Selective expression of one c-myc allele in two human myeloma cell lines

Chromosomal translocations or DNA rearrangements affecting c-myc occur in almost all murine plasmacytoma and human Burkitt's lymphoma tumors and are associated with a high incidence of exon 2 missense mutations and selective expression of the affected allele. Screening nine multiple myeloma cell lines, we identified no exon 2 missense mutations but did identify two lines with single, silent mutations in exon 1 and exon 2, respectively. Each of these informative multiple myeloma cell lines selectively expresses only one c-myc allele despite the apparent absence of chromosomal translocations or DNA rearrangements affecting c-myc.

Dysregulation of cyclin D1 by translocation into an IgH gamma switch region in two multiple myeloma cell lines

Translocations involving the IgH locus at chromosomal locus 14q32.3 are a common event in many B-cell malignancies. The translocations, which generally occur into JH and switch regions, are mediated by errors in the two developmentally regulated, lymphocyte-specific pathways: VDJ-and switch-mediated recombination. Dysregulation of cyclin D1 by a t(11;14)(q13;q32) translocation occurs in most cases of mantle-cell lymphoma and in approximately 30% of multiple myeloma (MM) tumors in which a 14q32 translocation can be detected. We show here that in two of three myeloma lines that overexpress cyclin D1, there is an 11;14 translocation into a gamma switch region, suggesting an error in switch recombination. By contrast, 11;14 translocations in mantlecell lymphoma are invariably into or near a JH segment, suggesting an error in VDJ recombination. This is consistent with the fact that myeloma cells have undergone lgH switch recombination, whereas mantle-cell lymphoma cells generally have not.

Sequence and expression of murine cDNAs encoding Xlr3a and Xlr3b, defining a new X-linked lymphocyte-regulated Xlr gene subfamily

Using a subtractive cDNA approach we have identified two nearly identical genes, Xlr3a and Xlr3b (X-linked lymphocyte regulated), expressed at a consistently high level in 14 out of 14 murine plasmacytoma cell lines, at a high level in 1 out of 8 B-lymphoma cell lines, and at a very low level in 2 out of the 8 B-lymphoma cell lines. The messages are not detected in 10 pre-B-lymphoma cell lines. These genes express 2.0-kb mRNAs that encode 226-amino-acid proteins that are extremely basic, with an estimated pI of 8.1 and 9.0, respectively. By sequence comparison they are homologous to Xlr1, an acidic nuclear protein that is produced in lymphoid cell lines corresponding to the late stages of lymphocyte differentiation. Xlr2 is a highly homologous gene that is expressed in differentiating male germ cells. Xlr3a and Xlr3b are members of a new subfamily in the Xlr multigene family. Like Xlr1, they are up-regulated during B-cell terminal differentiation in normal and neoplastic B-cells, and cross-hybridize with a message in testis RNA. Also, like Xlr1, they do not cross-hybridize with human genomic DNA.

The G protein-coupled receptor BLR1 is involved in murine B cell differentiation and is also expressed in neuronal tissues

The BLR1 gene, isolated initially from Burkitt's lymphoma cells (Eur. J. Immunol. 1992. 22: 2795), encodes a G protein-coupled receptor with significant relationship to receptors for chemokines (IL-8, MIP-1 alpha) and neuropeptides. The murine homologue of human BLR1 was cloned and used to investigate its expression in vivo. blr1-specific transcripts are observed in secondary lymphatic organs and to a lesser extent in brain of adult mice but not in other tissues. RNA in situ hybridization localizes blr1 transcription to primary follicles and to the mantle zone of secondary follicles. SCID mice in which mature B cell development is severely impaired exhibit a strongly reduced level of blr1-specific RNA in the spleen. The analysis of murine lymphoid tumor cell lines representing distinct stages of the B cell lineage reveals elevated expression of blr1 in B cell lymphomas but not in pre-B lymphomas or plasmacytomas. Induction of differentiation of resting B cells by cytokines or mitogens down-regulates expression of blr1. RNA in situ hybridization using brain sections of adult mice detects blr1 transcription in the granule and Purkinje cell layer of the cerebellum. Interestingly, the blr1 gene is also expressed during late embryogenesis in fetal liver and brain. In view of the remarkable expression pattern in the B cell lineage we suggest that murine BLR1 may represent a cytokine/neuropeptide receptor exerting regulatory functions on recirculating mature B lymphocytes.

Coordinate silencing of myeloma-specific genes in myeloma x T lymphoma hybrids

It has been well-established that Ig genes are transcriptionally silenced when Ig-producing myeloma lines are fused to non-B cells. In the present study, we analyzed the expression of several other myeloma-specific genes in fusions of myelomas with the T lymphoma, BW5147. Seven of the eight genes analyzed behaved coordinately with the Ig loci; they were silent in most myeloma x T hybrids but active in the rare hybrid that retained Ig gene expression. Cloned IgH genes introduced into the two types of hybrids behaved as their endogenous counterparts. The coordinate behavior of these several genes in the panel of "exceptional" and "extinguished" hybrids suggests a central and bimodal switch for alternately activating and de-activating the genetic program of the Ig-secreting plasmacyte. The switch between an active and an inactive transcriptional state involves, at some level, a change in the methylation status of the IgH genes. Methylation and transcriptional activity were inversely correlated. In Ig-extinguished hybrids the myeloma-derived locus was methylated de novo, whereas in the rare Ig-expressing hybrid, the T cell-derived locus was demethylated de novo.

Coordinate silencing of myeloma-specific genes in myeloma x T lymphoma hybrids

It has been well-established that Ig genes are transcriptionally silenced when Ig-producing myeloma lines are fused to non-B cells. In the present study, we analyzed the expression of several other myeloma-specific genes in fusions of myelomas with the T lymphoma, BW5147. Seven of the eight genes analyzed behaved coordinately with the Ig loci; they were silent in most myeloma x T hybrids but active in the rare hybrid that retained Ig gene expression. Cloned IgH genes introduced into the two types of hybrids behaved as their endogenous counterparts. The coordinate behavior of these several genes in the panel of "exceptional" and "extinguished" hybrids suggests a central and bimodal switch for alternately activating and de-activating the genetic program of the Ig-secreting plasmacyte. The switch between an active and an inactive transcriptional state involves, at some level, a change in the methylation status of the IgH genes. Methylation and transcriptional activity were inversely correlated. In Ig-extinguished hybrids the myeloma-derived locus was methylated de novo, whereas in the rare Ig-expressing hybrid, the T cell-derived locus was demethylated de novo.

Only the DNA binding and transactivation domains of c-Myb are required to block terminal differentiation of murine erythroleukemia cells

The c-Myb protein is a transcription factor with an apparent but poorly defined role in hematopoietic cell growth and differentiation. The DNA binding and several transcriptional regulatory domains of the c-Myb protein have been defined by transient transfections into nonhematopoietic cell lines. Although the relationship between these domains and transformation has been studied, little is known about the function of these domains during hematopoietic maturation. Up-regulation of stably transfected c-myb in murine erythroleukemia (MEL) cells blocks terminal differentiation when MEL cells are induced to differentiate with N,N'-hexamethylene bisacetamide. To determine which functional domains of c-Myb are necessary and sufficient to block differentiation, mutated c-myb constructs under the control of a murine metallothionein promoter were transfected into C19 MEL cells, and stable clonal cell lines were established. The ability of Myb mutants to block differentiation paralleled their ability to transactivate transcription of a reporter gene containing Myb-responsive elements, by transient transfection into a lymphoid cell line. The smallest c-Myb mutant able to block differentiation consisted of the DNA binding domain juxtaposed to the transactivation domain. Therefore, the DNA binding domain and the transactivation domain are necessary and sufficient for c-Myb to block differentiation in MEL cells.

Mechanism of c-myc regulation by c-Myb in different cell lineages

Activation of the murine c-myc promoter by murine c-Myb protein was examined in several cell lines by using a transient expression system in which Myb expression vectors activate the c-myc promoter linked to a chloramphenicol acetyltransferase reporter gene or a genomic beta-globin gene. S1 nuclease protection analyses confirmed that the induction of c-myc by c-Myb was transcriptional and affected both P1 and P2 start sites in a murine T-cell line, EL4, and a myelomonocytic line, WEHI-3. Mutational analyses of the c-myc promoter revealed that two distinct regions could confer Myb responsiveness in two T-cell lines, a distal site upstream of P1 and a proximal site within the first noncoding exon. In contrast, only the proximal site was required for other cell lineages examined. Five separate Myb-binding sites were located in this proximal site and found to be important for c-Myb trans activation. DNA binding was necessary for c-myc activation, as shown by the loss of function associated with mutation of Myb's DNA-binding domain and by trans-dominant repressor activity of the DNA binding, trans-activation-defective mutant. The involvement of additional protein factors was addressed by inhibiting protein synthesis with cycloheximide in a conditional expression system in which the activity of presynthesized Myb was under the control of estrogen. These experiments indicate that de novo synthesis of additional proteins was not necessary for c-myc trans activation. Together these data reveal two cell lineage-dependent pathways by which c-Myb regulates c-myc; however, both pathways are mechanistically indistinguishable in that direct DNA binding by Myb is required for activating c-myc whereas neither de novo protein synthesis nor other labile proteins are necessary.

Sequence and expression of a murine cDNA encoding PC326, a novel gene expressed in plasmacytomas but not normal plasma cells

Using a subtractive cDNA approach we have identified a gene, PC326, expressed in 13 of 14 murine plasmacytoma cell lines, but not in any B- or pre-B-lymphoma cell lines. It expresses 4.6-kb and 5.2-kb mRNAs that encode a 747 amino acid protein containing two highly acidic domains flanking a novel, moderately acidic 20 amino acid sequence that is repeated 7.5 times. Sequence comparison identifies an additional 43 amino acid domain that is homologous to a repeated sequence found in the members of the beta-transducin gene family. The PC326 mRNA is detectable in testis but in no other murine tissues, including plasma cells induced by lipopolysaccharide stimulation of splenocytes. Somatic cell hybrids derived from plasmacytomas and fibroblast or T-cell lines have a fibroblastic or T-cell phenotype respectively. Unlike B-cell-specific genes (e.g. immunoglobulin), the expression of which is extinguished in these hybrids, PC326 mRNA appears to be irreversibly turned on in these hybrids. Since PC326 is not expressed in normal plasma cells, it appears that its expression is a cause or consequence of the tumorigenic process that generates murine plasmacytomas.

A murine cDNA encodes a pan-epithelial glycoprotein that is also expressed on plasma cells

Using a subtractive cDNA approach, we have identified a number of genes expressed in murine plasmacytomas, but not B or pre-B lymphomas. One of these genes, 289A, expresses a 1.8-kb microsomally localized mRNA that encodes a 314-amino-acid protein containing a signal sequence and a hydrophobic transmembrane domain. Sequence comparison suggests that the predicted protein is the murine homologue of a human cell surface pan-epithelial glycoprotein known variously as EGP, GA733-2, KSA, and KS1/4, recognized by mAb HEA125, GA733, KS1/4, CO17-1A, M74, and 323/A3. The 289A mRNA is highly expressed in normal murine tissues containing epithelial cells, and at a low level in plasma cells induced by LPS stimulation of spleen B lymphocytes. It is expressed in 15 of 16 plasmacytomas, but at a much lower level, if at all, in pre-B or B lymphomas. In human B cell lines, 289A detects a 1.5-kb mRNA in the myeloma cell line 8226, but not in Burkitt's lymphoma or lymphoblastoid cell lines. Subsequent FACS analysis of human cell lines with the mAb GA733 and KS1/4 demonstrated concordant expression of the mRNA and the protein. We conclude that 289A is the murine homologue of EGP, GA733-2, KSA, and KS1/4 Ag. Although its expression was previously thought to be restricted to epithelial cells, it is also expressed in plasma cells and is a B lymphocyte differentiation Ag. Because of the multiplicity of names, we propose calling the human gene hEGP314, and the murine gene mEGP314.

A murine cDNA encodes a pan-epithelial glycoprotein that is also expressed on plasma cells

Using a subtractive cDNA approach, we have identified a number of genes expressed in murine plasmacytomas, but not B or pre-B lymphomas. One of these genes, 289A, expresses a 1.8-kb microsomally localized mRNA that encodes a 314-amino-acid protein containing a signal sequence and a hydrophobic transmembrane domain. Sequence comparison suggests that the predicted protein is the murine homologue of a human cell surface pan-epithelial glycoprotein known variously as EGP, GA733-2, KSA, and KS1/4, recognized by mAb HEA125, GA733, KS1/4, CO17-1A, M74, and 323/A3. The 289A mRNA is highly expressed in normal murine tissues containing epithelial cells, and at a low level in plasma cells induced by LPS stimulation of spleen B lymphocytes. It is expressed in 15 of 16 plasmacytomas, but at a much lower level, if at all, in pre-B or B lymphomas. In human B cell lines, 289A detects a 1.5-kb mRNA in the myeloma cell line 8226, but not in Burkitt's lymphoma or lymphoblastoid cell lines. Subsequent FACS analysis of human cell lines with the mAb GA733 and KS1/4 demonstrated concordant expression of the mRNA and the protein. We conclude that 289A is the murine homologue of EGP, GA733-2, KSA, and KS1/4 Ag. Although its expression was previously thought to be restricted to epithelial cells, it is also expressed in plasma cells and is a B lymphocyte differentiation Ag. Because of the multiplicity of names, we propose calling the human gene hEGP314, and the murine gene mEGP314.

Genes expressed selectively in plasmacytomas: markers of differentiation and transformation

We have analyzed a murine plasmacytoma minus highly differentiated B lymphoma subtractive cDNA library and identified eight genes that are expressed in most plasmacytomas but at a much lower level, or not at all, in most B lymphomas. Four of the genes are markers of the terminal differentiation of B lymphocytes into plasma cells: placental alkaline phosphatase, also expressed in pre-B lymphomas xlr-3, a new X-linked member of the xlr multi-gene family EGP314, a pan-epithelial glycoprotein with sequence features of an adhesion molecule PC315, a gene that is up-regulated by IL6, but without obvious sequence homologies. Two of the genes are not clearly related to normal B cell differentiation, appearing to be associated with malignant transformation of plasma cells: PC326 is a new member of the beta-transducin mosaic protein gene family. It is an X-linked gene, expressed at a very low level in testis, but in no other normal tissue, including LPS- or IL6-induced plasma cells. It has a high level of expression (apparently dysregulated) in most (> 85%) mineral oil induced plasmacytomas. However the likelihood that PC326 is expressed decreases as the tumor latency decreases when different retroviral agents are used to accelerate mineral oil induced plasmacytomagenesis. This suggests that PC326 expression may be a late event in a multi-step process of tumorigenesis. PC251 a new member of the hematopoietic growth factor receptor family, most homologous to IL5R alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional analysis of c-Myb protein in T-lymphocytic cell lines shows that it trans-activates the c-myc promoter

The function of c-Myb protein was revealed by transfecting an expression vector containing the entire c-Myb protein-coding sequence into the murine CTLL-2 T-cell line. Expressions of high levels of c-Myb protein did not alter the expression of several T-cell markers, c-fos mRNA expression, responses to interleukin-2, and growth characteristics of these cells. Interestingly, expression of the c-myc gene was drastically increased in this clone. Further, the c-myb expression plasmid, but not a frameshift mutant of c-myb, enhanced the expression of a hybrid construct of c-myc promoter linked to a reporter gene by 8- to 14-fold. These results demonstrate a role of c-Myb protein in c-myc gene expression.

Application for PCR technology to subtractive cDNA cloning: identification of genes expressed specifically in murine plasmacytoma cells

We describe a simple method for preparing a renewable source of subtractive cDNA which can be used as a hybridization probe or as insert which can be cloned into a variety of convenient vectors. This has been done by ligating a double-stranded oligonucleotide to each end of double-stranded subtractive cDNA, and then using this oligonucleotide sequence to amplify the heterogeneous population of cDNA molecules using the polymerase chain reaction and thermostable Taq DNA polymerase. This method improves the chances for identifying cDNA clones representing low abundance mRNAs that are expressed differentially. Using this approach, we have identified cDNA clones which detect three different low abundance mRNAs that are expressed in mouse plasmacytoma cell lines but not in mouse pre-B or B lymphoma cell lines.

Identification of consensus genes expressed in plasmacytomas but not B lymphomas

We have combined subtractive cDNA and PCR technologies to construct and analyze a plasmacytoma minus a highly differentiated B lymphoma subtractive cDNA library. We detected no plasmacytoma-specific clones by hybridization with differential cDNA probes or the subtractive insert. However, random selection of 115 clones has identified 16 quantitatively subtractive and 39 qualitatively subtractive clones. From these clones we have identified 8 potentially interesting genes. One quantitatively subtractive clone (clone 315) identifies an mRNA that is expressed in most plasmacytoma cell lines, but is expressed at an approximately 10-fold lower level in B and pre-B lymphoma cell lines; preliminary evidence suggests that the expression of this gene is increased by IL-6. From the 31 unrelated qualitatively subtractive clones, we have identified two classes of genes that are expressed in one or none of 8 B lymphomas examined: 1) those expressed in most plasmacytoma and pre-B lymphoma cell lines (clone 70 and clone 260); and 2) those expressed in most plasmacytoma cell lines, but not in any of the ten pre-B lymphomas examined (clones 251, 289A, 289B, 326, 291).

Differential expression of the c-myb proto-oncogene marks the pre-B cell/B cell junction in murine B lymphoid tumors

A series of murine B lymphoid tumor cell lines which are representative of the pre-B cell, immature and mature B cell, and plasma cell stages of B cell development have been examined for expression of c-myb proto-oncogene mRNA. The pre-B cell lymphoma cell lines express equivalent high steady state levels of c-myb mRNA. In contrast, the B cell lymphoma and plasmacytoma cell lines express steady state c-myb mRNA at levels which are 0.005 to 0.1 times that of the pre-B cell lymphoma lines. These results correlate high levels of c-myb mRNA expression with the pre-B cell stage of development. Subclones of the 1881 pre-B cell lymphoma which express K light chain and are surface IgM-positive as well as two types of hybrid B lymphoid cell lines have been used to demonstrate that surface immunoglobulin expression is not sufficient to result in the down-regulation of c-myb mRNA levels or changes in the expression N-myc mRNA, lambda 5 mRNA, or the BP-1 surface antigen which are markers of the pre-B cell stage of development. Thus, changes in the expression of genes which are independent of immunoglobulin expression are associated with transition from the pre-B cell to the immature B cell stage of development.

Differential expression of the c-myb proto-oncogene marks the pre-B cell/B cell junction in murine B lymphoid tumors

A series of murine B lymphoid tumor cell lines which are representative of the pre-B cell, immature and mature B cell, and plasma cell stages of B cell development have been examined for expression of c-myb proto-oncogene mRNA. The pre-B cell lymphoma cell lines express equivalent high steady state levels of c-myb mRNA. In contrast, the B cell lymphoma and plasmacytoma cell lines express steady state c-myb mRNA at levels which are 0.005 to 0.1 times that of the pre-B cell lymphoma lines. These results correlate high levels of c-myb mRNA expression with the pre-B cell stage of development. Subclones of the 1881 pre-B cell lymphoma which express K light chain and are surface IgM-positive as well as two types of hybrid B lymphoid cell lines have been used to demonstrate that surface immunoglobulin expression is not sufficient to result in the down-regulation of c-myb mRNA levels or changes in the expression N-myc mRNA, lambda 5 mRNA, or the BP-1 surface antigen which are markers of the pre-B cell stage of development. Thus, changes in the expression of genes which are independent of immunoglobulin expression are associated with transition from the pre-B cell to the immature B cell stage of development.

Differential expression of c-myb mRNA in murine B lymphomas by a block to transcription elongation

Expression of c-myb proto-oncogene messenger RNA (mRNA) and protein has been detected principally in tumors and in normal tissue of hematopoietic origin. In each hematopoietic lineage examined, expression of the c-myb gene is markedly downregulated during hematopoietic maturation. However, the mechanism by which differential expression of the c-myb gene is regulated is not known. In murine B-lymphoid tumor cell lines, the amount of steady-state c-myb mRNA is 10 to more than 100 times greater in pre-B cell lymphomas than in B cell lymphomas and plasmacytomas. The downregulation of c-myb mRNA correlates with events at the pre-B cell-B cell junction. Differential expression of c-myb mRNA levels detected between a pre-B cell lymphoma and a mature B cell lymphoma is now shown to be mediated by a block to transcription elongation in the first intron of the c-myb locus. In addition, this developmentally regulated difference in transcriptional activity is correlated with alterations in higher order chromatin structure as reflected by changes in the patterns of hypersensitivity to deoxyribonuclease I at the 5' end of the c-myb transcription unit. Regulation of transcription elongation may provide a more sensitive mechanism for rapidly increasing and decreasing mRNA levels in response to external stimuli than regulation of the initiation of transcription.

Expression of a transfected human c-myc oncogene inhibits differentiation of a mouse erythroleukaemia cell line

The Friend-virus-derived mouse erythroleukaemia (MEL) cell lines represent transformed early erythroid precursors that can be induced to differentiate into more mature erythroid cells by a variety of agents including dimethyl sulphoxide (DMSO). There is a latent period of 12 hours after inducer is added, when 80-90% of the cells become irreversibly committed to the differentiation programme, undergoing several rounds of cell division before permanently ceasing to replicate. After DMSO induction, a biphasic decline in steady-state levels of c-myc and c-myb messenger RNAs occurs. Following the initial decrease in c-myc mRNA expression, the subsequent increase occurs in, and is restricted to, the G1 phase of the cell cycle. We sought to determine whether the down-regulation is a necessary step in chemically induced differentiation. Experiments reported here indicate that expression in MEL cells of a transfected human c-myc gene inhibits the terminal differentiation process.

Murine myb protooncogene mRNA: cDNA sequence and evidence for 5' heterogeneity

We have sequenced two overlapping cDNA clones from a murine pro-B cell library to generate a composite sequence that includes 3413 bases of the murine c-myb mRNA. There is a single long open reading frame, beginning at the first base of this sequence, and continuing from the first methionine codon at nucleotide 265 to a TGA termination codon at nucleotide 2173. The predicted murine translation product contains 636 amino acid residues and is about 71 kDa long, which is in good agreement with the 75-kDa molecular size determined for the avian c-myb protein. The murine c-myb protein shows a striking 82% amino acid homology in the region (amino acids 71-444) where it can be compared to the published avian c-myb gene sequence. S1 nuclease protection analysis indicates extreme heterogeneity at the 5' end of steady-state murine c-myb mRNA.