Vertebrate DNAs contain nucleotide sequences related to the transforming gene of avian myeloblastosis virus

Conformation Dynamics of the Intrinsically Disordered Protein c-Myb with the ff99IDPs Force Field

The intrinsically disordered protein c-Myb plays a critical role in cellular proliferation and differentiation. Loss of c-myb function results in embryonic lethality due to failure of fetal hepatic hematopoiesis. The conformation dynamics of the intrinsically disordered c-Myb are still unknown. Here, molecular dynamics (MD) simulations with the intrinsically disordered protein force field ff99IDPs were used to study the conformation dynamics. In comparison with ff99SBildn, ff99IDPs can reproduce more diverse disordered conformers of c-Myb. The predicted secondary chemical shift under ff99IDPs is more close to that of experiment data than that under ff99SBildn. Therefore, ff99IDPs can sample native molten globule, native pre-molten globule and native coil conformers for c-Myb. These results are consistent with those of other intrinsically disordered proteins. Kinetic analysis of MD simulations shows that c-Myb folds via a two-state process and indicates that c-Myb folds in the order of tertiary folding and helical folding. The folding nucleus of KEL plays an essential role in stabilizing the folding state with dynamic correlation networks. The influences of solvent models for TIP3P, TIP4P-EW and TIP5P were also investigated and it was found that TIP3P and ff99IDPs are the best combination to research the conformer sampling of c-Myb. These results reveal the conformation dynamics of c-Myb and confirm that the ff99IDPs force field can be used to research the relationship between structure and function of other intrinsically disordered proteins.

Ras oncogene enhances the production of a recombinant protein regulated by the cytomegalovirus promoter in BHK-21 cells

In order to enhance recombinant protein productivity in animal cells, we developed the oncogene activated production (OAP) system. The OAP system is based on the premise that oncogenes are able to enhance promoter activity. To this end, we constructed reported plasmids by fusing various promoters to the human interleukin-6 (hIL-6) cDNA, and the effector plasmids by inserting individual oncogenes, for example c-myc, c-fos, v-jun, v-myb and c-Ha-ras, downstream from the human cytomegalovirus immediate early (CMV) promoter. Results of transient expression experiments with BHK-21 cells suggest that the CMV promoter is the most potent promoter examined and that the ras product is able to transactivate the beta-actin, CMV and SR alpha promoters. Recombinant BHK-21 cells producing hIL-6 under the control of the CMV promoter were contransfected with the ras oncogene and dihydrofolate reductase gene, then selected with 50 nM methotrexate to coamplify the ras oncogene. We were able to rapidly establish a stable and highly productive clone which exhibited a 35-times higher production rate as compared to the control value.

The detection of a breast cancer antigen on MCF-7 cells reactive with the TCR (alpha) of a specific killer T cell line

We established several immortalized human T cell lines by cotransfecting primary lymphocytes derived from breast cancer patients with human c-myc and human c-Ha-ras oncogenes. A CD8 positive (CD8+) killer T cell line, FT-8, exhibited in vitro specific cytotoxicity to a human breast cancer cell line, MCF-7. The FT-8 cells suppressed the growth of MCF-7 cells transplanted to athymic mice. The cytotoxic reaction was caused via T cell antigen receptor (TCR) on MCF-7 cells, because monoclonal antibodies against the TCR inhibited the cytotoxicity of FT-8 cells. The TCR (alpha) cDNA of FT-8 was cloned by using a PCR amplification technique and expressed by a cell-free in vitro translation system. The TCR (alpha) protein recognized a target antigen of 32 KDa on MCF-7 cells.

Purification and characterization of interferon-like antiviral protein derived from flatfish (Paralichthys olivaceus) lymphocytes immortalized by oncogenes

Flatfish leukocytes were transfected with the expression plasmids of the v-myc, c-myc, c-fos, v-myb and c-Ha-ras oncogenes. Only cotransfection of c-Ha-ras with c-myc or c-fos resulted in complete immortalization of the cells. Interferon-like anti-viral protein was found in the cultured medium of the immortalized lymphocytes. The protein was purified by DEAE-Toyopearl 650 M ion exchange chromatography and WGA agarose affinity chromatography. The protein was a glycoprotein of about 16 kDa. The antiviral activity of the protein was trypsin-sensitive and was fairly stable at pH values from 4 to 8. The protein retained about 60% of the activity even at 60 degrees C and showed a broad antiviral activity for various fish cells and viruses.

Immortalization of human T lymphocytes by oncogenes

Immortalized human T cell lines were established by cotransfecting c-Ha-ras and c-myc oncogenes to lymph node lymphocytes. The cell lines kept growing for 3 months after establishment without a decrease in growth rate. The cells did not require interleukin-2 (IL-2) for their growth, but addition of IL-2 stimulated the growth of these cells. Flow cytometric analysis revealed that these cells were T cells expressing CD4 or CD8 antigens. A CD4 positive (CD4+) cell line produced IL-6, indicating that the cell line belongs to helper T cells. The CD8 positive (CD8+) cell line possessed cytotoxicity to tumor cells, indicating that the cell line were killer T cells. Both cell lines were able to proliferate in serum-free medium indefinitely.

Oncogene expression in the liver tissue of patients with nonneoplastic liver disease

The expression of cellular oncogenes in nonneoplastic human liver tissue was examined to determine if there was a correlation between oncogene expression and physiologic regeneration in liver disease. Human liver tissue specimens from 70 patients with various histologic findings from almost normal to cirrhosis were examined (using northern blot analysis) for the expression of nine cellular oncogenes. With c-K-ras, four RNA bands (5.6-kilobase [kb], 2.1-kb, 1.5-kb, and 1.2-kb RNA species) were detected in all liver tissue examined. Expression of c-fos was also detected in a few samples examined when 50-micrograms samples of total RNA were applied. Other oncogenes such as H-ras, myc, erbB, raf, fms, fes, and myb were not detected. These results indicate that particular oncogene(s) may not be highly expressed during liver regeneration in human liver disease, or that populations of regenerating hepatocytes may be too small to show significant elevations of oncogene expression. The new finding of a constant expression of c-K-ras in human liver tissue suggests that it is linked to essential hepatocellular function rather than carcinogenesis.

Identification of functional domains in the maize transcriptional activator C1: comparison of wild-type and dominant inhibitor proteins

Genes encoding fusions between the maize regulatory protein C1 and the yeast transcriptional activator GAL4 and mutant C1 proteins were assayed for their ability to trans-activate anthocyanin biosynthetic genes in intact maize tissues. The putative DNA-binding region of C1 fused to the transcriptional activation domain of GAL4 activated transcription of anthocyanin structural gene promoters in c1 aleurones, c1 Rscm2 embryos, and c1 r embryogenic callus. Cells receiving these constructs accumulated purple anthocyanin pigments. The C1 acidic region fused to the GAL4 DNA-binding domain activated transcription of a GAL4-regulated promoter. An internal deletion of C1 also induced pigmentation; however, frameshifts in either the amino-terminal basic or carboxy-terminal acidic region blocked trans-activation, and the latter generated a dominant inhibitory protein. Fusion constructs between the wild-type C1 cDNA and the dominant inhibitor allele C1-I cDNA were used to identify the amino acid changes in C1 responsible for the C1-I inhibitory phenotype. Results from these studies establish that amino acids within the myb-homologous domain are critical for transcriptional activation.

c-myb gene analysis in T-cell malignancies with del(6q)

Three T-cell malignancies with del(6q) were analyzed for karyotypes and alteration of the oncogene c-myb that is assigned to 6q22-q24. Patients were diagnosed as having non-Hodgkin T-cell lymphoblastic lymphoma, adult T-cell leukemia, and acute T-cell lymphoblastic leukemia, and the deletions of chromosome 6 were del(6)(q21q25), del(6)(q21q23), and del(6)(q21) or del(6)(q21q27), respectively. Tumor cell DNAs were obtained from cultured pleural fluid or from fresh peripheral blood and marrow samples and were analyzed by Southern blot hybridization, using c-myb oncogene probes. Rearrangements, deletions, or amplifications were absent in these tumor DNAs, thereby indicating that the del(6q) breakpoint in these T-cell malignancies was located outside of the c-myb gene. Northern blot analysis revealed the elevated expression of c-myb in the non-Hodgkin lymphoma patient, in accord with lineage characteristics.

The cellular myb oncogene is amplified, rearranged and activated in human glioblastoma cell lines

The human c-myb gene which encodes a DNA binding protein and which is rarely amplified in neoplastic cells was found to be altered in four human glioblastoma cell lines. It exists in multiple copies in 2 out of 4 cases studied. The degree of amplification as determined by densitometry was about 10-fold, a rearrangement within the coding region and an enhanced gene activity of c-myb were noted. The observation of c-myb oncogene amplification and activity in glioblastoma cell lines presents the first report of this effect in human brain tumor cells.

Chromosomal reallocation of the chicken c-myb locus and organization of 3'-proximal coding exons

In the course of our studies concerning the tissue-specific expression of the c-myb proto-oncogene, we have established the nucleotide sequence of the chicken c-myb 3'-proximal coding exons. In situ hybridization performed with different genomic DNA probes corresponding to nearly all the c-myb gene allowed us to localize the corresponding locus on the large acrocentric chromosome 3 in chicken. Our sequencing data also indicate that the 3'-proximal noncoding sequences represented in c-myb mRNA species are derived from non-contiguous exons.

p75c-myb expression in leukemia-lymphoma cells correlated with proliferation and differentiation

Monoclonal antibodies (McAbs) against a part of v-myb gene product were prepared for the detection of human c-myb gene product (p75c-myb). Western blotting analyses with these McAbs were performed on human leukemia-lymphoma cells. All T-cell lines were positive in p75c-myb expression. B-cell lines were variable, myeloid and erythroid cells were positive although the amount of expressed p75c-myb was less than the T-cell lines. Cells isolated from patients were positive in expression except for cells from acute myeloblastic leukemia with maturation (AML M2), acute hypergranular promyelocytic leukemia (AML M3) and erythroleukemia (AML M6) developed from myelodysplastic syndromes. Differences in p75c-myb expression seemed to depend upon the differentiation stage and distinctive lineage from which each cell line had been established. The p75c-myb expression in HL60 (acute promyelocytic leukemia cell line) showed remarkably high at logarithmic growth. When examined with HL60, p75c-myb expression significantly decreased during the differentiation induced by 12-O-tetradecanoylphorbol-13-acetate or retinoic acid. These results suggest that p75c-myb expression plays a crucial role in hematopoietic cell proliferation and differentiation and that multiple mechanisms including aberrant expression of p75c-myb is involved in leukemogenesis.

Effects of the antisense v-myb' expression on K562 human leukemia cell proliferation and differentiation

Recombinant plasmids containing v-myb' (803 bp fragment of the 3' end of v-myb) were constructed to induce sense or antisense v-myb' RNA expression with dexamethasone in human cells. These plasmids were used as a tool for the investigation of the role of c-myb gene in human leukemia cell proliferation and differentiation. They were transfected by electroporation into the K562 human leukemia cell line derived from a patient with chronic myelogenous leukemia in blastic crisis. After induction of transcription by dexamethasone, the plasmid with antisense v-myb' repressed the expression of p75c-myb from the endogenous c-myb gene of K562 cells. It also reduced the proliferation rate of K562 cells to 50% of the control level, and induced these K562 cells to express the myelomonocytic differentiation cell surface marker CD13 and increased NBT reducing activity. The plasmid with sense v-myb' did not have an effect on p75c-myb expression, the proliferation of K562 cells or the expression of myelomonocytic differentiation phenotypes. These observations suggest that antisense v-myb' RNA represses p75c-myb expression and that a decrease of p75c-myb suppresses K562 cell proliferation and induces its differentiation towards the myelomonocytic lineage.

Establishment of a human malignant meningioma cell line with amplified c-myc oncogene

A new cell line (KT21-MG1) has been established from a human malignant meningioma transplanted into nude mice. The cultured cells showed epithelial cell-like morphology and were positive immunohistochemically for vimentin as the original tumor. They have been grown continuously in vitro for more than 2 years. The population doubling time was about 24 hours at the 30th passage. The cells are capable of proliferating in soft agar medium and produced tumors in nude mice, the histologies of which were similar to the original patient-derived tumor. Analysis of cellular oncogenes showed that myc and fps were amplified approximately tenfold and threefold, respectively, in this cell line, whereas N-myc, L-myc, N-ras, K-ras, H-ras, abl, erbB2, Blym, src, raf-1, myb, and sis were not changed significantly. The amplification of myc was accompanied by an enhanced expression. Chromosome studies of cultured cells showed the monosomy of chromosome 22 that has been reported to be a specific abnormality in meningiomas.

BAS1 has a Myb motif and activates HIS4 transcription only in combination with BAS2

The BAS1 and BAS2 proteins are both required for activation of GCN4-independent (basal) HIS4 transcription in yeast. BAS1 has an NH2-terminal region similar to those of the myb proto-oncogene family. BAS1 and BAS2, which contains a homeo box, bound to adjacent sites on the HIS4 promoter. The joint requirement of BAS1 and BAS2 for activation is probably not due to cooperative binding or the transcriptional control of one of the genes by the other. Although BAS1 and BAS2 were both required for activation of HIS4 transcription, BAS1 was not required for BAS2-dependent expression of the secreted acid phosphatases. The transcriptional activators of HIS4 have DNA binding domains that are conserved in evolution (BAS1 = Myb, BAS2 = homeo box, GCN4 = Jun). Their interactions, therefore, may be relevant to the control of gene expression in more complex systems.

Activation of endogenous c-fos proto-oncogene expression by human T-cell leukemia virus type I-encoded p40tax protein in the human T-cell line, Jurkat

We examined the ability of the trans-acting factor p40tax of human T-cell leukemia virus type I (HTLV-I), which is thought to be a crucial molecule in T-cell transformation by HTLV-I, to activate expression of a set of endogenous cellular genes related to T-cell proliferation. For this purpose we established a subclone (JPX-9) of Jurkat cells that was stably transfected with an expression plasmid containing the p40tax gene, whose expression is definitely dependent on heavy-metal ions. Expression of the interleukin-2 receptor alpha chain in JPX-9 cells was induced in response to the induction of p40tax expression, as has been demonstrated by others in transient transfection experiments with Jurkat cells. In addition, we found that significant enhancement of expression of the nuclear proto-oncogene c-fos was closely associated with expression of p40tax. Continous enhancement in the level of c-fos mRNA was observed in the presence of p40tax. In contrast, mRNA levels of other nuclear proto-oncogenes (c-myc, c-myb, and c-jun) were not appreciably effected by the expression of p40tax. These results suggest that (i) in addition to the interleukin-2-interleukin-2 receptor system, cellular genes such as c-fos, which regulate normal T-cell growth, are also activated directly or indirectly by p40tax and (ii) p40tax-induced modulation of gene expression plays a crucial role in T-cell transformation by HTLV-I.

Transcription of the chicken myb proto-oncogene starts within a CpG island

The nucleotide sequence of an 8.2-kb DNA fragment from the 5' proximal part of the chicken myb proto-oncogene spanning 1761 nucleotides upstream and 6436 nucleotides downstream from a presumed c-myb initiation codon was determined. A 3.3-kb G + C-rich region found in this sequence had also other features characterizing CpG islands, i.e. no CpG underrepresentation and lack of CpG methylation. In haematopoietic tissues c-myb mRNA synthesis starts in two major regions of the CpG island, namely 98 to 108 and 143 to 145 nucleotides upstream from the c-myb initiation codon. These two regions are in or close to the 124-bp evolutionarily conserved element located in the middle part of the CpG island. No alternative splicing of the 5' end of c-myb mRNA suggested earlier (1,2) was observed. The c-myb promoter contains neither TATA nor CAAT box-like structures at the usual positions. Instead, numerous potential Sp1 factor binding sites were found both upstream and downstream from the transcription initiation sites. Moreover, consensus v-myb protein DNA-binding sites were revealed in the promoter region and in sequences downstream from it.

Nucleotide sequence of chicken myb proto-oncogene promoter region: detection of an evolutionarily conserved element

The nucleotide sequence of the chicken myb proto-oncogene putative promoter region was determined and compared with the corresponding sequence of the mouse c-myb gene (1). 118 bp upstream from the initiation codon suggested by Gerondakis and Bishop (2) for the chicken c-myb protein, a 124-bp-long conserved element was found (92% identity in chicken and mouse sequences). Sequences homologous to this element were detected on Southern blots of restricted genomic DNAs from mouse, man, lizard, frog, and carp. No hybridization was observed with Drosophila, yeast, or Escherichia coli DNA. In human DNA, sequences homologous to this element were located at the 5' end of the c-myb gene, i.e. in the same position as in the chicken and mouse genes. Several lines of evidence suggest that the element is not a coding exon of a gene overlapping the c-myb gene. It may be of importance that one of the DNase I-sensitive sites and several c-myb mRNA cap sites localized recently in the mouse c-myb gene (3,4) lie within this region. It is suggested that this evolutionarily conserved element is involved in the regulation of myb proto-oncogene expression in vertebrates.

Isolation of human cDNA clones of myb-related genes, A-myb and B-myb

cDNA clones of the myb-related genes A-myb and B-myb were obtained by screening human cDNA libraries. The predicted open reading frame of B-myb could encode a protein of 700 amino acid residues. Although the C-terminal end has not been cloned yet, an almost entire coding region of A-myb, which is 745 amino acid long, was determined. The A-myb and B-myb proteins are highly homologous with the myb protein in three regions. Domain I, which is 161 amino acid long, is well conserved in the myb gene family. The homology between human-myb and A-myb in domain I is 90% at the amino acid level. Domain II, which is about 85 amino acid long, is less well conserved. Although it is a short stretch, domain III is found in the C-terminal region. The mRNAs of A-myb and B-myb were 5.0 and 2.6 kb, respectively. The mRNA expression pattern of the myb gene family in various tumors is presented.

Expression of cellular oncogenes in the myocardium during the developmental stage and pressure-overloaded hypertrophy of the rat heart

Proto-oncogenes have been revealed to participate in normal cell proliferation as well as in cell transformation. Since cardiac myocytes are terminally differentiated, they cannot divide except in the fetal period. To determine the role of cellular oncogenes in the growth of the heart, the expression pattern of eight cellular oncogenes during the developmental stage and pressure-overloaded hypertrophy of the rat hearts were examined in vivo. Northern blot analysis was performed with eight oncogene probes (myc, fos, Ha-ras, src, erbA, erbB, sis, myb). Pressure overload increased the levels of cellular (c-) fos, c-myc, and c-Ha-ras. An increase of c-fos and c-myc was detected at 30 minutes and 2 hours, respectively; the levels peaked at 8 hours, and they returned to baseline by 48 hours after aortic constriction. However, the level of c-Ha-ras showed a gradual increase. During the course of development, the expression of c-myc was detectable only in the embryonic stage, whereas the expression of c-fos was not detected in the fetal period, was increased after birth, and peaked in 200-day-old adults. The expression of c-Ha-ras was almost the same throughout the development. Cellular oncogenes were expressed in the heart in response to pressure overload and in a stage-specific manner. These results suggest that cellular oncogenes may participate in the normal developmental process and hypertrophy of hearts and that the cellular hypertrophy induced by pressure overload may share a similar mechanistic pathway with cell proliferation.

Structural and functional domains of the myb oncogene: requirements for nuclear transport, myeloid transformation, and colony formation

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in vivo and transforms only myeloid cells in vitro. Its product, p48v-myb, is a nuclear protein of unknown function. To determine structure-function relationships for this protein, we constructed a series of deletion mutants of v-myb, expressed them in retroviral vectors, and studied their biochemical and biological properties. We used these mutants to identify two separate domains of p48v-myb which had distinct roles in its accumulation in the cell nucleus. We showed that the viral sequences which normally encode both termini of p48v-myb were dispensible for transformation. In contrast, both copies of the highly conserved v-myb amino-terminal repeat were required for transformation. We also identified a carboxyl-terminal domain of p48v-myb which was required for the growth of v-myb-transformed myeloblasts in soft agar but not for morphological transformation.

Constitutive expression of a c-myb cDNA blocks Friend murine erythroleukemia cell differentiation

A full-length human c-myb cDNA clone has been isolated from a CCRF-CEM leukemia cell cDNA library. The plasmid vector contains simian virus 40-derived promotor, splice, and polyadenylation sequences as well as a transcription unit for a dihydrofolate reductase cDNA. We have introduced this construct into Friend erythroleukemia (F-MEL) cells and have isolated a number of clones which contain intact and transcriptionally active human c-myb sequences. F-MEL clones expressing the highest levels of the human c-myb mRNA differentiate poorly in response to dimethyl sulfoxide. Two clones which initially expressed low levels of human c-myb transcripts and which differentiated normally were subsequently inhibited in their ability to differentiate when grown in successively higher concentrations of methotrexate, due to amplification and enhanced expression of plasmid sequences. The inhibitory effect on F-MEL differentiation appeared to be independent of the early decline in c-myc transcripts which were normally regulated in all cases examined. Our results indicate that constitutive expression of a nontruncated human c-myb cDNA can exert profound effects on erythroid differentiation and argue for a causal role of c-myb in the F-MEL differentiation process.

Constitutive expression of a c-myb cDNA blocks Friend murine erythroleukemia cell differentiation

A full-length human c-myb cDNA clone has been isolated from a CCRF-CEM leukemia cell cDNA library. The plasmid vector contains simian virus 40-derived promotor, splice, and polyadenylation sequences as well as a transcription unit for a dihydrofolate reductase cDNA. We have introduced this construct into Friend erythroleukemia (F-MEL) cells and have isolated a number of clones which contain intact and transcriptionally active human c-myb sequences. F-MEL clones expressing the highest levels of the human c-myb mRNA differentiate poorly in response to dimethyl sulfoxide. Two clones which initially expressed low levels of human c-myb transcripts and which differentiated normally were subsequently inhibited in their ability to differentiate when grown in successively higher concentrations of methotrexate, due to amplification and enhanced expression of plasmid sequences. The inhibitory effect on F-MEL differentiation appeared to be independent of the early decline in c-myc transcripts which were normally regulated in all cases examined. Our results indicate that constitutive expression of a nontruncated human c-myb cDNA can exert profound effects on erythroid differentiation and argue for a causal role of c-myb in the F-MEL differentiation process.

High affinity interleukin 2 receptors in HTLV-1-infected T cells can mediate signals for gene expression

The expression of transcripts of the c-myb and c-myc protooncogenes and the interleukin 2 receptor (IL-2R) gene in human T cells infected with human T-cell leukemia virus type 1 (HTLV-1) after exposure to interleukin 2 (IL-2) were examined. Infection with HTLV-1 is known to be associated with constitutive expression of IL-2R, although infected cells do not require IL-2 for growth. Northern blot analysis showed that expression of the mRNAs of the c-myb, c-myc, and IL-2R genes were markedly increased by addition of IL-2 into the cultures, indicating that IL-2R transduced signals for gene expression in these cells as in normal T cells. Studies on distinct HTLV-1-infected T cell clones that differed in numbers of high-affinity IL-2R, showed that the extents of increase in mRNA expression by IL-2 were correlated with the number of high-affinity IL-2R. This correlation was confirmed by demonstration that the levels of mRNA expression were proportional to the numbers of IL-2-bound high-affinity but not low-affinity receptors. Thus, the signals induced by IL-2 for gene expression may be through high-affinity IL-2R.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

Human c-myb protooncogene: nucleotide sequence of cDNA and organization of the genomic locus

We have isolated cDNA clones of the human c-myb mRNA that contain approximately 3.4 kilobases of the approximately 3.8-kilobase mRNA sequence. Nucleotide sequence analysis shows that the c-myb mRNA contains an open reading frame of 1920 nucleotides, which could encode a 72-kDa protein. The cDNA nucleotide sequence and the predicted amino acid sequence of the c-myb protein are highly homologous to the corresponding chicken and mouse proteins. In particular, a region toward the NH2 terminus of the protein containing a 3-fold tandem repeat of 51 residues is evolutionarily conserved and is the only region of homology with the Drosophila c-myb protein. This region may represent a functionally important structure, most likely the DNA-binding domain. cDNA clones have been used to isolate genomic clones and to define a preliminary intron/exon organization of the c-myb gene. Identification of 5' and 3' coding and noncoding exons indicates that the human c-myb locus spans a 40-kilobase region.

Activation of c-myb expression by phytohemagglutinin stimulation in normal human T lymphocytes

The expression of c-myb in normal human T lymphocytes directly derived from a normal subject and not adapted to continuous growth in culture was found to be markedly increased after phytohemagglutinin stimulation. In the same cells, the expression of c-myc mRNA is a much earlier event compared with the appearance of c-myb mRNA, which takes place soon after that of histone H3 mRNA. The increase in c-myb expression was not due to a particular T-lymphocyte subset, as shown by in situ hybridization assays.

Identification of c-myb (chicken), c-myb (mouse) and v-myb (AMV) protein products by immunoprecipitation with antibodies directed against a synthetic peptide

A synthetic nonadecapeptide (IL 19) derived from a sequence of v-myb was covalently bound to haemocyanin and used for immunization. Anti-IL 19 serum immunoprecipitated a 75 kDa protein in the lysate of metabolically labelled chicken and murine thymus cells. Presaturation of the serum with IL 19 abolished this immunoprecipitation, thus indicating that the product of c-myb in both chicken and murine thymuses is the 75 kDa protein (p75c-myb). Anti IL 19 serum also precipitated p48v-myb in the lysate of nonproducer myeloblasts.

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.

Studies of the human c-myb gene and its product in human acute leukemias

The myb gene is the transforming oncogene of the avian myeloblastosis virus (AMV); its normal cellular homolog, c-myb, is conserved across a broad span of evolution. In humans, c-myb is expressed in malignant hematopoietic cell lines and in primary hematopoietic tumors. Partial complementary DNA clones were generated from blast cells of patients with acute myelogenous leukemia. The sequences of the clones were compared to the c-myb of other species, as well as the v-myb of AMV. In addition, the carboxyl terminal region of human c-myb was placed in an expression vector to obtain protein for the generation of antiserum, which was used to identify the human c-myb gene product. Like v-myb, this protein was found within the nucleus of leukemic cells where it was associated with the nuclear matrix. These studies provide further evidence that c-myb might be involved in human leukemia.

Antibodies to the evolutionarily conserved amino-terminal region of the v-myb-encoded protein detect the c-myb protein in widely divergent metazoan species

Antibodies directed against a bacterial fusion protein that contains the domain encoded by the highly evolutionarily conserved 5' one-third of the v-myb oncogene of avian myeloblastosis virus (AMV) detect the protein products of various members of the myb gene family. Immunoprecipitation or immunoblot analyses with these antibodies yielded the following information. First, the products of the v-myb oncogenes of AMV (p48v-myb) and of E26 virus (p135gag-myb-ets) contain this highly conserved amino acid sequence, as previously hypothesized. Second, p75c-myb, the product of the chicken c-myb protooncogene, also contains this protein domain. Third, these antibodies have identified the products of the human, murine, and Drosophila c-myb genes, which were all found to be nuclear proteins of Mr 75,000-80,000. The human c-myb protein product is present in immature cells of the erythroid, myeloid, and lymphoid lineages.

A cryptic transcription promoter in the myb oncogene of avian myeloblastosis virus

The potential regulatory signals contained in the v-myb oncogene of avian myeloblastosis virus have been inserted upstream to the herpes simplex type 1 thymidine kinase gene in order to test their promoter activity. The isolation of TK+ transformants after transfection of clone 1D(TK-) mouse cells with the resulting recombinant DNAs indicated that the expression of the TK gene was made possible by the myb-derived sequences. Analysis of the TK specific RNA expressed in different TK+ transformants revealed that the regulatory signals contained in v-myb correspond to a weak functional promoter.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

Biologically active proviral clone of myeloblastosis-associated virus type 1: implications for the genesis of avian myeloblastosis virus

A biologically active myeloblastosis-associated virus (MAV) provirus was cloned from a bacteriophage recombinant library constructed from leukemic chicken myeloblast DNA. The restriction endonuclease map of this clone was consistent with that of a type 1 MAV (MAV-1). Interference assays of virus recovered from cultured chicken embryo fibroblasts after DNA transfection established that the provirus was infectious and confirmed that it belonged to avian retrovirus subgroup A (type 1). Antipeptide antibodies raised against the env-encoded carboxyl terminus of p48myb, the transforming protein of avian myeloblastosis virus, specifically immunoprecipitated the gp37env from quail cells transfected with MAV-1 proviral DNA but not from cells infected with MAV-2. This suggests that MAV-1 rather than MAV-2 is the progenitor helper virus from which avian myeloblastosis virus arose by the transduction of cellular proto-oncogene sequences.

Activation of c-myb expression by phytohemagglutinin stimulation in normal human T lymphocytes

The expression of c-myb in normal human T lymphocytes directly derived from a normal subject and not adapted to continuous growth in culture was found to be markedly increased after phytohemagglutinin stimulation. In the same cells, the expression of c-myc mRNA is a much earlier event compared with the appearance of c-myb mRNA, which takes place soon after that of histone H3 mRNA. The increase in c-myb expression was not due to a particular T-lymphocyte subset, as shown by in situ hybridization assays.

Chromosome assignments of four mouse cellular homologs of sarcoma and leukemia virus oncogenes

Molecular probes for the oncogenes of Rous sarcoma virus (v-src), avian myeloblastosis virus (v-myb), Kirsten murine sarcoma virus (v-Ki-ras), and Harvey murine sarcoma virus (v-Ha-ras) were hybridized to the DNA from mouse-Chinese hamster somatic cell hybrids. The v-src, v-myb, v-Ki-ras, and v-Ha-ras genes each detected one or a few homologous mouse DNA fragments whose segregation was analyzed in cell hybrids. Mouse cellular homologs c-src, c-Ki-ras, c-Ha-ras, and c-myb segregated concordantly with chromosomes 2, 6, 7, and 10, respectively. Comparison with the known locations of human c-src (chromosome 20) and human c-Ha-ras1 (chromosome 11 short arm) suggests that the human and mouse homologs of these two viral oncogenes reside in conserved linkage groups. The c-Ki-ras gene on mouse chromosome 6 might reside also in a conserved linkage group, along with glyceraldehyde-3-phosphate dehydrogenase and triosephosphate isomerase. However, direct confirmation of this suggestion must await a demonstration that c-Ki-ras on mouse chromosome 6 is homologous to c-Ki-ras2 on the short arm of human chromosome 12.

RNA-tumorviruses, oncogenes, and their possible role in human carcinogenesis

The detection and characterization of oncogenes via RNA tumor viruses (or retroviruses) and the recognition of their location at breakpoints of chromosomal translocations which are frequently found in certain human neoplasms has promoted present understanding of molecular mechanisms underlying carcinogenesis. Oncogenes are cellular genes which can be transduced by RNA tumorviruses and induce malignant transformation under experimental conditions in vivo and in vitro. A role of retroviruses in human leukemogenesis is suggested by epidemiological observations and by the isolation of such viruses from several human T-cell leukemias and lymphomas (human T-cell leukemia/lymphoma virus or HTLV) as well as by biochemical association of retroviral markers with human leukemias. A role of HTLV has been suggested also in a human immune deficiency syndrome (AIDS). In view of the well known role of many factors in carcinogenesis the concept of carcinogenesis as a multistep process as well as the concept of cocarcinogenesis and the role of cofactors other than viruses, such as radiation and chemicals, aging, hormones, graft vs host reaction, environmental factors etc., will have to be carefully considered.

Structural organization and expression of human DNA sequences related to the transforming gene of avian myeloblastosis virus

Bacteriophage libraries of human DNA were screened for sequences homologous to the transforming gene (v-myb) of avian myeloblastosis virus. The three overlapping clones isolated were shown to contain a total of 1.0 kilobase pair (kbp) of sequence related to v-myb distributed over 6.2 kbp. Restriction enzyme mapping and heteroduplex analysis revealed the presence of five myb-related domains interrupted by four stretches of non-homology. To study the extent of human DNA coding sequences that constitute the myb gene homologue, c-myb (human), probes spanning about 30 kbp were prepared from the clones and used to study transcription in a human hematopoietic cell line (MOLT-4). Each of the probes hybridized a 4.5-kilobase transcript, which suggests that either the c-myb (human) gene encompasses 30 kbp or it contains two or more transcription units that each give rise to a mRNA of 4.5 kilobases.

Transduction of c-myb into avian myeloblastosis virus: locating points of recombination within the cellular gene

The oncogene (v-myb) of avian myeloblastosis virus apparently arose by transduction of nucleotide sequences from a cellular gene (c-myb). In c-myb the nucleotide sequences that formed v-myb exist at seven distinct regions separated by nontransduced stretches of sequence that are flanked by eucaryotic splice signals. By contrast, the sequences at the outside boundaries of the transduced region of c-myb do not resemble splice sites. We mapped the nucleotide sequences that are homologous to the ends of v-myb with respect to the exons and introns of c-myb. The results indicate that the leftward recombination between c-myb and the transducing retrovirus occurred within an intron of the cellular gene, whereas the rightward recombination took place in an exon of c-myb. Transduction of c-myb sequences may therefore have involved a DNA rearrangement.

The product of the retroviral transforming gene v-myb is a truncated version of the protein encoded by the cellular oncogene c-myb

Avian myeloblastosis virus (AMV) is an oncogenic retrovirus that rapidly causes myeloblastic leukemia in chickens and transforms myeloid cells in culture. AMV carries an oncogene, v-myb, that is derived from a cellular gene, c-myb, found in the genomes of vertebrate species. We constructed a plasmid vector that allows expression of a portion of the coding region for v-myb in a procaryotic host. We then used the myb-encoded protein produced in bacteria to immunize rabbits. The antisera obtained permitted identification of the proteins encoded by both v-myb and chicken c-myb. The molecular weights of the products of v-myb and c-myb (45,000 and 75,000 respectively) indicate that the v-myb protein is an appreciably truncated version of the c-myb protein.

Structure and transcription of the cellular homolog (c-myb) of the avian myeloblastosis virus transforming gene (v-myb)

We isolated and characterized molecular clones containing the chicken cellular homolog (c-myb) of the avian myeloblastosis virus oncogene (v-myb). Mapping of the c-myb clones using restriction endonucleases and hybridization to radiolabeled v-myb probes revealed that the sequences homologous to v-myb are contained within four separate regions, which have since been shown by nucleotide sequencing (Klempnauer et al., Cell 31:453-463, 1982) to carry seven exons. Analysis of c-myb transcripts showed the presence of several large precursor RNAs in addition to the 4.0-kilobase cytoplasmic mRNA. We also determined the approximate positions of the c-myb sequences that are present in the 4.0-kilobase c-myb mRNA but not present in v-myb. Some of these sequences are found in a separate region 5' to the v-myb-related sequences, whereas the remainder appear to be located immediately 3' to the v-myb-related sequences. The data presented here, in conjunction with nucleotide sequence analysis (Klempnauer et al., Cell 31:453-463, 1982), indicate that the c-myb gene contains at least eight exons which span a total of about 16 kilobase pairs. The presence of exon sequences in c-myb outside the regions of homology with v-myb raises the possibility that the v-myb and c-myb gene products may differ significantly.

Organization of chicken DNA sequences homologous to the transforming gene of avian myeloblastosis virus. II. Isolation and characterization of lambda proto-amv DNA recombinant clones from a library of leukemic chicken DNA

Several lambda proto-amv recombinants isolated from a lambda Charon 4A library of leukemic chicken DNA were analyzed by using various restriction endonucleases and hybridization with specific probes representing different regions of the transforming gene of avian myeloblastosis virus. The position of 30 sites for 11 different restriction endonucleases was established in the proto-amv region of chicken DNA. Identical restriction endonuclease maps were obtained for the normal and leukemic DNAs in the proto-amv domain, which covers 8 to 9 kilobases of DNA. The cellular genetic elements homologous to the cellular sequence (amv) inserted into the avian myeloblastosis virus genome are contained within six major proto-amv segments which are interrupted by at least five large DNA regions lacking homology with amv.

Nucleotide sequence of the retroviral leukemia gene v-myb and its cellular progenitor c-myb: the architecture of a transduced oncogene

The oncogene (v-myb) of avian myeloblastosis virus and a large portion of its cellular homolog (c-myb) have been molecularly cloned and sequenced. The portion of c-myb we analyzed contains seven interspersed segments (or exons). Fusion of these exons creates a continuous nucleotide sequence that is remarkably similar to the sequence of v-myb and that potentially encodes a protein very similar to that specified by v-myb. Comparisons between the sequences of v-myb and c-myb indicate that transduction of c-myb to form v-myb probably resulted from an initial DNA rearrangement and the subsequent use of a spliced RNA as an intermediate.

Organization of chicken DNA sequences homologous to the transforming gene of avian myeloblastosis virus. I. Restriction enzyme analysis of total DNA from normal and leukemic cells

Hybridization probes consisting of cloned DNA recombinants which represent different regions of the leukemogenic sequence (amv) from avian myeloblastosis virus were used to carry out a more detailed restriction endonuclease analysis of the homologous sequences (proto-amv) present in normal and leukemic chicken DNA. The results show that four large introns interrupt the normal cellular proto-amv sequences and that there is no major rearrangement of these sequences in leukemic myeloblasts.

Avian myeloblastosis provirus cloned in a lambda bacteriophage is leukemogenic

The avian myeloblastosis virus provirus inserted in a lambda bacteriophage, recombinant clone 11A1-1 (Souza et al., Proc. Natl. Acad. Sci. U.S.A. 77:3004-3008, 1980), was transfected into chicken embryo fibroblasts which had been preinfected with either Rous-associated virus type 61 or the transformation-defective avian sarcoma virus tdB77. Within 4 to 5 h after transfection, the cells were injected into 16-day-old chicken embryos or 1-day-old chicks. Acute myeloblastic leukemia developed after a long latent period. Filtered (0.22-micrometer pores) supernatant of transformed buffy-coat cell cultures from one leukemic chicken of the lambda 11A1-1 (tdB77) group rapidly transformed yolk sac cells in vitro. Results from an infectivity interference assay and analysis of proviral DNA fragments generated with restriction endonucleases were consistent with the presence in leukemic cells of defective avian myeloblastosis virus and tdB77 as the helper virus.

Detection and cloning of human DNA sequences related to the mouse mammary tumor virus genome

Sequences related to the mouse mammary tumor virus (MMTV) genome have been detected in fragments of restricted human cellular DNA. These results were obtained by using recombinant DNA containing the MMTV proviral genome and lowering the stringency of blot-hybridization conditions. The MMTV genome also reacts with unique families of fragments in restricted cellular DNA from other mammalian species but not with salmon sperm DNA. A clone that reacted with labeled MMTV proviral DNA was selected from a human DNA library in Charon 4A. Under stringent conditions, a 3.7-kilobase MMTV-related EcoRI fragment of this clone hybridized with many of the same EcoRI restriction fragments of human cellular DNA detectable with MMTV proviral DNA under low-stringency conditions. Specific fragments of the human clone were shown to contain sequences related to the molecularly cloned gag, pol, and env regions of the MMTV genome.