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

MYC and RAF: Key Effectors in Cellular Signaling and Major Drivers in Human Cancer

The prototypes of the human MYC and RAF gene families are orthologs of animal proto-oncogenes that were originally identified as transduced alleles in the genomes of highly oncogenic retroviruses. MYC and RAF genes are now established as key regulatory elements in normal cellular physiology, but also as major cancer driver genes. Although the predominantly nuclear MYC proteins and the cytoplasmic RAF proteins have different biochemical functions, they are functionally linked in pivotal signaling cascades and circuits. The MYC protein is a transcription factor and together with its dimerization partner MAX holds a central position in a regulatory network of bHLH-LZ proteins. MYC regulates transcription conducted by all RNA polymerases and controls virtually the entire transcriptome. Fundamental cellular processes including distinct catabolic and anabolic branches of metabolism, cell cycle regulation, cell growth and proliferation, differentiation, stem cell regulation, and apoptosis are under MYC control. Deregulation of MYC expression by rearrangement or amplification of the MYC locus or by defects in kinase-mediated upstream signaling, accompanied by loss of apoptotic checkpoints, leads to tumorigenesis and is a hallmark of most human cancers. The critically controlled serine/threonine RAF kinases are central nodes of the cytoplasmic MAPK signaling cascade transducing converted extracellular signals to the nucleus for reshaping transcription factor controlled gene expression profiles. Specific mutations of RAF kinases, such as the prevalent BRAF(V600E) mutation in melanoma, or defects in upstream signaling or feedback loops cause decoupled kinase activities which lead to tumorigenesis. Different strategies for pharmacological interference with MYC- or RAF-induced tumorigenesis are being developed and several RAF kinase inhibitors are already in clinical use.

c-MYC-induced genomic instability

MYC dysregulation initiates a dynamic process of genomic instability that is linked to tumor initiation. Early studies using MYC-carrying retroviruses showed that these viruses were potent transforming agents. Cell culture models followed that addressed the role of MYC in transformation. With the advent of MYC transgenic mice, it became obvious that MYC deregulation alone was sufficient to initiate B-cell neoplasia in mice. More than 70% of all tumors have some form of c-MYC gene dysregulation, which affects gene regulation, microRNA expression profiles, large genomic amplifications, and the overall organization of the nucleus. These changes set the stage for the dynamic genomic rearrangements that are associated with cellular transformation.

Retroviral oncogenes: a historical primer

Retroviruses are the original source of oncogenes. The discovery and characterization of these genes was made possible by the introduction of quantitative cell biological and molecular techniques for the study of tumour viruses. Key features of all retroviral oncogenes were first identified in src, the oncogene of Rous sarcoma virus. These include non-involvement in viral replication, coding for a single protein and cellular origin. The MYC, RAS and ERBB oncogenes quickly followed SRC, and these together with PI3K are now recognized as crucial driving forces in human cancer.

Expression of endogenous oncogenic V600EB-raf induces proliferation and developmental defects in mice and transformation of primary fibroblasts

Mutations of the human B-RAF gene are detected in approximately 8% of cancer samples, primarily in cutaneous melanomas (70%). The most common mutation (90%) is a valine-to-glutamic acid mutation at residue 600 (V600E; formerly V599E according to previous nomenclature). Using a Cre/Lox approach, we have generated a conditional knock-in allele of (V600E)B-raf in mice. We show that widespread expression of (V600E)B-Raf cannot be tolerated in embryonic development, with embryos dying approximately 7.5 dpc. Directed expression of mutant (V600E)B-Raf to somatic tissues using the IFN-inducible Mx1-Cre mouse strain induces a proliferative disorder and bone marrow failure with evidence of nonlymphoid neoplasia of the histiocytic type leading to death within 4 weeks of age. However, expression of mutant B-Raf does not alter the proliferation profile of all somatic tissues. In primary mouse embryonic fibroblasts, expression of endogenous (V600E)B-Raf induces morphologic transformation, increased cell proliferation, and loss of contact inhibition. Thus, (V600E)B-Raf is able to induce several hallmarks of transformation in some primary mouse cells without evidence for the involvement of a cooperating oncogene or tumor suppressor gene.

A revised and complete map of the chicken c-mil/raf-1 locus

The chicken c-mil/raf-1 gene (formerly also known as c-mht) was originally identified in the search for the cellular counterpart to the v-mil oncogene of the Mill Hill 2 retrovirus and was among the first cellular proto-oncogenes discovered. Although the c-mil/raf-1 promotor, as well as the exons transduced into v-mil, were characterized in detail, an entire map of this locus has never been published. Here, we now report the location of five previously unmapped exons. In addition, we have noticed inconsistent numbering of the c-mil/raf-1 exons in the literature and the GenBank database. Thus, we provide here a complete map of the c-mil/raf-1 gene and a revision of the exon numbers. Comparison of the chicken c-mil/raf-1 gene with those of other vertebrates suggests that the numbers and lengths of the translated exons of the raf-1 locus were established early in the vertebrate lineage and have been conserved during the divergent evolution of teleosts and tetrapods.

Host range restrictions of oncogenes: myc genes transform avian but not mammalian cells and mht/raf genes transform mammalian but not avian cells

The host range of retroviral oncogenes is naturally limited by the host range of the retroviral vector. The question of whether the transforming host range of retroviral oncogenes is also restricted by the host species has not been directly addressed. Here we have tested in avian and murine host species the transforming host range of two retroviral onc genes, myc of avian carcinoma viruses MH2 and MC29 and mht/raf of avian carcinoma virus MH2 and murine sarcoma virus MSV 3611. Virus vector-mediated host restriction was bypassed by recombining viral oncogenes with retroviral vectors that can readily infect the host to be tested. It was found that, despite high expression, transforming function of retroviral myc genes is restricted to avian cells, and that of retroviral mht/raf genes is restricted to murine cells. Since retroviral oncogenes encode the same proteins as certain cellular genes, termed protooncogenes, our data must also be relevant to the oncogene hypothesis of cancer. According to this hypothesis, cancer is caused by mutation of protooncogenes. Because protooncogenes are conserved in evolution and are presumed to have conserved functions, the oncogene hypothesis assumes no host range restriction of transforming function. For example, mutated human proto-myc is postulated to cause Burkitt lymphoma, because avian retroviruses with myc genes cause cancer in birds. But there is no evidence that known mutated protooncogenes can transform human cells. The findings reported here indicate that host range restriction appears to be one of the reasons (in addition to insufficient transcriptional activation) why known, mutated protooncogenes lack transforming function in human cells.

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

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

Casein kinase II inhibits the DNA-binding activity of Max homodimers but not Myc/Max heterodimers

Max is a heterodimeric partner of the Myc oncoprotein with sequence-specific DNA-binding activity. We found that the DNA-binding activity of bacterially expressed Max homodimers was inhibited in an ATP-dependent reaction by phosphorylation in vitro with purified bovine casein kinase II (CKII). In contrast, phosphorylation of Max and/or Myc by CKII had no inhibitory or stimulatory effect on the DNA-binding activity of Myc/Max heterodimers. By deletion analysis and site-directed mutagenesis, the inhibitory domain was localized to a CKII phosphorylation site in the amino terminus of Max. Finally, extracts prepared from NIH-3T3 cell lines that overexpress Max contained a phosphorylated Max protein which, following phosphatase treatment or heterodimerization with Myc, was capable of sequence-specific DNA-binding activity. Immunoprecipitation experiments confirmed that Max was also phosphorylated in NIH-3T3 cells, demonstrating that Max phosphorylation may have an important physiological function.

FH3, a v-myc avian retrovirus with limited transforming ability

We have isolated a new acute avian transforming virus which contains the oncogene myc. This virus, designated FH3, was isolated after injection of a 10-day-old chick embryo with avian leukosis virus. While FH3 shares many properties with other v-myc-containing avian retroviruses, it also has several unique properties. The primary target for transformation in vitro is chicken macrophages; infection of chicken fibroblasts does not lead to complete morphological transformation. FH3 also exhibits a limited host range, in that Japanese quail macrophages and fibroblasts are infected but are not completely transformed. FH3 induces in vivo a limited tumor type if injected into 10-day-old chick embryos; only a cranial myelocytoma, which does not appear to be metastatic, can be detected. The v-myc gene of FH3 is expressed predominantly as a P145 Gag-Myc protein which is encoded by a ca. 8-kilobase genomic RNA. This FH3-encoded polyprotein is localized in the nucleus of all infected cells, whether or not they are transformed.

raf involvement in the simultaneous genetic transfer of the radioresistant and transforming phenotypes

We examined a human Alu+ mouse tertiary transformant derived from a noncancerous skin fibroblast cell line which exhibits the unique characteristic of being resistant to the killing effects of ionizing radiation. This transformed cell line was found to contain activated human c-raf-1, and demonstrated an increased level of radioresistance indicating the simultaneous transfer of both the transforming and radiation-resistant phenotypes. We have also found a relationship between the presence of activated oncogenes, specifically those with serine/threonine kinase activity and the radioresistant phenotype.

v-cbl, an oncogene from a dual-recombinant murine retrovirus that induces early B-lineage lymphomas

Cas NS-1 is an acutely transforming murine retrovirus that induces pre-B and pro-B cell lymphomas. Molecular cloning showed it was generated from the ecotropic Cas-Br-M virus by sequential recombinations with endogenous retroviral sequences and a cellular oncogene. The oncogene sequence shows no homology with known oncogenes but some similarity to the yeast transcriptional activator GCN4. A 100-kDa gag-cbl fusion protein, with no detectable kinase activity, is responsible for the cellular transformation. The cellular homologue of v-cbl, present in mouse and human DNA, is expressed in a range of hemopoietic lineages.

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.

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.

Characterization of rat c-myc and adjacent regions

Rat genomic regions covering c-myc were cloned from the DNA of both normal liver and two lines of Morris hepatomas, one of which had c-myc amplification. The three restriction maps showed perfect agreement within the overlapping regions. The 7 kb regions, which included the entire normal rat c-myc and the region 2.2 kb upstream, and one from the hepatomas, were sequenced and found to be identical. The coding regions of exons 2 and 3 were highly conserved between rat, mouse and man, but some differences in amino acids were noted. Exon 1 and the non-coding region of exon 3 showed limited homology between the three species. Rat exon 1 contained several nonsense codons in each frame and no ATG codon, indicating there to be no coding capacity in this exon. The 2.2 kb upstream regions and the introns compared showed unusual conservation between the rat and human genes. Some motifs, previously proposed as having a functional role in human c-myc, were also found in equivalent positions of the rat sequence. Nucleas S1 protection mapping revealed the second promoter to be preferentially used in most tissues or in hepatoma cells, and the second poly A addition signal to be the only one functional in all the RNA sources examined.

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.

Fibroblast transformation parameters induced by the avian v-mil oncogene

An avian retrovirus containing only the v-mil oncogene (PA200-MH2) was analyzed for its ability to induce a transformed phenotype in chicken embryo fibroblasts. Infected cultures exhibited an altered morphology, disarranged actin cable filaments, and a decrease in the amount of cell surface fibronectin. In addition, these cells showed a high level of plasminogen activator protease activity and were also capable of growth in low serum concentrations. In contrast, PA200-MH2 was very inefficient at inducing foci under agar and colonies in semisolid medium relative to the Mill Hill 2 and Rous sarcoma viruses. This inefficiency was further reflected in vivo by the total inability of PA200-MH2 to induce wing tumors in young birds. However, 40% of the birds inoculated in the wing web with PA200-MH2-infected cells did develop slow-growing tumors at the site of injection, with no evidence of hematopoietic involvement. Our results indicate that the v-mil oncogene is transforming both in vitro and in vivo and that each of the oncogenes in the Mill Hill 2 virus, v-mil and v-myc, can independently transform fibroblasts. These data suggest that v-mil is functionally related to its homologous murine counterpart, v-raf, which also transforms fibroblasts.

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.

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.

Expression of molecular clones of v-myb in avian and mammalian cells independently of transformation

We demonstrated that molecular clones of the v-myb oncogene of avian myeloblastosis virus (AMV) can direct the synthesis of p48v-myb both in avian and mammalian cells which are not targets for transformation by AMV. To accomplish this, we constructed dominantly selectable avian leukosis virus derivatives which efficiently coexpress the protein products of the Tn5 neo gene and the v-myb oncogene. The use of chemically transformed QT6 quail cells for proviral DNA transfection or retroviral infection, followed by G418 selection, allowed the generation of cell lines which continuously produce both undeleted infectious neo-myb viral stocks and p48v-myb. The presence of a simian virus 40 origin of replication in the proviral plasmids also permitted high-level transient expression of p48v-myb in simian COS cells without intervening cycles of potentially mutagenic retroviral replication. These experiments establish that the previously reported DNA sequence of v-myb does in fact encode p48v-myb, the transforming protein of AMV.

Dispersed chromosomal localization of the proto-oncogenes transduced into the genome of Mill Hill 2 or E26 leukemia virus

Both Mill Hill 2 and E26 retroviruses have transduced two cellular genes--c-myc and c-mil/mht (Mill Hill 2) and c-myb and c-ets (E26). We localized the genes transduced by these viruses to different chromosomes: c-myc and c-myb to relatively large chromosomes and c-mil/mht and c-ets to microchromosomes. Thus, like avian erythroblastosis virus, each of these retroviruses has transduced two cellular genes unlinked in the chicken genome.

Cellular myc (c-myc) in fish (rainbow trout): its relationship to other vertebrate myc genes and to the transforming genes of the MC29 family of viruses

We have isolated, cloned, and sequenced the rainbow trout (Salmo gairdneri) c-myc gene. The presumptive coding region of the trout c-myc gene shows extensive homology to the c-myc genes of chicken, mouse, and human. Comparison of nucleotide sequences reveals that human, mouse, chicken, and trout c-myc genes contain at least two coding exons, interrupted by introns of decreasing size of 1.38 kilobases (kb), 1.2 kb, 0.97 kb, and 0.33 kb, respectively. The exons are clearly delineated by donor-acceptor splice signals. The degree of nucleotide homology between trout, chicken, and human exon II is less than that observed for exon III. However, the greatest homology among these three genes is localized to two specific regions within exon II (myc boxes A and B). At the predicted amino acid level, fish c-myc shows considerable homology to vertebrate c-myc gene products. Trout c-myc is expressed in normal trout cells as a single 2.3-kb mRNA species, similar in size to other vertebrate transcripts.

Concerted activation of two potential proto-oncogenes in carcinomas induced by mouse mammary tumour virus

The induction of tumours by retroviruses lacking transduced oncogenes can involve the transcriptional or functional activation of cellular proto-oncogenes by an integrated provirus. Thus, the two cellular genes int-1 and int-2, identified as common targets for activation by mouse mammary tumour virus (MMTV), may constitute previously unrecognized oncogenes. In tumours, proviral insertion at these loci leads to expression of messenger RNAs which are undetectable in normal mammary glands. Here we report that in a survey of the two transcriptional activity and structural integrity of the two int loci in 30 BR6 mouse mammary tumours, around 50% of the tumours expressed both of these genes, in ostensibly monoclonal cell populations. Our data suggest that int-1 and int-2 may act cooperatively in the genesis of mammary carcinomas. However, because three tumours (10%) involved neither gene, and because in five cases activation occurred in the apparent absence of an adjacent provirus, it is clear that other loci and mechanisms contribute to tumorigenesis.

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.

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.

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.

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.

Structure and biological activity of human homologs of the raf/mil oncogene

Two human genes homologous to the raf/mil oncogene have been cloned and sequenced. One, c-raf-2, is a processed pseudogene; the other, c-raf-1, contains nine exons homologous to both raf and mil and two additional exons homologous to mil. A 3' portion of c-raf-1 containing six of the seven amino acid differences relative to murine v-raf can substitute for the 3' portion of v-raf in a transformation assay. Sequence homologies between c-raf-1 and Moloney leukemia virus at both ends of v-raf indicate that the viral gene was acquired by homologous recombination. Although the data are consistent with the traditional model of retroviral transduction, they also raise the possibility that the transduction occurred in a double crossover event between proviral DNA and the murine gene.

Long terminal repeat sequences impart hematopoietic transformation properties to the myeloproliferative sarcoma virus

The myeloproliferative sarcoma virus not only transforms fibroblasts but also causes extensive expansion of the hematopoietic stem cell compartment on infection of adult mice. Similar to the Moloney sarcoma virus, it carries the mos oncogene. Moloney sarcoma virus, however, does not induce myeloproliferation and leukemia in adult mice. The difference between the two viruses was explored by using their molecularly cloned genomes and the cellular mos oncogene to construct recombinant genomes. It was shown that the U3 region of the viral long terminal repeat (LTR) has a decisive function in determining the target cell specificity of the myeloproliferative sarcoma virus. Any mos gene, whether of cellular or viral origin, is sufficient in conjunction with the proper LTR to induce myeloproliferation. Our results indicate that the pathogenicity of acutely transforming viruses is determined not only by the oncogene but also by sequences in the viral LTR.

Rapid induction of hemopoietic neoplasms in newborn mice by a raf(mil)/myc recombinant murine retrovirus

3611 MSV, a raf oncogene-transducing murine retrovirus, induced fibrosarcomas in newborn mice after a latency of 4 to 8 weeks. In contrast, newly constructed recombinant murine retroviruses carrying the myc oncogene did not induce tumors before greater than or equal to 9 weeks. A combination of both oncogenes in an infectious murine retrovirus induced hematopoietic neoplasms in addition to less prominent fibrosarcomas and pancreatic acinar dysplasia 1 to 3 weeks after inoculation. The hematological neoplasms consisted of immunoblastic lymphomas of T- and B-lineage cells and erythroblastosis. Cell lines from these tumors could be readily established in culture in regular medium, whereas culture of cells from raf oncogene-induced tumors required the addition of interleukin 3. In parallel to the synergistic action of both oncogenes on hematopoietic cells in vivo, we found that raf oncogene-induced transformation of fibroblast cell lines in culture was enhanced by the addition of myc, which by itself did not morphologically transform these permanent cell lines. We conclude that concomitant expression of raf and myc oncogenes in hematopoietic cells and fibroblastic cell lines enhances their respective transforming activities.

Structure and biological activity of human homologs of the raf/mil oncogene

Two human genes homologous to the raf/mil oncogene have been cloned and sequenced. One, c-raf-2, is a processed pseudogene; the other, c-raf-1, contains nine exons homologous to both raf and mil and two additional exons homologous to mil. A 3' portion of c-raf-1 containing six of the seven amino acid differences relative to murine v-raf can substitute for the 3' portion of v-raf in a transformation assay. Sequence homologies between c-raf-1 and Moloney leukemia virus at both ends of v-raf indicate that the viral gene was acquired by homologous recombination. Although the data are consistent with the traditional model of retroviral transduction, they also raise the possibility that the transduction occurred in a double crossover event between proviral DNA and the murine gene.

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.

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.

Cellular oncogenes (c-erb-A and c-erb-B) located on different chicken chromosomes can be transduced into the same retroviral genome

Avian erythroblastosis virus has transduced two cellular genes, c-erb-A and c-erb-B. Using fractionated chicken chromosomes, we found that the two genes are located on different chromosomes in the chicken genome: c-erb-A is on a microchromosome, and c-erb-B is on a large chromosome. The locations of two other cellular oncogenes (c-fps and c-myb) were also determined: c-fps is on a microchromosome, and c-myb is on chromosome of an intermediate size. Our results suggest that avian erythroblastosis virus had transduced the two cellular genes independently, conforming to previous indications that cellular oncogenes are dispersed among multiple chromosomes in every species that has been examined.

3'-Terminal region of avian carcinoma virus MH2 shares sequence elements with avian sarcoma viruses Y73 and SR-A

We determined the nucleotide sequence of the acute transforming avian retrovirus MH2 from an HgiAI site within the coding region of its oncogene, v-myc, to the KpnI site within the long terminal repeat. Comparison with published sequences from other retroviruses allowed us to identify all sequence elements in this region. We conclude that MH2 contains a unique assembly of 3'-terminal sequences, which includes part of the helper virus-derived SPC region of avian sarcoma virus Y73 and the complete F3 and F1 segments of Rous sarcoma virus strain SR-A.

Cellular homologs of the avian erythroblastosis virus erb-A and erb-B genes are syntenic in mouse but asyntenic in man

Avian erythroblastosis virus, a retrovirus that causes erythroblastosis and sarcomas in infected birds, possesses two host cell-derived genes [viral (v) erb-A and erb-B]. Although v-erb-B seems to be responsible for oncogenic transformation, v-erb-A might have an enhancing effect on transformation. In chickens, the natural host for avian erythroblastosis virus, cellular (c) erb-A and erb-B genes appear to be unlinked, but their chromosomal locations in other species are unknown. To ascertain the chromosomal location of c-erb genes in man and mouse, we analyzed interspecies somatic cell and microcell hybrids by Southern filter hybridization techniques using specific v-erb-A and v-erb-B probes. We found c-erb-A sequences on human chromosome 17 (17p11----qter) and located c-erb-B on human chromosome 7 (7pter----q22). In contrast, both c-erb-A and c-erb-B reside on mouse chromosome 11.

Cellular oncogenes (c-erb-A and c-erb-B) located on different chicken chromosomes can be transduced into the same retroviral genome

Avian erythroblastosis virus has transduced two cellular genes, c-erb-A and c-erb-B. Using fractionated chicken chromosomes, we found that the two genes are located on different chromosomes in the chicken genome: c-erb-A is on a microchromosome, and c-erb-B is on a large chromosome. The locations of two other cellular oncogenes (c-fps and c-myb) were also determined: c-fps is on a microchromosome, and c-myb is on chromosome of an intermediate size. Our results suggest that avian erythroblastosis virus had transduced the two cellular genes independently, conforming to previous indications that cellular oncogenes are dispersed among multiple chromosomes in every species that has been examined.

A human c-erbA oncogene homologue is closely proximal to the chromosome 17 breakpoint in acute promyelocytic leukemia

A human cDNA library was screened for sequences homologous to the erbA gene of avian erythroblastosis virus (AEV). One such clone, cHerbA-1, was used to map the chromosomal location of highly homologous human sequences that were found to be present on chromosome 17 as judged by Southern blot screening of a panel of mouse-human hybrid cell lines segregating human chromosomes. cHerbA-1 was hybridized in situ to metaphase chromosomes from a normal male subject and from a female patient with an acute promyelocytic leukemia (APL) having the typical t(15;17) translocation. The results localized the cellular c-erbA sequences on chromosome 17 to the q21-q24 region of normal chromosomes and indicated that the c-erbA sequences remained on the 17q- chromosome in the APL cells, suggesting that they could be assigned to the 17(q21-q22) region. For additional data, we hybridized human neoplastic cells derived from a poorly differentiated acute leukemia carrying a t(17;21) translocation with thymidine kinase (TK)-deficient LMTK- mouse cells. A resulting hybrid, containing only the 21q+ chromosome, did not have human c-erbA sequences. Since the breakpoint on 17q in this translocation was similar to that in the APL t(15;17) translocation, this supported the assignment of c-erbA to the q21-q22 region of chromosome 17. The apparent close proximity of the c-erbA sequences to the chromosomal breakpoints in these two leukemias suggests a possible role for this oncogene homologue in the development of these neoplasms.

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.