Viral integration near c-myc in 10-20% of mcf 247-induced AKR lymphomas

Endogenous feline leukemia virus long terminal repeat integration site diversity is highly variable in related and unrelated domestic cats

Endogenous retroviruses (ERV) are indicators of vertebrate evolutionary history and play important roles as homeostatic regulators. ERV long terminal repeat (LTR) elements may act as cis-activating promoters or trans-activating enhancer elements modifying gene transcription distant from LTR insertion sites. We previously documented that endogenous feline leukemia virus (FeLV)-LTR copy number variation in individual cats tracks inversely with susceptibility to virulent FeLV disease. To evaluate FeLV-LTR insertion characteristics, we assessed enFeLV-LTR integration site diversity in 20 cats from three genetically distinct populations using a baited linker-mediated PCR approach. We documented 765 individual integration sites unequally represented among individuals. Only three LTR integration sites were shared among all individuals, while 412 sites were unique to a single individual. When primary fibroblast cultures were challenged with exogenous FeLV, we found significantly increased expression of both exogenous and endogenous FeLV orthologs, supporting previous findings of potential exFeLV-enFeLV interactions; however, viral challenge did not elicit transcriptional changes in genes associated with the vast majority of integration sites. This study assesses FeLV-LTR integration sites in individual animals, providing unique transposome genotypes. Further, we document substantial individual variation in LTR integration site locations, even in a highly inbred population, and provide a framework for understanding potential endogenous retroviral element position influence on host gene transcription.

Conserved features of cancer cells define their sensitivity to HAMLET-induced death; c-Myc and glycolysis

HAMLET is the first member of a new family of tumoricidal protein-lipid complexes that kill cancer cells broadly, while sparing healthy, differentiated cells. Many and diverse tumor cell types are sensitive to the lethal effect, suggesting that HAMLET identifies and activates conserved death pathways in cancer cells. Here, we investigated the molecular basis for the difference in sensitivity between cancer cells and healthy cells. Using a combination of small-hairpin RNA (shRNA) inhibition, proteomic and metabolomic technology, we identified the c-Myc oncogene as one essential determinant of HAMLET sensitivity. Increased c-Myc expression levels promoted sensitivity to HAMLET and shRNA knockdown of c-Myc suppressed the lethal response, suggesting that oncogenic transformation with c-Myc creates a HAMLET-sensitive phenotype. Furthermore, HAMLET sensitivity was modified by the glycolytic state of tumor cells. Glucose deprivation sensitized tumor cells to HAMLET-induced cell death and in the shRNA screen, hexokinase 1 (HK1), 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 1 and hypoxia-inducible factor 1α modified HAMLET sensitivity. HK1 was shown to bind HAMLET in a protein array containing ∼8000 targets, and HK activity decreased within 15 min of HAMLET treatment, before morphological signs of tumor cell death. In parallel, HAMLET triggered rapid metabolic paralysis in carcinoma cells. Tumor cells were also shown to contain large amounts of oleic acid and its derivatives already after 15 min. The results identify HAMLET as a novel anti-cancer agent that kills tumor cells by exploiting unifying features of cancer cells such as oncogene addiction or the Warburg effect.

Mutational effects of retrovirus insertion on the genome of V79 cells by an attenuated retrovirus vector: implications for gene therapy

Attenuated retroviruses are currently the most widely used vectors in clinical gene therapy because of their potential to effect stable and permanent gene transfer. Since gene delivery is accompanied by random insertion of foreign genetic material into the recipient chromosomal DNA, the potential for insertional mutagenesis exists. In this study, we used a defective retrovirus vector containing a selectable marker, the hygromycin phosphotransferase gene, to investigate the mutagenic effects of vector integration on the mammalian genome. V79 Chinese hamster cells were infected with virus supernatants or by coculture with virus producer cells, and provirus insertion events occurred at low and high frequencies, respectively. The frequency of hprt mutagenesis was increased by a factor of 2.3 over the spontaneous hprt mutation frequency only following multiple provirus insertions/cell genome. Multiple provirus insertions (>3/genome) resulted in instability at the hprt locus in 63% of the virally induced hprt mutants, as indicated by rearrangements at the molecular level, whereas no rearrangements were found when the provirus copy number was 1-2/genome. To demonstrate direct proviral involvement in mutagenesis, the defective MLV vector was retrieved along with flanking genomic hprt sequences from one mutant, and localized within intron 5 of the hprt gene. These data suggest that provirus copy number is a key factor when considering the potential hazards of using retrovirus vectors for gene therapy.

A virus-virus interaction circumvents the virus receptor requirement for infection by pathogenic retroviruses

During ongoing C-type retrovirus infection, the probability of leukemia caused by insertional gene activation is markedly increased by the emergence of recombinant retroviruses that repeatedly infect host cells. The murine mink cell focus-inducing (MCF) viruses with this property have acquired characteristic changes in the N-terminal domain of their envelope glycoprotein that specify binding to a different receptor than the parental ecotropic virus. In this report, we show that MCF virus infection occurs through binding to this receptor (termed Syg1) and, remarkably, by a second mechanism that does not utilize the Syg1 receptor. By the latter route, the N-terminal domain of the ecotropic virus glycoprotein expressed on the cell surface in a complex with its receptor activates the fusion mechanism of the MCF virus in trans. The rate of MCF virus spread through a population of permissive human cells was increased by establishment of trans activation, indicating that Syg1 receptor-dependent and -independent pathways function in parallel. Also, trans activation shortened the interval between initial infection and onset of cell-cell fusion associated with repeated infection of the same cell. Our findings indicate that pathogenic retrovirus infection may be initiated by virus binding to cell receptors or to the virus envelope glycoprotein of other viruses expressed on the cell surface. Also, they support a broader principle: that cooperative virus-virus interactions, as well as virus-host interactions, shape the composition and properties of the retrovirus quasispecies.

Duplication of U3 sequences in the long terminal repeat of mink cell focus-inducing viruses generates redundancies of transcription factor binding sites important for the induction of thymomas

The ability of mink cell focus-inducing (MCF) viruses to induce thymomas is determined, in part, by transcriptional enhancers in the U3 region of their long terminal repeats (LTRs). To elucidate sequence motifs important for enhancer function in vivo, we injected newborn mice with MCF 1dr (supF), a weakly pathogenic, molecularly tagged (supF) MCF virus containing only one copy of a sequence that is present as two copies (known as the directly repeated [DR] sequence) in the U3 region of MCF 247 and analyzed LTRs from supF-tagged proviruses in two resulting thymomas. Tagged proviruses integrated upstream and in the reverse transcriptional orientation relative to c-myc provided the focus of our studies. These proviruses are thought to contribute to thymoma induction by enhancer-mediated deregulation of c-myc expression. The U3 region in a tagged LTR in one thymoma was cloned and sequenced. Relative to MCF 1dr (supF), the cloned U3 region contained an insertion of 140 bp derived predominantly from the DR sequence of the injected virus. The inserted sequence contains predicted binding sites for transcription factors known to regulate the U3 regions of various murine leukemia viruses. Similar constellations of binding sites were duplicated in two proviral LTRs integrated upstream from c-myc in a second thymoma. We replaced the U3 sequences in an infectious molecular clone of MCF 247 with the cloned proviral U3 sequences from the first thymoma and generated an infectious chimeric virus, MCF ProEn. When injected into neonatal AKR mice, MCF ProEn was more pathogenic than the parental virus, MCF 1dr (supF), as evidenced by the more rapid onset and higher incidence of thymomas. Molecular analyses of the resultant thymomas indicated that the U3 region of MCF ProEn was genetically stable. These data suggest that the arrangement and/or redundancy of transcription factor binding sites generated by specific U3 sequence duplications are important to the biological events mediated by MCF proviruses integrated near c-myc that contribute to transformation.

What retroviruses teach us about the involvement of c-Myc in leukemias and lymphomas

Overexpression of the cellular oncogene c-Myc frequently occurs during induction of leukemias and lymphomas in many species. Retroviruses have enhanced our understanding of the role of c-Myc in such tumors. Leukemias and lymphomas induced by retroviruses activate c-Myc by: (1) use of virally specified proteins that increase c-Myc transcription, (2) transduction and modification of c-Myc to generate a virally encoded form of the gene, v-Myc, and (3) proviral integration in or near c-Myc. Proviral integrations elevate transcription by insertion of retroviral enhancers found in the long terminal repeat (LTR). Studies of the LTR enhancer elements from these retroviruses have revealed the importance of these elements for c-Mycactivation in several cell types. Retroviruses also have been used to identify genes that collaborate with c-Myc during development and progression of leukemias and lymphomas. In these experiments, animals that are transgenic for c-Mycoverexpression (often in combination with the overexpression or deletion of known proto-oncogenes) have been infected with retroviruses that then insertionally activate novel co-operating cellular genes. The retrovirus then acts as a molecular 'tag' for cloning of these genes. This review covers several aspects of c-Myc involvement in retrovirally induced leukemias and lymphomas.

Selection for c-myc integration sites in polyclonal T-cell lymphomas

Type B leukemogenic virus (TBLV) is highly related to mouse mammary tumor virus but induces rapidly appearing T-cell lymphomas in mice. Unlike other T-cell tumors induced by retroviruses, only 5 to 10% of TBLV-induced lymphomas have detectable viral integrations near c-myc by Southern blotting, whereas Northern blotting has shown that most tumors have two- to sixfold overexpression of c-myc RNA. In this report, PCR was used to demonstrate that at least 30% of these lymphomas have TBLV insertions near c-myc. Some tumors contained multiple TBLV proviruses in different locations and orientations, suggesting that the tumors are polyclonal. The integrated proviruses near c-myc had different numbers (two to four) of long terminal repeat (LTR) enhancer repeats, although LTRs with three-repeat enhancers dominated the proviral population. Passage of polyclonal tumors in immunocompetent mice and semiquantitative PCR revealed that only cells with particular integrations were selected for growth. In three of six tumors tested, proviruses containing four-repeat enhancers near c-myc were selected during tumor passage. Since tumor cell selection may be accomplished by overexpression of c-myc RNA due to proximity to the unique TBLV LTR enhancer, we inserted LTRs at various locations within a plasmid containing the entire c-myc locus and cellular flanking sequences. To quantitatively measure effects on transcription, the Renilla luciferase gene was substituted for most of c-myc exon 2, and transient transfections were performed with c-myc reporter constructs in two different T-cell lines. As expected, insertion of a TBLV LTR with three-repeat enhancers in either orientation, 5" and 3", of the myc gene elevated reporter activity from 2- to 160-fold, consistent with enhancer function, but four-repeat LTRs had lower levels of expression compared to three-repeat LTRs. Surprisingly, LTR insertions that gave maximal c-myc expression in transient-transfection assays declined in tumor cells selected for growth in vivo. Selection for clonal growth may occur in tumor cells that have modest c-myc overexpression after proviral insertion to prevent apoptosis.

Combined histologic and molecular features reveal previously unappreciated subsets of lymphoma in AKXD recombinant inbred mice

Hematopoietic neoplasms developing in AKXD recombinant inbred, NFS.V(+) and ICSBP knockout mice were assessed using morphologic, cytologic and molecular criteria that relate these disorders to human lymphoma and leukemia. Lymphoma types included precursor T-cell and B-cell lymphoblastic, small lymphocytic, splenic marginal zone, follicular, and diffuse large cell (DLCL). In addition to previously defined subtypes of DLCL composed of centroblasts or immunoblasts, two additional subtypes are defined here: lymphoblastic lymphoma like (LL) and lymphoma characterized by a histiocytic reaction (HS). DLCL(HS) were distinguished from true histiocytic lymphomas by the presence of clonal Ig gene rearrangements.

In vitro infection of ovine cell lines by Jaagsiekte sheep retrovirus

Sheep pulmonary adenomatosis (SPA), also known as jaagsiekte or ovine pulmonary carcinoma, is a contagious lung cancer of sheep, originating from type II pneumocytes and Clara cells. Previous studies have implicated a type D retrovirus (jaagsiekte sheep retrovirus [JSRV]) as the causative agent of SPA. We recently isolated a proviral clone of JSRV from an animal with a spontaneous case of SPA (JSRV(21)) and showed that it harbors an infectious and oncogenic virus. This demonstrated that JSRV is necessary and sufficient to induce SPA. A major impediment in research on JSRV has been the lack of an in vitro tissue culture system for the virus. The experiments reported here show the first successful in vitro infection with this virus, using the JSRV(21) clone. JSRV(21) virus was obtained by transiently transfecting human 293T cells with a plasmid containing the JSRV(21) provirus driven by the human cytomegalovirus immediate-early promoter. Virus produced in this manner exhibited reverse transcriptase (RT) activity that banded at 1.15 g/ml in sucrose density gradients. Infection of concentrated JSRV(21) into ovine choroid plexus (CP), testes (OAT-T3), turbinate (FLT), and intestinal carcinoma (ST6) cell lines resulted in establishment of infection as measured by PCR amplification. Evidence that this reflected genuine infection included the fact that heat inactivation of the virus eliminated it, the levels of viral DNA increased with passage of the infected cells, and the infected cells released active RT as measured by the sensitive product enhancement RT assay. The RT activity released from the infected cells banded at 1.15 g/ml, and JSRV(21) provirus was transmitted from infected cells to uninfected ones by cocultivation. However, the amount of virus released from infected cells was low. These results suggest that the JSRV receptor is present on many ovine cell types and that the observed restriction of JSRV expression in vivo to tumor cells might be controlled by factors other than the viral receptor. Finally we tagged the U3 of pJSRV(21) with the bacterial supF gene, an amber suppressor tRNA gene. The resulting clone, termed pJSRV(supF), is infectious in vitro. It may be a useful tool for future studies on viral DNA integration, since the normal sheep genome contains 15 to 20 copies of highly JSRV-related endogenous sequences that cross-react with many JSRV hybridization probes.

Identification of a common site of provirus integration in radiation leukemia virus-induced T-cell lymphomas in mice

The BL/VL(3) Kaplan radiation leukemia virus (RadLV-VL(3)) is a nondefective retrovirus that induces T cell lymphomas in several strains of mice. By using DNA probes derived from RadLV/VL(3) provirus-flanking sequences cloned from the BL/VL(3) cell line, we identified a DNA region rearranged in 5 of 19 tumors analysed (25%). All proviruses were integrated in the same 5'-to-3' orientation in a small DNA region called Kis1 (Kaplan integration site 1). This region was localized on distal mouse chromosome 2 in a region not previously identified as important to lymphomagenesis. The cells rearranged at the Kis1 locus represent a clonal subpopulation of the clonal tumor masses examined, indicating a probable role of Kis1 in tumor progression.

Sequence-specific and/or stereospecific constraints of the U3 enhancer elements of MCF 247-W are important for pathogenicity

The oncogenic potential of many nonacute retroviruses is dependent on the duplication of the enhancer sequences present in the unique 3' (U3) region of the long terminal repeat (LTR). In a molecular clone (MCF 247-W) of the murine leukemia virus MCF 247, a leukemogenic mink cell focus-inducing (MCF) virus, the U3 enhancer sequences are tandemly repeated in the LTR. We mutated the enhancer region of MCF 247-W to test the hypothesis that the duplicated enhancer sequences of this virus have a sequence-specific and/or a stereospecific role in enhancer function required for transformation. In one virus, we inserted 14 nucleotide bp into the novel sequence generated at the junction of the two enhancers to generate an MCF virus with an interrupted enhancer region. In the second virus, only one copy of the enhancer sequences was present. This second virus also lacked the junction sequence present between the two enhancers of MCF 247-W. Both viruses were less leukemogenic and had a longer mean latency period than MCF 247-W. These data indicate that the sequence generated at the junction of the two enhancers and/or the stereospecific arrangement of the two enhancer elements are required for the full oncogenic potential of MCF 247-W. We analyzed proviral LTRs within the c-myc locus in tumor DNAs from mice injected with the MCF virus with the interrupted enhancer region. Some of the proviral LTRs integrated upstream of c-myc contain enhancer regions that are larger than those of the injected virus. These results are consistent with the suggestion that the virus with an interrupted enhancer changes in vivo to perform its role in the transformation of T cells.

The feline leukemia virus long terminal repeat contains a potent genetic determinant of T-cell lymphomagenicity

Feline leukemia virus (FeLV) is an important pathogen of domestic cats. The most common type of malignancy associated with FeLV is T-cell lymphoma. SL3-3 (SL3) is a potent T-cell lymphomagenic murine leukemia virus. Transcriptional enhancer sequences within the long terminal repeats (LTRs) of SL3 and other murine retroviruses are crucial genetic determinants of the pathogenicities of these viruses. The LTR enhancer sequences of FeLV contain identical binding sites for some of the transcription factors that are known to affect the lymphomagenicity of SL3. To test whether the FeLV LTR contains a genetic determinant of lymphomagenicity, a recombinant virus that contained the U3 region of a naturally occurring FeLV isolate, LC-FeLV, linked to the remainder of the genome of SL3 was generated. When inoculated into mice, the recombinant virus induced T-cell lymphomas nearly as quickly as SL3. Moreover, the U3 sequences of LC-FeLV were found to have about half as much transcriptional activity in T lymphocytes as the corresponding sequences of SL3. This level of activity was severalfold higher than that of the LTR of weakly leukemogenic Akv virus. Thus, the FeLV LTR contains a potent genetic determinant of T-cell lymphomagenicity. Presumably, it is adapted to be recognized by transcription factors present in T cells of cats, and this yields a relatively high level of transcription that allows the enhancer to drive the requisite steps in the process of lymphomagenesis.

Importance of a c-Myb binding site for lymphomagenesis by the retrovirus SL3-3

All murine leukemia viruses (MuLVs) and related type C retroviruses contain a highly conserved binding site for the Ets family of transcription factors within the enhancer sequences in the viral long terminal repeats (LTRs). The T-cell lymphomagenic MuLV SL3-3 (SL3-3) also contains a c-Myb binding site adjacent to the Ets site. The presence of this Myb site distinguishes SL3 from most other MuLVs. We tested the importance of these two sites for the lymphomagenicity of SL3-3. Mutation of the Ets site had little effect on viral pathogenicity, as it only slightly extended the latency period to disease onset. In contrast, mutation of the Myb site strongly inhibited pathogenicity, as only a minority of the inoculated mice developed tumors in the two mouse strains that were tested. All tumors that were induced by either mutant appeared to be lymphomas, and no evidence for reversion of either mutation was detected. The effects of the Ets and Myb site mutations on transcriptional activity of the SL3 LTR were tested by inserting the viral enhancer sequences into a plasmid containing the promoter region of the c-myc gene linked to a reporter gene. Mutation the Myb site almost eliminated enhancer activity in T lymphocytes, while mutation of the Ets site had smaller effects. Thus, the effects of the enhancer mutations on transcriptional activity in T cells paralleled their effects on viral lymphomagenicity. The absence of the c-Myb site in the LTR enhancer of the weakly lymphomagenic MuLV, Akv, likely contributes to the low pathogenicity of this virus relative to SL3-3. However, Moloney MuLV also lacks the Myb site in its LTR, although it induces T-cell lymphomas with a potency similar to that of SL3-3. Thus, it appears that SL3-3 and Moloney MuLV evolved genetic determinants of T-cell lymphomagenicity that are, at least in part, distinct.

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

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

Sequence tags of provirus integration sites in DNAs of tumors induced by the murine retrovirus SL3-3

The murine retrovirus SL3-3 is a potent inducer of T-cell lymphomas when inoculated into susceptible newborn mice. The proviral integration site sequences were surveyed in tumor DNAs by a simple two-step PCR method. From 20 SL3-3-induced tumors a total of 39 provirus-host junctions were amplified and sequenced. Seven showed homology to known sequences. These included the known common integration site c-myc as well as genes not previously identified as targets of provirus integration, namely N-ras and the genes coding for major histocompatibility complex class 11 E-beta, protein kinase C-eta, and T-cell receptor beta-chain. Among these genes, the integrations in c-myc as well as the one in N-ras were found to be clonal. One of the remaining 32 proviral integration site sequences that show no similarities to known sequences may represent a common integration site, as 2 of the 20 tumors demonstrated clonal provirus insertion into this region.

A multistep process of leukemogenesis in Moloney murine leukemia virus-infected mice that is modulated by retroviral pseudotyping and interference

Mixed retroviral infections frequently exhibit pseudotyping, in which the genome of one virus is packaged in a virion containing SU proteins encoded by another virus. Infection of mice by Moloney murine leukemia virus (M-MuLV), which induces lymphocytic leukemia, results in a mixed viral infection composed of the inoculated ecotropic M-MuLV and polytropic MuLVs generated by recombination of M-MuLV with endogenous retroviral sequences. In this report, we describe pseudotyping which occurred among the polytropic and ecotropic MuLVs in M-MuLV-infected mice. Infectious center assays of polytropic MuLVs released from splenocytes or thymocytes of infected mice revealed that polytropic MuLVs were extensively pseudotyped within ecotropic virions. Late in the preleukemic stage, a dramatic change in the extent of pseudotyping occurred in thymuses. Starting at about 5 weeks, there was an abrupt increase in the number of thymocytes that released nonpseudotyped polytropic viruses. A parallel increase in thymocytes that released ecotropic M-MuLV packaged within polytropic virions was also observed. Analyses of the clonality of preleukemic thymuses and thymomas suggested that the change in pseudotyping characteristics was not the result of the emergence of tumor cells. Examination of mice infected with M-MuLV, Friend erythroleukemia virus, and a Friend erythroleukemia virus-M-MuLV chimeric virus suggested that the appearance of polytropic virions late in the preleukemic stage correlated with the induction of lymphocytic leukemia. We discuss different ways in which pseudotypic mixing may facilitate leukemogenesis, including a model in which the kinetics of thymic infection, modulated by pseudotyping and viral interference, facilitates a stepwise mechanism of leukemogenesis.

Long terminal repeat enhancer core sequences in proviruses adjacent to c-myc in T-cell lymphomas induced by a murine retrovirus

The transcriptional enhancer in the long terminal repeat (LTR) of the T-lymphomagenic retrovirus SL3-3 differs from that of the nonleukemogenic virus Akv at several sites, including a single base pair difference in an element termed the enhancer core. Mutation of this T-A base pair to the C-G C-G sequence found in Akv significantly attenuated the leukemogenicity of SL3-3. Thus, this difference is important for viral leukemogenicity. Since Akv is an endogenous virus, this suggests that the C-G in its core is an adaptation to being minimally pathogenic. Most tumors that occurred in mice inoculated with the mutant virus, called SAA, contained proviruses with reversion or potential suppressor mutations in the enhancer core. We also found that the 72-bp tandem repeats constituting the viral enhancer could vary in number. Most tumors contained mixtures of proviruses with various numbers of 72-bp units, usually between one and four. Variation in repeat number was most likely due to recombination events involving template misalignment during viral replication. Thus, two processes during viral replication, misincorporation and recombination, combined to alter LTR enhancer structure and generate more pathogenic variants from the mutant virus. In SAA-induced tumors, enhancers of proviruses adjacent to c-myc had the largest number of core reversion or suppressor mutations of all of the viral enhancers in those tumors. This observation was consistent with the hypothesis that one function of the LTR enhancers in leukemogenesis is to activate proto-oncogenes such as c-myc.

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.

Susceptibility of AKXD recombinant inbred mouse strains to lymphomas

We analyzed the susceptibility of 10 AKXD recombinant inbred (RI) mouse strains to lymphomas. These strains were derived from crosses of AKR/J, a highly lymphomatous strain, and DBA/2J, a weakly lymphomatous strain. Of the 10 strains analyzed, nine showed a high incidence of lymphoma development. As with the other 13 AKXD strains analyzed previously (M. L. Mucenski, B. A. Taylor, N. A. Jenkins, and N. G. Copeland, Mol. Cell. Biol. 6:4236-4243, 1986), the mean age at onset of lymphomas and lymphoma types varied among the strains. Whereas some strains were susceptible to T-cell lymphomas, as was the AKR/J parent, other strains were susceptible to B-cell lymphomas or to a combination of T- and B-cell lymphomas. Somatic mink cell focus-forming proviruses appeared causally associated with T-cell lymphomas, whereas somatic ecotropic proviruses appeared causally associated with B-cell lymphomas. Mice with T-cell lymphomas died significantly earlier than mice with other lymphoma types (stem, pre-B, or B cell and myeloid). The numbers of effective loci influencing the mean age at onset of lymphomas, the presence or absence of mink cell focus-forming viruses in tumors, and the frequency of T-cell lymphomas were estimated to be 3.9, 1.8, and 2.7, respectively. Tests of association with marker loci already typed in the AKXD RI strains suggested that two loci, Rmcf and Pmv-25 (or a locus linked to Pmv-25), influence all three trait variables. Finally, D21S16h, a marker locus on distal chromosome 16, showed 50% probability of linkage to a locus that influences the mean age at onset of lymphomas. Additional studies in combination with classical genetic crosses should be helpful in confirming these linkages and in identifying other loci influencing tumor susceptibility in AKXD RI strains.

Retrovirus-induced lymphoproliferation as a model for developing diagnostic criteria for malignant lymphoma in mice

Several methods for evaluating lymphoproliferative lesions in mice were compared. The model systems included spontaneous lymphomas arising in CWD mice and NFS mice congenic for ecotropic murine leukemia virus (MuLV) induction loci and a series of transplants in mice with severe combined immunodeficiency disease mutation of cells derived from mice infected with LP-BM5 MuLV. Primary lymphomas and donor tissues and transplants were examined using histopathology, flow cytometry, and Southern blot analysis of DNA for rearrangements of immunoglobulin and T-cell receptor genes and for viral integrations. The use of flow cytometric analysis, to establish cell lineage and define population size, and DNA analysis, for cell lineage and clonality determination, allowed the identification of malignant lymphoproliferations. Histologic evaluation did not define clonal populations of particular lineage but did provide other indications of malignancy such as invasiveness and presence of a dominant morphologic cell type. Thus, the precision of diagnosis of mouse lymphomas can be considerably enhanced by augmenting histopathologic examination with antigenic and molecular characterizations that can define malignant populations.

Oncogenicity of AKR mink cell focus-inducing murine leukemia virus correlates with induction of chronic phosphatidylinositol signal transduction

Naturally occurring recombinant murine leukemia viruses (MuLVs), termed mink cell focus-inducing (MCF) viruses, are the proximal leukemogens in spontaneous thymic lymphomas of AKR mice. The mechanism by which these viruses transform lymphocytes is not clear. Previous studies have implicated either integrational activation of proto-oncogenes, chronic autocrine immune stimulation, and/or autocrine stimulation of growth factor receptors (e.g., interleukin 2 receptors) via binding of the viral env glycoprotein (gp70) to these receptors. Any one of these events could also involve activation of second messenger signaling pathways in the cell. We examined whether infection with oncogenic AKR-247 MCF MuLV induced transmembrane signaling cascades in thymocytes of AKR mice. Cyclic AMP levels were not changed, but there was enhanced turnover of phosphatidylinositol phosphates, with concomitant increases in diacyglycerol and inositol 1,4,5-triphosphate. Thus, phospholipase C activity was increased. Protein kinase C activity was also elevated in comparison to that in uninfected thymocytes. The above events occurred in parallel with MCF expression in the thymus and were chronically maintained thereafter. No changes in phospholipid turnover occurred in an organ which did not replicate the MCF virus (spleen) or in thymocytes of AKR mice infected with a thymotropic, nononcogenic MCF virus (AKV-1-C36). Therefore, only the oncogenic MCF virus induced phosphatidylinositol signal transduction. Flow cytometric comparison of cell surface gp70 revealed that AKR-247 MCF virus-infected thymocytes expressed more MCF virus gp70 than did thymocytes from AKV-1-C36 MCF virus-infected mice, suggesting that certain threshold quantities of MCF virus env glycoproteins may be involved in this signaling. This type of signal transduction is not induced by stimulation of the interleukin 2 receptor but is involved in certain oncogene systems (e.g., ras and fms). Its chronic induction by oncogenic MCF MuLV may thus initiate thymocyte transformation.

Insertional mutagenesis of flvi-2 in tumors induced by infection with LC-FeLV, a myc-containing strain of feline leukemia virus

LC-FeLV is a myc-containing strain of feline leukemia virus (FeLV) which exhibits only partial transforming activity in vitro and in vivo. LC-FeLV infection in kittens may induce, but does not necessarily induce, thymic lymphosarcoma in viremic animals after a short latency. These observations suggest that infection with LC-FeLV is not sufficient to induce complete transformation and that another genetic event(s) is required. One possibility for such an event is that the integrating provirus acts as an insertional mutagen and thereby disrupts the structure or function of another proto-oncogene. Using a strategy of transposon tagging, this possibility was examined in eight feline T-cell lymphosarcomas, including four induced by experimental infection with LC-FeLV, three induced by natural infection with FeLV, and one FeLV-negative tumor. The analysis demonstrated one locus, termed flvi-2, to be structurally altered in six of the tumors examined, including three induced by LC-FeLV and three in which no activated myc oncogene is apparent. Inverse polymerase chain reaction was used to demonstrate the presence and transcriptional orientation of proviruses integrated at flvi-2 in five of these tumors. The flvi-2 locus does not hybridize to cloned probes representing 21 previously identified proto-oncogenes or common domains of retroviral integration. Thus, the data suggest that interruption of the flvi-2 locus cooperates with the myc oncogene in the induction of T-cell lymphomas by LC-FeLV; indeed, the observations indicate that the insertional mutagenesis of flvi-2 plays a role in T-cell lymphomagenesis even in the absence of feline v-myc.

Influence of murine leukemia proviral integrations on development of N-methyl-N-nitrosourea-induced thymic lymphomas in AKR mice

The AKR mouse strain is characterized by a high incidence of spontaneous thymic lymphoma that appears in older animals (greater than 6 months of age) and is associated with novel provirus integrations of ecotropic and recombinant murine leukemia viruses (MuLVs). Treatment of 4- to 6-week-old AKR/J mice with the carcinogen N-methyl-N-nitrosourea (MNU) results in thymic lymphomas that arise as early as 3 to 4 months of age and contain novel somatically acquired MuLV provirus integrations. The AKR/J strain develops MNU-induced lymphoma with a higher incidence and shorter latency than has been observed for other inbred mouse strains. To determine whether provirus integrations of endogenous MuLV account for the enhanced susceptibility of the AKR strain, the incidence and latency of MNU-induced lymphoma development was compared in AKR/J and AKR.Fv-1b mice. The restrictive b allele of the Fv-1 locus restricts integration and replication of endogenous N-tropic MuLV; therefore, AKR-Fv-1b mice have a very low incidence of spontaneous lymphoma. In contrast, AKR.Fv-1b mice develop MNU-induced lymphomas with an incidence and latency similar to those of the AKR/J strain. Furthermore, thymic lymphomas from both strains express an immature CD4-8+ phenotype, indicating neoplastic transformation of the same thymocyte subset. Southern blot analysis confirmed that lymphoma DNA from AKR.Fv-1b mice did not contain somatically acquired provirus integrations. These results demonstrate that provirus integration does not contribute to the predisposition of AKR mice to develop a high incidence of early MNU-induced lymphomas. Nevertheless, MNU treatment stimulated high-level expression of infectious ecotropic MuLV in AKR.Fv-1b as well as in AKR/J mice, suggesting that viral gene products might enhance lymphoma progression.

Bone marrow depletion by 89Sr complements a preleukemic defect in a long terminal repeat variant of Moloney murine leukemia virus

We previously described a preleukemic state induced by Moloney murine leukemia virus (Mo-MuLV) characterized by hematopoietic hyperplasia in the spleen. Further experiments suggested that splenic hyperplasia results from inhibitory effects in the bone marrow, leading to compensatory extramedullary hematopoiesis. An enhancer variant of Mo-MuLV, Mo + PyF101 Mo-MuLV, fails to induce preleukemic hyperplasia and has greatly reduced leukemogenicity, indicating the importance of this state to efficient leukemogenesis. An alternative method for induction of preleukemic hyperplasia was sought. Treatment of mice with 89Sr causes specific ablation of bone marrow hematopoiesis and compensatory extramedullary hematopoiesis in spleen and nodes. NIH Swiss mice were inoculated neonatally with Mo + PyF101 Mo-MuLV and treated with 89Sr at 6 weeks of age. Approximately 85% developed lymphoid leukemia with a time course resembling that caused by wild-type Mo-MuLV. In contrast, very few animals treated with Mo + PyF101 Mo-MuLV or 89Sr alone developed disease. In approximately one-third of cases, the Mo + PyF101 Mo-MuLV proviruses were found at common sites for wild-type Mo-MuLV-induced tumors (c-myc, pvt-1, and pim-1), indicating that this virus is capable of performing insertional activation in T-lymphoid cells. These results support the proposal that splenic hyperplasia results from inhibitory effects in the bone marrow. They also indicate that Mo + PyF101 Mo-MuLV is blocked in early and not late events in leukemogenesis.

Provirus integration at the 3' region of N-myc in cell lines established from thymic lymphomas spontaneously formed in AKR mice and a [(BALB/c x B6)F1----AKR] bone marrow chimera

Among 18 thymic leukemia cell lines which have been established from spontaneous thymic lymphomas in AKR mice as well as in bone marrow chimeras which were constructed by transplanting allogeneic bone marrow cells into irradiated AKR mice, three proviral integration sites were identified; near c-myc, N-myc and pim-1 loci. No integration site specific for chimeric leukemia cell lines was found. In three thymic leukemia cell lines which contained rearranged N-myc genes, insertions of long terminal repeats (LTRs) of murine leukemia viruses were detected at 18 or 20 bp downstream of the translational termination codon. These results demonstrate that the 3' region of the N-myc gene is one of the integration targets for murine leukemia viruses in spontaneous thymic lymphomas. In these three cell lines, N-myc mRNA was stably transcribed and transcription of c-myc mRNA was down-regulated. The integrated murine leukemia viruses in AKR thymic leukemia were most likely AKV, though the DNA sequence of the LTR inserted in the genome of a leukemic cell line from [(BALB/c x B6)F1----AKR], CAK20, was different from LTRs of murine leukemia viruses so far reported.

Insertional mutagenesis: neoplasia arising from retroviral integration

The integration of retroviral proviruses near cellular genes can profoundly affect their expression. Painstaking analysis of insertion sites from a large number of tumors has revealed a number of previously unknown proto-oncogenes, and has elucidated new mechanisms whereby known proto-oncogenes can be activated. A number of these genes have been implicated in tumors of clinical relevance. At the time of writing a great deal remains to be learned of the normal function of these genes in the cell. While it has yet to be demonstrated that retroviral insertion mechanisms play some role in naturally occurring human neoplasms, they must be considered in the context of retroviral gene therapy protocols now being contemplated.

Evi-2, a common integration site involved in murine myeloid leukemogenesis

BXH-2 mice have the highest incidence of spontaneous retrovirally induced myeloid leukemia of any known inbred strain and, as such, represent a valuable model system for identifying cellular proto-oncogenes involved in myeloid disease. Chronic murine leukemia viruses often induce disease by insertional activation or mutation of cellular proto-oncogenes. These loci are identified as common viral integration sites in tumor DNAs. Here we report on the characterization of a novel common viral integration site in BXH-2 myeloid leukemias, designated Evi-2. Within the cluster of viral integration sites that define Evi-2, we identified a gene that has the potential for encoding a novel protein of 223 amino acids. This putative proto-oncogene possesses all of the structural features of a transmembrane protein. Within the transmembrane domain is a "leucine zipper," suggesting that Evi-2 is involved in either homopolymer or heteropolymer formation, which may play an important role in the normal functioning of Evi-2. Interestingly, the human homolog of Evi-2 has recently been shown to be tightly linked to the von Recklinghausen neurofibromatosis locus, suggesting a role for Evi-2 in human disease as well.

Evi-2, a common integration site involved in murine myeloid leukemogenesis

BXH-2 mice have the highest incidence of spontaneous retrovirally induced myeloid leukemia of any known inbred strain and, as such, represent a valuable model system for identifying cellular proto-oncogenes involved in myeloid disease. Chronic murine leukemia viruses often induce disease by insertional activation or mutation of cellular proto-oncogenes. These loci are identified as common viral integration sites in tumor DNAs. Here we report on the characterization of a novel common viral integration site in BXH-2 myeloid leukemias, designated Evi-2. Within the cluster of viral integration sites that define Evi-2, we identified a gene that has the potential for encoding a novel protein of 223 amino acids. This putative proto-oncogene possesses all of the structural features of a transmembrane protein. Within the transmembrane domain is a "leucine zipper," suggesting that Evi-2 is involved in either homopolymer or heteropolymer formation, which may play an important role in the normal functioning of Evi-2. Interestingly, the human homolog of Evi-2 has recently been shown to be tightly linked to the von Recklinghausen neurofibromatosis locus, suggesting a role for Evi-2 in human disease as well.

flvi-1, a common integration domain of feline leukemia virus in naturally occurring lymphomas of a particular type

A locus in feline DNA, termed flvi-1, which may play an important role in the natural induction of lymphomas by feline leukemia virus (FeLV) was identified. Examination of a bank of 21 naturally occurring FeLV-positive feline lymphomas revealed that FeLV proviral integration occurs at flvi-1 in four independent tumors (19%). Independent integrations occurred within a 2.4-kilobase region of flvi-1, the probability of which by random chance can be estimated as 10(-16). Several lines of evidence, including sequence analysis of the long terminal repeat, demonstrated that proviruses integrated at flvi-1 are exogenously acquired and are oriented in the same transcriptional direction with respect to the locus. Molecularly cloned flvi-1 did not hybridize with probes representing several previously described proviral integration domains or with probes representing 10 oncogenes. The natural feline lymphomas examined in this study were heterogeneous with respect to tissue of origin, cell type, and number of monoclonal proviral integrations. The four tumors in which flvi-1 is interrupted were classified as members of a phenotypic subgroup containing seven lymphomas, i.e., at least four (57%) of seven lymphomas of this type contained FeLV proviral integration at flvi-1. Members of this phenotypic subgroup are non-T-cell lymphomas isolated from the spleen and contain an average of three proviruses, compared with an average of eight among all of the tumors examined. The small number of proviral integrations in tumors of this subgroup suggests that an early proviral integration event into flvi-1 can induce malignant change.

Mutation of the core or adjacent LVb elements of the Moloney murine leukemia virus enhancer alters disease specificity

Transcriptional enhancers of replication-competent mouse C-type retroviruses are potent determinants of the distinct disease-inducing phenotypes of different viral isolates and can also strongly influence the incidence and latent period of disease induction. To study the contribution of individual protein-binding sites to viral pathogenicity, we introduced mutations into each of the known nuclear factor-binding sites in the enhancer region of the Moloney murine leukemia virus and injected viruses with these mutations into newborn NFS mice. All viruses induced disease. Viruses with mutations in both copies of the leukemia virus factor a (LVa) site, leukemia virus factor c (LVc) site, or in just the promoter proximal copy of the glucocorticoid response element (GRE) had a latent period of disease onset and disease specificity indistinguishable from that of the wild-type Moloney virus. Viruses with mutations in two or three of the GREs, in both copies of the leukemia virus factor b (LVb) site, in two of the four nuclear factor 1 (NF1) consensus motifs, or in both copies of the conserved viral core element showed a significant delay in latent period of disease induction. Strikingly, viruses with mutations in the core element induced primarily erythroleukemias, and mutations in the LVb site also resulted in a significant incidence of erythroleukemias. These and other genetic and biochemical studies suggest models for how subtle alterations in the highly conserved structure of mouse C-type retrovirus enhancers can produce a dramatic effect on disease specificity.

Dissociation of cellular proliferation and c-myc expression by buttercup extract

Buttercup extract (BE), an extract of the buttercup plant (Zanthoriza simplicissima), inhibits RNA and DNA synthesis by HL-60 promyelocytic leukemia cells. Exposure of these cells to 3% BE for 48 hours results in dramatic inhibition of RNA synthesis without loss of cell viability. The effect of BE is partially reversible over 12-24 hours with the level of RNA synthesis returning nearly to control levels during this time period. DNA synthesis is also reversibly inhibited by exposure to BE. Despite the inhibition of RNA synthesis in HL-60 cells, there is no decrease in the level of c-myc mRNA, even at high BE concentrations. The level of gene-specific mRNA for the c-Ha-ras, c-fms, and c-mos genes in these cells also remained constant during exposure to BE. Ribosomal RNA is not degraded during 24 hours of BE treatment in vitro, suggesting that BE does not maintain the relative mRNA level for these genes by selective degradation of other RNA species. The inhibition of RNA and DNA synthesis by BE without a corresponding alteration in the level of expression of the c-myc gene suggests that this agent dissociates c-myc expression and cellular proliferation in these cells.

Suppressive effects of the bacterial immunostimulant OK-432 on the incidence of spontaneous thymic lymphoma in AKR mice

The mean survival age of female AKR/J mice was significantly prolonged, the enlargement of thymus was markedly suppressed, and the proliferation of ecotropic and recombinant murine leukemia viruses was inhibited when 2-month-old female AKR/J mice were injected intraperitoneally with attenuated Streptococcus pyogenes, strain Su (OK-432) twice weekly for 8 weeks. However, these effects of OK-432 in 2-month-old female AKR/J mice were not seen in 5-month-old female AKR/J mice. The difference in the effectiveness of OK-432 in these animals probably depends on the difference in the degree of the proliferation of ecotropic and recombinant murine leukemia viruses in the thymus which consequently lead to thymic lymphoma.

Relationship between tumorigenicity and the dosage of lymphoma- vs. normal-parent-derived chromosome 15 in somatic cell hybrids between lymphoma cells with rearranged pvt-1 gene and normal cells

Somatic cell hybrids were generated between YACUT, a doubly drug-resistant subline of YAC-1 (a Moloney-virus-induced T-cell lymphoma of strain A/Sn origin with 2 proviral insertions near the pvt-1 locus) and normal diploid fibroblasts of CBAT6T6 origin. Three independent fusions were performed. Three uncloned hybrid cultures and 9 independently-derived clones were tested for tumorigenicity by the inoculation of graded cell numbers into syngeneic hosts. One of 3 uncloned hybrid cultures and 3 of 9 clones were weakly tumorigenic (take incidence 0%), and 1 of 3 uncloned hybrid cultures and 6 clones were highly tumorigenic (take incidence greater than 80%). One weakly tumorigenic hybrid and 3 weakly tumorigenic clones carried 3 copies of the tumor-derived chromosome 15 and 2 copies of the normal fibroblast-derived t(14;15) chromosomes. In contrast, 2 highly malignant hybrid clones lost one copy of the normal-fibroblast-derived t(14;15), but contained increased numbers (3.44-4.44) of the tumor-derived chromosome 15. Four tumorigenic segregants selected from the weakly tumorigenic fibroblast hybrids by in vivo inoculation showed the same cytogenetic change as the highly tumorigenic hybrid clones, in that the ratio of the normal:tumor-derived chromosomes 15 changed from 1.18-1.55 to 4.11-5.71. Tumorigenicity was thus associated with a modified balance between the tumor vs. the normal-parent-derived 15-chromosomes. Instead of the usual 3:2 ratio, the tumor-derived 15-chromosomes increased disproportionately, whereas the relative number of the normal-parent-derived 15-chromosome decreased, as a rule. These results suggest that amplification of the lymphoma-derived chromosome 15 favors tumorigenicity, but that this effect is counteracted by some influence emanating from the normal-parent-derived homologous chromosome.

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.

Retrovirally induced murine B-cell tumors rarely show proviral integration in sites common in T-cell tumors

The molecular etiology of retrovirally induced T-cell tumors has been shown in many cases to involve proviral integration near a cellular oncogene, c-myc, N-myc, Pim-1 and pvt-1 being frequent targets for insertional activation. Murine B-cell tumors induced by infection with murine leukemia virus have been studied for rearrangements in these and other loci. In contrast to the T-cell lymphomas, tumors of the B-cell lineage, either early B-cell tumors induced in nude mice or late B-cell tumors in immunocompetent mice, did not show disruption of N-myc or Pim-1 in any of the tumors studied, although those lymphomas had acquired many new proviruses. The loci c-abl, bcl-2, fis-1, c-erbB, c-myb, and neu were likewise not involved. Rearrangement of c-myc was seen in 1 out of 71 and rearrangement of the pvt-1 locus in 4 out of 73 (5%) of the B-cell tumors. Thus it appears that mechanistic differences exist in the development of T-cell tumors and B-cell tumors caused by the same etiological agent.

Establishment and characterization of mouse leukemia cell lines L615K and L7212K derived from transplantable murine leukemias maintained in China

Two culture cell lines L615K and L7212K were established from transplanted murine leukemias L615 and L7212, which had been established and maintained in China for years. Based on morphological, immunological and gene rearrangement analyses, L7212K cells are considered to be of T-cell origin while L615K cells might be immature T-cells. Immunofluorescence assays of viable leukemia cells and fluorescence focus assays of their culture supernate for infectious viruses suggested that recombinant mink cell focus-inducing viruses were significantly involved in both leukemic cell lines. Chromosome analysis of the L615K cells revealed a translocation t(12;17) which probably involved the c-fos locus on chromosome 12, since the DNA rearrangement of c-fos was demonstrated by Southern blot analysis with Hind III, and c-fos has been assigned to this chromosome. Although the expression of this gene was not detected by RNA Northern blot analysis, c-myc was slightly expressed in both L615K and L7212K cells.

Induction of nephroblastoma by myeloblastosis-associated virus type 1: state of proviral DNAs in tumor cells

Myeloblastosis-associated virus type 1 (MAV1) derived from a molecular clone of infectious proviral DNA (B. Perbal, J. S. Lipsick, J. Svoboda, R. F. Silva, and M. A. Baluda, J. Virol. 56:240-244, 1985) was shown to specifically induce nephroblastoma in chickens and therefore belongs to the MAV-N class. We show that nephroblastomas are polyclonal tumors containing rearranged proviral genomes. Rearrangements occur preferentially in the gag-pol region of the MAV1 proviral genome, and similar rearrangements can be detected in well-developed independent tumors.

The myc family of nuclear proto-oncogenes

Several members of the myc family of proto-oncogenes have been described, and some (c-, N-, and L-myc) have been characterized in considerable detail. They are united by a common gene structure and nucleotide homologies that were used to identify some of them initially. Their protein products also have scattered regions of amino acid identity or homology. Although the cellular activities of the various proteins are unknown, some members may play a role in regulating cell growth and differentiation. They share the ability to cooperate with an activated ras gene and cotransform embryonic rodent cells. In naturally occurring tumors, the members of the myc family of oncogenes appear to be activated by genetic changes (proviral insertion, chromosomal translocation, and gene amplification) that augment or otherwise disrupt normally regulated expression. The members of this family of genes differ markedly in their tissue specificity and developmental regulation of expression. This may account in part for the frequent appearance of activated c-myc genes in a wide variety of neoplasms and the limited appearance of activated N- and L-myc genes in tumors of embryonic or neural origin. The c-myc gene may be activated in tumors by a variety of mechanisms, whereas N- and L-myc appear to be activated only by gene amplification. Regulation of expression of the different myc genes also appears to occur by different mechanisms. Finally, the products of the different genes differ in may regions of the protein, and this divergence probably reflects their specific and individual functions.