature of the structural alterations in the ML-MuMTV LTR, we cloned a 4.0-kb HindIII fragment containing the 3' half of an acquired provirus. Sequence analysis of the ML-MuMTV LTR of this acquired provirus revealed a deletion of a 387-bp segment that maps between the 5' nucleotide 616 and the 3' nucleotide 1003 of the normal MuMTV LTR and duplication of a 102-bp fragment that mapped between 514 and 616. In addition to two point mutations in the direct repeat, the proviral ML-MuMTV LTR has also acquired 9- and 7-bp segments at the 5' and 3' sites of the duplicated 102-bp segment, respectively. Since direct repeats in the U3 regions of a number of LTRs have been found to be associated with enhancer function, we examined the enhancer function of the U3 region sequences of the ML-MuMTV LTR using enhancer-dependent transient expression assay of chloramphenicol acetyltransferase (CAT) gene in NIH 3T3 cells. Our studies have shown that the U3 region sequences of the rearranged ML-MuMTV LTR have the ability to enhance the expression of the CAT gene 12- to 15-fold more than the U3 region sequences from the normal MuMTV LTR. The presence of a direct repeat in the ML-MuMTV LTR and its ability to enhance the transcription of adjacent genes is analogous to the LTRs of certain murine leukemia viruses.

Mammary tumorigenesis in feral mice: identification of a new int locus in mouse mammary tumor virus (Czech II)-induced mammary tumors

A population of Mus musculus subsp. musculus (Czech II), recently isolated from the wild, lack endogenous mouse mammary tumor virus (MMTV) proviral genomes. Some of these mice carry an infectious MMTV [designated MMTV (Czech II)] that is transmitted in the milk and is associated with mammary tumor development. This virus is distinct from laboratory strains of MMTV present in inbred mice. An MMTV (Czech II) genome was found within a 0.5-kilobase region of the cellular genome in five of 16 Czech II mammary tumors. MMTV insertion at this site activates expression of a 2.4-kilobase species of RNA from a previously silent cellular gene. This region of the cellular genome was designated int-3 since it is unrelated to the int-1 and int-2 loci. The int-3 locus does not appear to correspond to other proto-oncogenes but is well conserved among mammalian species.

A human autosomal phosphoglycerate kinase locus maps near the HLA cluster

The human cDNA probe pPGK824 , of Singer-Sam et al. [Singer-Sam, J., Simmer , R. L., Keith, D. H., Shively , L., Teplitz , M., Itakura , K., Gartler , S. M. & Riggs, A. D. (1983) Proc. Natl. Acad. Sci. USA 80, 802-806] was used to isolate a genomic clone lambda PGK-1 containing a portion of an autosomal locus for phosphoglycerate kinase (PGK). A unique sequence subclone (pGK-1) of lambda PGK-1 was then used to map this locus to the region p23-q12 of human autosome 6--i.e., to the interval that contains the major human histocompatibility locus (HLA). Since an autosomal gene coding for testis-specific PGK in the mouse has been shown to be closely linked to the H2 locus and to the T/t-complex locus [ Eicher , E. M., Cherry, M. & Flaherty , L. (1978) Mol. Gen. Genet. 158, 225-228], it is suggested that the lambda PGK-1 recombinant clone contains part of the human gene for the testis-specific isozyme of PGK. In addition, the subcloned pGK-1 detects an EcoRI restriction fragment length variant and may therefore prove useful for further genetical analysis of the HLA region and specifically for testing the hypothesis that spina bifida and anencephaly may be the human equivalent of the murine defects due to the T/t-complex locus. Our findings support the generally held hypothesis that a large number of structural loci clustered around the histocompatibility genes have been conserved in a close linkage association throughout a large evolutionary interval.

Human interleukin 2 gene is located on chromosome 4

The interleukin-2 (IL2) gene is assigned to human chromosome 4. A synthetic oligonucleotide representing bases 285 to 324 of the IL2 cDNA clone described by Taniguchi et al. [Nature (London) 302:305-310, 1983] was used as hybridization probe in Southern blots. Eco RI digests of DNA derived from 11 somatic mouse-human cell hybrid clones were used. The IL2 gene was localized to human chromosome 4 based on the observed combinations of segregating chromosomes and bands. Under the conditions of stringency utilized, a single 3.6 kilobase Eco RI fragment hybridized to the oligonucleotide. Molecular weight standards were provided by rehybridizing the blots with an actin cDNA clone, pAct 1, which identified actin gene Eco RI fragments of defined size.

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.

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

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