C-myb Locus Research Articles

Epigenetic regulation of hematopoiesis by DNA methylation.

During embryonic development, cell type-specific transcription factors promote cell identities, while epigenetic modifications are thought to contribute to maintain these cell fates. Our understanding of how genetic and epigenetic modes of regulation work together to establish and maintain cellular identity is still limited, however. Here, we show that DNA methyltransferase 3bb.1 (dnmt3bb.1) is essential for maintenance of hematopoietic stem and progenitor cell (HSPC) fate as part of an early Notch-runx1-cmyb HSPC specification pathway in the zebrafish. Dnmt3bb.1 is expressed in HSPC downstream from Notch1 and runx1, and loss of Dnmt3bb.1 activity leads to reduced cmyb locus methylation, reduced cmyb expression, and gradual reduction in HSPCs. Ectopic overexpression of dnmt3bb.1 in non-hematopoietic cells is sufficient to methylate the cmyb locus, promote cmyb expression, and promote hematopoietic development. Our results reveal an epigenetic mechanism supporting the maintenance of hematopoietic cell fate via DNA methylation-mediated perdurance of a key transcription factor in HSPCs.

Core Transcriptional Regulatory Circuit Controlled by the TAL1 Complex in Human T Cell Acute Lymphoblastic Leukemia

The oncogenic transcription factor TAL1/SCL is aberrantly expressed in over 40% of cases of human Tcell acute lymphoblastic leukemia (T-ALL), emphasizing its importance in the molecular pathogenesis of T-ALL. Here we identify the core transcriptional regulatory circuit controlled by TAL1 and its regulatory partners HEB, E2A, LMO1/2, GATA3, and RUNX1. We show that TAL1 forms a positive interconnected autoregulatory loop with GATA3 and RUNX1 and that the TAL1 complex directly activates the MYB oncogene, forming a positive feed-forward regulatory loop that reinforces and stabilizes the TAL1-regulated oncogenic program. One of the critical downstream targets in this circuitry is the TRIB2 gene, which is oppositely regulated by TAL1 and E2A/HEB and is essential for the survival of T-ALL cells.

Distinct regulation of c-myb gene expression by HoxA9, Meis1 and Pbx proteins in normal hematopoietic progenitors and transformed myeloid cells

The proto-oncogenic protein c-Myb is an essential regulator of hematopoiesis and is frequently deregulated in hematological diseases such as lymphoma and leukemia. To gain insight into the mechanisms underlying the aberrant expression of c-Myb in myeloid leukemia, we analyzed and compared c-myb gene transcriptional regulation using two cell lines modeling normal hematopoietic progenitor cells (HPCs) and transformed myelomonocytic blasts. We report that the transcription factors HoxA9, Meis1, Pbx1 and Pbx2 bind in vivo to the c-myb locus and maintain its expression through different mechanisms in HPCs and leukemic cells. Our analysis also points to a critical role for Pbx2 in deregulating c-myb expression in murine myeloid cells cotransformed by the cooperative activity of HoxA9 and Meis1. This effect is associated with an intronic positioning of epigenetic marks and RNA polymerase II binding in the orthologous region of a previously described alternative promoter for c-myb. Taken together, our results could provide a first hint to explain the abnormal expression of c-myb in leukemic cells.

Regulation of c‐myb by NF‐kB family members during human induced pluripotent stem cell differentiation into hematopoietic precursors

c‐Myb is a transcription factor and proto‐oncogene that is important for hematopoietic cell development. This gene is predominately expressed in progenitor cells of adult bone marrow and down regulated during differentiation. Through gene knockout experimentation, the importance for c‐myb in developing cells has been determined. Null mutants show lethality in certain blood cell lines, strengthening the ideas that c‐myb plays an important role in cell line differentiation. Specifically, in murine embryonic cells, the absence of c‐myb results in the disruption of specific blood cell types. Previous studies have shown that NF‐κB family members are involved in regulating the activation of c‐myb by binding to Rel‐Related Proteins Binding Elements (RRBE) in the first intron. This leads to the hypothesis that NF‐kB regulates expression of c‐myb during development of hematopoietic cells. We have set up an in vitro differentiation protocol for differentiating induced pluripotent stem cells into blood cell precursors via a hemangioblast intermediate using defined cytokine and growth factors. Activation of c‐myb and NF‐kB was analyzed using qPCR to analyze the kinetics of c‐myb expression. Work is continuing using mobility shift assays and chromatin immunoprecipitations to determine the localization of NF‐kB to the c‐myb locus. Research support provided by a Providence College Committee on Aid to Faculty Research Grant.

P89c‐Myb is not required for fetal or adult hematopoiesis

The c-myb gene encodes two proteins, termed p75 and p89. Of these, the larger isoform is transcribed from an alternatively spliced message that contains an additional exon, exon 9A. Disruption of the c-myb locus in mice results in embryonic lethality due to defective hematopoiesis and in the adult, tissue-specific inactivation of c-myb in hematopoietic tissues blocks differentiation along several lineages. The c-myb knock-out mouse models described thus far result in the disruption of both the p75 and p89 isoforms, making it impossible to assign a definitive role to p89(c-Myb) in development and hematopoiesis. We have therefore generated a null-mutant mouse where exon 9A has been systemically deleted that results in the absence of only the p89-myb transcript and protein. Unlike disruption of both forms of the c-myb gene, loss of only the p89-encoding isoform does not have any deleterious effects on mammalian hematopoiesis and development.

A New Subtype of T-Cell Acute Leukemia in Very Young Children Is Defined by a Translocation Targeting the C-MYB Oncogene, and a Specific Gene Expression Signature.

Cytogenetics and expression studies have pointed out an increasing number of oncogenes in T-cell acute lymphoblastic leukemias (T-ALL), demonstrating the complexity of T-ALL oncogenesis and the requirement for a network of cooperative genomic events. Here, we identified two types of genomic alterations involving the C-MYB locus at 6q23 in human T-ALL, both associated with transcriptional deregulation. First, using a TCRB-based FISH screening, we found a new reciprocal translocation, t(6;7)(q23;q34), that juxtaposed the TCRB and C-MYB loci (n=6 cases). Breakpoints were fully characterized at the molecular level in 3 cases. Second, a genome wide copy-number analysis by array-CGH (customized 4K BAC/PAC arrays) identified short cryptic duplications which always include the C-MYB gene (MYBdup, n=11 out of 80 primary pediatric and adult T-ALL, 14%). Somatic origin of the genomic gain was demonstrated using paired leukemic and remission samples. The minimal region of gain was mapped to 230 Kb using 244K oligonucleotide a-CGH, and fiber-FISH demonstrated tandem duplication. Interestingly, these genomic events are reminiscent of the frequent myb retroviral insertions in murine leukemia. Expression analysis by RQ-PCR and microarrays data showed stronger C-MYB expression in the MYB-rearranged cases compared to other T-ALLs and normal controls (thymus, BM, and PBL). Moreover, allele-specific approaches showed a dramatically skewed allele expression in the TCRB-MYB cases, suggesting that the translocation-driven deregulated expression overcomes a cellular attempt to downregulate C-MYB. Considering that the C-MYB transcription factor has been involved at several key steps throughout normal thymic differentiation, these data strongly suggest that deregulation of C-MYB due to genomic events is oncogenic in T-ALL. Integrated analysis of clinical, genomic, and large-scale gene expression data showed that the MYB translocation defines a new T-ALL subtype. Strikingly, 5 out of 6 patients with the translocation had a very young age for T-cell leukemia (1.1, 1.3, 1.8, 2.5, and 2.9 year-old; median 2.2 versus 9.4 year-old for 355 pediatric T-ALL in the FRALLE 93/2000 pediatric trials, p<10−4). Gene expression profiling showed that these cases cluster in a new branch distinct from other subtypes and are characterized by a specific cell cycle/mitosis signature. By contrast, the MYBdup alteration was associated to previously defined T-ALL subtypes (TAL, TLX1/HOX11, TLX3/HOX11L2, MLL, CALM-AF10, and immature cases), suggesting an additional oncogenic event. NOTCH1 activating mutations and p16/CDKN2A/ARF deletions were found in both the TCRB-MYB and MYBdup cases, according to multi-step T-cell oncogenesis. This work is the first clear demonstration of the recurrent involvement of the C-MYB locus in a human neoplasia. Moreover the t(6;7) translocation defines a new T-ALL subtype associated with very young age for T-cell leukemia and a distinct biology.

The C-MYB locus is involved in chromosomal translocation and genomic duplications in human T-cell acute leukemia (T-ALL), the translocation defining a new T-ALL subtype in very young children

AbstractThe C-Myb transcription factor is essential for hematopoiesis, including in the T-cell lineage. The C-Myb locus is a common site of retroviral insertional mutagenesis, however no recurrent genomic involvement has been reported in human malignancies. Here, we identified 2 types of genomic alterations involving the C-MYB locus at 6q23 in human T-cell acute leukemia (T-ALL). First, we found a reciprocal translocation, t(6;7)(q23;q34), that juxtaposed the TCRB and C-MYB loci (n = 6 cases). Second, a genome-wide copy-number analysis by array-based comparative genomic hybridization (array-CGH) identified short somatic duplications that include C-MYB (MYBdup, n = 13 cases of 84 T-ALL, 15%). Expression analysis, including allele-specific approaches, showed stronger C-MYB expression in the MYB-rearranged cases compared with other T-ALLs, and a dramatically skewed C-MYB allele expression in the TCRB-MYB cases, which suggests that a translocation-driven deregulated expression may overcome a cellular attempt to down-regulate C-MYB. Strikingly, profiling of the T-ALLs by clinical, genomic, and large-scale gene expression analyses shows that the TCRB-MYB translocation defines a new T-ALL subtype associated with a very young age for T-cell leukemia (median, 2.2 years) and with a proliferation/mitosis expression signature. By contrast, the MYBdup alteration was associated with the previously defined T-ALL subtypes.

Expression of stress response protein glucose regulated protein-78 mediated by c-Myb

Glucose regulated protein-78, GRP78 has been implicated in the protection of tumor cells from cytotoxic damage and apoptosis. When protein profiles of colon cell lines were investigated we found remarkably high GRP78 expression in two cell lines. These cell lines express elevated levels of the transcription factor c-Myb due to genomic amplification of the c-myb locus and we hypothesized that c-Myb regulates GRP78 expression in colon cancer cells. The promoters of human and murine GRP78 and the related family member GRP94 were examined and potential c-Myb binding sites were identified and characterized. DNA binding studies with recombinant c-Myb and nuclear extracts together with ChIP assays on colon cell lines validated these sites. Endogenous GRP78 expression was further induced in these colon cells in response to Thapsigargin treatment, a potent inducer of the unfolded protein response. Transactivation studies with the human GRP78 promoter in colon cell lines showed reporter activity was dependent upon the presence of a conserved c-Myb binding site independent of sequences associated with the unfolded protein response. Finally, over-expression of c-Myb induced the endogenous GRP78 gene. These data suggest that amplification of c-myb in tumor cells may lead to robust GRP78 gene induction, which may in turn assist cells in survival under conditions of oxygen deprivation and nutrient stress.

Linkage on chromosome 10 of several murine retroviral integration loci associated with leukaemia.

Mml loci have been identified as provirus integration sites among a subset of monocytic tumours induced by murine leukaemia virus (MuLV) infection of BALB/c and DBA/2 mice. These myeloid leukaemias contain a retrovirus integrated on chromosome 10 in proximity to the c-myb locus; however, c-myb expression was not altered. Detailed physical mapping enabled placement of the retroviral integration sites approximately 25 kb (Mml1), approximately 51 kb (Mml2), and approximately 70 kb (Mml3) upstream of the c-myb locus. Furthermore, the Fti1 (fit-1) locus, a common integration site in feline leukaemia virus-induced T cell lymphomas, was mapped upstream of Mml3. Sequence analysis of Mml1, Mml2 and Mml3 loci (39.6, 16.4 and 5.9 kb, respectively) in conjunction with the BLAST (basic local alignment search tool) homology searches against the expressed sequence tag (EST) database and the use of gene/exon prediction programs revealed potential coding sequences that were not confirmed by Northern analysis or RT-PCR. The sequences between c-myb and Fti1, which were shown to include two potential scaffold/matrix attachment regions (S/MARs), are most likely regulatory in nature. An extended search for transcribed sequences far upstream of Mml3 revealed five genes, four of which were expressed in multiple tissues in mice. These genes could not be linked to tumour formation by the virus but their homologous sequences were found on human chromosome 6, thus allowing extension of the syntenic region on mouse chromosome 10 to approximately 250 kb.

Novel integration sites at the distal 3' end of the c-myb locus in retrovirus-induced promonocytic leukemias

In BALB/c nu/nu and sublethally irradiated DBA/2 mice, promonocytic leukemia was induced by intravenous inoculation of Friend murine leukemia virus (F-MuLV) strain C57 in conjunction with intraperitoneal injection of pristane. These tumors appear to be identical morphologically to previously reported ones induced by other MuLVs, such as Moloney, amphotropic 4070A, and F-MuLV FB29, which most commonly have provirus integrations in the 5' end of the c-myb locus. Interestingly, 2 of the 16 F-MuLV-induced tumors had viruses integrated in the distal 3' end of c-myb. To determine the precise locations of these integrations, it was necessary to clone sequences encoding the 3' c-myb exons and to prepare a physical map of this region. Exons 10 to 15 were positioned on the map, and it was found that the proviruses in the aforementioned tumors were located within narrow region in the beginning of the large (greater than 11 kb) intron 14. The predicted protein product encoded by the affected alleles is truncated by 38 amino acids. This represents a novel virus integration site which is most likely associated with oncogenic activation of the c-myb gene during leukemogenesis.

Lymphoid expression and regulation of A20, an inhibitor of programmed cell death.

A20 is a cytokine-inducible primary response gene that encodes a protein that inhibits apoptosis. We cloned and characterized a murine A20 cDNA and mapped the gene to mouse chromosome 10, 3.5 centimorgans proximal to the c-myb locus. Using the murine cDNA, we conducted in situ hybridization studies to examine patterns of A20 expression in mouse embryos and postnatal tissues. Both temporally restricted and tissue-specific patterns of A20 expression were observed, with strikingly high levels in lymphoid organs, including the thymus, spleen, and gut-associated lymphoid tissue. Northern analysis using purified human lymphocyte populations demonstrated that A20 is constitutively expressed in both immature and mature thymocyte subpopulations, as well as in resting peripheral T cells. Activation leads to a down-regulation of A20 expression in both mature thymocytes and peripheral T cells. This pattern of expression and regulation differs markedly from that observed in prior studies with cell lines in which stimulation led to an induction of A20 expression. Additionally, this is the first demonstration of constitutive expression of A20 in primary cells. The data suggest a role for A20 in the function of the lymphoid system.

Susceptibility and Resistance to Moloney Murine Leukemia Virus-Induced Promonocytic Leukemia

Moloney murine leukemia virus (M-MuLV) induces promonocytic leukemias, called MML, in pristane-treated adult mice. These tumors invariably express fused gag-myb mRNA as a consequence of virus integration and activation of the c-myb locus. In the present study it was determined that while BALB/c and DBA/2N mice are highly susceptible, C57BL/6, C3H/He, STS/A, NFS, NIH/Swiss, SJL/J, and NZB mice are strongly resistant to tumor induction. Although C57BL/6 mice were resistant because they were unable to support early virus replication in hematopoietic tissue, NFS and C3H/He mice supported replication and were shown, using RT-PCR, to have cells in the bone marrow and spleen that expressed the aberrant, leukemia-related gag-myb mRNA. This provided evidence that early stages of leukemia were permitted to develop in these mice, but preneoplastic cells were unable to progress to the acute phase. Experiments in which MML was induced by M-MuLV plus pristane treatment in immunodeficient C3H/He nu/nu and sublethally irradiated C3H/He mice suggested that the immune response may play a role in eliminating preleukemic cells in immunocompetent C3H/He. Tumors from these mice had rearrangements at the c-myb locus and expressed gag-myb RNA. It was concluded that, at least in the case of C3H/He mice, resistance is not due to an inability of virus to activate c-myb or to a lack of other tumor promoting events. Rather, leukemia development appears to be restricted by an immune response, presumably T-cell mediated. Evidence is provided that non-H-2 MHC genes are required for resistance in both C57BL/6 and C3H/He mice and that resistance is dominant. This provides an animal model for the study of tumor progression as it relates to the immune response.

Different abilities of Friend murine leukemia virus (MuLV) and Moloney MuLV to induce promonocytic leukemia are due to determinants in both psi-gag-PR and env regions.

Moloney murine leukemia virus (M-MuLV) is capable of inducing promonocytic leukemia in 50% of adult BALB/c mice that have received peritoneal injections of pristane, but Friend MuLV strain 57 (F-MuLV) is nonleukemogenic under similar conditions. It was shown earlier that these differences could not be mapped to the U3 region of the virus long terminal repeat, indicating the probable influence of structural genes and/or R-U5 sequences. In this study, reciprocal chimeras containing exchanged structural genes and R-U5 sequences from these two closely related viruses were analyzed for differences in ability to induce disease. Results showed that two regions of F-MuLV, psi-gag-PR and env, when substituted for those of M-MuLV were dramatically disease attenuating. The 5'-most region, which is widely distributed, overlaps with the 5' end of the env intron and includes the RNA packaging region, psi, the entire gag coding region, and the viral protease coding region (PR) of pol. It was also found that reciprocal constructs having substitutions of both of these regions of M-MuLV in an F-MuLV background allowed full reestablishment of promonocytic leukemia. These leukemias were positive for c-myb rearrangements which are characteristic of M-MuLV-induced promonocytic leukemias. Neither region alone, however, was sufficient to produce disease with a greater incidence than 13%. Further studies demonstrated that the inability of viruses with psi, gag, PR, or env sequences from F-MuLV to induce leukemia in this model system was not due to their inability to replicate in hematopoietic tissue, to integrate into the c-myb locus early on after infection in vivo, or to express gag-myb mRNA characteristic of M-MuLV-induced preleukemic cells and acute leukemia.

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 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.

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.

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

We have previously reported that infection of 9- to 13-day-old chicken embryos with RAV-1 results in rapid development of a novel B-cell lymphoma in which proviral insertion has activated expression of the c-myb gene (E. Pizer and E. H. Humphries, J. Virol. 63:1630-1640, 1989). The biological properties of these B-cell lymphomas are distinct from those associated with the B-cell lymphomas that develop following avian leukosis virus proviral insertion within the c-myc locus. In an extension of this study, more than 200 chickens, infected as 10- to 11-day-old embryos, were examined for development of lymphomas that possess disrupted c-myb loci. Fourteen percent developed disseminated B-cell lymphoma. In the majority of these tumors, the RAV-1 provirus had inserted between the first and second exons that code for p75c-myb. However, insertions between the second and third exons and between the third and fourth exons were also detected. In situ analysis of myb protein expression in tumor tissue revealed morphological features suggesting that the tumor originates in the bursa. Within the bursa, the lymphoma appeared to spread from follicle to follicle without compromising the structural integrity of the organ. Tumor masses in liver demonstrated heterogeneous levels of myb protein suggestive of biologically distinct subpopulations. In contrast to the morbidity data, immunohistological analysis of bursae from 4- to 6-week-old chickens at risk of developing lymphomas bearing altered c-myb loci revealed lesions expressing elevated levels of myb in 16 of 19 birds. The activated myb lymphoma displayed very poor capacity to proliferate outside its original host. Only 1 of 33 in vivo transfers of tumor to recipient hosts established a transplantable tumor. None of the primary tumor tissue nor the transplantable tumor exhibited the capacity for in vitro proliferation. Similar experimental manipulation has yielded in vitro lines established from avian B-cell lymphomas expressing elevated levels of c-myc or v-rel. The dependence on embryonic infection for development of activated-myb lymphoma suggests a requirement for a specific target cell in which c-myb is activated by proviral insertion. It is likely, moreover, that continued tumor development requires elevated expression of myb proteins within a specific cell population in a restricted stage of differentiation.

Proto-oncogene allelic variations in human squamous cell carcinomas of the larynx

Proto-oncogene restriction fragment length polymorphisms (RFLPs) were investigated in a group of 23 patients with squamous cell carcinomas of the larynx. The frequency of the rare 5 kb c-mos allele was significantly higher than that observed in control groups of patients with colorectal neoplasms or lymphoproliferative disorders. In addition, the 2 patients heterozygous at the c-mos locus (TC-8 and TC-10) were the only 2 of our series of develop multiple malignancies. Also, the 10 kb L-myc allele was remarkably more represented in patients with laryngeal carcinoma when compared to controls. These findings suggest that c-mos and L-myc RFLPs might be helpful in identifying those individuals who are at a higher risk of developing laryngeal carcinomas. Single allele amplification of L-myc, c-myb and c-mos proto-oncogenes, with no concomitant mRNA hyperexpression, were observed in 3 cases. The results obtained seem to rule out a direct pathogenetic role of these proto-oncogenes and suggest that the amplification of other closely linked genes, located on chromosomes 1, 6 and 8, respectively, may be causally associated with the development of these tumors. No allelic deletions at the c-myb locus were observed, whereas a loss of a c-Ha-ras-1 allele was demonstrated in one of the 11 heterozygous patients. Thus, the analysis of polymorphic proto-oncogenes in laryngeal carcinomas allowed us to identify a group of genetic abnormalities (chromosomes 1, 6 and 8 gene amplifications and c-Ha-ras-1 deletions) which may be involved in the development or progression of these tumors.

Acute myeloid leukemia induction by amphotropic murine retrovirus (4070A): clonal integrations involve c-myb in some but not all leukemias.

Amphotropic murine retrovirus 4070A was demonstrated to be highly leukemogenic when inoculated intravenously into adult DBA/2 mice that were undergoing an intense chronic inflammatory response, but was nonleukemogenic in the absence of inflammation. The virus-induced promoonocytic leukemias, designated AMPH-ML, are similar morphologically and in cell surface marker expression to monocytic leukemias, called MML and MF-ML, previously shown to be induced by Moloney murine leukemia virus and MF-3 virus (a recombinant between Friend murine leukemia virus and Moloney murine leukemia virus) and resemble certain mature acute monocytic leukemias in humans (AML subtype M5). Approximately two-thirds of the AMPH-MLs (subgroup I) were demonstrated to have alterations in the 5' end of the c-myb locus, an event which occurs in 100% of MML and MF-ML. Data indicate that proviral insertions in AMPH-ML subgroup I resulted in aberrant c-myb mRNA expression and truncation of its translation product at the amino terminus. Approximately one-third of the AMPH-MLs (subgroup II) had not undergone any DNA rearrangements at the c-myb locus. In addition, their transcripts and protein products were of normal size. These latter leukemias also had not undergone DNA rearrangements in c-myc, although retroviruses expressing myc have previously been shown to induce monocyte-macrophage tumors in mice undergoing a chronic inflammation. That subgroup II leukemias had at least one clonal viral insertion suggests that there may be other sites in the cellular genome that can be activated by insertional mutagenesis in these murine acute monocytic leukemias.

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.

Regions of the Moloney murine leukemia virus genome specifically related to induction of promonocytic tumors

Moloney murine leukemia virus (MuLV) can be a potent inducer of promonocytic leukemias in mice that are undergoing a chronic inflammatory response. The neoplasms are, at least in part, associated with insertional mutagenesis of the c-myb locus. Evidence is presented for the existence of at least two genetic elements of the virus that are crucial to induction of this disease but are not required for viral replication in hematopoietic tissues or induction of lymphoid disease. These genetic elements were detected by testing the pathogenicity of recombinants between Moloney and Friend MuLVs, the latter of which is nonleukemic to myeloid cells under these conditions, and by testing Moloney MuLV-based viruses that have nonretroviral sequences inserted at specific endonuclease sites in their long terminal repeats (LTRs). Analysis of the Moloney/Friend recombinants showed that there are sequences within the structural gene domain of Moloney, but not Friend, MuLV that are necessary for promonocytic leukemia, whereas the LTRs of the MuLVs are equally effective for promonocytic tumor formation and insertional mutagenesis of the c-myb gene. Experiments with viruses which were mutagenized in the LTR by insertions demonstrated that there is a specific genetic element in the U3 region of the LTR of Moloney MuLV, upstream of the 75-base-pair enhancer which, when interrupted, results in loss of leukemogenicity for cells in the monocytic lineage but not cells in the lymphoid lineage. We conclude, therefore, that promonocytic leukemia induction, in Moloney MuLV-infected mice undergoing a chronic inflammatory response, requires specific sequences in the structural gene region of Moloney MuLV as well as other sequences in the regulatory region of the virus.