Important role of the long terminal repeat of the helper Moloney murine leukemia virus in Abelson virus-induced lymphoma

AHI-1: a novel signaling protein and potential therapeutic target in human leukemia and brain disorders

Progress in the understanding of the molecular and cellular mechanisms of human cancer, including human leukemia and lymphomas, has been spurred by cloning of fusion genes created by chromosomal translocations or by retroviral insertional mutagenesis; a number of oncogenes and tumor suppressors involved in development of a number of malignancies have been identified in this manner. The BCR-ABL fusion gene, originating in a multipotent hematopoietic stem cell, is the molecular signature of chronic myeloid leukemia (CML). Discovery of this fusion gene has led to the development of one of the first successful targeted molecular therapies for cancer (Imatinib). It illustrates the advances that can result from an understanding of the molecular basis of disease. However, there still remain many as yet unidentified mutations that may influence the initiation or progression of human diseases. Thus, identification and characterization of the mechanism of action of genes that contribute to human diseases is an important and opportune area of current research. One promising candidate as a potential therapeutic target is Abelson helper integration site-1(Ahi-1/AHI-1) that was identified by retroviral insertional mutagenesis in murine models of leukemia/lymphomas and is highly elevated in certain human lymphoma and leukemia stem/progenitor cells. It encodes a unique protein with a SH3 domain, multiple SH3 binding sites and a WD40-repeat domain, suggesting that the normal protein has novel signaling activities. A new AHI-1-BCR-ABL-JAK2 interaction complex has recently been identified and this complex regulates transforming activities and drug resistance in CML stem/progenitor cells. Importantly, AHI-1 has recently been identified as a susceptibility gene involved in a number of brain disorders, including Joubert syndrome. Therefore, understanding molecular functions of the AHI-1 gene could lead to important and novel insights into disease processes involved in specific types of diseases. Ultimately, this knowledge will set the stage for translation into new and more effective diagnostic and treatment strategies.

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.

Ahi-1, a novel gene encoding a modular protein with WD40-repeat and SH3 domains, is targeted by the Ahi-1 and Mis-2 provirus integrations

The Ahi-1 locus was initially identified as a common helper provirus integration site in Abelson pre-B-cell lymphomas and shown to be closely linked to the c-myb proto-oncogene. Since no significant alteration of c-myb expression was found in Abelson murine leukemia virus-induced pre-B-lymphomas harboring a provirus inserted within the Ahi-1 locus, this suggested that it harbors another gene whose dysregulation is involved in tumor formation. Here we report the identification of a novel gene (Ahi-1) targeted by these provirus insertional mutations and the cloning of its cDNA. The Ahi-1 proviral insertions were found at the 3' end of the gene, in an inverse transcriptional orientation, with most of them located around and downstream of the last exon, whereas another insertion was within intron 22. In addition, another previously identified provirus insertion site, Mis-2, was found to map within the 16th intron of the Ahi-1 gene. The Ahi-1 cDNA encodes a 1,047-amino-acid protein. The predicted Ahi-1 protein is a modular protein that contains one SH3 motif and seven WD40 repeats. The Ahi-1 gene is conserved in mammals and encodes two major RNA species of 5 and 4.2 kb and several other shorter splicing variants. The Ahi-1 gene is expressed in mouse embryos and in several organs of the mouse and rat, notably at high levels in the brain and testes. In tumor cells harboring insertional mutations in Ahi-1, truncated Ahi-1/viral fused transcripts were identified, including some splicing variants with deletion of the SH3 domain. Therefore, Ahi-1 is a novel gene targeted by provirus insertion and encoding a protein that exhibits several features of a signaling molecule. Thus, Ahi-1 may play an important role in signal transduction in normal cells and may be involved in tumor development, possibly in cooperation with other oncogenes (such as v-abl and c-myc) or with a tumor suppressor gene (Nf1), since Ahi-1 insertion sites were identified in tumors harboring v-abl defective retroviruses or a c-myc transgene or in tumors exhibiting deletion of Nf1.

Nuclear factors that bind to the U3 region of two murine myeloid leukemia-inducing retroviruses, Cas-Br-E and Graffi

Cas-Br-E and Graffi are two myeloid leukemia-inducing murine viruses. Cas-Br-E induces, in NIH-Swiss mice, mostly non-T, non-B leukemia composed of very immature cells with no specific characteristics (Bergeron et al. (1993). Leukemia 7, 954-962). The Graffi murine leukemia virus causes exclusively myeloid leukemia, but the tumor cells are clearly of granulocytic nature (Ru et al. (1993). J. Virol. 67, 4722). We were interested to understand the role of the long terminal repeat (LTR) U3 region in the myeloid specificity of these two retroviruses. We used DNase I footprinting and gel mobility shift assays to identify a number of protein binding sites within Cas-Br-E and Graffi U3 regions. The pattern of protected regions is highly similar for the two viruses. Some factors identified in other murine leukemia viruses, like the core binding factor, also bind to Cas-Br-E and Graffi LTR; however, other binding sites seem specific for these two viruses. Only one difference between them was noted, at the 5' end of the U3 region. Transcriptional activity of both LTRs was also analyzed in various cell lines and compared with other murine leukemia viruses. The results show a slight myeloid specificity for the two LTRs, and indicate that the Graffi enhancer is quite strong in a broad range of cell types.

Members of the GATA family of transcription factors bind to the U3 region of Cas-Br-E and graffi retroviruses and transactivate their expression

Cas-Br-E and Graffi are two murine viruses that induce myeloid leukemia in mice: while Cas-Br-E induces mostly non-T, non-B leukemia composed of very immature cells, Graffi causes exclusively a granulocytic leukemia (E. Rassart, J. Houde, C. Denicourt, M. Ru, C. Barat, E. Edouard, L. Poliquin, and D. Bergeron, Curr. Top. Microbiol. Immunol. 211:201-210, 1995). In an attempt to understand the basis of the myeloid specificity of these two retroviruses, we used DNase I footprinting analysis and gel mobility shift assays to identify a number of protein binding sites within the Cas-Br-E and Graffi U3 regions. Two protected regions include potential GATA binding sites. Methylation interference analysis with different hematopoietic nuclear extracts showed the importance of the G residues in these GATA sites, and supershift assays clearly identified the binding factors as GATA-1, GATA-2, and GATA-3. Transient assays with long terminal repeat (LTR)-chloramphenicol acetyltransferase constructs showed that these three GATA family members are indeed able to transactivate Cas-Br-E and Graffi LTRs. Thus, the availability and relative abundance of the various members of the GATA family of transcription factors in a given cell type could influence the transcriptional tissue specificity of murine leukemia viruses and hence their disease specificity.

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

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

Establishment of leukemic T-cell lines from mice inoculated with the MAIDS defective virus

Mice inoculated with replication-competent stocks of the murine acquired immunodeficiency syndrome (MAIDS) virus are severely immunocompromised and proned to the development of T- and B-cell lymphomas. We have studied the development of T-cell lymphomas in C57BL/6 and RF/J mice inoculated with helper-free stocks of the MAIDS defective virus. We observed the expansion of T cell clones (detected by TCR gene rearrangements and by transplantation) only rarely in diseased C57BL/6 mice and slightly more frequently in RF/J mice. We succeeded in establishing four transplantable T cell tumors and malignant cell lines. The three cell lines from RF/J mice were immature T-cells (Thy-1+, CD3-, CD4+, CD8+, Mac-1+), while the line from the C57BL/6 mouse had the phenotype of mature T-cells (Thy-1+, CD3+, CD4+, CD8-). All lines were virus-producers despite the fact that helper-free stocks of the virus were inoculated. These helper MuLVs most likely originated from endogenous MuLV sequences. Also, the defective viral genome was clearly detectable in one cell line and was rearranged in two other lines. These established cell lines may be useful to determine whether they share some of the characteristics of the anergic T-cells in vivo and to study the role of the MAIDS defective virus in T cell transformation.

The Myb and Ahi-1 genes are physically very closely linked on mouse chromosome 10

Ahi-1 has previously been identified as a common helper provirus integration site on mouse Chromosome (Chr) 10 in 16% of Abelson pre-B-cell lymphomas and shown to be closely linked to the Myb protooncogene. By using long-range restriction mapping, we have mapped the Myb and Ahi-1 regions within a 120-kbp DNA fragment. The Ahi-1 region is located approximately 35 kbp downstream of the Myb gene. A further confirmation of this finding was obtained by screening a mouse YAC library. The three positive clones obtained contained both the Myb and Ahi-1 gene sequences. To test whether provirus integration in the Ahi-1 region enhances the expression of Myb by a cis-acting mechanism, we have also examined Myb gene expression in A-MuLV-induced pre-B-lymphomas. Our data have revealed that there is no clear evidence for such activation in the tumors we have tested, indicating that provirus insertion in the Ahi-1 region is activating a novel gene, apparently involved in tumor formation.

Long-range mapping of Mis-2, a common provirus integration site identified in murine leukemia virus-induced thymomas and located 160 kilobase pairs downstream of Myb

The nondefective Moloney murine leukemia virus (MuLV) induces clonal or oligoclonal T-cell tumors in mice or rats. The proviruses of these nondefective MuLVs have been shown to act as insertion mutagens most frequently activating an adjacent cellular gene involved in cell growth control. Mutations by provirus insertions, recognized as common provirus integration sites, have been instrumental in identifying novel cellular genes involved in tumor formation. We have searched for new common provirus integration sites in Moloney MuLV-induced thymomas. Using cellular sequences flanking a provirus cloned from one of these tumors, we found one region, designated Mis-2, which was the target of provirus integration in a low (3%) percentage of these tumors. Mis-2 was mapped on mouse chromosome 10, approximately 160 kbp downstream of myb. The Mis-2 region may contain a novel gene involved in tumor development.

BCR-ABL and v-abl oncogenes induce distinct patterns of thymic lymphoma involving different lymphocyte subsets

The human BCR-ABL oncogenes encoded by the Philadelphia chromosome (Ph) affect the pathogenesis of diverse types of leukemia and yet are rarely associated with T-lymphoid leukemia. To determine whether BCR-ABL kinases are inefficient in transforming T lymphocytes, BCR-ABL-expressing retroviruses were injected intrathymically into mice. Thymomas that expressed BCR-ABL kinase developed after a relatively long latent period. In most thymomas, deletion of 3' proviral sequences resulted in loss of tk-neo and occasionally caused expression of kinase-active carboxy-terminally truncated BCR-ABL oncoprotein. In contrast, deletion of 3' proviral sequences was not observed in thymomas induced with Abelson murine leukemia virus (A-MuLV). BCR-ABL viruses induced distinct patterns of disease and involved different thymocyte subsets than A-MuLV and Moloney murine leukemia virus (Mo-MuLV). While Mo-MuLV only induced Thy-1+ thymomas, v-abl- and BCR-ABL-induced thymomas often contained mixed populations of B220+ and Thy-1+ lymphocytes in the same tumor. In most v-abl and BCR-ABL tumors, Thy-1+ lymphoid cells expressed CD8 and a continuum of CD4 ranging from negative to positive. Conversely, Mo-MuLV thymomas contained distinct populations of CD4+ cells that were either CD8+ or CD8-. A-MuLV-transformed T-lymphoid cells did not express the CD3/T-cell receptor complex, while BCR-ABL tumors were CD3+. Thus, BCR-ABL viruses preferentially induce somewhat more differentiated T lymphocytes than are transformed by A-MuLV. Furthermore, rare B220+ lymphocytes may represent preferred v-abl and BCR-ABL transformation targets in the thymus.

Identification of a novel gene, Vin-1, in murine leukemia virus-induced T-cell leukemias by provirus insertional mutagenesis

The BL/VL3 radiation leukemia virus is a nondefective retrovirus which induces clonal or oligoclonal T-cell leukemia in mice. To study the role of provirus insertional mutagenesis in the development of these neoplasias, we searched for common provirus integration sites in BL/VL3 radiation leukemia virus-induced tumors. Using cellular sequences flanking a provirus cloned from one of these thymomas, we found that the viral genome was integrated into a common region, designated Vin-1, in a low percentage (5%) of these tumors. The proviruses found in this locus were integrated in the same orientation, close to a CpG-rich island, at proximity of a transcriptional unit encoding a 6-kb RNA. Vin-1 RNA was detected in several organs of the adult mouse. Vin-1 RNA levels were high in tumors having a provirus inserted within the Vin-1 region but were also high in some other tumors whose Vin-1 region was not found to be rearranged. Vin-1 was found to be well conserved among mammalian species and was mapped to mouse chromosome 6, between raf and K-ras-2. Vin-1 appears to be a novel gene which may be involved in tumor development.

The specificity of the disease induced by defective murine retroviruses containing abl, fos, or Ha-ras is usually not determined by their LTR

The long terminal repeats (LTR) of the defective murine sarcoma viruses (MSV) containing v-abl, v-Ha-ras, or v-fos were exchanged for LTRs from other retroviruses having different tissue tropism. The new chimeric MSV were found to induce the same diseases as the parental viruses, indicating that sequences outside the LTR, most likely those of the oncogene, are responsible for the disease specificity of these defective MSV.

Substitution of the U3 long terminal repeat region of the neurotropic Cas-Br-E retrovirus affects its disease-inducing potential

The Cas-Br-E and ts-Mo BA-1 murine leukemia viruses (MuLV) induce a spongiform neurodegenerative disease with different clinical manifestations, namely, either hind limb paralysis (Cas-Br-E) or tremors, spasticity, and hind limb weakness (ts-Mo Ba-1). We constructed the chimeric NEBA-1 MuLV by replacing the long terminal repeat of Cas-Br-E MuLV with that of ts-Mo BA-1 MuLV. In SWR/J or CFW/D mice, NEBA-1 MuLV induced an ataxic neurological disease characterized by clinical signs different from those induced by both parents. Although NEBA-1 MuLV did not induce lesions in novel brain areas, the spongiform lesions were more severe in deep cerebellar nuclei and in the spinal cord than those found in paralyzed mice inoculated with Cas-Br-E MuLV. By in situ hybridization, we found that the distribution of the spongiform lesions closely correlated with the distribution of the infected central nervous system cells. In the spinal cord, a close correlation was found between the number of infected cells and the severity of the spongiform degeneration. Sequencing of the substituted ts-BA-1 MuLV fragment and comparison with homologous sequences of Cas-Br-E and Moloney MuLV showed differences mainly in the U3 tandem direct repeats. Our results show that a few modifications within the U3 long terminal repeat allow the virus to cause more severe lesions in some central nervous system regions and that the severity of the spongiform degeneration correlates with the level of viral replication.

Regulation of IFN-gamma-induced host cell MHC antigen expression by Kirsten MSV and MLV. I. Effects on class I antigen expression

We have reported previously that the Kirsten murine sarcoma virus (Ki-MSV) that carries the v-Ki-ras oncogene prevents C3H10T 1/2 fibroblasts from being able to respond to interferon-gamma (IFN-gamma) with the expression of the class II major histocompatibility complex (MHC) antigen, H-2A. In this report we investigate further as to whether MSV or its parent virus Kirsten murine leukaemia virus (Ki-MLV) is able to reduce host class I MHC antigen expression. The results demonstrate that class I expression is diminished in MSV-infected cells over a time-course of 7 days after exposure to IFN-gamma and over a range of IFN-gamma concentrations. The optimal concentration of IFN-gamma for maximal class I expression remained unchanged. Cells infected with Ki-MLV, which failed to abolish the induction by IFN-gamma of class II antigens, also expressed lower levels of class I antigens, similar to those for cells infected with Ki-MSV, after exposure to IFN-gamma. It is likely therefore that the inhibition of class I induction is due to genetic material shared between the viruses, principally in the long terminal repeats (LTR), and hence that the mechanism of action is distinct from that responsible for the abolition of class II induction by Ki-MSV alone. Since class I antigens are required for CD8+ T cells (mainly cytotoxic T cells) to recognize (foreign) antigen this reduction in class I expression might lead to reduced visibility of infected cells to T cells and thus might contribute to the tumorigenicity of Ki-MSV-infected cells.

Regulation of IFN-gamma-induced host cell MHC antigen expression by Kirsten MSV and MLV. II. Effects on class II antigen expression

We have reported previously that the Kirsten murine sarcoma virus (Ki-MSV), which carries the v-Ki-ras oncogene, prevents the induction of the class II MHC antigen H-2A and reduces the induction of class I MHC antigens by interferon-gamma (IFN-gamma) on C3H10T 1/2 fibroblasts. It is here shown that the abolition by the virus of H-2A expression extends also to class II antigen H-2E and that this is maintained for at least 7 days after IFN treatment. In addition no concentration of IFN-gamma tested, including supra-optimal concentrations for class I antigen expression, induced class II antigens on MSV-infected cells. Thus MSV inhibits the induction by IFN-gamma of class II MHC antigens by a mechanism other than via a change in kinetics of response to, or in the sensitivity of the cells to, IFN. The possibility that transformation by MSV could result in the (selective) outgrowth of cells unresponsive to IFN was refuted by the observation that clones of C3H10T 1/2, when infected with Ki-MSV, expressed no or dramatically reduced levels of H-2A or H-2E. One C3H10T 1/2 clone chosen for high class II expression, when transformed with Ki-MSV, did express low levels of class II antigens at optimal concentrations of IFN-gamma, suggesting that the degree of the reduction of class II expression varies with the cells that are infected. Comparison with mechanisms whereby other viruses inhibit MHC antigen display revealed an interesting possibility: IFN response sequences (IRS) identified in the virus genomes might act in trans to (down) regulate MHC antigen expression. This could be an important mechanism determining the tumourigenicity of, and immune evasion by, Ki-MSV and other viruses.

Distinct helper virus requirements for Abelson murine leukemia virus-induced pre-B- and T-cell lymphomas

Abelson murine leukemia virus (A-MuLV) can induce pre-B- or T-cell lymphomas (thymomas) in mice depending on the route and time of injection. Previous studies have shown that the choice of the helper virus used to rescue A-MuLV greatly influences its ability to induce pre-B-cell lymphomas. In this study, we investigated the role of the helper virus in A-MuLV-induced thymomas. A-MuLV rescued with the helper Moloney MuLV, BALB/c endogenous N-tropic MuLV, and two chimeric MuLVs derived from these two parents were injected intrathymically in young adult NIH Swiss mice. All four A-MuLV pseudotypes were found to be equally efficient in the induction of thymomas, whereas drastic differences were observed in their pre-B-cell lymphomagenic potential. Thymoma induction by A-MuLV was independent of the replication potential of the helper virus in the thymus, and no helper proviral sequences could be detected in the majority of thymomas induced by A-MuLV rescued with parental BALB/c endogenous or chimeric MuLVs. In the thymomas in which helper proviruses were present, none of them were found integrated in the Ahi-1 region, a common proviral integration site found in A-MuLV-induced pre-B-cell lymphomas (Y. Poirer, C. Kozak, and P. Jolicoeur, J. Virol. 62:3985-3992, 1988). In addition, helper-free stocks of A-MuLV were found to be as lymphomagneic as other pseudotypes in inducing thymomas after intrathymic inoculation, in contrast to their inability to induce pre-B-cell lymphomas when injected intraperitoneally in newborn mice. Restriction enzyme analysis revealed one to three A-MuLV proviruses in each thymoma, indicating the oligoclonality of these tumors. Analysis of the immunoglobulin and T-cell receptor loci confirmed that the major population of cells of these primary thymomas belongs to the T-cell lineage. Together, these results indicate that the helper virus has no effect in the induction of A-MuLV-induced T-cell lymphomas, in contrast to its important role in the induction of A-MuLV-induced pre-B-cell lymphomas. Our data also revealed distinct biological requirements for transformation of these two target cells by v-abl.

Multiple steps are required for the induction of tumors by Abelson murine leukemia virus

Helper virus-free Abelson murine leukemia virus (A-MuLV) was used to induce monoclonal pre-B-cell tumors in mice. The clonality, patterns of immunoglobulin heavy-chain gene rearrangement, tumorigenicity, and v-abl oncogene expression in individual preleukemic and leukemic colonies were compared. Our results indicate that A-MuLV preleukemic cells with low or undetectable tumorigenic potential give rise to leukemic cells with high tumorigenic potential by a process of subclone selection. The levels of v-abl oncogene product in preleukemic and leukemic cell populations were not significantly different. These results suggest that an additional event(s) unrelated to the level of the v-abl protein product is required for A-MuLV-transformed cells to become fully malignant.

Identification of a common helper provirus integration site in Abelson murine leukemia virus-induced lymphoma DNA

Abelson murine leukemia virus induces oligoclonal pre-B lymphoma in mice. The expression of the v-abl oncogene in target cells does not appear to be sufficient for tumor induction in several mouse strains, and additional genetic events are thought to be required. We postulated that the helper Moloney murine leukemia virus might induce these events, and its potential role as an insertional mutagen was assessed by the search of a common helper provirus integration site in Abelson murine leukemia virus lymphomas. Molecular cloning of cellular sequences adjacent to Moloney proviruses enabled us to identify a cellular region, designated Ahi-1, which was found occupied by the helper proviruses in 16% of Abelson pre-B-cell lymphomas. All proviruses for which the precise integration site within Ahi-1 could be mapped were found to be in the same orientation. Ahi-1 has been mapped to mouse chromosome 10 and represents a new common proviral integration site. These data suggest that the helper virus contributes to the induction of secondary genetic events which may be important for the development of Abelson murine leukemia virus-induced pre-B-cell lymphoma.

Inability of Kaplan radiation leukemia virus to replicate on mouse fibroblasts is conferred by its long terminal repeat

The molecularly cloned infectious Kaplan radiation leukemia virus has previously been shown to be unable to replicate on mouse fibroblasts (E. Rassart, M. Shang, Y. Boie, and P. Jolicoeur, J. Virol. 58:96-106, 1986). To map the viral sequences responsible for this, we constructed chimeric viral DNA genomes in vitro with parental cloned infectious viral DNAs from the nonfibrotropic (F-) BL/VL3 V-13 radiation leukemia virus and the fibrotropic (F+) endogenous BALB/c or Moloney murine leukemia viruses (MuLV). Infectious chimeric MuLVs, recovered after transfection of Ti-6 lymphocytes with these recombinant DNAs, were tested for capacity to replicate on mouse fibroblasts in vitro. We found that chimeric MuLVs harboring the long terminal repeat (LTR) of a fibrotropic MuLV replicated well on mouse fibroblasts. Conversely, chimeric MuLVs harboring the LTR of a nonfibrotropic MuLV were restricted on mouse fibroblasts. These results indicate that the LTR of BL/VL3 radiation leukemia virus harbors the primary determinant responsible for its inability to replicate on mouse fibroblasts in vitro. Our results also show that the primary determinant allowing F+ MuLVs (endogenous BALB/c and Moloney MuLVs) to replicate on mouse fibroblasts in vitro resides within the LTR.

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 viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.