Abelson leukemia virus induces lymphoid and erythroid colonies in infected fetal cell cultures

Mapping and functional analysis of regulatory sequences in the mouse λ5-VpreB1 domain

The lambda5 and VpreB genes encode the components of the surrogate light-chain which forms part of the pre-B cell receptor and plays a key role in B cell development. In the mouse, the lambda5 and VpreB1 genes are closely linked and are co-regulated by a multi-component locus control region. To identify the sequences that regulate lambda5 and VpreB1 expression during B cell development, we have comprehensively mapped the DNaseI hypersensitive sites (HS) in the lambda5-VpreB1 functional domain. The active domain contains 12 HS that are distributed at high density across the 18.3 kb region that forms the lambda5 and VpreB1 functional unit. Analysis of a reporter gene driven by the VpreB1 promoter in transgenic mice identified a novel enhancer associated with two HS located upstream of lambda5. The lambda5-VpreB1 locus was also found to be closely linked to the ubiquitously expressed Topoisomerase-3beta (Topo3beta) gene. The VpreB1 and Topo3beta genes have entirely different expression patterns despite the fact that the two promoters are separated by a distance of only 1.5 kb.

The myeloproliferative sarcoma virus causes transformation or erythroid progenitor cells in vitro

The myeloproliferative sarcoma virus induces spleen focus formation in vivo and transforms fibroblasts in vitro. We showed in this study that in vitro infection of spleen or bone marrow cells from susceptible mice with the myeloproliferative sarcoma virus leads to the formation of erythroid bursts. Under optimal conditions erythroid bursts formed in the absence of added erythropoietin, but the addition of as little as 0.05 U of erythropoietin per ml to infected cultures resulted in a significant increase in numbers of erythroid bursts and the proportion of hemoglobinized cells. A comparison of the kinetics of burst formation and the size of the induced bursts with those induced with Friend virus suggested that either sarcoma virus such as the myeloproliferative sarcoma virus or the target cells for the two viruses were not the same. Density characterization and heat lability studies indicated that the increased erythroid proliferation in vitro was a virus-induced event, but the possibility that the induced erythroid burst formation is mediated via interaction with a nonerythroid target cell and subsequent release of a soluble factor cannot be ruled out.

The myeloproliferative sarcoma virus causes transformation or erythroid progenitor cells in vitro

The myeloproliferative sarcoma virus induces spleen focus formation in vivo and transforms fibroblasts in vitro. We showed in this study that in vitro infection of spleen or bone marrow cells from susceptible mice with the myeloproliferative sarcoma virus leads to the formation of erythroid bursts. Under optimal conditions erythroid bursts formed in the absence of added erythropoietin, but the addition of as little as 0.05 U of erythropoietin per ml to infected cultures resulted in a significant increase in numbers of erythroid bursts and the proportion of hemoglobinized cells. A comparison of the kinetics of burst formation and the size of the induced bursts with those induced with Friend virus suggested that either sarcoma virus such as the myeloproliferative sarcoma virus or the target cells for the two viruses were not the same. Density characterization and heat lability studies indicated that the increased erythroid proliferation in vitro was a virus-induced event, but the possibility that the induced erythroid burst formation is mediated via interaction with a nonerythroid target cell and subsequent release of a soluble factor cannot be ruled out.

Analysis of mice with single and multiple copies of transgenes reveals a novel arrangement for the lambda5-VpreB1 locus control region

The murine lambda5-VpreB1 locus encodes two proteins that form part of the pre-B-cell receptor and play a key role in B-lymphocyte development. We have identified a locus control region (LCR) which is responsible for coordinate activation of both genes in pre-B cells. Analysis of mice with single and multiple copies of transgenes shows a clear difference in the expression behavior of the genes depending on the transgene copy number. While expression of both lambda5 and VpreB1 in single- and two-copy integrations requires the presence of a set of DNase I hypersensitive sites located 3' of the lambda5 gene, small fragments containing the genes have LCR activity when arranged in multiple-copy tandem arrays, indicating that additional components of the LCR are located within or close to the genes. The complete LCR is capable of driving efficient copy-dependent expression of a lambda5 gene in pre-B cells even when it is integrated into centomeric gamma-satellite DNA. The finding that activation of expression of the locus by positively acting factors is fully dominant over the silencing effect of heterochromatin has implications for models for chromatin-mediated gene silencing during B-cell development.

Defective development of the embryonic liver in N-myc-deficient mice

The mechanisms involved in the formation and the differentiation of the liver remain unclear despite extensive studies. To investigate these events in mouse hepatic development, we have taken advantage of the N-myc mutant mouse line which exhibits abnormal liver development. N-myc mutant embryos die between 11.5 and 12.5 days postcoitum most probably from heart failure. In the present study, we report that at 11.5 days of gestation, extensive apoptosis restricted to the hepatocytes occurred in N-myc mutant liver when compared to wild-type samples. Moreover, the number of hematopoietic cells is reduced in the mutant liver. During early liver organogenesis, the N-myc gene is expressed in tissues involved in the induction and the differentiation of the hepatocytes. At 11.5 days postcoitum, both c-myc and N-myc genes are expressed in the liver. While c-myc is expressed at a high level in the organ per se, N-myc expression is mostly confined to the peripheral layer of the liver which will generate the Glisson's capsule. Taken together, the expression pattern of N-myc in the liver and the specific apoptosis of hepatocytes observed in N-myc mutants indicate that N-myc is required for hepatocyte survival and suggest that it is involved in the genetic cascade leading to normal liver development.

v-Abl activates c-myc transcription through the E2F site

The v-abl oncogene of Abelson murine leukemia virus encodes a deregulated form of the cellular nonreceptor tyrosine kinase. v-Abl activates c-myc transcription, and c-Myc is an essential downstream component in the v-Abl transformation program. To explore the mechanism by which v-Abl activates c-myc transcription, a cotransfection assay was developed. We show that transactivation of a c-myc promoter by v-Abl requires the SH1 (tyrosine kinase) and SH2 domains of v-Abl; the C-terminal domains are not required for transactivation. The assay also identified the E2F site in the c-myc promoter as a v-Abl-responsive element. In addition, multimerized E2F sites were shown to be sufficient to confer v-Abl-dependent activation on a minimal promoter. This is the first identification of a v-Abl response element for transcriptional activation. v-Abl tyrosine kinase-dependent changes in proteins binding the c-myc E2F site were also demonstrated, including induction of a complex containing DP1, p107, cyclin A, and cdk2. Identification of v-Abl-dependent changes in E2F-binding proteins provides an important link between v-Abl, transcription, cell cycle regulation, and control of cellular growth.

Absence of p53 allows direct immortalization of hematopoietic cells by the myc and raf oncogenes

The p53 tumor suppressor is implicated here as a crucial barrier to unlimited cell proliferation. Its role in transformation of hematopoietic cells was studied by infecting fetal liver cells from wild-type or p53-/- mice with oncogenic retroviruses. Transformed colonies arose with a raf and a myc-raf virus. Absence of p53 did not affect their frequency but proved critical for their continued propagation. Colonies of p53-/- cells bearing both myc and raf readily yielded continuous cell lines without apparent requirement for genetic alteration. The lines, mainly of erythroid or myelomonocytic origin, were diploid but highly tumorigenic from their inception. These findings imply that p53 loss contributes directly to immortalization and tumorigenesis, probably by abrogating an intrinsic senescence program.

Philadelphia chromosome-positive leukaemia: the translocated genes and their gene products

Overwhelming evidence indicates a role for the deregulated ABL protein tyrosine kinase in the aetiology of CML and Ph-positive acute leukaemia. These disorders are characterized by the generation of BCR/ABL fusion proteins with elevated tyrosine kinase activity. Although much is known concerning the transforming potential of ABL proteins in various systems, very little is understood of the normal function and mode of regulation of ABL activity. The mechanism of oncogenic activation is therefore also obscure. In spite of this, our understanding of the molecular details of these chromosomal translocations allows the design of therapies directed against their unique, leukaemia-specific proteins and RNA products.

In vitro phosphorylation of the erythropoietin receptor and an associated protein, pp130

The cytoplasmic domain of the cloned erythropoietin (EPO) receptor (EPOR) contains no protein kinase motif, yet addition of EPO to EPO-responsive cells causes an increase in protein-tyrosine phosphorylation. Here we show that addition of EPO or interleukin-3 (IL-3) to an IL-3-dependent cell line expressing the wild-type EPOR causes a small fraction (less than 5%) of total cellular EPOR to shift in gel mobility from 66 to 72 kDa, due at least in part to phosphorylation. Using biotinylated EPO as an affinity reagent, we show that the 72-kDa species is greatly enriched on the cell surface. To demonstrate that a protein kinase activity associates with cell surface EPOR, cells were incubated with biotinylated EPO and then cross-linked with a thiol-cleavable chemical cross-linker. The avidin-agarose-selected complexes were incubated with [gamma-32P]ATP. After in vitro phosphorylation and denaturation without reducing agent, both antiphosphotyrosine and anti-EPOR antibodies immunoprecipitated labeled 72-kDa EPOR and an unidentified 130-kDa phosphoprotein (pp130), indicating that a protein kinase is associated with cell surface EPOR and that a fraction of the EPOR was phosphorylated on tyrosine residues either in the cells or during the cell-free phosphorylation reaction. Under reducing conditions, the 72-kDa phosphorylated EPOR but not pp130 was immunoprecipitated with an anti-EPOR antibody, suggesting that the pp130 is bound to the EPOR by the thiol-cleavable chemical cross-linker. Previously, we showed that deletion of the 42 carboxy-terminal amino acids of the EPOR allows cells to grow in 1/10 the normal EPO concentration, without affecting receptor number or affinity. Two carboxy-terminal truncated EPO receptors that are hyperresponsive to EPO were poorly phosphorylated during the in vitro reaction, suggesting that the carboxy-terminal region of the EPOR contains a site for phosphorylation or a site for interaction with a protein kinase. Our data suggests that phosphorylation or interaction with a protein kinase in the carboxy-terminal region may down-modulate the proliferative action of the EPOR.

In vitro phosphorylation of the erythropoietin receptor and an associated protein, pp130

The cytoplasmic domain of the cloned erythropoietin (EPO) receptor (EPOR) contains no protein kinase motif, yet addition of EPO to EPO-responsive cells causes an increase in protein-tyrosine phosphorylation. Here we show that addition of EPO or interleukin-3 (IL-3) to an IL-3-dependent cell line expressing the wild-type EPOR causes a small fraction (less than 5%) of total cellular EPOR to shift in gel mobility from 66 to 72 kDa, due at least in part to phosphorylation. Using biotinylated EPO as an affinity reagent, we show that the 72-kDa species is greatly enriched on the cell surface. To demonstrate that a protein kinase activity associates with cell surface EPOR, cells were incubated with biotinylated EPO and then cross-linked with a thiol-cleavable chemical cross-linker. The avidin-agarose-selected complexes were incubated with [gamma-32P]ATP. After in vitro phosphorylation and denaturation without reducing agent, both antiphosphotyrosine and anti-EPOR antibodies immunoprecipitated labeled 72-kDa EPOR and an unidentified 130-kDa phosphoprotein (pp130), indicating that a protein kinase is associated with cell surface EPOR and that a fraction of the EPOR was phosphorylated on tyrosine residues either in the cells or during the cell-free phosphorylation reaction. Under reducing conditions, the 72-kDa phosphorylated EPOR but not pp130 was immunoprecipitated with an anti-EPOR antibody, suggesting that the pp130 is bound to the EPOR by the thiol-cleavable chemical cross-linker. Previously, we showed that deletion of the 42 carboxy-terminal amino acids of the EPOR allows cells to grow in 1/10 the normal EPO concentration, without affecting receptor number or affinity. Two carboxy-terminal truncated EPO receptors that are hyperresponsive to EPO were poorly phosphorylated during the in vitro reaction, suggesting that the carboxy-terminal region of the EPOR contains a site for phosphorylation or a site for interaction with a protein kinase. Our data suggests that phosphorylation or interaction with a protein kinase in the carboxy-terminal region may down-modulate the proliferative action of the EPOR.

Chronic myeloproliferative disease induced by site-specific integration of Abelson murine leukemia virus-infected hemopoietic stem cells

We recently showed that hemopoietic stem cells expressing the v-abl oncogene can cause leukemia when injected into lethally irradiated recipient mice. Progenitor cells expressing v-abl did not significantly contribute to disease development, and the leukemia was monoclonal in origin. By serially transplanting v-abl-transduced hemopoietic stem cells into normal, nonirradiated syngeneic recipients, we showed that multiple stem-cell clones do exist in some recipients. These cells fluctuated as normal stem cells do and could home to normal bone marrow. Based on the time course of disease, the recipients developed either an acute or a chronic phase of disorder. All recipients with the acute disease had stem-cell clones with random Abelson murine leukemia virus integration sites. All recipients with the chronic disorder had a specific Abelson murine leukemia virus integration site. We believe this abl-specific integration site, termed ASI, is important in abl-mediated stem-cell leukemogenesis.

Leukemia initiated by hemopoietic stem cells expressing the v-abl oncogene

We report a mouse model with which to study leukemogenesis initiated by a specific genetic change introduced into a primary lymphoid-myeloid pluripotent stem cell. Fetal liver hemopoietic cells were infected with a high titer of helper-free Abelson murine leukemia virus (A-MuLV) and were used to reconstitute lethally irradiated mice. Two weeks later, progenies of a single primitive hemopoietic stem cell carrying a specifically integrated A-MuLV proviral DNA could be detected in both colony-forming units in spleen and myeloid colony-forming cells in the bone marrow. Beginning at 3 weeks after transplantation, the recipients developed elevated leukocyte counts, splenomegaly, and increase of blast cells in the peripheral blood. Multiple clones of A-MuLV-infected cells were infused into each recipient. However, in the same animal, DNA extracted from various affected organs and from factor-independent lymphoid and myeloid immortalized cells all contained an identical, specifically integrated proviral genome. The A-MuLV-infected stem cells differentiated into various lineages of hemopoietic cells. Our data show that the expression of the v-abl oncogene in a primary lymphoid-myeloid hemopoietic stem cell directly initiates leukemogenesis by stimulating factor-independent growth. The monoclonal-type disease development seen in these animals may require the occurrence of an additional genetic event.

Multiple steps in the immortalization of cells of the B lineage

The ability of A-MuLV to transform bone marrow cells on in vitro culture in agarose is enhanced by inclusion of conditioned media during infection and culture. The conditioned medium of a non-virus producing A-MuLV transformed fibroblast cell line was synergistic with medium from Whitlock-Witte long-term bone marrow cultures, while conditioned medium from modified Dexter-type cultures was not active. These media all contained growth promoting activity for bone marrow cells. There are two types of transformed colonies produced, and transformation of only one type was enhanced by inclusion of conditioned media. Analysis of this type of transformed cell showed them to be pre-B cells. Limiting dilution analysis suggests the transformation process to be dependent on two types of cells, one presumably the target and the second an "accessory cell". Models are presented to account for the factor-dependent in vitro transformation of pre-B cells.

Conservation of function of Drosophila melanogaster abl and murine v-abl proteins in transformation of mammalian cells

The Drosophila melanogaster abl and the murine v-abl genes encode tyrosine protein kinases (TPKs) whose amino acid sequences are highly conserved. To assess functional conservation between the two gene products, we constructed Drosophila abl/v-abl-chimeric Abelson murine leukemia viruses. In these chimeric Abelson murine leukemia viruses, the TPK and carboxy-terminal regions of v-abl were replaced with the corresponding regions of D. melanogaster abl. The chimeric Abelson murine leukemia viruses were able to mediate morphological and oncogenic transformation of NIH 3T3 cells and were able to abrogate the interleukin-3 dependence of a lymphoid cell line. We also found that a virus that contained both TPK and carboxy-terminal Drosophila abl regions had no in vitro transforming activity for primary bone marrow cells and lacked the ability to induce tumors in susceptible mice. A virus that replaced only a portion of the v-abl TPK region with that of Drosophila abl had low activity in in vitro bone marrow transformation and tumorigenesis assays. These results indicate that the transforming functions of abl TPKs are only partially conserved through evolution. These results also imply that the TPK region of v-abl is a major determinant of its efficient lymphoid cell-transforming activity.

Identification of a novel bone marrow-derived B-cell progenitor population that coexpresses B220 and Thy-1 and is highly enriched for Abelson leukemia virus targets

A novel stage in early B-lymphocyte differentiation has been identified in normal mouse bone marrow cells. Earlier work had demonstrated that bone marrow cells characterized by low levels of Thy-1 and lack of a panel of lineage markers (Thy-1lo Lin- cells) were highly enriched for pluripotent hematopoietic stem cells. In this paper, we present evidence that another bone marrow population, which expressed low levels of Thy-1 and coexpressed B220, a B-lineage-specific form of the leukocyte common antigen, contained early and potent precursors for B lymphocytes upon in vivo transfer to irradiated hosts. These Thy-1lo B220+ cells, comprising 1 to 2% of bone marrow cells, were enriched for large cells in the mitotic cycle; the population lacked significant pluripotent hematopoietic stem cell activity and myeloid-erythroid progenitors. Most strikingly, Thy-1lo B220+ cells represented a highly enriched population of bone marrow cells that could be targets of Abelson murine leukemia virus transformation. We propose that Thy-1lo B220+ bone marrow cells represent the earliest stage of committed lymphocyte progenitors, intermediate in differentiation between Thy-1lo Lin- pluripotent stem cells and, in the B lineage, Thy-1- B220+ pre-B cells.

Expression of c-kit gene products in known cellular targets of W mutations in normal and W mutant mice--evidence for an impaired c-kit kinase in mutant mice

The proto-oncogene c-kit, a transmembrane tyrosine protein kinase receptor for an unknown ligand, was shown recently to map to the dominant white spotting locus (W) of the mouse. Mutations at the W locus affect various aspects of hematopoiesis, as well as the proliferation and/or migration of primordial germ cells and melanoblasts during development. Here, we show that c-kit is expressed in tissues known to be affected by W mutations in fetal and adult erythropoietic tissues, mast cells, and neural-crest-derived melanocytes. We demonstrate that the c-kit associated tyrosine-specific protein kinase is functionally impaired in W/WV mast cells, thus providing a molecular basis for understanding the developmental defects that result from these mutations.

Identification of a novel bone marrow-derived B-cell progenitor population that coexpresses B220 and Thy-1 and is highly enriched for Abelson leukemia virus targets

A novel stage in early B-lymphocyte differentiation has been identified in normal mouse bone marrow cells. Earlier work had demonstrated that bone marrow cells characterized by low levels of Thy-1 and lack of a panel of lineage markers (Thy-1lo Lin- cells) were highly enriched for pluripotent hematopoietic stem cells. In this paper, we present evidence that another bone marrow population, which expressed low levels of Thy-1 and coexpressed B220, a B-lineage-specific form of the leukocyte common antigen, contained early and potent precursors for B lymphocytes upon in vivo transfer to irradiated hosts. These Thy-1lo B220+ cells, comprising 1 to 2% of bone marrow cells, were enriched for large cells in the mitotic cycle; the population lacked significant pluripotent hematopoietic stem cell activity and myeloid-erythroid progenitors. Most strikingly, Thy-1lo B220+ cells represented a highly enriched population of bone marrow cells that could be targets of Abelson murine leukemia virus transformation. We propose that Thy-1lo B220+ bone marrow cells represent the earliest stage of committed lymphocyte progenitors, intermediate in differentiation between Thy-1lo Lin- pluripotent stem cells and, in the B lineage, Thy-1- B220+ pre-B cells.

Novel B-cell precursors blocked at the stage of DJH recombination

Abelson murine leukemia virus-transformed cells have provided the principal model for study of the early events in immunoglobulin gene rearrangements. In this communication, we describe a new type of Abelson virus-transformed pre-B-cell line that is arrested at the DJH stage of the recombination process. These cells differ from other pre-B transformants with respect to two properties associated with the immunoglobulin rearrangement process. First, in contrast to cell lines undergoing VH-to-DJH joining in vitro, none of these cell lines contained detectable levels of RNAs transcribed from their unrearranged VH genes. Second, only some of the cell lines recombined exogenous heptamer-nonamer sequences, indicating that many of them have lost at least a portion of the enzymatic machinery that mediates recombination. The correlation between the absence of unrearranged VH RNAs and the inability to rearrange endogenous immunoglobulin gene segments suggests that VH gene transcription is required both to maintain an active recombination system and for the final step in variable-region formation.

Multiple steps are required for the induction of tumors by Abelson murine leukemia virus

Helper virus-free Abelson murine leukemia virus (A-MuLV) was used to induce monoclonal pre-B-cell tumors in mice. The clonality, patterns of immunoglobulin heavy-chain gene rearrangement, tumorigenicity, and v-abl oncogene expression in individual preleukemic and leukemic colonies were compared. Our results indicate that A-MuLV preleukemic cells with low or undetectable tumorigenic potential give rise to leukemic cells with high tumorigenic potential by a process of subclone selection. The levels of v-abl oncogene product in preleukemic and leukemic cell populations were not significantly different. These results suggest that an additional event(s) unrelated to the level of the v-abl protein product is required for A-MuLV-transformed cells to become fully malignant.

Avian erythroblastosis virus transforms a novel mast cell-basophil precursor target in the Japanese quail

Hematopoietic cells of the Japanese quail were transformed by avian erythroblastosis virus in vivo and in vitro. In both circumstances, the infected hematopoietic tissues exhibited a dual oncogenic response of erythroid and mast cell-basophil elements. The erythroid transformants escaped the avian erythroblastosis virus block in differentiation and progressed to hemoglobinization. Resulting basophilic cells were morphologically, biochemically, and ultrastructurally identical to mast cell-basophils observed in other species. None of the virally transformed cells actively produced reverse transcriptase activity. Nonproducer cell lines synthesized viral RNA and both v-erbA and v-erbB proteins. These results indicate that the Japanese quail has a viral target cell different from that of the chicken. The implications of a single bipotential transformation target yielding both erythroid and mast cell-basophil colonies is discussed.

Novel B-cell precursors blocked at the stage of DJH recombination

Abelson murine leukemia virus-transformed cells have provided the principal model for study of the early events in immunoglobulin gene rearrangements. In this communication, we describe a new type of Abelson virus-transformed pre-B-cell line that is arrested at the DJH stage of the recombination process. These cells differ from other pre-B transformants with respect to two properties associated with the immunoglobulin rearrangement process. First, in contrast to cell lines undergoing VH-to-DJH joining in vitro, none of these cell lines contained detectable levels of RNAs transcribed from their unrearranged VH genes. Second, only some of the cell lines recombined exogenous heptamer-nonamer sequences, indicating that many of them have lost at least a portion of the enzymatic machinery that mediates recombination. The correlation between the absence of unrearranged VH RNAs and the inability to rearrange endogenous immunoglobulin gene segments suggests that VH gene transcription is required both to maintain an active recombination system and for the final step in variable-region formation.

Proliferation and differentiation of Abelson virus-infected murine myeloid leukemia cell lines does not involve an autocrine mechanism

Culture in agar of cloned promonocytic leukemia cell lines derived from Abelson virus-infected mice produced colonies of both a compact and diffuse morphology. Diffuse colonies contained fewer cells capable of forming colonies when recultured in agar than did compact colonies. Serial subcloning of cells from diffuse, but not compact, colonies ultimately led to the complete loss of colony-forming cells, i.e., to clonal extinction. The production of both compact and diffuse agar colonies was independent of the cell density of either the static liquid culture from which cells were taken for culture in agar, or the number of cells per agar culture. Furthermore, bioassays of culture supernatants indicated the leukemia cells did not secrete hemopoietic growth factors active on normal hemopoietic cells, transforming growth factors active on adherent cell lines, or factors that influenced the growth of the leukemic cells themselves. Collectively, these data suggest neither growth-factor independent replication nor the spontaneous differentiation of Abelson virus-infected myeloid cells involves autocrine secretion of growth regulators.

A simple method of examining retroviral species produced by infected cells

A simple and rapid method to examine the retroviral species present in the culture supernatant of productively infected cells is described. The method involves purification of viral genomic RNA directly from the culture supernatant and examination of that RNA by Northern or dot blot analysis. The method provides qualitative and quantitative information about the RNA species present and is particularly valuable for the detection of genetic variants in the population.

A nonimmunosuppressive helper virus allows high efficiency induction of B cell lymphomas by reticuloendotheliosis virus strain T

We have documented the effect of two nondefective helper viruses, reticuloendotheliosis virus A (REV-A) and chick syncytial virus (CSV) infection on bursal tissue. REV-A infection results in bursal atrophy, destroying both its structural and functional integrity. In contrast, the bursae in CSV-infected chicks, while reduced slightly in size, appear both structurally and functionally normal. REV-A-induced bursal atrophy is not a result of viral replication in the B-lymphocyte as (a) both viruses are capable of inducing, with equal efficiency, the formation of preneoplastic lesions containing proliferating B lymphocytes and (b) it appears that equivalent amounts of viral antigen are expressed in the bursae of chicks infected with either virus. We have examined the phenotype of tumors induced by the replication-defective virus REV-T when replicated by the two different helper viruses, REV-A and CSV. In REV-T(REV-A)-infected chicks, the majority of tumors that develop are negative for IgM expression. In contrast, the majority of tumors induced by REV-T(CSV) infection are IgM+. This finding is confirmed by recovery of IgM- cell lines from REV-T(REV-A)-infected chicks and IgM+ cell lines from REV-T(CSV)-infected chicks. In addition, repopulation studies show that bursal-derived cells that are IgM+ serve as target cells for REV-T(CSV)-induced lymphomas. This study demonstrates, therefore, that REV-T can induce IgM+, B cell lymphomas with high efficiency. We conclude that infections by the helper viruses, REV-A and CSV, differ dramatically in their effects on the composition of the population of cells that serve as targets for REV-T-induced neoplasia.

The viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.

Important role of the long terminal repeat of the helper Moloney murine leukemia virus in Abelson virus-induced lymphoma

The helper virus has been shown to play a critical role in the development of lymphoma induced by the defective Abelson murine leukemia virus (A-MuLV). Indeed, A-MuLV pseudotyped with some viruses, such as the Moloney MuLV, has been shown to be highly lymphogenic, whereas A-MuLV pseudotyped with other viruses, such as the BALB/c endogenous N-tropic MuLV, has been shown to be devoid of lymphogenic potential (N. Rosenberg and D. Baltimore, J. Exp. Med. 147:1126-1141, 1978; C. D. Scher, J. Exp. Med. 147: 1044-1053, 1978). To map the viral DNA sequences encoding the determinant of the lymphogenic potential of Moloney MuLV when complexed with A-MuLV, we constructed chimeric helper viral DNA genomes in vitro between parental cloned infectious viral DNA genomes from Moloney MuLV and from BALB/c endogenous N-tropic MuLV. Chimeric helper MuLVs, recovered after transfection of NIH 3T3 cells were used to rescue A-MuLV, and the pseudotypes were inoculated into newborn NIH Swiss, CD-1, and SWR/J mice to test their lymphogenic potential. We found that a 0.44-kilobase-pair PstI-KpnI long terminal repeat-containing fragment from the Moloney MuLV was sufficient to confer some, but not complete, lymphogenic potential to a chimeric virus (p7M2) in NIH Swiss and SWR/J mice, but not in CD-1 mice. The addition of the 3'-end env sequences (comprising the carboxy terminus of gp70 and all p15E) to the U3 long terminal repeat sequences restored the full lymphogenic potential of the Moloney MuLV. Our data indicate that the 3'-end sequences of the helper Moloney MuLV are somehow involved in the development of lymphoma induced by A-MuLV. The same sequences have previously been found to harbor the determinant of leukemogenicity and of disease specificity of Moloney MuLV when inoculated alone.

Oncogene activation by chromosomal rearrangement in chronic myelocytic leukemia

A considerable number of human tumors, especially leukemias and lymphomas is associated with a consistent specific chromosome translocation. At a number of the breakpoint regions of these specific aberrations are c-oncogenes (c-onc) located. The structure and/or expression of some of these c-onc is altered as a result of the specific translocation. In the CML-specific (9;22) translocation the transposition of the c-abl oncogene to the chromosome 22 bcr sequences results in the production of a chimeric bcr/c-abl fusion protein. This result strongly suggests that tumor-specific chromosomal aberrations can lead to the activation of cellular oncogenes.

Structure and origins of the HZ2-feline sarcoma virus

The HZ2-feline sarcoma virus (HZ2-FeSV) is a replication-defective acute transforming feline retrovirus with oncogene homology to Abelson murine leukemia virus (A-MuLV) (P. Besmer, W.D. Hardy,Jr., E. E. Zuckerman, P. J. Bergold, L. Lederman, and H. W. Snyder, Jr. (1983) Nature (London) 303, 825-828). In contrast to A-MuLV which was isolated from a hematopoietic tumor, the HZ2-FeSV derives from a multicentric fibrosarcoma. We have molecularly cloned the HZ2-FeSV provirus from mink HZ2-FeSV nonproducer cells. The molecularly cloned HZ2-FeSV provirus is biologically active upon transfection of NIH 3T3 indicator cells. The genetic structure of the HZ2-FeSV provirus was determined by EM heteroduplex and Southern blot analysis. The HZ2-FeSV has a 6.8 kb-viral genome with the structure: 5' delta gag-abl-delta pol-delta env 3'. The abl insert, which is 1.4 kb, is located 1.9 kb from the 5' end and 3.5 kb from the 3' end of the viral genome. The 5' 1.9 kb in the HZ2-FeSV are colinear with 5' FeLV sequences, and the 3' 3.5 kb are colinear with 3' FeLV sequences, with the exception of a 0.85-kb deletion in the env gene. HZ2-FeSV v-abl and A-MuLV v-abl share 1.2 kb of abl sequences which are known to specify the protein kinase domain of the abl gene product and are necessary for fibroblast transformation in vitro. The DNA from several tumor tissues of cat 3590 from which the HZ2-FeSV was obtained was found to contain several HZ2-FeSV-related proviruses including the HZ2-FeSV. The variant HZ2-FeSVs have indistinguishable 5' gag-abl sequences; however, they differ in 3' sequences which likely do not include any abl sequences. The DNAs from fibrosarcomas obtained by inoculation of kittens with tumor extract were found to contain variant HZ2-FeSV proviruses as well. Taken together these results indicate a role for the HZ2-FeSVs in sarcomagenesis.

Abelson virus potentiates long-term growth of mature B lymphocytes

Abelson murine leukemia virus (A-MuLV) infection of mouse bone marrow cells usually leads to transformation of pre-B cells. However, when the environment is modified by the continuous presence of lipopolysaccharide (LPS), two novel types of membrane immunoglobulin (mIg)-positive B cell lines are generated. Because the cells which give rise to these cell lines copurify with mIg-positive bone marrow cells, the cell lines arise as a result of A-MuLV interaction with a new type of in vitro target cell. The cell lines generated fall into two groups which differ in several phenotypic characteristics. Group 1 cells are more differentiated than the typical pre-B cell transformant in that they synthesize mIgM and appear to resemble virgin B cells. The group 1 cells do not secrete immunoglobulin and are independent of LPS for growth. In addition, these cell lines synthesize the Abelson P160 protein, contain integrated abl proviral DNA, and are highly tumorigenic in syngeneic animals. The group 2 cell lines differ markedly from both the group 1 cells and from typical, pre-B cell A-MuLV transformants. These cells are mIgG positive and secrete large amounts of immunoglobulin into the culture medium. The cell lines are comprised of both adherent and nonadherent cells and do not synthesize P160 or contain integrated v-abl sequences. The group 2 cells are nontumorigenic in syngeneic animals and require LPS for growth and viability. Both types of cells have remained in culture for over 2 years with no changes in their phenotypic characteristics. This A-MuLV infection system and the novel mIg-positive cell lines may serve as useful models for studying biochemical and molecular properties of mature B cells.

Gag-derived but not abl-derived determinants are exposed on the surface of Abelson virus-transformed cells

The organization of the transforming protein encoded by Abelson murine leukemia virus (A-MuLV) in transformed lymphoid and fibroblast cells was examined using immunofluorescent analysis. Antibodies specific for v-abl were capable of detecting cytoplasmic Abelson protein molecules in fixed cells, but none were able to stain the surface of live A-MuLV transformed cells. However, a series of monoclonal antibodies selected for the ability to bind to the surface of A-MuLV-transformed cells did stain live cells. These antibodies were shown to react with a determinant within the helper virus-derived p15 sequences that are present at the amino terminus of the Abelson protein, indicating that gag-derived determinants are exposed on the surface of transformed cells. The inability of a p12-specific monoclonal antibody to stain live cells indicates that only a small portion of the amino terminal sequences are exposed. Examination of the ability of these antibodies to react with Abelson protein encoded by a series of gag deletion mutants suggests that the determinant recognized by these antibodies lies between amino acids 38 and 114 of p15.

Expression of the mammalian c-fes protein in hematopoietic cells and identification of a distinct fes-related protein

The avian c-fps and mammalian c-fes proto-oncogenes are cognate cellular sequences. Antiserum raised against the P140gag-fps transforming protein of Fujinami avian sarcoma virus specifically recognized a 92,000-Mr protein in human and mouse hematopoietic cells which was closely related in structure to Snyder-Theilen feline sarcoma virus P87gag-fes. This polypeptide was apparently the product of the human c-fes gene and was therefore designated p92c-fes. Human p92c-fes was associated with a tyrosine-specific protein kinase activity in vitro and was capable of both autophosphorylation and phosphorylation of enolase as an exogenous protein substrate. The synthesis of human and mouse p92c-fes was largely, though not entirely, confined to myeloid cells. p92c-fes was expressed to relatively high levels in a multipotential murine myeloid cell line, in more mature human and mouse granulocyte-macrophage progenitors, and in differentiated macrophage like cells as well as in the mononuclear fraction of normal and leukemic human peripheral blood. p92c-fes was not found in erythroid cells, with the exception of a human erythroleukemia line which retains the capacity to differentiate into macrophage like cells. These results suggest a normal role for the p92c-fes tyrosine kinase in hematopoiesis, particularly in granulocyte-macrophage differentiation. In addition, a distinct 94,000-Mr polypeptide, antigenically related to p92c-fes, was identified in a number of hematopoietic and nonhematopoietic human and mouse cells and was also found to be associated with a tyrosine-specific protein kinase activity.

Transformation of T-lymphoid cells by Abelson murine leukemia virus

Abelson leukemia virus (A-MuLV) is an oncogenic murine retrovirus whose genome contains sequences homologous to those of a normal cellular gene, c-abl. It has been demonstrated to cause rapid transformation of several cell types, including pre-B lymphocytes, macrophages, and fibroblasts. More recently, A-MuLV has been reported to induce thymic tumors in a mouse strain (C57BL/Ka) previously thought to be resistant to disease induction. We showed that the masses occurring after intrathymic injection of the virus were composed of lymphocytes of a previously described immature T-cell phenotype. This phenotype has been defined here by flow cytometry of 10 primary tumor samples stained with antibodies to several thymocyte differentiation antigens. Hybridization of DNAs from these tumors with v-abl, immunoglobulin mu, and T-cell antigen receptor beta-chain probes confirmed the T-lymphoid, polyclonal nature of the primary tumor cells. The primary tumors were malignant, as clearly shown by reinjection into Thy-congenic host animals. Further, four Thy- in vitro cell lines derived from three tumors differed from the majority of primary tumor cells and were similar to previously described A-MuLV-transformed pre-B cells. The consistent T-lymphoid phenotype exhibited by primary A-MuLV thymomas may represent one stage of normal thymocyte differentiation.

Point mutations in the U3 region of the long terminal repeat of Moloney murine leukemia virus determine disease specificity of the myeloproliferative sarcoma virus

The myeloproliferative sarcoma virus (MPSV) is made up entirely of sequences derived from the Moloney murine leukemia virus (Mo-MuLV) and the cellular mos oncogene. As other members of the Moloney murine sarcoma virus (Mo-MuSV) family, MPSV transforms fibroblasts in vitro and causes sarcomas in vivo. In addition, however, MPSV also causes an acute myeloproliferative disease in adult mice. The mos oncogene is essential for its transforming capacity, but sequences specific to the long terminal repeat (LTR) U3 region of MPSV account for its expanded target specificity as compared to Mo-MuSV (C. Stocking, R. Kollek, U. Bergholz, and W. Ostertag, Proc. Natl. Acad. Sci. USA 82, 5746-5750 (1985)). The U3 region of the LTR of MPSV is, however, closely related to that of the Mo-MuLV, and it appeared likely that the difference between MPSV and Mo-MuSV was caused by a divergent evolution of Mo-MuSV LTRs. In this paper, we show that this is not the case. The few nucleotide differences in the LTR between Mo-MuLV and MPSV are crucial for the expanded host range of MPSV. Moreover, Mo-MuLV-related gag sequences retained in MPSV are not essential for the distinctive biological properties of MPSV.

Abelson virus drives the differentiation of Harvey virus-infected erythroid cells

Abelson murine leukemia virus (A-MuLV) and Harvey murine sarcoma virus (Ha-MSV) are retroviruses carrying unrelated onc genes. However, both of these viruses are capable of stimulating the growth and differentiation of erythroid precursor cells; the target cells for both appear at the same time during fetal development and follow a similar pattern throughout ontogeny. In addition, the colonies induced by each virus are morphologically similar and synthesize the adult form of hemoglobin. However, A-MuLV-infected cells are Epo-independent, whereas Ha-MSV-infected cells are Epo-dependent. Superinfection of Ha-MSV-infected cells with A-MuLV overrides their Epo-dependency. Thus, the consequences of the infection are determined by the interaction of the different onc gene products with identical or similar erythroid cells.

Abelson virus potentiates long-term growth of mature B lymphocytes

Abelson murine leukemia virus (A-MuLV) infection of mouse bone marrow cells usually leads to transformation of pre-B cells. However, when the environment is modified by the continuous presence of lipopolysaccharide (LPS), two novel types of membrane immunoglobulin (mIg)-positive B cell lines are generated. Because the cells which give rise to these cell lines copurify with mIg-positive bone marrow cells, the cell lines arise as a result of A-MuLV interaction with a new type of in vitro target cell. The cell lines generated fall into two groups which differ in several phenotypic characteristics. Group 1 cells are more differentiated than the typical pre-B cell transformant in that they synthesize mIgM and appear to resemble virgin B cells. The group 1 cells do not secrete immunoglobulin and are independent of LPS for growth. In addition, these cell lines synthesize the Abelson P160 protein, contain integrated abl proviral DNA, and are highly tumorigenic in syngeneic animals. The group 2 cell lines differ markedly from both the group 1 cells and from typical, pre-B cell A-MuLV transformants. These cells are mIgG positive and secrete large amounts of immunoglobulin into the culture medium. The cell lines are comprised of both adherent and nonadherent cells and do not synthesize P160 or contain integrated v-abl sequences. The group 2 cells are nontumorigenic in syngeneic animals and require LPS for growth and viability. Both types of cells have remained in culture for over 2 years with no changes in their phenotypic characteristics. This A-MuLV infection system and the novel mIg-positive cell lines may serve as useful models for studying biochemical and molecular properties of mature B cells.

Isolation and characterization of Abelson murine leukemia virus-transformed mast cell lines from midgestation embryonic placenta

Abelson murine leukemia virus was used to transform cells of the midgestation embryonic placenta. The frequency of transformed foci in semisolid agarose was highest when cells were isolated at 10 days of gestation and cell lines could be established in liquid culture. The continuous cell lines express characteristics of cultured mast cells, including surface antigens which are shared with lymphocytes and mononuclear phagocytes. These results imply a relationship between the transforming gene product and the mast cell growth factor interleukin 3.

Expression of the mammalian c-fes protein in hematopoietic cells and identification of a distinct fes-related protein

The avian c-fps and mammalian c-fes proto-oncogenes are cognate cellular sequences. Antiserum raised against the P140gag-fps transforming protein of Fujinami avian sarcoma virus specifically recognized a 92,000-Mr protein in human and mouse hematopoietic cells which was closely related in structure to Snyder-Theilen feline sarcoma virus P87gag-fes. This polypeptide was apparently the product of the human c-fes gene and was therefore designated p92c-fes. Human p92c-fes was associated with a tyrosine-specific protein kinase activity in vitro and was capable of both autophosphorylation and phosphorylation of enolase as an exogenous protein substrate. The synthesis of human and mouse p92c-fes was largely, though not entirely, confined to myeloid cells. p92c-fes was expressed to relatively high levels in a multipotential murine myeloid cell line, in more mature human and mouse granulocyte-macrophage progenitors, and in differentiated macrophage like cells as well as in the mononuclear fraction of normal and leukemic human peripheral blood. p92c-fes was not found in erythroid cells, with the exception of a human erythroleukemia line which retains the capacity to differentiate into macrophage like cells. These results suggest a normal role for the p92c-fes tyrosine kinase in hematopoiesis, particularly in granulocyte-macrophage differentiation. In addition, a distinct 94,000-Mr polypeptide, antigenically related to p92c-fes, was identified in a number of hematopoietic and nonhematopoietic human and mouse cells and was also found to be associated with a tyrosine-specific protein kinase activity.

Different genes control the susceptibility of mice to Moloney or Abelson murine leukemia viruses

The susceptibility of mice to lymphoma induction by Moloney or Abelson murine leukemia virus has been compared in BALB/c, C57BL/6, and BALB/cXC57BL/6 recombinant inbred strains. BALB/c mice were found to be susceptible to lymphoma induction by either virus, and C57BL/6 mice were found to be relatively resistant to lymphoma induction by either virus. The genes that control these patterns of susceptibility to each virus are not the same because susceptibility to each virus segregated independently in CXB recombinant inbred strains. We also found, as reported by Cook (W. Cook, Proc. Natl. Acad. Sci. U.S.A. 79:2917-2921, 1982), when injected intrathymically that Abelson murine leukemia virus rapidly induced thymomas in weanling B6 mice. Examination of the cellular phenotypes of the tumors induced by Abelson murine leukemia virus or by Moloney murine leukemia virus indicated that different lymphocyte subpopulations were the targets for tumor induction by each virus.

Differentiation of Abelson murine leukemia virus-infected promonocytic leukemia cells

Virus-producing, tumorigenic, promonocytic leukemia cell lines were derived from Abelson murine leukemia virus-infected mice. This study shows that, of these 25 cloned lines, 22 were capable of extensive differentiation. It also shows that granulocyte-macrophage colony-stimulating activity increased the proportion of differentiating cells in 17/22 lines. Cells from agar colonies with a diffuse colony morphology had an increased expression of mature macrophage phenotypic characteristics, and a reduced proliferative capacity in vitro, compared to cells from agar colonies with a compact colony morphology. Cells from diffuse colonies also produced less Abelson virus, and were less tumorigenic in vivo than cells from compact colonies. Together, these results suggest some Abelson virus-producing leukemic cells are not blocked in their capacity to differentiate and are capable of reversing the transformed phenotype.

Embryonic erythroid cells transformed by avian erythroblastosis virus may proliferate and differentiate

Embryonic chick cells from the primitive streak stage to later stages of the developing embryo were infected with avian erythroblastosis virus (AEV). The data indicate that the greatest number of target cells for AEV was observed in the 12-somite blastoderm and gradually decreased in hemopoietic tissues with the development of the embryo. The target cell for AEV is not in the BFU-E compartment, as it is in the adult bone marrow, but is probably recruited within the CFU-M compartment which precedes the BFU-E compartment. Our studies also show that a significant number of transformed colonies derived from embryonic hemopoietic tissues undergo hemoglobinization in contrast with what is observed in transformed colonies of bone marrow. A complete characterization of the embryonic and adult hemoglobin is at present under study.