Identification of cooperating oncogenes in E mu-myc transgenic mice by provirus tagging

Transformation by the Bmi-1 oncoprotein correlates with its subnuclear localization but not its transcriptional suppression activity

The bmi-1 oncogene cooperates with c-myc in transgenic mice, resulting in accelerated lymphoma development. Altering the expression of Bmi-1 affects normal embryogenesis. The protein product of bmi-1 is homologous to certain Drosophila Polycomb group proteins that regulate homeotic gene expression through alteration of chromatin structure. Chimeric LexA-Bmi-1 protein has previously been shown to repress transcription. How Bmi-1 functions in embryogenesis and whether this relates to the ability of Bmi-1 to mediate cellular transformation is unknown. We demonstrate here that Bmi-1 is able to transform rodent fibroblasts in vitro, providing a system that has allowed us to correlate its molecular properties with its ability to transform cells. We map functional domains of Bmi-1 involved in transcriptional suppression by using the GAL4 chimeric transcriptional regulator system. Deletion analysis shows that the centrally located helix-turn-helix-turn-helix-turn (HTHTHT) motif is necessary for transcriptional suppression whereas the N-terminal RING finger domain is not required. We demonstrate that nuclear localization requires KRMK (residues 230 to 233) and that the absence of nuclear entry ablates transformation. In addition, we find that the subnuclear localization of wild-type Bmi-1 to the rim of the nucleus requires the RING finger domain and correlates with its ability to transform. Our studies with Bmi-1 deletion mutants suggest that the ability of Bmi-1 to mediate cellular transformation correlates with its unique subnuclear localization but not its transcriptional suppression activity.

Genetic predisposition to lymphomas in mice

The spontaneous mouse lymphoma is a model of multifactorial genetic disease. It is induced by the endogenous murine leukemia virus (MuLV), whose genome is inherited as a Mendelian dominant trait. Lymphoma development takes place in multiple stages affected by many host genetic and epigenetic factors. An inbred strain SL/Kh with a high incidence of pre-B lymphomas has been established and the genetic predisposition of SL/Kh mice to lymphomas is being studied in the crosses with other inbred strains of mice. In the cross to the NFS/N lacking endogenous MuLV genome, it has been shown that lymphomas are induced by the expression of Emv-11 provirus (Chr. 7), and the types of B-lineage lymphomas are determined by combinations of the host genes, Esl-1 (Chr. 17) and Foc-1 (Chr. 4). Another gene, Tlsm-1 (Chr. 7) that determines the type of lymphomas to be T-lineage, is identified in the cross with AKR/Ms, with a high incidence of T-lymphomas. The role of the thymus in the development of T-lymphomas in the mouse, and the possible relevance of Tlsm-1 in this step, is discussed. The length of the latent period is determined by a gene Lia-1 (Chr. 17). A maternal resistance factor that is a maternal antibody to MuLV transmitted via milk and that epigenetically inhibits MuLV expression in SL/Ni-Eco-, one of subline of SL/NI mice, has been shown. Weak but definitive maternal resistance also operates in SL/Ni-Eco+, a subline lacking the maternal antibody to MuLV. In the latter, there is a recessive resistance gene Nir-1 (Chr. 4). In the cross with MSM/Ms, a wild mice-derived inbred strain, two resistance genes, Msmr-1 (Chr. 17) and Msmr-2 (Chr. 18), have been identified. In SL/Kh, all of these host genetic and epigenetic factors are favorable for lymphoma development. This model offers not only an understanding of the pathogenesis of virus-induced lymphomas but also may provide starting material for the comparative approach to homologous human diseases.

Gene trapping in differentiating cell lines: regulation of the lysosomal protease cathepsin B in skeletal myoblast growth and fusion

To identify genes regulated during skeletal muscle differentiation, we have infected mouse C2C12 myoblasts with retroviral gene trap vectors, containing a promoterless marker gene with a 5' splice acceptor signal. Integration of the vector adjacent to an actively transcribed gene places the marker under the transcriptional control of the endogenous gene, while the adjacent vector sequences facilitate cloning. The vector insertionally mutates the trapped locus and may also form fusion proteins with the endogenous gene product. We have screened several hundred clones, each containing a trapping vector integrated into a different endogenous gene. In agreement with previous estimates based on hybridization kinetics, we find that a large proportion of all genes expressed in myoblasts are regulated during differentiation. Many of these genes undergo unique temporal patterns of activation or repression during cell growth and myotube formation, and some show specific patterns of subcellular localization. The first gene we have identified with this strategy is the lysosomal cysteine protease cathepsin B. Expression from the trapped allele is upregulated during early myoblast fusion and downregulated in myotubes. A direct role for cathepsin B in myoblast growth and fusion is suggested by the observation that the trapped cells deficient in cathepsin B activity have an unusual morphology and reduced survival in low-serum media and undergo differentiation with impaired cellular fusion. The phenotype is reproduced by antisense cathepsin B expression in parental C2C12 myoblasts. The cellular phenotype is similar to that observed in cultured myoblasts from patients with I cell disease, in which there is diminished accumulation of lysosomal enzymes. This suggests that a specific deficiency of cathepsin B could contribute to the myopathic component of this illness.

The Polycomb-group homolog Bmi-1 is a regulator of murine Hox gene expression

Drosophila homeotic genes and vertebrate Hox genes are involved in the anteroposterior organization of the developing embryo. In Drosophila, the Polycomb- and trithorax-group genes are required to maintain the homeotic genes throughout development in the repressed or activated state, respectively. The murine Bmi-1 proto-oncogene was shown to exhibit homology to the Polycomb-group gene Posteior sex combs. Mice lacking the Bmi-1 gene revealed posterior transformations along the axial skeleton, whereas transgenic mice overexpressing Bmi-1 display anterior transformations. We have analysed the expression patterns of several Hox genes by RNA in situ hybridization on serial sections of 11.5- and 12.5-day Bmi-1 null mutant embryos. Furthermore, we have analysed the expression of a Hoxc-8/LacZ fusion gene in younger embryos. Our analyses show that Bmi-1 is involved in the repression of a subset of Hox genes from different clusters from at least day 9.5 onwards. We discuss the possibility that members of the murine Polycomb-group can form multimeric protein complexes of different compositions with varying affinity or specificity for different subsets of Hox genes.

Frequent provirus insertional mutagenesis of Notch1 in thymomas of MMTVD/myc transgenic mice suggests a collaboration of c-myc and Notch1 for oncogenesis

The MMTVD/myc transgenic mice spontaneously develop oligoclonal CD4+CD8+ T-cell tumors. We used provirus insertional mutagenesis in these mice to identify putative collaborators of c-myc. We found that Notch1 was mutated in a high proportion (52%) of these tumors. Proviruses were inserted upstream of the exon coding for the transmembrane domain and in both transcriptional orientations. These mutations led to high expression of truncated Notch1 RNAs and proteins (86-110 kD). In addition, many Notch1-rearranged tumors showed elevated levels of full-length Notch1 transcripts, whereas nearly all showed increased levels of full-length (330-kD) or close to full-length (280-kD) Notch1 proteins. The 5' end of the truncated RNAs were determined for some tumors by use of RT-PCR and 5' RACE techniques. Depending on the orientation of the proviruses, viral LTR or cryptic promoters appeared to be utilized, and coding potential began in most cases in the transmembrane domain. Pulse-chase experiments revealed that the 330-kD Notch1 proteins were processed into 110- and 280-kD cleavage products. These results suggest that Notch1 can be a frequent collaborator of c-myc for oncogenesis. Furthermore, our data indicate that Notch1 alleles mutated by provirus insertion can lead to increased expression of truncated and full-length (330/280-kD) Notch1 proteins, both being produced in a cleaved and uncleaved form.

Genetic mechanisms of estrogen-independence in breast cancer

Endocrine therapy is effective in the treatment of breast cancer. Adjuvant treatment with tamoxifen reduces tumor recurrence and achieves increased survival. In metastatic disease, tamoxifen treatment accomplishes objective responses in +/- 50% of the patients with estrogen receptor-positive primary tumors. However, the response duration is limited due to the inevitable development of metastases resistant to tamoxifen. The mechanisms leading to tamoxifen resistance are largely unknown. We have set out to identify genetic pathways in the tumor cells causing failure of tamoxifen therapy. We selected an estrogen-dependent human breast cancer cell line (ZR-75-1) and demonstrated that genetic and epigenetic alterations can change the hormone-response phenotype of these cells. Subsequently, we applied insertional mutagenesis with defective retroviruses to these ZR-75-1 breast cancer cells. Integration of a retrovirus in the cellular DNA alters the genome structure and may modify the expression of genes in its surroundings. As a result of the altered gene expression, the biological phenotype of the infected cell may be changed. The infected ZR-75-1 cells were subjected to tamoxifen selection and a panel of tamoxifen-resistant cell lines has been established. Screening for a common integration site for the retrovirus has provided, so far, compelling evidence for the involvement of at least one genetic locus (BCAR 1) in breast cancer antiestrogen resistance in vitro.

MoMuLV proviral integrations identified by Sup-F selection in tumors from infected myc/pim bitransgenic mice correlate with activation of the gfi-1 gene

Infecting mice with a mutant Moloney murine leukemia virus which contains the bacterial suppressor tRNA supF in its LTR allows rapid cloning of proviral integration sites from genomic tumour DNA. In a previous study Emu pim-1/Emu L-myc bitransgenic mice had been inoculated neonatally with MoMuLV supF virus. The retroviral infection led to acceleration of lymphomagenesis indicating the proviral activation of further oncogenes cooperating with myc and pim-1 in tumour development. Using a functional supF screen for analysis of genomic mouse tumour DNA libraries which had been constructed in the phage vector EMBL3A, a common proviral integration site on mouse chromosome 5 was cloned and found to be identical to the proviral integration site evi-5 which has recently been identified in an AKXD T-cell lymphoma and which is located 18 kb upstream of the gfi-1 gene. Tumours bearing evi-5 integrations showed an enhanced gfi-1 expression level suggesting that gfi-1 is the target gene for insertions at the evi-5 locus. Together with three other previously described Moloney integration clusters all responsible for enhanced gfi-1 expression the number of tumours from infected double transgenic Emu L-myc/Emu pim-1 transgenic mice with retrovirally activated gfi-1 added up to 53% underscoring the role of GFI-1 as an effective collaborator for MYC and PIM-1 in the process of lymphomagenesis.

Interaction of Vav with ENX-1, a putative transcriptional regulator of homeobox gene expression

The proto-oncogene product Vav plays a critical role in hematopoietic signal transduction. By using the yeast two-hybrid system, we identified a novel human protein, ENX-1, which interacts specifically with Vav both in vitro and in vivo. ENX-1 represents the human homolog of the Drosophila Enhancer of zeste gene, a member of the Polycomb group of genes, which are transcriptional regulators of homeobox gene expression. Interaction with ENX-1 suggests that Vav functions as an upstream element in the transcriptional regulation of homeobox genes, known to be important effectors in the hematopoietic system.

The t(10;11)(p13;q14) in the U937 cell line results in the fusion of the AF10 gene and CALM, encoding a new member of the AP-3 clathrin assembly protein family

The translocation t(10;11)(p13;q14) is a recurring chromosomal abnormality that has been observed in patients with acute lymphoblastic leukemia as well as acute myeloid leukemia. We have recently reported that the monocytic cell line U937 has a t(10;11)(p13;q14) translocation. Using a combination of positional cloning and candidate gene approach, we cloned the breakpoint and were able to show that AF10 is fused to a novel gene that we named CALM (Clathrin Assembly Lymphoid Myeloid leukemia gene) located at 11q14. AF10, a putative transcription factor, had recently been cloned as one of the fusion partners of MLL. CALM has a very high homology in its N-terminal third to the murine ap-3 gene which is one of the clathrin assembly proteins. The N-terminal region of ap-3 has been shown to bind to clathrin and to have a high-affinity binding site for phosphoinositols. The identification of the CALM/AF10 fusion gene in the widely used U937 cell line will contribute to our understanding of the malignant phenotype of this line.

A role for mel-18, a Polycomb group-related vertebrate gene, during theanteroposterior specification of the axial skeleton

Segment identity in both invertebrates and vertebrates is conferred by spatially restricted distribution of homeotic gene products. In Drosophila, the expression of Homeobox genes during embryogenesis is initially induced by segmentation gene products and then maintained by Polycomb group and Trithorax group gene products. Polycomb group gene homologs are conserved in vertebrates. Murine mel-18 and closely related bmi-1 are homologous to posterior sex combs and suppressor two of zeste. Mel-18 protein mediates a transcriptional repression via direct binding to specific DNA sequences. To gain further insight into the function of Mel-18, we have inactivated the mel-18 locus by homologous recombination. Mice lacking mel-18 survive to birth and die around 4 weeks after birth after exhibiting strong growth retardation. Similar to the Drosophila posterior sex combs mutant, posterior transformations of the axial skeleton were reproducibly observed in mel-18 mutants. The homeotic transformations were correlated with ectopic expression of Homeobox cluster genes along the anteroposterior axis in the developing paraxial mesoderm. Surprisingly, mel-18-deficient phenotypes are reminiscent of bmi-1 mutants. These results indicate that the vertebrate Polycomb group genes mel-18 and bmi-1, like Drosophila Polycomb group gene products, might play a crucial role in maintaining the silent state of Homeobox gene expression during paraxial mesoderm development.

Repression and activation by multiprotein complexes that alter chromatin structure

Recent studies have provided strong evidence that macromolecular complexes are used in the cell to remodel chromatin structure during activation and to create an inaccessible structure during repression, Although there is not yet any rigorous demonstration that modification of chromatin structure plays a direct, causal role in either activation or repression, there is sufficient smoke to indicate the presence of a blazing inferno nearby. It is clear that complexes that remodel chromatin are tractable in vitro; hopefully this will allow the establishment of systems that provide a direct analysis of the role that remodeling might play in activation. These studies indicate that establishment of functional systems to corroborate the elegant genetic studies on repression might also be tractable. As the mechanistic effects of these complexes are sorted out, it will become important to understand how the complexes are regulated. In many of the instances discussed above, the genes whose products make up these complexes were identified in genetic screens for effects on developmental processes. This implies a regulation of the activity of these complexes in response to developmental cues and further implies that the work to fully understand these complexes will occupy a generation of scientists.

Identification and characterization of a RING zinc finger gene (C-RZF) expressed in chicken embryo cells

To identify changes in gene expression that occur in chicken embryo brain (CEB) cells as a consequence of their binding to the extracellular matrix molecule cytotactin/tenascin (CT/TN), a subtractive hybridization cloning strategy was employed. One of the cDNA clones identified was predicted to encode 381 amino acids and although it did not resemble any known sequences in the nucleic acid or protein data bases, it did contain the sequence motif for the cysteine-rich C3HC4 type of zinc finger, also known as a RING-finger. This sequence was therefore designated the chicken-RING zinc finger (C-RZF). In addition to the RING-finger, the C-RZF sequence also contained motifs for a leucine zipper, a nuclear localization signal, and a stretch of acidic amino acids similar to the activation domains of some transcription factors. Southern analysis suggested that C-RZF is encoded by a single gene. Northern and in situ hybridization analyses of E8 chicken embryo tissues indicated that expression of the C-RZF gene was restricted primarily to brain and heart. Western analysis of the nuclear and cytoplasmic fractions of chicken embryo heart cells and immunofluorescent staining of chicken embryo cardiocytes with anti-C-RZF antibodies demonstrated that the C-RZF protein was present in the nucleus. The data suggest that we have identified another member of the RING-finger family of proteins whose expression in CEB cells may be affected by CT/TN and whose nuclear localization and sequence motifs predict a DNA-binding function in the nucleus.

Isolation and developmental expression analysis of Enx-1, a novel mouse Polycomb group gene

Members of the Polycomb group (Pc-G) of genes encode transcriptional regulators that control the expression of key developmental effector genes in Drosophila melanogaster. Although multiple Pc-G genes have been identified and characterized in Drosophila, information about these important regulatory proteins in vertebrates, including their precise expression patterns, has remained scarce. We report here the cloning of Enx-1, a novel vertebrate Pc-G gene, which encodes the murine homolog of the Drosophila Enhancer of zeste (E(z)) gene. Drosophila E(z) controls the expression of several homeobox genes as well as some segmentation genes and its disruption causes multiple phenotypes in Drosophila development. Analysis of the primary structure of murine Enx-1 reveals the conservation of several regions, including the previously described SET domain and a newly defined CXC domain. In addition, we find the SET domain to be conserved in evolutionarily distant species ranging from vertebrates to plants and fungi. The expression pattern analysis of Enx-1 reveals ubiquitous expression throughout early embryogenesis, while in later embryonic development Enx-1 expression becomes restricted to specific sites within the central and peripheral nervous system and to the major sites of fetal hematopoiesis. In adult stages we also find Enx-1 expression to be restricted to specific tissues, including spleen, testis and placenta.

Moloney murine leukemia virus-induced lymphomas in p53-deficient mice: overlapping pathways in tumor development?

The effect of Moloney murine leukemia virus (MoMLV) infection was examined in mice lacking a functional p53 gene. Virus-infected p53-/- mice developed tumors significantly faster than uninfected p53-/- or virus-infected p53+/+ littermates. However, the degree of synergy between MoMLV and the p53 null genotype was weaker than the synergy between either of these and c-myc transgenes. A similar range of T-cell tumor phenotypes was represented in all p53 genotype groups, including p53-/- mice, which developed thymic lymphomas as the most common of several neoplastic diseases. Lack of p53 was associated with higher rates of metastasis and the ready establishment of tumors in tissue culture. Loss of the wild-type allele was a common feature of tumors in p53+/- mice and was complete in tumor cells in vitro, but this appeared to occur by a mechanism other than proviral insertion at the wild-type allele. A lower average MoMLV proviral copy number was observed in tumors of the p53 null and heterozygote groups, suggesting that the absence of a functional p53 gene reduced the number of steps required to complete the malignant phenotype. Mink cell focus-forming virus-like proviruses were detected in tumors of all infected mice but were relatively rare in p53 null mice. Analysis of c-myc, pim-1, and pal-1 showed that these loci were occupied by proviruses in some cases but at similar frequencies in p53 wild-type and null mice. In conclusion, while inactivation of p53 in the germ line predisposes mice to tumors similar in phenotype to those induced by MoMLV, it appears that virus-induced tumors generally occur without p53 loss. We speculate that a bcl-2-like function carried or induced by MoMLV may underlie this p53-independent pathway.

Transgenic mouse models of breast cancer

Although valuable initial information can be gathered about transformation from in vitro studies, human cancer occurs in the context of a complex interaction with its environment and must ultimately be studied in living animals. Transgenic animal models have been used to study breast transformation for a number of years and have yielded valuable information on the subject. In this paper, we will summarize results from our laboratories, and others, regarding the use of transgenic mice to study breast tumorigenesis. We will also suggest future directions for the use of transgenic models to understand, and hopefully, one day to cure the disease.

Some characteristics of neoplastic cell transformation in transgenic mice

The role of the expression of different cellular genes and viral oncogenes in malignant cell transformation is discussed. We pay special attention to the role of the genes for growth factors and their receptors and homeobox genes in oncogenesis. Based on both the literature and our own data, specific features of tumors developed in transgenic mice are discussed. All of these data are used to analyze current theories of multistep oncogenesis and the stochastic component in this process. We suggest that all known evidence about the mechanisms of oncogenesis be used in studying the problem at various structural and functional levels in an organism. The chapter shows that transgenic mice are a most suitable model for studying various aspects of malignant transformation from the molecular to the organismal and populational levels.

Disruption of the murine homeobox gene Cdx1 affects axial skeletal identities by altering the mesodermal expression domains of Hox genes

Cdx1 is expressed along the embryonic axis from day 7.5 postcoitum until day 12, by which time the anterior limit of expression has regressed from the hindbrain level to the forelimb bud region. To assign a functional role for Cdx1 in murine embryonic development, we have inactivated the gene via homologous recombination. Viable fertile homozygous mutant mice were obtained that show anterior homeotic transformations of vertebrae. These abnormalities were concomitant with posterior shifts of Hox gene expression domains in the somitic mesoderm. The presence of putative Cdx1-binding sites in Hox gene control regions as well as in vitro transactivation of Hoxa-7 indicates a direct regulation.

mel-18, a Polycomb group-related mammalian gene, encodes a transcriptional negative regulator with tumor suppressive activity

The mammalian mel-18/bmi-1 gene products share an amino acid sequence and a secondary structure, including a RING-finger motif, with the Drosophila Polycomb group (PcG) gene products Psc and Su(z)2, implying that they represent a gene family with related functions. As Drosophila PcG gene products are thought to function as transcriptional repressors by modifying chromatin structure, Mel-18/Bmi-1 might be expected to have similar activities. Here we have analyzed the function of mel-18 and found that Mel-18 acts as a transcriptional repressor via its target DNA sequence, 5'-GACTNGACT-3'. Interestingly, this binding sequence is found within regulatory or non-coding regions of various genes, including the c-myc, bcl-2 and Hox genes, suggesting diverse functions of mel-18 as the mammalian homolog of the PcG gene. We also demonstrate that mel-18 has tumor suppressor activity, in contrast to bmi-1, which has been defined as a proto-oncogene.

Expression of Brca1 is associated with terminal differentiation of ectodermally and mesodermally derived tissues in mice

We have isolated genomic and cDNA clones of Brca1, a mouse homolog of the recently cloned breast cancer-associated gene, BRCA1. Brca1 encodes an 1812-amino-acid protein with a conserved zinc finger domain and significant homology to the human protein. Brca1 maps to Chromosome 11 within a region of conserved synteny with human chromosome 17, consistent with the mapping of the human gene to 17q21. Brca1 transcripts are expressed in a variety of cultured cells but reveal a specific and dynamic expression pattern during embryonic development. For example, expression is observed first in the otic vesicle of embryonic day 9.5 (E9.5) embryos. This expression diminishes and is replaced by expression in the neuroectoderm at E10.5. By E11-12.5, higher levels are observed in differentiating keratinocytes and in whisker pad primordia. Transcripts also become evident in epithelial cells of the E14-17 kidney. Brca1 expression occurs in differentiating epithelial cells of several adult organs as well, suggesting a general role in the functional maturation of these tissues. Consistent with this, Brca1 transcripts are expressed in both alveolar and ductal epithelial cells of the mammary gland. During pregnancy, there is a large increase in Brca1 mRNA in mammary epithelial cells, an increase that parallels their functional differentiation. Because high rates of breast cancer are associated with loss of BRCA1 in humans, it is possible that this gene provides an important growth regulatory function in mammary epithelial cells. In addition, increased transcription of mammary Brca1 during pregnancy might contribute, in part, to the reduced cancer risk associated with exposure to pregnancy and lactation.

Molecular cloning, structure, and expression of mouse estrogen-responsive finger protein Efp. Co-localization with estrogen receptor mRNA in target organs

We have previously identified a human estrogen-responsive gene, efp (estrogen-responsive finger protein), which encodes a putative transcription regulator (Inoue, S., Orimo, A., Hosoi, T., Kondo, S., Toyoshima, H., Kondo, T., Ikegami, A., Ouchi, Y., Orimo, H., and Muramatsu, M. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11117-11121). Here, we report isolation of mouse Efp cDNA and its structure containing three cysteine-rich domains (RING finger and B1 and B2 boxes), a coiled-coil domain, and a C-terminal domain. High levels of Efp mRNA were detected in uterus, ovary, and placenta by RNase protection assay. By in situ hybridization histochemistry the transcripts of efp were also detected in uterus, mammary gland, ovary, and brain, and the co-localization of Efp and estrogen receptor mRNA was particularly demonstrated in these female organs. Moreover, the level of Efp mRNA in uterus and brain, which are known as target organs for estrogen, was up-regulated in vivo by 17 beta-estradiol. Furthermore, both the Efp and estrogen receptor mRNA were stained in the brain vesicles of 11.5-day embryos by whole mount in situ hybridization. These findings raise the possibility that efp is an estrogen-responsive gene that mediates estrogen action in various target organs.

Alternative mechanisms of CAK assembly require an assembly factor or an activating kinase

We have cloned a mouse cDNA that encodes p36, a novel subunit of the CDK-activating kinase (CAK). p36 contains a C3HC4 zinc-binding domain or RING factor and is associated both with a TFIIH-bound form of CAK and with a free trimeric form. p36 promotes the assembly of CDK7 and cyclin H in vitro, stabilizing the transient CDK7-cyclin H complex. Stabilization and activation of CAK by p36 is independent of the phosphorylation state of T170, the conserved activating residue of CDK7. Assembly of active CDK7-cyclin H dimers can also occur through an alternative p36-independent pathway that requires phosphorylation of T170 by a CAK-activating kinase, or CAKAK. Thus, CDK7-cyclin H complex formation can be achieved by multiple mechanisms.

The Drosophila melanogaster gene lethal(3)73Ah encodes a ring finger protein homologous to the oncoproteins MEL-18 and BMI-1

The Drosophila melanogaster (Dm) gene lethal(3)73Ah, essential at the late pupal stage, encodes a protein with a novel Cys-rich sequence motif, typical for ring-finger proteins. Amino-acid sequence comparison revealed a striking homology of the entire lethal(3)73Ah sequence to the gene products of the mammalian oncogenes, mel-18 and bmi-1, and to the zinc-finger-containing N-terminal region of the Dm proteins encoded by the Posterior sex combs and Suppressor two of zeste genes. The lethal(3)73Ah gene is located in a densely transcribed region sharing 3'-untranslated sequences with the adjacent sex-determining gene, transformer. Its transcription is temporally and spatially regulated with maximal expression in adult females. In all stages the mRNA can be localized to the fat body and, in addition, to the ovaries of adult females.

Polycomb and bmi-1 homologs are expressed in overlapping patterns in Xenopus embryos and are able to interact with each other

The Polycomb group genes in Drosophila are involved in the stable and inheritable repression of gene expression. The Polycomb group proteins probably operate as multimeric complexes that bind to chromatin. To investigate molecular mechanisms of stable repression of gene activity in vertebrates we have begun to study Xenopus homologs of Polycomb group genes. We identified the Xenopus homologs of the Drosophila Polycomb gene and the bmi-1 gene. bmi-1 is a proto-oncogene which has sequence homology with the Polycomb group gene Posterior Sex Combs. We show that the XPolycomb and Xbmi-1 genes are expressed in overlapping patterns in the central nervous system of Xenopus embryos. However, XPolycomb is also expressed in the somites, whereas Xbmi-1 is not. We further demonstrate that the XPolycomb and Xbmi-1 proteins are able to interact with each other via conserved sequence motifs. These data suggest that also vertebrate Polycomb group proteins form multimeric complexes.

Transgenic mouse models for tumour-suppressor genes

Tumour-suppressor genes are negative regulators of cell division and growth. Over the past decade, multiple, distinct tumour-suppressor genes have been identified and cloned. In recent years, the ability to specifically manipulate the mouse genome via overexpression, underexpression or deletion of genes using transgenic expression systems and embryonic stem cell (ES) technology has led to the identification and definition of the precise function of several tumour suppressor genes in vivo. Included in this group are mice with mutations in the p53 and retinoblastoma (Rb) genes. p53 Mutant mice are highly susceptible to tumour development and will serve as excellent models to understand the aetiology and pathology of several human cancers. In contrast to the role of the Rb gene in human retinoblastomas, mice heterozygous for a mutant Rb allele do not develop retinoblastoma, but develop pituitary tumours instead. Similar ES cell technology has been used to generate alpha-inhibin deficient mice. Inhibin-deficient mice develop gonadal and adrenal tumours with nearly 100% penetrance. These studies have identified inhibin as a novel secreted tumour suppressor. In the future, many of the unidentified functions of tumour-suppressor genes can be tested using this powerful in vivo assay system.

The murine AIDS defective provirus acts as an insertional mutagen in its infected target B cells

In susceptible mice, the murine AIDS (MAIDS) defective virus can induce marked expansion of its target cells, the majority of which belong to the B-cell lineage. This expansion, which appears to be critical for the development of the immunodeficiency syndrome, is initially polyclonal but becomes oligoclonal late in the disease, suggesting the involvement of a secondary genetic event(s) during this proliferation. To determine whether integration of the MAIDS defective provirus into particular regions of the cellular genome contributes to this oligoclonal expansion, we searched for common provirus integration sites in enlarged lymphoid organs of MAIDS mice. We identified two common proviral integration sites, Dis-1 and Dis-2, which were occupied by a defective provirus at frequencies of 20 and 13%, respectively. Our analysis revealed that the Dis-1 region corresponds to the Sfpil1 (Spi-1, PU.1) locus, which maps on chromosome 2, and encodes a transcription factor. Insertion of the MAIDS defective provirus into this region led to a two- to threefold increase in the expression of Sfpi1 RNA. The Dis-2 locus was found to map to mouse chromosome 11, between Hox2 and Scya. It appears to be a novel locus probably harboring a gene involved in B-cell proliferation. The present study indicates that the MAIDS defective provirus can act as an insertional mutagen, thus contributing to the oligoclonal expansion of infected cells. The detection of two common proviral integration sites, each of which targetted at a low frequency in diseased organs, suggests that the deregulation of a unique gene through provirus insertion is essential for neither proliferation of infected B cells nor development of the immunodeficiency syndrome.

Proviral tagging in E mu-myc transgenic mice lacking the Pim-1 proto-oncogene leads to compensatory activation of Pim-2

The Pim-1 proto-oncogene is one of the most potent collaborators of the myc proto-oncogenes in inducing lymphomagenesis in mice. Contrary to the profound effects when overexpressed in vivo, Pim-1-deficient mice showed only subtle phenotypic alterations, which could indicate the presence of redundantly acting genes. In line with this, a PCR-based screen has led to the identification of a closely homologous gene, Pim-2. The X-linked Pim-2 gene is 53% identical to Pim-1 at the amino acid level and shares substrate preference and the usage of non-AUG initiation codons with Pim-1. We have used these data to test whether the strong synergistic interaction between Pim-1 and c-myc can be utilized to gain access to Pim-1 compensatory pathways. We reasoned that, upon proviral tagging in compound mutant mice (E mu-myc/Pim-1-/- mice), the selective advantage of cells carrying provirally activated genes, that act downstream from or parallel to Pim-1, would increase. We show here that this is the case. A dramatic increase (from 15 to 80%) was found in the frequency of proviral activation of the Pim-2 gene. These data show that the described strategy of 'complementation tagging' represents a powerful new tool to identify components of pathways involved in processes as complex as multistep tumorigenesis.

Endogenous env elements: partners in generation of pathogenic feline leukemia viruses

Feline leukemia viruses (FeLVs), which are replication-competent oncoretroviruses of the domestic cat species, are contagiously transmitted in natural environments. They are capable of inducing either acute antiproliferative disease or, after prolonged latency, lymphoid malignancies in this animal population. Current knowledge of the recombinational events between infectious FeLV and noninfectious endogenously inherited FeLV-like elements is reviewed, and the potential role of the derived recombinant viruses in pathogenesis is discussed. Major observations made are as follows: (1) Up to three fourths of the exogenous FeLV envelope glycoprotein (SU), beginning from the N-terminal end, can be replaced by sequences from an endogenous FeLV to produce biologically active chimeric FeLVs. The in vitro replication efficiency or cell tropism of the recombinants appears to be influenced by the amount of SU sequences replaced by the endogenous partner, as well as by the locus of origin of the endogenous sequences. (2) Generation of FeLV recombinants in tissue culture cells corresponds closely to the findings from natural tumors. There is direct evidence, based on molecular genetic analysis, for the prevalence of recombinant proviruses in naturally arising FeLV-induced lymphomas. (3) Certain recombinants harboring an altered primary neutralizing epitope in the middle of SU corresponding to the endogenous FeLV sequence can evade immunity developed against common FeLV infection. In several other recombinants, the epitope sequence is found to be frequently mutated during the process of recombination. (4) FeLV variants with altered epitope, although they may not be efficient in replication in vivo, apparently are capable of causing focal infection in target organs. Evidence is also presented that when coinfected with an exogenous FeLV, the epitope sequence in the variants is reverted to the exogenous type, providing an explanation why this sequence is found to be conserved in all natural isolates of FeLV. (5) A prototype chimeric polyprotein containing most of the SU from the endogenous source is abnormally processed and becomes trapped in the endoplasmic reticulum. A functional consequence of such trapping is interference with specific FeLV infection. (6) Some recombinants, while only poorly replicating in the host, may have the ability to infect target erythroid progenitor cells for the induction of strong cytopathic effect. (7) Some other recombinants appear to potentiate lymphomagenesis by exogenous FeLV and others to acquire properties to infect CNS endothelial cells, an event that could potentially be related to FeLV-induced neuropathogenicity. (8) Of multiple recombinant viruses, a specific recombinant species was found to occur in each of the three cats examined in which lymphoma was experimentally induced, and it was exclusively seen in one of these cats. This recombinant FeLV may potentially be a candidate for strong leukemogenic function. In addition to commonly encountered virus envelope changes, another prominent viral factor involved in tumorigenesis is mutated FeLV transcription regulatory sequences, most frequently with enhancer duplication or triplication. Although only a limited amount of information is available in the area of insertional mutagenesis in FeLV neoplastic disease, activation of certain key nuclear transcription factor genes has been documented.

Locking in stable states of gene expression: transcriptional control during Drosophila development

Cell fate decisions can be maintained during long periods of developmental time by stable states of gene expression. The Polycomb group and trithorax group proteins of Drosophila are key transcriptional regulators that maintain stable expression states during development. Recent advances in knowledge about individual Polycomb group and trithorax group proteins, their mechanisms of action, and potential homologs in mice and humans are contributing to a greater understanding of their roles in gene expression and development.

Transformation of axial skeleton due to overexpression of bmi-1 in transgenic mice

The oncogene bmi-1, which was originally found to be involved in B- and T-cell lymphoma formation encodes a protein with a domain of homology to the Drosophila protein Posterior sex combs (Psc) and its relative Suppressor 2 of Zeste (Su(z)2) (refs 4 and 5). Psc is a member of the Polycomb-group gene family, which is required to maintain the repression of homeotic genes that regulate the identities of Drosophila segments. The possibility that bmi-1 may play a similar role in vertebrates was suggested by our previous finding that mice lacking the bmi-1 gene show posterior transformations of the axial skeleton. Here we report that transgenic mice overexpressing Bmi-1 protein show the opposite phenotype, namely a dose-dependent anterior transformation of vertebral identity. The anterior expression boundary of the Hoxc-5 gene is shifted in the posterior direction, indicating that Bmi-1 is involved in the repression of Hox genes. We propose that Bmi-1 is a member of a vertebrate Polycomb complex that regulates segmental identity by repressing Hox genes throughout development.

Fgf-8, activated by proviral insertion, cooperates with the Wnt-1 transgene in murine mammary tumorigenesis

We have used mouse mammary tumor virus (MMTV) infection of Wnt-1 transgenic mice to accelerate mammary tumorigenesis and to molecularly tag insertionally activated proto-oncogenes that cooperate oncogenically with Wnt-1 (G. M. Shackleford, C. A. MacArthur, H. C. Kwan, and H. E. Varmus, Proc. Natl. Acad. Sci. USA 90:740-744, 1993). Here we report the identification and characterization of a 31-kb genomic locus that contains clonal MMTV integrations in 8 of 80 mammary tumors from MMTV-infected Wnt-1 transgenic mice. Two genes were identified within this locus, one of which was transcriptionally activated by MMTV insertions. This activated gene is identical to androgen-induced growth factor (AIGF/Fgf-8) (A. Tanaka, K. Miyamoto, N. Minamino, M. Takeda, B. Sato, H. Matsuo, and K. Matsumoto, Proc. Natl. Acad. Sci. USA 89:8928-8932, 1992), the eighth member of the fibroblast growth factor (FGF) family. Transcriptional activation of Fgf-8 was found in all tumors with MMTV insertions in this locus. Fgf-8 mRNA was absent in normal mammary glands and was detected only in adult testis and ovary and in midgestational embryos. The sequences of Fgf-8 genomic and cDNA clones revealed five coding exons, in contrast to the three coding exons found in other FGF genes. cDNAs encoding three isoforms of the FGF-8 protein were isolated. The three corresponding mRNAs resulted from the alternative use of two 5' splice sites and two 3' splice sites for the second and third exons, respectively. These results implicate Fgf-8 as the third FGF gene found to cooperate with Wnt-1 in MMTV-induced murine mammary tumorigenesis, suggesting that FGFs and Wnts are strong collaborators in this process.

Haematological disorders associated with feline retrovirus infections

Feline oncornavirus and lentivirus infections have provided useful models to characterize the virus and host cell factors involved in a variety of marrow suppressive disorders and haematological malignancies. Exciting recent progress has been made in the characterization of the viral genotypic features involved in FeLV-associated diseases. Molecular studies have clearly defined the causal role of variant FeLV env gene determinants in two disorders: the T-lymphocyte cytopathicity and the clinical acute immunosuppression induced by the FeLV-FAIDS variant and the pure red cell aplasia induced by FeLV-C/Sarma. Variant or enFeLV env sequences also appear to play a role in FeLV-associated lymphomas. Additional studies are required to determine the host cell processes that are perturbed by these variant env gene products. In the case of the FeLV-FAIDS variant, the aberrant env gene products appear to impair superinfection interference, resulting in accumulation of unintegrated viral DNA and cell death. In other cases it is likely that the viral env proteins interact with host products that are important in cell viability and/or proliferation. Understanding of these mechanisms will therefore provide insights to factors involved in normal lymphohaematopoiesis. Similarly, studies of FeLV-induced haematological neoplasms should reveal recombination or rearrangement events involving as yet unidentified host gene sequences that encode products involved in normal cell growth regulation. These sequences may include novel protoncogenes or sequences homologous to genes implicated in human haematological malignancies. The haematological consequences of FIV are quite similar to those associated with HIV. As with HIV, FIV does not appear to directly infect myeloid or erythroid precursors, and the mechanisms of marrow suppression likely involve virus, viral antigen, and/or infected accessory cells in the marrow microenvironment. Studies using in vitro experimental models are required to define the effects of each of these microenvironmental elements on haematopoietic progenitors. As little is known about the molecular mechanisms of FIV pathogenesis, additional studies of disease-inducing FIV strains are needed to identify the genotypic features that correlate with virulent phenotypic features. Finally, experimental FIV infection in cats provides the opportunity to correlate in vivo virological and haematological changes with in vitro observations in a large animal model that closely mimics HIV infection in man.

Akv murine leukemia virus enhances lymphomagenesis in myc-kappa transgenic and in wild-type mice

The contribution of endogenous retroviruses to the multistep process of lymphomagenesis was investigated in wild-type mice and in two different myc-kappa transgenic mouse lines by infection with Akv. This retrovirus is derived from the endogenous ecotropic provirus of the AKR mouse and was previously considered to be nonlymphomagenic. The mice of the two myc-k transgenic lines are predisposed to B-cell lymphomagenesis and were therefore considered to be more susceptible to Akv. For comparison, the same mouse strains were also infected with the exogenous Moloney murine leukemia virus (MoMuLV). Both MoMuLV and Akv increased the tumor incidence and shortened the tumor latency period in wild-type mice and in the transgenic mouse lines. The differences in pathogenicity, number of provirus integrations, and level of virus expression between MoMuLV and Akv indicate different mechanisms of lymphomagenesis: while MoMuLV induced tumors apparently by insertional mutagenesis involving common integration sites similar to previous reports, the enhancement of lymphomagenesis by Akv seems to be directed by other mechanisms.

Akv murine leukemia virus enhances bone tumorigenesis in hMT-c-fos-LTR transgenic mice

hMt-c-fos-LTR transgenic mice (U. Rüther, D. Komitowski, F. R. Schubert, and E. F. Wagner. Oncogene 4, 861-865, 1989) developed bone sarcomas in 20% (3/15) of females at 448 +/- 25 days and in 8% (1/12) of males at 523 days. After infection of newborns with Akv, an infectious retrovirus derived from the ecotropic provirus of the AKR mouse, 69% (20/28) of female animals and 83% (24/29) of males developed malignant fibrous-osseous tumors. The tumors in infected transgenics developed with higher frequency and a 200-days shorter mean tumor latency period. The hMt-c-fos-LTR transgene was expressed in all the fibrous-osseous tumors. They also showed newly integrated Akv proviruses, but in most tumors Akv was detected and expressed in only a small number of the tumor cells. Wild-type C3H mice infected with Akv developed benign osteomas with an incidence of 33% and a latency period of 474 days. The data indicate that Akv exerts distinct pathogenic effects on the skeleton. In hMt-c-fos-LTR transgenic mice, predisposed to bone sarcomagenesis, Akv acts synergistically with the fos transgene, resulting in the development of fibrous-osseous tumors.

Genetic determinants of feline leukemia virus-induced lymphoid tumors: patterns of proviral insertion and gene rearrangement

The genetic basis of feline leukemia virus (FeLV)-induced lymphoma was investigated in a series of 63 lymphoid tumors and tumor cell lines of presumptive T-cell origin. These were examined for virus-induced rearrangements of the c-myc, flvi-2 (bmi-1), fit-1, and pim-1 loci, for T-cell receptor (TCR) gene rearrangements, and for the presence of env recombinant FeLV (FeLV-B). The myc locus was most frequently affected in naturally occurring lymphomas (32%; n = 38) either by transduction (21%) or by proviral insertion (11%). Proviral insertions were also common at flvi-2 (24%). The two other loci were occupied in a smaller number of the naturally occurring tumors (fit-1, 8%; pim-1, 5%). Examination of the entire set of tumors showed that significant numbers were affected at two (19%) or three (5%) of the loci. Occupation of the fit-1 locus was observed most frequently in tumors induced by FeLV-myc strains, while flvi-2 insertions occurred with similar frequency in the presence or absence of obvious c-myc activation. These results suggest a hierarchy of mutational events in the genesis of feline T-cell lymphomas by FeLV and implicate insertion at fit-1 as a late progression step. The strongest links observed were with T-cell development, as monitored by rearrangement status of the TCR beta-chain gene, which was positively associated with activation of myc (P < 0.001), and with proviral insertion at flvi-2 (P = 0.02). This analysis also revealed a genetically distinct subset of thymic lymphomas with unrearranged TCR beta-chain genes in which the known target loci were involved very infrequently. The presence of env recombinant FeLV (FeLV-B) showed a negative correlation with proviral insertion at fit-1, possibly due to the rapid onset of these tumors. These results shed further light on the multistep process of FeLV leukemogenesis and the relationships between lymphoid cell maturation and susceptibility to FeLV transformation.

Myc-mediated apoptosis requires wild-type p53 in a manner independent of cell cycle arrest and the ability of p53 to induce p21waf1/cip1

Deregulated expression of the c-myc proto-oncogene can lead to apoptosis under certain physiological conditions. By introducing a conditionally active Myc allele into primary embryo fibroblasts null for p53, and into fibroblasts without endogenous p53 expression but ectopically expressing a temperature-sensitive p53 allele, we show that expression of wild-type p53 is required for susceptibility to Myc-mediated apoptosis. Although ectopic expression of wild-type p53 blocked cells in the G1 phase of the cell cycle, G1 arrest by isoleucine starvation, in a manner independent of p53, did not confer susceptibility to apoptosis. Thus, growth arrest per se is not sufficient to induce Myc-mediated apoptosis; instead, a property intrinsic to p53 is specifically required. Moreover, apoptosis did not require induction of p53 target proteins, including the cyclin-dependent kinase inhibitor p21waf1/cip1. Therefore, the role of p53 in apoptosis may be distinct from its role in cell cycle arrest.

SV40 T antigen directed by a powerful erythroid enhancer-promoter produced sarcomas and pancreatic tumors but not erythroid-specific tumors in transgenic mice

We have expressed the simian virus 40 (SV40) large T antigen oncogene in erythroid tissues of mice to test its ability to immortilize erythroid cells. A transgene construct was built in which the SV40 large T antigen structural gene was linked to erythroid-specific enhancer and promoter sequences. The enhancer employed was the human beta-globin family microlocus control region, and the promoter sequences were derived from the human beta-globin promoter. Transgenic mice were generated and they expressed T antigen in the bone marrow and spleen cells. Yet, no hematopoietic neoplasia arose in these mice. Instead, after a lag period of 2-6 months, the mice developed soft tissue sarcomas and pancreatic islet-cell tumors that expressed high levels of T antigen.

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.

Gene amplification in non-Hodgkin's lymphoma

Among 426 consecutively ascertained and karyotypically abnormal non-Hodgkin's lymphoma (NHL) tumours, cytological evidence for gene amplification in the form of homogeneously staining regions (HSRs) was encountered in nine cases of large cell diffuse lymphoma (LC-DL). The mean age of patients with HSRs was 62.9 years and four died within a year of diagnosis. To identify candidate gene(s) amplified in these tumours, we performed a Southern blot analysis of tumour DNA using probes for 23 known protooncogenes and the multidrug resistance gene, PGY1. Besides a two-fold amplification of the BCL2 gene in two cases, no evidence for overt amplification of any of the genes assayed was found. To confirm DNA amplification in these specimens we performed the DNA in-gel renaturation assay. Evidence for presence of amplified DNA fragments was obtained in four of seven specimens. These results suggest amplification of a novel gene(s). To our knowledge, this is the first formal study of gene amplification in a large consecutively ascertained series of fresh lymphoma biopsies.

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.

Posterior transformation, neurological abnormalities, and severe hematopoietic defects in mice with a targeted deletion of the bmi-1 proto-oncogene

The bmi-1 proto-oncogene has been implicated in B-cell lymphomagenesis in E mu-myc transgenic mice. Distinct domains of the Bmi-1 protein are highly conserved within the Drosophila protein Posterior Sex Combs, a member of the Polycomb group involved in maintaining stable repression of homeotic genes during development. We have inactivated the bmi-1 gene in the germ line of mice by homologous recombination in ES cells. Null mutant mice display three phenotypic alterations: (1) a progressive decrease in the number of hematopoietic cells and an impaired proliferative response of these cells to mitogens; (2) neurological abnormalities manifested by an ataxic gait and sporadic seizures; and (3) posterior transformation, in most cases along the complete anteroposterior axis of the skeleton. The observations indicate that Mbi-1 plays an important role in morphogenesis during embryonic development and in hematopoiesis throughout pre- and postnatal life. Furthermore, these data provide the first evidence of functional conservation of a mammalian Polycomb group homolog.

Transcriptional repression by Drosophila and mammalian Polycomb group proteins in transfected mammalian cells

The Polycomb group (Pc-G) genes are essential for maintaining the proper spatially restricted expression pattern of the homeotic loci during Drosophila development. The Pc-G proteins appear to function at target loci to maintain a state of transcriptional repression. The murine oncogene bmi-1 has significant homology to the Pc-G gene Posterior sex combs (Psc) and a highly related gene, Suppressor two of zeste [Su(z)2]. We show here that the proteins encoded by bmi-1 and the Pc-G genes Polycomb (Pc) and Psc as well as Su(z)2 mediate repression in mammalian cells when targeted to a promoter by LexA in a cotransfection system. These fusion proteins repress activator function by as much as 30-fold, and the effect on different activation domains is distinct for each Pc-G protein. Repression is observed when the LexA fusion proteins are bound directly adjacent to activator binding sites and also when bound 1,700 bases from the promoter. These data demonstrate that the products of the Pc-G genes can significantly repress activator function on transiently introduced DNA. We suggest that this function contributes to the stable repression of targeted loci during development.

The t(15;17) translocation alters a nuclear body in a retinoic acid-reversible fashion

Nuclear bodies (NBs) are ultrastructurally defined granules predominantly found in dividing cells. Here we show that PML, a protein involved in the t(15;17) translocation of acute promyelocytic leukaemia (APL), is specifically bound to a NB. PML and several NB-associated proteins, found as auto-antigens in primary biliary cirrhosis (PBC), are co-localized and co-regulated. The APL-derived PML-RAR alpha fusion protein is shown to be predominantly localized in the cytoplasm, whereas a fraction is nuclear and delocalizes the NB antigens to multiple smaller nuclear clusters devoid of ultrastructural organization. RA administration (which in APL patients induces blast differentiation and consequently complete remissions) causes the re-aggregation of PML and PBC auto-antigens onto the NB, while PML-RAR alpha remains mainly cytoplasmic. Thus, PML-RAR alpha expression leads to a RA-reversible alteration of a nuclear domain. These results shed a new light on the pathogenesis of APL and provide a molecular link between NBs and oncogenesis.

Transcriptional repression by Drosophila and mammalian Polycomb group proteins in transfected mammalian cells

The Polycomb group (Pc-G) genes are essential for maintaining the proper spatially restricted expression pattern of the homeotic loci during Drosophila development. The Pc-G proteins appear to function at target loci to maintain a state of transcriptional repression. The murine oncogene bmi-1 has significant homology to the Pc-G gene Posterior sex combs (Psc) and a highly related gene, Suppressor two of zeste [Su(z)2]. We show here that the proteins encoded by bmi-1 and the Pc-G genes Polycomb (Pc) and Psc as well as Su(z)2 mediate repression in mammalian cells when targeted to a promoter by LexA in a cotransfection system. These fusion proteins repress activator function by as much as 30-fold, and the effect on different activation domains is distinct for each Pc-G protein. Repression is observed when the LexA fusion proteins are bound directly adjacent to activator binding sites and also when bound 1,700 bases from the promoter. These data demonstrate that the products of the Pc-G genes can significantly repress activator function on transiently introduced DNA. We suggest that this function contributes to the stable repression of targeted loci during development.

Identification of Evi-3, a novel common site of retroviral integration in mouse AKXD B-cell lymphomas

We have identified a novel common site of ecotropic viral integration called ecotropic viral integration site 3 (Evi-3) in B-cell lineage lymphomas of the AKXD recombinant inbred strains of mice. A number of virally induced pre-B-, B-, myeloid, and T-cell lymphomas were screened for viral rearrangements at Evi-3; rearrangements were found in pre-B- and B-cell lymphomas but not in other hematopoietic tumors. Genetic mapping studies localized Evi-3 to mouse chromosome 18, distinct from proto-oncogene and common viral integration loci identified previously in the mouse. Each proviral integration at Evi-3 is contained within a 200-bp region that lies inside a CpG island. All but one of the proviruses have integrated in the same 5'-to-3' transcriptional orientation. Transcripts from Evi-3 are expressed in a developmentally regulated manner in B cells. Taken together, these data suggest that Evi-3 represents a novel proto-oncogene involved in mouse B-cell lymphomas.

In vivo and in vitro analyses of recombinant baculoviruses lacking a functional cg30 gene

The cg30 gene of Autographa californica nuclear polyhedrosis virus (AcMNPV) encodes two sequence motifs, a zinc finger-like motif and a leucine zipper, found in other polypeptides known to be involved in gene regulation. To gain insight into the function of the cg30 product, CG30, we constructed and characterized recombinant viruses lacking a functional cg30 gene. We found that cg30 mutants had no striking phenotype in cell lines derived from Spodoptera frugiperda or Trichoplusia ni or in T. ni larvae. Although cg30 is known to be transcribed as an early monocistronic RNA and as the second cistron of an abundant late bicistronic RNA, production of a CG30-beta-galactosidase fusion protein was observed mainly at early times postinfection. Viruses containing cg30 had a subtle growth advantage over those lacking cg30 after several viral passages in cell culture. We employed transient expression assays to determine whether cg30 and pe-38, an AcMNPV gene that encodes a polypeptide with zinc finger-like and leucine zipper motifs similar to those of cg30, have redundant functions. Although pe-38 may have a role in AcMNPV gene expression, there was no indication that cg30 and pe-38 are functionally redundant.

Mammary tumors expressing the neu proto-oncogene possess elevated c-Src tyrosine kinase activity

Amplification and overexpression of the neu (c-erbB2) proto-oncogene has been implicated in the pathogenesis of 20 to 30% of human breast cancers. Although the activation of Neu receptor tyrosine kinase appears to be a pivotal step during mammary tumorigenesis, the mechanism by which Neu signals cell proliferation is unclear. Molecules bearing a domain shared by the c-Src proto-oncogene (Src homology 2) are thought to be involved in signal transduction from activated receptor tyrosine kinases such as Neu. To test whether c-Src was implicated in Neu-mediated signal transduction, we measured the activity of the c-Src tyrosine kinase in tissue extracts from either mammary tumors or adjacent mammary epithelium derived from transgenic mice expressing a mouse mammary tumor virus promoter/enhancer/unactivated neu fusion gene. The Neu-induced mammary tumors possessed six- to eightfold-higher c-Src kinase activity than the adjacent epithelium. The increase in c-Src tyrosine kinase activity was not due to an increase in the levels of c-Src but rather was a result of the elevation of its specific activity. Moreover, activation of c-Src was correlated with its ability to complex tyrosine-phosphorylated Neu both in vitro and in vivo. Together, these observations suggest that activation of the c-Src tyrosine kinase during mammary tumorigenesis may occur through a direct interaction with activated Neu.