Tumor progression in murine leukemia virus-induced T-cell lymphomas: monitoring clonal selections with viral and cellular probes

Genetic Modeling of PIM Proteins in Cancer: Proviral Tagging and Cooperation with Oncogenes, Tumor Suppressor Genes, and Carcinogens

The PIM proteins, which were initially discovered as proviral insertion sites in Moloney-murine leukemia virus infection, are a family of highly homologous serine/threonine kinases that have been reported to be overexpressed in hematological malignancies and solid tumors. The PIM proteins have also been associated with metastasis and overall treatment responses and implicated in the regulation of apoptosis, metabolism, the cell cycle, and homing and migration, which makes these proteins interesting targets for anti-cancer drug discovery. The use of retroviral insertional mutagenesis and refined approaches such as complementation tagging has allowed the identification of myc, pim, and a third group of genes (including bmi1 and gfi1) as complementing genes in lymphomagenesis. Moreover, mouse modeling of human cancer has provided an understanding of the molecular pathways that are involved in tumor initiation and progression at the physiological level. In particular, genetically modified mice have allowed researchers to further elucidate the role of each of the Pim isoforms in various tumor types. PIM kinases have been identified as weak oncogenes because experimental overexpression in lymphoid tissue, prostate, and liver induces tumors at a relatively low incidence and with a long latency. However, very strong synergistic tumorigenicity between Pim1/2 and c-Myc and other oncogenes has been observed in lymphoid tissues. Mouse models have also been used to study whether the inhibition of specific PIM isoforms is required to prevent carcinogen-induced sarcomas, indicating that the absence of Pim2 and Pim3 greatly reduces sarcoma growth and bone invasion; the extent of this effect is similar to that observed in the absence of all three isoforms. This review will summarize some of the animal models that have been used to understand the isoform-specific contribution of PIM kinases to tumorigenesis.

Insertional oncogenesis by non-acute retroviruses: implications for gene therapy

Retroviruses cause cancers in a variety of animals and humans. Research on retroviruses has provided important insights into mechanisms of oncogenesis in humans, including the discovery of viral oncogenes and cellular proto-oncogenes. The subject of this review is the mechanisms by which retroviruses that do not carry oncogenes (non-acute retroviruses) cause cancers. The common theme is that these tumors result from insertional activation of cellular proto-oncogenes by integration of viral DNA. Early research on insertional activation of proto-oncogenes in virus-induced tumors is reviewed. Research on non-acute retroviruses has led to the discovery of new proto-oncogenes through searches for common insertion sites (CISs) in virus-induced tumors. Cooperation between different proto-oncogenes in development of tumors has been elucidated through the study of retrovirus-induced tumors, and retroviral infection of genetically susceptible mice (retroviral tagging) has been used to identify cellular proto-oncogenes active in specific oncogenic pathways. The pace of proto-oncogene discovery has been accelerated by technical advances including PCR cloning of viral integration sites, the availability of the mouse genome sequence, and high throughput DNA sequencing. Insertional activation has proven to be a significant risk in gene therapy trials to correct genetic defects with retroviral vectors. Studies on non-acute retroviral oncogenesis provide insight into the potential risks, and the mechanisms of oncogenesis.

Insertional mutagenesis reveals progression genes and checkpoints in MYC/Runx2 lymphomas

In this study, we have exploited the power of insertional mutagenesis to elucidate tumor progression pathways in mice carrying two oncogenes (MYC/Runx2) that collaborate to drive early lymphoma development. Neonatal infection of these mice with Moloney murine leukemia virus resulted in accelerated tumor onset with associated increases in clonal complexity and lymphoid dissemination. Large-scale analysis of retroviral integration sites in these tumors revealed a profound bias towards a narrow range of target genes, including Jdp2 (Jundm2), D cyclin, and Pim family genes. Remarkably, direct PCR analysis of integration hotspots revealed that every progressing tumor consisted of multiple clones harboring hits at these loci, giving access to large numbers of independent insertion events and uncovering the contrasting mutagenic mechanisms operating at each target gene. Direct PCR analysis showed that high-frequency targeting occurs only in the tumor environment in vivo and is specific for the progression gene set. These results indicate that early lymphomas in MYC/Runx2 mice remain dependent on exogenous growth signals, and that progression can be achieved by constitutive activation of pathways converging on a cell cycle checkpoint that acts as the major rate-limiting step for lymphoma outgrowth.

Epstein-Barr virus nuclear antigen-1 and Myc cooperate in lymphomagenesis

The lymphomagenic action of myc genes in conjunction with Epstein-Barr virus nuclear antigen-1 (EBNA-1) have been examined using transgenic mice in several separate tests. Synergy between Myc and EBNA-1 in lymphomagenesis was revealed in a cross breed study where co-expression of transgenic myc and EBNA-1 led to a tumor latency period reduced significantly in some crosses. In the resulting bitransgenic tumors, expression of the Emu-myc genes was not affected by EBNA-1 expression. MoMLV was utilized as a transposon tag to activate cellular oncogenes by infection of EmuEBNA-1 mice. Rearrangement at the c-myc locus in B cell tumors from these mice again suggests a cooperative action between myc and EBNA-1. Tumors arising in EmuEBNA-1 mice typically showed a trisomy of chromosome 15, upon which the c-myc locus resides. Bitransgenic tumors (EBNA-1/c-myc) did not show trisomy 15. This raises the possibility that amplification of c-myc is factorial in the selection of trisomy 15 in these tumors. These data indicate that myc and EBNA-1 act cooperatively and are not redundant in lymphomagenesis. Expression of EBNA-1 by EBV may provide a selection pressure in addition to translocation of the c-myc locus in the genesis of endemic Burkitt's lymphoma (BL).

Introduction of a cis-acting mutation in the capsid-coding gene of moloney murine leukemia virus extends its leukemogenic properties

Inoculation of newborn mice with the retrovirus Moloney murine leukemia virus (MuLV) results in the exclusive development of T lymphomas with gross thymic enlargement. The T-cell leukemogenic property of Moloney MuLV has been mapped to the U3 enhancer region of the viral promoter. However, we now describe a mutant Moloney MuLV which can induce the rapid development of a uniquely broad panel of leukemic cell types. This mutant Moloney MuLV with synonymous differences (MSD1) was obtained by introduction of nucleotide substitutions at positions 1598, 1599, and 1601 in the capsid gene which maintained the wild-type (WT) coding potential. Leukemias were observed in all MSD1-inoculated animals after a latency period that was shorter than or similar to that of WT Moloney MuLV. Importantly, though, only 56% of MSD1-induced leukemias demonstrated the characteristic thymoma phenotype observed in all WT Moloney MuLV leukemias. The remainder of MSD1-inoculated animals presented either with bona fide clonal erythroid or myelomonocytic leukemias or, alternatively, with other severe erythroid and unidentified disorders. Amplification and sequencing of U3 and capsid-coding regions showed that the inoculated parental MSD1 sequences were conserved in the leukemic spleens. This is the first report of a replication-competent MuLV lacking oncogenes which can rapidly lead to the development of such a broad range of leukemic cell types. Moreover, the ability of MSD1 to transform erythroid and myelomonocytic lineages is not due to changes in the U3 viral enhancer region but rather is the result of a cis-acting effect of the capsid-coding gag sequence.

Oncogene activation in myeloid leukemias by Graffi murine leukemia virus proviral integration

The Graffi murine leukemia virus (MuLV) is a nondefective retrovirus that induces granulocytic leukemia in BALB/c and NFS mice. To identify genes involved in Graffi MuLV-induced granulocytic leukemia, tumor cell DNAs were examined for genetic alterations at loci described as common proviral integration sites in MuLV-induced myeloid, lymphoid, and erythroid leukemias. Southern blot analysis revealed rearrangements in c-myc, Fli-1, Pim-1, and Spi-1/PU.1 genes in 20, 10, 3.3, and 3.3% of the tumors tested, respectively. These results demonstrate for the first time the involvement of those genes in granulocytic leukemia.

Retroviral insertions in Evi12, a novel common virus integration site upstream of Tra1/Grp94, frequently coincide with insertions in the gene encoding the peripheral cannabinoid receptor Cnr2

The common virus integration site (VIS) Evi11 was recently identified within the gene encoding the hematopoietic G-protein-coupled peripheral cannabinoid receptor Cnr2 (also referred to as Cb2). Here we show that Cnr2 is a frequent target (12%) for insertion of Cas-Br-M murine leukemia virus (MuLV) in primary tumors in NIH/Swiss mice. Multiple provirus insertions in Evi11 were cloned and shown to be located within the 3' untranslated region of the candidate proto-oncogene Cnr2. These results suggest that proviral insertion in the Cnr2 gene is an important step in Cas-Br-M MuLV-induced leukemogenesis in NIH/Swiss mice. To isolate Evi11/Cnr2 collaborating proto-oncogenes, we searched for novel common VISs in the Cas-Br-M MuLV-induced primary tumors and identified a novel frequent common VIS, Evi12 (14%). Interestingly, 54% of the Evi11/Cnr2-rearranged primary tumors contained insertions in Evi12 as well, which suggests cooperative action of the target genes in these two common VISs in leukemogenesis. By interspecific backcross analysis it was shown that Evi12 resides on mouse chromosome 10 in a region that shares homology with human chromosomes 12q and 19p. Sequence analysis demonstrated that Evi12 is located upstream of the gene encoding the molecular chaperone Tra1/Grp94, which was previously mapped to mouse chromosome 10 and human chromosome 12q22-24. Thus, Tra1/Grp94 is a candidate target gene for retroviral activation or inactivation in Evi12. However, Northern and Western blot analyses did not provide evidence that proviral insertion had altered the expression of Tra1/Grp94. Additional studies are required to determine whether Tra1/Grp94 or another candidate proto-oncogene in Evi12 is involved in leukemogenesis.

Low-frequency loss of heterozygosity in Moloney murine leukemia virus-induced tumors in BRAKF1/J mice

To identify potential involvement of tumor suppressor gene inactivation during leukemogenesis by Moloney murine leukemia virus (M-MuLV), a genome-wide scan for loss of heterozygosity (LOH) in tumor DNAs was made. To assess LOH, it is best to study mice that are heterozygous at many loci across the genome. Accordingly, we generated a collection of 52 M-MULV-induced tumor DNAs from C57BR/cdJ x AKR/J F1 (BRAKF1) hybrid mice. By using direct hybridization with oligonucleotides specific for three different classes of endogenous MuLV-related proviruses, 48 markers on 16 of 19 autosomes were simultaneously examined for allelic loss. No allelic losses were detected, with the exception of a common loss of markers on chromosome 4 in two tumors. The three autosomes that lacked informative endogenous proviral markers were also analyzed for LOH by PCR with simple-sequence length polymorphisms (SSLPs); one additional tumor showed LOH on chromosome 15. Further screening with chromosome 4 SSLPs identified one additional tumor with LOH on chromosome 4. Therefore, in total, the average fractional allelic loss was quite low (0.002), but the LOH frequency of 6% on chromosome 4 was highly statistically significant (P < 0.0005). Detailed SSLP mapping of the three tumors with LOH on chromosome 4 localized the region of common LOH to the distal 45 centimorgans, a region syntenic with human chromosomes 1 and 9. Candidate tumor suppressor genes, Mts1 (p16INK4a) and Mts2 (p15INK4b), have been mapped to this region, but by Southern blot analysis, no homozygous deletions were detected in either gene. One of three tumors with LOH on chromosome 4 also showed a proviral insertion near the c-myc proto-oncogene. These results suggested that tumor suppressor inactivation is generally infrequent in M-MuLV-induced tumors but that a subset of these tumors may have lost a tumor suppressor gene on chromosome 4.

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.

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.

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.

Spontaneous Mutation of Cell Oncogenes Plays a Minor Role in Neoplastic Transformation of Virus-Induced Murine T-Cell Lymphomas

Mink cell focus-forming viruses (MCF) are slow-transforming retroviruses that are able to accelerate the appearance of T-cell lymphomas when injected in newborn AKR mice. Activation of proto-oncogenes by proviral insertion is thought to be the major mechanism by which these viruses exert their oncogenic potential. However, molecular phenomena not strictly virus-determined, such as mutations in cellular oncogenes/tumor suppressor genes or chromosome aberrations, have been hypothesized to contribute to the achievement of the fully neoplastic phenotype in MCF-infected mice. To evaluate the role of spontaneous mutagenesis phenomena in murine virus-induced lymphomagenesis, we analyzed a series of 18 MCF247-induced thymic lymphomas and derived cell lines for the presence of p53 and c-ras gene mutations. Only 1 mutation at the p53 gene and 1 mutation at the ki-ras gene were detected in our study. Our results suggest that spontaneous mutagenesis plays a minor role in virus-induced lymphomagenesis and support the notion that multiple proviral insertions could be the prevalent mechanism of transformation in this experimental system.

Induction of leukemia in mice using a radiation leukemia virus-induced cell line: a model system for studying oncogenic progression

Leukemogenesis induced by slowly transforming retroviruses is a multistep process which is difficult to dissect because of its long latency and the problem of distinguishing oncogenic from differentiative events. A method for leukemia induction in mice has been developed using a cell line isolated following in vitro infection with the slowly transforming murine radiation leukemia virus (RadLV). The CI-V13D cell line represents a lymphoid precursor cell type at an early stage in cell transformation and can develop subcutaneous tumors in irradiated syngeneic hosts but not in allogeneic mice even after sublethal irradiation. Selective growth in allogeneic (CBA/H) mouse thymus has been demonstrated, but this requires preirradiation of the recipient. Upon reisolation from CBA/H thymus, C1-V13D progeny clones displayed increased tumorigenic potential in comparison to the 'parental' CI-V13D cell line. Tumorigenicity was shown to increase with serial passage through thymus and electron micrographs of clones also revealed increased production of C-type retroviruses. This new model for oncogenic progression should be more amenable to analysis of early genetic changes occurring during replication of leukemia in the thymus.

Escape from in vivo restriction of Moloney mink cell focus-inducing viruses driven by the Mo+PyF101 long terminal repeat (LTR) by LTR alterations

Mo+PyF101 M-MuLV is a variant Moloney murine leukemia virus containing polyomavirus F101 enhancers inserted just downstream from the M-MuLV enhancers in the long terminal repeat (LTR). The protein coding sequences for this virus are identical to those of M-MuLV. Mo+PyF101 M-MuLV induces T-cell disease with a much lower incidence and longer latency than wild-type M-MuLV. We have previously shown that Mo+PyF101 M-MuLV is defective in preleukemic events induced by wild-type M-MuLV, including splenic hematopoietic hyperplasia, bone marrow depletion, and generation of recombinant mink cell focus-inducing viruses (MCFs). We also showed that an M-MCF virus driven by the Mo+PyF101 LTR is infectious in vitro but does not propagate in mice. However, in these experiments, when a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF was inoculated into newborn NIH Swiss mice, they died of T-cell leukemia at times almost equivalent to those induced by wild-type M-MuLV. Tumor DNAs from Mo+PyF101 M-MuLV-Mo+PyF101 MCF-inoculated mice were examined by Southern blot analysis. The predominant forms of Mo+PyF101 MCF proviruses in these tumors contained added sequences in the U3 region of the LTR. The U3 regions of representative tumor-derived variant Mo+PyF101 MCFs were cloned by polymerase chain reaction amplification, and sequencing indicated that they had acquired an additional copy of the M-MuLV 75-bp tandem repeat in the enhancer region. NIH 3T3 cell lines infected with altered viruses were obtained from representative Mo+PyF101 M-MuLV-Mo+PyF101 MCF-induced tumors, and mice were inoculated with the recovered viruses. Leukemogenicity was approximately equivalent to that in the original Mo+PyF101 M-MuLV-Mo+PyF101 MCF viral stock. Southern blot analysis on the resulting tumors now predominantly revealed loss of the polyomavirus sequences. These results suggest that the suppressive effects of the PyF101 sequences on M-MuLV-induced disease and potentially on MCF propagation were overcome in two ways: by triplication of the M-MuLV direct repeats and by loss of the polyomavirus sequences.

Conditional expression and oncogenicity of c-myc linked to a CD2 gene dominant control region

Over-expression of the c-myc gene is widely implicated in the genesis of lymphoid neoplasia, including tumours of the T-cell lineage. To study the effects of deregulated c-myc expression on T-cell development and oncogenesis, we sought to generate a transgenic mouse model in which c-myc expression was targeted specifically to the T-cell lineage. A plasmid construct containing a dominant control region (DCR) from the human CD2 locus linked 5' to the human c-myc gene was used to generate 2 lines of transgenic mice. Both strains developed thymic lymphoma at low frequency, but thymic development and peripheral T-cell numbers were otherwise apparently normal. Low tumour penetrance was consistent with the observed lack of stable CD2-myc transgene mRNA in tissues of healthy transgenic mice. In contrast, transgene RNA was detected in all malignant tumours as well as in early lymphomatous lesions. RNase protection analyses confirmed these findings and showed that the PI human c-myc promoter was active in all neoplastic tissues but not in the thymus or other tissues of healthy transgenic mice. Despite the low spontaneous tumour incidence, the presence of the transgene markedly and uniformly accelerated the onset of tumours after neonatal infection with Moloney murine leukaemia virus. All tumours were rearranged for T-cell receptor beta-chain genes and were of T-cell origin from their surface phenotype (Thy-1+, CD3+, CD4+/-, CD8+, sIg-). Virus-accelerated tumours contained clonal integrations of Moloney murine leukaemia virus, suggesting that proviral insertional mutagenesis may have played a role in tumour development. Analysis of several candidate myc-cooperating genes failed to reveal any rearrangements apart from a low frequency involving proviral insertion at the pim-1 locus. The CD2-myc mouse should therefore be a valuable system in screening for novel myc-collaborating genes involved in T-cell lymphoma.

Insertional mutations in mammals and mammalian cells

The retroposon sequences, their mechanisms of transposition and the occurrence of insertional mutation in the mammalian genome are reviewed. Insertional mutations fall into two broad categories: those due to the disruption of a gene following the physical integration of a foreign DNA sequence result in loss of gene product and would be expected to be associated with a recessive mutation. A second class of insertional mutation is well documented in which upon integration the promoter/enhancer activities inherent in the retroposon genome exert their influence on neighboring genes. This promoter/enhancer activity of integrated retroposons may have effects over relatively long distances and thus limit the possibilities of establishing an association between retroposon integration and mutation. It is emphasized that a systematic search for insertional mutations in the mammalian genome involves an extensive two-dimensional array of possible retroposon sequences and mutant alleles. Present results represent only a small portion of the total array. Future studies promise to be fruitful in efforts to isolate genes through insertional tagging, to characterize the mechanisms of retroposon transposition, as well as to study the stability of the mammalian genome.

Pathogenesis of feline leukemia virus T17: contrasting fates of helper, v-myc, and v-tcr proviruses in secondary tumors

A naturally occurring feline thymic lymphosarcoma (T17) provided the unique observation of a T-cell antigen receptor beta-chain gene (v-tcr) transduced by a retrovirus. The primary tumor contained three classes of feline leukemia virus (FeLV) provirus, which have now been characterized in more detail as (i) v-tcr-containing recombinant proviruses, (ii) v-myc-containing recombinant proviruses, and (iii) apparently full-length helper FeLV proviruses. The two transductions appear to have been independent events, with distinct recombinational junctions and no sequence overlap in the host-derived inserts. The T17 tumor cell line releases large numbers of FeLV particles of low infectivity; all three genomes are encapsidated, but passage of FeLV-T17 on feline fibroblast and lymphoma cells led to selective loss of the recombinant viruses. The oncogenic potential of the T17 virus complex was, therefore, tested by infection of neonatal cats with virus harvested directly from the primary T17 tumor cell line. A single inoculation of FeLV-T17 caused persistent low-grade infection culminating in thymic lymphosarcoma and acute thymic atrophy, which was accelerated by coinfection with the weakly pathogenic FeLV subgroup A (FeLV-A)/Glasgow-1 helper. Molecularly cloned FeLV-tcr virus (T-31) rescued for replication by a weakly pathogenic FeLV-A/Glasgow-1 helper virus was similarly tested in vivo and induced thymic atrophy and thymic lymphosarcomas. Most FeLV-T17-induced tumors manifested either v-myc or an activated c-myc allele and had undergone rearrangement of endogenous T-cell antigen receptor beta-chain genes, supporting the proposition that the oncogenic effects of c-myc linked to the FeLV long terminal repeat are targeted to a specific window in T-cell differentiation. However, neither the FeLV-T17-induced tumors nor the T-31 + FeLV-A-induced tumors contained clonally represented v-tcr sequences. Only one of the FeLV-T17-induced tumors contained detectable v-tcr proviruses, at a low copy number. While v-tcr does not have a readily transmissible oncogenic function, a more restricted role is not excluded, perhaps involving antigenic peptide-major histocompatibility complex recognition by the T-cell receptor complex. Such a function could be obscured by the genetic diversity of the outbred domestic cat host.

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

Mo-MLV infection of E mu-myc transgenic mice results in a dramatic acceleration of pre-B cell lymphomagenesis. We have used provirus tagging to identify genes that cooperate with the E mu-myc transgene in B cell transformation. Here we report on the identification of four loci, pim-1, bmi-1, pal-1, and bla-1, which are occupied by proviruses in 35%, 35%, 28%, and 14% of the tumors, respectively. bmi-1, pal-1, and bla-1 represent novel common proviral insertion sites. The bmi-1 gene encodes a 324 amino acid protein with a predominantly nuclear localization. bmi-1 is highly conserved in evolution and contains several motifs frequently found in transcriptional regulators, including a new putative zinc finger motif. No genes have yet been assigned to pal-1 and bla-1. The distribution of proviruses over the four common insertion sites suggests that provirus tagging can be used not only to identify the cooperating oncogenes but also to assign these genes to distinct complementation groups in tumorigenesis.

Structural diversity and nuclear protein binding sites in the long terminal repeats of feline leukemia virus

The long terminal repeat U3 sequences were determined for multiple feline leukemia virus proviruses isolated from naturally occurring T-cell tumors. Heterogeneity was evident, even among proviruses cloned from individual tumors. Proviruses with one, two, or three repeats of the long terminal repeat enhancer sequences coexisted in one tumor, while two proviruses with distinct direct repeats were found in another. The enhancer repeats are characteristic of retrovirus variants with accelerated leukemogenic potential and occur between -155 and -244 base pairs relative to the RNA cap site. The termini of the repeats occur at or near sequence features which have been recognized at other retrovirus recombinational junctions. In vitro footprint analysis of the feline leukemia virus enhancer revealed three major nuclear protein binding sites, located at consensus sequences for the simian virus 40 core enhancer, the nuclear factor 1 binding site, and an indirect repeat which is homologous to the PEA2 binding site in the polyomavirus enhancer. Only the simian virus 40 core enhancer sequence is present in all of the enhancer repeats. Cell type differences in binding activities to the three motifs may underlie the selective process which leads to outgrowth of viruses with specific sequence duplications.

Provirus insertion in Tpl-1, an Ets-1-related oncogene, is associated with tumor progression in Moloney murine leukemia virus-induced rat thymic lymphomas

T-cell lymphomas induced in rats by Moloney murine leukemia virus acquire increasing numbers of proviruses in their genome during tumor progression in vivo and passage of tumor cells in vitro. To determine whether the proviruses progressively acquired during tumor progression play a causal role in this process, we cloned one of them from a cell line derived from the primary tumor 2772. A probe from the cellular DNA flanking the provirus was used to analyze 79 DNA samples from primary tumor tissues of 28 tumor-bearing rats and 80 DNA samples from 30 independent tumor cell lines. This analysis revealed a rearrangement in this region in the primary tumor derived from the thymus of one animal but not in a clone of the same tumor segregating in the spleen. Of the cell line DNA samples, three carried a provirus in this region. Two of these integration events had occurred independently in two clonally related sublines derived from tumor 2772, and they were followed by rapid selection in culture. On the basis of these findings this locus was named Tpl-1 (tumor progression locus 1). The Tpl-1 locus was mapped to rat chromosome 8 and to mouse chromosome 9 at a genetic distance of 1.2 +/- 0.9 centimorgans from the Ets-1 protooncogene. Although the genetic distance between Tpl-1 and Ets-1 indicates that they are different genes, analysis of Tpl-1 cDNA clones revealed that the two are closely related.

Distinct chromosomal abnormalities in murine leukemia virus-induced T- and B-cell lymphomas

We performed a cytogenetic study on 16 murine mature B-cell lymphomas and 10 T-cell lymphomas, using G-banding techniques. All tumors, with the exception of 3 spontaneous B-cell tumors, were induced by various slowly transforming murine leukemia viruses (MuLV). Metaphases were obtained from primary (10 B-cell tumors) and first or second transplant generation lymphomas (6 B-cell and 10 T-cell tumors), all of which were well characterized with respect to phenotypic, histologic and genotypic features. In the T-cell tumors we found relatively simple karyotypic abnormalities, including various numerical aberrations, such as trisomy 15, in line with many earlier reports. However, the majority of B-cell tumors showed a great variety of both structural and numerical chromosomal anomalies. Three B-cell lymphomas had an apparently normal karyotype. No single cytogenetic abnormality occurred commonly in the B-cell lymphomas, but some structural abnormalities were found in more than one stemline, in particular, ins (II) (A1; A2) in 3 tumors, and deletions involving the D-region of chromosome 14 in 3 other lymphomas. These cytogenetic results clearly indicate that the pathogenic mechanisms involved in MuLV-induced (long latency) B-cell lymphomagenesis and (short latency) T-cell lymphomagenesis differ considerably.

Lymphotropic papovavirus early region is specifically regulated transgenic mice and efficiently induces neoplasia

Transgenic mice have been generated which carry the early region of lymphotropic papovavirus (LPV). Eight of eleven founder animals died before 3 months of age after developing one or both of two distinct proliferative disorders. Of the three surviving animals, two are known to have rearranged or partial copies of the LPV genes. The majority of the founder animals (six) developed debilitating choroid plexus tumors by 26 to 42 days. Although this is the same tumor type induced by the simian virus 40 T-antigen gene, those induced by LPV appeared at a much younger age. The LPV early region was expressed in the brain tumors of these mice, as well as in the thymus and spleen. Expression in the latter two tissues reflects the cell-type specificity of the LPV enhancer demonstrated in cultured cells (i.e., lymphoid cells). Two founder animals (LP41 and LP50) gave rise to lines of mice that routinely develop lymphoproliferative disorders. LP50 and its LPV-positive offspring developed aggressive lymphomas and choroid plexus tumors. The transgenic offspring of LP41 also developed lymphomas. High levels of LPV RNA were expressed in the lymphomas of these mice as well as in the spleens and thymuses. The origin of the lymphomas from B- and T-cell lineages suggests that the LPV early genes are expressed in and can transform both of these cell types in vivo.

Predisposition to lymphomagenesis in pim-1 transgenic mice: cooperation with c-myc and N-myc in murine leukemia virus-induced tumors

Transgenic mice bearing the pim-1 gene supplemented with an upstream immunoglobulin enhancer and a downstream murine leukemia virus long terminal repeat express pim-1 mRNA at high levels in both B and T cells. Between 5% and 10% of the pim-1 transgenic mice develop clonal T cell lymphomas before 7 months of age, whereas none of the age-matched control mice do, providing direct evidence for the oncogenic potential of pim-1. Histological examination and FACS analysis revealed no abnormalities in hematopoietic tissues of disease-free pim-1 transgenic mice. When newborn pim-1 transgenic mice are infected with MuLV, T cell lymphomas develop much faster (latency 7-8 weeks) than in nontransgenic mice (latency 22 weeks). In all these T cell lymphomas either c-myc or N-myc was activated by proviral insertion, suggesting strong cooperation between pim-1 and myc in lymphomagenesis.

Characterization of cell markers in type B retroviral-induced thymic lymphomas--I. Surface antigen phenotype and karyotype in developing and primary lymphomas

CFW/D mice injected neonatally with a type B retrovirus, (DMBA-LV), rapidly develop thymic lymphomas. The present study examined simultaneously the karyotype and the expression of surface antigens on thymocytes from DMBA-LV treated mice at 28, 35 and 42 days of age during the development of lymphomas, and of cells from primary lymphomas. DMBA-LV treatment resulted in disturbances in the proportions of thymic Lyt 1+2-, Lyt 1-2+ and Lyt 1+2+ subpopulations. As a group, virus-treated mice showed a significant decrease in the size of the Lyt 1+2- subpopulation in thymuses during tumor development. However, developing tumors and primary tumors showed individual patterns of alteration in the proportions of thymocyte subpopulations rather than consistent trends for any one thymocyte subpopulation to increase or decrease. Cells with the abnormal chromosome complement, trisomy 15, were present early in the development of tumors and became the predominant karyotype in fully developed tumors. A correlation between the appearance of trisomy 15 and a particular thymocyte surface antigen phenotype was not detected. The results suggest that retrovirus infection disrupts, but does not eliminate the ability of developing thymocytes to form phenotypically distinct subpopulations.

Transgenic mice as a means to study synergism between oncogenes

Transgenic mice present a useful model to study the mechanisms underlying malignant transformation. (i) They can provide information on the oncogenic potential of genes as a function of tissue context. (ii) They allow the analysis of the primary effects of an oncogene on proliferation and differentiation before secondary mutations have occurred. (iii) Crossings between transgenic mice carrying different oncogenes can reveal their capacity to cooperate in transformation. (iv) Transgenic mice bearing a particular oncogene can be used to search for (new) (anti)oncogenes that synergize with the transgene. The non-acute transforming murine leukemia viruses (MuLV) appear very useful for this purpose. This has become clear from our studies with pim-I and c-myc transgenic mice. MuLV dramatically accelerates T-cell lymphomagenesis in transgenic mice overexpressing the pim-I oncogene in their lymphoid compartment. In all tumors induced by MuLV in pim-I transgenic mice, either the c-myc or the N-myc gene was activated by proviral insertion. Similarly, MuLV infection of transgenic mice overexpressing the c-myc gene in their B-cell compartment resulted in the acceleration of pre-B-cell lymphomagenesis. A significant fraction of the resulting pre-B-cell tumors showed proviral activation of pim-I. This shows that pim-I and myc synergize efficiently in both B- and T-cell lymphomagenesis. pim-I transgenic mice are also highly sensitive to tumor induction by N-ethyl-N-nitrosourea (ENU) and therefore represent an excellent in vivo model system to test the oncogenic potential of chemical compounds.

Oncogene expression in Rauscher murine leukemia virus induced erythroid, myeloid and lymphoid cell lines

A comparative study on the expression of nuclear and cytoplasmic oncogenes was carried out using the Northern blotting technique, in Rauscher virus induced primary leukemias and the more malignant transformed cell lines derived from them. The latter grow permanently in vitro. Hyperplastic spleens obtained from mice recovering from anemia were analysed as controls. In addition to the detection of mRNAs, Southern blotting was carried out to observe whether rearrangement or amplification of oncogenes had occurred. The results show that the nuclear oncogenes c-myc, c-myb and p53 are strongly expressed in leukemic tissue, whereas c-fos transcripts show a much weaker hybridization. The expression of two of these oncogenes, c-myc and c-myb was followed during differentiation in myeloid leukemic cells and showed a gradual decrease when compared with the actin gene, which is constitutively transcribed. A large number of cytoplasmic oncogenes is expressed in the leukemic cells lines, i.e. c-abl, c-fms, c-fes, c-src, c-ros, c-H-ras, c-K-ras and N-ras. Of these, transcripts coding for c-abl and c-src were absent in blast cells of acute erythroid leukemias. Transcripts coding for c-erb, c-mos and c-sis could also not be detected. A number of putative oncogenes which are reported to play a role in Moloney and Friend virus induced leukemias for instance pim-1, fis-1, fim-1 and fim-2 were also used for screening. Only expression of pim-1 in Rauscher virus induced myeloid leukemic cells and in primary acute erythroid leukemias could be observed. At the DNA level no rearrangement or amplification of any of the oncogenes investigated could be detected. The results show that a number of oncogenes are expressed simultaneously in the same leukemic tissue or cell lines. It therefore seems likely that the presence of transcripts of different oncogenes is associated with the progression of leukemia, but is not the primary cause of leukemogenesis or of the transformation of these cells into established cell lines.

Characterization of cell markers in type B retroviral-induced thymic lymphomas--II. Surface antigen phenotype, karyotype and proviral integration pattern in cultured lymphoma cells and cloned lines

This study examined the surface antigen phenotype, karyotype and proviral integration patterns of cultured cells from murine thymic lymphomas induced by injecting neonatal mice with the type B leukemogenic retrovirus (DMBA-LV). Cells from the primary thymic lymphomas were established in mass culture and from these, clonal tumor cell lines were derived. During in vitro culture of lymphoma cells, Lyt 1-2+ cells predominated with an apparent selection against cells of the Lyt 1+2- and Lyt 1+2+ phenotypes. Of 21 cloned lines established, five had a diploid chromosome complement and expressed the Lyt 1-2+ phenotype. The other 16 clones lines were characterized by trisomy of chromosome 15 and expressed the Lyt 1+2+ or Lyt 1-2+ phenotype. Cells characterized by either a diploid or trisomy chromosome complement were capable of growth in vivo. Southern blot analyses showed that during growth in culture, cells from the mass cultures and cloned lines continued to acquire low numbers of new integrated DMBA-LV proviral copies while maintaining the basic proviral integration pattern present in the DNA from cells of the primary lymphomas. These findings support the notion that the acquisition of new genetic information in cells from DMBA-LV-induced thymic lymphomas may contribute to the continual generation of tumor heterogeneity.

Integration of the BALB/c ecotropic provirus into the colony-stimulating factor-1 growth factor locus in a myc retrovirus-induced murine monocyte tumor

The development of tumors is thought to be a multistage process that requires an unknown number of genetic or epigenetic changes in a single cell. We previously described a murine monocyte tumor which was induced by a helper-free c-myc retrovirus and which also contained a DNA rearrangement at the colony-stimulating factor-1 (CSF-1) locus. The CSF-1 gene rearrangement gave rise to high levels of growth factor production and autocrine growth, implicating this secondary event in tumorigenesis. This CSF-1 gene rearrangement was found to be the result of integration of the BALB/c ecotropic retrovirus. Restriction enzyme mapping and DNA sequence analysis demonstrated that the novel provirus is identical to the BALB/c endogenous ecotropic provirus, indicating that infection was probably not due to the creation of a recombinant virus in vivo. The proviral integration site was mapped 3 kilobases 5' of the CSF-1 promoter and in an opposite transcriptional orientation, indicating that activation of CSF-1 expression was the result of the presence of the retroviral enhancer element.

N-methyl-N-nitrosourea-induced T-lymphomas of AKR/J mice contain somatically acquired ecotropic-like murine leukemia proviruses

We have studied somatically murine leukemia proviral integrations in primary N-methyl-N-nitrosourea (MNU)-induced thymic lymphomas of AKR/J mice. The majority of MNU-induced lymphomas contain newly acquired murine leukemia proviral sequences. In contrast to spontaneous AKR/J lymphomas which contain multiple integrations of mink cell focus-forming recombinant proviruses, MNU-induced lymphomas contain ecotropic-related proviruses. This conclusion was based on the demonstration that EcoRI- and PvuII-digested DNA from MNU-induced lymphomas contains new 3' proviral-cellular junction fragments that hybridize with the ecotropic-specific pAKV-4 and pAKV-5 hybridization probes. Also, EcoRI/PstI double digests of DNA from MNU-induced lymphomas revealed that the acquired proviruses do not contain an internal 3' EcoRI site characteristic of mink cell focus-forming recombinant viruses. The proviral integration patterns suggest that MNU-induced lymphomas are clonal or oligoclonal in nature. This conclusion is supported by comparison of proviral integration patterns in lymphomas obtained from thymus and spleen of individual mice, and by analyses of T-cell receptor beta-chain gene rearrangements. The frequent occurrence of ecotropic-related proviral sequences in MNU-induced lymphomas suggests that these newly acquired proviruses may play a role in tumor development.

Mechanism of selection of class II recombinant murine leukemia viruses in the highly leukemic strain CWD

The development of spontaneous lymphomas in CWD mice is associated with the expression of endogenous ecotropic murine leukemia viruses (MuLV) and the formation of recombinant viruses. However, the pattern of substitution of nonecotropic sequences within the envelope genes of the CWD class II recombinant viruses differs from that seen in class I recombinant MuLVs of AKR, C58, and HRS mice. To determine how CWD host genes might influence the envelope gene structure of the recombinant viruses, we characterized the responses of these mice to two different types of exogenous MuLVs. Neonatal mice injected the HRS class I recombinant PTV-1 became infected and developed T-cell lymphomas more rapidly than controls did. The inoculation of CWD mice with the leukemogenic AKR ecotropic virus SL3-3 led to the formation of recombinant MuLVs with a novel genetic structure and class II-like envelope genes, although SL3-3 generates class I recombinants in other strains. These results suggest that the absence of class I recombinant MuLVs in CWD mice is not related to the restriction of the replication or oncogenicity of class I viruses or to the absence of an appropriate ecotropic virus that can generate class I recombinants. More likely, the genes of CWD mice that direct the formation or selection of class II recombinant viruses affect the process of recombination between the ecotropic and nonecotropic envelope gene sequences.

Provirus tagging as an instrument to identify oncogenes and to establish synergism between oncogenes

Insertional mutagenesis is one of the mechanisms by which retroviruses can transform cells. Once a provirus was found in the vicinity of c-myc, with the concomitant activation of this gene, other proto-oncogenes were shown to be activated by proviral insertion in retrovirally-induced tumors. Subsequently, cloning of common proviral insertion sites led to the discovery of a series of new (putative) oncogenes. Some of these genes have been shown to fulfill key roles in growth and development. In this review I shall describe how proviruses can be used to identify proto-oncogenes, and list the loci, identified by this method. Furthermore, I shall illuminate the potential of provirus tagging by showing that it not only can mark new oncogenes, but can also be instrumental in defining sets of (onco)genes that guide a normal cell in a step-by-step fashion to its fully transformed, metatasizing, counterpart.

The approaching era of the tumor suppressor genes

Genes that can inhibit the expression of the tumorigenic phenotype have been detected by the fusion of normal and malignant cells, the phenotypic reversion of in vitro transformants, the induction of terminal differentiation of malignant cell lineages, the loss of "recessive cancer genes," the discovery of regulatory sequences in the immediate vicinity of certain oncogenes, and the inhibition of tumor growth by normal cell products. Such tumor suppressor genes will probably turn out to be as, if not more, diversified as the oncogenes. Consideration of both kinds of genes may reveal common or interrelated functional properties.

Characterization of early molecular biological events associated with thymic lymphoma induction following infection with a thymotropic type-B retrovirus

A highly leukemogenic virus (DMBA-LV) induces thymic lymphomas with a very short (40 days) latent period. All induced tumors contain low numbers of new integrated DMBA-LV type-B proviruses and tumorigenicity of DMBA-LV is completely abolished by a monoclonal antibody directed toward an envelope determinant present on a type-B mammary tumor-inducing viral isolate. While the DMBA-LV type-B genome is very highly related to mammary tumor-inducing isolates it does have unique gp52 and p28 proteins as well as unique restriction endonuclease sites. In the present study the target cell specificity of DMBA-LV was contrasted with that of the mammary tumor-inducing isolate MMTV (C3H). The results indicated that infection of CFW/D mice with DMBA-LV could be detected in the thymus only as early as 17 days postinfection and by 40 days postinfection all 40 thymuses examined contained new integrated proviral copies of DMBA-LV. In contrast, when mice were injected intrathymically with MMTV (C3H) virus infection was transiently detected in the thymus only at 28 days postinfection. By 35 and 42 days postinfection there was no indication that virus-infected cells were still present. Analysis of individual thymic lobes following DMBA-LV infection suggested that independent tumors may be initiated in each of the separate lobes. Furthermore, there appeared to be a correlation between the weight of the lobe and the number of new DMBA-LV proviral copies, the larger the lobe the greater the number of newly integrated proviral copies.

Frequent involvement of the fim-3 region in Friend murine leukemia virus-induced mouse myeloblastic leukemias

fim-3 is a new proviral integration region involved in 23% (16 of 68) of Friend murine leukemia virus-induced myeloblastic leukemias. This region is distinct from 20 oncogenes and from putative oncogenes tested so far. Proviruses are integrated in a 16-kilobase region, always in the same orientation. No RNA expression of fim-3 was detected.

The role of chromosome 15 in murine leukemogenesis. II. Relationship between tumorigenicity and the dosage of lymphoma vs. normal-parent-derived chromosomes 15 in somatic cell hybrids

Somatic cell hybrids were generated between an MCF-virus-induced 15-trisomic T-cell lymphoma of AKR origin with a proviral insertion near the c-myc locus, and normal diploid fibroblasts or lymphocytes of CBAT6T6 origin. Three lymphoma/fibroblast fusions were performed. Six independently-derived clones from 2 fusions were tested for tumorigenicity. Three of the 6 clones were weakly malignant (take incidence 20% below), and 3 were strongly malignant (take incidence over 80%). All 3 lymphoma/lymphocyte hybrids and 6 derived clones were strongly malignant. All hybrids contained a nearly complete chromosomal complement of both parental cells. This was confirmed at the molecular level by determining the ratio of germ-line (G) vs. rearranged (R) myc-carrying Eco RI fragments that showed the expected 1.9-2.7:1 proportion. Malignant segregants selected from the weakly malignant lymphoma/fibroblast hybrids by in vivo inoculation showed changed 15-chromosome ratios. Four out of the 6 clones showed amplification of the lymphoma-derived 15-chromosome that carries the R-myc fragment and a concomitant decrease in the average number of the G-myc-carrying chromosomes. This was deduced from the fact that the G:R ratio was between 2 and 3:1 in the in vitro hybrids but became inverted (1:2-3) in the tumors. Two tumors showed no amplification of R-myc. G-myc was decreased. One of these tumors showed a change in the G:R ratio from 2.5:1.0 to 1.2:1.0, while the other was essentially unchanged (1.9:1.0 in the in vitro clone and 2.2:1.0 in the derived tumor). These findings support the notion that both the amplification of the lymphoma-derived 15-chromosome with the retrovirally rearranged c-myc carrying fragment and/or the loss of the G-myc-carrying 15-chr can contribute to the tumorigenic potential of the hybrids.

Cell transformation and tumor induction by Abelson murine leukemia virus in the absence of helper virus

We investigated the role of the Moloney helper virus, Moloney murine leukemia virus (Mo-MuLV), in cell transformation and tumor induction by the defective Abelson murine leukemia virus (Ab-MuLV). A molecular clone of Ab-MuLV (P160 strain) was transfected into the psi 2 packaging cell line, and helper virus-free Ab-MuLV (psi 2) was harvested from the supernatant medium. Ab-MuLV (psi 2) was as efficient as helper virus-containing Ab-MuLV (Mo-MuLV) in the transformation of primary bone marrow cells in vitro. Inoculation of weanling BALB/c mice with Ab-MuLV (psi 2) induced nonthymic pre-B-cell lymphomas with high efficiency and short latency (28 days). Adult BALB/c mice were less sensitive to tumor induction by a factor of 100. Ab-MuLV (psi 2) did not induce tumors in weanling C57BL/6 mice, unlike Ab-MuLV (Mo-MuLV). Examination of the proviral integration pattern in Ab-MuLV (psi 2)-induced tumor cell DNA revealed that each of the tumors contained a single integrated provirus. Immunoprecipitation of viral-encoded proteins in helper virus-free tumor cell lines detected the P160 Ab-MuLV-transforming protein; however, no trace of the gag, pol, and env helper virus-encoded proteins was found. Our results indicate that integration and expression of a single Ab-MuLV genome is sufficient for efficient transformation of primary bone marrow cells by Ab-MuLV in vitro and tumor induction in susceptible mice. However, the helper virus may contribute to tumor induction in weanling resistant mice.

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

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