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

Identification of a novel protein in porcine astrovirus that is important for virus replication

Overlapping genes are common in some RNA viruses. It has been proposed that a potential overlapping gene is the ORFX, here termed ORF2b, which overlaps the ORF2 coding sequence in astroviruses. The aim of this study was to determine whether ORF2b is an overlapping gene that encodes a functional protein which is needed for viral replication. Sequence alignment showed that there was an ORF2b in a PAstV type 1 strain of astrovirus, PAstV1-GX1, which was embedded within the larger ORF2. The AUG codon for ORF2b is located 19 nucleotides downstream of the initiation site of ORF2 and contains 369 nucleotides and it codes for a predicted 122-amino-acid protein. A specific polyclonal antibody against the ORF2b protein was raised and used to demonstrate the expression of the new identified gene in virus-infected and pCAGGS-ORF2b-transfected cells. Analysis of purified virions revealed that the ORF2b protein was not incorporated into virus particles. Reverse genetics based on a PAstV type 1 infectious cDNA clone showed that the ORF2b protein was not essential but important for optimal virus infectivity. Knockout of the downstream potential stop codon candidate of ORF2b demonstrated that the C-terminus of the ORF2b protein can be extended by 170 amino acids, suggesting that the C-terminus of the newly identified ORF2b protein may be variable.

The glucocorticoid receptor is increased in Atm-/- thymocytes and in Atm-/- thymic lymphoma cells, and its nuclear translocation counteracts c-myc expression

We have previously demonstrated that spontaneous DNA synthesis in immature thymocytes of Atm-/- mice is elevated, and that treatment with the glucocorticoid dexamethasone (Dex) attenuates this increased DNA synthesis and prevents the development of thymic lymphomas. Deregulation of c-myc may drive the uncontrolled proliferation of Atm-/- thymocytes, since upregulation of c-myc parallels the elevated DNA synthesis in the cells. In this study, we show that the glucocorticoid receptor (GR) is expressed at high levels in Atm-/- thymocytes and in Atm-/- thymic lymphoma cells, although serum glucocorticoid (GC) levels in Atm-/- mice are similar to those in Atm+/+ mice. In cultured Atm-/- thymic lymphoma cells treated with Dex, GR nuclear translocation occurs, resulting in suppression of DNA synthesis and c-myc expression at both the mRNA and protein levels. Interestingly, the GR antagonist RU486 also causes GR nuclear translocation, but does not affect DNA synthesis and c-myc expression in Atm-/- thymic lymphoma cells. As expected, RU486 reverses the suppressive effects of Dex on DNA synthesis and c-myc expression. Administration of Dex to Atm-/- mice decreases the elevated c-Myc protein levels in their thymocytes. These findings suggest that GC/GR signaling plays an important role in regulating c-myc expression in Atm-/- thymocytes and thymic lymphoma cells.

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

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

Acutely transforming avian leukosis virus subgroup J strain 966: defective genome encodes a 72-kilodalton Gag-Myc fusion protein

Avian leukosis virus subgroup J (ALV-J), the most recent member of the avian retroviruses, is predominantly associated with myeloid leukosis in meat-type chickens. We have previously demonstrated that the acutely transforming virus strain 966, isolated from an ALV-J-induced tumor, transformed peripheral blood monocyte and bone marrow cells in vitro and induced rapid-onset tumors, suggesting transduction of oncogenes (L. N. Payne, A. M. Gillespie, and K. Howes, Avian Dis. 37:438-450, 1993). In order to understand the molecular basis for the rapid transformation and tumor induction, we have determined the complete genomic structure of the provirus of the 966 strain. The sequence of the 966 provirus clone revealed that its genome is closely related to that of HPRS-103 but is defective, with the entire pol and parts of the gag and env genes replaced by a 1,491-bp sequence representing exons 2 and 3 of the c-myc gene. LSTC-IAH30, a stable cell line derived from turkey monocyte cultures transformed by the 966 strain of ALV-J, expressed a 72-kDa Gag-Myc fusion protein. The identification of the myc gene in 966 virus as well as in several other ALV-J-induced tumors suggested that the induction of myeloid tumors by this new subgroup of ALV occurs through mechanisms involving the activation of the c-myc oncogene.

Transforming ability of Gag-Myc fusion proteins correlates with Gag-Myc protein stability and transcriptional repression

Avian retroviruses that have transduced c-myc are useful tools to study the conditions necessary for cellular transformation. FH3, one such retrovirus which encodes a Gag-Myc fusion protein, is not transforming in quail embryonic fibroblasts, but a late variant of FH3 that arose after passaging FH3-infected cells is transforming. Mutational analysis of FH3 revealed that the presence of a portion of the retroviral protease in FH3 inhibited transformation and that this inhibition was transferable to a more highly transforming retrovirus, MC29. Transforming and non-transforming FH3-derived and MC29-derived Gag-Myc proteins were used to further explore characteristics of Myc necessary for transformation. Gag-Myc proteins which were transforming were found to be the most stable in the cell. To distinguish whether transactivation and/or repression is correlated to transformation, the various Gag-Myc fusion proteins were tested for their ability to activate or repress c-Myc targets. Results indicated that a correlation exists between transforming Gag-Myc proteins and their ability to repress, whereas all Gag-Myc proteins could transactivate, regardless of their ability to transform. Taken together, these results suggest that protein stabilization of Myc and repression of target genes by Myc are important for cellular transformation.

The v-myc oncogene

v-myc is the viral homolog of c-myc transduced by several acute transforming retroviruses, many of which encode this gene as a Gag-Myc fusion protein. The v-myc oncogene can transform several lineages of mammalian and avian cells either alone or in cooperation with other oncogenes. While the Gag portion of the Gag-Myc fusion protein and the nuclear localization signal each appear to be dispensable for transformation, the N- and C-termini of the Myc sequence have been found to be essential for transformation. All v-myc genes contain point mutations which seem to confer a greater potency to v-myc in the process of transformation, proliferation, and apoptosis. In v-myc-transformed myelomonocytic cells, secondary events occur, such as the expression of colony stimulating factor-1 (CSF-1) which play a critical role in immortalization and subsequent tumor progression. Inhibition of the autocrine loop of CSF-1 was found to induce apoptosis in the immortalized cells. While overexpression of v-Myc blocks terminal differentiation of hematopoietic cells, this is not sufficient to block the differentiation of certain neural and skeletal muscle cells. Recent developments on the effects of v-myc on cell growth, transformation, differentiation and apoptosis are discussed in this review.

gag as well as myc sequences contribute to the transforming phenotype of the avian retrovirus FH3

The avian retrovirus FH3, like MC29 and CMII, encodes a Gag-Myc fusion protein. However, the FH3-encoded protein is larger, about 145 kDa, and contains almost the entire retroviral gag gene. In contrast to the other gag-myc avian retroviruses, FH3 fails to transform fibroblasts in vitro, although macrophages are transformed both in vitro and in vivo (C. Chen, B. J. Biegalke, R. N. Eisenman, and M. L. Linial, J. Virol. 63:5092-5100, 1989). We have used the polymerase chain reaction technique to obtain a molecular clone of FH3. Sequence analysis of the FH3 myc oncogene revealed a single proline----histidine change (position 223) relative to c-myc. However, substitution of the FH3 myc sequence with the chicken c-myc sequence did not alter the transformation potential of the virus. Hence, overexpression of the proto-oncogene as a Gag-Myc retroviral protein is sufficient for macrophage, but not fibroblast, transformation. After passage of FH3 in fibroblast cultures, a virus (FH3L) that is capable of rapidly transforming fibroblasts appears. The Gag-Myc protein encoded by FH3L is smaller (ca. 130 kDa) than that encoded by the original viral stock (FH3E). Sequencing of an FH3L molecular clone revealed a 212-amino-acid deletion within the Gag portion. Using FH3E/FH3L recombinants, we have demonstrated that the ability of encoded viruses to transform fibroblasts directly correlates with the presence of this deletion. Moreover, the addition of the Gag sequence deleted from FH3L to the MC29 oncoprotein significantly reduces its transforming activity as measured by focus assay. These data suggest that the C-terminal segment of Gag attenuates the oncogenic potential of Gag-Myc fusion proteins.

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.

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.

Two autonomous myc oncogenes in avian carcinoma virus OK10

The oncogenic avian retrovirus OK10 has the genetic structure gag-delta pol-myc-delta-env. The myc sequence is transduced from a cellular gene, proto-myc, while gag, pol, and env are essential retrovirus genes. By analogy with other directly oncogenic retroviruses, the specific myc sequence of OK10 is thought to be essential for transforming function. However, unlike the specific sequences of all other transforming retroviruses that encode unique transforming proteins, the myc sequence of OK10 encodes two potential transforming proteins, p58 and p200. p200 is translated from the gag-delta pol-myc region of genomic RNA, while p58 is thought to be translated from the gag leader and the open reading frame of myc via a subgenomic mRNA. In this paper, we ask whether both myc genes of OK10 are autonomous transforming genes. By differentially inactivating the p200 myc gene of OK10 provirus in vitro and analyzing transforming function in quail embryo cells, it was found that mutants expressing only p58 transformed like wild-type OK10. Further, it was shown that p58 with and without the gag leader had transforming function and that p58 of wild-type OK10 is initiated from the gag leader. Mutants expressing only p200 were also transforming but less efficiently than mutants that express only p58. A mutant OK10 virus in which the native frameshift of retroviruses between gag and pol was deleted expressed a shortened p200 (delta p200). Although this virus expressed more delta p200 than wild-type OK10 did, it transformed cells less efficiently. It follows that OK10 expresses two autonomous transforming genes, which is unique among retroviruses with onc genes.

Absence of missense mutations in activated c-myc genes in avian leukosis virus-induced B-cell lymphomas

We have determined the nucleotide sequences of two independent DNA clones which contained the activated c-myc genes from avian leukosis virus-induced B-cell lymphomas. Neither of these c-myc genes contained missense mutations. This strongly supports the notion that the c-myc proto-oncogene in avian leukosis virus-induced B-cell lymphomas can be oncogenically activated by altered expression of the gene without a change in the primary structure of the gene product.

Absence of missense mutations in activated c-myc genes in avian leukosis virus-induced B-cell lymphomas

We have determined the nucleotide sequences of two independent DNA clones which contained the activated c-myc genes from avian leukosis virus-induced B-cell lymphomas. Neither of these c-myc genes contained missense mutations. This strongly supports the notion that the c-myc proto-oncogene in avian leukosis virus-induced B-cell lymphomas can be oncogenically activated by altered expression of the gene without a change in the primary structure of the gene product.

myc protooncogene linked to retroviral promoter, but not to enhancer, transforms embryo cells

To define conditions under which the chicken protooncogene p-myc is converted to a viral and possibly to a cellular transforming gene, we assayed transforming function of hybrid genes put together from cloned retroviral and p-myc elements and of p-myc genes isolated from spontaneous viral lymphomas. Transforming function was measured in quail embryo cells transfected with cloned myc genes. We found that only myc genes with a promoter of a retroviral long terminal repeat (LTR) located between the native p-myc promoter and the second p-myc exon have transforming function. Transforming efficiencies decreased with increasing lengths of unspliced sequences between the LTR and p-myc exon 2. p-myc DNAs with LTRs downstream of the coding region or upstream but in the opposite transcriptional orientation failed to transform embryo cells. Likewise, only those retroviral-p-myc combinations from chicken B-cell lymphomas with a LTR positioned as promoter upstream of p-myc exon 2 had transforming function. We conclude that substitution of a retroviral LTR for the promoter and for as yet poorly defined, untranscribed regulatory elements of p-myc is sufficient to convert chicken p-myc to a transforming gene. However, retroviral LTRs can only convert p-myc genes to embryo-cell-transforming genes from a limited number of positions, and not as position-independent enhancers. Further, we deduce that there are two classes of viral chicken B-cell lymphomas, those with and those without embryo-cell-transforming p-myc genes.

Transforming potential of a myc-containing variant of feline leukemia virus in vitro in early-passage feline cells

We studied a naturally occurring variant of feline leukemia virus (FeLV) in which the oncogene myc has substituted for a portion of the viral structural genes (myc-FeLV). myc-FeLV was rescued by replication in the presence of FeLV as helper, and its biological activity was examined in early-passage feline cells in vitro. Infection of leukocytes from peripheral blood, spleen, or thymus, or of kitten fibroblasts did not immortalize these cells or alter them morphologically. Northern blot (RNA blot) analysis of virion RNA prepared from the supernatant of infected cells demonstrated the 8.2-kilobase genome of FeLV, but did not demonstrate the 5.0-kilobase genome of myc-FeLV. Apparently, the myc-FeLV genome was lost in the absence of the selective pressure of transformation. In contrast, infection of embryonic fibroblasts with myc-FeLV(FeLV) rendered these cells capable of greatly increased, if not infinite, proliferative potential. The cells were morphologically altered compared with controls and were only loosely adherent to the substrate. The cells failed to proliferate in semisolid medium and did not form tumors when inoculated subcutaneously into athymic mice. Blot analyses demonstrated the presence and expression of integrated proviral DNAs of both FeLV and myc-FeLV in these cells. They appear, then, to represent cells partially transformed by infection with myc-FeLV(FeLV). The action of feline v-myc in early-passage cells in vitro was compared to that of avian v-myc.

Cancer genes generated by rare chromosomal rearrangements rather than activation of oncogenes

The 20 known transforming onc genes of retroviruses are defined by sequences that are transduced from cellular genes, termed proto-oncogenes or cellular oncogenes. Based on these sequences, viral onc genes have been postulated to be transduced cellular cancer genes and proto-onc genes have been postulated to be latent cancer genes that can be activated from within the cell to cause virus-negative tumors. The hypothesis is popular because it promises direct access to cellular cancer genes. However, the existence of latent cancer genes presents a paradox since such genes are clearly undesirable. The hypothesis predicts (i) that viral onc genes and proto-onc genes are isogenic, (ii) that expression of proto-onc genes induces tumors, (iii) that activated proto-onc genes transform diploid cells upon transfection, like viral onc genes, and (iv) that diploid tumors exist that differ from normal cells only in transcriptionally or mutationally activated proto-onc genes. As yet, none of these predictions is confirmed. Moreover, the probability of spontaneous transformation in vivo is at least 10(9) times lower than predicted from the mechanisms thought to activate proto-onc genes. Therefore the hypothesis, that proto-onc genes are latent cellular oncogenes, appears to be an overinterpretation of sequence homology to structural and functional homology with viral onc genes. Here it is proposed that only rare truncations and illegitimate recombinations that alter the germline configuration of cellular genes, generate viral and possibly cellular cancer genes. The clonal chromosome abnormalities that are consistently found in tumor cells are microscopic evidence for rearrangements that may generate cancer genes. The clonality indicates that the tumors are initiated with, and possibly by, these abnormalities as predicted by Boveri in 1914 (Zur Frage der Entstehung maligner Tumoren, Jena, Fischer).

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.

Cancer genes: rare recombinants instead of activated oncogenes (a review)

The 20 known transforming (onc) genes of retroviruses are defined by sequences that are transduced from cellular genes termed protooncogenes or cellular oncogenes. Based on these sequences, viral onc genes have been postulated to be transduced cellular cancer genes, and proto-onc genes have been postulated to be latent cancer genes that can be activated from within the cell to cause virus-negative tumors. The hypothesis is popular because it promises direct access to cellular cancer genes. However, the existence of latent cancer genes presents a paradox, since such genes are clearly undesirable. The hypothesis predicts that viral onc genes and proto-onc genes are isogenic; that expression of proto-onc genes induces tumors; that activated proto-onc genes transform diploid cells upon transfection, like viral onc genes; and that diploid tumors exist. As yet, none of these predictions is confirmed. Instead: Structural comparisons between viral onc genes, essential retroviral genes, and proto-onc genes show that all viral onc genes are indeed new genes, rather than transduced cellular cancer genes. They are recombinants put together from truncated viral and truncated proto-onc genes. Proto-onc genes are frequently expressed in normal cells. To date, not one activated proto-onc gene has been isolated that transforms diploid cells. Above all, no diploid tumors with activated proto-onc genes have been found. Moreover, the probability of spontaneous transformation in vivo is at least 10(9) times lower than predicted from the mechanisms thought to activate proto-onc genes. Therefore, the hypothesis that proto-onc genes are latent cellular oncogenes appears to be an overinterpretation of sequence homology to structural and functional homology with viral onc genes. Here it is proposed that only rare truncations and illegitimate recombinations that alter the germ-line configuration of cellular genes generate viral and possibly cellular cancer genes. The clonal chromosome abnormalities that are consistently found in tumor cells are microscopic evidence for rearrangements that may generate cancer genes. The clonality indicates that the tumors are initiated with, and possibly by, these abnormalities, as predicted by Boveri in 1914.

Synergism of v-myc and v-Ha-ras in the in vitro neoplastic progression of murine lymphoid cells

Murine bone marrow was either singly or doubly infected with retroviral vectors expressing v-myc (OK10) or v-Ha-ras. The infected bone marrow was cultured in a system that supports the long-term growth of B-lineage lymphoid cells. While the v-myc vector by itself had no apparent effect on lymphoid culture establishment and growth, infection with the v-Ha-ras vector or coinfection with both v-myc and v-Ha-ras vectors led to the appearance of growth-stimulated cell populations. Clonal pre-B-cell lines stably expressing v-Ha-ras alone or both v-myc and v-Ha-ras grew out of these cultures. In comparison with cell lines expressing v-Ha-ras alone, cell lines expressing both v-myc and v-Ha-ras grew to higher densities, had reduced dependence on a feeder layer for growth, and had a marked increase in ability to grow in soft-agar medium. The cell lines expressing both oncogenes were highly tumorigenic in syngeneic animals. These experiments show that the v-myc oncogene in synergy with v-Ha-ras can play a direct role in the in vitro transformation of murine B lymphoid cells.

Synergism of v-myc and v-Ha-ras in the in vitro neoplastic progression of murine lymphoid cells

Murine bone marrow was either singly or doubly infected with retroviral vectors expressing v-myc (OK10) or v-Ha-ras. The infected bone marrow was cultured in a system that supports the long-term growth of B-lineage lymphoid cells. While the v-myc vector by itself had no apparent effect on lymphoid culture establishment and growth, infection with the v-Ha-ras vector or coinfection with both v-myc and v-Ha-ras vectors led to the appearance of growth-stimulated cell populations. Clonal pre-B-cell lines stably expressing v-Ha-ras alone or both v-myc and v-Ha-ras grew out of these cultures. In comparison with cell lines expressing v-Ha-ras alone, cell lines expressing both v-myc and v-Ha-ras grew to higher densities, had reduced dependence on a feeder layer for growth, and had a marked increase in ability to grow in soft-agar medium. The cell lines expressing both oncogenes were highly tumorigenic in syngeneic animals. These experiments show that the v-myc oncogene in synergy with v-Ha-ras can play a direct role in the in vitro transformation of murine B lymphoid 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.

A mouse c-myc retrovirus transforms established fibroblast lines in vitro and induces monocyte-macrophage tumors in vivo

Activation of the c-myc proto-oncogene, in the form of DNA rearrangements that lead to constitutive expression, has been implicated in the genesis of a wide range of tumors. Therefore, it is of great interest to determine the influence of c-myc oncogene activation on cellular growth control, especially in primary cells. To facilitate the efficient transfer of an activated c-myc oncogene, we developed a mouse retrovirus that contains the c-myc protein-coding sequences and which can be transmitted in the presence of a Moloney murine leukemia virus helper or established as a helper-free stock with a retrovirus-packaging cell line. The virus can transform established lines of mouse fibroblasts to anchorage-independent growth; the transformed cells are tumorigenic in nude mice. However, the virus was not capable of inducing foci of transformed cells on confluent monolayers. In addition to studies on established cell lines, the effect of the c-myc retrovirus on primary cells was examined. Infection of bone marrow cells gave rise to partially transformed mononuclear phagocytes which were entirely dependent upon an exogenous supply of the monocyte-specific colony-stimulating factor CSF-1 for proliferation. Infection in vivo induced monocyte-macrophage tumors with a latency period of 8 to 10 weeks.

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.

Harvey ras genes transform without mutant codons, apparently activated by truncation of a 5' exon (exon -1)

The hypothesis is tested that the ras gene of Harvey sarcoma virus (Ha-SV) and the proto-ras DNAs from certain tumor cells derive transforming function from specific codons in which they differ from normal proto-ras genes. Molecularly cloned Harvey proviral vectors carrying viral ras, normal rat proto-ras, and recombinant ras genes in which the virus-specific ras codons 12 and 59 were replaced by proto-ras equivalents each transformed aneuploid mouse 3T3 cells after latent periods that ranged from 4 to 10 days. Viruses with or without virus-specific ras codons all transformed diploid rat cells in 3-5 days equally well. However, in the absence of virus replication, mutant codons were beneficial for transforming function. Deletion of non-ras regions of Ha-SV did not affect transforming function. We conclude that specific ras codons are not necessary for transforming function. Comparisons of the ras sequences of Ha-SV, BALB SV, and Rasheed SV with sequences of proto-ras genes from rat and man revealed an upstream proto-ras exon, termed exon -1. The 3' end of this exon is present in all three viruses and in a ras pseudogene of the rat. Since ras genes transform without mutation and since exon -1 is truncated in viral ras genes and all transforming proto-ras DNAs of the Harvey and the Kirsten ras family, we propose that ras genes are activated by truncation of exon -1 either via viral transduction or artificially via cloning and transfection. The proposal implies that untruncated proto-ras genes with point mutations may not be cellular cancer genes.

Induction of c-sis mRNA and activity similar to platelet-derived growth factor by transforming growth factor beta: a proposed model for indirect mitogenesis involving autocrine activity

Treatment of quiescent cultures of mouse embryo-derived AKR-2B cells with transforming growth factor beta resulted in an early induction of c-sis mRNA. The increase in c-sis mRNA was followed by a corresponding increase in protein similar to platelet-derived growth factor (PDGF) in the culture medium. In addition, PDGF-regulated genes (c-fos and c-myc) were stimulated by transforming growth factor beta with delayed kinetics relative to that seen in other cell systems with direct PDGF stimulation. A model is proposed in which the monolayer mitogenicity of transforming growth factor beta is mediated by the induction of c-sis and PDGF and the subsequent autocrine stimulation of c-fos, c-myc, and other PDGF-inducible genes.

Defining the borders of the chicken proto-fps gene, a precursor of Fujinami sarcoma virus

The transforming (onc) genes of retroviruses contain specific sequences, derived from as yet poorly defined, normal cellular genes, termed proto-onc genes. Proto-onc genes must be defined to explain their docility compared to the oncogenicity of the viral derivatives. Here we set out to determine the borders of the chicken proto-fps gene from which the onc genes of avian Fujinami (FSV) and PRC sarcoma viruses (PRCSV) are derived. These onc genes are hybrids of an element from the gag gene of retroviruses (delta gag) linked to a 2.8-kb domain from proto-fps. To identify the 5' border of proto-fps we have sequenced 1.5 kb beyond the 5' border of overlap with viral fps utilizing a proto-fps clone derived previously. A possible promoter was identified that maps 736 nucleotides from this border. The 736 nucleotides contain two possible exons with 121 codons, and short regions of homology with the delta gag termini of FSV and PRCII. A translation stop codon and an adjacent polyadenylation signal were identified just prior to the 3' border of overlap with viral fps within a 1.15-kb sequence of a newly isolated proto-fps clone. Comparing four exons within this 1.15 kb proto-fps sequence with known fps equivalents of FSV and PRCSV, we have detected strain-specific, but no common point mutations in each viral genome. A 3.3-kb polyadenylated proto-fps mRNA was detected in chicken liver RNA by gel electrophoresis and hybridization with proto-fps DNA. We conclude that the coding capacity of proto-fps is just over 3 kb, consistent with the size of the putative proto-fps protein of 98 kDa and hence slightly larger than that of viral fps. Thus proto-fps and the viral delta gag-fps genes each contain distinct 5' regulatory and coding sequences and share the 3' terminal fps domains. It is suggested that this difference, rather than scattered point mutations, is responsible for the oncogenic function of the viral genes and the unknown cellular function of proto-fps.

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.

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.