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

Overexpression of Frat1 in transgenic mice leads to glomerulosclerosis and nephrotic syndrome, and provides direct evidence for the involvement of Frat1 in lymphoma progression

The proto-oncogene Frat1 was originally identified as a common site of proviral insertion in transplanted tumors of Moloney murine leukemia virus (M-MuLV)-infected Emu-Pim1 transgenic mice. Contrary to most common insertion sites implicated in mouse T cell lymphomagenesis, retroviral insertional mutagenesis of Frat1 constitutes a relatively late event in M-MuLV-induced tumor development, suggesting that proviral activation of Frat1 contributes to progression of T cell lymphomas rather than their genesis. To substantiate this notion we have generated transgenic mice that overexpress Frat1 in various organs, including lymphoid tissues. Frat1 transgenic mice develop focal glomerulosclerosis and a nephrotic syndrome, but they do not exhibit an increased incidence of spontaneous lymphomas. Conversely, these mice are highly susceptible to M-MuLV-induced lymphomagenesis, and Frat1/Pim1 bitransgenic animals develop lymphomas with increased frequency compared to Pim1 transgenic littermates. These data support a role for Frat1 in tumor progression.

Bmi-1 collaborates with c-Myc in tumorigenesis by inhibiting c-Myc-induced apoptosis via INK4a/ARF

The bmi-1 and myc oncogenes collaborate strongly in murine lymphomagenesis, but the basis for this collaboration was not understood. We recently identified the ink4a-ARF tumor suppressor locus as a critical downstream target of the Polycomb-group transcriptional repressor Bmi-1. Others have shown that part of Myc's ability to induce apoptosis depends on induction of p19arf. Here we demonstrate that down-regulation of ink4a-ARF by Bmi-1 underlies its ability to cooperate with Myc in tumorigenesis. Heterozygosity for bmi-1 inhibits lymphomagenesis in Emu-myc mice by enhancing c-Myc-induced apoptosis. We observe increased apoptosis in bmi-1(-/-) lymphoid organs, which can be rescued by deletion of ink4a-ARF or overexpression of bcl2. Furthermore, Bmi-1 collaborates with Myc in enhancing proliferation and transformation of primary embryo fibroblasts (MEFs) in an ink4a-ARF dependent manner, by prohibiting Myc-mediated induction of p19arf and apoptosis. We observe strong collaboration between the Emu-myc transgene and heterozygosity for ink4a-ARF, which is accompanied by loss of the wild-type ink4a-ARF allele and formation of highly aggressive B-cell lymphomas. Together, these results reinforce the critical role of Bmi-1 as a dose-dependent regulator of ink4a-ARF, which on its turn acts to prevent tumorigenesis on activation of oncogenes such as c-myc.

Disruption of the ARF-Mdm2-p53 tumor suppressor pathway in Myc-induced lymphomagenesis

Transgenic mice expressing the c-Myc oncogene driven by the immunoglobulin heavy chain enhancer (Emu) develop B-cell lymphoma and exhibit a mean survival time of approximately 6 months. The protracted latent period before the onset of frank disease likely reflects the ability of c-Myc to induce a p53-dependent apoptotic program that initially protects animals against tumor formation but is disabled when overtly malignant cells emerge. In cultured primary mouse embryo fibroblasts, c-Myc activates the p19(ARF)-Mdm2-p53 tumor suppressor pathway, enhancing p53-dependent apoptosis but ultimately selecting for surviving immortalized cells that have sustained either p53 mutation or biallelic ARF deletion. Here we report that p53 and ARF also potentiate Myc-induced apoptosis in primary pre-B-cell cultures, and that spontaneous inactivation of the ARF-Mdm2-p53 pathway occurs frequently in tumors arising in Emu-myc transgenic mice. Many Emu-myc lymphomas sustained either p53 (28%) or ARF (24%) loss of function, whereas Mdm2 levels were elevated in others. Its overexpression in some tumors lacking p53 function raises the possibility that Mdm2 can contribute to lymphomagenesis by interacting with other targets. Emu-myc transgenic mice hemizygous for ARF displayed accelerated disease (11-week mean survival), and 80% of these tumors lost the wild-type ARF allele. All ARF-null Emu-myc mice died of lymphoma within a few weeks of birth. About half of the tumors arising in ARF hemizygous or ARF nullizygous Emu-myc transgenic mice also overexpressed Mdm2. Therefore, Myc activation strongly selects for spontaneous inactivation of the ARF-Mdm2-p53 pathway in vivo, cancelling its protective checkpoint function and accelerating progression to malignancy.

Disruption of the ARF-Mdm2-p53 tumor suppressor pathway in Myc-induced lymphomagenesis

Transgenic mice expressing the c-Myc oncogene driven by the immunoglobulin heavy chain enhancer (Emu) develop B-cell lymphoma and exhibit a mean survival time of approximately 6 months. The protracted latent period before the onset of frank disease likely reflects the ability of c-Myc to induce a p53-dependent apoptotic program that initially protects animals against tumor formation but is disabled when overtly malignant cells emerge. In cultured primary mouse embryo fibroblasts, c-Myc activates the p19(ARF)-Mdm2-p53 tumor suppressor pathway, enhancing p53-dependent apoptosis but ultimately selecting for surviving immortalized cells that have sustained either p53 mutation or biallelic ARF deletion. Here we report that p53 and ARF also potentiate Myc-induced apoptosis in primary pre-B-cell cultures, and that spontaneous inactivation of the ARF-Mdm2-p53 pathway occurs frequently in tumors arising in Emu-myc transgenic mice. Many Emu-myc lymphomas sustained either p53 (28%) or ARF (24%) loss of function, whereas Mdm2 levels were elevated in others. Its overexpression in some tumors lacking p53 function raises the possibility that Mdm2 can contribute to lymphomagenesis by interacting with other targets. Emu-myc transgenic mice hemizygous for ARF displayed accelerated disease (11-week mean survival), and 80% of these tumors lost the wild-type ARF allele. All ARF-null Emu-myc mice died of lymphoma within a few weeks of birth. About half of the tumors arising in ARF hemizygous or ARF nullizygous Emu-myc transgenic mice also overexpressed Mdm2. Therefore, Myc activation strongly selects for spontaneous inactivation of the ARF-Mdm2-p53 pathway in vivo, cancelling its protective checkpoint function and accelerating progression to malignancy.

Transgenic models of lymphoid neoplasia and development of a pan-hematopoietic vector

The pathways to lymphoid neoplasia have been explored in a number of transgenic models. Because B lymphoid malignancies often involve translocation of an oncogene (e.g. myc, bcl-2, cyclin D1) to an immunoglobulin locus, resulting in its deregulated expression, the consequences of oncogene overexpression in lymphocytes can be evaluated with transgenes driven by an immunoglobulin regulatory element, such as an enhancer from the IgH locus. Mice bearing such transgenes have provided insight into the preneoplastic state, including alterations in the control of cellular proliferation, differentiation or apoptosis. They have also allowed studies on oncogene cooperation in vivo and the modulating effect of genetic background. Briefly reviewed here are the models studied in the authors' laboratories. Mice bearing myc and bcl-2 transgenes have received most attention but others studied include abl, ras, cyclin D1 and bmi-1 oncogenes. Also discussed is a new transgenic vector that should facilitate transgenic approaches to non-lymphoid leukemias. The vector bears elements from the promoter region of the vav gene, which is expressed almost exclusively in hematopoietic cells. It has proven capable of driving transgene expression throughout the hematopoietic compartment, including progenitor cells and their precursors. This novel vector should aid studies on many aspects of hematopoiesis, including the modeling of leukemogenesis.

Genes Encoding Three New Members of the Leukocyte Antigen 6 Superfamily and a Novel Member of Ig Superfamily, Together with Genes Encoding the Regulatory Nuclear Chloride Ion Channel Protein (hRNCC) and a Nω-Nω-Dimethylarginine Dimethylaminohydrolase Homologue, Are Found in a 30-kb Segment of the MHC Class III Region

Many of the genes in the class III region of the human MHC encode proteins involved in the immune and inflammatory responses. We have sequenced a 30-kb segment of the MHC class III region lying between the heat shock protein 70 and TNF genes as part of a program aimed at identifying genes that could be involved in autoimmune disease susceptibility. The sequence analysis has revealed the localization of seven genes, whose precise position and order is cen-G7-G6-G6A-G6B-G6C-G6D-G6E-tel, five of which are fully encoded in the sequence, allowing their genomic structures to be defined. Three of them (G6C, G6D, and G6E) encode putative proteins that belong to the Ly-6 superfamily, known to be GPI-anchored proteins attached to the cell surface. Members of the family are specifically expressed and are important in leukocyte maturation. A fourth gene, G6B, encodes a novel member of the Ig superfamily containing a single Ig V-like domain and a cytoplasmic tail with several signal transduction features. The G6 gene encodes a regulatory nuclear chloride ion channel protein, while the G6A gene encodes a putative homologue of the enzyme Nω,Nω-dimethylarginine dimethylaminohydrolase, which is thought to be involved in regulating nitric oxide synthesis. In addition, three microsatellite markers, 9N-1, 82-2, and D6S273 are contained within the sequence, the last two of which have been reported to be strongly associated with the autoimmune disease ankylosing spondylitis.

Cooperation of Myb and Myc proteins in T cell lymphomagenesis

The v-Myb oncogene causes late onset T cell lymphomas when expressed in the T cell lineage of transgenic mice. In order to define the cellular mutations cooperating with s-Myb to cause lymphomas, we have infected v-Myb transgenic mice with Moloney murine leukemia virus (M-MuLV). Tumor formation is significantly accelerated from a mean age of onset of 60 weeks in uninfected vMyb transgenics to 13 weeks in infected vMyb transgenics. We studied the loci into which the M-MuLV had inserted, and found that in 73% of animals, either the c-myc or the N-myc genes had been disrupted and deregulated. Therefore, v-myb and c-myb can cooperate to induce T cell lymphomas.

Reassessing the role of C-MYB in tumorigenesis

Hematopoietic tumors in both humans and mice frequently up-regulate expression of the c-myb gene, but it is unclear whether this is a cause or a consequence of the leukemic state. Recent results placing super-activation of the c-Myb protein at the bottom of a kinase-activated signal transduction pathway indicate that it may be a downstream effector of transformation induced by other oncogenes. The relationship between c-Myb and the serine-threonine kinase pim-1, its immediate activator, is discussed, together with the possibility that c-Myb, like pim-1, may be able to synergize with c-Myc to induce tumors.

SSX and the synovial-sarcoma-specific chimaeric protein SYT-SSX co-localize with the human Polycomb group complex

Chromosome translocation t(X;18)(p11.2;q11.2) is unique to synovial sarcomas and results in an 'in frame' fusion of the SYT gene with the SSX1 or closely-related SSX2 gene. Wild-type SYT and SSX proteins, and the SYT-SSX chimaeric proteins, can modulate transcription in gene reporter assays. To help elucidate the role of these proteins in cell function and neoplasia we have performed immunolabelling experiments to determine their subcellular localization in three cell types. Transient expression of epitope-tagged proteins produced distinctive nuclear staining patterns. The punctate staining of SYT and SYT-SSX proteins showed some similarities. We immunolabelled a series of endogenous nuclear antigens and excluded the SYT and SYT-SSX focal staining from association with these domains (e.g. sites of active transcription, snRNPs). In further experiments we immunolabelled the Polycomb group (PcG) proteins RING1 or BMI-1 and showed that SSX and SYT-SSX proteins, but not SYT, co-localized with these markers. Consistent with this we show that SSX and SYT-SSX associate with chromatin, and also associate with condensed chromatin at metaphase. Noteably, SSX produced a dense signal over the surface of metaphase chromosomes whereas SYT-SSX produced discrete focal staining. Our data indicate that SSX and SYT-SSX proteins are recruited to nuclear domains occupied by PcG complexes, and this provides us with a new insight into the possible function of wild-type SSX and the mechanism by which the aberrant SYT-SSX protein might disrupt fundamental mechanisms controlling cell division and cell fate.

The oncogene and Polycomb-group gene bmi-1 regulates cell proliferation and senescence through the ink4a locus

The bmi-1 gene was first isolated as an oncogene that cooperates with c-myc in the generation of mouse lymphomas. We subsequently identified Bmi-1 as a transcriptional repressor belonging to the mouse Polycomb group. The Polycomb group comprises an important, conserved set of proteins that are required to maintain stable repression of specific target genes, such as homeobox-cluster genes, during development. In mice, the absence of bmi-1 expression results in neurological defects and severe proliferative defects in lymphoid cells, whereas bmi-1 overexpression induces lymphomas. Here we show that bmi-1-deficient primary mouse embryonic fibroblasts are impaired in progression into the S phase of the cell cycle and undergo premature senescence. In these fibroblasts and in bmi-1-deficient lymphocytes, the expression of the tumour suppressors p16 and p19Arf, which are encoded by ink4a, is raised markedly. Conversely, overexpression of bmi-1 allows fibroblast immortalization, downregulates expression of p16 and p19Arf and, in combination with H-ras, leads to neoplastic transformation. Removal of ink4a dramatically reduces the lymphoid and neurological defects seen in bmi-1-deficient mice, indicating that ink4a is a critical in vivo target for Bmi-1. Our results connect transcriptional repression by Polycomb-group proteins with cell-cycle control and senescence.

RING1 interacts with multiple Polycomb-group proteins and displays tumorigenic activity

Polycomb-group (PcG) proteins form large multimeric protein complexes that are involved in maintaining the transcriptionally repressive state of genes. Previously, we reported that RING1 interacts with vertebrate Polycomb (Pc) homologs and is associated with or is part of a human PcG complex. However, very little is known about the role of RING1 as a component of the PcG complex. Here we undertake a detailed characterization of RING1 protein-protein interactions. By using directed two-hybrid and in vitro protein-protein analyses, we demonstrate that RING1, besides interacting with the human Pc homolog HPC2, can also interact with itself and with the vertebrate PcG protein BMI1. Distinct domains in the RING1 protein are involved in the self-association and in the interaction with BMI1. Further, we find that the BMI1 protein can also interact with itself. To better understand the role of RING1 in regulating gene expression, we overexpressed the protein in mammalian cells and analyzed differences in gene expression levels. This analysis shows that overexpression of RING1 strongly represses En-2, a mammalian homolog of the well-characterized Drosophila PcG target gene engrailed. Furthermore, RING1 overexpression results in enhanced expression of the proto-oncogenes c-jun and c-fos. The changes in expression levels of these proto-oncogenes are accompanied by cellular transformation, as judged by anchorage-independent growth and the induction of tumors in athymic mice. Our data demonstrate that RING1 interacts with multiple human PcG proteins, indicating an important role for RING1 in the PcG complex. Further, deregulation of RING1 expression leads to oncogenic transformation by deregulation of the expression levels of certain oncogenes.

Insights into cancer from transgenic mouse models

The generation of mice designed to overexpress activated forms of oncogenes or carrying targeted mutations in tumour suppressor genes, has allowed scientists to causally link the function of these genes with specific tumour processes, such as proliferation, apoptosis, angiogenesis or metastasis. In addition, these mice have been interbred to assess the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of the multi-stage nature of tumourigenesis. The effect of genetic mutations is often influenced by the genetic background of the mouse and by analysing strain-dependent phenotypes, modifier loci have been identified. Although genetic mutations in mouse and humans do not always lead to the same tumour spectrum, the underlying molecular mechanisms are frequently relevant to both species. Furthermore, new technical approaches creating conditional mouse mutants which develop tumours in a tissue-specific manner, will allow the effect of mutation of certain genes to be studied in specific tissues, free from the fatal effects of the mutation in other clinically less relevant tissues. Several exising mouse strains have already been used to develop and test new therapies and conditional mutagenesis will undoubtedly increase the potential use of transgenic mice in understanding and treating cancer.

A constitutively active epidermal growth factor receptor cooperates with disruption of G1 cell-cycle arrest pathways to induce glioma-like lesions in mice

The epidermal growth factor receptor (EGFR) gene is amplified or mutated in 30%-50% of human gliobastoma multiforme (GBM). These mutations are associated usually with deletions of the INK4a-ARF locus, which encodes two gene products (p16(INK4a) and p19(ARF)) involved in cell-cycle arrest and apoptosis. We have investigated the role of EGFR mutation in gliomagenesis, using avian retroviral vectors to transfer a mutant EGFR gene to glial precursors and astrocytes in transgenic mice expressing tv-a, a gene encoding the retrovirus receptor. TVA, under control of brain cell type-specific promoters. We demonstrate that expression of a constitutively active, mutant form of EGFR in cells in the glial lineage can induce lesions with many similarities to human gliomas. These lesions occur more frequently with gene transfer to mice expressing tv-a from the progenitor-specific nestin promoter than to mice expressing tv-a from the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter, suggesting that tumors arise more efficiently from immature cells in the glial lineage. Furthermore, EGFR-induced gliomagenesis appears to require additional mutations in genes encoding proteins involved in cell-cycle arrest pathways. We have produced these combinations by simultaneously infecting tv-a transgenic mice with vectors carrying cdk4 and EGFR or by infecting tv-a transgenic mice bearing a disrupted INK4a-ARF locus with the EGFR-carrying vector alone. Moreover, EGFR-induced gliomagenesis does not occur in conjunction with p53 deficiency, unless the mice are also infected with a vector carrying cdk4. The gliomagenic combinations of genetic lesions required in mice are similar to those found in human gliomas.

Molecular cloning, localization, and developmental expression of mouse brain finger protein (Bfp)/ZNF179: distribution of bfp mRNA partially coincides with the affected areas of Smith-Magenis syndrome

Bfp (brain finger protein) is a member of the RING finger protein family, which is highly expressed in the brain. We have previously shown that one copy of the human bfp gene, mapped at 17p11.2, was actually deleted in six of six Smith-Magenis syndrome (SMS) patients. Now we have isolated the mouse bfp cDNA. Using in situ hybridization and immunohistochemistry, the distribution of mouse bfp mRNA and protein was identified especially in neural cells of the cerebral cortex, hippocampus, lateral amygdaloid nucleus, and ventromedial hypothalamus. In primary culture of the whole brain in a neonatal mouse, the Bfp protein was detected in both neuron and glial cells, and its subcellular localization was predominantly in the nucleus, but some amounts were also found in the cytoplasm. The bfp mRNA was also expressed strongly in the marginal zone of brain vesicles, optic stalk, and cartilage primordium, which are part of the critical tissues frequently involved in SMS patients, and in such tissues as nasal epithelium and primordium of follicles in a 13. 5-dpc embryo. Subsequently, its amount in the developing brain further increased during embryogenesis, reaching the highest level in the adult brain. These findings suggest a possibility that Bfp might be involved in the pathogenesis of Smith-Magenis syndrome as a regulator protein related to neural differentiation and function.

Cloning of the rat proto-oncogene bmi-1

The bmi-1 gene was identified as a common proviral integration site in Moloney murine leukemia virus. In the present studies, we cloned and sequenced the rat bmi-1 gene by reverse transcriptase-polymerase chain reaction (RT-PCR) using degenerate PCR primers of homologous sequences between mouse and human. We found 93% identity to the mouse bmi-1 cDNA and 90% identity to the human bmi-1. The open reading frame encodes a protein of 324 amino acids. In the deduced amino acid sequence we observed 95% and 94% homology to the mouse and human, respectively. The structural motifs, a novel zinc finger motif and a putative helix-turn-helix motif, were conserved in the predicted rat BMI-1 protein. We also confirmed ubiquitous expression of bmi-1 in normal tissues except brain. These results suggest functional conservation of the bmi-1 gene in the rat.

The regional integration of retroviral sequences into the mosaic genomes of mammals

We have reviewed here three sets of data concerning the integration of retroviral sequences in the mammalian genome: (i) our experimental localization of a number of proviruses integrated in isochores characterized by different GC levels; (ii) results from other laboratories on the localization of retroviral sequences in open chromatin regions and/or next to CpG islands; and (iii) our compositional analysis of genes located in the neighborhood of integrated retroviral sequences. The three sets of data have provided a very consistent picture in that a compartmentalized, isopycnic integration of expressed proviruses appears to be the rule ('isopycnic' refers to the compositional match between viral and host sequences around the integration site). The results reviewed here suggest that: (i) integration of proviral sequences is targeted initially towards 'open chromatin regions'; while these exist in both GC-rich and GC-poor isochores, the 'open chromatin regions' of GC-rich isochores are the main targets for integration of retroviral sequences because of their much greater abundance; (ii) isopycnicity is associated with stability of integration; indeed, even non-expressed integrated retroviral sequences tend to show an isopycnic localization in the genome; (iii) transcription of integrated viral sequences (like transcription of host genes) appears to be associated, as a rule, with an isopycnic localization, as indicated by transcribed sequences that show an isopycnic integration and act in trans; (iv) selection plays a role in the choice of specific sites within an isopycnic region; in exceptional cases [such as mouse mammary tumor virus (MMTV) activating GC-rich oncogenes], selection may override isopycnicity.

Genetic interactions and dosage effects of Polycomb group genes in mice

In Drosophila and mouse, Polycomb group genes are involved in the maintenance of homeotic gene expression patterns throughout development. Here we report the skeletal phenotypes of compound mutants for two Polycomb group genes bmi1 and M33. We show that mice deficient for both bmi1 and M33 present stronger homeotic transformations of the axial skeleton as compared to each single Polycomb group mutant, indicating strong dosage interactions between those two genes. These skeletal transformations are accompanied with an enhanced shift of the anterior limit of expression of several Hox genes in the somitic mesoderm. Our results demonstrate that in mice the Polycomb group genes act in synergy to control the nested expression pattern of some Hox genes in somitic mesodermal tissues during development.

B-Cell lymphoma induction by akv murine leukemia viruses harboring one or both copies of the tandem repeat in the U3 enhancer

Akv is an endogenous, ecotropic murine leukemia virus (MuLV) of the AKR strain. It has served as a prototype nonpathogenic or weakly pathogenic reference virus for studies of closely related potent lymphomagenic viruses such as the T-lymphomagenic SL3-3. We here report that Akv and an Akv mutant (Akv1-99) with only one copy of the 99-bp transcriptional enhancer induce malignant lymphomas with nearly 100% incidence and mean latency periods of 12 months after injection into newborn NMRI mice. Molecular analysis of tumor DNA showed that the majority of the tumors were of the B-cell type. Sequence analysis of proviral transcriptional enhancers in DNA of B-cell lymphomas revealed conservation of the enhancer sequence, as well as a lack of sequence duplications of the Akv1-99 variant, while the repeat copy number in Akv was subject to fluctuations. In support of a B-cell specificity of the Akv enhancer, a murine plasmacytoma cell line was found to sustain three- to fivefold-higher transient transcriptional activity upon the Akv and Akv1-99 enhancers than upon the enhancer of the T-lymphomagenic SL3-3 MuLV. Thus, the overall picture is that Akv MuLV possesses a B- lymphomagenic potential and that the second copy of the 99-bp sequence seems to be of minor importance for this potential. However, in one animal the lymphomas induced by Akv1-99 were of the T-cell type. Among the 24 tumors analyzed only this one harbored a clonal proviral integration in the c-myc locus. This provirus had undergone a duplication of a 113-bp sequence of the enhancer region, partly overlapping with the 99-bp repeat of Akv, as well as a few single nucleotide alterations within and outside the repeats. Taken together with previous studies, our results suggest that T- versus B-lymphomagenic specificity of the enhancer is governed by more than one nucleotide difference and that alterations in binding sites for transcription factors of the AML1 and nuclear-factor-1 families may contribute to this specificity.

Mammalian Polycomb group genes are categorized as a new type of early response gene induced by B-cell receptor cross-linking

Polycomb group (PcG) genes were initially described in Drosophila melanogaster as regulators of the homeobox gene. Four mammalian homologues, mel-18, bmi-1, M33 and rae-28, are analyzed in this study. They not only regulate mammalian homeotic genes by analogy with their Drosophila counterparts, but also have some influence on the growth and differentiation of B lymphocytes. Here we report that these four mammalian PcG genes are rapidly induced after antigen-receptor cross-linking in B cells. Thus we would like to propose that mammalian PcG genes can be categorized as a new type of immediate early gene.

Ecotropic C-type retrovirus of B16 melanoma and malignant transformation of normal melanocytes

We reported previously that B16,JB/RH and JB/MS melanomas of C57BL/6 mice express the common melanoma-associated antigen (MAA) recognized by MM2-9B6 monoclonal antibody (MAb). This MAA is encoded by the env gene of an ecotropic MuLV-type retrovirus that somatically emerged in melanomas of C57BL/6 mice. The potential role of this melanoma-associated retrovirus (MelARV) in melanoma formation remains unknown and has not been previously investigated. To test this, normal melanocyte lines (melan-a and C57M) of C57BL/6 mice were infected with the MelARV produced by B16BL6 melanoma. Infection of these melanocytes with the MelARV was associated with the appearance of the MAA recognized by MM2-9B6 MAb. Most of the infected melanocyte sublines were able to grow only in the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA). Two infected melanocyte sublines showed morphological changes, were able to grow in the absence of TPA and, after inoculation into C57BL/6 mice, produced rapidly growing, highly pigmented tumors. These new melanomas, derived from the MelARV-infected melan-a and C57M melanocytes, were termed Meli-A1 and Meli-BL, respectively. Southern blot analysis of EcoRI- and HindIII-digested DNAs from these melanomas showed several retroviral insertion sites. One copy of MeIARV was found to be inserted at the end of the 6th leucine domain of the c-maf proto-oncogene, which encodes a basic region/leucine zipper transcription factor related to the AP-1 family that is able to form homodimers or heterodimers with Fos and Jun transcription factors. Our data indicate that c-maf is a common insertion site of MelARV in BL6, Meli-A1 and Meli-BL melanomas, whereas no such insertion site was found in the melanocytes infected with MelARV but not malignantly transformed. Thus, our data imply that the ecotropic MelARV that somatically emerged in B16 and other melanomas of C57BL/6 mice may play a role in malignant transformation.

Infection of WHV/c-myc transgenic mice with Moloney murine leukaemia virus and proviral insertion near the syndecan-4 gene in an early liver tumour

The capacity of Moloney murine leukaemia virus (MoMLV) to infect neonatal hepatocytes and to accelerate liver carcinogenesis was examined in a transgenic mouse model. WHV/c-myc mice which are highly susceptible to the development of liver tumours were infected with MoMLV shortly after birth, when expression of the murine ecotropic retroviral receptor gene was still detectable in the neonatal liver. All MoMLV-infected transgenic mice and non-transgenic littermates succumbed to T-cell lymphomas within 2-9 months; during this period of time, three infected transgenic animals developed primary hepatocellular carcinomas. Remarkably, one of these liver tumours arose significantly faster than tumours from uninfected WHV/c-myc controls, and it harboured a unique MoMLV provirus. The provirus integration site was located 5.5 kb upstream of the first exon of the syndecan-4 gene, which encodes a heparan sulphate proteoglycan implicated in growth factor activation and protein kinase C distribution in focal adhesions. Our data provide evidence for clonal MoMLV provirus integration in a hepatocellular carcinoma, and indicate that parenchymal liver cells may be susceptible to MoMLV infection following neonatal inoculation.

The functions of Myc in cell cycle progression and apoptosis

c-myc has emerged as one of the central regulators of mammalian cell proliferation. The gene encodes a transcription factor of the HLH/leucine zipper family of proteins that activates transcription as part of a heteromeric complex with a protein termed Max. In mammalian fibroblasts, Myc acts as an upstream regulator of cyclin-dependent kinases and functionally antagonises the action of at least one cdk inhibitor, p27. Myc also induces cells to undergo apoptosis, and the relationship between Myc-induced cell cycle entry and apoptosis is discussed.

Transgenic mouse models for tumor suppressor genes

The identification and cloning of tumor suppressor genes has mostly relied on familial human cancer predisposition syndromes and reverse genetics. Recent advances in manipulating the mouse genome by gene targeting techniques in embryonic stem (ES) cells has led to the generation of mutant mouse models mimicking many human syndromes. Mice lacking one or both alleles of known tumor suppressor genes have been generated to evaluate the normal function of these genes in vivo. These mice have proven to be highly susceptible to tumor development, indicating that the mouse is a potent in vivo assay system for tumor suppressor genes. The initiation of gonadal tumor development in mice lacking both copies of the alpha-inhibin gene demonstrates that this assay is also useful for identifying new tumor suppressor genes. In the future, murine ES cell/gene targeting strategies will continue to be used to identify novel tumor suppressors and analyze their in vivo roles in growth control.

Host cis-mediated extinction of a retrovirus permissive for expression in embryonal stem cells during differentiation

The use of retroviral vectors for gene transfer into animals has been severely hampered by the lack of provirus transcription in the early embryo and embryonic stem (ES) cells. This primary block in provirus expression is maintained in differentiated cells by a cis-acting mechanism that is not well characterized. Retroviral vectors based on the murine embryonal stem cell virus (MESV), which overcome the transcriptional block in ES cells, were constructed to investigate this secondary mechanism. These vectors transferred G418 resistance to ES cells with the same efficiency as to fibroblasts, but overall transcript levels were greatly reduced. A mosaic but stable expression pattern was observed when single cells from G418-resistant clones were replated in G418 or assayed for expression of LacZ or interleukin-3. The expression levels in independent clones were variable and correlated inversely with methylation. However, a second, more pronounced, block to transcription was found upon differentiation induction. Differentiation of the infected ES cells to cells permissive for retroviral expression resulted in repression and complete extinction of provirus expression. Extinction was not accompanied by increased levels of methylation. Provirus expression is thus regulated by two independent cis-acting mechanisms: (i) partial repression in the undifferentiated state, accompanied by increased methylation but compatible with long-term, low expression of retroviral genes, and (ii) total repression and extinction during early stages of differentiation, apparently independent of changes in methylation. These results indicate a time window early during the transition from an undifferentiated to a differentiated stage in which provirus expression is silenced. The mechanisms are presently unknown, but elucidation of these events will have an important impact on vector development for targeting stem cells and for gene therapy.

Craniofacial abnormalities induced by the ectopic expression of homeobox genes

In this paper I have tried to bring together work that highlights the role of homeobox genes in generating craniofacial form. I review both normal and disrupted embryogenesis and ask whether mis-expression of the homeobox genes outside their normal domains could be contributing to congenital facial abnormalities arising from either genetic or teratogenic actions. Experimentally generated transgenic mice carrying loss- or gain-of-function mutations in homeobox genes, in combination with their normal expression patterns, have allowed us to compile and test models of embryonic specification based around a Hox/homeobox code. These models form the basis on which the functional questions are considered. There are four major sections covering different experimental approaches designed to ectopically induce homeobox genes in the head. Transgenic mice, where heterologous promoters drive a given Hox gene in the head, have shown that the more posteriorly expressed Hox genes tend to have a significant effect only on the skull bones of mesodermal origin whereas those normally expressed more anteriorly, in the hindbrain and branchial arches, can affect more anterior branchial arch and neural crest-derived structures. Manipulation experiments which can induce homeobox genes in small, localised regions of the facial precursors show clear and dramatic effects of this expression on facial development. Null mutations in predicted repressors of Hox gene expression, however, do not appear to give rise to substantial craniofacial abnormalities. Retinoic acid, on the other hand, is well known for its teratogenic actions and its ability to induce Hox gene expression. Evidence is now accumulating that at least some of its teratogenic actions may be mediated by its regulation of the Hox and other homeobox genes in the head.

Hox homeobox genes as regulators of normal and leukemic hematopoiesis

Hox genes, first recognized for their role in embryonic development, may also play lineage-specific functions in a variety of somatic tissues including the hematopoietic system. Expression of these transcription factors has been demonstrated both in normal and leukemic human and hematopoietic cells, suggesting functional roles in hematopoietic cell growth and differentiation. Several recent studies have shown that Hox proteins are involved in controlling proliferation of primitive bone marrow cells and also in altering differentiation of myeloid as well as lymphoid progenitors, alterations that also can contribute to leukemic transformation. Hox genes, together with their upstream regulators and downstream target genes, may play key roles in fundamental processes controlling hematopoietic stem cell properties.

A novel RING finger protein, BFP, predominantly expressed in the brain

RING finger is a variant zinc finger motif present in a new family of proteins including transcription regulators. A genomic DNA fragment containing RING finger motifs was identified by the polymerase chain reaction using degenerate primers. Using this fragment as a probe, we have isolated a novel cDNA from rat brain library. The predicted open reading frame contains a RING finger domain at its N-terminal portion. The corresponding transcript was detected predominantly in the brain and therefore was designated brain finger protein (bfp). An antibody raised against a recombinant bfp reveals the presence of the bfp in the brain. Interestingly, the bfp is induced during retinoic acid-mediated differentiation of P19 embryonal carcinoma cells into neural cells. These findings suggest the possible involvement of bfp in some aspects of neural cell regulation.

Interference with the expression of a novel human polycomb protein, hPc2, results in cellular transformation and apoptosis

Polycomb (Pc) is involved in the stable and heritable repression of homeotic gene activity during Drosophila development. Here, we report the identification of a novel human Pc homolog, hPc2. This gene is more closely related to a Xenopus Pc homolog, XPc, than to a previously described human Pc homolog, CBX2 (hPc1). However, the hPc2 and CBX2/hPc1 proteins colocalize in interphase nuclei of human U-2 OS osteosarcoma cells, suggesting that the proteins are part of a common protein complex. To study the functions of the novel human Pc homolog, we generated a mutant protein, delta hPc2, which lacks an evolutionarily conserved C-terminal domain. This C-terminal domain is important for hPc2 function, since the delta hPc2 mutant protein which lacks the C-terminal domain is unable to repress gene activity. Expression of the delta hPc2 protein, but not of the wild-type hPc2 protein, results in cellular transformation of mammalian cell lines as judged by phenotypic changes, altered marker gene expression, and anchorage-independent growth. Specifically in delta hPc2-transformed cells, the expression of the c-myc proto-oncogene is strongly enhanced and serum deprivation results in apoptosis. In contrast, overexpression of the wild-type hPc2 protein results in decreased c-myc expression. Our data suggest that hPc2 is a repressor of proto-oncogene activity and that interference with hPc2 function can lead to derepression of proto-oncogene transcription and subsequently to cellular transformation.

Ring1A is a transcriptional repressor that interacts with the Polycomb-M33 protein and is expressed at rhombomere boundaries in the mouse hindbrain

In Drosophila, the products of the Polycomb group (Pc-G) of genes act as chromatin-associated multimeric protein complexes that repress expression of homeotic genes. Vertebrate Pc-G homologues have been identified, but the nature of the complexes they form and the mechanisms of their action are largely unknown. The Polycomb homologue M33 is implicated in mesoderm patterning in the mouse and here we show that it acts as a transcriptional repressor in transiently transfected cells. Furthermore, we have identified two murine proteins, Ring1A and Ring1B, that interact directly with the repressor domain of M33. Ring1A and Ring1B display blocks of similarity throughout their sequences, including an N-terminal RING finger domain. However, the interaction with M33 occurs through a region at the C-terminus. Ring1A represses transcription through sequences not involved in M33 binding. Ring1A protein co-localizes in nuclear domains with M33 and other Pc-G homologues, such as Bmi1. The expression of Ring1A at early stages of development is restricted to the neural tube, whereas M33 is expressed ubiquitously. Within the neural tube, Ring1A RNA is located at the rhombomere boundaries of the hindbrain. Taken together, these data suggest that Ring1A may contribute to a tissue-specific function of Pc-G-protein complexes during mammalian development.

Targeted disruption of the mouse homologue of the Drosophila polyhomeotic gene leads to altered anteroposterior patterning and neural crest defects

The rae28 gene is a mouse homologue of the Drosophila polyhomeotic gene (Nomura, M., Takihara, Y. and Shimada, K. (1994) Differentiation 57, 39–50), which is a member of the Polycomb group (Pc-G) of genes (DeCamillis, M., Cheng, N., Pierre, D. and Brock, H.W. (1992) Genes Dev. 6, 223–232). The Pc-G genes are required for the correct expression of the Homeotic complex genes and segment specification during Drosophila embryogenesis and larval development. To study the role of the rae28 gene in mouse development, we generated rae28-deficient mice by gene targeting in embryonic stem cells. The rae28−/− homozygous mice exhibited perinatal lethality, posterior skeletal transformations and defects in neural crest-related tissues, including ocular abnormalities, cleft palate, parathyroid and thymic hypoplasia and cardiac anomalies. The anterior boundaries of Hoxa-3, a-4, a-5, b-3, b-4 and d-4 expression were shifted rostrally in the paraxial mesoderm of the rae28−/− homozygous embryos, and those of Hoxb-3 and b-4 expression were also similarly altered in the rhombomeres and/or pharyngeal arches. These altered Hox codes were presumed to be correlated with the posterior skeletal transformations and neural crest defects observed in the rae28−/− homozygous mice. These results indicate that the rae28 gene is involved in the regulation of Hox gene expression and segment specification during paraxial mesoderm and neural crest development.

Proviral insertions induce the expression of bone-specific isoforms of PEBP2alphaA (CBFA1): evidence for a new myc collaborating oncogene

The til-1 locus was identified as a common retroviral integration site in virus-accelerated lymphomas of CD2-myc transgenic mice. We now show that viral insertions at til-1 lead to transcriptional activation of PEBP2alphaA (CBFA1), a transcription factor related to the Drosophila segmentation gene product, Runt. Insertions are upstream and in the opposite orientation to the gene and appear to activate a variant promoter that is normally silent in T cells. Activity of this promoter was detected in rodent osteogenic sarcoma cells and primary osteoblasts, implicating bone as the normal site of promoter activity. The isoforms encoded by the activated gene all encompass the conserved runt DNA-binding domain and share a novel N terminus different from the previously reported PEBP2alphaA products. Minor products include isoforms with internal deletions due to exon skipping and a novel C-terminal domain unrelated to known runt domain factors. The major isoform expressed from the activated til-1 locus (G1) was found to account for virtually all of the core binding factor activity in nuclear extracts from its corresponding lymphoma cell line. Another member of this gene family, AML1(CBFA2), is well known for its involvement in human hemopoietic tumors. These results provide evidence of a direct oncogenic role for PEBP2alphaA and indicate that the Myc and Runt family genes can cooperate in oncogenesis.

Functions of mammalian Polycomb group and trithorax group related genes

Genes of the Polycomb and trithorax groups (PcG and trxG) are part of a cellular memory system that maintains inactive and active states of homeotic gene expression in Drosophila. Recent genetic evidence indicates that several related loci in mammals are also involved in the regulation of Hox genes. Like their Drosophila counterparts, the vertebrate gene products are components of multiprotein complexes that regulate transcriptional activation, repression and aspects of chromatin structure. Initial indications suggest the existence of a large mammalian PcG and trxG family, with a potential to encode multiple specialised functions in cell fate and cell-cycle control.

RING1 is associated with the polycomb group protein complex and acts as a transcriptional repressor

The Polycomb (Pc) protein is a component of a multimeric, chromatin-associated Polycomb group (PcG) protein complex, which is involved in stable repression of gene activity. The identities of components of the PcG protein complex are largely unknown. In a two-hybrid screen with a vertebrate Pc homolog as a target, we identify the human RING1 protein as interacting with Pc. RING1 is a protein that contains the RING finger motif, a specific zinc-binding domain, which is found in many regulatory proteins. So far, the function of the RING1 protein has remained enigmatic. Here, we show that RING1 coimmunoprecipitates with a human Pc homolog, the vertebrate PcG protein BMI1, and HPH1, a human homolog of the PcG protein Polyhomeotic (Ph). Also, RING1 colocalizes with these vertebrate PcG proteins in nuclear domains of SW480 human colorectal adenocarcinoma and Saos-2 human osteosarcoma cells. Finally, we show that RING1, like Pc, is able to repress gene activity when targeted to a reporter gene. Our findings indicate that RING1 is associated with the human PcG protein complex and that RING1, like PcG proteins, can act as a transcriptional repressor.

The role of mel-18, a mammalian Polycomb group gene, during IL-7-dependent proliferation of lymphocyte precursors

mel-18 is a mammalian homolog of Drosophila melanogaster Polycomb group genes. Mice lacking the mel-18 gene show a posterior transformation of the axial skeleton, severe combined immunodeficiency, and a food-passing disturbance in the lower intestine due to hypertrophy of the smooth muscle layer. In this study, the severe combined immunodeficiency observed in mel-18 mutant mice is correlated with the impaired mitotic response of lymphocyte precursors upon interleukin-7 stimulation. Strikingly, the axial skeleton and lymphoid phenotypes are identical in both mel-18 and bmi-1 mutants, indicating that the Mel-18 and Bmi-1 gene products might act in the same genetic cascade. These results suggest that mammalian Polycomb group gene products are involved in cell cycle progression in the immune system.

P element insertion-dependent gene activation in the Drosophila eye

Insights into the function of a gene can be gained in multiple ways, including loss-of-function phenotype, sequence similarity, expression pattern, and by the consequences of its misexpression. Analysis of the phenotypes produced by expression of a gene at an abnormal time, place, or level may provide clues to a gene's function when other approaches are not illuminating. Here we report that an eye-specific, enhancer-promoter present in the P element expression vector pGMR is able to drive high level expression in the eye of genes near the site of P element insertion. Cell fate determination, differentiation, proliferation, and death are essential for normal eye development. Thus the ability to carry out eye-specific misexpression of a significant fraction of genes in the genome, given the dispensability of the eye for viability and fertility of the adult, should provide a powerful approach for identifying regulators of these processes. To test this idea we carried out two overexpression screens for genes that function to regulate cell death. We screened for insertion-dependent dominant phenotypes in a wild-type background, and for dominant modifiers of a reaper overexpression-induced small eye phenotype. Multiple chromosomal loci were identified, including an insertion 5' to hid, a potent inducer of apoptosis, and insertions 5' to DIAP1, a cell death suppressor. To facilitate the cloning of genes near the P element insertion new misexpression vectors were created. A screen with one of these vectors identified eagle as a suppressor of a rough eye phenotype associated with overexpression of an activated Ras1 gene.

Role for N-CoR and histone deacetylase in Sin3-mediated transcriptional repression

Normal mammalian growth and development are highly dependent on the regulation of the expression and activity of the Myc family of transcription factors. Mxi1-mediated inhibition of Myc activities requires interaction with mammalian Sin3A or Sin3B proteins, which have been purported to act as scaffolds for additional co-repressor factors. The identification of two such Sin3-associated factors, the nuclear receptor co-repressor (N-CoR) and histone deacetylase (HD1), provides a basis for Mxi1/Sin3-induced transcriptional repression and tumour suppression.

Expression of the vav oncogene in somatic cell hybrids

The vav oncogene is expressed primarily in tissues of hematopoietic origin. While much effort has been focused on determining the role of vav in various signal transduction pathways, little is known about the mechanism by which vav is regulated in a tissue-selective manner. This issue was examined by developing somatic cell hybrids between human U937 cells, which express vav, and mouse Balb/c 3T3 cells, which do not. If vav is primarily regulated by the presence of positive acting transcription factors, then vav expression should be maintained in hybrid cells. In contrast, if the regulation of vav is primarily negative in nature, then vav expression should be extinguished in most of the somatic cell hybrids. Of the hybrid cells that were obtained, 64% were positive by reverse transcriptase-polymerase chain reaction for the expression of the vav oncogene. Differences in the pattern of restriction enzyme cleavage sites between the mouse and human PCR products were used to determine that 6 of 11 of the positive clones expressed the normally dormant mouse gene. The other positive clones were found to express the human vav gene. In all cases, the hybrid cells preferentially retained the chromosomes and the cellular morphological appearance of the mouse Balb/c 3T3 fusion partner, which does not express the vav oncogene. Since vav is able to be transiently expressed by hybrid cells with a predominately mouse phenotype, these results support the hypothesis that vav is regulated primarily by the presence of transactivating factors which stimulate transcription, rather than by a gene silencing mechanism.

Identification and characterization of interactions between the vertebrate polycomb-group protein BMI1 and human homologs of polyhomeotic

In Drosophila melanogaster, the Polycomb-group (PcG) genes have been identified as repressors of gene expression. They are part of a cellular memory system that is responsible for the stable transmission of gene activity to progeny cells. PcG proteins form a large multimeric, chromatin-associated protein complex, but the identity of its components is largely unknown. Here, we identify two human proteins, HPH1 and HPH2, that are associated with the vertebrate PcG protein BMI1. HPH1 and HPH2 coimmunoprecipitate and cofractionate with each other and with BMI1. They also colocalize with BMI1 in interphase nuclei of U-2 OS human osteosarcoma and SW480 human colorectal adenocarcinoma cells. HPH1 and HPH2 have little sequence homology with each other, except in two highly conserved domains, designated homology domains I and II. They share these homology domains I and II with the Drosophila PcG protein Polyhomeotic (Ph), and we, therefore, have named the novel proteins HPH1 and HPH2. HPH1, HPH2, and BMI1 show distinct, although overlapping expression patterns in different tissues and cell lines. Two-hybrid analysis shows that homology domain II of HPH1 interacts with both homology domains I and II of HPH2. In contrast, homology domain I of HPH1 interacts only with homology domain II of HPH2, but not with homology domain I of HPH2. Furthermore, BMI1 does not interact with the individual homology domains. Instead, both intact homology domains I and II need to be present for interactions with BMI1. These data demonstrate the involvement of homology domains I and II in protein-protein interactions and indicate that HPH1 and HPH2 are able to heterodimerize.

bic, a novel gene activated by proviral insertions in avian leukosis virus-induced lymphomas, is likely to function through its noncoding RNA

The bic locus is a common retroviral integration site in avian leukosis virus (ALV)-induced B-cell lymphomas originally identified by infection of chickens with ALVs of two different subgroups (Clurman and Hayward, Mol. Cell. Biol. 9:2657-2664, 1989). Based on its frequent association with c-myc activation and its preferential activation in metastatic tumors, the bic locus is thought to harbor a gene that can collaborate with c-myc in lymphomagenesis and presumably plays a role in late stages of tumor progression. In the present study, we have cloned and characterized two novel genes, bdw and bic, at the bic locus. bdw encoded a putative novel protein of 345 amino acids. However, its expression did not appear to be altered in tumor tissues, suggesting that it is not involved in oncogenesis. The bic gene consisted of two exons and was expressed as two spliced and alternatively polyadenylated transcripts at low levels in lymphoid/hematopoietic tissues. In tumors harboring bic integrations, proviruses drove bic gene expression by promoter insertion, resulting in high levels of expression of a chimeric RNA containing bic exon 2. Interestingly, bic lacked an extensive open reading frame, implying that it may function through its RNA. Computer analysis of RNA from small exon 2 of bic predicted extensive double-stranded structures, including a highly ordered RNA duplex between nucleotides 316 and 461. The possible role of bic in cell growth and differentiation is discussed in view of the emerging evidence that untranslated RNAs play a role in growth control.

Activation of a novel proto-oncogene, Frat1, contributes to progression of mouse T-cell lymphomas

Acceleration of lymphomagenesis in oncogene-bearing transgenic mice by slow-transforming retroviruses has proven a valuable tool in identifying cooperating oncogenes. We have modified this protocol to search for genes that can collaborate effectively with the transgene in later stages of tumor development. Propagation of tumors induced by Moloney murine leukemia virus (M-MuLV) in E mu-Pim1 or H2-K-myc transgenic mice by transplantation to syngeneic hosts permitted proviral tagging of 'progression' genes. Molecular cloning of common proviral insertion sites that were detected preferentially in transplanted tumors led to the identification of a novel gene, designated Frat1. The initial selection for integrations near Frat1 occurs in primary tumor cells that have already acquired proviruses in other common insertion sites, yielding primary lymphomas that contain only a minor fraction of tumor cells with an activated Frat1 allele. Transplantation of such primary lymphomas allows for a further expansion of tumor cell clones carrying a proviral insertion near Frat1, resulting in detectable Frat1 rearrangements in 17% of the transplanted E mu-Pim1 tumors and 30% of the transplanted H2-K-myc tumors, respectively. We have cloned and sequenced both the mouse Frat1 gene and its human counterpart. The proteins encoded by Frat1 and FRAT1 are highly homologous and their functions are thus far unknown. Tumor cell lines with high expression of Myc and Pim1 acquired an additional selective advantage in vivo upon infection with a Frat1-IRES-lacZ retrovirus, thus underscoring the role of Frat1 in tumor progression, and the ability of Frat1 to collaborate with Pim1 and Myc in lymphomagenesis.

Upregulation of endogenous p53 and induction of in vivo apoptosis in B-lineage lymphomas of E(mu)-myc transgenic mice by deregulated c-myc transgene

E(mu)-myc transgenic mice carry a constitutively overexpressed c-myc oncogene and develop B-lineage lymphomas. Previous studies have shown that c-myc overexpression can lead to in vitro apoptosis. Here, we investigated the in vivo effects of altered c-myc expression on cell proliferation versus death in spontaneously arising E(mu)-myc tumors. E(mu)-myc tumors display extensive in vivo apoptosis confined to small clusters of cells with greatly increased expression of both the c-myc transgene and the endogenous p53 gene as compared with that in normal, pretumor, or surrounding tumor tissue. This restricted overexpression of both the c-myc transgene and the endogenous p53 gene in small clusters of apoptotic tumor cells indicates that overexpression of these genes and apoptosis are not obligatory or uniform during tumor development and suggests that further somatic mutations or microenvironmental influences may be responsible for these properties. Nevertheless, the clear ability of tumor cells to undergo apoptosis in vivo may be exploitable for therapeutic purposes.

Leukemogenesis by Moloney murine leukemia virus: a multistep process

Moloney murine leukemia virus is a prototypical simple retrovirus that has been an extremely useful model for leukemogenesis. Important steps in leukemogenesis include proviral activation of cellular proto-oncogenes, generation of mink cell focus-inducing recombinants, and early (preleukemic) virus-induced changes in hematopoiesis.

Identification of Bmi1-interacting proteins as constituents of a multimeric mammalian polycomb complex

The Bmi1 gene has been identified as a mouse Polycomb group (Pc-G) gene implicated in the regulation of Hox gene expression. Here we describe the characterization of a Bmi binding protein Mph1, which shares similarity to Drosophila polyhomeotic. Coimmunoprecipitation experiments indicate that Bmi1 and Mph1, as well as the Mel18 and M33 proteins described previously, are constituents of a multimeric protein complex in mouse embryos and human cells. A central domain of Bmi1 interacts with the carboxyl terminus of Mph1, whereas a conserved alpha-helical domain in the Mph1 protein is required for its homodimerization. Transgenic mice overexpressing various mutant Bmi1 proteins demonstrate that the central domain of Bmil is required for the induction of anterior transformations of the axial skeleton. Bmi1, M33, and Mph1 show an overlapping speckled distribution in interphase nuclei. These data provide molecular evidence for the existence of a mammalian Polycomb complex.

Characterization of pal-1, a common proviral insertion site in murine leukemia virus-induced lymphomas of c-myc and Pim-1 transgenic mice

Insertional mutagenesis with Moloney murine leukemia virus (MoMLV) in c-myc and Pim-1 transgenic mice permits the identification of oncogenes that collaborate with the transgenes in lymphomagenesis. The recently identified common insertion site pal-1, in MoMLV-induced lymphomas, is located in a region in which several independent integration clusters are found: eis-1, gfi-1, and evi-5. Proviral insertions of MoMLV in the different integration clusters upregulate the transcriptional activity of the Gfi-1 gene, which is located within the pal-1 locus. The eis-1/pal-1/gfi-1/evi-5 locus serves as a target for MoMLV proviral insertions in pre-B-cell lymphomas of Emu-myc transgenic mice (20%) and in T-cell lymphomas of H-2K-myc (75%) and Emu-pim-1 (93%) transgenic mice. Many tumors overexpress both Gfi-1 as well as Myc and Pim gene family members, indicating that Gfi-1 collaborates with Myc and Pim in lymphomagenesis. Proviral integrations in the previously identified insertion site bmi-1 are, however, mutually exclusive with integrations in the eis-1/pal-1/gfi-1/evi-5 locus. This finding suggests that Bmi-1 and Gfi-1 belong to the same complementation group in lymphoid transformation.

Myc: a single gene controls both proliferation and apoptosis in mammalian cells

c-myc was discovered as the cellular homologue of the transduced oncogene of several avian retroviruses. The gene encodes a transcription factor, which forms a heteromeric protein complex with a partner protein termed Max. In mammalian cells, Myc is a central regulator of cell proliferation and links external signals to the cell cycle machinery. Myc also induces cells to undergo apoptosis, unless specific signals provided either by cytokines or by oncogenes block the apoptotic pathway. Recent progress sheds light both on the factors regulating the function and expression of Myc and on the downstream targets in the cell cycle. Together, these findings suggest the existence of a novel signal transduction pathway regulating both apoptosis and proliferation.

Tumor suppressor gene mutations in mice

Over the past several years, a number of human tumor suppressor genes have been cloned and characterized. Germline mutations in tumor suppressor genes strongly predispose to cancer, and they are also mutated somatically in sporadic forms of the disease. In order to create animal models for the familial cancer syndromes caused by inherited mutations in these genes as well as to determine their role in embryogenesis, the homologues of several members of this class have been mutated in the mouse. The initial characterization of the heterozygous and homozygous phenotypes caused by these mutations has led to important insights into the mechanisms by which tumor suppressor genes participate in normal development and how their loss contributes to tumorigenesis.