Activation of the c-myb locus is insufficient for the rapid induction of disseminated avian B-cell lymphoma

Telomerase reverse transcriptase expression elevated by avian leukosis virus integration in B cell lymphomas

Simple retroviruses induce tumors by integrating into the host genome, activating cellular oncogenes and microRNAs, or inactivating tumor suppressor genes. The identification of these genes elucidates molecular mechanisms of tumorigenesis. In this study, we identified avian leukosis virus (ALV) proviral integration sites in rapid-onset B cell lymphomas arising <12 weeks after infection of chicken embryos. By using inverse PCR, 28 unique viral integration sites were identified in rapid-onset tumors. Integrations in the telomerase reverse transcriptase (TERT) promoter/enhancer region were observed in four different tumors, suggesting that this is a common integration site. These provirus integrations ranged from 217 to 2,584 bp upstream of the TERT transcription initiation site and were all in the opposite transcriptional orientation to TERT. Southern blots of tumor samples demonstrated that these integrations are clonal and therefore occurred early in the process of tumorigenesis. Real-time RT-PCR showed overexpression of TERT mRNA in tumors harboring viral integrations in the TERT promoter. Telomerase activity was also up-regulated in these tumors; however, telomere-length alterations were not detected. Furthermore, viral LTR sequences directly enhanced the expression of luciferase reporters containing the TERT promoter sequences. This study documents retroviral up-regulation of cellular TERT by insertional activation to initiate or enhance tumor progression.

Successful induction of ovine pulmonary adenocarcinoma in lambs of different ages and detection of viraemia during the preclinical period

Ovine pulmonary adenocarcinoma (OPA) can be reproduced consistently in neonatal lambs by intratracheal injection of inocula containing jaagsiekte sheep retrovirus (JSRV). In this study, clinical disease, confirmed pathologically as OPA, was induced in a high proportion of lambs that had been inoculated intratracheally with infectious lung fluid at 1, 3 and 6 months of age. The incubation periods, however, were longer in these three age groups than in 1-week-old lambs that were used as controls. Viraemia was detected in all age groups before onset of clinical signs, but occurred later in older animals. These results suggest an age-dependent susceptibility to OPA that could be determined by the availability of JSRV target cells in the ovine lung. The feasibility of inducing OPA in older lambs and detecting JSRV viraemia in preclinical stages enables improved studies on the pathogenesis, assessment of vaccines, diagnosis and control of the disease.

Neuromedin U: a Myb-regulated autocrine growth factor for human myeloid leukemias

The c-myb proto-oncogene has been implicated in leukemogenesis, but possible mechanisms remain ill defined. To gain further insight to this process, we used transcript profiling in K562 cells expressing a dominant-negative Myb (MERT) protein. A total of 105 potential Myb gene targets were identified. Neuromedin U (NmU), a peptide affecting calcium transport, underwent the greatest expression change ( approximately 5-fold decrease). To verify a linkage between c-myb and NmU, their mRNA levels were quantitated using real-time polymerase chain reaction in primary acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL), as well as normal hematopoietic cells. We found that c-myb was elevated in AML and ALL samples, but NmU expression was increased only in AML cells. Significantly, only AML cells expressed the cognate receptor of NmU, NMU1R, suggesting the presence of a novel autocrine loop. We examined this possibility in detail. Exogenous NmU "rescued" growth suppression in K562-MERT cells and stimulated the growth of primary AML cells. Short interfering RNA "knockdown" of NmU in K562 cells arrested cell growth. Exposing Indo-1-labeled K562 cells to NmU induced an intracellular Ca(++) flux consistent with engagement of the NMU1R. Combined, these results suggest that NmU expression is related to Myb and that the NmU/NMU1R axis constitutes a previously unknown growth-promoting autocrine loop in myeloid leukemia cells.

Silent point mutation in an avian retrovirus RNA processing element promotes c-myb-associated short-latency lymphomas

The avian leukosis virus DeltaLR-9 causes a high frequency of B-cell lymphomas within weeks after injection into 10-day-old chicken embryos. These lymphomas result from proviral integrations into the oncogene c-myb. In contrast, LR-9, which lacks the 42-nucleotide gag gene deletion of DeltaLR-9, does not cause a high frequency of c-myb-associated short-latency lymphomas. Although viral replication rates and spliced env mRNA levels were found to be similar for both viruses, DeltaLR-9 exhibited an increase in readthrough transcription compared to LR-9. The DeltaLR-9 deletion is located in the region of the gag gene corresponding to the matrix (MA) protein as well as in the negative regulator of splicing (NRS) element. To test whether disruption of the NRS or of the MA protein was responsible for inducing short-latency lymphomas, we generated viruses with NRS point mutations that maintained the wild-type Gag amino acid sequence. One of the mutant viruses induced an even higher incidence than DeltaLR-9 of short-latency lymphomas with viral integrations into c-myb. Thus, we propose that disruption of the NRS sequence promotes readthrough transcription and splicing to the downstream myb gene, causing overexpression of a slightly truncated Myb protein, which induces short-latency tumors.

Functional genomic analysis reveals distinct neoplastic phenotypes associated with c-myb mutation in the bursa of Fabricius

Avian retroviral integration into the c-myb locus is casually associated with the development of lymphomas in the bursa of Farbricius of chickens; these arise with a shorter latency than bursal lymphomas caused by deregulation of c-myc. This study indicates that c-myb mutation in embryonic bursal precursors leads to an oligoclonal population of developing bursal follicles, showing a variable propensity to form a novel lesion, the neoplastic follicle (NF). About half of such bursas rapidly developed lymphomas. Detection of changes in gene expression, during the development of neoplasms, was carried out by cDNA microarray analysis. The transcriptional signature of lymphomas with mutant c-myb was more limited than, and only partially shared with, those of bursal lymphomas caused by Myc or Rel oncogenes. The c-myb-associated lymphomas frequently showed overexpression of c-myc and altered expression of other genes involved in cell cycle control and proliferation-related signal transduction. Oligoclonal, NF-containing bursas lacked detectable c-myc overexpression and demonstrated a pattern of gene expression distinct from that of normal bursa and partially shared with the short-latency lymphomas. This functional genomic analysis uncovered several different pathways of lymphomagenesis by oncogenic transcription factors acting in a B-cell lineage.

Tumorigenic N-terminal deletions of c-Myb modulate DNA binding, transactivation, and cooperativity with C/EBP

Oncogenic activation of c-myb by retroviral insertion has been implicated in tumor formation in chicken and mice. These genetic alterations result in deregulated expression of the c-myb gene and frequently in N-terminal truncation of the c-Myb protein. We demonstrate that truncation of the c-Myb N-terminus affects DNA binding and reporter activation. However, all three mutants, Myb Delta N20, Myb Delta N47 and Myb Delta N71 cooperated with C/EBP beta in reporter assays. In contrast to Myb Delta N20 and Myb Delta N47, however, the Myb Delta N71 mutant failed to activate the chromatin embedded endogenous mim-1 gene together with C/EBP beta. This suggests that an N-terminal region (amino acids 47-71) within repeat 1 (R1) of the murine c-Myb DNA binding domain affects activation of chromosomal target genes in collaboration with C/EBP beta.

Transcription factors and translocations in lymphoid and myeloid leukemia

Chromosomal translocations involving transcription factors and aberrant expression of transcription factors are frequently associated with leukemogenesis. Transcription factors are essential in maintaining the regulation of cell growth, development, and differentiation in the hematopoietic system. Alterations in the mechanisms that normally control these functions can lead to hematological malignancies. Further characterization of the molecular biology of leukemia will enhance our ability to develop disease-specific treatment strategies, and to develop effective methods of diagnosis and prognosis.

Regulation of the resident chromosomal copy of c-myc by c-Myb is involved in myeloid leukemogenesis

c-myb is a frequent target of retroviral insertional mutagenesis in murine leukemia virus-induced myeloid leukemia. Induction of the leukemogenic phenotype is generally associated with inappropriate expression of this transcriptional regulator. Despite intensive investigations, the target genes of c-myb that are specifically involved in development of these myeloid lineage neoplasms are still unknown. In vitro assays have indicated that c-myc may be a target gene of c-Myb; however, regulation of the resident chromosomal gene has not yet been demonstrated. To address this question further, we analyzed the expression of c-myc in a myeloblastic cell line, M1, expressing a conditionally active c-Myb-estrogen receptor fusion protein (MybER). Activation of MybER both prevented the growth arrest induced by interleukin-6 (IL-6) and rapidly restored c-myc expression in nearly terminal differentiated cells that had been exposed to IL-6 for 3 days. Restoration occurred in the presence of a protein synthesis inhibitor but not after a transcriptional block, indicating that c-myc is a direct, transcriptionally regulated target of c-Myb. c-myc is a major target that transduces Myb's proliferative signal, as shown by the ability of a c-Myc-estrogen receptor fusion protein alone to also reverse growth arrest in this system. To investigate the possibility that this regulatory connection contributes to Myb's oncogenicity, we expressed a dominant negative Myb in the myeloid leukemic cell line RI-4-11. In this cell line, c-myb is activated by insertional mutagenesis and cannot be effectively down regulated by cytokine. Myb's ability to regulate c-myc's expression was also demonstrated in these cells, showing a mechanism through which the proto-oncogene c-myb can exert its oncogenic potential in myeloid lineage hematopoietic cells.

Minimal truncation of the c-myb gene product in rapid-onset B-cell lymphoma

Oncogenic activation of c-myb by insertional mutagenesis has been implicated in rapid-onset B-cell lymphomas induced by the nonacute avian leukosis virus EU-8. In these tumors, proviruses are integrated either upstream of the c-myb coding region or within the first intron of c-myb. Tumors with either type of integration contained identical chimeric mRNAs in which the viral 5' splice site was juxtaposed to the 3' splice site of c-myb exon 2 and myb exon 1 was eliminated. Both classes of integrations generated truncated Myb proteins that were indistinguishable by Western analysis. In contrast to most other examples of c-myb activation, the truncation consisted of only 20 N-terminal amino acids and did not disrupt either the DNA binding domain near the N terminus or the negative regulatory domain near the C terminus of Myb. The significance of the 20-amino-acid Myb truncation to tumorigenesis was tested by infection of chicken embryos with retroviral vectors expressing different myb gene products. While virus expressing either wild-type c-myb or c-myb mutated at the N-terminal casein kinase II sites was only weakly oncogenic at 10 weeks, the minimally truncated myb virus induced a high incidence of rapid-onset tumors, including B-cell lymphomas, sarcomas, and adenocarcinomas.

Genetic determinant of rapid-onset B-cell lymphoma by avian leukosis virus

Infection of 10 day-old chicken embryos with the recombinant avian leukosis virus (ALV) EU-8 induces a high incidence of rapid-onset B-cell lymphoma by insertional activation of the c-myb gene. LR-9, a related ALV with differences from EU-8 in the gag and pol genes, induces rapid-onset lymphoma at only a low incidence. To localize the viral determinant(s) responsible for this biologic difference, we constructed and tested a series of reciprocal chimeras between EU-8 and LR-9 ALVs. The ability to induce rapid-onset lymphoma efficiently was localized to a 925-nucleotide (nt) region of the EU-8 gag gene. Sequence analysis of the region revealed a 42-nt deletion in EU-8 relative to LR-9, as well as some single-nucleotide changes. A mutant virus, delta LR-9, constructed by deleting these 42 nt from LR-9, also induced rapid-onset lymphoma at a high frequency, confirming the biologic significance of this deletion. This deletion removed nt 735 to 776, which lies within a cis-acting RNA element that negatively regulates splicing (NRS). The deletion was shown to cause an increase in splicing efficiency, which may lead to increased production of a truncated myb gene product from an ALV-myb readthrough RNA.

Avian retroviruses

Avian leukosis virus (ALV) and reticuloendotheliosis virus (REV) are the most common naturally occurring avian retroviruses associated with neoplastic disease conditions in domesticated poultry. Avian leukosis virus infects primarily chickens, whereas REV infects chickens, turkeys, and other avian species. In addition to causing tumors, both ALV and REV can reduce productivity and induce immunosuppression and other production problems in affected flocks.

Characterization of a rearrangement in the c-MYB promoter in the acute lymphoblastic leukemia cell line CCRF-CEM

Despite the frequent description of 6q- structural abnormalities in human leukemias and lymphomas, rearrangements of the c-MYB locus have not been detected. We have detected a rearrangement in the c-MYB proto-oncogene in the cell line CCRF-CEM, an immature T-cell leukemia cell line which is not 6q-. Due to this rearrangement, a large portion of the c-MYB promoter conserved between the human and murine c-MYB genes is lost. The rearranged locus, which we have designated MRR (MYB rearranged region), has been cloned and mapped to chromosome 6. Field inversion gel electrophoresis (FIGE) studies reveal that the MRR sequence is linked to the c-MYB locus, suggesting that the rearrangement is due to a submicroscopic deletion. The rearrangement appears to have no effect on c-MYB promoter activity as analyzed in CCRF-CEM cells. The normal locus of the MRR sequence has been cloned from a human placental genomic library. Partial sequence analysis of this clone reveals that a portion of the DNA lost in the rearrangement shows a high degree of homology to a member of the myc family of oncogenes. Thus the characterization of this rearrangement has yielded a new set of probes for the study of chromosome 6q abnormalities in human leukemias and lymphomas and provides the first evidence for potential involvement of the c-MYB locus itself in submicroscopic deletions within chromosome 6.

Overexpression of C-terminally but not N-terminally truncated Myb induces fibrosarcomas: a novel nonhematopoietic target cell for the myb oncogene

The myb oncogene encodes a DNA-binding transcriptional transactivator which can become a hematopoietic cell-transforming protein following the deletion of amino acid sequences from either its amino or carboxyl terminus. Although a number of hematopoietic tumors express terminally deleted variants of Myb, the involvement of truncated Myb in nonhematopoietic tumors has not been adequately investigated. To assess the full spectrum of Myb's oncogenic capability, a replication-competent retroviral vector (RCAMV) was used to express a full-length protein (C-Myb), an amino-terminally truncated protein (VCC- or delta N-Myb), a carboxyl-terminally truncated protein (T-Myb), or a doubly truncated protein (VCT-Myb) in vivo. These viruses were injected intravenously into 10-day chicken embryos, and the infected chicks were monitored for tumors. Approximately 4 to 8 weeks after hatching, the majority (30 of 39 [77%]) of animals infected with the T-Myb retrovirus (without 214 carboxyl-terminal residues) developed nodular muscle tumors which could be identified by both morphologic and immunohistochemical criteria as fibrosarcomas. Identically appearing tumors could also be found in the kidney of some T-Myb-infected animals. The T-Myb-induced fibrosarcomas expressed the appropriately sized T-Myb protein, contained an unaltered proviral T-myb gene, and showed clonal proviral integration sites. In comparison, no sarcomas were observed in any of the animals infected with the amino-terminally truncated (VCC- and delta N-Myb) or doubly truncated (VCT-Myb) viruses. A loss of carboxyl-terminal but not amino-terminal sequences can thus convert Myb into a potent in vivo transforming protein for nonhematopoietic mesenchymal cells. In comparison, a truncation of either or both ends of the protein can activate Myb into a hematopoietic cell-transforming protein.

Overexpression of C-terminally but not N-terminally truncated Myb induces fibrosarcomas: a novel nonhematopoietic target cell for the myb oncogene

The myb oncogene encodes a DNA-binding transcriptional transactivator which can become a hematopoietic cell-transforming protein following the deletion of amino acid sequences from either its amino or carboxyl terminus. Although a number of hematopoietic tumors express terminally deleted variants of Myb, the involvement of truncated Myb in nonhematopoietic tumors has not been adequately investigated. To assess the full spectrum of Myb's oncogenic capability, a replication-competent retroviral vector (RCAMV) was used to express a full-length protein (C-Myb), an amino-terminally truncated protein (VCC- or delta N-Myb), a carboxyl-terminally truncated protein (T-Myb), or a doubly truncated protein (VCT-Myb) in vivo. These viruses were injected intravenously into 10-day chicken embryos, and the infected chicks were monitored for tumors. Approximately 4 to 8 weeks after hatching, the majority (30 of 39 [77%]) of animals infected with the T-Myb retrovirus (without 214 carboxyl-terminal residues) developed nodular muscle tumors which could be identified by both morphologic and immunohistochemical criteria as fibrosarcomas. Identically appearing tumors could also be found in the kidney of some T-Myb-infected animals. The T-Myb-induced fibrosarcomas expressed the appropriately sized T-Myb protein, contained an unaltered proviral T-myb gene, and showed clonal proviral integration sites. In comparison, no sarcomas were observed in any of the animals infected with the amino-terminally truncated (VCC- and delta N-Myb) or doubly truncated (VCT-Myb) viruses. A loss of carboxyl-terminal but not amino-terminal sequences can thus convert Myb into a potent in vivo transforming protein for nonhematopoietic mesenchymal cells. In comparison, a truncation of either or both ends of the protein can activate Myb into a hematopoietic cell-transforming protein.

Retrovirus-induced B cell neoplasia in the bursa of Fabricius

The chicken bursa provides a revealing experimental model system which has helped unravel some of the mysteries surrounding induction of neoplasia by retroviruses lacking dominant viral oncogenes. Analysis of this system continues to provide opportunities for further insight into mechanisms underlying some of the essential characteristics of neoplastic change including maturation arrest, prolonged cell survival, and genetic instability. The deregulation of c-myc expression induced by nearby proviral integration appears to initiate preneoplastic change in a specific window of development, i.e., the bursal stem cell. The generation of large numbers of these preneoplastic stem cells, and the ability for further amplification by transplantation technology, may provide an opportunity to address questions such as how and why myc oncogenes produce preneoplastic maturation arrest or why stem cells are selective targets for these effects. Among the unexplained consequences of this preneoplastic state appears to be genetic instability which leads, inevitably, to formation of invasive bursal neoplasms. It is at least conceivable that the observed myc-induced enhancement of the remarkable capacity for apoptotic cell death present in bursal cells plays a role in this instability. DNA strand breakage is a very early feature of bursal cell apoptosis. If such breakage could occur in sublethal form it might provide a mechanism for increased frequency of genetic change (deletions, rearrangement, and recombination). Among the changes that seem required for successful tumor cell growth outside of follicles is the suppression of cell death induced by loss of cell-cell contact which is characteristic of normal and preneoplastic bursal cells. Several genes in the bcl-2 family are potentially important in the modulation of cell death events central to the evolution of these neoplasms. Their role, if any, remains to be established.

Activation of c-myb is an early bone-marrow event in a murine model for acute promonocytic leukemia

Insertional mutagenesis of c-myb by Moloney murine leukemia virus occurs in 100% of promonocytic leukemias (MMLS) induced by the virus. These leukemias, which resemble acute monocytic leukemia-M5 in humans are induced only in mice undergoing a peritoneal chronic inflammatory response. We have found that two leukemia-specific gag-myb mRNAs in MML provide molecular markers for detection of preleukemic cells in hematopoietic tissue in vivo. The two aberrant RNAs result from splicing of gag to either exon 3 or 4 of c-myb, depending on the site of proviral integration. After reverse transcription-PCR with nested primers and hybridization with specific gag-myb junction probes, one cell, having aberrant c-myb message, could be detected in a minimum of 10(5) liver cells or 10(6) spleen or bone-marrow cells. This approach was used to examine hematopoietic tissues of mice after pristane injection to induce inflammation and virus inoculation. Cells with gag-myb mRNAs could be detected as early as 2 weeks after virus inoculation. In mice receiving both pristane and virus, there was evidence of preleukemic cells in 83% of the mice by 3 weeks after virus infection. Furthermore, 100% of the mice were positive for preleukemic cells by 8 weeks, even though only 50% of mice have been shown to succumb to MML (peak time for disease latency is 12-16 weeks). Cells with these aberrant c-myb messages were initially detected in the bone marrow, but during intermediate stages of disease development these cells disseminated to the spleen, liver, and granuloma. At preleukemic times, from 3 to 8 weeks after virus infection, a lower percentage of mice were positive in the group that did not receive pristane compared with mice in the group receiving pristane. However, at 18 weeks, 100% of the mice in the group receiving virus only had evidence of cells expressing gag-myb RNA in their spleens and/or bone marrow; it is of interest that mice inoculated with virus alone never develop MML. This approach for detecting preleukemic cells will now allow the study of mechanisms by which these preleukemic cells progress to a more transformed state and, perhaps, to a more differentiated state.

New sites of proviral integration associated with murine promonocytic leukemias and evidence for alternate modes of c-myb activation

Murine promonocytic leukemias involving insertional mutagenesis of the c-myb locus can be induced by replication-competent retroviruses. In previously studied promonocytic leukemic cells induced by Moloney murine leukemia virus (called MML), the provirus has been invariably integrated upstream of exons 3 or 4 and the leukemic cells expressed aberrant RNAs with fused virus-myb sequences. Furthermore, Myb expressed by these cells has been shown to be truncated by 47 or 71 amino acids. The present report examines the mechanisms of myb activation in leukemias induced by two other retroviruses, amphotropic virus 4070A and Friend strain FB29 (the leukemias are called AMPH-ML and FB-ML, respectively). This study revealed two additional c-myb proviral insertion sites in these promonocytic leukemias. One FB-ML had a proviral integration in exon 9, and expressed a C-terminally truncated Myb protein of 47 kDa similar to that previously demonstrated to be expressed in the myelomonocytic cell lines NFS60 and VFL-2. However, a sequence of reverse-transcribed and amplified RNA from this leukemia demonstrated that the truncation involved a loss of 248 amino acids compared with a loss of 240 amino acids in the myelomonocytic cell lines. Another leukemia had a provirus integrated in the 5' end of c-myb upstream of exon 2 (in the first intron) and produced a Myb protein that was indistinguishable on sodium dodecyl sulfate-polyacrylamide gel electrophoresis from normal Myb. This latter leukemia (FB-ML R1-4-10) expressed Myb with the smallest N-terminal truncation observed so far in promonocytic leukemias; translation begins at an ATG within c-myb exon 2, leading to loss of only 20 amino acids from the N terminus. Unlike the proteins produced in Moloney murine leukemia virus-induced promonocytic leukemias (MML) that have larger truncations, this protein has an intact DNA binding region and does not contain N-terminal amino acids encoded by gag. However, this protein is similar to all N-terminally truncated Mybs so far studied, in that the truncation resulted in deletion of a casein kinase II phosphorylation site which has been proposed to be involved in regulation of DNA binding.