RNA and protein encoded by MH2 virus: evidence for subgenomic expression of v-myc

c-myc protein distribution. Neoplastic tissues of the human colon

There is an extensive literature documenting the increased or deregulated expression of the c-myc oncogene in human malignancies. The authors have recently devised a sensitive immunocytochemical method for studying the tissue localization of c-myc protein in tissue sections of human colon. We have compared nuclear c-myc staining using a polyclonal rabbit anti-c-myc antibody and a mouse monoclonal myc antibody NCM II 274. Microscopic observation of the tissue specific pattern of c-myc protein distribution shows that nuclear staining intensity varies in normal and neoplastic crypt cell nuclei in parallel with morphologic criteria of neoplasia. These studies yield further information on the usefulness of c-myc protein as a prognostic indicator.

Mapping by in vitro constructs of the P100gag-mil region, accounting for induction of chicken neuroretina cell proliferation

The v-mil oncogene of the avian retrovirus MH2 is expressed as a fusion protein with viral gag determinants in infected cells. This P100gag-mil protein accounts for the proliferation of chicken embryo neuroretina cells (CNR) induced by MH2 in vitro. We constructed a series of mutants by in-frame deletions in different parts of the gag and mil domains and tested their ability to induce CNR growth. We show that gag sequences, as well as 200-base-pair 5' mil sequences, were not required to induce such a proliferation. However, gag sequences seem to contribute to a full proliferation of growing CNR. In contrast, deletions in the kinase domain abolish this induction. In particular, by deleting only 9 nucleotides localized around the unique SphI site of v-mil, we produced a totally inactive mutant (BalSp). This mutant directs the synthesis of a v-mil protein lacking the dipeptide Tyr-Leu, which is conserved in almost all the members of the large protein kinase family, and a histidine residue highly conserved in Ser-Thr protein kinase members.

Transformed and tumorigenic phenotypes induced by avian retroviruses containing the v-mil oncogene

Avian retrovirus MH2 contains two oncogenes, v-mil and v-myc. We have previously shown that a spontaneous mutant of MH2 (PA200-MH2), expressing only the v-mil oncogene, is able to induce proliferation of quiescent neuroretina cells. In this study, we investigated the transforming and tumorigenic properties of v-mil. PA200 induced fibrosarcomas in about 60% of the injected chickens, whereas inoculation of MH2 resulted mainly in the appearance of kidney carcinomas. Analysis of several parameters of transformation showed that PA200, in contrast to MH2, induced only limited in vitro transformation of fibroblasts and neuroretina cells. These results suggest that v-myc is the major transforming and tumorigenic gene in MH2-infected cells. This low in vitro transforming capacity differentiates v-mil not only from other avian oncogenes, but also from the homologous murine v-raf gene.

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.

Retention or loss of v-mil sequences after propagation of MH2 virus in vivo or in vitro

During propagation of the defective avian retrovirus MH2 in the presence of replication-competent helper virus, deletion of portions of the viral genome occurred frequently. After transformation of quail cells in vitro, v-mil sequences were lost, leading to populations of MH2 viruses which were highly deficient for mil gene expression but which could transform macrophage and fibroblast cells in vitro with high efficiency. In contrast, after induction of tumors in quail with mil-deficient MH2 viral stocks, a majority of the tumor DNAs contained mil+ proviruses, suggesting that there is selection for retention of the v-mil gene in vivo and that the mil protein may play a role in the oncogenicity of MH2 virus. We also isolated MH2-transformed cell lines which contained deleted proviruses arising from packaging and subsequent integration of the subgenomic v-myc-encoding mRNA. Some of these cell lines produced viruses which encoded abnormal v-myc proteins and had altered in vitro transforming properties. These altered phenotypes may be caused by mutations within the v-myc gene.

Proto-oncogene c-ros codes for a molecule with structural features common to those of growth factor receptors and displays tissue specific and developmentally regulated expression

A recombinant DNA clone containing cellular sequences homologous to the transforming sequence, v-ros, of avian sarcoma virus UR2 was isolated from a chicken genomic DNA library. Heteroduplex mapping and nucleotide sequencing reveal that the v-ros sequences are distributed in nine exons ranging from 65 to 204 nucleotides on cellular ros (c-ros) DNA over a range of 11 kilobases. Comparison of the deduced amino acid sequences of c-ros and v-ros shows two differences: v-ros contains a three-amino-acid insertion within the hydrophobic domain presumed to be involved in membrane association, and (ii) the carboxyl 12 amino acids of v-ros are completely different from those of the deduced c-ros sequence. The deduced amino acid sequence of c-ros bears striking structural features similar to those of insulin and epidermal growth factor receptors, including the presumed hydrophobic membrane binding domain, amino acids flanking the domain, and the distance between the domain and the catalytic region of the kinase activity. The expression of c-ros appears to be under a very stringent control. When tissues at various stages of chicken development were analyzed, only kidney was found to contain a significant level of c-ros RNA. The level of c-ros RNA in kidney tissue is most abundant in 7- to 14-day-old chickens. Finally, nucleotide sequences of c-ros DNA and UR2-associated helper viral genome at regions corresponding to the gag ros recombination site suggest that the junction has been formed by RNA splicing.

Protein product of proto-oncogene c-mil

Using antipeptide antibodies with specificity for the carboxyl termini of v-raf and v-mil protein products, two proteins with apparent molecular weights of approximately 71,000/73,000 and 215,000 were detected in immunoprecipitates from normal uninfected chicken cells. The 71,000/73,000-molecular-weight protein was identified as the product of the c-mil proto-oncogene by the close structural relationship of its 42,000-molecular-weight carboxyl-terminal domain to the v-mil-encoded domain of the hybrid protein p100gag-mil specified by the avian retrovirus MH2. The amino-terminal domain of the cellular protein is encoded by 5' c-mil sequences that have not been transduced into the genome of MH2. The c-mil protein (p71/73c-mil) was found to be phosphorylated in vivo, and homologous proteins were detected at variable levels in a variety of vertebrate cells, including human cells.

Transmission of (LTR, v-src, LTR) without recombination with a helper virus

We have analyzed transmission of (LTR, v-src, LTR) cryptic structure integrated in the H-19 mammalian tumor cell line. From this cell line different isolates of transforming virus were rescued in heterokaryons produced by fusion with chicken fibroblasts infected by replication-competent avian leukosis virus RAV-1. One of them (F6) was used for the transformation of avian cells in the absence of the helper virus. In four transformed cell lines studied, the (LTR, v-src, LTR) structure was again integrated at a unique position in the cell DNA of each line. This indicated that the (LTR, v-src, LTR) structure is transmitted by the helper virus without recombination. This point has been further supported by the finding that a src-containing species corresponding in size to the nonpolyadenylated src mRNA is present in the RNA isolated from the rescued F6 transforming virus which might serve as template for the synthesis of (LTR, v-src, LTR) structure by the reverse transcriptase provided by RAV-1.

Structure and transforming function of transduced mutant alleles of the chicken c-myc gene

A small retroviral vector carrying an oncogenic myc allele was isolated as a spontaneous variant (MH2E21) of avian oncovirus MH2. The MH2E21 genome, measuring only 2.3 kilobases, can be replicated like larger retroviral genomes and hence contains all cis-acting sequence elements essential for encapsidation and reverse transcription of retroviral RNA or for integration and transcription of proviral DNA. The MH2E21 genome contains 5' and 3' noncoding retroviral vector elements and a coding region comprising the first six codons of the viral gag gene and 417 v-myc codons. The gag-myc junction corresponds precisely to the presumed splice junction on subgenomic MH2 v-myc mRNA, the possible origin of MH2E21. Among the v-myc codons, the first 5 are derived from the noncoding 5' terminus of the second c-myc exon, and 412 codons correspond to the c-myc coding region. The predicted sequence of the MH2E21 protein product differs from that of the chicken c-myc protein by 11 additional amino-terminal residues and by 25 amino acid substitutions and a deletion of 4 residues within the shared domains. To investigate the functional significance of these structural changes, the MH2E21 genome was modified in vitro. The gag translational initiation codon was inactivated by oligonucleotide-directed mutagenesis. Furthermore, all but two of the missense mutations were reverted, and the deleted sequences were restored by replacing most of the MH2E21 v-myc allele by the corresponding segment of the CMII v-myc allele which is isogenic to c-myc in that region. The remaining two mutations have not been found in the v-myc alleles of avian oncoviruses MC29, CMII, and OK10. Like MH2 and MH2E21, modified MH2E21 (MH2E21m1c1) transforms avian embryo cells. Like c-myc, it encodes a 416-amino-acid protein initiated at the myc translational initiation codon. We conclude that neither major structural changes, such as in-frame fusion with virion genes or internal deletions, nor specific, if any, missense mutations of the c-myc coding region are necessary for activation of the basic oncogenic function of transduced myc alleles.

Proto-oncogene c-ros codes for a molecule with structural features common to those of growth factor receptors and displays tissue specific and developmentally regulated expression

A recombinant DNA clone containing cellular sequences homologous to the transforming sequence, v-ros, of avian sarcoma virus UR2 was isolated from a chicken genomic DNA library. Heteroduplex mapping and nucleotide sequencing reveal that the v-ros sequences are distributed in nine exons ranging from 65 to 204 nucleotides on cellular ros (c-ros) DNA over a range of 11 kilobases. Comparison of the deduced amino acid sequences of c-ros and v-ros shows two differences: v-ros contains a three-amino-acid insertion within the hydrophobic domain presumed to be involved in membrane association, and (ii) the carboxyl 12 amino acids of v-ros are completely different from those of the deduced c-ros sequence. The deduced amino acid sequence of c-ros bears striking structural features similar to those of insulin and epidermal growth factor receptors, including the presumed hydrophobic membrane binding domain, amino acids flanking the domain, and the distance between the domain and the catalytic region of the kinase activity. The expression of c-ros appears to be under a very stringent control. When tissues at various stages of chicken development were analyzed, only kidney was found to contain a significant level of c-ros RNA. The level of c-ros RNA in kidney tissue is most abundant in 7- to 14-day-old chickens. Finally, nucleotide sequences of c-ros DNA and UR2-associated helper viral genome at regions corresponding to the gag ros recombination site suggest that the junction has been formed by RNA splicing.

Characterization of c-myc proteins from avian bursal lymphoma cell lines

We have used a rabbit antiserum directed against a portion of the MC29 viral myc protein expressed in bacteria to characterize the cellular myc protein from three different avian bursal lymphoma cell lines (1104HI, 1104BI S13, BK25), and from normal chick embryo cells. The phosphorylated myc proteins immunoprecipitated from these cells varied in molecular weight from 58 to 62 kDa and localized to the cell nucleus, as shown by cell fractionation experiments. Pulse-chase experiments established that these proteins had short half-lives ranging from 12 min for the myc proteins from the 1104BI S13 cell line to 25 min for myc proteins from both the 1104HI and the BK25 cell lines. The structural relatedness of the proteins was established by comparing their partial proteolytic digestion products (Cleveland analysis) with the partial proteolytic digestion products of the MH2 viral myc protein. The anti-myc-serum also immunoprecipitated a 48-kDa protein from each of the bursal cell lines. We have identified this protein as a breakdown product of the bursal cell myc proteins. The different size and number of these bursal cell myc proteins may be a direct result of the specific site of integration as well as the orientation of the retrovirus LTR sequence relative to the adjacent cellular myc allele.

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.

Characterization of a myc-containing retrovirus generated by propagation of an MH2 viral subgenomic RNA

We have previously isolated, from wild-type MH2 virus that contains the two oncogenes mil and myc, mutants defective in one or the other oncogene product. We report here the molecular cloning and extensive characterization of MH2 CL25 provirus lacking the v-mil oncogene. Our results indicate that this virus corresponds to the propagation of the 2.8-kilobase subgenomic RNA of MH21.

Protein product of proto-oncogene c-mil

Using antipeptide antibodies with specificity for the carboxyl termini of v-raf and v-mil protein products, two proteins with apparent molecular weights of approximately 71,000/73,000 and 215,000 were detected in immunoprecipitates from normal uninfected chicken cells. The 71,000/73,000-molecular-weight protein was identified as the product of the c-mil proto-oncogene by the close structural relationship of its 42,000-molecular-weight carboxyl-terminal domain to the v-mil-encoded domain of the hybrid protein p100gag-mil specified by the avian retrovirus MH2. The amino-terminal domain of the cellular protein is encoded by 5' c-mil sequences that have not been transduced into the genome of MH2. The c-mil protein (p71/73c-mil) was found to be phosphorylated in vivo, and homologous proteins were detected at variable levels in a variety of vertebrate cells, including human cells.

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.

Mutagenesis of avian carcinoma virus MH2: only one of two potential transforming genes (delta gag-myc) transforms fibroblasts

Avian carcinoma virus MH2 contains two potential transforming genes, delta gag-mht and delta gag-myc. Thus, MH2 may be a model for two-gene carcinogenesis in which transformation depends on two synergistic genes. Most other directly oncogenic viruses contain single, autonomous transforming (onc) genes and are models for single-gene carcinogenesis. To determine which role each potential onc gene of MH2 plays in oncogenesis, we have prepared deletion and frameshift mutants of each of the two MH2 genes by in vitro mutagenesis of cloned proviral DNA and have tested transforming function and virus production in cultured primary quail cells. We have found that mht deletion mutants and wild-type virus transform primary cells and that myc deletion and frameshift mutants do not. The morphologies of cells transformed by the mht deletion mutants and by wild-type MH2 are similar yet vary considerably. Nevertheless, typical mutant transformed cells can often be distinguished from cells transformed by wild-type MH2. We conclude that the delta gag-myc gene transforms primary cells by itself, without the second potential onc gene. This myc-related gene is the smallest that has direct transforming function. delta gag-mht is without detectable transforming function but may affect transformation by delta gag-myc. Thus, MH2 behaves like a virus with a single onc gene, although it expresses two potential onc genes, and it appears not to be a model for two-gene carcinogenesis. Further work is necessary to determine whether the delta gag-mht gene possibly enhances oncogenic function of delta gag-myc or has independent oncogenic function in animals.

Induction of proliferation or transformation of neuroretina cells by the mil and myc viral oncogenes

The genome of the avian retrovirus MH2 contains, in addition to the v-myc oncogene shared with three other avian retroviruses (MC29, CMII and OK-10), a second cell-derived oncogene, v-mil (refs 1-3). Like the three other viruses, which contain only v-myc, MH2 induces mainly liver and kidney carcinomas in fowl and transforms fibroblasts and macrophages in vitro. However, MH2 and MC29 differ in their biological properties when assayed on cultures of chicken embryo neuroretina (NR) cells. Indeed, NR cells, which normally do not multiply in vitro, are induced to proliferate and become transformed upon infection with MH2, whereas infection with MC29 has no apparent effect on these cells. To analyse the functions of the two oncogenes of MH2, we isolated spontaneous and in vitro-constructed mutants of this virus and investigated their effects on NR cell multiplication and transformation. We report here that expression of v-mil is sufficient to induce NR cell proliferation, although it does not result in cell transformation. In addition, viruses expressing only the v-myc oncogene fail to induce any detectable change in NR cells. However, cooperation of the two oncogenes is required to achieve transformation of NR cells by MH2.

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.

Nucleotide sequence analysis of the chicken gene c-mil, the progenitor of the retroviral oncogene v-mil

The nucleotide sequence of the chicken gene c-mil was determined within and around all regions homologous to the oncogene v-mil of avian retrovirus MH2. The regions of homology to the previously determined v-mil sequence, ranging in size from 28 to 177 base pairs (bp), are distributed over 14 kilobase pairs (kbp) of the chicken genome and are organized in 11 exons. All exon-intron boundaries of c-mil, except the 5' boundary of exon 1 and the 3' boundary of exon 11, were unambiguously defined by the identification of consensus splice donor and acceptor sites precisely at positions where homology to v-mil ceases or resumes. The homology to v-mil starts within the coding sequence of exon 1 and ends within the 3' untranslated region of exon 11, 12 nucleotides downstream from the nonsense codon terminating the large open reading frame shared between c-mil and v-mil. The c-mil and v-mil sequences differ at only 7 out of 1153 nucleotide positions, and the predicted sequences of v-mil and c-mil proteins differ by one conservative and four nonconservative substitutions among 379 amino acid residues. Hence, the carboxy-terminal domains of the MH2 gag-mil hybrid protein and of the putative c-mil protein are very similar. However, the amino-terminal domain of the cellular protein is possibly encoded by additional 5' c-mil sequences not present in the transduced v-mil oncogene, while that of the MH2 hybrid protein is encoded by viral gag sequences. The sequence analysis also revealed that c-mil and c-myc derived sequences are immediately adjacent on the MH2 genome carrying both the v-mil and the v-myc oncogene. Hence, transduction of c-mil into MH2 involved recombination, at the 3' site, with either the c-myc locus or a previously transduced v-myc gene, and, at the 5' site, with gag sequences of the transducing virus. At both sites, no significant homologies were found between the sequence elements involved in the recombination.

Activated proto-onc genes: sufficient or necessary for cancer?

Proto-onc genes are normal cellular genes that are related to the transforming (onc) genes of retroviruses. Because of this relationship these genes are now widely believed to be potential cancer genes. In some tumors, proto-onc genes are mutated or expressed more than in normal cells. Under these conditions, proto-onc genes are hypothesized to be active cancer genes in one of two possible ways: The one gene-one cancer hypothesis suggests that one activated proto-onc gene is sufficient to cause cancer. The multigene-one cancer hypothesis suggests that an activated proto-onc gene is a necessary but not a sufficient cause of cancer. However, mutated or transcriptionally activated proto-onc genes are not consistently associated with the tumors in which they are occasionally found and do not transform primary cells. Further, no set of an activated proto-onc gene and a complementary cancer gene with transforming function has yet been isolated from a tumor. Thus, there is still no proof that activated proto-onc genes are sufficient or even necessary to cause cancer.

Molecular cloning of proviral DNA and structural analysis of the transduced myc oncogene of avian oncovirus CMII

Molecularly cloned proviral DNA of avian oncogenic retrovirus CMII was isolated by screening a genomic library of a CMII-transformed quail cell line with a myc-specific probe. On a 10.4-kilobase EcoRI fragment, the cloned DNA contained 4.4 kilobases of CMII proviral sequences extending from the 5' long terminal repeat to the EcoRI site within the partial (delta) complement of the env gene. The gene order of CMII proviral DNA is 5'-delta gag-v-myc-delta pol-delta env-3'. All three structural genes are partially deleted: the gag gene at the 3' end, the env gene at the 5' end, and the pol gene at both ends. The delta gag (0.83 kilobases)-v-myc (1.50 kilobases) sequences encode the p90gag-myc transforming protein of CMII. In comparison with the p110gag-myc protein of acute leukemia virus MC29, p90gag-myc lacks amino acids corresponding to additional 516 bases of gag sequences and 12 bases of 5' v-myc sequences present in the MC29 genome. Nucleotide sequence analysis of CMII proviral DNA at the delta gag-v-myc and the v-myc-delta pol junctions revealed significant homologies between avian retroviral structural genes and the cellular oncogene c-myc precisely at the positions corresponding to the gene junctions in CMII. Furthermore, the delta gag-v-myc junction in CMII corresponds to sequence elements in gag and C-myc that are possible splicing signals. The data suggest that transduction of cellular oncogenes may involve RNA splicing and recombination with homologous sequences on retroviral vectors. Different sequence elements of both the retroviral vectors and the c-myc gene recombined during genesis of highly oncogenic retroviruses CMII, MC29, or MH2.

Nucleotide sequence of two overlapping myc-related genes in avian carcinoma virus OK10 and their relation to the myc genes of other viruses and the cell

Avian carcinoma virus OK10 has the genetic structure gag-delta pol-myc-delta env. It shares the transformation-specific myc sequence with three other avian carcinoma viruses (MC29, MH2, CMII) and also with a normal chicken gene proto-myc and the gag, pol, and env elements with non-transforming retroviruses. Unlike the other myc-containing viruses, which synthesize singular myc proteins, OK10 synthesizes two different myc-related proteins of 200 and 57 kDa. Here we have sequenced the myc region of an infectious OK10 provirus to investigate how OK10 synthesizes two different proteins from the same myc domain and to identify characteristic differences between the normal proto-myc gene and the myc-related viral transforming genes. It was found that the 1.6-kilobase myc domain of OK10 is colinear and coterminal with the myc domains of MC29, MH2, and the terminal two exons of proto-myc. It is preceded by the same splice acceptor as the myc sequence of MH2 and as the second proto-myc exon. From this and the known structure of retroviruses, it follows that the OK10 gene encoding the 57-kDa protein is discontinuous with a small 5' exon that includes six gag codons and a large 3' myc exon (delta gag-myc). This gene and the delta gag-myc gene of MH2 are isogenic. The proto-myc-derived intron preceding the myc domain of OK10 is in the same reading frame as the adjacent delta pol and myc domains and, hence, is part of the gag-delta pol-myc gene encoding the 200-kDa protein. Sequence comparisons with proto-myc and MC29 and MH2 indicate that there are no characteristic mutations that set apart the viral myc domains from proto-myc. It is concluded that transforming function of viral myc-related genes correlates with the lack of a viral equivalent of the first proto-myc exon(s) and conjugation of the viral myc domains with large or small retroviral genetic elements rather than with specific point mutations. Because OK10 and MH2 each contain two genes with potential transforming function (namely, delta gag-myc and gag-delta pol-myc or delta gag-mht, respectively), it remains to be determined whether the delta gag-myc genes have transforming function on their own or need helper genes. The possible helper requirement cannot be very specific because the two potential helper genes are very different.

Molecular and biological properties of MH2D12, a spontaneous mil deletion mutant of avian oncovirus MH2

Avian oncogenic retrovirus MH2 carries two cell-derived oncogenes, v-mil and v-myc. From an infectious stock of MH2 a spontaneous deletion mutant, MH2D12, that has lost most of the v-mil gene but has retained a complete and functional v-myc gene, has been isolated. Nonproducer quail embryo cells transformed by MH2D12 in the absence of helper virus contain two virus-specific proteins: a gag-related protein of 53,000 Da (p53gag), and a v-myc gene product of 59,000/61,000 Da (p59/61v-myc) indistinguishable from the v-myc protein encoded by MH2. MH2D12 viral RNA contains all T1-oligonucleotides specific for the MH2 v-myc gene but none of those characteristic for the v-mil gene. The genetic structure of molecularly cloned proviral DNA of MH2D12 was revealed by restriction mapping, blot hybridization, heteroduplex analysis, and nucleotide sequencing. The MH2D12 provirus is homologous to the MH2 genome but has suffered a deletion of 1271 nucleotides from the central region encompassing the 3' end of delta gag and all of v-mil except the very 3' 31 nucleotides directly adjacent to the v-myc gene. A nine-nucleotide overlap of homology to gag or mil at the delta gag/delta mil junction suggests that recombination between homologous sequence elements of the delta gag and v-mil domains of MH2 was involved in the genesis of MH2D12. The nucleotide sequence analysis predicts that the carboxyterminal 17 amino acids of p53gag are encoded by the residual v-mil sequences and by intron-derived v-myc sequences. Transformation of quail embryo cells by MH2D12 can be assayed by focus and colony formation of transformed cells. This indicates that the v-mil gene is not essential for these activities. However, size and morphology of foci and colonies, and cellular morphology of cultured MH2D12-transformed cell lines can easily be distinguished from those observed in cell transformation by MH2 and resemble more those seen in cell transformation by viruses containing the myc oncogene only.

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.

MC29 virus-coded protein occurs as monomers and dimers in transformed cells

The MC29 virus-coded protein p110gag-myc was found exclusively in the nucleus of transformed Japanese quail (Q8) cells, and time course experiments indicated that the protein had a half-life of about 30 min. When extracts of either Q8 or chicken embryo cells infected with MC29 virus were prepared with nondenaturing detergents and then sedimented in sucrose gradients, p110 was found in the fractions expected to contain monomers (5.9S), dimers (9.3S), or mixtures of the two. The same extracts treated with denaturing detergent (0.2% sodium dodecyl sulfate) exhibited p110 only in fractions expected for the monomeric protein, but beta-mercaptoethanol had no effect on the original distribution. Gradients prepared with 0.5 or 1.0 M NaCl failed to dissociate the faster-sedimenting form. No other protein or polyribonucleotide which could increase the sedimentation rate of p110 was found, and neither RNase nor DNase altered the sedimentation pattern of p110 in nondenatured extracts. A reassociation of monomeric p110 into dimers discernible by gel electrophoresis was demonstrated.

Transcription of three c-myc exons is enhanced in chicken bursal lymphoma cell lines

The chicken c-myc gene, as defined by its homology to the v-myc gene of MC29 virus, is comprised of two exons. Using the techniques of runoff transcription, primer extension, and S1 nuclease protection, we demonstrate that there is a third c-myc exon of approximately equal to 345 base pairs (bp) located 0.7 kbp upstream of the 5' end of the v-myc homology. This first exon is transcribed and present in myc mRNA in normal chicken cells. We also examined RNA from five cell lines derived from avian leukosis virus-induced bursal lymphomas. In all these lines, the level of transcription of the 2.2- to 2.5-kbp myc mRNA is increased 30- to 60-fold over normal cells. The myc mRNA in four of these lines also contains increased levels of the first noncoding exon, and evidence is presented that the long terminal repeat (LTR) in the vicinity of c-myc is functioning as an enhancer of c-myc transcription rather than as a promoter in several of these cell lines. In two cell lines in which the viral LTR has integrated between the first and second exons in the proper orientation for downstream promotion of myc, the LTR does not exhibit promoter function. The pattern of c-myc transcription observed by others in a vast majority of avian leukosis virus-induced neoplasms is not observed in any of the five cell lines examined.

Serine- and threonine-specific protein kinase activities of purified gag-mil and gag-raf proteins

Retroviruses carry cell-derived oncogenes (v-onc) that have the potential to transform cells in culture and induce tumours in vivo. One of the few carcinoma-inducing viruses is the acutely transforming retrovirus MH2, which carries the putative oncogene v-mil and the known oncogene v-myc. Recently, a high degree of homology was discovered between v-mil and v-raf, the transforming gene of the murine retrovirus 3611 murine sarcoma virus (MSV), whereas homology to v-src is low. Both viruses express their oncogenes as the gag-fusion polyproteins p100gag-mil and p75gag-raf (of respective relative molecular mass (Mr) 100,000 and 75,000), while the myc oncogene of MH2 is expressed by means of a subgenomic messenger RNA. We have recently demonstrated that p100gag-mil is not a nuclear protein. Here we report that purified p100gag-mil and p75gag-raf exhibit protein kinase activities in vitro which, in contrast to the src-related p130gag-fps of Fujinami sarcoma virus (FSV) and all other characterized oncogene-encoded protein kinases, phosphorylate serine and threonine but not tyrosine. Both types of protein kinases phosphorylate lipids in vitro.

Proteins encoded by the long terminal repeat region of mouse mammary tumor virus: identification by hybrid-selected translation

The long terminal repeat (LTR) region of mouse mammary tumor virus (MMTV) is known to contain an open reading frame of sufficient length to code for a protein of 36,000 Mr. The coding capacity of the 3' sequences of MMTV genomic RNA has been demonstrated by in vitro translation studies, which have reported the synthesis of four related proteins: p36, p24, p21, and p18. These proteins are overlapping translation products of the same open reading frame, with the smaller ones initiating at internal methionine codons. From the predicted amino acid sequence of the LTR protein, we have selected a region likely to be antigenic, obtained a synthetic peptide of that region, and raised antiserum to the peptide. The antipeptide serum specifically immunoprecipitated all four proteins from in vitro translated genomic 3' MMTV RNA, plus an additional one of 32,000 Mr. Published sequence data of MMRV LTRs show an internal AUG codon at a position which could initiate a protein of 32,000 Mr. The three smaller in vitro translation products (p24, p21, and p18) were consistently synthesized in much greater amounts than the p36 or p32 protein. The relative amount of each in vitro synthesized protein from genomic MMTV RNA could be predicted and was in good agreement with the postulated effect of flanking nucleotides on the efficiency of the respective AUG initiation codon. Polyadenylated RNAs, isolated from various mouse tissues, were selected by hybridization to plasmid DNA containing MMTV LTR sequences immobilized on nitrocellulose. In vitro translation of hybrid-selected mRNAs isolated from BALB/c mouse lactating mammary glands and carcinogen-induced mammary tumors, followed by immunoprecipitation with antipeptide serum, revealed that only one polypeptide was synthesized by the MMTV LTR-specific mRNA, the 36,000 Mr species.

Immunological analysis of v-myc gene products using antibodies against a myc-specific synthetic peptide

Rabbit antisera were prepared against a synthetic peptide corresponding to the carboxyterminal amino acid sequences of the transforming protein p110gag-myc of avian oncovirus MC29. Analysis of immunoprecipitates formed with these sera (anti-mycC) demonstrated that the gag-myc hybrid proteins encoded by the avian leukemia viruses MC29, CMII, and OK10 were recognized by the anti-peptide sera, but not the gag or pol precursor proteins of helper viruses or the MH2-encoded p100gag-mil protein. In cells transformed by OK10 or MH2, putative v-myc proteins of 60K (OK10) and 59/61K (MH2) were also precipitated by anti-mycC. In addition, the anti-peptide sera reacted specifically with the gag-myc proteins encoded by three partially transformation-defective mutants of MC29, td10A, td10C, and td10H, and by MC29 variant HBI.

Nucleotide sequence of avian retroviral oncogene v-mil: homologue of murine retroviral oncogene v-raf

Eukaryotic cells contain genes termed proto-oncogenes (c-onc) which have the potential to transform cells in culture and induce tumours in vivo. Most of these genes have been identified by their occasional incorporation into retroviral genomes which can act as natural transducing vectors for these and perhaps other cellular genes. Cell-derived oncogenes of retroviruses (v-onc) are associated mostly with the induction of mesenchymal tumours whereas carcinoma induction is rare. One of these rare carcinoma-inducing viruses is the acutely transforming avian retrovirus MH2 (refs 3-5). Recently we and others have shown that this virus carries a novel putative oncogene, v- mil , in addition to the known oncogene v-myc. While the transforming ability of v- mil has not been directly established, we have recently discovered by hybridization analysis that v- mil is homologous to v-raf (ref. 9), the transforming gene of the murine retrovirus 3611 MSV (ref. 10). Both viruses express the mil /raf oncogene product as a gag-fusion polyprotein, while the myc oncogene of MH2 is expressed via a subgenomic mRNA. Here we report the complete nucleotide sequence of v- mil and compare it with that of v-raf. The 80% homology between the nucleotide sequences and the 94% homology between the predicted amino acid sequences of the two viral genes clearly indicate that these are the avian and murine forms of the same gene. Comparison of the two sequences with that of the human cellular homologue (T. I. Bonner et al., manuscript in preparation) indicates that v-raf has more 3' untranslated sequences while v- mil has additional sequences from two 5' exons of the cellular homologue. Although the mil /raf amino acid sequences reveal partial homology to that of the v-src product, no tyrosine-specific protein kinase activity is detected for the gag- mil and the gag-raf hybrid proteins.

Nucleotide sequence of avian carcinoma virus MH2: two potential onc genes, one related to avian virus MC29 and the other related to murine sarcoma virus 3611

The 5.2-kilobase (kb) RNA genome of avian carcinoma virus MH2 has the genetic structure 5'-delta gag (0.2 kb)- mht (1.2 kb)-myc (1.4 kb)-c (0.4 kb)-poly(A) (0.2 kb)-3'. delta gag is a partial retroviral core protein gene, mht and myc are cell-derived MH2-specific sequences, and c is the 3'-terminal retroviral vector sequence. Here we have determined the nucleotide sequence of 3.5 kb from the 3' end of delta gag to the 3' end of molecularly cloned proviral MH2 DNA, in order to elucidate the genetic structure of the virus and to compare it with other mht - and myc-containing oncogenic viruses as well as with the chicken proto-myc gene. The following results were obtained: (i) delta gag- mht forms a hybrid gene with a contiguous reading frame of 2682 nucleotides that terminates with a stop codon near the 3' end of mht . The 3' 969 nucleotides of mht up to the stop codon are 80% sequence related to the onc-specific raf sequence of murine sarcoma virus 3611 (94% homologous at the deduced amino acid level). (ii) The myc sequence is preceded by an RNA splice acceptor site shared with the cellular proto-myc gene, beyond which it is colinear up to a 3'-termination codon and 40 noncoding nucleotides with the myc sequences of avian retrovirus MC29 and chicken proto-myc. Thus, myc forms, together with a 5' retroviral exon, a second MH2-specific gene. (iii) myc is followed by the 3'-terminal c region of about 400 nucleotides, which is colinear with that of Rous sarcoma virus except for a substitution near the 5' end of the long terminal repeat. It is concluded that MH2 contains two genes with oncogenic potential, the delta gag- mht gene, which is closely related to the delta gag-raf transforming gene of MSV 3611, and the myc gene, which is related to the transforming gene of MC29. Furthermore, it may be concluded that the cellular proto-onc genes, which on sequence transduction become viral onc genes, are a small group because among the 19 known onc sequences, 5 are shared by different taxonomic groups of viruses of which the mht /raf homology is the closest determined so far.

A common onc gene sequence transduced by avian carcinoma virus MH2 and by murine sarcoma virus 3611

A common cellular sequence was independently transduced by avian carcinoma virus MH2 (v-mht) and murine sarcoma virus (MSV) 3611 (v-raf). Comparison of the nucleotide sequences of v-mht and v-raf revealed a region of homology that extends over 969 nucleotides. The homology between the corresponding amino acids was about 95 percent with only 19 of 323 amino acids being different. With this example, 5 of the 19 known different viral onc genes have been observed in viruses of different taxonomic groups. These data indicate that (i) the number of cellular proto-onc genes is limited because, like other viruses of different taxonomic groups, MH2 and MSV 3611 have transduced the same onc gene-specific sequences from different cell species and (ii) that specific deletion and linkage of the same proto-onc sequences to different viral vector elements affect the oncogenic potential of the resulting viruses. The difference in transformation capabilities of MH2 and MSV 3611 serves as an example.

Identification of nuclear proteins encoded by viral and cellular myc oncogenes

The myelocytomatosis viruses are a family of replication-defective avian retroviruses that cause a variety of tumours in chickens and transform both fibroblasts and macrophages in culture through the activity of their oncogene v-myc. A closely related gene (c-myc) is found in vertebrate animals and is thought to be the progenitor of v-myc. Changes in the expression and perhaps the structure of c-myc have been implicated in the genesis of avian, murine and human tumours (for a review, see ref. 15). Elucidation of the mechanisms by which v-myc and c-myc might elicit tumorigenesis requires identification of the proteins encoded by these genes. To this end, we have expressed a portion of v-myc in a bacterial host and used the resulting protein to raise antisera that react with myc proteins. We report here that v-myc and c-myc encode closely related proteins with molecular weights (MWs) of approximately 58,000. Integration of retroviral DNA near or within c-myc in avian lymphomas apparently enhances expression of the gene. Here we have used cells from one such tumour to identify the protein encoded by c-myc and find that the coding domain for the gene is probably intact.

Avian carcinoma virus MH2 contains a transformation-specific sequence, mht, and shares the myc sequence with MC29, CMII, and OK10 viruses

Avian carcinoma virus MH2 has been grouped together with MC29, CMII, and OK10, because all of these viruses share a transformation-specific sequence termed myc. A 5.2-kilobase (kb) DNA provirus of MH2 has been molecularly cloned. The complete genetic structure of MH2 is 5'-delta gag(1.9-kb)-mht(1.2-kb)-myc(1.3-kb)-delta env(?) and noncoding c-region (0.2-kb)-3'. delta gag, delta env, and c are genetic elements shared with nondefective retroviruses, whereas mht is a unique, possibly MH2 transformation-specific, sequence. Hybridizations with normal chicken DNA and cloned chicken c-myc DNA indicate that the mht sequence probably derives from a normal cellular gene that is distinct from the c-myc gene. The genetic structure of MH2 suggests that the delta gag and mht sequences function as a hybrid gene that encodes the p100 putative transforming protein. The myc sequence of MH2 appears to encode a second transforming function. Therefore, it seems that MH2 contains two genes with possible oncogenic function, whereas MC29, CMII, and OK10 each carries a single hybrid delta gag-myc transforming gene. It is remarkable that, despite these fundamental differences in their primary structures and mechanisms of gene expression, MH2 and MC29 have very similar oncogenic properties.

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.

Proteins encoded by v-myc and c-myc oncogenes: identification and localization in acute leukemia virus transformants and bursal lymphoma cell lines

We have prepared an antiserum against a synthetic dodecapeptide whose sequence corresponds to the C terminus of the MC29 v-myc protein. This antiserum (anti-v-myc 12C) specifically precipitates the known gag-myc fusion proteins produced by the defective leukemia viruses MC29, CMII, and OK10, but does not react with gag-precursor or product proteins. In addition, proteins of 62 kd and 61/63 kd are precipitated by anti-v-myc 12C from OK10 and MH2 transformants, respectively. The serum also recognizes comigrating 62 kd proteins from three chicken bursal lymphoma cell lines and from the products of in vitro translation of c-myc-specific mRNA. All of these myc-related proteins are phosphorylated and all appear to be localized in the cell nucleus. In uninfected quail cells, anti-v-myc 12C also recognizes a candidate c-myc protein of 60 kd, which does not appear to be phosphorylated and is present in low levels relative to v-myc and lymphoma c-myc proteins.

Expression of c-myc RNA in bursal lymphoma cell lines: identification of c-myc-encoded proteins by hybrid-selected translation

We examined expression of the c-myc locus in four cell lines established from bursal lymphomas induced by avian leukosis virus. In all four lines the level of myc-related RNA was elevated. In three lines a majority of the myc-containing RNAs lacked viral-LTR-related sequences, in contrast to results obtained with primary tumors. This suggests that LTR sequences are not required for maintenance of high level c-myc expression. One line, RP9, has a complex pattern of myc RNAs containing LTR sequences, and one of these RNAs is packaged into virions. Using hybrid selection of RNAs with myc DNA, followed by in vitro translation, we detected translation of myc-related proteins from RNA of all four cell lines. The sizes of these proteins differ among the cell lines. The major polypeptides detected were 64, 57, and 54 kilodaltons. Events leading to elevation of c-myc transcription may be accompanied by alterations in mRNA initiation or processing that generate different protein products.