Recovery of myc-specific sequences by a partially transformation-defective mutant of avian myelocytomatosis virus, MC29, correlates with the restoration of transforming activity

An avian retrovirus expressing chicken pp59c-myc possesses weak transforming activity distinct from v-myc that may be modulated by adjacent normal cell neighbors

We demonstrate that EF168, an avian retrovirus that expresses the chicken pp59c-myc proto-oncogene, transforms quail embryo fibroblasts in vitro. An EF168-transformed quail clone, EF168-28, containing a single provirus, synthesizes several hundred copies of c-myc RNA and expresses elevated levels of the pp59c-myc gene product. The EF168 provirus in EF168-28 was isolated as a molecular clone, and the nucleotide sequence of its c-myc allele was confirmed as identical to that of exons 2 and 3 of the chicken c-myc proto-oncogene. Extended infection of quail embryo fibroblast cultures with EF168 induced a number of in vitro transformation-associated parameters similar to those elicited by the oncogenic v-myc-encoding retrovirus MC29, including alteration of cellular morphology, anchorage-independent growth, and induction of immortalized cell lines. Despite the fact that EF168 and MC29 shared these biological activities, further analysis revealed that EF168 initiated transformation in quail embryo fibroblasts, bone marrow, or adherent peripheral blood cultures 100- to 1,000-fold less efficiently than did MC29. Further, in contrast to MC29-induced foci, EF168 foci were smaller, morphologically diffuse, and less prominent. Analysis of newly infected cells demonstrated efficient expression of EF168 viral RNA in the absence of transformation. These differences suggest that while the pp59v-myc gene product can exert dominant transforming activity on quail embryo fibroblasts, its ability to initiate transformation is distinct from that of the pp110gag-v-myc gene product encoded by MC29 and may be suppressed by adjacent nontransformed cell neighbors.

Mutations in src homology regions 2 and 3 of activated chicken c-src that result in preferential transformation of mouse or chicken cells

src homology regions 2 and 3 (SH2 and SH3) of proteins encoded by src and closely related genes are conserved domains believed to modulate the protein-tyrosine kinase activity of this class of proteins, perhaps through interactions with other proteins. To explore the possibility of using src mutants as probes for such interactions, we have compared mouse NIH 3T3 cells with chicken embryo fibroblasts as host cells for 24 previously described substitution and deletion mutants with lesions in the SH2- and SH3-encoding regions of a transformation-competent allele of chicken c-src. Although several of these mutants are equally competent or equally defective for transformation of the two cell types, four mutants (three of which map within SH3) preferentially transform NIH 3T3 cells, and seven mutants (all of which map within SH2) preferentially transform chicken cells. Some of the SH2 mutants least able to transform mouse cells exhibit augmented transforming activity in chicken cells. In general, the in vitro protein-tyrosine kinase activities of the mutants correlated with transforming activities. Thus, in many cases, the catalytic activity of a mutant protein depended upon the host cell in which the protein was made. Such host-dependent mutants may be especially useful reagents for biochemical and genetic studies of the src gene family.

The role of myc oncogenes in cell growth and differentiation

The myc oncoproteins are expressed in a wide range of normal adult and embryonic tissues. They are also found to be over-expressed in a variety of tumor types. All myc proteins are short-lived nuclear phosphoproteins thought to act as regulatory components of cell proliferation. The rapid induction of c-myc mRNA and protein following the addition of growth factors to quiescent cells, together with the short half-life of these molecules, suggests that they are sensitive and continuous indicators of external stimuli, consistent with a role in signal transduction. Furthermore, in untransformed cells, c-myc protein expression is tightly regulated, at least in part, by a mechanism of autoregulation. Deregulated expression of myc genes is a frequent observation in tumors and may lead to a cell becoming independent of one or more growth factors, with the concomitant potential for uncontrolled proliferation. Although the precise functions of the myc proteins are unknown, they all bear the hallmarks of multimeric DNA-binding proteins probably involved in the regulation of expression of specific genes.

MC29 deletion mutants which fail to transform chicken macrophages are competent for transformation of quail macrophages

A number of MC29 mutants with deleted myc genes have been previously characterized. Many of these mutants have been found to be defective for transformation of chicken macrophages in vitro and for tumor induction in chickens. Such mutants are capable of transforming Japanese quail macrophages in vitro and inducing a high incidence of tumors in Japanese quail. Thus, Japanese quail may contain a factor(s) capable of complementing the defective transforming proteins encoded by some deleted v-myc genes.

Isolation of an MH2 retrovirus mutant temperature sensitive for macrophage but not fibroblast transformation

A conditional mutant of the MH2 avian retrovirus, termed ts41MH2, was isolated. Unlike wtMH2, ts41MH2 permitted transformed macrophages to differentiate during a 5- to 7-day temperature shift from 37 to 42 degrees C. Mutant-infected cells incubated at 42 degrees C exhibited a flattened morphology and then fused to form giant multinucleated cells that closely resembled normal macrophage maturation in vitro. These differentiated cells reacted strongly with a myeloid-macrophage-specific monoclonal antibody. The process of differentiation was inhibited when ts41MH2-transformed nonproducer clones were superinfected before the temperature shift with the myc gene-containing MC29 or OK10 viruses. By contrast, no inhibition was observed in clones superinfected with the MH2-PA200 virus that contains only the mil gene. The mutant also demonstrated a reduced oncogenic potential relative to that of wtMH2 when it was inoculated intravenously into young birds. However, in contrast to the results obtained with hematopoietic cells, none of the five fibroblast transformation parameters tested for ts41MH2 were altered from those induced by wtMH2. These results suggest that the mutation in ts41MH2 is located in a region of myc required for macrophage transformation, but not required for fibroblast transformation.

Analysis of a deleted MC29 provirus: gag sequences are not required for fibroblast transformation

Recovered avian myelocytomatosis virus HBI is an MC29-related virus that induces lymphoid tumors in chickens rather than the predominant neoplastic disease induced by wild-type MC29 (namely, endotheliomas). An analysis of the structure of the HBI provirus(es) in the tumors demonstrated that the provirus(es) could be either full size or deleted. One tumor was found to be clonal in that it contained a single provirus which had been partially deleted; this raised a question concerning the role of this provirus in the maintenance of tumor growth. To characterize the detailed structure of this provirus and determine its biological activity, it was molecularly cloned from tumor DNA. Sequencing confirmed that the provirus contained a deletion which effectively removed the whole gag gene. However, the provirus was shown to encode a myc-specific protein, presumably initiating from within the myc gene, and to be biologically active when it was transfected onto quail embryo fibroblasts. Our results suggest that myc alone is sufficient to transform quail embryo fibroblasts and to maintain tumor growth in vivo.

Nucleotide sequence of HBI, a novel recombinant MC29 derivative with altered pathogenic properties

HBI is a recombinant avian retrovirus with novel pathogenic properties that was derived from the myc-containing virus MC29. In contrast to MC29, which causes endotheliomas in chickens, HBI induces lymphoid tumors. The results of molecular cloning and nucleotide sequencing of HBI reported here show that the virus contains sequences derived from both c-myc and ring-neck pheasant virus, in addition to MC29. The 3' half of the myc gene was largely replaced by c-myc sequences, and most of the long terminal repeat and gag regions were replaced by ring-neck pheasant virus sequences. The long terminal repeat contained a triplicate sequence which was homologous to the core enhancer sequence of the simian virus 40 72-base-pair repeat. The significance of these changes in relation to the unusual biological properties of the virus are discussed.

Molecular analysis of myc gene mutants

We describe the generation and characterization of a series of deletion mutants of the avian acute leukaemia virus MC29 which allow the study of the function of the myc in transformation of quail embryo fibroblasts in vitro and tumour induction in vivo. These mutants, which are deleted in the 3' portion of the myc gene, fail to transform macrophages in vitro or induce tumours in vivo but are still able to transform morphologically fibroblasts. From one of these mutants a 'recovered' MC29 virus was generated which, like wild type MC29, transformed fibroblasts and macrophages in vitro. When tested in vivo this virus induced lymphomas of T and B cells rather that the endotheliomas induced by wild type MC29. This system allows us to investigate another question which is the mechanism by which the virus (or oncogene it contains) preferentially transforms one cell type.

myc and src oncogenes have complementary effects on cell proliferation and expression of specific extracellular matrix components in definitive chondroblasts

The effects of the avian viral oncogenes src and myc were compared for their ability to alter the differentiated phenotype and the proliferative capacity of definitive chondroblasts. As previously demonstrated, viruses carrying the src oncogene suppressed the synthesis of the chondroblast-specific products, type II collagen and cartilage-specific sulfated proteoglycan. In contrast, infection with MC29 and HB1 viruses, which carry the myc oncogene, did not suppress the synthesis of these normal differentiated cell products, but the infected cells exhibited an increased proliferative potential. The MH2 virus, which carries both the myc and mil oncogenes, both induced the suppression of these chondroblast-specific products and increased cell proliferation. The implications of these results for cooperation between oncogenes and the multi-oncogene models for neoplastic transformation are discussed.

Homologous recombination between a defective virus and a chromosomal sequence in mammalian cells

Replacement of the early region of simian virus 40 results in virus that cannot replicate in a normal host, CV-1 cells, but can replicate in COS cells, a derivative of CV-1 cells that constitutively express simian virus 40 tumor antigen (T antigen). However, passage of such an early replacement simian virus 40 mutant in COS cells results in the emergence of virus that can propagate in CV-1 cells. Analysis of this virus revealed that the mutant rescued the integrated T-antigen gene from the COS cell genome. Comparison of the sequence of the recovered virus with that of the viral DNA resident in COS cells (strain 776) and the mutant used in our studies (derived from strain 777) proves that the mutant virus acquired the T-antigen gene from the COS cell chromosome via homologous recombination. Most probably this process was mediated by a direct genetic exchange.

myc and src oncogenes have complementary effects on cell proliferation and expression of specific extracellular matrix components in definitive chondroblasts

The effects of the avian viral oncogenes src and myc were compared for their ability to alter the differentiated phenotype and the proliferative capacity of definitive chondroblasts. As previously demonstrated, viruses carrying the src oncogene suppressed the synthesis of the chondroblast-specific products, type II collagen and cartilage-specific sulfated proteoglycan. In contrast, infection with MC29 and HB1 viruses, which carry the myc oncogene, did not suppress the synthesis of these normal differentiated cell products, but the infected cells exhibited an increased proliferative potential. The MH2 virus, which carries both the myc and mil oncogenes, both induced the suppression of these chondroblast-specific products and increased cell proliferation. The implications of these results for cooperation between oncogenes and the multi-oncogene models for neoplastic transformation are discussed.

A retroviral myc gene induces preneoplastic transformation of lymphocytes in a bursal transplantation assay

Treatment of chicken embryos with cyclophosphamide results in ablation of bursal lymphocytes. Bursal follicles can be reconstructed by infusion of embryonic bursal cells. Histologic examination of reconstituting bursal follicles showed that the first lymphocytes to appear were large pyrinophilic lymphoblasts that lined up adjacent to the bursal basement membrane and appeared to serve as progenitors for the differentiation of bursal medullary lymphocytes. When these cells were infected with the avian myelocytomatosis virus HB1 bearing a v-myc oncogene they appeared to home to the region of the bursal basement membrane but failed to differentiate. Instead, they formed structures indistinguishable from the preneoplastic transformed follicles that develop during bursal lymphomagenesis induced by lymphoid leukosis viruses. The DNA from these transformed follicles contained the HB1 v-myc gene but lacked the ability to transform NIH/3T3 mouse cells. Therefore these preneoplastic lesions were induced directly by HB1 myc and did not require the expression of Blym-1 or similar oncogenes. Exploitation of this transplantation technique with the chicken bursa will provide a useful method for assessing the stage-specific activity of oncogenes in vivo.

Do we understand the genetic mechanisms of oncogenesis? Keynote address for Honey Harbor meeting on cellular and molecular biology of neoplasia, October 2-6, 1983

Different experiments with viruses and transfection now support the classical view that cancer is the result of a multistep process. This analysis further indicates that some of these steps involve mutations affecting the qualitative and quantitative expression of dominant transforming genes or oncogenes. These mutations are spontaneous or induced and of various kinds, including base pair changes, deletions, translocations, and amplifications. The actions of the active transforming genes or oncogenes lead to the properties of the tumor cell. However, these activities are effective only in the appropriate cell with targets for the products of the oncogenes and without inhibitors. Because there will be multiple genetic changes in tumor cells, it is difficult to determine which changes are significant for the oncogenesis. Retrovirus vectors may be useful in this determination. In addition, our present methods of analysis may be missing certain of the multiple steps in oncogenesis, in particular, those involved with tissue-, organ-, and organism-specific controls.

Avian oncovirus MH2: molecular cloning of proviral DNA and structural analysis of viral RNA and protein

Viral RNA, molecularly cloned proviral DNA, and virus-specific protein of avian retrovirus MH2 were analyzed. The complexity and sequence conservation of the transformation-specific v-myc sequences of MH2 RNA were compared with those of the other members of the MC29 subgroup of acute leukemia viruses, MC29, CMII, and OK10, and with chicken cellular c-myc sequences. All T1 oligonucleotides mapping within the 1.3-kilobase coding region of MC29 v-myc have homologous counterparts in the RNAs of all MC29 subgroup viruses and in c-myc. These counterparts are either identical in composition or altered by single point mutations. Hence, the 47,000-dalton carboxy-terminal sequences of the transforming proteins of these viruses and of the cellular gene product are probably highly conserved but may contain single amino acid substitutions. T1 oligonucleotide mapping of MH2 RNA indicated that the MH2 v-myc sequences map close to the 3' end of viral RNA. A genomic library of an MH2-transformed quail cell line was prepared by using the Charon 4A vector system. By screening with an myc-specific probe, a clone containing the entire MH2 provirus (lambda MH2-1) was isolated. Digestion of cloned DNA with KpnI yielded a 5.1-kilobase fragment hybridizing to both gag- and myc-specific probes. Further restriction mapping of lambda MH2-1 DNA showed that about 1.6 kilobases of the gag gene are present near the 5' end of proviral DNA, and the conserved part of v-myc, i.e., 1.3 kilobases, is present near the 3' end of proviral DNA. These two domains are separated by a segment of at least 1 kilobase of different genetic origin, including additional unique sequences unrelated to virion genes. Tryptic peptide analysis of the gag-related protein of MH2, p100, revealed gag-specific peptides and several unique methionine-containing peptides. One of the latter is possibly shared with the polymerase precursor protein Pr180gag-pol, but no myc-specific peptides, defined for the MC29 protein p110gag-myc, appear to be present in MH2 p100. The data on viral RNA, proviral DNA, and protein of MH2 reveal a unique genetic structure for this virus of the MC29 subgroup and suggest that its v-myc gene is not expressed as a gag-related protein.

Retroviral transforming genes in normal cells?

The hallmark of retroviral transforming genes (onc genes) are specific sequences which are unrelated to essential virion genes but are closely related to sequences in normal cells. Viral onc genes probably originated from rare transductions of these cellular sequences by retroviruses without onc genes. Consequently, it has been suggested that retroviral transforming genes are present in normal cells in a latent form. However, recent structural analyses indicate that viral onc genes and cellular genes, which share specific sequences, are not isogenic. They differ from each other in scattered point mutations and in unique coding regions. The cellular genes containing onc-related sequences are expressed in normal cells compatible with a normal function. There is as yet no functional or consistent circumstantial evidence that these cellular genes cause cancer in animals that are not infected by viruses with onc genes. Therefore, it is still uncertain whether the onc-related cellular genes have oncogenic potential beyond their role as progenitors of retroviral onc genes.

Chromosomal assignment of a family of human oncogenes

A family of human transforming genes, previously shown to share homology with the ras family of viral oncogenes, maps to three different human chromosomes. A well-characterized mouse-human hybrid cell panel, combined with Southern blotting, was used in this study. The transforming gene of the T24 bladder carcinoma cell line maps to human chromosome 11. An oncogene isolated from the lung carcinoma cell line SK-Calu-1 maps to human chromosome 12. The third ras-related gene, cloned from SK-N-SH, a neuroblastoma cell line, maps to human chromosome 1.

Genome structure of HBI, a variant of acute leukemia virus MC29 with unique oncogenic properties

We have analyzed the viral RNA of a variant of avian acute leukemia virus MC29, termed HBI. This virus was isolated during in vitro passage of a partially transformation-defective (td) mutant of MC29 (td10H-MC29) in chicken macrophages. While td10H-MC29 has a reduced ability to transform macrophages in vitro or to induce tumors in vivo, HBI-MC29 transforms macrophages efficiently and induces in vivo a high incidence of lymphoid tumors. Electrophoretic analysis of HBI-MC29 genomic RNA revealed that it has a complexity of 5.7 kilobases, like the RNA of wild-type (wt) MC29, and that it is 0.6 kilobases longer than the 5.1-kilobase RNA of the deletion mutant td10H-MC29. Analysis of the viral RNAs of two clonal isolates of HBI-MC29 by T1 oligonucleotide fingerprinting showed that sequences from the viral transformation-specific region, v-myc, which are deleted in td10H RNA, are present in HBI RNA. Moreover, hybridization of HBI RNA to molecularly cloned subgenomic fragments of wtMC29 proviral DNA, followed by fingerprint analysis of hybridized RNA, showed that the entire v-myc-specific RNA sequences defined previously are present. Hybridization to cloned DNA of the normal chicken locus c-myc shows a close relationship between HBI v-myc RNA and c-myc DNA, especially in the sequences which were deleted from td10H-MC29. T1 oligonucleotide maps of HBI and td10H RNAs were prepared and compared. Total conservation of the oligonucleotide pattern is observed in the overlapping v-myc regions, while the partial structural genes gag and env show some variations, most of which can be directly proven to be due to point mutations or recombination with helper viral RNAs that were analyzed in parallel. Recombination of td10H-MC29 with c-myc, followed by recombinational and mutational changes in the structural genes during passage with helper virus, could be a possible explanation for the origin of HBI.

Homogeneously staining chromosomal regions contain amplified copies of an abundantly expressed cellular oncogene (c-myc) in malignant neuroendocrine cells from a human colon carcinoma

Two human neuroendocrine tumor cell lines derived from a colon carcinoma contain either numerous double minute chromosomes (COLO 320 DM) or a homogeneously staining marker chromosome (COLO 320 HSR). We found amplification and enhanced expression of the cellular oncogene c-myc in both COLO 320 DM and HSR cells, and we were able to show that the homogeneously staining regions of the COLO 320 HSR marker chromosome contain amplified c-myc. From previous and present karyotypes, it appears that the homogeneously staining regions reside on a distorted X chromosome. Therefore, amplification of c-myc has been accompanied by translocation of the gene from its normal position on chromosome 8 (8q24). Because double minute chromosomes were features of primary cultures from the original tumor, it seems reasonable to suspect that amplification of c-myc may have contributed to tumorigenesis.

Identification and characterization of the avian erythroblastosis virus erbB gene product as a membrane glycoprotein

Avian erythroblastosis virus causes erythroid leukemia and sarcomas in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions known as erbA and erbB, and recent evidence suggests that it is the erbB gene that is responsible for the transforming activity. From rats bearing avian erythroblastosis virus-induced sarcomas, we have obtained antisera which are specific for the erb gene products. Using such antisera, we have been able to characterize the erbB gene product as a 68,000 molecular weight protein. Pulse-chase and cell-free in vitro translation experiments show that the initial product is a 62,500 dalton protein which is initially modified to a 66,000 dalton protein, and then further modified to a 68,000 dalton form. These modifications could be shown to be associated with glycosylation and phosphorylation. Cell fractionation experiments revealed that the 66,000 and 68,000 dalton proteins were located in cell membrane fractions, and immunofluorescence results showed the erbB gene product to be expressed on the cell surface.

Nucleotide sequence to the v-myc oncogene of avian retrovirus MC29

Avian myelocytomatosis viruses are retroviruses whose oncogene (v-myc) induces an unusually wide variety of tumors, including carcinomas, endotheliomas, sarcomas, and myelocytomatoses. The viral gene v-myc arose by transduction of an undetermined portion of a cellular gene known as c-myc. In order to facilitate further studies of the functions of v-myc and c-myc and to permit detailed comparisons between the two genes, we have determined the nucleotide sequence of v-myc in the genome of the MC29 strain of myelocytomatosis virus. The v-myc domain in MC29 virus encodes a hydrophilic polypeptide with a molecular weight of 47,000, fused to a portion of the polyprotein encoded by the viral structural gene gag. The carboxyl-terminal half of the v-myc polypeptide is rich in basic amino acid residues. This feature may account for the DNA-binding properties of the hybrid gag-myc-encoded protein which would have a molecular weight of approximately 100,000, in accord with results from previous studies of the protein encoded by v-myc. The junctions between v-myc and the genome of the transducing virus are apparent but reveal no clues to the mechanism by which transduction might occur.